Executive Summary
Lung carcinoma represents a complex spectrum of diseases characterized by extensive genomic instability and dysregulation of fundamental cellular processes, making it the leading cause of cancer-related mortality worldwide[7]. The pathophysiology of lung cancer involves intricate interactions between oncogenic driver mutations, tumor suppressor gene inactivation, and the tumor microenvironment, creating a permissive cellular context for uncontrolled proliferation, invasion, and metastasis[2]. The disease is fundamentally driven by the accumulation of genetic and epigenetic alterations that activate growth-promoting pathways while simultaneously disabling mechanisms that normally restrict cell division and promote cell death. Understanding these mechanisms has revolutionized therapeutic approaches, transitioning from non-specific cytotoxic chemotherapy toward precision medicine strategies targeting specific molecular dependencies. This report comprehensively synthesizes current knowledge of lung cancer pathophysiology, examining molecular drivers, cellular mechanisms, disease progression trajectories, and the complex interactions between malignant cells and their tissue microenvironment that collectively define this heterogeneous disease.
Classification and Epidemiology of Lung Carcinoma
Lung cancer represents a complex spectrum of diseases divided into two major histological subtypes that account for the vast majority of clinical cases[7][10]. Non-small cell lung cancer (NSCLC) comprises approximately eighty to eighty-five percent of all lung cancer cases and includes several distinct histological subtypes including adenocarcinoma, squamous cell carcinoma, and large cell carcinoma[7][10]. Adenocarcinoma, the most common NSCLC subtype, typically arises in peripheral regions of the lung from mucus-secreting epithelial cells and occurs more frequently in non-smokers and women than other histological types[10]. Squamous cell carcinoma typically develops from the flat epithelial cells lining the central airways near the main bronchi and is strongly associated with tobacco smoking history[10]. Large cell carcinoma, the least common NSCLC subtype, can arise in any lung region and tends to grow and spread more rapidly than adenocarcinoma or squamous cell carcinoma[10].
Small cell lung cancer (SCLC) accounts for approximately ten to fifteen percent of all lung cancers and represents the most aggressive form of pulmonary malignancy, characterized by rapid growth, early metastatic dissemination, and poor prognosis in the absence of treatment[7][10]. SCLC is almost universally associated with cigarette smoking and demonstrates enhanced chemosensitivity compared to NSCLC, though long-term survival remains poor in most patients[7]. A third category, lung carcinoid tumors or neuroendocrine tumors of the lung, accounts for fewer than five percent of pulmonary malignancies and generally grow more slowly than other lung cancer types[7].
The development and progression of lung cancer involve intricate signaling pathways that regulate essential cellular processes including proliferation, survival, metastasis, and resistance to therapy[2]. These pathways become disrupted through genetic mutations, epigenetic alterations, and environmental factors including tobacco smoke, air pollution, asbestos, and radon exposure[2]. Over the past decade, remarkable progress has been achieved in elucidating the molecular mechanisms underlying lung cancer pathogenesis, enabling identification of key oncogenic drivers and facilitating development of targeted therapeutic interventions that have substantially improved treatment outcomes for molecularly defined patient subsets[2].
Core Pathophysiological Mechanisms: Fundamental Driver Alterations
The EGFR Signaling Pathway as a Prototypical Oncogenic Driver
The EGFR (epidermal growth factor receptor) pathway represents one of the most extensively studied and clinically significant signaling cascades in NSCLC, functioning as the canonical example of ligand-dependent receptor tyrosine kinase dysregulation in lung cancer[2]. This pathway proves crucial for regulating essential cellular processes including proliferation, survival, differentiation, and migration in normal lung epithelium[2]. In NSCLC, specific genetic alterations in the EGFR gene, most notably exon nineteen in-frame deletions and the L858R point mutation in exon twenty-one, lead to constitutive activation of this receptor tyrosine kinase[2]. These oncogenic mutations initiate ligand-independent receptor dimerization and subsequent autophosphorylation[2], fundamentally rewiring the cellular signaling context such that EGFR-mutant cancer cells become dependent upon this single kinase for survival and proliferation—a phenomenon termed "oncogene addiction."
The recognition of EGFR mutations as driver oncogenes in NSCLC has fundamentally reshaped the therapeutic landscape, with first-generation EGFR tyrosine kinase inhibitors including gefitinib and erlotinib demonstrating notable efficacy in patients harboring these mutations and producing substantial improvements in progression-free survival compared with conventional chemotherapy[2]. However, despite these initial therapeutic successes, the development of acquired resistance to EGFR tyrosine kinase inhibitors (TKIs) represents a major clinical challenge that limits long-term benefit[2]. The most common mechanism of acquired resistance involves the T790M gatekeeper mutation in exon twenty, which increases the ATP-binding affinity of the receptor and physically impedes TKI binding[2]. Additional resistance mechanisms include amplification of the MET proto-oncogene, amplification of HER2, histological transformation into SCLC, and activation of alternative signaling pathways[2], highlighting the remarkable plasticity of EGFR-mutant lung cancer cells in response to therapeutic pressure.
Activation of the EGFR pathway initiates downstream signal transduction through key cascade pathways including the PI3K/AKT/mTOR and RAS/RAF/MEK/ERK axes, which collectively drive uncontrolled cell proliferation, suppress apoptosis, and enhance tumor viability[2]. The MAP kinase and PI3K cascades represent the two major effector pathways through which EGFR transmits growth signals, with the latter pathway particularly critical for cell survival and metabolic reprogramming in the context of EGFR mutation[2]. Understanding how these mutations trigger constitutive pathway activation has enabled rational development of increasingly selective and potent EGFR inhibitors capable of overcoming specific resistance mutations.
KRAS Mutations and the RAS/MAPK Signaling Network
KRAS, a small GTPase that functions downstream of EGFR in the growth factor signaling hierarchy, also regulates cell growth, proliferation, and survival through signaling cascades involving the MAPK and PI3K/AKT pathways[3]. Early work identified the oncogenic potential of KRAS mutations, primarily attributed to G12 missense mutations at the guanine nucleotide binding site, which hinder GTP hydrolysis and maintain the protein in a constitutively active conformation that continuously transmits growth signals[3][5]. In contrast to EGFR mutations which occur predominantly in non-smokers and are correlated with non-inflamed tumor microenvironments, KRAS mutations are associated with tobacco smoking, high mutational burden, and immunologically more active tumors[3]. KRAS and EGFR mutations are almost completely mutually exclusive, suggesting that either pathway alone suffices to drive transformation of lung epithelial cells[38].
The molecular mechanisms through which oncogenic KRAS drives lung cancer involve complex signaling networks. Oncogenic KRAS signaling via the ERK-MAPK-AP1 pathway induces expression of immunosuppressive cytokines including TGF-β1 and IL-10[3], fundamentally reshaping the immune composition of the tumor microenvironment[3]. Additionally, high PD-L1 expression is frequently observed in KRAS-mutant tumors, and studies have demonstrated that this upregulation is driven by ERK signaling, contributing to CD3+ T cell apoptosis which can be reversed by dual inhibition of p-ERK and PD-1/PD-L1[3]. KRAS mutations, especially when co-occurring with mutant TP53, correlate with response to PD-1 checkpoint blockade, increased PD-L1 expression, and higher T cell infiltration according to multi-omics studies of patient samples[3].
When comparing KRAS mutation subtypes, notably KRAS G12D tumors exhibit significantly fewer cytotoxic T cells at the tumor-stroma interface compared to non-G12D subtypes[3]. In lung cancer, KRAS-mutant tumors upregulate regulatory T cells (Tregs) through IL-6 signaling[3], establishing a immunosuppressive tissue context. Consistent with these observations, IL-6 signaling has been associated with impaired cytotoxic T cell function and resistance to atezolizumab, a programmed death ligand-1 (PD-L1) inhibitor[3]. In chemotherapy-resistant lung cancer, the presence of M2 macrophages correlates with KRAS mutation and poor survival[3], indicating that KRAS-driven tumors establish a distinctly immunosuppressive microenvironment.
TP53 Mutations and Loss of Tumor Suppression
TP53 mutations represent the most common mutations in NSCLC, and several reports have highlighted their critical role in influencing prognosis and responsiveness to EGFR-targeted therapy[6]. More than seventy percent of TP53 alterations are represented by missense mutations localized along the DNA-binding domain, resulting in different consequences at cellular, organismal, and clinical levels[6]. Gene alterations affecting TP53 are proven to be a strong prognostic factor for NSCLC[6], and recent reports indicate that TP53 mutations predict EGFR-mutated NSCLC patients' responsiveness to TKIs.
Most frequently, TP53 mutations in cancer cells occur in a single allele, while the other allele has been lost or deleted following major chromosomal rearrangements, leading to loss of heterozygosity (LOH) such that the sole mutated TP53 allele is expressed[6]. However, a substantial fraction of tumors do not present with LOH for TP53, indicating that mutations might not be the primary driver of oncogenesis but rather occur at later disease stages as one among many critical pathological events accumulating during cancer cell evolution[6]. Moreover, it has recently been reported that activation of TP53 is involved in the EGFR-signaling pathway and in the apoptosis process induced by platinum-based chemotherapy[6]. These observations suggest that some EGFR-addicted tumors possess underlying biology defined by TP53 mutations as a key modifier of phenotype and therapeutic response[6].
TP53 mutations independently associate with progression-free survival in both first-, second-, and third-generation EGFR-TKIs[6], indicating that TP53 status represents an independent prognostic factor in the context of EGFR-mutant lung cancer. The mechanisms underlying this association likely involve the critical role of TP53 in regulating apoptosis, cell cycle control, and genomic stability—processes that prove essential for response to both targeted kinase inhibition and conventional chemotherapy.
ALK Rearrangements and Constitutive Tyrosine Kinase Activation
ALK (anaplastic lymphoma kinase) rearrangements, particularly the EML4-ALK fusion, constitute a clinically distinct molecular subgroup in NSCLC, occurring in three to seven percent of cases[2]. This genetic aberration originates from a chromosomal inversion on chromosome 2p, leading to fusion of the EML4 gene with the ALK gene[2]. The resulting EML4-ALK fusion protein acts as a constitutively active tyrosine kinase that drives oncogenic signaling primarily through the MAPK/ERK and PI3K/AKT pathways, which serve as key regulators of cell proliferation, survival, and metastasis[2]. EML4-ALK fusion was more prevalent in adenocarcinoma patients with younger age, no smoking history, stage IV disease, and no EGFR mutations[31], indicating that ALK-rearranged NSCLC represents a distinct molecular disease entity with characteristic demographics and clinical presentation.
In clinical studies, EML4-ALK fusion was more prevalent in EGFR-negative patients, with the ratio of EML4-ALK fusion in adenocarcinomas without EGFR mutation being significantly higher than in all adenocarcinomas[31]. Recent findings of coexistence of EGFR and ALK mutations have been reported at low frequency, with studies reporting that approximately one percent of patients positive for EML4-ALK fusion harbor EGFR mutations, suggesting that V1 type ALK fusion genes are compatible with certain EGFR mutations[31].
The PI3K/AKT/mTOR Pathway as a Central Regulator of Survival and Growth
In addition to well-characterized signaling pathways directly activated by EGFR and KRAS, the PI3K/AKT/mTOR axis serves a central role in tumor growth and survival in lung cancer[2]. Activation of this pathway is frequently driven by mutations or amplifications in upstream regulators such as EGFR, KRAS, and PI3K itself[2]. This signaling cascade fundamentally integrates growth factor signals with nutrient availability, coordinating cell survival decisions with metabolic state. The PTEN gene is responsible for translating an enzyme found in almost every tissue in the body that, through lipid phosphatase activity, converts PIP3 to PIP2 and prevents the regulation of growth factor signals modulated by PI3K/AKT[21]. The PTEN enzyme acts as a tumor suppressor by blocking PI3K signaling and inhibiting PIP3-dependent processes such as membrane uptake and AKT activation, thereby preventing cell survival, growth, and proliferation[21].
Loss of PTEN expression in cancer cells leads to ETS2 activation, which triggers matrix metalloproteinase 9 (MMP9) and CCL3 activation that consequently affect extracellular matrix remodeling and induces metastasis[21]. Furthermore, PTEN loss increases expression of CXCR4/CXCL12 and CXCR1/CXCL8, chemokine axes related to metastasis induction in tumors[21]. The PI3K/Akt/mTOR pathway activates a range of downstream molecules that downregulate epithelial proteins such as E-cadherin while upregulating mesenchymal proteins like N-cadherin and vimentin, accounting for epithelial-mesenchymal transition (EMT) induction[21]. Changes in cancer cell metabolism and intracellular nutrient levels can also contribute to sustained activation of mTORC1[21]. Increased mTOR activity affects protein synthesis and increases cell proliferation[21], indirectly supporting tumor growth through its anti-autophagic activity and increasing translation of HIF1A, which causes angiogenesis and oxygenation normalization[21].
Key Molecular Players: Genes, Proteins, and Cellular Components
Mutational Landscape and Oncogenic Alterations
The genomic changes characterizing lung cancer occur at different levels, from mutations in single or few nucleotides to gains or losses of entire chromosomes[38]. While some mutations prove completely innocuous, many genomic events drive dramatic functional changes and involve the core of lung carcinogenesis[38]. The most frequently mutated genes in lung cancer include TP53 and KRAS, with studies identifying more than twenty-six significantly mutated genes including several tumor suppressor genes and oncogenes known to be mutated in lung cancer[20]. Mutations in TP53 and KRAS are more common than mutations in other genes, and consistently, updates of the TP53 mutation database demonstrate that in lung tumors from smokers, G to T transversions are the most prevalent mutations, followed by G to A transitions[20]. Importantly, there are significantly more G to T transversions in smokers than in non-smokers, while G to A transitions are more common in non-smokers[20].
MET Signaling and Hepatocyte Growth Factor (HGF) Axis
The MET tyrosine kinase signaling pathway is upregulated in many cancers, including lung cancer, and the pathway normally promotes mitosis, cell motility, and cell survival[1]. In cancer, however, this pathway can promote cell proliferation, invasion, metastasis, and angiogenesis[1]. The activating ligand, hepatocyte growth factor (HGF), is normally secreted by fibroblasts and smooth muscle cells[1], and can also be produced by tumor cells themselves, with moderate expression observed in forty-five percent of lung cancer tumors according to one study[1]. Among the many effects of HGF signaling are increased cell movement and blood vessel formation[1].
When HGF binds to its receptor, MET autophosphorylates the tyrosine residues Y1230/1234/1235, which are located within the activating loop of the tyrosine kinase domain[1]. This autophosphorylation activates the intrinsic kinase activity of MET, causing downstream signaling molecules to be phosphorylated[1]. Other phosphorylation sites have been identified as well—when Y1313 is phosphorylated it binds and activates phosphatidylinositol-3-kinase (PI3K), which likely promotes cell viability and motility[1]. The Y1003 site, located in the juxtamembrane domain, serves as a negative regulatory site for MET signaling that acts by recruiting c-CBL[1]. Additionally, Y1365 regulates cell morphogenesis when phosphorylated[1]. Distant metastasis is promoted by MET via activation of Grb2, PI3K, or Shc pathways[1]. Additionally, HGF and MET promote angiogenesis, which proves crucial for tumor growth[1].
Investigators observed that one hundred percent of small-cell lung cancer samples showed phosphorylation at the Y1003 site, and fifty percent were phosphorylated at Y1230/1234/1235[1]. Non-small cell samples also showed activation/phosphorylation at Y1003 and Y1230/1234/1235: adenocarcinoma (forty-four and thirty-three percent respectively), large cell (eighty-six and fifty-seven percent), squamous cell (seventy-one and zero percent), carcinoid (forty and zero percent)[1]. Importantly, there was preferential expression of activated p-MET in tumor cells located at the invasive front of non-small cell lung cancer tumor tissues[1].
NKX2-1 and Developmental Pathways in Lung Cancer
It has been postulated that genetic alterations directly interfering with transcriptional networks regulating lung development may be more common in lung cancer than previously realized[38]. Supporting this was the recent finding of amplification of the homeobox transcription factor NKX2-1 (14q13.3), which plays a master role in induction and maintenance of lung and thyroid morphogenesis and differentiation of epithelial cell lineages[38]. Gain at 14q13.3 was present in more than ten percent of lung cancer specimens and was significantly more frequent in adenocarcinomas[38].
The Tumor Microenvironment: Stromal and Immune Components
Cancer-Associated Fibroblasts and Immune Architecture
Cancer-associated fibroblasts (CAFs), serving as the "architect" of the immune microenvironment in lung cancer, play a multidimensional role in tumor progression and immune regulation[8]. CAFs significantly influence tumor progression and immunomodulation through secretion of cytokines, remodeling of the extracellular matrix (ECM), and regulation of immune cell function[8]. CAFs significantly affect immune escape and treatment resistance of tumors[8]. The fibroblasts most responsible for T-cell exclusion are shown to be FAP+ αSMA+ phenotype[11]. When fibroblasts are activated, they make the environment around the tumor nest fibrotic, with mass fraction around the nest reaching twenty to thirty percent[11]. The tumor cells become trapped behind a barrier with very high friction which prevents the nest from expanding, and the surface area of the tumor nest decreases substantially[11]. At the same time, even in situations where T-cells are efficient, the fibrotic barrier precludes T-cells from the tumor[11]. In such cases, tumor integrity is maintained and CAFs play a tumor-promoting role, inhibiting the immune response and stabilizing the tumor nest, with a fibrotic zone reaching fifty percent[11].
Tumor-Associated Macrophages and Immune Polarization
During lung cancer initiation and progression, monocytes in the bloodstream are recruited to tumor sites and differentiate into macrophages under the influence of chemokines in the tumor microenvironment[55][58]. Key chemokines such as CCL2 and CSF-1 play pivotal roles in this process, with CCL2 mediating monocyte migration from bloodstream to tumor tissue through binding to CCR2, while CSF-1 promotes monocyte survival and differentiation into macrophages via CSF1R binding[55]. Once recruited, monocyte-derived macrophages (MDMs) polarize into different functional phenotypes according to local signals[55]. In the early stages of lung cancer, MDMs predominantly polarize into M1-type macrophages, which upon stimulation by IFN-γ and LPS exhibit strong pro-inflammatory and anti-tumor activities through secretion of high levels of TNF-α and IL-12, as well as generation of reactive oxygen species and reactive nitrogen species, which directly exert toxic effects on tumor cells[55].
However, as tumors progress, anti-inflammatory cytokines including IL-4, IL-10, and TGF-β gradually dominate, leading MDMs to polarize toward the M2-type macrophage phenotype[55]. M2 macrophages suppress inflammation through secretion of IL-10 and TGF-β, which promote tumor cell survival, proliferation, and immune evasion[55]. M2-polarized macrophages secrete IL-10 and TGF-β, which significantly inhibit the activation of effector T cells and reduce cytotoxic activity[55]. M2-type TAMs are implicated in both angiogenesis and lymphangiogenesis by inducing VEGF-A and VEGF-C in tumor cells[55]. Clinically, M2-polarized TAMs have been associated with poor prognosis due to their capacity to secrete growth factors and inhibit apoptotic pathways, thereby reducing tumor susceptibility to cytotoxic therapy[55]. In cisplatin-resistant NSCLC cell lines, elevated self-renewal capacity correlates with release of macrophage migration inhibitory factor (MIF), which skews macrophages toward M2 phenotype and fosters metastatic progression[55].
Hypoxia, HIF-1α, and Vascular Microenvironment
The hypoxia-inducible factor-1α (HIF-1α) plays a key role in facilitating cellular adaptation to hypoxia, profoundly influencing the immune vascular microenvironment and immunotherapy outcomes[19]. HIF-1α-mediated tumor hypoxia drives angiogenesis, immune suppression, and extracellular matrix remodeling, creating an environment that promotes tumor progression and resistance to immunotherapies[19]. HIF-1 is involved in both intrinsic and extrinsic activation of tumor-associated inflammatory signaling[22]. The growth of solid tumors eventually outpaces oxygen and nutrient supply, leading to necrosis[22]. Hypoxic and necrotic areas of tumors produce proinflammatory mediators that recruit more immune cells, resulting in suppression of immune response at the tumor site along with tumor cell proliferation, angiogenesis, and metastasis[22].
HIF-1 emerges as a critical mediator in signaling events culminating in lymphangiogenesis and lymphatic metastasis[22]. A cooperative induction of HIF-1 and STAT3 contributes to hypoxia-mediated immunoresistance in lung cancer cells[22]. STAT3 and NF-κB activation promote chemoresistance and radioresistance in various cancer cell lines[22]. The transcription factors c-Jun and AP-1 cooperate with HIF-1 to allow fine-tuned regulation of gene expression during hypoxia[22]. HIF-1 also mediates activation of several genes in response to IGF-1 that promote cell survival and motility[22].
Hypoxia promotes acidosis because HIF-1 induces expression of NHE1, MCT4, and carbonic anhydrase IX[22]. NHE1 directly manages free intracellular H+ when the buffering capacity of intracellular proteins is exhausted[22]. Tumor cell-specific expression of MCT1 and stromal cell-specific expression of MCT4 optimizes lactate utilization and associates with cancer progression[22]. Hypoxia significantly influences immune cell behavior and function within the immune vascular microenvironment, altering their roles in anti-tumor immunity and contributing to immune evasion[19]. Macrophages exhibit high plasticity and can polarize into two functional phenotypes: pro-inflammatory, anti-tumor M1 phenotype or immunosuppressive, pro-tumor M2 phenotype[19]. HIF-1α promotes macrophage polarization toward M2 phenotype, which supports tumor growth and suppresses immune responses[19].
The function of cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells, the primary effectors of anti-tumor immunity, is severely compromised in hypoxic immune vascular microenvironments[19]. Hypoxia-induced HIF-1α upregulates PD-L1 expression on tumor cells and antigen-presenting cells, leading to T cell exhaustion—a state characterized by reduced proliferation, cytokine production, and cytotoxicity[19]. The accumulation of lactate in the hypoxic tumor environment further inhibits NK cell function, reducing interferon-gamma and cytotoxic granule production[19]. By reducing hypoxia through vascular normalization, HIF-1α activity decreases, resulting in a more favorable microenvironment for immune cell activity and conditions less conducive to tumor progression[19].
Extracellular Matrix Composition and Remodeling
The extracellular matrix composition of the central lung is dominated by fibrillar collagens (primarily types I and II), while the interstitial ECM of alveoli in distal lung consists of a relaxed network of mainly type I and II collagens and elastin[27]. Compared with normal lung tissue, primary lung tumors display significant changes in core matrisomal proteins that maintain tissue structural and mechanical features[27]. Due to their different anatomical locations, adenocarcinoma and squamous cancer cells are exposed to different extracellular matrix environments, which likely shape tumor evolution and contribute to differences in etiology[27]. While direct comparisons of ECM landscape between adenocarcinoma and squamous carcinoma with respect to peripheral and central ECM composition in healthy lung have not yet been performed, consideration of ECM composition in different lung anatomical compartments remains important for identifying shared ECM remodeling programs contributing to lung tumorigenesis[27].
The ECM is composed of fibrillar proteins including collagens and elastin, as well as macromolecules including glycoproteins, proteoglycans, and hyaluronan[30]. There is a large number of regulatory components, such as enzymes, that cross-link matrix fibers and others that degrade matrix in a strongly regulated manner[30]. The lung ECM provides structural support for cells and tissues, but is highly dynamic and regulates diverse fundamental aspects of cell biology, including cell proliferation, migration, mechanosignaling, and inflammatory responses[30]. The ECM acts as a reservoir for growth factors and chemokines, facilitating chemotactic gradients and providing reserves of factors callable when needed in cell vicinity[30]. Through these roles, the ECM proves critical for organogenesis, homeostasis, repair, and regeneration[30].
Collagen composition and presence of other integrin ligands in the lung can act as a switch between dormant and proliferative states[27]. Increased fibrillar collagen levels promote proliferation, metastasis, and outgrowth[27]. Increased type IV collagen with decreased type III and type XVIII collagens promote liver metastasis and outgrowth[27]. Fibronectin increases promote lung colonization and outgrowth along with NSCLC proliferation and invasion[27]. Tenascin-C promotes adenocarcinoma metastasis and lung colonization[27]. Periostin high expression associates with poor prognosis and supports lung colonization and outgrowth[27].
Metabolic Reprogramming: The Warburg Effect and Beyond
Aerobic Glycolysis and Glucose Metabolism
Metabolic reprogramming represents a hallmark of cancer initiation, progression, and relapse[33]. From the initial observation that cancer cells preferentially ferment glucose to lactate, termed the Warburg effect, to emerging evidence indicating that metabolic heterogeneity and mitochondrial metabolism prove important for tumor growth, the complex mechanisms driving cancer metabolism remain vastly unknown[33]. The Warburg effect comprises three main aspects: enhanced glucose uptake, increased lactate secretion, and decreased oxidative metabolism[33]. Increased aerobic glycolysis characterized by glucose uptake and lactate secretion represents the most notable effect described by Warburg[33]. This phenomenon is observed in many cancers, though the mechanisms driving this phenotype are significantly more complex[33].
Several oncogenic pathways have been implicated in upregulation of glycolysis, including MYC, PI3K-AKT-mTOR, and stabilized HIF-1/2α[33]. EGFR mutations, which occur most often in lung adenocarcinomas, play important roles in mediating global metabolic reprogramming[33]. Alterations in EGFR commonly result in the Warburg effect through stabilization of glucose transporters[33]. Signaling through the PI3K/AKT/mTOR pathway promotes glycolysis by regulating glucose transporter GLUT1 localization to the plasma membrane in EGFR-mutated NSCLC[33].
To determine whether metabolic heterogeneity relates to increases in both glycolysis and TCA cycle intermediates, a study profiling eighty NSCLC human cell lines found that the ratio of glucose utilization and lactate secretion varied greatly between samples, indicating that the Warburg effect is not universal in NSCLC[33]. In fact, NSCLC can be divided into at least glycolysis-dependent and OXPHOS-dependent subtypes[33]. KRAS activating mutations are common and mutually exclusive to EGFR mutations, with in vivo lung tumors exhibiting depleted KRAS showing reduced glycolysis and lipid gene expression, consistent with reports showing KRAS overexpression upregulates these pathways[33].
Glutamine Metabolism and Amino Acid Dependence
Glutamine dependence has been therapeutically targeted with CB-839 in lung adenocarcinoma xenografts, revealing decreased tumor growth rates[33]. Interestingly, KEAP1 loss decreases production of reactive oxygen species and enhances resistance to oxidative stress[33]. This occurs through regulation of NRF2 protein stability, a mediator of pathways including cellular stress, autophagy, proliferation, and metabolism[33]. Together, KEAP1/NRF2 coordinate to reprogram cancer cells toward pathways that support glycolysis, mitochondrial respiration, and amino acid biosynthesis[33].
Metabolic Heterogeneity and Symbiotic Relationships
The metabolic heterogeneity of tumors allows glycolytic and oxidative cells to work symbiotically through bidirectional pyruvate-to-lactate conversion[33]. In NSCLC, glycolytic cells secrete lactate through MCT4 while oxidative cells uptake lactate through MCT1[33], maintaining acidic tumor microenvironments while providing fuel for de novo amino acid, nucleotide, and fatty acid synthesis[33]. The major product of the Warburg effect, lactate, is secreted into the tumor microenvironment by rapidly proliferating tumors[33]. This serves to acidify the microenvironment region, which fuels mitochondria, suppresses the immune system, and promotes metastasis through therapy resistance[33].
Disease Progression: From Transformation to Metastasis
Epithelial-Mesenchymal Transition and Cell Cooperativity
The role of epithelial-mesenchymal transition (EMT) in metastasis remains controversial, with EMT having been postulated as an absolute requirement for tumor invasion and metastasis[15]. However, recent evidence suggests that EMT cells are responsible for degrading surrounding matrix to enable invasion and intravasation of both EMT and non-EMT cells, while only non-EMT cells that have entered the bloodstream are able to reestablish colonies in secondary sites[15]. EMT has postulated models of incomplete EMT, mesenchymal-epithelial transition (MET), and collective migration as the role of EMT in cancer invasion and metastasis[15].
The main argument against EMT playing a critical role in metastasis is that metastases appear histopathologically similar to primary tumors from which they are derived[15]. To resolve this apparent contradiction, a mesenchymal-epithelial transition (MET) process in metastatic sites has been postulated as part of metastatic tumor formation[15]. However, EMT cells specifically fail to establish lung metastases even when directly inoculated into blood stream by tail vein injection[15]. Remarkably, overt lung metastases formed from non-EMT cells when injected via tail vein, indicating that p12-induced EMT is accompanied by decreased ability to establish metastatic tumors despite enhanced migratory and local invasive phenotypes[15].
Re-expression of E-cadherin by CMV promoter-controlled CDH1 cDNA transfection changed EMT cell morphology from fibroblastoid structure to polygonal shapes, confirming MET occurred from morphology viewpoint[15]. However, E-cadherin re-expressed MET cells also failed to establish lung metastasis when directly injected into tail veins[15]. Notably, lung metastases formed when mixtures of non-EMT and EMT cells were co-inoculated subcutaneously[15]. In this experiment, non-EMT and EMT cells were labeled with DsRed and GFP respectively, enabling metastatic tumor origin determination by fluorescence and immunohistochemistry[15]. Importantly, both EMT and non-EMT cells were detected in bloodstream, indicating intravasation of both cell types occurred[15]. Since non-EMT cell inoculation alone failed to intravasate, this result demonstrated cooperation between non-EMT and EMT cells in local invasion and intravasation processes[15].
In the EMT mechanism, microRNAs including miR-200 family, miR-27a, miR-95-3p, miR-195, and miR-133 may associate with lung metastasis[18]. Tumor-associated macrophages, major components of tumor microenvironment associated with metastasis, induce EMT for cancer cell migration and circulating tumor cell-mediated metastasis[18]. Understanding lung metastasis with EMT mechanism and RNA perspectives would lead to comprehensive cancer therapy[18].
Circulating Tumor Cells and Dissemination
Circulating tumor cells (CTCs) are tumor cells shed from primary sites that circulate in peripheral blood[43][46]. Recent studies have shown CTCs can serve as useful clinical markers in some solid tumors[43]. In primary lung cancer, clinical significance of CTCs remains unclear, but promising results have been recently reported[43]. Approximately forty percent of primary lung cancer patients initially present with distant metastases detectable with current diagnostic modalities including whole-body CT and PET scanning[43]. More importantly, in patients without clinically detectable distant metastasis at initial diagnosis, distant metastases frequently develop during or after treatment[43]. Tumor cells likely circulate in peripheral blood of most primary lung cancer patients, which may cause clinically apparent distant metastasis development[43].
CTCs demonstrate heterogeneity, disseminate early, and could represent only minor primary tumor subpopulations responsible for disease relapse[46]. Genotypic and phenotypic characterization of CTCs at single-cell level has shown heterogenous dissemination occurring early in tumor evolution[46]. While molecular profiling of CTCs has not yet translated clinically, CTC enumeration has been widely used as prognostic biomarker for monitoring treatment response and predicting disease relapse[46]. CTCs can travel as single cells or as cell aggregates called CTC clusters or circulating tumor microemboli, which have been reported for several cancer types[46]. Although detected at lower frequency with significantly shorter blood half-lives than single CTCs[46], CTC clusters prove more likely to form metastasis in mouse models[46]. CTC clusters can include non-tumor cell types including pericytes, immune cells, platelets, and cancer-associated fibroblasts[46], which may support clustered CTC survival[46].
Neuroendocrine Transformation and Histological Plasticity
One mechanism underlying acquired resistance to EGFR tyrosine kinase inhibitors is histological transformation from NSCLC to small cell lung cancer (SCLC)[4][25]. Interestingly, TP53/RB1 mutations alone are insufficient to induce one hundred percent transformation to SCLC[4], suggesting that additional molecular alterations or selective pressures, including therapy, microenvironment, or hypoxia, may play roles[4]. It is hypothesized that similar mechanisms may involve T-SCLC[4]. Notably, when RB1/TP53 deletion was combined with PTEN deletion, PTEN deletion enhanced neuroendocrine differentiation (NEtD), possibly via SOX2 upregulation[4]. Further exploration of mechanistic insights into RB1/TP53 loss in T-SCLC will help elucidate biology and ontogeny, potentially leading to identification of new therapeutic targets[4].
Several studies identified the PI3K/AKT/mTOR pathway as a key player in transformation of EGFR-mutation NSCLC to SCLC[4]. The PI3K/AKT/mTOR signaling pathway serves as major promoter of cell growth, survival, and invasion in various cancer types and has been implicated in SCLC phenotypic transformation and chemoresistance[4]. Activating mutations in PI3K/AKT signaling have been found at high frequency in T-SCLC, suggesting importance in transformation[4]. Cancer-associated fibroblast-derived exosome microRNA-20a has been shown to promote NSCLC progression and chemoresistance by inhibiting PTEN/PI3K-AKT pathway[4].
MYC expression directly promotes pulmonary neuroendocrine cell development into SCLC, while alveolar type II cells require both PTEN deletion and MYC expression for transformation[4]. These results suggest SCLC transformation and rapid disease progression may be driven by TP53/RB1 mutations combined with MYC amplification, commonly observed in adenocarcinomas prior to transformation[4]. MYC amplification also appears in T-SCLC and neuroendocrine-like tumors, underscoring MYC's critical role in transformation early stages[4].
Cell division cycle 7 (CDC7), a serine-threonine kinase, has recently been identified as an oncogenic driver of neuroendocrine transformation induced by targeted therapy in lung and prostate cancers[4]. Mechanistically, CDC7 upregulates early in NE transformation, with TP53 and E2F1 possibly directly regulating CDC7 transcription by binding gene promoters[4]. Therefore, CDC7 represents a promising therapeutic target for controlling lineage plasticity and neuroendocrine differentiation[4]. Its upregulation contributes to acquired resistance to targeted therapies[4].
While EZH2 expression does not significantly differ in large-cell carcinomas, lung adenocarcinomas, and squamous-cell carcinomas, it is markedly elevated in SCLC[4]. In SCLC, EZH2 has been shown to inhibit lineage-specific transformation by suppressing lineage-specific factors[4]. Overexpression of G9a has been observed in various cancers, with inhibitors targeting G9a being developed as potential therapeutic drugs[4]. Recently, Yang and colleagues identified G9a as an epigenetic driver of lung adenocarcinoma-to-SCLC transformation[4]. Mechanistically, EHMT2 increases methylation of SFRP1 promoter region, reducing SFRP1 expression, which activates Wnt/β-catenin pathway and triggers TKI-mediated neuroendocrine transformation[4].
Overall, these findings suggest that epigenetic modifications, particularly histone methylation, demethylation, and acetylation, are key players in neuroendocrine transformation regulation in lung adenocarcinomas and SCLC[4]. Targeting these pathways may provide valuable therapeutic opportunities for overcoming resistance and controlling histological plasticity[4].
Therapeutic Resistance: Mechanisms and Adaptive Responses
Multidrug Resistance and ABC Transporters
Multidrug resistance (MDR) represents a serious problem hampering cancer pharmacotherapy success[39]. A common mechanism involves overexpression of ATP-binding cassette (ABC) efflux transporters in cancer cells such as P-glycoprotein (P-gp/ABCB1), multidrug resistance-associated protein 1 (MRP1/ABCC1), and breast cancer resistance protein (BCRP/ABCG2) that limit anticancer drug exposure[39]. One approach to overcome MDR involves developing ABC efflux transporter inhibitors to sensitize cancer cells to chemotherapeutic drugs[39]. Complete clinical trials thus far have shown tested chemosensitizers only add limited or no benefits to cancer patients[39]. Some MDR modulators prove merely toxic, while others induce unwanted drug-drug interactions[39].
Importantly, many ABC transporters are also abundantly expressed in gastrointestinal tract, liver, kidney, brain, and other normal tissues, and they largely determine drug absorption, distribution, and excretion, affecting overall pharmacokinetic properties in humans[39]. ABC transporters such as P-gp, MRP1, and BCRP co-expressed in tumors show broad and overlapped substrate specificity[39]. Thus reliable preclinical assays and models are required for transporter-mediated flux assessment and potential pharmacokinetic effects[39].
Acquired or secondary MDR happens in survived tumor cells developing drug resistance capacity during pharmacotherapy[39]. Multiple mechanisms have been identified for MDR, classified as drag-dependent, target-dependent, and drag/target-independent[39]. Drag-dependent MDR results from cellular drug disposition alteration, particularly efflux transporter and drug-metabolizing enzyme overexpression[39]. Target-dependent MDR attributes to desensitization of drug targeting, including mutation, translocation, deletion, and target amplification[39]. Drag- or target-independent MDR results from drug targeting escape through genetic or epigenetic cell signaling pathway alterations[39].
Glycolysis and Lactylation in Chemoresistance
Glycolysis and lactylation exert synergistic effects, promoting chemoresistance development and creating positive feedback loops that continue resistance[36]. Glucose uptake mediated by facilitative glucose transporters (GLUTs) serves as foundational step in glucose metabolism and represents rate-limiting factor[36]. GLUTs, encoded by SLC2A genes, are dysregulated in various cancer types[36]. GLUT1 and GLUT3 dysregulation directly contributes to chemoresistance development[36]. Notably, IL-3 secretion in YAP1-overexpressing and 5-FU-resistant gastric cancer cells induces tumor-associated macrophages to undergo M2-type polarization, promoting GLUT3 expression and activating glycolysis[36]. The activated M2-type tumor-associated macrophages secrete increased CCL8 levels, ultimately inducing 5-FU resistance via JAK/STAT signaling[36].
GEM resistance commonly occurs in pancreatic cancer, potentially due to LDHA upregulation mediated by family with sequence similarity 83 (Fam83)[36]. Furthermore, LDHA expression stimulation by nuclear glucose-regulated protein (GRP78)-HIF-1α complex under glucose deprivation confers GEM resistance[36]. Additionally, post-transcriptional modifications and post-translational modifications (PTMs) of LDHA regulate glycolysis and drug resistance[36]. Methyltransferase 3 (METTL3) and insulin-like growth factor 2 mRNA-binding protein 2 (IGF2BP2) mediate m6A modification by activating LDHA transcription and translation, increasing LDHA-mediated chemoresistance[36].
Glycolysis is inextricably linked to lactylation modifications, providing substrate for protein lactylation by lactate production[36]. In turn, lactylation affects tumor metabolism and gene expression by altering protein function, playing critical roles in cancer chemoresistance[36]. Recent studies increasingly recognize that glycolysis and lactylation modifications synergistically promote chemoresistance[36]. Lung cancer-derived brain metastases exhibit pemetrexed resistance, with glycolysis and lactylation playing key roles[36]. Mechanistically, Aldo-keto reductase family 1 B10 (AKR1B10) elevates glycolytic levels by increasing LDHA expression, leading to lactate accumulation, which activates cyclin B1 (CCNB1) transcription through histone H4 K12 lactylation[36].
Autophagy and Apoptosis Regulation
Autophagy represents a catabolic cellular mechanism involving lysosomal degradation of unwanted cellular components[13]. Interaction between Beclin-1 and Bcl-2 proteins plays critical roles in autophagy initiation[13]. Malignantly transformed lung epithelial cells are resistant to autophagy and express lower basal autophagic protein levels, Beclin-1 and LC3-II, compared to non-tumorigenic cells[13]. No-mediated S-nitrosylation of Bcl-2 represents an important autophagy resistance mechanism[13], critical in driving cellular tumorigenesis[13]. Malignantly transformed lung epithelial cells are inherently resistant to both autophagy and apoptosis compared to non-tumorigenic lung cells[13]. NO-mediated Bcl-2 S-nitrosylation proved important for cellular processes due to its Bcl-2-Beclin-1 complex effects and subsequent autophagic process effects[13].
This autophagy resistance is significant because malignantly transformed cells express higher basal Bcl-2 levels compared to non-tumorigenic lung cells[13]. Overall, this study reveals a novel mechanism by which NO affects proteins downstream of JNK1 pathway in autophagy, first demonstrating Bcl-2 S-nitrosylation roles in autophagy initiation resistance[13]. The intrinsic apoptotic pathway, also known as the mitochondrial pathway, initiates through mitochondrial outer membrane permeabilization (MOMP) mediated by Bcl-2 protein family under DNA damage, energy starvation, and hypoxia[16]. The Bcl-2 family, pivotal in endogenous apoptosis regulation, categorizes based on Bcl-2 homology (BH) domain numbers[16].
Beclin-1, also a BH3-only protein, regulates both autophagy and apoptosis through Bcl-2 anti-apoptotic protein binding[16]. Bcl-2, sharing BH3 domains with Beclin-1, can bind via this domain, inhibiting autophagy when complexed[16]. Autophagy induction occurs when Bax competitively binds Bcl-2, dissociating Beclin-1[16]. Additionally, stressors like nutrient deficiency can activate c-Jun N-terminal Kinase 1 (JNK1), leading to Bcl-2 phosphorylation and subsequent autophagy promotion[16].
Signaling Pathways and Gene Regulatory Networks
Notch Signaling and Cell Fate Determination
Notch signaling regulates cell specification and homeostasis of stem cell compartments, and is counteracted by cell fate determinant Numb[45]. Notch signaling alteration occurs in approximately one-third of non-small-cell lung carcinomas: in approximately thirty percent of NSCLCs, Numb expression loss leads to increased Notch activity, while in smaller case fractions (around ten percent), NOTCH-1 gain-of-function mutations occur[45]. Notch activation correlates with poor clinical outcomes in NSCLC patients without TP53 mutations[45]. Primary epithelial cell cultures derived from NSCLC harboring constitutive Notch pathway activation are selectively killed by Notch inhibitors (γ-secretase inhibitors), showing these tumors' proliferative advantages depend upon Notch signaling[45].
The deregulation of Notch pathway represents relatively frequent events in NSCLCs, suggesting possible targeted molecular therapy targets[45]. NSCLCs harboring Notch pathway activation depend upon Notch signaling for growth potential[45]. Targeted Notch activation interference represents promising therapeutic avenues in NSCLC[45]. Notch activity remains significant in transition from developmental lung formation to participation in lung plasticity and repair[48]. Notch signaling revitalization has been linked to lung cancer development and progression, including SCLC and NSCLC[48]. Notch1 has been implicated in NSCLC onset and development and may prove useful in disease progression assessment[48].
SCLC, accounting for roughly fifteen percent of all lung cancers, is characterized by highly aggressive, poorly differentiated features[48]. Notch mutation prevalence in SCLC has been estimated at twenty-five to twenty-eight percent, resulting in Notch signaling function loss[48]. In SCLC, transcription factors ASCL1 and NEUROD1 play crucial roles in promoting malignant behavior and survival[28]. ASCL1 but not NEUROD1 is present in mouse pulmonary neuroendocrine cells and only ASCL1 is required in vivo for tumor formation in mouse SCLC models[28]. ASCL1 targets oncogenic genes including MYCL1, RET, SOX2, and NFIB, while NEUROD1 targets MYC[28].
Wnt/β-Catenin Signaling in Proliferation and Stemness
The Wnt/β-catenin pathway is abnormally activated in most lung cancer tissues and considered an accelerator of carcinogenesis and lung cancer progression[50]. The β-catenin-dependent pathway consists of three major components: Wnt signaling transduction at membranes, β-catenin stability modulation in cytoplasm, and Wnt target gene overactivation in nucleus[50]. Ligand-related Wnt/β-catenin signaling pathway activation is induced through Wnt ligand binding to co-receptor complexes including transmembrane protein frizzled (Fzd) and low-density lipoprotein receptor-related protein (LRP) 5/6[50]. β-catenin attaches to T-cell factors/lymphoid-enhancer factors (TCFs/LEFs) in nucleus and elicits Wnt target gene transcription, including c-myc, Cyclin D1 (CCND1), and survivin, regulating cell proliferation, transdifferentiation, apoptosis, and other life processes[50].
Wnt/β-catenin signaling is thought necessary for maintaining CSC phenotypes[50]. In CSCs, β-catenin-dependent Wnt signaling is extremely active and maintains putative lung CSC marker expression including CD44 and CD133[50]. Aberrant Wnt/β-catenin activation maintains lung cancer stem cell characteristics and promotes progression[50]. Control of cell cycles by Wnt signaling is responsible for most mechanisms promoting proliferation during cancer[50]. Wnt signaling involves lung cancer cell cycle regulation at multiple mechanistic layers[50]. Two key cycle regulators, Myc and CCND1, are β-catenin/TCF transcription complex direct target genes[50].
Lung cancer cells are tightly regulated by multiple negative regulators and avoiding these growth suppressors is necessary for tumorigenesis and facilitates progression[50]. Wnt signaling accelerates cycle progression by reducing many key cyclin-dependent kinase inhibitor (CDKN) expression[50]. Nuclear β-catenin accumulation in NSCLC accompanies increased cell proliferation[50]. β-catenin expression was inversely correlated with CDKN1B (p27KIP), and β-catenin activity dysregulation was shown to induce p53-dependent growth arrest in lung cancer cells[50]. Additionally, cancer cells acquire characteristic apoptosis resistance, preventing cell death and maintaining uncontrolled proliferation[50].
Hypoxia in lung adenocarcinoma cells can promote Wnt signaling through β-catenin stabilization and nuclear localization alteration[50]. Meanwhile, hypoxia-inducible factor overexpression increases β-catenin content and enhances lung cancer cell resistance to chronic hypoxia-induced stress[50]. VEGF, a major tumor microvasculature mediator, closely links to progression, metastasis, and recurrence of NSCLC[50]. Previous studies reported positive correlation between β-catenin and VEGF protein expression in NSCLC patients[50].
JAK/STAT3 Pathway and Immune Suppression
JAK1 is responsible for STAT3 activation in lung cancer cells[57]. Pronounced STAT3 activation occurs in cells with activating EGFR mutations yet EGFR activity inhibition had no effect on STAT3 activation[57]. JAK1 inhibition with small molecules or RNA interference resulted in STAT3 tyrosine phosphorylation loss and cell growth inhibition[57]. An interleukin-6 neutralizing antibody, siltuximab (CNTO 328), could inhibit STAT3 tyrosine phosphorylation in cell-dependent manner[57]. These results suggest JAK1 drives STAT3 activation in lung cancer cells, with indirect JAK1-STAT3 attacks using IL-6 neutralizing antibodies with or without EGFR inhibition capable of inhibiting lung cancer growth[57].
In canonical pathways, JAK-STAT signaling starts with cytokine (e.g., IL-6) binding to specific cell-surface receptors (e.g., gp130), leading receptor-associated JAK protein tyrosine phosphorylation[60]. Phosphorylated (activated) JAK kinase cross-phosphorylates cytoplasmic receptor tail tyrosine residues, recruiting latent STAT protein and resulting single STAT C-terminus phosphorylation around amino acid seven-hundred (Tyr705 in STAT3) by JAK[60]. STAT3 enables cancer cells to resist radiation and cytotoxic drugs through extrinsic and intrinsic apoptotic pathway inactivation[60]. STAT3 activation causes Fas down-regulation, a major extracellular apoptotic ligand[60]. STAT3 jointly acts with noncanonical NF-κB signaling pathways to increase NF-κB p100 proteolytic processing to NF-κB p50, an anti-apoptotic protein[60].
Regarding intrinsic apoptosis, STAT3 promotes Mcl-1 and Bcl-xL expression, Bcl-2 proteins preventing cytochrome c mitochondrial release[60]. STAT3 also enhances survivin levels, directly binding pro-apoptotic caspase 9 protein and inhibiting its activity in multiple cancers[60]. In several NSCLC cell lines, STAT3 activity blocking induces apoptosis[60]. STAT3 has been shown to prolong human PC-13 large-cell lung carcinoma cell survival upon serum withdrawal[60]. Additionally, in HeLa cervical cancer cells, STAT3 binds Skp2 promoters, an cell-cycle factor, and induces expression[60].
STAT3 promotes immune-suppressive gene expression in cancer cells whose products (e.g., IL-10) not only inhibit dendritic cell maturation but also induce dendritic cells and other immune cells' STAT3 signaling, including T cells and innate immune response mediating cells[60]. Through this mechanism, STAT3 maintains stable feed-forward loops between tumor cells and tumor-interacting immune cells[60]. STAT3 signaling significantly contributes to lung cancer antitumor immunity evasion[60]. Increased macrophages, lymphocytes, and neutrophils surround lung tumors in mice carrying epithelial cell-specific STAT3 deletion[60]. Additionally, antitumor inflammatory mediator levels including TNF-α and IFN-γ are enhanced in STAT3-deleted lung tumors[60].
Molecular and Cellular Pathways Integration
NF-κB Signaling and Inflammation-Driven Oncogenesis
The inappropriate induction or constitutive activation of protective responses in mutated or damaged cells appears to be major transformation and proliferation factors[40]. Two nuclear transcription factors involved in mediating cellular responses to oxidative cell stress and proinflammatory stimuli are activator protein-1 (AP-1) and nuclear factor-κB (NF-κB)[40]. These transcription factors' roles on cancer initiation and progression have been studied in cell culture and in vivo models[40]. AP-1 components c-Jun and c-Fos activation by JNK and by ERK1/2 or p38 MAPK, respectively, involves malignant cell transformation stimulated by tumor promoter phorbol 12-myristate 13-acetate (PMA)[40]. The proinflammatory and antiapoptotic response to tumor promotion is primarily mediated through NF-κB activation by IKK family serine/threonine kinases[40].
Many diverse stimuli utilize intracellular signaling pathways to activate NF-κB, a nuclear transcription factor regulating proinflammatory and cell survival pathways[40]. These stimuli include tumor necrosis factor (TNF-α), PMA and other tumor promoters, cigarette smoke extract (CSE), lipopolysaccharide (LPS), oxidants, and pathogenic bacteria[40]. The NF-κB family comprises five members: p50 (NF-κB1), p52 (NF-κB2), RelA (p65), RelB, and c-Rel[40]. p50 and p52 are cleaved from inactive precursor proteins, p105 and p100, respectively, prior to nuclear translocation[40]. The most widely studied NF-κB heterodimers are p50/p65 and p50/c-Rel (both associated with classical or canonical pathways) and p52/RelB (alternative pathway)[40]. The classical pathway activates inflammatory cytokines, bacterial and viral infections, and oxidative stimuli, inducing gene expression responsible for NF-κB antiapoptotic actions[40].
Cytoplasmic NF-κB is sequestered as an inactive complex with regulatory subunit IκB[40]. The most abundant IκB family member is IκBα[40]. Phosphorylation of two N-terminal domain conserved serine residues in NF-κB/IκB complex induces rapid dissociation and polyubiquitination of IκB followed by proteasome degradation[40]. Post-translational modifications dynamically shape NF-κB's role in cancer immunity, promoting macrophage polarization, controlling dendritic cell antigen presentation, regulating T cell exhaustion, and sustaining immunosuppressive networks within tumor microenvironments[37]. RelA phosphorylation at S276 enhances NF-κB transcriptional activity and promotes NF-κB-dependent IL-6 and IL-8 cytokine expression[37]. Phosphorylation at S536 activates canonical NF-κB pathways and mediates cancer cell malignant proliferation[37].
Environmental Carcinogens and Molecular Mutagenesis
Tobacco Smoke-Induced Carcinogenesis
Lung carcinogenesis occurs through a mechanistic framework whereby cigarette smoking causes lung cancer, focusing on genotoxic carcinogens and their DNA interactions[20]. Lung tumor multiplicity in NNK-treated mice was eighteen-point-four ± four-point-five and was not affected by nicotine given before, after, or concurrently with NNK[20]. Nicotine treatment did not alter tumor multiplicity or size, and similar results were obtained in mutant Kras-mediated lung tumor models[20]. This association likely results from smoking behavior increasing nicotine and lung carcinogen NNK uptake, as determined through nicotine metabolite and NNK metabolite (total NNAL) urine quantitation[20]. The association between increased NNK uptake levels and higher lung cancer risk proves completely plausible based on NNK's powerful genotoxic carcinogenicity for lung observed in studies with mice, rats, hamsters, and ferrets[20].
Metabolically activated polycyclic aromatic hydrocarbons (PAHs) bind DNA and cause TP53 and KRAS mutations[20]. NNK, also a genotoxic carcinogen, enzymatically converts in the body to highly electrophilic DNA-binding diazohydroxides and related intermediates causing KRAS mutations[20]. NNK readily and selectively induces lung tumors in all species tested, regardless of administration route, frequently at low doses[20]. Lung tumor induction by PAHs and NNK occurs smoothly in standard laboratory animal models without unrealistic doses or genetic modification needs[20].
Phenol and catechol, major smoke fraction constituents, do not possess significant tumor promoting activity when tested on mouse skin[20]. However, catechol proves potent co-carcinogen with classic PAH carcinogen benzo(a)pyrene (BaP) and its proximate carcinogen BaP-7,8-diol when tested on mouse skin[20]. Cigarette smoke condensate fractions enriched in catechol show similar activity[20]. Smoking increased five PAHs concentration including benzo(a)pyrene, which increased approximately twofold[23]. The risk for increasing carcinogenic PAHs (odds ratio, eight-point-twenty) occurred[23].
Greenman and colleagues reported on more than five-hundred protein kinase gene coding exon mutations in over two-hundred diverse human cancers including lung cancer[20]. Somatic point mutation numbers, including driver mutations, varied widely within and between cancer types[20]. Lung cancers were among those with highest somatic mutation numbers (four-point-twenty-one per megabase), attributed to recurrent exogenous mutagen exposure[20]. Twenty-six significantly mutated genes were identified, including several known lung cancer tumor suppressor genes and oncogenes: TP53, KRAS, CDKN2A, STK11, and others[20]. The greatest mutation numbers were found in TP53 and KRAS[20].
In lung tumors from smokers, G-to-T transversions represent the most prevalent mutations, followed by G-to-A transitions[20]. Significantly more G-to-T transversions occur in smokers than non-smokers, while G-to-A transitions are more common in non-smokers[20]. Hotspots for TP53 mutation in tobacco smoke-associated lung cancers occur at codons one-fifty-seven, one-fifty-eight, two-forty-five, two-forty-eight, two-forty-nine, and two-seventy-three[20].
Disease Phenotypes and Clinical Manifestations
Heterogeneity of Lung Cancer Subtypes
Each lung cancer type proves different, and knowing specific lung cancer type determines available treatment options depending on tumor type, size, stage, and overall health[7]. Lung carcinoid tumors, sometimes called lung neuroendocrine tumors, are uncommon and tend to grow slower than other lung cancer types[7]. These tumors comprise special cell types called neuroendocrine cells, specialized cells found throughout the body featuring both nerve and hormone-producing cell features[7]. Lung neuroendocrine tumors are usually classified as typical or atypical carcinoids, sometimes called carcinoid tumors[7]. Carcinoids are very rare, slow-growing, and most commonly treated with surgery[7].
Pancoast tumors, also known as superior sulcus tumors, grow in upper lung parts and interfere with surrounding structures[7]. These tumors are rare and most always NSCLC[7]. However, they can be tumors from other diseases like lymphoma or tuberculosis[7]. Pancoast tumors may be treated with chemotherapy, radiation and/or surgery[7].
Conclusion: Integrated Understanding of Lung Cancer Pathophysiology
Lung carcinoma represents a multifaceted disease characterized by the accumulation and integration of multiple pathophysiological alterations that collectively drive transformation, progression, and therapeutic resistance. The disease fundamentally arises through dysregulation of core signaling pathways including EGFR, KRAS, ALK, and PI3K/AKT/mTOR, which normally restrict cell proliferation and promote cell death, instead becoming aberrantly activated to sustain uncontrolled growth[2][3]. Simultaneously, tumor suppressor genes including TP53 and RB1 become inactivated, eliminating critical safeguards against cellular transformation[6][17]. These molecular alterations interact with complex tissue microenvironments composed of cancer-associated fibroblasts, tumor-associated macrophages, T cells, and extracellular matrix components that collectively create permissive conditions for malignant progression[8][11][55].
The metabolic reprogramming of lung cancer cells, characterized by increased aerobic glycolysis and lactate production, establishes hypoxic microenvironments that further promote angiogenesis, immune suppression, and metastatic dissemination[33][19]. Heterogeneous populations of cancer stem cells within tumors maintain capacity for indefinite self-renewal while disseminating early via circulating tumor cells that can establish distant metastases[32][43][46]. Environmental exposures including tobacco smoke directly cause DNA damage through polycyclic aromatic hydrocarbons and nitrosamines, driving accumulation of driving mutations in KRAS and TP53[20]. The remarkable plasticity of lung cancer cells enables histological transformation to aggressive neuroendocrine variants upon therapeutic pressure, representing an adaptive response to targeted kinase inhibition[4][25].
Understanding lung cancer pathophysiology at this comprehensive level has enabled rational development of targeted therapeutics addressing specific molecular dependencies, fundamentally improving outcomes for patients with molecularly defined tumors[2]. However, the continued emergence of therapeutic resistance mechanisms, the heterogeneity within individual tumors, and the critical roles of stromal and immune components in dictating treatment responses highlight that effective future strategies must simultaneously target multiple interconnected pathways while modulating the hostile tumor microenvironment. Continued advances in understanding these complex mechanisms will be essential for developing combination strategies capable of achieving durable remissions in lung cancer patients across diverse molecular subtypes.