2.1 Cancer Biology, Pathophysiology, & Carcinogenesis

2.1 Cancer Biology, Pathophysiology, & Carcinogenesis

Understanding the cellular, bioenergetic, and genetic alterations that define malignant transformation is foundational for the Board Certified Specialist in Oncology Nutrition (CSO). Cancer is not a single entity, but an expansive collection of over 200 distinct pathological disorders characterized by uncoordinated, autonomous cellular proliferation, evasion of programmed cell death (apoptosis), metabolic reprogramming, sustained angiogenesis, local tissue invasion, and distant metastatic spread.

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Multistage Carcinogenesis

The malignant transformation of a normal human somatic cell into an invasive neoplasm is a complex, multi-step process occurring over years to decades. Classically, carcinogenesis is categorized into three distinct, sequential phases: initiation, promotion, and progression.

+-----------------------+     +-----------------------+     +-----------------------+
|      INITIATION       | --> |       PROMOTION       | --> |      PROGRESSION      |
| Irreversible DNA damage|     | Clonal expansion of   |     | Karyotypic instability|
| Carcinogen exposure   |     | initiated cells       |     | Invasion & metastasis |
+-----------------------+     +-----------------------+     +-----------------------+

1. Initiation

• Mechanism: Somatic cell DNA is exposed to an exogenous or endogenous carcinogen (e.g., ionizing radiation, tobacco smoke nitrosamines, aflatoxin B1, ultraviolet radiation, reactive oxygen species), resulting in non-lethal, structural single- or double-strand DNA damage.
• Cellular Event: If endogenous DNA repair mechanisms—such as nucleotide excision repair, base excision repair, or homologous recombination—fail to rectify the genetic insult and the cell evades p53-mediated apoptosis, the permanent mutation becomes fixed into the host genome during subsequent mitotic S-phase replication.
• Reversibility: Irreversible. The initiated cell remains morphologically indistinguishable from surrounding healthy tissue but possesses altered responsiveness to growth-regulatory signals.

2. Promotion

• Mechanism: Prolonged, repeated exposure to promoting agents (e.g., chronic tissue inflammation, hyperinsulinemia, elevated bile acid concentrations, chronic alcohol consumption, phorbol esters) stimulates the selective clonal expansion of initiated cells.
• Cellular Event: Promoters do not directly mutate DNA structure; rather, they alter gene expression, activate mitogenic intracellular signaling cascades (such as MAPK/ERK and PI3K/Akt pathways), and suppress contact inhibition of cell growth.
• Reversibility: Potentially reversible in its early stages if the promoting stimulus is permanently removed (e.g., cessation of tobacco or alcohol use, resolution of chronic gastrointestinal inflammation like inflammatory bowel disease or Helicobacter pylori infection).

3. Progression

• Mechanism: Accumulated secondary, tertiary, and quaternary genetic and epigenetic alterations lead to profound chromosomal and karyotypic instability.
• Cellular Event: Tumor cells acquire the classical "hallmarks of cancer," including autonomous growth signaling, insensitivity to anti-growth signals, evasion of apoptosis, unlimited replicative potential (telomerase reactivation), sustained neo-angiogenesis, tissue invasion, and distant lymphatic or hematogenous metastasis.
• Reversibility: Irreversible. The established tumor displays extensive intra-tumoral genetic heterogeneity and autonomous growth independent of external growth factors.

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Tumor Metabolism & The Warburg Effect

Normal differentiated human somatic cells generate adenosine triphosphate (ATP) primarily through mitochondrial oxidative phosphorylation (OxPhos) under aerobic conditions, efficiently producing 36 to 38 molecules of ATP per molecule of oxidized glucose. Under anaerobic conditions, normal cells temporarily divert pyruvate to lactate via anaerobic glycolysis, yielding a net of only 2 ATP per glucose molecule.

In 1924, Nobel laureate Otto Warburg observed a fundamental bioenergetic paradigm shift in cancer cells: even in the presence of abundant ambient oxygen, cancer cells preferentially convert glucose to lactate. This metabolic hallmark is termed aerobic glycolysis or the Warburg Effect.

Evolutionary Advantages of Aerobic Glycolysis

Although producing only 2 ATP per glucose molecule appears bioenergetically inefficient, aerobic glycolysis confers critical evolutionary advantages upon rapidly proliferating malignant cells:

1. Macromolecular Biosynthesis: High glycolytic flux shunts carbon intermediates into side-branch biosynthetic pathways. Glucose-6-phosphate enters the pentose phosphate pathway (PPP) to generate ribose-5-phosphate (essential for nucleotide synthesis) and NADPH (for redox balance). 3-phosphoglycerate and pyruvate provide carbon skeletons for non-essential amino acids and acetyl-CoA required for de novo fatty acid synthesis for plasma membrane construction.
2. Microenvironment Acidification: Rapid extrusion of lactic acid and protons via monocarboxylate transporters (MCT1 and MCT4) and sodium-hydrogen exchangers (NHE1) acidifies the extracellular tumor microenvironment to a pH of 6.2 to 6.8. This localized extracellular acidity suppresses cytotoxic T-lymphocyte and natural killer (NK) cell activation, induces tumor-infiltrating macrophage polarization, degrades extracellular matrix (ECM) proteins via activated matrix metalloproteinases (MMP-2 and MMP-9), and promotes local tumor invasion and metastasis.
3. FDG-PET Diagnostic Imaging Rationale: Up-regulation of high-affinity glucose transporters (GLUT1, GLUT3) and hexokinase II forms the biochemical basis of 18F-fluorodeoxyglucose Positron Emission Tomography (FDG-PET). Cancer cells rapidly transport FDG across the cell membrane and phosphorylate it into 18F-FDG-6-phosphate. Because FDG-6-phosphate cannot be further metabolized by glucose-6-phosphate isomerase, it becomes trapped intracellularly, allowing precise radiographic visualization of primary tumors and distant metastases.

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Genetic Drivers: Oncogenes vs. Tumor Suppressor Genes

Malignant transformation is governed by genetic mutations occurring in two principal functional classes of genes: proto-oncogenes and tumor suppressor genes.

Proto-Oncogenes & Oncogenes

Proto-oncogenes encode normal cellular proteins that regulate growth factor signaling, intracellular signal transduction, gene transcription, and cell cycle progression. When mutated, amplified, or chromosomally translocated, proto-oncogenes become hyperactive oncogenes.

• Genetic Model: Gain-of-function mutation. Dominant pattern—a mutation in a single allele ("one hit") is sufficient to drive uncontrolled cell proliferation.
• Key Clinical Examples:
  • KRAS: Encodes a small GTPase protein acting downstream of the Epidermal Growth Factor Receptor (EGFR). Single point mutations in KRAS (codon 12, 13, or 61) lock the protein in an active GTP-bound conformation, continuously activating downstream RAF-MEK-ERK proliferative cascades independent of extracellular growth factors. Point mutations occur in >90% of pancreatic ductal adenocarcinomas and ~40% of colorectal carcinomas.
  • HER2/neu (ERBB2): Receptor tyrosine kinase gene amplification observed in 15–20% of invasive breast carcinomas and gastric adenocarcinomas, driving aggressive cell proliferation and signaling.
  • MYC: Transcription factor oncogene frequently overexpressed or translocated (e.g., t(8;14) in Burkitt lymphoma), driving global expression of genes involved in cell growth and Warburg metabolism.

Tumor Suppressor Genes

Tumor suppressor genes encode proteins that enforce cell cycle DNA damage checkpoints, initiate DNA repair, suppress inappropriate mitogenic signaling, and trigger programmed cell death when DNA damage is irreversible.

• Genetic Model: Loss-of-function mutation. Recessive pattern—operates under Knudson's two-hit hypothesis, requiring inactivation, deletion, or epigenetic silencing of both paternal and maternal alleles to abolish tumor suppressive capacity.
• Key Clinical Examples:
  • TP53 ("Guardian of the Genome"): Encodes the transcription factor p53. In response to DNA double-strand breaks, hypoxia, or oncogenic stress, p53 arrests the cell cycle at the G1/S checkpoint by up-regulating p21 (CDKN1A) to allow DNA repair, or induces apoptosis via BAX, PUMA, and NOXA activation if damage is unrepairable. Inactivation of TP53 occurs in >50% of all human cancers, abolishing cellular apoptotic control.
  • BRCA1 & BRCA2: Tumor suppressor genes encoding nuclear proteins critical for repairing double-stranded DNA breaks via homologous recombination. Germline loss-of-function mutations confer a 70–80% cumulative lifetime risk of invasive breast cancer and a 20–45% lifetime risk of epithelial ovarian cancer.
  • APC (Adenomatous Polyposis Coli): Regulates Wnt signaling by targeting beta-catenin for ubiquitin-mediated degradation. Germline or somatic loss of APC initiates familial adenomatous polyposis (FAP) and >80% of sporadic colorectal cancers.

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Serum Tumor Markers in Clinical Practice

Tumor markers are cell-surface glycoproteins, oncofetal antigens, enzymes, or hormones secreted by tumor cells or produced by host tissues in response to malignant growth. While generally lacking sufficient sensitivity and specificity for general population screening (with select exceptions like PSA), serum tumor markers are vital for monitoring therapeutic response, detecting occult recurrence, and establishing disease prognosis.