# Lecture 26: Cancer: The Cell Biology of Disease
## Cell Biology
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the hallmarks of cancer as defined by Hanahan and Weinberg
2. Explain the roles of oncogenes and tumor suppressor genes in cancer development
3. Describe the multi-step model of carcinogenesis
4. Explain the cell biological basis of invasion and metastasis
5. Discuss the principles of targeted cancer therapy based on cell biology knowledge
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## Lecture Content
### I. The Hallmarks of Cancer
Cancer is a disease of **uncontrolled cell proliferation** arising from **accumulation of genetic and epigenetic alterations**. **Hallmarks of cancer** (Hanahan and Weinberg, 2000; updated 2011, 2022): **Sustaining proliferative signaling**: constitutive growth factor signaling (autocrine loops, activating mutations in RTKs/Ras/MAPK) **Evading growth suppressors**: inactivation of Rb, p53, APC, and other tumor suppressors. **Resisting cell death**: overexpression of Bcl-2, loss of p53, upregulation of IAPs. **Enabling replicative immortality**: telomerase reactivation (TERT) or ALT (alternative lengthening of telomeres); bypass of senescence. **Inducing angiogenesis**: VEGF secretion to build new blood vessels that supply the tumor. **Activating invasion and metastasis**: EMT, protease secretion, loss of E-cadherin. **Deregulating cellular energetics**: Warburg effect (aerobic glycolysis — preference for glycolysis even in the presence of O2) **Avoiding immune destruction**: downregulation of MHC-I, PD-L1 expression to suppress T cell killing. **Genome instability and mutation**: defects in DNA repair, chromosomal instability. **Tumor-promoting inflammation**: chronic inflammation fuels tumor growth (cytokines, growth factors, ROS from immune cells) Additional emerging hallmarks: unlocking phenotypic plasticity, non-mutational epigenetic reprogramming, polymorphic microbiomes, senescent cells in the tumor microenvironment.
### II. Oncogenes
**Proto-oncogenes**: normal genes that promote cell growth/division. Become **oncogenes** through gain-of-function mutations. Dominant at the cellular level: mutation of one allele is sufficient. **Mechanisms of oncogene activation**: **Point mutation**: e.g., Ras (G12V — glycine to valine at position 12 locks Ras in GTP-bound active state) KRAS mutations found in ~25% of all human cancers (pancreatic, colorectal, lung) **Gene amplification**: extra copies of gene -> overexpression. HER2 (ERBB2) amplification in ~20% of breast cancers. MYC amplification in many cancers. **Chromosomal translocation**: places gene under a strong promoter or creates a fusion protein; t(9;22) BCR-ABL in chronic myeloid leukemia (CML): constitutively active tyrosine kinase; t(14;18) Bcl-2 in follicular lymphoma: Bcl-2 overexpression under IgH enhancer; t(8;14) MYC in Burkitt lymphoma: MYC under IgH enhancer. **Viral insertion**: retroviral promoter/enhancer activates nearby proto-oncogene. **Categories of oncogene products**: Growth factors: PDGF (Sis) Growth factor receptors: EGFR, HER2, Kit, RET. Signal transducers: Ras, Raf, PI3K, Src, Abl. Transcription factors: Myc, Jun, Fos. Cell cycle regulators: cyclin D1, CDK4. Anti-apoptotic: Bcl-2.
### III. Tumor Suppressor Genes
Normal function: restrain cell growth, promote apoptosis, maintain genome integrity. **Loss-of-function** mutations lead to cancer; typically requires **both alleles** to be lost (**Knudson's two-hit hypothesis**) First hit: inherited mutation (familial) or somatic mutation. Second hit: loss of heterozygosity (LOH), deletion, mutation, or epigenetic silencing. **Key tumor suppressors**: **p53 (TP53)**: "guardian of the genome". Mutated in >50% of all cancers. Functions: G1 arrest (p21), DNA repair, apoptosis (PUMA, NOXA, Bax), senescence. Many missense mutations are dominant negative (mutant p53 tetramerizes with wild-type p53 and inactivates it) Li-Fraumeni syndrome: germline TP53 mutations -> predisposition to many cancer types. **Rb (RB1)**: master regulator of G1/S checkpoint. Retinoblastoma: childhood eye tumor; familial (one inherited hit) or sporadic (two somatic hits) Functionally inactivated in most cancers (directly or via the CDK4-cyclin D-Rb pathway) **APC**: Wnt pathway negative regulator (component of beta-catenin destruction complex) Familial adenomatous polyposis (FAP): germline APC mutation -> thousands of colon polyps -> colorectal cancer. APC loss -> constitutive Wnt activation -> stem cell expansion in crypts. **BRCA1, BRCA2**: DNA double-strand break repair (homologous recombination) Germline mutations -> hereditary breast and ovarian cancer. Loss of HR repair -> genome instability -> sensitivity to PARP inhibitors (synthetic lethality) **PTEN**: phosphatase that degrades PIP3 -> antagonizes PI3K/Akt pathway. One of the most commonly lost tumor suppressors.
Oncogenes and tumor suppressors in cancer signaling. Panel A: Normal cell signaling — growth factor binds RTK -> Ras-MAPK and PI3K-Akt pathways (controlled by PTEN) -> transcription factors (Myc) -> cell cycle entry. Checkpoints: Rb (blocks E2F in G1), p53 (responds to DNA damage with arrest or apoptosis), APC (controls Wnt/beta-catenin). Panel B: Cancer cell — multiple alterations shown simultaneously: activating mutation in Ras (star, constitutively active), HER2 amplification (multiple receptors on the surface), loss of PTEN (crossed out), p53 mutated (crossed out), Rb inactivated, Bcl-2 overexpressed (blocking apoptosis), telomerase reactivated. Arrows emphasize uncontrolled proliferative signaling and blocked growth arrest/apoptosis pathways. Panel C: Knudson two-hit model — inherited retinoblastoma (one RB1 allele mutated at birth, second hit in somatic cell -> tumor) versus sporadic retinoblastoma (both hits occur somatically in the same cell -> much rarer).
### IV. Multi-Step Carcinogenesis
Cancer develops through the **gradual accumulation of mutations** over time (usually decades) **Colorectal cancer progression** (Vogelstein model): Normal epithelium -> APC loss -> small adenoma -> KRAS activation -> large adenoma -> SMAD4/p53 loss -> carcinoma -> additional mutations -> metastasis. Each step provides a selective growth advantage (clonal evolution) **Clonal evolution**: cancer progresses by Darwinian natural selection at the cellular level. Mutations that provide a proliferative/survival advantage are positively selected. Tumor heterogeneity: different subclones within a tumor carry different mutations. Branching evolution: subclones diverge -> intra-tumor heterogeneity -> therapy resistance. **Mutational burden**: most cancers have 40-80 protein-altering mutations (only 2-8 are "driver" mutations; the rest are "passenger" mutations) **Genome instability accelerates mutation accumulation**: Defects in DNA mismatch repair (MLH1, MSH2): microsatellite instability (MSI) -> Lynch syndrome (hereditary nonpolyposis colorectal cancer) Chromosomal instability (CIN): whole chromosome gains/losses; aneuploidy. Chromothripsis: catastrophic shattering and reassembly of chromosome segments.
### V. Invasion and Metastasis
Metastasis is the major cause of cancer death (~90% of cancer mortality) **Metastatic cascade**: **Local invasion**: tumor cells breach the basement membrane (MMPs, especially MMP-2, MMP-9, MT1-MMP) **Intravasation**: entry into blood vessels or lymphatics. **Survival in circulation**: anoikis resistance, platelet cloaking, immune evasion. **Extravasation**: arrest at distant site, exit from vessels. **Colonization**: survival and proliferation at the metastatic site (often rate-limiting) **Epithelial-to-mesenchymal transition (EMT)**: E-cadherin downregulation (Snail, Slug, Twist, ZEB1/2 transcription factors) Gain of mesenchymal markers (N-cadherin, vimentin, fibronectin) Increased motility, invasiveness, and resistance to apoptosis. Partial EMT (hybrid states) may be more metastatic than full EMT. **Seed and soil hypothesis** (Paget, 1889): metastasis depends on compatibility between the tumor cell (seed) and the distant organ microenvironment (soil) Pre-metastatic niche: tumor-secreted factors (exosomes, cytokines) prepare distant sites before tumor cells arrive. Organ tropism: breast cancer -> bone, lung, brain, liver; prostate cancer -> bone; colorectal cancer -> liver. **Tumor microenvironment**: cancer-associated fibroblasts (CAFs), tumor-associated macrophages (TAMs), immune cells, blood vessels, and ECM all support tumor growth and metastasis.
### VI. Targeted Therapy: Cell Biology Applied
**Imatinib (Gleevec)**: BCR-ABL tyrosine kinase inhibitor for CML. First molecularly targeted cancer therapy; transformed CML from fatal to chronic disease. Paradigm for targeted therapy based on understanding the driving oncogene. **Trastuzumab (Herceptin)**: anti-HER2 monoclonal antibody for HER2+ breast cancer. **EGFR inhibitors**: erlotinib, gefitinib, osimertinib for EGFR-mutant non-small cell lung cancer. **KRAS G12C inhibitors**: sotorasib, adagrasib — first drugs to directly target mutant KRAS (previously considered "undruggable") **PARP inhibitors** (olaparib): exploit synthetic lethality in BRCA1/2-mutant cancers. BRCA-deficient cells cannot repair DSBs by HR; PARP inhibition blocks backup repair -> cell death. **Immune checkpoint inhibitors**: Anti-PD-1 (pembrolizumab, nivolumab), anti-PD-L1, anti-CTLA-4 (ipilimumab) Block inhibitory checkpoints -> reactivate T cell killing of tumor cells. Remarkable responses in melanoma, lung cancer, renal cell carcinoma, and others. **Venetoclax**: Bcl-2 inhibitor (BH3 mimetic) for CLL and AML — directly restores apoptosis. **CAR-T cell therapy**: T cells engineered to express chimeric antigen receptors targeting tumor antigens (CD19 for B-cell lymphomas/leukemias) **Resistance**: nearly universal problem; pre-existing resistant subclones or acquired mutations under therapy selection pressure; drives combination therapy approaches.
The metastatic cascade and therapeutic targets. Panel A: Steps of metastasis — primary tumor (1) invading through the basement membrane, (2) intravasation into a blood vessel, (3) survival in circulation (tumor cells surrounded by platelets), (4) extravasation at a distant organ, (5) colonization and formation of a micrometastasis that grows into a secondary tumor. EMT transcription factors (Snail, Twist) indicated at the invasion step. Panel B: Targeted therapy map — a cancer cell with labeled molecular targets and their corresponding drugs: HER2 (trastuzumab), EGFR (erlotinib), BCR-ABL (imatinib), KRAS G12C (sotorasib), BRAF V600E (vemurafenib), Bcl-2 (venetoclax), PARP (olaparib in BRCA-mutant context), PD-L1 on tumor surface (anti-PD-L1 antibody blocking interaction with PD-1 on a T cell).
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