5.HOW CANCER ESCAPES APOPTOSIS (PROGRAMMED CELL DEATH)

1. What is Apoptosis?

Apoptosis = planned, controlled, tidy programmed cell death.

Key features:

• Cell shrinks and fragments into apoptotic bodies
• No swelling, leakage, or inflammation
• Cellular contents remain contained
• Dead cells removed silently by phagocytes

Analogy:

• Apoptosis = peaceful retirement of a cell
• Necrosis = messy accident causing inflammation

2. Pathways that initiate apoptosis

Two major activation routes:

A. Extrinsic Pathway (death receptor pathway)

Triggered by external ligands binding receptors.

Example:

• FAS ligand binds FAS receptor

→ activates downstream caspases

B. Intrinsic Pathway (mitochondrial pathway)

Triggered by cellular internal stress:

• DNA damage
• Growth factor deprivation
• Metabolic stress
• Hypoxia
• Chemotherapy and radiation (through DNA damage)

Intrinsic pathway is the main one blocked in cancers.

3. Why tumors resist apoptosis

Cancer cells acquire mutations that block the apoptosis machinery.

This allows:

• Survival despite DNA damage
• Resistance to hypoxia
• Resistance to chemotherapy

Main blocked route = intrinsic mitochondrial pathway

Mechanisms:

• Loss of p53 (guardian of genome)
• Alterations in BCL2 family proteins

4. Intrinsic Pathway Mechanism

4A. Trigger → Pore opening → Cytochrome c release

1. Internal stress signals

(DNA damage, low growth factors, metabolic stress, hypoxia)

2. Outer mitochondrial membrane becomes permeabilized
3. Cytochrome c leaks out into cytosol

4B. Control of mitochondrial pore formation (BCL2 family proteins)

Three functional groups:

Anti-apoptotic proteins (bodyguards)

These block pore formation:

• BCL2
• BCL-XL
• MCL1

Pro-apoptotic proteins (demolition team — directly makes pores)

• BAX
• BAK

BH3-only proteins (referees/handcuffs that neutralize bodyguards)

• BAD
• BID
• PUMA

How they interact:

BH3-only proteins bind anti-apoptotic proteins → disable them →

BAX & BAK oligomerize → form pores → cytochrome c release

4C. Downstream execution after cytochrome c release

Sequence:

Cytochrome c

↓

binds APAF-1

↓

forms apoptosome complex

↓

activates caspase-9 (initiator)

↓

activates caspase-3 (executioner)

↓

controlled dismantling of the cell

4D. IAPs (Inhibitors of Apoptosis Proteins)

Role:

• Block caspase-9 activation
• Can stop apoptosis even after cytochrome c is released

5. How cancer blocks apoptosis

Trick 1: Inactivate p53

Normal p53 function:

• Detects DNA damage
• Activates PUMA (BH3-only protein)
• PUMA inhibits BCL2 → allows BAX/BAK to form pores

Consequences of p53 loss:

• No PUMA produced
• BCL2 remains active
• Cells survive despite DNA mutations

Mechanisms of loss:

• TP53 gene mutation (very common; increased after therapy)
• MDM2 amplification:
• MDM2 binds p53 → blocks function

Trick 2: Overexpress anti-apoptotic BCL2 family proteins

Overexpression prevents pore formation by BAX/BAK

Mechanisms:

• Classic translocation:

t(14;18)(q32;q21) → moves BCL2 next to Ig heavy chain promoter

→ gene permanently activated

Clinical significance:

• Follicular lymphoma
• Long-lived lymphocytes → indolent lymphadenopathy

Other mechanisms of anti-apoptotic excess:

• Loss of microRNAs that normally suppress BCL2
• Gene amplification such as MCL1 amplification in lung/breast cancers

6. Therapies that target apoptosis evasion

A. Restore p53 activity

• Hard to replace TP53 directly
• Strategy: block MDM2 (its inhibitor)
• MDM2 inhibitors free p53 to function

Clinical relevance:

• Trials in sarcomas with MDM2 amplification

B. BH3 mimetics

• Mimic BH3-only proteins
• Directly inhibit BCL2
• Effective in BCL2-dependent cancers

Important example:

• CLL (chronic lymphocytic leukemia)
• BH3 mimetics entering mainstream use

7. Quick Memory Sheet

Pathways:

Intrinsic (mitochondrial)

Extrinsic (death receptor)

Intrinsic triggers:

DNA damage, growth factor loss, hypoxia, metabolic stress, chemo/radiation

Key players:

Pro-death: BAX, BAK

Anti-death: BCL2, BCL-XL, MCL1

BH3-only: BAD, BID, PUMA

Execution sequence:

Cytochrome c → APAF-1 → apoptosome → caspase-9 → caspase-3

Inhibitors:

IAPs block caspase-9

Cancer cheats:

TP53 loss, MDM2 amplification

BCL2 overexpression via t(14;18), microRNA loss, MCL1 amplification

Therapies:

MDM2 inhibitors (restore p53 function)

BH3 mimetics (block BCL2)

Clinical example:

Follicular lymphoma → t(14;18) → BCL2 ↑ → indolent lymph nodes

🧠 APOPTOSIS — EXAM REFLEX BLOCK (ZERO-OMISSION, HIGH-YIELD)

Use this as a rapid recall trigger in exams.

🔹 Core Definition (1-liner)

Apoptosis = programmed, energy-dependent, non-inflammatory cell death with membrane integrity preserved and phagocytic clearance.

🔹 Apoptosis vs Necrosis (Instant Contrast)

• Apoptosis → cell shrinks, fragments → no inflammation
• Necrosis → cell swells, ruptures → inflammation present

🔹 Pathways

• Intrinsic (mitochondrial) → MAIN pathway blocked in cancer
• Extrinsic (death receptor) → Fas–FasL → caspases

🔹 Intrinsic Pathway — Trigger List (must recall)

• DNA damage
• Growth factor deprivation
• Hypoxia
• Metabolic stress
• Chemotherapy / radiation

🔹 Mitochondrial Control = BCL-2 Family (EXAM FAVORITE)

1️⃣ Anti-apoptotic (❌ stop pore formation)

• BCL-2
• BCL-XL
• MCL-1

2️⃣ Pro-apoptotic (✅ pore formation)

• BAX
• BAK

3️⃣ BH3-only (⚖️ neutralize BCL-2)

• BAD
• BID
• PUMA

👉 Logic:

BH3-only proteins inhibit BCL-2 → BAX/BAK oligomerize → mitochondrial pores open

🔹 Execution Cascade (ORDER MATTERS)

Cytochrome c→ APAF-1→ Apoptosome→ Caspase-9 (initiator)→ Caspase-3 (executioner)→ Controlled cell dismantling

🔹 IAPs (Inhibitors of Apoptosis Proteins)

• Block caspase-9
• Can halt apoptosis even after cytochrome c release

🔹 How Cancer Evades Apoptosis (VERY HIGH-YIELD)

🧨 Mechanism 1: p53 Inactivation

Normal:

• DNA damage → p53 ↑
• p53 → PUMA ↑
• PUMA inhibits BCL-2 → apoptosis

Loss of p53:

• ❌ No PUMA
• ❌ BCL-2 unchecked
• ❌ DNA-damaged cells survive

How p53 lost:

• TP53 mutation (very common) 70%
• MDM2 amplification → p53 inhibited 30%

🧨 Mechanism 2: BCL-2 Overexpression

• Blocks BAX/BAK pore formation

Classic mechanism:

• t(14;18)(q32;q21)
• BCL-2 under Ig heavy chain promoter
• Constitutive BCL-2 expression
• keep the cell alive

Clinical association:

• Follicular lymphoma
• Long-lived lymphocytes
• Indolent lymphadenopathy

Other causes:

• Loss of BCL-2-suppressing microRNAs
• MCL-1 amplification (lung, breast cancers)

🔹 Targeted Therapies (Concept-Driven Questions)

🎯 MDM2 inhibitors

• Free endogenous p53
• Useful in MDM2-amplified tumors
• Studied in sarcomas

🎯 BH3 mimetics

• Mimic BH3-only proteins
• Directly inhibit BCL-2
• Effective in BCL-2-dependent cancers
• Key use: CLL

🔹 One-Glance Memory Locks

• Intrinsic pathway = cancer-blocked
• p53 → PUMA → BCL-2 inhibition
• t(14;18) → BCL-2 → follicular lymphoma
• Cytochrome c → APAF-1 → caspase-9 → caspase-3
• IAPs block caspase-9
• BH3 mimetics = modern apoptosis therapy