A 58-year-old man with colorectal cancer is enrolled in a clinical trial testing a drug that blocks caspase-8 activation at the death-inducing signalling complex (DISC). Which pathway of apoptosis would remain functional despite this blockade, and what is the key mechanistic reason?

Explanation

## Caspase-8 Blockade and Pathway Selectivity ### Clinical Scenario Analysis The intrinsic (mitochondrial) pathway would **remain fully functional** despite caspase-8 blockade because it is **mechanistically independent** of caspase-8 and the death-inducing signalling complex (DISC). ### Why the Intrinsic Pathway Is Spared **Key Point:** The intrinsic pathway: 1. Is triggered by **intracellular stress signals** (DNA damage, hypoxia, growth factor withdrawal, oxidative stress) 2. Activates **p53** and pro-apoptotic Bcl-2 family members (Bax, Bak) 3. Causes **mitochondrial outer membrane permeabilization (MOMP)** 4. Releases **cytochrome c** into the cytoplasm 5. Forms the **apoptosome** (cytochrome c + Apaf-1 + pro-caspase-9) 6. Activates **caspase-9** directly — **no caspase-8 involvement** 7. Caspase-9 then activates executioner caspases (caspase-3/7) **Clinical Pearl:** In colorectal cancer, DNA damage from chemotherapy or radiation triggers the intrinsic pathway via p53. A caspase-8 inhibitor would NOT prevent this apoptosis because the mitochondrial pathway is p53-dependent and caspase-9-dependent, not caspase-8-dependent. ### Why the Extrinsic Pathway Is Blocked The extrinsic pathway depends on: 1. Death receptor engagement (Fas, TNF-R1, TRAIL-R) 2. DISC formation 3. **Caspase-8 activation** ← **BLOCKED by the drug** 4. Caspase-8 → caspase-3/7 activation Without caspase-8, the extrinsic pathway cannot proceed (in Type I cells that do not amplify via the intrinsic pathway). ### Comparative Table: Caspase-8 Dependence | Pathway | Caspase-8 Dependence | Effect of Caspase-8 Blockade | |---------|----------------------|------------------------------| | **Intrinsic** | None (uses caspase-9) | **No effect** — pathway proceeds normally | | **Extrinsic** | **Obligatory** | **Blocked** — apoptosis cannot initiate | | **Type II cells** | Partial (amplification) | Reduced but not eliminated (intrinsic pathway still active) | **High-Yield:** Caspase-8 inhibitors are **selective blockers of the extrinsic pathway**. They do NOT prevent apoptosis triggered by intracellular stress because the intrinsic pathway is caspase-8-independent. **Mnemonic:** **"8 for extrinsic, 9 for intrinsic"** — caspase-8 initiates the extrinsic pathway; caspase-9 initiates the intrinsic pathway. Blocking caspase-8 leaves caspase-9 (and thus the intrinsic pathway) intact. ### Clinical Relevance This is why **caspase-8 inhibitors are not effective as single-agent anti-cancer drugs** — cancer cells can still undergo apoptosis via the intrinsic (mitochondrial) pathway in response to chemotherapy, radiation, or hypoxia. The drug would only block death receptor-mediated apoptosis (e.g., Fas-induced apoptosis in immune cells). [cite:Robbins 10e Ch 7]