A 58-year-old man with metastatic colorectal cancer is enrolled in a clinical trial of a monoclonal antibody (cetuximab) targeting epidermal growth factor receptor (EGFR). After the first infusion, tumour cells show increased surface expression of death receptors (Fas and TRAIL-R) and enhanced caspase-8 activation. Immunohistochemistry of a biopsy specimen reveals increased apoptosis in the tumour microenvironment. Which of the following best explains the mechanism by which EGFR blockade enhances death receptor-mediated apoptosis in these cancer cells?

Explanation

## EGFR Blockade and Extrinsic Apoptosis Pathway **Key Point:** EGFR inhibition enhances death receptor-mediated (extrinsic) apoptosis by **reducing survival signalling (PI3K/Akt)**, which normally suppresses both pro-apoptotic proteins and death receptor expression. ### Mechanism of EGFR Signalling in Cancer Cells ```mermaid flowchart TD A[EGFR activated by ligand]:::action --> B[PI3K/Akt pathway]:::action B --> C[Survival signals]:::action C --> D[Suppression of pro-apoptotic proteins<br/>Bcl-2, Bcl-xL upregulated]:::outcome C --> E[Downregulation of death receptors<br/>Fas, TRAIL-R]:::outcome F[EGFR blockade by cetuximab]:::action --> G[PI3K/Akt inhibition]:::action G --> H[Loss of survival signals]:::urgent H --> I[Upregulation of pro-apoptotic proteins<br/>Bax, Bad]:::outcome H --> J[Upregulation of death receptors<br/>Fas, TRAIL-R]:::outcome J --> K[Enhanced DISC formation<br/>Caspase-8 activation]:::action K --> L[Extrinsic apoptosis]:::outcome ``` ### Why This Is the Correct Mechanism **High-Yield:** EGFR and PI3K/Akt are **anti-apoptotic survival signals** in cancer cells. When EGFR is blocked: 1. **PI3K/Akt signalling is reduced** — Loss of survival kinase activity 2. **Akt-mediated suppression of pro-apoptotic proteins is relieved** — Bax, Bad, FoxO become active 3. **Akt-mediated suppression of death receptors is relieved** — Fas and TRAIL-R expression increases 4. **Death receptor signalling becomes dominant** — Enhanced DISC formation and caspase-8 activation ### Intrinsic vs. Extrinsic Pathways in This Context | Feature | Intrinsic Pathway | Extrinsic Pathway (This Case) | |---------|-------------------|-------------------------------| | **Trigger** | Intracellular stress (DNA damage, ROS) | Death receptor ligation (Fas, TRAIL-R) | | **Key step** | Mitochondrial outer membrane permeabilization (MOMP) | DISC formation at cell membrane | | **Initiator caspase** | Caspase-9 | Caspase-8 | | **Role of EGFR blockade** | Indirect (via reduced Akt-mediated Bcl-2 upregulation) | **Direct** — Upregulates death receptors and sensitizes to ligand | | **Mechanism in this case** | Secondary | **Primary** | **Clinical Pearl:** This principle underlies the use of **EGFR inhibitors + death receptor agonists** (e.g., TRAIL-R agonists) in combination therapy for colorectal cancer. EGFR blockade "primes" cells for death receptor-mediated apoptosis. **Mnemonic:** **EGFR OFF → Death ON** — When EGFR survival signals are blocked, death receptor sensitivity increases. [cite:Robbins 10e Ch 7] ### Why Extrinsic, Not Intrinsic? The vignette explicitly states **increased caspase-8 activation** and **enhanced death receptor expression**. Caspase-8 is the initiator caspase of the extrinsic pathway, not the intrinsic pathway (which uses caspase-9). This is the diagnostic clue.