A 52-year-old man with chronic hepatitis C develops hepatocellular carcinoma. Biopsy shows tumor cells with markedly reduced expression of Fas (CD95) on their surface, while mitochondrial Bcl-2 levels are elevated. Which pathway of apoptosis is the tumor primarily evading?

Explanation

## Clinical Scenario: Apoptosis Evasion in Hepatocellular Carcinoma **Key Point:** The clinical finding of **reduced Fas expression** with **elevated Bcl-2** indicates selective evasion of the **extrinsic pathway**; the intrinsic pathway remains intact but is defended by anti-apoptotic Bcl-2. ### Analysis of the Case **Findings:** 1. **Reduced Fas (CD95)** on tumor cell surface → **extrinsic pathway blocked** 2. **Elevated Bcl-2** (anti-apoptotic) → **intrinsic pathway defended** **Interpretation:** - Fas is the death receptor that initiates the extrinsic pathway via FasL binding - Loss of Fas expression prevents DISC formation and caspase-8 activation - The tumor has **selectively disabled the extrinsic pathway** - Elevated Bcl-2 protects against intrinsic apoptosis (from chemotherapy, radiation, hypoxia) - This is a **dual-hit strategy**: block extrinsic (immune-mediated), defend intrinsic (stress-induced) ### Why Each Pathway Is Affected Differently | Pathway | Status in This Tumor | Reason | | --- | --- | --- | | **Extrinsic** | **BLOCKED** | Fas downregulation prevents death receptor signaling; no DISC formation; no caspase-8 activation | | **Intrinsic** | **DEFENDED** | Elevated Bcl-2 inhibits Bax/Bak and prevents mitochondrial outer membrane permeabilization (MOMP); cytochrome c release blocked | **High-Yield:** In HCC and other cancers, **Fas loss is a hallmark of extrinsic pathway evasion**, especially in immune-rich environments (chronic hepatitis C involves immune infiltration). The tumor escapes immune-mediated apoptosis via FasL+ lymphocytes. **Clinical Pearl:** Chronic hepatitis C causes repeated hepatocyte injury and regeneration, driving selection for clones with apoptosis resistance. Fas downregulation is a common event in HCC because it prevents killing by Fas-ligand-expressing immune cells (CD8+ T cells, NK cells). Elevated Bcl-2 is a separate adaptation against intrinsic apoptosis triggers (chemotherapy, ischemia). **Mnemonic:** **"Fas Loss = Extrinsic Block"** — Loss of the death receptor = loss of the extrinsic pathway's entry point. [cite:Robbins 10e Ch 3, Ch 9]