Which of the following is the primary sensor of intracellular stress in the intrinsic apoptotic pathway?

Explanation

## Intrinsic Apoptotic Pathway — Role of p53 **Key Point:** p53 is the central 'guardian of the genome' and acts as the primary sensor of intracellular stress (DNA damage, hypoxia, oncogenic stress) in the intrinsic pathway. ### Mechanism of p53 in Intrinsic Apoptosis 1. **Stress Detection**: p53 accumulates in response to DNA damage, oxidative stress, or hypoxia. 2. **Transcriptional Activation**: p53 upregulates pro-apoptotic genes (BAX, PUMA, NOXA) and downregulates anti-apoptotic genes (BCL-2, MCL-1). 3. **Mitochondrial Outer Membrane Permeabilization (MOMP)**: Pro-apoptotic BCL-2 family members translocate to the mitochondrial membrane. 4. **Cytochrome c Release**: MOMP leads to cytochrome c efflux into the cytoplasm. 5. **Apoptosome Formation**: Cytochrome c binds to Apaf-1 and recruits procaspase-9, forming the apoptosome. 6. **Caspase Cascade**: Caspase-9 activates executioner caspases (caspase-3/7), leading to DNA fragmentation and cell death. **High-Yield:** p53 is upstream of the mitochondrial events; it is NOT the direct sensor of mitochondrial damage — rather, it senses nuclear/genomic stress and orchestrates the response. ### Intrinsic vs. Extrinsic Pathway Comparison | Feature | Intrinsic Pathway | Extrinsic Pathway | |---------|-------------------|-------------------| | **Primary Sensor** | p53 (intracellular stress) | Death receptors (Fas, TNF-R1) | | **Initiator Caspase** | Caspase-9 | Caspase-8 | | **Key Organelle** | Mitochondria (MOMP) | Plasma membrane | | **BCL-2 Family Role** | Central (BAX, BAK) | Minor/indirect | | **Apoptosome** | Yes (Apaf-1 + Cyt c) | No | **Clinical Pearl:** Mutations in p53 (Li–Fraumeni syndrome) impair apoptosis and predispose to multiple cancers because cells cannot respond to DNA damage signals. **Mnemonic:** **"p53 = Protector of genome"** — it detects damage and decides: repair or die.