| Pathway/component | Sequence of events involving CASP3 | Key regulators / molecular details | Functional outcome | Evidence |
|---|---|---|---|---|
| Intrinsic (mitochondrial) apoptosis | Intracellular stress signals trigger mitochondrial outer membrane permeabilization (MOMP), releasing cytochrome c into the cytosol; cytochrome c binds APAF1 to assemble the apoptosome, which activates caspase-9, and caspase-9 then cleaves and activates procaspase-3. | Stressors include DNA damage, ROS, ER stress, loss of adhesion, growth-factor withdrawal, and hypoxia. BAX/BAK are the key pore-forming effectors that drive MOMP. | Activation of executioner caspase-3 initiates the downstream proteolytic cascade that dismantles the cell. | (pqac-00000008, pqac-00000009, pqac-00000011, pqac-00000013) |
| MOMP as the commitment step | MOMP is described as the apoptotic “point of no return”; after membrane permeabilization, mitochondrial proteins enter the cytosol and enable caspase activation. | BH3-only proteins and p53 promote BAX/BAK activation directly or by neutralizing anti-apoptotic BCL-2 family members. | Commits the cell to apoptosis and licenses caspase-9 to activate caspase-3. | (pqac-00000009) |
| Cytochrome c–apoptosome axis | Released cytochrome c binds apoptotic protease activating factor-1 (APAF1), forming the apoptosome complex with procaspase-9; auto-activation of caspase-9 within this complex leads to downstream cleavage of executioner caspases-3/7. | Cytochrome c is normally in the mitochondrial intermembrane space and becomes apoptogenic upon cytosolic release. | Central biochemical route linking mitochondrial injury to CASP3 activation. | (pqac-00000003, pqac-00000011, pqac-00000013) |
| Extrinsic (death receptor) apoptosis | Extracellular death ligands bind cell-surface death receptors, leading to assembly of the death-inducing signaling complex (DISC), activation of initiator caspase-8 (and sometimes caspase-10), and direct cleavage/activation of procaspase-3. | Fas/CD95 and TNFR-family death receptors are highlighted; DISC-mediated proximity promotes initiator caspase activation. | Rapid activation of CASP3 from receptor-proximal signaling. | (pqac-00000000, pqac-00000003, pqac-00000008) |
| Extrinsic–intrinsic cross-talk via BID | Activated caspase-8 can also cleave BID, thereby engaging mitochondrial apoptosis and caspase-9 in addition to directly activating caspase-3. | BID is a BH3-interacting death agonist linking death receptor signaling to MOMP. | Amplifies apoptotic signaling and integrates extrinsic and intrinsic pathways. | (pqac-00000003, pqac-00000009) |
| Key upstream regulators: BCL-2 family | The balance between pro-apoptotic and anti-apoptotic BCL-2 family proteins determines whether MOMP occurs and therefore whether CASP3 can be activated through the intrinsic pathway. | Pro-apoptotic: BAX, BAK, BID, BIM, PUMA, NOXA, BIK. Anti-apoptotic: BCL-2, BCL-xL, MCL-1. | Governs apoptotic threshold upstream of CASP3. | (pqac-00000000, pqac-00000008, pqac-00000009) |
| Key upstream regulators: death receptors | Fas/CD95 and TNFR1-family receptors transduce extracellular apoptotic cues to initiator caspases that activate CASP3. | Ligand binding induces receptor complex assembly and initiator caspase activation at the DISC. | Couples immune/extracellular death signals to executioner caspase activation. | (pqac-00000000, pqac-00000008) |
| Key upstream regulators: IAPs and Smac/DIABLO | IAP proteins suppress caspase activity, while mitochondrial Smac/DIABLO released during MOMP antagonizes IAPs, thereby facilitating caspase-3 activation and function. | XIAP and related IAPs inhibit caspases; Smac/DIABLO relieves this inhibition after mitochondrial permeabilization. | Strengthens and sustains CASP3-mediated execution. | (pqac-00000009) |
| CASP3 as executioner protease | Initiator caspases activate CASP3, which then cleaves broad sets of structural, regulatory, DNA-repair, cytoskeletal, and chromatin-associated proteins. | CASP3 is an executioner cysteine-aspartate protease with strong DEVD-like substrate preference and a large substrate repertoire. | Produces the canonical biochemical and morphological features of apoptosis. | (pqac-00000001, pqac-00000003, pqac-00000004, pqac-00000007) |
| Downstream effect: PARP1 cleavage | CASP3 cleaves PARP1, a canonical hallmark substrate commonly used as a readout of executioner caspase activation. | PARP1 cleavage marks shutdown of DNA repair and progression of apoptosis. | Promotes irreversible apoptotic execution. | (pqac-00000007, pqac-00000012, pqac-00000013) |
| Downstream effect: DNA fragmentation | CASP3 cleaves ICAD, releasing CAD activity and driving genomic DNA fragmentation. | Cleavage of the inhibitor of caspase-activated DNase is a core mechanism for apoptotic DNA breakdown. | Produces a defining nuclear feature of apoptosis. | (pqac-00000007, pqac-00000011) |
| Downstream effect: cellular demolition | CASP3 activation causes membrane blebbing, chromatin condensation, cell shrinkage, phosphatidylserine exposure, and detachment from the extracellular matrix through cleavage of multiple substrates. | Executioner caspases-3/6/7 mediate the terminal demolition phase downstream of initiators. | Morphological and biochemical completion of apoptosis. | (pqac-00000000, pqac-00000007, pqac-00000009) |
| Pathway convergence | Both intrinsic and extrinsic pathways converge on CASP3 activation, making it a central integration point for diverse death stimuli. | Caspase-8 and caspase-9 represent the main upstream initiators for extrinsic and intrinsic apoptosis, respectively. | Explains why CASP3 is widely used as a core apoptosis effector/readout. | (pqac-00000000, pqac-00000003, pqac-00000008) |
| Additional route: granzyme B-mediated apoptosis | Cytotoxic lymphocyte granzyme B can promote caspase-dependent apoptosis by cleaving caspase-3, caspase-7, and BID. | Provides an immune-mediated apoptotic route that intersects with both direct CASP3 activation and mitochondrial amplification. | Supports target-cell killing by immune effectors. | (pqac-00000009) |
| Inflammatory restraint during apoptosis | During MOMP-associated apoptosis, executioner caspases-3/7 suppress mtRNA-driven type I interferon signaling, limiting inflammatory responses while cell death proceeds. | In CASP3/7-deficient settings, mtRNA activates the MDA5/MAVS/IRF3 pathway and type I IFN signaling. | Helps maintain the typically immunologically silent character of apoptosis. | (pqac-00000013) |


*Table: This table summarizes how human caspase-3 is activated through intrinsic and extrinsic apoptosis pathways, the main upstream regulators controlling those routes, and the principal downstream consequences of caspase-3 activation. It is useful for linking CASP3's biochemical function to its pathway context and cellular effects.*