Caspase-9, caspase-3 and caspase-7 have distinct roles during intrinsic apoptosis

TLDR

Apoptosis is regulated by Bcl‑2 and caspase families, and while the caspase cascade after cytochrome‑c release is well described, the distinct roles of caspases‑9, ‑3 and ‑7 remain incompletely defined. The study aims to delineate the unique functions of caspases‑9, ‑3 and ‑7 during intrinsic apoptosis. Caspase‑9 is required for Bid cleavage, mitochondrial morphological changes and ROS production; caspase‑3 suppresses ROS and is essential for efficient execution of apoptosis; caspase‑7 is necessary for apoptotic cell detachment, with caspase‑9 inhibition blocking mitochondrial changes but not cytochrome‑c release.

Abstract

Apoptosis is a form of programmed cell death that is regulated by the Bcl-2 family and caspase family of proteins. The caspase cascade responsible for executing cell death following cytochrome c release is well described; however the distinct roles of caspases-9, -3 and -7 during this process are not completely defined.Here we demonstrate several unique functions for each of these caspases during cell death. Specific inhibition of caspase-9 allows for efficient release of cytochrome c, but blocks changes in mitochondrial morphology and ROS production. We show that caspase-9 can cleave Bid into tBid at amino acid 59 and that this cleavage of Bid is required for ROS production following serum withdrawal. We also demonstrate that caspase-3-deficient MEFs are less sensitive to intrinsic cell death stimulation, yet have higher ROS production. In contrast, caspase-7-deficient MEFs are not resistance to intrinsic cell death, but remain attached to the ECM.Taken together, these data suggest that caspase-9 is required for mitochondrial morphological changes and ROS production by cleaving and activating Bid into tBid. After activation by caspase-9, caspase-3 inhibits ROS production and is required for efficient execution of apoptosis, while effector caspase-7 is required for apoptotic cell detachment.