Protein Kinase A Regulates Caspase-9 Activation by Apaf-1 Downstream of Cytochrome c*

The cyclic AMP signal transduction pathway modulates apoptosis in diverse cell types, although the mechanism is poorly understood. A critical component of the intrinsic apoptotic pathway is caspase-9, which is activated by Apaf-1 in the apoptosome, a large complex assembled in response to release of cytochrome c from mitochondria. Caspase-9 cleaves and activates effector caspases, predominantly caspase-3, resulting in the demise of the cell. Here we identified a distinct mechanism by which cyclic AMP regulates this apoptotic pathway through activation of protein kinase A. We show that protein kinase A inhibits activation of caspase-9 and caspase-3 downstream of cytochrome c in Xenopus egg extracts and in a human cell-free system. Protein kinase A directly phosphorylates human caspase-9 at serines 99, 183, and 195. However, mutational analysis demonstrated that phosphorylation at these sites is not required for the inhibitory effect of protein kinase A on caspase-9 activation. Importantly, protein kinase A inhibits cytochrome c-dependent recruitment of procaspase-9 to Apaf-1 but not activation of caspase-9 by a constitutively activated form of Apaf-1. These data indicate that extracellular signals that elevate cyclic AMP and activate protein kinase A may suppress apoptosis by inhibiting apoptosome formation downstream of cytochrome c release from mitochondria.