HtrA2/Omi, a serine protease, is released from mitochondria and promotes programmed cell death in response to apoptotic stimuli. The proapoptotic function of HtrA2 closely relates to its protease activity and binding and degrading XIAP, a member of IAP family proteins. However, the regulation of HtrA2 in the context of its physiologic function has not well been documented. Here we report that Akt1 and AKT2 phosphorylates mitochondria-released HtrA2 at serine-212 in vivo and in vitro, leading to inhibition of HtrA2 serine protease activity. Akt nonphosphoryable HtrA2-S212A and wild type HtrA2 exhibited the serine protease activity, whereas phosphomimic HtrA2-S212D lost its protease activity. Further, Akt significantly inhibited HtrA2 proapoptotic function. Ectopic expression of nonphosphoryable HtrA2-S212A induced more apoptosis than that of wild type HtrA2. By contrast, phosphomimic HtrA2-S212D inhibited programmed cell death induced by DNA damage agents (STS and VP16). However, Akt phosphorylation of HtrA2 did not disrupt its binding to XIAP rather inhibits its ability to cleavage XIAP and c-IAP. These results indicate that HtrA2 is a physiological substrate of Akt and that phosphorylation of HtrA2 by Akt directly inhibit its protease activity and proapoptotic function.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]