Akt Attenuation of the Serine Protease Activity of HtrA2/Omi through Phosphorylation of Serine 212

Abstract

The serine protease HtrA2/Omi is released from the mitochondria into the cytosol following apoptosis stimuli, leading to the programmed cell death in caspase-dependent and -independent manners. The function of HtrA2/Omi closely relates to its protease activity, which is required for cleavage of its substrate such as the members of the X-linked inhibitor of apoptotic protein family. However, the regulation of HtrA2/Omi by signaling molecule has not been documented. Here we report that serine/threonine kinases Akt1 and Akt2 phosphorylate mitochondria-released HtrA2/Omi on serine 212 in vivo and in vitro, which results in attenuation of its serine protease activity and pro-apoptotic function. Abolishing HtrA2/Omi phosphorylation by Akt through mutation of serine 212 to alanine (HtrA2/Omi-S212A) retains its serine protease activity and induces more apoptosis as compared with wild-type HtrA2/ Omi. Conversely, HtrA2/Omi-S212D, a mutant mimicking phosphorylation, lost the protease activity and failed to induce the programmed cell death. Furthermore, the phosphorylated HtrA2/Omi fails to cleave X-linked inhibitor of apoptotic protein without interfering with their complex formation. In addition, Akt inhibits the release of HtrA2/Omi from the mitochondria into the cytoplasm in response to cisplatin treatment. These data reveal for the first time that HtrA2/Omi is directly regulated by Akt and provide a mechanism by which Akt induces cell survival at post-mitochondrial level.

Phosphoinositide 3-Kinase/Akt Inhibits MST1-Mediated Pro-apoptotic Signaling through Phosphorylation of Threonine 120

“…The phosphorylation by Akt negatively regulates FOXO activity by relocalizing FOXO from the nucleus to the cytoplasm, where it is sequestered away from target genes through interacting with 14-3-3 (18). In addition, several pro-apoptotic and anti-apoptotic proteins are also phosphorylated by Akt, including ASK1 (19,20), XAIP (21), , BAX (23,24), and HtrA2 (25), which leads to direct activation of cell survival pathway. Moreover, Akt has been shown to activate NFB pro-survival signaling by phosphorylation of IKK␣ (26,27).…”

Syk Activation of Phosphatidylinositol 3-Kinase/Akt Prevents HtrA2-dependent Loss of X-linked Inhibitor of Apoptosis Protein (XIAP) to Promote Survival of Epstein-Barr Virus+ (EBV+) B Cell Lymphomas

“…Our data suggest instead an important role for the protease HtrA2 in the loss of XIAP as well as survival of EBVϩ PTLD-derived B cell lymphomas. This is consistent with findings that phosphorylation of HtrA2 by Akt attenuates its ability to degrade XIAP and induce apoptosis (49). However, a more detailed examination into the interplay between Akt, HtrA2, and XIAP is required.…”

“…XIAP was one of the first substrates identified for HtrA2, as increased HtrA2 expression results in decreased XIAP expression and vice versa (51,52). Akt phosphorylates HtrA2 at Ser 212 , which attenuates its serine protease activity and, consequently, its ability to degrade XIAP and induce apoptosis (49). We next directly asked if HtrA2 was involved in the loss of XIAP we observed after Syk and Akt inhibition.…”

“…Akt has been described as preventing the loss of XIAP by two distinct mechanisms (24,49), both involving regulation of XIAP at the protein level. First, phosphorylation of XIAP Ser 87 by Akt has been reported to prevent autoubiquitination and subsequent degradation of XIAP, thus stabilizing the levels of protein expression (24).…”

HtrA2 Regulates β-Amyloid Precursor Protein (APP) Metabolism through Endoplasmic Reticulum-associated Degradation

“…This could also be relevant regarding the role of cytosolic HtrA2 in apoptosis, especially in light of the antiapoptotic roles attributed to presenilin C-terminal fragments (54). Interestingly, proteolytic activity of HtrA2 can be regulated through phosphorylation at S212 by Akt kinase, a member of a key antiapoptotic pathway, adding another layer of complexity to HtrA2 biology (55).…”