Akt-mediated Valosin-containing Protein 97 Phosphorylation Regulates Its Association with Ubiquitinated Proteins^*

Abstract

Hypoxia is a common environmental stress that influences signaling pathways and cell function. Previous studies from our laboratory have identified significant differences in cellular responses to sustained or intermittent hypoxia with the latter proving more cytotoxic. We hypothesized that differences in susceptibility of neurons to intermittent (IH) and sustained hypoxia (SH) are mediated by altered Akt signaling. SH, but not IH, induced a significant increase in Akt activation in rat CA1 hippocampal region extracts compared with room air controls. Akt immunoprecipitations followed by proteomic analysis identified valosin-containing protein (VCP) as an Akt-binding protein. In addition, VCP expression and association with Akt was enhanced during SH, and this association was decreased upon phosphoinositide 3-kinase/Akt pathway blockade with LY294002. Active recombinant Akt phosphorylated recombinant VCP in vitro. Site-directed mutagenesis studies identified Ser³⁵², Ser⁷⁴⁶, and Ser⁷⁴⁸ as Akt phosphorylation sites on VCP. In addition, rat CA1 hippocampal tissue exposed to SH exhibited an acidic pI shift of VCP. Protein phosphatase 2A treatment inhibited this acidic shift consistent with SH-induced phosphorylation of VCP in vivo. PC-12 cells transfected with active Akt, but not dominant negative Akt or vector, induced VCP expression and an acidic shift in VCP pI, which was inhibited by protein phosphatase 2A treatment. Furthermore, VCP association with ubiquitinated proteins was demonstrated in vector-transfected PC-12 cell lysates, whereas active Akt-transfected cells demonstrated a marked decrease in association of VCP with ubiquitinated proteins. We concluded that Akt phosphorylates VCP in vitro and in vivo, and VCP phosphorylation releases it from ubiquitinated substrate protein(s) possibly allowing ubiquitinated protein(s) to be degraded by the proteosome.