Roles of 3-phosphoinositide-dependent kinase 1 in the regulation of endothelial nitric-oxide synthase phosphorylation and function by heat shock protein 90

The 90-kDa heat shock protein (Hsp90) plays an important role in endothelial nitric-oxide synthase (eNOS) regulation. Besides acting as an allosteric enhancer, Hsp90 was shown to serve as a module recruiting Akt to phosphorylate the serine 1179/1177 (bovine/human) residue of eNOS. Akt is activated by the phosphorylation of 3-phosphoinositide-dependent kinase 1 (PDK1). Whether PDK1 is involved in the actions of Hsp90 on eNOS phosphorylation and function remains unknown. To address this issue, we treated bovine eNOS stably transfected human embryonic kidney 293 cells with Hsp90 inhibitors and determined the alterations of phospho-eNOS, Akt, and PDK1. Both geldanamycin and radicicol, two structurally different Hsp90 inhibitors, selectively reduced serine 1179-phosphorylated eNOS, leading to decreased enzyme activity. In Hsp90-inhibited cells, eNOS-associated phospho-Akt was decreased, but the total amount of Akt associated with eNOS remained the same. Further studies showed that Hsp90 inhibition dramatically depleted intracellular PDK1. Proteasome but not caspase blockade prevented the loss of PDK1 caused by Hsp90 inhibition. Silencing the PDK1 gene by small interfering RNA was sufficient to induce reduction of phospho-Akt and consequent loss of serine 1179-phosphorylated eNOS. Moreover, overexpression of PDK1, but not Akt, reversed Hsp90 inhibition-induced loss of eNOS serine 1179 phosphorylation and salvaged enzymatic activity. Thus, in addition to functioning as a module to recruit Akt to eNOS, Hsp90 also critically stabilized PDK1 by preventing it from proteasomal degradation. Inhibition of Hsp90 function resulted in PDK1 depletion and thus triggered a cascade of Akt deactivation, loss of eNOS serine 1179 phosphorylation, and decrease of enzyme function.