1. ¹Interdepartmental Program in Vascular Biology and Transplantation and Department of Pathology, Yale University School of Medicine, 295 Congress Avenue, New Haven, CT 06510, USA
2. ²Department of Pharmacology, Boyer Center for Molecular Medicine, Yale University School of Medicine, 295 Congress Avenue, New Haven, CT 06510, USA
3. ³Cell Signaling Technology, Inc., Beverly, MA 01915, USA

Correspondence: W Min, E-mail: wang.min@yale.edu

Received 8 December 2004; Revised 10 January 2005; Accepted 10 January 2005; Published online 14 March 2005.

Abstract

Hsp90 client protein Akt has been shown to inhibit cell apoptosis in part by inhibiting proapoptotic kinase ASK1 (apoptosis signal-regulating kinase 1) activity. In the present study, we show that Hsp90 inhibits hydrogen peroxide (H₂O₂)-induced ASK1–p38 activation in endothelial cells (EC). The inhibitory effect of Hsp90 on ASK1–p38 activities is diminished when the Akt phosphorylation site on ASK1 (pSer83) is absent or when Akt is genetically deleted in cells, suggesting that Hsp90 and Akt function together to inhibit ASK1–p38 signaling. Thus, inhibition of Hsp90 by 17-allyamino-17-demethoxygeldanamycin (17-AAG) or phosphatidylinositol 3-kinase (PI3K) LY294002 induced and synergized ASK1 activation and ASK1-mediated EC apoptosis. Furthermore, we show that in resting EC Hsp90, Akt and ASK1 form a ternary complex in which both Akt and ASK1 bind to the middle domain of Hsp90, suggesting that Hsp90 may hold Akt and ASK1 in close proximity. The N-terminal domain of ASK1 containing the Akt phosphorylation site (pSer83) associates with Akt in resting state. However, Akt is released from the N-terminal domain concomitant with binding to the C-terminal domain of ASK1 in response to ASK1 activator H₂O₂, inhibitor of Hsp90 17-AAG and Akt inhibitor LY294002, leading to a more stable Hsp90-Akt–ASK1 complex. We conclude that Hsp90–Akt forms a complex with ASK1 and protect EC from stress-induced apoptosis.