Heat shock protein 90α (HSP90α), a substrate and chaperone of DNA-PK necessary for the apoptotic response

The "apoptotic ring" is characterized by the phosphorylation of histone H2AX at serine 139 (gamma-H2AX) by DNA-dependent protein kinase (DNA-PK). The gamma-H2AX apoptotic ring differs from the nuclear foci patterns observed in response to DNA-damaging agents. It contains phosphorylated DNA damage response proteins including activated Chk2, activated ATM, and activated DNA-PK itself but lacks MDC1 and 53BP1, which are required to initiate DNA repair. Because DNA-PK can phosphorylate heat shock protein 90 alpha (HSP90 alpha) in biochemical assays, we investigated whether HSP90 alpha is involved in the apoptotic ring. Here we show that HSP90 alpha is phosphorylated by DNA-PK on threonines 5 and 7 early during apoptosis and that both phosphorylated HSP90 alpha and DNA-PK colocalize in the apoptotic ring. We also show that DNA-PK is a client of HSP90 alpha and that HSP90 alpha is required for full DNA-PK activation, gamma-H2AX formation, DNA fragmentation, and apoptotic body formation. In contrast, HSP90 inhibition by geldanamycin markedly enhances TRAIL-induced DNA-PK and H2AX activation. Together, our results reveal that HSP90 alpha is a substrate and chaperone of DNA-PK in the apoptotic response. The response of phosphorylated HSP90 alpha to TRAIL and its localization to the gamma-H2AX ring represent epigenetic features of apoptosis that offer insights for studying and monitoring nuclear apoptosis.