Heat shock protein 27 (HSP27) is implicated in diverse biologic functions as a molecular chaperone. We found that HSP27 is involved in the protection of human cells against UVC lethality. To elucidate the molecular mechanisms underlying UVC resistance, we searched for HSP27-interacted proteins related to resistance in UVC-resistant human cells, APr-1. Three candidates for HSP27-interacted proteins were found from cell lysates using an affinity column coupled with GST-fused HSP27 protein. Interaction between HSP27 and two candidates, annexin II and HSP70, was confirmed by immunoprecipitation analysis. After UVC irradiation, the amount of the complex of HSP27 and annexin II decreased in the postnuclear fraction, while it increased in the nuclear fraction. Cells transfected with annexin II-siRNA were more susceptible to UVC lethality. These results suggest that annexin II is a novel HSP27-interacted protein which is involved in UVC resistance in human cells, at least those tested here.

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In this study, we found that refractoriness to ultraviolet (UVC) light-induced cell death was increased in UVC-radiation-sensitive cells derived from Cockayne syndrome patients when the cells were precultured in medium supplemented with recombinant annexin II (rANX II). In CS3BES cells, an immortal cell line derived from Cockayne syndrome patients, the rANX II supplementation-induced UVC-radiation resistance was suppressed by treatment with an anti-annexin II antibody and EGTA. The amount of biotinylated annexin II on the cell surface increased in the rANX II-supplemented cells but did not increase in the cells that were cotreated with rANX II and EGTA. The capacity to remove UVC-radiation-damaged DNA, (6-4) photoproducts and cyclobutane pyrimidine dimers, was the same in cells that were precultured with rANX II and in control cells that did not receive rANX II supplementation. The rANX II supplementation-induced UVC-radiation resistance was also observed in nucleotide excision repair-deficient cells and xeroderma pigmentosum group A-downregulated cells. The Bcl-xL to Bax protein ratios, an index of survival activity in cells exposed to lethal stresses, were increased in the cells that had been precultured in rANX II for 24 h prior to UVC irradiation. Treatment with a phosphatidylinositol 3-kinase inhibitor suppressed the increased UVC-radiation resistance and Bcl-xL to Bax ratios in the cells with rANX II supplementation. Furthermore, downregulation of Bcl-xL by siRNA transfection also suppressed the UVC-radiation resistance that was induced by rANX II supplementation. These results suggest that the increase in the Bcl-xL to Bax ratios may be associated with enhanced resistance to UVC-radiation-induced cell death.

Proteases have received attention as important cellular components responsible for stress response in human cells. However, little is known about the role of proteases in the early steps of cell response after X-ray irradiation. In the present study, we first searched for proteases whose activity levels are changed soon after X-ray irradiation in human RSa cells with a high sensitivity to X-ray cell-killing. RSa cells showed an increased level of fibrinolytic protease activity within 10 min after irradiation with X-ray (up to 3 Gy). The induced protease activity was proved to be inhibited by leupeptin. We next examined whether this protease inducibility is related to the X-ray susceptibility of cells. Treatment of RSa cells with leupeptin prior to X-ray irradiation resulted in lowered colony survival and an increased ratio of G(2)/M-arrested cells and apoptotic cells. These results suggest that leupeptin-sensitive proteases are involved in the resistance of human RSa cells to X-ray cell-killing.