Molecular chaperon HSP regulates apoptosis signaling

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In this study, the involvement of heat shock protein 90 in the anti-Fas antibody (CH-11)-induced apoptosis in HSG cells was investigated using geldanamycin (GDM), the specific inhibitor of HSP90. GDM itself caused the apoptosis in HSG cells, and moreover, it enhanced CH-11-induced apoptosis. RT-PCR and Westernblotting revealed that the expression of HSP90 was up-regulated by GDM, suggesting that the immediate complementation of HSP90 occurs after the functional inhibition of HSP90 by GDM. Further, the transfection of HSP90 protein into HSG cells significantly inhibited the enhancement of CH-11-induced apoptosis by GDM. Immunoprecipitation-Westernblotting analysis revealed FLIP, Caspase 8 and RIP as target proteins of HSP90. FLIP mRNA level was significantly decreased by GDM treatment. From these results, it was revealed that HSP90 negatively regulates Fas-induced apoptosis through its chaperoning effect on FLIP, the dominant-negative Caspase 8. Furthermore, we examined the transcription activation from the heat shock element-harboring luciferase reporter gene that was transfected into HSG cells. Treatment of the transfected cells with GDM, as well as heat shock treatment, increased the luciferase activity as compared with the untreated cells. This result suggested that GDM increases the expression of HSP90 through the dissociation of heat shock factor from HSPs and its following activation, translocation into nuclei, and then transcription activation of HSP90 gene.