Dexamethasone inhibits camptothecin-induced apoptosis in C6-glioma via activation of Stat5/Bcl-xL pathway.

Dexamethasone (DX) induces apoptosis resistance in most solid malignant tumors during co-treatment with chemotherapy agents, such as camptothecin (CAM). In this study, we investigated the mechanism by which DX reduces chemotherapy efficiency in C6-glioma. DX reduced CAM-increased DNA fragmentation and caspase-3 activation. The DX's protection was negated by RU486, an antagonist of glucocorticoid receptor (GR). DX itself increased anti-apoptotic gene, Bcl-xL expression, and its transcription factor, signaling transducer and activator of transcription 5 (Stat5), DNA binding activity and phospho-Stat5 expression. DX blocked the CAM-decreased Bcl-xL and phospho-Stat5 expression, and Stat5 binding activity. RU486 negated DX's actions. To determine whether Stat5 regulates Bcl-xL expression in CAM-induced cell death, C6-glioma was infected with an adenovirus containing a constitutively activated Stat5-GFP (Ad-Stat5ca). Overexpression of Stat5ca increased Bcl-xL and decreased CAM-induced cell death compared to control adenovirus infected cells; whereas Stat5 siRNA decreased DX-induced Bcl-xL and increased cell death. Phospho-Stat5 expression was observed in the nuclear extract by co-immunoprecipitation with an anti-GR antibody, indicating that Stat5 and GR were interactive and formed a complex in the nuclei. These results suggest that DX's prevention from CAM-induced apoptosis and RU486's antagonism of DX's protection may be through Stat5/Bcl-xL signal pathway regulated by a GR.