Deguelin, a Novel Hsp90 Inhibitor, Suppresses STAT3 and STAT5 Activity and Induces Apoptosis in the Human T-Cell Leukemia Virus Type 1-Transformed Cells.

Adult T-cell leukemia (ATL) cells are remarkably resistant to conventional chemotherapy. Therefore, new agents that effectively kill these cells are needed. Deguelin is a rotenoid from African plant Mundulea sericea which has been recently shown to suppress the lung tumorigenesis via inhibiting the activation of PI3kinase-Akt signaling. More recently, deguelin has been shown to bind to ATP-binding pocket of heat shock protein (Hsp) 90 and inhibit functions of Hsp90 client proteins. However, the effect of deguelin on ATL cells is not known. We investigated the effects of deguelin on the cell growth and cell death of human T-cell leukemia virus type 1 (HTLV-1)- transformed cells, KUT-1 and MT-2 (kindly provided by Dr. Hanada, Kagoshima Medical Center, Kagoshima and Dr. Kibata, Hayashibara Biochemical Laboratory Inc., Okayama, Japan, respectively) by trypan-blue exclusion and MTT assays, and annexin V/PI staining. Deguelin inhibited the proliferation of KUT-1 and MT-2 cells in a time-and a dose-dependent manner and had much less cytotoxic effects on normal human peripheral blood mononuclear cells. In addition, flow cytometric analysis revealed a higher frequency of annexin V-positive cells in deguelin treated cells compared to that in untreated cells. These results indicate that deguelin induces the cell-growth arrest and cell death in ATL cells. To better understand the mechanism of these effects, we examined the intracellular signaling and the expression of apoptosis associated proteins by immunoblot analysis. We found that deguelin prevented the tyrosine phosphorylation of the transcription factor signal transducer and activator of transcription (STAT) 3 and STAT5, both of which are constitutively activated in these cells. In addition, the inhibitor of apoptosis protein, XIAP expression decreased at 24 hours after deguelin treatment, indicating that deguelin induces the cell growth arrest and apoptosis via at least in part the inhibition of STAT3 and STAT5 phosphorylation and the downregulation of XIAP expression. We next examined whether deguelin suppresses janus kinase (JAK) 3 phosphorylation in KUT-1 cells using immunoprecipitation assay. However, deguelin treatment did not inhibit JAK3 phosphorylation, suggesting that the inhibition of STAT5 phosphorylation by deguelin is independent of JAK3 in KUT-1 cells. Since deguelin has been shown to inhibit the function of Hsp90 client proteins, we examined the protein expression of cyclin dependent kinase (Cdk) 4, which is known as one of the Hsp90 client proteins. Deguelin treatment reduced Cdk4 expression in both cells, suggesting that deguelin may destabilize Cdk4 via inhibiting the Hsp90 function. Taken together, deguelin appears to inhibit the proliferation and induce apoptosis of ATL cells via preventing the tyrosine phosphorylation of STAT proteins and destabilizing Cdk4. Thus, deguelin merits further investigation as a potential therapeutic agent for this incurable disease.