Deguelin Induces Cell Growth Arrest and Cell Death by Destabilizing Phosphorylated STAT3 and Survivin in Adult T-Cell Leukemia Cells.

Abstract 4784

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 effects of deguelin on ATL cells are poorly understood. To investigate the effects, we examined 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. Harashima, Hayashibara Institute Inc., Okayama, Japan, respectively) by trypan-blue exclusion and Annexin V/PI stain. Deguelin inhibited the proliferation of KUT-1 and MT-2 cells in a time- and dose-dependent manner (IC50=0.5μM). In addition, flow cytometric analysis revealed a higher frequency of Annexin V(+) cells in deguelin-treated cells compared to that in untreated cells (KUT-1 P< 0.01, MT-2 P< 0.01, respectively). These results indicate that deguelin induces the cell growth arrest and cell death of 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, which is constitutively activated in these cells. In addition, the inhibitor of apoptosis protein, survivin expression was decreased 24 hours after deguelin treatment, indicating that deguelin induces the cell growth arrest and apoptosis via at least in part the inhibition of STAT3 phosphorylation and the down-regulation of survivin expression. To determine the mechanism of inhibition of STAT3 phosphorylation and survivin expression, MT-2 cells were treated with proteasome inhibitor MG132 and/or deguelin. MG132 restored STAT3 dephosphorylation and down-regulation of survivin expression by deguelin. In addition, quantitative RT-PCR assay showed that deguelin treatment did not alter survivin gene expression. These results suggested that deguelin induces the degradation of phosphorylated STAT3 and survivin via the ubiquitination/proteasome pathway in ATL cells. Since deguelin has been shown to inhibit the function of Hsp90 client proteins, deguelin may destabilize phosphorylated STAT3 and survivin by inhibiting Hsp90 function. Taken together, deguelin appears to induce cell growth arrest and apoptosis of ATL cells via the suppression of the tyrosine phosphorylation of STAT3 and down-regulation of survivin. Thus, deguelin merits further investigation as a potential therapeutic agent for this incurable disease.