170 RADIOSENSITIZING EFFECT OF ZOLEDRONIC ACID ON RENAL CELL CARCINOMA CELLS BY DOWNREGULATING STAT1

Abstract

INTRODUCTION AND OBJECTIVES: Zoledronic acid (ZOL) is widely used for the treatment of bone metastasis from a variety of malignancies including renal cell carcinoma (RCC). We previously reported that ZOL clinically potentiates antitumor effects of radiotherapy (RT) on bone metastases from RCC (BJU Int. 2009). To date, however, it remains unknown whether and how ZOL radiosensitizes RCC. Overexpression of signal transducer and activator of transcription 1 (STAT1) has been shown to be associated with intrinsic RT resistance of tumors. The aim of this study is to investigate the role of STAT1 in ZOL-mediated radiosensitization of RCC cells. METHODS: Four human RCC cell lines were pretreated with ZOL at 10 M (786-O and Caki-1) or 20 M (A-498 and ACHN) for 48 h, and thereafter irradiated at a dose of 0, 4, and 8 Gy. The radiosensitizing effect of ZOL was evaluated by clonogenic cell survival assay. The effect of ZOL on STAT1 expression was evaluated by Western blot and real-time RT-PCR. To confirm the importance of STAT1 on radiosensitization of RCC cells, both functional siRNA knockdown and forced expression by cDNA transfection were performed. RESULTS: The radiosensitization by ZOL was observed in 786-O, A-498, and ACHN cells but not in Caki-1 cells. The three RCC cell lines, in which ZOL exerted a radiosensitizing effect, expressed STAT1 abundantly but Caki-1 cells did not. ZOL downregulated endogenous STAT1 expression in 786-O, A-498, and ACHN cells at protein level, but did not at mRNA level, indicating that ZOL attenuates STAT1 expression by a post-transcriptional modification. The knockdown of endogenous STAT1 by siRNA sensitized 786-O cells to RT equivalently to ZOL, and the introduction of exogenous STAT1 rendered Caki-1 cells more RT-resistant. CONCLUSIONS: We demonstrated that ZOL directly radiosensitizes RCC cells by post-transcriptionally downregulating STAT1. Because tumor cells commonly overexpress STAT1, ZOL warrants further clinical and translational studies as a potent radiosensitizer against RT-resistant tumors overexpressing STAT1.