Abstract 2564: Targeting IGF-1/STAT3 signaling crosstalk to inhibit prostate cancer growth and metastasis

Introduction: Prostate cancer is the most prevalent cancer in American men with unknown etiology. Signal transducer and activator of transcription 3 (STAT3) is a transcription factor, which has been implicated in the growth and metastasis of prostate cancer. Activation of STAT3 through phosphorylation modulates the expression of miR-21, an oncomir, by directly binding to their promoter region in response to various cytokines and growth factors. One such growth factor is insulin-like growth factor (IGF)-1, whose elevated levels are linked to the growth of prostate cancer. IGF-1 exerts its mitogenic effect by binding to its receptor, IGF-1R, whose expression on prostate cancer cells is also increased during neoplastic transformation. There is limited information concerning the crosstalk between IGF-1/IGF-1R and STAT3 signaling pathways which could explain their oncogenic roles in prostate cancer. In this study, we examined whether STAT3-mediated increase of miR-21 is critical for the proliferative effects of IGF-1. We further show that resveratrol, a polyphenolic antioxidant, attenuates prostate cancer growth and invasiveness through regulation of IGF-1/STAT3 pathway. Methods: In vitro studies were performed on DU145 cells, which are androgen receptor negative human prostate cancer cells. Cell viability was assessed by MTS assays, while invasion and migration of cells were tested by Boyden chamber assay and wound healing assay, respectively. Modulation of protein expression was determined by Western blotting. MiR-21 levels were determined by TaqMan assay. SiRNA transfections were performed to suppress the expression of specific proteins in order to elucidate the molecular mechanisms underlying the actions of IGF-1, STAT3 and resveratrol. Results: Acute IGF-1 treatment of DU145 cells increased phosphorylation of Akt which was correlated with the activation of STAT3. This effect of IGF-1 was through activation of IGF-1R, as siRNA-mediated knockdown of IGF-1R attenuated STAT3 activation and Akt phosphorylation. IGF-1 treatment also increased miR-21 levels and decreased the expression of its downstream protein, programmed cell death 4 (PDCD4). Effects of IGF-1 were completely attenuated by resveratrol. Furthermore, knockdown of IGF-1R and STAT3 mimicked the actions of resveratrol on DU145 cells suggesting that a signaling crosstalk exists between IGF-1 and STAT3 pathways which can be targeted to inhibit prostate cancer. Conclusion: This study demonstrates, for t