Luteinizing hormone facilitates angiogenesis in ovarian epithelial tumor cells and metformin inhibits the effect through the mTOR signaling pathway

Abstract

Abstract. High levels of gonadotropin are a risk factor for ovarian cancer development. Aberrant gonadotropin levels benefit tumor angiogenesis, but the detailed mechanism is not clear. Therefore, the aim of this study was to investigate the molecular mechanism of high levels of luteinizing hormone (LH) on the promotion of tumor angiogenesis and to outline a feasible therapeutic strategy. Western blotting and immunofluorescence staining were used to determine the effect of LH on VEGF and Slit2 expression and examine the signaling pathway involved in regulating the expression of both molecules. Real-time PCR was used to investigate the effect of metformin on LH induction of VEGF and Slit2 expression. It was found that 50 mIU/ml LH significantly upregulated VEGF and Slit2 expression, and activated the PI3K/AKT-mTOR signaling pathway. However, metformin inhibited the mTOR signaling pathway and further blocked LH-induced VEGF and Slit2 expression. In conclusion, high levels of LH promote angiogenesis in ovarian cancer via the PI3K/AKT-mTOR pathway. However, metformin could inhibit tumor angiogenesis by blocking the mTOR signaling pathway. IntroductionLuteinizing hormone (LH), one of the gonadotropins, is a pituitary glycoprotein hormone that regulates ovarian steroidogenesis. The gonadotropin theory of ovarian cancer proposes that accumulated serum gonadotropins, follicle-stimulating hormone (FSH) and LH, contribute to the development of ovarian cancer. A growing body of evidence has implicated the role of LH in the etiology of ovarian epithelial cancer (OEC). Previous studies show that LH inhibits Fas-induced apoptosis in ovarian cancer and supports the hypothesis of hormonal involvement in ovarian carcinogenesis (1). In addition, it reports that LH could enhance the invasion and migration of ovarian cancer via the PI3K/AKT signaling pathway. Recent evidence suggests that gonadotropins promote ovarian cancer OV207 and OVCAR-3 cell migration and proliferation by activation of ERK1/2 signaling in a calciumand PKC δ-dependent manner (2). However, little is known about the role of LH in tumor angiogenesis.Vascular endothelial growth factor (VEGF) is a glycoprotein which is associated with tumor angiogenesis (3). Higher levels of VEGF are consistent with poor prognosis and tumor progression in various types of tumors, including ovarian cancer (4). Elevated levels of gonadotropin are risk factors for ovarian cancer and have multiple bio-activations; a study addressed the role of LH on angiogenesis in ovarian cancer. Wang et al reported that LH induced VEGF expression in AO cells in a dose-dependent manner. This study suggests that VEGF may accelerate ovarian tumor growth and tumor recurrence under elevated gonadotropin levels after menopause or surgery (5). However, the detailed mechanism of the LH-induced VEGF expression is not clear. Except of VEGF, a recent study indicates that Slit2, a secreted protein known to function through the Roundabout (Robo) receptor as a chemorepellent in axon guidance and neuronal migration...

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