Abstract 2452: Silibinin inhibits lipid metabolism by primarily targeting the master regulator sterol response element binding protein 1 (SREBP1) in prostate cancer cells

Abstract

Abstract Aberrant lipid metabolism is a major factor in growth, progression and invasion/metastasis of prostate tumors. Prostate cancer (PCa) cells are unique in terms of their low glycolytic rate and weak expression of glucose transporters. Concomitantly, PCa cells overexpress fatty acid synthase (FASN) which is considered a metabolic oncogene. As glucose consumption is not elevated in PCa, a metabolic switch to lipogenesis and fatty acid oxidation provides energy and cellular building blocks for abnormal cell growth and proliferation. In the present study, we evaluated the effect and related molecular mechanisms of a natural flavonoid silibinin in inhibiting lipid metabolism in PCa cells. Results showed that silibinin (30-90 µM) inhibited lipid and cholesterol accumulation specifically in human prostate carcinoma LNCaP and DU145 cells but not in non-neoplastic prostate epithelial PWR-1E cells. This inhibition by silibinin was mediated via a decrease in the expression of key enzymes regulating fatty acid and cholesterol synthesis including FASN, acetyl CoA carboxylase, acetyl CoA synthetase and HMG CoA reductase. Since most of these lipogenic enzymes are transcriptionally controlled by SREBP, we next analyzed the effect of silibinin on this master transcriptional regulator of lipogenesis. Silibinin inhibited SREBP1 (as well as SREBP2) protein expression without affecting its mRNA level in PCa cells suggesting a post-translational mode of regulation. SREBP1 activation involves proteolytic cleavage of precursor protein (125 kDa), generating a 68 kDa fragment that translocates to the nucleus and binds to sterol response element on target genes. Confocal and Immunoblot analyses confirmed that silibinin inhibited the proteolytic cleavage and subsequent nuclear translocation of SREBP1. Importantly, silibinin activated AMPK that phosphorylates SREBP1 (at Ser-372 site) inhibiting its nuclear translocation and transcriptional activity. Moreover, AMPK inhibition, through Compound C, reversed silibinin-mediated inhibition of nuclear SREBP1 and lipid accumulation. The central role of SREBP1 in silibinin-mediated effects on lipid accumulation and cell growth inhibition was also established by using a specific synthetic SREBP inhibitor, fatostatin. Parallely, we also observed that silibinin attenuates fatty acid uptake through down regulating LDL receptor expression. Furthermore, silibinin also significantly reduced AMACR (α-methylacyl-CoA racemase) expression in PCa cells, suggesting that it could also negatively regulate fatty acid oxidation. Overall, we found that silibinin targets SREBP1 and metabolically starves PCa cells by reducing de novo lipogenesis and lipolysis leading to a strong growth inhibition. Considering the critical role of lipid metabolism in prostate carcinogenesis, non-toxic silibinin could be useful in PCa prevention and treatment. Citation Format: Dhanya K. Nambiar, Gagan Deep, Rana P. Singh, Chapla Agarwal, Rajesh Agarwal. Silibinin inhibits lipid metabolism by primarily targeting the master regulator sterol response element binding protein 1 (SREBP1) in prostate cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2452. doi:10.1158/1538-7445.AM2014-2452