The Role of Lipin-1 in the Regulation of Fibrogenesis and TGF-β Signaling in Hepatic Stellate Cells

Abstract

The adipogenic transcriptional regulation was reported to inhibit transdifferentiation of hepatic stellate cells (HSCs), which constitute the main fibrogenic cell type in the liver. Lipin-1 exhibits a dual function: an enzyme that catalyzes the conversion of phosphatidate to diacylglycerol and a transcriptional regulator. However, the involvement of Lipin-1 in the regulation of transforming growth factor-β (TGF-β) signaling and fibrogenesis in HSCs is not fully understood. Here, we showed that Lipin-1 was downregulated in activated primary HSCs and TGF-β-treated LX-2 cells, immortalized human HSC cell lines. The downregulation of Lipin-1 by TGF-β was not dependent on altered mRNA stability but rather on protein stability. Treatment of LX-2 cells with the proteasome inhibitor led to the accumulation of Lipin-1. Moreover, we observed a significant increase in Lipin-1 polyubiquitination. Overexpression of Lipin-1 attenuated TGF-β-induced fibrogenic gene expression. In addition, Lipin-1 inhibited TGF-β-mediated activation of Sma and Mad-related family (SMAD), a major transcription factor that transduces intracellular signals from TGF-β. Resveratrol, a well-known natural polyphenolic antioxidant, is known to inhibit liver fibrosis, although its mechanism of action remains unknown. Our data showed that resveratrol significantly increased the levels of Lipin-1 protein and mRNA in HSCs. Further investigation revealed that resveratrol blocked the polyubiquitination of Lipin-1. Resveratrol inhibited TGF-β-induced fibrogenic gene expression. TGF-β-induced SMAD binding element-luciferase reporter activity was significantly diminished by resveratrol with a simultaneous decrease in SMAD3 phosphorylation. Consistently, knockdown of the Lipin-1 gene using siRNA abolished the inhibitory effect of resveratrol. We conclude that Lipin-1 can antagonize HSC activation through the inhibition of TGF-β/SMAD signaling and that resveratrol may affect Lipin-1 gene induction and contribute to the inhibition of TGF-β-mediated hepatic fibrogenesis.

Keywords: Lipin-1; TGF-β; fibrogenesis.; hepatic stellate cells; resveratrol; ubiquitination.