Understanding of the role of SREBP-1c neddylation in hepatic lipogenesis and validation of a neddylation inhibitor as a therapeutic for hepatic steatosis

Abstract

Neural precursor cell expressed, developmentally down-regulated 8 (NEDD8) is a ubiquitin-like protein known to regulate protein stabilization and activity by binding to lysine residues of the substrate protein, a process known as neddylation. Neddylation is a type of post-translational modification, which has been recently shown to play a role in regulating lipid metabolism. Lipogenesis and lipid accumulation in the liver cause hepatic steatosis, and may lead to hepatitis, cirrhosis, and hepatic carcinoma. However, the effects of neddylation on lipogenesis and lipid accumulation in the liver have not previously been studied. The present study demonstrated that neddylation is essential for lipogenesis and lipid accumulation in the liver. According to the NCBI GEO dataset (GSE89632), NEDD8 is typically expressed in liver tissues of patients with hepatic steatosis. Furthermore, NEDD8 knockdown reduces lipid accumulation in HepG2 cells. Here I present that sterol regulatory element-binding protein-1c (SREBP-1c), a major transcription factor involved in lipogenesis and lipid accumulation in the liver, was neddylated. In addition, neddylation of SREBP-1c not only inhibited ubiquitination of SREBP-1c but also increased its transcriptional activity via increased protein stabilization. I also found that human double minute 2 (HDM2), a known as E3 ligase for peroxisome proliferator-activated receptor gamma (PPAR- neddylation, mediates neddylation of SREBP-1c. Then, since activity of SREBP-1c is a known cause of hepatic steatosis and the transcriptional activity of SREBP-1c is increased in the liver of high fat diet (HFD)-fed mice, HFD-fed mice were treated with MLN4924, a neddylation inhibitor, which reduced both SREBP-1c expression and its transcriptional activity in the liver. As expected, treatment of MLN4924 was effective in attenuating hepatic steatosis by reducing hepatic triglycerides and decreasing lipogenic genes expression. Taken together, these results suggest that neddylation of SREBP-1c contributes to hepatic steatosis formation and its inhibitor may be a novel therapeutic target for the treatment of hepatic steatosis.