N-glycosylation of CREBH improves lipid metabolism and attenuates lipotoxicity in NAFLD by modulating PPARα and SCD-1

Abstract

Introduction: Cyclic adenosine monophosphate (AMP)-responsive element-binding protein H (CREBH), an endoplasmic reticulum-anchored transcription factor essential for lipid metabolism and inflammation in nonalcoholic fatty liver disease (NAFLD), is covalently modified by N-acetylglucosamine. Glycosylation is a ubiquitous type of protein involved in posttranslational modifications, and plays a critical role in various biological processes. However, the mechanism of glycosylated CREBH remains poorly understood in NAFLD.

Methods: CREBH glycosylation mutants were obtained by site-mutation methods. After transfection with plasmids, AML-12, LO2, or HepG2 cells were treated with palmitic acid (PA) proteolysis, tunicamycin (Tm), or their combination. Glycosyltransferase V (GnT-V) was used induce hyperglycosylation to further understand the effect of CREBH. In addition, glycosylation mutant mice and hyperglycosylated mice were generated by lentivirus injection to construct two kinds of NAFLD animal models. The expression of NAFLD-related factors was detected to further verify the role of N-linked glycosylation of CREBH in lipid and sterol metabolism, inflammation, and lipotoxicity.

Results: N-glycosylation enhanced the ability of CREBH to activate transcription and modulated the production of peroxisome proliferator-activated receptor alpha (PPARα) and stearoyl-CoA desaturase-1 (SCD-1) activity by affecting their promoter-driven transcription activity and protein interactions, leading to reduce lipid deposition and attenuate lipotoxicity. Deglycosylation of CREBH induced by Tm could inhibit the proteolysis of CREBH induced by PA. The addition of unglycosylated CREBH to cells upregulates gene and protein expression of lipogenesis, lipotoxicity, and inflammation, and aggravates liver damage by preventing glycosylation in cells, as well as in mouse models of NAFLD. Furthermore, increased N-glycosylation of CREBH, as achieved by overexpressing GnT-V could significantly improve liver lesion caused by unglycosylation of CREBH.

Conclusion: These findings have important implications for the role of CREBH N-glycosylation in proteolytic activation, and they provide the first link between N-glycosylation of CREBH, lipid metabolism, and lipotoxicity processes in the liver by modulating PPARα and SCD-1. These results provide novel insights into the N-glycosylation of CREBH as a therapeutic target for NAFLD.

Keywords: N-glycosylation; cyclic AMP-responsive element-binding protein H; lipid metabolism; lipotoxicity; peroxisome proliferator-activated receptor alpha; stearoyl-CoA desaturase-1.