Am J Physiol Gastrointest Liver Physiol. 2025 May 07.
De novo lipogenesis (DNL) converts excess glucose into lipids, while the hexosamine biosynthetic pathway (HBP), a glycolytic branch, generates UDP-N-acetylglucosamine (UDP-GlcNAc) for protein glycosylation, including O-GlcNAcylation and N-linked glycosylation. Both pathways are active in hepatocytes and integral to glucose metabolism; however, their functional interplay remains unclear. Here, we investigated the role of HBP in hepatic DNL activation using both in vitro and in vivo models. AML12 hepatocytes were cultured in low- and high-glucose media with or without HBP blockade, both pharmacologically and genetically. For in vivo studies, male C57BL/6J mice were subjected to a fasting-refeeding regimen with or without intraperitoneal administration of azaserine, a competitive inhibitor of glutamine-fructose-6-phosphate transaminase 1 (GFPT1), the rate-limiting enzyme of the HBP. Our results demonstrated that, in AML12 cells, glucose exposure activated both DNL and HBP, leading to triacylglycerol (TAG) accumulation, while HBP inhibition ameliorates DNL and TAG accumulation. In mice, refeeding after a 24-hour fasting induced hepatic DNL, which was abolished by HBP inhibition, indicating its mechanistic involvement in glucose-driven lipogenesis. Mechanistically, we identified ATF4 as a key regulator of GFPT1 upregulation under high-glucose conditions. As expected, both glucose-treated hepatocytes and livers from fasting-refed mice exhibited increased protein glycosylation. Notably, blocking N-linked glycosylation, but not O-GlcNAcylation, abolished glucose-induced DNL activation, indicating that HBP is essential for glucose-induced DNL pathway activation via promoting N-linked glycosylation, independent of O-GlcNAcylation. In conclusion, our findings establish that an intact HBP is required for glucose-induced hepatic DNL activation, primarily through promoting protein N-linked glycosylation.
Keywords: ATF4; GFPT1; GlcNAcylation; HBP; lipogenesis