Glycerol metabolism regulates gluconeogenesis
Creators: name: Xu, Huiting
Publisher: name: No Publisher Supplied
Year: 2023

description: The liver plays an essential role in regulating glucose and lipid homeostasis. Elevated hepatic gluconeogenesis (GNG) mainly contributes to hyperglycemia in type II diabetes (T2D) patients. Our lab previously demonstrated that glycerol is the dominant substrate in hepatic gluconeogenesis in vivo and in vitro. However, substrates utilization and preference in glucagon-induced GNG, usually upregulated in T2D, is unclear. By employing primary hepatocytes as the golden standard for studying liver function in physiology and stable isotope-labelled tracers which allow us to track the carbon usage, we showed that glycerol, in the presence of other GNG substrates including pyruvate/lactate and glutamine at physiological concentration, is the preferred substrate in glucagon-induced GNG by labelling 80% and 50% of glucose in both mouse and human primary hepatocytes, respectively. Interestingly, glucagon increased glycerol and pyruvate/lactate contribution to glucose carbon by 1.6- and 1.8-fold, respectively, yet glutamine contribution to glucose carbon was increased 6.4-fold in mouse primary hepatocytes, suggesting glucagon preferentially increased glutamine use, although glycerol still remains the dominant GNG substrate. In vivo studies using a PKA-activation mouse model confirmed that most circulating lactate carbons originated from glycerol, but very little glycerol was derived from lactate carbons, reflecting glycerol’s importance as the primary carbon donor to glucagon-induced GNG. Glycerol, preferred GNG substrates in hepatocytes, also plays a critical role in the regulation of hepatic gluconeogenesis. We investigated the interplay among different GNG substrates and enzymes, which demonstrated the regulatory role of glycerol in the interaction of glucagon on the regulation of hepatic GNG enzymes and GNG. We showed that glycerol at physiological concentration suppresses PCK1 expression at both basal and glucagon-activated levels through P300-mediated acetylation, inhibiting pyruvate/lactate utilization in GNG. In contrast, glycerol interacted with glucagon synergistically activates G6PC expression and glucose production. Interestingly, the deletion of glycerol kinase (GK), which catalyzes the first step of glycerol metabolism, blocks glycerol’s regulation on both G6PC and PCK1, suggesting glycerol metabolism, not glycerol itself, accounts for the regulation of GNG enzymes and GNG. By generating high-fat diet (HFD)- and constitutively active PKA-induced diabetic mouse models, we showed that the knockdown of GK significantly ameliorated the diabetic phenotype in both HFD fed control (L-HFD-GFP) and liver specific PKA activation (L-HFD-PKA) mice. It is the first report to systematically describe the regulatory role of glycerol in controlling GNG, which broadens our understanding of GNG regulation, gives insight into the pathogenesis of T2D and offers a potential therapeutic target of T2D.Glycerol stands at the crossroads of lipid and glucose metabolism as it is not only the preferred substrate for GNG but also the backbone for free fatty acid re-esterification. Diabetic patients exhibit selective hepatic insulin resistance in which insulin fails to suppress gluconeogenesis and continues to activate lipogenesis, exacerbating the pathogenesis of hyperglycemia and hypertriglyceridemia. Interestingly, liver-specific insulin receptor knockout (LIRKO) mice showed hyperglycemia paradoxically without hepatic hypertriglyceridemia. The explanation for this paradox lies in the divergence of the regulatory steps of gluconeogenic and lipogenic pathways mediated by insulin. However, it is unclear to explain the lack of fatty liver in the HFD fed LIRKO mice that can directly utilize re-esterified triglycerides for lipid storage without insulin stimulation. In this study, we generated regular chow diet (RD) and high-fat diet (HFD) fed LIRKO mice by AAV8-TBG-CRE injection and found that hepatic GK protein levels were significantly reduced with a corresponding increase of PCK1 protein levels in LIRKO mice compared to the control mice, and HFD increased GK expression in the liver. Consistent with PCK1 and GK expression, HFD-LIRKO mice showed impaired glycerol usage and flux (Fcirc) concomitant with increased pyruvate/lactate usage and flux (Fcirc) and reduced 12h fasting glucose. GK converts glycerol to glycerol-3-phosphate, which is the backbone for triglycerides. Thus, we proposed that GK reduction in HFD-LIRKO mice fails to immobilize fatty acids and synthesize triglyceride, protecting HFD-LIRKO mice from fatty liver. Further studies were conducted in liver-specific GK knockout mouse models (LGKKO) fed on RD and HFD, which showed downregulation of lipogenic genes and protection from HFD-induced fatty liver even though the insulin signaling is intact. In conclusion, our finding emphasized the essential role of GK in hepatic GNG and fat accumulation under selective insulin resistance conditions, rendering it a promising therapeutic target of type II diabetes.