Glucagon Action in the Nucleus of Solitary Tract Regulates Hepatic Triglyceride Secretion
Creators: Grewal, Mantash S.
Publisher: name: University of Alberta Library
Year: 2023

<p>Background: Diabetes and obesity are major metabolic disorders that are increasing in prevalence around the whole world at an alarming rate. Both disorders are characterized by dyslipidemia, which is in part contributed to by elevated triglyceride (TG)-rich very low-density lipoprotein (VLDL-TGs). In addition to regulating glucose homeostasis, the pancreatic hormone, glucagon is involved in the regulation of hepatic lipid metabolism. Glucagon activates its receptor (GCGR) in the liver to affect hepatic triglyceride, fatty acid, and cholesterol metabolism. Increased circulating glucagon reduces hepatic lipoprotein production, triglyceride secretion, and plasma triglycerides. Glucagon can cross the blood brain barrier, specifically in the mediobasal hypothalamus (MBH) to modulate hepatic glucose metabolism and appetite. Another region that can respond to glucagon action to regulate hepatic glucose metabolism in the brain is the dorsal vagal complex (DVC). The DVC is located in the brainstem and is composed of 3 different nuclei: the nucleus of solitary tract (NTS), area postrema (AP), and the dorsal nucleus of the vagus (DMV). The DVC is a brain region which senses nutrients and hormones to coordinate metabolic homeostasis, including lipid metabolism. However, whether glucagon acts in the NTS to affect hepatic triglyceride secretion and plasma triglyceride levels in healthy, high-fat diet (HFD)-induced hypersecretion of TGs, and type 2 diabetic (T2D) animals remains unknown. Hence, in this study we aimed to elucidate a mechanism of direct glucagon action in the NTS to regulate hepatic triglyceride secretion in healthy, HFD-induced hypersecretion of TGs, and T2D animals. I hypothesize that NTS glucagon infusion will lower hepatic VLDL-TG secretion in healthy rats but not HFD-induced hypersecretion of TGs or in T2D animals.</p>

<p>Methods: Eight-week-old male Sprague Dawley rats underwent stereotaxic NTS cannulation and vascular catheterizations allowed for direct NTS infusions, intravenous injections, and blood sampling. A subset of the rats received injections of virally delivered, GcgrShRNA, PKAshRNA or control shRNA sequence for loss of function studies. In addition to assessing NTS glucagon in chow-fed rats, we also tested the efficacy of NTS glucagon to regulate hepatic VLDL-TG secretion in two additional animal models known to exhibit insulin resistance. As such, a set of rats were placed on HFD for 3 days to induce hepatic TG hypersecretion, and another set of rats were given intraperitoneal injections of nicotinamide and streptozotocin and placed on HFD for 7 days to induce type 2 diabetes. Plasma TGs were measured in 10h-fasted rats after intravenous poloxamer injection to assess the rate of hepatic VLDL-TG secretion in response to concurrent NTS infusions. At the end of experiment, tissue samples are collected for protein and gene analysis. </p>

<p>Results: In regular chow-fed rats (RC), NTS glucagon decreased TG secretion compared to NTS vehicle controls. This was mediated via GCGR and protein kinase A (PKA) since pharmacological and genetic inhibition of GCGR, or concurrent PKA inhibitor infusion, selectively in the NTS, blocked the TG-lowering effects of NTS glucagon. Additionally, inhibition of downstream kinase Mek/Erk1/2 blocked the lipid-lowering effect of NTS glucagon demonstrating the necessity of Mek/Erk1/2 to mediate the liporegulatory effects of NTS glucagon. Interestingly, NTS glucagon did not lower TG secretion in HFD-induced hyperlipidemic, or T2D, rats. However, direct NTS PKA activation, which was sufficient to recapitulate the reduction in VLDL-TG secretion induced by NTS glucagon, significantly lowered TG secretion in in both diet-induced TG hypersecretion and T2D rat models. We did not discover differences in liver TG content or changes in P-ACC, ACC, FAS, MTP protein levels or Srebf1c, Dgat1, Dgat2, Scd1, Lpin2, Arf1, Ppara, Cpt1a gene expression within the livers of the rats from these experiments that could account for the lipid-lowering effect of NTS glucagon. Of note, reduced VLDL-TG secretion rates were associated with significantly decreased plasma FFA levels in RC rats given NTS glucagon and in RC, HFD, and T2D rats that received NTS PKA activator, Sp-cAMPs, compared to their NTS vehicle control counterparts.</p>

<p>Conclusion: Hindbrain glucagon signalling in healthy RC rats modulates hepatic VLDL-TG secretion via NTS GCGRs, PKA and Mek/Erk1/2. Glucagon signalling in the NTS is abolished in models of diet-induced TG hypersecretion and T2D animals. However, direct activation of NTS PKA recapitulates glucagon’s effect to lower hepatic VLDL-TG secretion in these three rat models. These findings may provide insight on lowering lipids in hypertriglyceridemia. </p>