Diabetes and obesity are associated with dysregulated liver lipid metabolism. Glucagon has an essential role in lipid metabolism, in addition to regulating glucose homeostasis. The brain is a key coordinator of whole-body metabolism. Glucagon signalling in the mediobasal hypothalamus (MBH), a forebrain region that senses hormones and nutrients to regulate metabolism, lowers hepatic glucose production and appetite. However, whether glucagon action in the MBH affects lipid homeostasis in healthy and high-fat diet (HFD)-fed animals remains unknown. We assessed glucagon signaling in the MBH in regulating hepatic TG secretion in chow-fed normolipidemic and HFD-induced hyperlipidemic rats.
Stereotaxic MBH cannulation and vascular catheterizations in Sprague-Dawley rats allowed for direct MBH infusions, intravenous (IV) injections, and blood sampling. Plasma TGs were measured in 10h-fasted rats after IV poloxamer injection with concurrent MBH infusions. MBH glucagon decreased TG secretion compared to MBH vehicle control animals. This hypothalamic effect of glucagon was blocked by a simultaneous MBH co-infusion of an antagonist to the glucagon receptor (GCGR) and by protein kinase A (PKA) inhibition in the MBH in chow-fed rats. Direct MBH infusion of a PKA activator recapitulated the TG-lowering effects of MBH glucagon. Together, this suggests that MBH glucagon signals via a MBH GCGR-PKA pathway. Interestingly, both MBH glucagon and MBH PKA activation lowered TG secretion and plasma FFAs in HFD-induced hyperlipidemic animals. However, MBH glucagon did not affect hepatic TG content or liver lipogenic protein levels of MTP, FAS, or pACC:ACC of RC and HFD rats compared to their MBH vehicle controls.
We demonstrate that hypothalamic glucagon modulates hepatic TG secretion via a MBH GCGR-PKA signalling pathway, and activating MBH glucagon signalling improves lipid metabolism in hyperlipidemic rats. These findings may provide therapeutic insight on lowering lipids in hyperlipidemia.
M. S. Grewal: None. B. Vasilev: None. C. M. Soltys: None. B. Lum: None. J. T. Yue: None.
Canadian Institutes of Health Research (PJT-378765); Canada Foundation for Innovation (37267); Canada Research Chairs Tier 2 in Brain Regulation of Metabolism