Hypothalamic Glucocorticoid Action Regulates Lipid Metabolism
Creators: Cardoso, Miguel
Publisher: name: University of Alberta Library
Year: 2022

<p>Background: In metabolic disease statues, such as diabetes and obesity, dyslipidemia is characterized by a dysregulation of lipid homeostasis, due in part to elevated triglyceride (TG)-rich very low-density lipoprotein (VLDL-TG) production and secretion by the liver. Further, these metabolic diseases are associated with excessive levels and/or actions of glucocorticoids (GCs), which may contribute to dyslipidemia. The dysregulation of VLDL-TG secretion, and particularly its hypersecretion in metabolic disease states, remains largely unknown. The brain, including the medial basal hypothalamus (MBH), senses circulating nutrients and hormones to regulate lipid metabolism and VLDL-TG secretion. Whereas the peripheral effects of GCs are well known, including the direct effect of GCs to stimulate hepatic VLDL-TG secretion, the central effects of GCs acting in the MBH to regulate lipid metabolism is unknown. Given the link between GCs and metabolic disease states, that GCs act in the periphery to effect lipid metabolism, and the fact that the brain is a hormone-responsive liporegulatory site, the aim of this study was to explore if GCs can act directly in the MBH to regulate liver VLDL-TG secretion to better understand mechanisms by which blood lipid levels may be lowered in those with dyslipidemia. <br>
Hypothesis: (1) GCs act via the glucocorticoid receptors (GRs) in the MBH to modulate liver lipid homeostasis by increasing plasma TG and hepatic VLDL-TG secretion; (2) inhibition of MBH GC action (by blocking MBH GRs or downstream mediators of GC-GR action specifically within the MBH), will improve liver lipid metabolism in diet-induced hyperlipidemic rats.<br>
Methods: Male Sprague Dawley rats underwent stereotaxic MBH bilateral cannulation and vascular catheterization to enable direct-continuous MBH infusion, intravenous infusion, and arterial blood sampling. Following MBH cannulation, a subset of rats received either a MBH specific injection of a lentivirus containing a GRshRNA, to act as a GR knockdown, a heat shock protein 90 sh RNA (Hsp90 shRNA), to act as an Hsp90 knockdown, or a mismatch (MM) sequence to act as a control. A subset of rats were subjected to a 3-day high fat diet (HFD), as an acute model of diet-induced obesity. Following a 10-hour fast, free-moving, conscious rats were subjected to a direct, and continuous MBH infusion of a specific brain treatment (dexamethasone (DEX), as a synthetic GC, mifepristone, as a GR inhibitor, or a Hsp90 inhibitor) or saline paired with an intravenous poloxamer injection to inhibit lipoprotein lipase. Plasma samples were collected and TGs, glucose, and free fatty acids (FFAs) were quantified. Following experimentation rats were euthanized and brain and liver samples were collected for gene expression and protein level analysis. <br>
Results: Direct MBH GC infusion, in the form of DEX, increased plasma VLDL-TG secretion compared to MBH vehicle-infused controls, and this effect was negated via the inhibition of the MBH GR. These changes occurred independent of changes in plasma apolipoprotein B (apoB)- 48, apoB-100, glucose, FFAs, hepatic expression of Srebf1c, Scd1, Dgat1/2, Fasn, Lpin2, Cideb, Nr1h3, Nr1h2, Arf1, Cpt1a, Ppara, or hepatic protein levels of MTP, P-ACC/ACC, DGAT1, Arf1, or FAS relative to controls. In HFD-fed rats, which are characterized by elevated basal plasma GC and TGs, the chronic (13-day) inhibition of the GR via GRshRNA or the inhibition of Hsp90, in the MBH, significantly decreased plasma TGs and TG-rich lipoprotein secretion. The chronic (13-day) inhibition of Hsp90 in the MBH, using Hsp90 shRNA, in the 3-day HFD fed rats was associated with lower basal plasma FFAs, and lower plasma TG:apoB-100 ratio following experimentation compared to HFD controls. <br>
Conclusion: The results of these studies provide evidence that MBH GC action alters fasting lipid metabolism, by increasing TG-rich lipoprotein secretion. Further, we show for the first time that inhibition of GRs in the MBH negates diet-induced hypertriglyceridemia. The significance of this data is that it suggests that GC action in the hypothalamus may contribute to TG-rich lipoprotein overproduction and dysregulation of lipid homeostasis in diet-induced hypertriglyeridemia. </p>