Date of this Version
Roberts, JL (2015). Attenuation of mTORC1-driven secretion of lipoproteins and triacylglycerols by short chain fatty acids: mechanistic insight into the pathogenesis of hypertriglyceridemia (Master's Thesis). University of Nebraska-Lincoln, Lincoln, NE.
The mechanistic target of rapamycin complex 1 (mTORC1) is a serine/threonine kinase that drives several anabolic processes including lipid synthesis, protein synthesis, and adipogenesis. mTORC1 is highly active in the livers of obese rodents, in overnutrition, and is implicated in the development of obesity related metabolic disorders, including the overproduction of atherogenic lipoproteins. Direct inhibition of mTORC1 is not a viable treatment strategy because it prevents feedback inhibition of the insulin-signaling cascade, leading to increased lipid synthesis and secretion of lipoproteins. Thus, therapeutic approaches that drive catabolic pathways are considered promising mechanisms for overcoming mTORC1-driven anabolism. Human liver HepG2 cells were stably transduced with lentiviral shRNA targeting tuberous sclerosis complex 2 (TSC2, upstream inhibitor of mTORC1) to constitutively activate mTORC1 or lentiviral scramble shRNA as negative control. In TSC2 shRNA cells, TSC2 gene and protein expression were suppressed, and mTORC1 activity (phospho-p70 S6K/p70 S6K) was subsequently increased. Hyperactive mTORC1 upregulated lipogenic and lipid transport genes and significantly increased cellular triacylglycerol content when compared to scramble shRNA cells. Furthermore, the constitutive activation of mTORC1 increased apolipoprotein B (apoB) and triacylglycerol secretion into culture media. To determine if dietary agents would reverse mTORC1-mediated secretion of lipoproteins and triacylglycerides, TSC2 cells were treated with one of two short chain fatty acids, alpha-lipoic acid (LA; 200 μM) or 4-phenylbutyric acid (PBA; 8 mM). LA and PBA significantly decreased secreted apoB and triacylglyceride despite their differential effects on mTORC1 activity; LA repressed mTORC1 signaling in TSC2 shRNA cells; whereas PBA did not. In addition to lowering secreted triacylglycerol LA decreased cellular triacylglycerol content but not PBA, which increased cellular triacylglycerol. We conclude that LA and PBA have the potential to lower the secretion of atherogenic lipoproteins associated with mTORC1 signaling hyperactivation.
Advisor: Regis Moreau