Date of Award

8-2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Nutritional Sciences

Major Professor

Guoxun Chen

Committee Members

Jay Whelan, Ling Zhao, Michael Karlstad

Abstract

Skeletal muscle glucose metabolism can affect whole body glucose homeostasis significantly. Vitamin A (VA) plays a role in a number of physiological functions including glucose metabolism. However, its role in skeletal muscle glucose metabolism has not been well established. Insulin controls glucose metabolism in the skeletal muscle via the regulations of glucose uptake, glycogenesis, and glycolysis. We hypothesize that insulin and VA signaling pathways may converge to regulate glucose metabolism in skeletal muscle. Here, the effects of retinoic acid (RA) alone and in combination with insulin on glucose utilization in rat L6 muscle cells were studied. L6 cells were treated with 1 mM [micromole] RA and 10 nM [nanomole] insulin for a period of 6 days. Compared to control, cells treated with RA utilized significantly more glucose at days 4 and 6. RA synergized with insulin to increase glucose usage at 4 and 6 days after treatment. RA and insulin synergistically increased the protein expression levels of glycogen synthase and glycogen synthase kinase-3 phosphorylation, while decreasing glycogen synthase phosphorylation. Similar results were seen in VA deficient (VAD) and sufficient (VAS) rats with the VAS rats who received insulin injections having decreased levels of glycogen synthase phosphorylation and increased levels of glycogen synthase. To determine possible nuclear receptors responsible for the RA effect, L6 cells were treated with specific agonists for retinoic acid receptor, retinoid-X-receptor, or liver-X-receptor. Only the retinoic acid receptor specific agonist mimics the effects of RA to increase glucose usage and protein expression level of glycogen synthase in the presence of insulin. In addition, glycogen content was significantly increased in L6 cells treated with RA + insulin and TTNPB + insulin. Interestingly, this increased glucose usage in L6 cells was associated with a reduction of glucose transporter (GLUT) 1 and GLUT4 expression, and induction of GLUT3 and GLUT6 expression, a novel observation that has not been reported. We conclude that RA and insulin signaling pathways work cooperatively to enhance muscle cell glucose utilization through the induction of expression level of glycogen synthase, increase of glycogenesis, and alteration of expression profile of GLUTs in L6 cells.

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