Vitamin A Improves Hyperglycemia and Glucose-Intolerance through Regulation of Intracellular Signaling Pathways and Glycogen Synthesis in WNIN/GR-Ob Obese Rat Model

Abstract

Vitamin A and its metabolites modulate insulin resistance and regulate stearoyl-CoA desaturase 1 (SCD1), which are also known to affect insulin resistance. Here, we tested, whether vitamin A-mediated changes in insulin resistance markers are associated with SCD1 regulation or not. For this purpose, 30-week old male lean and glucose-intolerant obese rats of WNIN/GR-Ob strain were given either a stock or vitamin A-enriched diet, i.e. 2.6 mg or 129 mg vitamin A/kg diet, for 14 weeks. Compared to the stock diet, vitamin A-enriched diet feeding improved hyperglycemia and glucose-clearance rate in obese rats and no such changes were seen in lean rats receiving identical diets. These changes were corroborated with concomitant increase in circulatory insulin and glycogen levels of liver and muscle (whose insulin signaling pathway genes were up-regulated) in obese rats. Further, the observed increase in muscle glycogen content in these obese rats could be explained by increased levels of the active form of glycogen synthase, the key regulator of glycogen synthesis pathway, possibly inactivated through increased phosphorylation of its upstream inhibitor, glycogen synthase kinase. However, the unaltered hepatic SCD1 protein expression (despite decreased mRNA level) and increased muscle-SCD1 expression (both at gene and protein levels) suggest that vitamin A-mediated changes on glucose metabolism are not associated with SCD1 regulation. Chronic consumption of vitamin A-enriched diet improved hyperglycemia and glucose-intolerance, possibly, through the regulation of intracellular signaling and glycogen synthesis pathways of muscle and liver, but not associated with SCD1.

Keywords: adipose tissue; gene expression; glucose homeostasis; insulin; retinoids.

Fig. 1

Effect of vitamin A on insulin sensitivity-associated parameters. (A, B) Fasting plasma glucose and insulin levels, respectively. (C, D) Oral glucose tolerance test-area under the curve (OGTT-AUC) for glucose and insulin, respectively. (E, F) Glycogen levels of liver and muscle, respectively. Values are means±SEM of 6 rats, except for OGTT-AUC, 4 rats were from each group. Data were analyzed by one-way ANOVA-with post-hoc least significant difference test. Groups bearing different letters (a–c) are statistically different at P≤0.05. A-I & B-I, stock diet-fed lean and obese rats, respectively; A-II & B-II, vitamin A-enriched diet-fed lean and obese rats, respectively.

Fig. 2

Effect of vitamin A on intracellular signaling pathway proteins and genes of muscle and liver. (A) Western blots of insulin signaling pathway proteins and ratio of phosphorylated to non-phosphorylated protein for glycogen synthase kinase-3β (GSK-3β) and glycogen synthase (GS). (B, C) Relative mRNA levels of insulin signaling pathway genes of muscle and liver, respectively. Values are means±SEM of 3~4 rats from each group. Data were analyzed by one-way ANOVA-with post-hoc least significant difference test. Groups bearing different letters (a–c) are statistically different at P≤0.05. A-I & B-I, stock diet-fed lean and obese rats, respectively; A-II & B-II, vitamin A-enriched diet-fed lean and obese rats, respectively.

Fig. 3

Effect of vitamin A on triglyceride and stearoyl-CoA desaturase 1 (SCD1) expression levels of liver and muscle. (A, B) Triglyceride levels of liver and muscle, respectively. (C, D) Relative mRNA levels of SCD1 in liver and muscle, respectively. (E, F) Western blot of SCD1 protein in liver and muscle, respectively, and their densitometry values (expressed as fold change relative to stock diet-fed lean rats). Values are means±SEM of 3~4 rats from each group. Data were analyzed by one-way ANOVA-with post-hoc least significant difference test. Groups bearing different letters (a–c) are statistically different at P≤0.05. A-I & B-I, stock diet-fed lean and obese rats, respectively; A-II & B-II, vitamin A-enriched diet-fed lean and obese rats, respectively.

Fig. 4

Effect of vitamin A on fatty acid desaturase activity indices and transcriptional regulation of vitamin A metabolic pathway genes of liver and muscle. (A, B) Various fatty acid desaturase activity indices of liver and muscle, respectively. (C, D) Relative mRNA levels of vitamin A metabolic pathway in liver and muscle respectively. Values are means±SEM of 3~4 rats from each group. Data were analyzed by one-way ANOVA-with post-hoc least significant difference test. Groups bearing different letters (a–d) are statistically different at P≤0.05. A-I & B-I, stock diet-fed lean and obese rats, respectively; A-II & B-II, vitamin A-enriched diet-fed lean and obese rats, respectively. D9-16D, delta9-16desaturase; D9-18D, delta 9–18 desaturase; D6D, delta 6 desaturase; D5D, delta 5 desaturase.