Glucose and lipid metabolism in rats supplemented with glycyrrhizic acid exposed to short- or long- term stress and fed on a high-calorie diet

Stress and consumption of high-calorie diet are well-recognized as the primary contributor to various metabolic diseases such as the metabolic syndrome. Glycyrrhizic acid (GA), an active compound in the root extract of the licorice plant, Glycyrrhiza glabra has been shown to improve hyperglycaemia and dyslipidaemia in rats fed on a high- calorie diet. However, the effect of GA on glucose and lipid metabolism in rats under stress in combination with high- calorie diet has yet to be explored. Hence, the objective of this study was to investigate the effect of short- and long- term stress on glucose and lipid metabolism and the role of GA in improving these. Forty-eight Sprague Dawley rats were exposed to constant light illumination (300-400 lux) for either 14 or 28 days for short- or long-term stress and fed on a high-calorie diet. The blood and tissues including liver, kidney, heart, pancreas, subcutaneous and visceral adipose tissues (SAT and VAT), abdominal muscle (AM) and quadriceps femoris (QF) were collected for analysis. The stress, glucose metabolism and lipid metabolism states in rats were examined using different parameters. The systolic blood pressure, stress hormones i.e. adrenaline and corticosterone were used as the stress parameters. Together with blood glucose level, serum insulin and homeostatic index of insulin resistance, the activities of gluconeogenic enzymes included glucose-6-phosphatase (G6Pase), phosphoenolpyruvate carboxykinase (PEPCK) and hexose-6-phosphate dehydrogenase (H6PDH) were determined for the glucose metabolism state. Lastly, the state of lipid metabolism was identified using lipid profile and serum free fatty acids, fatty acids profile and the expression levels of various genes involved in glucose and lipid metabolism which are the peroxisome proliferator-activated receptors (PPAR-α & PPAR-γ), lipoprotein lipase (LPL), elongases (ELOVL5 & ELOVL6) and desaturases (D5D, D6D & D9D). Continuous light exposure for 14 days induced stress response in the short-term exposure group while the long- term exposure group adapted to stress induced by continuous light exposure for 28 days. When comparisons were made within the 14- or 28-day exposure group, the short-term exposure group had significant elevated blood glucose concentrations but GA lowered the concentrations significantly. However, neither stress nor GA affected the blood glucose concentration of the long-term exposure group. With regards to adrenaline and corticosterone, rats given GA from the short-term exposure group had elevated adrenaline level while those from the long-term exposure group had reduced level of corticosterone. Gluconeogenesis is the process of synthesis of glucose from non- carbohydrate source. In response to stress, gluconeogenic enzymes activities are increased to increase fuel supply to the important organs. Specifically, GA was found to reduce H6PDH activities in the VAT of the short-term exposure group and QF of the long-term exposure group. Serum free fatty acids and lipid profile are the key indicators for diabetes and cardiovascular disease. The development of these diseases is closely associated with disrupted lipid metabolism. PPAR and LPL are the key genes for lipid metabolism while the elongases and desaturases determine the chemical properties of fatty acids and hence their metabolic fate. Recent studies have demonstrated the importance of these genes in the development of MetS. Within the 14-day exposure group, neither stress nor GA affected all the lipid profile parameters and serum free fatty acids. As for the 28-day exposure group, stress was found to only increase triacylglycerol concentration significantly only. GA did not affect the lipid profile parameters and serum free fatty acids. For different types of fatty acids, the long-term exposure group generally had higher fatty acids content in the liver, SAT and VAT and lower content in the pancreas. Stress did not affect PPAR-α expression in both the 14- and 28-day exposure groups. However, GA-treated rats from the former group had increased PPAR-α expression only in the kidney while all other tissues from the latter group were unaffected. Stress did not affect PPAR-γ expressions in the 14-day exposure group but GA elevated PPAR-γ expression in the kidney significantly. As for the 28-day exposure group, stress increased its expression in the heart. GA was found to increase PPAR-γ expression in both the AM and QF. With regards to LPL, stress did not affect LPL expressions in all the studied tissues from both the 14- and 28-day exposure groups except for significant up-regulation in the QF of the latter group. GA did not affect LPL expressions in both the 14- and 28-day exposure groups except for significant up-regulation in the heart of the latter group. As for the expression of elongases and desaturases in the liver, stress down-regulated ELOVL5 in the long-term exposure group while up-regulated ELOVL6 in the short- term exposure group. On the other hand, hepatic desaturases and elongases and desaturases in both the SAT and VAT were unaffected by stress. Neither elongases nor desaturases expressions in all the studied tissues were affected by GA except for significant down-regulation of ELOVL5 in the VAT of the 28-day exposure group. Thus, our results indicate that GA could improve blood glucose concentration in rats exposed to short-term stress and were on high-calorie diet via selective action on gluconeogenic enzymes activities and modifications of fatty acids via regulation of PPAR, elongases and desaturases in different tissues. This research is the first report of GA on glucose and lipid metabolism in rats under stress and on high-calorie diet. The results gave evidence supporting the role of GA in ameliorating MetS via site-specific regulation of gene expressions involved in glucose and lipid metabolism and modification of fatty acids.