The Potential Effects And Mechanisms Of PPARα Agonist On The Pancreatic β Cell Function In Different Development Stages Of Type 2 Diabetes Mellitus Experimental Animal Models

Peroxisome proliferator-activated receptor alpha (PPARα) is an attractive potential mediator of fatty acids metabolism. It regulates the expression of genes encoding various enzymes involved in lipid metabolism, as a ligand-activated nuclear transcription factor expressed at high levels in tissues adapted to metabolize fatty acids such as liver, heart, and kidney. Many clinical and fundamental study reports suggested that PPAR alpha activation could improve the peripheral insulin resistance, vascular endothelial function and systemic inflammatory response by relieving lipid-mediated inhibition of insulin-stimulated glucose disposal. PPARa is known to be expressed at low levels in islets, but the direct effects of PPARa on beta cell function are unknown. Our laboratory has found that long-term fenofibrate treatment significant decreased plasma insulin in MSG obese rats without improving insulin sensitivity. It remains ambiguity whether PPARa activation has an adverse or beneficial effect on insulin secretion. According to the previous studies, we hypothesized that PPARa agonist causes both up-regulation of enzymes involved in lipid metabolism and increases some enzymes affecting insulin secretion. In order to investigate this hypothesis, we explored the effects of PPARa agonist fenofibrate and gemfibrozil on the modulation of insulin secretion and sensitivity that is observed in several animal models during diabetes progression.In the current study, we established two methods in vitro and in vivo in the first part for evaluating insulin sensitivity and the insulin secretion function of pancreatic beta cells -- hyperinsulinemic euglycemic/hyperglycemic clamp and primary islet isolation for the follow-up studies. The second part of the papers focused on the effects and mechanisms of PPAR alpha agonists (fenofibrate and gemfibrozil) on beta cell function in four kinds of animal model with different characteristics of type 2 diabetes, including normal ICR mice, MSG obese rats with metabolism syndrome, high-fat fed diabetic C57 mice and aged C57 mice with insulin resistance. The experiment performed in MSG obese rats firstly showed that 8-weeks treatments with fenofibrate and gemfibrozil respectively reduced serum triglyceride and cholesterol levels in a few days. However, fenofibrate and gemfibrozil exhibited no significant effects on glucose homeostasis or insulin sensitivity, but decreased fasting plasma insulin and impaired insulin secretory phases response to glucose stimulation during hyperglycemic clamp compared with Con group. These results suggested that fenofibrate and gemfibrozil could impaire the beta cell insulin secretion. Furthermore, fenofibrate treatment reduced activity of Na+-K+-ATPase in pancreatic mitochondrion in MSG obese rats in accordance with increased MDA content in pancreas and significantly increased iNOS and NF-K B mRNA and protein expression in pancreas islet. In addition fenofibrate treatment significantly increased mRNA level of UCP2 expression in pancreas of MSG obese rats. It also found that fenofibrate and gemfibrozil decreased oral glucose tolerance and insulin secretion in normal ICR mice islet with increased UCP2 expression in islet. As for the two C57 animal models, we also found the same points that fenofibrate and gemfibrozil can increase NF-κB or iNOS expression levels and increase UCP2 gene expression to a certain extent although fenofibrate can improve insulin sensitivity and insulin secretion in the aged C57 mice. These results suggested that PPARa agonist may activate NF-κB pathway and elevate the expression of UCP2 in pancreas, and it may be the molecular mechanism of impaired beta cell insulin secretion after PPARa agonist treatment in three animal models. Finally, The third part of papers used in vitro method to inspect the effect of the fenofibrate on NF-κB, the results showed that fenofibrate at 10-7M can elevate NF-κB activity dependent upon PPARa activation, and its role is equivalent to TPA at 10-6M.Taken together, this study established beta cell evaluating methods in vivo and in vitro. At the same time we investigated the effect of two PPARa agonists on beta cell insulin secretion function in four animal models. We preliminarily draw the conclusion that PPARa agonist fenofibrate and gemfibrozil impaired in vivo insulin two-phase secretion stimulated by high glucose in normal ICR mice, MSG obese rats and high-fat fed diabetic C57 mice. A more pronounced negative effect on the two phase of insulin secretion especially the first phase secretion suggests that ATP production-dependent glucose sensing is impaired after drugs treatment, which may be due to the elevated UCP2 expression in pancreatic mitochondrion. It also concluded that after administration of fenofibrate and gemfibrozil the expression of iNOS and NF-κB in pancreas increased significantly. The excessive islet inflammatory mediators such as IL6, TNFa production might explain some if not all of the impairments of islet insulin secretion and beta cell survival. At this point, we suggest that in clinical practice much more attention should be turned toward the induction of inflammation in pancreas generated after PPAR alpha agonist treatment, which may lead to impaired beta cell function.