Small molecule kaempferol, a novel regulator of glucose homeostasis in diabetes

Abstract

Diabetes mellitus is a growing public health concern, presently affecting 25.8 million or 8.3% of the American population. While the availability of novel drugs, techniques, and surgical intervention has improved the survival rate of individuals with diabetes, the prevalence of diabetes is still rising. Type 2 diabetes (T2D) is a result of chronic insulin resistance and loss of -cell mass and function, and it is is always associated with the impairment in energy metabolism, causing increased intracellular fat content in skeletal muscle (SkM), liver, fat, as well as pancreatic islets. As such, the search for novel agents that simultaneously promotes insulin sensitivity and 𝜷-cell survival may provide a more effective strategy to prevent the onset and progression of this disease. Kaempferol is a flavonol that has been identified in many plants and used in traditional medicine. It has been shown to elicit various pharmacological activities in epidemiological and preclinical studies. However, to date, the studies regarding its effect on the pathogenesis of diabetes are very limited. In this dissertation, I explored the anti-diabetic potential of the dietary intake of kaempferol in diet-induced obese mice and insulin-deficient diabetic mice. For the first animal study, kaempferol was supplemented in the diet to determine whether it can prevent insulin resistance and hyperglycemia in high fat (HF) diet-induced obese mice or STZ-induced obese diabetic mice. For the second animal study, kaempferol was administrated once daily via oral gavage to diet-induced obese and insulin-resistant mice or lean STZ-induced diabetic mice to evaluate its efficacy for treating diabetes and further determining the underlying mechanism. The results demonstrated that dietary intake of kaempferol for 5 months (mo) improved insulin sensitivity and glucose tolerances, which were associated with increased Glut4 and AMPKα expression in muscle and adipose tissues in middle-aged mice fed a high-fat (HF) diet. In vitro, kaempferol increased lipolysis and restored chronic high fatty acid-impaired glucose uptake and glycogen synthesis in SkM cells, which were associated with improved AMPKα activity and Glut4 expression. In addition, dietary kaempferol treatment preserved functional pancreatic 𝜷-cell mass and prevented hyperglycemia and glucose intolerance in STZ-induced diabetic mice. Data from the second study show that oral administration of kaempferol significantly improved blood glucose control in obese mice, which was associated with reduced hepatic glucose production and improved whole body insulin sensitivity without altering body weight gain, food consumption, or the adiposity. In addition, kaempferol treatment increased Akt and hexokinase activity, but decreased pyruvate carboxylase and glucose-6 phosphatase activity in the liver homogenate without altering their protein expression. Consistently, kaempferol decreased pyruvate carboxylase activity and suppressed gluconeogenesis in HepG2 cells as well as primary hepatocytes isolated from the livers of obese mice. Kaempferol directly blunted the activity of purified pyruvate carboxylase. In the last study, we found that kaempferol stimulates basal glucose uptake in primary human SkM. In C2C12 mouse myotubes, kaempferol also increased insulin stimulated glycogen synthesis and preserved insulin dependent glycogen synthesis and glucose uptake in the presence of fatty acids. Kaempferol stimulated Akt phosphorylation in a similar time-dependent manner as insulin in human SkM cells. Consistent with this, kaempferol increased Akt and AMPK phosphorylation in isolated murine red SkM tissue. The effect of kaempferol on glucose uptake was blunted in the presence of chemical inhibitors of glucose transporter 4 (Glut4), phosphoinositide 3-kinase (PI3K), glucose transporter 1 (Glut1), and AMPK. The AMPK inhibitor also prevented kaempferol-stimulated Akt phosphorylation. Further, kaempferol improved the stability of insulin receptor substrate-1. Taken together, these studies suggest that the kaempferol is a naturally occurring compound that may be of use in the regulation of glucose homeostasis and diabetes by improving insulin sensitivity and glucose metabolism, as well as by preserving functional 𝜷-cell mass.