| Summary: | 碩士 === 國立陽明大學 === 生理學研究所 === 100 === Obesity and insulin resistance are the major risk factors for the development of metabolic disorders, such as type 2 diabetes. Previous studies indicated that endocannabinoid system regulates homeostasis and plays an important role in the pathogenesis of metabolic diseases. Overactivation of endocannabinoid system has been observed in obese patients. Activation of cannabinoid receptor type 1 (CB1 receptor) increased lipid accumulation and insulin resistance. On the contrary, CB1 receptor antagonists have been proved to exert therapeutic effects including weight loss and improved glucose and lipid metabolism. However, the regulatory mechanism of CB1 receptor in metabolic diseases has not been elucidated. Liver is one of the important organ involves in regulation of glucose and lipid homeostasis. In our present study, we investigated the regulation of CB1 receptor expression in liver cells. It is well known that hyperinsulinemia is a common feature in insulin-resistant patients. Therefore, we observed the effects of insulin on CB1 receptor expression in AML12 hepatocytes. Result showed that insulin increased CB1 receptor expression in AML12 hepatocytes, and the stimulatory effect was mediated by Extracellular signal regulated kinase (ERK) pathway. To analyze the mRNA expression, however, CB1 receptor mRNA was not stimulated by insulin treatment, indicating that insulin did not increase CB1 receptor gene transcription. On the other hand, we used cycloheximide to inhibit de novo protein synthesis and observed the rate of protein degradation after insulin treatment. We found that insulin decreased protein degradation rate through inhibiting lysosomal degradation, and this effect was also mediated by ERK pathway. To confirm the physiological effects of insulin-stimulated CB1 receptor, AML12 hepatocytes were treated with cannabinoid receptor agonists, AEA and ACEA. Cannabinoid-induced triglyceride accumulation was further enhanced after insulin stimulation. Besides, insulin treatment also increased CB2 receptor expression and CB2 receptor-mediated triglyceride accumulation. Finally, we used primary rat hepatocytes and insulin-infused rats to confirm our finding, respectively. Although hepatic CB1 receptor expression was not significantly increased after 8-hour insulin infusion in vivo, CB1 receptor was increased in insulin-treated primary rat hepatocytes. To conclude our findings, insulin stimulates CB1 receptor expression in hepatocytes. In insulin-resistant patients with hyperinsulinemia, elevated plasma insulin level further enhances activation of endocannabinoid system, and consequently leads to further deterioration of insulin sensitivity and abnormal triglyceride accumulation.
|
|---|
