Extracellular5’-AMP Signal In The Pathogenesis Of Type2Diabetes

Type2diabetes is one of the most common chronic diseases affecting people and the prevalence of diabetes increases with age. The number of diabetes cases has increased dramatically in recent decades, and type2diabetes has become one of the world’s top health problems. High-fat diet and obesity play an important role in leading to insulin resistance and type2diabetes. The most widely accepted pathogenesis of type2diabetes is that expanded fat mass releases high amount of free fatty acids (FFA), abnormal elevating of plasma FFA levels, can produce insulin resistance. However, precise mechanism by which FFA cause insulin resistance has not yet been fully elucidated. Exploring mechanisms underlying FFA-induced insulin resistance in order to find a therapeutic target is very important in prevention and treatment of insulin resistance and type2diabetes.First, abnormally elevated levels of plasma5’-AMP (pAMP) were found in mouse models of type2diabetes and in patients with type2diabetes, forming a new and specific characteristic of type2diabetes. HPLC analysis were performed to investigate the plasma nucleotides in two different mouse models of type2diabetes and in patients with type2diabetes. The results demonstrated that pAMP and UA levels were elevated in spontaneous diabetic db/db mice; In high-fat diet-induced type2diabetic mice, the levels of pAMP and plasma UA were elevated; and the pAMP level was also increased in patients with type2diabetes. Through strengthen the5’-AMP signals in normal mice, we found that5’-AMP is an important signaling molecule that elevated blood sugar and cause insulin resistance. Glucose tolerance test and insulin tolerance test indicated that5’-AMP treatment reduced glucose tolerance and insulin sensitivity in normal mice, and induced insulin resistance; pyruvate tolerance test showed that5’-AMP increased pyruvate-induced gluconeogenesis; and5’-AMP treatment caused hyperinsulimia and lowered oxygen consumption in normal mice. All these phenotypes are similar to diabetic phenotypes in diabetic mice.We further found that vein endothelial cells injury and apoptosis induced by high levels of FFA contributed to the increase in pAMP. In vitro, FFA induced damage and apoptosis in human umbilical vein endothelial cells (HUVECs) and contributed to an increase in pAMP. In vivo, surgery-induced injury increased pAMP level and also blood glucose level in mice. Our data indicated that cell injury caused the release of intracellular5’-AMP and increased the extracellular5’-AMP level. Studies on adenosine receptor knockout mice show that pAMP-induced hyperglycemia was achieved by increasing the concentration of intracellular adenosine.5’-AMP elevated blood glucose in mice deficient in adenosine receptors (A1-/-, A2a-/-, A2b-/-, A3-/-) with equal efficiency as wild-type mice. The findings suggested that5’-AMP-induced hyperglycemia was not directly related to adenosine receptor pathways. At1h after5’-AMP injection, HPLC analysis demonstrated that the function of5’-AMP was initiated by the elevation of cellular adenosine levels. It was followed by the decreased dimethylation of histone H3K9related Foxol, G6Pase and PEPCK promoter through declining ratio of S-adenosylmethionine (AdoMet) to S-adenosylhomocysteine (AdoHcy), resulting in high blood glucose levels. At the early stage (15min) of5’-AMP injection, our results demonstrated that5’-AMP treatment dramatically increased G6Pase activity but had no effect on G6Pase mRNA abundance. And HPLC data indicated that adenosine levels increased after5’-AMP treatment. Our in vitro experiments showed that adenosine directly increased the activity of G6Pase. Our results revealed that the function of pAMP was initiated by the elevation of cellular adenosine levels, directly stimulating G6Pase enzyme activity. On the other side,5’-AMP treatment decreased the expression of GLUT4and attenuated insulin-dependent GLUT4translocation in skeletal muscle, resulting in decreased glucose utilization. Collectively, mice treated with5’-AMP displayed a rapid and steep increase in blood glucose.Metformin improved5’-AMP-induced hyperglycemia by a new mechanism of reducing5’-AMP-induced increase in adenosine content. HPLC data showed that metformin reduced5’-AMP-induced increase in adenosine content, and QRT-PCR analysis showed that mRNA levels of Foxol, G6Pase and PEPCK were significantly decreased in5’-AMP-treated mice, resulting in lower blood glucose level. These results suggested a new potential mechanism for metformin action. By reducing fat absorption and plasma FFA level, Salecan reduced cell injury and apoptosis to weaken high-fat diet-enhanced extracellular5’-AMP signals. Salecan is a novel glucan that isolated by our group recently. Using a high-fat diet-induced obese mouse model, we found that Salecan dose-dependent improved high-fat diet-induced increases in body weight, body fat, liver and adipose weight, lipid profiles of plasma and liver, and a decrease in glucose tolerance. HPLC analysis showed that Salecan reduced high-fat diet-induced increase in plasma UA level in mice. Dietary Salecan intake caused an obvious elevation of fat in feces. Presence of Salecan disturbed bile acid-promoted emulsification and reduced the size of emulsion droplets in vitro. These results indicated that Salecan decreases fat absorption by disturbing bile acid-promoted emulsification of fat and reduced fat accumulation in mice, resulting in a decrease in FFA-induced cell injury and plasma UA level. These findings suggested that Salecan qualify to be useful for treating high fat-induced obesity, indicating a potential application in treating high-fat diet-induced type2diabetes.