Defective branched-chain amino acids (BCAAs) catabolism alters glucose metabolism and induces pancreatic β-cell dysfunction in type 2 diabetes

Abstract

Blood glucose homeostasis is tightly controlled by the secretion and actions of insulin, a hormone that is specifically produced and released by pancreatic β-cells. Pancreatic β-cell dysfunction, characterized by defective insulin secretion and reduced β-cell mass, leads to type 2 diabetes (T2D). However, the driving force for β-cell dysfunction remains elusive. Circulating levels of branched-chain amino acid (BCAA, including leucine, isoleucine and valine) is strikingly increased in human with T2D. This elevation has recently been identified to precede the onset of T2D and associate with β-cell dysfunction. However, the underlying mechanisms have not been explored before. The primary purpose of this study is to examine whether and how BCAA catabolism contributes to β-cell dysfunction. Our in vivo and in vitro studies showed that glucose metabolism in β-cells is impaired by defective BCAA catabolism, contributing to β-cell dysfunction and T2D. Key findings: 1. Pancreatic β-cells consist of an intact BCAA catabolic pathway. However, this BCAA catabolic capacity is impaired in db/db T2D mice, as evidenced by an accumulation of intracellular BCAA related catabolites and reduced activity of BCKDH-A (the key enzyme of BCAA metabolism). 2. In vitro studies revealed that branched-chain α-keto acids (BCKA, the first metabolites of BCAA catabolism), but not BCAA or downstream acylcarnitines, induces defective glucose-induced insulin secretion (GSIS) in MIN6 β-cells. C57BL/6 male mice with chronic BCKA feeding displayed defective GSIS and glucose intolerance when compared to those treated with vehicle. 3. Promoting BCAA catabolism using 3,6-dichlorobenzo[b]thiophene-2-carboxylic acid (BT2, pharmacological activator of BCKA oxidation) alleviated glucose intolerance by enhancing GSIS in db/db diabetic mice. 4. The expression of PPM1K (a positive regulator of BCKDH-A activity) was decreased in islet from db/db diabetic mice compared with those from healthy controls. Genetic deletion of PPM1K or siRNA mediated silencing of PPM1K deficiency led to excessive BCKA and impaired GSIS in pancreatic islets and INS-1E β-cells, respectively. 5. Aberrant BCKA catabolism (induced by chronic treatment with a high concentration of BCKA or silencing of PPM1K) impaired glycolysis, leading to reduced production of ATP for induction of insulin secretion. 6. The inhibitory effects of BCKA on GSIS is glucose specific, because potassium chloride or methyl pyruvate (end-product of glycolysis)-stimulated insulin secretion was normal in MIN6 β-cells treated with BCKA. Taken together, these data suggest that aberrant BCAA catabolic pathway results in excessive accumulation of BCKA in pancreatic β-cells, which in turn further exaggerates β-cell dysfunction by impairing glucose metabolism and subsequent GSIS, leading to T2D. Additionally, my second project is to investigate how the adaptor protein APPL2 (a downstream regulator of adiponectin and insulin signaling) controls GSIS. Our data demonstrated that APPL2 interacts with RacGAP1 to antagonize the inhibitory effect of RacGAP1 on Rac1 activation, enhancing glucose-induced F-actin remodeling and GSIS in pancreatic β-cells.