Blocking the Entrance to Open the Gate

Abstract

Almost 80% of postprandial glucose uptake resides in skeletal muscle (1). Hence, development of skeletal muscle insulin resistance is a hallmark of type 2 diabetes. Muscle is an ambiguous organ with respect to substrate selection. To fuel muscle contraction, both glucose and fatty acids can be oxidized. Glucose first needs to enter the muscle cells via insulin-mediated GLUT4-dependent transmembrane transport and then be converted to acetyl-CoA via the pyruvate dehydrogenase (PDH) complex. Oxidation of fatty acids, especially long-chain fatty acids, requires mitochondrial entrance via carnitine palmitoyltransferase-1 (CPT-1) prior to subsequent β-oxidation. To explain the well-known relationship between obesity and type 2 diabetes, Sir Philip Randle (2) proposed in 1963 that high uptake of fatty acids in skeletal muscle resulting from high free fatty acid levels observed in obesity would result in high fatty acid oxidation rates (Fig. 1). In turn, this would reduce glucose oxidation, thereby rendering the muscle insulin resistant. At the cellular level, high rates of fatty acid oxidation would result in accumulation of acetyl-CoA and citrate, thereby inhibiting PDH and glycolysis, ultimately resulting in reduced glucose oxidation. However, in the previous 2 decades, the concept of the Randle cycle in skeletal muscle has been challenged. Elegant studies by Shulman and colleagues (3–5) showed that in type 2 diabetes, reduced uptake of glucose due to compromised GLUT4 translocation, not a reduced glycolytic flux, is the main culprit in development of skeletal muscle insulin resistance. Moreover, fat accumulation in muscle, and particularly accumulation of muscle diacylglycerol (DAG), was suggested to impair …