Control of hepatic glucose metabolism by islet and brain

Dysregulation of hepatic glucose uptake (HGU) and inability of insulin to suppress hepatic glucose production (HGP) contribute to hyperglycaemia in patients with type 2 diabetes (T2D). Growing evidence suggests that insulin can inhibit HGP not only through a direct effect on the liver but also through a mechanism involving the brain. Yet, the notion that insulin action in the brain plays a physiological role in the control of HGP continues to be controversial. Although studies in dogs suggest that the direct hepatic effect of insulin is sufficient to explain day-to-day control of HGP, a surprising outcome has been revealed by recent studies in mice, investigating whether the direct hepatic action of insulin is necessary for normal HGP: when the hepatic insulin signalling pathway was genetically disrupted, HGP was maintained normally even in the absence of direct input from insulin. Here, we present evidence that points to a potentially important role of the brain in the physiological control of both HGU and HGP in response to input from insulin as well as other hormones and nutrients.