Following a glucose challenge, both insulin-dependent and -independent mechanisms contribute to the return to baseline blood glucose concentrations. Referred to as glucose effectiveness (GE), the insulin- independent component contributes as much to overall glucose homeostasis as insulin, but it has been viewed as a fixed and largely unregulated process and hence has not been a research focus for investigators. Several recent observations, however, offer evidence of the brain's capacity to potently induce glucose lowering via insulin-independent mechanisms. Adding to this work is our preliminary data showing that in leptin-deficient ob/ob mice, intracerebroventricular (icv) injection of the ani-diabetic hormone fibroblast growth factor-19 (FGF19) rapidly normalizes glucose tolerance despite having no effect on either insulin secretion or insulin sensitivity. Instead, this effect i mediated entirely by a selective, 3-fold increase of GE. Although the peripheral mechanism underlying this effect is unknown, our data strongly implicate a process whereby glucose is taken up into peripheral tissues via an insulin-independent mechanism, followed by its metabolism to lactate that is subsequently released back into circulation. With this background, we propose Specific Aim 1: To determine how FGF19 increases insulin-independent glucose disposal. Studies in this aim will 1) quantify this brain- mediated increase of glucose uptake and metabolism to lactate in response to FGF19, 2) determine the extent to which it explains the associated increase of GE, and 3) identify the tissues in which it occurs. These goals will be accomplished in mice using a combination of methods ranging from hyperglycemic clamp to metabolomics and biochemical analyses. Could a similar process contribute to the anti-diabetic effects of bariatric surgery? Rodent data implicate the brain in the glucose-lowering effects of bariatric procedures, and in some cases, glucose lowering involves insulin-independent as well as insulin-dependent mechanisms. Moreover, bariatric procedures increase FGF19 secretion from the GI tract. Based on these considerations, we propose Specific Aim 2: To determine if increased GE contributes to the anti-diabetic effect of bariatric surgery. Studies in this aim will determine if bariatric surgery activates CNS mechanisms analogous to those engaged by FGF19, including stimulation of insulin-independent glucose uptake, followed by conversion to lactate, which is then released into circulation. Our finding that FGF19 action in the brain rapidly, potently and selectively increases insulin-independent glucose disposal identifies a novel, CNS-driven mechanism with translational implications for both the pathogenesis and treatment of human diabetes. Studies in this proposal seek to clarify how this occurs and the extent to which it explains the anti-diabetic effect of bariatric surgical procedures.
Although a great amount has been learned over the years regarding the control of insulin secretion and insulin sensitivity, two key determinants of glucose tolerance, treatment of type 2 diabetes has advanced relatively little in recent years. In addition to insulin-mediated glucose lowering, insulin-independent mechanisms also play a crucial role in glucose homeostasis. This proposal seeks funding to extent our exciting new demonstration that the anti-diabetic hormone fibroblast growth factor 19 (FGF19) lowers blood glucose levels through a novel, insulin-independent, CNS-driven mechanism. In addition to clarifying how this FGF19 effect occurs, we will determine if the anti-diabetic effect of bariatric surgery in rodent models involves a similar mechanism.
|Rojas, J M; Schwartz, M W (2014) Control of hepatic glucose metabolism by islet and brain. Diabetes Obes Metab 16 Suppl 1:33-40|