It is notable that pharmacological or

It is notable that pharmacological or genetic inhibition of GCGR signaling results in the engagement of a number of compensatory mechanisms that potentially impact glucose control. These include alpha-cell hyperplasia [2], [11], [12], [13] and increased beta-cell proliferation under low insulin conditions [10]. Several mechanisms have been proposed to contribute to the alpha-cell proliferation, including increased ANGPL4 [14], although this has been questioned [15]. More convincingly, the hyperaminoacidemia that follows GCGR interruption leads to activation of the mTOR pathway in the alpha cell [16], [17], [18]. Reduced GCGR signaling also leads to altered levels of multiple humoral factors important in the control of glucose. Hence, Gcgr mice have increased ghrelin levels even during insulinopenic conditions due to STZ treatment [19], and this may contribute to the prevention of hypoglycemia. On the other hand, loss of Gcgr results in supraphysiological increases in Fibroblast Growth factor 21 (FGF21) [6] and GLP-1 [2], hormones that each have glucose-lowering properties. The development of compensatory mechanisms for the loss of GCGR signaling suggests a more complex role of glucagon action in diabetic hyperglycemia than simply increased hepatic glucose production. To evaluate the relative impact of the GCGR on blood glucose we crossed Gcgr “flox” mice with a line expressing TMX-inducible cre-recombinase under the control of the universally expressed gene Rosa26. This allowed for time-controlled disruption of Gcgr gene CCK Octapeptide, non-sulfated receptor following tamoxifen injections and the ability to compare acute and chronic loss of the GCGR during insulinopenic diabetes. Our results demonstrate that engagement of compensatory signals, specifically GLP-1 receptor signaling, rather than loss of GCGR activation per se, attenuates the development of hyperglycemia during insulinopenic conditions.
Material and methods
Results
Discussion The engagement of multiple compensatory mechanisms limits assessment of the direct contribution of GCGR signaling to glycemic control in models of chronic loss of GCGR signaling. By temporally controlling the disruption of Gcgr, we found that acute loss of Gcgr in a model of insulinopenic diabetes provides only modest protection from hyperglycemia and that a large component of this is mediated by the GLP-1R. These data suggest that loss of GCGR signaling and the effects typically attributed to acute glucagon action, glycogenolysis and gluconeogenesis, have only a mild impact on hyperglycemia associated with insulinopenia. Instead, it appears that the engagement of compensatory mechanisms over time, including increased GLP-1R signaling, plays the critical role in protection against diabetes that accompanies GCGR knockout. Glucagon is a cornerstone of the physiological control of blood glucose and is generally believed to be the principle mediator of counter-regulation, opposing the effect of insulin and preventing hypoglycemia. Thus, defective stimulation of glucagon secretion is acknowledged as a key determinant of the incidence and severity of hypoglycemic episodes in T1DM patients [31]. On the other hand, pathophysiological conditions characterized by increased circulating levels of glucagon, including patients with glucagonoma, may result in hyperglycemia [32]. These findings support a beneficial effect of reducing glucagon receptor signaling as a potential antidiabetic treatment. Consistent with this, disruption of GCGR signaling in rodent models, either by or pharmacological or genetic means, results in significant improvements in glycemic control. Indeed, congenital mouse models of loss of Gcgr expression (Gcgr mice) exhibit reduced baseline blood glucose, improved glucose tolerance, and even more strikingly, the maintenance of normoglycemia and increased survival following the destruction of beta-cells with STZ [3], [5]. However, they also exhibit resistance to body weight gain when fed a HFD [3], a finding that is somewhat counterintuitive considering that conditions characterized by increased endogenous glucagon levels [33], [34] or treatment with Gcgr agonists [23] result in reduced food intake and increased weight loss. The apparently conflicting results are likely the result of the multiple compensatory mechanisms exhibited by congenital Gcgr mice. Some of those compensations include increased circulating levels of proglucagon-derived peptides including glucagon and GLP-1 [2] as well as increased FGF21 [6], all factors with a significant contribution to glycemic control.