Paracrine regulation of glucagon secretion from pancreatic islets

Abstract

Diabetes mellitus has been a disease of increasing prevalence for nearly a century and is attributed to a dysregulation of hormones secreted from the pancreatic Islets of Langerhans; insulin from β-cells and glucagon from α-cells. Dysregulated α-cell glucagon secretion is responsible for the chronic hyperglycemia that accompanies diabetes and may be an important therapeutic avenue. Despite its importance, the normal molecular regulation of glucagon secretion is poorly understood. This work characterized a novel mechanism by which somatostatin and insulin coordinate to lower cAMP and phosphorylated PKA in the α-cells with rising glucose to suppress glucagon secretion in a Ca2+-independent manner. This decrease in cAMP/PKA normally arises from somatostatin preventing cAMP production by adenylyl cyclases via the Gαi subunit of the SSTR2 and from insulin receptor activation of phosphodiesterase 3B to drive degradation of cAMP in a glucose-dependent manner. Our data indicate that both somatostatin and insulin signaling is required to decrease cAMP and PKA sufficiently to inhibit glucagon secretion from islets and isolated α-cells. We conclude that somatostatin and insulin together are critical paracrine mediators of glucose-inhibited glucagon secretion and function by lowering cAMP/PKA signaling with increasing glucose. The complex inter-relationships in these results demonstrate the need for simultaneous measurements of multiple signaling pathways. For example, the roles of Ca2+ and cAMP in regulating glucose-stimulated insulin secretion from β-cells have been long known. However, it has been challenging to study temporal relationships between these signaling molecules due to the spectral overlap of most [Ca2+]i and cAMP biosensors. We have developed a hyperspectral image mapping spectrometry technique for simultaneously monitoring these biosensors in real time. Using the IMS, we can resolve the effects of glucose and known stimulating drugs on these signaling molecules simultaneously and show that their glucose-induced oscillations are anti-correlated. We have also recently demonstrated the capability of this method for monitoring two Forster resonance energy transfer based biosensors simultaneously, a feat rarely attempted by other spectral imaging systems. This was used to study the relationship between cAMP signaling and caspase-3 mediated β-cell apoptosis, a critical event in developing diabetes.