Characterization of the expression, localization, regulation and role of pancreatic derived factor (PANDER, FAM3B) in pancreatic alpha cells

Pancreatic Islets of Langerhans are critical regulators of serum glucose, exerting substantial control over the balance of systemic energy availability and fluctuating demand. The release of the antagonistic hormones insulin and glucagon as well as other paracrine messengers elicits responses within the islet and at peripheral tissues to maintain euglycemia. PANcreatic DERived Factor (PANDER; FAM3B), a recently identified islet-specific protein, has been extensively characterized by our group and others almost entirely with respect to insulin-secreting β-cells. An in-vivo role for PANDER in glucose homeostasis is suggested by a number of homeostatic aberrations in two PANDER mouse models. In a transgenic mouse islet-specifically over-expressing PANDER, we observe impaired glucose tolerance and elevated hepatic glucose production (HGP), while in a global PANDER knockout mouse model; there is reduced HGP and gluconeogenic enzyme expression as compared with wild type control animals. These aberrations suggest a biological role for PANDER in regulating glucose homeostasis similar to glucagon. Glucagon-secreting α-cells comprise the second largest islet cell population and influence serum glucose levels via the stimulation of Hepatic Glucose Production (HGP). The aim of this dissertation is to evaluate PANDER expression in α-cells, evaluate the effect of typical glucagon secretagogues on the regulation of PANDER secretion from these cells, and investigate the influence of PANDER on serum glucagon levels in mice. pander mRNA and protein are detected in α-cell lines and in the β- and α-cell-enriched fractions of FACS sorted islets, and is predominantly localized to glucagon-negative secretory granules within the cytosol. Sharing both regulatory similarities and differences with glucagon, the amino acid L-arginine has a robust positive effect on PANDER release while there is no effect of either hyperglycemia or hypoglycemia. Interestingly low levels of insulin known to suppress glucagon have a robust stimulatory effect on PANDER release, mediated by signaling through the insulin receptor (IR) downstream of PI3-kinase (PI3K) via the Akt/PKB node. The observed similarities and differences are consistent with the minimal co-localization of PANDER and glucagon, with predominant sequestration of PANDER to a separate granular cytoplasmic compartment. Despite absence of observed effect of PANDER over-expression in-vitro on hypoglycemia-induced glucagon release from α-TC1-6 cells, there is a complete loss of the typical glucagon response to fasting in PANDER transgenic mice, associated with mild basal hyperglucagonemia (that is in the fed state) as compared with WT controls. This may reflect a paracrine/autocrine effect of PANDER on glucagon release, also with a potential direct effect of PANDER on Hepatic Glucose Production. Together the data demonstrate nutritional and paracrine regulation of PANDER release from a granular cytoplasmic compartment within α-cells, which directly or indirectly influences post-absorptive glucagon, and the normal glucagon response to hypoglycemia in-vivo