Disruption of G Protein-Coupled Receptor 39 Impairs Insulin Secretion in Vivo

Abstract

GPR39 is a G protein-coupled receptor expressed in liver, gastrointestinal tract, adipose tissue, and pancreas. We have recently shown that young GPR39(-/-) mice have normal body weight, food intake, and fasting glucose and insulin levels. In this study, we examined the role of GPR39 in aging and diet-induced obese mice. Body weight and food intake were similar in wild-type and GPR39(-/-) mice as they aged from 12 to 52 wk or when fed a low-fat/high-sucrose or high-fat/high-sucrose diet. Fifty-two-week-old GPR39(-/-) mice showed a trend toward decreased insulin levels after oral glucose challenge. When fed either a low-fat/high-sucrose or high-fat/high-sucrose diet, GPR39(-/-) mice had increased fed glucose levels and showed decreased serum insulin levels during an oral glucose tolerance test in the face of unchanged insulin tolerance. Pancreas morphology and glucose-stimulated insulin secretion in isolated islets from wild-type and GPR39(-/-) mice were comparable, suggesting that GPR39 is not required for pancreas development or ex vivo insulin secretion. Small interfering RNA-mediated knockdown of GPR39 in clonal NIT-1 beta-cells revealed that GPR39 regulates the expression of insulin receptor substrate-2 and pancreatic and duodenal homeobox-1 in a cell-autonomous manner; insulin receptor substrate-2 mRNA was also significantly decreased in the pancreas of GPR39(-/-) mice. Taken together, our data indicate that GPR39 is required for the increased insulin secretion in vivo under conditions of increased demand, i.e. on development of age-dependent and diet-induced insulin resistance. Thus, GPR39 agonists may have potential for the treatment of type 2 diabetes.

The diabetes-susceptible gene SLC30A8/ZnT8 regulates hepatic insulin clearance

“…Zinc secreted from β cells is known to affect neighboring endocrine cells (7,11). Consistent with previous reports (10,(14)(15)(16), addition of zinc to the perfusate (30 μM) markedly suppressed insulin hypersecretion in ZnT8KO mice, almost to control levels ( Figure 2D). Furthermore, the addition of zinc chelator calcium EDTA (2.5 mM) (7) significantly increased insulin secretion from isolated C57BL/6J islets ( Figure 5C and Supplemental Figure 2B).…”

“…However, in that study, the discrepancy between these 2 observations was not fully explored. Previous studies reported that insulin secretion from pancreatic islets was suppressed by zinc through enhanced outward current of the K-ATP channel (8)(9)(10)(14)(15)(16). Therefore, we rigorously examined whether zinc could suppress insulin secretion in various experimental settings, using isolated islets from ZnT8KO mice ( Figure 2B and Figure 5D), the addition of zinc chelator to C57BL/6J islets ( Figure 5C), and pancreas perfusion ( Figure 2D), in addition to simple comparison between control and ZnT8KO mice.…”

“…These results suggest that zinc inhibits insulin-stimulated IR internalization. Furthermore, the zinc ionophore pyrithione enhanced, whereas the zinc chelator TPEN abolished, zinc-induced inhibition of IR internalization ( Figure 6F), which suggests that the zinc-dependent inhibition of IR internalization is induced by intracellular zinc influx, rather than by zinc-sensing receptors such as GPR39 (10). To investigate the transporter responsible for the zinc influx, we examined ZIP14-deficient mice (41,48).…”

“…SLC30A8 encodes zinc transporter-8 (ZnT8), which delivers zinc ion from the cytoplasm of pancreatic β cells to insulin granules (6). Insulin granules contain high amounts of zinc, and zinc that is cosecreted with insulin affects neighboring endocrine cells in the islets of Langerhans in both paracrine and autocrine fashions (7)(8)(9)(10)(11). While studies of ZnT8 deletion or overexpression in insulinoma cells have suggested that it contributes to the maintenance of glucose-stimulated insulin secretion (GSIS) (12,13), others have reported that zinc suppresses insulin secretion from pancreatic β cells (8)(9)(10)(14)(15)(16).…”

“…Insulin granules contain high amounts of zinc, and zinc that is cosecreted with insulin affects neighboring endocrine cells in the islets of Langerhans in both paracrine and autocrine fashions (7)(8)(9)(10)(11). While studies of ZnT8 deletion or overexpression in insulinoma cells have suggested that it contributes to the maintenance of glucose-stimulated insulin secretion (GSIS) (12,13), others have reported that zinc suppresses insulin secretion from pancreatic β cells (8)(9)(10)(14)(15)(16). In addition, recent loss-of-function studies of Slc30a8 in mice demonstrated that ZnT8 is important for the crystallization of insulin molecules and efficient insulin processing in insulin granules, but there is no agreement on the precise role of ZnT8 in increased susceptibility to type 2 diabetes (17)(18)(19)(20)(21).…”