Receptor-operated signaling pathways in normal and diabetic pancreatic islet cell function

Abstract

Loss of glucose-dependent insulin release is an early pathogenetic event in human type 2 diabetes. Impaired glucose-mediated insulin secretion from pancreatic islet β-cells leads to insulin deficiency and thus hyperglycemia. Insulin release from the β-cell is a highly regulated and finely tuned Ca2+-dependent process, glucose being the major stimulator of insulin secretion under physiological conditions. A rise in the cytoplasmic free Ca2+ concentration ([Ca2+]i) is a crucial step in insulin secretion, involving Ca2+ influx through voltage-gated L-type Ca2+ channels and Ca2+ mobilization from intracellular Ca 2+ stores. The latter process is mediated by the action of specific proteins, collectively termed the exocytotic machinery. A number of hormones are involved in regulating β-cell function, among which growth hormone (GH) and insulin-like growth factors (IGF) play an important role in β-cell regeneration and function, although the mechanisms are still far from understood. IGF-binding protein (IGFBP)-1 plays an important role in glucose homeostasis and its serum level is elevated in diabetes. A direct role of IGFBP-1 on β-cell function has not been clarified. In addition, the drug pioglitazone, an agonist of the peroxisome proliferator-activated receptor-γ (PPARγ), improves insulin sensitivity in type 2 diabetic patients through regulating glucose and lipid metabolism in insulin target tissues. In contrast, the role of the PPARγ class of drugs on pancreatic β-cell function remains elusive.

Paper I: In the insulin-secreting BRIN-BD11 cells, human GH (hGH) rapidly evoked a rise in [Ca2+]i j, an effect that was prevented by removal of extracellular Ca2+, by diazoxide or L-type Ca2+ channel blockers. However, hGH had no effect on membrane potential or L-type Ca2+ channel activity. The effect of hGH was blocked by inhibition of the ER Ca2+-ATPase or the ER Ca2+ release channels. In addition, the hGH-stimulated rise in [Ca2+]i was completely abolished by ruthenium red, an inhibitor of mitochondrial Ca2+ transport, and by caffeine. hGH induced tyrosine phosphorylation of ryanodine receptors (RyRs). The effect of hGH on [Ca2+]i was completely blocked by the tyrosine kinase inhibitors genistein and lavendustin A. Hence, hGH-stimulated rise in [Ca2+]i is dependent on extracellular Ca2+ and is mediated by Ca2+-induced Ca2+ release in BRIN-BD11 cells. This process is mediated by tyrosine phosphorylation of RyRs and may play a regulatory role in physiological Ca2+ handling in insulin-secreting cells.

Paper II: Stimulation of BRIN-BD11 cells with hGH caused rapid tyrosine phosphorylation of the non-receptor tyrosine kinases JAK2 and c-Src. Treatment of cells with hGH enhanced the [Ca2+]i response, while specific inhibitors of JAK2 and c-Src abolished the effect of hGH. The rise in [Ca2+]i elicited by hGH was associated with enhanced insulin secretion, which was mimicked by ovine prolactin (PRL) but not by bovine GH. The effects of hGH on [Ca2+]i and insulin secretion are mediated mainly through activation of the prolactin receptor and JAK2 and Src kinases in insulin-secreting cells.

Paper III: Putative direct effects of the PPARγ agonist pioglitazone on β-cell function were studied in human islets from both non-diabetic and diabetic subjects. In low glucose, pioglitazone caused a slight and transient increase in insulin secretion in non-diabetic, but not diabetic, islets. Continuous presence of the drug suppressed insulin release in non-diabetic islets. In islets from non-diabetic subjects, both high glucose- and tolbutamide-stimulated insulin secretions were inhibited by pioglitazone. When islets from either non-diabetic or diabetic subjects were perifused with low glucose, short-term pretreatment with pioglitazone caused approximately 2-fold increase in insulin secretion but only after drug withdrawal. Pioglitazone pre-treatment of diabetic islets restored their glucose sensitivity. Examination of [Ca2+]i in non-diabetic islets revealed a slight Ca2+ transient by pioglitazone at low glucose. In contrast, the suppressed insulin secretion of the drug at higher glucose concentrations was not associated with any discernable changes in [Ca2+]i. Our data indicate bimodal acute effects of pioglitazone directly on human islets and suggest that short-term pretreatment with pioglitazone primes both healthy and diabetic human islets for enhanced glucose-sensitive insulin secretion. This mechanism might be involved in the β-cell sparing effects of pioglitazone and other thiazolidinedione PPARγ agonists in clinical use.

Paper IV: The presence of different cell types in pancreatic islets has complicated the evaluation of IGFBP-1 on β-cell function. Pre-incubation of β-cells isolated from ob/ob mouse islets with IGFBP-1 resulted in a suppressed insulin secretion in response to glucose, glucagon-like peptide-1 (GLP-1) or the hypoglycemic sulfonylurea tolbutamide. However, WF13P-1 treatment did not significantly influence glucose-stimulated [Ca2+]i or NAD(P)H. In contrast, treatment of intact pancreatic islets with IGFBP-1 caused an enhanced glucose-stimulated insulin secretion, associated with a decreased secretion of somatostatin. This study suggests a negative effect of IGFBP-1 on stimulus-secretion coupling in β-cells through a mechanism directly involved in insulin exocytosis.