Glucose dependent stimulation by prostaglandin D2 of glucagon and insulin in perfused rat pancreas

GPR44 as a Target for Imaging Pancreatic Beta-Cell Mass

PURPOSE OF REVIEW

Quantitative markers for beta-cell mass (BCM) in human pancreas are currently lacking. Medical imaging using positron emission tomography (PET) markers for beta-cell restricted targets may provide an accurate and non-invasive measurement of BCM, to assist diagnosis and treatment of metabolic disease. GPR44 was recently discovered as a putative marker for beta cells and this review summarizes the developments so far.

RECENT FINDINGS

Several small molecule binders targeting GPR44 have been radiolabeled for PET imaging and evaluated in vitro and in small and large animal models. ¹¹C-AZ12204657 and ¹¹C-MK-7246 displayed a dose-dependent and GPR44-mediated binding to beta cells both in vitro and in vivo, with negligible uptake in exocrine pancreas. GPR44 represents an attractive target for visualization of BCM. Further progress in radioligand development including clinical testing is expected to clarify the role of GPR44 as a surrogate marker for BCM in humans.

An atlas and functional analysis of G-protein coupled receptors in human islets of Langerhans

G-protein coupled receptors (GPCRs) regulate hormone secretion from islets of Langerhans, and recently developed therapies for type-2 diabetes target islet GLP-1 receptors. However, the total number of GPCRs expressed by human islets, as well as their function and interactions with drugs, is poorly understood. In this review we have constructed an atlas of all GPCRs expressed by human islets: the 'islet GPCRome'. We have used this atlas to describe how islet GPCRs interact with their endogenous ligands, regulate islet hormone secretion, and interact with drugs known to target GPCRs, with a focus on drug/receptor interactions that may affect insulin secretion. The islet GPCRome consists of 293 GPCRs, a majority of which have unknown effects on insulin, glucagon and somatostatin secretion. The islet GPCRs are activated by 271 different endogenous ligands, at least 131 of which are present in islet cells. A large signalling redundancy was also found, with 119 ligands activating more than one islet receptor. Islet GPCRs are also the targets of a large number of clinically used drugs, and based on their coupling characteristics and effects on receptor signalling we identified 107 drugs predicted to stimulate and 184 drugs predicted to inhibit insulin secretion. The islet GPCRome highlights knowledge gaps in the current understanding of islet GPCR function, and identifies GPCR/ligand/drug interactions that might affect insulin secretion, which are important for understanding the metabolic side effects of drugs. This approach may aid in the design of new safer therapeutic agents with fewer detrimental effects on islet hormone secretion.

Prostaglandin E2 inhibits phosphoinositide metabolism in isolated pancreatic islets

Isolated islets of the rat labelled with myo-[3H]inositol showed decreased accumulation of total inositol phosphates (InsPs) and [3H]polyphosphoinositide hydrolysis in response to glucose after preincubation with prostaglandin E2 (PGE2). The response was concentration-dependent and specific for PGE2. PGE2 did not affect basal [3H]phosphoinositide hydrolysis or InsPs accumulation. Pertussis-toxin pretreatment antagonized the response to PGE2, whereas 8-bromo cyclic AMP was without effect. The PGE2-induced decrease in InsPs may contribute to the suppression of insulin secretion.

Opiate-prostaglandin interactions in the regulation of insulin secretion from rat islets of Langerhans in vitro

The inadequate insulin secretory response to glucose stimulation in non-insulin dependent diabetes has been attributed to many factors including high PGE2 levels blunting the secretory response, and to the existence of inhibitory opiate activity in vivo. The purpose of the present work was to see if there was a connection between these two independent theories. Radioimmunoassayable PGE2 in islets of Langerhans was found to be proportional to islet number and protein content and was typically 4 to 5pg/micrograms islet protein. Indomethacin (2.8 X 10(-5) M), sodium salicylate (1.25 X 10(-3) M) and chlorpropamide (7.2 X 10(-5) M) all lowered islet PGE2 levels and stimulated insulin release in vitro. Dynorphin (1-13), stimulated insulin release at a concentration of 6 X 10(-9) M, while lowering islet PGE2. Conversely, at a higher concentration, (6 X 10(-7) M), dynorphin had no stimulatory effect on insulin secretion and did not lower PGE2 levels in islets or in the incubation media. The stimulatory effects of dynorphin and sodium salicylate on insulin secretion were blocked by exogenous PGE2 (10(-5) M). PGE2 at a lower concentration (10(-9) M) did not exert any inhibitory effect on dynorphin- or sodium salicylate-induced insulin release. This concentration of exogenous PGE2 stimulated insulin release in the presence of 6mM glucose. Results from these experiments suggest that since an opioid peptide can lower endogenous PGE2 production in islets and since the stimulatory effects of the opioid peptide are reversed by exogenous PGE2 there may be interactions between these two modulators of insulin secretion.

Presence of prostaglandin D2, E2 and F2 alpha in rat pancreatic islets

The presence of prostaglandin (PG) D2, PGE2 and PGF2 alpha was demonstrated in rat pancreas and pancreatic islets by specific radioimmunoassay and their contents were measured separately. The amounts of PGD2, PGE2 and PGF2 alpha in rat pancreas were 16.1 +/- 1.1, 9.48 +/- 2.05 and 4.43 +/- 0.62(2) ng/g tissue, respectively. The amounts of PGD2, PGE2 and PGF2 alpha in rat pancreatic islets were 427 +/- 66, 482 +/- 281 and 479 +/- 41 pg/mg protein, respectively. The syntheses of several PGs from PGH2, a common intermediate for PG production, in islet homogenates were also studied. PGH2 was rapidly transformed into PGD2, PGE2 and PGF2 alpha in almost equal proportion in islet homogenates, and the transformation were proportional to the amounts of the homogenates added in the reaction.

Effects of inhibitors of eicosanoid synthesis on insulin release by neonatal pancreatic islets

In the pancreatic islet, eicosanoids may arise from both cyclooxygenase- and lipoxygenase-dependent metabolism of arachidonic acid. The inclusion of inhibitors of selective steps in these pathways indicated that in cultured neonatal rat islets, arachidonic acid may be metabolised through both pathways, concurrent with insulin release stimulated by D-glucose, D-glyceraldehyde and 2-ketoisocaproate. The effects of the inhibitors suggested that the products of the lipoxygenase pathway were necessary for the stimulatory effects of nutrients to be observed. In contrast to glucose, where insulin release was stimulated in the presence of inhibitors of cyclooxygenase, the stimulatory action of D-glyceraldehyde, 2-ketoisocaproate and melittin was only minimally affected by these inhibitors, although it was inhibited by lipoxygenase inhibition. These findings support a major stimulatory role for products of the lipoxygenase pathway of arachidonic acid metabolism in nutrient-induced secretion, and a negative or modulatory role of cyclooxygenase pathway products on glucose-stimulated insulin release in the neonatal islet.

Activity of prostaglandin biosynthetic pathways in rat pancreatic islets

Isolated pancreatic islets of the rat were either prelabeled with [3H]arachidonic acid, or were incubated over the short term with the concomitant addition of radiolabeled arachidonic acid and a stimulatory concentration of glucose (17mM) for prostaglandin (PG) analysis. In prelabeled islets, radiolabel in 6-keto-PGF1 alpha, PGE2, and 15-keto-13,14-dihydro-PGF2 alpha increased in response to a 5 min glucose (17mM) challenge. In islets not prelabeled with arachidonic acid, label incorporation in 6-keto-PGF1 alpha increased, whereas label in PGE2 decreased during a 5 min glucose stimulation; after 30-45 min of glucose stimulation labeled PGE levels increased compared to control (2.8mM glucose) levels. Enhanced labelling of PGF2 alpha was not detected in glucose-stimulated islets prelabeled or not. Isotope dilution with endogenous arachidonic acid probably occurs early in the stimulus response in islets not prelabeled. D-Galactose (17mM) or 2-deoxyglucose (17mM) did not alter PG production. Indomethacin inhibited islet PG turnover and potentiated glucose-stimulated insulin release. Islets also converted the endoperoxide [3H]PGH2 to 6-keto-PGF1 alpha, PGF2 alpha, PGE2 and PGD2, in a time-dependent manner and in proportions similar to arachidonic acid-derived PGs. In dispersed islet cells, the calcium ionophore ionomycin, but not glucose, enhanced the production of labeled PGs from arachidonic acid. Insulin release paralleled PG production in dispersed cells, however, indomethacin did not inhibit ionomycin-stimulated insulin release, suggesting that PG synthesis was not required for secretion. In confirmation of islet PGI2 turnover indicated by 6-keto-PGF1 alpha production, islet cell PGI2-like products inhibited platelet aggregation induced by ADP. These results suggest that biosynthesis of specific PGs early in the glucose secretion response may play a modulatory role in islet hormone secretion, and that different pools of cellular arachidonic acid may contribute to PG biosynthesis in the microenvironment of the islet.