Activity of prostaglandin biosynthetic pathways in rat pancreatic islets

Regulation of pancreatic β-cell function and mass dynamics by prostaglandin signaling

Prostaglandins (PGs) are signaling lipids derived from arachidonic acid (AA), which is metabolized by cyclooxygenase (COX)-1 or 2 and class-specific synthases to generate PGD₂, PGE₂, PGF_2α, PGI₂ (prostacyclin), and thromboxane A₂. PGs signal through G-protein coupled receptors (GPCRs) and are important modulators of an array of physiological functions, including systemic inflammation and insulin secretion from pancreatic islets. The role of PGs in β-cell function has been an active area of interest, beginning in the 1970s. Early studies demonstrated that PGE₂ inhibits glucose-stimulated insulin secretion (GSIS), although more recent studies have questioned this inhibitory action of PGE₂. The PGE₂ receptor EP3 and one of the G-proteins that couples to EP3, Gα_Z, have been identified as negative regulators of β-cell proliferation and survival. Conversely, PGI₂ and its receptor, IP, play a positive role in the β-cell by enhancing GSIS and preserving β-cell mass in response to the β-cell toxin streptozotocin (STZ). In comparison to PGE₂ and PGI₂, little is known about the function of the remaining PGs within islets. In this review, we discuss the roles of PGs, particularly PGE₂ and PGI₂, PG receptors, and downstream signaling events that alter β-cell function and regulation of β-cell mass.

Mechanism of prostacyclin-induced potentiation of glucose-induced insulin secretion

Arachidonic acid metabolites are crucial mediators of inflammation in diabetes. Although eicosanoids are established modulators of pancreatic β-cell function, the role of prostacyclin (prostaglandin I2) is unknown. Therefore, this study aimed to analyze the role of prostacyclin in β-cell function. Prostacyclin synthase (PGIS) was weakly expressed in rat islet cells but nevertheless significantly increased by incubation with 30 mM glucose, especially in non-β-cells. PGIS was overexpressed in INS1E cells, and the regulation of insulin secretion was analyzed. PGIS overexpression strongly potentiated glucose-induced insulin secretion along with increased insulin content and ATP production. Importantly, overexpression of PGIS potentiated only nutrient-induced insulin secretion. The effect of PGIS overexpression was mediated by prostacyclin released from insulin-secreting cells and dependent on prostacyclin receptor (IP receptor) activation, with concomitant cAMP production. The cAMP-mediated potentiation of glucose-induced insulin secretion by prostacyclin was independent of the protein kinase A pathway but strongly attenuated by the knockdown of the exchange protein directly activated by cAMP 2 (Epac2), pointing to a crucial role for Epac2 in this process. Thus, prostacyclin is a powerful potentiator of glucose-induced insulin secretion. It improves the secretory capacity by inducing insulin biosynthesis and probably by stimulating exocytosis. Our findings open a new therapeutical perspective for an improved treatment of type 2 diabetes.

Calcium mobilization, prostaglandin E2 and alpha 2-adrenoceptor modulation of glucose utilization and insulin secretion in pancreatic islets

alpha 2-Adrenoceptor agonists inhibit glucose-stimulated insulin release and glucose utilization in pancreatic islets. In isolated pancreatic islets of the rat, the Ca2+ channel agonists CGP-28392 and BAY-K-8644 increased insulin release in the presence of clonidine. Neither CGP-28392 nor BAY-K-8644 antagonized the effect of clonidine on glucose utilization. The Ca2+ ionophore, ionomycin, also did not affect glucose utilization in the presence or absence of clonidine. Glucagon partly reversed the effects of clonidine on insulin release, and it potentiated glucose-stimulated insulin release in the absence of clonidine. Glucagon reversed the effects of clonidine on glucose utilization. Amiloride antagonized the effects of clonidine on insulin secretion but did not enhance markedly glucose utilization in the presence or absence of clonidine. Carbamylcholine and arecoline reversed the effects of clonidine on glucose utilization and partly reversed the effects on insulin release in the absence of extracellular Ca2+. Prostaglandin (PG) E2, but not PGF2 alpha, inhibited glucose utilization in a time- and concentration-dependent manner. PGE2 also inhibited glucose-stimulated insulin release. Pertussis toxin blocked both actions of PGE2. The cyclooxygenase inhibitor indomethacin did not affect insulin release or glucose utilization in the presence of clonidine. Thus, elevated intracellular Ca2+ levels antagonize the effects of clonidine on insulin release, whereas other mediators appear to be required to alter glucose utilization.

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.

Pancreatic islet arachidonic acid turnover and metabolism and insulin release in response to delta-9-tetrahydrocannabinol

Isolated pancreatic islets from the rat secrete insulin in response to glucose or delta-9-tetrahydrocannabinol (THC). THC stimulated the basal release of insulin and also potentiated the secretory response to glucose. The exposure of control or glucose-stimulated islets to THC inhibited the incorporation of [14C]arachidonic acid (AA) into phospholipids. However, in islets prelabeled with [14C]AA, THC enhanced the glucose-induced loss of AA from phospholipids. The enhanced AA release from islet phospholipids in response to glucose and THC was accompanied by increased synthesis of 12-L-[5,6,8,9,11,12,14,15-3H(N)]-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE) and prostaglandin E2. The lipoxygenase inhibitor 3-amino-1-(3-trifluoromethylphenyl)-2-pyrazoline hydrochloride (BW755C) inhibited 12-HETE synthesis and insulin release in glucose and THC-challenged islets; nordihydroguaiaretic acid also inhibited insulin release in THC-treated islets. In contrast, the cyclooxygenase inhibitor, indomethacin, stimulated insulin release. In homogenized islet preparations, THC inhibited acyl-CoA acyltransferase, while it stimulated phospholipase A2 activity. The stimulatory effects of THC on islet cell AA hydrolysis from phospholipids, lipoxygenase product formation, and secretion suggests that these biochemical sequelae in cell activation are important modulators of insulin release.

A re-interpretation of lipoxygenase-dependent insulin release: which metabolites of arachidonic acid, or none?

There are considerable data implicating a pancreatic islet 12-lipoxy-genase in glucose-induced insulin secretion. This enzyme traditionally is conceived as converting unesterified arachidonic acid to "free" hydroperoxyeicosatetraenoic acid and metabolites thereof. However, studies employing the provision of exogenous metabolites of arachidonic acid to islet tissue fail to identify convincingly the mediator of insulin release. It is proposed that the islet lipoxygenase directly peroxidizes unsaturated fatty acids esterified within membrane phospholipids, leading to changes in ion flux and enzyme activity (particularly phospholipase A2) at the membrane level. The release of unesterified metabolites of arachidonate, although reflecting islet lipoxygenase activity, may be an epiphenomenon.