Receptor-Mediated Adenylate Cyclase—Coupled Mechanism for PGE2 Inhibition of Insulin Secretion in HIT Cells

Abstract

Prostaglandin E2 (PGE2) inhibits glucose-induced insulin secretion, and inhibitors of PGE2 synthesis augment this event. However, there has been confusion regarding prostaglandin regulation of insulin secretion, partly because no mechanism has been demonstrated for the inhibitory action of PGE2 on β-cell function. These studies were performed with a clonai cell line of glucose-responsive β-cells (HIT cells) to determine whether PGE2 effects on insulin secretion are receptor mediated and, if so, whether the postreceptor effects are mediated by inhibitory regulatory components (N1) of adenylate cyclase. Saturable [3H]PGE2 binding to HIT cells was demonstrated. This binding was dissociable and specific for prostaglandins of the E series. Scatchard analyses of binding data indicated a single class of sites with a Kd of ∼1 × 10−9 M. Guinea pig islets were also demonstrated to have a single class of binding sites with a similar Kd but only 22% as many binding sites (0.060 vs. 0.013 pmol/mg protein, HIT cells vs. guinea pig islet). HIT cells were demonstrated to synthesize PGE2, and this synthesis was inhibitable by acetylsalicylic acid. Accumulation of cAMP by HIT cells was inhibited in a concentration-dependent manner by PGE2 with an IC50 of 1 × 10−9 M. Insulin secretion by HIT cells during static incubations with 11.1 mM glucose was also inhibited by PGE2 in a concentration-dependent manner with an IC50 of 1 × 10 −9 M. PGE2 was more potent than epinephrine but less potent than somatostatin in this regard. Maximum inhibition of glucose-induced insulin secretion was 26, 37, and 29% of control values for somatostatin, PGE2, and epinephrine, respectively. Incubation of HIT cell membranes with pertussis toxin and [α-32P]NAD with subsequent electrophoresis via sodium dodecyl sulfate–polyacrylamide gels revealed ADP-ribosylation of a component consistent with the N, component of adenylate cyclase. Pretreatment of intact HIT cells with pertussis toxin prevented subsequent labeling of this component when membranes were incubated with pertussis toxin and [32P]NAD. Pertussis toxin partially prevented the inhibitory effects of PGE2, but not NiCI2, on glucose-induced insulin secretion. These findings uniquely demonstrate inhibition by PGE2 of glucose-induced insulin secretion from HIT cells. The mechanism of PGE2 action appears to involve specific PGE receptors that activate the N1, component of adenylate cyclase, resulting in diminished cAMP accumulation.