Novel role for prostaglandin E2 in fish hepatocytes: Regulation of glucose metabolism

Abstract

Prostaglandin E2 (PGE2) potently activated glycogenolysis and gluconeogenesis in isolated rockfish (Sebastes caurinus) hepatocytes. The average degree of activation for glycogenolysis was 6.4 ± 0.67-fold (mean ± S.E.M.; n=37), and could be as much as 19-fold. Analysis of dose-concentration relationships between glycogenolytic actions and PGE2 concentrations yielded an EC50 around 120 nM in hepatocyte suspensions and 2 nM for hepatocytes immobilized on perifusion columns. For the activation of gluconeogenesis (1.74 ± 0.14-fold; n=10), the EC50 for suspensions was 60 nM. Intracellular targets for PGE2 actions are adenylyl cyclase, protein kinase A and glycogen phosphorylase. Concentrations of cAMP increased with increasing concentrations of PGE2, and peaked within 2 min of hormone application. In the presence of the phosphodiesterase inhibitor, isobutyl-3-methylxanthine, peak height was increased and peak duration extended. The protein kinase A inhibitor, Rp-cAMPS, counteracted the activation of glycogenolysis by PGE2, implying that the adenylyl cyclase/protein kinase A pathway is the most important, if not exclusive, route of message transduction. PGE2 activated plasma membrane adenylyl cyclase and hepatocyte glycogen phosphorylase in a dose-dependent manner. The effects were specific for PGE2; smaller degrees of activation of glycogenolysis were noted for PGE1, 11-deoxy PGE1, 19-R-hydroxy-PGE2, and prostaglandins of the A, B and Fα-series. The selective EP2-receptor agonist, butaprost, was as effective as PGE2, suggesting that rockfish liver contains prostaglandin receptors pharmacologically related to the EP2 receptors of non-hepatic tissues of mammals. Rockfish hepatocytes quickly degraded added PGE2 (t1/2= 17-26 min). A similar ability to degrade PGE2 has been noted in catfish (Ameiurus nebulosus) hepatocytes, but no glycogenolytic or gluconeogenic actions of the hormone are noted for this species. We conclude that PGE2 is an important metabolic hormone in fish liver, with cAMP-mediated actions on glycogen and glucose metabolism, and probably other pathways regulated by cAMP and protein kinase A. The constant presence of EP2-like receptors is a unique feature of the fish liver, with interesting implications for function and evolution of prostaglandin receptors in vertebrates.