Glycogenolytic and antiglycogenolytic prostaglandin E₂ actions in rat hepatocytes are mediated via different signalling pathways

Abstract

Prostaglandin E, has been reported both to stimulate glycogen-phosphorylase activity (glycogenolytic effect) and to inhibit the glucagon-stimulated glycogen-phosphorylase activity (antiglycogenolytic effect) in rat hepatocytes. It was the purpose of this study to resolve this apparent contradiction and to characterize the signalling pathways and receptor subtypes involved in the opposing prostaglandin E, actions.Prostaglandin E, (10 pM) increased glucose output, glycogen-phosphorylase activity and inositol trisphosphate formation in hepatocyte cell culture andor suspension. In the same systems, prostaglandin E, decreased the glucagon-stimulated (1 nM) glycogen-phosphorylase activity and cAMP formation.The signalling pathway leading to the glycogenolytic effect of PGE, was interrupted by incubation of the hepatocytes with 4P-phorbol 12-myristate 13-acetate (100 nM) for 10 min, while the antiglycogenolytic effect of prostaglandin E, was not attenuated.The signalling pathway leading to the antiglycogenolytic effect of prostaglandin E, was interrupted by an incubation of cultured hepatocytes with pertussis toxin (100 ng/ml) for 18 h, whereas the glycogenolytic effect of prostaglandin E, was enhanced.The EP,/EP, prostaglandin-E,-receptor-specific prostaglandin E, analogue Sulproston had a stronger glycogenolytic potency than the EP, prostaglandin-E,-receptor-specific prostaglandin E, analogue Misoprostol. The antiglycogenolytic potency of both agonists was equal.It is concluded that the glycogenolytic and the antiglycogenolytic effects of prostaglandin E, are mediated via different signalling pathways in hepatocytes possibly involving EP, and EP, prostaglandin E, receptors, respectively.Prostaglandins have been implicated to participate in cell to cell signal propagation between non-parenchymal and parenchymal cells in the liver. They are synthesized only in non-parenchymal liver cells [ 11, primarily Kupffer cells, but also in endothelial cells and Ito cells. They are degraded mainly by the parenchymal cells, i.e. hepatocytes [2-61, which also have been shown to possess binding sites for prostaglandins [2,[7][8][9][10]. The role of prostaglandins in the regulation of liver metabolism has been discussed controversially in previous studies. The use of cyclooxygenase inhibitors have revealed the involvement of prostaglandins in signalling pathways leading to an increase in glycogenolysis and glucose output elicited by such diverse stimuli as zymoCorrespondence to G. P.

Stimulation of glycogen phosphorylase in rat hepatocytes via prostanoid release from Kupffer cells by recombinant rat anaphylatoxin C5a but not by native human C5a in hepatocyte/Kupffer cell co‐cultures

“…Thus, the increase in glycogen phosphorylase activity in co-cultures must have been largely mediated by the release of prostanoids from Kupffer cells. Prostanoids in turn can be directly activate glycogen phosphorylase in hepatocytes via a phospholipase C linked signal chain [7,8,10,11]. In line with this conclusion, the increase in glycogen phosphorylase activity by noradrenaline, which acts primarily directly on hepatocytes, was not attenuated by incubation with acetylsalicylic acid (Fig.…”

“…Hepatocytes were isolated according to Meredith [23] without the use of collagenase as described previously [11]. The liver was perfused without recirculation via the portal vein with a CaZ+-free Krebs-Henseleit buffer containing 15 mM glucose, 2 mM lactate, 0.2 mM pyruvate and 2 mM EDTA.…”

“…ylase activity in hepatocytes [7][8][9][10][11], these results suggested that anaphylatoxins stimulated hepatic glycogenolysis by an eicosanoid-mediated intercellular communication between nonparenchymal cells and hepatocytes. Such a mechanism had been shown before for other mediators that reach the liver during inflammatory processes such as endotoxins [9], immune complexes [12], platelet activating factor [13][14][15] and also for zymosan [16], a yeast cell wall preparation.…”

Induction by interleukin 6 of Gs-coupled prostaglandin E2 receptors in rat hepatocytes mediating a prostaglandin E2-dependent inhibition of the hepatocyte’s acute phase response

“…Yet another possible reason for the discrepancy might be the purity of the hepatocyte cultures. The hepatocytes in this and previous studies that failed to show a PGE 2 -mediated increase in cAMP formation 22 were almost absolutely devoid of contaminating nonparenchymal liver cells. Hepatocyte cultures prepared according to the classical protocol of Berry and Friend 32 contain small but significant amounts of nonparenchymal cells that might release IL-6 during the culture and hence induce EP2-R and EP4-R in hepatocytes, rendering the hepatocyte sensitive to a PGE 2 -induced cAMP formation.…”

The C‐terminal domain of the Gs‐coupled EP4 receptor confers agonist‐dependent coupling control to Gi but no coupling to Gs in a receptor hybrid with the Gi‐coupled EP3 receptor

“…According to their affinity to receptor-specific agonists and antagonists and to the intracellular signal chains to which they are coupled, these receptors can be divided into 4 subclasses [4]: EP^ are coupled to G q , EP 2 R and EP 4 R to G s and EP 3 R receptors to G;. Binding of PGE 2 to EP 3 R on rat hepatocytes decreased glucagon-stimulated cAMP formation and glucose output [5] while binding of PGE 2 to EP4R on human T-lymphocytes increased cAMP formation involved in differentiation and proliferation processes [6].…”