Down-regulation of prostaglandin F2 alpha receptors in rat liver during chronic endotoxemia

Prostaglandin F2α Facilitates Hepatic Glucose Production Through CaMKIIγ/p38/FOXO1 Signaling Pathway in Fasting and Obesity

Gluconeogenesis is drastically increased in patients with type 2 diabetes and accounts for increased fasting plasma glucose concentrations. Circulating levels of prostaglandin (PG) F_2α are also markedly elevated in diabetes; however, whether and how PGF_2α regulates hepatic glucose metabolism remain unknown. Here, we demonstrated that PGF_2α receptor (F-prostanoid receptor [FP]) was upregulated in the livers of mice upon fasting- and diabetic stress. Hepatic deletion of the FP receptor suppressed fasting-induced hepatic gluconeogenesis, whereas FP overexpression enhanced hepatic gluconeogenesis in mice. FP activation promoted the expression of gluconeogenic enzymes (PEPCK and glucose-6-phosphatase) in hepatocytes in a FOXO1-dependent manner. Additionally, FP coupled with G_q in hepatocytes to elicit Ca²⁺ release, which activated Ca²⁺/calmodulin-activated protein kinase IIγ (CaMKIIγ) to increase FOXO1 phosphorylation and subsequently accelerate its nuclear translocation. Blockage of p38 disrupted CaMKIIγ-induced FOXO1 nuclear translocation and abrogated FP-mediated hepatic gluconeogenesis in mice. Moreover, knockdown of hepatic FP receptor improved insulin sensitivity and glucose homeostasis in ob/ob mice. FP-mediated hepatic gluconeogenesis via the CaMKIIγ/p38/FOXO1 signaling pathway, indicating that the FP receptor might be a promising therapeutic target for type 2 diabetes.

Influence of endotoxin on contractility in the rat gastric fundus

The influence of in vivo treatment with E. coli lipopolysaccharide endotoxin on the contractility of the rat gastric fundus was studied. Four h after lipopolysaccharide treatment (20 mg/kg i.p.), the contractile responses to prostaglandin F2alpha in longitudinal muscle strips from the gastric fundus were not different from those in control animals, while the well-known decreased response to noradrenaline in rings of the thoracic aorta was confirmed. Incubation of the tissues with L-arginine did not depress the response to prostaglandin F2alpha in fundus strips of lipopolysaccharide-treated rats. Twelve h after lipopolysaccharide treatment (6.7 mg/kg i.p.), the prostaglandin F2alpha-induced contractions were consistently depressed. The impairment of the prostaglandin F2alpha-induced responses by lipopolysaccharide treatment was not reversed by the nitric oxide synthase inhibitors NG-nitro-L-arginine (L-NNA, 10(-4) M), NG-nitro-L-arginine methyl ester (L-NAME, 3 x 10(-4) M), aminoguanidine (10(-4) M) and L-N6-l-iminoethyl-lysine (L-NIL, 10(-4) M) nor by the cyclooxygenase inhibitor indomethacin (10(-5) M). The impairment was prevented by pretreating the animals with dexamethasone (5 mg/kg i.p.), which had no effect per se on the contractile response to prostaglandin F2alpha. Lipopolysaccharide treatment did not influence the contractile responses to KCl and serotonin. The nonadrenergic noncholinergic relaxant responses to transmural electrical stimulation were not influenced 4 h after lipopolysaccharide treatment but were moderately reduced after 12 h. The results illustrate that the selective impairment of prostaglandin F2alpha-induced contractions in the rat gastric fundus by lipopolysaccharide treatment is not mediated via generation of nitric oxide; downregulation of the prostaglandin F2alpha-receptor by lipopolysaccharide treatment might be involved.

Stimulation of glycogen degradation by prostaglandin E2 in primary cultured rat hepatocytes

Hepatocytes isolated from rats by the collagenase perfusion method were cultured as monolayers at concentrations of 0.4-1.1 x 10(6) attached cells/dish (9 cm2) for 1-3 days and the effect of prostaglandins on their glycogenolysis was studied. By use of [14C]glycogen-labeled cells, prostaglandin E2 (PGE2) was found to have a stimulatory effect on glycogen degradation at high cell density (more than 0.8 x 10(6) cells/dish) in 1-day cultures. PGE2 was maximally effective at 10(-7) M, increasing [14C]release from cellular [14C]glycogen to 2-3 times the basal level after 1 h incubation, and to plateau level within 2 h. PGE1, 16,16-dimethyl PGE2 and PGF2 alpha had similar effects, but PGD2 and dinor-PGE1 (a metabolite of PGE1 and PGE2 in hepatocytes) had no effect. This prostaglandin-induced glycogen degradation was observed in 1-day cultures, with a maximum between 20-30 h, but not in 2-day and later cultures. Treatment of hepatocytes with pertussis toxin potentiated PGE2-stimulated glycogen degradation, indicating that the effect involves a different pathway from that for inhibition of glucagon- and epinephrine-stimulated glycogenolysis by E series prostaglandins reported previously.

Characterization of prostaglandin-F2 alpha-binding sites on rat hepatocyte plasma membranes

Prostaglandin (PG) F2 alpha has previously been shown to increase glucose output from perfused livers and isolated hepatocytes, where it stimulated glycogen phosphorylase via an inositol-trisphosphate-dependent signal pathway. In this study, PGF2 alpha binding sites on hepatocyte plasma membranes, that might represent the putative receptor, were characterized. Binding studies could not be performed with intact hepatocytes, because PGF2 alpha accumulated within the cells even at 4 degrees C. The intracellular accumulation was an order of magnitude higher than binding to plasma membranes. Purified hepatocyte plasma membranes had a high-affinity/low-capacity and a low-affinity/high-capacity binding site for PGF2 alpha. The respective binding constants for the high-affinity site were Kd = 3 nM and Bmax = 6 fmol/mg membrane protein, and for the low-affinity site Kd = 426 nM and Bmax = 245 fmol/mg membrane protein. Specific PGF2 alpha binding to the low-affinity site, but not to the high-affinity site, could be enhanced most potently by GTP[gamma S] followed by GDP[beta S] and GTP, but not by ATP[gamma S] or GMP. PGF2 alpha competed most potently with [3H]PGF2 alpha for specific binding to hepatocyte plasma membranes, followed by PGD2 and PGE2. Since the low-affinity PGF2 alpha-binding site had a Kd in the concentration range in which PG had previously been shown to be half-maximally active, and since this binding site showed a sensitivity to GTP, it is concluded that it might represent the receptor involved in the PGF2 alpha signal chain in hepatocytes. A biological function of the high-affinity site is currently not known.