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

PGE2 synthesis and signaling in the liver physiology and pathophysiology: An update

The liver plays a central role in systemic metabolism and drug degradation. However, it is highly susceptible to damage due to various factors, including metabolic imbalances, excessive alcohol consumption, viral infections, and drug influences. These factors often result in conditions such as fatty liver, hepatitis, and acute or chronic liver injury. Failure to address these injuries could promptly lead to the development of liver cirrhosis and potentially hepatocellular carcinoma (HCC). Prostaglandin E2 (PGE2) is a metabolite of arachidonic acid that belongs to the class of polyunsaturated fatty acids (PUFA) and is synthesized via the cyclooxygenase (COX) pathway. By binding to its G protein coupled receptors (i.e., EP1, EP2, EP3 and EP4), PGE2 has a wide range of physiological and pathophysiology effects, including pain, inflammation, fever, cardiovascular homeostasis, etc. Recently, emerging studies showed that PGE2 plays an indispensable role in liver health and disease. This review focus on the research progress of the role of PGE2 synthase and its receptors in liver physiological and pathophysiological processes and discuss the possibility of developing liver protective drugs targeting the COXs/PGESs/PGE2/EPs axis.

The Crosstalk between Mesenchymal Stromal/Stem Cells and Hepatocytes in Homeostasis and under Stress

Liver diseases, characterized by high morbidity and mortality, represent a substantial medical problem globally. The current therapeutic approaches are mainly aimed at reducing symptoms and slowing down the progression of the diseases. Organ transplantation remains the only effective treatment method in cases of severe liver pathology. In this regard, the development of new effective approaches aimed at stimulating liver regeneration, both by activation of the organ's own resources or by different therapeutic agents that trigger regeneration, does not cease to be relevant. To date, many systematic reviews and meta-analyses have been published confirming the effectiveness of mesenchymal stromal cell (MSC) transplantation in the treatment of liver diseases of various severities and etiologies. However, despite the successful use of MSCs in clinical practice and the promising therapeutic results in animal models of liver diseases, the mechanisms of their protective and regenerative action remain poorly understood. Specifically, data about the molecular agents produced by these cells and mediating their therapeutic action are fragmentary and often contradictory. Since MSCs or MSC-like cells are found in all tissues and organs, it is likely that many key intercellular interactions within the tissue niches are dependent on MSCs. In this context, it is essential to understand the mechanisms underlying communication between MSCs and differentiated parenchymal cells of each particular tissue. This is important both from the perspective of basic science and for the development of therapeutic approaches involving the modulation of the activity of resident MSCs. With regard to the liver, the research is concentrated on the intercommunication between MSCs and hepatocytes under normal conditions and during the development of the pathological process. The goals of this review were to identify the key factors mediating the crosstalk between MSCs and hepatocytes and determine the possible mechanisms of interaction of the two cell types under normal and stressful conditions. The analysis of the hepatocyte-MSC interaction showed that MSCs carry out chaperone-like functions, including the synthesis of the supportive extracellular matrix proteins; prevention of apoptosis, pyroptosis, and ferroptosis; support of regeneration; elimination of lipotoxicity and ER stress; promotion of antioxidant effects; and donation of mitochondria. The underlying mechanisms suggest very close interdependence, including even direct cytoplasm and organelle exchange.

Suppression of apoptotic signaling in rat hepatocytes by non-dioxin-like polychlorinated biphenyls depends on the receptors CAR and PXR

Non-dioxin-like polychlorinated biphenyls (NDL-PCBs) represent a sub-group of persistent organic pollutants found in food, environmental samples and human and animal tissues. Promotion of pre-neoplastic lesions in rodent liver has been suggested as an indicator for a possible increased risk of liver cancer in humans exposed to NDL-PCBs. In rodent hepatocytes, suppression of DNA damage-triggered apoptosis is a typical mode of action of liver tumor promoters. Here, we report that NDL-PCBs suppress apoptosis in rat hepatocytes treated in culture with an apoptogenic dose of UV light. Suppression became less pronounced when the constitutive androstane receptor (CAR) and/or the pregnane-X-receptor (PXR) where knocked-out using siRNAs, while knocking-out both receptors led to a full reconstitution of apoptosis. In contrast, suppression of apoptosis by the CAR or PXR activators phenobarbital or dexamethasone were CAR- or PXR-specific. Induction and suppression of apoptosis were paralleled by changes in caspase 3/7, 8 and 9 activities. Our findings indicate that NDL-PCBs can suppress UV-induced apoptosis in rat hepatocytes by activating CAR and PXR. It needs further investigation if these mechanisms of action are also of relevance for human liver.

Metabolic Functions of G Protein-Coupled Receptors in Hepatocytes-Potential Applications for Diabetes and NAFLD

G protein-coupled receptors (GPCRs) are cell surface receptors that mediate the function of extracellular ligands. Understanding how GPCRs work at the molecular level has important therapeutic implications, as 30–40% of the drugs currently in clinical use mediate therapeutic effects by acting on GPCRs. Like many other cell types, liver function is regulated by GPCRs. More than 50 different GPCRs are predicted to be expressed in the mouse liver. However, knowledge of how GPCRs regulate liver metabolism is limited. A better understanding of the metabolic role of GPCRs in hepatocytes, the dominant constituent cells of the liver, could lead to the development of novel drugs that are clinically useful for the treatment of various metabolic diseases, including type 2 diabetes, nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH). In this review, we describe the functions of multiple GPCRs expressed in hepatocytes and their role in metabolic processes.

Bioactive lipids, inflammation and chronic diseases

Endogenous bioactive lipids are part of a complex network that modulates a plethora of cellular and molecular processes involved in health and disease, of which inflammation represents one of the most prominent examples. Inflammation serves as a well-conserved defence mechanism, triggered in the event of chemical, mechanical or microbial damage, that is meant to eradicate the source of damage and restore tissue function. However, excessive inflammatory signals, or impairment of pro-resolving/anti-inflammatory pathways leads to chronic inflammation, which is a hallmark of chronic pathologies. All main classes of endogenous bioactive lipids - namely eicosanoids, specialized pro-resolving lipid mediators, lysoglycerophopsholipids and endocannabinoids - have been consistently involved in the chronic inflammation that characterises pathologies such as cancer, diabetes, atherosclerosis, asthma, as well as autoimmune and neurodegenerative disorders and inflammatory bowel diseases. This review gathers the current knowledge concerning the involvement of endogenous bioactive lipids in the pathogenic processes of chronic inflammatory pathologies.

Silencing Retinoid X Receptor Alpha Expression Enhances Early-Stage Hepatitis B Virus Infection In Cell Cultures

Multiple steps of the life cycle of hepatitis B virus (HBV) are known to be coupled to hepatic metabolism. However, the details of involvement of the hepatic metabolic milieu in HBV infection remain incompletely understood. Hepatic lipid metabolism is controlled by a complicated transcription factor network centered on retinoid X receptor alpha (RXRα). Here, we report that RXRα negatively regulates HBV infection at an early stage in cell cultures. The RXR-specific agonist bexarotene inhibits HBV in HepG2 cells expressing the sodium taurocholate cotransporting polypeptide (NTCP) (HepG2-NTCP), HepaRG cells, and primary Tupaia hepatocytes (PTHs); reducing RXRα expression significantly enhanced HBV infection in the cells. Transcriptome sequencing (RNA-seq) analysis of HepG2-NTCP cells with a disrupted RXRα gene revealed that reduced gene expression in arachidonic acid (AA)/eicosanoid biosynthesis pathways, including the AA synthases phospholipase A2 group IIA (PLA2G2A), is associated with increased HBV infection. Moreover, exogenous treatment of AA inhibits HBV infection in HepG2-NTCP cells. These data demonstrate that RXRα is an important cellular factor in modulating HBV infection and implicate the participation of AA/eicosanoid biosynthesis pathways in the regulation of HBV infection.IMPORTANCE Understanding how HBV infection is connected with hepatic lipid metabolism may provide new insights into virus infection and its pathogenesis. By a series of genetic studies in combination with transcriptome analysis and pharmacological assays, we here investigated the role of cellular retinoid X receptor alpha (RXRα), a crucial transcription factor for controlling hepatic lipid metabolism, in de novo HBV infection in cell cultures. We found that silencing of RXRα resulted in elevated HBV covalently closed circular DNA (cccDNA) formation and viral antigen production, while activation of RXRα reduced HBV infection efficiency. Our results also showed that silencing phospholipase A2 group IIA (PLA2G2A), a key enzyme of arachidonic acid (AA) synthases, enhanced HBV infection efficiency in HepG2-NTCP cells and that exogenous AA treatment reduced de novo HBV infection in the cells. These findings unveil RXRα as an important cellular factor in modulating HBV infection and may point to a new strategy for host-targeted therapies against HBV.

Hepatic Overexpression of CD36 Improves Glycogen Homeostasis and Attenuates High-Fat Diet-Induced Hepatic Steatosis and Insulin Resistance

The common complications in obesity and type 2 diabetes include hepatic steatosis and disruption of glucose-glycogen homeostasis, leading to hyperglycemia. Fatty acid translocase (FAT/CD36), whose expression is inducible in obesity, is known for its function in fatty acid uptake. Previous work by us and others suggested that CD36 plays an important role in hepatic lipid homeostasis, but the results have been conflicting and the mechanisms were not well understood. In this study, by using CD36-overexpressing transgenic (CD36Tg) mice, we uncovered a surprising function of CD36 in regulating glycogen homeostasis. Overexpression of CD36 promoted glycogen synthesis, and as a result, CD36Tg mice were protected from fasting hypoglycemia. When challenged with a high-fat diet (HFD), CD36Tg mice showed unexpected attenuation of hepatic steatosis, increased very low-density lipoprotein (VLDL) secretion, and improved glucose tolerance and insulin sensitivity. The HFD-fed CD36Tg mice also showed decreased levels of proinflammatory hepatic prostaglandins and 20-hydroxyeicosatetraenoic acid (20-HETE), a potent vasoconstrictive and proinflammatory arachidonic acid metabolite. We propose that CD36 functions as a protective metabolic sensor in the liver under lipid overload and metabolic stress. CD36 may be explored as a valuable therapeutic target for the management of metabolic syndrome.

Lipid mediators are critical in resolving inflammation: a review of the emerging roles of eicosanoids in diabetes mellitus

The biosynthesis pathway of eicosanoids derived from arachidonic acid, such as prostaglandins and leukotrienes, relates to the pathophysiology of diabetes mellitus (DM). A better understanding of how lipid mediators modulate the inflammatory process may help recognize key factors underlying the progression of diabetes complications. Our review presents recent knowledge about eicosanoid synthesis and signaling in DM-related complications, and discusses eicosanoid-related target therapeutics.

The mammalian INDY homolog is induced by CREB in a rat model of type 2 diabetes

Reduced expression of the INDY (I'm not dead yet) tricarboxylate carrier increased the life span in different species by mechanisms akin to caloric restriction. Mammalian INDY homolog (mIndy, SLC13A5) gene expression seems to be regulated by hormonal and/or nutritional factors. The underlying mechanisms are still unknown. The current study revealed that mIndy expression and [(14)C]-citrate uptake was induced by physiological concentrations of glucagon via a cAMP-dependent and cAMP-responsive element-binding protein (CREB)-dependent mechanism in primary rat hepatocytes. The promoter sequence of mIndy located upstream of the most frequent transcription start site was determined by 5'-rapid amplification of cDNA ends. In silico analysis identified a CREB-binding site within this promoter fragment of mIndy. Functional relevance for the CREB-binding site was demonstrated with reporter gene constructs that were induced by CREB activation when under the control of a fragment of a wild-type promoter, whereas promoter activity was lost after site-directed mutagenesis of the CREB-binding site. Moreover, CREB binding to this promoter element was confirmed by chromatin immunoprecipitation in rat liver. In vivo studies revealed that mIndy was induced in livers of fasted as well as in high-fat-diet-streptozotocin diabetic rats, in which CREB is constitutively activated. mIndy induction was completely prevented when CREB was depleted in these rats by antisense oligonucleotides. Together, these data suggest that mIndy is a CREB-dependent glucagon target gene that is induced in fasting and in type 2 diabetes. Increased mIndy expression might contribute to the metabolic consequences of diabetes in the liver.

Eicosanoids, β-cell function, and diabetes

Arachidonic acid (AA) is metabolized by cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 (CYP) enzymes into eicosanoids, which are involved in diverse diseases, including type 1 and type 2 diabetes. During the last 30 years, evidence has been accumulated that suggests important functions for eicosanoids in the control of pancreatic β-cell function and destruction. AA metabolites of the COX pathway, especially prostaglandin E(2) (PGE(2)), appear to be significant factors to β-cell dysfunction and destruction, participating in the pathogenesis of diabetes and its complications. Several elegant studies have contributed to the sorting out of the importance of 12-LOX eicosanoids in cytokine-mediated inflammation in pancreatic β cells. The role of CYP eicosanoids in diabetes is yet to be explored. A recent publication has demonstrated that stabilizing the levels of epoxyeicosatrienoic acids (EETs), CYP eicosanoids, by inhibiting or deleting soluble epoxide hydrolase (sEH) improves β-cell function and reduces β-cell apoptosis in diabetes. In this review we summarize recent findings implicating these eicosanoid pathways in diabetes and its complications. We also discuss the development of animal models with targeted gene deletion and specific enzymatic inhibitors in each pathway to identify potential targets for the treatment of diabetes and its complications.

Effects of acetyl salycilic acid and ibuprofen in chronic liver damage induced by CCl4

Non-steroidal anti-inflammatory drugs (NSAIDs) are drugs used primarily to treat inflammation, pain and fever. Their main mechanism of action is cyclooxygenase (COX) inhibition, and this enzyme has been linked to hepatotoxicity. The association of COX and liver injury has been, in part, due to the presence of COX-2 isoform in damaged liver and the possible induction of this enzyme by profibrotic molecules like Transforming Growth Factor-β (TGF-β). The aim of this work was to evaluate the effects of two of the most used NSAIDs, acetyl salicylic acid (ASA) and ibuprofen (IBP), on experimental liver fibrosis. We formed experimental groups of rats including vehicle and drug controls, damage induced by chronic CCl4 (0.4 g kg(-1) , i.p., three times per week, for 8 weeks) administration, and CCl4 plus ASA (100 mg kg(-1) , p.o., daily) or IBP (30 mg kg(-1) , p.o., daily). Both drugs showed important antifibrotic properties. They inhibited COX-2 activity, prevented oxidative stress measured as lipid peroxidation and glutathione content, and ASA inhibited partially and IBP totally increased TGF-β expression and collagen content. ASA and IBP prevented translocation of NFκB to the nucleus and, interestingly, ASA induced MMP-2 and MMP-13 whereas IBP induced MMP-2, MMP-9 and MMP-13. As a whole, these effects explain the beneficial effects of ASA and IBP on experimental liver fibrosis.

Antifibrotic and fibrolytic properties of celecoxib in liver damage induced by carbon tetrachloride in the rat

BACKGROUND

Transforming growth factor-beta (TGF-beta) plays a pivotal role in liver fibrosis, because it activates hepatic stellate cells, stimulating extracellular matrix deposition. Cyclooxygenase-2 (COX-2) has been associated with TGF-beta because its inhibition decreases TGF-beta expression and collagen production in some cultured cell types.

AIM

The aim of this work was to evaluate the ability of celecoxib (a selective COX-2 inhibitor) to prevent and to reverse the liver fibrosis induced by CCl(4).

METHODS

We established experimental groups of rats including vehicle and drug controls, damage induced by chronic CCl(4) administration and CCl(4) plus pharmacological treatment in both prevention and reversion models. We determined: alanine aminotransferase, alkaline phosphatase, gamma-glutamyl transpeptidase, COX and metalloproteinase-2 and -9 activities, lipid peroxidation, glutathione levels, glycogen and collagen content and TGF-beta expression.

RESULTS

Celecoxib prevented and aided to the recovery of livers with necrotic and cholestatic damage. Celecoxib exhibited anti-oxidant properties by restoring the redox equilibrium (lipid peroxidation and glutathione levels). Glycogen was decreased by CCl(4), while celecoxib partially prevented and reversed this effect. Celecoxib inhibited COX-2 activity, decreased TGF-beta expression, induced metalloproteinase-2 activity and, consequently, prevented and reversed collagen accumulation.

CONCLUSION

Our findings indicate that celecoxib exerts strong antifibrogenic and fibrolytic effects in the CCl(4) model of cirrhosis.

Aggravation by prostaglandin E2 of interleukin-6-dependent insulin resistance in hepatocytes

Hepatic insulin resistance is a major contributor to fasting hyperglycemia in patients with metabolic syndrome and type 2 diabetes. Circumstantial evidence suggests that cyclooxygenase products in addition to cytokines might contribute to insulin resistance. However, direct evidence for a role of prostaglandins in the development of hepatic insulin resistance is lacking. Therefore, the impact of prostaglandin E(2) (PGE(2)) alone and in combination with interleukin-6 (IL-6) on insulin signaling was studied in primary hepatocyte cultures. Rat hepatocytes were incubated with IL-6 and/or PGE(2) and subsequently with insulin. Glycogen synthesis was monitored by radiochemical analysis; the activation state of proteins of the insulin receptor signal chain was analyzed by western blot with phosphospecific antibodies. In hepatocytes, insulin-stimulated glycogen synthesis and insulin-dependent phosphorylation of Akt-kinase were attenuated synergistically by prior incubation with IL-6 and/or PGE(2) while insulin receptor autophosphorylation was barely affected. IL-6 but not PGE(2) induced suppressors of cytokine signaling (SOCS3). PGE(2) but not IL-6 activated extracellular signal-regulated kinase 1/2 (ERK1/2) persistently. Inhibition of ERK1/2 activation by PD98059 abolished the PGE(2)-dependent but not the IL-6-dependent attenuation of insulin signaling. In HepG2 cells expressing a recombinant EP3-receptor, PGE(2) pre-incubation activated ERK1/2, caused a serine phosphorylation of insulin receptor substrate 1 (IRS1), and reduced the insulin-dependent Akt-phosphorylation.

CONCLUSION

PGE(2) might contribute to hepatic insulin resistance via an EP3-receptor-dependent ERK1/2 activation resulting in a serine phosphorylation of insulin receptor substrate, thereby preventing an insulin-dependent activation of Akt and glycogen synthesis. Since different molecular mechanisms appear to be employed, PGE(2) may synergize with IL-6, which interrupted the insulin receptor signal chain, principally by an induction of SOCS, namely SOCS3.

PPARalpha-dependent induction of the energy homeostasis-regulating nuclear receptor NR1i3 (CAR) in rat hepatocytes: potential role in starvation adaptation

A tight hormonal control of energy homeostasis is of pivotal relevance for animals. Recent evidence suggests an involvement of the nuclear receptor NR1i3 (CAR). Fasting induces CAR by largely unknown mechanisms and CAR-deficient mice are defective in fasting adaptation. In rat hepatocytes CAR was induced by WY14643, a PPARalpha-agonist. A DR1 motif in the CAR promoter was necessary and sufficient for this control. The PPARalpha-dependent increase in CAR potentiated the phenobarbital-induced transcription of the prototypical CAR-dependent gene CYP2B1. Since free fatty acids are natural ligands for PPARalpha, a fasting-induced increase in free fatty acids might induce CAR. In accordance with this hypothesis, CAR induction by fasting was abrogated in PPARalpha-deficient mice.

Effects of the prostaglandins PGF2alpha and PGE2 on calcium signaling in rat hepatocyte doublets

Coordination of intercellular Ca2+ signals is important for certain hepatic functions including biliary flow and glucose output. Prostaglandins, such as PGF2alpha and PGE2, may modify these hepatocyte functions by inducing Ca2+ increase, but very little is known about the organization of the Ca2+ signals induced by these agonists. We studied Ca2+ signals induced by PGF2alpha and PGE2 in fura-2 AM-loaded hepatocyte doublets. Even though both prostaglandins induced Ca2+ oscillations, neither PGF2alpha nor PGE2 induced coordinated Ca2+ oscillations in hepatocyte doublets. Gap junction permeability (GJP), assessed by fluorescence recovery after photobleaching, showed that this absence of coordination was not related to a defect in GJP. Inositol (1,4,5)trisphosphate [Ins(1,4,5)P3] assays and the increase in Ins(1,4,5)P3 receptor sensitivity to Ins(1,4,5)P3 observed in response to thimerosal suggested that the absence of coordination was a consequence of the very small quantity of Ins(1,4,5)P3 formed by these prostaglandins. Furthermore, when PGE2 and PGF2alpha were added just before norepinephrine, they favored the coordination of Ca2+ signals induced by norepinephrine. However, GJP between hepatocyte doublets was strongly inhibited by prolonged (>or=2 h) treatment with PGF2alpha, thereby preventing the coordination of Ca2+ oscillations induced by norepinephrine in these cells. Thus, depending on the time window, prostaglandins, specially PGF2alpha, may enhance or diminish the propagation of Ca2+ signals. They may therefore contribute to the fine tuning of Ca2+ wave-dependent functions, such as nerve stimulation, hormonal regulation of liver metabolism, or bile secretion, in both normal and pathogenic conditions.

Distribution of constitutive (COX-1) and inducible (COX-2) cyclooxygenase in postviral human liver cirrhosis: a possible role for COX-2 in the pathogenesis of liver cirrhosis

AIMS

Prostaglandins produced by the action of cyclooxygenases (COX) are important mediators of systemic vasodilatation and inflammation in liver cirrhosis. The aim of this study was to investigate the distribution of COX-1 and COX-2 in postviral cirrhosis.

METHODS

The immunohistochemical expression of the constitutive (COX-1) and the inducible (COX-2) isoenzymes was investigated in 15 patients with cirrhosis after hepatitis B and C infection; three normal control livers were also analysed.

RESULTS

COX-2 was absent from normal liver but was highly expressed in cirrhosis, mainly in the inflammatory, sinusoidal, vascular endothelial, and biliary epithelial cells. Low amounts of COX-1 were expressed in both normal and cirrhotic livers, exclusively in sinusoidal and vascular endothelial cells, with no differences seen between normal and cirrhotic livers.

CONCLUSIONS

COX-2 is overexpressed in liver cirrhosis, and possibly contributes to prostaglandin overproduction, which may be a major component of the inflammation and hyperdynamic circulation associated with cirrhosis. Because COX-2 is thought to contribute to tumour development, high COX-2 production could be a contributor to hepatocellular carcinoma development in cirrhosis. The finding of COX-2 and not COX-1 upregulation in cirrhosis could provide a possible new role for selective COX-2 inhibitors in reducing inflammation and minimising the occurrence of hepatocellular carcinoma in patients with cirrhosis.

Influence of interleukin-1alpha and COX-2 over the metabolism of arachidonic acid and glucose in isolated uterus of restricted diet rats

Isolated uteri from rats fed with a normal diet convert [14C]arachidonate into eicosanoids: PGE(2), PGF(2alpha), TXB(2) and 6-keto-F(1alpha). Restricted diet (50% of the normal diet, during 25 days) diminishes the levels of PGE(2), PGF(2alpha) and TXB(2). The addition of Interleukin-1alpha to the Krebs-Ringer bicarbonate medium increases sharply the production of eicosanoids. Inhibitors of nitric oxide synthase, Nomega-nitro-L-arginine methyl ester or aminoguanidine, do not prevent eicosanoids increase. Conversely, NS-398 (a selective inhibitor of COX-2) blocks the increase of eicosanoids while PGE(2) blocks eicosanoids production mediated by IL-1alpha. Other experiments with uteri of underfed rats confirm that interleukin-1alpha produces an increase in the glucose metabolism. The addition of Nomega-nitro-L-arginine methyl ester, aminoguadinine or NS-398 blocked such stimulation. It is concluded that Interleukin-1alpha produces an increase of glucose metabolism in uteri isolated from underfed rats by two different mechanisms, both involving COX-2: (1) nitric oxide independent and (2) nitric oxide dependent.

Sensitization by interleukin-6 of rat hepatocytes to tumor necrosis factor alpha-induced apoptosis

BACKGROUND/AIMS

Tumor necrosis factor (TNF) elicits hepatocyte apoptosis in toxic liver injury and is also central in hepatocyte proliferation after partial hepatectomy. In both circumstances interleukin (IL)-6 levels are also elevated. In mouse liver IL-6 attenuated Fas receptor-mediated apoptosis indicating its interference with pro-apoptotic signal chains. It was, therefore, the aim to examine the modulation by IL-6 of TNFalpha-induced apoptosis in rat hepatocytes.

METHODS

Primary rat hepatocytes were treated with IL-6 prior to induction of apoptosis with TNFalpha/actinomycin D or anti-Fas antibody M-20. Apoptosis was detected by determination of caspase-3 activation and bisbenzimide staining of condensed nuclei. Expression of TNFalpha receptors was analyzed by semi-quantitative polymerase chain reaction and ligand binding studies with [125I]-TNFalpha.

RESULTS

IL-6 treatment doubled TNFalpha/actinomycin D-induced caspase-3 activity and significantly enhanced chromatin condensation. By contrast IL-6 inhibited Fas-induced increase in caspase-3 activity by 45% and significantly reduced chromatin condensation. IL-6 increased the mRNA level of TNF-R1 1.35-fold and augmented cell surface binding of [125I]-TNFalpha 3-fold. The latter and TNFalpha-mediated caspase activation was attenuated by prostaglandin E(2).

CONCLUSIONS

IL-6 - in contrast to its anti-apoptotic modulation of the Fas-induced pathway - exerted a pro-apoptotic effect on the TNFalpha/actinomycin D-induced apoptosis by increasing the number of TNF-R on hepatocytes.

Activation of prostaglandin E2-receptor EP2 and EP4 pathways induces growth inhibition in human gastric carcinoma cell lines

The effect of prostaglandin E2 (PGE2) on the proliferation of gastric cancer cells is still unclear. PGE2 receptors are divided into four subtypes - EP1, EP2, EP3, and EP4 - which are coupled to three different intracellular signal-transduction systems. Stimulation of EP2 and EP4 is linked with cyclic adenosine 3', 5'-monophosphate (cAMP)-dependent protein kinase A (PKA). In some human gastric cancer cells, PGE2 has been suggested to have an antiproliferative effect by way of increased cAMP production. Expression of EP2 and EP4 in human gastric carcinoma cells, however, has not been examined. We examined the expression of EP2 and EP4 and the antiproliferative effects of specific EP2 and EP4 agonists on four different human gastric cancer cell lines. Our data clarified that all the cell lines investigated in this study expressed EP2 and EP4 and that the specific agonists of these receptors induced growth inhibition with an accompanying increase in cAMP production. In summary, gastric cancer cells have EP2 and EP4 receptors, and their selective activation is linked with the decreased cell proliferation.

Inhibition by prostaglandin E(2) of anaphylatoxin C5a- but not zymosan-induced prostanoid release from rat Kupffer cells

The proinflammatory anaphylatoxin C5a induces the release of prostanoids, ie, prostaglandins (PG) and thromboxane (TX), from the resident liver macrophages (Kupffer cells [KC]). Because KC themselves express prostanoid receptors, prostanoids--besides having paracrine functions--might regulate their own release in an autocrine loop. So far, such a possible feedback regulation has not been investigated systematically, probably because of methodological difficulties to measure newly synthesized prostanoids in the presence of added prostanoids. Here, after prelabeling of phospholipids with [(14)C]arachidonate, cellularly formed [(14)C]prostanoids were determined in the presence of added unlabelled prostanoids by thin layer chromatography. In cultured KC, recombinant rat C5a (rrC5a) rapidly increased PGD(2), PGE(2), and TXA(2) release, which was strongly reduced by PGE(2), but neither by PGD(2) nor by the TXA(2) analog U46619. The inhibitory effect of PGE(2) was mimicked by cAMP, indicating that the G(s)-coupled PGE(2) receptors type 2 or 4 were involved. Zymosan also enhanced prostanoid release from KC, but with slightly slower kinetics; this action was neither inhibited by PGE(2) nor by cAMP. Also in perfused rat livers, rrC5a enhanced prostanoid release from KC as shown by prostanoid overflow and thereby indirectly increased glucose output from hepatocytes. Again, PGE(2), but not PGD(2), inhibited rrC5a-elicited prostanoid overflow. This resulted in a complete inhibition of rrC5a-induced, prostanoid-mediated glucose output. Thus, PGE(2) can inhibit specifically the C5a-induced prostanoid release from KC via a feedback mechanism and thereby limit prostanoid-mediated hepatocellular defense reactions, eg, glucose release.

Contribution of the two Gs-coupled PGE2-receptors EP2-receptor and EP4-receptor to the inhibition by PGE2 of the LPS-induced TNFalpha-formation in Kupffer cells from EP2-or EP4-receptor-deficient mice. Pivotal role for the EP4-receptor in wild type Kupffer cells

BACKGROUND/AIMS

Prostaglandin E2 (PGE2) is known to inhibit the lipopolysaccharide (LPS)-induced tumor necrosis factor alpha (TNFalpha) formation in Kupffer cells via an increase in cAMP. Four receptor-subtypes have been cloned for PGE2 so far. Two of them, the EP2-receptor and the EP4-receptor are linked to stimulatory Gs-proteins and could mediate the inhibition by PGE2 of TNFalpha-formation.

METHODS

The significance of both receptors for PGE2-dependent inhibition of LPS-induced TNFalpha-formation was studied using Kupffer cells of mice in which either one of the two receptors had been eliminated by homologous recombination.

RESULTS

The mRNAs of both receptors were expressed in wild type mouse Kupffer cells. Exogenous PGE2 inhibited TNFalpha-formation in Kupffer cells lacking either EP2-receptor or EP4-receptor to a similar extent as in control cells, however, 10-fold higher PGE2 concentrations were needed for half maximal inhibition in cells lacking the EP4-receptor than in control or EP2-receptor-deficient cells. The response to endogenous PGE2 was blunted in EP4-receptor-deficient mice only and especially after prolonged incubation.

CONCLUSIONS

The data indicate, that PGE2 can inhibit TNFalpha-formation via both the EP2- and the EP4-receptor and that, however, the EP4-receptor appears to be physiologically more relevant in Kupffer cells since it conferred a high affinity response to PGE2.

Opioid effects on glucose and eicosanoid metabolism in isolated uterus of ovariectomized and non-ovariectomized restricted diet rats

The effect of a 25-day restricted diet (50% of the normal food intake) on uterine glucose metabolism of ovariectomized (25 days) and non-ovariectomized rats, was studied. Underfeeding reduces (14)CO(2) production from U(14)C-glucose in intact animal. However, in spayed rats, results are the opposite. In intact rats receiving a low food intake, the effect of the addition to the KRB medium of various agonist opioids, was studied. Dinorphin A did not bring about any change. On the other hand, beta endorphin increased glucose metabolism. Also, the addition of Dago and Dadle increased (14)CO(2) production, while their corresponding specific blockers, beta-FNA and Naltrindole, reversed it. Ovariectomized rats subjected to food restriction are not affected by opioid agonists. In vitro morphine, like endogenous opioids, increased (14)CO(2) in intact restricted diet rats. Arachidonic acid metabolism in these rats show that underfeeding brings about a decrease in PGF(2 alpha) and PGE(2), but the addition of morphine does not alter this situation, for which eicosanoids metabolites are not related to the effect of morphine. The morphine effect was not altered by naloxone. The subcutaneous injection of morphine increased glucose metabolism in intact underfed animals, while naloxone reduced (14)CO(2) in spayed rats subjected to underfeeding. It can be concluded that uteri from ovariectomized rats receiving a restricted diet are influenced by a mechanism of upregulation related to endogenous opioids. These likely originate in other tissues, and so prevent us from seeing the morphine effect.

cAMP inhibits inducible nitric oxide synthase expression and NF-kappaB-binding activity in cultured rat hepatocytes

BACKGROUND

The inducible nitric oxide synthase (iNOS) is strongly expressed following inflammatory stimuli. Adenosine 3',5'-cyclic monophosphate (cAMP) increases iNOS expression and activity in a number of cell types but decreases cytokine-stimulated iNOS expression in hepatocytes. The mechanisms for this effect are unknown.

METHODS

Rat hepatocytes were stimulated with cytokines to induce iNOS and cultured with cAMP agonists dibutyryl-cAMP (dbcAMP), 8-bromo-cAMP, and forskolin (FSK). Nitric oxide synthesis was assessed by supernatant nitrite levels and iNOS expression was measured by Northern and Western blot analyses. Nuclear factor kappaB binding was assessed by electromobility shift assay.

RESULTS

Cyclic AMP dose dependently decreased NO synthesis in response to a combination of proinflammatory cytokines or interleukin-1beta (IL-1beta) alone. The adenylate cyclase inhibitor SQ 22,536 increased cytokine- or IL-1beta-stimulated NO synthesis. dbcAMP decreased iNOS mRNA expression and iNOS protein expression. Both dbcAMP and glucagon decreased iNOS promoter activity in rat hepatocytes transfected with the murine iNOS promoter and decreased DNA binding of the transcription factor NF-kappaB.

CONCLUSION

These data suggest that cAMP is important in hepatocyte iNOS expression and agents that alter cAMP levels may profoundly alter the response of hepatocytes to inflammatory stimuli through effects onthe iNOS promoter region and NF-kappaB.

Functions of anaphylatoxin C5a in rat liver: direct and indirect actions on nonparenchymal and parenchymal cells

Growing evidence obtained in recent years indicates that anaphylatoxin C5a receptors (C5aR) are not restricted to myeloid cells but are also expressed on nonmyeloid cells in different tissues such as brain, lung, skin and liver. In contrast to its well-defined systemic functions, the actions of anaphylatoxins in these organs are poorly characterized. The liver can be a primary target organ for the C5a anaphylatoxin since the liver is directly connected to the gut, via the mesenteric veins and portal vein which is a main source of complement activating lipopolysaccharides (LPS). In the normal rat liver, the C5aR is only expressed by nonparenchymal cells, i.e. strongly by Kupffer cells (KC) and hepatic stellate cells (HSC) and weakly by sinusoidal endothelial cells (SEC), but not expressed by the parenchymal hepatocytes (HC). Accordingly, direct effects of C5a were only found in the C5aR-expressing KC and HSC: C5a induced the release of prostanoids from KC and HSC and enhanced the LPS-dependent release of interleukin-6 from KC. These soluble mediators indirectly influenced effector functions of the C5aR-free HC. C5a enhanced the glycogen phosphorylase activity and thus the glucose output from HC indirectly via prostanoids released from KC and HSC. Glucose can serve as an energy substrate as well as an electron donor for the synthesis of reactive oxygen intermediates by KC. Moreover, C5a also enhanced transcription of the gene for the type-2 acute phase protein alpha 2-macroglobulin in HC indirectly by increasing LPS-dependent IL-6 release from KC. Under pathological conditions, C5aR was found to be upregulated in various organs including the liver. Simulation of inflammatory conditions by treatment of rats with IL-6, a main inflammatory mediator in the liver, caused a de novo expression of functional C5aR in HC. In livers of IL-6-treated rats, C5a initiated glucose output from HC and perhaps other HC-specific defense reactions directly without the intervention of soluble mediators from nonparenchymal cells.

Zymosan-induced changes in glucose release and fatty acid oxidation in the perfused rat liver

The aim of the present study was to investigate the actions of zymosan on glucose release and fatty acid oxidation in perfused rat livers and to determine if Kupffer cells and Ca2+ ions are implicated in these actions. Zymosan caused stimulation of glycogenolysis in livers from fed rats. In livers from fasted rats zymosan caused gradual inhibition of glucose production and oxygen consumption from lactate plus pyruvate. Ketogenesis, oxygen consumption, and [14C-]-CO2 production were inhibited by zymosan when the [1-14C]-palmitate was supplied exogenously. However, ketogenesis and oxygen consumption from endogenous sources were not inhibited. An interference with substrate-uptake by the liver may be the cause of the changes in gluconeogenesis and oxidation of fatty acids from exogenous sources. The pretreatment of the rats with gadolinium chloride and the removal of Ca2+ ions did not suppress the effects of zymosan on glucose release, a finding that argues against the participation of Kupffer cells or Ca2+ ions in the liver responses. The hepatic metabolic changes caused by zymosan could play a role in the systemic metabolic alterations reported to occur after in vivo zymosan administration.

Influence of insulin on the metabolism of glucose in uteri isolated from ovariectomized and non ovariectomized underfed rats

The effects of insulin on the metabolism of U14C-glucose in uteri isolated from ovariectomized and non-ovariectomized rats receiving a restricted diet (50% of the normal food intake) for 25 days, were studied. As a result of food restriction, the production of 14CO2 diminishes in intact rats, while results are reversed in ovariectomized ones. Various concentrations of insulin were added to the medium, but only 0.50 IU. ml(-1)was effective in increasing glucose metabolism in intact rats receiving a restricted diet; neither underfed castrated animals nor control ones receiving a normal diet, reacted to this concentration. The increase of 14CO2 produced by insulin is not affected by acetyl salicylic acid. Insulin does not alter the effect of underfeeding over arachidonic acid metabolism. On the contrary, the increase in glucose metabolism was blocked by N(G)methyl-L-arginine or by hemoglobin, increased with the addition of L arginine and is not affected by acetyl salicylic acid. Hemoglobin and L-arginine show no effects without insulin. We can conclude that the stimulating effect of insulin on glucose metabolism in uteri isolated from intact rats subjected to dietary restriction, is nitric oxide dependent.

Effects of morphine and naloxone on glucose metabolism in uterine strips from ovariectomized and non-ovariectomized restricted diet rats

The effect of underfeeding over glucose metabolism in uteri isolated from ovariectomized and non-ovariectomized rats subjected to a restricted diet for 25 days (50% of the normal food intake), was studied. Underfeeding decreases (14)CO(2) formation from U(14) C-glucose in intact animal uteri. While in ovariectomized rats (25 days), the effect is the opposite. The addition of morphine 10(-6) M to the medium does not affect rats fed ad libitum. However, (14)CO(2) levels increase significantly in intact animals receiving a restricted diet. In ovariectomized rats morphine does not show any activity, regardless of the type of diet rats were subjected to. None of the rat groups seems to be sensitive to naloxone 10(-6) M. The s.c. injection of morphine (4 mg.kg (-1)) increases glucose metabolism only in intact rats provided with a restricted diet, while naloxone (2.5 mg.kg (-1) ) produces a decrease of ( 14)CO(2) in ovariectomized underfed animals. To conclude, morphine either 'in vivo' or 'in vitro' is active only in uteri from intact rats subjected to underfeeding. Naloxone produces a decrease in (14)CO(2) production, particularly when it is s.c. injected to ovariectomized rats undergoing a dietary restriction. Since the uterus does not react to naloxone, the effect of the opiod blocker may be the result of endogenous opioids originated in other tissues.

Structure of the 5'-flanking region of the rat prostaglandin F(2alpha) receptor gene and its transcriptional control functions in hepatocytes

Prostaglandin F(2alpha) (PGF(2alpha)), modulates hepatocyte functions via a heptahelical G(q)-coupled PGF(2alpha)-receptor (FP-R) which in liver is expressed exclusively in hepatocytes. The aim of the present study was to isolate the 5'-flanking region of the rat FP-R gene and to elucidate its basal and IL-6-modulated transcription control function in rat hepatocytes. The 5'-non-translated region of the rat hepatocyte FP-R mRNA differed from the corresponding region in rat fetal astrocyte or corpus luteum. It was encoded by exons 1a and 2 which were separated by a 1. 4 kb intron containing the exons 1b and 1c coding for the 5'-untranslated region of rat fetal astrocyte and corpus luteum FP-R mRNA, respectively. The transcription initiation site in hepatocytes was localized 263 bp upstream of the start ATG by 5'-RACE. A DNA-fragment covering the 5'-flanking region of the rFP-R gene from -1 of the transcription initiation site to -2590 bp was cloned and sequenced. Its 3'-two thirds had a 65% sequence identity to the mouse FP-R promoter however no homology to the bovine FP-R promoter. In the overlapping sequence most of the putative transcription factor binding sites were conserved between mouse and rat. The rat promoter contained no classical TATA- or CAAT-boxes but putative binding sites for the transcription factors C/EBP, GATA-1, HNF-1, HNF-3beta, SP-1, and USF. Luciferase reporter gene constructs containing portions of the 5'-flanking region were transfected into rat hepatocytes. Luciferase expression ranked -181 >/= -608 < -1418 > -1821 >/= -2590. The strongest transcriptional activity was conferred by the region between -608 and -1418 containing a cluster of potential HNF-1 and HNF-3beta binding sites that might allow the exclusive expression of FP-R mRNA in hepatocytes. The amount of FP-R mRNA and the luciferase expression under control of the -2590 promoter fragment were reduced by IL-6 in hepatocytes.

Kupffer cell-derived prostaglandin E(2) is involved in alcohol-induced fat accumulation in rat liver

Destruction of Kupffer cells with gadolinium chloride (GdCl(3)) and intestinal sterilization with antibiotics diminished ethanol-induced steatosis in the enteral ethanol feeding model. However, mechanisms of ethanol-induced fatty liver remain unclear. Accordingly, the role of Kupffer cells in ethanol-induced fat accumulation was studied. Rats were given ethanol (5 g/kg body wt) intragastrically, and tissue triglycerides were measured enzymatically. Kupffer cells were isolated 0-24 h after ethanol, and PGE(2) production was measured by ELISA, whereas inducible cyclooxygenase (COX-2) mRNA was detected by RT-PCR. As expected, ethanol increased liver triglycerides about threefold. This increase was blunted by antibiotics, GdCl(3), the dihydropyridine-type Ca(2+) channel blocker nimodipine, and the COX inhibitor indomethacin. Ethanol also increased PGE(2) production by Kupffer cells about threefold. This increase was also blunted significantly by antibiotics, nimodipine, and indomethacin. Furthermore, tissue triglycerides were increased about threefold by PGE(2) treatment in vivo as well as by a PGE(2) EP(2)/EP(4) receptor agonist, whereas an EP(1)/EP(3) agonist had no effect. Moreover, permeable cAMP analogs also increased triglyceride content in the liver significantly. We conclude that PGE(2) derived from Kupffer cells, which are activated by ethanol, interacts with prostanoid receptors on hepatocytes to increase cAMP, which causes triglyceride accumulation in the liver. This mechanism is one of many involved in fatty liver caused by ethanol.

Involvement of Sp1 in the transcriptional regulation of the rat insulin-like growth factor-1 gene

Most insulin-like growth factor-I (IGF-I) transcripts are initiated in exon 1, but mechanisms of regulation are not well understood. Since potential Sp1 sites are found in footprinted regions within approximately 360 bp upstream and downstream from the major initiation sites in exon 1, we explored the involvement of Sp1 and Sp3 in regulation of IGF-1 expression. Gel shift assays showed strong Sp1 binding to the downstream site, but binding to the upstream site was weak; Sp1 bound to a CCTGCCCA sequence in downstream footprint region V, and Sp3 binding was centered on the same sequence. IGF-I basal promoter constructs containing a mutation in the downstream Sp1 site exhibited a 32% decrease in expression in CHO cells and a 75% decrease in HepG2 cells, indicating the importance of Sp1 for expression in vivo. Sp1 and Sp3 expression vectors provided three- to five-fold stimulation of wild-type IGF-I constructs, but had little effect on a construct containing a mutation in the downstream Sp1 site, and Sp1 had comparable effects in Drosophila SL2 cells. IGF-I heterologous promoter constructs exhibited similar responses: in both SL2 cells and CHO cells, stimulation by Sp1 was enhanced with constructs containing downstream region V. Since Sp1 also stimulated expression of concatamers of putative cis-acting sites fused to the SV40 promoter enhancer in pGL3, the results in combination indicate that the presence of IGF-I region V is sufficient to permit stimulation by Sp1.

CONCLUSION

Sp1 and related factors may play an important role in the regulation of IGF-I gene transcription, through interactions with region V downstream from the major initiation sites in exon 1.

Induction by interleukin 6 of G(s)-coupled prostaglandin E(2) receptors in rat hepatocytes mediating a prostaglandin E(2)-dependent inhibition of the hepatocyte's acute phase response

Prostanoids, that are released from nonparenchymal liver cells in response to proinflammatory stimuli, are involved in the regulation of hepatic functions during inflammation. They exert their effects on their target cells via heptahelical receptors in the plasma membrane. For the 5 prostanoids prostaglandin E(2) (PGE(2)), prostaglandin F(2alpha), prostaglandin D(2) (PGD(2)), prostacyclin, and thromboxane A(2) there exist 8 receptors that are coupled to different heterotrimeric G proteins. These receptors are expressed differentially in the 4 principal liver cell types, i.e., hepatocytes, Kupffer cells, sinusoidal endothelial cells, and hepatic stellate cells. It was intriguing, that the messenger RNA (mRNA) of none of the G(s)-coupled prostanoid receptors (DP-R, EP2-R, EP4-R, and IP-R) that can attenuate the inflammatory reaction were present in hepatocytes. The current study shows that the expression of the G(s)-coupled prostanoid receptors EP2-R, EP4-R, and DP-R, but not the IP-R, was efficiently and rapidly up-regulated by treatment of hepatocytes in vitro or rats in vivo with the key acute phase cytokine interleukin 6 (IL-6). In IL-6-treated hepatocytes PGE(2) in turn attenuated the IL-6-induced alpha(2)-macroglobulin formation via a cyclic adenosine monophosphate (cAMP)-dependent signal chain. The data indicate that an IL-6-mediated induction of the previously not expressed EP2-R and EP4-R on hepatocytes might establish a prostanoid-mediated feedback inhibition loop for the attenuation of the acute phase response.

PGE(2) stimulates O(2) uptake in hepatic parenchymal cells: involvement of the cAMP-dependent protein kinase

The aim of this study was to determine which PGE(2) receptors and signal transduction pathways are responsible for the stimulation of oxygen uptake in liver. Hepatic parenchymal cells isolated from female Sprague-Dawley rats were incubated either with PGE(2), 17-phenyl-omega-trinor PGE(2) (an EP(1)-specific agonist), or 11-deoxy PGE(1) (an EP(2)/EP(4)-specific agonist), and oxygen consumption was measured. Both PGE(2) and 11-deoxy PGE(1) stimulated oxygen consumption. However, an EP(1) agonist was without effect. Although PGE(2) elevated intracellular calcium, this occurred at concentrations approximately 500-fold lower than that required to stimulate oxygen uptake. PGE(2)-stimulated increases in cAMP formation correlated well with the increase in oxygen consumption. Dibutyryl cAMP also increased oxygen consumption. Furthermore, N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide, a cell-permeable inhibitor of protein kinase A (PKA), reduced the stimulation of oxygen uptake by PGE(2). Incubation of isolated parenchymal cell mitochondria with the purified catalytic subunit of PKA and ATP increased both state 3 rates of oxygen uptake and the respiratory control ratio by approximately 50%. Activation of these events was prevented by incubation with the PKA inhibitory peptide, PKI. These findings are consistent with the hypothesis that PGE(2) stimulates oxygen consumption via an EP(2) and/or EP(4) subclass of receptors through the actions of cAMP on a cAMP-dependent protein kinase.

Increase by anaphylatoxin C5a of glucose output in perfused rat liver via prostanoids derived from nonparenchymal cells: direct action of prostaglandins and indirect action of thromboxane A(2) on hepatocytes

In the perfused rat liver the anaphylatoxin C5a enhanced glucose output, reduced flow, and elevated prostanoid overflow. Because hepatocytes (HCs) do not express C5a receptors, the metabolic C5a actions must be indirect, mediated by e.g. prostanoids from Kupffer cells (KCs) and hepatic stellate cells (HSCs), which possess C5a receptors. Surprisingly, the metabolic C5a effects were not only impaired by the prostanoid synthesis inhibitor, indomethacin, but also by the thromboxane A(2) (TXA(2)) receptor antagonist, daltroban, even though HCs do not express TXA(2) receptors. TXA(2) did not induce prostaglandin (PG) or an unknown factor release from KCs or sinusoidal endothelial cells (SECs), which express TXA(2) receptors, because (1) daltroban did neither influence the C5a-induced release of prostanoids from cultured KCs nor the C5a-dependent activation of glycogen phosphorylase in KC/HC cocultures and because (2) the TXA(2) analog, U46619, failed to stimulate prostanoid release from cultured KCs or SECs or to activate glycogen phosphorylase in KC/HC or SEC/HC cocultures. In the perfused liver, Ca(2+)-deprivation inhibited not only flow reduction but also glucose output elicited by C5a to similar extents as daltroban. Similarly, in the absence of extracellular Ca(2+), flow reduction and glucose output induced by U46619 were almost completely prevented, whereas glucose output induced by the directly acting PGF(2alpha) was only slightly lowered. Thus, in the perfused rat liver PGs released after C5a-stimulation from KCs and HSCs directly activated glycogen phosphorylase in HCs, and TXA(2) enhanced glucose output indirectly mainly by causing hypoxia as a result of flow reduction.

Differential expression of prostanoid receptors in hepatocytes, Kupffer cells, sinusoidal endothelial cells and stellate cells of rat liver

BACKGROUND/AIMS

Prostanoids produced by nonparenchymal cells modulate the function of parenchymal and nonparenchymal liver cells during homeostasis and inflammation via eight classes of prostanoid receptors coupled to different G-proteins. Prostanoid receptor expression in parenchymal and nonparenchymal cells was studied in order to get a better insight into the complex prostanoid-mediated intrahepatic signaling network.

METHODS

RNA was isolated from freshly purified parenchymal and nonparenchymal rat liver cells and the mRNA level of all eight prostanoid receptor classes was determined by newly developed semiquantitative reverse transcription-polymerase chain reaction protocols.

RESULTS

The mRNAs for the prostanoid receptors were differentially expressed. Hepatocytes were the only cell type which contained the mRNA of the Gq-linked prostaglandin F2alpha receptor; they were devoid of any mRNA for the Gs-linked prostanoid receptors. Kupffer cells possessed the largest amount of mRNA for the Gs-linked prostaglandin E2 receptor subtype 2. Endothelial cells expressed high levels of mRNA for the Gq-linked thromboxane receptor and medium levels of mRNA for the Gs-linked prostacyclin receptor, while stellate cells had the highest levels of mRNA for the prostacyclin receptor. The mRNAs for the Gq-linked prostaglandin E2 receptor subtype 1 and the Gi-linked prostaglandin E2 receptor subtype 3 were expressed in hepatocytes and all nonparenchymal cell types at similar high levels, whereas the mRNA of the Gs-linked prostaglandin D2 receptor was expressed in all nonparenchymal cells at very low levels.

CONCLUSIONS

In hepatocytes the prostaglandin F2alpha receptor can mediate an increase in glucose output via an increase of intracellular InsP3 while cAMP-dependent glucose output can be inhibited via the subtype 3 prostaglandin E2 receptor. The subtype 2 prostaglandin E2 receptor can restrain the inflammatory response of Kupffer cells via an increase in intracellular cAMP The thromboxane receptor and the prostacyclin receptor in sinusoidal endothelial and the prostacyclin receptor in stellate cells may be involved in the regulation of sinusoidal blood flow and filtration.

Stimulation of oxygen uptake by prostaglandin E2 is oxygen dependent in perfused rat liver

The aim of this study was to determine if the effect of prostaglandin E2 (PGE2) on hepatic oxygen uptake was affected by oxygen tension. Livers from fed female Sprague-Dawley rats were perfused at normal or high flow rates (4 or 8 ml . g-1 . min-1) to vary local oxygen tension within the liver lobule. During perfusion at normal flow rates, PGE2 (5 microM) infusion increased oxygen uptake by about 50 micromol . g-1 . h-1; however, when livers were perfused at high flow rates, the increase was nearly twice as large. Simultaneously, glucose output was increased rapidly by about 50%, whereas glycolysis was decreased about 60%. When flow rate was held constant, increases in oxygen uptake due to PGE2 were proportional to oxygen delivery. Infusion of PGE2 into livers perfused at normal flow rates increased state 3 rates of oxygen uptake of subsequently isolated mitochondria by about 25%; however, rates were increased 50-75% in mitochondria isolated from livers perfused at high flow rates. Thus it is concluded that PGE2 stimulates oxygen uptake via mechanisms regulated by oxygen tension in perfused rat liver. High flow rates also increased basal rates of oxygen uptake: this increase was prevented by inactivation of Kupffer cells with GdCl3. In addition, conditioned medium from Kupffer cells incubated at high oxygen tension (75% oxygen) stimulated oxygen uptake of isolated parenchymal cells by >30% and elevated PGE2 production about twofold compared with Kupffer cells exposed to normal air-saturated buffer (21% oxygen). These effects were blocked completely by both indomethacin and nisoldipine. These data support the hypothesis that oxygen stimulates Kupffer cells to release mediators such as PGE2 which elevate oxygen consumption in parenchymal cells, possibly by mechanisms involving cyclooxygenase and calcium channels.

Stepwise activation of the gonadotropic signal transduction pathway, and the ability of prostaglandin F2alpha to inhibit this activated pathway

Through selective activation of the gonadotropic signal transduction pathway, we have determined the probable site of the antigonadotropic effects of prostaglandin F2alpha (PGF2alpha) in the human granulosa-luteal cell (hGLC). The gonadotropic signal transduction pathway was activated at the level of the receptor (luteinizing hormone and beta-adrenergic), stimulatory G protein (Gs), adenylate cyclase (AC), and protein kinase A (PKA) by human chorionic gonadotropin (hCG) and isoproterenol (Iso), cholera toxin (CTX), forskolin, and dibutryl cAMP (Db cAMP), respectively. Concomitantly, the ability of PGF2alpha to inhibit progesterone production in response to the activation of this cascade at these different levels was examined. hGLCs were obtained from in vitro fertilization patients and were precultured for 8 d in Medium 199 supplemented with fetal bovine serum (M199; 10% FBS). Following the preculture period, cells were treated with either vehicle or one of the above activators of the gonadotropic pathway, either in the absence or presence of PGF2alpha (in M199; No FBS). Following the treatment period, media were collected and assayed for progesterone by RIA. Prostaglandin F2alpha (10(-6) M) significantly inhibited hCG (1 IU/mL), Iso (10(-5) M), CTX (1 microg/mL), and forskolin- (10(-5) M) stimulated progesterone production. Conversely, PGF2alpha did not inhibit progesterone production stimulated by a saturating concentration of Db cAMP (10(-6) M). The ability of PGF2alpha to inhibit hCG- or CTX-stimulated progesterone production was attenuated by pertussis toxin (PTX; 50 ng/mL). In conclusion, through a pertussis toxin-sensitive G protein, PGF2alpha inhibits progesterone production at a level below AC, and above the activation of PKA by cAMP.

Molecular cloning and characterization of the four rat prostaglandin E2 prostanoid receptor subtypes

We have characterized the rat prostanoid EP1, EP2, EP3alpha and EP4 receptor subtypes cloned from spleen, hepatocyte and/or kidney cDNA libraries. Comparison of the deduced amino acid sequences of the rat EP receptors with their respective homologues from mouse and human showed 91% to 98% and 82% to 89% identity, respectively. Radioreceptor binding assays and functional assays were performed on EP receptor expressing human embryonic kidney (HEK) 293 cells. The KD values obtained with prostaglandin E2 for the prostanoid receptor subtypes EP1, EP2, EP3alpha and EP4 were approximately 24, 5, 1 and 1 nM, respectively. The rank order of affinities for various prostanoids at the prostanoid receptor subtypes EP2, EP3alpha and EP4 receptor subtypes was prostaglandin E2 = prostaglandin E1 > iloprost > prostaglandin F2alpha > prostaglandin D2 > U46619. The rank order at the prostanoid EP1 receptor was essentially the same except that iloprost had the highest affinity of the prostanoids tested. Of the selective ligands, butaprost was selective for prostanoid EP2, M&B28767 and sulprostone were selective for EP3alpha and enprostil displayed dual selectivity, interacting with both prostanoid receptor subtypes EP1 and EP3alpha. All four receptors coupled to their predominant signal transduction pathways in HEK 293 cells. Notably, using a novel aequorin luminescence assay to monitor prostanoid EP1 mediated increases in intracellular calcium, both iloprost and sulprostone were identified as partial agonists. Finally, by Northern blot analysis EP3 transcripts were most abundant in liver and kidney whereas prostanoid EP2 receptor mRNA was expressed in spleen, lung and testis and prostanoid EP1 receptor mRNA transcripts were predominantly expressed in the kidney. The rat prostanoid EP1 probes also detected additional and abundant transcripts present in all the tissues examined. These were found to be related to the expression of a novel protein kinase gene and not the prostanoid EP1 gene [Batshake, B., Sundelin, J., 1996. The mouse genes for the EP1 prostanoid receptor and the novel protein kinase overlap. Biochem. Biophys. Res. Commun. 227. 1329-1333].

Substitution of charged amino acid residues in transmembrane regions 6 and 7 affect ligand binding and signal transduction of the prostaglandin EP3 receptor

Expression of the rabbit EP3 receptor isoform 77A in COS1 and HEK293tsA201 cells demonstrated specific binding of [3H]prostaglandin (PG)E2 and receptor-evoked decreases in intracellular cAMP levels. Competition binding with PGE2, PGE2 methyl ester, misoprostol-free acid, misoprostol, and sulprostone suggested that a negative charge at the C1 position is essential for high affinity ligand binding and that the charge at this position is more important than steric bulk. Charged amino acid residues within the transmembrane (TM) domains of the receptor were mutated, and the resulting receptor proteins were analyzed for the effects of these mutations on receptor structure and/or function. Positively charged TM residues are candidates for interaction with the C1 carboxylic acid moiety of prostanoid ligands. Substitution of R329 (TM VII) with either alanine or glutamate resulted in a loss of both detectable [3H]PGE2 binding and receptor activation despite expression of the receptor protein as determined by immunoprecipitation and immunofluorescence. Substitution of K300 (TM V) with alanine had no effect on binding or signal transduction. Substitution of the conserved aspartic acid at position 338 (TM VII) with alanine caused a loss of detectable receptor-evoked inhibition of cAMP generation, although this mutation did not appreciably affect ligand binding. These studies suggest that R329 but not K300 is a key determinant in receptor/ligand interaction. Furthermore, D338 plays a critical role in G1 activation by the EP3 receptor.

Comitogenicity of eicosanoids and the peroxisome proliferator ciprofibrate in cultured rat hepatocytes

Several hypolipidemic drugs and environmental contaminants induce hepatic peroxisome proliferation and hepatic tumors when administered to rodents. These chemicals increase the expression of the peroxisomal beta-oxidation pathway and the cytochrome P-450 4A family, which metabolize lipids, including eicosanoids and their precursor fatty acids. We previously found that the peroxisome proliferator ciprofibrate decreases the level of eicosanoids in the liver and in cultured hepatocytes. In this study, we examined the effect of prostaglandins E2 and F2 alpha (PGE2 and PGF2 alpha), leukotriene C4 (LTC4) and the peroxisome proliferator ciprofibrate on DNA synthesis in cultured hepatocytes. Primary rat hepatocytes were cultured on collagen gels in serum-free L-15 medium with varying concentrations of eicosanoids and ciprofibrate, and the absence or presence of growth factors. Ciprofibrate lowered hepatocyte eicosanoid concentrations; the addition of eicosanoids restored their levels. After a 48-h exposure with [3H]-thymidine, DNA synthesis was determined by measuring [3H]-thymidine incorporation into DNA. The addition of PGE2, PGF2 alpha, and LTC4 to cultures along with ciprofibrate increased DNA synthesis, whereas treatment with ciprofibrate or eicosanoids alone resulted in a much smaller increase. The addition of epidermal growth factor (EGF) to the eicosanoid-ciprofibrate combination increased DNA synthesis more than EGF or the eicosanoid-ciprofibrate combination alone. The PGF2 alpha-ciprofibrate combination also was comitogenic with transforming growth factor-alpha and hepatocyte growth factor. The addition of both ciprofibrate and prostaglandins also blocked the growth inhibitory effect of transforming growth factor-beta on DNA synthesis induced by EGF. These results show that the eicosanoids PGE2, PGF2 alpha, and LTC4 are comitogenic with the peroxisome proliferator ciprofibrate in cultured rat hepatocytes.

Growth-promoting effects of Ca(2+)-mobilizing agents in hepatocytes: lack of correlation between the acute activation of phosphoinositide-specific phospholipase C and the stimulation of DNA synthesis by angiotensin II, vasopressin, norepinephrine, and prostaglandin F2 alpha

Although several hormones that promote hepatocyte proliferation also activate phosphoinositide-specific phospholipase C (PI-PLC) and mobilize Ca2+, the role of PI-PLC in the growth-stimulating effect of these agents is not clear. We have investigated this issue further, by exposing freshly isolated adult rat hepatocytes to vasopressin, angiotensin II, norepinephrine (in the presence of the beta-adrenoceptor blocker timolol) or PGF2 alpha, and examined both acute responses and the subsequent DNA synthesis when the cells were grown in monolayer culture. All the agonists elevated the level of inositol 1,4,5-trisphosphate (InsP3) and enhanced the DNA synthesis, amplifying the response to epidermal growth factor (EGF), and this comitogenic effect could be exerted by a single exposure of the cells 24 h prior to the addition of EGF. The acute activation of PI-PLC, measured as the early rise (peak 15-60 s) in InsP3, was 8-10-fold with vasopressin or angiotensin II, 3-4-fold with norepinephrine, and approximately 2-fold with PGF2 alpha. For all the agonists, a rise in cytosolic free Ca2+ in 100% of the cells and a maximal increase in glycogen phosphorylase activity were evoked at concentrations that approximately doubled the level of InsP3. However, the growth-stimulatory effects of these agonists showed a different order of efficacy as compared to the activation of PI-PLC; in terms of the maximal stimulation of DNA synthesis, the effects were: norepinephrine approximately PGF2 alpha > angiotensin II > vasopressin. Also, norepinephrine, PGF2 alpha, and angiotensin II, but not vasopressin, further enhanced the DNA synthesis when their concentrations were increased above those yielding maximal elevation of InsP3. In experiments where vasopressin and angiotensin II were combined, their effects on the DNA synthesis were additive while the InsP3 responses were not. The results show that the extent of the initial activation of PI-PLC is not the determinant for the magnitude of the growth effects of Ca(2+)-mobilizing hormones in hepatocytes. This suggests either (a) that the proliferative response to these agents is determined by the activity of PI-PLC at a later time, or its integral over an extended part of the prereplicative period, rather than by the acute activation, or (b) that additional, PI-PLC-independent, mechanisms are required.

PGE2 and LTB4 inhibit cytokine-stimulated nitric oxide synthase type 2 expression in isolated rat hepatocytes

Prostaglandins have been shown to have a wide range of effects on nitric oxide synthesis when studied in different cell populations. The proximity of hepatocytes to eicosanoid-producing endothelial cells and Kupffer cells prompted us to determine the effects of PGE2 and LTB4 on hepatocyte NO production by the inducible nitric oxide synthase (iNOS, NOS-2) in vitro. PGE2 decreased hepatocyte NO synthesis in a concentration-dependent manner when the cells were stimulated with a combination of cytokines or IL-1 alone. LTB4 had a similar effect. PGE2 had to be present at the time of cytokine exposure to produce maximal inhibition of NO synthesis. Reduced synthesis of NO2- was associated with reduced NOS-2 mRNA levels suggesting that the induction of NOS-2 was inhibited. These findings demonstrate that eicosanoids can regulate hepatocyte NO synthesis in vitro.

The role of intracellular Ca2+ in the regulation of gluconeogenesis

A hypothesis for the hormonal regulation of gluconeogenesis, in which increases in cytosolic free-Ca2+ levels ([Ca2+]i) play a major role, is presented. This hypothesis is based on the observation that gluconeogenic hormones evoke a common pattern of Ca2+ redistribution, resulting in increases in [Ca2+]i. Current concepts of hormonally evoked Ca2+ fluxes are presented and discussed. It is suggested that the increase in [Ca2+]i is functionally linked to stimulation of gluconeogenesis. The stimulation of gluconeogenesis is accomplished in two ways: (1) by increasing the activities of the Krebs cycle and the electron-transfer chain, thereby supplying adenosine triphosphates (ATP) and reducing equivalents to the process; and (2) by stimulating the activities of key gluconeogenic enzymes, such as pyruvate carboxylase. The hypothesis presents a conceptual framework that ties together two interrelated manifestations of hormone action: signal transduction and metabolism.

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

Human anaphylatoxin C3a had previously been shown to increase glycogenolysis in perfused rat liver and prostanoid formation in rat liver macrophages. Surprisingly, human C5a, which in other systems elicited stronger responses than C3a, did not increase glycogenolysis in perfused rat liver. Species incompatibilities within the experimental system had been supposed to be the reason. The current study supports this hypothesis: (1) In rat liver macrophages that had been maintained in primary culture for 72 h recombinant rat anaphylatoxin C5a in concentrations between 0.1 and 10 micrograms/ml increased the formation of thromboxane A2, prostaglandin D2, E2 and F2 alpha 6- to 12-fold over basal within 10 min. In contrast, human anaphylatoxin C5a did not increase prostanoid formation in rat Kupffer cells. (2) The increase in prostanoid formation by recombinant rat C5a was specific. It was inhibited by a neutralizing monoclonal antibody. (3) In co-cultures of rat hepatocytes and rat Kupffer cells but not in hepatocyte mono-cultures recombinant rat C5a increased glycogen phosphorylase activity 3-fold over basal. This effect was inhibited by incubation of the co-cultures with 500 microM acetylsalicyclic acid. Thus, C5a generated either locally in the liver or systemically e.g. in the course of sepsis, may increase hepatic glycogenolysis by a prostanoid-mediated intercellular communication between Kupffer cells and hepatocytes.

On the mechanisms of the growth-promoting effect of prostaglandins in hepatocytes: the relationship between stimulation of DNA synthesis and signaling mediated by adenylyl cyclase and phosphoinositide-specific phospholipase C

While many observations indicate that prostaglandins may act as positive regulators of hepatocyte proliferation, the underlying mechanisms are not known. We have examined some of the signal pathways in the growth response induced by prostaglandins in hepatocytes, with particular focus on adenylyl cyclase and phosphoinositide-specific phospholipase C. Adult rat hepatocytes were cultured as primary monolayers in serum-free medium in the presence of EGF and insulin. PGE2 or PGF2 alpha (added 0-3 h after plating) enhanced the incorporation of [3H]-thymidine into DNA (measured at 50 h); at 100 microM the stimulation was about threefold PGI2 and PGD2 also showed significant but smaller stimulatory effects. No significant increase in the level of cyclic AMP (cAMP) was detected in response to any of the prostaglandins. Low concentrations of glucagon (0.1-10 nM), a potent activator of hepatic adenylyl cyclase, or 8-bromo-cAMP (0.1-10 microM) enhanced the DNA synthesis. When 8-bromo-cAMP was used in maximally effective concentrations, no further stimulation was obtained by combining it with glucagon, whereas the effects of PGE2 and 8-bromo-cAMP were completely additive. All the prostaglandins also showed additivity with the effect of glucagon on the DNA synthesis. PGE2, PGF2 alpha, PGI2, and PGD2 increased intracellular inositol-1,4,5-trisphosphate (InsP3), with a relative order of efficacy roughly corresponding to their activity as stimulators of DNA synthesis. Increases in cytosolic free Ca2+, as measured in single cells, were elicited in a majority of the hepatocytes by all these prostaglandins at 1 microM. Supramaximal concentrations of vasopressin, a strong activator of phospholipase C in hepatocytes, acted additively with PGE2 on the DNA synthesis. Pretreatment of the hepatocytes with a concentration of pertussis toxin that prevented the inhibitory effect of PGE2 on glucagon-induced cAMP accumulation did not abolish the ability of PGE2 to stimulate the DNA synthesis. The results do not support a role for adenylyl cyclase activation in the stimulatory effect of prostaglandins on hepatocyte growth. While the data are compatible with an involvement of phosphoinositide-specific phospholipase C in the growth-promoting effect of prostaglandins in cultured rat hepatocytes, they suggest this may not be the sole mechanism.

Inhibition by PGE2 of glucagon-induced increase in phosphoenolpyruvate carboxykinase mRNA and acceleration of mRNA degradation in cultured rat hepatocytes

In cultured rat hepatocytes the key gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PCK) is known to be induced by glucagon via an elevation of cAMP. Prostaglandin E2 has been shown to antagonize the glucagon-activated cAMP formation, glycogen phosphorylase activity and glucose output in hepatocytes. It was the purpose of the current investigation to study the potential of PGE2 to inhibit the glucagon-induced expression of PCK on the level of mRNA and enzyme activity. PCK mRNA and enzyme activity were increased by 0.1 nM glucagon to a maximum after 2 h and 4 h, respectively. This increase was completely inhibited if 10 microM PGE2 was added concomitantly with glucagon. This inhibition by PGE2 of glucagon-induced PCK activity was abolished by pertussis toxin treatment. When added at the maximum of PCK mRNA at 2 h, PGE2 accelerated the decay of mRNA and reduced enzyme activity. This effect was not reversed by pertussis toxin treatment. Since in liver PGE2 is derived from Kupffer cells, which play a key role in the local inflammatory response, the present data imply that during inflammation PGE2 may reduce the hepatic gluconeogenic capacity via a Gi-linked signal chain.

Molecular cloning and expression of a prostaglandin E2 receptor of the EP3 beta subtype from rat hepatocytes

Rat hepatocytes have previously been reported to possess prostaglandin E2 receptors of the EP3-type (EP3-receptors) that inhibit glucagon-stimulated glycogenolysis by decreasing cAMP. Here, the isolation of a functional EP3 beta receptor cDNA clone from a rat hepatocyte cDNA library is reported. This clone can be translated into a 362-amino-acid protein, that displays over 95% homology to the EP3 beta receptor from mouse mastocytoma. The amino- and carboxy-terminal region of the protein are least conserved. Transiently transfected HEK 293 cells expressed a single binding site for PGE2 with an apparent Kd of 15 nM. PGE2 > PGF2 alpha > PGD2 competed for [3H]PGE2 binding sites as did the EP3 receptor agonists M&B 28767 = sulprostone > misoprostol but not the EP1 receptor antagonist SC 19220. In stably transfected CHO cells M&B 28767 > sulprostone = PGE2 > misoprostol > PGF2 alpha inhibited the forskolin-elicited cAMP formation. Thus, the characteristics of the EP3 beta receptor of rat hepatocytes closely resemble those of the EP3 beta receptor of mouse mastocytoma.