Induction of Ornithine Decarboxylase in Rat Liver by Phorbol Ester Is Mediated by Prostanoids from Kupffer Cells

Involvement of cyclooxygenase-2 in the potentiation of allyl alcohol-induced liver injury by bacterial lipopolysaccharide

Bacterial endotoxin (lipopolysaccharide; LPS) augments the hepatotoxicity of a number of xenobiotics including allyl alcohol. The mechanism for this effect is known to involve the inflammatory response elicited by LPS. Upregulation of cyclooxygenase-2 (COX-2) and production of eicosanoids are important aspects of inflammation, therefore studies were undertaken to investigate the role of COX-2 in LPS-induced enhancement of liver injury from allyl alcohol. Rats were pretreated (iv) with a noninjurious dose of LPS or sterile saline vehicle and 2 h later were treated (ip) with a noninjurious dose of allyl alcohol or saline vehicle. COX-2 mRNA was determined by semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR), and liver injury was assessed from activities in serum of alanine and aspartate aminotransferases (ALT and AST, respectively) and from histology. Liver injury was observed only in rats cotreated with LPS and allyl alcohol. Serum ALT activity was increased by 4 h after administration of LPS and continued to increase through 8 h. COX-2 mRNA was detectable at low levels in livers from rats receiving only the vehicles at any time up to 8 h. Expression of COX-2 mRNA was increased by 30 min after administration of LPS and remained elevated through 6 h. Allyl alcohol treatment alone caused an increase in COX-2 mRNA at 4 h (2 h after allyl alcohol) that lasted less than 2 h. In livers from rats cotreated with LPS and allyl alcohol, levels of COX-2 mRNA were greater than levels seen with either LPS or allyl alcohol alone. The increased expression of COX-2 mRNA was accompanied by an increase in the concentration of prostaglandin (PG) D(2) in plasma. Plasma PGD(2) concentration was increased to a greater extent in rats treated with LPS plus allyl alcohol compared to allyl alcohol or LPS alone. Pretreatment with the COX-2 selective inhibitor, NS-398, abolished the increase in plasma PGD(2) and reduced the increase in ALT and AST activities observed in rats cotreated with LPS and allyl alcohol. NS-398 did not affect liver injury from allyl alcohol alone administered at a larger, hepatotoxic dose. In addition, ibuprofen, a nonselective inhibitor of cyclooxygenases, did not protect against liver injury from LPS plus allyl alcohol. In isolated hepatocytes PGD(2), but not PGE(2), reduced the concentration of allyl alcohol required to cause half-maximal cytotoxicity. These results suggest that products of COX-2 play a role in the augmentation of allyl alcohol-induced liver injury by LPS.

The role of scavenger receptor class A in the adhesion of cells is dependent on cell type and cellular activation state

Scavenger receptor class A (SR-A) facilitates the development of atherosclerosis, which might be due to its role in the uptake of modified low-density lipoproteins. However, the receptor is also suggested to be important for cell adhesion, thereby potentially influencing the residence time of cells in vivo. Using SR-A-deficient mice, we investigated the role of SR-A in the adhesion of peritoneal macrophages (PM) and tissue macrophages (Kupffer cells). In resident PM no effect of the absence or presence of SR-A on cell adhesion was observed, either in the presence or in the absence of serum. However, in thioglycollate-induced PM, SR-A is important for adhesion both in the presence and in the absence of serum and more than 85% of the divalent-cation-independent adhesion in the presence of serum is mediated by SR-A. In unactivated Kupffer cells, like in resident PM, adhesion is not influenced by the absence or presence of SR-A. In vivo administration of phorbol 12-myristate 13-acetate leads to the activation of Kupffer cells, and it appears that under these conditions SR-A does contribute to adhesion, since both in the absence and in the presence of serum SR-A is responsible for about 35% of cell adhesion. It is concluded that SR-A is important for the divalent-cation-independent adhesion of activated PM and Kupffer cells, suggesting that SR-A may influence the residence time of cells at sites of cellular activation, e.g., in atherosclerotic plaques and during liver infection.

Reduction of bile secretion by prostaglandins in the rat in vivo

Bile secretion has been reported to be regulated by circulating hormones and by autonomic liver nerves. In the in situ perfused rat liver, prostaglandins reduce bile flow and bile acid secretion. The aim of this study was to investigate the regulation of bile secretion by prostaglandins in the in vivo situation. The bile duct and portal vein of anaesthetised Wistar rats were cannulated by polyethylene tubes. Bile flow was determined gravimetrically. Bile acids were quantified by the 3-alpha-hydroxy-steroid-dehydrogenase method and by high-pressure-liquid-chromatography (HPLC) separation. Administration of 1 microM prostaglandin F2 alpha into the portal vein over 5 minutes reduced bile flow from 1.57 microliter/min.g liver to 0.95 microliter/min.g liver and bile acids secretion from 148 to 81 nmol/100g/min. The administration of different doses (0.1 microM, 1 microM, 10 microM) of prostaglandin F2 alpha reduced hepatic bile secretion in a dose-dependent manner. Similar effects were observed after infusion of prostaglandin D2. However, the ratio of the bile acids (alpha-tauromuricholic acid), beta-tauromuricholic acid, taurocholic acid, taurochenodeoxycholic acid, and taurodeoxycholic acid) was unchanged by prostaglandin F2 alpha. In conclusion, infusion of prostaglandin F2 alpha into the portal vein results in a reduction of bile flow and bile acid secretion in a dose-dependent manner. These results suggest that the effect is linked to canicular bile secretion.

In vivo effect of berenil on rat liver polyamine metabolism

1. Berenil, administered to rats in vivo, promoted a decrease in liver SAMDC activity, but an increase in ODC and SAT activity. 2. Its effect on ODC was completely prevented by cycloheximide, that on SAT only partially. 3. Berenil had no effect on ODC activity in adrenalectomized rats. Adrenergic antagonists counteracted the effect of Berenil on ODC activity. 4. Polyamine content was increased. The maximum modification was observed for putrescine and N1-acetylspermidine.

Effects of prostaglandins on ornithine decarboxylase activity in rat small intestine

Role of prostaglandins on feeding-associated induction of ornithine decarboxylase in small intestine was studied. Rats received intraperitoneal injection of either saline, or 16,16-dimethyl prostaglandin E2, or TRY-200 (a stable prostaglandin I2 analog), or refeeding, after a 44 hr-fast. Four hours later, mucosae from duodenum, jejunum, and ileum were scraped for subsequent measurements of enzyme activity of ornithine decarboxylase by a radiometric technique. Refeeding resulted in a profound induction of enzyme activity throughout the small intestine. Parenteral administration of prostaglandin I2 also led to a significant induction with the level similar to refeeding. The stimulatory effect of prostaglandin I2 was completely abolished by a specific and irreversible enzyme inhibitor, difluoromethylornithine. Prostaglandin E2 had a similar but lesser effect than prostaglandin I2 on the induction of the enzyme activity. Pretreatment with indomethacin, a cyclooxygenase inhibitor had no effect on feeding-associated enzyme induction. These results indicate that although exogenous prostaglandin I2 appears to be a potent stimulant for ornithine decarboxylase activity in rat small intestine, endogenous prostaglandins seem to play little or no role in feeding-associated induction of ornithine decarboxylase.

Transient activation of hepatic glycogenolysis by thrombin in perfused rat livers

Thrombin, a peptide with native protease activity, caused a rapid (less than 1 min) increase in glycogenolysis of about 30%, assessed from rates of production of glucose+lactate+pyruvate, and in oxygen uptake in perfused rat liver. These increases were followed by a rapid return to basal values within 5 min. The effect of thrombin on glycogenolysis was dose-dependent and was maximal at perfusate concentrations around 1 U/ml. Interestingly, the effect of thrombin on glycogenolysis could be elicited only once in any given liver. The activation of glycogenolysis by thrombin was diminished nearly 50% by prior infusion of the protease inhibitor, diisopropyl fluorophosphate (10 microM), and over 90% when thrombin was treated with diisopropyl fluorophosphate prior to infusion. The stimulation of glycogenolysis by thrombin could be detected in isolated hepatocytes or in livers stored for 24 h in cold Euro-Collins solution, a treatment which destroys endothelial cells. Further, thrombin stimulated production of prostaglandin D2 from arachidonic acid in cultured hepatic endothelial but not Kupffer cells. The effect of thrombin on carbohydrate output was also blocked by a phospholipase A2 inhibitor (quinacrine, 50 microM) and by an inhibitor of the cyclooxygenase (indomethacin, 20 microM), suggesting the involvement of cyclooxygenase in the mechanism of action of thrombin. In support of this idea, the transient kinetics of stimulation of glycogenolysis by thrombin and arachidonic acid was nearly identical to release of thromboxane B2 (80-420 pg/ml) and prostaglandin D2 (300-900 pg/ml) from the perfused liver. Further, a second addition of thrombin failed to increase thromboxane and prostaglandin D2 release as well as carbohydrate production, supporting a causal link between these phenomena. Taken together, these data support the hypothesis that thrombin interacts with receptors in the liver, possibly on endothelial cells, leading to activation of phospholipase A2 and subsequent transient production of prostaglandins and thromboxanes. These mediators subsequently interact with receptors on parenchymal cells, leading to a transient stimulation of glycogenolysis.