Different preparations of zymosan induce glycogenolysis independently in the perfused rat liver. Involvement of mannose receptors, peptide-leukotrienes and prostaglandins

Leukotrienes: underappreciated mediators of innate immune responses

Leukotrienes are bronchoconstrictor and vasoactive lipid mediators that are targets in the treatment of asthma. Although they are increasingly recognized to exert broad proinflammatory effects, their role in innate immune responses is less well appreciated. These molecules are indeed synthesized by resident and recruited leukocytes during infection. Acting via cell surface G protein-coupled receptors and subsequent intracellular signaling events, they enhance leukocyte accumulation, phagocyte capacity for microbial ingestion and killing, and generation of other proinflammatory mediators. Interestingly, a variety of acquired states of immunodeficiency, such as HIV infection and malnutrition, are characterized by a relative deficiency of leukotriene synthesis. The data reviewed herein point to leukotrienes as underappreciated yet highly relevant mediators of innate immunity.

Differences in the involvement of prostanoids from Kupffer cells in the mediation of anaphylatoxin C5a-, zymosan-, and lipopolysaccharide-dependent hepatic glucose output and flow reduction

Various inflammatory stimuli such as anaphylatoxin C5a, zymosan, and lipopolysaccharides (LPSs) have been reported both to enhance glucose output in the perfused rat liver and to induce prostanoid (ie, prostaglandin and thromboxane) release from Kupffer cells, the resident liver macrophages. Because prostanoids can enhance glucose output from hepatocytes, it was the aim of this study to compare the possible roles of prostanoids released after C5a, zymosan, and LPS in the mediation of hepatic glucose output. In perfused livers both C5a and zymosan immediately enhanced glucose output, reduced flow, and induced prostanoid overflow into the hepatic vein, but with different quantities and kinetics. Only the C5a-induced but not the zymosan-induced effects were abrogated by inhibitors of prostanoid signaling as the prostanoid synthesis inhibitor indomethacin and the thromboxane receptor antagonist daltroban. In contrast to C5a and zymosan, LPS had no effect on glucose output, flow rate, or prostanoid overflow. In isolated Kupffer cells, C5a and zymosan induced maximal release of prostaglandins D(2) and E(2) and of thromboxane A(2) within a period of 0 to 2 minutes and 5 to 15 minutes, respectively. In pulse-chase experiments, maximal prostanoid release was already observed after 2 minutes of continuous stimulation with C5a, but only after 10 to 15 minutes of continuous stimulation with zymosan. LPS-dependent prostanoid release was not seen before 1 hour. Thus, even though C5a, zymosan, and LPS induced prostanoid release from Kupffer cells, only C5a quickly regulated hepatic glucose metabolism in a prostanoid-dependent manner (due to the kinetics and quantities of prostanoids released).

Changes in hepatic nitrogen metabolism in isolated perfused liver during the development of thioacetamide-induced cirrhosis in rats

Changes in hepatic nitrogen metabolism in isolated perfused liver were studied during the induction of experimental cirrhosis by thioacetamide in female Sprague-Dawley rats. Cirrhosis of the micronodular type developed during 12-week administration of thioacetamide. Despite an increase in food consumption for 4 weeks after the end of administration, the physiological changes characteristic of cirrhosis were maintained. The rate of urea excretion per unit liver weight was significantly decreased compared with pair-fed control rats both during and after thioacetamide treatment. During 4 weeks of thioacetamide treatment, the rate of urea production in perfused liver from a combination of 0.25 mM NH4Cl and 1 mM glutamine decreased slightly, without a decrease in the maximum rate of urea production from 10 mM NH4Cl. In cirrhotic rats, the rate of urea production in perfused liver from NH4Cl and/or glutamine decreased, with a decrease in the maximum rate of urea production. The Km of ureagenesis for NH3 was unchanged in cirrhotic livers. During 4 weeks of thioacetamide treatment, glutamate dehydrogenase activity decreased, but the thioacetamide-induced cirrhotic state had no effect on glutamate dehydrogenase or glutaminase activity. Glutamine synthetase activity was decreased in rats treated with thioacetamide for 4 or 12 weeks. These results are consistent with the hypothesis that the capacity for urea production from NH3 and amino acids is decreased in the development of cirrhosis.

Differential effects of exogenous and endogenously generated H2O2 on phagocytic activity and glucose release of normal and cirrhotic livers

BACKGROUND/AIMS

Reactive oxygen species play an essential role in necro-inflammatory processes. Therefore, the aim of the present studies was to investigate the effect of exogenous and endogenously produced H2O2 on the phagocytic capacity and glucose release of perfused cirrhotic rat livers in comparison with that on the controls.

METHODS

Complete septal cirrhosis was achieved by oral treatment of rats with thioacetamide for 6 months. The phagocytic capacity of the perfused livers was measured by the uptake of colloidal carbon. During the continuous perfusion with colloidal carbon, either H2O2 or benzylamine was added to the perfusion medium for a limited time period. The latter functioned as an endogenous H2O2 donor.

RESULTS

In control rats exogenous and endogenously produced H2O2 caused a transient stimulation of the hepatic colloidal carbon uptake as well as of the glucose release. Inhibition of the catalase by aminotriazol doubled the changes evoked by H2O2, whereas blockade of the Kupffer cells by GdCl3 drastically reduced its stimulatory effect. Cirrhotic livers took up less colloidal carbon and released lower amounts of glucose than the controls when stimulated by exogenous H2O2. The inhibition of the nitric oxide synthetase augmented the H2O2-induced effect in controls as well as in the cirrhotic livers by 250% and 620% (colloidal carbon uptake) and 340% and 760% (glucose release), respectively. The blockade of the eicosanoid production by indomethacin and caffeic acid drastically increased the glucose release and the colloidal carbon uptake in controls and, in absolute terms, to a lesser extent in cirrhotic livers. Endogenous H2O2 produced by the addition of benzylamine stimulated the colloidal carbon uptake and glucose release in livers from both groups. The inhibition of the lipoxygenase increased both parameters, whereas different effects were elicited by the addition of superoxide dismutase in controls and cirrhotic livers.

CONCLUSION

The maximum uptake of colloidal carbon and glucose release, measured after stimulation by H2O2, was lower in cirrhotic livers than in controls, thus indicating a lowered phagocytic capacity of Kupffer cells and altered glycogenolytic response of the hepatocytes in cirrhotic livers. The use of various effectors provided evidence that superoxide anions, nitric oxide and, possibly, arachidonic acid are involved in the signal transduction between Kupffer cells and hepatocytes when stimulated by exogenous or endogenously produced H2O2. This signalling mechanism seems to be impaired in cirrhotic livers.

Activated Kupffer cells attenuate the liver response to the peroxisome proliferator perfluorooctanoic acid

It has been suggested that peroxisome proliferators stimulate Kupffer cells, an effect which may be involved in their mechanism of action. To evaluate this hypothesis, this study was designed to investigate the effect of stimulating Kupffer cells on basal as well as induced peroxisomal enzyme activity. Twenty four hours following treatment of male Sprague-Dawley rats with the peroxisome proliferating agent perfluorooctanoic acid (PFOA), in corn oil or with corn oil alone, hepatic peroxisomal beta-oxidation was 4.6 +/- 0.2 and 1.8 +/- 0.1 U/g liver, respectively. As expected, PFOA did not influence the catalase activity. Stimulating Kupffer cells in vivo by zymosan A (25 mg/kg, i.v.) prior to treatment with corn oil or PFOA diminished basal as well as PFOA-induced peroxisomal beta-oxidation by 20-35%. Activation of Kupffer cells by zymosan A also diminished catalase activity by over 60%. Furthermore, PFOA reduced blood colloidal carbon clearance by 35% within 2 h of its administration. The data suggest that activation of Kupffer cells exerts a negative effect on basal as well as PFOA-induced peroxisomal enzyme activities. Data also suggest that PFOA inhibits Kupffer cells. Activated Kupffer cells may indeed produce factors which interfere with normal hepatic peroxisomal functions and responses.

Phagocytic function and metabolite production in thioacetamide-induced liver cirrhosis: a comparative study in perfused livers and cultured Kupffer cells

BACKGROUND/AIMS

The aim of the study presented here was to evaluate the basal and stimulated phagocytic activities and the metabolite production of isolated perfused livers, and also the phagocytic capacity of cultured Kupffer cells from rats with macronodular cirrhosis.

METHODS

Rats were made cirrhotic by oral administration of thioacetamide. The phagocytic activity was assessed by the rate of removal of colloidal carbon. The Kupffer cells were prepared by a pronase/collagenase digestion method followed by elutriation.

RESULTS

The phagocytic activity and production of glucose, lactate and pyruvate were reduced in cirrhotic livers when calculated per g liver. Due to hyperplastic-regenerative processes the mass of the cirrhotic livers was markedly augmented so that the colloidal carbon uptake calculated per cirrhotic liver was not significantly different from the controls. Colloidal carbon-induced glucose release increased more markedly in the controls than in cirrhotic livers. Isoproterenol considerably stimulated phagocytosis and glucose production in controls, whereas the response was clearly reduced in cirrhotic livers when calculated either per g liver or per total liver weight. The cyclic AMP analogue elicited a marked glycogenolytic response in the controls, whereas there was only a slight increase in glucose production in cirrhotic livers. Phagocytosis of cirrhotic livers was only moderately stimulated by opsonized zymosan when compared with the controls. Freshly isolated Kupffer cells exhibited a reduced phagocytic activity. Stimulation by zymosan was observed only in cell suspensions of the controls. In contrast, Kupffer cells from cirrhotic livers did not differ from controls with respect to basal or zymosan-stimulated phagocytic activity after 48-h cultivation.

CONCLUSION

The stimulated phagocytic function was disturbed in perfused macronodular-cirrhotic livers as compared to controls. In contrast, 48-h cultured Kupffer cells from cirrhotic livers exhibited the same basal and stimulated phagocytic capacity as controls. The glucose release from perfused livers, initiated by stimulation of Kupffer cells or hepatocytes, was significantly reduced in cirrhotic livers. Therefore, we postulate an impaired intra- and/or intercellular signalling in macronodular-cirrhotic livers.

Ethanol alters the metabolic response of isolated perfused rat liver to a phagocytic stimulus

Intercellular communication in the liver is a potentially important mechanism for the regulation of hepatic metabolism. Since alcohol (ethanol, ETOH) can interact with both parenchymal and nonparenchymal cells, the present study was performed to assess the possible effects of ETOH on the nonparenchymal cell-to-hepatocyte signal traffic by studying the glycogenolytic and glycolytic response of the perfused rat liver to colloidal carbon, a phagocytic stimulus for Kupffer and sinusoidal endothelial cells. Livers from fed rats were perfused with hemoglobin-free Krebs Ringer bicarbonate buffer containing ETOH (20 mM) or acetaldehyde (1 mM). Twenty minutes after initiating the infusion of ETOH or acetaldehyde, colloidal carbon was infused and the rate of carbon uptake, glucose, lactate and pyruvate output, and oxygen consumption were determined. In control livers, carbon stimulated the output of glucose (60%), lactate (25%), and pyruvate (53%), without affecting the lactate/pyruvate ratio. ETOH, but not acetaldehyde, enhanced the carbon effect on glucose output (38%), but suppressed the increased lactate and pyruvate output (48% and 91% respectively) resulting in a dramatic 10-fold increase in the lactate/pyruvate ratio. By using inhibitors of cyclooxygenase or alcohol dehydrogenase (indomethacin and 4-methylpyrazole, respectively) in the presence of carbon and/or ETOH, we determined that: (1) following carbon stimulation prostaglandins are the likely mediators secreted by nonparenchymal cells that increase carbohydrate output; and (2) the ETOH-induced enhancement of carbon-stimulated glycogenolysis is also mediated by prostaglandins and is not dependent on the oxidative metabolism of ETOH.