De novo Induktion von Anaphylatoxin C5a-Rezeptoren in Hepatocyten der Ratte durch den Entzündungsauslöser Lipopolysaccharid in vivo

Abstract

In normal rat liver, anaphylatoxin C5a receptors (C5aR) are only expressed by nonparenchymal cells, mainly Kupffer cells and hepatic stellate cells, weakly by sinusoidal endothelial cells but not by hepatocytes. An up-regulation of C5aR expression in liver was observed previously after in vivo treatment of rats and mice with bacterial lipopolysaccharide (LPS). However, it was not investigated in which cell type the enhanced expression occurred. Thus, it was postulated that this enhanced expression, at least in part, might be caused by a de novo induction of C5aR on hepatocytes. Indeed, a time-dependent C5aR mRNA and protein expression in hepatocytes has been demonstrated under inflammatory conditions simulated by in vivo treatment of rats with LPS. At the time of the maximal C5aR protein expression, i.e. 8-10 h after the LPS administration, the newly expressed C5aR were also functional. In contrast to its effect on cells and livers from control animals, rat recombinant C5a (rrC5a) directly activated the glycogen phosphorylase activity in isolated hepatocytes and enhanced prostanoid independent the glucose output in perfused livers from LPS treated rats. In contrast to control to the in vivo treatment of rats, an in vitro stimulation with LPS failed to induce C5aR mRNA and protein in isolated hepatocytes from normal liver. As possible mediators, the proinflammatory cytokines IL-6, IL-1ß and TNFα, which are known to be released after LPS stimulation from monocytes /macrophages in the circulation but also from nonparenchymal liver cells, were suggested. Indeed, an in vivo treatment of rats with IL-6 led to a time-dependent C5aR expression in hepatocytes similar to this, induced by LPS administration in vivo. Even in this case, rrC5a elicited the activity of the hepatocellular glycogen phosphorylase in isolated cells as well as the prostanoid-independent glucose output in perfused livers. In contrast to control animals, in isolated HC from LPS-treated rats rrC5a directly activated the glycogen phosphorylase and enhanced prostanoid-independent the glucose output from perfused livers. Furtermore, an in vitro treatment with IL-6 induced also the expression of functional C5aR. Because of their high toxicity in vivo the influence of other cytokine like IL-1ß and TNFα was tested on isolated hepatocytes only. Similar to IL-6, the stimulation with IL-1ß led to a strong induction of functional C5aR expression. In contrast, no significant levels of C5aR mRNA and protein were detected after TNFα stimulation. The present study demonstrated that under inflammatory conditions C5a receptors are de novo expressed by hepatocytes. This up-regulation might be mediated by in terleukins IL-6 and IL-1ß released from macrophages like Kupffer cells. These results indicate that hepatocytes-specific defence reactions like short-time glucose-output, which serve both as fuel for energy-consumptive defence reactions and as an electron donor for the generation of reactive oxygen intermediates, can be differently mediated under normal and under inflammatory conditions. Under normal conditions when hepatocytes are devoid of C5aR, glucose output is stimulated by C5a indirectly via prostanoids released from Kupffer cells and hepatic stellate cells. Under inflammatory conditions, hepatocytes have been induced to express C5aR themselves and glucose output has been elicited by C5a directly without the intervention of nonparenchymal cells. These findings confirm the crucial role of anaphylatoxins like C5a for the regulation not only of systemic but also of hepatic defence reactions.