The effects of the anaphylatoxins C5a and C3a on the liver are only poorly characterized in contrast to their well known systemic actions. Recently, it has been demonstrated that the anaphylatoxin C5a enhanced glucose output from hepatocytes (HC) indirectly via prostanoid release from Kupffer cells (KC). In the present study, it is shown that recombinant rat C5a (rrC5a), together with LPS, activated the gene of the acute phase protein alpha(2)-macroglobulin (alpha(2)MG) in HC also indirectly via IL-6 release from KC. RrC5a alone increased neither IL-6 mRNA in nor IL-6 release from KC, whereas LPS alone did so. However, rrC5a synergistically enhanced the LPS-dependent increase in IL-6 mRNA and IL-6 release. Only rIL-6, but not TNF-alpha or IL-1beta, enhanced alpha(2)MG mRNA in HC. In line with the actions of rrC5a and LPS on KC, conditioned medium of KC stimulated only with rrC5a did not increase alpha(2)MG mRNA in HC. However, medium of KC stimulated with rrC5a plus LPS induced alpha(2)MG mRNA expression in HC more strongly than medium from cells stimulated only with LPS; thus, C5a acted synergistically with LPS. The stimulatory effects of KC-conditioned medium could partially be inhibited by a neutralizing anti-IL-6 Ab, indicating that KC-derived IL-6 was a major mediator in C5a- plus LPS-elicited alpha(2)MG gene expression. These results suggest that C5a, besides enhancing glucose output via prostanoids, is involved in the initiation of the acute phase response in HC via proinflammatory cytokines from KC. This provides evidence for another important function of C5a in the regulation of hepatocellular defense reactions.