Serum amyloid A transcription in Atlantic salmon (Salmo salar L.) hepatocytes is enhanced by stimulation with macrophage factors, recombinant human IL-1 beta, IL-6 and TNF alpha or bacterial lipopolysaccharide.
Serum amyloid A (A-SAA) has previously been reported to be an acute-phase protein in salmonids. Hepatocytes represent a major source of A-SAA in salmonids, but nothing is known about hepatocyte SAA synthesis in fish. In the present work, the expression of A-SAA transcripts in primary cultures of Atlantic salmon hepatocytes in response to macrophage derived cytokines, human recombinant cytokines and bacterial lipopolysaccharide (LPS) was studied by Northern blot analysis. The macrophage supernatants were prepared by stimulating Atlantic salmon head kidney macrophages with LPS, yeast glucan or a leukocyte derived macrophage activating factor (MAF). The supernatants from glucan- or MAF-stimulated macrophages had no effect on A-SAA expression of the hepatocytes, while supernatants from LPS-stimulated macrophages gave about a 2-fold increase in expression. The combination of either glucan and MAF, or LPS and MAF were more effective and these supernatants gave a 3.4- and 5.2-fold increase in A-SAA expression, respectively. The hepatocytes were also treated with the human recombinant cytokines TNFalpha, IL-1beta and IL-6, alone or in combination. The A-SAA response to each of them alone was modest, but TNFalpha and IL-6 or IL-1beta and IL-6 in combination gave a higher response than each cytokine alone. These data suggest that the expression of A-SAA by hepatocytes from Atlantic salmon is induced by cytokine-like molecules. Interestingly, hepatocytes treated directly with LPS gave a more than 10-fold increase in SAA mRNA expression, but it is not known if this is a direct effect of LPS on the hepatocytes or if it is mediated by other contaminating cell types.