Nitric oxide production by hepatocytes contributes to the inhibitory effect of endotoxin on insulin-like growth factor I gene expression

Abstract

We tested whether endotoxin (lipopolysaccharide, LPS) inhibits IGF-I gene expression in hepatocytes and the possible role of Kupffer cells and nitric oxide (NO) in this effect. LPS decreased IGF-I mRNA in hepatocyte cultures and increased the nitrite + nitrate levels in the culture medium. Furthermore, there was a negative correlation between the IGF-I mRNA and the nitrite+nitrate levels. When hepatocytes were cocultured with Kupffer cells, the inhibitory effect of LPS on IGF-I mRNA was higher than in hepatocyte cultures, but the stimulatory effect on nitrite+nitrate was similar in both conditions. The exogenous NO donated by S-nitroso-n-acetyl-d,l-penicillamide also decreased the IGF-I gene expression in hepatocyte cultures. In addition, two specific inducible NO synthase (iNOS) inhibitors, l-N6-(1-iminoethyl)lysine (l-NIL) and aminoguanidine, prevented the effect of LPS on nitrite+nitrate levels and on IGF-I gene expression in hepatocyte cultures. These data indicate that iNOS-derived NO may cause downregulation of IGF-I gene expression in hepatocytes. However, in cocultures, the iNOS inhibitor l-NIL prevented the effect of LPS on nitrite+nitrate levels, but only attenuated the LPS-induced decrease in IGF-I gene expression. We conclude that in hepatocytes, LPS-induced decrease in IGF-I is mainly due to induction of iNOS, whereas in the presence of Kupffer cells LPS inhibits IGF-I through NO release and through other inhibitory pathways.