Effect of phosphorylated rat fetuin on the growth of hepatocytes in primary culture in the presence of human hepatocyte-growth factor. Evidence that phosphorylated fetuin is a natural modulator of hepatocyte-growth factor.

Rat fetuin, a counterpart of human alpha 2-HS glycoprotein and bovine fetuin, that is synthesized and secreted by hepatocytes is mostly phosphorylated, though rat fetuin isolated from bone matrix does not contain phosphorus. A rat 63-kDa phosphorylated N-glycoprotein (pp63) is the phosphorylated form of rat fetuin and pp63 has been shown to inhibit insulin-receptor tyrosine kinase activity. Therefore, we examined the effect of phosphorylated rat fetuin (phosphofetuin) on DNA synthesis in rat hepatocytes in culture in the presence of human hepatocyte-growth factor (HGF), since the human receptor of HGF, c-Met, is known to contain a tyrosine-kinase domain in its intracellular domain. We found that phosphofetuin from conditioned medium of rat-hepatocyte cultures dose-dependently decreased HGF-stimulated DNA synthesis in hepatocytes, whereas addition of non-phosphorylated rat fetuin had no effect. Addition of anti-(rat fetuin) Ig to the culture medium increased HGF-stimulated DNA synthesis by hepatocytes. Immunoprecipitation and cross-linking experiments showed that phosphofetuin bound to human HGF. We found that phosphofetuin interfered with binding of HGF to its specific receptor(s). These observations suggest that phosphofetuin synthesized by hepatocytes may be a natural modulator of HGF as a chalone, and that regulation of expression of phosphofetuin by growth factors and cytokines may be involved in liver regeneration under inflammatory conditions, such as in hepatitis.