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 pho sphorylated, though rat fetuin isolated from bone matrix does not cont ain phosphorus. A rat 63-kDa phosphorylated N-glycoprotein (pp63) is t he phosphorylated form of rat fetuin and pp63 has been shown to inhibi t insulin-receptor tyrosine kinase activity. Therefore, we examined th e effect of phosphorylated rat fetuin (phosphofetuin) on DNA synthesis in rat hepatocytes in culture in the presence of human hepatocyte-gro wth 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 hepatocyte s, whereas addition of non-phosphorylated rat fetuin had no effect. Ad dition of anti-(rat fetuin) Ig to the culture medium increased HGF-sti mulated DNA synthesis by hepatocytes; Immunoprecipitation and cross-li nking experiments showed that phosphofetuin bound to human HGF. We fou nd that phosphofetuin interfered with binding of HGF to its specific r eceptor(s). These observations suggest that phosphofetuin synthesized by hepatocytes may be a natural modulator of HGF as a chalone, and tha t regulation of expression of phosphofetuin by growth factors and cyto kines may be involved in liver regeneration under inflammatory conditi ons, such as in hepatitis.