Long-term cultures of human hepatocytes were maintained serum-free in a chemically defined medium in the presence of HGF and EGF for up to 30 days. STAT 1alpha/1beta, STAT 3, and STAT 5 were present and tyrosine phosphorylated throughout the culture period in the cytosol as well as the nucleus. We show by co-immunoprecipitation that a portion of the cellular pools of STAT 1alpha/1beta and STAT 5 is physically associated with c-MET and EGF-receptor. Co-immunoprecipitation of STAT 3 with STAT 5 did occur in the cytosol but not in the nucleus, suggesting dimerization of the two STAT family members. The observed differences of protein amounts and tyrosine phosphorylation between cytosol and nucleus, the association of STAT proteins with EGF-receptor and c-MET and with each other may all be involved in regulating the activity of the STAT transcription factors. It is intriguing to speculate that STAT 5 may have a modulating role in the regulation of STAT 3 activity.
Copyright 1999 Academic Press.