EXPRESSION OF HELIX-LOOP-HELIX FACTOR ID-1 IS DEPENDENT ON THE HEPATOCYTE PROLIFERATION AND DIFFERENTIATION STATUS IN RAT-LIVER AND IN PRIMARY CULTURE

Id proteins are known as negative regulators of differentiation in var ious cell types. In this report, we show that the Id-1 gene was downre gulated during the development of rat liver. No Id-1 transcripts were detected in terminal differentiated hepatocytes. We have studied Id-1 expression in proliferating hepatocytes using an in vivo model of live r regeneration after partial hepatectomy and an in vitro growth factor -stimulated hepatocyte culture system. Strong activation of Id-1 was o bserved in mid-late G(1) of the hepatocyte cell cycle at a time corres ponding to a mitogen restriction point. These observations suggest tha t Id-1 is involved in the control of proliferation and differentiation in liver cells.