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 various cell types. In this report, we show that the Id-1 gene was down-regulated 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 liver regeneration after partial hepatectomy and an in vitro growth factor-stimulated hepatocyte culture system. Strong activation of Id-1 was observed in mid-late G1 of the hepatocyte cell cycle at a time corresponding to a mitogen restriction point. These observations suggest that Id-1 is involved in the control of proliferation and differentiation in liver cells.