Differential Patterns of Expression of Three C/EBP Isoforms, HNF-1, and HNF-4 after Partial Hepatectomy in Rats

Regenerating liver provides a system for studying the mechanisms controlling regulated proliferation of differentiated hepatocytes. A set of transcription factors termed hepatocyte nuclear factors (HNF-1, -3, -4) and CCAAT/enhancer binding protein (C/EBP) isoforms are known to regulate several genes predominantly expressed in the liver. To assess whether these factors might be involved in the hepatocyte proliferation program, we have studied the expression of the three C/EBP isoforms C/EBPα, C/EBPβ, and C/EBPδ and the two hepatocyte-enriched transcription factors, HNF-1 and HNF-4, in rat liver at various time points after partial hepatectomy and sham operations using transcriptional "run-on" assays and Northern blot and Western blot experiments. We report here that partial hepatectomy in rats leads to dramatic changes in the pattern of expression of some of these genes. The three C/EBP isoforms are differentially regulated in response to partial hepatectomy and are likely to play different roles in determining the proliferation/differentiation state of hepatocytes. In particular, C/EBPα expression is rapidly down-regulated, whereas C/EBPδ is induced. C/EBPβ expression is also increased, although an increase is also observed after sham operation. The drastic decrease in C/EBPα under these conditions of active DNA synthesis and rapid cell proliferation further supports the concept of a potential incompatibility between high C/EBPα protein levels and cell proliferation. The patterns of transcriptional rates of HNF-1 and HNF-4 during the different stages of the regenerative process are similar. However, HNF-1 steady-state mRNA and protein levels are significantly changed while HNF-4 remains virtually unaffected, indicating that post-transcriptional mechanisms are also involved in the regulation of HNF-1 gene expression.