MATURATION-DEPENDENT GENE-EXPRESSION IN A CONDITIONALLY TRANSFORMED LIVER PROGENITOR-CELL LINE

We have isolated a conditionally transformed liver progenitor cell lin e with phenotypic similarities to both hepatoblasts (bipotent embryoni c liver cells that give rise to hepatocytes and intrahepatic biliary e pithelial cells) and liver epithelial cells (primitive hepatic cells i solated from adult livers capable of generating both hepatocytic and b iliary lineages). Cell line L2039 was derived from E14 fetal mouse liv er after transformation with temperature-sensitive SV-40 large T antig en. At 33 degrees C, these cells have an epithelial morphology with a high nucleocytoplasmic ratio and express both hepatocytic and biliary genes, including albumin, alpha-fetoprotein, glutamine synthetase, ins ulinlike growth factor II receptor, fibronectin and laminin, and cytok eratins 8 and 19, a set of markers characteristic for hepatoblasts. Th e presence of cytokeratin 14, vimentin, and several oval-cell antigens link cell line L2039 to nonparenchymal liver epithelial cell populati ons thought to contain progenitor cells. Serum-free, hormonally define d media conditions and extracellular matrix requirements were determin ed for growth and differentiation of this cell line. During culture on type IV collagen at 39 degrees C, L2039 cells cease dividing and demo nstrate hepatocytic differentiation with the assumption of a hepatocyt elike morphology and glucocorticoid-dependent regulation of liver-spec ific genes, including albumin, alpha-fetoprotein, phosphoenolpyruvate carboxykinase, and liver-enriched transcription factors. The number of albumin-positive cells increases during culture at 39 degrees C, indi cating that L2039 cells convert from a prehepatocytic to a hepatocytic phenotype. Under conditions specific for hepatocytic differentiation, C/EBPs were expressed and differentially regulated, with C/EBP beta a nd C/EBP delta upregulated early and C/EBP alpha only slightly express ed after 7 d, indicating that C/EBP alpha may not be a crucial factor in commitment to the hepatocytic phenotype.