Hepatoma variants (C2) are defective for transcriptional and post-transcriptional actions from both endogenous and viral genomes.

A series of rat hepatoma cell lines was infected with a recombinant adenovirus bearing the rat albumin promoter. Transcription from this promoter was scored directly and was highest in FAO, the differentiated parent, undetectable in C2, a cell variant that has lost almost all hepatocytic characteristics, and high again in C2-Rev7, a 'revertant' cell line derived from C2 that has regained the ability to produce many proteins characteristic of hepatocytes. The endogenous albumin gene is not transcribed in C2 cells, and at a very low rate in C2-Rev7 cells, which accumulate endogenous albumin mRNA at close to normal amounts. Thus the C2-Rev7 'recovery' of albumin mRNA concentration for the endogenous gene is based mainly on post-transcriptional events while the ability of C2-Rev7 to transcribe the albumin promoter in the viral genome is based on a transcriptional factor(s). We also showed that the C2 phenotype included post-transcriptional effects for other genes: transcription of phenylalanine hydroxylase and phosphoenolpyruvate carboxykinase mRNA sequences continue in C2 at rates equivalent to FAO but these C2 cells have no mRNA for these proteins while FAO does. In addition, C2 cells transcribed certain early adenovirus transcription units (E2 and 4) as well as FAO cells but accumulated E2 mRNAs poorly if at all. The changes that led to the C2-Rev7 cell line produced a return to normal of the ability to accumulate these viral mRNAs. Thus a major event in the C2 to C2-Rev7 transition involves post-transcriptional processes as well as the ability to transcribe the albumin promoter positioned in the virus genome.