Enhanced expression of a new class of liver-enriched b-Zip transcription factors, hepatocarcinogenesis-related transcription factor, in hepatocellular carcinomas of rats and humans.

Rat hepatocarcinogenesis-related transcription factor (HTF) was earlier identified as a b-Zip transcription factor in chemically induced rat hepatocellular carcinoma (HCC) by cDNA subtraction, and its structure was found to be different from that of the conventional b-Zip proteins. We investigated htf gene expression in rat tissues by Northern analysis and found that HTF expression was ubiquitous but was enriched in the liver. HTF expression increased concomitantly with HCC development in rat liver, and the HTF-containing DNA-binding factor also increased. Stimulated HTF gene expression also was observed in rat regenerating livers. From the results of various assays, X-box-binding protein 1/Tax-response element binding factor 5 was suggested to be a human homologue of rat HTF. In humans, HTF gene expression was also abundant in the liver and was revealed to be specifically stimulated in HCCs, but not in other types of cancers. To our knowledge, HTF is the first example of a liver-enriched transcription factor that exhibits HCC-associated gene expression. Injection of anti-HTF antibody decreased the growth rate of cultured HCC cells. Consequently, HTF is thought to participate in hepatocyte growth as well as in hepatocarcinogenesis.