Enzymology of highly differentiated hepatocellular carcinomas

Modulation of the liver specific phenotype in the human hepatoblastoma line Hep G2

The human hepatoblastoma line Hep G2 modulates gene expression in vitro in response to increasing culture density. Two stages of growth can be defined. At low density (less than 200,000 cells/cm2) the cultures have a doubling time of 24 h and exhibit several characteristics of fetal liver cells, including increased synthesis of alphafetoprotein, reduced synthesis of albumin, a predominance of the fetal isoenzymes of both aldolase and pyruvate kinase and a reduced level of the cell surface receptor for asialoglycoproteins. Confluent, high density cultures of Hep G2 (greater than 1 X 10(6) cells/cm2) have a doubling time of 193 h, a four fold higher level of albumin production, increased levels of the adult isoenzymes of aldolase and pyruvate kinase and increased asialoglycoprotein receptor. The alteration in albumin and alphafetoprotein synthesis was reflected by changes in the messenger RNA levels and the relative transcription of these two genes. Hep G2 provides a cell culture model for the modulation of the liver phenotype which occurs during fetal/adult development or during liver regeneration.

Cellular events during hepatocarcinogenesis in rats and the question of premalignancy

The cellular, biochemical, and genetic changes that occur in the liver of rats exposed to chemical hepatocarcinogens are reviewed. Multiple new cell types appear in the liver of carcinogen-treated rats including foci, nodules, ducts, oval cells, and atypical hyperplastic areas. The application of phenotypic markers for these cell types suggests that hepatocellular carcinomas may arise from more than one cell type, including a putative liver stem cell that proliferates following carcinogen exposure. Study of DNA, RNA, and proteins produced by hepatocellular carcinomas and putative premalignant cells has so far failed to identify a gene or gene product clearly associated with the malignant or premalignant phenotype. Understanding the cellular lineage from normal cell through putative premalignant cell to cancer is critical to understanding the process of carcinogenesis. Application of new immunological (monoclonal antibody, transplantation) and molecular biological (gene cloning, oncogene identification) approaches to this problem holds promise that the process of hepatocarcinogenesis will be better known in the near future.

An evaluation of cellular lineages in the pathogenesis of experimental hepatocellular carcinoma

Analysis of liver from rats exposed to chemical hepatocarcinogens has led to a model that postulates sequential premalignant changes, culminating in hepatoma formation from neoplastic nodules. Several experimental protocols devised during the last quarter century have focused upon this lineage model. But proof that neoplastic nodules are the definitive premalignant lesions has not been achieved. Recent work, using alpha-fetoprotein as a marker for liver cell alterations induced by different carcinogen-feeding regimens, suggests that chemically induced hepatomas may also arise from nonnodular cell populations. Therefore, the extensive biochemical and biological studies of presumed "premalignant" cells may have utilized the wrong cells. Unequivocal identification of the cell population at risk for malignancy is needed to delineate mechanisms by which chemicals cause hepatocellular carcinoma.