Abstract
The retinoblastoma tumor suppressor gene product, RB protein, is a negative regulator of cell growth and antagonizes c-Myc oncoprotein. We have previously reported that the c-myc gene expression is aberrantly elevated in rat hepatocellular carcinomas induced by aflatoxin B1 (AFB1). In this paper, to elucidate the significance of the overexpression of c-myc gene in AFB1 hepatocarcinogenesis, we analyzed the effects of a human RB cDNA expression vector, pSV2·RB, on the growth and tumorigenicity of Kagura-2 (K2) cells, which have been established from AFB1-induced rat hepatoma and overexpressing c-myc gene just as an original tumor. Even though any effects of exogeneous expression of RB gene on the transcriptional levels of c-myc and glutathione-S-transferase P (GST-P) genes were not detected, pSV2·RB-transfected K2 cells significantly reduced the ability to form colonies in semi-solid medium, and their growth rate was markedly decreased in a lower concentration of serum. Moreover, tumors that formed by the transfectants in nude mice were much smaller, but were histologically indistinguishable from those of parental K2 cells. These results indicate that the enforced expression of RB gene suppresses the growth of K2 cells in vitro and in vivo without any effects on the expression level of c-myc mRNA.
