Development of an in vitro model of tumor progression using v‐raf and v‐raf/v‐myc transformed rat liver epithelial cells: Correlation of tumorigenicity with the downregulation of specific proteins

A series of clonal cell lines were derived from rat liver epithelial cells after being infected with a defective retrovirus containing either v-raf (3611-MSV) or v-raf/v-myc(J2) together with a helper virus. These clones exhibited a different morphology from the regular cuboid shape of the control cells, infected only with the helper virus. All of the infected cell lines contained at least one full length copy of appropriate proviral DNA and expressed comparable levels of v-raf mRNA, although only the cells transformed with the v-raf/v-myc combination were capable of anchorage-independent growth in soft agar. All of the clones except the controls formed tumors in nude mice but with markedly different latency periods and growth rates. Thus, these cell lines represent an in vitro model for tumor progression. Two-dimensional polyacrylamide electrophoresis was used to investigate changes in cellular protein expression related to malignant conversion. The expression of three proteins of pI/Mr x 10(-3) 5.9-7.2/205 (RP1), 6.5-7.5/160 (RP2) and 4.0/85 (RP3) consistently matched the transformed phenotype. In particular the expression of RP1 and RP2 correlated with the relative tumorigenicity of the cell lines. Rat liver epithelial cell lines transformed by other protocols that did not involve v-raf also showed downregulation of these three polypeptides. Crude fractionation studies determined RP1 to be soluble and RP2 and RP3 to be membrane associated. RP2 was shown to be a glycoprotein containing mannose and galactose residues. These three proteins are consistent markers for the tumorigenic potential of rat liver epithelial cells.