Expression and phenotypic alterations caused by an inducible transforming ras oncogene introduced into rat liver epithelial cells.

Although transforming ras oncogenes have been implicated as causative factors in liver cell transformation, the exact function and phenotypic alterations generated by the expression of such transforming genes in liver epithelial cells has yet to be defined. We have utilized a retroviral vector system to deliver an inducible transforming ras gene into normal, anchorage dependent rat liver epithelial cells. The Moloney murine sarcoma virus based vector is composed of a dominant selectable marker, Neo, which is transcriptionally driven from the 5' proviral long terminal repeat (LTR) and a transforming Ha-ras gene under the transcriptional control of a glucocorticoid inducible LTR of the mouse mammary tumor virus. Subsequent to infection, G418 resistant, tumorigenic cell lines were isolated and one particular cell line, designated REL-Ras3, was extensively characterized. Single copies of a full length as well as a truncated provirus were integrated into REL-Ras3 cells. The integrated ras gene was transcribed into poly(A+) RNA with dexamethasone treatment increasing both the steady state level of ras mRNA as well as transcription initiated from the MMTV LTR. Western blot analysis confirmed the presence of P21 containing a transforming mutation in position 12. Phenotypic alterations associated with ras expression in REL-Ras3 cells include: gross morphological alterations; loss of contact inhibition of growth; becoming lethally tumorigenic and anchorage independent; alterations in growth kinetics involving a diminished lag phase of the growth curve; and increases in glucose transport. Differences in growth kinetics and glucose transport could be directly correlated with the levels of ras expression.