Development of serum independence in Kirsten murine sarcoma virus-infected rat adrenal cells

Cells cultured from explants of adult rat adrenal cortex are spindle shaped and divide rapidly in media with 10-25% fetal bovine serum (FBS), but are epithelial-like and stationary in 3-6% horse serum (HS). Fully transformed Kirsten murine sarcoma virus (KiMSV)-infected cells lose this differential response to serum. To determine at what stage in the transformation process this loss occurs, cultures in passages 1-2 were infected with KiMSV, propagated for up to 30 weeks in FBS, and parallel cultures were transferred to HS prior to transformation, within 4 weeks after appearance of foci, or after complete transformation (exhibiting altered culture morphology, increased growth rate and tumorigenicity). In 7 lines grown in FBS, foci appeared 1-14 weeks post-infection and most cultures were completely transformed 1-3 weeks thereafter. Substitution of HS for FBS prior to focus formation caused partial reversion to epithelial-like morphology and reduction in growth rate. Transformation was delayed or prevented altogether, but could be elicited by addition of 1% FBS to HS. HS-substitution within 4 weeks after appearance of foci caused regression of foci, slowing of growth, and delays of complete transformation lasting up to 5 months. In three lines, HS effects on cell shape and on proliferation were dissociated, suggesting separate controls of these two parameters. HS-substitution after complete transformation did not reduce growth or reverse morphologic changes. The results indicate that, in some cases, transformation by acute oncogenic retroviruses is a multistep process involving epigenetic regulation, and that autonomy from environmental controls is acquired gradually by the infected cells. The results also demonstrate that acute transforming retroviruses can cause prolonged latent infections, the phenotypic expression of which depends on environmental factors.