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

Cells cultured from explants of adult rat adrenal cortex arespindle shaped and divide rapidly in media with 10–25%fetal bovine serum (FBS), but are epithelial-like and stationaryin 3–6% horse serum (HS). Fully transformed Kirsten murinesarcoma virus (KiMSV)-infected cells lose this differentialresponse to serum. To determine at what stage in the transformationprocess this loss occurs, cultures in passages 1–2 wereinfected with KiMSV, propagated for up to 30 weeks in FBS, andparallel 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 rateand tumorigenicity). In 7 lines grown in FBS, foci appeared1–14 weeks post-infection and most cultures were completelytransformed 1–3 weeks thereafter. Substitution of HS forFBS prior to focus formation caused partial reversion to epithelial-likemorphology and reduction in growth rate. Transformation wasdelayed or prevented altogether, but could be elicited by additionof 1% FBS to HS. HS-substitution within 4 weeks after appearanceof foci caused regression of foci, slowing of growth, and delaysof complete transformation lasting up to 5 months. In threelines, HS effects on cell shape and on proliferation were dissociated,suggesting separate controls of these two parameters. HS-substitutionafter complete transformation did not reduce growth or reversemorphologic changes. The results indicate that, in some cases,transformation by acute oncogenic retroviruses is a multistepprocess involving epigenetic regulation, and that autonomy fromenvironmental controls is acquired gradually by the infectedcells. The results also demonstrate that acute transformingretroviruses can cause prolonged latent infections, the pheno-typicexpression of which depends on environmental factors.