V-SRC TRANSFORMATION OF RAT EMBRYO FIBROBLASTS - INEFFICIENT CONVERSION TO ANCHORAGE-INDEPENDENT GROWTH INVOLVES HETEROGENEITY OF PRIMARY CULTURES

To clarify whether a single oncogene can transform primary cells in cu lture, we compared the transforming effect of a recombinant retrovirus (ZSV) containing the v-src gene in rat embryo fibroblasts (REFs) to t hat in the rat cell line 3Y1. In the focus assay, REFs exhibited resis tance to transformation as only six foci were observed in the primary cultures as opposed to 98 in 3Y1 cells. After G418 selection, efficien cy of transformation was again somewhat lower with REFs compared to th at with 3Y1 cells, but the number of G418-resistant REF colonies was m uch greater than the number of foci in REF cultures. Furthermore, whil e 98% of G418-resistant colonies of ZSV-infected REFs were morphologic ally transformed, only 25% were converted to anchorage-independent gro wth, as opposed to 100% conversion seen in ZSV-infected 3Y1 cells. The poor susceptibility of REFs to anchorage-independent transformation d id not involve differences in expression and subcellular distribution of p60(v-src), or its kinase activity in vitro and in vivo. It rather reflected a property of the primary cultures, as cloning of REFs befor e ZSV infection demonstrated that only 2 out of 6 REF clones tested we re permissive for anchorage-independent growth. The nonpermissive phen otype was dominant over the permissive one in somatic hybrid cells, an d associated with organized actin filament bundles and a lower growth rate, both before and after ZSV infection. These results indicate that the poor susceptibility of REFs to anchorage-independent transformati on by p60(v-src) reflects the heterogeneity of the primary cultures. R EFs can be morphologically transformed by p60(v-src) with high efficie ncy but only a small fraction is convertible to anchorage-independent growth. REF resistance seems to involve the presence of a suppressor f actor which may emerge from REF differentiation during embryonic devel opment.