Simple system for isolation of cellular and viral mutants for transformation by retrovirus

Suppression of v-src transformation by the drs gene

Previously, we isolated a novel gene, drs, which was downregulated by retroviral oncogenes such as v-src and v-K-ras, from a cDNA library of primary rat embryo fibroblasts. Experiments using a temperature-sensitive mutant of the v-src gene indicated that downregulation of drs mRNA was dependent on functional expression of v-Src. In addition, expression of drs mRNA was also reduced by serum stimulation of G0-arrested normal rat fibroblast cells. To clarify the function of the drs gene in cell transformation and proliferation, we introduced drs linked to a potent promoter into a normal rat cell line, F2408, and examined the effect of ectopic expression of exogenous drs on the transformation by the v-src gene and growth properties. Cells expressing exogenous drs gene showed significantly decreased efficiency of transformation by v-src irrespective of functional expression of v-Src kinase, while the growth rate and G1/S progression of the cells were not suppressed by expression of exogenous drs gene, indicating that drs has the ability to suppress v-src transformation without disturbing cell proliferation.

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 culture, we compared the transforming effect of a recombinant retrovirus (ZSV) containing the v-src gene in rat embryo fibroblasts (REFs) to that in the rat cell line 3Y1. In the focus assay, REFs exhibited resistance to transformation as only six foci were observed in the primary cultures as opposed to 98 in 3Y1 cells. After G418 selection, efficiency of transformation was again somewhat lower with REFs compared to that with 3Y1 cells, but the number of G418-resistant REF colonies was much greater than the number of foci in REF cultures. Furthermore, while 98% of G418-resistant colonies of ZSV-infected REFs were morphologically transformed, only 25% were converted to anchorage-independent growth, as opposed to 100% conversion seen in ZSV-infected 3Y1 cells. The poor susceptibility of REFs to anchorage-independent transformation did not involve differences in expression and subcellular distribution of p60v-src, or its kinase activity in vitro and in vivo. It rather reflected a property of the primary cultures, as cloning of REFs before ZSV infection demonstrated that only 2 out of 6 REF clones tested were permissive for anchorage-independent growth. The nonpermissive phenotype was dominant over the permissive one in somatic hybrid cells, and 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 transformation by p60v-src reflects the heterogeneity of the primary cultures. REFs can be morphologically transformed by p60v-src with high efficiency but only a small fraction is convertible to anchorage-independent growth. REF resistance seems to involve the presence of a suppressor factor which may emerge from REF differentiation during embryonic development.

Rat cellular mutants for expression of mRNA from the long terminal repeat of murine retrovirus

Previously we isolated revertants from a rat cell line transformed by recombinant murine retrovirus containing the v-src gene. These mutant cell lines, R78 and R107, showed low src-kinase activity, but retained wild-type transforming retrovirus, suggesting that a cellular gene involved in viral gene expression was mutated. Southern and Northern hybridization analyses showed that the expression of viral mRNAs from the integrated proviral DNA was reduced in these mutant cells. DNA transfection experiments with various transforming genes and promoters revealed that the mutant cell lines were resistant to transformation by transforming genes expressed under the long terminal repeat (LTR) of Moloney murine leukemia virus (Mo-MuLV). In contrast, these cell lines could be efficiently transformed by the same transforming genes with human metallothionein promoter, polyomavirus promoter-enhancer, and c-H-ras promoter. Transient expression assays using plasmids containing the CAT gene under the LTR of Mo-MuLV also showed that CAT activity expressed under the LTR in these mutant cells was lower than that in the parental cell line, No. 7. These results suggest that cellular mutations of R78 and R107 cells affect specific transcription from the LTR of Mo-MuLV. Studies using various constructs of the LTR CAT indicated that the region responsible for the repression was located in a fragment (-328 to -150) of the LTR containing the 72-bp repeat enhancer. Somatic cell hybridization experiments showed that the mutant phenotype of these mutant cell lines is dominant to that of the parental cell line.