TRANSFORMATION-RESTORING FACTOR - A LOW-MOLECULAR-WEIGHT SECRETED FACTOR REQUIRED FOR ANCHORAGE-INDEPENDENT GROWTH OF ONCOGENE-RESISTANT MUTANT-CELL LINES

We have previously described two independent mutant rat fibroblast cel l lines that fail to form colonies in soft agar when infected with a v -H-ras-expressing retrovirus, yet still undergo transformation-related morphological alterations in response to this oncogene, We report her e that conditioned medium (CM) from non-transformed rat fibroblasts co ntains an activity that specifically corrects this defect in the mutan t cell lines, rendering them capable of anchorage-independent growth i n response to uas, The major activity in CM, designated transformation -restoring factor (TRF), is similar to 1300 molecular weight, lipid in soluble, and heat, protease, acid and base stable, Latent activity, di stinct from TRF, is also present in CM; several lines of evidence indi cate that transforming growth factor (TGF) beta is responsible for thi s activity, TRF, however, cannot substitute for TGF beta in the phenot ypic transformation of NRK cells, TRF activity is decreased in CM of c ontrol cells transformed by uas and this response to ras is retained b y the mutant cell lines, We propose that whereas wild-type cells trans formed by ras may constitutively activate a TRF-regulated pathway, thu s becoming independent of TRF for growth in soft agar, these mutants h ave acquired dependence on an exogenous supply of TRF for this aspect of the transformed phenotype, Cellular activities regulated, directly or indirectly, by TRF may be effecters of the anchorge-independent gro wth property that is a hallmark of transformed rodent fibroblasts.