Modulation by normal serum factors of Kirsten murine sarcoma virus-induced transformation in adult rat cells infected in early passage.

Adult rat adrenal cells, infected with Kirsten murine sarcoma virus in early passage, transform consistently in 10% fetal bovine serum (FBS)-supplemented medium. Substitution of 3% horse serum (HS) for FBS reverses early foci and delays transformation. The influence of the serum on DNA synthesis, anchorage dependence, tumorigenicity, and subcellular Mr 21,000 transforming protein (p21) distribution was followed from infection in passage 1 to complete transformation. In FBS, increased expression of p21 preceded other evidence of transformation. Subsequently, p21-positive cells transformed morphologically, but initially their growth parallelled that of coexisting untransformed cells. Foci formed at passages 5 to 10, and the cells became anchorage independent and tumorigenic at passages 10 to 20. As transformation in FBS progressed, p21 relocated from a diffuse distribution to sites of retraction from substrata and then to ruffles and lamellae on cellular processes. Early in transformation, HS-medium reduced proliferation of morphologically normal and morphologically transformed p21-positive cells. This effect was counteracted by the addition of FBS or the Mr 50,000 to 100,000 fraction of FBS. Fully transformed, tumorigenic cells grew rapidly in both sera but, if transferred from FBS to HS, became more anchorage and density dependent, and p21 relocated from cell processes to the cell bodies. In immortal lines, the substitution of HS for FBS accelerated rather than delayed the progression of Kirsten murine sarcoma virus-induced transformation. These results show that Kirsten murine sarcoma virus-induced transformation of adult presenescent cells is controlled by physiological factors to which immortal cells appear refractory. The changes in subcellular distribution of p21 during transformation parallel the expression of some, but not other, transformation parameters and suggest a possible association of p21 with increased membrane activity.