p21(ras)-mediated decrease of the retinoblastoma protein in fibroblasts occurs through growth factor-dependent mechanisms

Stable coexpression of the human retinoblastoma protein (pRB) cDNA and EJ c-Ha-ras oncogene in murine fibroblasts leads to loss of pRB expression with concomitant transformation of the cells (1). We show here that conditional expression of p21(ras) in mouse fibroblasts expressing human pRB leads to a rapid decrease of pRB expression at both protein and mRNA levels. The decrease of pRB mRNA is blocked by cycloheximide, suggesting the requirement of ongoing protein synthesis. p21(ras) expression leads also to decreases of c-myc and tissue metalloproteinase inhibitor-2 mRNAs, whereas cyclin-dependent kinase 4, cyclin D1, E2F-1, and ornithine decarboxylase are unaffected. The decrease in pRB is accompanied by progressive morphological transformation of the cells. The effect of p21(ras) on pRB expression was serum and growth factor dependent. A shift of the cells to low serum (0.2% FCS) abolished the effects of p21(ras) on pRB, but this effect was reconstituted by the addition of growth factors epidermal growth factor, fibroblast growth factor-2, transforming growth factor β1, and platelet- derived growth factor to the cells. The results suggest a complex interaction between p21(ras), pRB, and growth factors in the control of cell growth. p21(ras) appears to drive the cell cycle by deregulation of key cell cycle regulators, the functions of which in low serum become redundant or require the presence of growth factors positively driving the cell cycle.