Transformation of mortal human fibroblasts and activation of a growth inhibitory pathway by the bovine papillomavirus E5 oncoprotein.

The 44-amino acid bovine papillomavirus E5 protein induces tumorigenic transformation of immortal rodent fibroblasts by binding to and activating the platelet-derived growth factor beta receptor (PDGFbetaR). Here E5 was expressed in mortal human diploid fibroblasts (HDFs), which lack the accumulated genetic changes that are present in immortal rodent cells. E5 induced focus formation and morphological transformation of HDFs without inducing anchorage independence or immortalization. Similar effects were observed with the v-sis and neu* oncogenes. E5-PDGFbetaR complexes were observed in the E5-expressing HDFs, as was constitutive PDGFbetaR activation, which was required for the transforming activity of E5. The E5 HDFs attained a higher saturation density than the control cells, expressing increased levels of hyperphosphorylated retinoblastoma protein at subconfluent densities. However, when these cells reached confluence, growth inhibition accompanied by dramatic down-regulation of the PDGFbetaR, and retinoblastoma protein was induced apparently by a factor secreted into the medium. This may represent a novel negative feedback mechanism controlling PDGFbetaR-induced proliferation and thereby protecting against complete transformation.