Inactivation of retinoblastoma family proteins by SV40 T antigen results in creation of a hepatocyte growth factor/scatter factor autocrine loop associated with an epithelial-fibroblastoid conversion and invasiveness.

ABSTRACT

SV40 T antigen (LT) is an oncoprotein that inactivates nuclear regulators such as retinoblastoma (RB) family proteins and p53. We recently reported that in Madin-Darby canine kidney (MDCK) epithelial cells the binding of LT to RB family proteins results in a massive apoptosis and a concomitant down-regulation of c-myc. Here, we show that LT causes loss of epithelial differentiation and induces invasiveness. MDCK cells expressing wild-type LT, but not mutants unable to bind RB, exhibit a fibroblast-like morphology, show a strong down-regulation of the vHNF1 transcription factor and acquire invasive properties. The stable retransformation of MDCK(LT) with a RB and/or c-myc-expressing vector restores the expression of epithelial characteristics. Our data therefore suggest an important role for RB and c-myc in modulating the epithelial phenotype both during normal tissue development and in invasive processes. In addition, when grown in collagen gels, the MDCK(LT) cells form branching tubules, and their conditioned media produce the scattering of monolayer cultured MDCK cells. These last properties are reminiscent of those induced by hepatocyte growth factor/scatter factor (HGF/SF). Moreover, the HGF/SF protein was detected by Western blotting in the MDCK(LT)-conditioned medium. The production of HGF/SF is specifically induced by LT-RB inactivation, because Ras transformation of MDCK cells fails to induce the production of this factor. These results demonstrate that inactivation of RB family proteins in these cells is at the origin of a HGF/SF autocrine loop.