Abstract
Transforming growth factors beta (TGF-beta s) are potent inhibitors of epithelial cell growth in culture and might play a similar role in vivo. Several studies have suggested that acquisition of TGF-beta resistance is an important step in epithelial tumor development. Here, we show that resistance to TGF-beta 1 growth inhibition can be induced by transformation of keratinocytes with the E1A, but not the ras, oncogene. Mutational analysis revealed that these effects closely correlate with the ability of E1A proteins to bind to the retinoblastoma gene product (p105) as well as to three other cellular proteins (p60, p107, and p300). Only partial resistance to TGF-beta 1 growth inhibition was elicited by E1A mutants that bind to a subset of proteins, whereas complete resistance was induced by E1A mutants that bind to all four proteins together. Total protection against TGF-beta growth inhibition was also induced by concomitant introduction into cells of an E1A mutant binding to the p60/p105/p107 proteins and one binding to p300. In parallel with these effects, epidermal transglutaminase, a marker of keratinocyte differentiation, was induced by TGF-beta in control but not in E1A-transformed cells. TGF-beta 1 receptor levels were only partially down-modulated by an intact E1A gene and not significantly affected by the various truncated mutants. Thus, the ability of E1A to induce TGF-beta resistance depends on its ability to bind, and presumably inactivate, several cellular proteins that may be involved in transmission of the TGF-beta signal and seem to act downstream from its receptor(s).