Mechanism of E1A-Induced Transforming Growth Factor-β (TGF-β) Resistance in Mouse Keratinocytes Involves Repression of TGF-β Type II Receptor Transcription*

Cellular transformation driven by the E1A oncogene is associated with the development of cellular resistance to the growth inhibitory effects of transforming growth factor-β (TGF-β). We demonstrate that development of resistance occurs simultaneously with decreased expression of TGF-β type II receptor (TGF-β RII) mRNA and protein. To determine whether changes in transcriptional regulation are responsible for the decreased receptor expression in E1A-transformed cells, a series of mobility shift assays was performed utilizing nuclear extracts from E1A-transformed and untransformed murine keratinocytes using radiolabeled positive regulatory elements (PRE1 and PRE2) of the TGF-β RII promoter. The results from these assays suggest that E1A-transformed cells express markedly lower levels of nuclear proteins that bind specifically to PRE1 and PRE2. Transfection of both E1A-transformed and untransformed cell lines with a series of mutant promoter constructs confirmed that both PREs contribute significantly to basal expression of TGF-β RII and that inactivation of either element leads to markedly reduced promoter activity. We conclude that development of TGF-β resistance in E1A-transformed cells is achieved in part through transcriptional down-regulation of the TGF-β RII gene and that this down-regulation is the result of decreased expression of unidentified transcription factor complexes that interact with PRE1 and PRE2.