Mechanism of E1A-induced transforming growth factor-beta (TGF-beta) resistance in mouse keratinocytes involves repression of TGF-beta 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-beta (TGF-beta). We demonstrate that development of resistance occurs simultaneously with decreased expression of TGF-beta type II receptor (TGF-beta 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-beta 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-beta RII and that inactivation of either element leads to markedly reduced promoter activity. We conclude that development of TGF-beta resistance in E1A-transformed cells is achieved in part through transcriptional down-regulation of the TGF-beta RII gene and that this down-regulation is the result of decreased expression of unidentified transcription factor complexes that interact with PRE1 and PRE2.