The retinoblastoma gene product, Rb, represses neu expression through two regions within the neu regulatory sequence

The neu oncogene is frequently overexpressed in breast, ovarian and lung cancers, and its overexpression correlates with poor disease prognosis. The exact mechanism of deregulation of neu expression is not well understood. Our previous studies indicate that the tumor suppressor retinoblastoma gene product, Rb, represses transcription of neu through the GTG enhancer (-243 to -234 relative to initiation of translation site of rat neu). We carried out further deletion analysis of the regulatory sequences of neu and found that Rb also represses neu close to transcription initiation sites (-172 to -79). Bal 31 deletions downstream of nucleotide -172 show that the sequence TCGAGGAA (-172 to -165) is important for efficient transcription from the neu promoter and also for repression by Rb. Rb mutants with mutations in the large T/E1a binding domain repress transcription from transcription initiation sites but not the GTG enhancer, suggesting that Rb modulates different regions of the regulatory sequence of neu by different pathways. The net effect of the Rb mutants is to repress not only transcription but also the transforming activity of activated neu in focus-forming assays. Thus, one mechanism whereby Rb may act as a tumor suppressor is to repress transcription of the strongly transforming neu oncogene.