Interaction of the Rb tumor suppressor protein with the c-fos promoter in c-fos transfected cells overexpressing c-fos and Rb

The Rb tumor suppressor protein is overexpressed in HeLa cell lines permanently transfected with a constitutively expressed c-fos gene. CP17-14 cell overexpression of Rb may be due to a balancing response to overexpression of the stimulatory effects of c-fos overexpression on transcription. The cis-acting retinoblastoma control element (RCE, -97 to -86 bp) in the human c-fos promoter is thought to allow regulation of c-fos by Rb. Gel-shift assays were performed with a 168 bp fragment encoding the c-fos RCE. Competition assays with increasing mass of unlabeled probe or dose-dependence assays using increasing mass of nuclear proteins, demonstrated sequence-specific complex formation. Indistinguishable complexes were formed between the c-fos RCE fragment in transfected cells, but at higher levels (>50%), compared to proteins from parental cells. Supershift analysis utilizing epitope-specific Rb-monoclonal antibodies indicated the presence of Rb protein bound to the RCE-containing DNA fragment In contrast, polyclonal anti-Rb antibodies enhanced the amounts of nuclear protein-DNA complexes detected but did not result in a supershift. These results suggested the presence of Rb and/or Rb-like peptides involved in complex formation and the presence of multiple varients of RCE-binding complexes in response to c-fos over-expression.