ErbB3-binding protein EBP1 decreases ErbB2 levels via a transcriptional mechanism

Ectopic expression of EBP1, an ErbB3-interacting protein, reduces the expressionof the ErbB2 protein and mRNA. However, the mechanism of EBP1-induced decreasein ErbB2 mRNA levels has not yet been determined. Since EBP1 affects both transcriptionaland post-transcriptional processes, we evaluated the ability of EBP1 to regulateErbB2 transcription and RNA stability. We discovered that while wild-type EBP1decreased the activity of a proximal ErbB2 promoter, EBP1 mutants unable to interactwith the Sin3A transcriptional repressor inhibited activity to a lesser extent.EBP1 also decreased the activity of distal ErbB2 promoters. Chromatin immunoprecipitationanalysis indicated that EBP1 bound both distal and proximal endogenous ErbB2 promotersin serum-starved conditions. The ErbB3 ligand heregulin (HRG) at growth-promotingconcentrations reduced EBP1 binding to the ErbB2 promoter. Although endogenousEBP1 bound ErbB2 mRNA, EBP1 overexpression or ablation of EBP1 protein by shRNAfailed to alter ErbB2 mRNA stability. These results suggest that the major effectof EBP1 on ErbB2 mRNA levels is at the transcriptional level.