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Introduction  Activator protein-2 (AP-2) α and AP-2γ transcription factors contribute to  ERBB2  gene overexpression in breast cancer. In order to understand the mechanism by which the  ERBB2  gene is overexpressed we searched for novel AP-2 interacting factors that contribute to its activity.    Methods  Ku proteins were identified as AP-2α interacting proteins by glutathione serine transferase (GST)-pull down followed by mass spectrometry. Transfection of the cells with siRNA, expression vectors and reporter vectors as well as chromatin immunoprecipitation (ChIP) assay were used to ascertain the implication of Ku proteins on ERBB2 expression.    Results  Nuclear proteins from BT-474 cells overexpressing AP-2α and AP-2γ were incubated with GST-AP2 or GST coated beads. Among the proteins retained specifically on GST-AP2 coated beads Ku70 and Ku80 proteins were identified by mass spectrometry. The contribution of Ku proteins to  ERBB2  gene expression in BT-474 and SKBR3 cell lines was investigated by downregulating Ku proteins through the use of specific siRNAs. Depletion of Ku proteins led to downregulation of ERBB2 mRNA and protein levels. Furthermore, reduction of Ku80 in HCT116 cell line decreased the AP-2α activity on a reporter vector containing an AP-2 binding site linked to the  ERBB2  core promoter, and transfection of Ku80 increased the activity of AP-2α on this promoter. Ku siRNAs also inhibited the activity of this reporter vector in BT-474 and SKBR3 cell lines and the activity of the  ERBB2  promoter was further reduced by combining Ku siRNAs with AP-2α and AP-2γ siRNAs. ChIP experiments with chromatin extracted from wild type or AP-2α and AP-2γ or Ku70 siRNA transfected BT-474 cells demonstrated Ku70 recruitment to the  ERBB2  proximal promoter in association with AP-2α and AP-2γ. Moreover, Ku70 siRNA like AP-2 siRNAs, greatly reduced PolII recruitment to the  ERBB2  proximal promoter.    Conclusions  Ku proteins in interaction with AP-2 (α and γ) contribute to increased ERBB2 mRNA and protein levels in breast cancer cells.

Ku proteins interact with activator protein-2 transcription factors and contribute to ERBB2overexpression in breast cancer cell lines