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Overexpression of the  ERBB2  oncogene is observed in about 20% of human breast tumors and is the consequence of increased transcription rates frequently associated with gene amplification. Several studies have shown a link between activator protein 2 (AP-2) transcription factors and  ERBB2  gene expression in breast cancer cell lines. Moreover, the Yin Yang 1 (YY1) transcription factor has been shown to stimulate AP-2 transcriptional activity on the  ERBB2  promoter  in vitro . In this report, we examined the relationships between  ERBB2 , AP-2α, and YY1 both in breast cancer tissue specimens and in a mammary cancer cell line.    Methods  ERBB2, AP-2α, and YY1 protein levels were analyzed by immunohistochemistry in a panel of 55 primary breast tumors.  ERBB2  gene amplification status was determined by fluorescent  in situ  hybridization. Correlations were evaluated by a χ 2  test at a  p  value of less than 0.05. The functional role of AP-2α and YY1 on  ERBB2  gene expression was analyzed by small interfering RNA (siRNA) transfection in the BT-474 mammary cancer cell line followed by real-time reverse transcription-polymerase chain reaction and Western blotting.    Results  We observed a statistically significant correlation between ERBB2 and AP-2α levels in the tumors ( p  < 0.01). Moreover, associations were found between ERBB2 protein level and the combined high expression of AP-2α and YY1 ( p  < 0.02) as well as between the expression of AP-2α and YY1 ( p  < 0.001). Furthermore, the levels of both AP-2α and YY1 proteins were inversely correlated to  ERBB2  gene amplification status in the tumors ( p  < 0.01). Transfection of siRNAs targeting AP-2α and AP-2γ mRNAs in the BT-474 breast cancer cell line repressed the expression of the endogenous  ERBB2  gene at both the mRNA and protein levels. Moreover, the additional transfection of an siRNA directed against the YY1 transcript further reduced the ERBB2 protein level, suggesting that AP-2 and YY1 transcription factors cooperate to stimulate the transcription of the  ERBB2  gene.    Conclusion  This study highlights the role of both AP-2α and YY1 transcription factors in  ERBB2  oncogene overexpression in breast tumors. Our results also suggest that high  ERBB2  expression may result either from gene amplification or from increased transcription factor levels.

The combined immunodetection of AP-2α and YY1 transcription factors is associated with ERBB2 gene overexpression in primary breast tumors