Abstract 939: ErbB-2 inhibits Notch-4 expression and activity in breast cancer independent of ER function

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Amplification, over-expression, and/or hyperactivity of ErbB-2 occur in 50% of breast tumors which include both luminal A/B and the HER2+ subtypes. Metastatic breast tumors that overexpress ErbB-2 are generally resistant to anti-hormonal or anti-HER2-targeted therapy. We have published that either 17beta-estradiol or overexpression of ErbB-2 suppresses Notch activity and this is reversed by inhibitors of estrogen receptor (ER) or ErbB-2. Furthermore, we demonstrated that when breast tumors are treated with a Notch inhibitor (gamma-secretase inhibitor or GSI) in combination with tamoxifen (ER+ cells) or trastuzumab (ErbB-2+ cells), ER+ breast tumor xenografts regress or ErbB-2+ xenografts do not recur. Based on these published findings, we asked in the current study by what mechanism does ErbB-2 inhibit Notch signaling. We used both genetic and pharmacologic approaches to address the hypothesis that ErbB-2 hyperactivity was sufficient to suppress Notch independent of ER function. Our results showed that Notch-4 transcription is specifically and significantly decreased when ErbB-2 or Heregulin-beta1 is stably-overexpressed in MCF-7 cells. Furthermore, the data demonstrated that while 17beta-estradiol inhibits Notch-4 mRNA and protein expression, which was reversed by fulvestrant, the inhibition on Notch-4 by ErbB-2 or by Heregulin-beta1 overexpression was independent of ER activity. In addition, we confirmed that ER status was an independent factor by specifically targeting ErbB-2 using lapatinib in ER negative SKBr3 breast cancer cells and the results showed that Notch-4 mRNA and protein increased to more than 100 fold compared to vehicle treatment. We have recently published that c-Fos is a potential transcriptional repressor of Notch-4 in breast cancer cells and in agreement, we identified that c-Fos levels were significantly elevated in breast cancer cells stably-overexpressing either ErbB-2 or Heregulin-beta1. The biological significance of Notch-4 overexpression was investigated in two relevant resistant models in vitro and in vivo. Resistance to lapatinib was prevented by co-treatment of a GSI plus lapatinib. Resistance to anti-hormonal therapy using long-term estrogen deprived MCF-7 cells was reversed by a GSI treatment. MCF-7/HER2 breast tumor xenografts regressed to undetectable levels with the combination of a GSI plus trastuzumab under conditoions where estrogen was deprived. In conclusion, results from the current study suggest that ErbB-2 overexpression or hyperactivity via Heregulin-beta1 is a potent inhibitor of Notch-4 expression possibly by increasing c-Fos expression. Furthermore, the regulation of Notch-4 expression is independent of ER function. More importantly, anti-ER or anti-EbB-2 targeted therapy de-repress Notch-4 expression and thus sensitize breast cancer cells to a Notch inhibitor such as a GSI to prevent or reverse resistance. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 939. doi:1538-7445.AM2012-939