Abstract P6-04-06: Inverse regulation of Neuregulin1 and HER-3 during treatment with aromatase inhibitors of estrogen receptor-positive breast cancer

Background: Resistance to endocrine therapy may be promoted by deregulation of growth factor signaling pathways in addition to its intimate bidirectional crosstalk with the estrogen receptor (ER) signaling pathway. We have previously shown HER-2/neu mRNA upregulation in HER-2/neu non-amplified tumors responding to therapy with aromatase inhibitors (AIs) [1]. Targeting HER-downstream signaling pathways may represent a strategy to overcome acquired endocrine resistance. Here, we aim at exploring the effect of treatment with AIs on the HER-receptor family members and the HER-3/-4 ligand neuregulin1 (NRG1) in ER+ breast cancers. Methods: Matched tumor biopsies were collected from 85 ER+ breast cancers before and after three months of neo-adjuvant treatment with the AIs letrozole or anastrozole. For 64 of the patients, tumor biopsies were also available at two weeks of treatment. The patients were classified as either responders or non-responders depending on more or less than 50% reduction in tumor size, respectively. RNA was extracted from tumors and mRNA expression analyses of HER-1-4 and NRG1 were performed by real-time PCR using gene-specific primers. Changes in mRNA levels during treatment were estimated using the non-parametric Wilcoxon sign-rank test, while the correlations between mRNA levels were analyzed using 2-tailed Spearman rank test. Results: Among all the ER+ HER-2/neu non-amplified tumors we observed a significant increase in mRNA expression of EGFR/HER-1 (51/64 tumors, p NRG1 (49/63, p HER-3 decreased significantly during treatment (p = 0.023, 27 up and 39 down). Consequently, changes in NRG1 during treatment correlated negatively with HER-3 (r = −0.274, p = 0.036) and positively with EGFR/HER-1 (r = 0.557, p HER-3 increased during the first two weeks of treatment (21 up and 8 down; p = 0.031) followed by a decrease (10 up and 19 down; p = 0.033) from two weeks to three months of treatment. Discussion: We report an increase in NRG1 expression during estrogen deprivation. This may be explained by a biological reprogramming causing the cells to be more sensitive to mitogenic signals. Elevated NRG1 level activates the HER-2/neu:HER-3 complex in tumors which in ovarian cancer lead to downregulation of HER-3 [2]. In line with this, we observed downregulation of HER-3 in tumors with increased NRG1 expression. As EGFR/HER-1 is known to be transcriptionally repressed by estrogen, it is conceivable that the stimulation of EGFR/HER-1 is caused by estrogen depletion. We propose that treatment with AIs leads to increased signaling through the HER-2/neu:HER-3 complex in ER+ HER2/neu non-amplified breast tumors. 1. Flageng, M.H., L.L. Moi, J.M. Dixon, J. Geisler, E.A. Lien, W.R. Miller, P.E. Lonning, and G. Mellgren, Br J Cancer, 2009.101: 1253–60. 2. Makhija, S., L.C. Amler, D. Glenn, F.R. Ueland, M.A. Gold, D.S. Dizon, V. Paton, C.Y. Lin, T. Januario, K. Ng, A. Strauss, S. Kelsey, M.X. Sliwkowski, and U. Matulonis, J Clin Oncol, 2010. 28:1215–23. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-04-06.