Investigating tamoxifen resistance in the luminal B estrogen receptor positive breast cancer subtype: Tailoring treatment in hormone responsive breast cancer.

Loi S, Sotiriou C, Haibe-Kains B, Lallemand F, Conus N, Piccart MJ, Speed T and McArthur GA

Background: We recently reported that the two estrogen receptor (ER)-positive breast cancer (BC) molecular subtypes can be defined by their expression of proliferation genes using a gene expression index (GGI): the luminal A and B subtypes have low and high levels respectively. When treated with adjuvant tamoxifen, luminal A tumors have a good prognosis, however the clinical outcome of the luminal B subtype was poor. This study aimed to explain the biological basis for these observations using gene expression profiling and an in vitro model of ER+ BC.

Methods: 246 ER+ BC samples from women treated with adjuvant tamoxifen monotherapy were analyzed with Affymetrix gene expression arrays and evaluated using gene set enrichment analysis (GSEA). ER+ MCF-7 BC cells (control) treated with tamoxifen (TAM) and heregulin (HRG) were used to investigate molecular pathways identified using GSEA.

Results: We found that a gene set suggesting ERBB2 pathway activation was significantly enriched in the luminal B subtype (p=0.02). Only 10% of samples overexpressed HER2 by immunohistochemistry, suggesting that activation of HER2 signaling pathways is independent of HER2 overexpression in this subtype. MCF-7 cell-lines were treated with HRG (HRG-MCF7) to create a model of ERBB2 pathway activation. HRG-MCF7 cells displayed phosphorylation of HER2/3 without HER2 overexpression. Treatment with HRG overcame TAM induced cell cycle arrest with higher S-phase fraction (p<0.01) and increased anchorage-independent colony formation (p<0.01). Gene expression profiling confirmed significant enrichment of the ERBB2 gene set (p<0.01) and higher GGI levels (p=0.02) in HRG-MCF7 cells compared with control.

Conclusions: HRG-MCF7 cells may be useful as an in vitro model of the luminal B subtype. In this group, targeting activated HER2 signaling may be a helpful treatment strategy despite the lack of HER2 overexpression. Our data suggest that agents like lapatinib may be effective only in the luminal B and not the luminal A tumors, demonstrating the importance of stratifying by subtype in future clinical trials of ER+ disease.