Chlorpyrifos and tamoxifen co-pretreatment promotes stem-like phenotype and upregulation of anti-estrogen therapy resistance markers in ERα breast cancer cells.

Endocrine therapy (ET) is the standard first-line treatment for hormone receptor positive breast cancer (BC) but about 30 % of patients develop resistance. Chlorpyrifos (CPF), is an endocrine disruptor pesticide that promotes a stemness phenotype and triggers molecular mechanisms involved in ET resistance. The goal of this study is to assess whether CPF induces tamoxifen (TAM) resistance by modulating estrogen receptor α (ERα) expression through histone deacetylase 1 (HDAC1) regulation. We analyzed public databases of BC patients and performed in vitro experiments using the MCF-7 cell line. We evaluated whether the co-pretreatment with CPF and TAM (CPF + TAM) modulates stemness and ET resistance markers. We found that CPF + TAM selects cells that exhibit increased clonogenicity, enhanced mammosphere-forming ability, and upregulates the stemness markers CD44, NANOG and POU5F1 while decreasing CD24. This selected cell subpopulation shows higher levels of mRNA and elevated protein expression of ERα and HDAC1. Finally, we identified a resistance and stemness marker signature induced by CPF + TAM that closely resembles the profile observed in the dataset of patients who acquired TAM resistance. Our findings show that CPF promotes an undifferentiated basal-like cell phenotype that contributes to TAM resistance, reinforcing the need for global restrictions to safeguard public health.