Increased Expression of Interleukin-1 Receptor Characterizes Anti-estrogen-Resistant ALDH + Breast Cancer Stem Cells.

Estrogen-receptor-positive breast tumors are treated with anti-estrogen (AE) therapies but frequently develop resistance. Cancer stem cells (CSCs) with high aldehyde dehydrogenase activity (ALDH(+) cells) are enriched following AE treatment. Here, we show that the inter-leukin-beta (IL-beta) signaling pathway is activated in ALDH(+) cells, and data from single cells reveals that AE treatment selects for IL-1 recep-tor (IL1R1)-expressing ALDH(+) cells. Importantly, CSC activity is reduced by an IL1R1 inhibitor in AE-resistant models. Moreover, IL1R1 expression is increased in the tumors of patients treated with AE therapy and predicts treatment failure. Single-cell gene expression anal-ysis revealed that at least two subpopulations exist within the ALDH(+) population, one proliferative and one quiescent. Following AE ther-apy the quiescent population is expanded, which suggests CSC dormancy as an adaptive strategy that facilitates treatment resistance. Targeting of ALDH(+)IL1R1(+) cells merits testing as a strategy to combat AE resistance in patients with residual disease.