Abstract A45: Targeting breast cancer stem cells using cancer preventive rexinoids

Abstract

Abstract Background: Recent research in breast biology supports the cancer stem cell hypothesis which asserts that malignancies arise in tissue stem cells through dysregulation of self-renewal. The cancer stem cell hypothesis has important implications for early detection, prevention and treatment of breast cancer. If breast cancers arise from the transformation of normal stem cells, it may be possible to target stem cells for the prevention of cancer. We have previously shown that rexinoids prevent the development of ER-negative breast cancer in transgenic mouse models. Previous studies have demonstrated a role of retinoid signaling in the regulation of normal breast stem cell self-renewal and differentiation. Bexarotene and LG100268 are rexinoids that selectively bind RXR receptors. Based on the effects of these molecularly targeted agents, we hypothesize that rexinoids prevent breast cancer development by suppressing the growth and transformation of the breast stem cell population. Methods: We performed in vivo and in vitro studies to address our hypothesis. Cell proliferation assays were used to determine whether rexinoids can inhibit the growth of normal, pre-malignant and malignant breast cell lines (HMECs, MCF10A, DCIS.COM, SUM225, HCC1937 and HCC38 cell lines). To determine whether these agents affect stem cells, we performed mammosphere assays and FACS analysis after treatment with vehicle or rexiniods. We next conducted in vivo studies using MMTV-Wnt transgenic mice treated with LG100268 (50mg/kg) or vehicle, and studied the stem cell properties of mammary epithelial cells from these mice. The stem cell population in mammary glands was quantified by staining of CD24, CD49f and using FACS analysis and mammosphere assays. Results: Our results showed that the rexionids LG100268 and bexarotene inhibit the growth of breast cell lines only at a high concentrations (the IC50 (concentration that reaches a 50% growth inhibiton) in most of the tested cell lines exceeded 10uM). Results of in vitro mammosphere assays showed that 100nM LG100268 reduced mammosphere formation efficiency by more than 40% in all breast cancer cell lines. Results from our in vivo experiments showed that LG100268 reduced the mammary epithelial stem cell population by 57% as defined by the CD24+/CD49f++ population identified by FACS and by 47% as measured by in vitro mammosphere assays. Conclusion: These results from in vitro and in vivo studies demonstrate that the rexinoid LG100268 is a potent mammary stem cell inhibitor. This study shows that rexinoids suppress stem cell expansion and suggest that mammary stem cell inhibitors will be useful breast cancer preventive agents. This work is supported by a grant from the Breast Cancer Research Foundation (to PHB) and from an NIH/NCI RO1 grant (CA078480) (to PHB). Citation Information: Cancer Prev Res 2011;4(10 Suppl):A45.