[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Clinical trials and studies show that post-menopausal women undergoing hormone replacement therapy containing a combination of estrogen and progestin (P) have an increased risk of breast cancer compared with women taking estrogen alone or placebo. Using animal models, we previously showed that both natural and synthetic P accelerate the development of breast tumors in vivo and increase their metastasis to lymph nodes. Our studies included an assessment of medroxyprogesterone acetate (MPA), a synthetic P that is widely used clinically. Having established the deleterious effects associated with P, we sought to better understand the mechanisms underlying breast tumor growth and metastasis. We show that exposure of human breast cancer cells to synthetic P causes the overexpression of several cancer stem cell (CSC) markers, which we further demonstrate to be functionally significant, since mammosphere formation increases. Based on our observations, we contend that exposure of breast cancer cells to synthetic P, including MPA, leads to an enrichment of CSCs, which would likely support the development of P-accelerated tumors in vivo. An enriched CSC pool greatly increases the likelihood that resistance to therapy will arise, as well as raising the chance of metastasis. Our findings suggest that clinicians may be able to combat P-dependent tumor growth by blocking PR-mediated induction of CSC markers by immunotherapy, tissue-selective anti-Ps, or through a combination approach involving both immunotherapy against CD44 and small-molecule targeting of PR. We carried out a series of studies to determine whether RO 48-8076, an oxidosqualene cyclase (OSC) inhibitor, might reduce P-induced CSC expansion. In previous studies, we showed that RO diminishes tumor formation in vivo and found that it exerts its potent anti-tumor effects in part through PR degradation. This in turn reduces P-induced expression of CD44, a process we showed to be PR-dependent. Importantly, treatment of hormone-responsive breast cancer cells with RO abolished MPA-induced mammosphere formation. We therefore contend that RO may represent a novel means by which to prevent MPA-induced CSC expansion. RO treatment may also represent a novel strategy of reducing resistance to anti-hormone therapies, given that survival of stem cells following chemotherapy is the major reason why such treatments fail. In addition to being a novel treatment option for hormone-dependent breast cancers, we show that RO might also be used to treat highly aggressive triple negative breast cancers (TNBC). Herein we present in vivo data showing that RO reduces the metastasis of TNBCs to the lungs. We hypothesize that by virtue of its ability to downregulate MetAP2 protein, RO might target the metastatic cascade by inhibiting angiogenesis and cell cycle progression, or by inducing apoptosis. Further mechanistic studies are needed to elucidate the importance of RO-mediated MetAP2 downregulation. We also demonstrate that RO inhibits TNBC cell migration and invasion, though the mechanism through which RO exerts these effects requires more in-depth study. Nevertheless, we show that RO, a compound hitherto considered as simply an inhibitor of cholesterol biosynthesis, clearly possesses potent anti-cancer effects. Its potential as an agent which might be used to combat both hormone-dependent and hormone-independent breast cancers, including TNBC, warrants further investigation.
Estrogens and Progestogens in Triple Negative Breast Cancer: Do They Harm?
