Abstract
The breast cancer stem cells (BCSC) play important roles in breast cancer occurrence, recurrence and metastasis. However, the role of estrogen signaling, a signaling pathway important in development and progression of breast cancer, in regulation of BCSC has not been well established. Previously, we identified and cloned a variant of estrogen receptor α, ER-α36, with a molecular weight of 36 kDa. ER-α36 lacks both transactivation domains AF-1 and AF-2 of the 66 kDa full-length ER-α (ER-α66) and mediates rapid estrogen signaling to promote proliferation of breast cancer cells. In this study, we aim to investigate the function and the underlying mechanism of ER-α36-mediated rapid estrogen signaling in growth regulation of the ER-positive breast cancer stem/progenitor cells. ER-positive breast cancer cells MCF7 and T47D as well as the variants with different levels of ER-α36 expression were used. The effects of estrogen on BCSC's abilities of growth, self-renewal, differentiation and tumor-seeding were examined using tumorsphere formation, flow cytometry, indirect immunofluorence staining and in vivo xenograft assays. The underlying mechanisms were also studied with Western-blot analysis. We found that 17-β-estradiol (E2β) treatment increased the population of ER-positive breast cancer stem/progenitor cells while failed to do so in the cells with knocked-down levels of ER-α36 expression. Cells with forced expression of recombinant ER-α36, however, responded strongly to E2β treatment by increasing growth in vitro and tumor-seeding efficiency in vivo. The rapid estrogen signaling via the AKT/GSK3β pathway is involved in estrogen-stimulated growth of ER-positive breast cancer stem/progenitor cells. We concluded that ER-α36-mediated rapid estrogen signaling plays an important role in regulation and maintenance of ER-positive breast cancer stem/progenitor cells.
Highlights
Accumulating experimental and clinical evidence supports that breast cancer may arise from mammary stem/progenitor cells that possess the ability to self-renew [1,2,3,4]
We found that estrogen treatment significantly increased the CD44+/ CD242 cell population in both MCF7 and T47D cells (Figure 1A)
The breast cancer stem/progenitor cells enriched from estrogen receptor (ER)-positive breast cancer MCF7 and T47D cells were used as models to investigate their responses to estrogen
Summary
Accumulating experimental and clinical evidence supports that breast cancer may arise from mammary stem/progenitor cells that possess the ability to self-renew [1,2,3,4]. The breast cancers with ALDH1high cancer stem-like cells are often associated with more aggressive phenotypes such as estrogen receptor (ER) negativity, high histological grade, HER2 positivity, as well as poor prognosis [6]. It was reported that stem-like cells isolated from normal mammary gland and breast cancer tissues lack expression of the full-length ER-a [11,12]. Clarke et al reported that ER-a is expressed in putative normal breast stem/progenitor cells enriched by the ‘‘side population’’ method [13]. Despite the fact that ER expression in mammary stem cells is not clear, the significance of estrogen signaling for normal development and growth of the mammary gland is well established by studies in human and animal, which was explained as though indirect paracrine pathways [14,15,16,17]
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