Abstract
Aromatase inhibitors (AIs) effectively treat hormone receptor-positive postmenopausal breast cancer, but some patients do not respond to treatment or experience recurrence. Mechanisms of AI resistance include ligand-independent activation of the estrogen receptor (ER) and signaling via other growth factor receptors; however, these do not account for all forms of resistance. Here we present an alternative mechanism of AI resistance. We ectopically expressed aromatase in MCF-7 cells expressing green fluorescent protein as an index of ER activity. Aromatase-overexpressing MCF-7 cells were cultured in estrogen-depleted medium supplemented with testosterone and the AI, letrozole, to establish letrozole-resistant (LR) cell lines. Compared with parental cells, LR cells had higher mRNA levels of steroid sulfatase (STS), which converts estrone sulfate (E1S) to estrone, and the organic anion transporter peptides (OATPs), which mediate the uptake of E1S into cells. LR cells proliferated more in E1S-supplemented medium than did parental cells, and LR proliferation was effectively inhibited by an STS inhibitor in combination with letrozole and by ER-targeting drugs. Analysis of ER-positive primary breast cancer tissues showed a significant correlation between the increases in the mRNA levels of STS and the OATPs in the LR cell lines, which supports the validity of this AI-resistant model. This is the first study to demonstrate the contribution of STS and OATPs in E1S metabolism to the proliferation of AI-resistant breast cancer cells. We suggest that E1S metabolism represents a new target in AI-resistant breast cancer treatment.
Highlights
Aromatase inhibitors (AIs) block estrogen production from androgens and are routinely administered to postmenopausal women with estrogen receptor (ER)-positive breast cancer
As monitored by green fluorescent protein (GFP) expression, ER activity increased in TS-treated E10arom cells and decreased in cells co-treated with TS and Let compared with ethanol-treated control cells (Fig 1A, fluorescent images)
Via establishment of LR cell lines, we show that E1S contributes to the growth of ER-positive, AI-resistant breast cancer cells
Summary
Aromatase inhibitors (AIs) block estrogen production from androgens and are routinely administered to postmenopausal women with estrogen receptor (ER)-positive breast cancer. AI efficacy was validated by several clinical trials [1,2,3], but some patients do not respond to this treatment and experience recurrence. The mechanisms of AI resistance in ER-positive postmenopausal breast cancer are incompletely identified. Far, they include ligand-independent activation of ERs [4,5,6,7] and signaling via human epidermal growth factor receptor 2 (HER2) [8, 9]. Mechanistic studies have identified mammalian target of rapamycin as a molecular target in AI resistance [10]; an inhibitor of this molecule was developed and a clinical study supports its efficacy [11]. Exploring genes of tissues from neoadjuvant clinical trials and patient-derived xenograft studies suggest that the somatic mutation of multiple genes and ESR1 mutation can induce AI resistance [12, 13]
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