Endocrine therapy resistance can be associated with high estrogen receptor α (ERα) expression and reduced ERα phosphorylation in breast cancer models

Abstract

B Kuske and C Naughton contributed equally to this work. AbstractHormone-dependent estrogen receptor (ER)-positive breast cancer cells may adapt to low estrogen environments such as produced by aromatase inhibitors. In many instances, cells become insensitive to the effects of estrogen but may still retain dependence on ER. We have investigated the expression, function, and activation of ERa in two endocrine-resistant MCF-7 models to identify mechanisms that could contribute to resistance. While MCF-7/LCC1 cells are partially estrogen dependent, MCF-7/LCC9 cells are fully estrogen insensitive and fulvestrant and tamoxifen resistant. In both MCF-7/LCC1 and MCF-7/LCC9 cell lines, high expression of ERa was associated with enhanced binding to the trefoil factor 1 (TFF1) promoter in the absence of estrogen and increased transcription of TFF1 and progesterone receptor. In contrast to the observations derived from hypersensitive and supersensitive models, these cells were truly estrogen independent; nevertheless, removal of ERa by siRNA, or fulvestrant, a specific ER downregulator, inhibited growth indicating dependence on ERa. In the absence of estrogen, neither ERa Ser 118 nor Ser 167 were phosphorylated as frequently found in other ligandindependent cell line models. Addition of estrogen activated ERa Ser 118 in MCF-7 and LCC1 cells but not in LCC9 cells. We suggest that the estrogen-independent growth within these cell lines is accounted for by high levels of ERa expression driving transcription and full estrogen independence explained by lack of ERa activation through Ser 118 .

Selective recruitment of breast cancer anti-estrogen resistance genes and relevance for breast cancer progression and tamoxifen therapy response

“…The probability of recovery of a particular gene under different selection conditions may depend on the integration position modulating its level of expression and on the consequences of the particular anti-estrogen on the biology of the ZR-75-1 cells. Our observations that the mechanisms underlying resistance against diverse anti-estrogens differ are in line with previous observations in breast cancer cell models (Soulez & Parker 2001, Faridi et al 2003, Frasor et al 2004, Martin et al 2005, Fan et al 2006, Kuske et al 2006, Scafoglio et al 2006, Shaw et al 2006, Osipo et al 2007. Accordingly, patients with breast tumors failing on tamoxifen have been shown to respond to the pure anti-estrogen fulvestrant (Howell et al 2002, Osborne et al 2002, Howell 2006, indicating subtle differences in the underlying mechanisms of tumor growth control.…”

Endocrine Therapy of Breast Cancer

“…In this cell line, the negligible changes of ERa Ser 118 phosphorylation obtained on estrogen or tamoxifen addition contrasted with observations in the other cell lines. Markedly reduced phosphorylation is likely to affect cofactor binding and our initial findings suggest that p160 binding (specifically AIB1) is reduced in this cell line, again consistent with endocrine insensitivity (Kuske et al 2004). However, it is quite clear that fulvestrant can downregulate the receptor and even extremely high levels of fulvestrant (10 mM) were unable to influence growth (data not shown).…”

The Emerging Role of the LIV-1 Subfamily of Zinc Transporters in Breast Cancer

“…Cells were harvested, RNA prepared, RT-PCR performed, and DNA for individual genes was amplified by PCR in the presence of actin, to allow normalization of the results. The cell samples were first characterized for pS2 levels, and showed the expected decrease in the FasR cells (45). Representative gels are given for the three LIV-1 family members that appeared most altered, LIV-1, HKE4, and ZIP8.…”