Characterisation of a tamoxifen-resistant variant of the ZR-75-1 human breast cancer cell line (ZR-75-9a1) and stability of the resistant phenotype

Abstract

Summary A 6-month exposure of ZR-75-1 human breast cancer cells to tamoxifen (1 UM rising to 2 PM), resulted in a fall in oestrogen receptor (ER) levels from 225 fmol mg protein-1 to 56 fmol mg protein-1 while progesterone receptor (PGR) concentration fell from 63 fmol mg protein-1 to undetectable levels. Sensitivity to the anti-proliferative effects of tamoxifen was unchanged. A further 6 months' exposure to 4 MM tamoxifen resulted in loss of detectable ER and PGR and development of resistance to tamoxifen. Resistant al., 1978). A large subset of ER-positive tumours therefore do not respond, and patients may also relapse following an initial response to the anti-oestrogen. Paradoxically, patients resistant to tamoxifen may respond to alternative hormonal therapy and among patients acquiring 'resistance' to tamoxifen some may show a further response on subsequent re-challenge with the drug.The availability of a number of cell lines derived from human breast cancer has greatly aided studies on the mechanism of oestrogen control of breast cancer cell proliferation and its inhibition by anti-oestrogens. The development of anti-oestrogen resistant variants from such lines should provide further insight into the mechanism of action of and development of resistance to anti-oestrogens. The majority of studies to date have shown that resistance is not associated with the acquisition of an ER-negative phenotype. Thus both the R3 (Nawata et al., 1981b) and R27 (Nawata et al., 1981a) tamoxifen-resistant sub-lines derived from MCF-7 retain ER, although they differ in a number of responses to.oestrogen stimulation. Neither of these lines, however, maintains the resistant phenotype in the absence of continuing tamoxifen exposure (Bronzert et al., 1986). LY2, an MCF-7 variant, was selected for its ability to grow in the presence of the potent anti-oestrogen LY 117018, and is cross-resistant to tamoxifen. This line also expresses ER, although to a lesser extent than MCF-7, and seems to be stably resistant to anti-oestrogens (Bronzert et al., 1985). A tamoxifen and oestrogen insensitive variant of the T47D line, lacking ER but expressing high levels of progesterone receptor (PGR), has also been described but this line arose as a result of changes in culture conditions rather than under selective pressure of anti-oestrogen exposure (Horwitz et al., 1982).In this paper we describe what is, to our knowledge, the first tamoxifen-resistant variant of the ZR-75-1 human breast cancer cell line. This variant arose during prolonged Correspondence: H.W. van den Berg. Received 22 September 1988, and in revised form, 22 October 1988. exposure of the parent line to increasing concentrations of tamoxifen and appears to represent a novel resistant phenotype having lost both ER and PGR. We also describe ultrastructural characteristics of this variant and report on phenotypic stability in the absence of continuing selective pressure. Materials and methodsCell culture and development of the ZR-75-9a1 subline The ZR-75-1 human breast cancer ...

Expression of receptors for epidermal growth factor and insulin-like growth factor I by ZR-75-1 human breast cancer cell variants is inversely related: the effect of steroid hormones on insulin-like growth factor I receptor expression

“…As expected, oestradiol was without effect in the ZR-75-9al line, which lacks oestrogen receptors (Van den Berg et al, 1989). Although the mechanism is unknown, the observation that low concentrations of oestradiol reduces ['25I]IGF-I binding by ZR-PR-LT cells would be consistent with our earlier findings that oestradiol inhibits ZR-PR-LT cell proliferation (Van den Berg et al, 1990).…”

“…We have previously shown that acquired tamoxifen resistance accompanied by loss of detectable ERs and progesterone receptors (PGRs) in the ZR-75-1 human breast cancer cell line (Van den Berg et al, 1989) is associated with an increase in EGFR expression (Long et al, 1992), in agreement with clinical findings (Harris, 1989). Conversely, an oestrogen-independent variant of the same cell line, which constitutively expresses high numbers of PGRs (Van den Berg et al, 1990), has a much reduced EGFR content (Long et al, 1992).…”

Changes in epidermal growth factor receptor expression and response to ligand associated with acquired tamoxifen resistance or oestrogen independence in the ZR-75-1 human breast cancer cell line

“…If the tamoxifen resistant line is transferred to drug and phenol red free medium supplemented with 5% FCSdcc the ER and PGR negative phenotype is maintained for at least 12 weeks (van den Berg et al, 1989). Table III shows that under these culture conditions EGFR receptor expression over a 4 week period remained elevated in comparison to parent line values.…”

Modification of gene expression induced by siRNA targeting of estrogen receptor α in MCF7 human breast cancer cells

“…Restoration of tamoxifen sensitivity was achieved in the ER-negative MDAMB231 cells with demethylating agents (24) although ectopic expression of ER failed to achieve this (25). ER positivity alone could be modulated in ZR-75 derived 9a1 cells by addition and removal of tamoxifen (26). Several contrasting models of fulvestrant resistant cells have been reported that show: i) complete ER protein loss (27); ii) 90% decrease in ER mRNA and protein accompanied by functional changes in ER target genes (28); iii) 30% reduction of ER and maintenance of its function (29); and iv) additional cross-resistance to tamoxifen but with elevated levels of ER compared to the sensitive parent line (30).…”