The inter-relationships between ovarian-independent growth, tumorigenicity, invasiveness and antioestrogen resistance in the malignant progression of human breast cancer

Abstract

Among the processes contributing to the progressive acquisition of the highly malignant phenotype in breast cancer are ovarian-independent growth, antioestrogen resistance and increased metastatic potential. We have previously observed that increased invasiveness and development of ovarian-independent growth occur independently. In an attempt to define the inter-relationships between these processes further, we have compared the phenotypes of ovarian-independent, invasive and antioestrogen-resistant sublines of the ovarian-dependent human breast cancer cell line MCF-7. Cells acquiring ovarian-independent growth can retain sensitivity to anti-oestrogens. One clone of MCF-7 cells selected for stable antioestrogen resistance has become non-tumorigenic but its invasive potential remains unaltered. Thus, acquisition of some characteristics of the progressed phenotype can occur independently. This phenomenon of independent parameters in phenotypic progression could partly explain the considerable intra- and intertumour heterogeneity characteristic of breast tumours.

Molecular Mechanisms of Estrogen and Antiestrogen Resistance

“…The MCF-7 variant LY-2 has become nontumorigenic. 62 MCF-7 cells selected in vivo can become dependent on or stimulated by tamoxifen.…”

“…62 In addition to exhibiting a significant shift in their dose-response relationship for antiestrogens, LY-2 cells exhibit a blunted mitogenic response to E2.…”

“…124 However, LY-2 cells have lost their ability to form proliferating tumors in oophorectomized or E2-supplemented nude mice, 62 limiting their use to in vitro studies.…”

Molecular Mechanisms of Estrogen and Antiestrogen Resistance

“…Matrigel has been used in two different assays to examine in vitro invasiveness of breast cancer cells. 208 The Boyden chamber chemoinvasion assay 209 compares the ability of cells to traverse a Matrigel-coated polycarbonate filter as they migrate toward different chemical attrac- 61 This increased metastatic potential is reflected in increased activity in the Boyden chamber 62 (but not in Matrigel outgrowth assays), increased local invasiveness in vivo, and an ability to produce occasional distant metastases in nude mice. 61 To examine the hypothesis that ER-human breast cancer cell lines are constitutively more invasive than their ER+ counterparts, a large number of the human breast cancer cell lines described above have been examined for invasiveness in the Boyden chamber assay in vitro and for metastatic potential in the nude mouse.…”

“…We routinely wash cells at least three times with phenol red-free media supplemented • with 5% dextran/charcoal-stripped serum and maintain the washed monolayers in this medium for a further 3 to 5 days to ensure adequate removal of endogenous steroids. 14,62 The cell-seeding density also can have considerable effects on cellular growth and metabolism. Cells seeded at different densities have previously been demonstrated to exhibit both different cell population doubling times and differences in methotrexate poly-y-glutamate formation."…”