ESTROGEN-RECEPTOR EXPRESSION AND FUNCTION IN LONG-TERM ESTROGEN-DEPRIVED HUMAN BREAST-CANCER CELLS

Hormone-dependent breast cancer responds to primary therapies that blo ck estrogen production or action, but tumor regrowth often occurs 12-1 8 months later. Additional hormonal treatments that further reduce est rogen synthesis or more effectively block its action cause additional remissions, but the mechanisms responsible for these secondary respons es are not well understood. As a working hypothesis, we postulated tha t primary hormonal therapy induces adaptive changes, resulting in enha nced estrogen receptor (ER) expression and target gene activation and, further, that secondary treatment modalities interfere with these rec eptor-mediated transcriptional pathways. To test this hypothesis, we u sed an MCF-7 breast cancer model system involving deprivation of estra diol in culture for a prolonged period. These long-term estradiol-depr ived (LTED) cells adapt by acquiring the ability to regrow in the abse nce of added estradiol. The experimental paradigm involved the compari son of wild-type cells with LTED cells. As endpoints, we directly asse ssed ER expression at the messenger RNA-, protein-, and ligand-binding levels and ER functionality by quantitating reporter gene activation and expression of endogenous estrogen target gene messenger RNA, as we ll as ER coactivator levels. Our data demonstrated an adaptive increas e in ER expression and in basal ER functionality, as assessed by read- out of three different transfected reporters in LTED, as opposed to wi ld-type MCF-7 cells. Increased reporter gene read-out was dramatically inhibited by the pure antiestrogen ICI 182,780. As verification that endogenous las well as transfected) estrogen target genes had enhanced transcription, we found that the basal levels of c-myb and c-myc mess age were substantially increased in LTED cells and could be inhibited by antiestrogen. Interestingly, the levels of c-myb and c-myc message in the LTED cells seemed to be increased out of proportion to the degr ee of ER reporter gene activation and were similar to these in wild-ty pe cells maximally stimulated with estradiol. In addition, not all est rogen-responsive genes were activated, because transforming growth fac tor-a: message level was not increased in LTED cells. Up-regulation of the steroid receptor coactivator SRC-1 did not seem to mediate the pr ocess of enhanced ER-induced transcription. Considering these observat ions together, we suggest that long-term estradiol deprivation causes adaptive processes that not only involve up-regulation of the ER but a lso influence the specificity and magnitude of activation of estrogen- responsive genes.