NEW T47D BREAST-CANCER CELL-LINES FOR THE INDEPENDENT STUDY OF PROGESTERONE B-RECEPTORS AND A-RECEPTORS - ONLY ANTIPROGESTIN-OCCUPIED B-RECEPTORS ARE SWITCHED TO TRANSCRIPTIONAL AGONISTS BY CAMP

Because progesterone antagonists are growth inhibitors, they are in Ph ase III clinical trials for the treatment of breast cancer. However, w hen cellular cAMP levels are elevated, some antiprogestins inappropria tely activate transcription. We have proposed that hormone ''resistanc e'' may result from such unintended stimulation of breast cancer by an tagonists. In transient expression systems, the two natural isoforms o f human progesterone receptors (PR), B-receptors and truncated A-recep tors, have dissimilar effects on agonist-mediated transcription. We sh ow here that in the presence of 8-Br-cAMP, antiprogestin-occupied B-re ceptors but not A-receptors become transcriptional activators. Therefo re, we developed new model systems to study each PR isoform independen tly in a breast cancer setting: (a) a stable PR-negative monoclonal su bline (T47D-Y) of PR-positive T47D breast cancer cells was selected by flow cytometric PR screening. T47D-Y cells are PR-negative by immunoa ssays, by ligand binding assay, by growth resistance to progestins, by failure to bind a progesterone response element (PRE) in vitro, and b y failure to transac tivate PRE-regulated promoters; and (b) T47D-Y ce lls were stably transfected with expression vectors encoding one or th e other PR isoform, and two monoclonal cell lines were selected that e xpress either B-receptors (T47D-YB) or A-receptors (T47D-YA) at levels equal to those seen in natural T47D cells. The ectopically expressed receptors are properly phosphorylated, and like endogenously expressed receptors, they undergo ligand-dependent down-regulation. The expecte d B:B or A:A homodimers are present in cell extracts from each cell li ne, but A:B heterodimers are missing in both. In the presence of agoni sts, cAMP-dependent, transcriptional synergism of PRE-regulated promot ers is seen in both cell lines. By contrast, in the presence of the an tiprogestins RU486 or ZK112993, inappropriate transactivation occurs i n YB cells but not in YA cells. The class of antiprogestins represente d by ZK98299, which blocks PR binding to DNA, does not activate transc ription in either cell line. We propose that these new cell lines are physiological models for the study of PR isoform-specific antiprogesti n resistance in breast cancer.