New human breast cancer cells to study progesterone receptor isoform ratio effects and ligand-independent gene regulation.

All known progesterone target cells coexpress two functionally different progesterone receptor (PR) isoforms: 120-kDa B-receptors (PR-B) and N-terminally truncated, 94-kDa A-receptors (PR-A). Their ratio varies in normal and malignant tissues. In human breast cancer cells, homodimers of progesterone-occupied PR-A or PR-B regulate different gene subsets. To study PR homo- and heterodimers, we constructed breast cancer cell lines in which isoform expression is controlled by an inducible system. PR-negative cells or cells that stably express one or the other isoform were used to construct five sets of cells: (i) PR-negative control cells (Y iNull), (ii) inducible PR-A cells (Y iA), (iii) inducible PR-B cells (Y iB), (iv) stable PR-B plus inducible PR-A cells (B iA), and (v) stable PR-A plus inducible PR-B cells (A iB). Expression levels of each isoform and/or the PR-A/PR-B ratios could be tightly controlled by the dose of inducer as demonstrated by immunoblotting and transcription studies. Induced PRs underwent normal progestin-dependent phosphorylation and down-regulation and regulated exogenous promoters as well as endogenous gene expression. Transcription of exogenous promoters was dependent on the PR-A/PR-B ratio, whereas transcription of endogenous genes was more complex. Finally, we have described several genes that are regulated by induced PR-A even in the absence of ligand.