Progesterone-Independent Effects of Human Progesterone Receptors (PRs) in Estrogen Receptor-Positive Breast Cancer: PR Isoform-Specific Gene Regulation and Tumor Biology

Abstract

Progesterone receptors (PRs) are prognostic markers in breast cancers irrespective of the patient's progestational status. However, there are two PR isoforms, PR-A and PR-B, that are equimolar in the normal breast but dysregulated in advanced disease. Postmenopausal, tamoxifen-treated patients with estrogen receptor (ER)-positive, PR-A-rich tumors have much faster disease recurrence than patients with PR-B-rich tumors. To study the mechanisms we engineered ER+ breast cancer cells that express each PR isoform under control of an inducible promoter. We identified 79 genes regulated by progesterone (P), mainly by PR-B, and 51 genes regulated without progesterone, mainly by PR-A. Only nine genes were regulated with and without ligand, leading to definition of three classes: I) genes regulated only by liganded PR; II) genes regulated only by unliganded PR; III) genes regulated by both. Unliganded PR-A and PR-B differentially regulate genes that coordinate extracellular signaling pathways and influence tumor cell biology. Indeed, in the absence of P, compared with ER+/PR-B+ or PR- cells, ER+, PR-A+ cells exhibit an aggressive phenotype, are more adhesive to an extracellular matrix, and are more migratory. Additionally, unliganded PR-A and PR-B both inhibit cell growth and provoke resistance to Taxol-induced apoptosis. We propose that PR-A:PR-B ratios, even in the absence of P, influence the biology and treatment response of ER+ tumors, that PR-A isoforms are functionally dominant in P-deficient states, and that PR-A rich tumors are especially aggressive.

Linkage of progestin and epidermal growth factor signaling: Phosphorylation of progesterone receptors mediates transcriptional hypersensitivity and increased ligand-independent breast cancer cell growth

“…The plating efficiency of S294A PR-B cell clones is roughly 25-30% lower than that of wt PR-B clones. This may indicate that PR Ser294 is important for the targeting of PRs to a subset of genes involved in cell shape and/or adhesion (36,37). T47D cells stably expressing S294D phospho-mimic PR appear morphologically comparable to cells expressing wt PR-B, and exhibit an equivalent plating efficiency (data not show).…”

“…Such studies have been limited by our incomplete knowledge of PR/progestin-regulated gene expression, especially for genes whose promoters lack consensus PRE sequences (19,36). A surprising diversity of genes whose promoters lack clear PRE sequences are differentially regulated by PR-A and PR-B isoforms (36), including a large number of genes regulated by unliganded PR-A or PR-B, but not their liganded derivatives (37). Notably, PR-A is not phosphorylated on Ser294 in vivo (57), suggesting that direct phosphorylation of PRs may contribute to differences in PR isoform function.…”

“…However, nearly half of all genes identified using gene-array approaches encode cell adhesion and membranebound proteins or proteins involved in membrane-initiated signaling (36,37). PR is known to upregulate c-myc, STAT5A, cyclin D1, TGF-α, and EGFR mRNAs (39), and progestins synergize with EGF to induce increased expression of c-myc, c-fos, p21 and cyclins D1 and E (58).…”

“…However, Jacobsen et. al (37) reported at least 51 genes regulated by unliganded PRs, only 9 of which were also regulated by liganded PRs. Further, a large subset of the genes regulated by unliganded PRs were involved in membrane-initiated signaling events or coordination of extracellular signaling, and known to influence tumor cell biology (36,37).…”

“…al (37) reported at least 51 genes regulated by unliganded PRs, only 9 of which were also regulated by liganded PRs. Further, a large subset of the genes regulated by unliganded PRs were involved in membrane-initiated signaling events or coordination of extracellular signaling, and known to influence tumor cell biology (36,37). For example, basal expression of IRS-1 in T47D cells is insensitive to progestins, but requires Ser294 phosphorylation of unliganded PR-B, but not PR-A (19).…”