ck2-Dependent Phosphorylation of Progesterone Receptors (PR) on Ser81 Regulates PR-B Isoform-Specific Target Gene Expression in Breast Cancer Cells

Abstract

Progesterone receptors (PR) are critical mediators of mammary gland development and contribute to breast cancer progression. Progestin-induced rapid activation of cytoplasmic protein kinases leads to selective regulation of growth-promoting genes by phospho-PR species. Herein, we show that phosphorylation of PR Ser81 is ck2 dependent and progestin regulated in intact cells but also occurs in the absence of PR ligands when cells enter the G₁/S phase of the cell cycle. T47D breast cancer cells stably expressing a PR-B mutant receptor that cannot be phosphorylated at Ser79/81 (S79/81A) formed fewer soft agar colonies. Regulation of selected genes by PR-B, but not PR-A, also required Ser79/81 phosphorylation for basal and/or progestin-regulated (BIRC3, HSD11β2, and HbEGF) expression. Additionally, wild-type (wt) PR-B, but not S79/81A mutant PR, was robustly recruited to a progesterone response element (PRE)-containing transcriptional enhancer region of BIRC3; abundant ck2 also associated with this region in cells expressing wt but not S79/81A PR. We conclude that phospho-Ser81 PR provides a platform for ck2 recruitment and regulation of selected PR-B target genes. Understanding how ligand-independent PRs function in the context of high levels of kinase activities characteristic of breast cancer is critical to understanding the basis of tumor-specific changes in gene expression and will speed the development of highly selective treatments.

ACKNOWLEDGMENTS

We thank Khalil Ahmed (University of Minnesota) for providing test aliquots of ck2 inhibitors. We thank Andrea R. Daniel (Lange lab) for helpful comments on the manuscript and Todd Knutson (Lange lab) for supplying cDNA isolated from the inducible control and PR-B isogenic cell system.

This work was supported by NIH/NCI grant number R01 CA123763 (C.A.L.), Department of Defense Post-Doctoral Fellowship number USDOD ARMY/W81XWH-09-1-0639 PK0001 (C.R.H.), and NIH Institutional Training Grant number T32 CA009138 (C.R.H. and G.E.D.).