Protein kinases mediate ligand-independent derepression of sumoylated progesterone receptors in breast cancer cells.

In advanced breast tumors, protein kinases are upregulated and steroid hormone receptors often function independently of ligand. Herein, we explored mechanisms of ligand-independent progesterone receptor (PR) activity. We showed previously that growth factor-induced phosphorylation of PR Ser-294 blocks PR Lys-388 sumoylation. SUMO-deficient mutant PR-B (K388R) thus provides a model receptor for the study of PR function in the context of high kinase activities. T47D cells stably expressing K388R PR-B exhibited increased ligand-independent proliferation and growth in soft agar relative to cells expressing wt PR-B or phospho-mutant (sumoylated) S294A PR-B. Expression of selected PR target genes (HB-EGF, IRS-1, and STC1) was significantly elevated in cells containing desumoylated (K388R) PR-B. Basal PR transcriptional activity occurred independently of progestins, was increased by activated CDK2, and attenuated by RU486. Notably, ChIP assays demonstrated that K388R PR-B and SRC1 were constitutively recruited to the STC1 promoter in the absence of progestin; PR Lys-388 sumoylation was required for HDAC3 recruitment. Knock-down of STC1 inhibited proliferation of cells expressing K388R PR-B. These data suggest a mechanism whereby phosphorylated, and thus desumoylated, PRs mediate increased expression of growth promoting genes. Our data explain why breast cancer models often remain insensitive to progestins, but are growth-inhibited by antiprogestins, and underscore the need to target PR-B and associated kinase activities as part of breast cancer therapy.