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.