Abstract 5280: O-GlcNAcylation of progesterone receptor promotes mammary tumorigenesis

The role of hormone receptors in breast cancer has been well established as a key contributor to breast cancer tumorigenesis. Estrogen receptor and progesterone receptors are present in nearly 70% of breast cancers. The role of estrogen receptor and estrogens in breast cancer has been extensively studied and targeted effectively through endocrine therapy, however the impact of progesterone and the progesterone receptor (PR) independent of estrogen is not well understood. We have previously demonstrated that PR promotes tumor growth and drives an immune suppressive environment, therefore understanding the biology of PR in breast cancer is critical. We have also demonstrated that PR attenuates type 1 interferon signaling via inhibition of STAT1 phosphorylation and increased degradation of STAT2. Post-translational modifications such as phosphorylation and SUMOylation can influence PR target gene promoter selectivity thereby altering its function. Our recent work indicates PR is modified by O-GlcNAc, a single N-acetyl-glucosamine sugar that cycles on and off and serine or threonine amino acids in nuclear, cytoplasmic and mitochondrial proteins. Levels of total O-GlcNAc staining are higher in breast cancer tissue compared to adjacent normal tissue. Active O-GlcNAcylation, as evidenced by immunohistochemistry staining of O-GlcNAc-transferase (OGT), is elevated in patients with PR+ tumors compared to PR-. Using T47D breast cancer cells (an ER/PR-positive tumor line), mass spectrometry analysis revealed an O-GlcNAc site at S499 on PR. We then used a naturally occurring PR-negative variant of T47D cells to introduce stable expression of a mutant PR with serine to alanine substitution at the O-GlcNAc site (S499A), thereby blocking O-GlcNAc flux, in addition to wt PR as a control. RNA-Seq analysis of these cells following treatment with progesterone revealed several significant differential gene expression patterns, including rescue of PR dependent interferon signaling attenuation. T47D-mutant PR cell lines implanted into immune deficient mice had significantly decreased growth as compared to WT-PR control tumors. In syngeneic mice harboring PR-positive and PR-negative tumors, loss of O-GlcNAc flux and increased O-GlcNac levels increased tumor growth only when tumors were expressing PR, corroborating the influence of O-GlcNAc in immunodeficient mice. Together these findings indicate O-GlcNAcylation of PR is potentially a mechanism of PR driven breast cancer tumorige