EZH2 IS A KEY PLAYER IN PROGESTIN-INDUCED BREAST CANCER GROWTH AND TAMOXIFEN RESISTANCE

CENCIARINI ME, GOMEZ POVIÑA JC, MERCOGLIANO MF, DI GIORGIO NP, SCHILLACI R, ELIZALDE PV, IZZO F, PROIETTI CJ

Congreso; 98th Annual Meeting of the Endocrine Society; 2016

Abstract: More than 70% of breast cancers (BC) expressestrogen and progesterone receptors (ER and PR, respectively), and patientspositive for ER expression are treated with tamoxifen (TAM) as endocrinetherapy. Nevertheless, more than one third of patients are not responsive totherapy. Even though PR involvement in BC progression has been wellacknowledged, the molecular mechanisms involved remain unclear. We haverecently described that the synthetic progestin medroxiprogesterone acetate(MPA) induced the interaction between PR and the epigenetic regulator Enhancerof Zeste Homolog 2 (EZH2), which catalyzes the trimethylation of lysine 27 ofhistone H3 (H3K27me3), an epigenetic mark associated with transcriptionalrepression. This results in the downregulation of tumor suppressor GATA3 andthe increase in cell proliferation of the human breast cancer cell line T47D(1). Since EZH2 has been implicated in the progression of several types ofcancer, including those of the breast, and our own previous results indicate afunctional relationship between PR and EZH2 in BC cells, we hypothesized thatEZH2 could function as a mediator in the pro-tumorigenic effects of progestins,targeting specific tumor suppressor and differentiating genes to allowER/PR-positive BC growth. We inoculated a progesterone pellet in ovariectomizedBALB/c mice and observed an increase in EZH2 expression, in H3K27me3 globallevels, and in the proliferation marker Ki67, and a decrease in GATA3 mRNAlevels in the mammary gland. In addition, we found that EZH2 expression ishigher in the progestin-induced C4HD murine breast tumor than in the normalmammary gland. A meta-analysis of available datasets of breast cancer patientsusing the software Oncomine showed that EZH2 is overexpressed in human ductalbreast cancer when compared to normal tissue. In order to assess the importanceof EZH2 in breast cancer growth, we transfected T47D cells with EZH2 siRNAs andfound that MPA-induced in vitro cell proliferation and GATA3transcriptional repression were prevented. These results confirm EZH2requirement for progestin-induced proliferative effects. On the other hand, wefound that MPA-pretreatment induced TAM resistance in TAM-sensible T47D cells,and that EZH2 silencing averted this resistance switch, suggesting a linkbetween TAM resistance and PR activation through EZH2-mediated epigeneticreprogramming. Furthermore, by performing in silico analysis through thepublicly available software KM-plotter, we found that increased EZH2 expressionin ER/PR-positive BC patients correlates with reduced response to endocrinetherapy. Taken together, these results show a key role of EZH2 inprogestin-induced breast cancer and provide a novel therapeutic target forTAM-resistant breast cancer.