Abstract 4680: H3K9 methyltransferase G9a epigenetically regulates breast cancer cell iron homeostasis via repressing ferroxidase hephaestin expression, promotes breast cancer growth

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Abstract G9a is a H3K9 methyltransferase with elevated expression observed in many types of human cancers, seems to be required for the maintenance of the malignant phenotype. However, the tumorigenic role of G9a in breast cancer is still far from clear. In addition, growing evidence revealed that the disruption of iron homeostasis may contribute to the development of breast cancer (BC). Pathways of iron acquisition, efflux, storage and regulation are all perturbed in BC, suggesting that reprogramming of iron metabolism is a central aspect of tumor cells survival. Hephaestin (HEPH) is a ceruloplasmin homologue that converts iron (II) to iron (III), and mediates iron efflux in concert with ferroportin to transport iron across the basolateral membrane. It retains unclear whether HEPH concentration has any impact on the iron status of breast tissue, and breast cancer growth. In this study, we demonstrate that G9a exerts its oncogenic function by destruction cellular iron homeostasis. We revealed that G9a knockdown, and G9a specific inhibitors significantly up-regulated HEPH expression, decreased intracellular labile iron content, and then suppressed BC cells proliferation both in vitro and in vivo. This idea is further validated by the finding that the enhanced iron content and decreased HEPH expression are required for the increased proliferation of G9a-overexpressed breast cancer cells. Further, our CHIP and co-IP data demonstrate a traditional role of G9a as a transcriptional repressor that co-existed with YY1 and HDAC1, contribute to the reduction of HEPH transcription. In addition, depletion of HEPH with an increase in labile iron reversed cells growth in G9a silencing BC cells. These observations prompted us to investigate the relevance of G9a-HEPH pathway to human disease. The immunohistochemistry analysis of 75 breast cancer specimens revealed inverse staining patterns between G9a and HEPH expression in breast cancer tissues independent of BC type (Spearman's nonparametric correlation test, correlation coefficient: -0.678, p<0.05). Patients who owned a G9ahigh tumor had an even worse prognosis in this retrospective analysis, with a median overall survival of 56.9 months compared to 103.4 months in the G9alow tumors. And we revealed for the first time that low level of HEPH also correlated with overall lower disease-free survival. Finally, expression of high G9a and low HEPH correlated a worse prognosis. In summary, we propose that G9a has an upstream regulatory role in HEPH-mediated cellular iron hemostasis, leads to iron accumulation and stimulates breast cancer progression through its epigenetic silencing machinery. Our observations raise exciting possibilities regarding G9a and HEPH might be potential prognostic markers of breast cancer progression and targets for therapeutic intervention. Citation Format: Ya-fang Wang, Jie Zhang, Yi Su, Yan-yan Shen, Mei-yu Geng, Jian Ding, Yi Chen. H3K9 methyltransferase G9a epigenetically regulates breast cancer cell iron homeostasis via repressing ferroxidase hephaestin expression, promotes breast cancer growth [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4680. doi:10.1158/1538-7445.AM2017-4680