Abstract 2739: EZH2 promotes breast cancer bone metastasis

Abstract

Abstract Metastasis causes about 90% of cancer deaths and approximately 55% of breast cancer patients develop bone metastases. Although significant advances in the cure of breast cancer, effective therapies targeted bone metastases are still not available. To develop novel therapies for patients with bone metastasis, better understanding of underlying mechanisms of bone metastasis is critical. EZH2 is a classic histone methyl transferase that generally suppresses downstream target genes through tri-methylation of H3K27. EZH2 is considered as a breast cancer oncogene and is frequently over expressed in breast cancer, but the effect of EZH2 on bone metastasis is unknown. To explore the function of EZH2 in bone metastasis, we intracardially injected into nude mice control MDA.MB.231 breast cancer cells expressing readily detectable EZH2 or a MDA.MB.231 subline having EZH2 knocked out (231.EZH2.ko). We found that the mice injected with 231.EZH2.ko had prolonged bone metastasis-free survival compared to the control group of mice. To gain initial insight on the underlying mechanism of EZH2-mediated bone metastasis, we co-cultured MDA.MB.231 cells or 231.EZH2.ko with pre-osteoclasts (RAW264.7) and osteoblasts (MC3T3) under TGF-β treatment (triple co-culture) to mimic bone metastasis tumor microenvironment. We examined osteoclast maturation and breast cancer cell proliferation because osteolysis induced by mature osteoclasts is critical for breast cancer cell growth in the bone metastasis. In triple co-culture of MDA.MB.231 cells, more RAW264.7 pre-osteoclasts were differentiated to mature osteoclasts, as measured by TRAP+ staining, a marker of mature osteoclasts, and MDA.MB.231 cells grew faster than the triple co-culture of 231.EZH2.ko cells. Furthermore, we found that the mRNA level of PTHrP, a master regulator of bone metastasis and a TGF-β downstream cytokine, was expressed at significantly higher level in MDA.MB.231 cells than that in 231.EZH2.ko cells under TGF-β stimulation. PTHrP was known to promote the “vicious cycle” of bone destruction in bone metastasis. The data suggests EZH2 promotes breast cancer bone metastasis through PTHrP. Currently, we are further exploring how EZH2 regulates PTHrP in breast cancer cells to promote bone metastasis. Our studies may reveal novel biomarkers and/or therapeutic targets for bone metastasis. Citation Format: Jingkun Qu, Lin Zhang, Zhifen Zhou, Dihua Yu. EZH2 promotes breast cancer bone metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2739.