Edwards Courtney M, Clements Miranda E, Vecchi Lawrence A, Johnson Jasmine A, Johnson Rachelle W
Graduate Program in Cancer Biology, Vanderbilt University, Nashville, TN, USA.
Vanderbilt Center for Bone Biology, Vanderbilt University Medical Center, Nashville, TN, USA.
J Bone Oncol. 2021 Dec 1;31:100407. doi: 10.1016/j.jbo.2021.100407. eCollection 2021 Dec.
Breast cancer cells frequently disseminate to the bone marrow, where they either induce osteolysis or enter a dormant state. Downregulation of leukemia inhibitory factor receptor (LIFR), a known breast tumor suppressor, enables otherwise dormant MCF7 human breast cancer cells to become aggressively osteolytic. Hypoxia (low oxygen tensions), which may develop in tumors as a pathological response to the metabolic demands of the proliferating cells and as a physiological state in the bone, downregulates LIFR in breast cancer cells independent of hypoxia-inducible factor (HIF) signaling. However, the mechanism by which LIFR is repressed in hypoxia is unknown. Histone deacetylase (HDAC) inhibitors stimulate LIFR by increasing histone acetylation in the proximal promoter and induce a dormancy phenotype in breast cancer cells inoculated into the mammary fat pad. We therefore aimed to determine whether hypoxia alters histone acetylation in the LIFR promoter, and whether HDAC inhibitors effectively stimulate LIFR in breast cancer cells residing in hypoxic microenvironments. Herein, we confirmed that disseminated MCF7 cells became hypoxic in the bone and that hypoxia increased the epigenetic transcriptional repressor H3K9me3 in the distal LIFR promoter while H3K9ac, which promotes transcription, was significantly reduced. Furthermore, HDAC inhibitor treatment rescued hypoxic repression and dramatically increased expression of LIFR, p38β, and p21, which regulate tumor dormancy. In a second model of LIFR repression, in which parathyroid hormone-related protein (PTHrP) suppresses LIFR expression, we found that PTHrP binds to the distal LIFR promoter, and that PTHrP suppression of LIFR protein is similarly reversed by HDAC inhibitor treatment. Together, these data suggest that HDAC inhibitors stimulate LIFR regardless of the way it is repressed by the microenvironment.
乳腺癌细胞常常扩散至骨髓，在那里它们要么诱发骨质溶解，要么进入休眠状态。白血病抑制因子受体（LIFR）是一种已知的乳腺肿瘤抑制因子，其下调会使原本处于休眠状态的MCF7人乳腺癌细胞变得具有强烈的溶骨性。缺氧（低氧张力）可能作为对增殖细胞代谢需求的病理反应在肿瘤中出现，也可能作为骨骼中的一种生理状态，它不依赖缺氧诱导因子（HIF）信号通路而使乳腺癌细胞中的LIFR下调。然而，缺氧时LIFR被抑制的机制尚不清楚。组蛋白脱乙酰酶（HDAC）抑制剂通过增加近端启动子中的组蛋白乙酰化来刺激LIFR，并在接种到乳腺脂肪垫的乳腺癌细胞中诱导出休眠表型。因此，我们旨在确定缺氧是否会改变LIFR启动子中的组蛋白乙酰化，以及HDAC抑制剂是否能有效刺激处于缺氧微环境中的乳腺癌细胞中的LIFR。在此，我们证实扩散至骨髓的MCF7细胞出现缺氧，且缺氧会增加LIFR远端启动子中表观遗传转录抑制因子H3K9me3，而促进转录的H3K9ac则显著减少。此外，HDAC抑制剂处理可挽救缺氧抑制作用，并显著增加调节肿瘤休眠的LIFR、p38β和p21的表达。在LIFR抑制的第二种模型中，甲状旁腺激素相关蛋白（PTHrP）抑制LIFR表达，我们发现PTHrP与LIFR远端启动子结合，并且HDAC抑制剂处理同样能逆转PTHrP对LIFR蛋白的抑制作用。总之，这些数据表明，无论LIFR被微环境抑制的方式如何，HDAC抑制剂均可刺激LIFR。
