H3K9 組蛋白甲基轉移酶 G9a 促進癌症侵襲及轉移之探討

Abstract

G9a 是一種哺乳動物的組蛋白甲基轉移酶，有助於抑癌基因的非遺傳層次基因靜默。新的證據顯示，G9a 是維持腫瘤惡性型態所必須的，但G9a 對腫瘤轉移功能的調控並未被探討。本研究證實，在高度侵襲能力的肺癌細胞，G9a 的表現有增高的趨勢，並且高表現的G9a 和病人預後差呈現相關性。在高G9a 表現的肺癌細胞株，利用RNA 干擾技術抑制內生性的G9a 表現，在細胞實驗和動物實驗中均觀察到癌細胞移動和侵襲轉移能力受到抑制。另一方面，在G9a 表現低的肺癌細胞株，外送G9a 表現載體使肺癌細胞的G9a 過度表現，亦能促進癌細胞的侵襲和轉移。機制研究顯示，G9a 所造成的效應主要是經由抑制細胞粘附分子 (Ep-CAM)所導致。首先，RNA 干擾所造成的Ep-CAM 抑制，部分回復G9a 抑制所導致的轉移抑制作用。第二、在臨床病人的原位肺癌組織，G9a 和Ep-CAM 的表現呈現負相關性。第三、Ep-CAM 的抑制是與啟動子甲基化和組蛋白H3K9 的二甲基化累積有關。G9a 抑制造成H3K9 甲基化程度的減緩，降低了HP1、DNMT1 和HDAC1的轉錄輔助因子被聚集到的Ep-CAM 的啟動子。我們的研究結果建立一個G9a 過度表現所導致的非基因層次路徑的失調在肺癌的癌症進程中的功能性角色。此外，G9a 可能在癌細胞的移動力、存活及血管新生活性中具有功能。我們的研究結果顯示，發展G9a 抑制劑為治療標的在控制腫瘤生長及轉移的價值。本研究中，我們也建立了篩選G9a 抑制劑的快速篩選平台。

G9a is a mammalian histone methyltransferase that contributes to the epigenetic silencing of tumor suppressor genes. Emerging evidence suggests that G9a is required to maintain the malignant phenotype, but the role of G9a function in mediating tumor metastasis has not been explored. Here, we show that G9a is expressed in aggressive lung cancer cells, and its elevated expression correlates with poor prognosis. RNAi-mediated knockdown of G9a in highly invasive lung cancer cells inhibited cell migration and invasion in vitro and metastasis in vivo. Conversely, ectopic G9a expression in weakly invasive lung cancer cells increased motility and metastasis. Mechanistic investigations suggested that repression of the cell adhesion molecule Ep-CAM mediated the effects of G9a. First, RNAi-mediated knockdown of Ep-CAM partially relieved metastasis suppression imposed by G9a suppression. Second, an inverse correlation between G9a and Ep-CAM expression existed in primary lung cancer. Third, Ep-CAM repression was associated with promoter methylation and an enrichment for dimethylated histone H3K9. G9a knockdown reduced the levels of H3K9 dimethylation and decreased the recruitment of the transcriptional cofactors HP1, DNMT1, and HDAC1 to the Ep-CAM promoter. Our findings establish a functional contribution of G9a overexpression with concomitant dysregulation of epigenetic pathways in lung cancer progression. In addition, G9a may also have functions in cancer cell motility, survival and angiogenic activity. Our results underscore the utility of developing G9a inhibitors as a potentially powerful therapeutic target. We also established the HTS platform for inhibitors against the G9a.