乙醯基轉移酶hNaa10p/hARD1協助DNA甲基轉移酶DNMT1抑制抑癌基因活化之機制探討

Abstract

抑癌基因啟動子的高度甲基化抑制抑癌基因轉錄於腫瘤形成過程扮演重要角色，然而主導基因甲基化的甲基轉移酶活性調控及其結合至目標基因等機制未明。我們發現甲基轉移酶DNA methyltransferase 1 (DNMT1)可與負責催化蛋白質alpha-及epsilon-乙醯化反應的人類蛋白hNaa10p/hARD1產生交互作用。多項證據明確指出hNaa10p/hARD1可透過DNMT1使細胞癌化。我們發現hNaa10p的過量表現與肺癌患者的預後不佳具有正相關性。與此現象相符的是，當細胞大量表現外生性hNaa10p時，細胞出現轉型現象(cell transformation)。利用小片段干擾RNA減少肺癌細胞hNaa10p基因表現，可使癌細胞增殖能力受損，並減少其於小鼠體內形成腫瘤能力。hNaa10p的致癌能力主要與DNMT1的結合相關，而非其造成DNMT1乙醯化所致。我們更發現，hNaa10p可促進DNMT1與其受質DNA結合，並藉此增加DNMT1活性。細胞實驗亦證實hNaa10p可吸引DNMT1與抑癌基因(例如: E-cadherin) 啟動子結合，增加E-cadherin啟動子甲基化，並降低其基因表現。這些現象完全依賴hNaa10p與DNMT1結合。我們更進一步指出，E-cadherin並非唯一受影響基因。根據上述觀察，我們不僅確認hNaa10p為致癌基因，並釐清hNaa10p乃藉由調控DNMT1的功能促進腫瘤形成。

Hypermethylation-mediated tumor suppressor gene silencing plays a crucial role in tumorigenesis. Understanding its underlying mechanism is essential for cancer treatment. Previous studies on the hNaa10p/hARD1 (human arrest-defective 1), a human protein capable of catalyzing both N-alpha- and epsilon-acetylation, have generated conflicting results with regard to its role in tumorigenesis. Here we provide multiple lines of evidence supporting its oncogenic function. We show that hNaa10p overexpression correlates with poor survival of human lung cancer patients. Consistently, enforced expression of hNaa10p is sufficient to cause cellular transformation and siRNA-mediated depletion of hNaa10p impairs cancer cell proliferation in colony assays and xenograft studies. The oncogenic potential of hNaa10p seems to depend on its interaction with the DNA methyltransferase DNMT1, but not the hNaa10p-mediated DNMT1 acetylation. Mechanistically, hNaa10p positively regulates the enzymatic activity of DNMT1 by facilitating its binding to DNA in vitro and its recruitment to promoters of tumor suppressor genes, such as E-cadherin, in vivo. Importantly, interaction between hNaa10p and DNMT1 is required for E-cadherin silencing through promoter CpG methylation and E-cadherin repression contributes to the hNaa10p-mediated oncogenic property. Together our studies not only establish hNaa10p as an oncoprotein, but also reveal that hNaa10p contributes to oncogenesis through modulation of DNMT1 function.