根據衛生署所公布的九八年度國人十大死因，肺癌位居國人癌症死亡率之首，而其中癌細胞的轉移是造成肺癌病患存活率降低的主因，因此針對癌細胞轉移的分子機制之研究，對於肺癌病患的治療上佔有重要的地位。DNA methyltransferase 1(DNMT1) 蛋白質的主要作用是催化DNA進行甲基化修飾，先前研究指出DNMT1蛋白質會在癌細胞中大量表現，並與腫瘤的生成及轉移有著很大的關聯性；在許多癌細胞裡發現，腫瘤抑制基因上的啟動子都具有高度的甲基化修飾，點出了DNMT1的大量表現以及高度酵素活性，可能造成腫瘤抑制基因的不表現，因而促進腫瘤細胞的生成與轉移。在本篇研究中，我們發現轉錄因子Krüppel-like factor 10 (KLF10)會結合dnmt1啟動子上的SP1結合序列(SP1 binding site)，抑制基因的轉錄。當我們利用誘導或轉染的方式，使KLF10蛋白質在非小細胞肺癌細胞中大量表現，會減低細胞中DNMT1的含量，並進一步地抑制癌細胞的增生與爬行。除此之外，我們還發現KLF10會與Specificity protein 1(SP1)競爭以調節DNMT1的表現量；已知SP1在癌細胞中具有高度表現量，並能結合dnmt1啟動子上的SP1結合序列，促進基因的轉錄，從我們的實驗中發現，當減低細胞中SP1的含量時，可以提升KLF10抑制DNMT1表現的能力，因此推測KLF10與SP1會競爭結合dnmt1啟動子上的SP1結合序列，調控dnmt1基因的轉錄。故在本篇研究中，我們發現KLF10會扮演抑制肺癌惡化的角色，並可以藉由調控DNMT1的表現量，來抑制肺癌細胞的生長與轉移，此外，我們也認為這個負向調控機轉可做為日後肺癌治療的參考。

Lung cancer is the leading cancer-related death in Taiwan. The major cause accounting for this severe disease is merely contributed by development of distal metastasis in its disease progression. Hence, it is important to investigate the underlying mechanism(s) of tumor metastasis for the therapeutic purpose of lung cancer. DNA methyltransferase 1 (DNMT1), a member of DNMT family, catalyzes the methylation processes of genomic DNA in somatic cells. Many studies have reported that overexpression of DNMT1 is found in a variety of cancers and associated with their tumorigenesis and tumor metastasis. Its well-known pathological function is involved in silence or attenuation of the transcriptional activation of many tumor suppressor genes (TSGs) by hypermethylation on their promoters. However, it is completely obscure how the DNMT1 gene expression is regulated and what nuclear factors(s) modulates its gene activation. In the Chip-chip analysis and the present study, Krüppel-like factor 10 (KLF10), dominantly expressed in G1 phase, was found to be a putative nuclear factor binding to the Sp1 binding sites on the dnmt1 promoter, and statistically downregulated in lung cancer tissues in immunohistochemistry study. Experiments using transiently or stably modulated the expression status of DNMT1 showed that KLF10 was an upstream negative regulator of its gene activation in non-small cell lung cancer cells. Moreover, suppression of cell proliferation and mobility by KLF10 was primarily mediated by regulation of its downstream target, DNMT1, gene expression. Increased expression of Sp1 markedly upregulated its gene expression. Consistently, attenuation of endogenous Sp1 by its gene-specific siRNA significantly decreased its expression. Interestingly, overexpression of KLF10 could further enhance the downregulated effect on dnmt1 transcriptional activity mediated by attenuation of Sp1, suggesting that KLF10 may regulate the transcription activity of Sp1. Collectively, the present study discloses that KLF10 is a novel negative regulator of the dnmt1 gene, suppresses DNMT1-medaited cell proliferation and cell migration, as well as modulates Sp1-mediated dnmt1 gene activation. Thus, KLF10 can be potential diagnostic and/or prognostic biomarkers for lung cancer.

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