探討組蛋白甲基轉移酶G9a在調控頭頸癌化療藥物抗性之角色

Abstract

頭頸部鱗狀細胞癌是世界上最常見的癌症之一，近年來主要治療方式為手術切除及合併放射線治療和化學治療，但於中晚期病患之治療預後依舊不甚理想。治療過程中癌細胞對於化療藥物產生抗性，進而導致癌症復發及轉移仍然是治療上的最大難題。而抗藥性之產生近年來被確認與癌細胞惡性程度相關。表觀基因修飾異常為癌症發生與惡化之重要機制，其中負責催化組蛋白H3離胺酸9 (H3K9)甲基化之組蛋白甲基轉移酶G9a已在數種癌症中被證實會過量表現，並於促進癌症進程中扮演關鍵角色。因此研究G9a在頭頸部鱗狀細胞癌調控化療藥物抗性之可能角色及機轉將可助於改善化學治療策略並防止腫瘤復發及轉移。此研究中我們於頭頸部鱗狀細胞癌組織中發現，G9a的表現與病患之化療效力及無疾病存活期皆呈現負相關性。並且於體外篩選之具順鉑 (cisplatin)藥物抗性之細胞株中發現G9a的表現量及酵素活性皆有上升現象。利用RNA干擾技術抑制G9a基因表現或以專一性抑制劑抑制其酵素活性皆顯著使藥物抗性細胞對藥物敏感化，並增加藥物誘發之細胞凋亡。進一步發現，此機制係因G9a能藉由轉錄活化谷氨酸-半胱氨酸合成酶催化次體(glutamate-cysteine ligase catalytic subunit)增加細胞中谷胱甘肽(glutathione)含量，而谷胱甘肽已知為代謝順鉑藥物之主要物質，因而促使癌細胞產生藥物抗性。於頭頸部鱗狀細胞癌組織中也證實G9a之表現量和谷氨酸-半胱氨酸合成酶催化次體具顯著正相關性。綜合以上，上述研究結果說明了G9a於頭頸部鱗狀細胞癌中會促使癌細胞對於化療藥物產生抗性，顯示阻斷其相關網絡可做為改善預後之策略，期望此研究發現之G9a於抗藥性角色可進一步提供癌症治療一新指標。

Transient chemotherapeutic response is a major obstacle to treating head and neck squamous cell carcinomas (HNSCC), even though there have been substantial advances in current therapeutic strategies. It is widely recognized that a wide variety of epigenetic changes are prevalent in cancer, including histone modification. Histone methyltransferase G9a has recently been shown to be abundantly expressed in HNSCC, and is required to maintain the malignant phenotype. Thus, to investigate the role of G9a in HNSCC chemoresistance may provide benefit for improving cancer therapeutic strategies. In this study, we found that high G9a expression is significantly associated with poor chemotherapeutic response and disease-free survival in HNSCC patients. Similarly, G9a expression and enzymatic activity were elevated in cisplatin-resistant HNSCC cells. Genetic or pharmacological inhibition of G9a sensitized the resistant cells to cisplatin and increased drug-induced cell apoptosis. Mechanistic investigations indicated that G9a contributes to transcriptional activation of the glutamate-cysteine ligase catalytic subunit (GCLC), which results in upregulation of cellular glutathione (GSH) content and drug resistance. Additionally, we observed a significant positive correlation between G9a and GCLC expression in tumor tissue of HNSCC patients. Taken together, our findings provide evidence that G9a protects HNSCC cells against chemotherapy by increasing the biosynthesis of GSH, and imply G9a as a promising target for overcoming cisplatin resistance in HNSCC.