人類肺癌抑癌基因之探討

Abstract

肺癌高居世界癌症相關死因的首位，每年有超過150萬人死於肺癌。罹患肺癌的病人預後情況相當不理想，五年存活率僅約15%。據研究指出在肺癌中表皮生長因子受體 (epidermal growth factor receptor；EGFR) 常受到不正常的調控，如此異常表現將導致訊息傳遞改變造成肺癌病人不良的預後。之前的研究發現一轉錄因子LCTS1 (Lung Cancer Tumor Suppressor 1) 可作為一個腫瘤抑制基因。然而，目前尚未出現此因子對於人類肺癌細胞抑癌作用的研究，因此本論文以人類肺癌細胞株為研究模式，探討此轉錄因子是否對於人類肺癌細胞株具有抑癌作用，而其抑癌作用是否透過影響EGFR訊息傳遞路徑進行調控。我們首先建立可被四環黴素 (doxycycline) 誘導表現此轉錄因子的各種肺癌細胞，探討它的表現對肺癌細胞的生長與基因表現的影響。我們發現在肺癌細胞中誘導此轉錄因子表現，會使細胞群落形成的能力顯著減弱，同時伴隨著EGFR蛋白質的明顯下降。接著，我們釐清EGFR蛋白質表現量下降的原因，發現高度表現此轉錄因子對EGFR的訊息傳遞核糖核酸 (mRNA) 表現量影響的比例不高，且由報導基因分析技術 (reporter gene analysis) 中，發現此轉錄因子並不影響EGFR的轉錄作用。接著，我們進一步證明此轉錄因子影響EGFR蛋白的穩定度，高度表現此轉錄因子後，肺癌細胞EGFR較不穩定，容易被降解。而EGFR下游訊號傳遞路徑，磷酸化Akt的表現量也明顯地減少。綜合以上結果，我們認為此轉錄因子在肺癌中能當作一種潛在性的腫瘤抑制基因，然而其詳細的功能與機制仍需日後進一步的探討。

Lung cancer, the leading cause of cancer-related deaths worldwide, is responsible for more than 1.5 million deaths annually. The overall 5-year survival rate for patients with lung cancer is less than 15%. It has been reported that epidermal growth factor receptor (EGFR) is usually abnormal in lung cancer and its expression is correlated with poor prognosis. Previously, we identified LCTS1 (Lung Cancer Tumor Suppressor 1) as a candidate gene associated with disease progress of lung cancer. However, the precise role and molecular mechanisms of the factor in lung cancer remain unclear. Therefore, the objective of this study is to identify the functional role and potential contribution of the LCTS1 in lung cancer. We overexpressed the LCTS1 in several lung cancer cell lines under the control of doxycycline and found that overexpression of the protein in lung cancer cell lines dramatically decreased the ability of colony formation as well as the EGFR protein level. In addition, we showed that down-regulation of EGFR protein level was not due to the change of EGFR transcriptional ability but resulted from the instability of EGFR protein. Our results suggested that the LCTS1 may function as a tumor suppressor in lung cancer. However, the detailed mechanisms and functions of the LCTS1 need to be further elucidated in the future.