Gefitinib誘發細胞凋亡經由表皮生長因子接受器非依賴性路徑的分子機制

Abstract

表皮生長因子接受器屬於酪胺酸激酶受體的ErbB/HER家族之成員，活化後會促進腫瘤的生長。Gefitinib (Iressa™, ZD1839)是一種小分子的酪胺酸激酶抑制劑，已經被證實具有抗癌的活性，主要經由標靶到表皮生長因子接受器的催化部位與ATP去競爭鍵結位置。然而，gefitinib在癌細胞中誘發表皮生長因子接受器非依賴性的細胞凋亡機制仍然不清楚。在本篇研究中，我們主要探討由gefitinib誘發的表皮生長因子接受器非依賴性之癌細胞凋亡訊息傳遞路徑。Gefitinib會在RKO (大腸癌)、A549 (肺癌)、BFTC905 (膀胱癌)、MCF7 (乳癌)及A375 (皮膚癌)細胞中造成細胞毒性，以10-60 μM gefitinib處理24小時後，細胞毒性的敏感度依序為A375 > MCF7 > BFTC905 > A549 > RKO細胞。有趣地，A375、MCF7 以及RKO細胞表現非常低量的磷酸化表皮生長因子接受器與總量表皮生長因子接受器蛋白；相反地，BFTC905與A549細胞則高度的表現。再者，A375與BFTC905細胞在gefitinib處理後都能誘發細胞凋亡的產生。Securin又稱作腦下垂體腫瘤轉形基因，大量地表現在許多的癌症，可以促進腫瘤的形成。我們觀察到癌細胞處理gefitinib後，明顯降低securin蛋白的表達。進一步研究發現，缺乏securin的細胞會比功能正常的細胞對gefitinib誘發的細胞毒性更為敏感，相反地若轉殖大量表現securin的質體到癌細胞，則對gefitinib誘發的細胞毒性產生抗性。ATF3是一種轉錄因子，可藉由調控細胞存活與死亡的訊息平衡影響癌症的形成。在我們的研究中發現，gefitinib處理後可以誘發ATF3蛋白的表達與轉移到細胞核。綜合以上結果，我們推測gefitinib所誘發的癌細胞凋亡可透過表皮生長因子接受器非依賴性的途徑，而釐清securin與ATF3在gefitinib誘發的癌細胞死亡訊息的調控機轉，可提供未來癌症治療的新策略。

The epidermal growth factor receptor (EGFR) belongs to the ErbB/HER family of tyrosine kinase receptors. The activation of EGFR is important for promoting tumor growth. Gefitinib (Iressa™, ZD1839), a small molecule tyrosine kinase inhibitor, has been demonstrated the promising antitumor activity. It targets the catalytic domain of EGFR to compete with the ATP binding site. However, the precise EGFR-independent apoptotic mechanisms by gefitinib remain incompletely clear. In this study, we investigated the effects of gefitinib on the EGFR-independent cell death signaling pathways in human cancer cells. Direct cytotoxicity was observed in RKO (colon cancer), A549 (lung cancer), BFTC905 (bladder cancer), MCF7 (breast cancer) and A375 (skin cancer) cells by gefitinib. The order of cytotoxic sensitivity was A375 > MCF7 > BFTC905 > A549 > RKO cells following treatment with 10-60 μM gefitinib for 24 h. Interestingly, A375, MCF7 and RKO cells expressed very low protein level of the phosphorylated-EGFR and total EGFR; in contrast, BFTC905 and A549 cells expressed the high level. Moreover, treatment with gefitinib induced apoptosis in A375 and BFTC905 cells. Securin, also known as pituitary tumor-transforming gene (PTTG), overexpresses in a variety of tumors and promotes tumorigenesis. We also observed that gefitinib inhibited the securin protein expression in cancer cells. Furthermore, loss of securin enhanced the gefitinib-induced cell death; conversely, an overexpression of securin resisted the gefitinib-induced cell death. Activating transcription factor 3 (ATF3), a transcription factor, may regulate the delicate balance between proliferative and apoptotic signals that control the development of cancer. We found that gefitinib induced ATF3 protein expression and translocated to nuclei. As a whole, we suggest that gefitinib can induce apoptosis that may be through the EGFR-independent pathways in cancer cells. Understanding the mechanisms which securin and ATF3 signal transduction on the regulation of apoptosis following treatment with gefitinib may contribute to the novel therapeutic strategies in cancers.