FOXO3 於結直腸癌中雙面功能機制探討

Abstract

Forkhead box O3 (FOXO3) 為一轉錄因子，參與許多生理功能的調控，包含抑制細胞週期、促進細胞凋亡、參與細胞分化以及幫助細胞抵抗壓力。FOXO3 的這些生理功能使其常被認為是腫瘤抑制基因。在許多腫瘤中，FOXO3 會被過量表現或是高度活化的 Protein Kinase B (Akt) 所磷酸化並被 14-3-3 蛋白帶出細胞核，使其無法實現其轉錄因子的功能，此現象符合其作為一腫瘤抑制基因的角色。然而，也有文獻指出當 Wnt–β-catenin 訊息傳遞路徑高度活化時，累積在細胞核當中的 FOXO3 能幫助癌細胞轉移。而超過 90% 的大腸癌是因為基因突變造成異常的 Wnt–β-catenin 活化所形成的。 這些研究使 FOXO3 在大腸癌中的功能非常矛盾，而 FOXO3 的雙重功能雖然已被許多文獻報導，但對於其如何影響細胞的機制仍然不清楚。於本研究中，我利用 FOXO3 過量表現或是基因削弱的方式發現 FOXO3 會正向調控癌細胞的惡性以及癌症幹細胞特性，此發現與其作為腫瘤抑制基因的功能並不相符。由於 Tsai, K.-L. 的團隊藉由 X 射線晶體學發現 FOXO3 的乙醯化與否能改變其轉錄活性，為了釐清其在大腸癌細胞中的作用機制以及功能，我在大腸癌細胞中過量表現模擬乙醯化 FOXO3 (FOXO3-KQ) 以及去已醯化 FOXO3 (FOXO3-KR) 的點突變FOXO3，觀察此轉錄後修飾是否藉由調控 FOXO3 轉錄基因的方式而影響癌細胞的惡性以及癌症幹細胞特性。在我的結果中發現，過量表現 FOXO3-KQ 的癌細胞相較於過量表現 FOXO3-KR 的細胞具有更高的癌細胞惡性以及癌症幹細胞特性。這些結果指出，FOXO3 的乙醯化可能是調控其雙重角色的關鍵因子，提供一種新的結直腸癌的治療方式。

Forkhead box O3 (FOXO3), known as a transcription factor, plays versatile roles in regulating physiology of cells, including cell cycle arrest, differentiation, apoptosis, and stress resistance. Considering the function of FOXO3 in cells, it was widely regarded as a tumor suppressor. Indeed, FOXO3 proteins can be translocated from nucleus to cytoplasm via conjugating with 14-3-3 proteins after phosphorylated by activated Akt, which hinders the function of FOXO3. Moreover, Akt has been shown to be hyperactivated or overexpressed in various types of cancers. Considered altogether, these studies supported the role of FOXO3 as a tumor suppressor. However, the accumulation of FOXO3 in nucleus has been reported to promote metastasis when Wnt–β-catenin is concurrently hyperactivated. Coincidently, over 90% of colon cancer originated out of aberrant Wnt–β-catenin by mutations. Taken together, the role of FOXO3 seems paradoxical in colon cancer. This dual role of FOXO3 has been reported, yet little is known regarding the mechanism of how FOXO3 is modulated in nucleus. By manipulating expression of FOXO3, I found that FOXO3 is essential to the maintenance of cancer cell malignancy and stemness, which contradicts the role as tumor suppressor. To further address the enigma of FOXO3’s function, we overexpressed acetylated FOXO3 (FOXO3-KQ) or deacetylated FOXO3 (FOXO3-KR) mimics in cancer cells since Tsai, K.-L. et al. (2007) points out that acetylation of FOXO3 can regulate its transcriptional activity from the view of X-ray crystallography structure. Surprisingly, we found that cancer cells overexpressed with FOXO3-KQ did show higher malignancy and stemness properties. These results suggested that acetylation might be the key regulation of FOXO3’s dual role in colorectal cancer and provided a new therapeutic approach against colorectal cancer.