MAF1是廣為人知的核糖核酸聚合酶III之抑制子，由於核醣核酸聚合酶III之產物為促進腫瘤生長的重要原料，MAF1身為其抑制物，亦被認為參與在調節腫瘤生長的過程當中。過去文獻報導，於神經膠母細胞瘤中之MAF1主要座落於細胞核中，並扮演著抑制腫瘤生長的角色。然而在實驗室過去的研究中發現，在乳癌檢體中之MAF1於腫瘤組織相較於一般組織不但有過度表現的情形，並且主要表現於細胞質中。這個發現顯示MAF1在乳癌中的細胞質表現可能存在著重要的功能。此外，實驗室先前研究也在乳癌細胞中發現文獻未曾報導過之MAF1結合細胞質蛋白質EB1，並發現降低MAF1表現量會減少EB1的蛋白質表現。EB1是一個具高度保留性的蛋白質，其功能和調節細胞當中微管的動態相關。EB1已知在乳癌中有過量表現的情形，並且具有促進腫瘤生長的作用。故本研究的目的在於確認MAF1在於乳癌中是否具有致癌性的功能，以及探討在乳癌當中MAF1與EB1的互動所扮演的角色。透過分析乳癌組織陣列染色結果發現，在細胞質中有MAF1高度表現的病人相較於MAF1均勻表現於細胞中的病人具有較差的預後，顯示於乳癌中MAF1在細胞中表現的位置會影響其功能。為了找尋MAF1和EB1蛋白之間互動的區段，將EB1高度保留性且具有轉譯後修飾的位點進行點突變以進行免疫共沉澱。實驗結果顯示：突變型的EB1會明顯降低和MAF1之間的結合。更進一步，免疫螢光染色的結果發現，表現突變型的EB1會影響部分MAF1蛋白其亞細胞表現位置，增加MAF1於細胞核中表現的比例。這個結果暗示了一般型的EB1於細胞無承受環境壓力的情況下有助於將MAF1留在細胞質中。COPS5為調節EB1之泛素依賴型蛋白質水解的重要基因，而MAF1的過表現會降低COPS5的mRNA表現，暗示著MAF1對EB1的影響可能透過直接的互動或是經由COPS5間接的影響EB1的表現。這些發現顯示了乳癌中MAF1與EB1在於細胞質中的互動可能對於腫瘤的發展具有重要的影響。

MAF1 is known as a repressor of RNA polymerase III. Since Pol III products are important materials for tumor progression, the RNA polymerase III repressor MAF1 is expected to regulate tumor progression. It has been reported that MAF1 is mainly located in nucleus and functions as a tumor suppressor in glioblastoma. However, our previous results demonstrated that MAF1 was overexpressed in breast cancer tumor parts and mainly stayed in cytosol. It suggested that MAF1 might exhibit important cytosolic functions in breast cancer. Besides, our previous research discovered that EB1 is a novel binding partner of MAF1 and knockdown of MAF1 reduced EB1 protein level in vitro. EB1 is a highly conserved microtubule-associated protein which is reported to be upregulated and functions oncogenically in breast cancer. The aim of this study was to confirm MAF1’s oncogenic function and investigate the role of EB1 and MAF1 interaction in breast cancer. In breast cancer tissue array, patients with higher cytosolic MAF1 expression had worse prognosis than those with MAF1 equally distributed in cells. It indicated that MAF1’s subcellular distribution would influence its functions in breast cancer. To identify the interaction region between MAF1 and EB1, we created EB1 mutant constructs based on its post translational modification sites in evolutionarily conserved CH and EB1 domain. The co-immunoprecipitation results revealed that mutant EB1 would reduce its binding ability to MAF1 compared to the wild type. It suggested that EB1 might interact with MAF1 through its highly conserved regions. Moreover, the immunofluorescence results demonstrated that the mutant EB1 would partially change MAF1 subcellular distribution, increasing the proportion of MAF1 in nucleus. It indicated that wild type EB1 might help MAF1 retain in cytosol in unstressed conditions. Furthermore, MAF1 inhibited the mRNA expression of COPS5, which is known to regulate EB1’s ubiquitin dependent proteolysis. The effects of MAF1 on EB1 may result from a direct interaction and an indirect effect through COPS5. Altogether, the cytosolic interaction between MAF1 and EB1 might have important impacts on tumor progression in breast cancer.

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