eIF3k 及 miR-103/107 在癌症相關特性上之功能探討

Abstract

迴避死亡及癌細胞組織浸潤轉移是癌症的兩大特徵，在我論文的第一個部 份，主要是針對一種抑制癌症的細胞死亡形式—細胞凋亡的機探討，我們發現真 核轉譯起始因子 3k (eIF3k) 在表皮細胞中具有調控細胞凋亡的角色。eIF3k 已知 為 eIF3 複合體中的一個次單位，除此之外，在我們的研究中發現 eIF3k 會表現 於角質中間絲細胞骨架（keratin intermediate filaments）上，並與 18 號細胞角質 蛋白（K18）作結合，一旦啟動細胞凋亡的機制，eIF3k 會藉由與 keratin的作用進 入含有 K8 及 K18 的包含體中。先前已知這些含有 K8 及 K18 之包含體具有延 緩細胞凋亡進行的功能，可以擒住一部份的半胱氨酸蛋白（caspases），降低其與 受質接觸的機率，進而延緩細胞凋亡之進行。本研究發現 eIF3k 藉使 caspases 自 包含體釋放出來，進而促進細胞凋亡的進行。因此，本研究發現因此，本研究發 現 eIF3k 在細胞中不僅會與 eIF3 複合體結合，亦會與 keratin 結合，展現其不 同於 eIF3 複合體之特性，並藉由此種交互作用執行其特殊之生理功能，進而調控 細胞凋亡之進行。 接下來，在我論文的第二部份，主要是針對在結直腸癌 (CRC) 中，微型核糖 核酸 (microRNA) 對腫瘤轉移的調控機制探討，我們發現微型核糖核酸 103 號及 107 號 (miR-103/107) 具有促進腫瘤轉移的功能。首先，在一系列結直腸癌細胞 株中，我們發現 miR-103/107 的表現量與各細胞株的轉移潛能有正相關的現象， 且 miR-103/107 促進細胞移動能力，促進細胞與基質的黏附以及抑制細胞與細胞 之間的黏附等，在在皆顯示出 miR-103/107 可促進各種腫瘤轉移相關的特性，且 這些特性也被發現是透過抑制死亡相關蛋白激酶 (DAPK) 及 Krüppel 樣因子4 (KLF4) 所達成的。更重要的是，在缺氧情況下能促進細胞移動轉移的現象這條訊 息途逕也參予其中。最後，我們利用原位 CRC 腫瘤模型，再次應證了 miR-103/107 可透過抑制 DAPK 及 KLF4 的交互作用而達到促進 CRC 之腫瘤轉移。 總結，我們的研究發現了 eIF3k 具有調控細胞凋亡的功能以及探討了 miR-103/107 對腫瘤轉移的影響，期望對於腫瘤進程的研究能帶來幫助。

Resistance to cell death and activation of invasion-metastasis cascade are two of the hallmarks of cancer. In the first part of this thesis, I focus on the molecular mechanism of apoptosis, a form of cell death that has profound impacts on tumor suppression. I identify eIF3k as a novel regulator of apoptosis in simple epithelial cells. Despite being identified as a component of the eIF3 complex, a large portion of eIF3k is present in the keratin 8 and 18 (collectively called K8/K18) intermediate filaments through its physical association with K18. Upon induction of apoptosis, eIF3k colocalizes with K8/K18 in the cytoplasmic inclusions. Depletion of eIF3k de-sensitizes simple epithelial cells to various types of apoptosis through a K8/K18-dependent manner. Mechanistically, this attenuation of apoptosis is due to the retention of active caspase 3 in K8/K18-containing cytoplasmic inclusions by increasing its binding to keratins. Consequently, the cleavage of caspase cytosolic and nuclear substrates, such as ICAD and PARP, respectively, is reduced in eIF3k-depleted cells. Hence, this study identifies an apoptosis-promoting function of eIF3k in simple epithelial cells by relieving the caspase-sequestration effect of K8/K18, thereby increasing the availability of caspases to their non-keratin-residing substrates. The aim of the second part of this thesis is to unravel the function of microRNAs (miRNAs) in metastasis process of colorectal cancer (CRC). I identify miR-103 and miR-107 (miR-103/107) as potential metastamirs in CRC. First, miR-103/107 expression correlates with metastatic potential of CRC cell line. Second, miR-103/107 protentiate a number of metastasis-relevant traits in vitro, such as increasing motility and cell-matrix adhesion and suppressing cell-cell contact assembly. These functions are mediated at least in part by the repression of two metastasis suppressors, death-associated protein kinase (DAPK) and Krüppel-like factor 4 (KLF4). Importantly, miR-103-1 and miR-107 are transcriptional targets of HIF-1 and their repression of DAPK and KLF4 mediates hypoxia-induced migration and invasion. In orthotopic CRC model, overexpression of miR-103/107 potentiates CRC metastasis and this effect is blocked by re-expression of DAPK or KLF4. These data indicate that miR-103/107 coordinately repress DAPK and KLF4 to potentiate CRC metastasis and this regulatory scheme contributes in part to hypoxia-stimulated tumor metastasis. In conclusion, the uncovering of eIF3k apoptotic function and miR-103/107 metastatic effect would shed light on the tumor progression mechanism.