EIF3k 及 miR-103/107 在癌症相關特性上之功能探討

Abstract

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 Kruppel-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.