| Summary: | 博士 === 國立成功大學 === 基礎醫學研究所 === 101 === Fas ligand (FasL, CD95L) is a well-known critical protein of the tumor necrosis factor (TNF) family that initiates Fas (CD95) engagement and programmed cell death in a variety of susceptible cells to mediate cytotoxicity and immune cell homeostasis. In addition, FasL transmits a reverse signal via the intracellular region to modulate T lymphocyte activation. In addition to normal cells, many late-stage cancer cells also express FasL and show high metastatic potential. Although FasL-related signals can modulate lymphocyte development, the correlation between FasL expression and cancer malignancy is still inconclusive. To verify the uncertain role of FasL intracellular domains, we have previously expressed full-length and deletion variants of human FasL, Δ33 (motif for CK1 binding of the cytoplasmic tail deleted) and Δ70 (motif for CK1 binding and the proline-rich domain deleted) in NIH3T3 cells. Although the ectopic expression of FasL does not induce endoplasmic reticulum (ER) stress or affect cell proliferation and apoptosis, it enhances cell motility and invasion. Truncated FasL, lacking most of the intracellular domain, profoundly enhances lung tumor nodules in nude mice. The present study further investigates FasL-signal-mediated tumor metastasis. The results of confocal imaging and the immunoprecipitation assay obviously show that FasL forms complexes with Met in lipid raft even without the FasL intracellular region and Fas binding sites. In addition, the FasL/Met complexes trigger Met and downstream Stat3 activation that is positively correlated with the levels of FasL expression. The FasL-elevated invasive phenotypes are effectively blocked by Met and Stat3 inhibition. Knocking down the FasL by RNA interference (RNAi) technology appreciably suppresses cell motility of various human cancer cell lines, including A549, Huh7, PLC/PRF/5, SiHa, and U118. In addition, knocking down Met gene expression reverts the FasL-associated motility to the basal level. By diverse truncation FasL constructions, the FasL105-130 extracellular region is identified as the necessary site for FasL interaction with Met in lipid rafts, which consequently leads to Met activation. Furthermore, treatment with a synthetic peptide corresponding to FasL117-126 significantly reduces the FasL/Met interaction, Met pathway activation, and cell motility. Collectively, our results establish the FasL-Met-Stat3 signaling mechanism and explain the metastatic phenotype of FasL-expressing tumors.
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