Fas is a cell surface molecule belonging to the tumor necrosis factor (TNF) receptor family. It acts as a receptor molecule responsible for intracellular transmission of apoptosis-inducing signals. Fas ligand (FasL) is a protein homologous with the TNF family. FasL binding to membrane-type Fas has been reported to induce apoptosis. To develop an effective treatment for oral cancer, we immunohistologically examined the localization of Fas and FasL in morphologically normal oral mucosal epithelium, epithelial dysplasia, and oral squamous cell carcinoma. We also examined the expression of these molecules in cultured cell lines derived from squamous cell carcinoma of the head and neck (HSC-2, 3, 4, KB, Ca 9-22) by immunocytochemical procedures, Western blotting, and reverse transcriptase-polymerase chain reaction (RT-PCR).
We studied 42 primary invasive oral squamous cell carcinomas, 36 epithelial dysplasias, and 13 samples of normal oral mucosal epithelium. Tissuc specimens were fixed in buffered formalin and embedded in paraffin wax.
Fas expression was observed in the spinous and parakeratotic layers of normal oral mucosal epithelium. In contrast, FasL expression was observed in the basal layers of normal oral mucosal epithelium. Fas and FasL expression was not consistently edvident in epithelial dysplasias. Primary invasive oral squamous cell carcinoma showed decreased expression of Fas and enhanced expression of FasL. Coexpression of Fas and FasL in cancer nests was also observed in some areas. In squamous carcinoma cell lines from head and neck cancers, immunocytochemical analysis revealed expression of both Fas and FasL in all specimens examined, whereas weak reactions were observed in some cell lines, such as HSC-4 and KB. Fas expression (45kDa) was detected in HSC-2, 3, and 4 and Ca 9-22 by Western blotting. A high-molecular-weight reaction product was found in the extract from KB.
FasL expression was detected in HSC-2, 3, and 4. Expression of both Fas and FasL was detected in all cell lines by RT-PCR. In HSC-2, 3, and 4, a shorter PCR product (87bp) was obtained with the 150-bp PCR product from normal Fas, suggesting the presence of soluble Fas (sFas). These findings clearly indicate that up-regulation of FasL frequently occurs in oral squamous cell carcinomas and suggest that downregulation of Fas expression may be a mechanism by which oral squamous carcinoma cells escape from attack by the host's immune system. sFas antagonizes cell surface Fas function, and sFas expression may increase the resistance of cells against apoptosis.
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