Thyroid carcinoma cells are resistant to FAS-mediated apoptosis but sensitive to tumor necrosis factor-related apoptosis-inducing ligand

Fas (APO-1/CD95) is a transmembrane protein of the tumor necrosis factor (T NF)/nerve growth factor receptor superfamily that induces apoptosis in susc eptible normal and neoplastic cells upon cross-linking by its ligand (FasL) . TNF-related apoptosis-inducing ligand (TRAIL) is a more recently identifi ed member of the TNF superfamily that has been shown to selectively kill ne oplastic cells by engaging two cell-surface receptors, DR4 and DR5. Two add itional TRAIL receptors (DcR1 and DcR2) do not transmit an apoptotic signal and have been proposed to confer protection from TRAIL-induced apoptosis. We addressed the expression of Fas, DR4, and DR5 in thyroid carcinoma cell lines and in 31 thyroid carcinoma specimens by Western blot analysis and im munohistochemistry, respectively, and tested the sensitivity of thyroid car cinoma cell lines to Fas- and TRAIL-induced apoptosis. Fas was found to be expressed in most thyroid carcinoma cell lines and tissue specimens. Althou gh cross-linking of Fas did not induce apoptosis in thyroid carcinoma cell lines, Fas-mediated apoptosis did occur in the presence of the protein synt hesis inhibitor cycloheximide, suggesting the presence of a short-hived inh ibitor of the Fas pathway in these cells. Cross-linking of Pas failed to in duce recruitment and activation of caspase 8, whereas transfection of a con stitutively active caspase 8 construct effectively killed the SW579 papilla ry carcinoma cell line, arguing that the action of the putative inhibitor o ccurs upstream of caspase 8. By contrast, recombinant TRAIL induced apoptos is in 10 of 12 thyroid carcinoma cell lines tested, by activating caspase-1 0 at the receptor level and triggering a caspase-mediated apoptotic cascade . Resistance to TRAIL did not correlate with DcR1 or DcR2 protein expressio n and was overcome by protein synthesis inhibition in 50% of the resistant cell lines. One medullary carcinoma cell line was resistant to Fas- and TRA IL-induced apoptosis, even in the presence of cycloheximide, and to transfe ction of constitutively active caspase-8, suggesting a different regulation of the apoptotic pathway. Our observations indicate that TRAIL effectively kills carcinomas that originate from the follicular epithelium of the thyr oid gland, by inducing caspase-mediated apoptosis, and may provide a potent ially potent therapeutic reagent against thyroid cancer.