zeta-Associated Protein of 70 kDa(ZAP-70),but Not Syk,Tyrosine Kinase Can Mediate Apoptosis of T Cells through the Fas/Fas Ligand,Caspase-8 and Caspase-3 Pathways

摘要

The TCR zeta-chain-associated protein of 70 kDA(ZAP-70)and Syk tyrosine kinases play critical roles in regulating TCR-mediated signal transduction.They not only share some overlapped functions but also may play unique roles in regulating the function and development of T cells.However,it is not known whether they have different effects on the activation and activation-induced cell death of T cells.To address this question,we generated cDNAs encoding chimeric molecules that a tailless TCR zeta-chain was directly linked to truncated ZAP-70(Z/ZAP)or Syk(Z/Syk)molecules lacking the two Src homology 2 domains.Transfection of these molecules into zeta-chain-deficient cells restored their TCR expression.In addition,Z/ZAP and Z/Syk transfectants but not control cells demonstrated kinase activities in phosphorylating an exogenous substrate specific for ZAP-70 and Syk kinases.Z/ZAP transfectants activated through TCRs underwent a faster time course of apoptosis and had a greater percentage of apoptotic cells than that of Z/Syk and control cells.Activated Z/ZAP transfectants increased Fas and Fas ligand(FasL)expression 3- and 40-fold,respectively.Blocking of the Fas/FasL interaction could inhibit the apoptosis of Z/ZAP transfectants.In contrast,although activated Z/Syk transfectants could increase FasL expression,their Fas expression actually decreased and the percentage of apoptotic cells did not increase.Further studies of the mechanisms revealed that activation of Z/ZAP but not Z/Syk transfectants resulted in rapid activation of caspase-3 and caspase-8 that could also be inhibited by blocking Fas/FasL interaction.These results demonstrated that ZAP-70 and Syk play distinct roles in T cell activation and activation-induced cell death.