Cell cycle-dependent regulation of FLIP levels and susceptibility to Fas-mediated apoptosis

Activation-induced cell death of peripheral T cells results from the intera ction between Pas and Fas ligand. Resting peripheral T cells are resistant to Pas-induced apoptosis and become susceptible only after their activation . We have investigated the molecular mechanism mediating the sensitization of resting peripheral T cells to Pas-mediated apoptosis following TCR stimu lation. TCR activation decreases the steady state protein levels of FLIP (F LICE-like inhibitory protein), an inhibitor of the Pas signaling pathway. R econstitution of intracellular FLIP levels by the addition of a soluble HIV transactivator protein-FLIP chimera completely restores resistance to Fas- mediated apoptosis in TCR primary T cells. Inhibition of IL-2 production hy cyclosporin A, or inhibition of IL-2 signaling by rapamycin or anti-IL-2 n eutralizing Abs prevents the decrease in FLIP levels and confers resistance to Pas-mediated apoptosis following T cell activation. Using cell cycle-bl ocking agents, we demonstrate that activated T cells arrested in G(1) phase contain high levels of FLIP protein, whereas activated T cells arrested in S phase have decreased FLIP protein levels, These findings Link regulation of FLIP protein levels with cell cycle progression and provide an explanat ion for the increase in TCR-induced apoptosis observed during the S phase o f the cell cycle.