T-CELL RECEPTOR STIMULATION ELICITS AN EARLY PHASE OF ACTIVATION AND A LATER PHASE OF DEACTIVATION OF THE TRANSCRIPTION FACTOR NFAT1

We show here that NFAT1 is rapidly activated, then slowly deactivated, by stimulation of T cells through their antigen receptor. Within minu tes of T-cell receptor stimulation, NFAT1 is dephosphorylated, translo cates from the cytoplasm into the nucleus, and shows an increase in it s ability to bind to DNA. These changes are dependent on calcium mobil ization and calcineurin activation, since they are also elicited by io nomycin and are blocked by the immunosuppressive drug cyclosporin A. A fter several hours of T-cell receptor stimulation, the majority of the NFAT1 in the cell reverts to its original phosphorylated form, reappe ars in the cytoplasm, and again displays a low affinity for DNA. Deact ivation of NFAT1 is facilitated by phorbol 12-myristate 13-acetate and inhibitors of capacitative calcium entry and most likely reflects the slow return of intracellular free calcium concentrations towards rest ing levels. Our results suggest that calcineurin-dependent signalling pathways mediate the early activation of NFAT1, while phorbol 12-myris tate W-acetate-dependent feedback pathways contribute to the late deac tivation. Persistent NFAT-dependent cytokine gene transcription in act ivated T cells may be mediated by other NFAT family proteins in additi on to NFAT1 during the immune response.