CKIP-1 Is an Intrinsic Negative Regulator of T-Cell Activation through an Interaction with CARMA1

The transcription factor NF-kappa B plays a key regulatory role in lymphocyte activation and generation of immune response. Stimulation of T cell receptor (TCR) induces phosphorylation of CARMA1 by PKC theta, resulting in formation of CARMA1-Bcl10-MALT1 (CBM) complex at lipid rafts and subsequently leading to NF-kappa B activation. While many molecular events leading to NF-kappa B activation have been reported, it is less understood how this activation is negatively regulated. We performed a cell-based screening for negative regulators of TCR-mediated NF-kappa B activation, using mutagenesis and complementation cloning strategies. Here we show that casein kinase-2 interacting protein-1 (CKIP-1) suppresses PKCh-CBM-NF-kappa B signaling. We found that CKIP-1 interacts with CARMA1 and competes with PKC theta for association. We further confirmed that a PH domain of CKIP-1 is required for association with CARMA1 and its inhibitory effect. CKIP-1 represses NF-kappa B activity in unstimulated cells, and inhibits NF-kappa B activation induced by stimulation with PMA or constitutively active PKC theta, but not by stimulation with TNFa. Interestingly, CKIP-1 does not inhibit NF-kappa B activation induced by CD3/CD28 costimulation, which caused dissociation of CKIP-1 from lipid rafts. These data suggest that CKIP-1 contributes maintenance of a resting state on NF-kappa B activity or prevents T cells from being activated by inadequate signaling. In conclusion, we demonstrate that CKIP-1 interacts with CARMA1 and has an inhibitory effect on PKCh-CBM-NF-kappa B signaling.