Combined spatial and enzymatic regulation of Csk by cAMP and protein kinase A inhibits T cell receptor signaling

Raft-associated Csk controls signaling through the T cell receptor (TCR) and was mainly anchored to Cbp/PAG (phosphoprotein associated with glycosphingolipid-enriched membrane domains). Treatment of cells with the cAMP-elevating agent prostaglandin E-2 (PGE(2)) augmented the level of Cbp/PAG phosphorylation with a concomitant increase in amounts of Csk bound to Cbp/PAG. While TCR-triggering resulted in transient dissociation of Csk from Cbp/PAG/rafts allowing TCR-induced tyrosine phosphorylation to occur, pretreatment with PGE(2) reduced Csk dissociation upon TCR triggering. This correlated with lowered TCR-induced phosphorylation of CD3 zeta-chain and linker for activation of T cells. Moreover, competition of endogenous Csk from lipid rafts abolished PGE(2)-mediated inhibition of TCR-induced zeta-chain phosphorylation and activation of the nuclear factor of activated T cells (NFAT) activator protein 1 (AP-1). Finally, raft-associated Csk already activated via Cbp/PAG binding, gained additional increase in phosphotransferase activity upon protein kinase A-mediated phosphorylation of Csk. We propose that cAMP regulates Csk via both spatial and enzymatic mechanisms, thereby inhibiting signaling through the TCR.