INHIBITION OF T-LYMPHOCYTE ACTIVATION BY CAMP IS ASSOCIATED WITH DOWN-REGULATION OF 2 PARALLEL MITOGEN-ACTIVATED PROTEIN-KINASE PATHWAYS, THE EXTRACELLULAR SIGNAL-RELATED KINASE AND C-JUN N-TERMINAL KINASE

The induction of T cell proliferation requires signals from the TCR an d a co-receptor molecule, such as CD28, that activate parallel and par tially cross-reactive signaling pathways. These pathways are disrupted by agonists that utilize adenylate cyclase and cAMP-dependent protein kinase A (PKA). We found that the adenylate cyclase activator, forsko lin, inhibits anti-CD3-induced shift in Lck electrophoretic mobility, suggesting an intervention at the TCR-coupled phosphoinositide turnove r that precedes the activation of PKC. The shift of Lck following dire ct PKC activation by 12-O-tetradecanoyl phorbol 13-acetate, which bypa sses early receptor-triggered biochemical events, is insensitive to fo rskolin. Nevertheless, forskolin also inhibits PKC downstream events, such as c-jun expression, which is critical for the activation process of T cells. To Further analyze potential cross points between positiv ely and negatively regulating signaling pathways in T cells, we tested the effects of activators of the adenylate cyclase or PKA on two para llel mitogen-activated protein kinase signaling pathways mediated by e xtracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase . Using a PKC-specific inhibitor, GF109203X, or PKC-depleted T cells, we found that a large part of the anti-CD3-induced ERK activation is P KC dependent. Both PKC-dependent and -independent activation of ERK we re sensitive to inhibition by forskolin or a cell-permeable cAMP analo gue, dbcAMP. Furthermore, the effect of 12-O-tetradecanoyl phorbol 13- acetate and ionomycin, which synergized to fully activate c-Jun N-term inal kinase, was also sensitive to inhibition by forskolin. Our result s suggest that PKA inhibits T cell activation by interfering with mult iple events along the two signaling pathways operating downstream of t he TCR and the CD28 co-receptor molecules.