Protein Kinase C and Calcineurin Synergize to Activate IκB Kinase and NF-κB in T Lymphocytes*

The nuclear factor of κB (NF-κB) is a ubiquitous transcription factor that is key in the regulation of the immune response and inflammation. T cell receptor (TCR) cross-linking is in part required for activation of NF-κB, which is dependent on the phosphorylation and degradation of IκBα. By using Jurkat and primary human T lymphocytes, we demonstrate that the simultaneous activation of two second messengers of the TCR-initiated signal transduction, protein kinase C (PKC) and calcineurin, results in the synergistic activation of the IκBα kinase (IKK) complex but not of another putative IκBα kinase, p90 rsk . We also demonstrate that the IKK complex, but not p90 rsk , is responsible for thein vivo phosphorylation of IκBα mediated by the co-activation of PKC and calcineurin. Each second messenger is necessary, as inhibition of either one reverses the activation of the IKK complex and IκBα phosphorylation in vivo. Overexpression of dominant negative forms of IKKα and -β demonstrates that only IKKβ is the target for PKC and calcineurin. These results indicate that within the TCR/CD3 signal transduction pathway both PKC and calcineurin are required for the effective activation of the IKK complex and NF-κB in T lymphocytes.