Protein Kinase C-θ Participates in NF-κB Activation Induced by CD3-CD28 Costimulation through Selective Activation of IκB Kinase β

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Abstract
The NF-κB/Rel family of eukaryotic transcription factors plays an essential role in the regulation of inflammatory, antiapoptotic, and immune responses. NF-κB is activated by many stimuli including costimulation of T cells with ligands specific for the T-cell receptor (TCR)-CD3 complex and CD28 receptors. However, the signaling intermediates that transduce these costimulatory signals from the TCR-CD3 and CD28 surface receptors leading to nuclear NF-κB expression are not well defined. We now show that protein kinase C-θ (PKC-θ), a novel PKC isoform, plays a central role in a signaling pathway induced by CD3-CD28 costimulation leading to activation of NF-κB in Jurkat T cells. We find that expression of a constitutively active mutant of PKC-θ potently induces NF-κB activation and stimulates the RE/AP composite enhancer from the interleukin-2 gene. Conversely, expression of a kinase-deficient mutant or antisense PKC-θ selectively inhibits CD3-CD28 costimulation, but not tumor necrosis factor alpha-induced activation of NF-κB in Jurkat T cells. The induction of NF-κB by PKC-θ is mediated through the activation of IκB kinase β (IKKβ) in the absence of detectable IKKα stimulation. PKC-θ acts directly or indirectly to stimulate phosphorylation of IKKβ, leading to activation of this enzyme. Together, these results implicate PKC-θ in one pathway of CD3-CD28 costimulation leading to NF-κB activation that is apparently distinct from that involving Cot and NF-κB-inducing kinase (NIK). PKC-θ activation of NF-κB is mediated through the selective induction of IKKβ, while the Cot- and NIK-dependent pathway involves induction of both IKKα and IKKβ.