Abstract

NF-kappaB is a family of essential transcription factors involved in both embryonic development and inflammatory responses of the immune system. NF-kappaB can be activated by two pathways, i.e. the canonical (NF-kappaB1) pathway, which acts through the catalytic components of the IkappaB kinase complex and leads to IkappaB phosphorylation, degradation, and subsequent NF-kappaB nuclear translocation, or the non-canonical (NF-kappaB2) pathway, which involves NF-kappaB-induced kinase-dependent proteolytic processing of p100/p52 to yield translocation-competent p52-containing NF-kappaB complexes. We examined the relative roles of the NF-kappaB1 and NF-kappaB2 pathways in TCR/CD28 costimulation. We found that TCR/CD28 costimulation activates the canonical but not the non-canonical NF-kappaB pathway and that the serine/threonine kinase protein kinase C (PKC) is essential for TCR/CD28-mediated canonical NF-kappaB activation in T cells. Importantly, TCR/CD28 costimulation induces higher p52 protein levels in T cells, but this effect is secondary to enhanced de novo synthesis of p100, not to enhanced processing of extant p100; PKC deficiency impairs signal-dependent p52 accumulation because of defects in p100 production. Finally, we found that TCR/CD28 costimulation induces IkappaBalpha, IkappaBbeta, and IkappaBepsilon degradation, and PKC is required for IkappaBalpha and IkappaBepsilon but not IkappaBbeta degradation. PKC acts solely within the canonical pathway to activate NF-kappaB, and PKC deficiency impacts upon p100/p52 processing in a manner that is independent of NF-kappaB-induced kinase.