CD28 mediates a potent costimulatory signal for rapid degradation of IκBβ which is associated with accelerated activation of various NF-κB/Rel heterodimers

Optimal activation of T cells requires at least two signals delivered by the T-cell receptor complex and costimulatory molecules such as CD28. The CD28 signaling participates in the transcription of the interleukin-2 gene through activation of an enhancer termed the CD28-responsive element (CD28RE). Stimulation of CD28 enhances mitogen-mediated induction of CD28RE- binding proteins including members of the NF-κB/Rel transcription factor family, although the underlying mechanism remains elusive. In this report, we show that CD28 costimulation leads to biphasic induction of NF-κB/Rel heterodimers, including early-phase induction of p50/RelA and c-Rel/RelA and late-phase induction of p50/c-Rel. Interestingly, activation of these NF- κB/Rel complexes by the CD28 signal is associated with the rapid degradation of both IκBα and IκBβ, two major cytoplasmic inhibitors of NF-κB/Rel. Although IκBα degradation can be induced by phorbol ester alone, degradation of IκBβ is largely dependent on the CD28 costimulatory signal. We further demonstrate that CD28-mediated transactivation of the CD28RE enhancer is potently inhibited by an N-terminal truncation mutant of IκBα that is incapable of responding to the degradation signals. Together, these results suggest that the CD28 costimulatory signal augments activation of NF- κB/Rel by promoting degradation of IκBβ as well as enhancing degradation of IκBα and that induction of NF-κB/Rel serves as an essential step in the signal-mediated activation of the CD28RE enhancer.