Enhanced ΙκΒα Degradation Is Responsible for Constitutive NF-κΒ Activity in Mature Murine B-Cell Lines

Nuclear factor kappa B (NF-kappa B) is a ubiquitous transcription factor which binds to decameric DNA sequences (kappa B sites) and regulates transcription of multiple genes. The activity of NF-kappa B is regulated by an inhibitor protein, I kappa B, which sequesters NF-kappa B in the cytoplasm. Release of I kappa B and subsequent nuclear translocation of NF-kappa B generally require activating signals. However, in mature murine B cells, the DNA-binding activity of NF-kappa B is constitutively nuclear and activates the Ig kappa gene, a marker for mature B cells. To understand the basis for the constitutive NF-kappa B activation, we examined the properties of NF-kappa B and I kappa B in both pre-B and mature B cells, the regulated and constitutive states, respectively. We found that expression of I kappa B alpha and p105, members of the I kappa B family, and Rel, a member of the NF-kappa B family, is augmented in mature B cells. Both I kappa B alpha and p 105 are associated with NF-kappa B proteins and sequester most of these proteins in the cytoplasm of mature B cells. However, rapid I kappa B alpha dissociation and degradation lead to continuous nuclear translocation of a small fraction of NF-kappa B proteins, which represent the constitutively active NF-kappa B in mature B cells. We estimate that the protease activity is at least 35-fold greater in mature B cells than in pre-B cells. Rapid degradation of I kappa B alpha is directly involved in constitutive NF-kappa B activation, because stabilization of I kappa B alpha by a protease inhibitor causes loss of NF-kappa B activity in mature B cells. These results provide evidence that continuous and rapid degradation of I kappa B alpha in the absence pf external stimuli is causally involved in the constitutive activation of NF-kappa B in mature murine B cells.