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

Abstract

Nuclear factor κB (NF-κΒ) is a ubiquitous transcription factor which binds to decameric DNA sequences (κB sites) and regulates transcription of multiple genes. The activity of NF-κΒ is regulated by an inhibitor protein, IκB, which sequesters NF-κΒ in the cytoplasm. Release of IκB and subsequent nuclear translocation of NF-κΒ generally require activating signals. However, in mature murine B cells, the DNA-binding activity of NF-κΒ is constitutively nuclear and activates the Igκ gene, a marker for mature B cells. To understand the basis for the constitutive NF-κΒ activation, we examined the properties of NF-κΒ and IκB in both pre-B and mature B cells, the regulated and constitutive states, respectively. We found that expression of ΙκΒα and pl05, members of the IκB family, and Rel, a member of the NF-κΒ family, is augmented in mature B cells. Both ΙκΒα and pl05 are associated with NF-κΒ proteins and sequester most of these proteins in the cytoplasm of mature B cells. However, rapid ΙκΒα dissociation and degradation lead to continuous nuclear translocation of a small fraction of NF-κΒ proteins, which represent the constitutively active NF-κΒ 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 ΙκΒα is directly involved in constitutive NF-κΒ activation, because stabilization of ΙκΒα by a protease inhibitor causes loss of NF-κΒ activity in mature B cells. These results provide evidence that continuous and rapid degradation of ΙκΒα in the absence of external stimuli is causally involved in the constitutive activation of NF-κΒ in mature murine B cells.