Constitutive RelB activation in v‐Src‐transformed fibroblasts: Requirement for IκB degradation

Abstract

RelB, an NF-κB/Rel-related transacting factor, was initially identified as an immediate-early gene product in fibroblasts and subsequently shown to exhibit constitutive DNA binding activity in lymphoid cells. The data presented in this report show that RelB is also constitutively active, as monitored by electrophoretic mobility shift assay, in the v-Src-transformed fibroblast cell line, SR1. By contrast, nontransformed parental (3Y1) cells displayed inducible NF-κB activity; RelB activity was also observed, although to a lesser extent, in two additional v-Src-transformed fibroblast lines. RelB activation in SR1 cells did not require an increase in RelB expression or result from a decrease in the levels of IκBα or p105, proteins previously shown to bind to and inhibit the activity of the Rel proteins. Numerous studies have shown that stimulus-dependent Rel activation requires degradation of IκBα, p105 or other member of the IκB family, and that this process is precluded by agents that inhibit proteasome activity. We show that treatment of SR1 cells with proteasome inhibitors abolishes RelB activity and thus suggest that RelB in these cells is associated with IκB and that v-Src transformation activates RelB by accelerating IκB proteolysis. Additional data show that serum and tumor necrosis factor-α (TNF-α) increase RelB protein levels in 3Y1 cells and that this process is blocked by proteasome inhibitors. J. Cell. Biochem. 73:237–247, 1999. © 1999 Wiley-Liss, Inc.