Constitutive RelB activation in v-Src-transformed fibroblasts: Requirementfor I kappa B degradation

RelB, an NF-kappa B/Rel-related transacting factor, was initially identifie d as an immediate-early gene product in fibroblasts and subsequently shown to exhibit constitutive DNA binding activity in lymphoid cells. The data pr esented in this report show that RelB is also constitutively active, as mon itored by electrophoretic mobility shift assay, in the v-Src-transformed fi broblast cell line, SR1. By contrast, nontransformed parental (3Y1) cells d isplayed inducible NF-kappa B activity; RelB activity was also observed, al though to a lesser extent, in two additional v-Src-transformed fibroblast l ines. RelB activation in SR1 cells did not require an increase in RelB expr ession or result from a decrease in the levels of I kappa B alpha or p105, proteins previously shown to bind to and inhibit the activity of the Rel pr oteins. Numerous studies have shown that stimulus-dependent Rel activation requires degradation of I kappa B alpha, p105 or other member of the I kapp a B family, and that this process is precluded by agents that inhibit prote asome activity. We show that treatment of SR1 cells with proteasome inhibit ors abolishes RelB activity and thus suggest that RelB in these cells is as sociated with I kappa B and that v-Src transformation activates RelB by acc elerating I kappa B proteolysis. Additional data show that serum and tumor necrosis factor-alpha (TNF-alpha) increase RelB protein levels in 3Y1 cells and that this process is blocked by proteasome inhibitors. (C) 1999 Wiley- Liss, Inc.