IkappaB alpha overexpression in human breast carcinoma MCF7 cells inhibits nuclear factor-kappaB activation but not tumor necrosis factor-alpha-induced apoptosis.

Nuclear factor-kappaB (NF-kappaB) is one of major component induced by tumor necrosis factor-alpha (TNF), and its role in the signaling of TNF-induced cell death remains controversial. In order to delineate whether the involvement of NF-kappaB activation is required for triggering of the apoptotic signal of TNF, we inhibited the nuclear translocation of this transcription factor in TNF-sensitive MCF7 cells by introducing a human MAD-3 mutant cDNA coding for a mutated IkappaB alpha that is resistant to both phosphorylation and proteolytic degradation and that behaves as a potent dominant negative IkappaB alpha protein. Our results demonstrated that the mutated IkappaB alpha was stably expressed in the transfected MCF7 cells and blocked the TNF-induced NF-kappaB nuclear translocation. Indeed, TNF treatment of these cells induced the proteolysis of only the endogenous IkappaB alpha but not the mutated IkappaB alpha. The nuclear NF-kappaB released from the endogenous IkappaB alpha within 30 min of TNF treatment was rapidly inhibited by the mutated IkappaB alpha. There was no significant difference either in cell viability or in the kinetics of cell death between control cells and the mutated IkappaB alpha transfected cells. Furthermore, electron microscopic analysis showed that the cell death induced by TNF in both control and mutated IkappaB alpha transfected cells was apoptotic. The inhibition of NF-kappaB translocation in mutated IkappaB alpha-transfected cells persisted throughout the same time course that apoptosis was occurring. Our data provide direct evidence that the inhibition of NF-kappaB did not alter TNF-induced apoptosis in MCF7 cells and support the view that TNF-mediated apoptosis is NF-kappaB independent.