Mechanism for biphasic Rel A·NF-κB1 nuclear translocation in tumor necrosis factor α-stimulated hepatocytes

The proinflammatory cytokine, tumor necrosis factor a (TNFα), is a potent activator of angiotensinogen gene transcription in hepatocytes by activation of latent nuclear factor-κB (NF-κB) DNA binding activity. In this study, we examine the kinetics of TNFα-activated translocation of the 65-kDa (Rel A) and 50-kDa (NF-κB1) NF-κB subunits mediated by inhibitor (IκB) proteolysis in HepG2 hepatoblastoma cells. HepG2 cells express the IκB members IκBa, IκBβ, and IκBγ. In response to TNFα, Rel A·NF-κB1 translocation and DNA binding activity follows a biphasic profile, with an 'early' induction (15-30 min), followed by a nadir to control levels at 60 min, and a 'late' induction (>120 min). The early phase of Rel A·NF-κB1 translocation depends on simultaneous proteolysis of both IκBα and IκBβ isoforms; IκBγ is inert to TNFα treatment. The 60-min nadir is due to a rapid IκBα resynthesis that reassociates with Rel A and completely inhibits its DNA binding activity; the 60-min nadir is not observed when IκBα resynthesis is prevented by cycloheximide treatment. By contrast, selective inhibition of IκBβ proteolysis by pretreatment of HepG2 cells with the peptide aldehyde N-acetyl-Leu-Leu-nor-leucinal completely blocks the late phase of Rel A-NFκB1 translocation. These studies indicate the presence of inducible and constitutive cytoplasmic NF-κB pools in hepatocytes. TNFα induces a coordinated proteolysis and resynthesis of IκB isoforms to produce dynamic changes in NF-κB nuclear abundance.