Interleukin-1-induced nuclear factor-κB-IκB α autoregulatory feedback loop in hepatocytes. A role for protein kinase Cα in post-transcriptional regulation of IκBα resynthesis

The IκB inhibitors regulate the activity of the potent transcription factor nuclear factor-κB (NF-κB). Following signal-induced IκB proteolysis, NF-κB translocates into the nucleus to activate transcription of target genes, including IκBα itself, initiating the 'NF-κB-IκBα autoregulatory feedback loop.' Upon IκBα resynthesis, NF-κB is subsequently inactivated and redistributed back into the cytoplasm. We have previously reported a robust NF-κB-IκBα autoregulatory feedback loop in HepG2 hepatocytes. Sixty minutes after tumor necrosis factor (TNF-α) stimulation, IκBα is resynthesized to ~2-fold greater level than in control cells and completely inhibits NF-κB binding. Here we investigate the mechanism for IκBα resynthesis comparing the effect of stimulation of TNF- α with that of interleukin-1 (IL-1α). Although either TNF-α or IL-1α stimulation of protein kinase C (PKC)-down-regulated cells equivalently induces NF-κB translocation, the kinetics of IκBα resynthesis is slowed. Moreover, pretreatment with selective calcium-dependent PKC inhibitors selectively slowed the kinetics of the IL-1α-induced overshoot without affecting that produced by TNF-α. Down-regulation of PKCα by antisense phosphorothioate oligonucleotides and expression vectors selectively blocked the IL-1α-induced IκBα overshoot. In the absence of PKCα, although IL- 1α induced similar amounts of IκBα transcription and changes in steady- state mRNA, a greater component of IκBα mRNA was retained in the nucleus. These data indicate a selective role for PKCα in IL-1α-induced IκBα resynthesis, which is mediated, at least in part, by post-transcriptional control of mRNA export.