Interleukin-1-induced nuclear factor-kappaB-IkappaBalpha autoregulatory feedback loop in hepatocytes. A role for protein kinase calpha in post-transcriptional regulation of ikappabalpha resynthesis.

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