Interleukin-1-induced nuclear factor-kappa B-I kappa B alpha autoregulatory feedback loop in hepatocytes - A role for protein kinase C alpha in post-transcriptional regulation of I kappa B alpha resynthesis

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