A redox switch and phosphorylation are involved in the post-translational up-regulation of the adenosine-uridine binding factor by phorbol ester and ionophore

Messenger RNAs coding for cytokines and lymphokines are extremely unstable due to an AU-rich cis element located in their 3′-untranslated region. Cell activation with phorbol ester 12-O-tetradecanoylphorbol-13-acetate or calcium ionophore has been shown to markedly stabilize these normally labile messages. We have recently described a cytoplasmic protein, denoted the adenosine-uridine binding factor (AUBF) which complexes in vitro to a variety of labile RNAs containing multiple reiterations of the pentamer AUUUA. In order to determine if AUBF plays a role in the stabilization of cytokine and lymphokine mRNAs, we have investigated the mechanisms which control AUBF activity in peripheral blood mononuclear cells as well as Jurkat cells. AUBF is inactive in resting peripheral blood mononuclear cells but can be activated by brief treatment with 12-O-tetradecanoylphorbol-13-acetate or ionophore. Up-regulation is independent of protein synthesis or RNA transcription, suggesting pre-existing AUBF is subject to post-translational modification. AUBF activity can be reversibly blocked by diamide but irreversibly inhibited by n-ethylmaleimide, suggesting that AUBF contains a redox switch as described for other RNA-binding proteins. Finally, AUBF activity is abolished by potato acid phosphatase, demonstrating that AUBF is a phosphoprotein. These data demonstrate that AUBF activity is subject to at least two levels of post-translational regulation and is enhanced by mitogens previously shown to induce the stabilization of AUUUA mRNAs. Based upon these data, we propose that AUBF binding may mediate 12-O-tetradecanoylphorbol-13-acetate and ionophore-mediated labile message stabilization.