Protein synthesis inhibitors differentially superinduce c‐fos and c‐jun by three distinct mechanisms: lack of evidence for labile repressors.

Protein synthesis inhibitors strongly augment and prolong the usually transient induction of c‐fos and c‐jun by growth factors, phorbol esters etc., a phenomenon termed superinduction which is conventionally regarded as a secondary consequence of translational arrest. Our recent demonstration that some inhibitors can act positively as nuclear signalling agonists compromises this view and necessitates a re‐evaluation of superinduction. First, we show that labile repressors, widely postulated to act negatively on diverse superinducible genes, are not involved in regulating c‐fos and c‐jun. Secondly, two components of c‐fos and c‐jun superinduction, namely the delay in shutting off transcription and stabilization of their mRNAs, arise from translational arrest and are common to all protein synthesis inhibitors. Thirdly, the recently described capacity to act positively as nuclear signalling agonists to stimulate pp33/pp15 phosphorylation is restricted to compounds such as anisomycin and cycloheximide; these, but not emetine or puromycin, will induce c‐fos/c‐jun on their own. Fourthly, the translational arrest‐related components of superinduction are dissociable from the signalling agonist effects at sub‐inhibitory concentrations of anisomycin, under which conditions a new type of c‐fos/c‐jun superinduction with ‘spike’ kinetics is observed. Finally, we show that in response to EGF plus anisomycin, the nuclear signalling responses are themselves augmented and prolonged in a manner that corresponds to c‐fos/c‐jun superinduction under these conditions.