JNK inhibition arrests cotranslational degradation

Adhesion to fibronectin stimulates protein synthesis (translation) of fibroblasts. Protein synthesis stimulation is dependent from the activation of β1-integrin. β1-Integrin elicits a PI3K cascade that modulates eIF4F (eukaryotic initiation factor 4F) complex formation. In the attempt to further dissect elements of the PI3K cascade that might be responsible for fibronectin-stimulated translation, we used pharmacological inhibitors of known kinases. We found that JNK inhibition, by SP600125 treatment, increased 35S-methionine incorporation. Paradoxically, the increase in methionine incorporation was associated to a reduction of initiation of translation. These data imply that, during the adhesion of fibroblasts to fibronectin, conspicuous protein degradation occurs. Indeed, we found that inhibition of the proteasome by MG132 also increased methionine incorporation. Cotranslational degradation depended on PI3K activation. In spite of this, serum promoted translation, but not cotranslational degradation. The crosstalk between translation and degradation was further analyzed by studying the phosphorylation of initiation factors. Briefly, inhibition of JNK leads to eIF2α phosphorylation, which may account for the decrease in initiation of translation. In conclusion, β1-integrin-activated translation causes the synthesis of short-lived proteins, whose degradation is controlled by the JNK pathway. We hypothesize that JNK is a general regulator of cotranslational degradation.