Calcineurin-mediated Dephosphorylation Enhances the Stability and Transactivation of C-Myc

c-Myc, a transcription factor, induces cell proliferation and is often aberrantly or highly expressed in cancers. However, molecular mechanisms underlying this aberrantly high expression remain unclear. Here, we found that intracellular Ca concentration regulates c-Myc oncoprotein stability. We identified that calcineurin, a Ca-dependent protein phosphatase, is a positive regulator of c-Myc expression. Calcineurin depletion suppresses c-Myc targeted gene expression and c-Myc degradation. Calcineurin directly dephosphorylates Thr and Ser in c-Myc, which inhibit binding to the ubiquitin ligase Fbxw7. Mutations within the autoinhibitory domain of calcineurin, most frequently observed in cancer, may increase phosphatase activity, increasing c-Myc transcriptional activity in turn. Notably, calcineurin inhibition with FK506 decreased c-Myc expression with enhanced Thr and Ser phosphorylation in a mouse xenograft model. Thus, calcineurin can stabilize c-Myc, promoting tumor progression. Therefore, we propose that Ca signaling dysfunction affects cancer-cell proliferation via increased c-Myc stability and that calcineurin inhibition could be a new therapeutic target of c-Myc-overexpressing cancers.