MyoD-dependent regulation of NF-κB activity couples cell-cycle withdrawal to myogenic differentiation

Primary myoblasts isolated from wild type and MyoD-/- mutant mice were examined by microarray analysis and further characterized by cell and molecular experiments in cell culture.We found that NF-κB, a key regulator of cell-cycle withdrawal and differentiation, aberrantly maintains nuclear localization and transcriptional activity in MyoD-/- myoblasts. As a result, expression of cyclin D is maintained during serum withdrawal, inhibiting expression of muscle-specific genes and progression through the differentiation program. Sustained nuclear localization of cyclin E, and a concomitant increase in cdk2 activity maintains S-phase entry in MyoD-/- myoblasts even in the absence of mitogens. Importantly, this deficit was rescued by forced expression of IκBαSR, a non-degradable mutant of IκBα, indicating that inhibition of NF-κB is sufficient to induce terminal myogenic differentiation in the absence of MyoD.MyoD-induced cytoplasmic relocalization of NF-κB is an essential step in linking cell-cycle withdrawal to the terminal differentiation of skeletal myoblasts. These results provide important insight into the unique functions of MyoD in regulating the switch from progenitor proliferation to terminal differentiation.Cell survival and differentiation is regulated by NF-κB, a family of ubiquitously expressed transcription factors comprising RelA/p65, c-Rel, RelB, p50 (processed form of p105), and p52 (processed form of p100) [1]. NF-κB proteins function as homo- or heterodimers, the most common of which is the p50/p65 heterodimer. All family members contain a DNA-binding domain, a protein-protein dimerization domain, and a nuclear localization sequence (NLS). However, only RelA/p65, c-Rel, and RelB have a transactivation domain [2].Sub-cellular localization of NF-κB is regulated by ‘inhibitor of κB’ proteins: IκBα, IκBβ, and IκB？ [3]. IκB proteins bind NF-κB, mask the nuclear localization signal, and sequester NF-κB in the cytoplasm as an inactive protein. Upon induction,