NF-kappa B controls cell growth and differentiation through transcriptional regulation of cyclin D1

Accumulating evidence implicates the transcription factor NF-kappa B as a p ositive mediator of cell growth, but the molecular mechanism(s) involved in this process remains largely unknown. Here we use both a skeletal muscle d ifferentiation model and normal diploid fibroblasts to gain insight into ho w NF-kappa B regulates cell growth and differentiation. Results obtained wi th the C2C12 myoblast cell line demonstrate that NF-kappa B functions as an inhibitor of myogenic differentiation. Myoblasts generated to lack NF-kapp a B activity displayed defects in cellular proliferation and cell cycle exi t upon differentiation. An analysis of cell cycle markers revealed that NF- kappa B activates cyclin D1 expression, and the results showed that this re gulatory pathway is one mechanism by which NF-kappa B inhibits myogenesis. NF-kappa B regulation of cyclin D1 occurs at the transcriptional level and is mediated by direct binding of NF-kappa B to multiple sites in the cyclin D1 promoter. Using diploid fibroblasts, we demonstrate that NF-kappa B is required to induce cyclin D1 expression and pRb hyperphosphorylation and pr omote G(1)-to-S progression. Consistent with results obtained with the C2C1 2 differentiation model, we show that NF-kappa B also promotes cell growth in embryonic fibroblasts, correlating with its regulation of cyclin D1. The se data therefore identify cyclin D1 as an important transcriptional target of NF-kappa B and reveal a mechanism to explain how NF-kappa B is involved in the early phases of the cell cycle to regulate cell growth and differen tiation.