Involvement of mitogen- and stress-activated protein kinase 1 in BMP-6-induced chondrocyte differentiation.

Bone morphogenetic proteins (BMPs) are involved in several cellular responsive actions, such as development, cell differentiation, and apoptosis, via their specific transmembrane receptors. In particular, BMPs promote the differentiation and maturation of bone and cartilage from mesenchymal stem cells. Based on comprehensive analyses performed with a large number of antibodies, mitogen- and stress-activated protein kinase (MSK)1 was found to be immediately phosphorylated in the mouse chondrocyte precursor cell line, ATDC5, upon BMP-6 stimulation. The overexpression and knockdown of MSK1 in ATDC5 cells also enhanced and suppressed BMP-6-induced chondrocyte differentiation, respectively. Similar to ATDC5 cells, an ex vivo organ culture system using mouse embryonic metatarsal bones also demonstrated that BMP-6-mediated MSK1 activation might play a role in chondrocyte differentiation. Using several inhibitors, the p38 kinase pathway was confirmed to be implicated in BMP-6-induced phosphorylation of MSK1. Furthermore, MSK1 mutants lacking kinase activities and those lacking serine/threonine residues targeted by p38 kinase severely impaired their ability to potentiate BMP-6-induced chondrogenic differentiation of ATDC5 cells. Interestingly, a loss-of-function study for Smad4 perturbed BMP-6-induced phosphorylation of p38 kinase to inhibit BMP-6-mediated chondrocyte differentiation via MSK1 activation. Overall, both Smad-dependent and independent pathways require BMP-6-induced chondrocyte differentiation via MSK1 activation in ATDC5 cells.