Characterization of the bone morphogenetic protein-2 as a neurotrophic factor - Induction of neuronal differentiation of PC12 cells in the absence of mitogen-activated protein kinase activation.

Rat pheochromocytoma PC12 cells are shown to express a single class of high affinity binding sites for bone morphogenetic protein (BMP)-2 (1,300 receptors/cell, K-d = 31.3 pM). Affinity cross-linking using radiolabeled BMP-2 demonstrated the presence of six components with apparent molecular masses of 170, 155, 105, 90, 80, and 70 kDa. BMP-2 induced morphological changes in PC12 cells with the concomitant expression of three neurofilament proteins. Thus, BMP-2 would appear to be another neurotrophic factor that, like nerve growth factor or basic fibroblast growth factor, stimulates the neuronal differentiation of PC12 cells. Unlike nerve growth factor and basic fibroblast growth factor, how ever, BMP-2 failed to induce the activation of either 41- and 43-kDa mitogen-activated protein (MAP) kinases or the MAP kinase/extracellular signal-regulated kinase kinase (MEK). Also, BMP-2 did not induce the expression of the c-fos gene in PC12 cells. Activin A was also capable of inducing the neuronal differentiation of PC12 cells without activating MAP kinases and MEK. These findings show a clear dissociation between the requirement for the activation of the MAP kinase cascade and the ability of BMP-2 and activin A to induce PC12 cell neuronal differentiation. In addition, these results suggest that the activation of MAP kinases and MEK is not an absolute requirement for PC12 cell differentiation.