Distinct pathways of extracellular signal-regulated kinase activation by growth factors, fibronectin and parathyroid hormone 1-34

Growth factors, hormones, and matrix proteins regulate osteoblast proliferation and differentiation, acting through cognate receptors. Since each of the receptors are coupled to a variety of distinct signal transduction pathways, in this report we evaluated whether there is a common convergent intermediate step that allows cross-talk among the various pathways. Since extracellular signal-regulated kinases 1 and 2 (Erk1/2) play a role in mitogenesis and differentiation processes, we evaluated the effects of various osteotrophic factors on Erk1/2 phosphorylation in osteoblasts. Osteoblasts isolated from the metaphyseal marrow (MM) and diaphyseal marrow (DM) of 4-6 week old male rat longitudinal bones were grown to confluency and Erk1/2-phosphorylation was evaluated using antibodies that recognized either the total or the phosphorylated form of the kinase. There was very little Erk1/2 phosphorylation in cells kept in suspension. Both MM and DM cells attached to fibronectin (FN), demonstrated Erk1/2 phosphorylation that persisted for at least up to 8 h. Platelet-derived growth factor AB (PDGF-AB) induced a transient and robust Erk 1/2 phosphorylation that was attenuated by 2 h. Studies with specific inhibitors indicated that the effects of these factors were mediated by protein kinase C, by receptor tyrosine kinase, as well as by protein phosphatases. Parathyroid hormone (PTH 1-34), a bone anabolic agent however, caused a down-regulation of FN stimulated Erk1/2 phosphorylation in MM derived cells. The inhibitory effect of PTH was mediated through cAMP-dependent protein kinase A (PKA) activation. The data collectively suggest that a combination of diverse extracellular stimuli regulates Erk1/2 phosphorylation that may ultimately influence osteoblast proliferation and/or differentiation. (C) 2003 Elsevier Science (USA). All rights reserved.