IDENTIFICATION AND ACTIVATION OF MITOGEN-ACTIVATED PROTEIN (MAP) KINASE IN NORMAL HUMAN OSTEOBLASTIC AND BONE-MARROW STROMAL CELLS - ATTENUATION OF MAP KINASE ACTIVATION BY CAMP, PARATHYROID-HORMONE AND FORSKOLIN

The mitogen-activated protein (MAP) kinases (p44(mapk) and p42(mapk)), also known as extracellular signal-regulated kinases 1 and 2 (ERK1 an d ERK2), are activated in response to a variety of extracellular signa ls, including growth factors, hormones and, neurotransmitters. We have investigated MAP kinase signal transduction pathways in normal human osteoblastic cells. Normal human bone mallow stromal (HBMS), osteoblas tic (HOB), and human (TE85, MG-63, SaOS-2), rat (ROS 17/2.8, UMR-106) and mouse (MC3T3-E1) osteoblastic cell lines contained immunodetectabl e p44(mapk)/ERK1 and p42(mapk)/ERK2. MAP kinase activity was measured by 'in-gel' assay using myelin basic protein as the substrate. Mainly ERK2 was rapidly activated (within 10 min) by bFGF, IGF-I and PDGF-BB in normal HOB, HEMS and human osteosarcoma cells, whereas both ERK1 an d ERK2 were activated by growth factors in rat osteoblast-like cell li nes, ROS 17/2.8 and UMR-106. The ERK1 activation was greater than the ERK2 in ROS 17/2.8 cells. Furthermore, ERK2 was also activated by bFGF and PDGF-BB in the mouse osteoblastic cell line, MC3T3-E1. This is th e first demonstration of inter-species differences in the activation o f MAP kinases in osteoblastic cells. Cyclic AMP derivatives or cAMP ge nerating agents such as PTH and forskolin inhibited ERK2 activation by bFGF and PDGF-BB suggesting a 'cross-talk' between the two different signalling pathways activated by receptor tyrosine kinases and cAMP-de pendent protein kinase. The accumulated results also suggest that the MAP kinases may be involved in mediating mitogenic and other biologica l actions of bFGF, IGF-I and PDGF-BB in normal human osteoblastic and bone marrow stromal cells.