THE ROLE OF GLUCOCORTICOIDS AND PROSTAGLANDIN E(2) IN THE RECRUITMENTOF BONE-MARROW MESENCHYMAL CELLS TO THE OSTEOBLASTIC LINEAGE - POSITIVE AND NEGATIVE EFFECTS

The role of glucocorticoids in bone formation presents a problem becau se although pharmacological doses in vivo give rise to osteoporosis, p hysiological concentrations are required for osteoblast (OB) different iation in vitro. To try and rationalize this dichotomy, we investigate d the effect of dexamethasone on the recruitment of OB precursors pres ent in bone marrow. Using the GFU-f assay, we can measure (1) total co lony formation; (2) the osteoblastic differentiation of the colonies d efined as their ability to express alkaline phosphatase, synthesize co llagen, and to calcify; and (3) colony expansion as either average col ony surface area or average colony number. In control cultures and in the presence of 10(-10)-10(-9) M dexamethasone, colony formation and t otal cell number was maximal, but the addition of PGE, had no effect o n colony number and very few colonies expressed the OB phenotype. In t he presence of 10(-8)-10(-7) M dexamethasone, colony numbers and total cell numbers were reduced but were increased by the addition of PGE(2 ), the average colony cell number and surface area were relatively unc hanged and a proportion of the colonies expressed APase, calcified and synthesized collagen. In cultures containing 10(-6)-10(-5) M dexameth asone, colony numbers were further reduced but were stimulated by the addition of PGE(2) and some colonies differentiated; however, colony e xpansion was dramatically reduced by up to 80%. These results suggest that physiological levels of glucocorticoids are necessary for OB diff erentiation and allow the control of OB recruitment by PGE(2). High le vels of glucocorticoids drastically reduce proliferation of the OB pre cursors leading to glucocorticoid-induced osteoporosis.