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J.Health Sci., 50(3), 309-314, 2004

B-Lymphocytes are Elevated in Mouse Bone Marrow by Estrogen Deficiency, and Induce Receptor Activator of Nuclear Factor kappa B Ligand (RANKL) Expression in Osteoblasts via Cell Adhesion

Chiho Matsumoto, Chisato Miyaura,* and Akira Ito

Department of Biochemistry, School of Pharmacy, Tokyo University of Pharmacy and Life Science, 1432-1, Horinouchi, Hachioji Tokyo 192-0392, Japan

Loss of estrogen caused by ovariectomy (OVX) stimulates bone resorption and bone marrow B-lymphopoiesis, resulting in a marked bone loss and an accumulation of B cells in mouse bone marrow. In OVX mice, the expression of receptor activator of nuclear factor kappa B ligand (RANKL) mRNA was elevated in trabecular bone and bone marrow compared with sham mice. To examine the roles of B-lymphocytes in bone resorption, B cells were purified from bone marrow, fixed, and co-cultured with mouse osteoblasts. Most of the fixed B cells adhered to cell surface of osteoblasts. The expression of RANKL mRNA in osteoblasts was markedly elevated by the contact with the fixed B cells, and the induction rate of RANKL was correlated with the number of B cells added. Treatment with inhibitors of ERK 1/2 and p38 MAP kinases suppressed the B cell-induced RANKL expression in osteoblasts, suggesting the involvement of these kinases in the signals via the cell-to-cell contact. These findings emphasize the roles of B-lymphocytes in RANKL-induced osteoclastogenesis and in pathogenesis of bone loss due to estrogen deficiency.