THE ROLE OF OSTEOBLAST DENSITY AND ENDOGENOUS INTERLEUKIN-6 PRODUCTION IN OSTEOCLAST FORMATION FROM THE HEMATOPOIETIC STEM-CELL LINE FDCP-MIX C(2)GM IN COCULTURE WITH PRIMARY OSTEOBLASTS

Osteoclast formation from the hemopoietic stem cell line FDCP-mix C(2) GM was shown to be strongly dependent on osteoblast density. In cocult ures of C(2)GM cells with fetal mouse osteoblasts seeded at high densi ty (i.e., 2.5 x 10(4) cells/cm(2)), we found a significantly lower ost eoclast formation compared with cocultures with osteoblasts seeded at low density (i.e., 1 x 10(4) cells/cm2). The differentiation state of osteoblasts in high-density cultures resembled more than that of osteo blasts in low-density cultures, the differentiation state of mature os teoblasts, since the cells in the former cultures showed higher alkali ne phosphatase (APase) activity than the cells in the latter cultures, and nodules were formed in high-density cultures but not in low-densi ty cultures. Endogenous interleukin-6 (IL-6) production was found to b e significantly lower in high-density cultures, which may partly expla in the impaired osteoclast formation in high-density cocultures. Addit ion of IL-6 to the high-density cocultures indeed restored osteoclast formation. There appeared to be no overt difference in IL-6 receptor m RNA expression between high-density and low-density cultures. In concl usion, this paper suggests that mature, highly differentiated osteobla sts are not directly involved in osteoclastogenesis. In contrast, oste oblast-like cells lacking mature osteoblast markers induce osteoclast formation. Whether these low-density osteoblast-like cells represent a n immature differentiation state or the lining cell phenotype is uncle ar.