The role of osteoblast density and endogenous interleukin-6 production in osteoclast formation from the hemopoietic stem cell line FDCP-MIX C2GM 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 cocultures of C(2)GM cells with fetal mouse osteoblasts seeded at high density (i.e., 2.5 x 10(4) cells/cm(2)), we found a significantly lower osteoclast 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 osteoblasts in low-density cultures, the differentiation state of mature osteoblasts, since the cells in the former cultures showed higher alkaline phosphatase (APase) activity than the cells in the latter cultures, and nodules were formed in high-density cultures but not in low-density cultures. Endogenous interleukin-6 (IL-6) production was found to be significantly lower in high-density cultures, which may partly explain the impaired osteoclast formation in high-density cocultures. Addition of IL-6 to the high-density cocultures indeed restored osteoclast formation. There appeared to be no overt difference in IL-6 receptor mRNA expression between high-density and low-density cultures. In conclusion, this paper suggests that mature, highly differentiated osteoblasts are not directly involved in osteoclastogenesis. In contrast, osteoblast-like cells lacking mature osteoblast markers induce osteoclast formation. Whether these low-density osteoblast-like cells represent an immature differentiation state or the lining cell phenotype is unclear.