Stimulation of the proliferation of human bone cells in vitro by human monocyte products with interleukin-1 activity.

The process of induction of bone formation, which follows bone resorption during normal and pathological bone turnover, is well documented. However, the mechanisms responsible for this process are unclear. Mononuclear phagocytes present at the sites of bone remodeling could play a role in this "coupling" of bone formation to bone resorption. This study was designed to investigate such a possibility. By measuring both the increase in [3H]thymidine incorporation and in cell number, we found that human monocytes in culture released factors capable of stimulating the proliferation of osteoblast-like cells derived from human bone. Rapidly dividing cells exhibited a greater response to interleukin 1 (IL-1) than confluent cells. The factors are similar to IL-1 in that they exhibited the same molecular weight and isoelectric point, were present in fractions that contained IL-1 activity after gel filtration chromatography and isoelectric focusing, and showed similar dose-response characteristics. Proliferation was more marked when prostaglandin production by the cells, which was also stimulated by these factors, was inhibited by indomethacin. A factor produced by monocytes that affects osteoblast activity may be important in the coupling of osteoclast and osteoblast actions.