| Summary: | Osteoblasts, the bone-forming cells, derive from multipotential bone marrow stromal precursors called colony-forming units-fibroblastic (CFU-F). CFU-F rapidly adhere to plastic upon culture ex vivo, adhesion of such stromal precursors to bone in vivo is likely to be an early event in the anabolic response to bone stimulatory factors. It has been suggested that osteoclasts are involved in the activation of bone formation during bone remodelling. In order to test this, osteoclast conditioned medium was prepared from osteoclasts cultured on either plastic or bone. It was then used as culture medium for bone marrow cells. It was found that the conditioned medium was unable to increase the adherence of bone marrow cells and therefore the number of CFU-F when cultured in 6-well plates. The ability of parathyroid hormone (PTH) to enhance bone formation has recently been exploited in the treatment of osteoporosis. However, the underlying mechanisms are unknown. PTH and other possible osteoblast activating factors were tested for the ability to activate adhesion of CFU-F in vitro. For this, bone marrow cells were incubated in PTH for varying times. Non-adherent cells were then removed, and the adherent cells were incubated in PTH-free medium for 14 days to assess, as colony formation, the number of CFU-F that had adhered in the preceding period. Incubation in PTH caused a substantial increase in the number of CFU-F that adhered within 24 h. This increase was abrogated by peptidic inhibitors of integrins. The increase did not appear to be mediated through a PTHinduced increase in interleukin-6, since interleukin-6 had no effect on CFU-F numbers when substituted for PTH. Similarly, adhesion was unaffected by incubation of bone marrow cells in dibutyryl cyclic AMP, nor by inhibitors or donors of nitric oxide. However, activation of CFU-F in vitro by PTH was strongly inhibited by indomethacin and mimicked by Prostaglandin E2 (PGE2). To test the effects of PTH in vivo, the number of CFU-F that could be extracted from murine bone marrow after administration of an anabolic dose of PTH were measured. A dramatic reduction in the number of CFU-F that could be extracted from their bone marrow commenced within 2 h of treatment. It was also found that indomethacin reversed the PTH mediated reduction of CFU-F that could be extracted from mouse bone marrow. Intermittent PTH administered over a 6 day period increased the dynamic parameters associated with bone formation and there was a concomitant increase in the number of osteoblasts on bone surfaces. These results suggested that PTH rapidly activates adhesion of CFU-F to plastic or bone surfaces. This activation may represent an early event in the anabolic response of bone cells to PTH.
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