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. Osteoblasts, the bone-forming cells, derive from multipotential bone marrow stromal precursors called colony-forming units-fibroblastic (CFU-F) upon culture ex vivo. Adhesion of such stromal precursors to bone is likely to be an early event in the anabolic response of bone to PTH. To test this, we measured the number of CFU-F that could be extracted from murine bone marrow after administration of an anabolic dose of PTH. We found that a very early response is a dramatic reduction, starting within 2 h, in the number of CFU-F that could be extracted from their bone marrow. We then tested whether PTH has 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. We found that 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 seem to be mediated through a PTH-induced 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 E(2), and indomethacin reversed the PTH-mediated reduction of CFU-F that could be extracted from mouse bone marrow. 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.

It is believed that parathyroid hormone (PTH) increases the resorptive activity of pre-existing osteoclasts through a primary interaction with cells of the osteoblastic lineage. Much less is known, however, of the mechanisms by which PTH induces osteoclast formation. It is known that osteoclast formation occurs through a contact-dependent interaction between stromal cells and haemopoietic precursors, but it is not known whether PTH acts on stromal cells or precursors to induce osteoclast formation. To address this issue, we compared the ability of haemopoietic cultures to generate osteoclasts, identified as calcitonin receptor positive (CTRP) cells, and to resorb bone in response to PTH and 1,25(OH)2 vitamin D3 (1,25(OH)2D3). We found that when murine haemopoietic tissues were incubated at densities sufficiently high to support haemopoiesis, both PTH and 1,25(OH)2D3 induced bone resorption in bone marrow cells, but in cultures of haemopoietic spleen only 1,25(OH)2D3 induced CTRP cells, and neither hormone induced bone resorption. To determine whether these differences were attributable to differences in stromal cells or haemopoietic precursors, lower densities of haemopoietic spleen cells were incubated on osteoblastic (UMR 106), splenic or bone marrow stromal cells. We found that the behaviour of the cocultures reflected the characteristics and origin of the stromal cells. Thus, the ability of both osteoblastic and splenic stromal cells to induce CTRP cells with 1,25(OH)2D3, while only osteoblastic cells induced osteoclasts with PTH, from the same precursors, suggests that the ability of PTH to induce osteoclastic differentiation cannot be attributed to a hormonal action on osteoclast precursors, but depends on a response in stromal cells.