Bone resorption caused by three periodontal pathogens in vivo in mice is mediated in part by prostaglandin.

Gingival inflammation, bacterial infection, alveolar bone destruction, and subsequent tooth loss are characteristic features of periodontal disease, but the precise mechanisms of bone loss are poorly understood. Most animal models of the disease require injury to gingival tissues or teeth, and the effects of microorganisms are thus complicated by host responses to tissue destruction. To determine whether three putative periodontal pathogens, Porphyromonas gingivalis, Campylobacter rectus, and Fusobacterium nucleatum, could cause localized bone resorption in vivo in the absence of tissue injury, we injected live or heat-killed preparations of these microorganisms into the subcutaneous tissues overlying the calvaria of normal mice once daily for 6 days and then examined the bones histologically. We found that all three microorganisms (both live and heat killed) stimulated bone resorption and that the strain of F. nucleatum used appeared to be the strongest inducer of osteoclast activity. Treatment of the mice concomitantly with indomethacin reduced but did not completely inhibit bone resorption by these microorganisms, suggesting that their effects were mediated, in part, by arachidonic acid metabolites (e.g., prostaglandins). Our findings indicate that these potential pathogens can stimulate bone resorption locally when placed beside a bone surface in vivo in the absence of prior tissue injury and support a role for them in the pathogenesis of bone loss around teeth in periodontitis.

Tumor necrosis factor enhances parathyroid hormone-related protein-induced hypercalcemia and bone resorption without inhibiting bone formation in vivo.

Humoral hypercalcemia of malignancy results from the effects of tumor-produced factors on bone, kidney, and intestine that disrupt normal calcium homeostasis. Although parathyroid hormone-related protein (PTHrP) is a major mediator of the syndrome, tumors also produce other hypercalcemic factors, such as tumor necrosis factor (TNF), which may modulate the effects of PTHrP. It has been postulated that TNF may counteract the stimulatory effects of PTHrP on bone formation. To examine the effects of TNF on PTHrP-induced changes in calcium and bone metabolism, a murine tumor model of hypercalcemia was used. Nude mice were inoculated with Chinese hamster ovarian (CHO) cells expressing human TNF (CHO/TNF) or nontransfected CHO cells (CHO/-) and further treated with injections of human PTHrP(1-34) or vehicle. The effects of TNF, PTHrP, and the combination of the two factors on blood ionized calcium, osteoclast recruitment, and bone histomorphometry were evaluated. Mice bearing CHO/TNF tumors that were injected with PTHrP had significantly higher calcium concentrations, increased committed osteoclast progenitors, and mature osteoclasts as well as enhanced bone resorption compared with mice bearing CHO/TNF tumors injected with vehicle or those bearing CHO/- tumors injected with PTHrP or vehicle. A 2-fold increase in new woven bone formed in the calvaria at sites of previous bone resorption was observed in CHO/TNF mice treated with PTHrP. Bone formation rates in the vertebrae were similar in both CHO/- and CHO/TNF mice treated with PTHrP. These data demonstrate that the hypercalcemic effects of PTHrP are enhanced by TNF and that this effect is due to the increased production of committed osteoclast precursors with a subsequent increase in osteoclastic bone resorption. Furthermore, PTHrP caused a coupled increase in osteoclastic bone resorption and new bone formation that was not inhibited by TNF. These findings highlight the complex interactions that may occur between tumor-produced factors on bone that result in malignancy-associated hypercalcemia and suggest that TNF may not be responsible for the decreased bone formation seen in some patients with this condition.

Effects of parathyroid hormone (PTH)-related protein and PTH on osteoclasts and osteoclast precursors in vivo.

Increased production of PTH-related protein (PTHrP) and PTH is frequently responsible for hypercalcemia and its associated morbidity. However, it is unclear whether these peptides produce identical effects on cells in the osteoclast lineage in vivo. To examine the effects of continuous in vivo exposure to these factors on both the osteoclast precursors and mature osteoclasts, we inoculated Chinese hamster ovarian cells expressing PTH-(1-84), PTHrP-(1-141), or nontransfected Chinese hamster ovarian cells into nude mice. The effects of these tumors on blood ionized calcium, plasma PTH and PTHrP concentrations, and osteoclast formation were then determined. PTH and PTHrP tumor-bearing mice became hypercalcemic (1.90 +/- 0.04 and 1.97 +/- 0.16 mmol/liter, respectively) compared with control mice (1.29 +/- 0.015 mmol/liter). After 4 days of hypercalcemia, mice were killed, and bone marrow cells were harvested to examine cells at three discrete stages of osteoclast development: multipotent osteoclast precursors, the granulocyte/macrophage colony-forming unit; more committed marrow mononuclear osteoclast precursors; and mature osteoclasts. Neither PTH nor PTHrP had an effect on granulocyte/macrophage colony-forming unit, but similarly increased the number of more committed mononuclear osteoclast progenitors as well as mature osteoclasts in the calvaria. No differences were detected between the effects of PTH and PTHrP on cells in the osteoclast lineage in vivo. Thus, PTH and PTHrP appear to affect only more differentiated cells in the osteoclast lineage, and the differences in osteoclastic bone resorption between primary hyperparathyroidism and humoral hypercalcemia of malignancy are probably due to mechanisms other than effects on osteoclast precursor cells in vivo.