Nitric Oxide Synthase Expression in Bone Cells

We have localized the expression of the three main nitric oxide synthases (eNOS, bNOS, and iNOS) in bone cells of rats and humans using immunohistochemistry. The predominant isoform expressed in normal adult bone was the constitutive isoform, eNOS, mainly in cells of osteoblastic lineage. In adult bone, the osteoblast lineage cells exhibiting eNOS expression were flat bone lining cells and osteocytes, but cuboidal osteoblasts were consistently negative. Expression for bNOS was not detected in any bone cells. iNOS expression was not detected in any cells of osteoblastic lineage in normal adult rat or human bone, but was observed in cuboidal osteoblasts of adult rats with experimental colitis, in which the suppression in bone formation may be cytokine mediated. Osteoclasts in normal rat tissue showed expression for both eNOS and iNOS, but these were patchy. As for cells of the osteoblast lineage, osteoclasts were negative for bNOS. Thus, our findings support evidence, from in vitro studies and from animal experiments, that nitric oxide may play an important role in the physiology of bone.