Osteocytic expression of constitutive NO synthase isoforms in the femoral neck cortex: a case-control study of intracapsular hip fracture.

Journal article

UNLABELLED: NO is an osteocytic signaling molecule that can inhibit osteoclasts. The NO synthases eNOS and nNOS were expressed by >50% of osteonal osteocytes in controls. Hip fracture cases showed +NOS osteocytes only in deep osteonal bone, and 25-35% reduced expression overall. These data are consistent with increased osteonal vulnerability to deep osteoclastic attack. INTRODUCTION: Osteocytes may regulate the response to mechanical stimuli in bone through the production of local signaling molecules such as NO derived from the NO synthase eNOS. Because NO is inhibitory to osteoclastic resorption, it has been suggested that osteocytes expressing eNOS act as sentinels, confining resorption within single osteons. Recently, nNOS has been shown to be present in osteocytes of adult human bone. MATERIALS AND METHODS: Cross-sections of the femoral neck (eight female cases of intracapsular hip fracture and seven postmortem controls; age, 68-91 years) were analyzed by immunohistochemistry. The percentages of osteocytes expressing each of these two isoforms were calculated, and their distances to the nearest canal surface were measured. RESULTS: The percentage of +nNOS osteocytes was lower in the fracture cases than in the controls (cases: 43.12 +/- 1.49, controls: 56.68 +/- 1.45; p < 0.0001). Compared with nNOS, eNOS expression was further reduced (p = 0.009) in the cases but was not different in the controls (cases: 36.41 +/- 1.53, controls: 56.47 +/- 2.41; p < 0.0001). The minimum distance of +eNOS or +nNOS osteocytes to a canal surface was higher in the cases compared with controls (eNOS: controls; 44.4 +/- 2.2 microm, cases: 61.7 +/- 2.0 microm; p < 0.0001; nNOS: controls: 52.4 +/- 1.7 microm, cases: 60.2 +/- 2.1 microm; p = 0.0039). +eNOS osteocytes were closer to the canal surfaces than +nNOS osteocytes in the controls by 8.00 +/- 4.0 microm (p = 0.0012). CONCLUSION: The proportions of osteocytes expressing nNOS and eNOS were both reduced in the fracture cases, suggesting that the capacity to generate NO might be reduced. Furthermore, the reduction in NOS expression occurs in those osteocytes closest to the canal surface, suggesting that the ability of NO to minimize resorption depth might be impaired. Further studies are needed on the regulation of the expression and activity of these distinct NOS isoforms.

Authors: Caballero-Alías, A; Loveridge, N; Pitsillides, A; Parker, M; Kaptoge, S

• Publication status: Published
• Journal: Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research
• Volume: 20
• Issue: 2
• Pages: 268-273
• Publication date: 2005-02-01
• DOI: 10.1359/jbmr.041103
• EISSN: 1523-4681
• ISSN: 0884-0431
• Language: English
• Keywords: Nitric Oxide Synthase Type III; Hip Joint; Bone Resorption; Immunohistochemistry; Nerve Tissue Proteins; Nitric Oxide Synthase Type I; Time Factors; Case-Control Studies; Osteoclasts; Osteocytes; Femoral Neck Fractures; Nitric Oxide Synthase; Humans; Aged; Protein Isoforms; Female; Image Processing, Computer-Assisted; Aged, 80 and over