Nitric oxide mediates 17beta-estradiol-stimulated human and rodent osteoblast proliferation and differentiation

Oestradiol can stimulate osteoblast activity. Osteoblast function is thought to be regulated by nitric oxide (NO). We hypothesised that the effect of 17beta-oestradiol (17beta-E(2)) on osteoblast activity is mediated by NO. This hypothesis was tested using osteoblasts isolated from human trabecular bone, calvariae of rats, endothelial NO synthase (eNOS) gene-deficient mice, and their wild-type counterparts. Our results show that 17beta-E(2) dose-dependently stimulated proliferation and differentiation of primary human, rat and wild-typeosteoblasts. The presence of N(G)-monomethyl-l-arginine (10(-3) M), an inhibitor of NOS activity, blocked the 17beta-E(2)-(10(-7) M)-induced increases in thymidine incorporation (P < 0.01), alkaline phosphatase activity (P < 0.01) and bone nodule formation (P < 0.01) of wild-type, human and rat osteoblasts, respectively. Moreover, 17beta-E(2) did not induce a response in eNOS gene-deficient osteoblasts. 17beta-E(2) also increased total eNOS enzyme expression in rat osteoblasts. These findings indicate 17beta-E(2) modulates osteoblast function by NO-dependent mechanisms mediated via the eNOS isoform.

Bioglass 45S5 stimulates osteoblast turnover and enhances bone formation In vitro: implications and applications for bone tissue engineering

We investigated the concept of using bioactive substrates as templates for in vitro synthesis of bone tissue for transplantation by assessing the osteogenic potential of a melt-derived bioactive glass ceramic (Bioglass 45S5) in vitro. Bioactive glass ceramic and bioinert (plastic) substrates were seeded with human primary osteoblasts and evaluated after 2, 6, and 12 days. Flow cytometric analysis of the cell cycle suggested that the bioactive glass-ceramic substrate induced osteoblast proliferation, as indicated by increased cell populations in both S (DNA synthesis) and G2/M (mitosis) phases of the cell cycle. Biochemical analysis of the osteoblast differentiation markers alkaline phosphatase (ALP) and osteocalcin indicated that the bioactive glass-ceramic substrate augmented osteoblast commitment and selection of a mature osteoblastic phenotype. Scanning electron microscopic observations of discrete bone nodules over the surface of the bioactive material, from day 6 onward, further supported this notion. A combination of fluorescence, confocal, transmission electron microscopy, and X-ray microprobe (SEM-EDAX) examinations revealed that the nodules were made of cell aggregates which produced mineralized collagenous matrix. Control substrates did not exhibit mineralized nodule formation at any point studied up to 12 days. In conclusion, this study shows that Bioglass 45S5 has the ability to stimulate the growth and osteogenic differentiation of human primary osteoblasts. These findings have potential applications for tissue engineering where this bioactive glass substrate could be used as a template for the formation of bioengineered bone tissue.

Developmental regulation of nitric oxide synthase expression in rat skeletal bone

Nitric oxide (NO) has been implicated in bone growth and remodeling by studies showing that inhibition of NO-synthase (NOS) activity retards normal gain in bone mineral density both during skeletal development and after sexual maturity. In the present study, we aimed to assess the level of expression and cellular localization of the three NOS isoforms during skeletal bone development from neonatal to sexual maturity in female Wistar rats. Reverse transcription polymerase chain reaction (RT-PCR) was used to analyze the presence of NOS1 (neuronal), NOS2 (inducible), and NOS3 (endothelial) transcripts in femoral bone from neonatal, 4-, 8-, and 12-week-old rats. RT-PCR amplified NOS1, NOS2, and NOS3 transcripts of 472-, 807-, and 289-bp, respectively. There were no detectable differences in the levels of NOS1 mRNA between the groups; however, NOS2 mRNA was more abundant in the neonatal group compared with 4-, 8-, and 12-week groups. Expression of NOS1 protein could not be detected in bones by either Western blotting or immunocytochemistry in any of the age groups investigated. Western blots for NOS2 revealed expression in the neonatal group only and it was not detected in any of the older age groups. Immunostaining for NOS2 was also most evident in the neonatal group and was localized specifically to trabecular osteoblasts and osteoclasts. In all age groups studied, NOS3 mRNA and protein were found in bone-resorbing osteoclasts, cuboidal active osteoblasts, and osteocytes. Semiquantitative RT-PCR provided evidence of down-regulation of NOS3 transcripts during the skeletal development. This was confirmed using in situ hybridization, which showed higher expression in neonatal and 4-week groups than in other groups. Western blots and counting the ratio of trabecular osteoblasts that were NOS3 immunoreactive showed parallel down-regulation of NOS3 protein during skeletal development. Taken together, these data show that there is regulation of NOS2 and in particular NOS3 expression during skeletal development and this may be significant to trabecular bone growth and remodeling.

Cytokine-induced prostaglandin E2 synthesis and cyclooxygenase-2 activity are regulated both by a nitric oxide-dependent and -independent mechanism in rat osteoblasts in vitro

Osteoblasts respond to stimulation with interleukin-1 (IL-1), tumor necrosis factor-alpha (TNF-alpha), and interferon-gamma (IFN-gamma) by production of nitric oxide and prostaglandins (PGs). In this study the relationship between nitric oxide and PG synthesis was investigated after cytokine stimulation of cultured rat osteoblasts. IL-1, TNF-alpha, IFN-gamma, and exogenous sodium nitroprusside, a nitric oxide donor, all stimulated PGE2 production in a dose-dependent manner. PGE2 production was blocked by L-nitro-arginine methyl ester, an inhibitor of nitric oxide production, after IFN-gamma stimulation and was partially blocked after TNF-alpha stimulation. However, IL-1-induced PGE2 was unaffected. Similarly, expression of the cyclooxygenase-2 protein was stimulated by cytokines, and IFN-gamma-induced expression was again blocked by L-nitro-arginine methyl ester. In contrast, all cytokines induced the cyclooxygenase-2 mRNA expression independently of nitric oxide production, although exogenous sodium nitroprusside was able to induce the cyclooxygenase-2 mRNA in the absence of cytokines. The results show that nitric oxide can induce PG synthesis and cyclooxygenase-2 expression and may regulate cyclooxygenase-2 expression at both transcriptional and post-transcriptional levels. In addition, the data show the existence of both nitric oxide-dependent and -independent pathways of PG synthesis after cytokine stimulation of osteoblasts. The results suggest that nitric oxide may be an important mediator of PG production in inflammatory bone diseases.

Decreased nitric oxide levels and bone turnover in amenorrheic athletes with spinal osteopenia

Amenorrheic athletes have been likened to postmenopausal women, with low estrogen levels and osteopenia. It has been suggested that estrogen exerts its antiresorptive actions on bone via a nitric oxide (NO)-dependent mechanism. This study investigated whether the mechanism of bone loss in amenorrheic athletes is similar to that of postmenopausal women with reduced NO levels and high bone turnover. Eleven amenorrheic athletes, 15 eumenorrheic athletes, and 10 sedentary controls were studied. Spine and hip bone mineral density was measured using dual-energy x-ray absorptiometry. Bone turnover was assessed by biochemical markers of formation (osteocalcin and bone-specific alkaline phosphatase) and resorption (deoxypyridinoline). NO metabolites were measured from 24-h urine samples using a chemiluminescence assay. Spine, but not hip, bone mineral density was reduced in the amenorrheic group, compared with the eumenorrheic (P = 0.0001) and control (P = 0.04) groups. Osteocalcin, bone-specific alkaline phosphatase, and deoxypyridinoline were similar in all groups. NO metabolites were lower in the amenorrheic group, compared with controls (P = 0.035), despite a higher dietary intake of nitrates. Unlike postmenopausal women, amenorrheic athletes do not have raised bone turnover but do have reduced NO metabolites and spinal osteopenia. The results show, however, that reduced NO production is a common denominator in both conditions and further support the importance of NO in estrogen-mediated protection of skeletal mass and strength.

Sensory and sympathetic innervation of the vertebral endplate in patients with degenerative disc disease

We obtained intervertebral discs with cartilage endplates and underlying cancellous bone at operation from patients with degenerative disc disease and then used immunohistochemical techniques to localise the nerves and nerve endings in the specimens. We used antibodies for the ubiquitous neuronal protein gene product 9.5 (PGP 9.5). Immunoreactivity to neuropeptide Y was used to identify autonomic nerves and calcitonin gene-related peptide (CGRP) and substance P to identify sensory nerves. Blood vessels were identified by immunoreactivity with platelet-endothelial cell-adhesion molecule (CD31; PECAM). In a control group with no known history of chronic back pain, nerve fibres immunoreactive to PGP 9.5 and neuropeptide Y were most closely related to blood vessels, with occasional substance P and CGRP immunoreactivity. In patients with severe back pain and markedly reduced disc height, proliferation of blood vessels and accompanying nerve fibres was observed in the endplate region and underlying vertebral bodies. Many of these nerves were immunoreactive to substance P or CGRP, and in addition, substance P- and CGRP-immunoreactive nociceptors were seen unrelated to blood vessels. Quantification by image analysis showed a marked increase in CGRP-containing sensory nerve fibres compared with normal control subjects. We speculate that a chemotactic response to products of disc breakdown is responsible for the proliferation of vascularity and CGRP-containing sensory nerves found in the endplate region and vertebral body adjacent to degenerate discs. The neuropeptides substance P and CGRP have potent vasodilatory as well as pain-transmitting effects. The increase in sensory nerve endings suggests increase in blood flow, perhaps as an attempt to augment the nutrition of the degenerate disc. The increase in the density of sensory nerves, and the presence of endplate cartilage defects, strongly suggest that the endplates and vertebral bodies are sources of pain; this may explain the severe pain on movement experienced by some patients with degenerative disc disease.