Effects of nitric oxide synthase inhibitors on bone formation in rats

Protein Kinase G2 Is Essential for Skeletal Homeostasis and Adaptation to Mechanical Loading in Male but Not Female Mice

We previously showed that the NO/cGMP/protein kinase G (PKG) signaling pathway positively regulates osteoblast proliferation, differentiation, and survival in vitro, and that cGMP-elevating agents have bone-anabolic effects in mice. Here, we generated mice with an osteoblast-specific (OB) knockout (KO) of type 2 PKG (gene name Prkg2) using a Col1a1(2.3 kb)-Cre driver. Compared to wild type (WT) littermates, 8-week-old male OB Prkg2-KO mice had fewer osteoblasts, reduced bone formation rates, and lower trabecular and cortical bone volumes. Female OB Prkg2-KO littermates showed no bone abnormalities, despite the same degree of PKG2 deficiency in bone. Expression of osteoblast differentiation- and Wnt/β-catenin-related genes was lower in primary osteoblasts and bones of male KO but not female KO mice compared to WT littermates. Osteoclast parameters were unaffected in both sexes. Since PKG2 is part of a mechano-sensitive complex in osteoblast membranes, we examined its role during mechanical loading. Cyclical compression of the tibia increased cortical thickness and induced mechanosensitive and Wnt/β-catenin-related genes to a similar extent in male and female WT mice and female OB Prkg2-KO mice, but loading had a minimal effect in male KO mice. We conclude that PKG2 drives bone acquisition and adaptation to mechanical loading via the Wnt/β-catenin pathway in male mice. The striking sexual dimorphism of OB Prkg2-KO mice suggests that current U.S. Food and Drug Administration-approved cGMP-elevating agents may represent novel effective treatment options for male osteoporosis. © 2022 American Society for Bone and Mineral Research (ASBMR).

Levels of Biological Markers of Nitric Oxide in Serum of Patients with Mandible Fractures

Background: Nitric oxide is a small gaseous molecule with significant bioactivity. It has been observed that NO may have a dual role dependent on its production and concentrations in the bone microenvironment. The objective of the study was to assess the concentration of total nitric oxide malonyldialdehyde, nitrotyrosine, and asymmetric dimethylarginine in the serum of patients with mandibular fractures and to understand the relationship between these compounds, in order to expand the knowledge base of the role of nitric oxide and its activity indicators in the process of bone fracture healing. Material and Methods: The study included 20 patients with mandibular fractures who were undergoing inpatient and outpatient treatments and a control group of 15 healthy people. Results were analyzed with respect to the measurement time. Total nitric oxide concentration in the blood serum was determined according to the Griess reaction, while the concentration of malonyldialdehyde, nitrotyrosine, and asymmetric dimethylarginine was estimated using the immunoenzymatic method (i.e., enzyme-linked immunosorbent assay). Results: Before the procedure, as well as on the first day and 2 and 6 weeks after the procedure, higher concentrations of total nitric oxide and lower concentrations of malonyldialdehyde were observed in the blood serum of patients with mandibular fractures compared to the control group. No statistically significant differences were found in nitrotyrosine concentrations in the blood serum of patients throughout the measurement period. However, a significantly higher asymmetric dimethylarginine concentration was observed in the patient serum before the procedure and on the first day of operation as compared with the control group. Analysis of the results observed in patient serum with respect to the number of fractures within the mandible demonstrated the same trend of concentrations for the tested compounds for the entire study group. Conclusions: In summary, our results revealed that the intensity of local processes resulting from mandibular fractures is associated with the concentration of nitric oxide, confirming its significant role, as well as that of its indicators, in the process of bone fracture healing in this patient population.

Skeletal Functions of Voltage Sensitive Calcium Channels

Voltage-sensitive calcium channels (VSCCs) are ubiquitous multimeric protein complexes that are necessary for the regulation of numerous physiological processes. VSCCs regulate calcium influx and various intracellular processes including muscle contraction, neurotransmission, hormone secretion, and gene transcription, with function specificity defined by the channel's subunits and tissue location. The functions of VSCCs in bone are often overlooked since bone is not considered an electrically excitable tissue. However, skeletal homeostasis and adaptation relies heavily on VSCCs. Inhibition or deletion of VSCCs decreases osteogenesis, impairs skeletal structure, and impedes anabolic responses to mechanical loading. RECENT FINDINGS: While the functions of VSCCs in osteoclasts are less clear, VSCCs have distinct but complementary functions in osteoblasts and osteocytes. PURPOSE OF REVIEW: This review details the structure, function, and nomenclature of VSCCs, followed by a comprehensive description of the known functions of VSCCs in bone cells and their regulation of bone development, bone formation, and mechanotransduction.

Phenomenon of osteocyte lacunar mineralization: indicator of former osteocyte death and a novel marker of impaired bone quality?

An increasing number of patients worldwide suffer from bone fractures that occur after low intensity trauma. Such fragility fractures are usually associated with advanced age and osteoporosis but also with long-term immobilization, corticosteroid therapy, diabetes mellitus, and other endocrine disorders. It is important to understand the skeletal origins of increased bone fragility in these conditions for preventive and therapeutic strategies to combat one of the most common health problems of the aged population. This review summarizes current knowledge pertaining to the phenomenon of micropetrosis (osteocyte lacunar mineralization). As an indicator of former osteocyte death, micropetrosis is more common in aged bone and osteoporotic bone. Considering that the number of mineralized osteocyte lacunae per bone area can distinguish healthy, untreated osteoporotic and bisphosphonate-treated osteoporotic patients, it could be regarded as a novel structural marker of impaired bone quality. Further research is needed to clarify the mechanism of lacunar mineralization and to explore whether it could be an additional target for preventing or treating bone fragility related to aging and various endocrine diseases.

Malnutrition and Fracture Healing: Are Specific Deficiencies in Amino Acids Important in Nonunion Development?

With the increasing incidence of fractures now, and in the future, the absolute number of bone-healing complications such as nonunion development will also increase. Next to fracture-dependent factors such as large bone loss volumes and inadequate stabilization, the nutritional state of these patients is a major influential factor for the fracture repair process. In this review, we will focus on the influence of protein/amino acid malnutrition and its influence on fracture healing. Mainly, the arginine-citrulline-nitric oxide metabolism is of importance since it can affect fracture healing via several precursors of collagen formation, and through nitric oxide synthases it has influences on the bio-molecular inflammatory responses and the local capillary growth and circulation.

Nitric oxide and cyclic GMP functions in bone

Nitric oxide plays a central role in the regulation of skeletal homeostasis. In cells of the osteoblastic lineage, NO is generated in response to mechanical stimulation and estrogen exposure. Via activation of soluble guanylyl cyclase (sGC) and cGMP-dependent protein kinases (PKGs), NO enhances proliferation, differentiation, and survival of bone-forming cells in the osteoblastic lineage. NO also regulates the differentiation and activity of bone-resorbing osteoclasts; here the effects are largely inhibitory and partly cGMP-independent. We review the skeletal phenotypes of mice deficient in NO synthases and PKGs, and the effects of NO and cGMP on bone formation and resorption. We examine the roles of NO and cGMP in bone adaptation to mechanical stimulation. Finally, we discuss preclinical and clinical data showing that NO donors and NO-independent sGC activators may protect against estrogen deficiency-induced bone loss. sGC represents an attractive target for the treatment of osteoporosis.

A Novel, Direct NO Donor Regulates Osteoblast and Osteoclast Functions and Increases Bone Mass in Ovariectomized Mice

Most US Food and Drug Administration (FDA)-approved treatments for osteoporosis target osteoclastic bone resorption. Only PTH derivatives improve bone formation, but they have drawbacks, and novel bone-anabolic agents are needed. Nitrates, which generate NO, improved BMD in estrogen-deficient rats and may improve bone formation markers and BMD in postmenopausal women. However, nitrates are limited by induction of oxidative stress and development of tolerance, and may increase cardiovascular mortality after long-term use. Here we studied nitrosyl-cobinamide (NO-Cbi), a novel, direct NO-releasing agent, in a mouse model of estrogen deficiency-induced osteoporosis. In murine primary osteoblasts, NO-Cbi increased intracellular cGMP, Wnt/β-catenin signaling, proliferation, and osteoblastic gene expression, and protected cells from apoptosis. Correspondingly, in intact and ovariectomized (OVX) female C57Bl/6 mice, NO-Cbi increased serum cGMP concentrations, bone formation, and osteoblastic gene expression, and in OVX mice, it prevented osteocyte apoptosis. NO-Cbi reduced osteoclasts in intact mice and prevented the known increase in osteoclasts in OVX mice, partially through a reduction in the RANKL/osteoprotegerin gene expression ratio, which regulates osteoclast differentiation, and partially through direct inhibition of osteoclast differentiation, observed in vitro in the presence of excess RANKL. The positive NO effects in osteoblasts were mediated by cGMP/protein kinase G (PKG), but some of the osteoclast-inhibitory effects appeared to be cGMP-independent. NO-Cbi increased trabecular bone mass in both intact and OVX mice, consistent with its in vitro effects on osteoblasts and osteoclasts. NO-Cbi is a novel direct NO-releasing agent that, in contrast to nitrates, does not generate oxygen radicals, and combines anabolic and antiresorptive effects in bone, making it an excellent candidate for treating osteoporosis. © 2016 American Society for Bone and Mineral Research.

A neglected requirement for optimizing treatment of age-related osteoporosis: Replenishing the skeleton's base reservoir with net base-producing diets

Osteoporosis is a disorder of bone in which the mass of the bone is reduced and the bone's architecture at the microscopic level is disordered. Together those abnormalities predispose affected individuals to experience fractures despite only minimal trauma (i.e., fragility fractures). Age related osteoporosis is a common type of osteoporosis that occurs with aging in both men and women usually beginning after the age of peak bone mass. Research has found that the disorder can be partially reversed by reducing the net amount of acid that is produced when consuming typical Western diets. However, the amelioration that results has not been so dramatic or so consistent that physicians have adopted the procedure as part of the standard treatment for age-related osteoporosis. We propose that reducing the net acid load from the diet is not sufficient to reverse age related osteoporosis because it fails to supply base needed to restore the large amount of base in bone that had been lost by reacting with the net acid load of the diet that had been consumed for years or decades. Reducing the net acid load from the diet might be expected to have little ameliorative effect or merely slow the progression of the disorder. We hypothesize that both to restore osteoporotic bone to, or nearly to, its pre-disease state, as well as to eliminate the risk of fragility fractures, requires consuming diets that produce net amounts of base to restore the base lost from years to decades of consuming diets that produce net amounts of acid. We hypothesize also that the excess base and attendant subclinical metabolic alkalosis will both stimulate the cellular process of bone formation and suppress the cellular process of bone resorption, and thereby implement the restorative process.

The anabolic action of intermittent parathyroid hormone on cortical bone depends partly on its ability to induce nitric oxide-mediated vasorelaxation in BALB/c mice

There is strong evidence that vasodilatory nitric oxide (NO) donors have anabolic effects on bone in humans. Parathyroid hormone (PTH), the only osteoanabolic drug currently approved, is also a vasodilator. We investigated whether the NO synthase inhibitor L‐NAME might alter the effect of PTH on bone by blocking its vasodilatory effect. BALB/c mice received 28 daily injections of PTH[1–34] (80 µg/kg/day) or L‐NAME (30 mg/kg/day), alone or in combination. Hindlimb blood perfusion was measured by laser Doppler imaging. Bone architecture, turnover and mechanical properties in the femur were analysed respectively by micro‐CT, histomorphometry and three‐point bending. PTH increased hindlimb blood flow by >30% within 10 min of injection (P < 0.001). Co‐treatment with L‐NAME blocked the action of PTH on blood flow, whereas L‐NAME alone had no effect. PTH treatment increased femoral cortical bone volume and formation rate by 20% and 110%, respectively (P < 0.001). PTH had no effect on trabecular bone volume in the femoral metaphysis although trabecular thickness and number were increased and decreased by 25%, respectively. Co‐treatment with L‐NAME restricted the PTH‐stimulated increase in cortical bone formation but had no clear‐cut effects in trabecular bone. Co‐treatment with L‐NAME did not affect the mechanical strength in femurs induced by iPTH. These results suggest that NO‐mediated vasorelaxation plays partly a role in the anabolic action of PTH on cortical bone. © 2016 The Authors. Cell Biochemistry and Function published by John Wiley & Sons, Ltd.

Mechanical Stimulation and IGF-1 Enhance mRNA Translation Rate in Osteoblasts Via Activation of the AKT-mTOR Pathway

Insulin-like growth factor-1 (IGF-1) is anabolic for muscle by enhancing the rate of mRNA translation via activation of AKT and subsequent activation of the mammalian target of rapamycin complex 1 (mTOR), thereby increasing cellular protein production. IGF-1 is also anabolic for bone, but whether the mTOR pathway plays a role in the rate of bone matrix protein production by osteoblasts is unknown. We hypothesized that anabolic stimuli such as mechanical loading and IGF-1 stimulate protein synthesis in osteoblasts via activation of the AKT-mTOR pathway. MC3T3-E1 osteoblasts were either or not subjected for 1 h to mechanical loading by pulsating fluid flow (PFF) or treated with or without human recombinant IGF-1 (1-100 ng/ml) for 0.5-6 h, to determine phosphorylation of AKT and p70S6K (downstream of mTOR) by Western blot. After 4 days of culture with or without the mTOR inhibitor rapamycin, total protein, DNA, and gene expression were quantified. IGF-1 (100 ng/ml) reduced IGF-1 gene expression, although PFF enhanced IGF-1 expression. IGF-1 did not affect collagen-I gene expression. IGF-1 dose-dependently enhanced AKT and p70S6K phosphorylation at 2 and 6 h. PFF enhanced phosphorylation of AKT and p70S6K already within 1 h. Both IGF-1 and PFF enhanced total protein per cell by ∼30%, but not in the presence of rapamycin. Our results show that IGF-1 and PFF activate mTOR, thereby stimulating the rate of mRNA translation in osteoblasts. The known anabolic effect of mechanical loading and IGF-1 on bone may thus be partly explained by mTOR-mediated enhanced protein synthesis in osteoblasts.

Nontuberculous mycobacterial osteomyelitis

Osteomyelitis caused by nontuberculous mycobacteria (NTM) can have severe consequences and a poor prognosis. Physicians therefore need to be alert to this condition, especially in immunocompromised patients. Although the pathogenesis of NTM osteomyelitis is still unclear, studies in immunodeficient individuals have revealed close relationships between NTM osteomyelitis and defects associated with the interleukin-12-interferon-γ-tumor necrosis factor-α axis, as well as human immunodeficiency virus infection, various immunosuppressive conditions, and diabetes mellitus. Culture and species identification from tissue biopsies or surgical debridement tissue play crucial roles in diagnosing NTM osteomyelitis. Suitable imaging examinations are also important. Adequate surgical debridement and the choice of appropriate, combined antibiotics for long-term anti-mycobacterial chemotherapy, based on in vitro drug susceptibility tests, are the main therapies for these bone infections. Bacillus Calmette-Guerin vaccination might have limited prophylactic value. The use of multiple drugs and long duration of treatment mean that the therapeutic process needs to be monitored closely to detect potential side effects. Adequate duration of anti-mycobacterial chemotherapy together with regular monitoring with blood and imaging tests are key factors determining the recovery outcome in patients with NTM osteomyelitis.

BMP Signaling Regulates Bone Morphogenesis in Zebrafish through Promoting Osteoblast Function as Assessed by Their Nitric Oxide Production

Bone morphogenetic proteins (BMPs) control many developmental and physiological processes, including skeleton formation and homeostasis. Previous studies in zebrafish revealed the crucial importance of proper BMP signaling before 48 h post-fertilization (hpf) for cartilage formation in the skull. Here, we focus on the involvement of the BMP pathway between 48 and 96 hpf in bone formation after 96 hpf. Using BMP inhibitors and the expression of a dominant-negative BMP receptor, we analyze whether the loss of BMP signaling affects osteoblastogenesis, osteoblast function and bone mineralization. To this end, we used the transgenic zebrafish line Tg(osterix:mCherry), detection of nitric oxide (NO) production, and alizarin red staining, respectively. We observed that inhibition of BMP signaling between 48 and 72 hpf led to a reduction of NO production and bone mineralization. Osteoblast maturation and chondrogenesis, on the other hand, seemed unchanged. Osteoblast function and bone formation were less affected when BMP signaling was inhibited between 72 and 96 hpf. These results suggest that for the onset of bone formation, proper BMP signaling between 48 and 72 hpf is crucial to ensure osteoblast function and ossification. Furthermore, detection of NO in developing zebrafish larvae appears as an early indicator of bone calcification activity.

Nitric oxide signaling in mechanical adaptation of bone

One of the most serious healthcare problems in the world is bone loss and fractures due to a lack of physical activity in elderly people as well as in bedridden patients or otherwise inactive youth. Crucial here are the osteocytes. Buried within our bones, these cells are believed to be the mechanosensors that stimulate bone formation in the presence of mechanical stimuli and bone resorption in the absence of such stimuli. Intercellular signaling is an important physiological phenomenon involved in maintaining homeostasis in all tissues. In bone, intercellular communication via chemical signals like NO plays a critical role in the dynamic process of bone remodeling. If bones are mechanically loaded, fluid flows through minute channels in the bone matrix, resulting in shear stress on the cell membrane that activates the osteocyte. Activated osteocytes produce signaling molecules like NO, which modulate the activity of the bone-forming osteoblasts and the bone-resorbing osteoclasts, thereby orchestrating bone adaptation to mechanical loading. In this review, we highlight current insights in the role of NO in the mechanical adaptation of bone mass and structure, with emphasis on its role in local bone gain and loss as well as in remodeling supervised by osteocytes. Since mechanical stimuli and NO production enhance bone strength and fracture resistance, these new insights may facilitate the development of novel osteoporosis treatments.

IL-6 Alters Osteocyte Signaling toward Osteoblasts but Not Osteoclasts

Mechanosensitive osteocytes regulate bone mass in adults. Interleukin 6 (IL-6), such as present during orthodontic tooth movement, also strongly affects bone mass, but little is known about the effect of IL-6 on osteocyte function. Therefore we aimed to determine in vitro whether IL-6 affects osteocyte mechanosensitivity, and osteocyte regulation of osteoclastogenesis and osteoblast differentiation. MLO-Y4 osteocytes were incubated with/without IL-6 (1 or 10 pg/mL) for 24 hr. Subsequently, osteocytes were subjected to mechanical loading by pulsating fluid flow (PFF) for 1 hr. Mouse osteoclast precursors were cultured for 7 days on top of IL-6-treated osteocytes. Conditioned medium from osteocytes treated with/without IL-6 was added to MC3T3-E1 pre-osteoblasts for 14 days. Exogenous IL-6 (10 pg/mL) did not alter the osteocyte response to PFF. PFF significantly enhanced IL-6 production by osteocytes. IL-6 enhanced Rankl expression but reduced caspase 3/7 activity by osteocytes, and therefore did not affect osteocyte-stimulated osteoclastogenesis. Conditioned medium from IL-6-treated osteocytes reduced alkaline phosphatase (ALP) activity and Runx2 expression in osteoblasts, but increased expression of the proliferation marker Ki67 and osteocalcin. Our results suggest that IL-6 is produced by shear-loaded osteocytes and that IL-6 may affect bone mass by modulating osteocyte communication toward osteoblasts.

Endothelial nitric oxide synthase is not essential for nitric oxide production by osteoblasts subjected to fluid shear stress in vitro

Endothelial nitric oxide synthase (eNOS) has long been held responsible for NO production by mechanically stimulated osteoblasts, but this has recently been disputed. We investigated whether one of the three known NOS isoforms is essential for NO production by mechanically stimulated osteoblasts in vitro and revisited the bone phenotype of the eNOS-/- mouse. Osteoblasts, obtained as outgrowths from mouse calvaria or long bones of wild-type (WT), eNOS-/-, inducible NOS-/- (iNOS-/-), or neuronal NOS-/- (nNOS-/-) mice, were subjected to mechanical stimulation by means of pulsating fluid flow (PFF); and NO production was determined. Tibiae and femora from 8-week-old mice were subjected to μCT and three-point bending tests. Deletion of single NOS isoforms did not lead to significant upregulation of alternate isoforms in cultured osteoblasts from WT, eNOS-/-, iNOS-/-, or nNOS-/- mice. Expression of eNOS mRNA in osteoblasts was below our detection limit, and no differences in growth between WT and eNOS-/- osteoblasts were found. PFF increased NO production by approximately fourfold in WT and eNOS-/- osteoblasts and significantly stimulated NO production in iNOS-/- and nNOS-/- osteoblasts. Tibiae and femora from WT and eNOS-/- mice showed no difference in bone volume and architecture or in mechanical parameters. Our data suggest that mechanical stimuli can enhance NO production by cultured osteoblasts singly deficient for each known NOS isoform and that lack of eNOS does not significantly affect bone mass and strength at 8 weeks of age. Our data challenge the notion that eNOS is a key effector of mechanically induced bone maintenance.

Organic nitrates for osteoporosis: an update

The number of osteoporotic fractures is increasing worldwide as populations age. An inexpensive and widely available treatment is necessary to alleviate this increase in fractures. Current treatments decrease fractures at trabecular bone sites (spine) but have limited effects at cortical sites (hip, legs, forearm and upper arm)-the most common sites of osteoporotic fracture. Treatments are also limited by costs, side effects and lack of availability. Nitric oxide (NO) is a novel agent that has the potential to influence cortical bone, is inexpensive, widely available and has limited side effects. In this review, we will evaluate the in vitro and in vivo data that support the concept that NO is important in bone cell function, review the observational, case control and randomized trial data on organic nitrates and the effects of these agents on bone turnover, geometry and strength.

Epigenetic regulation during fetal femur development: DNA methylation matters

Epigenetic modifications are heritable changes in gene expression without changes in DNA sequence. DNA methylation has been implicated in the control of several cellular processes including differentiation, gene regulation, development, genomic imprinting and X-chromosome inactivation. Methylated cytosine residues at CpG dinucleotides are commonly associated with gene repression; conversely, strategic loss of methylation during development could lead to activation of lineage-specific genes. Evidence is emerging that bone development and growth are programmed; although, interestingly, bone is constantly remodelled throughout life. Using human embryonic stem cells, human fetal bone cells (HFBCs), adult chondrocytes and STRO-1⁺ marrow stromal cells from human bone marrow, we have examined a spectrum of developmental stages of femur development and the role of DNA methylation therein. Using pyrosequencing methodology we analysed the status of methylation of genes implicated in bone biology; furthermore, we correlated these methylation levels with gene expression levels using qRT-PCR and protein distribution during fetal development evaluated using immunohistochemistry. We found that during fetal femur development DNA methylation inversely correlates with expression of genes including iNOS (NOS2) and COL9A1, but not catabolic genes including MMP13 and IL1B. Furthermore, significant demethylation was evident in the osteocalcin promoter between the fetal and adult developmental stages. Increased TET1 expression and decreased expression of DNA (cytosine-5-)-methyltransferase 1 (DNMT1) in adult chondrocytes compared to HFBCs could contribute to the loss of methylation observed during fetal development. HFBC multipotency confirms these cells to be an ideal developmental system for investigation of DNA methylation regulation. In conclusion, these findings demonstrate the role of epigenetic regulation, specifically DNA methylation, in bone development, informing and opening new possibilities in development of strategies for bone repair/tissue engineering.

BONE ADAPTATION AND REGENERATION – NEW DEVELOPMENTS

Bone is a dynamic tissue that is constantly renewed and adapts to its local loading environment. Mechanical loading results in adaptive changes in bone size and shape that strengthen bone structure. The mechanisms for adaptation involve a multistep process called mechanotransduction, which is the ability of resident bone cells to perceive and translate mechanical energy into a cascade of structural and biochemical changes within the cells. The transduction of a mechanical signal to a biochemical response involves pathways within the cell membrane and cytoskeleton of the osteocytes, the professional mechansensor cells of bone. During the last decade the role of mechanosensitive osteocytes in bone metabolism and turnover, and the lacuno-canalicular porosity as the structure that mediates mechanosensing, is likely to reveal a new paradigm for understanding the bone formation response to mechanical loading, and the bone resorption response to disuse. Strain-derived fluid flow of interstitial fluid through the lacuno-canalicular porosity seems to mechanically activate the osteocytes, as well as ensures transport of cell signaling molecules, nutrients and waste products. Cell-cell signaling from the osteocyte sensor cells to the effector cells (osteoblasts or osteoclasts), and the effector cell response – either bone formation or resorption, allow an explanation of local bone gain and loss as well as remodeling in response to fatigue damage as processes supervised by mechanosensitive osteocytes.

The osteogenic activity of cultured bone cells has been quantitatively correlated with varying stress stimulations highlighting the importance of the rate of loading. Theoretically a possible mechanism for the stress response by osteocytes is due to strain amplification at the pericellular matrix. Single cell studies on molecular responses of osteocytes provide insight on local architectural alignment in bone during remodeling. Alignment seems to occur as a result of the osteocytes sensing different canalicular flow patterns around cutting cone and reversal zone during loading, thus determining the bone's structure. Disturbances in architecture and permeability of the 3D porous network will affect transduction of mechanical loads to the mechanosensors.

Uncovering the cellular and mechanical basis of the osteocyte's response to loading represents a significant challenge to our understanding of cellular mechanotransduction and bone remodeling. In view of the importance of mechanical stress for maintaining bone strength, mechanical stimuli have great potential for providing a therapeutic approach for bone (re)generation.

Sodium/myo-inositol cotransporter 1 and myo-inositol are essential for osteogenesis and bone formation

myo-Inositol (MI) plays an essential role in several important processes of cell physiology, is involved in the neural system, and provides an effective treatment for some psychiatric disorders. Its role in osteogenesis and bone formation nonetheless is unclear. Sodium/MI cotransporter 1 (SMIT1, the major cotransporter of MI) knockout (SMIT1(-/-)) mice with markedly reduced tissue MI levels were used to characterize the essential roles of MI and SMIT1 in osteogenesis. SMIT1(-/-) embryos had a dramatic delay in prenatal mineralization and died soon after birth owing to respiratory failure, but this could be rescued by maternal MI supplementation. The rescued SMIT1(-/-) mice had shorter limbs, decreased bone density, and abnormal bone architecture in adulthood. Deletion of SMIT1 resulted in retarded postnatal osteoblastic differentiation and bone formation in vivo and in vitro. Continuous MI supplementation partially restored the abnormal bone phenotypes in adult SMIT1(-/-) mice and strengthened bone structure in SMIT1(+/+) mice. Although MI content was much lower in SMIT1(-/-) mesenchymal cells (MSCs), the I(1,4,5)P(3) signaling pathway was excluded as the means by which SMIT1 and MI affected osteogenesis. PCR expression array revealed Fgf4, leptin, Sele, Selp, and Nos2 as novel target genes of SMIT1 and MI. SMIT1 was constitutively expressed in multipotential C3H10T1/2 and preosteoblastic MC3T3-E1 cells and could be upregulated during bone morphogenetic protein 2 (BMP-2)-induced osteogenesis. Collectively, this study demonstrated that deficiency in SMIT1 and MI has a detrimental impact on prenatal skeletal development and postnatal bone remodeling and confirmed their essential roles in osteogenesis, bone formation, and bone mineral density (BMD) determination.

Is nitric oxide a mediator of the effects of low-intensity electrical stimulation on bone in ovariectomized rats?

Low-intensity electrical stimulation (LIES) may counteract the effects of ovariectomy (OVX) on nitric oxide synthase (NOS) expression, osteocyte viability, bone structure, and microarchitecture in rats (Lirani-Galvão et al., Calcif Tissue Int 84:502-509, 2009). The aim of the present study was to investigate if these effects of LIES could be mediated by NO. We analyzed the effects of NO blockage (by L-NAME) in the response to LIES on osteocyte viability, bone structure, and microarchitecture in OVX rats. Sixty rats (200-220 g) were divided into six groups: sham, sham-L-NAME (6 mg/kg/day), OVX, OVX-L-NAME, OVX-LIES, and OVX-LIES-L-NAME. After 12 weeks, rats were killed and tibiae collected for histomorphometric analysis and immunohistochemical detection of endothelial NOS (eNOS), inducible NOS (iNOS), and osteocyte apoptosis (caspase-3 and TUNEL). In the presence of L-NAME, LIES did not counteract the OVX-induced effects on bone volume and trabecular number (as on OVX-LIES). L-NAME blocked the stimulatory effects of LIES on iNOS and eNOS expression of OVX rats. Both L-NAME and LIES decreased osteocyte apoptosis. Our results showed that in OVX rats L-NAME partially blocks the effects of LIES on bone structure, turnover, and expression of iNOS and eNOS, suggesting that NO may be a mediator of some positive effects of LIES on bone.

Expression of cartilage developmental genes in Hoxc8- and Hoxd4-transgenic mice

Hox genes encode transcription factors, which regulate skeletal patterning and chondrocyte differentiation during the development of cartilage, the precursor to mature bone. Overexpression of the homeobox transcription factors Hoxc8 and Hoxd4 causes severe cartilage defects due to delay in cartilage maturation. Matrix metalloproteinases (MMPs), bone morphogenetic proteins (BMPs) and fibroblastic growth factors (FGFs) are known to play important roles in skeletal development and endochondral bone formation and remodeling. In order to investigate whether these molecules are aberrantly expressed in Hoxc8- and/or Hoxd4-transgenic cartilage, we performed quantitative RT-PCR on chondrocytes from Hox-transgenic mice. Gene expression levels of Bmp4, Fgf8, Fgf10, Mmp9, Mmp13, Nos3, Timp3, Wnt3a and Wnt5a were altered in Hoxc8-transgenic chondrocytes, and Fgfr3, Ihh, Mmp8, and Wnt3a expression levels were altered in Hoxd4-transgenic chondrocytes, respectively. Notably, Wnt3a expression was elevated in Hoxc8- and reduced in Hoxd4-transgenic cartilage. These results suggest that both transcription factors affect cartilage maturation through different molecular mechanisms, and provide the basis for future studies into the role of these genes and possible interactions in pathogenesis of cartilage defects in Hoxc8- and Hoxd4-transgenic mice.

From streaming-potentials to shear stress: 25 years of bone cell mechanotransduction

Mechanical loads are vital regulators of skeletal mass and architecture as evidenced by the increase in bone formation following the addition of exogenous loads and loss of bone mass following their removal. While our understanding of the molecular mechanisms by which bone cells perceive changes in their mechanical environment has increased rapidly in recent years, much remains to be learned. Here, we outline the effects of interstitial fluid flow, a potent biophysical signal induced by the deformation of skeletal tissue in response to applied loads, on bone cell behavior. We focus on the molecular mechanisms by which bone cells are hypothesized to perceive interstitial fluid flow, the cell signaling cascades activated by fluid flow, and the use of this signal in tissue engineering protocols.

Mechanical signaling for bone modeling and remodeling

Proper development of the skeleton in utero and during growth requires mechanical stimulation. Loading results in adaptive changes in bone that strengthen bone structure. Bone's adaptive response is regulated by the ability of resident bone cells to perceive and translate mechanical energy into a cascade of structural and biochemical changes within the cells a process known as mechanotransduction. Mechanotransduction pathways are among the most anabolic in bone, and consequently, there is great interest in elucidating how mechanical loading produces its observed effects, including increased bone formation, reduced bone loss, changes in bone cell differentiation and lifespan, among others. A molecular understanding of these processes is developing, and with it comes a profound new insight into the biology of bone. In this article, we review the nature of the physical stimulus to which bone cells mount an adaptive response, including the identity of the sensor cells, their attributes and physical environment, and putative mechanoreceptors they express. Particular attention is allotted to the focal adhesion and Wnt signaling, in light of their emerging role in bone mechanotransduction. Te cellular mechanisms for increased bone loss during disuse, and reduced bone loss during loading are considered. Finally, we summarize the published data on bone cell accommodation, whereby bone cells stop responding to mechanical signaling events. Collectively, these data highlight the complex yet finely orchestrated process of mechanically regulated bone homeostasis.

Genetic disruption of all NO synthase isoforms enhances BMD and bone turnover in mice in vivo: involvement of the renin-angiotensin system

INTRODUCTION

NO is synthesized by three different NO synthase (NOS) isoforms, including neuronal (nNOS), inducible (iNOS) and endothelial NOS (eNOS). The roles of NO in bone metabolism have been extensively investigated in pharmacological studies and in studies with NOS isoform-deficient mice. However, because of the nonspecificity of agents and compensation among the NOS isoforms, the ultimate roles of endogenous NO are still poorly understood. To address this point, we successfully generated mice in which all three NOS genes are completely disrupted. In this study, we examined whether bone metabolism is abnormal in those mice.

MATERIALS AND METHODS

Experiments were performed in 12-wk-old male wildtype, singly nNOS(-/-), iNOS(-/-), and eNOS(-/-) and triply n/i/eNOS(-/-) mice. BMD was assessed by DXA. The kinetics of osteoblastic bone formation and those of osteoclastic bone resorption were evaluated by measurements of morphological and biochemical markers.

RESULTS

BMD was significantly higher only in the triply NOS(-/-) mice but not in any singly NOS(-/-) mice compared with the wildtype mice. Markers of osteoblastic bone formation, including bone formation rate, mineral apposition rate, and serum alkaline phosphatase concentration, were also significantly larger only in the triply NOS(-/-) mice compared with wildtype mice. Furthermore, markers of osteoclastic bone resorption, including osteoclast number, osteoclast surface, and urinary deoxypyridinoline excretion, were again significantly greater only in the triply NOS(-/-) mice. Importantly, the renin-angiotensin system in bone was significantly activated in the triply NOS(-/-) mice, and long-term oral treatment with an angiotensin II type 1 (AT(1)) receptor blocker normalized this pathological bone remodeling in those mice.

CONCLUSIONS

These results provide the first direct evidence that genetic disruption of the whole NOS system enhances BMD and bone turnover in mice in vivo through the AT(1) receptor pathway, showing the critical role of the endogenous NO/NOS system in maintaining bone homeostasis.

Cbfa-1 mediates nitric oxide regulation of MMP-13 in osteoblasts

During bone development, osteoblast differentiation requires remodeling of the extracellular matrix. Although underlying mechanisms have not been elucidated, evidence points to the participation of the nitric oxide (NO) and cyclic guanosine 3',5'-monophosphate (cGMP) system. Here, we detected increased matrix metalloproteinase (MMP)-13 mRNA, protein and activity, as well as increased inducible NO synthase (iNOS) and NO production during the differentiation of MC3T3-E1 osteoblasts. Transcriptional activity of the MMP-13 promoter was augmented by NO, 8-bromo-cGMP (8-Br-cGMP), and by a dominant-positive form of protein kinase G (PKG1-alpha). The stimulatory effect on the MMP-13 promoter was partially inhibited by mutation of the osteoblast-specific element 2 (OSE-2) binding site. Core binding factor-1 (Cbfa-1) expression peaked at 7 days of differentiation, and was phosphorylated by PKG in vitro. Cbfa-1 was localized to cell nuclei, and its translocation was inhibited by the iNOS inhibitor 1400W. Immunohistological examination revealed that MMP-13 and Cbfa-1 expression levels are both reduced in 17-day-old embryos of iNOS-deficient mice. Silencing of Cbfa-1 mRNA blocked MMP-13 expression without interfering with endogenous NO production, confirming its role in NO-induced MMP-13 expression by MC3T3-E1 cells. The results described here suggest a mechanism by which NO regulates osteogenesis.

Study on the effects of mechanical pressure to the ultrastructure and secretion ability of mandibular condylar chondrocytes

During mandibular movement, condyle is subjected to repetitive compression and the mandibular condylar chondrocytes (MCCs) can detect and respond to this biomechanical environment by altering their metabolism. The present study was undertaken to investigate the effects of pressure to the ultrastructure, aggrecan synthesis, nitric oxide (NO) and prostaglandin F(1)alpha(PGF(1)alpha) secretion in MCCs. In vitro cultured rabbit MCCs were incubated and pressed under continuous pressure of 90kPa for 60min and 360min by hydraulic pressure controlled cellular strain unit. The ultrastructure, aggrecan mRNA expression, activity of nitric oxide synthase (NOS) and PGF(1)alpha secretion were investigated. Besides, nitric oxide inhibitor was used together with pressure to investigate the role of NO in mechanical effects. The appearance of MCC on TEM showed that after been pressed under 90kPa for 60min, the cellular processes became elongated and voluminous, together with aggrecan mRNA increasing. Under 90kPa for 360min, some of the cells showed distinct sign of apotosis and the aggrecan mRNA decreased. Pressure of 90kPa could cause increase of NOS activity and decrease of PGF(1)alpha composition. Inhibitor experiments indicated that pressure-induced upregulation of aggrecan mRNA and inhibition of PGF(1)alpha synthesis was partly mediated by NO. Continuous pressure could cause changes on the ultrastructure and function of MCC, as well as up-regulation of aggrecan synthesis, increase of NO secretion and decrease of PGF(1)alpha composition. NO was the upstream molecule, which mediated the response of aggrecan and PGF(1)alpha to mechanical pressure.

Bio imaging of intracellular NO production in single bone cells after mechanical stimulation

We show the intracellular upregulation of NO production after mechanical stimulation, an essential chemical signal in bone remodeling. This is done in real time using the fluorescent chromophore DAR-4M AM. Differences in cellular response to mechanical stimulation of different regions of a single cell were observed.

INTRODUCTION

Osteocytes are the most abundant bone cells that are believed to be the mechanosensors of bone, responding to mechanical stresses in interstitial fluid flow through the canaliculi. Under mechanical load, chemical signals such as NO play a key role in the activity of osteoblasts/osteoclasts that regulate bone remodeling. Despite the importance of NO in signaling, its real-time detection has proved challenging. This is largely because of the short NO half-life (typically approximately 0.1-5 s). Here, we show the upregulation of intracellular NO production in single osteocytes under localized mechanical stimulation.

MATERIALS AND METHODS

We used the chromophore DAR-4M AM for NO detection. This is loaded into surface-attached MLO-Y4 osteocyte-like and MC3T3-E1 osteoblast-like cells that are subjected to a localized mechanical stimulation using optical tweezers or a microneedle tip. DAR-4M AM is membrane-permeable and chelates NO, forming a stable, fluorescent compound, which is visible with a rhodamine filter.

RESULTS

Nonstimulated MLO-Y4 and MC3T3-E1 cells showed basal NO production levels, as indicated by a gradual increase in their fluorescence intensity. Localized mechanical stimulation of single MC3T3-E1 cells and MLO-Y4 cells by optical tweezers (150-550 pN, 0.5-3 Hz, 1 minute) showed a nearly 15-30% increase, whereas MLO-Y4 cells stimulated by a microneedle (10-20 nN, 1 minute) showed nearly 15-16% increase relative to their nonstimulated state. Furthermore, stimulation of a single cell process by a microneedle resulted in a 2-10% increase in the fluorescence intensity.

CONCLUSIONS

NO is essential for mechanically induced bone remodeling and is a meaningful parameter for measuring bone cell activation after mechanical loading. Here we show NO upregulation in individual bone cells after a localized mechanical stimulation. We also show that both the cell body and the cell processes might be involved in mechanosensing. This technique allows characterization of the mechanosensitivity of different parts of a single osteocyte. This opens up the possibility to uncover the complexities and function of single osteocytes in the dynamic process of bone remodeling.

Release of nitric oxide, but not prostaglandin E2, by bone cells depends on fluid flow frequency

Loading frequency is an important parameter for the stimulation of bone formation in vivo. It is still unclear how the information of external loading characteristics is conveyed to osteoblasts and osteoclasts. Osteocytes are thought to detect mechanical loads by sensing fluid flow through the lacuno-canalicular network within bone and to translate this information into chemical signals. The signaling molecules nitric oxide (NO) and prostaglandin E2 (PGE2) are known to play important roles in the adaptive response of bone to mechanical loads. We have investigated the effects of fluid flow frequency on the production of PGE2 and NO in bone cells in vitro. Pulsatile fluid flow with different frequencies stimulated the release of NO by MC3T3-E1 osteoblasts in a dose-dependent manner. In contrast, PGE2 production was enhanced consistently by all fluid flow regimes, independent of flow frequency. This implies that the NO response may play a role in mediating the differential effects of the various loading patterns on bone.

Roles of MAP kinases in the regulation of bone matrix gene expressions in human osteoblasts by oscillatory fluid flow

We investigated the effects of oscillatory flow in regulating the gene expressions of type I collagen (COL1, the main component of human bone tissues) and osteopontin (OPN, the key gene for calcium deposition) in human osteoblast-like (MG-63) cells, and the roles of mitogen-activated protein kinases (MAPKs) in this regulation. The cells were subjected to oscillatory flow (0.5 +/- 4 dyn/cm(2)) or kept under static condition for various time periods (15 min, 30 min, 1 h, 2 h, 4 h, 8 h, and 16 h). Oscillatory flow caused significant up-regulations of both COL1 and OPN gene expressions over the 16 h of study, and a transient activation of MAPKs was starting at 15 min and declining to basal level in 2 h. The flow-induction of COL1 was blocked by an ERK inhibitor (PD98059) and reduced by a JNK inhibitor (SP600125), whereas that of OPN was abolished by PD98059. Analysis of the cis-elements in the COL1 and OPN promoters suggests the involvement of transacting factors Elk-1 and AP-1 in the transcription regulation. The ERK inhibitor (PD98059) blocked Elk-1 phosphorylation, as well as COL1 and OPN gene expression. The JNK inhibitor (SP600125) abolished c-jun phosphorylation and COL1 expression. These results suggest that the flow-induction of OPN was mediated through the ERK-Elk1-OPN pathway, and that COL1 was regulated by both the ERK-Elk1-COL1 and JNK-c-JUN-COL1 pathway.

The effects of arg-vasopressin on osteoblast-like cells in endothelial nitric oxide synthase-knockout mice and their wild type counterparts

In the present study, we investigated whether nitric oxide (NO) could be involved in the effects of arg-vasopressin (AVP) on osteoblast-like cells. Cells derived from endothelial nitric oxide synthase (eNOS)-knockout mice and their wild type (WT) counterparts, and an osteosarcoma cell line (SaOS-2) were used. AVP (10-100 pmol/l) increased proliferation of osteoblast-like cells from WT mice. The effect was abolished by an AVP V1-receptor antagonist. AVP increased proliferation of cells from eNOSKO mice only when a NO donor, SNAP, was added. A nitric oxide synthase-inhibitor, L-NAME, antagonized the increase in cell proliferation in response to AVP in SaOS-2 cells. In conclusion, this study indicates that NO is involved in the effects of AVP on cell proliferation in osteoblast-like cells.

Group III metabotropic glutamate receptor activation inhibits Ca2+ influx and nitric oxide synthase activity in bone marrow stromal cells

Nitric oxide (NO) is pivotal to bone physiology. In the central nervous system constitutive, Ca(2+)-calmodulin regulated NO synthase activity and glutamate signalling are intimately linked. Since L-glutamate signalling occurs in bone and is implicated in bone regulation, we have investigated the effect of L-glutamate on NO synthase in bone-derived cells. Treatment of marrow stromal cells with L-glutamate reduced basal NO synthase activity by 40%. Imaging showed that L-glutamate caused a rapid, usually localised and slowly-reversible fall in [Ca(2+)](i). This effect was resistant to disruption of intracellular Ca(2+) stores but sensitive to extracellular La(3+) or omission of extracellular Ca(2+), demonstrating that glutamate acts by inhibition of membrane Ca(2+) influx. The only previous description of such an effect of L-glutamate is via activation of the group III receptor, mGluR6, in the retina. Using Western blotting and RT-PCR we detected mGluR6 protein and transcripts in marrow stromal cells. The effects of L-glutamate on NOS activity and [Ca(2+)](i) in marrow stromal cells were abolished by a group III mGluR inhibitor, (S)-2-amino-2-methyl-4-phosphonobutyric acid. Recording of membrane potential showed that, similarly to the effects of retinal mGluR6 activation, L-glutamate induced membrane hyperpolarisation (-16 +/- 2 mV), which was also sensitive to group III mGluR inhibition. L-glutamate had no effect on cAMP levels. We conclude that activation of a group III mGluR in bone marrow stromal cells inhibits a Ca(2+)-permeable plasma membrane channel, reducing [Ca(2+)](i) and suppressing generation of NO. These observations directly link bone L-glutamate signalling to processes central to bone growth and regulation.

Deletion of iNOS gene impairs mouse fracture healing

Nitric oxide (NO) is a signaling molecule synthesized from l-arginine by nitric oxide synthases (NOSs). NOS isoforms are either constitutive (endothelial NOS [eNOS] and neuronal NOS [nNOS]) or inducible NOS (iNOS). Previously, our group has reported that NO is expressed during and modulates fracture healing. In this study, we evaluated the specific contribution of iNOS to fracture healing by using iNOS gene therapy in iNOS-deficient mice. Twelve-week-old female wild-type mice and iNOS-KO mice had a right femoral midshaft osteotomy fixed with an intramedullary 0.5-mm-diameter needle. A gelatine sponge was implanted across the fracture site. The gelatine sponge received either Ad5-CMViNOS (in iNOS-deficient mice; n=16) or Ad5-CMVempty (in wild-type mice; n=15, and iNOS-deficient mice; n=15) at a dose of 10(7) pfu. Mice were sacrificed at day 14, and their right and left hind limbs were harvested. Cross-sectional area (CSA) was determined by measuring the callus diameter across the mediolateral and anteroposterior plane using a vernier caliper. Specimens were loaded to failure torsionally in a biaxial INSTRON testing system, and maximum torque, torsional stiffness, and maximal and total energy were determined. Deletion of the iNOS gene decreased the total and maximum energy absorption of the healing femoral fracture by 30% and by 70% (P<0.01), respectively, in comparison to the wild-type mice. This reduction in energy absorption was reversed by iNOScDNA administration via adenovirus vector. Furthermore, iNOScDNA caused an increase in torsional failure by 20% (P=0.01) in comparison to iNOS(-/-) mice that did not receive the iNOScDNA. There were no significant differences in the biomechanical properties of intact femora. These data indicate that iNOS is important in mouse fracture healing. However, the clinical utility of NOS gene therapy to enhance fracture healing will need further evaluation.

Inhibited anabolic effect of insulin-like growth factor-I on stromal bone marrow cells in endothelial nitric oxide synthase-knockout mice

AIMS

Insulin-like growth factor-I (IGF-I), parathyroid hormone (PTH) and PTH-related protein (PTHrP) are hormones that have anabolic effects on bone formation. The aim of this study was to investigate whether production of nitric oxide (NO) is involved in the effect of IGF-I and PTH/PTHrP on osteoblast-like cells.

METHODS

Bone marrow stromal cells from adult endothelial nitric oxide synthase (eNOS)-knockout (eNOSKO) mice and wild type (WT) counterparts were cultivated with osteogenic substances. The cells showed an osteoblastic phenotype measured as osteocalcin production and alkaline phosphatase activity. DNA synthesis was measured as [3H] thymidine incorporation in the bone marrow cells and in a human osteosarcoma cell-line (SaOS-2).

RESULTS

The stimulatory effect of IGF-I on thymidine incorporation seen in WT animals was absent in eNOSKO mice. Addition of a NO donor to eNOSKO cells recovered the effect of IGF-I on thymidine incorporation. PTH/PTHrP stimulated cell proliferation in both WT and eNOSKO mice. In SaOS-2 cells, incubation with IGF-I together with a NOS inhibitor resulted in an inhibition of the anabolic effect of IGF-I on cell proliferation.

CONCLUSIONS

The stimulatory effect of IGF-I on WT cell proliferation was abolished in eNOSKO cells, recovered by an NO donor and inhibited in osteosarcoma cells by a NOS inhibitor. The results indicate that the effect of IGF-I is dependent on NO production. The impaired IGF-I response may contribute to the bone defect formation seen in eNOSKO animals.

Are Cardiovascular Disease and Osteoporosis Directly Linked?

For years, osteoporosis and cardiovascular disease were thought to be two independent consequences of aging; however, mounting evidence supports an association between these diseases. Recently, a widespread class of cholesterol-lowering drugs known as statins have demonstrated (in rodents and cell cultures) the ability to induce bone formation. This finding is significant since current therapies are limited to the prevention or slowing down of bone loss rather than (enhancing/improving) bone formation. In humans, the ability of statins to generate new bone has not been consistent; however, several investigations have demonstrated a dramatic decrease in fracture risk. Although it has been proposed that statins induce new bone via increased bone morphogenetic protein-2, other conditions affected by statins such as dyslipidaemia, vascular calcification, endothelial dysfunction and impaired nitric oxide expression, may also contribute to the cardiovascular and bone health paradigm. Furthermore, the role of physical activity and its influence on cardiovascular and bone health, especially in postmenopausal women, may contribute to the discrepancy of findings in human data. In summary, it remains to be determined if statins contribute to bone health via improvements in vascular health or by pleiotropic properties unique to their pharmacology. This review provides information on our current understanding of the bone and cardiovascular association, as well as on novel areas of research to further our current understanding of these conditions.

NO plays a role in LPS-induced decreases in circulating IGF-I and IGFBP-3 and their gene expression in the liver

In this study, we administered aminoguanidine, a relatively selective inducible nitric oxide synthase (iNOS) inhibitor, to study the role of nitric oxide (NO) in LPS-induced decrease in IGF-I and IGFBP-3. Adult male Wistar rats were injected intraperitoneally with LPS (100 microg/kg), aminoguanidine (100 mg/kg), LPS plus aminoguanidine, or saline. Rats were injected at 1730 and 0830 the next day and killed 4 h after the last injection. LPS administration induced an increase in serum concentrations of nitrite/nitrate (P < 0.01) and a decrease in serum concentrations of growth hormone (GH; P < 0.05) and IGF-I (P < 0.01) as well as in liver IGF-I mRNA levels (P < 0.05). The LPS-induced decrease in serum concentrations of IGF-I and liver IGF-I gene expression seems to be secondary to iNOS activation, since aminoguanidine administration prevented the effect of LPS on circulating IGF-I and its gene expression in the liver. In contrast, LPS-induced decrease in serum GH was not prevented by aminoguanidine administration. LPS injection decreased IGFBP-3 circulating levels (P < 0.05) and its hepatic gene expression (P < 0.01), but endotoxin did not modify the serum IGFBP-3 proteolysis rate. Aminoguanidine administration blocked the inhibitory effect of LPS on both IGFBP-3 serum levels and its hepatic mRNA levels. When aminoguanidine was administered alone, IGFBP-3 serum levels were increased (P < 0.05), whereas its hepatic mRNA levels were decreased. This contrast can be explained by the decrease (P < 0.05) in serum proteolysis of this binding protein caused by aminoguanidine. These data suggest that iNOS plays an important role in LPS-induced decrease in circulating IGF-I and IGFBP-3 by reducing IGF-I and IGFBP-3 gene expression in the liver.

A case for strain-induced fluid flow as a regulator of BMU-coupling and osteonal alignment

Throughout life, human bone is renewed continuously in a tightly controlled sequence of resorption and formation. This process of bone remodeling is remarkable because it involves cells from different lineages, collaborating in so-called basic multicellular units (BMUs) within small spatial and temporal boundaries. Moreover, the newly formed (secondary) osteons are aligned to the dominant load direction and have a density related to its magnitude, thus creating a globally optimized mechanical structure. Although the existence of BMUs is amply described, the cellular mechanisms driving bone remodeling-particularly the alignment process-are poorly understood. In this study we present a theory that explains bone remodelling as a self-organizing process of mechanical adaptation. Osteocytes thereby act as sensors of strain-induced fluid flow. Physiological loading produces stasis of extracellular fluid in front of the cutting cone of a tunneling osteon, which will lead to osteocytic disuse and (continued) attraction of osteoclasts. However, around the resting zone and the closing cone, enhanced extracellular fluid flow occurs, which will activate osteocytes to recruit osteoblasts. Thus, cellular activity at a bone remodeling site is well related to local fluid flow patterns, which may explain the coordinated progression of a BMU.

Signaling by mechanical strain involves transcriptional regulation of proinflammatory genes in human periodontal ligament cells in vitro

Intracellular signals generated by mechanical strain profoundly affect the metabolic function of osteoblast-like periodontal ligament (PDL) cells, which reside between the tooth and alveolar bone. In response to applied mechanical forces, PDL cells synthesize bone-resorptive cytokines to induce bone resorption at sites exposed to compressive forces and deposit bone at sites exposed to tensile forces in an environment primed for catabolic processes. The intracellular mechanisms that regulate this bone remodeling remain unclear. Here, in an in vitro model system, we show that tensile strain is a critical determinant of PDL-cell metabolic functions. Equibiaxial tensile strain (TENS), when applied at low magnitudes, acts as a potent antagonist of interleukin (IL)-1beta actions and suppresses transcriptional regulation of multiple proinflammatory genes. This is evidenced by the fact that TENS at low magnitude: (i) inhibits recombinant human (rh)IL-1beta-dependent induction of cyclooxygenase-2 (COX-2) mRNA expression and production of prostaglandin estradiol (PGE2); (ii) inhibits rhIL-1beta-dependent induction matrix metalloproteinase-1 (MMP-1) and MMP-3 synthesis by suppressing their mRNA expression; (iii) abrogates rhIL-1beta-induced suppression of tissue inhibitor of metalloprotease-II (TIMP-II) expression; and (iv) reverses IL-1beta-dependent suppression of osteocalcin and alkaline phosphatase synthesis. Nevertheless, these actions of TENS were observed only in the presence of IL-1beta, as TENS alone failed to affect any of the aforementioned responses. The present findings are the first to show that intracellular signals generated by low-magnitude mechanical strain interfere with one or more critical step(s) in the signal transduction cascade of rhIL-1beta upstream of mRNA expression, while concurrently promoting the expression of osteogenic proteins in PDL cells.

Effect of asymmetric dimethylarginine on osteoblastic differentiation

BACKGROUND

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthase (NOS) that accumulates in renal insufficiency and may be a uremic toxin. To determine whether ADMA inhibits bone metabolism, we investigated the in vitro effect of ADMA on osteoblastic differentiation in mouse bone marrow-derived mesenchymal stem cells (BMSCs).

METHODS

The effect of ADMA on nitric oxide (NO) production was determined by measuring the stable end product of NO, nitrite, in the culture medium using commercial NO kit. The temporal sequence of osteoblastic differentiation in BMSCs was assessed in the presence and absence of ADMA by measuring alkaline phosphatase (ALP) activity, mineralization, and osteoblast gene expression at 0, 4, 8, 12 days of culture.

RESULTS

ADMA (5, 50, 500 micromol. L-1) resulted in a dose-dependent decrease in nitrite formation in conditioned media of BMCS cultures, consistent with inhibition of NOS. ADMA treatment was associated with reduced ALP activity, calcium deposition and osteoblast-related gene expression in BMSCs cultures. Concurrent treatment with l-arginine (3600 micromol. L-1) reversed the ADMA (500 micromol. L-1)-mediated decrease in NO production, restored the differentiation potential of BMSCs, and significantly attenuated the down-regulation of Cbfa1 and osteocalcin gene expression by ADMA.

CONCLUSIONS

ADMA inhibition of the NO-NOS pathway in BMSCs impairs osteoblastic differentiation of mouse BMSC cultures. These studies further support a role of NO in the local regulation of bone metabolism and the possibility that ADMA may act as uremic toxin on bone through its effect to inhibit NO actions in osteoblasts.

The production of nitric oxide and prostaglandin E(2) by primary bone cells is shear stress dependent

Loading-induced flow of interstitial fluid through the lacuno-canalicular network is a likely signal for bone cell adaptive responses. However, the nature of the stimulus that activates the cell is debated. Candidate stimuli include wall shear stress, streaming potentials, and chemotransport. We have addressed the nature of the flow-derived cell stimulus by comparing variations in fluid transport with variations in wall shear stress, using nitric oxide (NO) and prostaglandin E(2) (PGE(2)) production as a parameter of bone cell activation. Adult mouse long bone cell cultures were treated for 15min with or without pulsating fluid flow using the following regimes: Low PFF, mean flow rate 0.20 cm(3)/s, 3 Hz, shear stress 0.4+/-0.12 Pa; Medium PFF, 0.33 cm(3)/s, 5 Hz, 0.6+/-0.27 Pa; and High PFF, 0.63 cm(3)/s, 9Hz, 1.2+/-0.37 Pa. In some Low PFF experiments, 2.8% neutral dextran (mol. wt. 4.98x10(4)) was added to the flow medium to increase the viscosity, thereby increasing the wall shear stress 3-fold to a level similar of the High PFF stimulus, but without affecting streaming potentials or chemotransport. NO and PGE(2) production were stimulated by Low, Medium, and High PFF in a dose-dependent manner. Application of Low PFF using dextran-supplemented medium, enhanced both the NO and PGE(2) response by 3-fold, to a level mimicking the response to High PFF at normal viscosity. These results show that the production of NO and PGE(2) by bone cells can be enhanced in a dose-dependent manner by fluid flow of increasing wall shear stress. Therefore, the stimulus leading to NO and PGE(2) production is the flow-derived shear stress, and not streaming potentials or chemotransport.

Effect of L-lysine and L-arginine on primary osteoblast cultures from normal and osteopenic rats

A therapeutic role of amino acids L-lysine (Lys) and L-arginine (Arg) in osteoporosis and fracture healing was demonstrated previously by in vivo studies. In the present study, primary cultures of osteoblasts were used to investigate the effect of amino acids on gene expression (alkaline phosphatase activity, ALP; osteocalcin, OC; type I collagen), nitric oxide production (NO) and proliferation (MTT) of cells. Cells were isolated from the distal femurs of normal and osteopenic rats. Normal and osteopenic bone-derived cells were divided into four groups: control, Lys (0.587 mg/mL/d), Arg (0.625 mg/mL/d), and Lys + Arg (0.587 + 0.625 mg/mL/d). No evidence of differences between normal and osteopenic bone-derived cultures in basal conditions was observed. A significant (P = 0.002) increase of 10.4% in NO production was observed in normal bone-derived osteoblasts treated with Lys + Arg when compared to the control group at 7 days. At the same time, normal bone-derived osteoblasts treated with Arg and Lys + Arg showed significant increases in type I collagen synthesis of 25.3% and 28.4%, respectively, when compared to the control group. Osteopenic bone-derived osteoblasts showed significant (P = 0.002) increases of 27.6% in MTT and 28.7% in cell count at 48 hours when treated with Lys + Arg in comparison with the control group. At 7 days, NO production and type I collagen synthesis increased significantly (P< 0.005) both in osteopenic bone-derived osteoblasts treated with Arg (NO: 18.5%; type I collagen: 34.4%) and Lys + Arg (NO: 23.7%; type I collagen: 20.9%) compared to the control group. Finally, a significant (P = 0.025) decrease of 5.8% in OC level was observed in osteopenic bone-derived osteoblasts treated with Arg. Results suggest that the potential therapeutic effect of Lys and Arg on bone could be related, at least in part, to an improvement of NO production and type I collagen synthesis by osteoblasts both in normal and in osteopenic bone. In osteopenic bone-derived osteoblasts this synthetic phase is preceded by an initial increase of cell proliferation.

Defective bone formation and anabolic response to exogenous estrogen in mice with targeted disruption of endothelial nitric oxide synthase

Nitric oxide (NO) is a pleiotropic signaling molecule that is produced by bone cells constitutively and in response to diverse stimuli such as proinflammatory cytokines, mechanical strain, and sex hormones. Endothelial nitric oxide synthase (eNOS) is the predominant NOS isoform expressed in bone, but its physiological role in regulating bone metabolism remains unclear. Here we studied various aspects of bone metabolism in female mice with targeted disruption of the eNOS gene. Mice with eNOS deficiency (eNOS KO) had reduced bone mineral density, and cortical thinning when compared with WT controls and histomorphometric analysis of bone revealed profound abnormalities of bone formation, with reduced osteoblast numbers, surfaces and mineral apposition rate. Studies in vitro showed that osteoblasts derived from eNOS KO mice had reduced rates of growth when compared with WT and were less well differentiated as reflected by lower levels of alkaline phosphatase activity. Mice with eNOS deficiency lost bone normally following ovariectomy but exhibited a significantly blunted anabolic response to high dose exogenous estrogen. We conclude that the eNOS pathway plays an essential role in regulating bone mass and bone turnover by modulating osteoblast function.

Endothelial nitric oxide synthase gene-deficient mice demonstrate marked retardation in postnatal bone formation, reduced bone volume, and defects in osteoblast maturation and activity

Nitric oxide (NO) has been implicated in the local regulation of bone metabolism. However, the contribution made by specific NO synthase (NOS) enzymes is unclear. Here we show that endothelial NOS gene knockout mice (eNOS-/-) have marked abnormalities in bone formation. Histomorphometric analysis of eNOS-/- femurs showed bone volume and bone formation rate was reduced by up to 45% (P: < 0.01) and 52% (P: < 0.01), respectively. These abnormalities were prevalent in young (6 to 9 weeks old) adults but by 12 to 18 weeks bone phenotype was restored toward wild-type. Dual energy X-ray absorptiometry analysis confirmed the age-related bone abnormalities revealing significant reductions in femoral (P: < 0.05) and spinal bone mineral densities (P: < 0.01) at 8 weeks that were normalized at 12 weeks. Reduction in bone formation and volume was not related to increased osteoclast numbers or activity but rather to dysfunctional osteoblasts. Osteoblast numbers and mineralizing activity were reduced in eNOS-/- mice. In vitro, osteoblasts from calvarial explants showed retarded proliferation and differentiation (alkaline phosphatase activity and mineral deposition) that could be restored by exogenous administration of a NO donor. These cells were also unresponsive to 17ss-estradiol and had an attenuated chemotactic response to transforming growth factor-beta. In conclusion, eNOS is involved in the postnatal regulation of bone mass and lack of eNOS gene results in reduced bone formation and volume and this is related to impaired osteoblast function.

Cyclic tensile stretch inhibition of nitric oxide release from osteoblast-like cells is both G protein and actin-dependent

Recent reports indicate the alteration of nitric oxide (NO) synthesis with mechanical stress loaded on the osteoblast and NO is considered to have a significant role in mechanotransduction. We found the involvement of guanine-nucleotide-binding regulatory proteins (G proteins), especially Gi, in stress-inhibited NO release of osteoblast-like cells (JOR:17;593-597, 1999). To determine further the mechanism involved in this process, we measured c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) activity under cyclic tensile stretch loaded on osteoblast-like cells. Cyclic stretch significantly enhanced JNK/SAPK activity and pertussis toxin clearly reversed stress-enhanced JNK/SAPK activity. Cytochalasin D, actin microfilament disrupting reagent, also abolished the stress activation of JNK/SAPK. We propose a model for signaling events induced by cyclic tensile stretch, namely a transmembrane mechanosensor which couples Gi-protein, actin cytoskeleton and finally activates JNK/SAPK activity of osteoblasts.

Insulin stimulates production of nitric oxide via ERK in osteoblast cells

We explored to determine if iNOS could be induced by insulin in osteoblast-like UMR-106 cells. Insulin (100 nM) stimulated nitric oxide production by twofold and significantly increased iNOS mRNA and protein levels. Insulin also increased collagen synthesis, but had little effect on alkaline phosphatase activity. In contrast, IGF-1 had little effect on NO production below 10 nM and it stimulated NO production by only 57% at 100 nM. IGF-1 had little effect on collagen levels, whereas it inhibited alkaline phosphatase activities in a dose-dependent manner. When an MEK inhibitor was preincubated, insulin failed to stimulate NO production, whereas insulin dramatically increased NO production in the ERK1 overexpressed cells. Taken together, it is proposed that insulin increases iNOS mRNA, iNOS protein, and NO production, possibly via activation of ERK. These may play an important role in osteoblast functions such as collagen synthesis.