Postmenopausal osteoporosis is a metabolic disease associated with estrogen deficiency. The results of numerous studies have revealed the positive correlation between iron accumulation and postmenopausal osteoporotic status. Although the results of previous studies have indicated that estrogen or iron alone have an effect on bone metabolism, their combined effects are not well defined. Using an in vivo mouse model, we found that bone mass was minimally affected by an excess of iron in the presence of estrogen. Once the source of estrogen was removed (ovariectomy), iron accumulation significantly decreased bone mass. These effects were accompanied by fluctuations in the level of oxidative stress. To determine whether these effects were related to bone formation or bone resorption, primary osteoblasts (OBs), RAW264.7 cells, and bone-marrow-derived macrophages were used for i n vitro experiments. We found that iron accumulation did inhibit the activity of OBs. However, estrogen had little effect on this inhibition. In contrast, iron promoted osteoclast differentiation through the production of reactive oxygen species. Estrogen, a powerful reactive oxygen scavenger, suppressed this effect in osteoclasts. Our data provided direct evidence that iron affected the bone mass only in the absence of estrogen. The inhibitory effect of estrogen on iron-induced osteopenia was particularly relevant to bone resorption rather than bone formation.
Wang Xiao, Fei Beibei, Shen Guangsi, Jiang Yu, Zhang Wen, Huang Xi, and Xu Youjia
Urszula T Iwaniec and Russell T Turner
Weight-dependent loading of the skeleton plays an important role in establishing and maintaining bone mass and strength. This review focuses on mechanical signaling induced by body weight as an essential mechanism for maintaining bone health. In addition, the skeletal effects of deviation from normal weight are discussed. The magnitude of mechanical strain experienced by bone during normal activities is remarkably similar among vertebrates, regardless of size, supporting the existence of a conserved regulatory mechanism, or mechanostat, that senses mechanical strain. The mechanostat functions as an adaptive mechanism to optimize bone mass and architecture based on prevailing mechanical strain. Changes in weight, due to altered mass, weightlessness (spaceflight), and hypergravity (modeled by centrifugation), induce an adaptive skeletal response. However, the precise mechanisms governing the skeletal response are incompletely understood. Furthermore, establishing whether the adaptive response maintains the mechanical competence of the skeleton has proven difficult, necessitating the development of surrogate measures of bone quality. The mechanostat is influenced by regulatory inputs to facilitate non-mechanical functions of the skeleton, such as mineral homeostasis, as well as hormones and energy/nutrient availability that support bone metabolism. Although the skeleton is very capable of adapting to changes in weight, the mechanostat has limits. At the limits, extreme deviations from normal weight and body composition are associated with impaired optimization of bone strength to prevailing body size.
Guillaume Mabilleau, Aleksandra Mieczkowska, Nigel Irwin, Peter R Flatt, and Daniel Chappard
Bone is permanently remodeled by a complex network of local, hormonal, and neuronal factors that affect osteoclast and osteoblast biology. Among these factors, a role for gastrointestinal hormones has been proposed based on the evidence that bone resorption dramatically falls after a meal. Glucagon-like peptide-1 (GLP1) is one of these gut hormones, and despite several reports suggesting an anabolic effect of GLP1, or its stable analogs, on bone mass, little is known about the effects of GLP1/GLP1 receptor on bone strength. In this study, we investigated by three-point bending, quantitative X-ray microradiography, microcomputed tomography, qBEI, and FTIRI bone strength and bone quality in male Glp1r knockout (Glp1r KO) mice when compared with control WT animals. Animals with a deletion of Glp1r presented with a significant reduction in ultimate load, yield load, stiffness, and total absorbed and post-yield energies when compared with WT animals. Furthermore, cortical thickness and bone outer diameter were significantly decreased in deficient animals. The mineral quantity and quality were not significantly different between Glp1r KO and WT animals. On the other hand, the maturity of the collagen matrix was significantly reduced in deficient animals and associated with lowered material properties. Taken together, these data support a positive effect of GLP1R on bone strength and quality.
Corine Martineau, Louise Martin-Falstrault, Louise Brissette, and Robert Moreau
A positive correlation between plasma levels of HDL and bone mass has been reported by epidemiological studies. As scavenger receptor class B, type I (SR-BI), the gene product of Scarb1, is known to regulate HDL metabolism, we recently characterized bone metabolism in Scarb1-null mice. These mice display high femoral bone mass associated with enhanced bone formation. As gender differences have been reported in HDL metabolism and SR-BI function, we investigated gender-specific bone alterations in Scarb1-null mice by microtomography and histology. We found 16% greater relative bone volume and 39% higher bone formation rate in the vertebrae from 2-month-old Scarb1-null females. No such alteration was seen in males, indicating gender- and region-specific differences in skeletal phenotype. Total and HDL-associated cholesterol levels, as well as ACTH plasma levels, were increased in both Scarb1-null genders, the latter being concurrent to impaired corticosterone response to fasting. Plasma levels of estradiol did not differ between null and WT females, suggesting that the estrogen metabolism alteration is not relevant to the higher vertebral bone mass in female Scarb1-null mice. Constitutively, high plasma levels of leptin along with 2.5-fold increase in its expression in white adipose tissue were measured in female Scarb1-null mice only. In vitro exposure of bone marrow stromal cells to ACTH and leptin promoted osteoblast differentiation as evidenced by increased gene expression of osterix and collagen type I alpha. Our results suggest that hyperleptinemia may account for the gender-specific high bone mass seen in the vertebrae of female Scarb1-null mice.
A Tumber, S Papaioannou, J Breckon, MC Meikle, JJ Reynolds, and PA Hill
The aims of this study were to identify the role and sites of action of serine proteinases (SPs) in bone resorption, a process which involves a cascade of events, the central step of which is the removal of bone matrix by osteoclasts (OCs). This resorbing activity, however, is also determined by recruitment of new OCs to future resorption sites and removal of the osteoid layer by osteoblasts (OBs), which enables OCs to gain access to the underlying mineralized bone. The resorption systems we have studied consisted of (i) neonatal calvarial explants, (ii) isolated OCs cultured on ivory slices, (iii) mouse OBs cultured on either radiolabelled type I collagen films or bone-like matrix, (iv) bone marrow cultures to assess OC formation and (v) 17-day-old fetal mouse metatarsal bone rudiments to assess OC migration and fusion. Two separate SP inhibitors, aprotinin and alpha(2)-antiplasmin dose-dependently inhibited (45)Ca release from neonatal calvarial explants: aprotinin (10(-6) M) was the most effective SP inhibitor, producing a maximum inhibitory effect of 55.9%.Neither of the SP inhibitors influenced either OC formation or OC resorptive activity. In contrast, each SP inhibitor dose-dependently inhibited OB-mediated degradation of both type I collagen fibrils and non-mineralized bone matrix. In 17-day-old metatarsal explants aprotinin produced a 55% reduction in the migration of OCs from the periosteum to the mineralized matrix after 3 days in culture but after 6 days in culture aprotinin was without effect on OC migration. Primary mouse osteoblasts expressed mRNA for urokinase type plasminogen activator (uPA), tIssue type plasminogen activator (tPA), the type I receptor for uPA, plasminogen activator inhibitor types I and II and the broad spectrum serine proteinase inhibitor, protease nexin I. In situ hybridization demonstrated expression of tPA and uPA in osteoclasts disaggregated from 6-day-old mouse long bones. We propose that the regulation of these various enzyme systems within bone tIssue determines the sites where bone resorption will be initiated.
Joanna S. Price, Babatunde O. Oyajobi, Richard O. C. Oreffo, and R. Graham G. Russell
Deer antler growth provides a unique natural model of rapid and complete bone regeneration. In this study, the distal antler tips of male red deer (Cervus elaphus) were collected post-mortem during the annual growth period (April-August), and an in vitro system established for the culture of cells from three regions; the inner layer of the perichondrium, the reserve mesenchyme and the cartilage zone. Alkaline phosphatase (ALP) expression by cultured cells, as demonstrated by enzyme histochemistry and biochemical assay, reflected the stage of cellular differentiation. ALP activity was highest in cells cultured from the hypertrophic cartilage region (3.6 ± 0.2 μmol/μg cell protein/minute), and lowest in undifferentiated mesenchymal cells (0.3 ± 0.01 μmol/μg cell protein/minute). ALP expression was lost with passage in culture. Levels of ALP activity in cultured cells correlated with the pattern and extent of enzyme expression in tissue sections as demonstrated by histochemical staining. Insulin-like growth factor (IGF)-I (10−9M-10−7M) was found to be mitogenic for cultured cells from all three zones as shown by increased incorporation of [3H]thymidine into DNA. These results demonstrate that cells from three different regions of the antler tip can be maintained in culture, and that antler cells share certain phenotypic characteristics of growth plate chondrocytes. These data provide further evidence of a role for IGF-1 in the regulation of antler growth. Antler regrowth is a potentially useful model for the study of the factors that regulate bone formation.
V E MacRae, T Burdon, S F Ahmed, and C Farquharson
Proinflammatory cytokines inhibit growth plate development. However, their underlying mechanisms of action are unclear. These effects may be mediated by ceramide, a sphingosine-based lipid second messenger, which is elevated in a number of chronic inflammatory diseases. To test this hypothesis, we determined the effects of C2-ceramide, a cell permeable ceramide analogue, on the growth of the ATDC5 chondrogenic cell line and on cultured fetal mice metatarsals. In ATDC5 cells, C2-ceramide significantly induced apoptosis at both 40 (82%; P < 0.05) and 25 μM (53%; P < 0.05). At 40 μM, C2-ceramide significantly reduced proliferation ([3H]-thymidine uptake/mg protein) (62%; P < 0.05). C2-ceramide did not markedly alter the differentiation state of the cells as judged by the expression of markers of chondrogenesis and differentiation (sox 9, collagen II and collagen X). The IGF-I signalling pathway is the major autocrine/paracrine regulator of bone growth. Both in the presence and absence of IGF-I, C2-ceramide (25 μM) induced an equivalent reduction in proliferation (60%; P < 0.001). Similarly, C2-ceramide (40 μM) induced a 31% reduction in fetal metatarsal growth both in the presence and absence of IGF-I (both P < 0.001). Furthermore, C2-ceramide reduced ADCT5 proliferation in the presence of AG1024, an IGF-I and insulin receptor blocker. Therefore, C2-ceramide-dependent inhibition appears to be independent of IGF-mediated stimulation of bone growth. Indeed, biochemical studies demonstrated that C2-ceramide (25 μM) pretreatment did not alter IGF-I-stimulated phosphorylation of insulin receptor substrate-1, Akt or P44/42 MAP kinase. In conclusion, C2-ceramide inhibits proliferation and induces apoptosis in growth plate chondrocytes through an IGF-I independent mechanism.
Anna E Bollag, Tianyang Guo, Ke-Hong Ding, Vivek Choudhary, Xunsheng Chen, Qing Zhong, Jianrui Xu, Kanglun Yu, Mohamed E Awad, Mohammed Elsalanty, Maribeth H Johnson, Meghan E McGee-Lawrence, Wendy B Bollag, and Carlos M Isales
Osteoporosis, low bone mass that increases fracture susceptibility, affects approximately 75 million individuals in the United States, Europe and Japan, with the number of osteoporotic fractures expected to increase by more than three-fold over the next 50 years. Bone mass declines with age, although the mechanisms for this decrease are unclear. Aging enhances production of reactive oxygen species, which can affect bone formation and breakdown. The multiple sclerosis drug Tecfidera contains dimethylfumarate, which is rapidly metabolized to monomethylfumarate (MMF); MMF is thought to function through nuclear factor erythroid-derived-2-like-2 (NRF2), a transcription factor activated by oxidative stress which induces the expression of endogenous anti-oxidant systems. We hypothesized that MMF-elicited increases in anti-oxidants would inhibit osteopenia induced by ovariectomy, as a model of aging-related osteoporosis and high oxidative stress. We demonstrated that MMF activated NRF2 and induced anti-oxidant NRF2 target gene expression in bone marrow-derived mesenchymal stem cells. Sham-operated or ovariectomized adult female mice were fed chow with or without MMF and various parameters were monitored. Ovariectomy produced the expected effects, decreasing bone mineral density and increasing body weight, fat mass, bone marrow adiposity and serum receptor activator of nuclear factor-kappa-B ligand (RANKL) levels. MMF decreased fat but not lean mass. MMF improved trabecular bone microarchitecture after adjustment for body weight, although the unadjusted data showed few differences; MMF also tended to increase adjusted cortical bone and to reduce bone marrow adiposity and serum RANKL levels. Because these results suggest the possibility that MMF might be beneficial for bone, further investigation seems warranted.
Toshihiro Sugiyama, Toshiaki Takaki, Kenya Sakanaka, Hiroki Sadamaru, Koji Mori, Yoshihiko Kato, Toshihiko Taguchi, and Takashi Saito
Long-term warfarin use has been reported to increase fracture risk of rib and vertebra but not hip in elderly patients, but the mechanisms remain unknown. We hypothesized that warfarin would impair bone material quality but could not weaken bone strength under conditions with higher mechanical stimuli. To test this hypothesis, rats were randomized to vehicle or warfarin group at 4 weeks of age and subsequently weight matched into a sedentary or jumping exercise group at 12 weeks of age. At 6 months of age, osteocalcin content, bone mineral density (BMD), mineral size, material properties, morphological parameters, and biomechanical properties of cortical bones were evaluated. In order to seek evidence for a common mechanism of action, effects of nucleation rate of mineral crystals on their rigidity were also investigated using computer simulation. In humeral cortical bones, warfarin did not change BMD, but markedly decreased osteocalcin content, diminished mineral size, and impaired material hardness. Consistent with these results, our computer-simulation model showed that osteocalcin-induced delay of mineral crystal nucleation decreased mineral formation rate, increased mean and distribution of mineral sizes, and strengthened mineral rigidity. In tibial cortical bones, warfarin decreased material ultimate stress; however, under jumping exercise, warfarin increased cross-sectional total and bone areas of these tibiae and completely maintained their biomechanical properties including work to failure. Collectively, our findings suggest that long-term warfarin therapy weakens rib and vertebra by impairing cortical bone material quality due to a marked decrease in osteocalcin content but could not reduce hip strength through compensatory adaptation of cortical bone structure to higher mechanical stimuli.
H. M. Docherty and D. A. Heath
Over the past years the explanation of the hypercalcaemia associated with malignancy has changed repeatedly. Theories have come and gone and, in some cases, come again. The search for new hypercalcaemic factors is gaining momentum and major new breakthroughs seem imminent.
The original explanation of the hypercalcaemia of malignancy was that it was due to bone metastases physically causing the release of calcium from bone. This simple theory, though widely believed, was not supported by any evidence. Many patients with extensive bone disease had normal serum calcium values, many were hypercalcaemic in the absence of bone metastases, and certain malignancies had high or low incidences of hypercalcaemia despite similar rates of bone metastases, c.f. squamous cell and small cell carcinoma of the lung. That certain malignancies might produce a humoral factor was suggested by the work of Gordan, Cantino, Erhardt et al. (1966) who claimed to have isolated vitamin D-like