The use of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) to reduce serum cholesterol is well described. However, the recent finding that statins have direct effects on bone was unexpected. A number of epidemiological studies have recently been published that explore the effects of statins on bone mineral density and risk of fracture in humans. Statins may act by directly stimulating the expression of bone morphogenetic protein-2 and increasing osteoblast differentiation or, like nitrogen-containing bisphosphonates, may have effects on the mevalonate pathway that leads to inhibition of osteoclast activity and osteoblast apoptosis. In addition, the demonstration that statins can inhibit inflammation and encourage angiogenesis offers other possibilities for action.

Background Hydroxymethylglutaryl coenzyme A reductase inhibitors increase new bone formation in vitro and in rodents. Results of epidemiologic analyses evaluating the association between use of these cholesterol-lowering drugs, bone mineral density and fracture have been mixed. Methods Women (n = 24) with osteopenia, assessed by broad band ultrasound attenuation, were randomized to simvastatin 20 mg, 40 mg or identical-appearing placebo for 12 weeks. Fasting lipid profiles and biochemical markers of bone formation (bone-specific alkaline phosphatase) and resorption (N-telopeptides and C-terminal propeptide of type 1 collagen) were measured at baseline, 6 and 12 weeks. Results Plasma low density lipoprotein-cholesterol concentration fell 7%, 39% (p < 0.01 vs baseline) and 47% (p < 0.01 vs baseline) after 12 weeks of treatment with placebo, simvastatin 20 mg and 40 mg, respectively. At baseline, bone marker concentrations were similar in the three treatment groups. At 6 and 12 weeks, bone marker concentrations were not different from baseline, and no significant differences in bone marker concentrations were observed between treatment groups at either 6 or 12 weeks. Conclusion Among osteopenic women, treatment with simvastatin for 12 weeks did not affect markers of bone formation or resorption.

Bone-resorbing osteoclasts are formed from hemopoietic cells of the monocyte–macrophage lineage under the control of bone-forming osteoblasts. We have cloned an osteoblast-derived factor essential for osteoclastogenesis, the receptor activator of NF-κB ligand (RANKL). Synovial fibroblasts and activated T lymphocytes from patients with rheumatoid arthritis also express RANKL, which appears to trigger bone destruction in rheumatoid arthritis as well. Recent studies have shown that T lymphocytes produce cytokines other than RANKL such as IL-17, granulocyte–macrophage colony-stimulating factor and IFN-γ, which have powerful regulatory effects on osteoclastogenesis. The possible roles of RANKL and other cytokines produced by T lymphocytes in bone destruction are described.

Background Bone morphogenetic proteins (BMPs) and transforming growth factor-βs (TGF-βs) are important regulators of bone repair and regeneration. BMP-2 and TGF-β1 have been shown to inhibit gap junctional intercellular communication (GJIC) in MC3T3-E1 cells. Connexin 43 (Cx43) has been shown to mediate GJIC in osteoblasts and it is the predominant gap junctional protein expressed in these murine osteoblast-like cells. We examined the expression, phosphorylation, and subcellular localization of Cx43 after treatment with BMP-2 or TGF-β1 to investigate a possible mechanism for the inhibition of GJIC. Results Northern blot analysis revealed no detectable change in the expression of Cx43 mRNA. Western blot analysis demonstrated no significant change in the expression of total Cx43 protein. However, significantly higher ratios of unphosphorylated vs. phosphorylated forms of Cx43 were detected after BMP-2 or TGF-β1 treatment. Immunofluorescence and cell protein fractionation revealed no detectable change in the localization of Cx43 between the cytosol and plasma membrane. Conclusions BMP-2 and TGF-β1 do not alter expression of Cx43 at the mRNA or protein level. BMP-2 and TGF-β1 may inhibit GJIC by decreasing the phosphorylated form of Cx43 in MC3T3-E1 cells.

Background Although the role of the osteoclast in bone resorption is becoming better understood, much remains to be learned about osteoclastogenesis and the exact mechanism of action of anti-resorbing agents such as 17β-estradiol. This study investigated bone and morphologic osteoclast alterations following long-term estrogen administration to the B6D2F1 mouse. B6D2F1 mice aged 4-5 weeks were exposed to high levels of estrogen via implanted silastic tubing for at least 12 weeks; controls received empty tubing. Femurs of control and treated mice were assessed with radiology, quantitative histomorphometry and transmission electron microscopy. Results After 8 weeks of treatment, there was radiologic evidence of severe osteosclerosis and 86% of femoral marrow space was replaced with bone. After 12 weeks histologic studies of treated animals revealed that osteoclasts were positive for tartrate-resistant acid phosphatase but showed markedly abnormal ultrastructure which prevented successful bone resorption. Conclusions Findings extend our understanding of osteoclast structure and function in the mouse exposed in vivo to high doses of estrogen. Ultrastructural examination showed that osteoclasts from estrogen-treated mice were unable to seal against the bone surface and were unable to form ruffled borders.

Inflammatory arthritides are commonly characterized by localized and generalized bone loss. Localized bone loss in the form of joint erosions and periarticular osteopenia is a hallmark of rheumatoid arthritis, the prototype of inflammatory arthritis. Recent studies have highlighted the importance of receptor activator of nuclear factor-κB ligand (RANKL)-dependent osteoclast activation by inflammatory cells and subsequent bone loss. In this article, we review the pathogenesis of inflammatory bone loss and explore the possible therapeutic interventions to prevent it.

The advances in biomedicine over the past decade have provided revolutionary insights into molecules that mediate cell proliferation and differentiation. Findings on the complex interplay of cells, growth factors, matrix molecules and cell adhesion molecules in the process of tissue patterning have vitalized the revolutionary approach of bioregenerative medicine and tissue engineering. Here we review the impact of recent work in this interdisciplinary field on the treatment of musculoskeletal disorders. This novel concept combines the transplantation of pluripotent stem cells, and the use of specifically tailored biomaterials, arrays of bioactive molecules and gene transfer technologies to direct the regeneration of pathologically altered musculoskeletal tissues.

The contribution of osteoclasts to the process of bone loss in inflammatory arthritis has recently been demonstrated. Studies in osteoclast biology have led to the identification of factors responsible for the differentiation and activation of osteoclasts, the most important of which is the receptor activator of NF-κB ligand/osteoclast differentiation factor (RANKL/ODF), a tumor necrosis factor (TNF)-like protein. The RANKL/ODF receptor, receptor activator of NF-κB (RANK), is a TNF-receptor family member present on both osteoclast precursors and mature osteoclasts. Like other TNF-family receptors and the IL-1 receptor, RANK mediates its signal transduction via TNF receptor-associated factor (TRAF) proteins, suggesting that the signaling pathways activated by RANK and other inflammatory cytokines involved in osteoclast differentiation and activation are interconnected.

Prolonged skeletal unloading through bedrest results in bone loss similar to that observed in elderly osteoporotic patients but with an accelerated timeframe. This rapid effect on weight-bearing bones is also observed in astronauts who lose up to 2% of their bone mass per month spent in Space. Despite important implications for Spaceflight travellers and bedridden patients on Earth the exact mechanisms involved in disuse osteoporosis have not been elucidated. Parathyroid hormone-related protein (PTHrP) regulates many physiological processes including skeletal development and has been proposed as a gravisensor. To investigate the role of PTHrP in microgravity-induced bone loss trabecular osteoblasts (TOs) from Pthrp+/+ and -/- mice were exposed to simulated microgravity for 6 days. Viability of TOs decreased in inverse proportion to PTHrP expression levels. Microarray analysis of Pthrp+/+ TOs after 6 days at 0g revealed expression changes in genes encoding prolactins,apoptosis and survival molecules bone metabolism and extra-cellular matrix composition proteins chemokines IGF family and Wnt-related signalling molecules. Importantly 88% of 0g-induced expression changes in Pthrp+/+ cells overlap those observed in Pthrp-/- cells in normal gravity. Pulsatile treatment with PTHrP1-36 peptide during microgravity exposure reversed a large proportion of 0g-induced changes in Pthrp+/+ TOs. Our results confirm PTHrP efficacy as an anabolic agent to prevent microgravity-induced cell death in TOs. Total RNA samples extracted from Pthrp+/+and -/- trabecular osteoblasts (TOs) exposed for 6 days to simulated 0g in Synthecon rotating cell or left 6 days in culture at 1g. Cells had either been treated with a pulsatile treatment (2 h/day) of PTHrP1-36 peptide (10-8M) or received a change in growth medium. In total: 8 different conditions with 2 replicates each i.e. Pthrp+/+ TOs at 0g or 1g with or without PTHrP1-36 treatment and Pthrp-/- TOs at 0g or 1 g,with or without PTHrP1-36 treatment.