Integrins, insulin like growth factors, and the skeletal response to load

Alzheimer's disease and its associated risk of bone fractures: a narrative review

Background

Alzheimer's disease (AD) is a neurodegenerative disorder that is the major cause of dementia in the aged population. Recent researches indicate that patients with AD have a significantly increased fracture risk, but the pathological mechanisms are still unclear.

Objective

We systematically reviewed studies regarding bone fracture risk in AD to uncover links between the pathologies of osteoporosis and AD.

Methods

We searched the literature using the databases of PubMed, Web of Science, Embase and Cochrane Library. Studies were included if they evaluated bone fracture risk in AD patients and if they explored the pathogenesis and prevention of bone fractures in these patients.

Results

AD patients had a significantly higher risk of bone fractures than age-matched controls. Multiple factors contributed to the increased risk of bone fractures in AD patients, including the direct effects of amyloid pathology on bone cells, abnormal brain-bone interconnection, Wnt/β-catenin signalling deficits, reduced activity, high risk of falls and frailty, and chronic immune activity. Exercise, prevention of falls and fortified nutrition were beneficial for reducing the fracture risk in AD patients. However, the efficacy of anti-osteoporotic agents in preventing bone fractures should be further evaluated in AD patients as corresponding clinical studies are very scarce.

Conclusion

Alzheimer's disease patients have increased bone fracture risk and decreased bone mineral density owing to multiple factors. Assessment of anti-osteoporotic agents' efficacy in preventing bone fractures of AD patients is urgently needed.

miR-181c-5p mediates simulated microgravity-induced impaired osteoblast proliferation by promoting cell cycle arrested in the G2 phase

Impaired osteoblast proliferation plays fundamental roles in microgravity‐induced bone loss, and cell cycle imbalance may result in abnormal osteoblast proliferation. However, whether microgravity exerts an influence on the cell cycle in osteoblasts or what mechanisms may underlie such an effect remains to be fully elucidated. Herein, we confirmed that simulated microgravity inhibits osteoblast proliferation. Then, we investigated the effect of mechanical unloading on the osteoblast cell cycle and found that simulated microgravity arrested the osteoblast cell cycle in the G₂ phase. In addition, our data showed that cell cycle arrest in osteoblasts from simulated microgravity was mainly because of decreased cyclin B1 expression. Furthermore, miR‐181c‐5p directly inhibited cyclin B1 protein translation by binding to a target site in the 3′UTR. Lastly, we demonstrated that inhibition of miR‐181c‐5p partially counteracted cell cycle arrest and decreased the osteoblast proliferation induced by simulated microgravity. In conclusion, our study demonstrates that simulated microgravity inhibits cell proliferation and induces cell cycle arrest in the G₂ phase in primary mouse osteoblasts partially through the miR‐181c‐5p/cyclin B1 pathway. This work may provide a novel mechanism of microgravity‐induced detrimental effects on osteoblasts and offer a new avenue to further investigate bone loss induced by mechanical unloading.

Molecular signaling in bone cells: Regulation of cell differentiation and survival

The achievement of proper bone mass and architecture, and their maintenance throughout life requires the concerted actions of osteoblasts, the bone forming cells, and osteoclasts, the bone resorbing cells. The differentiation and activity of osteoblasts and osteoclasts are regulated by molecules produced by matrix-embedded osteocytes, as well as by cross talk between osteoblasts and osteoclasts through secreted factors. In addition, it is likely that direct contact between osteoblast and osteoclast precursors, and the contact of these cells with osteocytes and cells in the bone marrow, also modulates bone cell differentiation and function. With the advancement of molecular and genetic tools, our comprehension of the intracellular signals activated in bone cells has evolved significantly, from early suggestions that osteoblasts and osteoclasts have common precursors and that osteocytes are inert cells in the bone matrix, to the very sophisticated understanding of a network of receptors, ligands, intracellular kinases/phosphatases, transcription factors, and cell-specific genes that are known today. These advances have allowed the design and FDA-approval of new therapies to preserve and increase bone mass and strength in a wide variety of pathological conditions, improving bone health from early childhood to the elderly. We have summarized here the current knowledge on selected intracellular signal pathways activated in osteoblasts, osteocytes, and osteoclasts.

Parkinson's disease and risk of hip fracture: systematic review and meta-analysis

The relationship between Parkinson's disease (PD) and risk of hip fracture yielded inconsistent results. Therefore, we conducted the present systematic review and meta-analysis of published observational studies to assess the association between PD and risk of hip fracture. PubMed, ISI, EMBASE, and Cochrane databases were searched systematically to identify studies assessing the relationship between PD and the risk of hip fracture up to July 01, 2017. In addition, to find related articles, the reference section of retrieved articles was checked. Random-effects model was used for calculation of pooled hazard ratio (HR) and 95% confidence intervals (CI). Thirteen independent studies containing 564,947 participants were included in the meta-analysis. The overall results of included studies showed PD to be associated with the risk of hip fracture (HR_overall = 3.13, 95% CI 2.53-3.87) in women 3.11 (2.51-3.86) and men 2.60 (2.19-3.09). Our meta-analysis showed the direct association between PD and the risk of hip fracture in both men and women. However, due to the limitations of this study, further well-designed studies are required to confirm our findings.

miR-208a-3p Suppresses Osteoblast Differentiation and Inhibits Bone Formation by Targeting ACVR1

Emerging evidence indicates that many microRNAs (miRNAs) are indispensable regulators of osteoblast differentiation and bone formation. However, the role of miRNAs in mechanotransduction of osteoblasts remains to be elucidated. This study aimed to identify a mechanosensitive miRNA that regulates Activin A receptor type I (ACVR1)-induced osteogenic differentiation. After 4 weeks of hindlimb unloading (HLU) suspension of 6-month-old male C57BL/6J mice, femurs and tibias were harvested to extract total bone RNAs. Elevated levels of miR-208a-3p correlated with a lower degree of bone formation in whole-bone samples of HLU mice. However, in vitro overexpression of miR-208a-3p inhibited osteoblast differentiation, whereas silencing of miR-208a-3p by antagomiR-208a-3p promoted expression of osteoblast activity, bone formation marker genes, and matrix mineralization under mechanical unloading condition. Bioinformatics analysis and a luciferase assay revealed that ACVR1 is a target gene of miR-208a-3p that negatively regulates osteoblast differentiation under mechanical unloading environment. Further, this study also demonstrates that in vivo pre-treatment with antagomiR-208a-3p led to an increase in bone formation and trabecular microarchitecture and partly rescued the bone loss caused by mechanical unloading. Collectively, these results suggest that in vivo, inhibition of miRNA-208a-3p by antagomiR-208a-3p may be a potential therapeutic strategy for ameliorating bone loss.

Bone mass and vitamin D levels in Parkinson's disease: is there any difference between genders?

[Purpose] The aim of this study was to determine the bone mineral density, vitamin D level, and frequencies of osteopenia and osteoporosis in patients with Parkinson's disease and to compare male and female patients with the controls separately. [Subjects and Methods] One hundred fifteen Parkinson's disease patients (47 males, 68 females; age range: 55-85 years) and 117 age- and gender-matched controls (47 males, 70 females) were enrolled in the study. Bone mineral density measured by dual-energy X-ray absorptiometry and serum D vitamin levels of each participant were recorded. [Results] The mean lumbar spine, femur neck, and total femur bone mineral density levels, T-scores, and vitamin D levels were found to be significantly lower in Parkinson's disease patients in both genders. Furthermore, osteoporosis rates were found be significantly higher only in female Parkinson's disease patients compared with female controls. [Conclusion] Data from the present study revealed that while osteoporosis was significantly higher only in female Parkinson's disease patients, all Parkinson's disease patients had lower bone mineral density scores and vitamin D levels compared with the controls regardless of gender, suggesting that clinicians should pay attention to the osteoporosis risk in Parkinson's disease and that adequate preventive measures should be taken in order to limit the future risk due to osteoporotic fractures.

Interfaces with Tunable Mechanical and Radiosensitizing Properties

We report the fabrication of a composite containing nanostructured GaOOH and Matrigel with tunable radiosensitizing and stiffness properties. Composite characterization was done with microscopy and rheology. The utility of the interface was tested in vitro using fibroblasts. Cell viability and reactive oxygen species assays quantified the effects of radiation dosages and GaOOH concentrations. Fibroblasts' viability decreased with increasing concentration of GaOOH and composite stiffness. During ionizing radiation experiments the presence of the scintillating GaOOH triggered a different cellular response. Reactive oxygen species data demonstrated that one can reduce the amount of radiation needed to modulate the behavior of cells on interfaces with different stiffness containing a radiosensitizing material.

The role of selenium in insulin-like growth factor I receptor (IGF-IR) expression and regulation of apoptosis in mouse osteoblasts

Selenium (Se) is an essential component for animals and human beings. The chemoprotective role of Se, via the regulation of the cell cycle, stimulation of apoptosis and activation of some cytokines among others, is well known; however, the comprehensive effects of Se on the expression of IGF-IR and its regulation of apoptosis have not been investigated. Thus the aim of this study was to report on the effects that different concentrations of Se extert on body weight, blood serum IGF-IR levels and histopathology in mice; and on IGF-IR expression, proliferation and apoptosis in mouse osteoblasts. In vivo experiments showed a significant decrease in body weight, serum levels of IGF-IR and prominent toxicant effects on the liver, kidney, heart and spleen following the administration of defined concentrations of Se for 30 d. However, moderate levels (0.1 mg/kg) of Se gradually improved weight and serum IGF-IR. In vitro osteoblast experiments revealed that at concentrations of 5 × 10(-6) and 10(-5) mol/L Se, MTT activity decreased in comparison with control cells. Cell cycle, TEM and caspase-3 activity supported these observations including an increase in the sub-G1 phase and notable apoptosis in osteoblasts, along with a decrease in the expression of mRNA and protein levels of IGF-IR. Moreover, the MTT activity, mRNA and protein levels of IGF-IR in osteoblasts were decreased and caspase-3 activity was increased in siRNA groups as compared with non-siRNA groups. These data suggest that Se significantly affects IGF-IR expression, and that it contributes to the proliferation and regulation of apoptosis in osteoblasts.

MiR-103 inhibits osteoblast proliferation mainly through suppressing Cav1.2 expression in simulated microgravity

Emerging evidence indicates that microRNAs (miRNAs) play important roles in modulating osteoblast function and bone formation. However, the influence of miRNA on osteoblast proliferation and the possible mechanisms underlying remain to be defined. In this study, we aimed to investigate whether miR-103 regulates osteoblast proliferation under simulated microgravity condition through regulating Cav1.2, the primary subunit of L-type voltage sensitive calcium channels (LTCCs). We first investigated the effect of simulated microgravity on osteoblast proliferation and the outcomes clearly demonstrated that the mechanical unloading inhibits MC3T3-E1 osteoblast-like cell proliferation. Using quantitative Real-Time PCR (qRT-PCR), we provided data showing that miR-103 was up-regulated in response to simulated microgravity. In addition, we observed that up-regulation of miR-103 inhibited and down-regulation of miR-103 promoted osteoblast proliferation under simulated microgravity condition. Furthermore, knocking-down or over-expressing miR-103, respectively, up- or down-regulated the level of Cav1.2 expression and LTCC currents, suggesting that miR-103 acts as an endogenous attenuator of Cav1.2 in osteoblasts under simulated microgravity condition. More importantly, we showed that the effect of miR-103 on osteoblast proliferation was diminished in simulated microgravity, when co-transfecting miR-103 mimic or inhibitor with Cav1.2 siRNA. Taken together, our data suggest that miR-103 inhibits osteoblast proliferation mainly through suppression of Cav1.2 expression under simulated microgravity condition. This work may provide a novel mechanism of microgravity-induced detrimental effects on osteoblast proliferation, identifying miR-103 as a novel possible therapeutic target in bone remodeling disorders in this mechanical unloading.

NELL-1 in the treatment of osteoporotic bone loss

NELL-1 is a secreted, osteoinductive protein whose expression rheostatically controls skeletal ossification. Overexpression of NELL-1 results in craniosynostosis in humans and mice, whereas lack of Nell-1 expression is associated with skeletal undermineralization. Here we show that Nell-1-haploinsufficient mice have normal skeletal development but undergo age-related osteoporosis, characterized by a reduction in osteoblast:osteoclast (OB:OC) ratio and increased bone fragility. Recombinant NELL-1 binds to integrin β1 and consequently induces Wnt/β-catenin signalling, associated with increased OB differentiation and inhibition of OC-directed bone resorption. Systemic delivery of NELL-1 to mice with gonadectomy-induced osteoporosis results in improved bone mineral density. When extended to a large animal model, local delivery of NELL-1 to osteoporotic sheep spine leads to significant increase in bone formation. Altogether, these findings suggest that NELL-1 deficiency plays a role in osteoporosis and demonstrate the potential utility of NELL-1 as a combination anabolic/antiosteoclastic therapeutic for bone loss.

microRNA-103a functions as a mechanosensitive microRNA to inhibit bone formation through targeting Runx2

Emerging evidence indicates that microRNAs (miRNAs) play essential roles in regulating osteoblastogenesis and bone formation. However, the role of miRNA in osteoblast mechanotransduction remains to be defined. In this study, we aimed to investigate whether miRNAs regulate mechanical stimulation-triggered osteoblast differentiation and bone formation through modulation of Runx2, the master transcription factor for osteogenesis. We first investigated the role of mechanical loading both in a mouse model and in an osteoblast culture system and the outcomes clearly demonstrated that mechanical stimuli can regulate osteogenesis and bone formation both in vivo and in vitro. Using bioinformatic analyses and subsequent confirmation by quantitative real-time PCR (qRT-PCR), we found that multiple miRNAs that potentially target Runx2 were responding to in vitro mechanical stimulation, among which miR-103a was fully characterized. miR-103a and its host gene PANK3 were both downregulated during cyclic mechanical stretch (CMS)-induced osteoblast differentiation, whereas Runx2 protein expression was upregulated. Overexpression of miR-103a significantly decreased and inhibition of miR-103a increased Runx2 protein level, suggesting that miR-103a acts as an endogenous attenuator of Runx2 in osteoblasts. Mutation of putative miR-103a binding sites in Runx2 mRNA abolishes miR-103a-mediated repression of the Runx2 3'-untranslated region (3'UTR) luciferase reporter activity, suggesting that miR-103a binds to Runx2 3'UTR. Osteoblast marker gene profiling and osteogenic phenotype assays demonstrated that miR-103a negatively correlates with CMS-induced osteogenesis. Further, the perturbation of miR-103a also has a significant effect on osteoblast activity and matrix mineralization. More importantly, we found an inhibitory role of miR-103a in regulating bone formation in hindlimb unloading mice, and pretreatment with antagomir-103a partly rescued the osteoporosis caused by mechanical unloading. Taken together, our data suggest that miR-103a is the first identified mechanosensitive miRNA that regulates osteoblast differentiation by directly targeting Runx2, and therapeutic inhibition of miR-103a may be an efficient anabolic strategy for skeletal disorders caused by pathological mechanical loading.

Effects of the masticatory demand on the rat mandibular development

The influence of masticatory loading stimulus on mandibular development is not fully clear. In this paper, experimental alterations in the daily muscle use, caused by a changed diet consistency, were continuously monitored, while adaptations in bone and cartilage were examined. It is hypothesised that decreased muscular loading will result in a decrease in the growth factor expression and mandible growth. Fourteen 21-day-old Wistar strain male rats were randomly divided into two groups and fed on either a hard or soft diet for 14 weeks. An implanted radio-telemetric device recorded continuously muscle activity of the superficial masseter muscle. Chondroblast proliferation in the condylar cartilage was identified by insulin-like growth factor-1 receptor (IGF-1r) immunostaining. Furthermore, an X-ray was taken for cephalometric analysis. In the soft-diet group, the duty time of the superficial masseter muscle at higher activity levels was significantly lower than that in the hard-diet group. This decrease in muscular loading of the jaw system was accompanied by: a significant reduction in (i) articular cartilage thickness, (ii) expression of IGF-1r immunopositive cells and (iii) mandible ramus height. In conclusion, a decrease in masticatory demand during the growth period leads to insufficient mandibular development.

Dynamic fluid flow stimulation on cortical bone and alterations of the gene expressions of osteogenic growth factors and transcription factors in a rat functional disuse model

Recently we have developed a dynamic hydraulic stimulation (DHS) as a loading modality to induce anabolic responses in bone. To further study the functional process of DHS regulated bone metabolism, the objective of this study was to evaluate the effects of DHS on cortical bone and its alterations on gene expressions of osteogenic growth factors and transcription factors as a function of time. Using a model system of 5-month-old hindlimb suspended (HLS) female Sprague-Dawley rats, DHS was applied to the right tibiae of the stimulated rats with a loading frequency of 2Hz with 30mmHg (p-p) dynamic pressure, 5days/week, for a total of 28days. Midshafts of the tibiae were analyzed using μCT and histology. Total RNA was analyzed using RT-PCR on selected osteogenic genes (RUNX2, β-catenin, osteopontin, VEGF, BMP2, IGF-1, and TGF-β) on 3-, 7-, 14- , and 21-day. Results showed increased Cort.Th and Ct.BV/TV as well as a time-dependent fashion of gradual changes in mRNA levels upon DHS. While DHS-driven fold changes of the mRNA levels remained low before Day-7, its fold changes started to elevate by Day-14 and then dropped by Day-21. This study further delineates the underlying molecular mechanism of DHS-derived mechanical signals, and its time-dependent optimization.

Parkinson's disease and osteoporosis

BACKGROUND

patients with Parkinson's disease (PD) have a high risk of sustaining osteoporotic fractures as a result of falls and reduced bone mass.

OBJECTIVE

to summarise the underlying pathophysiological mechanisms of bone loss in PD by reviewing the available literature.

METHODS

a Medline search was performed for articles published between January 1975 and January 2011, using the keywords 'bone mineral density', 'bone loss', 'bone metabolism', 'osteoporosis', 'osteopenia', 'Parkinson's disease' and 'Parkinsonism'.

RESULTS

PD patients have a lower bone mineral density (BMD) than age-matched controls. Bone loss in PD is multifactorial, resulting from immobility, decreased muscle strength, and low body weight. Vitamin D deficiency is also important, not only because it reduces BMD, but also because cell function in the substantia nigra depends on vitamin D. Lastly, hyperhomocysteinaemia, an independent risk factor for osteoporosis, is common in PD, due to levodopa use, as well as vitamin B12 and folic acid deficiency. A few studies have demonstrated that treatment with bisphosphonates, vitamin D and calcium can increase BMD and reduce fractures in PD patients.

CONCLUSION

bone loss in PD is multifactorial. It is clinically important because of the concomitant risk of fractures. Screening for osteoporosis should be considered more often, and therapeutic interventions should be initiated.

Hedgehog signaling and osteoblast gene expression are regulated by purmorphamine in human mesenchymal stem cells

Several biological events are controlled by Hedgehog (Hh) signaling, including osteoblast phenotype development. This study aimed at evaluating the gene expression profile of human mesenchymal stem cells (hMSCs) treated with the Hh agonist, purmorphamine, focusing on Hh signaling and osteoblast differentiation. hMSCs from bone marrow were cultured in non-osteogenic medium with or without purmorphamine (2 µM) for periods of up to 14 days. Purmorphamine up-regulated gene expression of the mediators of Hh pathway, SMO, PTCH1, GLI1, and GLI2. The activation of Hh pathway by purmorphamine increased the expression of several genes (e.g., RUNX2 and BMPs) related to osteogenesis. Our results indicated that purmorphamine triggers Hh signaling pathway in hMSCs, inducing an increase in the expression of a set of genes involved in the osteoblast differentiation program. Thus, we conclude that Hh is a crucial pathway in the commitment of undifferentiated cells to the osteoblast lineage.

Mechanobiological modulation of cytoskeleton and calcium influx in osteoblastic cells by short-term focused acoustic radiation force

Mechanotransduction has demonstrated potential for regulating tissue adaptation in vivo and cellular activities in vitro. It is well documented that ultrasound can produce a wide variety of biological effects in biological systems. For example, pulsed ultrasound can be used to noninvasively accelerate the rate of bone fracture healing. Although a wide range of studies has been performed, mechanism for this therapeutic effect on bone healing is currently unknown. To elucidate the mechanism of cellular response to mechanical stimuli induced by pulsed ultrasound radiation, we developed a method to apply focused acoustic radiation force (ARF) (duration, one minute) on osteoblastic MC3T3-E1 cells and observed cellular responses to ARF using a spinning disk confocal microscope. This study demonstrates that the focused ARF induced F-actin cytoskeletal rearrangement in MC3T3-E1 cells. In addition, these cells showed an increase in intracellular calcium concentration following the application of focused ARF. Furthermore, passive bending movement was noted in primary cilium that were treated with focused ARF. Cell viability was not affected. Application of pulsed ultrasound radiation generated only a minimal temperature rise of 0.1°C, and induced a streaming resulting fluid shear stress of 0.186 dyne/cm(2), suggesting that hyperthermia and acoustic streaming might not be the main causes of the observed cell responses. In conclusion, these data provide more insight in the interactions between acoustic mechanical stress and osteoblastic cells. This experimental system could serve as basis for further exploration of the mechanosensing mechanism of osteoblasts triggered by ultrasound.

Dwarfism in mice lacking collagen-binding integrins α2β1 and α11β1 is caused by severely diminished IGF-1 levels

Mice with a combined deficiency in the α2β1 and α11β1 integrins lack the major receptors for collagen I. These mutants are born with inconspicuous differences in size but develop dwarfism within the first 4 weeks of life. Dwarfism correlates with shorter, less mineralized and functionally weaker bones that do not result from growth plate abnormalities or osteoblast dysfunction. Besides skeletal dwarfism, internal organs are correspondingly smaller, indicating proportional dwarfism and suggesting a systemic cause for the overall size reduction. In accordance with a critical role of insulin-like growth factor (IGF)-1 in growth control and bone mineralization, circulating IGF-1 levels in the sera of mice lacking either α2β1 or α11β1 or both integrins were sharply reduced by 39%, 64%, or 81% of normal levels, respectively. Low hepatic IGF-1 production resulted from diminished growth hormone-releasing hormone expression in the hypothalamus and, subsequently, reduced growth hormone expression in the pituitary glands of these mice. These findings point out a novel role of collagen-binding integrin receptors in the control of growth hormone/IGF-1-dependent biological activities. Thus, coupling hormone secretion to extracellular matrix signaling via integrins represents a novel concept in the control of endocrine homeostasis.

The interaction between nanoscale surface features and mechanical loading and its effect on osteoblast-like cells behavior

Osteoblasts respond to mechanical stimulation by changing morphology, gene expression and matrix mineralization. Introducing surface topography on biomaterials, independently of mechanical loading, has been reported to give similar effects. In the current study, using a nanotextured surface, and mechanical loading, we aimed to develop a multi-factorial model in which both parameters interact. Mechanical stimulation to osteoblast-like cells was applied by longitudinal stretch in parallel direction to the nanotexture (300 nm wide and 60 nm deep grooves), with frequency of 1 Hz and stretch magnitude varying from 1% to 8%. Scanning electron microscopy showed that osteoblast-like cells subjected to mechanical loading oriented perpendicularly to the stretch direction. When cultured on nanotextured surfaces, cells aligned parallel to the texture. However, the parallel cell direction to the nanotextured surface was lost and turned to perpendicular when parallel stretch to the nanotexture, greater than 3% was applied to the cells. This phenomenon could not be achieved when a texture with micro-sized dimensions was used. Moreover, a significant synergistic effect on upregulation of fibronectin and Cfba was observed when dual stimulation was used. These findings can lead to a development of new biomimetic materials that can guide morphogenesis in tissue repair and bone remodeling.

Cyclic GMP and protein kinase G control a Src-containing mechanosome in osteoblasts

Mechanical stimulation is crucial for bone growth and remodeling, and fluid shear stress promotes anabolic responses in osteoblasts through multiple second messengers, including nitric oxide (NO). NO triggers production of cyclic guanosine 3',5'-monophosphate (cGMP), which in turn activates protein kinase G (PKG). We found that the NO-cGMP-PKG signaling pathway activates Src in mechanically stimulated osteoblasts to initiate a proliferative response. PKGII was necessary for Src activation, a process that also required the interaction of Src with β₃ integrins and dephosphorylation of Src by a complex containing the phosphatases SHP-1 (Src homology 2 domain-containing tyrosine phosphatase 1) and SHP-2. PKGII directly phosphorylated and stimulated SHP-1 activity, and fluid shear stress triggered the recruitment of PKGII, Src, SHP-1, and SHP-2 to a mechanosome containing β₃ integrins. PKGII-null mice showed defective Src and ERK (extracellular signal-regulated kinase) signaling in osteoblasts and decreased ERK-dependent gene expression in bone. Our findings reveal a convergence of NO-cGMP-PKG and integrin signaling and establish a previously unknown mechanism of Src activation. These results support the use of PKG-activating drugs to mimic the anabolic effects of mechanical stimulation of bone in the treatment of osteoporosis.

A kinase-independent role for unoccupied insulin and IGF-1 receptors in the control of apoptosis

Insulin and insulin-like growth factor 1 (IGF-1) act as antiapoptotic hormones. We found that, unexpectedly, double-knockout (DKO) cells that lacked both insulin and IGF-1 receptors (IR and IGF1R, respectively) were resistant to apoptosis induced through either the intrinsic or the extrinsic pathway. This resistance to apoptosis was associated with decreased abundance of the proapoptotic protein Bax and increases in abundance of the antiapoptotic proteins Bcl-2, Bcl-xL, XIAP, and Flip. These changes in protein abundance involved primarily posttranscriptional mechanisms. Restoration of IR or IGF1R to DKO cells also restored their sensitivity to apoptosis. Notably, expression of a catalytically inactive mutant form of the IR also restored susceptibility to apoptosis. Thus, IR and IGF1R have bidirectional roles in the control of cell survival and can be viewed as dependence receptors. Insulin and IGF-1 binding stimulates receptor tyrosine kinase activity and blocks apoptosis, whereas unliganded IR and IGF1R, acting through a mechanism independent of their catalytic activity, exert a permissive effect on cell death.

Passive mechanical forces upregulate the fast myosin heavy chain IId/x via integrin and p38 MAP kinase activation in a primary muscle cell culture

We have studied the mechanism by which a previously described primary muscle culture growing on microcarriers predominantly expresses fast myosin heavy chain (MHC) IId/x. We have measured MHC IId/x mRNA and protein levels, mRNA of MHC I and markers of muscle metabolism, insulin-like growth factor (IGF)-1 and mechano-growth factor (MGF) transcripts, indicators of the activation of the Akt-mammalian target of rapamycin (mTOR) axis, the p38-, ERK1/2-, and JNK-mitogen-activated protein kinase (MAP) kinase pathways, and of protein phosphatase PP2A, and we have assessed the involvement of integrin. By placing the culture flasks on a rotary shaker, we induce a continuous motion of the culture medium in which the carrier-myotube aggregates are suspended. This motion exerts passive forces on the myotubes that are decisive for the predominance of MHC II expression. These forces act via integrin, which transduces the mechanical signal into activation of PP2A and of p38 MAP-Kinase. The latter presumably is directly responsible for a drastic upregulation of MHC IId/x, whereas MHC I and metabolic markers remain unaffected. At the same time, despite an elevated level of IGF-1 transcription under passive forces, the IGF-1 receptor-Akt-mTOR axis is switched off as evident from the lack of an effect of inhibition of the IGF-1 receptor and from the PP2A-mediated low degree of phosphorylation of Akt and 4E-BP1. Similarly, the ERK1/2- and JNK-MAP kinase pathways are repressed. We conclude that passive stretch exerted on the myotubes by the rotary fluid motion induces a rather selective upregulation of fast MHC II, which goes along with a mild muscle hypertrophy as judged from the amount of protein per cell and is caused by p38 MAP kinase activity elevated via integrin sensing. The direct link between passive stretch and MHC II expression constitutes a novel mechanism, which is expected to become effective physiologically under passive stretch and eccentric contractions of skeletal muscles.

Cancellous bone properties and matrix content of TGF-beta2 and IGF-I in human tibia: a pilot study

Transforming and insulin-like growth factors are important in regulating bone mass. Thus, one would anticipate correlations between matrix concentrations of growth factors and functional properties of bone. We therefore investigated the relationships of (1) TGF-beta2 and (2) IGF-I matrix concentrations with the trabecular microstructure, stress distribution, and mechanical properties of tibial cancellous bone from six male human cadavers. Trabecular stress amplification (VMExp/sigma(app)) and variability (VMCOV) were calculated using microcomputed tomography (muCT)-based finite element simulations. Bone volume fraction (BV/TV), surface/volume ratio (BS/BV), trabecular thickness (Tb.Th), number (Tb.N) and separation (Tb.Sp), connectivity (Eu.N), and anisotropy (DA) were measured using 3-D morphometry. Bone stiffness and strength were measured by mechanical testing. Matrix concentrations of TGF-beta2 and IGF-I were measured by ELISA. We found higher matrix concentrations of TGF-beta2 were associated with higher Tb.Sp and VMExp/sigma(app) for pooled data and within subjects. Similarly, a higher matrix concentration of IGF-I was associated with lower stiffness, strength, BV/TV and Tb.Th and with higher BS/BV, Tb.Sp, VMExp/sigma(app) and VMCOV for pooled data and within subjects. IGF-I and Tb.N were negatively associated within subjects. It appears variations of the stress distribution in cancellous bone correlate with the variation of the concentrations of TGF-beta2 and IGF-I in bone matrix: increased local matrix concentrations of growth factors are associated with poor biomechanical and architectural properties of tibial cancellous bone.

Signaling networks and transcription factors regulating mechanotransduction in bone

Mechanical stimulation has a critical role in the development and maintenance of the skeleton. This function requires the perception of extracellular stimuli as well as their conversion into intracellular biochemical responses. This process is called mechanotransduction and is mediated by a plethora of molecular events that regulate bone metabolism. Indeed, mechanoreceptors, such as integrins, G protein-coupled receptors, receptor protein tyrosine kinases, and stretch-activated Ca(2+) channels, together with their downstream effectors coordinate the transmission of load-induced signals to the nucleus and the expression of bone-related genes. During the past decade, scientists have gained increasing insight into the molecular networks implicated in bone mechanotransduction. In the present paper, we consider the major signaling cascades and transcription factors that control bone and cartilage mechanobiology and discuss the influence of the mechanical microenvironment on the determination of skeletal morphology.

The role of liver-derived insulin-like growth factor-I

IGF-I is expressed in virtually every tissue of the body, but with much higher expression in the liver than in any other tissue. Studies using mice with liver-specific IGF-I knockout have demonstrated that liver-derived IGF-I, constituting a major part of circulating IGF-I, is an important endocrine factor involved in a variety of physiological and pathological processes. Detailed studies comparing the impact of liver-derived IGF-I and local bone-derived IGF-I demonstrate that both sources of IGF-I can stimulate longitudinal bone growth. We propose here that liver-derived circulating IGF-I and local bone-derived IGF-I to some extent have overlapping growth-promoting effects and might have the capacity to replace each other (= redundancy) in the maintenance of normal longitudinal bone growth. Importantly, and in contrast to the regulation of longitudinal bone growth, locally derived IGF-I cannot replace (= lack of redundancy) liver-derived IGF-I for the regulation of a large number of other parameters including GH secretion, cortical bone mass, kidney size, prostate size, peripheral vascular resistance, spatial memory, sodium retention, insulin sensitivity, liver size, sexually dimorphic liver functions, and progression of some tumors. It is clear that a major role of liver-derived IGF-I is to regulate GH secretion and that some, but not all, of the phenotypes in the liver-specific IGF-I knockout mice are indirect, mediated via the elevated GH levels. All of the described multiple endocrine effects of liver-derived IGF-I should be considered in the development of possible novel treatment strategies aimed at increasing or reducing endocrine IGF-I activity.

Mechanotransduction in osteoblast regulation and bone disease

Osteoblasts are key components of the bone multicellular unit and have a seminal role in bone remodeling, which is an essential function for the maintenance of the structural integrity and metabolic capacity of the skeleton. The coordinated function of skeletal cells is regulated by several hormones, growth factors and mechanical cues that act via interconnected signaling networks, resulting in the activation of specific transcription factors and, in turn, their target genes. Bone cells are responsive to mechanical stimuli and this is of pivotal importance in developing biomechanical strategies for the treatment of osteodegenerative diseases. Here, we review the molecular pathways and players activated by mechanical stimulation during osteoblastic growth, differentiation and activity in health, and consider the role of mechanostimulatory approaches in treating various bone pathophysiologies.