Diabetic and age-related enhancement of collagen-linked fluorescence in cortical bones of rats

Aging impairs the osteocytic regulation of collagen integrity and bone quality

Poor bone quality is a major factor in skeletal fragility in elderly individuals. The molecular mechanisms that establish and maintain bone quality, independent of bone mass, are unknown but are thought to be primarily determined by osteocytes. We hypothesize that the age-related decline in bone quality results from the suppression of osteocyte perilacunar/canalicular remodeling (PLR), which maintains bone material properties. We examined bones from young and aged mice with osteocyte-intrinsic repression of TGFβ signaling (TβRIIocy-/-) that suppresses PLR. The control aged bone displayed decreased TGFβ signaling and PLR, but aging did not worsen the existing PLR suppression in male TβRIIocy-/- bone. This relationship impacted the behavior of collagen material at the nanoscale and tissue scale in macromechanical tests. The effects of age on bone mass, density, and mineral material behavior were independent of osteocytic TGFβ. We determined that the decline in bone quality with age arises from the loss of osteocyte function and the loss of TGFβ-dependent maintenance of collagen integrity.

Role of advanced glycation endproducts in bone fragility in type 1 diabetes

The excess fracture risk observed in adults with type 1 diabetes (T1D) is inexplicable in the presence of only modest reductions in areal bone mineral density (BMD). Accumulation of advanced glycation endproducts (AGEs) in bone has been invoked as one explanation for the increased bone fragility in diabetes. The evidence linking AGEs and fractures in individuals with T1D is sparse, although the association has been observed in individuals with type 2 diabetes. Recent data show that in T1D, AGEs as measured by skin intrinsic fluorescence, are a risk factor for lower BMD. Further research in T1D is needed to ascertain whether there is a causal relationship between fractures and AGEs. If confirmed, this would pave the way for finding interventions that can slow AGE accumulation and thus reduce fractures in T1D.

Effect of in vitro ribosylation on the dynamic fracture behavior of mature bovine cortical bone

In this study, the fracture behavior of ribosylated bovine cortical bone is investigated under loading conditions simulating a fall event. Single edge notched specimens, separated into a control group (n = 11) and a ribosylated group (n = 8), were extracted from the mid-diaphysis of a single bovine femur harvested from a mature cow. A seven-day ribosylation process results in the accumulation of Advanced-Glycation End Products (AGEs) cross-links and AGE adducts. Specimens were subjected to symmetric three point bending (opening mode) and an impact velocity of 1.6 m/s using a drop tower. Near-crack displacement fields up to fracture initiation are determined from high-speed images post-processed using digital image correlation. A constrained over-deterministic least squares regression and orthotropic material linear elastic fracture mechanics theory are used to extract the in-plane critical stress intensity factors at fracture initiation (i.e., fracture initiation toughness values). Statistically significant differences were not observed when comparing the in-plane fracture initiation toughness values (p≥0.96) or energy release rate (p=0.90) between the control and seven-day ribosylated groups. The intrinsic variability of bone may require high sample numbers in order to achieve an adequately powered experiment when assessing dynamic fracture behavior. While there are no detectable differences due to the ribosylation treatment investigated, this is likely due to the limited sample sizes utilized.

Novel Fluorometric Assay of Antiglycation Activity Based on Methylglyoxal-Induced Protein Carbonylation

Advanced glycation end products (AGEs), which can have multiple structures, are formed at the sites where the carbonyl groups of reducing sugars bind to the free amino groups of proteins through the Maillard reaction. Some AGE structures exhibit fluorescence, and this fluorescence has been used to measure the formation and quantitative changes in carbonylated proteins. Recently, fluorescent AGEs have also been used as an index for the evaluation of compounds that inhibit protein glycation. However, the systems used to generate fluorescent AGEs from the reaction of reducing sugars and proteins used for the evaluation of antiglycation activity have not been determined through appropriate research; thus, problems remain regarding sensitivity, quantification, and precision. In the present study, using methylglyoxal (MGO), a reactive carbonyl compound to induce glycation, a comparative analysis of the mechanisms of formation of fluorescent substances from several types of proteins was conducted. The analysis identified hen egg lysozyme (HEL) as a protein that produces stronger fluorescent AGEs faster in the Maillard reaction with MGO. It was also found that the AGE structure produced in MGO-induced in HEL was argpyrimidine. By optimizing the reaction system, we developed a new evaluation method for compounds with antiglycation activity and established an efficient evaluation method (HEL-MGO assay) with greater sensitivity and accuracy than the conventional method, which requires high concentrations of bovine serum albumin and glucose. Furthermore, when compounds known to inhibit glycation were evaluated using this method, their antiglycation activities were clearly and significantly measured, demonstrating the practicality of this method.

Age-Influenced Receptors of Advanced Glycation End Product Overexpression Associated With Osteogenic Differentiation Impairment in Patients With Type 2 Diabetes

Preclinical studies have found impaired osteogenic differentiation to be associated with type 2 diabetes (T2DM), which is related to skeletal accumulation of advanced glycation end products (AGEs). Our previous study also showed impaired osteogenic differentiation in peripheral blood-derived mononuclear cells (PBMC) isolated from patients with long-standing T2DM, which is conceivably due to the overexpression of receptor of advance glycation end products (RAGE) and the enhancement of cellular apoptosis. However, the existence of RAGE overexpression in earlier stages of diabetes remains unclear, as do the factors influencing that RAGE overexpression. This cross-sectional study enrolled 40 patients with T2DM treated with metformin monotherapy and 30 age-matched non-diabetic controls (NDM) to investigate the overexpression of RAGE in PBMC derived from patients with earlier stage diabetes, as well as to explore its determining factors. Almost all (90%) PBMC-isolated from NDM (NDM-pD) expressed osteoblast-specific genes including ALPL, BGLAP, COL1A1, and RUNX2/PPAR while only 40% of PBMC-derived from diabetic patients (DM-pD) expressed those genes. By using age- and pentosidine-matched NDM-pD as a reference, AGER and BAX/BCL2 expression in PBMC isolated from diabetic patients showing impaired osteoblast-specific gene expression (DM-iD) were 6.6 and 5 folds higher than the reference while AGER and BAX/BCL2 expression in DM-pD were comparable to the reference. AGER expression showed a significant positive correlation with age (r=0.470, p=0.003). The multivariate analysis demonstrated that both age and AGER expression correlated with the potential for osteogenic differentiation in the PBMC isolated from patients with diabetes. In conclusion, this study showed osteogenic differentiation impairment in approximately half of PBMC derived from type 2 diabetic patients receiving metformin monotherapy. Both AGER and BAX/BCL2 overexpression were demonstrated only in PBMC-isolated from diabetic patients with poor osteogenic differentiation. Therefore, this study not only illustrated the existence of RAGE overexpression in PBMC derived from patients with early stages of T2DM but also strengthened the linkage between that RAGE overexpression and the retardation of osteogenic differentiation. Age was also shown to be a positive influencing factor for RAGE overexpression. Furthermore, both age and RAGE overexpression were demonstrated as independent risk factors for determining osteogenic differentiation potential of the PBMC-isolated from T2DM.

Impaired Gastric Hormone Regulation of Osteoblasts and Lysyl Oxidase Drives Bone Disease in Diabetes Mellitus

Diabetic bone disease is a complication of type I and type II diabetes, both of which are increasing in the United States and elsewhere. Increased hip and foot fracture rates do not correlate well with changes in bone mineral density (BMD), whereas studies support the importance of collagen structure to bone strength. Extracellular lysyl oxidase (LOX) catalyzes the oxidative deamination of hydroxylysine and lysine residues in collagens resulting in aldehydes that subsequently form critically important biosynthetic crosslinks that stabilize functional collagens. Although LOX-dependent biosynthetic crosslinks in bone collagen are deficient in diabetic bone, the expression and regulation of bone LOXs in diabetes have not been comprehensively studied. Here, we found that LOX is profoundly downregulated in bone in diabetes. Moreover, we have identified a novel metabolic regulatory relationship that is dysregulated in diabetes using mouse models. Data indicate that the incretin (gastric hormone) known as glucose-dependent insulinotropic polypeptide (GIP) that is anabolic to osteoblasts strongly upregulates LOX, and that this regulation is disrupted in the streptozotocin-induced model of diabetes in mice. In vivo and in vitro studies support that diabetes results in elevated circulating peripheral dopamine, likely also derived from the gut, and is responsible for blocking GIP signaling and LOX levels in osteoblasts. Moreover, peripheral administration of the dopamine D2 receptor antagonist amisulpride to diabetic mice restored trabecular bone structure to near normal and partially reversed downregulation of LOX. Taken together our data identifies a novel metabolic relationship between the gut-derived hormone GIP and bone-derived LOX, and points to the importance of LOX dysregulation in the pathology of diabetic bone disease. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

Nondestructive assessment of collagen hydrogel cross-linking using time-resolved autofluorescence imaging

We investigate the use of a fiber-based, multispectral fluorescence lifetime imaging (FLIm) system to nondestructively monitor changes in mechanical properties of collagen hydrogels caused by controlled application of widely used cross-linking agents, glutaraldehyde (GTA) and ribose. Postcross-linking, fluorescence lifetime images are acquired prior to the hydrogels being processed by rheological or tensile testing to directly probe gel mechanical properties. To preserve the sterility of the ribose-treated gels, FLIm is performed inside a biosafety cabinet (BSC). A pairwise correlation analysis is used to quantify the relationship between mean hydrogel fluorescence lifetimes and the storage or Young's moduli of the gels. In the GTA study, we observe strong and specific correlations between fluorescence lifetime and the storage and Young's moduli. Similar correlations are not observed in the ribose study and we postulate a reason for this. Finally, we demonstrate the ability of FLIm to longitudinally monitor dynamic cross-link formation. The strength of the GTA correlations and deployment of our fiber-based FLIm system inside the aseptic environment of a BSC suggests that this technique may be a valuable tool for the tissue engineering community where longitudinal assessment of tissue construct maturation in vitro is highly desirable.

Age-related changes in the knee meniscus

BACKGROUND

Aging is the most prominent risk factor for the development of osteoarthritis (OA), which affects knees and causes major health burdens. Meniscal dysfunction mostly based on degeneration contributes to the development and progression of knee OA. Meniscal degeneration is caused by various extrinsic factors, such as repetitive trauma or leg malalignment, while meniscal aging is considered as internal changes, such as molecular or cellular changes. Little is known about age-related changes in the meniscus. Therefore, this review aimed to summarize and clarify the understanding of the aged meniscus.

METHODS

There are few articles about natural aging in the meniscus, because most reports only demonstrate the effects of OA on the meniscus. We searched PubMed (1948 to November 2016) to identify and summarize all English-language articles evaluating natural aging in the meniscus.

RESULTS

There is evidence of compositional change in the meniscus with aging, involving cells, collagens, and proteoglycans. In addition, as recent reports on the natural aging of cartilage have indicated, senescence of the meniscal cells may also lead to disruption of meniscal cells and tissue homeostasis. Due to the low turnover rate of collagen, accumulation of advanced glycation end-products largely contributes to tissue stiffness and vulnerability, and finally results in degenerative changes or tears. Furthermore, environmental factors such as joint fluid secreted by inflamed synovium could also contribute to meniscal tissue deterioration.

CONCLUSIONS

Age-related changes induce meniscal tissue vulnerability and finally lead to meniscal dysfunction.

Impaired osteogenic differentiation and enhanced cellular receptor of advanced glycation end products sensitivity in patients with type 2 diabetes

Preclinical studies have demonstrated impaired osteoblast differentiation in type 2 diabetes (T2DM), which is related to skeletal accumulation of advanced glycation end products (AGEs). However, the role of AGE in osteoblast differentiation in patients with T2DM is unclear. This cross-sectional study was performed to investigate osteoblast differentiation and its association with serum pentosidine and soluble receptor of AGEs (sRAGE). Twenty-seven patients with T2DM and 15 age-matched controls were included to measure sRAGE and osteogenic differentiation in mononuclear cells derived from peripheral blood. The mononuclear cells isolated from patients with T2DM showed a significantly lower rate of osteogenic differentiation (7.4% vs 86.7%, p < 0.0001) with a lower level of ALPL, COL1A1, and BGLAP expression than those of controls by 11-, 44-, and 15-fold respectively, together with nonvisualized mineralization by alizarin red S staining. The levels of pentosidine and sRAGE were comparable in both groups. AGER expression was significantly higher in the T2DM group. BAX expression was also significantly higher in the T2DM group, and showed a strong correlation with AGER expression (r = 0.86, p < 0.0001). Fasting plasma glucose (FPG) level, AGER expression, and BAX expression showed a strong correlation with osteogenic differentiation defects on univariate analysis. However, only FPG showed a correlation with this defect in a multivariate analysis. In conclusion, patients with T2DM showed impairment of osteoblast differentiation, and FPG was an independent risk factor for this impairment. Moreover, T2DM showed a higher cellular sensitivity for activation of receptor of AGEs and higher cellular apoptosis, which may contribute to the defect in osteoblast differentiation.

Effects of raloxifene and alendronate on non-enzymatic collagen cross-links and bone strength in ovariectomized rabbits in sequential treatments after daily human parathyroid hormone (1-34) administration

This study investigated the effects of raloxifene and alendronate to follow parathyroid hormone (PTH) on bone collagen and biomechanical properties in ovariectomized rabbits. Sequential treatments of raloxifene and alendronate after hPTH(1-34) treatment improved biomechanical properties with and without bone collagen improvement, respectively.

INTRODUCTION

The standard sequential treatment to follow human parathyroid hormone (hPTH) (1-34) therapy for osteoporosis has yet to be determined. The objective of this study was to compare the effects of raloxifene and alendronate treatments to follow daily hPTH(1-34) treatment on non-enzymatic collagen cross-links, bone mass, and bone strength in ovariectomized (OVX) rabbits.

METHODS

From 3 months after ovariectomy, seven month-old female New Zealand white rabbits were given either vehicle or hPTH(1-34) (8 μg/kg/day), once daily for 5 months. After hPTH(1-34) treatment, the hPTH(1-34)-treated animals were divided into two groups, and given raloxifene (10 mg/kg, daily) orally or alendronate (100 μg/kg, twice weekly) subcutaneously for 5 months. We evaluated bone mineral density (BMD), bone structural parameters, advanced glycation end product (AGE) content in collagen, and bone mechanical parameters including intrinsic parameters in the femur.

RESULTS

Raloxifene (hPTH/RLX) and alendronate (hPTH/ALN) to follow hPTH(1-34) increased cortical thickness, maximum load, and maximum stress and decreased endocortical surface in the diaphysis, in addition to increasing total BMD in the distal metaphysis. Decreased trabecular AGE, pentosidine, and homocysteine contents and increased toughness and breaking energy were noted with hPTH/RLX treatment only. With hPTH/ALN treatment, no effects on non-enzymatic collagen cross-link AGEs were noted although increases in stiffness and elastic modulus were observed.

CONCLUSION

These results suggest that sequential treatments with hPTH(1-34) and antiresorptive drugs (raloxifene and alendronate) have a beneficial effect on bone mass and biomechanical properties in OVX rabbits.

Age-related changes in the fracture resistance of male Fischer F344 rat bone

In addition to the loss in bone volume that occurs with age, there is a decline in material properties. To test new therapies or diagnostic tools that target such properties as material strength and toughness, a pre-clinical model of aging would be useful in which changes in bone are similar to those that occur with aging in humans. Toward that end, we hypothesized that similar to human bone, the estimated toughness and material strength of cortical bone at the apparent-level decreases with age in the male Fischer F344 rat. In addition, we tested whether the known decline in trabecular architecture in rats translated to an age-related decrease in vertebra (VB) strength and whether non-X-ray techniques could quantify tissue changes at micron and sub-micron length scales. Bones were harvested from 6-, 12-, and 24-month (mo.) old rats (n=12 per age). Despite a loss in trabecular bone with age, VB compressive strength was similar among the age groups. Similarly, whole-bone strength (peak force) in bending was maintained (femur) or increased (radius) with aging. There was though an age-related decrease in post-yield toughness (radius) and bending strength (femur). The ability to resist crack initiation was actually higher for the 12-mo. and 24-mo. than for 6-mo. rats (notch femur), but the estimated work to propagate the crack was less for the aged bone. For the femur diaphysis region, porosity increased while bound water decreased with age. For the radius diaphysis, there was an age-related increase in non-enzymatic and mature enzymatic collagen crosslinks. Raman spectroscopy analysis of embedded cross-sections of the tibia mid-shaft detected an increase in carbonate subsitution with advanced aging for both inner and outer tissue.

Effect of type 2 diabetes-related non-enzymatic glycation on bone biomechanical properties

There is clear evidence that patients with type 2 diabetes mellitus (T2D) have increased fracture risk, despite having high bone mineral density (BMD) and body mass index (BMI). Thus, poor bone quality has been implicated as a mechanism contributing to diabetic skeletal fragility. Poor bone quality in T2D may result from the accumulation of advanced glycation end-products (AGEs), which are post-translational modifications of collagen resulting from a spontaneous reaction between extracellular sugars and amino acid residues on collagen fibers. This review discusses what is known and what is not known regarding AGE accumulation and diabetic skeletal fragility, examining evidence from in vitro experiments to simulate a diabetic state, ex vivo studies in normal and diabetic human bone, and diabetic animal models. Key findings in the literature are that AGEs increase with age, affect bone cell behavior, and are altered with changes in bone turnover. Further, they affect bone mechanical properties and microdamage accumulation, and can be inhibited in vitro by various inhibitors and breakers (e.g. aminoguanidine, N-Phenacylthiazolium Bromide, vitamin B6). While a few studies report higher AGEs in diabetic animal models, there is little evidence of AGE accumulation in bone from diabetic patients. There are several limitations and inconsistencies in the literature that should be noted and studied in greater depth including understanding the discrepancies between glycation levels across reported studies, clarifying differences in AGEs in cortical versus cancellous bone, and improving the very limited data available regarding glycation content in diabetic animal and human bone, and its corresponding effect on bone material properties in T2D.

Effects of Collagen Crosslinking on Bone Material Properties in Health and Disease

Data have accumulated to show that various types of collagen crosslinking are implicated in the health of individuals, as well as in a number of disease states, such as osteoporosis, diabetes mellitus, chronic kidney disease, inflammatory bowel disease, or in conditions of mild hyperhomocysteinemia, or when glucocorticoid use is indicated. Collagen crosslinking is a posttranslational modification of collagen molecules and plays important roles in tissue differentiation and in the mechanical properties of collagenous tissue. The crosslinking of collagen in the body can form via two mechanisms: one is enzymatic crosslinking and the other is nonenzymatic crosslinking. Lysyl hydroxylases and lysyl oxidases regulate tissue-specific crosslinking patterns and quantities. Enzymatic crosslinks initially form via immature divalent crosslinking, and a portion of them convert into mature trivalent forms such as pyridinoline and pyrrole crosslinks. Nonenzymatic crosslinks form as a result of reactions which create advanced glycation end products (AGEs), such as pentosidine and glucosepane. These types of crosslinks differ in terms of their mechanisms of formation and function. Impaired enzymatic crosslinking and/or an increase of AGEs have been proposed as a major cause of bone fragility associated with aging and numerous disease states. This review focuses on the effects of collagen crosslinking on bone material properties in health and disease.

An update on diabetes related skeletal fragility

There are several mechanisms by which diabetes could affect bone mass and strength. These mechanisms include insulin deficiency; hyperglycemia; the accumulation of advanced glycation end products that may influence collagen characteristics; marrow adiposity and bone inflammation. Furthermore, associated diabetic complications and treatment with thaizolidinediones may also increase risk of fracturing. The following article provides its readers with an update on the latest information pertaining to diabetes related bone skeletal fragility. In the authors' opinion, future studies are needed in order to clarify the impact of different aspects of diabetes metabolism, glycemic control, and specific treatments for diabetes on bone. Given that dual energy x-ray absorptiometry is a poor predictor of bone morbidity in this group of patients, there is a need to explore novel approaches for assessing bone quality. It is important that we develop a better understanding of how diabetes affects bone in order to improve our ability to protect bone health and prevent fractures in the growing population of adults with diabetes.

Diabetes, collagen, and bone quality

Diabetes increases risk of fracture, although type 2 diabetes is characterized by normal or high bone mineral density (BMD) compared with the patients without diabetes. The fracture risk of type 1 diabetes as well as type 2 diabetes increases beyond an explained by a decrease of BMD. Thus, diabetes may reduce bone strength without change in BMD. Whole bone strength is determined by bone density, structure, and quality, which encompass the micro-structural and tissue material properties. Recent literature showed that diabetes reduces bone material properties rather than BMD. Collagen intermolecular cross-linking plays an important role in the expression of bone strength. Collagen cross-links can be divided into beneficial enzymatic immature divalent and mature trivalent cross-links and disadvantageous nonenzymatic cross-links (Advanced glycation end products: AGEs) induced by glycation and oxidation. The formation pathway and biological function are quite different. Not only hyperglycemia, but also oxidative stress induces the reduction in enzymatic cross-links and the formation of AGEs. In this review, we describe the mechanism of low bone quality in diabetes and the usefulness of the measurement of plasma or urinary level of AGEs for estimation of fracture risk.

Beneficial Effect of Cissus quadrangularis Linn. on Osteopenia Associated with Streptozotocin-Induced Type 1 Diabetes Mellitus in Male Wistar Rats

Petroleum ether fraction of Cissus quadrangularis (PECQ) impact on the development of osteopenia in type 1 diabetic rat model has been evaluated. Diabetic rats were treated orally with two doses of PECQ. Another group of diabetic rats were treated with subcutaneous injection of synthetic human insulin. The cortical and trabecular bone thickness and bone strength were significantly decreased in diabetic rats. Treatment with two doses of PECQ significantly prevented these changes in diabetic rats. However, PECQ treatment (two doses) did not alter the glycemic levels in these diabetic rats. Increased levels of serum alkaline phosphatase (ALP), tartrate-resistant acid phosphatase (TRAP), and hydroxyproline were noted in diabetic rats when compared to normal control rats. The two doses of PECQ treatment further improved the serum ALP levels and significantly decreased the serum levels of TRAP and hydroxyproline. The effects of PECQ treatment on histological, biomechanical, and biochemical parameters are comparable to those of insulin. Since PECQ improves the bone health in hyperglycemic conditions by enhancing the cortical and trabecular bone growth and altering the circulating bone markers, it could be used as an effective therapy against diabetes-associated bone disorders.

Nanoscale changes in collagen are reflected in physical and mechanical properties of bone at the microscale in diabetic rats

Diabetes detrimentally affects the musculoskeletal system by stiffening the collagen matrix due to increased advanced glycation end products (AGEs). In this study, tibiae and tendon from Zucker diabetic Sprague-Dawley (ZDSD) rats were compared to Sprague-Dawley derived controls (CD) using Atomic Force Microscopy. ZDSD and CD tibiae were compared using Raman Spectroscopy and Reference Point Indentation (RPI). ZDSD bone had a significantly different distribution of collagen D-spacing than CD (p=0.015; ZDSD n=294 fibrils; CD n=274 fibrils) which was more variable and shifted to higher values. This shift between ZDSD and CD D-spacing distribution was more pronounced in tendon (p<0.001; ZDSD n=350; CD n=371). Raman revealed significant increases in measures of bone matrix mineralization in ZDSD (PO4(3-) ν1/Amide I p=0.008; PO4(3-) ν1/CH2 wag p=0.047; n=5 per group) despite lower bone mineral density (aBMD) and ash fraction indicating diabetes may preferentially reduce the Raman signature of collagen. Decreased indentation distance increase (p=0.010) and creep indentation distance (p=0.040) measured by RPI (n=9 per group) in ZDSD rats suggest a matrix more resistant to indentation under the high stresses associated with RPI at this length scale. There were significant correlations between Raman and RPI measurements in the ZDSD population (n=18 locations) but not the CD population (n=16 locations) indicating that while RPI is relatively unaffected by biological noise, it is sensitive to disease-induced compositional changes. In conclusion, diabetes in the ZDSD rat causes changes to the nanoscale morphology of collagen that result in compositional and mechanical effects in bone at the microscale.

Multiscale analysis of morphology and mechanics in tail tendon from the ZDSD rat model of type 2 diabetes

Type 2 diabetes (T2D) impacts multiple organ systems including the circulatory, renal, nervous and musculoskeletal systems. In collagen-based tissues, one mechanism that may be responsible for detrimental mechanical impacts of T2D is the formation of advanced glycation end products (AGEs) leading to increased collagen stiffness and decreased toughness, resulting in brittle tissue behavior. The purpose of this study was to investigate tendon mechanical properties from normal and diabetic rats at two distinct length scales, testing the hypothesis that increased stiffness and strength and decreased toughness at the fiber level would be associated with alterations in nanoscale morphology and mechanics. Individual fascicles from female Zucker diabetic Sprague-Dawley (ZDSD) rats had no differences in fascicle-level mechanical properties but had increased material-level strength and stiffness versus control rats (CD). At the nanoscale, collagen fibril D-spacing was shifted towards higher spacing values in diabetic ZDSD fibrils. The distribution of nanoscale modulus values was also shifted to higher values. Material-level strength and stiffness from whole fiber tests were increased in ZDSD tails. Correlations between nanoscale and microscale properties indicate a direct positive relationship between the two length scales, most notably in the relationship between nanoscale and microscale modulus. These findings indicate that diabetes-induced changes in material strength and modulus were driven by alterations at the nanoscale.

Bone quality in diabetes

Diabetes is associated with increased risk of fracture, although type 2 diabetes is characterized by normal bone mineral density (BMD). The fracture risk of type 1 diabetes increases beyond an explained by a decrease of BMD. Thus, diabetes may be associated with a reduction of bone strength that is not reflected in the measurement of BMD. Based on the present definition, both bone density and quality, which encompass the structural and material properties of bone, are important factors in the determination of bone strength. Diabetes reduces bone quality rather than BMD. Collagen cross-linking plays an important role in bone strength. Collagen cross-links can be divided into lysyl hydroxylase and lysyl oxidase-mediated enzymatic immature divalent cross-links, mature trivalent cross-links, and glycation- or oxidation-induced non-enzymatic cross-links (Advanced Glycation End-products: AGEs) such as pentosidine. These types of cross-links differ in the mechanism of formation and in function. Not only hyperglycemia, but also oxidative stress induces the reduction in enzymatic beneficial cross-links and the accumulation of disadvantageous AGEs in bone. In this review, we describe the mechanism of low bone quality in diabetes.

Review: The diabetic bone: a cellular and molecular perspective

With the increasing worldwide prevalence of diabetes the resulting complications, their consequences and treatment will lead to a greater social and financial burden on society. One of the many organs to be affected is bone. Loss of bone is observed in type 1 diabetes, in extreme cases mirroring osteoporosis, thus a greater risk of fracture. In the case of type 2 diabetes, both a loss and an increase of bone has been observed, although in both cases the quality of the bone overall was poorer, again leading to a greater risk of fracture. Once a fracture has occurred, healing is delayed in diabetes, including nonunion. The reasons leading to such changes in the state of the bone and fracture healing in diabetes is under investigation, including at the cellular and the molecular levels. In comparison with our knowledge of events in normal bone homeostasis and fracture healing, that for diabetes is much more limited, particularly in patients. However, progress is being made, especially with the use of animal models for both diabetes types. Identifying the molecular and cellular changes in the bone in diabetes and understanding how they arise will allow for targeted intervention to improve diabetic bone, thus helping to counter conditions such as Charcot foot as well as preventing fracture and accelerating healing when a fracture does occur.

Increasing duration of type 1 diabetes perturbs the strength-structure relationship and increases brittleness of bone

Type 1 diabetes (T1DM) increases the likelihood of a fracture. Despite serious complications in the healing of fractures among those with diabetes, the underlying causes are not delineated for the effect of diabetes on the fracture resistance of bone. Therefore, in a mouse model of T1DM, we have investigated the possibility that a prolonged state of diabetes perturbs the relationship between bone strength and structure (i.e., affects tissue properties). At 10, 15, and 18 weeks following injection of streptozotocin to induce diabetes, diabetic male mice and age-matched controls were examined for measures of skeletal integrity. We assessed 1) the moment of inertia (I(MIN)) of the cortical bone within diaphysis, trabecular bone architecture of the metaphysis, and mineralization density of the tissue (TMD) for each compartment of the femur by micro-computed tomography and 2) biomechanical properties by three-point bending test (femur) and nanoindentation (tibia). In the metaphysis, a significant decrease in trabecular bone volume fraction and trabecular TMD was apparent after 10 weeks of diabetes. For cortical bone, type 1 diabetes was associated with decreased cortical TMD, I(MIN), rigidity, and peak moment as well as a lack of normal age-related increases in the biomechanical properties. However, there were only modest differences in material properties between diabetic and normal mice at both whole bone and tissue-levels. As the duration of diabetes increased, bone toughness decreased relative to control. If the sole effect of diabetes on bone strength was due to a reduction in bone size, then I(MIN) would be the only significant variable explaining the variance in the maximum moment. However, general linear modeling found that the relationship between peak moment and I(MIN) depended on whether the bone was from a diabetic mouse and the duration of diabetes. Thus, these findings suggest that the elevated fracture risk among diabetics is impacted by complex changes in tissue properties that ultimately reduce the fracture resistance of bone.

Osteoporosis in diabetes mellitus: Possible cellular and molecular mechanisms

Osteoporosis, a global age-related health problem in both male and female elderly, insidiously deteriorates the microstructure of bone, particularly at trabecular sites, such as vertebrae, ribs and hips, culminating in fragility fractures, pain and disability. Although osteoporosis is normally associated with senescence and estrogen deficiency, diabetes mellitus (DM), especially type 1 DM, also contributes to and/or aggravates bone loss in osteoporotic patients. This topic highlight article focuses on DM-induced osteoporosis and DM/osteoporosis comorbidity, covering alterations in bone metabolism as well as factors regulating bone growth under diabetic conditions including, insulin, insulin-like growth factor-1 and angiogenesis. Cellular and molecular mechanisms of DM-related bone loss are also discussed. This information provides a foundation for the better understanding of diabetic complications and for development of early screening and prevention of osteoporosis in diabetic patients.

Advanced glycation end products downregulates peroxisome proliferator-activated receptor γ expression in cultured rabbit chondrocyte through MAPK pathway

Accumulation of advanced glycation end products (AGEs) which are known to adversely affect cartilage turnover and mechanical properties, provides a molecular mechanism by which aging contributes to the development of osteoarthritis. The objective of the present study was to investigate the role of peroxisome proliferator-activated receptor γ (PPARγ) in AGEs-mediated chondrocytes damage. In the cultured rabbit chondrocytes, our results show that the PPARγ agonist pioglitazone can concentration-dependently inhibit the AGEs-induced expression of TNF-α and MMP-13. Several studies have shown that activation of PPARγ may interfere with several signaling pathways regulating the proinflammatory genes in vivo and vitro experiments, but little is known regarding their expression and regulation in cartilage. Thus the effect of AGEs on PPARγ expression was next examined. Reverse transcription (RT)-PCR analysis revealed that AGEs treatment of chondrocytes downregulated PPARγ expression in a time- and concentration-dependent manner. AGEs-induced a significant downregulation in PPARγ mRNA at 48 h and the maximum effect was found at 100 μg/ml AGEs. This effect was significantly depressed by the anti-RAGE antibody. Specific inhibitors of the mitogen-activated protein kinases (MAPK) p38 (SB203580) and c-Jun N-terminal kinase (SP600125), but not of extracellular signal-regulated kinase (PD98059), prevented AGEs-induced downregulation of PPARγ expression. In conclusion, AGEs may be responsible for PPARγ downregulation via a mechanism involving activation of the MAPK (p38 and JNK), and this downregulation might play a key role in AGEs-induced production of TNF-α and MMP-13.

Collagen cross-links as a determinant of bone quality: a possible explanation for bone fragility in aging, osteoporosis, and diabetes mellitus

Collagen cross-linking, a major post-translational modification of collagen, plays important roles in the biological and biomechanical features of bone. Collagen cross-links can be divided into lysyl hydroxylase and lysyloxidase-mediated enzymatic immature divalent cross-links,mature trivalent pyridinoline and pyrrole cross-links, and glycation- or oxidation-induced non-enzymatic cross-links(advanced glycation end products) such as glucosepane and pentosidine. These types of cross-links differ in the mechanism of formation and in function. Material properties of newly synthesized collagen matrix may differ in tissue maturity and senescence from older matrix in terms of crosslink formation. Additionally, newly synthesized matrix in osteoporotic patients or diabetic patients may not necessarily be as well-made as age-matched healthy subjects. Data have accumulated that collagen cross-link formation affects not only the mineralization process but also microdamage formation. Consequently, collagen cross-linking is thought to affect the mechanical properties of bone. Furthermore,recent basic and clinical investigations of collagen cross-links seem to face a new era. For instance, serum or urine pentosidine levels are now being used to estimate future fracture risk in osteoporosis and diabetes. In this review, we describe age-related changes in collagen cross-links in bone and abnormalities of cross-links in osteoporosis and diabetes that have been reported in the literature.

Type 1 diabetes in young rats leads to progressive trabecular bone loss, cessation of cortical bone growth, and diminished whole bone strength and fatigue life

People with diabetes have increased risk of fracture disproportionate to BMD, suggesting reduced material strength (quality). We quantified the skeletal effects of type 1 diabetes in the rat. Fischer 344 and Sprague-Dawley rats (12 wk of age) were injected with either vehicle (Control) or streptozotocin (Diabetic). Forelimbs were scanned at 0, 4, 8, and 12 wk using pQCT. Rats were killed after 12 wk. We observed progressive osteopenia in diabetic rats. Trabecular osteopenia was caused by bone loss: volumetric BMD decreased progressively with time in diabetic rats but was constant in controls. Cortical osteopenia was caused by premature arrest of cortical expansion: cortical area did not increase after 4-8 wk in diabetic rats but continued to increase in controls. Postmortem muCT showed a 60% reduction in proximal tibial trabecular BV/TV in diabetic versus control rats, whereas moments of inertia of the ulnar and femoral diaphysis were reduced approximately 30%. Monotonic bending tests indicated that ulna and femora from diabetic animals were approximately 25% less stiff and strong versus controls. Estimates of material properties indicated no changes in elastic modulus or ultimate stress but modest ( approximately 10%) declines in yield stress for diabetic bone. These changes were associated with a approximately 50% increase in the nonenzymatic collagen cross-link pentosidine. Last, cyclic testing showed diminished fatigue life in diabetic bones at the structural (force) level but not at the material (stress) level. In summary, type 1 diabetes, left untreated, causes trabecular bone loss and a reduction in diaphyseal growth. Diabetic bone has greatly increased nonenzymatic collagen cross-links but only modestly reduced material properties. The loss of whole bone strength under both monotonic and fatigue loading is attributed mainly to reduced bone size.

Advanced Glycation End-products and Bone Fractures

Bone does not turn over uniformly, and becomes susceptible to post-translational modification by non-enzymatic glycation (NEG). NEG of bone causes the formation of advanced glycation end-products (AGEs) and this process is accelerated with aging, diabetes and antiresorptive postmenopausal osteoporosis therapy. Due to the elevated incidence of fracture associated with aging and diabetes, several studies have attempted to measure and evaluate AGEs as biomarkers for fracture risk. Here current methods of estimating AGEs in bone by liquid chromatography and fluorometric assay are summarized and the relationships between AGEs and fracture properties at whole bone, apparent tissue and matrix levels are discussed.

Diabetes and osteoporosis

Care of patients with diabetes should include assessment of bone health. The extension of the average life expectancy of people with diabetes, which has accompanied improvements in medical care, has also increased the significance of osteoporosis. In addition to the usual causes of osteoporosis associated with aging, bone health is also compromised by diabetes. Studies on bone involvement in patients with diabetes mellitus have generated conflicting results, largely because of the pathogenetic complexity of the condition. It is now clear that patients with type 1 diabetes have lower bone mineral density (BMD) and a higher risk of fractures. Evidence is emerging that patients with type 2 diabetes who have complications are also at increased risk of certain types of osteoporotic fractures, despite having a higher BMD when compared to patients with type 1 diabetes. Although many factors, including number and type of falls, visual impairment, neuropathy, and reduced muscle strength, influence the probability of fractures, the most significant factor seems to be the strength of the bone itself. Thus, sarcopenia, a reduction in muscle mass and muscle strength, is considered one of the main determinants of bone fragility. The aim of this review is to examine the occurrence of osteoporosis in type 1 and type 2 diabetes.

Age-related effect on the concentration of collagen crosslinks in human osteonal and interstitial bone tissue

Collagen crosslinks are important to the quality of bone and may be contributors to the age-related increase in bone fracture. This study was performed to investigate whether age and gender effects on collagen crosslinks are similar in osteonal and interstitial bone tissues. Forty human cadaveric femurs were collected and divided into two age groups: middle-aged (42-63 years of age) and elderly (69-90 years of age) with ten males and ten females in each group (n = 10). Micro-cores of bone tissue from both secondary osteons and interstitial regions in the medial quadrant of the diaphysis were extracted using a custom-modified, computer-controlled milling machine. The bone specimens were then analyzed using high performance liquid chromatography to determine the effects of age and gender on the concentration of mature, enzymatic crosslinks (hydroxylysyl-pyridinoline-HP and lysyl-pyridinoline-LP) and a non-enzymatic crosslink (pentosidine-PE) at these two microstructural sites. The results indicate that age has a significant effect on the concentration of LP and PE, while gender has a significant effect on HP and LP. In addition, the concentration of the crosslinks in the secondary osteons is significantly different from that in the interstitial bone regions. These results suggest that the amount of non-enzymatic crosslinking may increase while that of mature enzymatic crosslinking may decrease with age. Such changes could potentially reduce the inherent quality of the bone tissue in the elderly skeleton.

Extracellular post-translational modifications of collagen are major determinants of biomechanical properties of fetal bovine cortical bone

Mechanical behavior of bone depends on its mass and architecture, and on the material properties of the matrix, which is composed of a mineral phase and an organic component mainly constituted of type I collagen. Mineral accounts largely for the stiffness of bone, whereas type I collagen provides bone its ductility and toughness, i.e., its ability to undergo deformation and absorb energy after it begins to yield. The molecular mechanisms underlying the effect of alterations in type I collagen on bone mechanical properties are unclear. We used an in vitro model of fetal bovine cortical bone specimens (n = 44), where the extent of type I collagen cross-linking was modified by incubation at 37 degrees C for 0, 60, 90 and 120 days, keeping constant the architecture and the mineral content. At each incubation time, the following parameters were determined: (1) the bone concentration of enzymatic (pyridinoline; PYD and deoxypyridinoline, DPD) and non-enzymatic (pentosidine) crosslinks by HPLC, (2) the extent of aspartic acid isomerization of the type I collagen C-telopeptide (CTX) by ELISA of native (alpha CTX) and isomerized (beta CTX) forms, (3) the mineral density by DXA, (4) the porosity by micro-computed tomography and (5) the bending and compressive mechanical properties. Incubation of bone specimens at 37 degrees C for 60 days increased the level (per molecule of collagen) of PYD (+98%, P = 0.005), DPD (+42%, P = 0.013), pentosidine (+55-fold, P = 0.005), and the degree of type I collagen C-telopeptide isomerization (+4.9-fold, P = 0.005). These biochemical changes of collagen were associated with a 30% decrease in bending and compressive yield stress and a 2.5-fold increase in compressive post-yield energy absorption (P < 0.02 for all), with no significant change of bone stiffness. In multivariate analyses, the level of collagen cross-linking was associated with yield stress and post-yield energy absorption independently of bone mineral density, explaining up to 25% of their variance. We conclude that the extent and nature of collagen cross-linking contribute to the mechanical properties of fetal bovine cortical bone independently of bone mineral density.

Osteogenic actions of the anti-diabetic drug metformin on osteoblasts in culture

An association has been previously established between uncompensated diabetes mellitus and the loss of bone mineral density and/or quality. In this study, we evaluated the effects of metformin on the growth and differentiation of osteoblasts in culture. Treatment of two osteoblast-like cells (UMR106 and MC3T3E1) with metformin (25-500 microM) for 24 h led to a dose-dependent increase of cell proliferation. Metformin also promoted osteoblastic differentiation: it increased type-I collagen production in both cell lines and stimulated alkaline phosphatase activity in MC3T3E1 osteoblasts. In addition, metformin markedly increased the formation of nodules of mineralization in 3-week MC3T3E1 cultures. Metformin induced activation and redistribution of phosphorylated extracellular signal-regulated kinase (P-ERK) in a transient manner, and dose-dependently stimulated the expression of endothelial and inducible nitric oxide synthases (e/iNOS). These results show for the first time a direct osteogenic effect of metformin on osteoblasts in culture, which could be mediated by activation/redistribution of ERK-1/2 and induction of e/iNOS.

The role of collagen in bone strength

Bone is a complex tissue of which the principal function is to resist mechanical forces and fractures. Bone strength depends not only on the quantity of bone tissue but also on the quality, which is characterized by the geometry and the shape of bones, the microarchitecture of the trabecular bones, the turnover, the mineral, and the collagen. Different determinants of bone quality are interrelated, especially the mineral and collagen, and analysis of their specific roles in bone strength is difficult. This review describes the interactions of type I collagen with the mineral and the contribution of the orientations of the collagen fibers when the bone is submitted to mechanical forces. Different processes of maturation of collagen occur in bone, which can result either from enzymatic or nonenzymatic processes. The enzymatic process involves activation of lysyl oxidase, which leads to the formation of immature and mature crosslinks that stabilize the collagen fibrils. Two type of nonenzymatic process are described in type I collagen: the formation of advanced glycation end products due to the accumulation of reducible sugars in bone tissue, and the process of racemization and isomerization in the telopeptide of the collagen. These modifications of collagen are age-related and may impair the mechanical properties of bone. To illustrate the role of the crosslinking process of collagen in bone strength, clinical disorders associated with bone collagen abnormalities and bone fragility, such as osteogenesis imperfecta and osteoporosis, are described.

Advanced glycation end products and bone loss during aging

It is well known that bone mass density decreases with age. Age-related bone mass loss is ascribed to several factors. Nonenzymatic glycation has been proposed as a new potential factor in the loss of bone during aging. In this study we evaluated the concentration of pentosidine, an advanced glycation end product, in cortical and trabecular bone and in the plasma of subjects undergoing orthopedic surgery. The relationship between these parameters and a clinical index of osteoporosis was also studied. Samples of bone and plasma of 104 nondiabetic subjects (74 women and 30 men), 72 +/- 1 years old, were studied. Pentosidine was determined by HPLC after decalcification and hydrolysis. The radiologic Singh index was evaluated blindly by orthopedic surgeons to provide the degree of osteoporosis. Pentosidine concentration of cortical bone shows a significant exponential increase with age (r = 0.610, P < 0.001). This increase, however, is not seen in the trabecular bone, which is characterized by a large spread in the data. Interestingly the concentration of cortical pentosidine is also related to the Singh score (r(s) = -0.274, P < 0.01). Plasma pentosidine has a significant exponential correlation with age (r = +0.339, P < 0.001) and a linear correlation with the cortical bone pentosidine (r = +0.248, P < 0.05). This study demonstrates that pentosidine increases exponentially in cortical bone during aging, and is thus a good biomarker for the degree of bone mass density loss. The trabecular bone concentration of pentosidine is more variable, probably because of the turnover rate and the local environment; plasma pentosidine might provide information on the bone turnover rate.

Modulating the Effects of Diabetes on Osseointegration With Aminoguanidine and Doxycycline

BACKGROUND

The current knowledge of wound healing around implant surfaces is quite limited, particularly as it relates to the effects of systemic diseases such as diabetes. The purpose of our research is to histologically evaluate the effects of aminoguanidine and doxycycline in the modification of peri-implant wound healing around endosseous implants in diabetic rats.

METHODS

Thirty-two Sprague-Dawley rats were randomly assigned to four different treatment groups. One group served as the non-diabetic control, while diabetes was induced in other groups. Titanium plasma-sprayed (TPS) implants were placed in the femora of each animal 2 weeks following diabetic induction. One group of diabetic rats was given aminoguanidine via intraperitoneal injection, and another given doxycycline via oral gavage for 28 days beginning on the day of implantation. The third group of diabetic rats received no medication (controls). All animals were sacrificed following 28 days of healing.

RESULTS

The results were measured by marrow bone-to implant contact (MBIC) between the groups. Values for MBIC were greater for the non-diabetic control group than the diabetic control group (P < 0.001). Aminoguanidine-treated diabetic animals had a significantly greater MBIC than the diabetic control group (P < 0.01). Diabetic animals receiving doxycycline did not differ significantly from the diabetic control group (P > 0.05).

CONCLUSIONS

The results of this study using a rat model con- firm previous reports that diabetes inhibits osseointegration, as defined by MBIC. In addition, this study demonstrates that the detrimental effects of diabetes on osseointegration can be modified using aminoguanidine systemically. However, systemic administration of doxycycline only slightly enhances osseointegration.

Accumulation of advanced glycation end products as a molecular mechanism for aging as a risk factor in osteoarthritis

OBJECTIVE

Osteoarthritis (OA) is one of the most prevalent and disabling chronic conditions affecting the elderly. Its etiology is largely unknown, but age is the most prominent risk factor. The current study was designed to test whether accumulation of advanced glycation end products (AGEs), which are known to adversely affect cartilage turnover and mechanical properties, provides a molecular mechanism by which aging contributes to the development of OA.

METHODS

The hypothesis that elevated AGE levels predispose to the development of OA was tested in the canine anterior cruciate ligament transection (ACLT) model of experimental OA. Cartilage AGE levels were enhanced in young dogs by intraarticular injections of ribose. This mimics the accumulation of AGEs without the interference of other age-related changes. The severity of OA was then assessed 7 weeks after ACLT surgery in dogs with normal versus enhanced AGE levels.

RESULTS

Intraarticular injections of ribose enhanced cartilage AGE levels approximately 5-fold, which is similar to the normal increase that is observed in old dogs. ACLT surgery resulted in more-pronounced OA in dogs with enhanced AGE levels. This was observed as increased collagen damage and enhanced release of proteoglycans. The attempt to repair the matrix damage was impaired; proteoglycan synthesis and retention were decreased at enhanced AGE levels. Mankin grading of histology sections also revealed more-severe OA in animals with enhanced AGE levels.

CONCLUSION

These findings demonstrate increased severity of OA at higher cartilage AGE levels and provide the first in vivo experimental evidence for a molecular mechanism by which aging may predispose to the development of OA.

Advanced glycation endproducts interfere with integrin-mediated osteoblastic attachment to a type-I collagen matrix

The adhesion of osteoblasts to bone extracellular matrix, of which type-I collagen constitutes >85%, can modulate diverse aspects of their physiology such as growth, differentiation and mineralisation. In this study we examined the adhesion of UMR106 rat osteoblast-like cells either to a control (Col) or advanced-glycation-endproduct-modified (AGEs-Col) type I collagen matrix. We investigated the possible role of different integrin receptors in osteoblastic adhesion, by co-incubating these cells either with beta-peptide (conserved sequence 113-125 of the beta subunit of integrins) or with two other peptides, RGD (Arg-Gly-Asp) and DGEA (Asp-Gly-Glu-Ala), which are recognition sequences for the alpha-subunits of alpha(1,5)beta(1) and alpha(2)beta(1) integrins. Collagen glycation inhibited the adhesion of UMR106 osteoblasts to the matrix (40% reduction versus Col, P > 0.001). beta-Peptide showed a dose- and glycation-dependent inhibitory effect on adhesion, and at a concentration of 100 microM decreased the attachment of UMR106 cells to both matrices (42% to Col, P<0.001and 25% to AGEs-Col, P<0.01). The synthetic peptides RGD (1mM) and DGEA (5mM) inhibited the attachment of UMR106 cells to Col (30 and 20%, P > 0.01 and P< 0.001, respectively), but not to AGEs-Col. beta-Peptide induced an increase in UMR106 cell clumping and a decrease in cellular spreading, while DGEA increased spreading with cellular extensions in multiple directions. These results indicate that both alpha and beta integrin subunits participate in osteoblastic attachment to type-I collagen, probably through the alpha(1,5)beta(1) and alpha(2)beta(1) integrins. AGEs-modification of type-I collagen impairs the integrin-mediated adhesion of osteoblastic cells to the matrix, and could thus contribute to the pathogenesis of diabetic osteopenia.

Advanced glycation end-products (AGEs) induce concerted changes in the osteoblastic expression of their receptor RAGE and in the activation of extracellular signal-regulated kinases (ERK)

An increase in the interaction between advanced glycation end-products (AGEs) and their receptor RAGE is believed to contribute to the pathogenesis of chronic complications of Diabetes mellitus, which can include bone alterations such as osteopenia. We have recently found that extracellular AGEs can directly regulate the growth and development of rat osteosarcoma UMR106 cells, and of mouse calvaria-derived MC3T3E1 osteoblasts throughout their successive developmental stages (proliferation, differentiation and mineralisation), possibly by the recognition of AGEs moieties by specific osteoblastic receptors which are present in both cell lines. In the present study we examined the possible expression of RAGE by UMR106 and MC3T3E1 osteoblastic cells, by immunoblot analysis. We also investigated whether short-, medium- or long-term exposure of osteoblasts to extracellular AGEs, could modify their affinity constant and maximal binding for AGEs (by 125I-AGE-BSA binding experiments), their expression of RAGE (by immunoblot analysis) and the activation status of the osteoblastic ERK 1/2 signal transduction mechanism (by immunoblot analysis for ERK and P-ERK). Our results show that both osteoblastic cell lines express readily detectable levels of RAGE. Short-term exposure of phenotypically mature osteoblastic UMR106 cells to AGEs decrease the cellular density of AGE-binding sites while increasing the affinity of these sites for AGEs. This culture condition also dose-dependently increased the expression of RAGE and the activation of ERK. In proliferating MC3T3E1 pre-osteoblasts, 24-72 h exposure to AGEs did not modify expression of RAGE, ERK activation or the cellular density of AGE-binding sites. However, it did change the affinity of these binding sites forAGEs, with both higher- and lower-affinity sites now being apparent. Medium-term ( 1 week) incubation of differentiated MC3T3E1 osteoblasts with AGEs, induced a simultaneous increase in RAGE expression and in the relative amount of P-ERK. Mineralising MC3T3E1 cultures grown for 3 weeks in the presence of extracellular AGEs showed a decrease both in RAGE and P-ERK expression. These results indicate that, in phenotypically mature osteoblastic cells, changes in ERK activation closely follow the AGEs-induced regulation of RAGE expression. Thus, the AGEs-induced biological effects that we have observed previously in osteoblasts, could be mediated by RAGE in the later stages of development, and mediated by other AGE receptors in the earlier pre-osteoblastic stage.

Glucose-mediated in vitro glycation modulates biomechanical integrity of the soft tissues but not hard tissues

Glycation induced crosslinking of connective tissue collagen is thought to be involved in the pathogenesis of various disorders associated with diabetes and aging. Although the formation of the glycation related collagen crosslinks appears to be universal for all tissues, currently it is unknown whether differences exist between soft and hard tissue biomechanics in response to glucose-mediated in vitro glycation. In this study, the impact of non-enzymatic glycation was investigated on tendons and bones, using them as models for soft and hard tissue respectively. Achilles tendons from rabbits, and femur and tibia from rats, were subjected to in vitro glycation with glucose. Sixty days following glycation, the matrix integrity of the tendons and bones was evaluated and compared with the respective non-glycated tissue (n=10 in each group). The results revealed that the impact of glycation was significant on the tendon but not on the bone. Measurements of the biomechanical stability of glycated tendons indicated a significant increase in maximum load (21%), Young's modulus of elasticity (72%), energy to yield (35%) and toughness (68%) compared to the non-glycated tendons. No significant differences were found in breaking strength, bending stiffness, energy to yield and toughness between glycated and non-glycated femurs or tibias. The deformation of both soft and hard tissue was unaffected by the glycation. Measurements at ultimate tissue failure (break point) revealed that glycated tendons bore significantly higher load and energy absorption than non-glycated tendons. In contrast, the deformation of the glycated tendons at break point was considerably reduced as compared to control tendons. However, glycation had no significant effects on the hard tissue biomechanical properties at break point. The results of this study demonstrate that in vitro glycation influences the biomechanical properties of soft tissue but not hard tissue.

Realistic three-dimensional epithelial tissue phantoms for biomedical optics

We introduce new realistic three-dimensional tissue phantoms which can help to understand the optical properties of human epithelium as well as the optical signatures associated with the dysplasia to carcinoma sequence. The phantoms are based on a step by step multilayer reconstitution of the epithelial tissue using main components characteristic for the human epithelium. Each consecutive step is aimed to increase the similarity between real tissue and a phantom. We began by modeling the stromal layer which predominantly consists of a network of collagen bundles. Phantoms consisting of a collagen matrix alone and in the presence of embedded cervical cells were created. Their morphology and fluorescence properties were studied and were compared with those of cervical epithelium. We show that the phantoms resemble the microstructure and the optical properties of the human epithelial tissue. We also demonstrate that the proposed phantoms provide an opportunity to study changes in optical properties of different tissue components as a result of their interactions with each other or exogenous factors.

Bone morphogenetic proteins, extracellular matrix, and mitogen-activated protein kinase signaling pathways are required for osteoblast-specific gene expression and differentiation in MC3T3-E1 cells

Osteoblasts secrete a complex extracellular matrix (ECM) containing collagenous and noncollagenous proteins, bone morphogenetic proteins (BMPs), and growth factors. Osteoblast-specific gene expression requires ascorbic acid (AA)-dependent assembly of a collagenous ECM. Matrix responsiveness requires an alpha2beta1 integrin-collagen interaction and mitogen-activated protein kinase (MAPK) activity, which phosphorylates and activates the osteoblast-specific transcription factor Cbfa1. This study examines interactions between this integrin/MAPK-mediated pathway and signals initiated by BMPs contained in the osteoblast matrix. MC3T3-E1 cells were shown to constitutively express BMP-2, BMP-4, and BMP-7. Noggin, a specific BMP inhibitor, reversibly blocked AA-induced gene expression, indicating that BMP production by MC3T3-E1 cells was necessary for differentiation. The ability of exogenously added BMP-2, BMP-4, or BMP-7 to stimulate osteocalcin (OCN) and bone sialoprotein (BSP) mRNAs or OCN promoter activity was synergistically increased in cells that were actively synthesizing an ECM (i.e., were grown in the presence of AA). A minimum of 4 days of ECM accumulation was required for this synergistic response to be observed. Neither BMP-7, AA, nor a combination of these two treatments had major effects on Cbfa1 messenger RNA (mRNA) or protein levels, as would be expected if regulation was mainly at the posttranscriptional level. U0126, a specific inhibitor of MAPK/extracellular signal-regulated kinase (MEK), blocked AA- or BMP-7/AA-dependent gene expression in a time- and dose-dependent manner that was closely correlated with inhibition of extracellular signal-regulated kinase (ERK) phosphorylation. This work establishes that autocrine BMP production as well as integrin-mediated cell-collagen interactions are both required for osteoblast differentiation, and both these pathways require MAP kinase activity.

Non-enzymatic glycosylation of a type I collagen matrix: effects on osteoblastic development and oxidative stress

BACKGROUND

The tissue accumulation of protein-bound advanced glycation endproducts (AGE) may be involved in the etiology of diabetic chronic complications, including osteopenia. The aim of this study was to investigate the effect of an AGE-modified type I collagen substratum on the adhesion, spreading, proliferation and differentiation of rat osteosarcoma UMR106 and mouse non-transformed MC3T3E1 osteoblastic cells. We also studied the role of reactive oxygen species (ROS) and nitric oxide synthase (NOS) expression on these AGE-collagen mediated effects.

RESULTS

AGE-collagen decreased the adhesion of UMR106 cells, but had no effect on the attachment of MC3T3E1 cells. In the UMR106 cell line, AGE-collagen also inhibited cellular proliferation, spreading and alkaline phosphatase (ALP) activity. In preosteoblastic MC3T3E1 cells (24-hour culture), proliferation and spreading were significantly increased by AGE-collagen. After one week of culture (differentiated MC3T3E1 osteoblasts) AGE-collagen inhibited ALP activity, but had no effect on cell number. In mineralizing MC3T3E1 cells (3-week culture) AGE-collagen induced a decrease in the number of surviving cells and of extracellular nodules of mineralization, without modifying their ALP activity. Intracellular ROS production, measured after a 48-hour culture, was decreased by AGE-collagen in MC3T3E1 cells, but was increased by AGE-collagen in UMR106 cells. After a 24-hour culture, AGE-collagen increased the expression of endothelial and inducible NOS, in both osteoblastic cell lines.

CONCLUSIONS

These results suggest that the accumulation of AGE on bone extracellular matrix could regulate the proliferation and differentiation of osteoblastic cells. These effects appear to depend on the stage of osteoblastic development, and possibly involve the modulation of NOS expression and intracellular ROS pathways.

Degradation of the cartilage collagen matrix associated with changes in chondrocytes in osteoarthrosis. Assessment by loss of background fluorescence and immunodetection of matrix components

Articular cartilage damage and eventual loss is the primary pathological change seen in osteoarthrosis (OA). In this study we have investigated the link between turnover of the collagen matrix and changes in chondrocytes. The background fluorescence of articular cartilage, as indicated by its emission spectrum and resistance to extraction was generated by the slow non-enzymic modification of the collagen matrix by advanced glycation end products (AGEs). Assessment of changes in background fluorescence in sections of articular cartilage provided a narrative of collagen degradation. Patients without OA pathology typically had a uniform strong background fluorescence throughout the depth of the cartilage. Cartilage from OA patients showed a range of changes in background fluorescence dependent on depth from the articular surface and proximity to overt lesions. Loss of background fluorescence was centered on chondrocytes, more extensive near the surface and associated with detection of the proteoglycan epitope 7D4. Expression of type X collagen was seen in articular cartilage in the region of the interface of with subchondral bone in most OA patients but was not associated with prominent, pericellular, loss of background fluorescence. These observations are consistent with progressive cartilage damage in OA, whereby collagen turnover and loss of surface integrity is associated with chondrocyte activity similar to that seen in immature articular cartilage.

Effect of age and menopause on serum concentrations of pentosidine, an advanced glycation end product

BACKGROUND

Pentosidine is an advanced glycation end product. Our aim is to investigate (a) the age-related change of serum pentosidine and (b) the effect of menopause on serum pentosidine.

METHODS

Using the high-performance liquid-chromatography method with column switching, we measured serum pentosidine in 140 healthy women aged 20-93 years. Serum creatinine was also measured. The samples of 13 young and 13 old subjects were used for the measurements of free pentosidine and fractions of pentosidine. Free pentosidine was measured without hydrolysis, and the fractions were measured with a 10,000 mol wt cutoff filter. To investigate the effect of menopause on pentosidine, two biochemical markers for bone turnover (CTx and osteocalcin) were measured in age-matched premenopausal and postmenopausal women (16 in each group).

RESULTS

Serum pentosidine significantly increased with age (r = .702, p < .0001 ). The values of serum pentosidine for the groups beyond the age of 50 were significantly higher than those for the younger groups. The value for the group aged 80-93 years was three times higher than that for the group aged 20-29 years. Serum pentosidine moderately and significantly correlated to serum creatinine (r = .483, p = .0001). Free pentosidine was detected in only 3 of 13 young subjects and 2 of 13 old subjects. The ratio of free to total pentosidine was 2.9% and 1.2% in young and old subjects, respectively. Pentosidine <10,000 mol wt was not detected in all subjects. Pentosidine >10,000 mol wt was detected in all subjects. Serum CTx and osteocalcin significantly increased in postmenopausal women compared with those of pre-menopausal women. There was no significant change in serum pentosidine between the premenopause group and the postmenopause group.

CONCLUSION

Serum pentosidine significantly increased with age in healthy subjects aged 20-93 years and correlated to serum creatinine. The changes of fractions of pentosidine with aging were not observed. There was no effect of menopause on pentosidine.

Effects of advanced glycation end-products on the proliferation and differentiation of osteoblast-like cells

Two different lines of osteoblast-like cells were used to investigate the effect of advanced glycation end-products of bovine serum albumin on cell proliferation and differentiation. These parameters were found to be both dose- and time-dependent. Cell proliferation remained unchanged after a 24 h incubation period, it increased after intermediate periods of incubation with advanced glycation end-products, but was found to be depressed after several days incubation. Cellular alkaline phosphatase activity followed a similar pattern: an initial increase induced by advanced glycation end-products was generally followed, after relatively long incubation periods, by a slight but significant decrease in this parameter. 45Ca2+ uptake was only significantly inhibited by advanced glycation end-products after 24 h incubation. These results suggest that advanced glycation end-products directly regulate osteoblast proliferation and differentiation in a dose and time dependent manner.

Advanced glycation endproducts stimulate interleukin-6 production by human bone-derived cells

Advanced glycation endproducts (AGEs), which result from nonenzymatic reactions of glucose with tissue proteins, have been shown to accumulate on long-lived proteins in advanced aging and diabetes mellitus. Thus, AGEs have been implicated in some of the chronic complications associated with these disorders. In this study, we investigated the effects of the glucose-modified protein on the production of the potent bone resorption factors by cells derived from explants of human bone. AGEs stimulated the release of interleukin-6 (IL-6) in the culture supernatants from the bone-derived cells and increased the levels of IL-6 mRNA in the cells. By contrast, the levels of IL-11 in the culture supernatants were not altered by AGEs, and the other bone resorption factors IL-1 alpha and IL-1 beta were undetectable (< 1.0 pg/ml) either without or with the treatment of AGEs. Electrophoretic mobility-shift assays revealed that the transcription nuclear factor-kappa B, which is critical for the inducible expression of IL-6, was activated in the nuclear extracts from mouse osteoblastic MC3T3-E1 cells treated with AGEs. These results suggest that AGEs are involved in bone remodeling modulation by stimulating IL-6 production in human bone-derived cells.

Glycation of collagen in hypertriglyceridemic rats

Nonenzymatic collagen glycation and modification with lipid derived metabolites was studied in rat skin and tail tendon collagen of control and hypertriglyceridemic (HTG) rats. Age-dependent changes typical for lipid and sugar derived adducts were evaluated by measuring fluorescence of these collagens at wavelengths typical for sugar (335/385 and 370/440 nm) and lipid derived adducts (356/460 and 390/460 nm). In addition pentosidine assay (corresponding to the fluorescence parameters 335/385 nm) was performed as well. It was found that pentosidine concentration as well as fluorescence intensities in skin collagen was the same for control and HTG rats and significantly increased with age. On the other hand, no significant age-dependent changes in fluorescence intensities were observed in tail tendon collagen. Pentosidine concentration in tail tendon collagen was much lower than that in skin and it was decreased in young HTG rats compared to control ones. It increased with age, more distinctly in HTG rats than in their control counterparts, in such a way that at the age of 19 months the pentosidine levels were undistinguishible in both rat strains. Possible mechanisms underlying these results are discussed.

Role of nonenzymatic glycosylation of type I collagen in diabetic osteopenia

Formation of advanced glycation end products (AGEs) in extracellular matrix (ECM) is implicated in the development of chronic diabetic complications. However, the involvement of AGEs in diabetic bone disease has not been well established. We have examined whether AGEs are increased in the bone collagen of streptozotocin-induced diabetic rats in vivo and whether glycation of type I collagen affects the functions of osteoblastic cells in vitro. During 12 weeks of observation, AGEs in collagen extracted from the tibiae of diabetic rats increased in a time-dependent manner and were significantly higher than controls at every time point. In vitro, the incubation of collagen with glucose-6-phosphate resulted in a time-dependent increase of AGEs. When osteoblastic cells isolated from fetal rat calvaria were cultured on AGE-modified type I collagen, it dose-dependently inhibited phenotypic expressions of osteoblasts. Among osteoblastic parameters, nodule formation was the most sensitive, being inhibited by approximately 70% by the glycation of collagen for only 1 week. Alkaline phosphatase activity and osteocalcin secretion were inhibited by 20-30% and 15-70%, respectively, by the glycation of collagen for 1-5 weeks. These results indicate that AGE-modified collagen affects osteoblastic cell differentiation and function in vitro and suggest that similar changes occurring in vivo may contribute to diabetic osteopenia.