Amphoterin stimulates myogenesis and counteracts the antimyogenic factors basic fibroblast growth factor and S100B via RAGE binding

S100b treatment overcomes RAGE signaling deficits in myoblasts on advanced glycation end-product cross-linked collagen and promotes myogenic differentiation

Advanced glycation end-products (AGEs) stochastically accrue in skeletal muscle and on collagen over an individual's lifespan, stiffening the muscle and modifying the stem cell (MuSC) microenvironment while promoting proinflammatory, antiregenerative signaling via the receptor for advanced glycation end-products (RAGEs). In the present study, a novel in vitro model was developed of this phenomenon by cross linking a 3-D collagen scaffold with AGEs and investigating how myoblasts responded to such an environment. Briefly, collagen scaffolds were incubated with d-ribose (0, 25, 40, 100, or 250 mM) for 5 days at 37°C. C2C12 immortalized mouse myoblasts were grown on the scaffolds for 6 days in growth conditions for proliferation, and 12 days for differentiation and fusion. Human primary myoblasts were also used to confirm the C2C12 data. AGEs aberrantly extended the DNA production stage of C2C12s (but not in human primary myoblasts) which is known to delay differentiation in myogenesis, and this effect was prevented by RAGE inhibition. Furthermore, the differentiation and fusion of myoblasts were disrupted by AGEs, which were associated with reductions in integrins and suppression of RAGE. The addition of S100b (RAGE agonist) recovered the differentiation and fusion of myoblasts, and the addition of RAGE inhibitors (FPS-ZM1 and Azeliragon) inhibited the differentiation and fusion of myoblasts. Our results provide novel insights into the role of the AGE-RAGE axis in skeletal muscle aging, and future work is warranted on the potential application of S100b as a proregenerative factor in aged skeletal muscle.NEW & NOTEWORTHY Collagen cross-linked by advanced glycation end-products (AGEs) induced myoblast proliferation but prevented differentiation, myotube formation, and RAGE upregulation. RAGE inhibition occluded AGE-induced myoblast proliferation, while the delivery of S100b, a RAGE ligand, recovered fusion deficits.

Pathophysiology of RAGE in inflammatory diseases

The receptor for advanced glycation end products (RAGE) is a non-specific multi-ligand pattern recognition receptor capable of binding to a range of structurally diverse ligands, expressed on a variety of cell types, and performing different functions. The ligand-RAGE axis can trigger a range of signaling events that are associated with diabetes and its complications, neurological disorders, cancer, inflammation and other diseases. Since RAGE is involved in the pathophysiological processes of many diseases, targeting RAGE may be an effective strategy to block RAGE signaling.

Advanced Glycation End Products (AGEs) and Chronic Kidney Disease: Does the Modern Diet AGE the Kidney?

Since the 1980s, chronic kidney disease (CKD) affecting all ages has increased by almost 25%. This increase may be partially attributable to lifestyle changes and increased global consumption of a “western” diet, which is typically energy dense, low in fruits and vegetables, and high in animal protein and ultra-processed foods. These modern food trends have led to an increase in the consumption of advanced glycation end products (AGEs) in conjunction with increased metabolic dysfunction, obesity and diabetes, which facilitates production of endogenous AGEs within the body. When in excess, AGEs can be pathological via both receptor-mediated and non-receptor-mediated pathways. The kidney, as a major site for AGE clearance, is particularly vulnerable to AGE-mediated damage and increases in circulating AGEs align with risk of CKD and all-cause mortality. Furthermore, individuals with significant loss of renal function show increased AGE burden, particularly with uraemia, and there is some evidence that AGE lowering via diet or pharmacological inhibition may be beneficial for CKD. This review discusses the pathways that drive AGE formation and regulation within the body. This includes AGE receptor interactions and pathways of AGE-mediated pathology with a focus on the contribution of diet on endogenous AGE production and dietary AGE consumption to these processes. We then analyse the contribution of AGEs to kidney disease, the evidence for dietary AGEs and endogenously produced AGEs in driving pathogenesis in diabetic and non-diabetic kidney disease and the potential for AGE targeted therapies in kidney disease.

Inflammatory Caspase Activity Mediates HMGB1 Release and Differentiation in Myoblasts Affected by Peripheral Arterial Disease

Introduction: We previously showed that caspase-1 and -11, which are activated by inflammasomes, mediate recovery from muscle ischemia in mice. We hypothesized that similar to murine models, inflammatory caspases modulate myogenicity and inflammation in ischemic muscle disease. Methods: Caspase activity was measured in ischemic and perfused human myoblasts in response to the NLRP3 and AIM2 inflammasome agonists (nigericin and poly(dA:dT), respectively) with and without specific caspase-1 or pan-caspase inhibition. mRNA levels of myogenic markers and caspase-1 were assessed, and protein levels of caspases-1, -4, -5, and -3 were measured by Western blot. Results: When compared to perfused cells, ischemic myoblasts demonstrated attenuated MyoD and myogenin and elevated caspase-1 mRNA. Ischemic myoblasts also had significantly higher enzymatic caspase activity with poly(dA:dT) (p < 0.001), but not nigericin stimulation. Inhibition of caspase activity including caspase-4/-5, but not caspase-1, blocked activation effects of poly(dA:dT). Ischemic myoblasts had elevated cleaved caspase-5. Inhibition of caspase activity deterred differentiation in ischemic but not perfused myoblasts and reduced the release of HMGB1 from both groups. Conclusion: Inflammatory caspases can be activated in ischemic myoblasts by AIM2 and influence ischemic myoblast differentiation and release of pro-angiogenic HMGB1. AIM2 inflammasome involvement suggests a role as a DNA damage sensor, and our data suggest that caspase-5 rather than caspase-1 may mediate the downstream mediator of this pathway.

Satellite Cell Expression of RAGE (Receptor for Advanced Glycation end Products) Is Important for Collateral Vessel Formation

Background The growth and remodeling of vascular networks is an important component of the prognosis for patients with peripheral artery disease. One protein that has been previously implicated to play a role in this process is RAGE (receptor for advanced glycation end products). This study sought to determine the cellular source of RAGE in the ischemic hind limb and the role of RAGE signaling in this cell type. Methods and Results Using a hind limb ischemia model of vascular growth, this study found skeletal muscle satellite cells to be a novel major cellular source of RAGE in ischemic tissue by both staining and cellular sorting. Although wild-type satellite cells increased tumor necrosis factor-α and monocyte chemoattractant protein-1 production in response to ischemia in vivo and a RAGE ligand in vitro, satellite cells from RAGE knockout mice lacked the increase in cytokine production both in vivo in response to ischemia and in vitro after stimuli with the RAGE ligand high-mobility group box 1. Furthermore, encapsulated wild-type satellite cells improved perfusion after hind limb ischemia surgery by both perfusion staining and vessel quantification, but RAGE knockout satellite cells provided no improvement over empty capsules. Conclusions Thus, RAGE expression and signaling in satellite cells is crucial for their response to stimuli and angiogenic and arteriogenic functions.

Advanced Glycation End-Products in Skeletal Muscle Aging

Advanced age causes skeletal muscle to undergo deleterious changes including muscle atrophy, fast-to-slow muscle fiber transition, and an increase in collagenous material that culminates in the age-dependent muscle wasting disease known as sarcopenia. Advanced glycation end-products (AGEs) non-enzymatically accumulate on the muscular collagens in old age via the Maillard reaction, potentiating the accumulation of intramuscular collagen and stiffening the microenvironment through collagen cross-linking. This review contextualizes known aspects of skeletal muscle extracellular matrix (ECM) aging, especially the role of collagens and AGE cross-linking, and underpins the motor nerve’s role in this aging process. Specific directions for future research are also discussed, with the understudied role of AGEs in skeletal muscle aging highlighted. Despite more than a half century of research, the role that intramuscular collagen aggregation and cross-linking plays in sarcopenia is well accepted yet not well integrated with current knowledge of AGE’s effects on muscle physiology. Furthermore, the possible impact that motor nerve aging has on intramuscular cross-linking and muscular AGE levels is posited.

Elevated level of the soluble receptor for advanced glycation end-products involved in sarcopenia: an observational study

BACKGROUND

The soluble receptor for advanced glycation end products (sRAGE) has been proposed to serve as a marker for disease severity, but its role in sarcopenia, an age-related progressive loss of muscle mass and function, remains elusive. This study examines the association between sRAGE and sarcopenia.

METHODS

A total of 314 community-dwelling elderly adults who had their health examination at Tri-Service General Hospital from 2017 to 2019 underwent protein analysis with enzyme-linked immunosorbent assay. The relationship with sarcopenia and its detailed information, including components and diagnosis status, were examined using linear and logistic regressions.

RESULTS

As for sarcopenia components, low muscle mass (β = 162.8, p = 0.012) and strength (β = 181.31, p = 0.011) were significantly correlated with sRAGE, but not low gait speed (p = 0.066). With regard to disease status, confirmed sarcopenia (β = 436.93, p < 0.001), but not probable (p = 0.448) or severe sarcopenia (p = 0.488), was significantly correlated with sRAGE. In addition, females revealed a stronger association with sRAGE level by showing significant correlations with low muscle mass (β = 221.72, p = 0.014) and low muscle strength (β = 208.68, p = 0.043).

CONCLUSIONS

sRAGE level showed a positive association with sarcopenia, illustrating its involvement in the evolution of sarcopenia. This association is more evident in female groups, which may be attributed to the loss of protection from estrogen in postmenopausal women. Utilizing sRAGE level as a prospective marker for sarcopenia deserves further investigation in future studies.

Advanced Glycation End Products Are Retained in Decellularized Muscle Matrix Derived from Aged Skeletal Muscle

Decellularized tissues are biocompatible materials that engraft well, but the age of their source has not been explored for clinical translation. Advanced glycation end products (AGEs) are chemical cross-links that accrue on skeletal muscle collagen in old age, stiffening the matrix and increasing inflammation. Whether decellularized biomaterials derived from aged muscle would suffer from increased AGE collagen cross-links is unknown. We characterized gastrocnemii of 1-, 2-, and 20-month-old C57BL/6J mice before and after decellularization to determine age-dependent changes to collagen stiffness and AGE cross-linking. Total and soluble collagen was measured to assess if age-dependent increases in collagen and cross-linking persisted in decellularized muscle matrix (DMM). Stiffness of aged DMM was determined using atomic force microscopy. AGE levels and the effect of an AGE cross-link breaker, ALT-711, were tested in DMM samples. Our results show that age-dependent increases in collagen amount, cross-linking, and general stiffness were observed in DMM. Notably, we measured increased AGE-specific cross-links within old muscle, and observed that old DMM retained AGE cross-links using ALT-711 to reduce AGE levels. In conclusion, deleterious age-dependent modifications to collagen are present in DMM from old muscle, implying that age matters when sourcing skeletal muscle extracellular matrix as a biomaterial.

Hypoxic Signaling in Skeletal Muscle Maintenance and Regeneration: A Systematic Review

In skeletal muscle tissue, oxygen (O₂) plays a pivotal role in both metabolism and the regulation of several intercellular pathways, which can modify proliferation, differentiation and survival of cells within the myogenic lineage. The concentration of oxygen in muscle tissue is reduced during embryogenesis and pathological conditions. Myogenic progenitor cells, namely satellite cells, are necessary for muscular regeneration in adults and are localized in a hypoxic microenvironment under the basal lamina, suggesting that the O₂ level could affect their function. This review presents the effects of reduced oxygen levels (hypoxia) on satellite cell survival, myoblast regeneration and differentiation in vertebrates. Further investigations and understanding of the pathways involved in adult muscle regeneration during hypoxic conditions are maybe clinically relevant to seek for novel drug treatments for patients with severe muscle damage. We especially outlined the effect of hypoxia-inducible factor 1-alpha (HIF1A), the most studied transcriptional regulator of cellular and developmental response to hypoxia, whose investigation has recently been awarded with the Nobel price.

The S100 Protein Family as Players and Therapeutic Targets in Pulmonary Diseases

The S100 protein family consists of over 20 members in humans that are involved in many intracellular and extracellular processes, including proliferation, differentiation, apoptosis, Ca₂⁺ homeostasis, energy metabolism, inflammation, tissue repair, and migration/invasion. Although there are structural similarities between each member, they are not functionally interchangeable. The S100 proteins function both as intracellular Ca²⁺ sensors and as extracellular factors. Dysregulated responses of multiple members of the S100 family are observed in several diseases, including the lungs (asthma, chronic obstructive pulmonary disease, idiopathic pulmonary fibrosis, cystic fibrosis, pulmonary hypertension, and lung cancer). To this degree, extensive research was undertaken to identify their roles in pulmonary disease pathogenesis and the identification of inhibitors for several S100 family members that have progressed to clinical trials in patients for nonpulmonary conditions. This review outlines the potential role of each S100 protein in pulmonary diseases, details the possible mechanisms observed in diseases, and outlines potential therapeutic strategies for treatment.

Identification of Withania somnifera-Silybum marianum-Trigonella foenum-graecum Formulation as a Nutritional Supplement to Contrast Muscle Atrophy and Sarcopenia

Background: Muscle atrophy, i.e., the loss of skeletal muscle mass and function, is an unresolved problem associated with aging (sarcopenia) and several pathological conditions. The imbalance between myofibrillary protein breakdown (especially the adult isoforms of myosin heavy chain, MyHC) and synthesis, and the reduction of muscle regenerative potential are main causes of muscle atrophy. Methods: Starting from one-hundred dried hydroalcoholic extracts of medical plants, we identified those able to contrast the reduction of C2C12 myotube diameter in well-characterized in vitro models mimicking muscle atrophy associated to inflammatory states, glucocorticoid treatment or nutrient deprivation. Based on their ability to rescue type II MyHC (MyHC-II) expression in atrophying conditions, six extracts with different phytochemical profiles were selected, mixed in groups of three, and tested on atrophic myotubes. The molecular mechanism underpinning the effects of the most efficacious formulation, and its efficacy on myotubes obtained from muscle biopsies of young and sarcopenic subjects were also investigated. Results: We identified WST (Withania somnifera, Silybum marianum, Trigonella foenum-graecum) formulation as extremely efficacious in protecting C2C12 myotubes against MyHC-II degradation by stimulating Akt (protein kinase B)-dependent protein synthesis and p38 MAPK (p38 mitogen-activated protein kinase)/myogenin-dependent myoblast differentiation. WST sustains trophism in C2C12 and young myotubes, and rescues the size, developmental MyHC expression and myoblast fusion in sarcopenic myotubes. Conclusion: WST strongly counteracts muscle atrophy associated to different conditions in vitro. The future validation in vivo of our results might lead to the use of WST as a food supplement to sustain muscle mass in diffuse atrophying conditions, and to reverse the age-related functional decline of human muscles, thus improving people quality of life and reducing social and health-care costs.

Targeting RAGE prevents muscle wasting and prolongs survival in cancer cachexia

BACKGROUND

Cachexia, a multifactorial syndrome affecting more than 50% of patients with advanced cancer and responsible for ~20% of cancer-associated deaths, is still a poorly understood process without a standard cure available. Skeletal muscle atrophy caused by systemic inflammation is a major clinical feature of cachexia, leading to weight loss, dampening patients' quality of life, and reducing patients' response to anticancer therapy. RAGE (receptor for advanced glycation end-products) is a multiligand receptor of the immunoglobulin superfamily and a mediator of muscle regeneration, inflammation, and cancer.

METHODS

By using murine models consisting in the injection of colon 26 murine adenocarcinoma (C26-ADK) or Lewis lung carcinoma (LLC) cells in BALB/c and C57BL/6 or Ager^-/- (RAGE-null) mice, respectively, we investigated the involvement of RAGE signalling in the main features of cancer cachexia, including the inflammatory state. In vitro experiments were performed using myotubes derived from C2C12 myoblasts or primary myoblasts isolated from C57BL/6 wild type and Ager^-/- mice treated with the RAGE ligand, S100B (S100 calcium-binding protein B), TNF (tumor necrosis factor)α±IFN (interferon) γ, and tumour cell- or masses-conditioned media to analyse hallmarks of muscle atrophy. Finally, muscles of wild type and Ager^-/- mice were injected with TNFα/IFNγ or S100B in a tumour-free environment.

RESULTS

We demonstrate that RAGE is determinant to activate signalling pathways leading to muscle protein degradation in the presence of proinflammatory cytokines and/or tumour-derived cachexia-inducing factors. We identify the RAGE ligand, S100B, as a novel factor able to induce muscle atrophy per se via a p38 MAPK (p38 mitogen-activated protein kinase)/myogenin axis and STAT3 (signal transducer and activator of transcription 3)-dependent MyoD (myoblast determination protein 1) degradation. Lastly, we found that in cancer conditions, an increase in serum levels of tumour-derived S100B and HMGB1 (high mobility group box 1) occurs leading to chronic activation/overexpression of RAGE, which induces hallmarks of cancer cachexia (i.e. muscle wasting, systemic inflammation, and release of tumour-derived pro-cachectic factors). Absence of RAGE in mice translates into reduced serum levels of cachexia-inducing factors, delayed loss of muscle mass and strength, reduced tumour progression, and increased survival.

CONCLUSIONS

RAGE is a molecular determinant in inducing the hallmarks of cancer cachexia, and molecular targeting of RAGE might represent a therapeutic strategy to prevent or counteract the cachectic syndrome.

Insulin-Like Growth Factor-I Protects Against the Detrimental Effects of Advanced Glycation End Products and High Glucose in Myoblastic C2C12 Cells

Previous studies suggested that advanced glycation end products (AGEs) and insulin-like growth factor-I (IGF-I) are involved in the mechanism of diabetes-induced sarcopenia. In this study, we examined effects of treatments with AGEs and/or IGF-I for 24 h on myogenic differentiation and apoptosis in mouse myoblastic C2C12 cells. Real-time PCR and Western blot were performed to investigate mRNA and protein expressions, and apoptosis was examined by using a DNA fragment detection ELISA kit. AGE3 significantly decreased mRNA and protein expressions of MyoD and Myogenin, whereas IGF-I significantly increased them and attenuated the effects of AGE3. AGEs significantly decreased endogenous IGF-I mRNA expression and suppressed IGF-I-induced Akt activation. High glucose (22 mM) significantly increased mRNA expression of Rage, a receptor for AGEs, while IGF-I significantly decreased it. DNA fragment ELISA showed that AGE2 and AGE3 significantly increased apoptosis of C2C12 cells, whereas IGF-I significantly suppressed the AGE2- and AGE3-induced apoptosis. In contrast, high glucose enhanced AGE3-induced apoptosis. IGF-I significantly attenuated the effects of high glucose plus AGE3 on the mRNA and protein expressions of MyoD and Myogenin as well as the apoptosis. These findings indicate that AGEs inhibit myogenic differentiation and increase apoptosis in C2C12 cells, and that high glucose increases RAGE and enhances the AGE3-induced apoptosis, suggesting that AGEs and high glucose might contribute to the reduction of muscle mass and function. Moreover, IGF-I attenuated the detrimental effects of AGEs and high glucose in myoblastic cells; thus, IGF-I-Akt signal could be a therapeutic target of DM-induced sarcopenia.

Cellular and molecular mechanisms of sarcopenia: the S100B perspective

Primary sarcopenia is a condition of reduced skeletal muscle mass and strength, reduced agility, and increased fatigability and risk of bone fractures characteristic of aged, otherwise healthy people. The pathogenesis of primary sarcopenia is not completely understood. Herein, we review the essentials of the cellular and molecular mechanisms of skeletal mass maintenance; the alterations of myofiber metabolism and deranged properties of muscle satellite cells (the adult stem cells of skeletal muscles) that underpin the pathophysiology of primary sarcopenia; the role of the Ca²⁺ -sensor protein, S100B, as an intracellular factor and an extracellular signal regulating cell functions; and the functional role of S100B in muscle tissue. Lastly, building on recent results pointing to S100B as to a molecular determinant of myoblast-brown adipocyte transition, we propose S100B as a transducer of the deleterious effects of accumulation of reactive oxygen species in myoblasts and, potentially, myofibers concurring to the pathophysiology of sarcopenia.

RAGE in the pathophysiology of skeletal muscle

Emerging evidence suggests that the signalling of the Receptor for Advanced Glycation End products (RAGE) is critical for skeletal muscle physiology controlling both the activity of muscle precursors during skeletal muscle development and the correct time of muscle regeneration after acute injury. On the other hand, the aberrant re-expression/activity of RAGE in adult skeletal muscle is a hallmark of muscle wasting that occurs in response to ageing, genetic disorders, inflammatory conditions, cancer, and metabolic alterations. In this review, we discuss the mechanisms of action and the ligands of RAGE involved in myoblast differentiation, muscle regeneration, and muscle pathological conditions. We highlight potential therapeutic strategies for targeting RAGE to improve skeletal muscle function.

S100 Proteins As an Important Regulator of Macrophage Inflammation

The S100 proteins, a family of calcium-binding cytosolic proteins, have a broad range of intracellular and extracellular functions through regulating calcium balance, cell apoptosis, migration, proliferation, differentiation, energy metabolism, and inflammation. The intracellular functions of S100 proteins involve interaction with intracellular receptors, membrane protein recruitment/transportation, transcriptional regulation and integrating with enzymes or nucleic acids, and DNA repair. The S100 proteins could also be released from the cytoplasm, induced by tissue/cell damage and cellular stress. The extracellular S100 proteins, serving as a danger signal, are crucial in regulating immune homeostasis, post-traumatic injury, and inflammation. Extracellular S100 proteins are also considered biomarkers for some specific diseases. In this review, we will discuss the multi-functional roles of S100 proteins, especially their potential roles associated with cell migration, differentiation, tissue repair, and inflammation.

Effect of calprotectin subunit S100A9 on the expression and methylation of OCLN in human melanoma cell line A-375

Increased levels of calprotectin subunits S100A8 and S100A9 have been detected in human cancers. Melanoma is the most aggressive type of skin cancer, and its treatment is challenging because of its brain metastasis. OCLN encodes occluding, which plays a major role in the formation and regulation of tight junctions. The aim of this study was to evaluate the methylation status of the OCLN promoter and its expression in A-375 melanoma cells treated with or without various concentrations of S100A9 for 24, 48, and 72 h. Total RNA was extracted, and synthesized cDNA was assessed by performing real-time PCR. MSP-PCR was performed after treatment with bisultfie. Recombinant S100A9 inhibited the proliferation of the A-375 cell line and the expression of the OCLN gene was downregulated in a time- and concentration-dependent manner. Results of MSP-PCR showed that the OCLN gene promoter in a human melanoma cell line (A-375) was semimethylated.

Perpetual change: autophagy, the endothelium, and response to vascular injury

Current studies of vascular health, aging, and autophagy emphasize how the endothelium adapts to stress and contributes to disease. The endothelium is far from an inert barrier to blood-borne cells, pathogens, and chemical signals; rather, it actively translates circulating mediators into tissue responses, changing rapidly in response to physiologic stressors. Macroautophagy-the cellular ingestion of effete organelles and protein aggregates to provide anabolic substrates to fuel bioenergetics in times of stress-plays an important role in endothelial cell homeostasis, vascular remodeling, and disease. These roles include regulating vascular tone, sustaining or limiting cell survival, and contributing to the development of atherosclerosis secondary to infection, inflammation, and angiogenesis. Autophagy modulates these critical functions of the endothelium in a dynamic and perpetual response to tissue and intravascular cues.

Maillard reaction in food allergy: Pros and cons

Food allergens have a notable potential to induce various health concerns in susceptible individuals. The majority of allergenic foods are usually subjected to thermal processing prior to their consumption. However, during thermal processing and long storage of foods, Maillard reaction (MR) often takes place. The MR is a non-enzymatic glycation reaction between the carbonyl group of reducing sugars and compounds having free amino groups. MR may sometimes be beneficial by damaging epitope of allergens and reducing allergenic potential, while exacerbation in allergic reactions may also occur due to changes in the motifs of epitopes or neoallergen generation. Apart from these modulations, non-enzymatic glycation can also modify the food protein(s) with various type of advance glycation end products (AGEs) such as Nϵ-(carboxymethyl-)lysine (CML), pentosidine, pyrraline, and methylglyoxal-H1 derived from MR. These Maillard products may act as immunogen by inducing the activation and proliferation of various immune cells. Literature is available to understand pathogenesis of glycation in the context of various diseases but there is hardly any review that can provide a thorough insight on the impact of glycation in food allergy. Therefore, present review explores the pathogenesis with special reference to food allergy caused by non-enzymatic glycation as well as AGEs.

S100 Proteins As an Important Regulator of Macrophage Inflammation

The S100 proteins, a family of calcium-binding cytosolic proteins, have a broad range of intracellular and extracellular functions through regulating calcium balance, cell apoptosis, migration, proliferation, differentiation, energy metabolism, and inflammation. The intracellular functions of S100 proteins involve interaction with intracellular receptors, membrane protein recruitment/transportation, transcriptional regulation and integrating with enzymes or nucleic acids, and DNA repair. The S100 proteins could also be released from the cytoplasm, induced by tissue/cell damage and cellular stress. The extracellular S100 proteins, serving as a danger signal, are crucial in regulating immune homeostasis, post-traumatic injury, and inflammation. Extracellular S100 proteins are also considered biomarkers for some specific diseases. In this review, we will discuss the multi-functional roles of S100 proteins, especially their potential roles associated with cell migration, differentiation, tissue repair, and inflammation.

DNA-aptamers raised against AGEs as a blocker of various aging-related disorders

A non-enzymatic reaction between sugars or aldehydes and the amino groups of proteins, lipids and nucleic acids contributes to the aging of macromolecules, which could impair their structural integrity and function. This process begins with the conversion of reversible Schiff base adducts, and then to more stable, covalently-bound Amadori rearrangement products. Over a course of days to weeks, these early glycation products undergo further reactions, such as rearrangements and dehydration to become irreversibly crossed-linked, fluorescent protein derivatives termed advanced glycation end products (AGEs). The formation and accumulation of AGEs have been known to progress in a physiological aging process and at an accelerated rate under hyperglycemic, inflammatory and oxidative stress conditions. There is a growing body of evidence that AGEs and their receptor RAGE interaction play a role in the pathogenesis of various devastating disorders, including cardiovascular disease, Alzheimer's disease, insulin resistance, osteoporosis and cancer growth and metastasis. Furthermore, diet has been recently recognized as a major environmental source of AGEs that could also elicit pro-inflammatory reactions, thereby being involved in organ damage in vivo. Therefore, inhibition of AGE formation and/or blockade of the interaction of AGEs with RAGE may be a novel therapeutic target for aging-related disorders. This article discusses a potential utility of DNA-aptamers raised against AGEs for preventing aging and/or diabetes-associated organ damage, especially focusing on diabetic microvascular complications, vascular remodeling, metabolic derangements, and melanoma growth and expansion in animal models.

Key Role of DAMP in Inflammation, Cancer, and Tissue Repair

PURPOSE

This review aimed to take stock of the current status of research on damage-associated molecular pattern (DAMP) protein. We discuss the Janus-faced role of DAMP molecules in inflammation, cancer, and tissue repair. The high-mobility group box (HMGB)-1 and adenosine triphosphate proteins are well-known DAMP molecules and have been primarily associated with inflammation. However, as we shall see, recent data have linked these molecules to tissue repair. HMGB1 is associated with cancer-related inflammation. It activates nuclear factor kB, which is involved in cancer regulation via its receptor for advanced glycation end-products (RAGE), Toll-like receptors 2 and 4. Proinflammatory activity and tissue repair may lead to pharmacologic intervention, by blocking DAMP RAGE and Toll like receptor 2 and 4 role in inflammation and by increasing their concentration in tissue repair, respectively.

METHODS

We conducted a MEDLINE search for articles pertaining to the various issues related to DAMP, and we discuss the most relevant articles especially (ie, not only those published in journals with a higher impact factor).

FINDINGS

A cluster of remarkable articles on DAMP have appeared in the literature in recent years. Regarding inflammation, several strategies have been proposed to target HMGB1, from antibodies to recombinant box A, which interacts with RAGE, competing with the full molecule. In tissue repair, it was reported that the overexpression of HMGB1 or the administration of exogenous HMGB1 significantly increased the number of vessels and promoted recovery in skin-wound, ischemic injury.

IMPLICATIONS

Due to the bivalent nature of DAMP, it is often difficult to explain the relative role of DAMP in inflammation versus its role in tissue repair. However, this point is crucial as DAMP-related treatments move into clinical practice.

S100B protein in skeletal muscle regeneration: regulation of myoblast and macrophage functions

Not available.

Receptor for Advanced Glycation End Products (RAGE) Serves a Protective Role during Klebsiella pneumoniae - Induced Pneumonia

Klebsiella species is the second most commonly isolated gram-negative organism in sepsis and a frequent causative pathogen in pneumonia. The receptor for advanced glycation end products (RAGE) is expressed on different cell types and plays a key role in diverse inflammatory responses. We here aimed to investigate the role of RAGE in the host response to Klebsiella (K.) pneumoniae pneumonia and intransally inoculated rage gene deficient (RAGE^-/-) and normal wild-type (Wt) mice with K. pneumoniae. Klebsiella pneumonia resulted in an increased pulmonary expression of RAGE. Furthermore, the high-affinity RAGE ligand high mobility group box-1 was upregulated during K. pneumoniae pneumonia. RAGE deficiency impaired host defense as reflected by a worsened survival, increased bacterial outgrowth and dissemination in RAGE^-/- mice. RAGE^-/- neutrophils showed a diminished phagocytosing capacity of live K. pneumoniae in vitro. Relative to Wt mice, RAGE^-/- mice demonstrated similar lung inflammation, and slightly elevated—if any—cytokine and chemokine levels and unchanged hepatocellular injury. In addition, RAGE^-/- mice displayed an unaltered response to intranasally instilled Klebsiella lipopolysaccharide (LPS) with respect to pulmonary cell recruitment and local release of cytokines and chemokines. These data suggest that (endogenous) RAGE protects against K. pneumoniae pneumonia. Also, they demonstrate that RAGE contributes to an effective antibacterial defense during K. pneumoniae pneumonia, at least partly via its participation in the phagocytic properties of professional granulocytes. Additionally, our results indicate that RAGE is not essential for the induction of a local and systemic inflammatory response to either intact Klebsiella or Klebsiella LPS.

DAMPs from Cell Death to New Life

Our body handles tissue damage by activating the immune system in response to intracellular molecules released by injured tissues [damage-associated molecular patterns (DAMPs)], in a similar way as it detects molecular motifs conserved in pathogens (pathogen-associated molecular patterns). DAMPs are molecules that have a physiological role inside the cell, but acquire additional functions when they are exposed to the extracellular environment: they alert the body about danger, stimulate an inflammatory response, and finally promote the regeneration process. Beside their passive release by dead cells, some DAMPs can be secreted or exposed by living cells undergoing a life-threatening stress. DAMPs have been linked to inflammation and related disorders: hence, inhibition of DAMP-mediated inflammatory responses is a promising strategy to improve the clinical management of infection- and injury-elicited inflammatory diseases. However, it is important to consider that DAMPs are not only danger signals but also central players in tissue repair. Indeed, some DAMPs have been studied for their role in tissue healing after sterile or infection-associated inflammation. This review is focused on two exemplary DAMPs, HMGB1 and adenosine triphosphate, and their contribution to both inflammation and tissue repair.

Stem cell activation in skeletal muscle regeneration

Muscle stem cell (satellite cell) activation post muscle injury is a transient and critical step in muscle regeneration. It is regulated by physiological cues, signaling molecules, and epigenetic regulatory factors. The mechanisms that coherently turn on the complex activation process shortly after trauma are just beginning to be illuminated. In this review, we will discuss the current knowledge of satellite cell activation regulation.

HMGB1 in health and disease

Complex genetic and physiological variations as well as environmental factors that drive emergence of chromosomal instability, development of unscheduled cell death, skewed differentiation, and altered metabolism are central to the pathogenesis of human diseases and disorders. Understanding the molecular bases for these processes is important for the development of new diagnostic biomarkers, and for identifying new therapeutic targets. In 1973, a group of non-histone nuclear proteins with high electrophoretic mobility was discovered and termed high-mobility group (HMG) proteins. The HMG proteins include three superfamilies termed HMGB, HMGN, and HMGA. High-mobility group box 1 (HMGB1), the most abundant and well-studied HMG protein, senses and coordinates the cellular stress response and plays a critical role not only inside of the cell as a DNA chaperone, chromosome guardian, autophagy sustainer, and protector from apoptotic cell death, but also outside the cell as the prototypic damage associated molecular pattern molecule (DAMP). This DAMP, in conjunction with other factors, thus has cytokine, chemokine, and growth factor activity, orchestrating the inflammatory and immune response. All of these characteristics make HMGB1 a critical molecular target in multiple human diseases including infectious diseases, ischemia, immune disorders, neurodegenerative diseases, metabolic disorders, and cancer. Indeed, a number of emergent strategies have been used to inhibit HMGB1 expression, release, and activity in vitro and in vivo. These include antibodies, peptide inhibitors, RNAi, anti-coagulants, endogenous hormones, various chemical compounds, HMGB1-receptor and signaling pathway inhibition, artificial DNAs, physical strategies including vagus nerve stimulation and other surgical approaches. Future work further investigating the details of HMGB1 localization, structure, post-translational modification, and identification of additional partners will undoubtedly uncover additional secrets regarding HMGB1's multiple functions.

Defective RAGE activity in embryonal rhabdomyosarcoma cells results in high PAX7 levels that sustain migration and invasiveness

Rhabdomyosarcoma is a muscle-derived malignant tumor mainly affecting children. The most frequent variant, embryonal rhabdomyosarcoma (ERMS) is characterized by overexpression of the transcription factor, PAX7 which prevents ERMS cells from exiting the cell cycle and terminally differentiating. However, a role for PAX7 in the invasive properties of ERMS cells has not been investigated in detail thus far. Here we show that ectopic expression of receptor for advanced glycation end-products (RAGE) in human ERMS cells results in the activation of a RAGE/myogenin axis which downregulates PAX7 by transcriptional and post-translational mechanisms, as in normal myoblasts, and reduces metastasis formation. High PAX7 sustains migration and invasiveness in ERMS cells by upregulating EPHA3 and EFNA1 and downregulating NCAM1 thus decreasing the neural cell adhesion molecule (NCAM)/polysialylated-NCAM ratio. Microarray gene expression analysis shows that compared with the RAGE(-ve) TE671/WT cells and similarly to primary human myoblasts, TE671/RAGE cells show upregulation of genes involved in muscle differentiation and cell adhesion, and downregulation of cell migration related and major histocompatibility complex class I genes. Our data reveal a link between PAX7 and metastasis occurrence in ERMSs, and support a role for the RAGE/myogenin axis in metastasis suppression. Thus, low RAGE expression in ERMS primary tumors may be predictive of metastatic behavior.

Expression of high mobility group box 1 in inflamed dental pulp and its chemotactic effect on dental pulp cells

High mobility group box 1 protein (HMGB1) is a chromatin protein which can be released extracellularly, eliciting a pro-inflammatory response and promoting tissue repair process. This study aimed to examine the expression and distribution of HMGB1 and its receptor RAGE in inflamed dental pulp tissues, and to assess its effects on proliferation, migration and cytoskeleton of cultured human dental pulp cells (DPCs). Our data demonstrated that cytoplasmic expression of HMGB1 was observed in inflamed pulp tissues, while HMGB1 expression was confined in the nuclei in healthy dental pulp. The mRNA expression of HMGB1 and RAGE were significantly increased in inflamed pulps. In in vitro cultured DPCs, expression of HMGB1 in both protein and mRNA level was up-regulated after treated with lipopolysaccharide (LPS). Exogenous HMGB1 enhanced DPCs migration in a dose-dependent manner and induced the reorganization of f-actin in DPCs. Our results suggests that HMGB1 are not only involved in the process of dental pulp inflammation, but also play an important role in the recruitment of dental pulp stem cells, promoting pulp repair and regeneration.

Active vitamin D possesses beneficial effects on the interaction between muscle and bone

Vitamin D deficiency and advanced glycation end products (AGEs) are suggested to be involved in the pathogenesis of osteoporosis and sarcopenia. However, the effects of vitamin D and AGEs on myogenesis and the interaction between muscle and bone remains still unclear. We previously showed that osteoglycin (OGN) is secreted from myoblasts and stimulates osteoblastic differentiation, suggesting that it plays important roles in the interaction between muscle and bone. The aim of this study is thus to examine the effects of vitamin D and AGEs on myoblastic differentiation of C2C12 cells and osteoblastic differentiation of osteoblastic MC3T3-E1 cells through OGN expression. 1α,25-dihydroxyvitamin D3 (1,25D) and eldecalcitol, an active vitamin D analog, induced the expression of MyoD, myogenin and OGN, and these effects were abolished by vitamin D receptor (VDR) suppression by siRNA in C2C12 cells. Moreover, conditioned medium from 1,25D-pretreated C2C12 cells stimulated the expression of type 1 collagen and alkaline phosphatase in MC3T3-E1 cells, compared to control medium from 1,25D-untreated C2C12 cells. In contrast, conditioned medium from VDR-suppressed and 1,25D-pretreated C2C12 cells showed no effects. AGE2 and AGE3 suppressed the expression of MyoD, myogenin and OGN in C2C12 cells. Moreover, 1,25D blunted the AGEs' effects. In conclusion, these findings showed for the first time that active vitamin D plays important roles in myogenesis and muscle-induced osteoblastogenesis through OGN expression. Active vitamin D treatment may rescue the AGEs-induced sarcopenia as well as - suppressed osteoblastic differentiation via OGN expression in myoblasts.

Regulation of myogenic activation of p38 MAPK by TACE-mediated TNFα release

The activation of p38 MAPK in myogenic precursor cells (MPCs) is a key signal for their exit of cell cycle and entry of the myogenic differentiation program. Therefore, identification of the signaling mechanism that activates p38 MAPK during this process is important for the understanding of the regulatory mechanism of muscle regeneration. This article reviews recent findings regarding the role of inflammatory cytokine tumor necrosis factor-α (TNFα) as a key activator of p38 MAPK during myogenesis in an autocrine/paracrine fashion, and the signaling mechanisms that converge upon TNFα converting enzyme (TACE) to release TNFα from differentiating MPCs in response to diverse regenerative stimuli.

HuR and miR-1192 regulate myogenesis by modulating the translation of HMGB1 mRNA

Upon muscle injury, the high mobility group box 1 (HMGB1) protein is upregulated and secreted to initiate reparative responses. Here we show that HMGB1 controls myogenesis both in vitro and in vivo during development and after adult muscle injury. HMGB1 expression in muscle cells is regulated at the translational level: the miRNA miR-1192 inhibits HMGB1 translation and the RNA-binding protein HuR promotes it. HuR binds to a cis-element, HuR binding sites (HuRBS), located in the 3'UTR of the HMGB1 transcript, and at the same time miR-1192 is recruited to an adjacent seed element. The binding of HuR to the HuRBS prevents the recruitment of Argonaute 2 (Ago2), overriding miR-1192-mediated translation inhibition. Depleting HuR reduces myoblast fusion and silencing miR-1192 re-establishes the fusion potential of HuR-depleted cells. We propose that HuR promotes the commitment of myoblasts to myogenesis by enhancing the translation of HMGB1 and suppressing the translation inhibition mediated by miR-1192.

RAGE signaling deficiency in rhabdomyosarcoma cells causes upregulation of PAX7 and uncontrolled proliferation

Embryonal rhabdomyosarcomas (ERMSs) show elevated levels of PAX7, a transcription factor that marks quiescent adult muscle stem (satellite) cells and is important for proliferation and survival of activated satellite cells and whose timely repression is required for myogenic differentiation. However, the mechanism of PAX7 accumulation in ERMSs and whether high PAX7 causes uncontrolled proliferation in ERMS remains to be elucidated. The receptor for advanced glycation end-products (RAGE, encoded by AGER) transduces a myogenic and anti-proliferative signal in myoblasts, and stable transfection of the ERMS cell line TE671, which does not express RAGE, with AGER results in reduced proliferation and formation of tumor masses in vivo, and enhanced apoptosis and myogenic differentiation. Herein, we show that RAGE expression is low or absent in human ERMSs. We also show that in ERMS cells (1) PAX7 accumulates owing to absent or low RAGE signaling; (2) elevated PAX7 levels reduce RAGE expression and levels of MyoD and myogenin, muscle-specific transcription factors required for myoblast proliferation arrest and differentiation, respectively; (3) PAX7 supports myoblast proliferation by reducing the levels of MyoD, primarily by promoting its degradation; and (4), when ectopically expressed in ERMS cells, that RAGE upregulates myogenin which upregulates MyoD and downregulates PAX7, with consequent inhibition of proliferation and stimulation of differentiation. Thus, failure to express RAGE and, hence, MyoD and myogenin above a critical level in ERMS cells might result in deregulated PAX7 expression leading to uncontrolled proliferation and, potentially, to rhabdomyosarcomagenesis.

Many faces of DAMPs in cancer therapy

A new concept of immunogenic cell death (ICD) has recently been proposed. The immunogenic characteristics of this cell death mode are mediated mainly by molecules called 'damage-associated molecular patterns' (DAMPs), most of which are recognized by pattern recognition receptors. Some DAMPs are actively emitted by cells undergoing ICD (e.g. calreticulin (CRT) and adenosine triphosphate (ATP)), whereas others are emitted passively (e.g. high-mobility group box 1 protein (HMGB1)). Recent studies have demonstrated that these DAMPs play a beneficial role in anti-cancer therapy by interacting with the immune system. The molecular pathways involved in translocation of CRT to the cell surface and secretion of ATP from tumor cells undergoing ICD are being elucidated. However, it has also been shown that the same DAMPs could contribute to progression of cancer and promote resistance to anticancer treatments. In this review, we will critically evaluate the beneficial and detrimental roles of DAMPs in cancer therapy, focusing mainly on CRT, ATP and HMGB1.

The immunobiology of the receptor of advanced glycation end-products: trends and challenges

Pattern-recognition receptors have been highly conserved in evolution. They recognize danger signals including both pathogen- and damage-associated molecular patterns, also known as alarmins. Several signaling pathways leading to an inflammatory reaction as part of an effective defensive response, are thus triggered. RAGE, a receptor initially considered for advanced glycation end-products, is also known to be activated by several danger signals, thus functioning as a pattern-recognition receptor. As a new member of this family, attempts to unraveling its functioning show that RAGE activation not only results in innate immune response but also contributes to promote and shape the acquired immune reaction. As reported for other members of the family, RAGE presents many polymorphic variants and additional studies are needed to elucidate its significance in immune response and disease susceptibility. Here we describe recent advances unraveling RAGE functions, as well as its significance and challenges in immunobiology.

S100B protein in tissue development, repair and regeneration

The Ca²⁺-binding protein of the EF-hand type, S100B, exerts both intracellular and extracellular regulatory activities. As an intracellular regulator, S100B is involved in the regulation of energy metabolism, transcription, protein phosphorylation, cell proliferation, survival, differentiation and motility, and Ca²⁺ homeostasis, by interacting with a wide array of proteins (i.e., enzymes, enzyme substrates, cytoskeletal subunits, scaffold/adaptor proteins, transcription factors, ubiquitin E3 ligases, ion channels) in a restricted number of cell types. As an extracellular signal, S100B engages the pattern recognition receptor, receptor for advanced glycation end-products (RAGE), on immune cells as well as on neuronal, astrocytic and microglial cells, vascular smooth muscle cells, skeletal myoblasts and cardiomyocytes. However, RAGE may not be the sole receptor activated by S100B, the protein being able to enhance bFGF-FGFR1 signaling by interacting with FGFR1-bound bFGF in particular cell types. Moreover, extracellular effects of S100B vary depending on its local concentration. Increasing evidence suggests that at the concentration found in extracellular fluids in normal physiological conditions and locally upon acute tissue injury, which is up to a few nM levels, S100B exerts trophic effects in the central and peripheral nervous system and in skeletal muscle tissue thus participating in tissue homeostasis. The present commentary summarizes results implicating intracellular and extracellular S100B in tissue development, repair and regeneration.

Advanced-glycation-end-product-induced formation of immunoproteasomes: involvement of RAGE and Jak2/STAT1

AGEs (advanced glycation-end products) accumulate during aging and several pathologies such as Alzheimer's disease and diabetes. These protein products are known to inhibit proteolytic pathways. Moreover, AGEs are known to be involved in the activation of immune responses. In the present study we demonstrate that AGEs induce the expression of immunoproteasomal subunits. To elucidate a molecular basis underlying the observed effects we were able to demonstrate an activation of the Jak2 (Janus kinase 2)/STAT1 (signal transducer and activator of transcription 1) pathway. Inhibition of Jak2 by AG-490 and STAT1 by specific siRNA (small interfering RNA) abolished AGE-induced expression of immunoproteasomal subunits. Furthermore, silencing of RAGE (receptor for AGEs) revealed that AGE-induced up-regulation of the immunoproteasome is mediated by a RAGE signalling process. Thus we have described for the first time that the signalling pathway of Jak2 and STAT1 activated by AGEs via RAGE is involved in the induction of the immunoproteasome.

High mobility group box protein-1 in wound repair

High-mobility group box 1 protein (HMGB1), a member of highly conserved non-histone DNA binding protein family, has been studied as transcription factor and growth factor. Secreted extracellularly by activated monocytes and macrophages or passively released by necrotic or damaged cells, extracellular HMGB1 is a potent mediator of inflammation. Extracellular HMGB1 has apparently contrasting biological actions: it sustains inflammation (with the possible establishment of autoimmunity or of self-maintaining tissue damage), but it also activates and recruits stem cells, boosting tissue repair. Here, we focus on the role of HMGB1 in physiological and pathological responses, the mechanisms by which it contributes to tissue repair and therapeutic strategies base on targeting HMGB1.

Clinical chorioamnionitis is characterized by changes in the expression of the alarmin HMGB1 and one of its receptors, sRAGE

OBJECTIVE

High mobility group box-1 (HMGB1) protein is an alarmin, a normal cell constituent, which is released into the extracellular environment upon cellular stress/damage and capable of activating inflammation and tissue repair. The receptor for advanced glycation end products (RAGE) can bind HMGB1. RAGE, in turn, can induce the production of pro-inflammatory cytokines; this may be modulated by the soluble truncated forms of RAGE, including soluble RAGE (sRAGE) and endogenous secretory RAGE (esRAGE). The objectives of this study were to determine whether: 1) clinical chorioamnionitis at term is associated with changes in amniotic fluid concentrations of HMGB1, sRAGE and esRAGE; and 2) the amniotic fluid concentration of HMGB1 changes with labor or as a function of gestational age.

METHODS

Amniotic fluid samples were collected from the following groups: 1) mid-trimester (n = 45); 2) term with (n = 48) and without labor (n = 22) without intra-amniotic infection; and 3) term with clinical chorioamnionitis (n = 46). Amniotic fluid concentrations of HMGB1, sRAGE and esRAGE concentrations were determined by ELISA.

RESULTS

1) the median amniotic fluid HMGB1 concentration was higher in patients at term with clinical chorioamnionitis than in those without this condition (clinical chorioamnionitis: median 3.8 ng/mL vs. term in labor: median 1.8 ng/mL, p = 0.007; and vs. term not in labor: median 1.1 ng/mL, p = 0.003); 2) in contrast, patients with clinical chorioamnionitis had a lower median sRAGE concentration than those without this condition (clinical chorioamnionitis: median 9.3 ng/mL vs. term in labor: median 18.6 ng/mL, p = 0.001; and vs. term not in labor median: 28.4 ng/mL, p < 0.001); 3) amniotic fluid concentrations of esRAGE did not significantly change in patients with clinical chorioamnionitis at term (clinical chorioamnionitis: median 5.4 ng/mL vs. term in labor: median 6.1 ng/mL, p = 0.9; and vs. term not in labor: median 9.5 ng/mL, p = 0.06); and 4) there was no significant difference in the median AF HMGB1 concentration between women at term in labor and those not in labor (p = 0.4) and between women in the mid-trimester and those at term not in labor (mid-trimester: median 1.5 ng/mL; p = 0.2).

CONCLUSION

An increase in the amniotic fluid HMGB1 concentration and a decrease in sRAGE were observed in clinical chorioamnionitis at term. This finding provides evidence that an alarmin, HMGB1, and one of its receptors, sRAGE, are engaged in the process of clinical chorioamnionitis at term. These changes are quite different from those observed in cases of intra-amniotic infection/inflammation in preterm gestations.

HMGB1-RAGE regulates muscle satellite cell homeostasis through p38-MAPK- and myogenin-dependent repression of Pax7 transcription

Expression of the paired-box 7 (PAX7) transcription factor is regulated during both myoblast proliferation and differentiation: high levels of PAX7 compromise myogenic differentiation because of excess and prolonged proliferation, whereas low levels of PAX7 result in precocious differentiation. We showed that myogenin repressed Pax7 transcription in differentiating myoblasts by binding to specific recognition sites in the Pax7 promoter, and that high-mobility group box 1 (HMGB1)-receptor for advanced glycation end-products (RAGE) signaling was required for myogenin induction and myogenin-dependent repression of Pax7 transcription. In addition, PAX7 negatively and myogenin positively regulated RAGE expression. RAGE, a multiligand receptor of the immunoglobulin superfamily, was not expressed in adult skeletal muscles, and was transiently expressed in activated, proliferating and differentiating satellite cells (SCs) in injured muscles. Compared with wild-type muscles, Rage(-/-) muscles exhibited increased numbers of basal SCs that were further increased in injured Rage(-/-) muscles following elevated myoblast asymmetric division; complete regeneration of injured Rage(-/-) muscles was found to be delayed by ~1 week. Thus, RAGE signaling physiologically repressed Pax7 transcription in SCs by upregulating myogenin, thereby accelerating muscle regeneration and limiting SC self-renewal.

S100B engages RAGE or bFGF/FGFR1 in myoblasts depending on its own concentration and myoblast density. Implications for muscle regeneration

In high-density myoblast cultures S100B enhances basic fibroblast growth factor (bFGF) receptor 1 (FGFR1) signaling via binding to bFGF and blocks its canonical receptor, receptor for advanced glycation end-products (RAGE), thereby stimulating proliferation and inhibiting differentiation. Here we show that upon skeletal muscle injury S100B is released from myofibers with maximum release at day 1 post-injury in coincidence with satellite cell activation and the beginning of the myoblast proliferation phase, and declining release thereafter in coincidence with reduced myoblast proliferation and enhanced differentiation. By contrast, levels of released bFGF are remarkably low at day 1 post-injury, peak around day 5 and decline thereafter. We also show that in low-density myoblast cultures S100B binds RAGE, but not bFGF/FGFR1 thereby simultaneously stimulating proliferation via ERK1/2 and activating the myogenic program via p38 MAPK. Clearance of S100B after a 24-h treatment of low-density myoblasts results in enhanced myotube formation compared with controls as a result of increased cell numbers and activated myogenic program, whereas chronic treatment with S100B results in stimulation of proliferation and inhibition of differentiation due to a switch of the initial low-density culture to a high-density culture. However, at relatively high doses, S100B stimulates the mitogenic bFGF/FGFR1 signaling in low-density myoblasts, provided bFGF is present. We propose that S100B is a danger signal released from injured muscles that participates in skeletal muscle regeneration by activating the promyogenic RAGE or the mitogenic bFGF/FGFR1 depending on its own concentration, the absence or presence of bFGF, and myoblast density.

Damage-associated molecular patterns (DAMPs) in preterm labor with intact membranes and preterm PROM: a study of the alarmin HMGB1

OBJECTIVE

Preterm parturition is a syndrome caused by multiple etiologies. Although intra-amniotic infection is causally linked with intrauterine inflammation and the onset of preterm labor, other patients have preterm labor in the absence of demonstrable infection. It is now clear that inflammation may be elicited by activation of the Damage-Associated Molecular Patterns (DAMPs), which include pathogen-associated molecular patterns (PAMPs) as well as "alarmins" (endogenous molecules that signal tissue and cellular damage). A prototypic alarmin is high-mobility group box 1 (HMGB1) protein, capable of inducing inflammation and tissue repair when it reaches the extracellular environment. HMGB1 is a late mediator of sepsis, and blockade of HMGB1 activity reduces mortality in an animal model of endotoxemia, even if administered late during the course of the disorder. The objectives of this study were to: (1) determine whether intra-amniotic infection/inflammation (IAI) is associated with changes in amniotic fluid concentrations of HMGB1; and (2) localize immunoreactivity of HMGB1 in the fetal membranes and umbilical cord of patients with chorioamnionitis.

METHODS

Amniotic fluid samples were collected from the following groups: (1) preterm labor with intact membranes (PTL) with (n=42) and without IAI (n=84); and (2) preterm prelabor rupture of membranes (PROM) with (n=38) and without IAI (n=35). IAI was defined as either a positive amniotic fluid culture or amniotic fluid concentration of interleukin-6 (IL-6) ≥ 2.6ng/mL. HMGB1 concentrations in amniotic fluid were determined by ELISA. Immunofluorescence staining for HMGB1 was performed in the fetal membranes and umbilical cord of pregnancies with acute chorioamnionitis.

RESULTS

(1) Amniotic fluid HMGB1 concentrations were higher in patients with IAI than in those without IAI in both the PTL and preterm PROM groups (PTL IAI: median 3.1 ng/mL vs. without IAI; median 0.98 ng/mL; p <0.001; and preterm PROM with IAI median 7.3 ng/mL vs. without IAI median 2.6 ng/mL; p=0.002); (2) patients with preterm PROM without IAI had a higher median amniotic fluid HMGB1 concentration than those with PTL and intact membranes without IAI (p <0.001); and (3) HMGB1 was immunolocalized to amnion epithelial cells and stromal cells in the Wharton's jelly (prominent in the nuclei and cytoplasm). Myofibroblasts and macrophages of the chorioamniotic connective tissue layer and infiltrating neutrophils showed diffuse cytoplasmic HMGB1 immunoreactivity.

CONCLUSIONS

(1) intra-amniotic infection/inflammation is associated with elevated amniotic fluid HMGB1 concentrations regardless of membrane status; (2) preterm PROM was associated with a higher amniotic fluid HMGB1 concentration than PTL with intact membranes, suggesting that rupture of membranes is associated with an elevation of alarmins; (3) immunoreactive HMGB1 was localized to amnion epithelial cells, Wharton's jelly and cells involved in the innate immune response; and (4) we propose that HMGB1 released from stress or injured cells into amniotic fluid may be responsible, in part, for intra-amniotic inflammation due to non-microbial insults.

Human muscle satellite cells show age-related differential expression of S100B protein and RAGE

During aging, skeletal muscles show reduced mass and functional capacity largely due to loss of the regenerative ability of satellite cells (SCs), the quiescent stem cells located beneath the basal lamina surrounding each myofiber. While both the external environment and intrinsic properties of SCs appear to contribute to the age-related SC deficiency, the latter ones have been poorly investigated especially in humans. In the present work, we analyzed several parameters of SCs derived from biopsies of vastus lateralis muscle from healthy non-trained young (28.7 ± 5.9 years; n = 10) and aged (77.3 ± 6.4 years; n = 11) people. Compared with young SCs, aged SCs showed impaired differentiation when cultured in differentiation medium, and exhibited the following: (1) reduced proliferation; (2) higher expression levels of S100B, a negative regulator of myoblast differentiation; (3) undetectable levels in growth medium of full-length RAGE (receptor for advanced glycation end products), a multiligand receptor of the immunoglobulin superfamily, the engagement of which enhances myoblast differentiation; and (4) lower expression levels of the transcription factors, MyoD and Pax7. Also, either overexpression of full-length RAGE or knockdown of S100B in aged SCs resulted in enhanced differentiation, while overexpression of either a non-transducing mutant of RAGE (RAGEΔcyto) or S100B in young SCs resulted in reduced differentiation compared with controls. Moreover, while aged SCs maintained the ability to respond to mitogenic factors (e.g., bFGF and S100B), they were no longer able to secrete these factors, unlike young SCs. These data support a role for intrinsic factors, besides the extracellular environment in the defective SC function in aged skeletal muscles.

Genetically-determined hyperfunction of the S100B/RAGE axis is a risk factor for aspergillosis in stem cell transplant recipients

Invasive aspergillosis (IA) is a major threat to the successful outcome of hematopoietic stem cell transplantation (HSCT), although individual risk varies considerably. Recent evidence has established a pivotal role for a danger sensing mechanism implicating the S100B/receptor for advanced glycation end products (RAGE) axis in antifungal immunity. The association of selected genetic variants in the S100B/RAGE axis with susceptibility to IA was investigated in 223 consecutive patients undergoing HSCT. Furthermore, studies addressing the functional consequences of these variants were performed. Susceptibility to IA was significantly associated with two distinct polymorphisms in RAGE (-374T/A) and S100B (+427C/T) genes, the relative contribution of each depended on their presence in both transplantation counterparts [patient SNP_RAGE, adjusted hazard ratio (HR), 1.97; P = 0.042 and donor SNP_RAGE, HR, 2.03; P = 0.047] or in donors (SNP_S100B, HR, 3.15; P = 7.8e-⁴) only, respectively. Functional assays demonstrated a gain-of-function phenotype of both variants, as shown by the enhanced expression of inflammatory cytokines in RAGE polymorphic cells and increased S100B secretion in vitro and in vivo in the presence of the S100B polymorphism. These findings point to a relevant role of the danger sensing signaling in human antifungal immunity and highlight a possible contribution of a genetically-determined hyperfunction of the S100B/RAGE axis to susceptibility to IA in the HSCT setting.