Arsenic inhibits myogenic differentiation and muscle regeneration

Chronic arsenite exposure induced skeletal muscle atrophy by disrupting angiotensin II-melatonin axis in rats

Arsenic is a well-known environmental toxicant and emerging evidence suggests that arsenic exposure has potential skeletal muscle toxicity; however, the underlying mechanism has not yet been clarified. The aim of this study was to investigate the correlation among adverse effects of subchronic and chronic environmental arsenic exposure on skeletal muscle as well as specific myokines secretion and angiotensin II (AngII)-melatonin (MT) axis in rats. Four-week-old rats were exposed to arsenite (iAs) in drinking water at environmental relevant concentration of 10 ppm for 3 or 9 months. Results indicated that the gastrocnemius muscle had atrophied and its mass was decreased in rats exposed to arsenite for 9 months, whereas, they had no significant changes in rats exposed to arsenite for 3 months. The levels of serum-specific myokine irisin and gastrocnemius muscle insulin-like growth factor-1 (IGF-1) were increased in 3-month exposure group and decreased in 9-month exposure group, while serum myostatin (MSTN) was increased significantly in 9-month exposure group. In addition, serum AngII level increased both in 3- and 9-month exposure groups, while serum MT level increased in 3-month exposure group and decreased in 9-month exposure group. Importantly, the ratio of AngII to MT level in serum increased gradually with the prolongation of arsenite exposure. It showed a certain correlation between AngII-MT axis and gastrocnemius muscle mass, gastrocnemius muscle level of IGF-1 or serum levels of irisin and MSTN. In conclusion, the disruption of AngII-MT axis balance may be a significant factor for skeletal muscle atrophy induced by chronic environmental arsenic exposure.

Bisphenol-A Abrogates Proliferation and Differentiation of C2C12 Mouse Myoblasts via Downregulation of Phospho-P65 NF-κB Signaling Pathway

Previous studies showed that bisphenol-A (BPA), a monomer of polycarbonate plastic, is leached out and contaminated in foods and beverages. This study aimed to investigate the effects of BPA on the myogenesis of adult muscle stem cells. C2C12 myoblasts were treated with BPA in both proliferation and differentiation conditions. Cytotoxicity, cell proliferation and differentiation, antioxidant activity, apoptosis, myogenic regulatory factors (MRFs) gene expression, and mechanism of BPA on myogenesis were examined. C2C12 myoblasts exposed to 25-50 µM BPA showed abnormal morphology, expressing numerous and long cytoplasmic extensions. Cell proliferation was inhibited and was accumulated in subG1 and S phases of the cell cycle, subsequently leading to apoptosis confirmed by nuclear condensation and the expression of apoptosis markers, cleaved caspase-9 and caspase-3. In addition, the activity of antioxidant enzymes, catalase, superoxide dismutase, and glutathione peroxidase was significantly decreased. Meanwhile, BPA suppressed myoblast differentiation by decreasing the number and size of multinucleated myotubes via the modulation of MRF gene expression. Moreover, BPA significantly inhibited the phosphorylation of P65 NF-κB in both proliferation and differentiation conditions. Altogether, the results revealed the adverse effects of BPA on myogenesis leading to abnormal growth and development via the inhibition of phospho-P65 NF-κB.

Arsenic and adipose tissue: an unexplored pathway for toxicity and metabolic dysfunction

Arsenic-contaminated drinking water can induce various disorders by disrupting lipid and glucose metabolism in adipose tissue, leading to insulin resistance. It inhibits adipocyte development and exacerbates insulin resistance, though the precise impact on lipid synthesis and lipolysis remains unclear. This review aims to explore the processes and pathways involved in adipogenesis and lipolysis within adipose tissue concerning arsenic-induced diabetes. Although arsenic exposure is linked to type 2 diabetes, the specific role of adipose tissue in its pathogenesis remains uncertain. The review delves into arsenic's effects on adipose tissue and related signaling pathways, such as SIRT3-FOXO3a, Ras-MAP-AP-1, PI(3)-K-Akt, endoplasmic reticulum stress proteins, CHOP10, and GPCR pathways, emphasizing the role of adipokines. This analysis relies on existing literature, striving to offer a comprehensive understanding of different adipokine categories contributing to arsenic-induced diabetes. The findings reveal that arsenic detrimentally impacts white adipose tissue (WAT) by reducing adipogenesis and promoting lipolysis. Epidemiological studies have hinted at a potential link between arsenic exposure and obesity development, with limited research suggesting a connection to lipodystrophy. Further investigations are needed to elucidate the mechanistic association between arsenic exposure and impaired adipose tissue function, ultimately leading to insulin resistance.

Hexavalent chromium inhibits myogenic differentiation and induces myotube atrophy

Hexavalent chromium [Cr(VI)] is extensively used in many industrial processes. Previous studies reported that Cr(VI) exposures during early embryonic development reduced body weight with musculoskeletal malformations in rodents while exposures in adult mice increased serum creatine kinase activity, a marker of muscle damage. However, the impacts of Cr(VI) on muscle differentiation remain largely unknown. Here, we report that acute exposures to Cr(VI) in mouse C2C12 myoblasts inhibit myogenic differentiation in a dose-dependent manner. Exposure to 2 μM of Cr(VI) resulted in delayed myotube formation, as evidenced by a significant decrease in myotube formation and expression of muscle-specific markers, such as muscle creatine kinase (Mck), Myocyte enhancer factor 2 (Mef2), Myomaker (Mymk) and Myomixer (Mymx). Interestingly, exposure to 5 μM of Cr(VI) completely abolished myotube formation in differentiating C2C12 cells. Moreover, the expression of key myogenic regulatory factors (MRFs) including myoblast determination protein 1 (MyoD), myogenin (MyoG), myogenic factor 5 (Myf5), and myogenic factor 6 (Myf6) were significantly altered in Cr(VI)-treated cells. The inhibitory effect of Cr(VI) on myogenic differentiation was further confirmed in freshly isolated mouse satellite cells, a stem cell population essential for adult skeletal muscle regeneration. Furthermore, Cr(VI) exposure to fully differentiated C2C12 myotubes resulted in a decrease in myotube diameter, which was exacerbated upon co-treatment with dexamethasone. Together, our results demonstrate that Cr(VI) inhibits myogenic differentiation and induces myotube atrophy in vitro.

Subchronic Arsenite Exposure Induced Atrophy and Erythropoietin Sensitivity Reduction in Skeletal Muscle Were Relevant to Declined Serum Melatonin Levels in Middle-Aged Rats

Arsenic is a kind of widespread environmental toxicant with multiorgan-toxic effects, and arsenic exposure is associated with the occurrence and development of many chronic diseases. The influence of environmental arsenic exposure on skeletal muscle, which is a vital organ of energy and glucose metabolism, has received increasing attention. This study aimed to investigate the types of inorganic arsenic-induced skeletal muscle injury, and the potential regulatory effects of melatonin (MT) and erythropoietin (EPO) in young (3-month-old) and middle-aged (12-month-old) rats. Our results showed that 1 mg/L sodium arsenite exposure for 3 months could accelerate gastrocnemius muscle atrophy and promote the switch of type II fibers to type I fibers in middle-aged rats; however, it did not cause significant pathological changes of gastrocnemius muscle in young rats. In addition, arsenite could inhibit serum MT levels, and promote serum EPO levels but inhibit EPO receptor (EPOR) expression in gastrocnemius muscle in middle-aged rats, while serum MT levels and EPOR expression in gastrocnemius muscle showed an opposite effect in young rats. Importantly, exogenous MT antagonized the arsenite-induced skeletal muscle toxic effect and restored serum EPO and gastrocnemius muscle EPOR expression levels in middle-aged rats. There was a positive correlation among gastrocnemius muscle index, serum MT level, and gastrocnemius muscle EPOR protein level in arsenite-exposed rats. This study demonstrated that inorganic arsenic could accelerate skeletal muscle mass loss and type II fiber reduction in middle-aged rats, which may be related to decreased MT secretion and declined EPO sensitivity in skeletal muscle.

Arsenic in Drinking Water and Diabetes

Arsenic is ubiquitous in soil and water environments and is consistently at the top of the Agency for Toxic Substances Disease Registry (ATSDR) substance priority list. It has been shown to induce toxicity even at low levels of exposure. One of the major routes of exposure to arsenic is through drinking water. This review presents current information related to the distribution of arsenic in the environment, the resultant impacts on human health, especially related to diabetes, which is one of the most prevalent chronic diseases, regulation of arsenic in drinking water, and approaches for treatment of arsenic in drinking water for both public utilities and private wells. Taken together, this information points out the existing challenges to understanding both the complex health impacts of arsenic and to implementing the treatment strategies needed to effectively reduce arsenic exposure at different scales.

Arsenic impairs stem cell differentiation via the Hippo signaling pathway

Arsenic is a ubiquitous toxic metalloid, with over 150 million people exposed to arsenic concentrations above the current 10 ppb drinking water standard through contaminated food and water. Arsenic is a known developmental toxicant as neuronal and muscle development are disrupted following arsenic exposure during embryogenesis. In this study, murine embryonic stem cells were chronically exposed to 0.1 μM (7.5 ppb) arsenic for 32 weeks. RNA sequencing showed that the Hippo signaling pathway, which is involved in embryonic development and pluripotency maintenance, is impaired following arsenic exposure. Thus, temporal changes in the Hippo pathway's core components and its downstream target genes Ctgf and c-Myc were investigated. Protein expression of the pathway's main effector YAP in its active form was significantly upregulated by 3.7-fold in arsenic-exposed cells at week 8, while protein expression of inactive phosphorylated YAP was significantly downregulated by 2.5- and 2-fold at weeks 8 and 16. Exposure to arsenic significantly increased the ratio between nuclear and cytoplasmic YAP by 1.9-fold at weeks 16 and 28. The ratio between nuclear and cytoplasmic transcriptional enhancer factor domain was similarly increased in arsenic-treated samples by 3.4- and 1.6-fold at weeks 16 and 28, respectively. Levels of Ctgf and c-Myc were also upregulated following arsenic exposure. These results suggest that chronic exposure to an environmentally relevant arsenic concentration might hinder cellular differentiation and maintain pluripotency through the impairment of the Hippo signaling pathway resulting in increased YAP activation.

Arsenic-induced toxicity and the ameliorative role of antioxidants and natural compounds

Arsenic (As) poisoning has proven to be a major threat worldwide because of its toxic effects on the human body. As toxicity through drinking water is a global health concern. The toxicity of As is known to affect the liver, kidney, lungs, muscles, cardiovascular system, and nervous system and can even induce diabetes. Further As can cause skin lesions leading to notable diseases in the skin like Bowen's disease. Chronic exposure to As has caused many tragedies in Eastern, and several Southeast Asian and Latin American countries. Long-term exposure to As makes it an immediate threat that should be dealt with as a priority, and one of the ways to handle it may be with the use of antioxidants. In this review, we have discussed the natural and anthropogenic sources of As, its metabolism, pathophysiology, and mechanism of toxicity. Besides, we have also discussed some of the synthetic chelators and the ameliorative role of antioxidants and natural compounds in reducing As toxicity.

ATP6AP1 is a potential prognostic biomarker and is associated with iron metabolism in breast cancer

Cancer occurrence and progression may be facilitated by aberrant expression of ATPase H+ transporting accessory protein 1 (ATP6AP1). However, the clinical relevance of ATP6AP1 in breast cancer remains unclear. In this study, we investigated the association between ATP6AP1 and breast cancer. Data collected from patients with breast cancer from the Gene Expression Omnibus (GEO) and The Cancer Genome Atlas (TCGA) were used in this study. To determine the relationship between ATP6AP1 and breast cancer survival rates, Kaplan-Meier analysis was used. To determine the prognostic value of ATP6AP1, a receiver operating characteristic (ROC) curve was constructed. To identify the major pathways involving ATP6AP1, we performed functional enrichment analysis using gene set enrichment analysis (GSEA). We analyzed the association between ATP6AP1 expression and tumor immunity using the ESTIMATE algorithm and single-sample GSEA (ssGSEA). A nomogram based on a Cox regression analysis was constructed to predict the impact of ATP6AP1 on prognosis. ATP6AP1 expression was significantly upregulated in breast cancer tissues. Moreover, patients with elevated ATP6AP1 expression had shorter total survival rates than those with lower expression levels (p = 0.032). The area under the receiver operating characteristic curve for ATP6AP1 was 0.939. Gene set enrichment analysis revealed that reaction iron uptake and transport, proteasome degradation, glutathione metabolism, and pyruvate metabolism were enriched in the ATP6AP1 high expression phenotype. The relationship between immune infiltration cells and ATP6AP1 expression, including macrophages, B cells, dendritic cells, cytotoxic cells, NK cells, and T cells, was found to be negative, suggesting that ATP6AP1 overexpression results in immunosuppression. Based on the Cox regression analyses, the calibration plot of the nomogram demonstrated effective performance in predicting breast cancer patients. ATP6AP1 may facilitate breast cancer progression by inhibiting antitumor immunity and promoting iron metabolism and may be a biomarker for breast cancer prognosis.

Arsenic Secondary Methylation Capacity Is Inversely Associated with Arsenic Exposure-Related Muscle Mass Reduction

Skeletal muscle mass reduction has been implicated in insulin resistance (IR) that promotes cardiometabolic diseases. We have previously reported that arsenic exposure increases IR concomitantly with the reduction of skeletal muscle mass among individuals exposed to arsenic. The arsenic methylation capacity is linked to the susceptibility to some arsenic exposure-related diseases. However, it remains unknown whether the arsenic methylation capacity affects the arsenic-induced reduction of muscle mass and elevation of IR. Therefore, this study examined the associations between the arsenic methylation status and skeletal muscle mass measures with regard to IR by recruiting 437 participants from low- and high-arsenic exposure areas in Bangladesh. The subjects' skeletal muscle mass was estimated by their lean body mass (LBM) and serum creatinine levels. Subjects' drinking water arsenic concentrations were positively associated with total urinary arsenic concentrations and the percentages of MMA, as well as inversely associated with the percentages of DMA and the secondary methylation index (SMI). Subjects' LBM and serum creatinine levels were positively associated with the percentage of DMA and SMI, as well as inversely associated with the percentage of MMA. HOMA-IR showed an inverse association with SMI, with a confounding effect of sex. Our results suggest that reduced secondary methylation capacity is involved in the arsenic-induced skeletal muscle loss that may be implicated in arsenic-induced IR and cardiometabolic diseases.

Associations between rice consumption, arsenic metabolism, and insulin resistance in adults without diabetes

Rice consumption is an important source of arsenic exposure. Little has known about the impact of rice consumption on arsenic metabolism, which is related to insulin resistance. In this study, we examined the associations between rice consumption and arsenic metabolism, and between arsenic metabolism and insulin resistance in non-diabetic U.S adults who participated in the National Health and Nutrition Examination Survey (NHANES) 2003-2016. Rice consumer was defined as ≥0.25 cups of cooked rice/day. HOMA2-IR was calculated using HOMA2 Calculator software based on participant's fasting glucose and insulin values. Urinary arsenic concentrations below limits of detection were imputed first, and then arsenic metabolism (the proportions of inorganic arsenic (iAs), monomethylarsonate (MMA), and dimethylarsinate (DMA) to their sum) were calculated (expressed as iAs%, MMA%, and DMA%). Using the leave-one-out approach, rice consumers compared with non-consumers had a 1.71% (95% CI: 1.12%, 2.29%) higher DMA% and lower MMA% when iAs% fixed; a 1.55% (95% CI: 0.45%, 2.66%) higher DMA% and lower iAs% when MMA% fixed; and a 1.62% (95% CI: 0.95%, 2.28%) higher iAs% and lower MMA% when DMA% fixed, in multivariable adjustment models. With every 10% decrease in MMA%, the geometric mean ratio of HOMA2-IR was 1.06 (95% CI: 1.03,1.08) and 1.05 (95% CI: 1.02, 1.09) when DMA% and iAs% was fixed, respectively; however, the associations were attenuated after adjusting for body mass index. In stratified analysis, we found that lower MMA% was associated with higher HOMA2-IR in participants with obesity: a 10% increase in iAs% with a 10% decrease in MMA% was associated with higher HOMA2-IR with the geometric mean ratio of 1.05 (95% CI: 1.01, 1.09). Our findings suggest that rice consumption may contribute to lower MMA% that was further associated with higher insulin resistance, especially in individuals with obesity. Future prospective studies are needed to confirm our results in different populations.

Long-term arsenic exposure impairs differentiation in mouse embryonal stem cells

Arsenic is a contaminant found in many foods and drinking water. Exposure to arsenic during development can cause improper neuronal progenitor cell development, differentiation, and function, while in vitro studies have determined that acute arsenic exposure to stem and progenitor cells reduced their ability to differentiate. In the current study, P19 mouse embryonal stem cells were exposed continuously to 0.1-μM (7.5 ppb) arsenic for 32 weeks. A cell lineage array examining messenger RNA (mRNA) changes after 8 and 32 weeks of exposure showed that genes involved in pluripotency were increased, whereas those involved in differentiation were reduced. Therefore, temporal changes of select pluripotency and neuronal differentiation markers throughout the 32-week chronic arsenic exposure were investigated. Sox2 and Oct4 mRNA expression were increased by 1.9- to 2.5-fold in the arsenic-exposed cells, beginning at Week 12. Sox2 protein expression was similarly increased starting at Week 16 and remained elevated by 1.5-fold to sixfold. One target of Sox2 is N-cadherin, whose expression is a hallmark of epithelial-mesenchymal transitions (EMTs). Exposure to arsenic significantly increased N-cadherin protein levels beginning at Week 20, concurrent with increased grouping of N-cadherin positive cells at the perimeter of the embryoid body. Expression of Zeb1, which helps increase the expression of Sox2, was also increased started at Week 16. In contrast, Gdf3 mRNA expression was reduced by 3.4- to 7.2-fold beginning at Week 16, and expression of its target protein, phospho-Smad2/3, was also reduced. These results suggest that chronic, low-level arsenic exposure may delay neuronal differentiation and maintain pluripotency.

Arsenic Directs Stem Cell Fate by Imparting Notch Signaling Into the Extracellular Matrix Niche

Compromise of skeletal muscle metabolism and composition may underlie the etiology of cardiovascular and metabolic disease risk from environmental arsenic exposures. We reported that arsenic impairs muscle maintenance and regeneration by inducing maladaptive mitochondrial phenotypes in muscle stem cells (MuSC), connective tissue fibroblasts (CTF), and myofibers. We also found that arsenic imparts a dysfunctional memory in the extracellular matrix (ECM) that disrupts the MuSC niche and is sufficient to favor the expansion and differentiation of fibrogenic MuSC subpopulations. To investigate the signaling mechanisms involved in imparting a dysfunctional ECM, we isolated skeletal muscle tissue and CTF from mice exposed to 0 or 100 μg/l arsenic in their drinking water for 5 weeks. ECM elaborated by arsenic-exposed CTF decreased myogenesis and increased fibrogenic/adipogenic MuSC subpopulations and differentiation. However, treating arsenic-exposed mice with SS-31, a mitochondrially targeted peptide that repairs the respiratory chain, reversed the arsenic-promoted CTF phenotype to one that elaborated an ECM supporting normal myogenic differentiation. SS-31 treatment also reversed arsenic-induced Notch1 expression, resulting in an improved muscle regeneration after injury. We found that persistent arsenic-induced CTF Notch1 expression caused the elaboration of dysfunctional ECM with increased expression of the Notch ligand DLL4. This DLL4 in the ECM was responsible for misdirecting MuSC myogenic differentiation. These data indicate that arsenic impairs muscle maintenance and regenerative capacity by targeting CTF mitochondria and mitochondrially directed expression of dysfunctional regulators in the stem cell niche. Therapies that restore muscle cell mitochondria may effectively treat arsenic-induced skeletal muscle dysfunction and compositional decline.

Arsenic exposure-related hyperglycemia is linked to insulin resistance with concomitant reduction of skeletal muscle mass

BACKGROUND

Alargebodyof evidence has shown a link between arsenic exposure and diabetes, but the underlying mechanisms have not yet been clarified.

OBJECTIVE

We explored the association between arsenic exposure and the reduction of skeletal muscle mass as a potential mechanism of insulin resistance for developing arsenic-related hyperglycemia.

METHODS

A total of 581 subjects were recruited from arsenic-endemic and non-endemic areas in Bangladesh and their fasting blood glucose (FBG), serum insulin, and serum creatinine levels were determined. Subjects' arsenic exposure levels were assessed by arsenic concentrations in water, hair, and nails. HOMA-IR and HOMA-β were used to calculate insulin resistance and β-cell dysfunction, respectively. Serum creatinine levels and lean body mass (LBM) were used as muscle mass indicators.

RESULTS

Water, hair and nail arsenic concentrations showed significant positive associations with FBG, serum insulin and HOMA-IR and inverse associations with serum creatinine and LBM in a dose-dependent manner both in males and females. Water, hair and nail arsenic showed significant inverse associations with HOMA-β in females but not in males. FBG and HOMA-IR were increased with the decreasing levels of serum creatinine and LBM. Odds ratios (ORs)of hyperglycemia were significantly increased with the increasing concentrations of arsenic in water, hair and nails and with the decreasing levels of serum creatinine and LBM. Females' HOMA-IR showed greater susceptibility to the reduction of serum creatinine and LBM, possibly causing the greater risk of hyperglycemia in females than males. Path analysis revealed the mediating effect of serum creatinine level on the relationship of arsenic exposure with HOMA-IR and hyperglycemia.

CONCLUSION

Arsenic exposure elevates FBG levels and the risk of hyperglycemia through increasing insulin resistance with greater susceptibility in females than males. Additionally, arsenic exposure-related reduction of skeletal muscle mass may be a mechanism underlying the development of insulin resistance and hyperglycemia.

Inorganic Arsenic Exposure Decreases Muscle Mass and Enhances Denervation-Induced Muscle Atrophy in Mice

Arsenic is a toxic metalloid. Infants with a low birth-weight have been observed in areas with high-level arsenic in drinking water ranging from 463 to 1025 μg/L. A distal muscular atrophy side effect has been observed in acute promyelocytic leukemia patients treated with arsenic trioxide (As₂O₃) for therapy. The potential of As₂O₃ on muscle atrophy remains to be clarified. In this study, the myoatrophic effect of arsenic was evaluated in normal mice and sciatic nerve denervated mice exposed with or without As₂O₃ (0.05 and 0.5 ppm) in drinking water for 4 weeks. We found that both 0.05 and 0.5 ppm As₂O₃ increased the fasting plasma glucose level; but only 0.5 ppm arsenic exposure significantly decreased muscle mass, muscle endurance, and cross-sectional area of muscle fibers, and increased muscle Atrogin-1 protein expression in the normal mice. Both 0.05 and 0.5 ppm As₂O₃ also significantly enhanced the inhibitory effects on muscle endurance, muscle mass, and cross-sectional area of muscle fibers, and increased the effect on muscle Atrogin-1 protein expression in the denervated mice. These in vivo results suggest that inorganic arsenic at doses relevant to humans may possess myoatrophic potential.

Effects of Inorganic Arsenic on Human Prostate Stem-Progenitor Cell Transformation, Autophagic Flux Blockade, and NRF2 Pathway Activation

BACKGROUND

Inorganic arsenic (iAs) is an environmental toxicant associated with an increased risk of prostate cancer in chronically exposed populations worldwide. However, the biological mechanisms underlying iAs-induced prostate carcinogenesis remain unclear.

OBJECTIVES

We studied how iAs affects normal human prostate stem-progenitor cells (PrSPCs) and drives transformation and interrogated the molecular mechanisms involved.

METHODS

PrSPCs were enriched by spheroid culture from normal human primary or immortalized prostate epithelial cells, and their differentiation capability was evaluated by organoid culture. Microarray analysis was conducted to identify iAs-dysregulated genes, and lentiviral infection was used for stable manipulation of identified genes. Soft agar colony growth assays were applied to examine iAs-induced transformation. For in vivo study, PrSPCs mixed with rat urogenital sinus mesenchyme were grafted under the renal capsule of nude mice to generate prostatelike tissues, and mice were exposed to 5 ppm (∼65μM) iAs in drinking water for 3 months.

RESULTS

Low-dose iAs (1μM) disturbed PrSPC homeostasis in vitro, leading to increased self-renewal and suppressed differentiation. Transcriptomic analysis indicated that iAs activated oncogenic pathways in PrSPCs, including the KEAP1-NRF2 pathway. Further, iAs-exposed proliferative progenitor cells exhibited NRF2 pathway activation that was sustained in their progeny cells. Knockdown of NRF2 inhibited spheroid formation by driving PrSPC differentiation, whereas its activation enhanced spheroid growth. Importantly, iAs-induced transformation was suppressed by NRF2 knockdown. Mechanistically, iAs suppressed Vacuolar ATPase subunit VMA5 expression, impairing lysosome acidification and inhibiting autophagic protein degradation including p62, which further activated NRF2. In vivo, chronic iAs exposure activated NRF2 in both epithelial and stroma cells of chimeric human prostate grafts and induced premalignant events.

CONCLUSIONS

Low-dose iAs increased self-renewal and decreased differentiation of human PrSPCs by activating the p62-NRF2 axis, resulting in epithelial cell transformation. NRF2 is activated by iAs through specific autophagic flux blockade in progenitor cells, which may have potential therapeutic implications. https://doi.org/10.1289/EHP6471.

Exposure of low-concentration arsenic induces myotube atrophy by inhibiting an Akt signaling pathway

Arsenic, a widely distributed toxic metalloid, has been found to be associated with the low-birth-weight infants and the impairment of muscle regenerative capacity in areas with high levels of arsenic in drinking water. The distal muscular atrophy is one of side effects of arsenic trioxide (As₂O₃) for acute promyelocytic leukemia therapy. We hypothesized that arsenic may be a potential risk factor for skeletal muscle atrophy. Here, we investigated the action and molecular mechanism of low-dose arsenic on the induction of skeletal muscle atrophy in a skeletal muscle cell model. The differentiated C2C12 myotubes were treated with As₂O₃ (0.25-1 μM) for 48 h without apparent effects on cell viability. The signaling molecules for myotube atrophy were assessed. Submicromolar-concentration As₂O₃ dose-dependently triggered C2C12 myotube atrophy and increased the protein expressions of atrogenes Atrogin1 and MuRF1 and inhibited the upstream phosphorylated proteins Akt and FoxO1, while As₂O₃ dose-dependently increased AMPK phosphorylation in myotubes. Akt activator SC79 could significantly reverse the As₂O₃-induced myotube atrophy. These results suggest that arsenic is capable of inducing myotube atrophy by inhibiting an Akt signaling pathway.

Arsenic induces human chondrocyte senescence and accelerates rat articular cartilage aging

Arsenic-contaminated drinking water is known to be a serious human health problem. A previous epidemiological study has indicated that arsenic levels in blood were higher in arthritis patients compared to age-matched control subjects. Bone is known as an important arsenic store compartment in the body. Arsenic exposure has been suggested to promote senescence in human mesenchymal stem cells that may affect the balance of adipogenic and osteogenic differentiation. The toxicological effect and mechanism of arsenic exposure on articular chondrocytes still remain unclear. Here, we investigated the arsenic-induced senescence in cultured human articular chondrocytes and long-term arsenic-exposed rat articular cartilage. Arsenic trioxide (As₂O₃; 1-5 μM) significantly induced senescence in human articular chondrocytes by increasing senescence-associated β-galactosidase (SA-β-Gal) activity and protein expression of p16, p53, and p21. Arsenic induced the phosphorylation of p38 and c-Jun N-terminal kinase (JNK) proteins. The inhibitors of p38 and JNK significantly reversed the arsenic-induced chondrocyte senescence. Arsenic could also trigger the induction of GATA4-NF-κB signaling and senescence-associated secretory phenotype (SASP) by increasing IL-1α, IL-1β, TGF-β, TNF-α, CCL2, PAI-1, and MMP13 mRNA expression. The increased cartilage senescence and abrasion were also observed in a rat model long-term treatment with arsenic (0.05 and 0.5 ppm) in drinking water for 36 weeks as compared to age-matched control rats. The phosphorylation of p38 and JNK and the induction of GATA4-NF-κB signaling and SASP were enhanced in the rat cartilages. Taken together, these findings suggest that arsenic exposure is capable of inducing chondrocyte senescence and accelerating rat articular cartilage aging and abrasion.

Methylmercury modifies temporally expressed myogenic regulatory factors to inhibit myoblast differentiation

Methylmercury (MeHg) is a pervasive environmental toxicant, with known detrimental effects on neurodevelopment. Despite a longstanding paradigm of neurotoxicity, where motor deficits are prevalent among those developmentally exposed, consideration of muscle as a MeHg target has received minimal investigation. Recent evidence has identified muscle-specific gene networks that modulate developmental sensitivity to MeHg toxicity. One such network is muscle cell differentiation. Muscle cell differentiation is a coordinated process regulated by the myogenic regulatory factors (MRFs): Myf5, MyoD, MyoG, and MRF4. A previous study demonstrated that MeHg inhibits muscle cell differentiation in vitro, concurrent with reduced MyoG expression. The potential for MeHg to modify the temporal expression of the MRFs to alter differentiation, however, has yet to be fully explored. Using the C2C12 mouse myoblast model, we examined MRF expression profiles at various stages subsequent to MeHg exposure to proliferating myoblasts. MeHg was seen to persistently alter myoblast differentiation capacity, as myod, myog, and mrf4 gene expression were all affected. Myog exhibited the most robust changes in expression across the various culture conditions, while myf5 was unaffected. Following MeHg exposure to myoblasts, where elevated p21 expression indicated departure from proliferation, cells failed to subsequently differentiate, even in the absence of MeHg, as reflected by a concurrent reduction in MRF4 and myosin heavy chain (MHC), markers of terminal differentiation. Our results indicate that within a brief window of exposure MeHg can disrupt the intrinsic myogenic differentiation program of proliferative myoblasts.

Non-monotonic dose-response effects of arsenic on glucose metabolism

BACKGROUND

Inorganic arsenic (iAs) is a widespread environmental toxin. In addition to being a human carcinogen, its effect on diabetes has started to gain recognition recently. Insulin is the key hormone regulating systemic glucose metabolism. The in vivo effect of iAs on insulin sensitivity has not been directly addressed.

OBJECTIVES

Here we use mouse models to dissect the dose-dependent effects of iAs on glucose metabolism in vivo.

METHODS

We performed hyperinsulinemic-euglycemic clamp, the gold standard analysis of systemic insulin sensitivity. We also performed dynamic metabolic testings and RNA-seq analysis.

RESULTS

We found that a low-dose exposure (0.25 ppm iAs in drinking water) caused glucose intolerance in adult male C57BL/6 mice, likely by disrupting glucose-induced insulin secretion without affecting peripheral insulin sensitivity. However, a higher-dose exposure (2.5 ppm iAs) had diminished effects on glucose tolerance despite disrupted pancreatic insulin secretion. Insulin Clamp analysis showed that 2.5 ppm iAs actually enhanced systemic insulin sensitivity by simultaneously enhancing insulin-stimulated glucose uptake in skeletal muscles and improved insulin-mediated suppression of endogenous glucose production. RNA-seq analysis of skeletal muscles revealed that 2.5 ppm iAs regulated expression of many genes involved in the metabolism of fatty acids, pyruvate, and amino acids.

CONCLUSION

These findings suggest that iAs has opposite glycemic effects on distinct metabolic tissues at different dose thresholds. Such non-monotonic dose-response effects of iAs on glucose tolerance shed light on the complex interactions between iAs and the systemic glucose metabolism, which could potentially help reconcile some of the conflicting results in human epidemiological studies.

Adverse effects of acrolein, a ubiquitous environmental toxicant, on muscle regeneration and mass

BACKGROUND

Acrolein is an extremely electrophilic aldehyde. Increased urinary acrolein adducts have been found in type 2 diabetic patients and people with a smoking habit. The increased blood acrolein was shown in patients who received the cancer drug cyclophosphamide. Both diabetes and smoking are risk factors for skeletal muscle wasting or atrophy. Acrolein has been found to induce myotube atrophy in vitro. The in vitro and in vivo effects and mechanisms of acrolein on myogenesis and the in vivo effect of acrolein on muscle wasting still remain unclear.

METHODS

C2C12 myoblasts were used to assess the effects of low-dose acrolein (0.125-1 μM) on myogenesis in vitro. Mice were exposed daily to acrolein in distilled water by oral administration (2.5 and 5 mg/kg) for 4 weeks with or without glycerol-induced muscle injury to investigate the effects of acrolein on muscle wasting and regeneration.

RESULTS

Non-cytotoxic-concentration acrolein dose dependently inhibited myogenic differentiation in myoblasts (myotube formation inhibition: 0.5 and 1 μM, 66.25% and 46.25% control, respectively, n = 4, P < 0.05). The protein expression for myogenesis-related signalling molecules (myogenin and phosphorylated Akt: 0.5 and 1 μM, 85.15% and 51.52% control and 62.63% and 56.57% control, respectively, n = 4, P < 0.05) and myosin heavy chain (MHC: 0.5 and 1 μM, 63.64% and 52.53% control, n = 4, P < 0.05) were decreased in acrolein-treated myoblasts. Over-expression of the constitutively active form of Akt in myoblasts during differentiation prevented the inhibitory effects of acrolein (1 μM) on myogenesis (MHC and myogenin protein expression: acrolein with or without constitutively active Akt, 64.65% and 105.21% control and 69.14% and 102.02% control, respectively, n = 5, P < 0.05). Oral administration of acrolein for 4 weeks reduced muscle weights (5 mg/kg/day: 65.52% control, n = 6, P < 0.05) and cross-sectional area of myofibers in soleus muscles (5 mg/kg/day: 79.92% control, n = 6, P < 0.05) with an up-regulation of atrogin-1 and a down-regulation of phosphorylated Akt protein expressions. Acrolein retarded soleus muscle regeneration in a glycerol-induced muscle regeneration mouse model (5 mg/kg/day: 49.29% control, n = 4, P < 0.05). Acrolein exposure reduced muscle endurance during rotarod fatigue performance in mice with or without glycerol-induced muscle injury (5 mg/kg/day without glycerol: 30.43% control, n = 4, P < 0.05). Accumulation of acrolein protein adducts could be detected in the soleus muscles of acrolein-treated mice.

CONCLUSIONS

Low-dose acrolein significantly inhibited myogenic differentiation in vitro, which might be through inhibition of Akt signalling. Acrolein induced muscle wasting and retarded muscle regeneration in mice. These results suggest that acrolein may be a risk factor for myogenesis and disease-related myopathy.

Islet-like clusters derived from skeletal muscle-derived stem/progenitor cells for autologous transplantation to control type 1 diabetes in mice

A population of muscle-derived stem/progenitor cells (MDSPCs) contained in skeletal muscle is responsible for muscle regeneration. MDSPCs from mouse muscle have been shown to be capable of differentiating into pancreatic islet-like cells. However, the potency of MDSPCs to differentiate into functional islet-like cluster remains to be confirmed. The therapeutic potential of autologous MDSPCs transplantation on type 1 diabetes still remains unclear. Here, we investigated a four-stage method to induce the differentiation of MDSPCs into insulin-producing clusters in vitro, and tested the autologous transplantation to control type 1 diabetes in mice. MDSPCs isolated from the skeletal muscles of mice possessed the ability to form islet-like clusters through several stages of differentiation. The expressions of pancreatic progenitor-related genes, insulin, and islet-related genes were significantly upregulated in islet-like clusters determined by the quantitative reverse transcription polymerase chain reaction. The autologous islet-like clusters transplantation effectively improved hyperglycaemia and glucose intolerance and increased the survival rate in streptozotocin-induced diabetic mice without the use of immunosuppressants. Taken together, these results provide evidence that MDSPCs from murine muscle tissues are capable of differentiating into insulin-producing clusters, which possess insulin-producing ability in vitro and in vivo, and have the potential for autologous transplantation to control type 1 diabetes.

Preventing muscle wasting by osteoporosis drug alendronate in vitro and in myopathy models via sirtuin-3 down-regulation

BACKGROUND

A global consensus on the loss of skeletal muscle mass and function in humans refers as sarcopenia and cachexia including diabetes, obesity, renal failure, and osteoporosis. Despite a current improvement of sarcopenia or cachexia with exercise training and supportive therapies, alternative and specific managements are needed to discover for whom are unable or unwilling to embark on these treatments. Alendronate is a widely used drug for osteoporosis in the elderly and postmenopausal women. Osteopenic menopausal women with 6 months of alendronate therapy have been observed to improve not only lumbar bone mineral density but also handgrip strength. However, the effect and mechanism of alendronate on muscle strength still remain unclear. Here, we investigated the therapeutic potential and the molecular mechanism of alendronate on the loss of muscle mass and strength in vitro and in vivo.

METHODS

Mouse myoblasts and primary human skeletal muscle-derived progenitor cells were used to assess the effects of low-dose alendronate (0.1-1 μM) combined with or without dexamethasone on myotube hypertrophy and myogenic differentiation. Moreover, we also evaluated the effects of low-dose alendronate (0.5 and 1 mg/kg) by oral administration on the limb muscle function and morphology of mice with denervation-induced muscle atrophy and glycerol-induced muscle injury.

RESULTS

Alendronate inhibited dexamethasone-induced myotube atrophy and myogenic differentiation inhibition in mouse myoblasts and primary human skeletal muscle-derived progenitor cells. Alendronate significantly abrogated dexamethasone-up-regulated sirtuin-3 (SIRT3), but not SIRT1, protein expression in myotubes. Both SIRT3 inhibitor AKG7 and SIRT3-siRNA transfection could also reverse dexamethasone-up-regulated atrogin-1 and SIRT3 protein expressions. Animal studies showed that low-dose alendronate by oral administration ameliorated the muscular malfunction in mouse models of denervation-induced muscle atrophy and glycerol-induced muscle injury with a negative regulation of SIRT3 expression.

CONCLUSIONS

The putative mechanism by which muscle atrophy was improved with alendronate might be through the SIRT3 down-regulation. These findings suggest that alendronate can be a promising therapeutic strategy for management of muscle wasting-related diseases and sarcopenia.

Embryonic-only arsenic exposure alters skeletal muscle satellite cell function in killifish (Fundulus heteroclitus)

Arsenic is a contaminant found worldwide in drinking water and food. Epidemiological studies have correlated arsenic exposure with reduced weight gain and improper muscular development, while in vitro studies show that arsenic exposure impairs myogenic differentiation. The purpose of this study was to use Fundulus heteroclitus or killifish as a model organism to determine if embryonic-only arsenic exposure permanently reduces the number or function of muscle satellite cells. Killifish embryos were exposed to 0, 50, 200, or 800 ppb arsenite (As^III) until hatching, and then juvenile fish were raised in clean water. At 28, 40, and 52 weeks after hatching, skeletal muscle injuries were induced by injecting cardiotoxin into the trunk of the fish just posterior to the dorsal fin. Muscle sections were collected at 0, 3 and 10 days post-injury. Collagen levels were used to assess muscle tissue damage and recovery, while levels of proliferating cell nuclear antigen (PCNA) and myogenin were quantified to compare proliferating cells and newly formed myoblasts. At 28 weeks of age, baseline collagen levels were 105% and 112% greater in 200 and 800 ppb groups, respectively, and at 52 weeks of age, were 58% higher than controls in the 200 ppb fish. After cardiotoxin injury, collagen levels tend to increase to a greater extent and take longer to resolve in the arsenic exposed fish. The number of baseline PCNA(+) cells were 48-216% greater in 800 ppb exposed fish compared to controls, depending on the week examined. However, following cardiotoxin injury, PCNA is reduced at 28 weeks in 200 and 800 ppb fish at day 3 during the recovery period. By 52 weeks, there are significant reductions in PCNA in all exposure groups at day 3 of the recovery period. Based on these results, embryonic arsenic exposure increases baseline collagen levels and PCNA(+) cells in skeletal muscle. However, when these fish are challenged with a muscle injury, the proliferation and differentiation of satellite cells into myogenic precursors is impaired and instead, the fish appear to be favoring a fibrotic resolution to the injury.

Arsenic Induces Members of the mmu-miR-466-669 Cluster Which Reduces NeuroD1 Expression

Chronic arsenic exposure can result in adverse development effects including decreased intellectual function, reduced birth weight, and altered locomotor activity. Previous in vitro studies have shown that arsenic inhibits stem cell differentiation. MicroRNAs (miRNAs) are small noncoding RNAs that regulate multiple cellular processes including embryonic development and cell differentiation. The purpose of this study was to examine whether altered miRNA expression was a mechanism by which arsenic inhibited cellular differentiation. The pluripotent P19 mouse embryonal carcinoma cells were exposed to 0 or 0.5 μM sodium arsenite for 9 days during cell differentiation, and changes in miRNA expression was analyzed using microarrays. We found that the expression of several miRNAs important in cellular differentiation, such as miR-9 and miR-199 were decreased by 1.9- and 1.6-fold, respectively, following arsenic exposure, while miR-92a, miR-291a, and miR-709 were increased by 3-, 3.7-, and 1.6-fold, respectively. The members of the miR-466-669 cluster and its host gene, Scm-like with 4 Mbt domains 2 (Sfmbt2), were significantly induced by arsenic from 1.5- to 4-fold in a time-dependent manner. Multiple miRNA target prediction programs revealed that several neurogenic transcription factors appear to be targets of the cluster. When consensus anti-miRNAs targeting the miR-466-669 cluster were transfected into P19 cells, arsenic-exposed cells were able to more effectively differentiate. The consensus anti-miRNAs appeared to rescue the inhibitory effects of arsenic on cell differentiation due to an increased expression of NeuroD1. Taken together, we conclude that arsenic induces the miR-466-669 cluster, and that this induction acts to inhibit cellular differentiation in part due to a repression of NeuroD1.

Methylmercury exposure causes a persistent inhibition of myogenin expression and C2C12 myoblast differentiation

Methylmercury (MeHg) is a ubiquitous environmental toxicant, best known for its selective targeting of the developing nervous system. MeHg exposure has been shown to cause motor deficits such as impaired gait and coordination, muscle weakness, and muscle atrophy, which have been associated with disruption of motor neurons. However, recent studies have suggested that muscle may also be a target of MeHg toxicity, both in the context of developmental myogenic events and of low-level chronic exposures affecting muscle wasting in aging. We therefore investigated the effects of MeHg on myotube formation, using the C2C12 mouse myoblast model. We found that MeHg inhibits both differentiation and fusion, in a concentration-dependent manner. Furthermore, MeHg specifically and persistently inhibits myogenin (MyoG), a transcription factor involved in myocyte differentiation, within the first six hours of exposure. MeHg-induced reduction in MyoG expression is contemporaneous with a reduction of a number of factors involved in mitochondrial biogenesis and mtDNA transcription and translation, which may implicate a role for mitochondria in mediating MeHg-induced change in the differentiation program. Unexpectedly, inhibition of myoblast differentiation with MeHg parallels inhibition of Notch receptor signaling. Our research establishes muscle cell differentiation as a target for MeHg toxicity, which may contribute to the underlying etiology of motor deficits with MeHg toxicity.

Assessment of arsenic trioxide toxicity on cock muscular tissue: alterations of oxidative damage parameters, inflammatory cytokines and heat shock proteins

To evaluate the toxicity of arsenic trioxide (As₂O₃) in the muscular tissues (wing, thigh and pectoral) of birds, 72 one-day-old Hy-line cocks were selected and randomly divided into four groups. They were fed either a commercial diet or an arsenic-supplemented diet containing 7.5, 15 or 30 mg/kg As₂O₃. The experiment lasted for 90 days and the samples of muscular tissues were collected at 30, 60 and 90 days. The results showed that As₂O₃ exposure significantly lowered the activities of antioxidant enzymes (catalase (CAT), glutathione peroxidase (GSH-Px)) and inhibition ability of hydroxyl radicals (OH) and increased the malondialdehyde (MDA) contents. Furthermore, the mRNA levels of inflammatory cytokines (tumor necrosis factor-α (TNF-α), nuclear factor-kappa B (NF-κB), cyclooxygenase-2 (COX-2), inducible NO synthase (iNOS), prostaglandin E synthase (PTGEs)) and heat shock proteins (HSPs) in muscular tissue were significantly upregulated in the As₂O₃ exposure groups. The results indicated that As₂O₃ exposure resulted in oxidative damage, induced the inflammatory response, and influenced the mRNA levels of HSPs in muscular tissue of cocks. Additionally, the results suggested that HSPs possibly resisted due to the As₂O₃ exposure-induced oxidative stress and inflammatory response, which provided a favorable environment and played protective roles in the muscular tissues of cocks. The information presented in this study is helpful to understand the mechanism of As₂O₃ toxicity in bird muscular tissues.

Exposures to arsenite and methylarsonite produce insulin resistance and impair insulin-dependent glycogen metabolism in hepatocytes

Environmental exposure to inorganic arsenic (iAs) has been shown to disturb glucose homeostasis, leading to diabetes. Previous laboratory studies have suggested several mechanisms that may underlie the diabetogenic effects of iAs exposure, including (i) inhibition of insulin signaling (leading to insulin resistance) in glucose metabolizing peripheral tissues, (ii) inhibition of insulin secretion by pancreatic β cells, and (iii) dysregulation of the methylation or expression of genes involved in maintenance of glucose or insulin metabolism and function. Published studies have also shown that acute or chronic iAs exposures may result in depletion of hepatic glycogen stores. However, effects of iAs on pathways and mechanisms that regulate glycogen metabolism in the liver have never been studied. The present study examined glycogen metabolism in primary murine hepatocytes exposed in vitro to arsenite (iAs³⁺) or its methylated metabolite, methylarsonite (MAs³⁺). The results show that 4-h exposures to iAs³⁺ and MAs³⁺ at concentrations as low as 0.5 and 0.2 µM, respectively, decreased glycogen content in insulin-stimulated hepatocytes by inhibiting insulin-dependent activation of glycogen synthase (GS) and by inducing activity of glycogen phosphorylase (GP). Further investigation revealed that both iAs³⁺ and MAs³⁺ inhibit insulin-dependent phosphorylation of protein kinase B/Akt, one of the mechanisms involved in the regulation of GS and GP by insulin. Thus, inhibition of insulin signaling (i.e., insulin resistance) is likely responsible for the dysregulation of glycogen metabolism in hepatocytes exposed to iAs³⁺ and MAs³⁺. This study provides novel information about the mechanisms by which iAs exposure impairs glucose homeostasis, pointing to hepatic metabolism of glycogen as one of the targets.

Effects of arsenic on adipocyte metabolism: Is arsenic an obesogen?

The environmental obesogen model proposes that in addition to a high-calorie diet and diminished physical activity, other factors such as environmental pollutants and chemicals are involved in the development of obesity. Although arsenic has been recognized as a risk factor for Type 2 Diabetes with a specific mechanism, it is still uncertain whether arsenic is also an obesogen. The impairment of white adipose tissue (WAT) metabolism is crucial in the onset of obesity, and distinct studies have evaluated the effects of arsenic on it, however only in some of them for obesity-related purposes. Thus, the known effects of arsenic on WAT/adipocytes were integrated based on the diverse metabolic and physiological processes that occur in WAT and are altered in obesity, specifically: adipocyte growth, adipokine secretion, lipid metabolism, and glucose metabolism. The currently available information suggests that arsenic can negatively affect WAT metabolism, resulting in arsenic being a potential obesogen.

Genetic and epigenetic mechanisms underlying arsenic-associated diabetes mellitus: a perspective of the current evidence

Chronic exposure to arsenic has been associated with the development of diabetes mellitus (DM), a disease characterized by hyperglycemia resulting from dysregulation of glucose homeostasis. This review summarizes four major mechanisms by which arsenic induces diabetes, namely inhibition of insulin-dependent glucose uptake, pancreatic β-cell damage, pancreatic β-cell dysfunction and stimulation of liver gluconeogenesis that are supported by both in vivo and in vitro studies. Additionally, the role of polymorphic variants associated with arsenic toxicity and disease susceptibility, as well as epigenetic modifications associated with arsenic exposure, are considered in the context of arsenic-associated DM. Taken together, in vitro, in vivo and human genetic/epigenetic studies support that arsenic has the potential to induce DM phenotypes and impair key pathways involved in the regulation of glucose homeostasis.

Embryonic-only arsenic exposure in killifish (Fundulus heteroclitus) reduces growth and alters muscle IGF levels one year later

Arsenic is a contaminant of drinking water and crops in many parts of the world. Epidemiological studies have shown that arsenic exposure is linked to decreased birth weight, weight gain, and proper skeletal muscle function. The goal of this study was to use killifish (Fundulus heteroclitus) as a model to determine the long-term effects of embryonic-only arsenic exposure on muscle growth and the insulin-like growth factor (IGF) pathway. Killifish embryos were exposed to 0, 50, 200 or 800ppb As^III from fertilization until hatching. Juvenile fish were reared in clean water and muscle samples were collected at 16, 28, 40 and 52 weeks of age. There were significant reductions in condition factors, ranging from 12 to 17%, in the fish exposed to arsenic at 16, 28 and 40 weeks of age. However, by 52 weeks, no significant changes in condition factors were seen. Alterations in IGF-1R and IGF-1 levels were assessed as a potential mechanism by which growth was reduced. While there no changes in hepatic IGF-1 transcripts, skeletal muscle cells can also produce their own IGF-1 and/or alter IGF-1 receptor levels to help enhance growth. After a 200 and 800ppb embryonic exposure, fish grown in clean water for 16 weeks had IGF-1R transcripts that were 2.8-fold and 2-fold greater, respectively, than unexposed fish. Through 40 weeks of age, IGF1-R remained elevated in the 200ppb and 800ppb embryonic exposure groups by 1.8-3.9-fold, while at 52 weeks of age, IGF-1R levels were still significantly increased in the 800ppb exposure group. Skeletal muscle IGF-1 transcripts were also significantly increased by 1.9-5.1 fold through the 52 weeks of grow-out in clean by water in the 800ppb embryonic exposure group. Based on these results, embryonic arsenic exposure has long-term effects in that it reduces growth and increases both IGF-1 and IGF-1R levels in skeletal muscle even 1year after the exposure has ended.

Zebrafish: A Model for the Study of Toxicants Affecting Muscle Development and Function

The rapid progress in medicine, agriculture, and allied sciences has enabled the development of a large amount of potentially useful bioactive compounds, such as drugs and pesticides. However, there is another side of this phenomenon, which includes side effects and environmental pollution. To avoid or minimize the uncontrollable consequences of using the newly developed compounds, researchers seek a quick and effective means of their evaluation. In achieving this goal, the zebrafish (Danio rerio) has proven to be a highly useful tool, mostly because of its fast growth and development, as well as the ability to absorb the molecules diluted in water through its skin and gills. In this review, we focus on the reports concerning the application of zebrafish as a model for assessing the impact of toxicants on skeletal muscles, which share many structural and functional similarities among vertebrates, including zebrafish and humans.

Photobiomodulation Protects and Promotes Differentiation of C2C12 Myoblast Cells Exposed to Snake Venom

BACKGROUND

Snakebites is a neglected disease and in Brazil is considered a serious health problem, with the majority of the snakebites caused by the genus Bothrops. Antivenom therapy and other first-aid treatments do not reverse local myonecrose which is the main sequel caused by the envenomation. Several studies have shown the effectiveness of low level laser (LLL) therapy in reducing local myonecrosis induced by Bothropic venoms, however the mechanism involved in this effect is unknown. In this in vitro study, we aimed to analyze the effect of LLL irradiation against cytotoxicity induced by Bothrops jararacussu venom on myoblast C2C12 cells.

METHODOLOGY

C2C12 were utilized as a model target and were incubated with B. jararacussu venom (12.5 μg/mL) and immediately irradiated with LLL at wavelength of red 685 nm or infrared 830 nm with energy density of 2.0, 4.6 and 7.0 J/cm2. Effects of LLL on cellular responses of venom-induced cytotoxicity were examined, including cell viability, measurement of cell damage and intra and extracellular ATP levels, expression of myogenic regulatory factors, as well as cellular differentiation.

RESULTS

In non-irradiated cells, the venom caused a decrease in cell viability and a massive release of LDH and CK levels indicating myonecrosis. Infrared and red laser at all energy densities were able to considerably decrease venom-induced cytotoxicity. Laser irradiation induced myoblasts to differentiate into myotubes and this effect was accompanied by up regulation of MyoD and specially myogenin. Moreover, LLL was able to reduce the extracellular while increased the intracellular ATP content after venom exposure. In addition, no difference in the intensity of cytotoxicity was shown by non-irradiated and irradiated venom.

CONCLUSION

LLL irradiation caused a protective effect on C2C12 cells against the cytotoxicity caused by B. jararacussu venom and promotes differentiation of these cells by up regulation of myogenic factors. A modulatory effect of ATP synthesis may be suggested as a possible mechanism mediating cytoprotection observed under laser irradiation.

Arsenic Promotes NF-Κb-Mediated Fibroblast Dysfunction and Matrix Remodeling to Impair Muscle Stem Cell Function

Arsenic is a global health hazard that impacts over 140 million individuals worldwide. Epidemiological studies reveal prominent muscle dysfunction and mobility declines following arsenic exposure; yet, mechanisms underlying such declines are unknown. The objective of this study was to test the novel hypothesis that arsenic drives a maladaptive fibroblast phenotype to promote pathogenic myomatrix remodeling and compromise the muscle stem (satellite) cell (MuSC) niche. Mice were exposed to environmentally relevant levels of arsenic in drinking water before receiving a local muscle injury. Arsenic-exposed muscles displayed pathogenic matrix remodeling, defective myofiber regeneration and impaired functional recovery, relative to controls. When naïve human MuSCs were seeded onto three-dimensional decellularized muscle constructs derived from arsenic-exposed muscles, cells displayed an increased fibrogenic conversion and decreased myogenicity, compared with cells seeded onto control constructs. Consistent with myomatrix alterations, fibroblasts isolated from arsenic-exposed muscle displayed sustained expression of matrix remodeling genes, the majority of which were mediated by NF-κB. Inhibition of NF-κB during arsenic exposure preserved normal myofiber structure and functional recovery after injury, suggesting that NF-κB signaling serves as an important mechanism of action for the deleterious effects of arsenic on tissue healing. Taken together, the results from this study implicate myomatrix biophysical and/or biochemical characteristics as culprits in arsenic-induced MuSC dysfunction and impaired muscle regeneration. It is anticipated that these findings may aid in the development of strategies to prevent or revert the effects of arsenic on tissue healing and, more broadly, provide insight into the influence of the native myomatrix on stem cell behavior.

Arsenic inhibits stem cell differentiation by altering the interplay between the Wnt3a and Notch signaling pathways

data indicates that arsenic exposure inhibits stem cell differentiation. This study investigated whether arsenic disrupted the Wnt3a signaling pathway, critical in the formation of myotubes and neurons, during the differentiation in P19 mouse embryonic stem cells. Cells were exposed to 0, 0.1, or 0.5 μM arsenite, with or without exogenous Wnt3a, for up to 9 days of differentiation. Arsenic exposure alone inhibits the differentiation of stem cells into neurons and skeletal myotubes, and reduces the expression of both β-catenin and GSK3β mRNA to ~55% of control levels. Co-culture of the arsenic-exposed cells with exogenous Wnt3a rescues the morphological phenotype, but does not alter transcript, protein, or phosphorylation status of GSK3β or β-catenin. However, arsenic exposure maintains high levels of Hes5 and decreases the expression of MASH1 by 2.2-fold, which are anti- and pro-myogenic and neurogenic genes, respectively, in the Notch signaling pathway. While rescue with exogenous Wnt3a reduced Hes5 levels, MASH1 levels stay repressed. Thus, while Wnt3a can partially rescue the inhibition of differentiation from arsenic, it does so by also modulating Notch target genes rather than only working through the canonical Wnt signaling pathway. These results indicate that arsenic alters the interplay between multiple signaling pathways, leading to reduced stem cell differentiation.

Arsenic trioxide alters the differentiation of mouse embryonic stem cell into cardiomyocytes

Chronic arsenic exposure is associated with increased morbidity and mortality for cardiovascular diseases. Arsenic increases myocardial infarction mortality in young adulthood, suggesting that exposure during foetal life correlates with cardiac alterations emerging later. Here, we investigated the mechanisms of arsenic trioxide (ATO) cardiomyocytes disruption during their differentiation from mouse embryonic stem cells. Throughout 15 days of differentiation in the presence of ATO (0.1, 0.5, 1.0 μM) we analysed: the expression of i) marker genes of mesoderm (day 4), myofibrillogenic commitment (day 7) and post-natal-like cardiomyocytes (day 15); ii) sarcomeric proteins and their organisation; iii) Connexin 43 and iv) the kinematics contractile properties of syncytia. The higher the dose used, the earlier the stage of differentiation affected (mesoderm commitment, 1.0 μM). At 0.5 or 1.0 μM the expression of cardiomyocyte marker genes is altered. Even at 0.1 μM, ATO leads to reduction and skewed ratio of sarcomeric proteins and to a rarefied distribution of Connexin 43 cardiac junctions. These alterations contribute to the dysruption of the sarcomere and syncytium organisation and to the impairment of kinematic parameters of cardiomyocyte function. This study contributes insights into the mechanistic comprehension of cardiac diseases caused by in utero arsenic exposure.

Low-Concentration Arsenic Trioxide Inhibits Skeletal Myoblast Cell Proliferation via a Reactive Oxygen Species-Independent Pathway

Myoblast proliferation and differentiation are essential for skeletal muscle regeneration. Myoblast proliferation is a critical step in the growth and maintenance of skeletal muscle. The precise action of inorganic arsenic on myoblast growth has not been investigated. Here, we investigated the in vitro effect of inorganic arsenic trioxide (As₂O₃) on the growth of C2C12 myoblasts. As₂O₃ decreased myoblast growth at submicromolar concentrations (0.25–1 μM) after 72 h of treatment. Submicromolar concentrations of As₂O₃ did not induce the myoblast apoptosis. Low-concentration As₂O₃ (0.5 and 1 μM) significantly suppressed the myoblast cell proliferative activity, which was accompanied by a small proportion of bromodeoxyuridine (BrdU) incorporation and decreased proliferating cell nuclear antigen (PCNA) protein expression. As₂O₃ (0.5 and 1 μM) increased the intracellular arsenic content but did not affect the reactive oxygen species (ROS) levels in the myoblasts. Cell cycle analysis indicated that low-concentrations of As₂O₃ inhibited cell proliferation via cell cycle arrest in the G1 and G2/M phases. As₂O₃ also decreased the protein expressions of cyclin D1, cyclin E, cyclin B1, cyclin-dependent kinase (CDK) 2, and CDK4, but did not affect the protein expressions of p21 and p27. Furthermore, As₂O₃ inhibited the phosphorylation of Akt. Insulin-like growth factor-1 significantly reversed the inhibitory effect of As₂O₃ on Akt phosphorylation and cell proliferation in the myoblasts. These results suggest that submicromolar concentrations of As₂O₃ alter cell cycle progression and reduce myoblast proliferation, at least in part, through a ROS-independent Akt inhibition pathway.

Arsenic Induces Insulin Resistance in Mouse Adipocytes and Myotubes Via Oxidative Stress-Regulated Mitochondrial Sirt3-FOXO3a Signaling Pathway

Chronic exposure to arsenic via drinking water is associated with an increased risk for development of type 2 diabetes mellitus (T2DM). This study investigates the role of mitochondrial oxidative stress protein Sirtuin 3 (Sirt3) and its targeting proteins in chronic arsenic-induced T2DM in mouse adipocytes and myotubes. The results show that chronic arsenic exposure significantly decreased insulin-stimulated glucose uptake (ISGU) in correlation with reduced expression of insulin-regulated glucose transporter type 4 (Glut4). Expression of Sirt3, a mitochondrial deacetylase, was dramatically decreased along with its associated transcription factor, forkhead box O3 (FOXO3a) upon arsenic exposure. A decrease in mitochondrial membrane potential (Δψm) was observed in both 3T3L1 adipocytes and C2C12 myotubes treated by arsenic. Reduced FOXO3a activity by arsenic exhibited a decreased binding affinity to the promoters of both manganese superoxide dismutase (MnSOD) and peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1α, a broad and powerful regulator of reactive oxygen species (ROS) metabolism. Forced expression of Sirt3 or MnSOD in mouse myotubes elevated Δψm and restored ISGU inhibited by arsenic exposure. Our results suggest that Sirt3/FOXO3a/MnSOD signaling plays a significant role in the inhibition of ISGU induced by chronic arsenic exposure.

Arsenic induces sustained impairment of skeletal muscle and muscle progenitor cell ultrastructure and bioenergetics

Over 4 million individuals in the United States, and over 140 million individuals worldwide, are exposed daily to arsenic-contaminated drinking water. Human exposures can range from below the current limit of 10 μg/L to over 1mg/L, with 100 μg/L promoting disease in a large portion of those exposed. Although increased attention has recently been paid to myopathy following arsenic exposure, the pathogenic mechanisms underlying clinical symptoms remain poorly understood. This study tested the hypothesis that arsenic induces lasting muscle mitochondrial dysfunction and impairs metabolism. Compared to nonexposed controls, mice exposed to drinking water containing 100 μg/L arsenite for 5 weeks demonstrated impaired muscle function, mitochondrial myopathy, and altered oxygen consumption that were concomitant with increased mitochondrial fusion gene transcription. There were no differences in the levels of inorganic arsenic or its monomethyl and dimethyl metabolites between controls and exposed muscles, confirming that arsenic does not accumulate in muscle. Nevertheless, muscle progenitor cells isolated from exposed mice recapitulated the aberrant myofiber phenotype and were more resistant to oxidative stress, generated more reactive oxygen species, and displayed autophagic mitochondrial morphology, compared to cells isolated from nonexposed mice. These pathological changes from a possible maladaptive oxidative stress response provide insight into declines in muscle functioning caused by exposure to this common environmental contaminant.

Effects of arsenic on osteoblast differentiation in vitro and on bone mineral density and microstructure in rats

BACKGROUND

Arsenic is a ubiquitous toxic element and is known to contaminate drinking water in many countries. Several epidemiological studies have shown that arsenic exposure augments the risk of bone disorders. However, the detailed effect and mechanism of inorganic arsenic on osteoblast differentiation of bone marrow stromal cells and bone loss still remain unclear.

OBJECTIVES

We investigated the effects and mechanism of arsenic on osteoblast differentiation in vitro and evaluated bone mineral density (BMD) and bone microstructure in rats at doses relevant to human exposure from drinking water.

METHODS

We used a cell model of rat primary bone marrow stromal cells (BMSCs) and a rat model of long-term exposure with arsenic-contaminated drinking water, and determined bone microstructure and BMD in rats by microcomputed tomography (μCT).

RESULTS

We observed significant attenuation of osteoblast differentiation after exposure of BMSCs to arsenic trioxide (0.5 or 1 μM). After arsenic treatment during differentiation, expression of runt-related transcription factor-2 (Runx2), bone morphogenetic protein-2 (BMP-2), and osteocalcin in BMSCs was inhibited and phosphorylation of enhanced extracellular signal-regulated kinase (ERK) was increased. These altered differentiation-related molecules could be reversed by the ERK inhibitor PD98059. Exposure of rats to arsenic trioxide (0.05 or 0.5 ppm) in drinking water for 12 weeks altered BMD and microstructure, decreased Runx2 expression, and increased ERK phosphorylation in bones. In BMSCs isolated from arsenic-treated rats, osteoblast differentiation was inhibited.

CONCLUSIONS

Our results suggest that arsenic is capable of inhibiting osteoblast differentiation of BMSCs via an ERK-dependent signaling pathway and thus increasing bone loss.

Environmental endocrine disruption of energy metabolism and cardiovascular risk

Rates of metabolic diseases have increased at an astounding rate in recent decades. Even though poor diet and physical inactivity are central drivers, these lifestyle changes alone fail to fully account for the magnitude and rapidity of the epidemic. Thus, attention has turned to identifying novel risk factors, including the contribution of environmental endocrine disrupting chemicals. Epidemiologic and preclinical data support a role for various contaminants in the pathogenesis of diabetes. In addition to the vascular risk associated with dysglycemia, emerging evidence implicates multiple pollutants in the pathogenesis of atherosclerosis and cardiovascular disease. Reviewed herein are studies linking endocrine disruptors to these key diseases that drive significant individual and societal morbidity and mortality. Identifying chemicals associated with metabolic and cardiovascular disease as well as their mechanisms of action is critical for developing novel treatment strategies and public policy to mitigate the impact of these diseases on human health.

Di-(2-ethylhexyl)-phthalate reduces MyoD and myogenin expression and inhibits myogenic differentiation in C2C12 cells

The purpose of this study was to investigate the effects of di-(2-ethylhexyl) phthalate (DEHP) treatment on MyoD and myogenin expression and myotube formation in the murine C2C12 cells. Myogenic differentiation is principally regulated by activities of myogenic regulatory factors, such as MyoD and myogenin, leading the elongation and fusion of mononucleated myoblasts into multinucleated myotubes. In the present study, myogenic differentiation of C2C12 cells was induced by serum deprivation with medium containing vehicle or DEHP (10, 100, 1,000 μg/ml) for 5 days. Using 3-(4,5-dimethylthiazol-2-yl) 2,5-diphenyltetrazolium bromide (MTT) assay clearly demonstrated cell viability was not affected by DEHP at any given dose. At the dose of 1,000 μg/ml DEHP, the elongation of multinucleated myotubes, and the percent of nuclei incorporated into myosin heavy chain (MyHC)-stained myotubes were markedly reduced. In addition, immunoblotting revealed expression of muscle specific marker MyHC, as well as myogenic regulatory factors MyoD and myogenin, were reduced in DEHP-treated myotubes during myogenic differentiation. Taken together, the results showed that DEHP may impair myogenic differentiation through repression of myogenic regulatory factors, such as MyoD and myogenin, resulting in a reduction of MyHC expression. This in vitro study suggests that DEHP may be an environmental risk factor for myogenesis.

The hijacking of cellular signaling and the diabetes epidemic: mechanisms of environmental disruption of insulin action and glucose homeostasis

The burgeoning epidemic of metabolic disease causes significant societal and individual morbidity and threatens the stability of health care systems around the globe. Efforts to understand the factors that contribute to metabolic derangements are critical for reversing these troubling trends. While excess caloric consumption and physical inactivity superimposed on a susceptible genetic background are central drivers of this crisis, these factors alone fail to fully account for the magnitude and rapidity with which metabolic diseases have increased in prevalence worldwide. Recent epidemiological evidence implicates endocrine disrupting chemicals in the pathogenesis of metabolic diseases. These compounds represent a diverse array of chemicals to which humans are exposed via multiple routes in adulthood and during development. Furthermore, a growing ensemble of animal- and cell-based studies provides preclinical evidence supporting the hypothesis that environmental contaminants contribute to the development of metabolic diseases, including diabetes. Herein are reviewed studies linking specific endocrine disruptors to impairments in glucose homeostasis as well as tying these compounds to disturbances in insulin secretion and impairments in insulin signal transduction. While the data remains somewhat incomplete, the current body of evidence supports the hypothesis that our chemically polluted environment may play a contributing role in the current metabolic crisis.

Low-dose benzo(a)pyrene and its epoxide metabolite inhibit myogenic differentiation in human skeletal muscle-derived progenitor cells

The risk of low birth weights is elevated in prenatal exposure to polycyclic aromatic hydrocarbons (PAHs), which are ubiquitous environmental pollutants generated from combustion of organic compounds, including cigarette smoke. We hypothesized that benzo(a)pyrene (BaP), a member of PAHs existing in cigarette smoke, may affect the myogenesis to cause low birth weights. We investigated the effects of BaP and its main metabolite, benzo(a)pyrene-7,8-dihydrodiol-9,10-epoxide (BPDE), on the myogenic differentiation of human skeletal muscle-derived progenitor cells (HSMPCs). HSMPCs were isolated by a modified preplate technique and cultured in myogenic differentiation media with or without BaP and BPDE (0.25 and 0.5 μM) for 4 days. The multinucleated myotube formation was morphologically analyzed by hematoxylin and eosin staining. The expressions of myogenic differentiation markers and related signaling proteins were determined by Western blotting. Both BaP and BPDE at the submicromolar concentrations (0.25 and 0.5 μM) dose-dependently repressed HSMPCs myogenic differentiation without obvious cell toxicity. Both BaP and BPDE inhibited the muscle-specific protein expressions (myogenin and myosin heavy chain) and phosphorylation of Akt (a known modulator in myogenesis), which could be significantly reversed by the inhibitors for aryl hydrocarbon receptor (AhR), estrogen receptor (ER), and nuclear factor (NF)-κB. BaP- and BPDE-activated NF-κB-p65 protein phosphorylation could also be attenuated by both AhR and ER inhibitors. The inhibitory effects of BaP and BPDE on myogenesis were reversed after withdrawing BaP exposure, but not after BPDE withdrawal. These results suggest that both BaP and BPDE are capable of inhibiting myogenesis via an AhR- or/and ER-regulated NF-κB/Akt signaling pathway.

Embryonic arsenic exposure reduces the number of muscle fibers in killifish (Fundulus heteroclitus)

Arsenic is a contaminant of drinking water and has been correlated with adverse developmental outcomes such as low birth weight, reduced weight gain, and altered locomotor activity. Previous research has shown that killifish (Fundulus heteroclitus) exposed to high arsenic levels during embryogenesis had smaller muscle fiber diameters. The current study was designed to determine whether changes in muscle fibers persisted, were exacerbated, or resolved over time. Killifish embryos were exposed to 0-5 ppm arsenite and, upon hatching, were transferred into either clean water or continued receiving the same exposure to arsenic for up to 16 weeks. Arsenic significantly decreased the weight of both embryonic-only exposed juveniles and continuously exposed juveniles between 4 and 16 weeks of development at concentrations as low as 0.8 ppm. Although arsenite exposure increased the percentage of small diameter fibers during the early weeks, fiber diameters returned to control levels in the embryonic-only exposed fish. However, muscle fiber density was still reduced to 85.7%, 80.3%, and 73.8% of control for the 0.8, 2, and 5 ppm embryonic-only exposure groups, respectively, even after 16 weeks of development. These results indicate that while continuous exposure to arsenic may alter the size of muscle fibers, embryonic-only exposure to arsenic has lasting effects on the number of muscle fibers formed.

Role of pericytes in skeletal muscle regeneration and fat accumulation

Stem cells ensure tissue regeneration, while overgrowth of adipogenic cells may compromise organ recovery and impair function. In myopathies and muscle atrophy associated with aging, fat accumulation increases dysfunction, and after chronic injury, the process of fatty degeneration, in which muscle is replaced by white adipocytes, further compromises tissue function and environment. Some studies suggest that pericytes may contribute to muscle regeneration as well as fat formation. This work reports the presence of two pericyte subpopulations in the skeletal muscle and characterizes their specific roles. Skeletal muscle from Nestin-GFP/NG2-DsRed mice show two types of pericytes, Nestin-GFP-/NG2-DsRed+ (type-1) and Nestin-GFP+/NG2-DsRed+ (type-2), in close proximity to endothelial cells. We also found that both Nestin-GFP-/NG2-DsRed+ and Nestin-GFP+/NG2-DsRed+ cells colocalize with staining of two pericyte markers, PDGFRβ and CD146, but only type-1 pericyte express the adipogenic progenitor marker PDGFRα. Type-2 pericytes participate in muscle regeneration, while type-1 contribute to fat accumulation. Transplantation studies indicate that type-1 pericytes do not form muscle in vivo, but contribute to fat deposition in the skeletal muscle, while type-2 pericytes contribute only to the new muscle formation after injury, but not to the fat accumulation. Our results suggest that type-1 and type-2 pericytes contribute to successful muscle regeneration which results from a balance of myogenic and nonmyogenic cells activation.