Aluminum trichloride induces bone impairment through TGF-β1/Smad signaling pathway
Introduction
Aluminum (Al) is well-known environmental contaminant that raises serious health issues all over the world (Bondy, 2014, Gilbert and Avenant-Oldewage, 2016). It is released due to anthropogenic activities such as mining and industrial uses in the production of Al metal and other Al compounds, putting animals and humans in contact with absorbable cationic Al (Sharma et al., 2016). Additionally, ingestion of Al from food is a mainly form of human exposure by which Al applied as food additives (Ribes et al., 2008). The wide distribution, extensive exposure to humans and inherent toxicity of Al is indeed a global public health problem that affects millions of people (Zhang et al., 2014). Increasing amount of evidences reported in recent years has confirmed the fact that Al has severe bone toxic effects. Al accumulation disrupts bone formation, ultimately causes bone disease which defined as “Al-induced bone disease” (e.g., osteoporosis) (Aaseth et al., 2012). In dialyzed patients, as bone Al concentrations increased from 46 ± 7 to 175 ± 22 mg/kg (dry weight), the severity of Al-induced bone disease increased (Hodsman et al., 1982). Exposure to Al leads to bone impairment and the concentration of Al has been found to be increased in rats bone (Sun et al., 2015). But the molecular mechanism of AlCl3-induced bone impairment are still not completely clarified.
One of the major mechanisms behind Al induced toxicity has been attributed to oxidative stress. Oxidative stress occurs when the generation of reactive oxygen species (ROS) in a system exceeds the system's capacity to neutralize and eliminate them. Diverse studies have shown that exposure to Al can induce oxidative stress by stimulating ROS production in cells, altering the activity of diverse antioxidant enzymes including superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), and promoting content of malondialdehyde (MDA) (Dawood et al., 2012, Kumar and Gill, 2014 Kumar and Gill, 2014). Furthermore, our previous research found that aluminum trichloride (AlCl3) induces the oxidative stress of osteoblasts (Li et al., 2012). However, whether there is a similar effect in rats bone need to be explored.
During the last four decades, many experimental animal and cells culture studies, have been shown that imbalance of bone formation and resorption is partially response to the toxic effects of Al on bone. Osteoblasts and osteoclasts are main functional cells for bone formation and bone resorption (Kanehisa and Heersche, 1988, Abdelmagid et al., 2014, Kanazawa et al., 2015). Exposure to Al inhibits osteoblastic bone formation and improves osteoclastic bone resorption, thereby resulting in osteoporosis (Aaseth et al., 2012, Crisponi et al., 2013, Cao et al., 2016, Sun et al., 2016). Bone formation and resorption is regulated by Transforming growth factor β1 (TGF-β1) secreted by osteoblasts (Janssens et al., 2005, Ochiai et al., 2012, Jia et al., 2013). In postmenopausal women, TGF-β1 gene polymorphism is important in increasing bone mineral density (BMD) and reducing bone loss (Hinke et al., 2001). The risk of osteoporosis in native Chinese women increased as circulating TGF-β1 was reduced (Xie et al., 2013). These effects of TGF-β1 are produced by the activation of the Smad-dependent pathway. Briefly, TGF-β1 signals through two transmembrane receptors, TβRI and TβRII, producing the phosphorylation of Smad2 and Smad3, and further allowing the association of these proteins with Smad4. This complex subsequently translocates into the nucleus and regulates downstream genes expression (Chen et al., 2016). Smad7 is an intracellular inhibitor, which negatively controls TGF-β1/Smad signaling pathway by competing with Smad2 and Smad3 for binding to activated TβRI (Arnott et al., 2008). On that basis, our previous research further found that different dose of AlCl3 suppress key biological molecules of TGF-β1/Smad signaling pathway expression in osteoblasts (Sun et al., 2016). However, it remains unknown whether TGF-β1/Smad signaling pathway is involved in the toxic effects of AlCl3 on osteoblasts and bone.
The aim of this study is to evaluate whether TGF-β1/Smad signaling pathway is involved in the toxic effects of AlCl3 on osteoblasts and bone. We found that AlCl3 induced BMD decline, ultrastructural abnormalities, oxidative damage, imbalance of bone formation and resorption, and down-regulation TGF-β1/Smad signaling pathway in bone. Moreover, the inhibitory effect of AlCl3 on TGF-β1/Smad signaling pathway was reversed by exogenous TGF-β1 in rat osteoblasts. Our results suggest that AlCl3 induces bone impairment through inactivation of TGF-β1/Smad signaling pathway. This study can provide a theoretical foundation for revealing the mechanism of bone impairment caused by AlCl3.
Section snippets
Animals and treatments
This experimental protocol was approved by the Ethics Committee on the Use and Care of Animals, Northeast Agricultural University, China. The housing conditions were maintained at a constant temperature (24 ± 1 °C), relative humidity at 55 ± 5%, and in a 12/12-h light/dark cycle. Food (standard diet) and water were available ad libitum. Forty 4-week-old healthy male Sprague-Dawley rats weighed 72–95 g were acclimatized for 1 week then the rats were randomly divided into two groups, each 20 rats. The
AlCl3 decreases rats body weight and increases the contents of Al in femora and serum
As shown in Fig. 1, the body weight of rats was lower in AG than in CG. The contents of Al in femora and serum were significantly higher in AG than in CG (Fig. 2A and B). These results indicate that Al could accumulate in bone and inhibit the growth of rats.
AlCl3 reduces BMD
To investigate the toxic effects of AlCl3 on rats bone, the BMD of the lumbar spines (L4-L6), femoral metaphysis and tibia were measured by dual-energy X-ray absorptiometry. As shown in Fig. 3, the BMD of the lumbar spines (L4-L6), femoral
Disussion
Al is recognized worldwide as serious inorganic contaminants in drinking water and food (Ribes et al., 2008, Djouina et al., 2016). Increasing amount of evidences over the last decade have been shown that exposure to Al is associated with low BMD and an increased risk of osteoporosis (Aaseth et al., 2012, Crisponi et al., 2013). However, the precise molecular mechanisms by which Al exerts its catabolic action on bone are still unclear. Our previous study had confirmed that AlCl3 exposure
Conclusion
In the light of the results obtained, we can conclude that AlCl3 exposure induced bone impairment, presenting as the decreased BMD, abnormal bone ultrastructure, and oxidative damage. Moreover, AlCl3 inhibited TGF-β1/Smad signaling pathway in bone, and administration of exogenous TGF-β1 significantly relieved the inhibitory effects of AlCl3 on this pathway in osteoblasts. These results indicate that AlCl3 induces bone impairment via inactivation of TGF-β1/Smad signaling pathway. The TGF-β1 may
Conflict of interest
The authors declare that there are no conflicts of interest.
Acknowledgments
This study was supported by a grant from the National Natural Science Foundation Project (contract grant number: 31372496 and 31302147)
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2020, Journal of EthnopharmacologyCitation Excerpt :It has been reported that the compromised MAPK signaling pathway contributed to the pathology of neurodegeneration such as AD (Kim and Choi, 2015) as well as inhibiting osteoblasts directly (Xiao et al., 2019). As for TGF-β family members, they played different roles in the skeleton with direct effects on bone impairment (Sun et al., 2016), whereas the activation of neuronal TGF-beta signaling increases neurodegenerative disorders and AD-like disease (Tesseur et al., 2006). Thus, MAPK and TGF-beta signaling pathways may be expected to become shared mechanisms for revealing the pathogenesis of OP and AD or slowing the progressions.
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Both authors contributed equally to this study.