Lipoic acid increases the expression of genes involved in bone formation in mice fed a high-fat diet
Introduction
Low bone mineral density (BMD) is closely associated with high-fat diet (HFD) in humans and animals [1], [2], [3], [4]. High-fat diets are pervasive in North America, Europe, and China [4], [5], [6]. In addition, oxidative stress caused by the overproduction of reactive oxygen species (ROS) has been associated with obesity and hyperlipidemia [7], [8]. Interestingly, oxidative damage has also been recognized to play a key role in abnormal bone metabolic processes [9], [10], [11]. It has previously been shown in vitro and in vivo that free radicals are involved in osteoclastogenesis and bone resorption [11], and that excessive hydrogen peroxide production inhibits osteoblastic differentiation [12]. In addition, other risk factors for osteoporosis, such as smoking and hypertension, are also associated with increased oxidative damage and free radical levels [13]. Our previous study suggested that oxidative stress in bone could be an important mediator of hyperlipidemia-induced bone loss [14].
Experimental studies have demonstrated that antioxidants prevent, or at least attenuate, oxidative damage throughout the body [7], [15]. α-Lipoic acid (LA), also known as 1,2-dithiolane-3-pentanoic acid or thioctic acid (Fig. 1), is found in animal tissues, such as the liver and heart, and plant sources, such as spinach, broccoli, and tomatoes [16]. Lipoic acid is recognized as a universal antioxidant that not only scavenges free radicals directly but also provides the reducing medium for the regeneration of the antioxidant from its oxidized form [17], [18], [19].
In in vitro osteoclast cultures, LA potently suppressed osteoclastogenesis induced by either a high dose of receptor activator of NF-κB ligand (RANKL) or a low dose of RANKL alongside tumor necrosis factor (TNF)–α. The suppression occurs through inhibitory NF-κB activation and is associated with a reduction in ROS generation [20], [21]. Furthermore, we previously found that LA is effective at attenuating bone oxidative stress and abnormal bone metabolism in HFD-fed mice [14]. However, these studies did not address the global molecular mechanisms by which LA ameliorates HFD-induced low bone mass.
Although the mechanism by which oxidative stress has deleterious effects on cellular function, such as gene transcription and signal transduction, is not clear, some mechanisms have been proposed. Reactive oxygen species that are recognized as secondary messengers can influence gene expression [22], [23]. On the other hand, a group of nuclear transcription factors is regulated by redox states; and excessive ROS cause redox imbalance to activate or inactivate these molecules [24]. The HFD-induced oxidative stress within bone also affects the expression of genes involved in bone metabolism, resulting in imbalance involving insufficient bone formation and overresorption; and LA supplementation could prevent the oxidative stress and maintain the redox balance within bone through scavenging ROS [14]. Lipoic acid supplementation may regulate expression of genes engaged in bone metabolism and related transcription factors in bones of mice fed HFD by modulating the ROS level. Furthermore, it is well known that oxidative stress inhibits bone formation but enhances bone resorption [9], [11]. Therefore, we hypothesized that the expression of genes associated with bone formation may be upregulated and bone resorption may be downregulated in mice fed HFD supplemented with LA, preventing a shift in the bone metabolism balance toward resorption. To test this hypothesis, the cDNA microarray technology, which provides a broad view of gene expression, was used to identify bone metabolism–related genes differentially expressed between the HFD-fed mice and the supplemental LA-fed mice.
In this study, C57BL/6 mice were used as a model for HFD-induced low bone because the lipid metabolism and skeletal structure changes in these animals are similar to those that occur in humans [25], [26]. In addition, it has been found that HFD induces dyslipidemia, oxidative damage, and decreased bone mass in growing C57BL/6 mice [2], [14].
Section snippets
Animals
Male C57BL/6 mice (4 weeks old) were purchased from Shanghai Slac Laboratory animal Co Ltd (Shanghai, China). The animals were housed under conditions of controlled temperature (23°C ± 2°C) and humidity (60%) with a 12-hour light/12-hour dark cycle. The experimental protocol was developed according to the national institution's guidelines for the care and use of laboratory animals. All mice studies were approved by the Jiangan University Animal Care and Use Committee.
Experimental design and diets
All mice were fed a normal
Effects of LA on body weight and bone status during HFD consumption
The food intake and energy intake of mice are presented in Table 2. High-fat diet consumption induced a significant decrease in food intake but a significant increase in energy intake (P < .05) compared with the normal diet. There was no significant difference in food intake and energy intake between the HFD and supplemental LA groups (P > .05).
Body weight, abdominal fat content, femoral bone mass, and femoral maximum load for each group after 12 weeks of feeding are shown in Figure 2. Lipoic
Discussion
Bone is a dynamic organ that undergoes significant turnover. The process includes bone resorption by osteoclasts followed by bone formation by osteoblasts [35]. In several previous studies, the HFD-induced imbalance involving insufficient bone formation and overresorption was reflected by low bone mass [2], [18]. Our microarray data showed that HFD supplemented with LA led to the upregulation of genes engaged in bone formation, such as Col1a1, osteocalcin, and Fos, and downregulation of genes
Acknowledgment
This work was supported by the 111 project-PCSIRT0627 of the Minister of Education of the PR China. The authors thank Xiao-Jia Pu (Nuclear Medicine Section, Wuxi People's Hospital) for providing technical support and equipment for BMD measurement. We are also thankful to Jie Wang and Ming Xu (Jiangsu Institute of Parasitic Diseases) for providing the ABI Prism 7000 Sequence Detection System.
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