Significant increases in body weight and serum cholesterol levels, together with significant decreases in bone quantity and quality, were found in HFD-induced obese mice. In the trabecular bone of these mice, deterioration of the trabecular bone architecture resulted in an overall decrease in mandibular BV/TV, as determined by micro-CT. Although cortical bone formation was slower in HFD-fed than in control mice, bone formation on the periosteal surface increased with age in both groups. Additionally, bone resorption on the endosteal surface was slightly higher in HFD-fed mice than controls, as seen on micro-CT. The HFD-fed mice at 19 weeks also had a significantly lower in Ct.BD than that of the controls, consistent with a significant increase in the porosity of cortical bone at the end of the experiment.
This study demonstrated a significant increase in serum leptin levels in HFD-fed mice compared with their age-matched controls, although the levels also increased in the latter.
Leptin is known as an important circulating signal that inhibits food intake and enhances energy expenditure through its actions in the brain . However, several studies have shown that a HFD plays key role in the development of leptin resistance in animals [17, 18]. In another study, energy expenditures were lower in mice fed a high-fat versus a low-fat diet, even though intake was similar between two groups . Energy expenditure inhibition due to leptin resistance leads to abnormal accumulation of triglycerides in the liver and other organs, since ingested triglycerides are not being used as an energy source.
We found that serum triglyceride levels tended to be lower and serum HDL cholesterol levels higher in HFD-fed mice than in control mice. These findings are in contrast to those of previous studies [20, 21] but are consistent with those of Graham et al. . The differences may be related to whether serum leptin levels exceed the capacity of the body’s transport system, including the entry of leptin into the cerebrospinal fluid. Additionally, in HFD-fed mice, an increase in serum total cholesterol levels is probably followed by an increase in serum HDL cholesterol levels.
The causes of leptin resistance are unclear, but hyper-nutrition leads to endoplasmic reticulum stress, and thus to inflammation, in adipose tissue [23–25]. Oxidative stress was also shown to induce leptin resistance in HFD-fed mice . The inflammation in HFD-fed mice is accompanied by increased expression of inflammatory cytokines, such as interleukin (IL)-6, IL-1β, and tumor necrosis factor-α, in adipocytes and macrophages via activation of the c-Jun N-terminal kinase and nuclear factor-κB pathways [24, 26]. Recently, Dib et al. reported that leptin acts as a pro-inflammatory adipocytokine in peripheral tissues . These studies suggested that endoplasmic reticulum stress induces inflammation by mediating leptin signals in the adipose tissue of HFD-fed mice.
Other studies have shown a selective increase in the production of reactive oxygen species (ROS) in the adipose tissues of obese mice. These ROS cause both oxidative and endoplasmic reticulum stress, including inflammatory changes in adipose tissue during the course of adipocyte hypertrophy . Moreover, ROS and oxidative stress inhibit osteoblastogenesis [29, 30], suggesting that ROS also inhibit periosteal cortical bone formation in growing HFD-fed growing mice.
The increased expression of inflammatory cytokines and leptin elicit osteoclast activity by regulating the RANKL/RANK/OPG pathway, resulting in increased bone resorption [31, 32].
Thus, in this study, the deterioration of bone structure in the HFD-fed mice could have been due to multiple forms of oxidative stress, which induced leptin resistance and increased inflammation in adipose tissue. However, an age-dependent increase in serum leptin levels within the physiological range did not affect gradual bone growth.
We also showed that HFD-induced obesity during growth increases the risk of mandibular bone osteoporosis and spontaneous periodontal disease. A recent report proposed a link between systemic osteoporosis and periodontal bone loss based on significant up-regulation of inflammatory cytokines in bone and in the bone marrow cells of rats with osteoporosis .
In this study, HFD-induced alveolar bone loss may have reflected a state of non-invasive and non-infective inflammation, such as that characteristic of autoimmune disorders. This is in contrast to previous studies that used models of experimental periodontitis [8, 9], as periodontal disease develops as a result of the continuous interaction between host cells and subgingival pathogenic bacteria .
The HFD-induced alveolar bone loss in our mice may have been triggered by bacterial endotoxin [lipopolysaccharide (LPS)], a potential inflammatory mediator in mice with HFD-induced obesity . In a recent study, a HFD increased alveolar bone loss in mice injected with LPS . In general, bacterial endotoxins are present in large quantities in the gut , and clinical studies have reported the development of postprandial endotoxemia following a high-fat meal [4, 38]. These findings suggest that dietary fats promote the translocation of bacterial endotoxins from the gut into the circulation, where they stimulate periodontal inflammation and alveolar bone loss.
Recently, Suganami et al. proposed the concept of “homeostatic inflammation” in the pathogenesis of non-infectious inflammatory diseases . This may account for the HFD-induced alveolar bone loss in our mice, in which systemic inflammatory changes in bone and other tissues may have developed in association with metabolic stress.
In a previous study, rats fed a high-cholesterol diet showed a modest increase in the distance between the cement-enamel junctions and the alveolar bone crest. The authors suggested that osteoclastic function plays a major role in alveolar bone resorption during increased oxidative stress . However, at a gross histological level, there was no evidence of alveolar bone crest resorption in any of the groups, despite significant increases in serum total cholesterol levels in the HFD-fed mice. These results likely reflected cortical bone formation on the periosteal surface during the growth period. By contrast, the PDL fibres in HFD-fed mice were disrupted, with loss of orientation with respect to the bone surface, and the normal narrowing of the PDL space was inhibited. Narrowing of the periodontium narrows with age, as seen in the control mice, is accompanied by increasing acellular cementum formation and alveolar bone formation. The increased vascular permeability due to inflammatory changes in the blood vessels of the periodontium may promote monocyte adhesion to endothelial cells and migration. In addition, osteoclasts differentiated from those monocytes may have then attached to the alveolar bone surface, resulting in increased alveolar bone resorption in the HFD-fed mice.
Together, these findings suggest that the spontaneous deterioration of periodontal bone is a consequence of HFD-induced obesity during growth.
Two limitations to our study must be noted. First, in the histological evaluation, HFD-fed mice had clear alveolar bone resorption and inflammatory structural changes in the PDL, but systemic bone metabolism was not assessed using serum analyses, such as those measuring bone resorption markers and inflammatory cytokines. Second, bone resorption due to insulin resistance was not considered. Further studies are needed to elucidate the mechanisms of inflammatory alveolar bone resorption induced by a HFD.