Psychosocial stress, particularly chronic stress, is a nontraditional risk factor for atherosclerosis in humans [17, 18]. Experimental studies have also demonstrated that chronic stress accelerates atherosclerosis [19, 20], and occlusal disharmony induces psychological stress [21–24]. However, the mechanisms by which occlusal disharmony accelerates the initiation of atherosclerosis are not clearly understood. To the best of our knowledge, this is the first study to assess the causal relationship between occlusal disharmony and initiation of atherosclerosis in the rat descending aorta. In this study, the occlusal disharmony group showed higher plasma levels of corticosterone than the control group. Furthermore, in the occlusal disharmony group, the percentages of total aortic lumen area occupied by plaques and lipid deposition were significantly higher than those in the control group. These findings suggest that psychological stress induced by occlusal disharmony accelerated atherosclerosis.
TLR4 plays an important role in the initiation of atherosclerosis . Increased expression of TLR4 within lipid-rich atherosclerotic plaques in both human and animal models has been reported [33, 34]. While TLR4 was originally described as a pattern receptor that recognizes lipopolysaccharide, endogenous ligands, such as ox-LDL, fibronectin and heat shock protein, are known to be TLR4 activators . Modifications of LDL, icluding ceramide-enriched LDL can induce TLR4 expression . In this study, serum levels of LDL and aortic TLR4 gene expression in the occlusal disharmony were significantly higher than those in the control group. TLR4 expression following occlusal disharmony may be up-regulated by LDL.
VCAM1 and ICAM1 are endothelial adhesion molecules of the Ig gene superfamily that may participate in atherogenesis by promoting monocyte accumulation in the arterial intima . Although expression of both VCAM-1 and ICAM-1 is up-regulated in atherosclerotic lesions, VCAM1, but not ICAM1, plays a dominant role in the initiation of atherosclerosis . On the other hand, LDL induces expression of VCAM1 in human vascular endothelial cells . In this study, the protein and gene expression of VCAM1 in the aorta, the area of lipid deposition, and the serum levels for LDL were significantly higher in the occlusal disharmony group when compared to the control group. These findings suggest that LDL induced by occlusal disharmony induces VCAM1 expression and accelerates the initiation of atherosclerosis.
Although it remains unclear how corticosterone induced by occlusal disharmony contributes to up-regulation of total cholesterol or LDL in this study, there may be a possible relationship between corticosterone and regulation of lipid and lipoprotein metabolism. Exposure to low levels of corticosterone for long terms significantly exacerbates atherosclerosis in ApoE knockout mice . Synthetic glucocorticoid treatment increases the expression of the enzyme 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMG-CoA reductase) . HMG-CoA reductase is a key regulatory enzyme in the conversion of HMG-CoA to mevalonic acid, which is a precursor for cholesterol synthesis . Competitive inhibitors of HMG-CoA reductase increase the expression of LDL receptors in the liver, which decreases serum total cholesterol and LDL concentrations . Taken together, these findings suggest that corticosterone exacerbates atherosclerosis by primarily up-regulating circulating levels of total cholesterol and LDL .
There were no significant differences in serum levels of ox-LDL and ROM between the control and occlusal disharmony groups. An increasing number of studies have demonstrated that oxidative stress plays a pivotal role in the pathogenesis of atherosclerosis [17, 44]. ROS peroxidizes lipid components, leading to the formation of ox-LDL, which plays a role in the development and progression of atherosclerosis and its complications . Ox-LDL is formed by oxidative stress and leads to endothelial activation and injury resulting in an inflammatory response that induces recruitment, activation and migration of monocytes through inter-endothelial gaps to the sub-endothelial region . These observations have been confirmed in stress models, in which restraint stress up-regulates lectin-like ox-LDL receptor-1 via formation of ROS in the aorta of apolipoprotein E-deficient mice . However, our findings differed from the results of a previous study. The reasons for this discrepancy are not clear, but may depend on the type of psychological stress (restraint stress vs. occlusal disharmony) and species (mouse vs. rat).
A chronic inflammatory response contributes to atherogenesis and plays a critical role in the initiation of this process . CRP is a marker of inflammation and is a strong marker for cardiovascular morbidity . In this study, there were no significant differences in serum levels of CRP between the control and occlusal disharmony groups. However, in a previous study, chronic unpredictable stress accelerated atherosclerosis by increasing serum levels of CRP in apolipoprotein E knockout mice . This chronic unpredictable stress included heat stimulation, cage tilting, wet bedding, lights on overnight, tail pinch, high-speed agitation, cold stimulation, overhang, water deprivation and food deprivation. These conditions significantly reduced body weight and induced 2.5-fold higher levels of aortic VCAM1 expression . The psychological stress induced by occlusal disharmony in our study is milder than the unpredictable stress in the previous study. In fact, there was no body weight loss in our stress model.
Psychosocial stress is one of the risk factors for atherosclerosis in humans [17, 18]. Although occlusal disharmony induces psychological stress in animal [22–24] and in human  models, the relationship between psychological stress from occlusal disharmony and atherosclerosis is not clear in humans. Recently, cross-sectional epidemiological studies have reported on the relationship between tooth loss and atherosclerosis [49, 50]. Number of teeth was related to atherosclerotic plaque in the carotid arteries in an elderly Swedish population . A positive relationship between increased tooth loss in the posterior region and accumulation of arterial atheroma has been shown in a Korean population . These results suggest that occlusal disharmony from tooth loss induces psychological stress and is a risk factor for atherosclerosis in humans. Thus, improvement of occlusal disharmony, such as through prosthodontic therapy, may contribute to the prevention and treatment of atherosclerosis. However, further studies are necessary to clarify the relationship between improved occlusal disharmony and atherosclerosis.
Our study has several limitations. In this model, only the initial stage of atherosclerosis was observed, rather than advanced lesions. Although our occlusal disharmony model that induces psychological stress was established for an 8-week experimental period , longer-term follow-up may provide additional information on the development of atherosclerosis. Furthermore, there is no data whether atherosclerosis is diminished by suppressing the plasma corticosterone level in our occlusal disharmony rats.
In conclusion, in the apoE knockout rats, occlusal disharmony may induce VCAM1, ICAM1 and TLR4 expression and accelerate the initiation of atherosclerosis.