Simvastatin prevents alveolar bone loss in an experimental rat model of periodontitis after ovariectomy

Periodontitis is a destructive disease that targets tooth-supporting structures through complex and multifactorial pathogenic processes [1]. The resorption of the alveolar crest bone initiated by periodontitis can gradually lead to tooth loss and edentulousness, which in turn results in continued alveolar bone loss (ABL) and resorption of the remaining residual ridge. There is increasing evidence that periodontitis is associated with a number of chronic diseases such as osteoporosis [1],[2]. Osteoporosis is a skeletal disorder characterized by compromised bone strength, predisposing the bone to an increased risk of fracture [3]. Dramatic advances in the understanding of bone metabolism has brought us a new prospective that possible pathways by which systemic bone loss, such as osteoporosis, may contribute to more rapid alveolar bone loss in periodontitis [2],[4]. A possible mechanism could relate to a more susceptible environment for bacterial deposit in oral cavity caused by the hormonal changes associated with postmenopausal osteoporosis. Since osteoporosis could increase some inflammation factors, and some of these factors are also participated in the progression of periodontitis [5] [7]. Therefore, the administration of compounds that can prevent bone loss and inhibit inflammatory cytokines has the potential to treat both periodontitis and osteoporosis.

Statins are specific inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme reductase that are traditionally used to inhibit the production of cholesterol in cardiovascular diseases [8] [12]. However, statins have pleiotropic therapeutic effects including vasodilatory, antithrombotic, antioxidant, anti-inflammatory, and immunosuppressive actions [13] [15]. In 1999, Mundy G et al. [16] first reported that statins were potent stimulators of bone formation in vitro. In 2011, Moon HJ et al. [17] proved that simvastatin acted as an osteoclastogenesis inhibitor by suppressing reactive oxygen species-mediated signaling pathways. Therefore, simvastatin has both anti-catabolic and anabolic effect on bone metabolism. Indeed, Vaziri H et al. [18] have evaluated the effect of simvastatin on ligature-induced bone resorption in the mandible of ovariectomized rats. Their results demonstrated the protective effects of local simvastatin administration on a periodontal attachment apparatus and ABL. Based on their research, we aimed to explore the effect of simvastatin on maxillary and compare the effects of local and oral simvastatin administration in a rat model with both periodontitis and osteoporosis.

The worldwide prevalence of periodontitis is between 10% and 15%, and it may reach an astonishing 80% in certain regions [23]. Both periodontitis and osteoporosis are of high prevalence in the elderly. Tezal M et al. [24] found osteoporosis was a risk factor for the loss of periodontal attachment, loss of alveolar crest height, and even loss of tooth. Gomes-Filho IS et al. [25] found that postmenopausal women with osteoporosis were more susceptible to periodontitis. Genco RJ et al. [26] regarded osteoporosis as one of the risk factors for periodontal diseases. Besides, based on our previous animal experiment results (unpublished data), OVX could deteriorate the alveolar bone loss and reduce alveolar crest height of experimental periodontitis in rats. Thus, finding therapeutic agents to alleviate periodontitis, as well as osteoporosis has significant clinical value.

Simvastatin is widely used for cardiovascular diseases. It also has anti-inflammatory, bone-formation promoting, and bone-resorption inhibiting effects. Here, our study further demonstrated the protective effect of simvastatin on an animal model with OVX and experimental periodontitis, suggesting a potential therapeutic benefit of simvastatin for osteoporosis patients with periodontitis. ABL is considered as an indicator of the periodontitis process. In the current study, analysis of the line distance data of CEJ-ABC clearly demonstrated that either local or local combined with oral simvastatin administration had the potential to improve the periodontitis-induced decrease in alveolar crest height. This finding is consistent with previous report, which local simvastatin administration brought a comfortable improvement in treating periodontitis and enhanced bone formation [18],[27]. It is noteworthy that oral simvastatin administration failed to prevent the loss of alveolar crest height. This may be due to the systemic metabolism of simvastatin after oral administration. The concentration of simvastatin in the oral administration group may have not reached a high enough level to stimulate alveolar crest bone formation and inhibit inflammation-induced alveolar crest bone resorption, whereas the local administration could easily reach the effective level. Consistent with this finding, histology observation of the region between M1 and M2 in the maxillae showed that the ligature placement caused periodontium destruction, but local simvastatin administration alleviated this process.

In the present study, micro-CT measurement showed that simvastatin treatment moderately increased the BV/TV in OVX rats with periodontitis. Moreover, the structural analysis of Tb.Th and Tb.Sp by micro-CT indicated that local or oral simvastatin administration could only protect against the OVX-induced deterioration of trabecular bone structure to some extent. In 2008, Lee Y et al. [28] demonstrated that simvastatin could effectively augment bone thickness in the mandible. Our findings confirmed this phenomenon and further indicated that local or oral administration of simvastatin could improve the alveolar bone index in the maxilla. When combining local and oral administration, the effect of simvastatin was even more effective and beneficial. All of the above may explain why simvastatin of local or oral administration was able to reduce the periodontitis-associated ABL.

Bone is a metabolically active tissue regulated by osteoblasts and osteoclasts [22],[29]. Thus, it was not surprising that the levels of the serum bone resorption marker TRAP5b and the serum bone formation marker OCN increased in OVX rats. Furthermore, our results showed that oral simvastatin administration could increase the level of OCN and reduce the level of TRAP5b. This is in agreement with our findings in the TRAP-stained paraffin sections. Hence, these findings demonstrate that simvastatin might function by stimulating osteoblasts and inhibiting osteoclasts. However, the effects of statins on biochemical markers of bone turnover are still being debated [30]. Different experimental models, animal age, or therapy duration might explain the differences. In addition, it is known that microstructural changes of the bone can affect its biomechanical properties. Bending stress, bending strain, and Young s modulus are important intrinsic biomechanical properties to describe the bone state [31]. Our data suggested that oral simvastatin administration could enhance systemic bone biomechanical properties, while local administration could not.

Our study confirmed that simvastatin had the potential of recovering alveolar bone loss following experimental periodontitis, and further demonstrated its role in restoring alveolar bone mass induced by OVX. Besides, we also compared the difference between local and oral administration of simvastatin and showed that local simvastatin administration increased alveolar crest height and prevented local alveolar bone loss, but could not change systemic osteoporotic changes, while oral administration improving local or systemic bone osteoporotic changes and bone mass, enhanced bone biomechanical properties, but had no effect on alveolar crest height. Nevertheless, repeated local injections are not suitable for clinical patients; however, a new application method that utilizes methylcellulose gel is promising [32].

In summary, local and/or oral administration of simvastatin had a positive effect on OVX rats with experimentally induced periodontitis. Whilst the animal study suggests that the combination of local and oral simvastatin administration could be an alternative therapy for periodontitis and osteoporosis, more clinical studies are necessary to explore the effect on these patients with periodontitis and osteoporosis.

XCX carried out the study design, participated in the micro-CT analysis and drafted the manuscript. HC participated in animal surgery, serum biochemical analysis and biomechanical testing. XZ participated in animal surgery, simvastatin treatment and histomorphometric analysis. ZJZ participated in animal surgery, simvastatin treatment and H&E staining. XQL participated in the collection of specimens, the statistical analysis and TRAP staining. AQ participated in study design and helped to draft and polish the manuscript. EYL conceived of the study, participated in its design and helped to draft the manuscript. All authors read and approved the final manuscript.