Bone Remodeling Associated Salivary Biomarker MIP-1α Distinguishes Periodontal Disease from Health

Periodontal disease results from interaction between oral bacteria and the host inflammatory response. This interaction triggers a cascade of inflammatory events, which in turn promote connective tissue destruction and alveolar bone remodeling.1-7 These unique biological events contain signatures of the microbial ecology, as well as downstream events involving inflammation, attachment loss and bone destruction. It is likely that identification of the dominant signatures for each of these biological phases could provide insight into biomarkers of periodontal disease in oral fluids. To this end, several investigators have identified salivary biomarkers of biological events associated with aspects of periodontal disease8-12 and reviews on this topic are available.13-15 However, lacking to date is a focused panel of biomarkers that encompass the early and later biological phases that would yield, at least theoretically, the specificity for the development of a real-time, accurate, biologically-based, periodontal disease diagnostic device. Progress continues to be made towards the achievement of a salivary diagnostic device based on the knowledge that saliva is a rich pool of proteins and molecules that reflects aspects of oral health. In one recent example, we found that biomarkers of inflammation, connective tissue destruction and bone remodeling were elevated in concentrations in the saliva of patients with periodontal disease.11 In that study, we observed that salivary levels of IL-1β and MMP-8 were significantly associated with clinical parameters of periodontal disease, and levels of IL-1β and MMP-8 elevated above a threshold (i.e., 2 standard deviations above the mean of healthy controls), together demonstrated an odds ratio of 45 for periodontal disease. While these results demonstrated proof of principle that biomarkers from two distinct biological phases could aid in distinguishing periodontal disease from health, the identification of a biomarker associated with aspects of bone remodeling – a late biological event that could improve the accuracy of salivary diagnostics - remains elusive. Bone resorption is mediated by osteoclasts that exhibit specific abilities to degrade organic and inorganic components of bone. Different mediators such as interleukin-1β, prostaglandin E2 (PGE2), tumor necrosis factor-alpha (TNF-α), macrophage inflammatory protein (MIP)-1α, interleukin-6 (IL-6), interleukin-11 (IL-11), and interleukin-17 (IL-17) act upstream as activators of osteoclastogenesis.16-22 Within the resorption lacunae, receptor activator of nuclear factor-kappa B ligand (RANKL) and osteoprotegerin (OPG) are important cytokines belonging to the TNF family that regulate differentiation of osteoclast progenitor cells into active osteoclasts or inhibiting the differentiation process, respectively.23, 24 As a result, type 1 collagen is degraded during bone destruction by proteolytic enzymes such as matrix metalloproteinases (MMPs) and cathepsin K which lead to release of cross-linked telopeptides into the circulation (serum, saliva, and urine) as stable fragments such as pyridinoline cross-linked carboxyterminal telopeptide of type 1 collagen (ICTP)25-27 and C-terminal type 1 collagen telopeptide (β-CTX).28-31 We and others have investigated salivary levels of several of these important molecules associated with cytokine signaling and alveolar bone resorption32-39 however no conclusive information has been yet reported on the best biomarker associated with alveolar bone remodeling in adults. Also, there is a lack of knowledge whether upstream pathways, midstream osteoclastogenic factors, or downstream degradation products are better salivary biomarkers of periodontal disease. Therefore, the purpose of this study was to test the hypothesis that a specific salivary biomarker associated with bone remodeling could be identified that would distinguish healthy and periodontal disease subjects. Proteins associated with the upstream, midstream and downstream processes of osteoclastogenesis (i.e., MIP-1α, OPG, β-CTX, and ICTP) were selected for evaluation.