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DISCOVERY! |
1 Department of Oral Biology & Pathology, School of Dental Medicine, SUNY at Stony Brook, Stony Brook, NY;
2 Department of Surgical Specialties, College of Dentistry, University of Nebraska Medical Center, Lincoln, NE; and
3 Department of Surgery, SUNY Upstate Medical University, Syracuse, NY, USA
* corresponding author, lgolub{at}epo.hsc.sunysb.edu
Martin Taubman, Editor
KEY WORDS: Periodontitis • systemic diseases • hypothetical model
The connection between systemic diseases and chronic destructive periodontitis (CDP) has received increasing attention in recent years. A major unanswered question is how disease in one part of the body (e.g., the joints and skeletal tissues) can transmit signals to the periodontium to enhance or, as we hypothesize in this report, to co-induce CDP. The inflammatory mediators and effector molecules described below, carried by the circulation, are likely conduits. The purpose of this paper is to explore a proposed "two-hit" model of CDP (Fig
.), by interpreting the results of experiments with animal models and supported by evidence from human clinical studies. This model conceptualizes how bone- and connective-tissue-destructive diseases in one location (e.g., the joints in patients with rheumatoid arthritis, the skeletal system during post-menopausal osteoporosis, and others) may communicate with the tissues in the periodontium (the 2nd "hit"), together with the microbial products (e.g., endotoxin) generated by the subgingival biofilm (the 1st "hit"), to co-induce periodontitis (Fig.). A similar "two-hit" model has recently been proposed to explain the pathogenesis of acute lung injury during acute respiratory distress syndrome (ARDS), mediated by essentially the same inflammatory mediators and effector molecules as proposed in this paper (Carney et al., 1999; Steinberg et al., 2005a). Our hypothesis and model are further supported by a wealth of evidence, accumulated over the past several decades, suggesting that inhibitors of these mediators and effector molecules (particularly pleotropic MMP-inhibitor drugs, notably the non-antimicrobial formulations of tetracycline compounds) can reduce the severity of several of these systemic diseases (e.g., ARDS, rheumatoid arthritis, post-menopausal osteoporosis), as well as periodontitis, in experimental animals and, in some cases, in humans as well (Golub et al., 1998, 1999; Payne and Reinhardt, 1998; Carney et al., 1999; Elliott et al., 2003; Brandt et al., 2005; Steinberg et al., 2005b).
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Evidence from human studies also suggests that both adult rheumatoid arthritis (RA) and juvenile idiopathic arthritis (JIA) can increase the incidence of periodontitis (Kasser et al., 1997; Miranda et al., 2003). One possible link between the systemic and local diseases includes systemic osteoporosis as a complication of arthritis (Greenwald and Kirkwood, 1999), which may potentiate accelerated periodontal bone loss (Payne et al., 1999). Another link is the elevated levels of inflammatory mediators, both locally in the diseased synovium and systemically in the circulation. In fact, elevated levels of pro-inflammatory cytokines such as interleukin (IL)-1, IL-6, and tumor necrosis factor (TNF)-
have been observed in the serum of RA and JIA patients (Vreugdenhil et al., 1990; Yilmaz et al., 2001). These cytokines are thought to stimulate resident cells in the synovium and the periodontium to produce MMPs mediating connective tissue destruction, and to induce the differentiation and activity of osteoclasts to destroy bone (Lotz et al., 1995; McGee et al., 1998). One such cytokine in particular, TNF-, also promotes bone resorption: (i) by up-regulating inducible nitric oxide synthase (iNOS) and the production of nitric oxide (NO) (Soory, 2002); and (ii) by modulating the receptor activator of nuclear factor 
B (NFB) ligand (RANKL) in osteoblasts, and its antagonist osteoprotegerin (OPG), thus altering the RANKL/OPG ratio, which enhances osteoclast activity (Hofbauer et al., 2000; Haynes, 2004).
We are not suggesting that circulating (serum levels of) pro-inflammatory cytokines by themselves are having a proportionately direct effect on synovial or periodontal target cells to produce MMPs, NO, or RANKL, since their local concentrations in RA and CDP are thousands of times higher than those in serum (Miranda et al., 2005). However, elevated serum levels of IL-1, IL-6, and TNF- (the 2nd "hit"), originating from RA lesions, may be early initiators of an amplifying inflammatory cascade that (together with an additional factor, such as endotoxin, functioning as a 1st "hit"; see below) could lead to: increased local inflammatory mediators, including cytokines and prostaglandins; then RANKL and effector molecules, including MMPs and other tissue- and bone-destructive proteinases; and, finally, osteoporosis and periodontal breakdown. In the first large-cohort prospective study of early RA (which eliminated the confounding factors of immobility and systemic steroid usage), IL-6 and, especially, C-reactive protein (CRP) were correlated with bone mineral density (BMD) loss during active arthritic disease (Gough et al., 1998). Since IL-6 is one of the cytokines known to induce production of CRP (an acute-phase protein produced by the liver and a biomarker of systemic inflammation), these findings support the authors’ concept that pro-inflammatory cytokines released from inflamed joints, which induce a systemic inflammatory response, may stimulate osteoclastic bone resorption throughout the skeletal system, including the alveolar bone. Additional evidence supporting this concept was provided by recent studies from O’Dell and his colleagues (Elliott et al., 2003). They found that a non-antimicrobial formulation of doxycycline (sub-antimicrobial-dose doxycycline, or SDD, an MMP inhibitor), when combined with methotrexate (a disease-modifying anti-arthritic drug), was two to three times more effective than methotrexate plus placebo in reducing the severity of RA in patients over a two-year time period. SDD has repeatedly been shown to reduce the severity of periodontal disease in multi-center clinical trials and, as described below, may also be effective in reducing systemic bone loss during post-menopausal osteoporosis.
As a second example of our hypothesis, the evidence for an interaction between local inflammatory disease (periodontitis) and systemic osteoporosis has been strengthened by human clinical studies demonstrating a relationship among post-menopausal (PM) osteoporosis, tooth loss, and alveolar bone loss (Krall et al., 1996; Payne et al., 1999). Golub and colleagues (1999), using a standard animal model of PM osteoporosis, the ovariectomized (OVX) aged rat, shed some light on potential pathways by observing increased MMP (collagenase and gelatinase) activity in the gingiva in response to ovariectomy and estrogen deficiency. The increased gingival collagenase activity paralleled the increase in alveolar bone loss locally as well as trabecular bone density loss in the long bones systemically. Moreover, treating these OVX rats with a non-antimicrobial CMT not only reduced the severity of osteoporosis systemically, but also reduced the severity of periodontal destruction, including a reduction in alveolar bone loss (see below).
Estrogen deficiency in post-menopausal women is the major cause of osteoporosis in humans (Pacifici, 1996). It has been well-established that estrogen has anti-inflammatory effects and inhibits the production of pro-inflammatory and bone-resorptive cytokines, such as IL-1, TNF-, IL-6, and GM-CSF (Jilka et al., 1992; Kitazawa et al., 1994; Kimble et al., 1997). Conversely, estrogen deficiency results in elevated bone marrow cell production of these pro-osteoclastogenic cytokines. As described above, recent publications suggest that TNF- is the key candidate molecule in the pathogenesis of bone loss observed in post-menopausal estrogen deficiency (Roggia et al., 2001; Cenci et al., 2003). TNF- stimulates osteoclastogenesis and suppresses recruitment of osteoblasts from progenitor cells (Nanes, 2003). Since TNF- is known to induce collagenase activity (Uitto et al., 1998), this molecule may also be responsible for the increased collagenase activity, even in the local gingival tissues, observed in the OVX rat study published by Golub and co-workers.
A proposed model linking systemic osteoporosis and periodontal bone loss is suggested as follows: Estrogen deficiency results in a significant up-regulation of bone-resorptive cytokines in bone and bone marrow cells. Increases in circulating levels of IL-1, IL-6, and TNF- ensue. Even small elevations in serum levels of these circulating cytokines may initiate the cytokine/prostaglandin (PG)/MMP/RANKL cascade locally in the periodontium, as suggested earlier with RA (2nd "hit"), especially when co-induced by bacterial factors such as lipopolysaccharide (LPS) (the 1st "hit"). For example, sensitizing periodontal fibroblasts with IL-1 or TNF-, along with triggering doses of LPS, has been shown to enhance NO (Lavnikova and Laskin, 1995) and IL-6 (Kent et al., 1999) production. In another study, systemic administration of TNF-, combined with ligature-induced periodontitis in rats, caused greater periodontal breakdown than did either treatment alone (Gaspersic et al., 2003). Furthermore, inhibition of PGE2 synthesis by indomethacin has been shown, in vitro, to inhibit RANKL expression in osteoblasts induced by periodontal pathogens (Choi et al., 2005). Thus, therapies that target inhibition of these upstream bone-resorptive cytokines or inflammatory mediators may prove useful in mitigating osteoporosis and periodontal bone loss. Alternatively, therapies that target inhibition of collagenase downstream may also prove beneficial in simultaneously reducing systemic and local bone loss. Common degradative pathways are shared by arthritis, osteoporosis, and periodontitis, and we propose that elevations in serum levels of bone-resorptive cytokines provide one likely conduit that links these three diseases. In support of this hypothesis, Golub et al.(1999) found that treating the ovariectomized aged female rat with CMT-8 (a chemically modified non-antimicrobial analog of doxycycline) reduced, in a dose-response fashion, the severity of systemic osteoporosis, and reduced "markers" of local periodontal destruction, including a "normalization" of both pathologically excessive mammalian (host) collagenase activity in the gingiva and alveolar bone loss.
Finally, extensive literature published over the past several decades supports additional diseases, such as diabetes and cardiovascular disease (CVD), that share these links between biomarkers of systemic inflammation (C-reactive protein or CRP, plus other acute-phase proteins and cytokines such as IL-6 and TNF-) and CDP. For example, Craig et al.(2003) reported that patients with CDP exhibited 100% higher levels of CRP in their serum than those with mild, less destructive periodontitis. Tonetti’s group has also observed similar associations (D’Aiuto et al., 2004). Ridker et al.(2002) and other cardiology groups have repeatedly demonstrated, in large-scale clinical studies, that CRP and other biomarkers, such as IL-6 and MMP-9 in plasma, are major predictors of future cardiovascular disease, including fatal heart attacks (Blankenberg et al., 2003). In fact, our group recently reported that SDD, an MMP and cytokine-inhibitory drug approved as adjunctive treatment for CPD, reduced these biomarkers of systemic inflammation (i.e., CRP, IL-6, and MMP-9) in the plasma of patients with cardiovascular disease (Brown et al., 2004).
In addition, experimentally inducing diabetes mellitus (e.g., by injecting a ß-cell toxin such as alloxan or streptozotocin) in the rat has long been known to induce local biomarkers of periodontal breakdown (elevated production and activity of collagenase and gelatinase in gingiva, and severe alveolar bone loss), as well as systemic markers of both inflammation (elevated serum PGE2) and connective tissue breakdown (elevated collagenase and collagen loss in skin, and skeletal osteoporosis). Similar changes have recently been seen in diabetic humans (Ryan et al., 2003).
Of central importance to our hypothesis, Nieman’s group and other groups have repeatedly used the term "two-hit model" for their animal studies on ARDS (Patrick et al., 1996; Carney et al., 1999; Steinberg et al., 2005a,b). ARDS, an often (40–50%) fatal destructive lung (and multi-organ) disease associated with ventilator-induced lung injury (VILI), is mediated by inflammatory mediators (cytokines, prostanoids, NO) and effector molecules (host-derived MMPs and PMN serine proteinases, especially elastase) similar to those which mediate periodontal destruction. In the 1st "hit", the animal, typically a Yorkshire pig, is subjected to cardiopulmonary bypass, which induces VILI (ventilator-induced lung injury). This procedure does not predictably result in lung tissue destruction, but does promote the accumulation of PMN leukocytes in the lung alveoli. However, the 2nd "hit", generated by injection of a relatively low dose of endotoxin, induces PMNL degranulation, irreversible lung destruction (mediated by MMPs and elastase), and death. In all of the examples of tissue-destructive medical and oral diseases described above, pleiotropic proteinase-inhibitor drugs—most notably the chemically modified non-antimicrobial tetracycline, CMT-3—have been found to prevent both irreversible tissue damage and (in the case of ARDS in experimental animals) death (Steinberg et al., 2005b).
In summary, we propose that the periodontopathic subgingival microflora, organized as a biofilm, provides one "hit" in the cascade of destructive events in chronic destructive periodontitis. An additional "hit" in this two-hit model is provided by a systemic inflammatory response (characterized by elevated biomarkers, such as CRP, IL-6, and MMP-9 in serum or plasma), induced by various medical disorders (in fact, many of these are known to be associated with enhanced periodontal destruction). Targeting both "hits" provides the optimal therapeutic strategy, with benefits for both the local periodontal and associated systemic medical diseases. Current treatment approaches available to the periodontist and dentist include: (i) antimicrobial therapy, including mechanical debridement and surgical reduction of probing depth to reduce the bacterial "load" in the periodontal pocket, combined (as needed) with topical and systemically administered antimicrobials; and (ii) host-modulation therapy (using FDA-approved, MMP-inhibitor sub-antimicrobial-dose doxycycline by itself or, after confirmation by additional research, using other pharmaceuticals, such as non-steroidal anti-inflammatory drugs [NSAIDs] and bisphosphonates, or combinations of these). Above all, assuming that this "two-hit" model is supported by future research, this concept could facilitate the incorporation of systemic/medical/pharmacologic treatment with the current mechanical/surgical approach in the management of chronic destructive periodontitis.
ACKNOWLEDGMENTS
This work is supported in part by grant #R01DE12872 (JBP, LMG, and RAR) from the NIDCR and by the Office of Research for Women’s Health (NIH) and by grant #2R42HLO65030-02A1 (GN and LMG) from the National Heart, Lung, and Blood Institute (NIH). LMG and GN are listed as inventors on several patents for drugs mentioned in this article, and these patents have been fully assigned to their institutions: SUNY at Stony Brook and SUNY Upstate Medical University. LMG is a consultant to Collagenex Pharmaceuticals, Inc. and the Fund for Autoimmune Diseases Research.
Received August 10, 2005; Last revision October 3, 2005; Accepted November 15, 2005
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