Vitamin D and periodontal health: Current concepts.

Vitamin D is crucial for a wide variety of organ systems; nevertheless, its deficiency is highly prevalent, present in 30-50% of the general population. Evidence has demonstrated that vitamin D deficiency may place subjects at risk for not only low mineral bone density/osteoporosis and osteopenia, but also infectious and chronic inflammatory diseases. Through its effect on bone and mineral metabolism, innate immunity, and several vitamin D receptor gene polymorphisms, vitamin D has been reported to be associated with the periodontal disease.

INTRODUCTION

In tradition, vitamin D has been associated with bone health and it is well-understood that vitamin D deficiency leads to rickets in children and osteomalacia/osteoporosis in adults.[1] However, it is now known that adequate vitamin D is important for optimal functioning of many organs and tissues throughout the body.[2] Vitamin D deficiency or insufficiency is prevalent in practically every segment of the population, including children and young adults. This world-wide pandemic remains generally unrecognized and untreated. Evolving data indicate that vitamin D deficiency in addition to playing a significant role in the genesis of coronary risk factors also pre-disposes to hypertension, diabetes, the metabolic syndrome, left ventricular hypertrophy, congestive heart failure, and chronic vascular inflammation.[12] Vitamin D is classified as a secosteroid in which one of the rings has been broken by ultraviolet B (UVB) sunlight and the main source of vitamin D is de novo synthesis in the skin. Although vitamin D is consumed in food, dietary intake alone is often insufficient, supplying only 20% of the body's requirements. In recent years, the discovery of the vitamin D receptor (VDR) in the cells of the immune system and the fact that several of these cells produce the vitamin D hormone suggested that it could have immunoregulatory properties.

PHYSIOLOGY AND METABOLISM

Vitamin D comes in two forms: Vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D2, found in plants, is the product of UVB-290-315 mm irradiation of ergosterol, and can be consumed as a supplement or in fortified foods. Vitamin D3, a product of UVB irradiation of 7-dehydrocholesterol, is synthesized in the human epidermis or consumed in the form of oily fish, fortified foods or a supplement [Figure 1].[1] Vitamin D is converted in the liver to 25(OH) D, which is the major circulating metabolite of vitamin D.

Figure 1

Vitamin D synthesis

In the kidney, 25(OH) D is converted by 1-hydroxylase to its active form, 1, 25-dihydroxy vitamin D (1, 25(OH)2D), which plays a vital role in maintaining bone and muscle health by regulating calcium metabolism.

Vitamin D in the form of 1, 25(OH)2D is a hormone, because it is produced primarily in the kidney and then circulates throughout the body, where it exerts wide ranging effects [Figures 2 and 3].

Figure 2

Bioactions of vitamin D

Figure 3

Systemic effects of vitamin D deficiency

The VDR-bound 1, 25(OH)2D in turn binds to the retinoic acid x-receptor and serves as a nuclear transcription factor, altering gene function and inducing protein synthesis.[1] Directly or indirectly, 1, 25(OH)2D regulates over 200 genes, including those involved in renin production in the kidney, insulin production in the pancreas, release of cytokines from lymphocytes, production of cathelicidin in macrophages and growth and proliferation of both vascular smooth muscle cells and cardiomyocytes.[1]

VITAMIN D – AN IMMUNOMODULATING AGENT

Innate immunity

The innate immune system can also be inhibited by vitamin D. Here, vitamin D has been shown to inhibit the differentiation, maturation, and immune-stimulating ability of dendritic cells by down-regulating the expression of Major Histocompatibility Complex class II molecules.[3] Dendritic cells have important functions in maintaining both protective immunity and self-tolerance. Immature dendritic cells promote T-cell tolerance, whereas mature dendritic cells activate naive T-cells. Physiologic levels of vitamin D inhibit the maturation of dendritic cells and maintain an immature and tolerogenic phenotype with inhibition of activation markers such as MHC class II, Cluster of D ifferentiation (CD) 40, CD80, and CD86 up-regulation of inhibitory molecules. Vitamin D concurrently suppresses interleukin (IL)-12 and enhances IL-10 production in these dendritic cells.[4]

Interestingly, vitamin D has also been shown to have a stimulatory effect on monocytes in vitro, suggesting a complex role in immune hemostasis rather than a purely suppressive effect on the immune system [Figure 4a].[3] The extent of this physiologic balance has yet to be fully elucidated. Vitamin D also influences the immune response through the regulation of cathelicidin, which is the only antimicrobial peptide produced by humans.[5] Cathelicidin is produced by neutrophils, macrophages, and the cells lining the epithelial surfaces of the skin, respiratory tract, and gastrointestinal tract – the sites, which are constantly exposed to potential pathogens. Cathelicidin has a broad antimicrobial activity against gram-positive and gram-negative bacteria, as well as certain viruses and fungi. Vitamin D treatment up-regulated cathelicidin mRNA in several cell lines and primary cultures including keratinocytes, neutrophils, and macrophages [Figure 4b].[6]

Figure 4

(a) Vitamin D effect on immune cells; (b) Effect on cathelicidin

Humoral immunity

The activation of CD4 + T-cells results in a five-fold increase in VDR expression, enabling calcitriol to regulate at least 102 identified genes.[7] This regulatory effect has a downstream impact on the levels of circulating chemokines and cytokines. T helper 1 (Th1) cells secrete interferon gamma (IFNγ), IL-2, IL-12, and tumor necrosis factor alpha, all of which augment the cell-mediated defense against intracellular pathogens. Th2 cells express IL-4, IL-5, and IL-13, which further propagate the Th2 response. These Th2-derived cytokines modulate the immune response against parasites and are also associated with the regulation of atopy and asthma.[89] Vitamin D exerts a strong suppressive effect on the expression of IL-2 and IFNγ in a VDR-regulated mechanism. The suppression of IL-2 production, in turn, inhibits T-cell proliferation. Evaluation of T-lymphocyte subpopulations demonstrates that vitamin D blocks the induction of Th1 cytokines, especially IFNγ, although simultaneously enhancing Th2 responses through the enhancement of IL-4 production.[89] Overall, vitamin D decreases cell-mediated immune responses. This suppressive effect on humoral immunity is facilitated through the effect of vitamin D3 on Antigen Presenting Cells (APC). In APCs, calcitriol inhibits the production of IL-12, a cytokine that normally enhances the Th1 response. In effect, vitamin D acts as a physiologic “brake” on humoral immunity.

Auto immunity

VDR agonists not only favor induction of CD4 + CD25 + Treg cells and enhance their suppressive activity, but can also promote their recruitment at inflammatory sites. Furthermore, 1, 25(OH)2D3 treatments induced natural killer (NK) T-cell functions in vitro and in vivo[10] [Figure 1]. NK T-cells are early innate regulatory cells that can alter the outcome of autoimmunity. Therefore, two types of cells are induced by 1, 25(OH)2D3, the Treg and the NK T-cells; induction of these regulatory cells and direct inhibition of Th1 cells are the mechanisms by which 1, 25(OH)2D3 suppresses experimental autoimmunity.[10] In addition, treatment with VDR agonists inhibits the T-cell production of IL-17, a pro-inflammatory cytokine that is produced by pathogenic T-cells (Th17) in various models of organ-specific autoimmunity in the brain, heart, synovium, and intestines.[11] Interestingly, IL-17 production is sustained by IL-23, an IL-12 family member consisting of p19 and p40 chains, the latter of which is strongly inhibited by VDR agonists.[11]

Anti-cancer effect

Recently, 1, 25(OH)2D3 treatment induced a significant inhibition of normal lymphoid cell progenitors growth of both T and B lineage and inhibited significantly also the growth of malignant B-cell lineage lymphoid progenitors, without inducing cytotoxic effect [Figure 4a].[12]

More recently, by testing the effects of 1, 25(OH)2D3 on B-cell responses and it was found that it inhibited the ongoing proliferation of activated B-cells and induced their apoptosis, whereas initial cell division was unimpeded.[13] The generation of plasma cells and post-switch memory B-cells was significantly inhibited by 1, 25(OH)2D3 although the up-regulation of genetic programs involved in B-cell differentiation was only modestly affected [Figure 4b]. B-cells expressed mRNAs for proteins involved in vitamin D activity, including 1 α-hydroxylase, 24-hydroxylase and the VDR, each of which was regulated by 1,25(OH)2D3 and/or activation. Interestingly, 1, 25(OH)2D3 up-regulated the expression of p27, but not of p18 and p21, which may be important in regulating the proliferation of activated B-cells and their subsequent differentiation in plasma cells[13] vitamin D.

VITAMIN D AND BONE METABOLISM

As an important component of interactions among the kidney, bone, parathyroid gland, and intestine, vitamin D helps to modulate skeletal and mineral homeostasis. Through its role in maintaining proper extracellular calcium levels, vitamin D is essential in maintaining skeletal integrity. Several molecules namely receptor activator of NF-kB ligand (RANKL), and a RANKL antagonist (osteoprotegerinor [OPG]) are produced by osteoblasts and other cells such as activated CD4 + T lymphocytes and are important regulators of bone remodeling.[14] Binding of RANKL to RANK, expressed on the surface of osteoclast progenitor cells, causes them to differentiate into mature osteoclasts. OPG acts as a soluble receptor for RANKL, inhibiting RANK-RANKL interaction and the maturation of osteoclast progenitor cells. The relative ratio of RANKL to OPG in the osteoclast precursor microenvironment can thus determine mature osteoclast formation. The RANKL gene promotor structure contains vitamin D and glucocorticoid response elements, and studies have shown that vitamin D-VDR stimulates RANKL expression in cells such as osteoblasts and bone marrow-derived stromal cells.[15] Vitamin D has been shown to down regulate OPG, and this combination of increased RANKL expression and decreased expression of OPG caused by vitamin D would favor differentiation and activation of osteoclasts and increased bone resorption.[16] However, However kitazaw et al reported that while vitamin d initially releases OPG Kitazaw et al., reported that while vitamin D initially represses OPG, long-term exposure to vitamin D led to a recovery of OPG expression. This suggested that the catabolic effects of vitamin D can be transient. Indeed, vitamin D has several anabolic effects on osteoblasts, including stimulation of osteopontin and alkaline phosphatase. Therefore, vitamin D appears to stimulate bone resorption, which is necessary for bone remodeling and formation of new bone, but after longer periods of exposure and it may facilitate osteoblast proliferation and differentiation.

VITAMIN D AND PERIODONTAL HEALTH

More recent studies showed significant associations between periodontal health and intake of vitamin D and calcium,[17] and that dietary supplementation with calcium and vitamin D may improve periodontal health, increase bone mineral density in the mandible and inhibit alveolar bone resorption [Figure 5].[1819] In a recently published longitudinal study, Garcia et al. reported that calcium and vitamin D supplementation may reduce the severity of periodontal disease if used at doses higher than 800-1,000 IU daily and supported the rational for testing the potential beneficial role of vitamin D on periodontal disease in randomized clinical trials. They also noted that vitamin D, in addition to its role in bone and calcium homeostasis, acts as an anti-inflammatory agent because it inhibits immune cell cytokine expression and causes monocyte/macrophages to secrete molecules that have a strong antibiotic effect. Indeed, vitamin D deficiency may be linked to increased risk of infectious diseases. This suggests that vitamin D may be of benefit in the treatment of periodontitis, not only because of its direct effects on bone metabolism, but also because it may have antibiotic effects on periodontopathogens and inhibit inflammatory mediators that contribute to the periodontal destruction.

Figure 5

Vitamin D receptor polymorphism

VDR POLYMORPHISM AND PERIODONTAL DISEASE

The associations of several VDR restriction fragment length polymorphism (RFLPs) with several diseases namely secondary hyperparathyroidism in the renal failure, osteoporosis, cancer, nephrolithiasis, diabetes, and periodontal disease have been reported [Figure 5].[20] The RFLPs, Bsml, Tru9I, TaqI, EcoRV and ApaI are located between exon 8 and 9 and may influence mRNA stability. The RFLP Fokl creates a start codon in exon 2, resulting in an alternative start site and an association between the Taql RFLP and periodontitis has been reported.[21] An association between the less frequent T allele and localized early onset periodontitis (aggressive periodontitis) in Caucasian subjects was reported.[22] The thymine phenotype allele (TT) genotype and the T allele are associated with chronic periodontitis in Japanese and Caucasian subjects, while the TT genotype and T allele are associated with early onset periodontitis (aggressive periodontitis) in Chinese subjects. A strong association between Chinese female patients with aggressive periodontitis and the TT genotype is suggested[23] while the TT genotype is associated with fewer occurrences on tuberculosis and chronic hepatitis B virus infection. The TT genotype and the T allele are associated with decreases in bone mineral density and the incidence of osteoporosis. These finding suggest that the TaqI RFLP is associated with both immune function and bone metabolism. Ethnic differences and different mechanisms in pathogenesis between aggressive periodontitis and chronic periodontitis may influence the results of TaqI RFLP analysis.

The BsmI RFLP in combination with other RFLPs are associated with early onset periodontitis and chronic periodontitis in Korean.[1524] The ApaI, BsmI and FokI RFLPs have been reported to confer elevated risk of severe chronic periodontitis in Japanese men. These RFLPs are associated with bone and mineral disease and the TaqI and FokI RFLPs are associated with increased cancer risk, such as prostate and breast malignancy. Further, studies are required to elucidate the functional relevance of VDR RFLPs and disease pathogenensis. An inverse association between the serum 25-hydroxyvitamin D3 concentrations and periodontal disease has been reported[26] and these findings indicate that 1, 25(OH)2D3 plays a role in prevention of periodontal disease and that hypomorphic VDR alleles and reduced levels of 1,25(OH)2D3 - I,25-dihydroxy vitamin D3 may be associated with the periodontal disease.

VITAMIN D DEFICIENCY

Although there is no consensus regarding optimal serum 25(OH) D level, they are inversely associated with Para thormone (PTH) levels until 25(OH) D reaches 30-40 ng/mL, at which point PTH levels begin to level off.[25] Generally, 25(OH) D levels of <20 ng/mL is considered Vitamin D deficient, whereas the level of 21-29 ng/mL is considered insufficient [Table 1]. Vitamin D deficiency is now regarded as an epidemic, with 1 billion people world-wide estimated as Vitamin D deficient or insufficient.

Table 1

Vitamin D levels

A recent study found that more than 70% of individuals aged ≥12 years have serum 25(OH) D levels <32 ng/mL. Usually, 25(OH) D deficiency results from lack of sun exposure or inadequate nutrition, and the prevalence of vitamin D deficiency was higher in women, elderly persons, and ethnic minorities.

TREATMENT STRATEGY

In traditional, up to 95% of the body's vitamin D requirement comes from the synthesis in the epidermis on sun exposure, with the remainder ingested from dietary sources. The U.S. government's current recommendation for oral vitamin D is 200 IU daily for individual's age ≤50 years, 400 IU daily for individuals between age 50 years and 70 years, and 600 IU for those older than age 70 years. The most potent sources of vitamin D are sunlight (about 3,000 IU vitamin D3/5-10 min of mid-day, midyear exposure of arms and legs for a light-skinned Caucasian) or prescription oral supplements of 50,000 IU capsule of either vitamin D2 or D3 every 2 weeks.

Among foods, oily fish have the highest content of vitamin D3, which ranges from 100 to 1,000 IU/3.5 oz,[25] whereas other sources such as milk or orange juice fortified with vitamin D contain up to 100 IU per serving. As a general rule, every 100 IU vitamin D ingested daily increases the 25(OH) D level by about 1 ng/ml. Over-the-counter dietary supplements of vitamin D2 and D3 typically contain 400-5,000 IU/capsule. Oral supplementation with either vitamin D2 or D3 initially will increase vitamin D levels equally well, although the increases in serum 25(OH) D levels seem to persist longer after a bolus dose of vitamin D3 than D2. Treatment of vitamin D-deficient individuals should be initiated with 50,000 IU of vitamin D2 or D3 weekly for a period of 8-12 weeks. Once the initial repletion phase is complete, maintenance therapy can be continued in 1 of 3 ways: (1) 50,000 IU vitamin D2 or D3 every 2 weeks; (2) 1,000-2,000 IU vitamin D3 daily; and (3) sunlight exposure for 5-10 min for Caucasians (longer times required for people with increased skin pigmentation) between the hours of 10 AM to 3 PM (spring, summer, and fall) [Figure 6].[26]

Figure 6

Treatment regimen

CONCLUSION

Although historically vitamin D has been associated with the regulation of musculoskeletal health recent research has indicated its cardiovascular, anti-cancer, immunomodulatory, and various systemic effects through VDR activation. The 1, 25(OH)2D3-VDR system plays a significant role in oral homeostasis and its dysfunction leads to periodontal disease. Hence, Vitamin D research should make important contributions to the understanding of periodontal diseases and may benefit in the treatment due to its direct effect on bone metabolism and its anti-inflammatory properties.