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Ji M, Xiao L, Xu L, Huang S, Zhang D. How pH is regulated during amelogenesis in dental fluorosis. Exp Ther Med 2018; 16:3759-3765. [PMID: 30402142 DOI: 10.3892/etm.2018.6728] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2017] [Accepted: 12/01/2017] [Indexed: 12/14/2022] Open
Abstract
Amelogenesis is a complicated process that concerns the interaction between growing hydroxyapatite crystals and extracellular proteins, which requires the tight regulation of pH. In dental fluorosis, the balance of pH regulation is broken, leading to abnormal mineralization. The current review focuses on the electrolyte transport processes associated with pH homeostasis, particularly regarding the changes in ion transporters that occur during amelogenesis, following exposure to excessive fluoride. Furthermore, the possible mechanism of fluorosis is discussed on the basis of acid hypothesis. There are two main methods by which F- accelerates crystal formation in ameloblasts. Firstly, it induces the release of protons, lowering the pH of the cell microenvironment. The decreased pH stimulates the upregulation of ion transporters, which attenuates further declines in the pH. Secondly, F- triggers an unknown signaling pathway, causing changes in the transcription of ion transporters and upregulating the expression of bicarbonate transporters. This results in the release of a large amount of bicarbonate from ameloblasts, which may neutralize the pH to form a microenvironment that favors crystal nucleation. The decreased pH stimulates the diffusion of F- into the cytoplasm of amelobalsts along the concentration gradient formed by the release of protons. The retention of F- causes a series of pathological changes, including oxidative and endoplasmic reticulum stress. If the buffering capacity of ameloblasts facing F- toxicity holds, normal mineralization occurs; however, if F- levels are high enough to overwhelm the buffering capacity of ameloblasts, abnormal mineralization occurs, leading to dental fluorosis.
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Affiliation(s)
- Mei Ji
- Department of Stomatology, Shandong Provincial Hospital Affiliated to Shandong University, Jinan, Shandong 250021, P.R. China
| | - Lili Xiao
- Department of Stomatology, Shandong Provincial Hospital Affiliated to Shandong University, Jinan, Shandong 250021, P.R. China
| | - Le Xu
- Department of Stomatology, Shandong Provincial Hospital Affiliated to Shandong University, Jinan, Shandong 250021, P.R. China
| | - Shengyun Huang
- Department of Stomatology, Shandong Provincial Hospital Affiliated to Shandong University, Jinan, Shandong 250021, P.R. China
| | - Dongsheng Zhang
- Department of Stomatology, Shandong Provincial Hospital Affiliated to Shandong University, Jinan, Shandong 250021, P.R. China
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Jalali R, Guy F, Ghazanfari S, Lyaruu D, van Ruijven L, DenBesten P, Martignon S, Castiblanco G, Bronckers ALJJ. Mineralization-defects are comparable in fluorotic impacted human teeth and fluorotic mouse incisors. Arch Oral Biol 2017; 83:214-221. [PMID: 28806687 DOI: 10.1016/j.archoralbio.2017.07.018] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2017] [Revised: 07/14/2017] [Accepted: 07/25/2017] [Indexed: 12/20/2022]
Abstract
OBJECTIVE Fluoride excess of 0.05-0.07mgF/kgbw/day in water or food additives like salt is the principal cause of endemic dental fluorosis. How fluoride causes these defects is not clear yet. Recent studies in rodents suggest that development of enamel fluorosis is associated with insufficient neutralization of protons released during the formation of hypermineralized lines. DESIGN Here we examined whether hypermineralization could also be assessed by MicroCT in developing molar enamel of humans exposed to fluoride. RESULT Micro-CT analysis of hypomineralized enamel from human fluorotic molars graded by the Thylstrup-Fejerskov (TF) Index as III-IV showed weak hypermineralized lines and hypermineralized patches not seen in TF-I/II grade enamel. The mesio-distal sides of these molar teeth were significantly smaller (∼18%, p=0.02) than in TF-I/II teeth. CONCLUSION The patterns of changes observed in human fluorotic teeth were similar to those in fluorotic rodent incisors. The data are consistent with the hypothesis that also in developing human teeth fluoride-stimulated local acidification of enamel could be a mechanism for developing fluorotic enamel.
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Affiliation(s)
- Rozita Jalali
- Department of Oral Cell Biology, Academic Centre for Dentistry Amsterdam, University of Amsterdam, and MOVE Research Institute, VU University Amsterdam, Amsterdam, Netherlands.
| | - Franck Guy
- CNRS INEE UMR 7262 - IPHEP, Institut de Paléoprimatologie et Paléontologie Humaine, Evolution et Paléoenvironnements, Université de Poitiers - Faculté des Sciences, Poitiers, Cedex 9, France
| | - Samaneh Ghazanfari
- Department of Orthopaedic Surgery - VU Medical Centre, MOVE Research Institute Amsterdam, Amsterdam, Netherlands and the Aachen-Maastricht Institute for Biobased Materials, Faculty of Humanities and Sciences, Maastricht University, Maastricht, Netherlands
| | - Don Lyaruu
- Department of Oral Cell Biology, Academic Centre for Dentistry Amsterdam, University of Amsterdam, and MOVE Research Institute, VU University Amsterdam, Amsterdam, Netherlands
| | - Leo van Ruijven
- Department of Oral Cell Biology, Academic Centre for Dentistry Amsterdam, University of Amsterdam, and MOVE Research Institute, VU University Amsterdam, Amsterdam, Netherlands
| | - Pamela DenBesten
- Department of Orofacial Sciences, School of Dentistry, University of California, San Francisco, CA, 94143-0422, USA
| | - Stefania Martignon
- UNICA - Caries Research Unit, Research Vice-rectory, Universidad El Bosque, Bogotá, Colombia
| | - Gina Castiblanco
- UNICA - Caries Research Unit, Research Vice-rectory, Universidad El Bosque, Bogotá, Colombia
| | - Antonius L J J Bronckers
- Department of Oral Cell Biology, Academic Centre for Dentistry Amsterdam, University of Amsterdam, and MOVE Research Institute, VU University Amsterdam, Amsterdam, Netherlands
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3
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Appropriate real-time PCR reference genes for fluoride treatment studies performed in vitro or in vivo. Arch Oral Biol 2015; 62:33-42. [PMID: 26615575 DOI: 10.1016/j.archoralbio.2015.11.004] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2015] [Revised: 09/15/2015] [Accepted: 11/05/2015] [Indexed: 01/06/2023]
Abstract
OBJECTIVE Quantitative real-time PCR (qPCR) is routinely performed for experiments designed to identify the molecular mechanisms involved in the pathogenesis of dental fluorosis. Expression of reference gene(s) is expected to remain unchanged in fluoride-treated cells or in rodents relative to the corresponding untreated controls. The aim of this study was to select optimal reference genes for fluoride experiments performed in vitro and in vivo. DESIGN Five candidate genes were evaluated: B2m, Eef1a1, Gapdh, Hprt and Tbp. For in vitro experiments, LS8 cells derived from mouse enamel organ were treated with 0, 1, 3 and/or 5mM sodium fluoride (NaF) for 6 or 18 h followed by RNA isolation. For in vivo experiments, six-week old rats were treated with 0 or 100 ppm fluoride as NaF for six weeks at which time RNA was isolated from enamel organs. RNA from cells and enamel organs were reverse-transcribed and stability of gene expression for the candidate reference genes was evaluated by qPCR in treated versus non-treated samples. RESULTS The most stably expressed genes in vitro according to geNorm were B2m and Tbp, and according to Normfinder were Hprt and Gapdh. The most stable genes in vivo were Eef1a1 and Gapdh. Expression of Ddit3, a gene previously shown to be induced by fluoride, was demonstrated to be accurately calculated only when using an optimal reference gene. CONCLUSIONS This study identifies suitable reference genes for relative quantification of gene expression by qPCR after fluoride treatment both in cultured cells and in the rodent enamel organ.
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Liu L, Zhang Y, Gu H, Zhang K, Ma L. Fluorosis induces endoplasmic reticulum stress and apoptosis in osteoblasts in vivo. Biol Trace Elem Res 2015; 164:64-71. [PMID: 25434583 DOI: 10.1007/s12011-014-0192-4] [Citation(s) in RCA: 29] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/17/2014] [Accepted: 11/20/2014] [Indexed: 11/26/2022]
Abstract
The present study investigated the effects of fluoride on endoplasmic reticulum (ER) stress (ERS) and osteoblast apoptosis in vivo. Forty-eight Wistar rats were randomly divided into four groups (12/group) and exposed to 0, 50, 100, and 150 mg/L of fluoride in drinking water for 8 weeks, respectively. Peripheral blood samples and bilateral femurs were used to monitor the progression of fluorosis in the animals. Hematoxylin and eosin (H&E) staining of the bone tissues was used to determine the severity of osteofluorosis. The expression of ERS chaperones (glucose-regulated protein 78 (GRP78), X-box binding protein l (XBP1), cysteine aspartate specific protease-12 (caspase-12), and growth arrest and DNA damage-inducible gene 153 (Gadd153/CHOP) was analyzed by immunohistochemistry staining, and osteoblast apoptosis was determined by TUNEL staining and flow cytometry. Accumulation of fluoride in bone was associated with the severity of osteofluorosis. The expression of GRP78, XBP1, caspase-12, and CHOP was increased in a dose-dependent manner. Fluoride-induced apoptosis in osteoblasts was also dose-dependent. High concentrations of fluoride induced ERS and osteoblast apoptosis in vivo. The increased expression of GRP78 and XBP1 increased the adaptation of osteoblasts to ERS to a certain extent. Caspase-12 and CHOP activation was associated with ERS and osteoblast apoptosis.
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Affiliation(s)
- Lu Liu
- Department of Preventive Dentistry, School of Stomatology, China Medical University, ShenYang, China
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5
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Abstract
Human enamel development of the permanent teeth takes place during childhood and stresses encountered during this period can have lasting effects on the appearance and structural integrity of the enamel. One of the most common examples of this is the development of dental fluorosis after childhood exposure to excess fluoride, an elemental agent used to increase enamel hardness and prevent dental caries. Currently the molecular mechanism responsible for dental fluorosis remains unknown; however, recent work suggests dental fluorosis may be the result of activated stress response pathways in ameloblasts during the development of permanent teeth. Using fluorosis as an example, the role of stress response pathways during enamel maturation is discussed.
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Suzuki M, Bartlett JD. Sirtuin1 and autophagy protect cells from fluoride-induced cell stress. Biochim Biophys Acta Mol Basis Dis 2013; 1842:245-55. [PMID: 24296261 DOI: 10.1016/j.bbadis.2013.11.023] [Citation(s) in RCA: 57] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2013] [Revised: 11/12/2013] [Accepted: 11/25/2013] [Indexed: 12/11/2022]
Abstract
Sirtuin1 (SIRT1) is a nicotinamide adenine dinucleotide (NAD(+))-dependent deacetylase functioning in the regulation of metabolism, cell survival and organismal lifespan. Active SIRT1 regulates autophagy during cell stress, including calorie restriction, endoplasmic reticulum (ER) stress and oxidative stress. Previously, we reported that fluoride induces ER-stress in ameloblasts responsible for enamel formation, suggesting that ER-stress plays a role in dental fluorosis. However, the molecular mechanism of how cells respond to fluoride-induced cell stress is unclear. Here, we demonstrate that fluoride activates SIRT1 and initiates autophagy to protect cells from fluoride exposure. Fluoride treatment of ameloblast-derived cells (LS8) significantly increased Sirt1 expression and induced SIRT1 phosphorylation resulting in the augmentation of SIRT1 deacetylase activity. To demonstrate that fluoride exposure initiates autophagy, we characterized the expression of autophagy related genes (Atg); Atg5, Atg7 and Atg8/LC3 and showed that both their transcript and protein levels were significantly increased following fluoride treatment. To confirm that SIRT1 plays a protective role in fluoride toxicity, we used resveratrol (RES) to augment SIRT1 activity in fluoride treated LS8 cells. RES increased autophagy, inhibited apoptosis, and decreased fluoride cytotoxicity. Rats treated with fluoride (0, 50, 100 and 125ppm) in drinking water for 6weeks had significantly elevated expression levels of Sirt1, Atg5, Atg7 and Atg8/LC3 in their maturation stage enamel organs. Increased protein levels of p-SIRT1, ATG5 and ATG8/LC3 were present in fluoride-treated rat maturation stage ameloblasts. Therefore, the SIRT1/autophagy pathway may play a critical role as a protective response to help prevent dental fluorosis.
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Affiliation(s)
- Maiko Suzuki
- Department of Mineralized Tissue Biology, The Forsyth Institute, 245 First Street, Cambridge, MA 02142, USA; Harvard School of Dental Medicine, 188 Longwood Avenue, Boston, MA 02115, USA
| | - John D Bartlett
- Department of Mineralized Tissue Biology, The Forsyth Institute, 245 First Street, Cambridge, MA 02142, USA; Harvard School of Dental Medicine, 188 Longwood Avenue, Boston, MA 02115, USA.
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Yamada A, Iwamoto T, Fukumoto E, Arakaki M, Miyamoto R, Sugawara Y, Komatsu H, Nakamura T, Fukumoto S. Epithelial-mesenchymal interaction reduces inhibitory effects of fluoride on proliferation and enamel matrix expression in dental epithelial cells. PEDIATRIC DENTAL JOURNAL 2012. [DOI: 10.1016/s0917-2394(12)70253-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
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Mofatto LS, Frozoni MRS, do Espírito Santo AR, Guimarães GN, de Souza AP, de Campos Vidal B, Line SRP. Fluoride effect on the secretory-stage enamel organic extracellular matrix of mice. Connect Tissue Res 2011; 52:212-7. [PMID: 21117895 DOI: 10.3109/03008207.2010.511731] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
The formation of an ordered enamel organic extracellular matrix (EOECM) seems to be a crucial step for the proper formation of the enamel mineral phase. The ordered supramolecular structure of the EOECM in the secretory stage can be analyzed using polarizing microscopy, as it is strongly birefringent. Excessive fluoride (F) ingestion during tooth development can cause enamel fluorosis, leading to increased porosity in mature enamel. We analyzed the effects of F on the birefringence of the EOECM in the A/J, CBA, and DBA/2 strains of mice given 0, 11.25, and 45 ppm of fluoride in drinking water. In the CBA and DBA/2 strains, the 11.25 and 45 ppmF groups presented a significant decrease in optical retardation (OR) when compared with the respective 0 (CBA 11.25 ppmF p = 0.0056 and 45 ppmF p < 0.0001; DBA/2 11.25 and 45 ppmF p < 0.05). ORs in A/J 0 ppmF were significantly higher than in 45 (p < 0.0001). The enamel of the A/J strain was more severely affected by fluoride than it was in the other strains of mice and exhibited the lowest levels of fluoride in plasma, whereas its normal secretory enamel presented a significantly higher protein absorbance than it did in CBA and DBA mice (p = 0.0099 and p = 0.0025, respectively). The results showed that experimental fluorosis can alter the supramolecular organization of EOECM in the secretory stage of amelogenesis and that the susceptibility to dental fluorosis seems to be influenced by the inherent characteristics of the developing enamel.
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Affiliation(s)
- Luciana Souto Mofatto
- Department of Morphology, Piracicaba Dental School, University of Campinas-UNICAMP, Piracicaba, SP, Brazil
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Sharma R, Tsuchiya M, Skobe Z, Tannous BA, Bartlett JD. The acid test of fluoride: how pH modulates toxicity. PLoS One 2010; 5:e10895. [PMID: 20531944 PMCID: PMC2878349 DOI: 10.1371/journal.pone.0010895] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2010] [Accepted: 05/09/2010] [Indexed: 01/04/2023] Open
Abstract
Background It is not known why the ameloblasts responsible for dental enamel formation are uniquely sensitive to fluoride (F−). Herein, we present a novel theory with supporting data to show that the low pH environment of maturating stage ameloblasts enhances their sensitivity to a given dose of F−. Enamel formation is initiated in a neutral pH environment (secretory stage); however, the pH can fall to below 6.0 as most of the mineral precipitates (maturation stage). Low pH can facilitate entry of F− into cells. Here, we asked if F− was more toxic at low pH, as measured by increased cell stress and decreased cell function. Methodology/Principal Findings Treatment of ameloblast-derived LS8 cells with F− at low pH reduced the threshold dose of F− required to phosphorylate stress-related proteins, PERK, eIF2α, JNK and c-jun. To assess protein secretion, LS8 cells were stably transduced with a secreted reporter, Gaussia luciferase, and secretion was quantified as a function of F− dose and pH. Luciferase secretion significantly decreased within 2 hr of F− treatment at low pH versus neutral pH, indicating increased functional toxicity. Rats given 100 ppm F− in their drinking water exhibited increased stress-mediated phosphorylation of eIF2α in maturation stage ameloblasts (pH<6.0) as compared to secretory stage ameloblasts (pH∼7.2). Intriguingly, F−-treated rats demonstrated a striking decrease in transcripts expressed during the maturation stage of enamel development (Klk4 and Amtn). In contrast, the expression of secretory stage genes, AmelX, Ambn, Enam and Mmp20, was unaffected. Conclusions The low pH environment of maturation stage ameloblasts facilitates the uptake of F−, causing increased cell stress that compromises ameloblast function, resulting in dental fluorosis.
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Affiliation(s)
- Ramaswamy Sharma
- Department of Cytokine Biology, Forsyth Institute, and Department of Developmental Biology, Harvard School of Dental Medicine, Boston, Massachusetts, United States of America
| | - Masahiro Tsuchiya
- Division of Aging and Geriatric Dentistry, Tohoku University, Sendai, Japan
| | - Ziedonis Skobe
- Department of Cytokine Biology, Forsyth Institute, and Department of Developmental Biology, Harvard School of Dental Medicine, Boston, Massachusetts, United States of America
| | - Bakhos A. Tannous
- Departments of Neurology and Radiology, Massachusetts General Hospital, and Program in Neuroscience, Harvard Medical School, Boston, Massachusetts, United States of America
| | - John D. Bartlett
- Department of Cytokine Biology, Forsyth Institute, and Department of Developmental Biology, Harvard School of Dental Medicine, Boston, Massachusetts, United States of America
- * E-mail:
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Hassunuma RM, Zen Filho EV, Ceolin DS, Cestari TM, Taga R, de Assis GF. Ultrastructural and immunohistochemical study of the influence of fluoride excess on the development of rat incisor tooth buds. J Appl Oral Sci 2009; 15:292-8. [PMID: 19089147 PMCID: PMC4327432 DOI: 10.1590/s1678-77572007000400010] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2006] [Accepted: 07/04/2007] [Indexed: 11/22/2022] Open
Abstract
Little information is available on the pathogenesis of fluorosis during the fetal and initial postnatal period. In the present study, female rats received 0 (control), 7 or 100 ppm of sodium fluoride in drinking water, one week before breeding and throughout gestation and nursing periods. The hemimandibles of the offspring were collected at 0, 7 and 14 days of postnatal life (n = 5) and processed for morphological analyses by light and electron microscopy, immunohistochemical analysis for amelogenin and morphometric study of enamel matrix and ameloblasts of incisors. The results showed a decrease in matrix production at the secretory phase at all study periods for the 100 ppm group. In this same group, the secretory ameloblasts showed reduction of enamel matrix secretion, disorganization of mitochondrial crests, large vacuoles at the apical portion of the cytoplasm, retention of intracisternal material and dilatation of some cisterns in the rough endoplasmic reticulum. In the groups of animals aged 7 and 14 days, analysis of variance showed significant reduction (p<0.05) in cytoplasmic volume of 23.80% and 24.75%, respectively, in relation to the control group. The smooth-ended maturation ameloblasts exhibited a large number of vacuoles with electron-dense endocytic matrix, suggesting a delay in the resorption process. Immunohistochemical analysis showed no difference in the intensity and labeling pattern of the enamel matrix in any study group. Interestingly, in offspring at the age of 14 days for the 7 ppm group, there was an increase in the matrix length at the secretory phase. Therefore, part of the excessive dose of sodium fluoride given to the mother in drinking water can reach the offspring through the placenta and mother’s milk, causing morphological changes in ameloblasts and suggesting a reduction in secretion and a delay in matrix resorption.
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Bronckers ALJJ, Lyaruu DM, DenBesten PK. The impact of fluoride on ameloblasts and the mechanisms of enamel fluorosis. J Dent Res 2009; 88:877-93. [PMID: 19783795 DOI: 10.1177/0022034509343280] [Citation(s) in RCA: 121] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
Intake of excess amounts of fluoride during tooth development cause enamel fluorosis, a developmental disturbance that makes enamel more porous. In mild fluorosis, there are white opaque striations across the enamel surface, whereas in more severe cases, the porous regions increase in size, with enamel pitting, and secondary discoloration of the enamel surface. The effects of fluoride on enamel formation suggest that fluoride affects the enamel-forming cells, the ameloblasts. Studies investigating the effects of fluoride on ameloblasts and the mechanisms of fluorosis are based on in vitro cultures as well as animal models. The use of these model systems requires a biologically relevant fluoride dose, and must be carefully interpreted in relation to human tooth formation. Based on these studies, we propose that fluoride can directly affect the ameloblasts, particularly at high fluoride levels, while at lower fluoride levels, the ameloblasts may respond to local effects of fluoride on the mineralizing matrix. A new working model is presented, focused on the assumption that fluoride increases the rate of mineral formation, resulting in a greater release of protons into the forming enamel matrix.
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Affiliation(s)
- A L J J Bronckers
- Department of Oral Cell Biology, ACTA, University of Amsterdam and VU University Amsterdam, Research Institute MOVE, Amsterdam, The Netherlands
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12
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Sharma R, Tsuchiya M, Bartlett JD. Fluoride induces endoplasmic reticulum stress and inhibits protein synthesis and secretion. ENVIRONMENTAL HEALTH PERSPECTIVES 2008; 116:1142-1146. [PMID: 18795154 PMCID: PMC2535613 DOI: 10.1289/ehp.11375] [Citation(s) in RCA: 70] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 02/15/2008] [Accepted: 05/15/2008] [Indexed: 05/26/2023]
Abstract
BACKGROUND Exposure to excessive amounts of fluoride (F(-)) causes dental fluorosis in susceptible individuals; however, the mechanism of F(-)-induced toxicity is unclear. Previously, we have shown that high-dose F(-) activates the unfolded protein response (UPR) in ameloblasts that are responsible for dental enamel formation. The UPR is a signaling pathway responsible for either alleviating endoplasmic reticulum (ER) stress or for inducing apoptosis of the stressed cells. OBJECTIVES In this study we determined if low-dose F(-) causes ER stress and activates the UPR, and we also determined whether F(-) interferes with the secretion of proteins from the ER. METHODS We stably transfected the ameloblast-derived LS8 cell line with secreted alkaline phosphatase (SEAP) and determined activity and localization of SEAP and F(-)-mediated induction of UPR proteins. Also, incisors from mice given drinking water containing various concentrations of F(-) were examined for eucaryotic initiation factor-2, subunit alpha (eIF2alpha) phosphorylation. RESULTS We found that F(-) decreases the extracellular secretion of SEAP in a linear, dose-dependent manner. We also found a corresponding increase in the intracellular accumulation of SEAP after exposure to F(-). These changes are associated with the induction of UPR proteins such as the molecular chaperone BiP and phosphorylation of the UPR sensor PKR-like ER kinase, and its substrate, eIF2alpha. Importantly, F(-)-induced phosphorylation of eIF2alphawas confirmed in vivo. CONCLUSIONS These data suggest that F(-) initiates an ER stress response in ameloblasts that interferes with protein synthesis and secretion. Consequently, ameloblast function during enamel development may be impaired, and this may culminate in dental fluorosis.
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Affiliation(s)
- Ramaswamy Sharma
- Department of Cytokine Biology, Forsyth Institute, Boston, Massachusetts, USA
- Department of Developmental Biology, Harvard School of Dental Medicine, Boston, Massachusetts, USA
| | - Masahiro Tsuchiya
- Division of Aging and Geriatric Dentistry, Graduate School of Dentistry, Tohoku University, Sendai, Japan
| | - John D. Bartlett
- Department of Cytokine Biology, Forsyth Institute, Boston, Massachusetts, USA
- Department of Developmental Biology, Harvard School of Dental Medicine, Boston, Massachusetts, USA
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13
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Ribeiro DA, Hirota L, Cestari TM, Ceolin DS, Taga R, Assis GF. Ultrastructural morphometric analysis of ameloblasts exposed to fluoride during tooth development. J Mol Histol 2006; 37:361-7. [PMID: 17077986 DOI: 10.1007/s10735-006-9065-2] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2006] [Accepted: 10/02/2006] [Indexed: 10/24/2022]
Abstract
Since a considerable amount of the world population is exposed to high doses of fluoride, it is of special concern to investigate its action mechanisms during dental enamel development. In this study, the toxicity of fluoride in ameloblasts during enamel development was evaluated by means of ultrastructural morphometric analysis. A total of 18 male Wistar rats were distributed into three groups. In Group I, the animals received deionized drinking water ad libitum (negative control) and in Groups II and III, they received sodium fluorided (NaF) drinking water at doses of 7 and 100 ppm ad libitum, respectively, for 6 weeks. Morphometric data were expressed as volume density of the most significant organelles present in the secretory and maturation phases of amelogenesis such as RER, granules, lysosomes, phagic vacuoles, microfilaments and mitochondria. The results showed that the volume density of mitochondria in the 100 ppm experimental group was 29% (P < 0.05) higher than the control group in secretory ameloblasts. No remarkable differences were found in maturation ameloblasts for all organelles evaluated. Taken together, these data indicate that NaF at high doses is able to induce cellular damage in secretory ameloblasts, whereas no noxious effect was observed during maturation stage of amelogenesis as depicted by ultrastructural analysis.
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Affiliation(s)
- Daniel A Ribeiro
- Department of Health Sciences, Federal University of Sao Paulo, Av. Ana Costa 95, Santos 110601-001, SP, Brazil.
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14
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Bartlett JD, Dwyer SE, Beniash E, Skobe Z, Payne-Ferreira TL. Fluorosis: a new model and new insights. J Dent Res 2005; 84:832-6. [PMID: 16109993 DOI: 10.1177/154405910508400910] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
Abstract
Fluoride is an effective agent for the prevention of dental caries. However, the mechanism of how excessive fluoride exposure causes fluorosis remains uncertain. Zebrafish (Danio rerio) exhibit periodic tooth replacement throughout their lives, thereby providing continuous access to teeth at developmental stages susceptible to fluoride exposure. Zebrafish teeth do not contain true enamel, but consist of a hard enameloid surface. Therefore, we asked whether zebrafish could be used as a model organism for the study of dental fluorosis. Scanning electron microscopy of fluoride-treated teeth demonstrated that the enameloid was pitted and rough, and FTIR analysis demonstrated that the teeth also contained a significantly higher organic content when compared with untreated controls. Furthermore, we demonstrate for the first time that decreased expression of an important signaling molecule (Alk8) in tooth development may contribute to the observed fluorotic phenotype, and that increased cell apoptosis may also play a role in the mechanism of fluorosis.
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Affiliation(s)
- J D Bartlett
- Department of Biomineralization, The Forsyth Institute, Boston, MA 02115, USA
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15
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Kubota K, Lee DH, Tsuchiya M, Young CS, Everett ET, Martinez-Mier EA, Snead ML, Nguyen L, Urano F, Bartlett JD. Fluoride induces endoplasmic reticulum stress in ameloblasts responsible for dental enamel formation. J Biol Chem 2005; 280:23194-202. [PMID: 15849362 DOI: 10.1074/jbc.m503288200] [Citation(s) in RCA: 126] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The mechanism of how fluoride causes fluorosis remains unknown. Exposure to fluoride can inhibit protein synthesis, and this may also occur by agents that cause endoplasmic reticulum (ER) stress. When translated proteins fail to fold properly or become misfolded, ER stress response genes are induced that together comprise the unfolded protein response. Because ameloblasts are responsible for dental enamel formation, we used an ameloblast-derived cell line (LS8) to characterize specific responses to fluoride treatment. LS8 cells were growth-inhibited by as little as 1.9-3.8 ppm fluoride, whereas higher doses induced ER stress and caspase-mediated DNA fragmentation. Growth arrest and DNA damage-inducible proteins (GADD153/CHOP, GADD45alpha), binding protein (BiP/glucose-responsive protein 78 (GRP78), the non-secreted form of carbonic anhydrase VI (CA-VI), and active X-box-binding protein-1 (Xbp-1) were all induced significantly after exposure to 38 ppm fluoride. Unexpectedly, DNA fragmentation increased when GADD153 expression was inhibited by short interfering RNA treatment but remained unaffected by transient GADD153 overexpression. Analysis of control and GADD153(-/-) embryonic fibroblasts demonstrated that caspase-3 mediated the increased DNA fragmentation observed in the GADD153 null cells. We also demonstrate that mouse incisor ameloblasts are sensitive to the toxic effects of high dose fluoride in drinking water. Activated Ire1 initiates an ER stress response pathway, and mouse ameloblasts were shown to express activated Ire1. Ire1 levels appeared induced by fluoride treatment, indicating that ER stress may play a role in dental fluorosis. Low dose fluoride, such as that present in fluoridated drinking water, did not induce ER stress.
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Affiliation(s)
- Kaori Kubota
- Department of Cytokine Biology, Forsyth Institute, Harvard School of Dental Medicine, Boston, Massachusetts 02115, USA
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Li Y, Decker S, Yuan ZA, Denbesten PK, Aragon MA, Jordan-Sciutto K, Abrams WR, Huh J, McDonald C, Chen E, MacDougall M, Gibson CW. Effects of sodium fluoride on the actin cytoskeleton of murine ameloblasts. Arch Oral Biol 2005; 50:681-8. [PMID: 15958199 DOI: 10.1016/j.archoralbio.2004.11.021] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2004] [Accepted: 11/22/2004] [Indexed: 12/24/2022]
Abstract
Fluoride is associated with a decrease in the incidence of dental caries, but excess fluoride can lead to enamel fluorosis, a defect that occurs during tooth enamel formation. In fibroblasts, the Arhgap gene encodes a RhoGAP, which regulates the small G protein designated RhoA. Fluoride treatment of fibroblasts inactivates RhoGAP, thereby activating RhoA, which leads to elevation of filamentous actin (F-actin). Since RhoA is a molecular switch, our hypothesis is that in ameloblasts, fluoride may alter the cytoskeleton through interference with the Rho signaling pathway. Our objective was to measure the effects of sodium fluoride on F-actin using tooth organ culture and confocal microscopy. The results indicated that cellular responses to fluoride include elevation of F-actin in ameloblasts. It was concluded from immunohistochemistry, RT-PCR and confocal approaches that the components of the Rho pathway are present in ameloblasts, and that the response to fluoride involves the Rho/ROCK pathway.
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Affiliation(s)
- Yong Li
- Department of Anatomy and Cell Biology, University of Pennsylvania School of Dental Medicine, Philadelphia, 19104-6030, USA
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17
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DenBesten PK. Biological mechanisms of dental fluorosis relevant to the use of fluoride supplements. Community Dent Oral Epidemiol 1999; 27:41-7. [PMID: 10086925 DOI: 10.1111/j.1600-0528.1999.tb01990.x] [Citation(s) in RCA: 93] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Fluorosis occurs when fluoride interacts with mineralizing tissues, causing alterations in the mineralization process. In dental enamel, fluorosis causes subsurface hypomineralizations or porosity, which extend toward the dentinal-enamel junction as severity increases. This subsurface porosity is most likely caused by a delay in the hydrolysis and removal of enamel proteins, particularly amelogenins, as the enamel matures. This delay could be due to the direct effect of fluoride on the ameloblasts or to an interaction of fluoride with the proteins or proteinases in the mineralizing matrix. The specific mechanisms by which fluoride causes the changes leading to enamel fluorosis are not well defined; though the early-maturation stage of enamel formation appears to be particularly sensitive to fluoride exposure. The development of fluorosis is highly dependent on the dose, duration, and timing of fluoride exposure. The risk of enamel fluorosis is lowest when exposure takes place only during the secretory stage, but highest when exposure occurs in both secretory and maturation stages. The incidence of dental fluorosis is best correlated with the total cumulative fluoride exposure to the developing dentition. Fluoride supplements can contribute to the total fluoride exposure of children, and if the total fluoride exposure to the developing teeth is excessive, fluorosis will result.
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Affiliation(s)
- P K DenBesten
- Department of Growth and Development, University of California at San Francisco, 94019, USA.
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Benson GP, Zaki AE, Eisenmann DR. The use of autoradiography and cycloheximide to determine the origin of enamel proteins in the maturation ameloblasts of the rat incisor. Arch Oral Biol 1998; 43:771-7. [PMID: 9796778 DOI: 10.1016/s0003-9969(98)00055-7] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
Morphological and cytochemical studies have suggested that maturation ameloblasts participate in protein loss by absorbing and degrading enamel proteins as the enamel matures. Several immunocytochemical and autoradiographic studies have suggested other possible explanations for the presence of enamel matrix proteins in maturation ameloblasts. The weakness of these autoradiographic studies is the uncontrolled distribution of systemically injected radioactive amino acids, making it impossible to trace the source of the visualized intracellular isotope. This study used a localized technique to control the targets of the applied isotope and to identify enamel matrix proteins in the maturation ameloblasts with more confidence about their origin. The amount of labelled enamel protein was higher in maturation ameloblasts than transitional ameloblasts. When cycloheximide, a protein-synthesis inhibitor, was applied, there was no effect on the amount of labelled protein in the maturation ameloblasts. These findings support the hypothesis that maturation ameloblasts actively resorb and degrade enamel matrix proteins during enamel formation in the mandibular incisor of the rat.
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Affiliation(s)
- G P Benson
- Department of Oral Biology, University of Illinois, Chicago 60612, USA
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