Cytotoxic effects of polycarbonate-based orthodontic brackets by activation of mitochondrial apoptotic mechanisms
Introduction
Polycarbonate resins have been increasingly used as dental biomaterials due to their biocompatibility, exceptional esthetics and tailored mechanical attributes [1]. An interesting and growing application of polycarbonate resins was the production of esthetic orthodontic brackets. Early attempts to produce orthodontic brackets from unfilled polycarbonates were unsuccessful, due to excessive in-service distortion, discoloration and staining [2], [3], [4]. To improve water resistance, new glass-particle or glass-fiber reinforced materials were introduced with metallic-strengthened slots, in an attempt to diminish the undesirable features.
Bisphenol-A (BPA) is the main raw material used in the production of the aromatic polycarbonate (ArPCB) matrix of many plastic esthetic brackets. The benzene rings and the quaternary carbon atoms of the BPA structure create a bulk, stiff chain which offers rigidity, strength and less susceptibility to biodegradation in comparison with aliphatic polycarbonates [5]. Moreover, ArPCBs offer temperature and impact resistance, excellent optical properties, large plastic deformations without cracking and easy molding and thermoforming capacity, making this material attractive for component manufacturing.
The widespread use of polycarbonate-based orthodontic brackets has caused concerns on the possible biological and systemic health side-effects of the eluents of these materials released intraorally by physical and chemical processes. Brackets made of ArPCBs have demonstrated sensitivity to water plasticization and water cracking, resulting in water degradation and release of traceable amounts of BPA in the oral cavity of patients and in aqueous environments after long term immersion [6], [7], [8], [9]. Cytotoxic responses of plastic bracket eluents in human gingival fibroblasts have been presented so far, leading to reduced viability, plasma membrane damage, DNA fragmentation and increased cell death [10].
BPA and BPA derivatives, increase the levels of reactive oxygen species [9], [11] that are known mediators of signaling cascades under physiological conditions. Elevated levels of such compounds can disrupt the cellular redox equilibrium, causing oxidative DNA damage and apoptosis in mammalian cells. BPA specifically, has been previously shown to activate multiple cytotoxic mechanisms and induce DNA damage by activating oxidative stress, p53 [12] and other cell cycle proteins [13], mitochondrial [14] and endoplasmic reticulum proteins [15] and mTOR pathways [16]. The role of BPA in the canonical apoptotic pathways has been poorly examined and there is limited data associating its role in mitochondrial cell death of T cell lines [17] and germ cells after UV irradiation and hydroquinone treatment [18]. At the same time, epidemiological and genetic studies have shown that BPA is an environmental estrogenic compound that can exert proliferative responses and more specifically can induce hormonal-related effects including altered peripubertal mammary gland development in mice [19]; early puberty in females [20] and feminization in males; higher risk for breast cancer in females and prostate cancer in males [21]; induction of calcium influx, which leads to prolactin release and associated behavioral effects [22], [23]; development of hyperglycemia and insulin tolerance [24]; elevation of oxidative stress mediators [25] and upregulation of the cAMP response element-binding factor, which inhibits apoptosis [26].
Due to the significant use of polycarbonate particle- and fiber-reinforced esthetic brackets in the orthodontic practice and the implication of their constituents with contrasting biological activities, the present study was designed aiming to investigate possible cytotoxic and estrogenic effects of eluents from three types of polycarbonate brackets on NIH 3T3 fibroblasts, Hela cells and the estrogen receptive MCF7 cell lines. The hypothesis tested was that there are no statistically significant differences in the performance of the brackets.
Section snippets
Brackets
The brackets used in this study are listed in Table 1. According to the manufacturers’ product information sheets, EL, EG and SL are all polycarbonate composite brackets reinforced with filler-particles (EL) or glass-fibers (EG, SL). The polycarbonate-free bracket (BR), composed of unfilled polyoxymethylene, was used as a control, whereas triple distilled water was used as the immersion medium for brackets in all the experiments performed.
Composition
The molecular composition of the plastic brackets was
Composition
Representative ATR-FTIR spectra of the plastic brackets are presented in Fig. 1A–D.
Table 2 summarizes the FTIR peak assignments. BR demonstrated a typical spectrum of polyoxymethylene. A copolymer of aromatic polycarbonate and polyethylene terepthalate was the major component of EL, whereas EG and SL were composed of aromatic polycarbonates.
EG eluent induces cell death and disrupts cell proliferation
EG eluent induced significant release of LDH in the culture medium at 24 h in all three lines tested (Fig. 2). Similar results were obtained at 48 h (data not
Discussion
Although plastic brackets have been marketed for many years, they are still subjected to continuous developments aiming to improve strength, durability and esthetics. Besides, there is a significant interest on the biological and systemic health effects, mainly attributed to the starting raw bracket materials, since the latter have been implicated with multiple and often opposing biological effects [31].
As previous reports have shown that polycarbonate based orthodontic brackets release
Conclusions
In the present study evidence is provided for cytotoxic effects of particle- and fiber-reinforced polycarbonate orthodontic brackets in fibroblast and breast cancer cells through the activation of mitochondrial cell death mechanisms. These were independent of the described estrogenic effects of these materials. This is the first study that provides data for an apoptotic effect of these materials and can serve as a basis for further studies to implicate alternative cell death mechanisms induced
References (49)
- et al.
In vitro torque-deformation characteristics of orthodontic polycarbonate brackets
American Journal of Orthodontics and Dentofacial Orthopedics
(1994) - et al.
Torsional creep of polycarbonate orthodontic brackets
Dental Materials
(1997) - et al.
In-vitro study of the cellular viability and nitric oxide production by J774 macrophages with ceramic, polycarbonate, and polyoxymethylene brackets
American Journal of Orthodontics and Dentofacial Orthopedics
(2010) - et al.
The role of reactive oxygen species and hemeoxygenase-1 expression in the cytotoxicity, cell cycle alteration and apoptosis of dental pulp cells induced by BisGMA
Biomaterials
(2010) - et al.
Effect of bisphenol-A on the expression of selected genes involved in cell cycle and apoptosis in the OVCAR-3 cell line
Toxicology Letters
(2011) - et al.
Mitochondrial signaling pathway is also involved in bisphenol A induced germ cell apoptosis in testes
Toxicology Letters
(2010) - et al.
Bisphenol A induces endoplasmic reticulum stress-associated apoptosis in mouse non-parenchymal hepatocytes
Life Sciences
(2010) - et al.
Life with or without AIF
Trends in Biochemical Sciences
(2010) - et al.
The role of Bisphenol A in shaping the brain, epigenome and behavior
Hormones and Behavior
(2011) - et al.
Bisphenol A is released from polycarbonate drinking bottles and mimics the neurotoxic actions of estrogen in developing cerebellar neurons
Toxicology Letters
(2008)