Development of low- and high-serum culture conditions for use of human oral fibroblasts in toxicity testing of dental materials

Use of siRNA in dental tissue-derived cell cultures: integrin knockdown in fibroblasts

Short (or small) interfering RNAs (siRNAs) are double-stranded RNA molecules about 21-25 nucleotides long that have the capacity to disrupt the activity of genes on a posttranscriptional level. This sequence homology-driven gene silencing capacity has been utilized by researchers to selectively block the translation of mRNA to proteins in order to study specific gene functions and identify target molecules. Importantly, siRNAs have the potential to be used in treatment of disease. Here, we describe how the siRNA technology can be used to knock down genes in dental tissue-derived cells using integrin α11 knockdown as an example.

A Role for α11β1 Integrin in the Human Periodontal Ligament

We previously demonstrated a role for α11β1 integrin in periodontal ligament (PDL)-driven tooth eruption in the mouse. To explore a possible role for α11β1 in the human periodontium, we have characterized the expression and function of α11 in human PDL tissue, in human PDL fibroblasts (hPDLF), and in human gingival fibroblasts (hGF). α11 expression was detected in PDL tissue, in hPDLF, and in hGF cells. Platelet-derived growth factor-BB and insulin-like growth factor II stimulated contraction of collagen lattices by both types of fibroblasts. α2 integrin blocking antibodies and the use of α11 siRNA demonstrated a role for both α2β1 and α11β1 in collagen lattice remodeling. Analysis of the proximal ITGA11 promoter from persons with chronic periodontal disease failed to reveal any polymorphism. Analysis of our data shows that α11β1 is a major collagen receptor on cultured human PDL cells and implies that it is also functionally important in the PDL in vivo.

The application of normal, SV40 T-antigen-immortalised and tumour-derived oral keratinocytes, under serum-free conditions, to the study of the probability of cancer progression as a result of environmental exposure to chemicals

In vitro models are currently not considered to be suitable replacements for animals in experiments to assess the multiple factors that underlie the development of cancer as a result of environmental exposure to chemicals. An evaluation was conducted on the potential use of normal keratinocytes, the SV40 T-antigen-immortalised keratinocyte cell line, SVpgC2a, and the carcinoma cell line, SqCC/Y1, alone and in combination, and under standardised serum-free culture conditions, to study oral cancer progression. In addition, features considered to be central to cancer development as a result of environmental exposure to chemicals, were analysed. Genomic expression, and enzymatic and functional data from the cell lines reflected many aspects of the transition of normal tissue epithelium, via dysplasia, to full malignancy. The composite cell line model develops aberrances in proliferation, terminal differentiation and apoptosis, in a similar manner to oral cancer progression in vivo. Transcript and protein profiling links aberrations in multiple gene ontologies, molecular networks and tumour biomarker genes (some proposed previously, and some new) in oral carcinoma development. Typical specific changes include the loss of tumour-suppressor p53 function and of sensitivity to retinoids. Environmental agents associated with the aetiology of oral cancer differ in their requirements for metabolic activation, and cause toxic effects to cells in both the normal and the transformed states. The results suggest that the model might be useful for studies on the sensitivity of cells to chemicals at different stages of cancer progression, including many aspects of the integrated roles of cytotoxicity and genotoxicity. Overall, the properties of the SVpgC2a and SqCC/Y1 cell lines, relative to normal epithelial cells in monolayer or organotypic culture, support their potential applicability to mechanistic studies on cancer risk factors, including, in particular, the definition of critical toxicity effects and dose-effect relationships.

Using optical tweezers for measuring the interaction forces between human bone cells and implant surfaces: System design and force calibration

Optical tweezers were used to study the interaction and attachment of human bone cells to various types of medical implant materials. Ideally, the implant should facilitate cell attachment and promote migration of the progenitor cells in order to decrease the healing time. It is therefore of interest, in a controlled manner, to be able to monitor the cell adhesion process. Results from such studies would help foresee the clinical outcome of integrating medical implants. The interactions between two primary cell culture models, human gingival fibroblasts and bone forming human osteoblast cells, and three different implant materials, glass, titanium, and hydroxyapatite, were studied. A novel type of optical tweezers, which has a newly designed quadrant detector and a powerful 3 W laser was constructed and force calibrated using two different methods: one method in which the stiffness of the optical trap was obtained by monitoring the phase lag between the trap and the moved object when imposing a forced oscillation on the trapped object and another method in which the maximum trapping force was derived from the critical velocity at which the object escapes the trap. Polystyrene beads as well as cells were utilized for the calibrations. This is the first time that cells have been used directly for these types of force calibrations and, hence, direct measurements of forces exerted on cells can be performed, thus avoiding the difficulties often encountered when translating the results obtained from cell measurements to the calibrations obtained with reference materials. This more straightforward approach represents an advantage in comparison to established methods.

Cytotoxicity of new resin-, calcium hydroxide- and silicone-based root canal sealers on fibroblasts derived from human gingiva and L929 cell lines

AIM

To assess ex vivo the cytotoxic effects of five new root canal sealers (RC Sealer, Epiphany, EndoREZ, GuttaFlow and Acroseal) and three existing products (AH Plus, RoekoSeal and Apexit) using primary human gingival fibroblasts (HGF) and a mouse fibroblast cell line, L929.

METHODOLOGY

Eight samples of each sealer were fabricated in sterile cylindrical Teflon blocks, 4.4 mm diameter and 2 mm height and then divided into two groups, fresh and aged specimens. Extraction of fresh specimens was carried out after setting whilst aged specimens were placed in Petri dishes and kept in a humid chamber at 37 degrees C for 7 days before extraction in cell culture medium using the ratio 1.25 cm(2) mL(-1). Undiluted eluates were used for the dimethylthiazol diphenyltetrazolium bromide (MTT) assay with HGF and L-929. Morphology of HGF cells was also examined by an inverted microscope using undiluted eluates of the sealers. The results were analysed using a two-tailed t-test (alpha = 0.05) between groups.

RESULTS

Resin-based (Epiphany and EndoREZ) and calcium hydroxide-based (Apexit and Acroseal) sealers were significantly more cytotoxic than other sealers (P<0.05). However, L929 cells were more sensitive to Apexit and EndoREZ than HGF cells. RC Sealer showed mild cytotoxicity to HGF at both setting times. AH Plus did not exert any cytotoxic effect to HGF and aged specimens appeared to induce cellular proliferation. RoekoSeal and GuttaFlow also demonstrated mild cytotoxicity. GuttaFlow was slightly more cytotoxic to both cultures, especially when tested fresh.

CONCLUSIONS

Toxicity varied but RC Sealer and GuttaFlow were the least toxic new sealers.

Laser therapy accelerates initial attachment and subsequent behaviour of human oral fibroblasts cultured on titanium implant material. A scanning electron microscope and histomorphometric analysis

The aim of the study was to investigate the effect of low-level laser therapy (LLLT) on attachment and proliferation of human gingival fibroblasts (HGF) cultured on titanium implant material. HGF were exposed to gallium-aluminum-arsenide diode laser at dosages of 1.5 or 3 J/cm(2) and then cultured on commercially pure titanium discs. Cell profile areas were measured after 1, 3 and 24 h, using scanning electron microscopy and an automatic image analyzer. The results were expressed as percentage of attachment. In order to investigate the effect of LLLT on cellular growth after 8 and 10 days, HGF were cultured on titanium discs for 24 h and then exposed to laser irradiation on 3 consecutive days. Colony-forming efficiency (CFE) and clonal growth rates (CGR) were measured. Cell viability was determined by Hoechst and prodidium iodide staining. Non-lased cultures served as controls. Morphologically, the cells spread well on all titanium surfaces, indicating good attachment by both irradiated and non-irradiated cells. Fibroblasts exposed to laser irradiation had significantly higher percentages of cell attachment than the non-exposed cells (P<0.05). CFE and CGR were also enhanced for the irradiated cells (P<0.05). Cell viability was high (>90%) in the irradiated and control groups, without significant differences. It is concluded that in vitro LLLT enhances the attachment and proliferation of HGF on titanium implant material.

Determining optimal dose of laser therapy for attachment and proliferation of human oral fibroblasts cultured on titanium implant material

The purpose of this study was to investigate the influence of single or multiple doses of low-level laser therapy (LLLT) on attachment and proliferation of human gingival fibroblasts in a standardized, reproducible in vitro model. Titanium discs were randomly allotted to one of three groups: group I served as a control, group II was exposed to a single laser dose of 3 J/cm2, and the three subgroups in group III were exposed to laser doses of 0.75, 1.5, and 3 J/cm2. To examine the possible thermal effects of laser exposure on the cell culture, the temperature in the Petri dish was measured for every dose used, before and during irradiation. For attachment assays, groups II and III were exposed to laser irradiation and then seeded onto titanium discs. In group III, the exposures were repeated after 3 and 6 h. Cells were cultured for 6 and 24 h and stained with Hoechst and Propidium. Attached cells were counted under a light microscope. To investigate the effect of LLLT on cell proliferation after 48 h, 72 h, and 7 days, cells were cultured on titanium discs for 24 h and then exposed to laser irradiation for 1 day and 3 consecutive days, respectively. Cell proliferation was determined by counting cells under the microscope and by a cell proliferation enzyme-linked immunosorbent assay system. No increase of temperature of the cell cultures occurred before or during laser exposure at any of the doses tested. Both single and multiple doses of LLLT significantly enhanced cellular attachment (p<0.05). The proliferation assays showed higher cell proliferation (p<0.05) in group III at doses of 1.5 and 3 J/cm2 after 72 h and 7 days, with agreement between staining and enzyme-linked immunosorbent assay. It is concluded that, in this cellular model, the attachment and proliferation of human gingival fibroblasts are enhanced by LLLT in a dose-dependent manner.

The influence of surface topography of ceramic abutments on the attachment and proliferation of human oral fibroblasts

As different implant abutments are introduced to obtain a sufficient soft tissue barrier, the aim of this study was to determine the effect of three different surface modifications of densely sintered high-purity aluminium oxide on morphology, attachment and proliferation of human gingival fibroblasts. Fibroblasts were cultured on pressed aluminium oxide, milled, and then sintered to full density (1), on pressed, densely sintered (2), and on pressed, densely sintered and then polished surfaces (3). The different surfaces were analyzed using a confocal laser scanner, an atomic force microscope and a scanning electron microscope. The cell profile areas were measured using a semiautomatic interactive image analyzer and the figures were expressed as percent of attachment. The polished specimens had the smoothest surfaces and the roughest were the milled surfaces in terms of height deviation. No difference was found in the spacing between the peaks on the polished surfaces compared to the milled surfaces. Fibroblasts on the milled ceramic appeared to follow the direction of the fine irregularities on the surface. The analyses showed the polished surfaces had significantly higher percentages of initial cell attachment than the other surfaces (P < 0.05). After 3 days of cell culture, significantly more cells were attached to the milled and sintered surfaces than to the polished one, possibly indicating higher proliferation capacity on those types of surfaces.

Uniform expression of alcohol dehydrogenase 3 in epithelia regenerated with cultured normal, immortalised and malignant human oral keratinocytes

The human oral epithelium is a target for damage from the inhalation of formaldehyde. However, most experimental studies on this chemical have relied on laboratory animals that are obligatory nose breathers, including rats and mice. Therefore, in vitro model systems that mimic the structure of the human oral epithelium and which retain normal tissue-specific metabolic competence are desirable. Based on the established role of alcohol dehydrogenase 3 (ADH3), also known as glutathione-dependent formaldehyde dehydrogenase, as the primary enzyme catalysing the detoxification of formaldehyde, the aim of this study was to investigate the expression of ADH3 in organotypic epithelia regenerated with normal (NOK), immortalised (SVpgC2a) and malignant (SqCC/Y1) human oral keratinocytes. Organotypic epithelia, usually consisting of 5-10 cell layers, were produced at the air-liquid interface of collagen gels containing human oral fibroblasts, after culture for 10 days in a standardised serum-free medium. Immunochemical staining demonstrated uniform expression of ADH3 in these organotypic epithelia, as well as in the epithelial cells of oral tissue. The specificity of the ADH3 antiserum was ascertained from the complete neutralisation of the immunochemical reaction with purified ADH3 protein. Assessment of the staining intensities indicated that the expression levels were similar among the regenerated epithelia. Furthermore, the regenerated epithelia showed similar ADH3 expression to the epithelium in oral tissue. Therefore, a tissue-like expression pattern for ADH3 can be generated from the culture of various oral keratinocyte lines in an organotypic state. Similar expression levels among the various cell lines indicate the preservation of ADH3 during malignant transformation, and therefore that NOK, SVpgC2a and SqCC/Y1 represent functional models for in vitro studies of formaldehyde metabolism in human oral mucosa.

Expression of alcohol dehydrogenase 3 in tissue and cultured cells from human oral mucosa

Because formaldehyde exposure has been shown to induce pathological changes in human oral mucosa, eg, micronuclei, the potential enzymatic defense by alcohol dehydrogenase 3 (ADH3)/glutathione-dependent formaldehyde dehydrogenase was characterized in oral tissue specimens and cell lines using RNA hybridization and immunological methods as well as enzyme activity measurements. ADH3 mRNA was expressed in basal and parabasal cell layers of oral epithelium, whereas the protein was detected throughout the cell layers. ADH3 mRNA and protein were further detected in homogenates of oral tissue and various oral cell cultures, including, normal, SV40T antigen-immortalized, and tumor keratinocyte lines. Inhibition of the growth of normal keratinocytes by maintenance at confluency significantly decreased the amount of ADH3 mRNA, a transcript with a determined half-life of 7 hours. In contrast, decay of ADH3 protein was not observed throughout a 4-day period in normal keratinocytes. In samples from both tissue and cells, the ADH3 protein content correlated to oxidizing activity for the ADH3-specific substrate S:-hydroxymethylglutathione. The composite analyses associates ADH3 mRNA primarily to proliferative keratinocytes where it exhibits a comparatively short half-life. In contrast, the ADH3 protein is extremely stable, and consequently is retained during the keratinocyte life span in oral mucosa. Finally, substantial capacity for formaldehyde detoxification is shown from quantitative assessments of alcohol- and aldehyde-oxidizing activities including K:(m) determinations, indicating that ADH3 is the major enzyme involved in formaldehyde oxidation in oral mucosa.

Toxicity of formaldehyde to human oral fibroblasts and epithelial cells: influences of culture conditions and role of thiol status

The toxicity of formaldehyde, a monomer released from certain polymeric dental materials, was studied in cultured human oral fibroblasts and epithelial cells. The influences of growth conditions were evaluated for both cell types, as well as the role of the internal and external thiol states. A one-hour exposure to formaldehyde decreased the colony-forming efficiency (CFE) of both cell types in a concentration-dependent manner, although the toxicity varied up to 100-fold with the conditions. Clearly, the presence of serum and the thiol cysteine counteracted the toxicity in fibroblasts. Similarly, pituitary extract and cysteine, or a mixture of amino acids and ethanolamines, counteracted the formaldehyde toxicity in serum-free cultures of epithelial cells. In contrast, a growth-promoting surface matrix of fibronectin and collagen did not influence the formaldehyde toxicity, as shown by both the CFE assay and a dye reduction assay. Further, a short-term change to the various growth media per se with or without the supplements serum or cysteine did not significantly alter the CFE. Analysis of the thiol state demonstrated significant differences between epithelial cells and fibroblasts, i.e., comparatively lower cellular levels of the free low-molecular-weight thiols glutathione and cysteine in fibroblasts. This result correlated to significantly higher formaldehyde toxicity in the fibroblasts than in the epithelial cells. Taken together, the results indicated the cytoprotective function of both intracellular and extracellular thiols toward formaldehyde, as well as the usefulness of thiol-free and chemically defined conditions for toxicity assessments in oral epithelial cells and fibroblasts. We conclude that the combined use of a controlled external milieu and the presumed target cell type may be advantageous in evaluations of oral toxicity mechanisms or the toxic potency of dental materials, particularly those which, like formaldehyde, may react with thiols or amines.

Cytotoxicity of impression materials

The purpose of this study was to assess the potential cytotoxicity of impression materials. The impression material brands tested included Impregum, Reprosil, Surgident, Permlastic, Jeltrate Regular and Jeltrate Fast Set. These impression materials and their components were tested for their possible cytotoxic effects by three different methods. The results showed that all impression materials were cytotoxic to some degree by all three assay methods and some of the components were also cytotoxic. These results support the literature showing in vivo adverse effects of certain impression materials and/or their components in patients and practitioners.

Evaluation of biocompatibility of various ceramic powders with human fibroblasts in vitro

Cell reaction to powders of ceramics was studied in vitro. Cultured human fibroblasts were exposed to different types of ceramic powders: zirconia (ZP), alumina (A), tricalcium phosphate (TCP) and hydroxyapatite (HA), at various concentrations. The cell viability at the different exposure times was measured by the colony formation (expressed as colony forming efficiency, CFE), neutral red uptake (NR) and colorimetric tetrazolium (MTT) reduction. Alumina and hydroxyapatite showed no cytotoxic effects at studied doses (1-500 mug/ml) while zirconia and tricalcium phosphate inhibited cell viability, with 50% of CFE reduction at the concentration of about 50 mug/ml. In order to study the cytotoxic mechanism of zirconia powder, two further experiments were included, viz. the cellular response to the sintered zirconia ceramic powders (CZP) which were obtained by crushing the sintered ceramic material; and the measurement of the degradation of zirconia ceramic plate in the different solutions, i.e., either in saline or in 0.02 M lactic acid (pH 2.72). Similar cell reactions were obtained for the CZP and ZP by using MTT and NR assays. Slow releases of ions from zirconia ceramic plate, yttrium in both solutions and zirconium and yttrium in lactic acid, were detected.

The mechanism of Hg2+ toxicity in cultured human oral fibroblasts: the involvement of cellular thiols

To study amalgam-related toxicity in a primary target cell type, human oral fibroblasts were grown in a low-serum medium containing 1.25% fetal bovine serum and exposed to Hg2+, a corrosion product of amalgam. A 1-h exposure to various concentrations of Hg2+ resulted in a dose-dependent loss of colony forming efficiency. Removal of the low-molecular-weight thiol cysteine from the medium increased the toxicity of Hg2+ almost 50-fold in comparison with complete medium or medium without fetal bovine serum. Accordingly, fetal bovine serum was not found to contain detectable levels of low-molecular-weight thiols. The levels of cellular free protein thiols were shown to be depleted Hg2+ at significantly lower concentrations of the metal ion than those required to decrease the levels of the major cellular low-molecular weight thiol glutathione. These decreases were dependent on the exposure conditions, i.e. the presence of serum and thiols, in a manner similar to the effect on colony forming efficiency. Other functions commonly related to cell viability, including the accumulation of the vital dye neutral red, the cytosolic retention of deoxyglucose and the mitochondrial reduction of tetrazolium were also inhibited by Hg2+, albeit at higher concentrations. Finally, the depletion of cellular glutathione, by pre-exposure of the cells to the glutathione synthesis inhibitor buthionine sulfoximine, somewhat increased the toxicity of Hg2+ and potentiated the depletion of protein thiols. Taken together, the toxicity of Hg2+ in human oral fibroblasts was demonstrated in several assays of which colony forming efficiency was the most sensitive, cell killing by this agent was related to its high affinity for protein thiols, whereas glutathione showed a significant, but limited, ability to protect the cells from Hg2+ toxicity.