The Effects of Dental Resin Polymerization Initiators on Cell Lipid Metabolism

Abstract

Benzoyl peroxide and camphorquinone, initiators of heat and light polymerized dental resins, are considered cytotoxic and the mechanism of cytotoxicity suggested is lipid peroxidation-induced membrane damage. The mechanism of such damage is not clear. The objectives of our current study were I) To study the effects of the various concentrations of initiators benzoyl peroxide and camphorquinone on cell lipid metabolism, 2) To study the effects of peroxidation-inducing concentrations of benzoyl peroxide on turnover of major lipids, 3) To study the effects of the materials on the lipid second messenger ceramide and on apoptotic responses in cells. Methods. Lipid metabolism i.e. synthesis as well as turnover, was measured using l4C acetate in HCP and THP-l cells. The lipids were extracted using the Bligh & Dyer method of lipid extraction and separated using one and two-dimensional thin-layer chromatography. The lipid peroxidation was measured using thio-barbituric acid reactive substance (T-BARS) produced in response to benzoyl peroxide combined with ferric chloride and camphorquinone with, or without activation with light, when combined with an enhancer dimethylaminoethyl ethyl methacrylate (DMAEMA). Ceramides were detected by extracting neutral lipids using chloroform/methanol extraction and separated by high performance thin-layered chromatography (HPTLC). DNA fragmentation assay was used to detect apoptosis. Results. Benzoyl peroxide and camphorquinone at minimally inhibitory concentrations induced similar changes in neutral lipids such as increased triglycerides and decreased cholesterol synthesis. Sphingomyelin changes were specific to HCP cells exposed to camphorquinone. The changes were mostly related to altered synthesis rather than turnover. The changes were also cell-type specific. Toxic concentrations induced peroxidation as measured by T-BARS in a time and dose dependent manner only in HCP cells while THP-1 showed different responses. Major lipid profiles were unaltered at peroxidation-inducing concentrations. Sub-toxic concentrations of benzoyl peroxide induced ceramide elevation at 24 hours, after an initial inhibition at 10 minutes, in both cell types. DNA fragmentation was, however, evident only in THP-l cells at sub-toxic concentration. Conclusion. Both initiators, benzoyl peroxide and camphorquinone, induced changes in neutral lipids. Their mechanism of peroxidation-inducing membrane damage was not dependent on the quantitative alteration in major polar lipids. Benzoyl peroxide induced changes in ceramides in both HCP and THP-l cells. Induction of apoptosis was clearly seen only in THP-l cells in response to benzoyl peroxide while HCP cells lacked this response.