Methacrylates are common constituents of resin-based biomaterials. These materials cure by polymerization of methacrylate monomers, but incomplete polymerization causes leakage and patient exposure to the electrophile monomers.

2-Hydroxyethylmethacrylate (HEMA) is a widely used methacrylate in resin-based dental biomaterials. In vitro studies demonstrate dose-dependent toxicity of HEMA. Increased oxidative stress is a suggested key event in the onset of the toxic response.

The Nrf2 level increases in HEMA-exposed cell cultures. The Nrf2 level is important in the regulation of many cytoprotective genes, e.g., several enzymatic antioxidants. In this study, we aim to determine if the in vitro toxicity to HEMA depends on Nrf2 activity. The ability of the Nrf2-system to influence HEMA-induced toxicity may give information on the underlying mechanism of HEMA toxicity.

We compared the effect of HEMA on two human epithelial airway cell lines differing in the Nrf2 signaling pathway integrity: the SV40 transformed human bronchial epithelial cell line BEAS-2B and the human alveolar epithelial cancer cell line A549. The latter contains a mutation in the Keap1 gene, an important regulator of Nrf2 level and activity. We also tested the effect of adaptation to “lowdose” HEMA exposure on the cytotoxic response to a second exposure. Cell viability was measured by the MTT-assay, and western blotting was used to analyze the levels of specific proteins in HEMA (0-8 mM)-exposed cells.

The two cell lines exhibited a similar pattern of HEMA toxicity as measured by MTT. In BEAS-2B cells, the levels of several Nrf2-regulated proteins, including proteins with antioxidant activity, increased after HEMA exposure for 24 hours, whereas no change was seen in A549 cells. The levels of these proteins in A549 cells were comparable to the levels in HEMA exposed BEAS-2B cells. Although treatment with HEMA for 24 hours increased the levels of several proteins with cytoprotective/antioxidant potential in BEAS-2B cells, pretreatment with a “non-toxic” HEMA dose for 24 hours did not reduce the toxic response to a subsequent HEMA exposure.

In summary, the results indicate that HEMA induced the Nrf2 activity in BEAS-2B cells, while the Nrf2 pathway was continuously active in A549 cells. However, the results do not support a hypothesis suggesting that increased Nrf2 activity protects cells against HEMA-induced toxicity. This may point to mechanisms independent of oxidative stress to explain HEMA toxicity.

The Official Journal of the Society of Toxicology, Volume 162, Issue1, March 2018, Abstract Number 2162, page 282
J. Samuelsen, E. Morisbak, S. Uvsløkk and B. P. Olderbø.