Interaction between LPS and a dental resin monomer on cell viability in mouse macrophages

Lipopolysaccharide (LPS) from cariogenic microorganisms and resin monomers like HEMA (2-hydroxyethyl methacrylate) included in dentin adhesive are present in a clinical situation in deep dentinal cavity preparations. Here, cell survival, expression of proteins related to redox homeostasis, and viability of mouse macrophages exposed to LPS and HEMA were analyzed with respect to the influence of oxidative stress.

Cell survival of RAW264.7 mouse macrophages was determined using a crystal violet assay, protein expression was detected by Western blotting, and HEMA- or LPS-induced apoptosis (cell viability) was analyzed by flow cytometry. Cells were exposed to HEMA (0-8mM), LPS (0.1μg/ml) or combinations of both substances for 24h. The influence of mitogen-activated protein kinases (MAPK) was analyzed using the specific inhibitors PD98059 (ERK1/2), SB203580 (p38) or SP600125 (JNK), and oxidative stress was identified by the antioxidant N-acetylcysteine (NAC).

Cell survival was reduced by HEMA. LPS, however, increased cell survival from 29% in cultures exposed to 8mM HEMA, to 46% in cultures co-exposed to 8mM HEMA/LPS. Notably, LPS-induced apoptosis was neutralized by 4-6mM HEMA but apoptosis caused by 8mM HEMA was counteracted by LPS. Expression of NOS (nitric oxide synthase), p47^phox and p67^phox subunits of NADPH oxidase, catalase or heme oxygenase (HO-1) was associated with HEMA- or LPS-induced apoptosis. While no influence of MAPK was detected, NAC inhibited cytotoxic effects of HEMA.

HEMA- and LPS-triggered pathways may induce apoptosis and interfere with physiological tissue responses as a result of the differential formation of oxidative stress.