An autophagy deficiency promotes methylmercury-induced multinuclear cell formation.

Methylmercury (MeHg) is a highly toxic pollutant, and is considered hazardous to human health. In our previous study, we found that MeHg induces autophagy and that Atg5-dependent autophagy plays a protective role against MeHg toxicity. To further characterize the role of autophagy in MeHg-induced toxicity, we examined the impact of autophagy on microtubules and nuclei under MeHg exposure using Atg5KO mouse embryonic fibroblasts (MEFs). Low concentrations of MeHg induced a decrease in α-tubulin and acetylated-tubulin in both wild-type and Atg5KO cells. While α-tubulin acetylation was promoted by treatment with tubacin, a selective inhibitor of histone deacetylase 6, MeHg treatment inhibits the increase of tubacin-induced acetylated-tubulin. However, similar effects were observed for treatment with either tubacin or tubacin + MeHg in wild-type and Atg5KO cells. We also found a significant increase in the number of multinuclear cells upon MeHg exposure in Atg5KO MEFs compared to wild-type MEFs. In addition, DNA double strand breaks (DSBs), measured by phosphorylation of the core histone H2A variant (H2AX) on serine 139 (γH2AX), markedly increased in Atg5KO MEFs compared to wild-type MEFs. Our results therefore suggest that autophagy is not a simple elimination pathway of MeHg-induced damaged proteins, but that it also plays a protective role in the context of MeHg-associated DSBs.