MtDNA Copy Number in Oral Epithelial Cells Serves as a Potential Biomarker of Mitochondrial Damage by Neonicotinoid Exposure: A Cross-Sectional Study

As the mitochondrial DNA copy number (mtDNAcn) has been reported to be a biomarker for mtDNA damage in honeybees when exposed to sublethal neonicotinoids, the feasibility of using human mitochondria as a predictor upon neonicotinoid exposure remains elusive. This study investigated the association between the urinary neonicotinoid and the relative mtDNAcn (RmtDNAcn) of oral epithelial cells collected in a cross-sectional study with repeated measurements over 6 weeks. The molecular mechanism underlying neonicotinoid-caused mitochondrial damage was also examined by in vitro assay. Herein, the average integrated urinary neonicotinoid (IMIRPF) concentration ranged from 8.01 to 13.70 μg/L (specific gravity-adjusted) during the sampling period. Concomitantly, with an increase in the urinary IMIRPF, the RmtDNAcn significantly increased from 1.20 (low group) to 1.93 (high group), indicating potential dose-dependent mitochondrial damage. Furthermore, the linear regression analysis confirmed the significant correlation between the IMIRPF and RmtDNAcn. Results from in vitro assays demonstrated that neonicotinoid exposure led to the inhibition of the genes encoding mitochondrial oxidative phosphorylation (OXPHOS) complexes I and III (e.g., ND2, ND6, CytB, and CYC1), accompanied by increased reactive oxygen species production in SH-SY5Y cells. Conjointly, neonicotinoid exposure led to mitochondrial dysfunction and a resulting increase in the RmtDNAcn, which may serve as a plausible biomarker in humans.