Function of DNA methyltransferase 3a in lead (Pb²⁺)-Induced Cyclooxygenase-2 gene

Lead ions (Pb²⁺) are toxic industrial pollutants associated with chronic inflammatory diseases in humans and animals. Previously, we found that Pb²⁺ ions induce COX-2 gene expression via the EGF receptor/nuclear factor-κB signal transduction pathway in epidermoid carcinoma cell line A431. In this study, to see whether Pb²⁺ ions affect COX-2 expression by epigenetic mechanisms, we looked at the mRNAs of DNA methyltransferases (DNMTs) using real-time PCR of total RNA from these cells. Cells exposed to Pb²⁺ had low levels of DNMT3a mRNA, whereas the levels of DNMT1 and DNMT3b mRNAs remained unchanged. Pretreatment of cells with DNMT inhibitor 5-aza-2'-deoxycytidine (5 μM) followed by Pb²⁺ (1 μM) significantly increased levels of COX-2 mRNA compared with cells treated with Pb²⁺ alone. Overexpression of tumor suppressor gene Rb correlated with an increase in COX-2 mRNA and a decrease in DNMT3a mRNA. Conversely, overexpression of transcription factor E2F1 correlated with a decrease in COX-2 mRNA and an increase in DMNT3a mRNA. Pretreatment with EGFR inhibitors AG1478 and PD153035 significantly limited Pb²⁺-induced reduction in DNMT3a mRNA. In addition, gene knockdown of DNMT3a with short hairpin RNA correlated with increased COX-2 mRNA induced by Pb²⁺. Our findings suggest Pb²⁺ ions induce COX-2 expression indirectly by reducing DNMT3a methylation of the COX-2 promoter via transcription factors Rb and E2F1.