Involvement of the epidermal growth factor receptor in Pb²⁺-induced activation of cPLA₂/COX-2 genes and PGE₂ production in vascular smooth muscle cells

Lead (Pb²⁺) is one of the most common heavy metal pollutants, which can cause chronic cardiovascular diseases. To clarify the mechanism by which Pb²⁺ induces inflammatory reactions, we examined the expression of inflammatory genes including encoding cyclooxygenase-2 (COX-2), cytosolic phospholipase A₂ (cPLA₂), and their down stream product prostaglandin E₂ (PGE₂) in CRL1999 cells that is a vascular smooth muscle cell line from human aorta. The expression of COX-2/cPLA₂ genes and PGE₂ secretion was increased markedly after cells were exposed to 1μM Pb²⁺. PD098059, a MEK inhibitor, suppressed Pb²⁺-mediated inflammatory reactions; this indicates the involvement of the phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2). Furthermore, Pb²⁺-induced activation of the COX-2/cPLA₂ genes was inhibited by both epidermal growth factor receptor (EGFR) inhibitors (AG1478 and PD153035) and EGFR siRNA. Short-term stimulation with Pb²⁺ induced EGFR phosphorylation at the Tyr residue (position, 1173). Importantly, overexpression of EGFR resulted in a significant potentiation effect on Pb²⁺-induced gene expression. Taken together, our results indicate that 1μM Pb²⁺ can induce PGE₂ secretion by upregulating the transcription of COX-2/cPLA₂ genes. EGFR is the key target in the plasma membrane responsible for transmitting Pb²⁺ signals in order to trigger downstream inflammatory cascades.