Calcium-mediated activation of c-jun NH2-terminal kinase (JNK) and apoptosis in response to cadmium in murine macrophages

Cadmium is a well-known carcinogenic and immunotoxic metal commonly found in cigarette smoke and industrial effluent. An altered intracellular calcium ([Ca2+](i)) level has been implicated in the pathophysiology of immune dysfunction. The present study was designed to determine the possible involvement of calcium (Ca2+) and mitogen-activated protein kinases (MAPKs) signaling pathways on cadmium-induced cell death in J774A.1 murine macrophage cells. Cadmium caused a low-amplitude [Ca2+](i) elevation at 20 muM and rapid and high-amplitude [Ca2+](i) elevation at 500 muM. Exposure to cadmium dose-dependently induced phosphorylation of c-Jun NH2-terminal kinase (JNK) and deactivated p38 MAPK. Use of the selective JNK inhibitor SP600125 suggested that activation of JNK is pro-apoptotic and pro-necrotic. Buffering of the calcium response with 1,2-bis-(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxy-methyl) ester (BAPTA-AM) and ethylene glycol-bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) completely blocked cadmium-induced apoptotic response. The pretreatment of cells with BAPTA-AM and EGTA suppressed the cadmium-induced cell injury, including growth arrest, mitochondrial activity impairment, and necrosis, and it also recovered the cadmium-altered JNK and p38 MAPK activity. Chelating [Ca2+](i) also reversed cadmium-induced hydrogen peroxide generation, suggesting that production of reactive oxygen species (ROS) is related to [Ca2+](i). The present study showed that cadmium induces a [Ca2+](i)-ROS-JNK-caspase-3 signaling pathway leading to apoptosis. Furthermore, cadmium-induced [Ca2+](i) regulates phosphorylation/dephosphorylation of JNK and p38, and it modulates signal transduction pathways to proliferation, mitochondrial activity, and necrosis.