Cadmium toxicity involves oxidative stress, calcium and map kinases
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Cadmium is a well-known carcinogen and immunotoxic metal commonly found in cigarette smoke and industrial effluents. It has been reported that smokers have 4-5 times higher level of cadmium in blood than non-smokers. Inhalation of cadmium will directly effect on macrophages in respiratory system without first-pass elimination and be a factor of immunodepression of smokers. The objectives of the present study are to define the important signaling mediators in cadmium-induced cell death and growth arrest in murine macrophages. Cadmium elevated intracellular calcium ([Ca2+]i) and reactive oxygen species (ROS) at early time point of 6 h in J774A.1 murine macrophage cells. Caspase-3 activation at 8 h and thereafter DNA fragmentation were detected and initiator caspase-8 and -9 were involved to activate executor caspase-3 in cadmium-induced apoptosis. Phosphorylation of c-Jun NH2-terminal kinase (JNK) and extracellular signal-related kinase (ERK) were activated and p38 mitogen-activated protein kinase (MAPK) was down-regulated by cadmium. We show here that [Ca2+]i elevation and oxidative stress by cadmium are interrelated each other. Both Ca2+ chelators and antioxidants inhibited cadmium-induced caspase-3 activation, DNA fragmentation and growth arrest. Chelating intracellular and extracellular Ca2+ inhibited cadmium-induced JNK activation and inhibition of JNK reduced apoptotic response suggesting elevated [Ca2+]i-JNK-caspase-3 signaling pathway leading to apoptosis by cadmium. Antioxidants decreased cadmium-induced ERK activation and cell death. We found that cadmium induces ROS-ERK-p21WAF1/CIP1 signaling pathway leading to G2/M arrest and cell death. Inhibition of ERK recovered cadmium-induced necrosis but did not show any effect on caspase-3 activation and apoptosis. Activation of ERK was dependent on free Cd2+, while JNK and p38 MAPK were not. Chelating Cd2+ was able to inhibit cadmium-induced necrosis but failed to recover cadmium-induced apoptosis. Chelating Cd2+ was still able to elevate [Ca2+]i and hydrogen peroxide generation. These results demonstrate that free Cd2+ plays an important role in ERK-necrosis signaling. Altered [Ca2+]i level or redox system by cadmium complexes is capable of initiating toxic action leading to apoptosis, mitochondrial impairment and growth arrest.