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The critical role of autophagy in cadmium-induced immunosuppression regulated by endoplasmic reticulum stress-mediated calpain activation in RAW264.7 mouse monocytes

So, Keum-Young, Lee, Byung-Hoon, Oh, Seon-Hee
Toxicology 2017
AMP-activated protein kinase, NAD ADP-ribosyltransferase, acetylcysteine, antioxidants, apoptosis, autophagy, cadmium, calcium, calpain, caspase-3, caspase-8, cell viability, chelating agents, cytotoxicity, dose response, endoplasmic reticulum, immunosuppression, immunotoxicity, mice, monocytes, oxidative stress, plasmids, rapamycin, transfection, tumor necrosis factor-alpha
Cadmium (Cd) has toxic and suppressive effects on the immune system, but the underlying mechanisms remain poorly understood. Here, we show that autophagy plays a critical role in regulation of Cd-induced immunosuppression in RAW264.7 cells. Cd decreased cell viability in a dose-dependent manner; cleaved caspase-8, caspase-3, and poly (ADP-ribose) polymerase (PARP)-1; increased DNA laddering; induced CCAAT-enhancer-binding protein homologous protein (CHOP); and reduced tumor necrosis factor (TNF)-α expression; indicating that caspase-dependent and endoplasmic reticulum (ER)-mediated apoptosis are involved in Cd-induced immunotoxicity. Furthermore, Cd induced autophagy, as demonstrated by microtubule-associated protein 1 light chain 3B (LC3B) plasmid DNA transfection and its conversion from LC3-I to the LC3-II form by autophagy inhibitors, via AMP-activated protein kinase (AMPK)-mammalian target of rapamycin (mTOR) signaling. Pharmacological and genetic inhibition of autophagy suppressed Cd-induced apoptosis, as evidenced by inhibition of caspase-8, caspase-3, and PARP-1 cleavage, indicating that autophagy promotes apoptosis. The pan-caspase inhibitor zVAD inhibited Cd-induced apoptosis, but increased autophagy and decreased cell viability, indicating that autophagy can compensate for reduced apoptotic cell death. Calpain inhibitors blocked Cd-induced apoptosis and autophagy, indicating that calpain plays a critical role in Cd cytotoxicity. Treatment with Ca2+ chelators completely recovered Cd-induced cell viability and inhibited Cd-induced apoptosis and autophagy. Treatment with N-acetyl-L-cysteine (NAC) suppressed Cd-induced antioxidant enzyme levels, apoptosis, and autophagy. Collectively, Cd-induced oxidative stress triggers ER stress, leading to Ca2+−dependent calpain activation and subsequent activation of autophagy and apoptosis, resulting in immune suppression.