Cadmium is one of the most toxic metal compounds found in the environment. It is well established that Cd induces hepatotoxicity in humans and multiple animal models. Melatonin, a major secretory product of the pineal gland, has been reported to protect against Cd-induced hepatotoxicity. However, the mechanism behind this protection remains to be elucidated. We exposed HepG2 cells to different concentrations of cadmium chloride (2.5, 5, and 10 μM) for 12 h. We found that Cd induced mitochondrial-derived superoxide anion-dependent autophagic cell death. Specifically, Cd decreased SIRT3 protein expression and activity and promoted the acetylation of SOD2, superoxide dismutase 2, mitochondrial, thus decreasing its activity, a key enzyme involved in mitochondrial ROS production, although Cd did not disrupt the interaction between SIRT3 and SOD2. These effects were ameliorated by overexpression of SIRT3. However, a catalytic mutant of SIRT3 (SIRT3(H248Y)) lacking deacetylase activity lost the capacity to suppress Cd-induced autophagy. Notably, melatonin treatment enhanced the activity but not the expression of SIRT3, decreased the acetylation of SOD2, inhibited mitochondrial-derived O2(•-) production and suppressed the autophagy induced by 10 μM Cd. Moreover, 3-(1H-1,2,3-triazol-4-yl)pyridine, a confirmed selective SIRT3 inhibitor, blocked the melatonin-mediated suppression of autophagy by inhibiting SIRT3-SOD2 signaling. Importantly, melatonin suppressed Cd-induced autophagic cell death by enhancing SIRT3 activity in vivo. These results suggest that melatonin exerts a hepatoprotective effect on mitochondrial-derived O2(•-)-stimulated autophagic cell death that is dependent on the SIRT3/SOD2 pathway.
Keywords: 3-MA, 3-methyladenine; 3-TYP, 3-(1H-1,2,3-triazol-4-yl)pyridine; ACTB, actin, β; Baf A1, bafilomycin A1; Cd, cadmium; CdCl2, cadmium chloride; GPT/ALT, glutamic-pyruvate transaminase (alanine aminotransferase); H2O2, hydrogen peroxide; LC3, microtubule-associated protein 1 light chain 3; O2•−, superoxide anion; SIRT1, sirtuin 1; SIRT3; SIRT3, sirtuin 3; SOD2; SOD2, superoxide dismutase 2, mitochondrial; SQSTM1/p62, sequestosome 1; autophagy; cadmium; hepatotoxicity; mROS, mitochondrial reactive oxygen species; mel, melatonin; melatonin; mitochondrial ROS; tf-LC3, tandem fluorescent mRFP-GFP-LC3B.