Metformin Alleviates Cadmium-Induced Autophagic Flux Impairment-Dependent Apoptosis by Activating AMPK in Neuronal Cells

Cadmium (Cd), a common environmental and occupational toxicant, is an important risk factor for neurodegenerative diseases. Metformin has been found to have neuroprotective effect, in addition to antidiabetic function. Our recent studies have identified that metformin ameliorates Cd neurotoxicity via blocking ROS-dependent PP5/AMPK-JNK signaling pathway. Here we further show that metformin protected PC12 cells and primary neurons from Cd-poisoning by mitigating Cd-induced increases in ATG5/LC3-II/p62 levels and autophagosomes. Knockdown of ATG5 dramatically potentiated the inhibitory effects of metformin on Cd-induced LC3-II, cleavage of caspase-3, accumulation of autophagosomes and apoptosis in PC12 cells. Addition of chloroquine (CQ) strengthened the basic and Cd-elevated ATG5/LC3-II/p62 levels, autophagosome accumulation and cell apoptosis, whereas metformin powerfully blocked the events, implying a metformin-promoted autophagic flux-dependent mechanism involved. Further research revealed that metformin prevented Cd-induced autophagic flux impairment and cell apoptosis, which was attributed to restraining Cd inactivation of AMPK. This is supported by the findings that activation of AMPK with AICAR or ectopic expression of constitutively active AMPKα (AMPKα-ca) reinforced the inhibitory effects of metformin on Cd-evoked ATG5/LC3-II/p62/autophagosomes and apoptosis in PC12 cells and/or primary neurons. Taken together, the results indicate that metformin protects neuronal cells from Cd-induced autophagic flux impairment-dependent apoptosis by activating AMPK. Our studies highlight that metformin has a great potential for prevention of Cd toxicity related to neurodegenerative diseases.