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Epidemiological studies have suggested a positive association between environmental cadmium (Cd) exposure and type 2 diabetes mellitus (T2DM). Skeletal muscle insulin resistance (IR) plays a critical role in the pathogenesis of T2DM. This study aimed to investigate the effects of chronic low-level Cd exposure on skeletal muscle IR and its potential mechanism. Rats were exposed to drinking water containing 2 or 10 mg/L Cd for 24 weeks. Differentiated L6 myotubes were treated with Cd for 72 h. Immunofluorescence, flow cytometry assay, RNA-sequencing, and Seahorse analysis were conducted to determine the effects of Cd and its underlying mechanism on relevant parameters, including insulin sensitivity, glucose uptake, oxidative stress, mitophagy, and mitochondrial function in skeletal muscle and L6 myotubes. N-acetyl-cysteine (NAC), a scavenger of reactive oxygen species (ROS), and mitophagy inhibitor Cyclosporin A (CsA) were used to confirm the role of oxidative stress in mitophagy and mitochondrial dysfunction caused by Cd. We found that rats exposed to 10 mg/L Cd exhibited hyperglycemia and skeletal muscle IR. Cd markedly increased IRS-1 phosphorylation at Ser612, while decreased levels of phosphorylated PI3K, Akt, AS160, inhibited GLUT4 translocation and glucose uptake. Mechanistically, Cd increased the intracellular ROS, hydrogen peroxide, and malondialdehyde levels and decreased antioxidase activity in L6 myotubes. Furthermore, Cd upregulated the mRNA and protein levels of LC3II/I, PINK1, and Parkin. In addition, Cd induced the formation of mitophagosomes, reduced the mitochondrial membrane potential, decreased the adenosine triphosphate content, and impaired the mitochondrial respiratory capacity. Strikingly, NAC ameliorated oxidative stress, excessive mitophagy, and the associated reduction in myotube insulin sensitivity, while inhibition of mitophagy by CsA alleviated skeletal muscle IR. In conclusion, this study reveals a previously unrecognized mechanism that chronic low-level Cd exposure may induce mitophagy by activating the PINK1/Parkin signal pathway by increasing ROS, thus causing skeletal muscle IR and elevated blood glucose.
Chen et al. (Wed,) studied this question.