Zidovudine-mediated autophagy inhibition enhances mitochondrial toxicity in muscle cells.

Nucleoside reverse transcriptase inhibitors (NRTI) such as zidovudine (AZT) are constituents of HIV-1 therapy and prevention of mother to child transmission. Prolonged thymidine analogues exposure has been associated with mitochondrial toxicities to heart, liver, and skeletal muscle. We hypothesized that the thymidine analogue AZT might interfere with autophagy in myocytes, a lysosomal degradation pathway implicated in regulation of mitochondrial recycling, cell survival and the pathogenesis of myodegenerative diseases. The impact of AZT and lamivudine (3TC) on C2C12 myocyte autophagy was studied using various methods based on LC3-GFP overexpression or LC3 staining in combination with western blotting, flow cytometry and confocal and electron microscopy. Lysosomal and mitochondrial functions were studied using appropriate staining for lysosomal mass, acidity, cathepsin activity as well as mitochondrial mass and membrane potential in combination with flow cytometry and confocal microscopy. AZT, but not 3TC, exerted a significant dose and time-dependent inhibitory effect on late stages of autophagosome maturation, which was reversible upon mTOR inhibition. Inhibition of late autophagy at therapeutic drug concentrations led to dysfunctional mitochondria accumulation with membrane hyperpolarization and increased ROS generation, and ultimately compromised cell viability. These AZT effects could be readily replicated by pharmacological and genetic inhibition of myocyte autophagy and most importantly rescued by pharmacological stimulation of autophago-lysosomal biogenesis. Our data suggest that the thymidine analogue AZT inhibits autophagy in myocytes, which in turn leads to accumulation of dysfunctional mitochondria with increased ROS generation and compromised cell viability. This novel mechanism could contribute to our understanding of the long-term side effects of antiviral agents.