Zidovudine causes early increases in mitochondrial ribonucleic acid abundance and induces ultrastructural changes in cultured mouse muscle cells

BACKGROUND: Zidovudine (azidothymidine, AZT) causes a toxic mitochondrial myopathy in AIDS patients with changes in mitochondrial (mt) DNA and mitochondrial structure. To determine early subcellular events in AZT myopathy in vitro we examined C2C12 myotubes treated with AZT (0.2, 2, 10 μg/ml) for up to 4 weeks. We identified AZT-induced effects on muscle intracellular compartments by determining the comparative abundance of selected myotube cytoplasmic and mtRNAs and mtDNA. EXPERIMENTAL DESIGN: RNA and DNA were extracted from cultured C2C12 myotubes, blotted and probed with cDNAs specific for the mitochondrial gene products cytochrome b and cytochrome c oxidase I, nuclear-encoded sarcomeric troponin C, and cytoplasmic glyceraldehyde-3-phosphate dehydrogenase. Transmission electron microscopy was performed on parallel samples. RESULTS: After 4 weeks of AZT, cytochrome b and cytochrome c oxidase I mtRNA abundance increased to 64% and to 31% above respective control levels. No change occurred in mtDNA abundance or nuclear encoded glyceraldehyde-3-phosphate dehydrogenase mRNA compared with the nuclear encoded sarcomeric troponin C control. Transmission electron microscopy showed focal disruption of mitochondrial cristae with intramyocytic lipid droplets after 14 days of AZT treatment. CONCLUSIONS: Increased mtRNA abundance in absence of changes in mtDNA abundance suggests that early AZT toxicity to mitochondria may relate to defects in mtDNA replication or mtRNA transcription.