Effect of L-carnitine on the zidovudine-induced destruction of human myotubes. Part I: L-carnitine prevents the myotoxicity of AZT in vitro

Abstract

Background: Zidovudine (AZT) as used in the treatment of AIDS, causes a mitochondrial myopathy characterized by depletion of mitochondrial DNA, enzymatic defects in the respiratory chain system, and accumulation of lipid droplets. Most of these changes are also seen in normal human myotubes treated with AZT. Because L-carnitine plays a major role in the transport of long chain fatty acids across the inner mitochondrial membrane and facilitates the beta-oxidation of fatty acids, we examined the effect of L-carnitine in preventing the destructive effect of AZT on the mitochondria and the myotubes of human muscle in tissue culture.

Experimental design: Myotubes, prepared from human muscle biopsies, were exposed to various concentrations of AZT for up to 3 weeks. One-third of the flasks were treated with AZT alone, another third with AZT plus L-carnitine and another third were untreated. The cultures were evaluated with: (a) immunocytochemistry counting the number of myotubes stained with antibodies to Leu-19; (b) enzyme histochemistry for NADH reaction and oil-red-O stain to assess mitochondrial enzymatic activity and lipid droplet accumulation; and (c) electron microscopy counting all the organelles within representative sections of the myotubes, at x24,000, and calculating the volumetric density of each organelle/unit volume of tissue.

Results: AZT, at concentrations 250 microM and above, caused depopulation of the Leu-19-positive myotubes, destructive changes in the mitochondria consisting of swelling, lamellar inclusions and multiple concentric cristae, accumulation of lipid droplets, and increase lysosomes. L-Carnitine increased the number of Leu-19-positive myotubes from 3.4 +/- 0.6 to 9.4 +/- 1.2, preserved the morphology of the mitochondria, increased their volumetric density from 2.5 +/- 0.4 to 6.0 +/- 0.7, and reduced the volumetric density of the lipid droplets from 12.2 +/- 4.9 to 1.4 +/- 0.7 and of the lysosomes from 15.6 +/- 3.6 to 3.9 +/- 1.4 (p < 0.001).

Conclusions: L-Carnitine, used concurrently with AZT, prevents the human myotubes from the AZT-associated destruction, preserves the structure and volume of mitochondria and prevents the accumulation of lipids. The findings may have potential clinical implications in preventing the myotoxicity of AZT in patients with AIDS.