Experimental and clinical studies support the view that the semisynthetic
tetracycline minocycline exhibits neuroprotective roles in several models of
neurodegenerative diseases, including
ischemia, Huntington, Parkinson diseases, and
amyotrophic lateral sclerosis. However, recent evidence indicates that
minocycline does not always present beneficial actions. For instance, in an in vivo model of
Huntington's disease, it fails to afford protection after
malonate intrastriatal injection. Moreover, it reverses the
neuroprotective effect of
creatine in nigrostriatal dopaminergic neurons. This apparent contradiction prompted us to analyze the effect of this
antibiotic on
malonate-induced cell death. We show that, in rat cerebellar granular cells, the
succinate dehydrogenase inhibitor
malonate induces cell death in a concentration-dependent manner. By using DFCA,
monochlorobimane and 10-N-nonyl-Acridin Orange to measure, respectively, H2O2-derived
oxidant species and reduced forms of GSH and
cardiolipin, we observed that
malonate induced
reactive oxygen species (ROS) production to an extent that surpasses the
antioxidant defense capacity of the cells, resulting in GSH depletion and
cardiolipin oxidation. The pre-treatment for 4 h with
minocycline (10-100 microM) did not present cytoprotective actions. Moreover,
minocycline failed to block ROS production and to abrogate
malonate-induced oxidation of GSH and
cardiolipin. Additional experiments revealed that
minocycline was also unsuccessful to prevent the mitochondrial swelling induced by
malonate. Furthermore,
malonate did not induce the expression of the iNOS,
caspase-3, -8, and -9 genes which have been shown to be up-regulated in several models where
minocycline resulted cytoprotective. In addition,
malonate-induced down-regulation of the antiapoptotic gene Bcl-2 was not prevented by
minocycline, controversially the mechanism previously proposed to explain
minocycline protective action. These results suggest that the
minocycline protection observed in several
neurodegenerative disease models is selective, since it is absent from cultured cerebellar granular cells challenged with
malonate.