Inhibition of
glycogen synthase kinase (GSK)-3 reduces
ischemia/reperfusion injury by mechanisms that involve the mitochondria. The goal of this study was to explore possible molecular targets and mechanistic basis of this cardioprotective effect. In perfused rat hearts, treatment with GSK inhibitors before
ischemia significantly improved recovery of function. To assess the effect of GSK inhibitors on mitochondrial function under ischemic conditions, mitochondria were isolated from rat hearts perfused with GSK inhibitors and were treated with uncoupler or
cyanide or were made anoxic. GSK inhibition slowed
ATP consumption under these conditions, which could be attributable to inhibition of
ATP entry into the mitochondria through the
voltage-dependent anion channel (VDAC) and/or
adenine nucleotide transporter (ANT) or to inhibition of the
F(1)F(0)-ATPase. To determine the site of the inhibitory effect on
ATP consumption, we measured the conversion of
ADP to
AMP by
adenylate kinase located in the intermembrane space. This assay requires
adenine nucleotide transport across the outer but not the inner mitochondrial membrane, and we found that GSK inhibitors slow
AMP production similar to their effect on
ATP consumption. This suggests that GSK inhibitors are acting on outer mitochondrial membrane transport. In sonicated mitochondria, GSK inhibition had no effect on
ATP consumption or
AMP production. In intact mitochondria,
cyclosporin A had no effect, indicating that
ATP consumption is not caused by opening of the
mitochondrial permeability transition pore. Because GSK is a
kinase, we assessed whether
protein phosphorylation might be involved. Therefore, we performed Western blot and 1D/2D gel phosphorylation site analysis using
phos-tag staining to indicate
proteins that had decreased phosphorylation in hearts treated with GSK inhibitors. Liquid chromatographic-mass spectrometric analysis revealed 1 of these
proteins to be VDAC2. Taken together, we found that GSK-mediated signaling modulates transport through the outer membrane of the mitochondria. Both proteomics and
adenine nucleotide transport data suggest that GSK regulates VDAC and that VDAC may be an important regulatory site in
ischemia/reperfusion injury.