Yoshioka et al. definitively prove the hypothesis that decreasing TXNIP expression improves recovery after I-R injury (4
). Using mice in which Txnip
was deleted in every cell, as well as those in which the gene was deleted in a cardiac-specific manner, the authors showed that after I-R injury, left ventricular function was improved in TXNIP-deficient versus wild-type mice. Unexpectedly, the improvement in left ventricular function was accompanied by a decrease, not an increase, in mitochondrial respiratory function. Yoshioka et al. went on to determine the mechanism underlying the beneficial effect of TXNIP
deletion by several approaches. First, they performed bioinformatics analyses of the transcriptome and proteome in TXNIP-deficient mouse hearts to identify pathways altered by Txnip
deletion. The mRNA expression approach revealed 390 genes regulated by TXNIP, of which 306 were downregulated. In addition, they showed an altered gene expression that primarily led to decreased expression of proteins important for energy metabolism. More specifically, proteins that are involved in OXPHOS or fatty acid/carbohydrate metabolism associated with metabolic pathways in the mitochondria. The proteomics approach revealed 21 proteins that were differentially regulated, of which 13 had described cellular functions, and all were decreased in TXNIP-deficient mouse hearts. Among the 13 proteins, 12 were related to mitochondrial metabolism, which supports the notion that TXNIP is a key regulator of mitochondrial function in the heart.
Second, Yoshioka et al. used skinned cardiac myocyte fibers and isolated mitochondria to show that deletion of Txnip in mice was associated with reduced mitochondrial function. This was not due to a loss of respiratory complexes, but resulted from a functional loss of ADP-stimulated respiration. There was also no change in mitochondrial permeability transition pore opening. Third, the authors showed that, surprisingly, there was no change in the number of mitochondria or in their structure. However, electron microscopy showed large perimitochondrial lipid droplets and matrix granules in the mitochondria of TXNIP-deficient hearts, suggestive of altered fatty acid and lipid metabolism. Fourth, they measured the activity of TRX1 and TRX2. Although there was no change in TRX1 activity, there was a substantial increase in TRX2 activity and a decrease in ROS generation in TXNIP-deficient mice that was apparent after I-R injury. Fifth, they determined cellular ATP content after I-R, which was 2-fold greater in TXNIP-KO hearts. This was due to a shift from aerobic to anaerobic metabolism, as shown by blocking glycolysis. Finally, the authors studied the role of the pyruvate dehydrogenase complex, which regulates glycolytic flux relative to mitochondrial respiration. They found that TXNIP binds to pyruvate dehydrogenase (PDH), specifically pyruvate dehydrogenase E1 component, subunit α (Figure ). In TXNIP-deficient hearts, PDH activity was inhibited, and there was a shift in the use of glycolytically derived pyruvate away from its metabolism in mitochondria toward cytosolic lactate production and ATP generation.
In summary, Yoshioka et al. demonstrated that TXNIP acts as a metabolic switch to enhance ATP production by anaerobic pathways via its ability to inhibit mitochondrial respiration (4
). Moreover, TXNIP was shown to mediate this function by both decreasing expression of mitochondrial OXPHOS proteins and by redirecting pyruvate away from mitochondria.