The thioredoxin system in aging muscle: key role of mitochondrial thioredoxin reductase in the protective effects of caloric restriction?

Cellular redox balance is maintained by various antioxidative systems. Among those is the thioredoxin system, consisting of thioredoxin, thioredoxin reductase, and NADPH. In the present study, we examined the effects of caloric restriction (2 mo) on the expression of the cytosolic and mitochondrial thioredoxin system in skeletal muscle and heart of senescent and young rats. Mitochondrial thioredoxin reductase (TrxR2) is significantly reduced in aging skeletal and cardiac muscle and renormalized after caloric restriction, while the cytosolic isoform remains unchanged. Thioredoxins (mitochondrial Trx2, cytosolic Trx1) are not influenced by caloric restriction. In skeletal and cardiac muscle of young rats, caloric restriction has no effect on the expression of thioredoxins or thioredoxin reductases. Enforced reduction of TrxR2 (small interfering RNA) in myoblasts under exposure to ceramide or TNF-alpha causes a dramatic enhancement of nucleosomal DNA cleavage, caspase 9 activation, and mitochondrial reactive oxygen species release, together with reduced cell viability, while this TrxR2 reduction is without effect in unstimulated myoblasts under basal conditions. Oxidative stress in vitro (H2O2 in C2C12 myoblasts and myotubes) results in different changes: TrxR2, Trx2, and Trx1 are induced without alterations in the cytosolic thioredoxin reductase isoforms. Thus aging is associated with a TrxR2 reduction in skeletal muscle and heart, which enhances susceptibility to apoptotic stimuli but is renormalized after short-term caloric restriction. Exogenous oxidative stress does not result in these age-related changes of TrxR2.