Effects of two longevity interventions, calorie restriction and rapamycin treatment, on the kynurenine-aryl hydrocarbon receptor pathway in aging skeletal muscle

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is thought to play important roles in aging, oxidative stress, and cellular senescence. We have previously shown that the AhR agonist kynurenine (Kyn), a tryptophan metabolite that increases with age, can induce muscle atrophy in young mice. AhR overexpression can also lead to muscle atrophy and neuromuscular junction degradation. Here we utilized existing GEO data sets from skeletal muscles of aged mice to examine the impact of two longevity-related interventions, calorie restriction (CR) or treatment with the drug rapamycin (RM), on the expression of genes in the Kyn-AhR pathway. Data were examined in four skeletal muscles: soleus, gastrocnemius, tibialis anterior and triceps brachii. Results show that AhR expression increased with age in the triceps but was decreased with CR in the soleus and gastrocnemius. RM treatment did not significantly alter AhR expression in any of the four muscles of aged mice. Three enzymes that convert kynurenine to kynurenic acid in skeletal muscle, Kyat1, Kyat3 and Got2/Kyat4, are known to increase with endurance exercise and all three increased significantly with CR in aged skeletal muscle. In contrast, RM treatment did not increase Kyat1 expression in aged muscle and RM significantly decreased Kyat3 expression levels in muscles from aged mice. Together these data point to kynurenine aminotransferases as mediating some of the positive effects of CR on skeletal muscle with aging, and support prior research suggesting that CR and RM modulate different patterns of muscle-specific gene expression.