• Oxidation of Dietary Amino Acids Disrupts their Anabolic Effects on Bone Marrow-Derived Mesenchymal Stem Cells

      El Refaey, Mona M.; Department of Cellular Biology and Anatomy (2016-07)
      Age-dependent bone loss has been well documented in both human and animal models. Since it has been proposed that aging is associated with an increase in the generation of damaging reactive oxygen species (ROS), our hypothesis was that the oxidized products of dietary amino acids could play a role in age-induced bone loss by altering osteoprogenitor cell differentiation and function or activating osteoclastic activity. We first examined the effects of the oxidized nutrients on the bone marrow-derived mesenchymal stem cells and our data showed a decrease in the protein and gene expression of osteogenic markers normally stimulated by nutrients. Aromatic amino acids activated signaling pathways involved in protein synthesis in vitro, and thus, in contrast, the oxidized metabolites of these aromatic amino acids had no effect on the activation of these anabolic pathways. We then examined the bone marrow concentration of the oxidized aromatic amino acids in mature (12 months) vs. aged (24 months) C57BL/6 mice and found that kynurenine, the oxidized product of the aromatic amino acid tryptophan, was found in the highest concentration in 12 months mice. Thus, we tested the effects of kynurenine, fed as a dietary supplement, on the bone mass of twelve-month-old C57BL/6 mice compared to a normal protein diet to see if the oxidized amino acid would induce a pattern consistent with age-related bone loss. Twelve-month-old, male C57BL/6 mice were fed one of four diets; 18% protein diet (normal protein diet); 8% protein diet + tryptophan; 8% protein diet + kynurenine (50 μM) and 8% protein diet + kynurenine (100 μM) for 8 wks. Bone densitometry and micro-CT analyses demonstrated bone loss following the kynurenine diet. Histological and histomorphometric studies showed a decreased bone formation and an increased MONA M. EL REFAEY Oxidation of Dietary Amino Acids Disrupts Their Anabolic Effects on Bone Marrow-Derived Mesenchymal Stem Cells (Under the direction of DR. CARLOS M. ISALES) osteoclastic activity in the kynurenine groups; these animals also exhibited an increase in serum pyridinoline, a marker of bone breakdown. Thus, these data demonstrate that feeding an oxidized product of an essential amino acid induces bone loss in a pattern consistent with accelerated aging, and we propose that one of the mechanisms involved in age-induced bone loss may be from alterations of dietary nutrients by the increased generation of ROS associated with aging.
    • THE ROLE OF KYNURENINE, A TRYPTOPHAN METABOLITE THAT INCREASES WITH AGE, IN MUSCLE ATROPHY AND LIPID PEROXIDATION)

      Kaiser, Helen E.; Department of Cellular Biology and Anatomy (Augusta University, 2020-05)
      Loss of mobility and independence are risk factors for falls and mortality, and drastically reduce the quality of life among older adults. The cellular and molecular mechanisms underlying loss of muscle mass and strength with age (sarcopenia) are not well-understood; however, heterochronic parabiosis experiments show that circulating factors are likely to play a role. Kynurenine (KYN) is a circulating tryptophan metabolite that is known to increase with age and is implicated in several age-related pathologies. Here I tested the hypothesis that KYN contributes directly to muscle loss with aging. Results indicate that that KYN treatment of mouse and human myoblasts increased levels of reactive oxygen species (ROS) two-fold, and significantly increased lipid peroxidation enzymes. Small-molecule inhibition of the Aryl hydrocarbon receptor (Ahr), an endogenous KYN receptor, in vitro did not prevent KYN-induced increases in ROS, and homozygous Ahr knockout in vivo did not protect mice from KYN-induced stress, suggesting that KYN can directly increase ROS independent of Ahr activation. In vivo, wild-type mice treated with KYN had reduced skeletal muscle strength, size, and increased oxidative stress and lipid peroxidation. Old wild-type mice treated with 1MT, a small molecule that suppresses KYN production by IDO1, showed an increase in muscle fiber size, peak muscle strength, and oxidative stress. Protein analysis identified mitochondrial lipid peroxidation as a downstream mechanism that is increased upon KYN treatment. Lipid peroxidation enzymes increased with KYN have been shown to produce H2O2 outside of the electron transport chain. Our data suggest that IDO inhibition may represent a novel therapeutic approach for the attenuation of sarcopenia and possibly other age-associated conditions associated with KYN accumulation such as bone loss and neurodegeneration.