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

Abstract

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.