New Insights into Tryptophan and Its Metabolites in the Regulation of Bone Metabolism

Osteoporosis, a debilitating disease caused by an imbalance between the action of osteoblasts and osteoclasts, is becoming an increasing problem in today's aging population. Although many advances in this field have addressed certain aspects of disease progression and pain management, new approaches to treatment are required. This review focuses on the influence of tryptophan, its metabolites and their influence on bone remodeling. Tryptophan is a precursor to serotonin, melatonin, kynurenines and niacin. Changes of tryptophan levels were noticed in bone metabolic diseases. Moreover, some works indicate that tryptophan plays a role in osteoblastic differentiation. Serotonin can exert different effects on bones, which depend on site of serotonin synthesis. Gut-derived serotonin inhibits bone formation, whereas brain-derived serotonin enhances bone formation and decreases bone resorption. Melatonin, increased differentiation of human mesenchymal stem cells into the osteoblastic cell lineage. Results of melatonin action on bone are anabolic and antiresorptive. Activation of the second tryptophan metabolic pathway, the kynurenine pathway, is associated with osteoblastogenesis and can be implicated in the occurrence of bone diseases. Oxidation products like kynurenine stopped proliferation of bone marrow mesenchymal stem cells. This may result in inhibition of osteoblastic proliferation and differentiation. Kynurenic acid acts as an antagonist at glutamate receptors, which are expressed on osteoclasts. Quinolinic acid activates N-methyl-D-aspartate receptors. 3-hydroxyanthranilic acid exhibits pro-oxidant and antioxidant activity. Decreased concentration of 3-hydroxyanthranilic acid can be one of the causes of osteoporosis. 3-hydroxykynurenine reduced the viability of osteoblast-like cells. Picolinic acid exerted osteogenic effect in vitro. Kynurenine derivatives exert various effects on bones. Discovery of the exact mechanism of action of tryptophan metabolites on bones may take us a step closer to understanding the complicated mechanism of bone metabolism, which in turn may result in finding a new, effective therapy for treating bone diseases.