The multiple faces of tryptophan in bone biology

Osteoporosis is highly prevalent in older persons. While many advances have been made in the field of osteoporosis, current treatments have been affected by unexpected side effects and limited efficacy; therefore, new approaches to identify disease mechanisms and pathways are required. This review focuses on the influence of tryptophan metabolites, particularly kynurenines and serotonin on bone. The kynurenine (KYN) pathway is associated with osteoblastogenesis and can be linked to the pathophysiology of osteoporosis. The activity of osteoblasts is reduced by 3-hydroxykynurenine (3-HKYN), a product of KYN. In addition, decreasing concentrations of 3-hydroxyanthranilic acid with aging can be one of the causes of bone loss. In contrast, picolinic acid, an end-product of the KYN pathway, acts as a bone anabolic. On the other hand, gut-derived serotonin (GDS) inhibits bone formation, whereas brain-derived serotonin enhances bone formation and decreases bone resorption. Overall, understanding the exact mechanisms of action of tryptophan metabolites on bone could have great potential to develop effective treatments for osteoporosis and other bone diseases.