The Kynurenine Pathway of Tryptophan Degradation is Activated During Osteoblastogenesis

ABSTRACT The mechanisms involved in the anabolic effect of IFNγ on bone have not been carefully examined. Using microarray expression analysis, we found that IFNγ upregulates a set of genes associated with a tryptophan degradation pathway, known as the kynurenine pathway, in osteogenic differentiating human mesenchymal stem cells. We therefore hypothesized that activation of the kynurenine pathway plays a role in osteoblastogenesis even in the absence of IFNγ. Initially, we observed a strong increase in tryptophan degradation during osteoblastogenesis with and without IFNγ in the media. We next blocked indoleamine 2,3‐dyoxygenase‐1 (IDO1), the most important enzyme in the kynurenine pathway, using a siRNA and pharmacological approach and observed a strong inhibition of osteoblastogenesis with a concomitant decrease in osteogenic factors. We next examined the bone phenotype of Ido1 knock‐out (Ido1‐/‐) mice. Compared to their wild‐type littermates, Ido1‐/‐ mice exhibited osteopenia associated with low osteoblast and high osteoclast numbers. Finally, we tested whether the end‐products of the kynurenine pathway have an osteogenic effect on hMSC. We identified that picolinic acid had a strong and dose‐dependent osteogenic effect in vitro. In summary, we demonstrate that the activation of the kynurenine pathway plays an important role during the commitment of hMSC into the osteoblast lineage in vitro, and that this process can be accelerated by exogenous add...
Source: Stem Cells - Category: Stem Cells Authors: Tags: Translational and clinical research Source Type: research