Legumain is a paracrine regulator of osteoblast differentiation and mediates the inhibitory effect of TGF-β1 on osteoblast maturation
1.Transforming growth factor-beta 1 (TGF-{beta}1) is a critical regulator of skeletal homeostasis and has diverse promoting effects on osteoblastogenesis. However, the mechanisms behind the intriguing inhibitory effect of TGF-{beta}1 on osteoblast maturation are not fully understood. Here, we demonstrate a novel mechanism by which TGF-{beta}1 modulates osteoblast maturation through lysosomal protease legumain. We observed that presence of TGF-{beta}1 in osteogenic cultures of human bone marrow derived mesenchymal stromal (stem) cells enhanced legumain activity and secretion, in-spite of decreased legumain mRNA expression, suggesting post-transcriptional regulation. We further showed that osteogenic cells internalize and activate prolegumain, associated with inhibited osteoblast maturation, revealing legumain as a paracrine regulator of osteoblast maturation. Interestingly, TGF-{beta}1 treatment exacerbated legumain internalization and activity, and showed an additive effect on legumain-induced inhibition of osteoblast maturation. Importantly, legumain inhibition abolished the inhibitory effect of TGF-{beta}1 on osteoblast maturation. Our findings reveal that TGF-{beta}1 inhibits osteoblast maturation through stimulating secretion and activity of endogenous legumain, as well as increasing the internalization and activation of extracellular legumain. Therefore, our study provides a deeper understanding of the complex regulation of osteoblastogenesis and unveils a novel TGF-{beta}1-legumain axis in regulation of osteoblast maturation and offer novel insights for possible therapeutic interventions related to bone diseases associated with aberrant TGF-{beta}1 signaling.