Bone Abstracts

The healing of bone fractures is a tightly regulated process where released growth factors and cytokines interplay within an inflammatory environment in order to reestablish the functional bone. Recent studies have suggested that endothelial cells may dedifferentiate into mesenchymal multipotent cells via a mechanism called endothelial-to-mesenchymal-transition (EndoMT). Transforming growth factor-β (TGF-β) plays a critical role inducing EndoMT. Subsequent differentiation into mineralizing osteoblast-like is triggered by bone morphogenetic proteins (BMPs). TGF-β and BMPs are part of the TGF-β family of cytokines binding to type I and type II serine/threonine kinase receptors at the plasma membrane, which upon activation signal via Smad and non-Smad signaling pathways, including MAP kinases. Interestingly, they are targeted by the pro-inflammatory cytokines released upon a fracture as well, suggesting a convergence between BMPs and inflammation. Hereby we investigate how BMPs trigger osteoblast trans-differentiation under inflammatory conditions in endothelial cells, therefore uncovering their contribution to bone healing.

We demonstrate that BMP-6 and BMP-9 induce very potently the trans-differentiation of endothelial cells into osteoblast-like cells. Noteworthy, the activity of BMP-9 was dramatically enhanced in combination with the pro-inflammatory cytokine TNF-α. Among different pro-inflammatory cascades activated in endothelial cells, down-regulation of the JNK MAP-kinase increased the mineralization of human and murine endothelial cells. Whereas BMP-6 potentiated TNF-α-induced-JNK activation, BMP-9 showed no effect. Finally, we compared the activation of JNK in endothelial cells from the capillaries in bone sections from normal versus delayed-healing patients. JNK and its downstream target c-jun were significantly more activated in fractures with delayed-healing, which also were weakly stained for BMP-9, in comparison to normal-healing fractures.

The results presented here suggest a key role for non-canonical BMP pathways, and in particular JNK, on the differentiation of endothelial into osteoblast-like mineralizing cells. Furthermore, they may have application for bone tissue engineering and healing of bone fractures.