Osteoinductive effect of soluble transforming growth factor beta receptor 3 on human osteoblast lineage

The development of bone requires carefully choregraphed signaling to bone progenitors to form bone. Our group recently described the requirement of transforming growth factor beta receptor 3 (TGFβR3), a receptor involved in TGFβ pathway signaling, during osteoblast lineage commitment in mice. The TGFβ pathway is known to play multiple osteo-inductive and osteo-inhibitory roles during osteoblast development and TGFβR3 human mutations are associated with reduced bone mineral density, making TGFβR3 a unique target for bone inductive therapy. In this article, we demonstrated increased mineralization of human pediatric bone-derived osteoblast-like cells (HBO) when treated with soluble TGFβR3 (sR3) using Alizarin Red staining. Osteogenic commitment of HBO cells was demonstrated by induction of osteogenic genes RUNX2, osteocalcin, osteopontin, and osterix. Evaluation of the canonical TGFβ pathway signaling demonstrated that sR3 was able to induce bone formation in HBO cells, mainly through activation of noncanonical targets of TGFβ pathway signaling including AKT, ERK, and p38 MAP kinases. Inhibition of these osteogenic noncanonical pathways in the HBO cells also inhibited mineralization, suggesting they are each required. Although no induction of SMAD1, 5, and 9 was observed, there was the activation of SMAD2 and 3 suggesting that sR3 is primarily signaling via the noncanonical pathways during osteogenic induction of the HBO. Our results highlight the important role of TGFβR3 in osteoblast induction of mineralization in human bone cells through noncanonical targets of TGFβ signaling. Future studies will focus on the ability of sR3 to induce bone regeneration in vivo using animal models.