Runx2, an inducer of osteoblast and chondrocyte differentiation

Runx2 is a transcription factor that is essential for osteoblast differentiation and chondrocyte maturation. Ihh, expressed in prehypertrophic and hypertrophic chondrocytes, is required for the specification of Runx2⁺ osteoprogenitors in endochondral bone development. Runx2 induces Sp7, an essential transcription factor for osteoblast differentiation. Canonical Wnt signaling is also required for osteoblast differentiation. Runx2⁺ osteoprogenitors retain the capacity to differentiate into chondrocytes, and Sp7 and canonical Wnt signaling direct cells to osteoblasts, thereby inhibiting chondrocyte differentiation. The function of Runx2 after the commitment to osteoblasts remains controversial. Runx3 has a redundant function with Runx2 in chondrocyte maturation. Runx2 regulates the expression of Ihh, Col10a1, Spp1, Ibsp, Mmp13, and Vegfa in the respective layers in growth plates. Runx2 enhances chondrocyte proliferation through the induction of Ihh. Ihh induces Pthlh, which inhibits Runx2 and chondrocyte maturation, forming a negative feedback loop for chondrocyte maturation. Runx2 is one of the genes responsible for the pathogenesis of osteoarthritis (OA) because RUNX2 is up-regulated in chondrocytes in OA cartilage and a germline haplodeficiency or deletion of Runx2 in articular chondrocytes decelerates OA progression. Runx2 plays an important role in the bone metastasis of breast and prostate cancers by up-regulating Spp1, Ibsp, Mmp9, Mmp13, Vegfa, Tnfsf11, and Ihh expression and down-regulating Tnfrsf11b expression. Cbfb forms a heterodimer with Runx2 and is required for the efficient DNA binding of Runx2. Cbfb stabilizes Runx proteins at different levels among Runx family proteins by inhibiting their ubiquitination-mediated degradation. Cbfb plays more important roles in endochondral ossification than in intramembranous ossification.