Induction of Osteoclast Differentiation by Runx2 through Receptor Activator of Nuclear Factor-κB Ligand (RANKL) and Osteoprotegerin Regulation and Partial Rescue of Osteoclastogenesis in Runx2–/– Mice by RANKL Transgene

Abstract

Receptor activator of nuclear factor-B ligand (RANKL), osteoprotegerin (OPG), and macrophage-colony stimulating factor play essential roles in the regulation of osteoclastogenesis. Runx2-deficient (Runx2 ؊/؊ ) mice showed a complete lack of bone formation because of maturational arrest of osteoblasts and disturbed chondrocyte maturation. Further, osteoclasts were absent in these mice, in which OPG and macrophage-colony stimulating factor were normally expressed, but RANKL expression was severely diminished. We investigated the function of Runx2 in osteoclast differentiation. A Runx2 ؊/؊ calvaria-derived cell line (CA120 -4), which expressed OPG strongly but RANKL barely, severely suppressed osteoclast differentiation from normal bone marrow cells in co-cultures. Adenoviral introduction of Runx2 into CA120 -4 cells induced RANKL expression, suppressed OPG expression, and restored osteoclast differentiation from normal bone marrow cells, whereas the addition of OPG abolished the osteoclast differentiation induced by Runx2. Addition of soluble RANKL (sRANKL) also restored osteoclast differentiation in co-cultures. Forced expression of sRANKL in Runx2 ؊/؊ livers increased the number and size of osteoclast-like cells around calcified cartilage, although vascular invasion into the cartilage was superficial because of incomplete osteoclast differentiation. These findings indicate that Runx2 promotes osteoclast differentiation by inducing RANKL and inhibiting OPG. As the introduction of sRANKL was insufficient for osteoclast differentiation in Runx2 ؊/؊ mice, however, our findings also suggest that additional factor(s) or matrix protein(s), which are induced in terminally differentiated chondrocytes or osteoblasts by Runx2, are required for osteoclastogenesis in early skeletal development.In the process of endochondral ossification, chondrocytes mature to hypertrophic chondrocytes, matrix around terminally differentiated chondrocytes (terminal hypertrophic chondrocytes) is mineralized, blood vessels invade into the calcified cartilage, and cartilage is replaced by bone (1). Osteoclasts accelerate these processes by resorption of the calcified matrix leading to bone marrow formation. Osteoclasts differentiate from hematopoietic precursor cells through direct contact with osteoblastic/stromal cells (2). Recently, osteoprotegerin (OPG) 1 / osteoclastogenesis inhibitory factor, which is an inhibitor of osteoclast differentiation (3, 4), and receptor activator of NF-B (RANK) ligand (RANKL)/tumor necrosis factor-related activation-induced cytokine/OPG ligand/osteoclast differentiation factor, which is an inducer of osteoclast differentiation (5-8), were identified. RANKL, which is expressed on the surface of osteoblastic/stromal cells or released as a soluble factor, binds to its receptor RANK (9, 10), which is expressed on the surface of osteoclast precursors and osteoclasts, and induces osteoclast differentiation and activation. OPG, which binds RANKL with higher affinity than RANK, acts as a decoy receptor for RANKL and in...

Periodontal Ligament Stem Cells Modulate Root Resorption of Human Primary Teeth via Runx2 Regulating RANKL/OPG System

“…As previously reported, osteoclasts were absent in Runx2 -/ -mice, due to diminished RANKL expression [19]. Besides, CA120-4 (a Runx2…”

“…However, adenoviral introduction of Runx2 into CA120-4 cells was able to induce RANKL expression while inhibit OPG expression to induce osteoclast differentiation [19]. These findings indicated that Runx2 promotes osteoclast differentiation by inducing RANKL and inhibiting OPG.…”

“…Previous studies reported that Runx2, which is the master osteogenic transgenic factor regulating osteogenesis metabolism, also induces differentiation of osteoclast through regulation of RANKL and OPG [17][18][19][20][21]. However, the mechanism of Runx2 mutation of CCD patient leading to no root resorption of primary teeth is still not answered yet.…”

Overexpression of osteoprotegerin promotes preosteoblast differentiation to mature osteoblasts

“…5 The effect of Runx2 function on osteoblast differentiation and OPG expression is controversial. [21][22][23] Some studies show that Runx2 induces RANKL expression and suppresses OPG expression, 24 and osteoblast differentiation was arrested in Runx2 overexpression cells. 22 Another study shows that Runx2 contributes to OPG activities.…”

Homing of Cancer Cells to the Bone

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