Osteoblasts Provide a Suitable Microenvironment for the Action of Receptor Activator of Nuclear Factor-κB Ligand

Abstract

Deficiency of osteoprotegerin (OPG), a soluble decoy receptor for receptor activator of nuclear factor-kappaB ligand (RANKL), in mice induces osteoporosis caused by enhanced bone resorption. Serum concentrations of RANKL are extremely high in OPG-deficient (OPG(-/-)) mice, suggesting that circulating RANKL is involved in osteoclastogenesis. RANKL(-/-) mice exhibit osteopetrosis, with the absence of osteoclasts. We examined the requirements for osteoclastogenesis using OPG(-/-) mice, RANKL(-/-) mice, and a system involving bone morphogenetic protein 2 (BMP-2)-induced ectopic bone formation. When collagen disks containing BMP-2 (BMP-2-disks) or vehicle were implanted into OPG(-/-) mice, osteoclast-like cells (OCLs) and alkaline phosphatase-positive OCLs appeared in BMP-2-disks but not the control disks. F4/80-positive osteoclast precursors were similarly distributed in both BMP-2- and control disks. Cells expressing RANKL were detected in the BMP-2-disks, and the addition of OPG to the disk inhibited OCL formation. Muscle cells in culture differentiated into alkaline phosphatase-positive cells in the presence of BMP-2 and accordingly expressed RANKL mRNA in response to PTH. This suggests that RANKL expressed by osteoblasts is a requirement for osteoclastogenesis. We then examined how osteoblasts are involved in osteoclastogenesis other than RANKL expression, using RANKL(-/-) mice. BMP-2- and control disks were implanted into RANKL(-/-) mice, which were injected with RANKL for 7 d. Many OCLs were observed in the BMP-2-disks and bone tissues but not the control disks. These results suggest that osteoblasts also play important roles in osteoclastogenesis through offering the critical microenvironment for the action of RANKL.

c-Fos plays an essential role in the up-regulation of RANK expression in osteoclast precursors within the bone microenvironment

“…We previously reported that osteoclasts were generated in bone in response to an injection of RANKL in RANKL 2/2 mice, as they were in wild-type mice (Yamamoto et al, 2006). Osteoclasts were not observed in the soft tissues around the bone in RANKLinjected RANKL 2/2 mice, indicating that the expression of RANKL is not involved in determining the correct site for osteoclastogenesis.…”

Vitamin D endocrine system and osteoclasts

“…When RANKL was injected into these RANKL À / À mice, osteoclasts appeared in BMP-2-containing disks but not in the control disk. 51 These results suggested that some QOPs circulated in the blood and settled in the bone (Figure 6a). The distribution of QOPs was similar to that of ALP þ osteoblasts in RANKL À / À mice, 49 suggesting that ALP þ bone-forming osteoblasts are somehow involved in homing of QOPs to the bone.…”

Biocompatibility and Bone Tissue Compatibility of Alumina Ceramics Reinforced with Carbon Nanotubes

“…RNA of all cells was extracted from osteoblasts following the method of Yamamoto et al [26] and complementary DNA was synthesized from the RNA using a reverse transcription (RT) device (ReverTra Ace, Toyobo, Osaka, Japan).…”