Activation of Receptor Activator of NF-κB Ligand Gene Expression by 1,25-Dihydroxyvitamin D₃ Is Mediated through Multiple Long-Range Enhancers

Abstract

RANKL is a tumor necrosis factor (TNF)-like factor secreted by mesenchymal cells, osteoblast derivatives, and T cells that is essential for osteoclastogenesis. In osteoblasts, RANKL expression is regulated by two majorSkeletal remodeling in adults occurs through the coupled actions of bone-forming osteoblasts and bone-resorbing osteoclasts (17). The latter are terminally differentiated, multinucleated cells of the monocyte-macrophage lineage (63). The process of osteoclastogenesis is highly complex and is orchestrated by a number of growth factors, steroidal components, and cytokines, all of which exert their actions in a highly temporal fashion (57). Many of these regulatory factors are produced and secreted by adjacent support cells that include stroma, B and T cells, and cells of the osteoblast lineage. The production of such factors by osteoblasts highlights the extrinsic role that these cells play in the process of bone resorption. Importantly, while many of these secreted components are critical for normal adult bone remodeling, their aberrant secretion can be pathological and lead to either focal or systemic bone disease (45).Although many factors participate in osteoclastogenesis, the molecule that is now considered to be both necessary and sufficient in vivo and in vitro is the receptor activator of NF-B ligand (RankL). RankL is a tumor necrosis factor (TNF)-like factor that is produced by stromal cells and osteoblasts as well as a variety of other cell types (38). This factor not only actively promotes the process of osteoclast differentiation, but also is required for the cell's bone-resorbing activity and for its survival (22). The interaction of RankL with receptor activator of NF-B (Rank), an integral receptor protein located on the surface of osteoclast precursors, triggers a number of signaling cascades that include the IKK/IK␤/NF-B transduction pathway and the mitogen-activated protein kinase (MAPK), Src, and phosphatidylinositol 3-kinase (PI3K)/AKT pathways as well (64). A novel calcium oscillation pathway that involves ITAM coreceptors is also involved in the downstream effects of RankL (35). Importantly, stimulation of these pathways culminates in the activation of multiple transcription factors, including c-fos, NF-B, and NFATc1, all of which play strategic roles in the differentiation process at the genetic level (44, 61). Overall, the timely activation of these transcription factors initiates growth arrest and promotes osteoclast differentiation, fusion, activation, and survival (63). The evidence that supports the essentiality of both RankL and its receptor in osteoclast formation is most strongly supported by the skeletal phenotypes of both RankL-and Rank-null mice, neither of which are capable of producing osteoclasts in vivo and thus are phenotypically osteopetrotic (20,37).RankL is synthesized and expressed on the surface of regulatory cells in response to a myriad of both local and systemic factors, many of which are essential to physiologic bone turnover. These include the two hormo...

Control of bone resorption in mice by Schnurri-3

“…Transcription factors, such as vitamin D receptor (VDR), Runx2, cAMP response element binding protein (CREB), and STAT3, are known to associate with this RANKL gene regulatory region (24,27). We found that Shn3 does not activate transcription from this reporter when the CREB binding sites are deleted (Fig.…”

“…TNFSF11 gene expression is controlled by a variety of distal and proximal regulatory regions (21,24,25). We focused on a conserved regulatory region located 76 kb upstream of the transcriptional start site that had been described by two independent groups as important for calciotropic agent responsiveness in vitro and in vivo (26).…”

Targeted deletion of Sost distal enhancer increases bone formation and bone mass

“…Currently, there is only one other report of a distal transcriptional enhancer: the RankL distal regulatory enhancer (DCR) that affects bone mass. When deleted, DCR caused HBM due to reduced osteoclast activity, leading to a lower rate of bone remodeling similar to that observed in humans and mice with hypoparathyroidism (25,26).…”

CXCL5 stimulation of RANK ligand expression in Paget's disease of bone

“…42 Mouse RANKL expression has been shown to be upregulated by 1,25-dihydroxyvitamin D3 and PTH through long-range distal enhancer regions. 43,44 Further, Runx2/Cbfa1 has been shown to potentiate PTH stimulation of RANKL through a site in the distal control region. 45 Recent evidence also suggests that RANKL expression in T cells is regulated by c-Fos through a distal enhancer region.…”