RANKL biology: bone metabolism, the immune system, and beyond

Abstract

Receptor activator of NF-κB (RANK) ligand (RANKL) induces the differentiation of monocyte/macrophage-lineage cells into the bone-resorbing cells called osteoclasts. Because abnormalities in RANKL, its signaling receptor RANK, or decoy receptor osteoprotegerin (OPG) lead to bone diseases such as osteopetrosis, the RANKL/RANK/OPG system is essential for bone resorption. RANKL was first discovered as a T cell-derived activator of dendritic cells (DCs) and has many functions in the immune system, including organogenesis, cellular development. The essentiality of RANKL in the bone and the immune systems lies at the root of the field of "osteoimmunology." Furthermore, this cytokine functions beyond the domains of bone metabolism and the immune system, e.g., mammary gland and hair follicle formation, body temperature regulation, muscle metabolism, and tumor development. In this review, we will summarize the current understanding of the functions of the RANKL/RANK/OPG system in biological processes.

Keywords: Bone; Immune system; OPG; Organ development; Osteoimmunology; RANK; RANKL; Tumor.

Fig. 1

RANKL in bone metabolism. a The RANKL–RANK interaction in bone development and remodeling. Hypertrophic chondrocytes and osteoblasts function as the source of RANKL during growth. After the growth period, osteocytes are the major source of RANKL. RANKL induces the differentiation of osteoclasts, which resorb bone matrix. b RANKL–RANK interaction in bone and joint diseases related to immobility and aging. The bone loss induced by unloading is induced by osteocyte RANKL. B cell RANKL is reported to partially contribute to the bone loss in postmenopausal osteoporosis, as well. c In the lesion that occur in rheumatoid arthritis, synovial fibroblasts stimulated with pro-inflammatory cytokines, including IL-17, express RANKL and enhance osteoclastogenesis. In periodontitis, RANKL is mainly provided by PDL cells and osteoblasts. (see also Table 2). The IL-17 in these processes is produced by T_H17 cells stimulated by IL-6. T_H17 cells (exFoxp3 T_H17 cells, in particular) express RANKL as well. RANKL receptor activator of NF-κB ligand, RANK receptor activator of NF-κB, T_H17 cell T helper 17 cell, PDL periodontal ligament

Fig. 2

RANKL in immunity. a RANKL–RANK interaction in the development of the thymus. RANKL is produced by LTi cells, T cells, and iNKT cells and interacts with the RANK expressed on mTECs. This interaction induces the expression of Aire, resulting in the expression of TSAs on MHC molecules. The TSA–MHC complex is necessary for negative selection, the key process for establishing self-tolerance. b RANKL–RANK interaction in the lymph node development. Lymph node development begins with the interaction between LTi cells and LTo cells. LTα1β2 is expressed by LTi cells and interacts with LTβR on LTo cells, which in turn leads to the expression of RANKL on LTo cells. The expressed RANKL stimulates LTi cells to induce more LTα1β2, forming a positive feedback loop. With the stimulation of LTα1β2, some LTo cells mature into MRCs. The RANKL on LTo cells and MRCs binds to the RANK on lymphatic endothelial cells, resulting in the recruitment of macrophages. c RANKL–RANK interaction in the gastrointestinal tract. (Left) ILC3s interact with each other through RANKL and RANK. The interaction leads to the decrease of the proliferation and IL-17/IL-22 production of these cells, resulting in the suppression of excessive inflammation. (Right) RANKL–RANK interaction in M cell development. Mesenchymal cells beneath the epithelium of the gastrointestinal tract express RANKL and interact with RANK–expressing epithelial cells. These cells differentiate into morphologically and functionally unique cells called M cells. These cells enable the transfer of antigens from the lumen of gastrointestinal tract to DCs, leading to IgA production. d RANKL–RANK interaction in the skin. Keratinocytes express RANKL upon UV–irradiation. The RANKL binds to LCs in the skin. These LCs contribute to the generation of Treg cells, which decrease the skin inflammation and resolution of dermatitis in psoriasis and atopic dermatitis. e RANKL–RANK interaction in the CNS inflammation. (Left) T_H17 cell cells induce the CCL20 expression of astrocytes at the blood–brain barrier via RANKL–RANK signaling. CCL20 recruits CCR6-expressing cells, including T_H17 cell cells. These accumulated cells penetrate the barrier and infiltrate into the CNS to elicit inflammation. (Right) In the context of ischemic stroke, dead cells in the brain release DAMPs, which are recognized by TLRs. TLR stimulation of microglial cells leads to the production of pro-inflammatory cytokines including IL-6 and TNF-α, leading to inflammation and further cell death. RANKL–RANK signal in the microglial cells inhibits the production of these cytokines, resulting in the protection of the brain. RANKL receptor activator of NF-κB ligand, RANK receptor activator of NF-κB, LTi cell lymphoid tissue inducer cell, iNKT cell invariant natural killer T cell, mTEC medullary thymic epithelial cell, Aire autoimmune regulator, TSA tissue-specific antigen, MHC major histocompatibility complex, LTo cell lymphoid tissue organizer cell, LT lymphotoxin, LTβR lymphotoxin β receptor, MRC marginal reticular cell, ILC3 group 3 innate lymphoid cell, IL interleukin, DC dendritic cell, UV ultra violet, LC Langerhans cell, Treg cell regulatory T cell, CNS central nervous system, T_H17 cell T helper 17 cell, CCL20 C-C motif chemokine ligand 20, CCR6 C-C motif chemokine receptor 6, DAMP damage-associated molecular pattern, TLR Toll-like receptor

Fig. 3

RANKL in biological processes other than bone metabolism and the immune systems. a RANKL–RANK interaction in the development of the mammary gland. The LECs of the mammary gland are divided into two subpopulations based on the expression of PR. PR-expressing LECs express RANKL in response to Pg. RANKL interacts with LECs and MECs, resulting in the proliferation of these epithelial cells and the morphogenesis of the gland. b RANKL–RANK interaction in thermogenesis. Certain types of cells of the LSn of the forebrain express RANKL, which interacts with neurons and astrocytes in the POA and the MSn. These nuclei produce PGE2 via COX-2, which leads to both shivering and non-shivering thermogenesis. c RANKL–RANK signaling in the blood vessel. Both RANKL and RANK are expressed on vascular cells including VSMCs. RANKL induces the expression of BMP2 and 4, which promotes the osteogenic gene expression of these cells, resulting in the vascular calcification. The signal is suppressed by estrogen and its receptor ERα. Expression of RANKL and RANK in this context is enhanced by Ang II. Production of Ang II is increased in turn by RANKL and RANK. d RANKL–RANK interaction in the hair cycle. Cells in the inner root sheath of the HF express RANKL. Cells in the outer root sheath, the bulge and the IFE express RANK. The interaction of these cells induces the growth of the epidermis and activates the hair cycle. e RANKL–RANK interaction in the liver. Hepatocytes stimulated with RANKL express pro-inflammatory cytokines that stimulate Kupffer cells, leading to T2DM. f RANKL–RANK interaction in the skeletal muscle. RANKL–RANK signaling in muscle fibers is involved in the strength and glucose metabolism of the skeletal muscle. RANKL receptor activator of NF-κB ligand, RANK receptor activator of NF-κB, Pg progesterone, PR progesterone receptor, LEC luminal epithelial cell, MEC myoepithelial cell, LSn lateral septal nucleus, POA preoptic area, MSn medial septal nucleus, PGE2 prostaglandin E2, COX-2 cyclooxygenase-2, VSMC vascular smooth muscle cell, BMP bone morphogenetic protein, ER estrogen receptor, Ang angiotensin, ATR angiotensin receptor, HF hair follicle, IFE interfollicular epidermis, T2DM type 2 diabetes mellitus

Fig. 4

RANKL in tumorigenesis and metastasis. a RANKL–RANK interaction in breast cancer. Mutations in BRCA1 lead to the increased expression of RANK in luminal progenitor cells of the mammary gland. The RANKL expressed on PR-expressing LECs (see Fig. 3a) stimulates the proliferation and survival of the mutant cells and DNA repair is impaired in these cells, resulting in the tumorigenesis. b RANKL–RANK interaction in lung cancer. KRAS mutations in the lung epithelial cells increase RANK expression on these cells. These cells undergo excessive proliferation upon RANKL stimulation, leading to tumor development. c RANKL–RANK interaction in multiple myeloma. Myeloma cells enhance RANKL expression on the stromal cells of tumors in the bone, resulting in osteoclastic bone resorption and the release of myeloma cells from dormancy. Together, these processes lead to an expansion of the tumors in the bone. d RANKL–RANK interaction in bone metastasis. Cancer cells metastasized to the bone marrow produce molecules, including PTHrP. Some of these induce RANKL expression on the tumor stromal cells. This RANKL induces osteoclastic bone resorption, and the degraded bone releases growth factors embedded in the matrix, such as IGF-1 and TGF-β. These factors increase the tumor size and the enlarged tumor further contributes to the amount of RANKL expression, forming a vicious cycle. The soluble form of RANKL contributes to the chemotaxis of the tumor cells expressing RANK toward the site of metastasis. Tumoral RANKL is also involved in the angiogenesis and the permeability of the blood vessels, facilitating tumor invasion. RANKL receptor activator of NF-κB ligand, RANK receptor activator of NF-κB, Pg progesterone, PR progesterone receptor, LEC luminal epithelial cell, PTHrP parathyroid hormone-related peptide, IGF insulin-like growth factor, TGF-β transforming growth factor-β