Receptor for activator of nuclear factor kappa-B (RANKL), also known as OPGL (CD254), is a member of the tumor necrosis factor ligand superfamily, member 11 (TNFSF11). It is a factor that differentiates and activates osteoclasts. Because abnormalities in RANKL, its signaling receptor RANK, or its decoy receptor osteoprotegerin (OPG) lead to skeletal diseases such as osteophyte formation, the RANKL/RANK/OPG system is crucial for bone resorption. RANKL enhances the ability of dendritic cells to stimulate the proliferation of naive T cells and plays a key role in enhanced bone resorption in humoral hypercalcemia of malignancy.

RANKL expression distribution

RANKL is mainly expressed in late spermatids, mammary glandular cells, early spermatids, ductal cells, NK cells, and pancreatic endocrine cells.

(Data source: Uniprot)

Structure and signal transduction regulation of RANKL

The RANKL gene is located on chromosome 13 (13q14.11) and encodes a 317-amino acid glycoprotein. Human and mouse RANKL share 85% amino acid sequence homology. RANKL belongs to the TNF cytokine superfamily. RANKL is a type II transmembrane protein whose carboxy-terminal extracellular domain can be cleaved by enzymes such as matrix metalloproteinases and released into the extracellular environment to form soluble RANKL. Both membrane-bound and soluble RANKL can bind to RANK. RANKL monomers oligomerize to form trimers, a conformation essential for TNF superfamily cytokines that requires three clefts for receptor binding; signaling in target cells occurs through RANK trimers.

(Data source: uniprot)

Binding of the RANK receptor to RANKL (a RANK agonist) activates downstream signaling pathways, including canonical and noncanonical NF-κB, MAPK, and PI3K-AKT pathways. RANK itself lacks kinase activity, and its signaling is initially mediated by adaptor molecules such as tumor necrosis factor (TNF) receptor-associated factor (TRAF) proteins (including TRAF6), GRB-associated binding protein 2 (GAB2), and SRC195-197. The RANKL cytoplasmic tail contains multiple proline-rich repeat motifs that mediate interactions with SRC homology 2 (SH2) domain-containing proteins.

(Data source: Ahern E, et al. Nat Rev Clin Oncol. 2018)

The role of RANKL in disease

RANKL and RANK are expressed in tumor tissues, and RANKL/RANK expression levels in cancer tissues are associated with prognosis in multiple cancer types, including breast, lung, endometrial, renal cell, and gastric cancers. The RANKL/RANK axis may influence the development and progression of cancer, with the specific effects of RANKL/RANK varying depending on the cancer type.

The role of RANKL-RANK in breast cancer: BRCA1 gene mutations lead to increased RANK expression in mammary luminal progenitor cells. RANKL expressed on PR-expressing LECs stimulates the proliferation and survival of mutant cells, which have impaired DNA repair and lead to tumorigenesis.

RANKL-RANK in lung cancer: KRAS mutations in lung epithelial cells increase RANK expression in these cells. These cells overproliferate under RANKL stimulation, leading to tumorigenesis.

The role of RANKL-RANK in multiple myeloma: Myeloma cells enhance RANKL expression in tumor stromal cells within the bone, leading to osteoclast bone resorption and the release of myeloma cells from dormancy. These processes together lead to the expansion of the tumor within the bone.

The role of RANKL-RANK in bone metastasis: Cancer cells that metastasize to the bone marrow produce molecules including PTHrP. Some of these molecules induce RANKL expression in tumor stromal cells. This RANKL induces osteoclastic bone resorption, and the degraded bone releases matrix-embedded growth factors such as IGF-1 and TGF-β. These factors promote tumor growth, which in turn further promotes RANKL expression, creating a vicious cycle. Soluble RANKL facilitates the chemotaxis of RANK-expressing tumor cells to metastatic sites. Tumor-expressed RANKL also participates in angiogenesis and vascular permeability, promoting tumor invasion.

(Data source: Ono T, et al. Inflamm Regen. 2020)

Targeted therapy for RANKL

Currently, the primary treatment approach targeting RANKL is monoclonal antibodies. Two monoclonal antibodies have been approved for marketing: Narlumosbart (developed by Shanghai Jmt-bio Technology Co., Ltd.) and Denosumab (developed by Amgen). Numerous other drugs are in clinical development.

Denosumab , a fully human IgG2 monoclonal antibody, was approved for marketing in 2010. Denosumab specifically binds to RANKL with high affinity, blocking the interaction between RANKL and its sole receptor, RANK. This inhibits osteoclast formation, function, and survival, reduces bone resorption, and interrupts cancer-induced bone destruction. It is primarily used to treat glucocorticoid-induced osteoporosis , malignant tumors, humoral hypercalcemia , rheumatoid arthritis , bone cancer , multiple myeloma , bone diseases , giant cell tumors of bone, bone metastases, and osteoporosis.

HLO5 is a monoclonal antibody drug targeting RANKL developed by HUALAN BIOLOGICAL. It is currently in Phase 3 clinical trials for the treatment of solid tumors and bone metastases.

(Data source: New Drug Intelligence Database)