The RANK Signaling Pathway: Unraveling its Complex Roles in Health and Disease

Introduction

The RANK signaling pathway, composed of receptor activator of nuclear factor κB (RANK), RANK ligand (RANKL), and osteoprotegerin (OPG), plays a crucial role in various physiological processes, particularly in bone remodeling and immune regulation. This pathway is a part of the tumor necrosis factor (TNF) and TNF receptor superfamilies, and its dysregulation has been implicated in pathological conditions such as osteoporosis and cancer-induced bone destruction.

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Fig. 1 Pleiotropic effects of the RANK pathway in cancer (González-Suárez E., et al. 2016).

Members of the RANK Pathway and Signaling

RANK, RANKL, and OPG are central players in the RANK signaling pathway. RANK, a type I glycoprotein consisting of 616 amino acids, lacks enzymatic activity and relies on adaptor molecules for signaling. RANKL, the sole ligand for RANK, exists in transmembrane and soluble forms, both actively participating in downstream signaling. OPG, a soluble decoy receptor, competes with RANK for RANKL binding, regulating RANK activation tightly. Upon RANKL binding to RANK, receptor trimerization occurs, recruiting TNF receptor-associated factor (TRAF) adaptor proteins, predominantly TRAF6. This leads to the activation of downstream signaling pathways, including nuclear factor κB (NF-κB), AKT/PKB, JNK, ERK, and p38.

The RANK receptor has three distinct cytoplasmic domains (motifs 1, 2, and 3), each mediating specific functions in osteoclast formation and function. Motif 1 binds TRAF6, activating NF-κB and MAPK pathways. Motifs 2 and 3, more potent than motif 1, activate NF-κB alone or NF-κB and p38 simultaneously. TRAF2, TRAF5, and TRAF6 bind the C-terminal region of RANK, essential for NF-κB activation. Additionally, RANK/RANKL activates AKT/PKB, forming a complex with TRAF6 and c-Src.

RANKL, a type II transmembrane protein, undergoes cleavage by metalloproteinases to yield a soluble form. RANKL binding to RANK induces NFATc1 activation via TRAF6 and c-Fos, crucial for osteoclast precursor differentiation. Bidirectional signaling has been observed for RANKL in T cells and chronic lymphoid leukemia cells.

OPG, a 401 amino acid protein, acts as a decoy receptor for RANKL and regulates osteoclastogenesis. OPG also interacts with other ligands, such as TRAIL, syndecan-1, glycosaminoglycans, von Willebrand factor, and factor VIII–von Willebrand factor complex.

The RANK Pathway in Homeostasis and Disease

RANK, RANKL, and OPG are pivotal in bone remodeling, and regulating osteoclast differentiation, activation, and survival. Mice deficient in RANKL or RANK exhibit osteopetrosis, while OPG knockout mice develop early-onset osteoporosis. The dysregulation of the RANK pathway is implicated in conditions like postmenopausal osteoporosis and cancer-induced bone destruction.

Postmenopausal osteoporosis is associated with increased RANKL production due to decreased estrogen levels, leading to elevated osteoclast activity and diminished bone mass. In cancer-induced bone destruction, metastatic tumors in the bone microenvironment induce RANKL production, activating osteoclasts and creating a detrimental feedback loop.

Denosumab, a monoclonal antibody blocking RANKL activity, has been developed and approved for clinical use to reduce the risk of osteoporosis and prevent skeletal-related events in cancer patients with bone metastasis. New receptor-selective RANKL inhibitors show promise in providing alternative therapeutic strategies.

The RANK Pathway in Immune Cells and Chronic Inflammation

RANK and RANKL play crucial roles in immune tissues, influencing immune responses, immunosuppression, and the innate and adaptive immune systems. RANKL treatment enhances inflammatory cytokine expression in mature dendritic cells, promoting CD4+ T cell differentiation into Th1 cells. The RANK-RANKL interaction is essential for central tolerance, thymic epithelial cell development, and the regulation of autoreactive T cells.

The RANK pathway is implicated in inflammatory conditions such as arthritis, inflammatory bowel disease, and chronic colitis. RANKL inhibition has shown efficacy in reducing colitis development, while its induction in immunosuppressive Treg cells is suggested for inflammatory bowel disease treatment. In rheumatoid arthritis, RANKL expression at the pannus-bone interface is targeted for therapeutic intervention.

RANK Signaling in Cancer

The RANK signaling pathway plays a crucial role in the development and progression of various cancers. The expression of RANKL can be detected in multiple solid tumors, including breast, lung, prostate, and ovarian cancers. Increased levels of RANKL are associated with adverse prognoses in certain cancers.

In breast cancer, RANKL expression is observed in tumor cells, while RANK is present in immune cells infiltrating the tumor and adjacent stromal cells. RANKL promotes the proliferation, survival, migration, and invasion of breast cancer cells. Ongoing clinical trials aim to assess the effectiveness of denosumab in breast cancer treatment, a drug that targets the RANK signaling pathway.

In lung cancer, overexpression of RANKL is linked to tumor progression and metastasis. The RANK signaling pathway is involved in the process of bone metastasis, making it a potential therapeutic target.

Prostate cancer exhibits increased expression of RANKL, contributing to the occurrence of osteosclerotic metastasis. Targeting the RANK signaling pathway may provide a novel approach to managing bone metastasis in prostate cancer.

Conclusion

The RANK signaling pathway is a complex and versatile network that regulates diverse physiological processes. Its involvement in bone remodeling, immune regulation, and epithelial cell differentiation highlights its importance in maintaining homeostasis. Dysregulation of this pathway contributes to various diseases, including osteoporosis, cancer-induced bone destruction, inflammation, and aberrant epithelial cell proliferation.

Targeting the RANK pathway holds therapeutic potential for a range of conditions. Current interventions, such as denosumab, demonstrate the clinical efficacy of RANKL inhibition. As research continues to unravel the intricacies of RANK signaling, novel therapeutic strategies may emerge, offering new hope for patients with conditions influenced by this intricate molecular pathway.

References

1. Ono T., et al. RANKL biology: bone metabolism, the immune system, and beyond. Inflammation and Regeneration. 2020, 40(1): 1-16.
2. González‐Suárez E., Sanz‐Moreno A. RANK as a therapeutic target in cancer. The FEBS Journal. 2016, 283(11): 2018-2033.