Introduction
Cancer progression and metastasis involve a complex interplay of signaling pathways within the tumor microenvironment (TME). Among these, the receptor activator of nuclear factor kappa-B ligand (RANKL) and its receptor, RANK, have emerged as pivotal players. Originally recognized for its role in bone remodeling, the RANK/RANKL signaling pathway is now known to influence various aspects of tumor biology, including cancer cell survival, proliferation, migration, and metastasis.
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Fig. 1 RANK/RANKL signaling in cancer cells (Renema N., et al. 2016).
Discovery and Characterization of RANK-RANKL
RANKL, a member of the tumor necrosis factor (TNF) superfamily, was first identified as a key regulator of bone remodeling. It binds to RANK, a type I transmembrane protein receptor also belonging to the TNF receptor superfamily, to mediate various biological processes. RANKL exists in three isoforms due to alternative splicing, with RANKL1 being the full-length membrane-bound form, RANKL2 lacking part of the cytoplasmic domain, and RANKL3 being a soluble form. The primary source of RANKL in bone tissue is osteocytes, although it is also expressed in other tissues such as the brain, skin, and immune system.
RANK, on the other hand, is expressed in a variety of tissues, including the thymus, mammary glands, and bones. The interaction between RANK and RANKL triggers intracellular signaling cascades that lead to the activation of key transcription factors such as NF-κB, AP-1, and NFATc1. These transcription factors regulate the expression of genes involved in bone resorption, cell survival, proliferation, and immune modulation.
Mechanisms of RANK-RANKL Signaling in Cancer
RANK signaling is primarily mediated through the recruitment of TNF receptor-associated factors (TRAFs), particularly TRAF6, which is critical for the activation of downstream signaling pathways. These include the NF-κB, MAPK, and PI3K/Akt pathways, all of which play essential roles in cell survival, proliferation, and migration. The activation of these pathways by RANKL in cancer cells can lead to various oncogenic processes, including epithelial-to-mesenchymal transition (EMT), a key step in the metastatic cascade.
The regulatory role of RANK-RANKL signaling in cancer is further modulated by osteoprotegerin (OPG), a decoy receptor that binds to RANKL and prevents its interaction with RANK. OPG is produced by various cells within the TME, including osteoblasts, endothelial cells, and immune cells. The balance between RANKL and OPG levels in the TME is critical for determining the extent of RANK signaling, with an excess of RANKL promoting tumor progression and metastasis.
In addition to OPG, recent studies have identified leucine-rich repeat-containing G-protein-coupled receptor 4 (LGR4) as another regulator of RANKL activity. LGR4 can bind to RANKL and negatively regulate osteoclastogenesis, thereby acting as a feedback loop to control RANK signaling. While the role of LGR4 in cancer is not yet fully understood, it is known to promote the proliferation of various tumor cells, including those in breast and prostate cancers, through activation of the Wnt/β-catenin signaling pathway.
RANK-RANKL in Tumor Initiation and Progression
The role of RANK-RANKL signaling in tumor initiation is best exemplified in breast cancer. During mammary gland development, RANKL promotes the survival and proliferation of epithelial cells, processes that are crucial for the formation of lobo-alveolar structures necessary for lactation. However, dysregulation of RANK-RANKL signaling can lead to the development of pre-neoplastic lesions and tumor foci. In mouse models, overexpression of RANK in mammary glands leads to the formation of ductal hyperplasias and adenocarcinomas, providing direct evidence of RANKL's involvement in tumor initiation.
RANK-RANKL signaling also plays a critical role in tumor progression by promoting cell migration and metastasis. RANKL acts as a chemoattractant for RANK-expressing cancer cells, facilitating their migration to distant sites such as bone, lungs, and liver. This mechanism is particularly relevant in bone metastases, where RANKL produced by osteoblasts and bone marrow stromal cells attracts metastatic cancer cells, leading to the formation of secondary tumor sites. Furthermore, RANKL-induced migration is associated with the activation of MAP kinase pathways, which are essential for the invasive behavior of cancer cells.
In addition to its direct effects on cancer cells, RANKL can modulate the TME by promoting angiogenesis. RANK expression has been detected in endothelial cells, and RANKL signaling can enhance the survival and proliferation of these cells, contributing to the formation of new blood vessels. This angiogenic effect of RANKL is further amplified by vascular endothelial growth factor (VEGF), a key mediator of angiogenesis often upregulated in cancer.
RANK-RANKL and Cancer Cell Dormancy
Cancer cell dormancy is a state in which tumor cells enter a quiescent phase, evading immune surveillance and resisting conventional therapies. The TME, including the RANK-RANKL axis, plays a pivotal role in maintaining cancer cell dormancy. For instance, RANKL has been implicated in the dormancy of breast cancer cells in bone metastases. In a mouse model, VCAM-1, aberrantly expressed in dormant micrometastases, promotes the recruitment and activation of osteoclastic cells via RANKL, leading to the reactivation of dormant tumor cells and the transition to an actively proliferating state.
This interaction between RANKL and dormant cancer cells underscores the importance of the RANK-RANKL axis in determining the fate of micrometastases. The balance between dormancy and reactivation is influenced by various factors within the TME, including the levels of RANKL and its regulators, OPG and LGR4. Disrupting this balance can lead to the re-emergence of dormant cancer cells, contributing to tumor recurrence and metastasis.
Therapeutic Implications of Targeting RANK-RANKL
Given its central role in cancer progression, the RANK-RANKL axis presents an attractive target for therapeutic intervention. Denosumab, a monoclonal antibody that binds to RANKL and prevents its interaction with RANK, has been approved for the treatment of bone metastases in patients with solid tumors. By inhibiting RANKL, Denosumab reduces osteoclast-mediated bone resorption, thereby preventing skeletal-related events such as fractures and pain.
Beyond its use in bone metastases, Denosumab has shown potential in other cancer types, including breast and prostate cancers, where RANK-RANKL signaling is implicated in tumor progression. Clinical trials are ongoing to explore the efficacy of Denosumab in these settings, as well as in combination with other therapies such as chemotherapy and immune checkpoint inhibitors.
In addition to Denosumab, other therapeutic strategies targeting the RANK-RANKL axis are being investigated. These include small-molecule inhibitors of RANK signaling and agents that modulate the expression of OPG or LGR4 to fine-tune the RANKL-RANK interaction. As our understanding of the complex roles of RANK-RANKL signaling in cancer deepens, these approaches may offer new avenues for the treatment of various malignancies.
Conclusion
The RANK-RANKL axis is a key regulator of bone remodeling and immunity, but its roles in cancer are equally significant. From tumor initiation and progression to metastasis and dormancy, RANK-RANKL signaling influences nearly every stage of cancer development. As research continues to unravel the complexities of this pathway, targeting RANK-RANKL offers promising potential for therapeutic intervention in cancer, particularly in the management of bone metastases and other malignancies where this signaling axis plays a critical role.
References
- Renema N., et al. RANK-RANKL signaling in cancer. Bioscience Reports. 2016, 36 (4): e00366.
- Lacey D. L., et al. Osteoprotegerin ligand is a cytokine that regulates osteoclast differentiation and activation. Cell. 1998, 93 (2): 165-176.
- Tan W., et al. Tumour-infiltrating regulatory T cells stimulate mammary cancer metastasis through RANKL–RANK signalling. Nature. 2011, 470 (7335): 548-53.
- Li X., et al. Potential role of the OPG/RANK/RANKL axis in prostate cancer invasion and bone metastasis. Oncology Reports. 2014, 32 (6): 2605-11.
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