Roles of the RANKL-RANK Axis in Immunity-Implications for Pathogenesis and Treatment of Bone Metastasis

Abstract

A substantial amount patients with cancer will develop bone metastases, with 70% of metastatic prostate and breast cancer patients harboring bone metastasis. Despite advancements in systemic therapies for advanced cancer, survival remains poor for those with bone metastases. The interaction between bone cells and the immune system contributes to a better understanding of the role that the immune system plays in the bone metastasis of cancer. The immune and bone systems share various molecules, including transcription factors, signaling molecules, and membrane receptors, which can stimulate the differentiation and activation of bone-resorbing osteoclasts. The process of cancer metastasis to bone, which deregulates bone turnover and results in bone loss and skeletal-related events (SREs), is also controlled by primary cancer-related factors that modulate the intratumoral microenvironment as well as cellular immune process. The nuclear factor kappa B ligand (RANKL) and the receptor activator of nuclear factor kappa B (RANK) are key regulators of osteoclast development, bone metabolism, lymph node development, and T-cell/dendritic cell communication. RANKL is an osteoclastogenic cytokine that links the bone and the immune system. In this review, we highlight the role of RANKL and RANK in the immune microenvironment and bone metastases and review data on the role of the regulatory mechanism of immunity in bone metastases, which could be verified through clinical efficacy of RANKL inhibitors for cancer patients with bone metastases. With the discovery of the specific role of RANK signaling in osteoclastogenesis, the humanized monoclonal antibody against RANKL, such as denosumab, was available to prevent bone loss, SREs, and bone metastases, providing a unique opportunity to target RANKL/RANK as a future strategy to prevent bone metastases.

Keywords: RANKL/RANK; bone metastasis; denosumab; immune cells; osteoblast; osteoclasts.

Figure 1

Immune system in a long journey to develop bone metastasis. To establish the metastatic tumor, cancer cells escape from the tumoricidal immune response that is mediated by killer cells, such as CD8+ T cells and natural killer (NK) cells, and then invade through the surrounding stroma and intrude into blood vessels (intravasation). At the metastatic site, the arrested tumor cells in microvessels escape from the blood vessel (extravasation). Metastatic tumor cells can interact with both osteoclasts and osteoblasts in the bone microenvironment and release factors, such as TNF, TGF-β, and RANKL, to promote osteoclastogenesis and osteoblastogenesis. CD4+ T cells can produce not only osteoclastogenic cytokines such as IL17 and TNF but also anti-osteoclastogenic cytokines such as IFNγ and IL4. The figure was designed using Adobe Illustrator CC.

Figure 2

Contribution of the RANKL–RANK axis in immune system and bone metastasis. Among tumor-infiltrating immune cells, the expression of RANKL has been observed on all immune cell types, such CD8+ T, CD4+ T, and Treg cells, and RANKL can act on dendritic cells (DCs) to promote their survival and to prolong T–DC interactions. Bone-resorbing factors, such as vitamin D3, PTHrP, IL-1, IL-11, IL-17, and TNF-α, act on osteoblasts to induce RANKL, which binds to RANK present at the surface of osteoclast progenitors (pre-osteoclasts), which results in bone resorption by mature osteoclasts. The figure was designed using Adobe Illustrator CC.

Figure 3

RANKL/RANK signaling and its inhibitors. RANKL is a type II transmembrane molecule that contains a small N-terminal intracellular domain, a transmembrane region, and a c-terminal extracellular domain consisting of a stalk and a receptor-binding region [111]. sRANKL is derived from the membrane-bound form through alternative splicing or proteolytic cleavage. Binding between RANKL and RANK induces the recruitment of TNF receptor associated (TRAF) proteins (including TRAF6), GRB-associated-binding protein 2 (GAB2), and SRC, which activates downstream signaling pathways, such as NF-κB, MAPK, and PI3K–AKT pathways. Pharmacological inhibitors targeting both soluble and membrane-bound forms of RANKL that have been used in humans include the anti-RANKL monoclonal antibody denosumab. The figure was designed using Adobe Illustrator CC.