The Role of LGR4 (GPR48) in Normal and Cancer Processes

Abstract

Leucine-rich repeats containing G protein-coupled receptor 4 (LGR4) is a receptor that belongs to the superfamily of G protein-coupled receptors that can be activated by R-spondins (RSPOs), Norrin, circLGR4, and the ligand of the receptor activator of nuclear factor kappa-B (RANKL) ligands to regulate signaling pathways in normal and pathological processes. LGR4 is widely expressed in different tissues where it has multiple functions such as tissue development and maintenance. LGR4 mainly acts through the Wnt/β-catenin pathway to regulate proliferation, survival, and differentiation. In cancer, LGR4 participates in tumor progression, invasion, and metastasis. Furthermore, recent evidence reveals that LGR4 is essential for the regulation of the cancer stem cell population by controlling self-renewal and regulating stem cell properties. This review summarizes the function of LGR4 and its ligands in normal and malignant processes.

Keywords: CSCs; GPR48; LGR4; cancer.

Figure 1

Structure and domains of LGR4. LGR4 is a transmembrane receptor with a long N-terminal extracellular domain constituted by 17 leucine-rich repeats, flanked by cysteine-rich regions. In addition, a seven-transmembrane helix domain and a C-terminal intracellular domain are found in LGR4. The signal peptide of LGR4 is found in the N-terminal region.

Figure 2

LGR4-induced Wnt/β-catenin signaling pathway. (a) RSPOs are a family of secreted proteins that can activate LGR4-induced Wnt/β-catenin signaling. Structurally, all the RSPOs have a signal peptide in the N-terminal domain, two furin-like cysteine-rich domains, a thrombospondin 1 repeat domain (TSR), and a basic amino acid-rich domain, which varies in size according to the RSPO member (b) In the absence of RSPO, ZNF3/RNF43 ubiquitinates the frizzled (Fzd)/LRP5-6 receptor complex for degradation. Wnt signal is blocked and the β-catenin destruction complex (formed by CK1, GSK3β, APC, and AXIN) is activated. GSK3β and CK1 phosphorylate β-catenin, inducing its ubiquitination and consequent proteasomal degradation. When LGR4 is activated by RSPOs, Norrin, or circLGR4 ligands, it stabilizes the frizzled/Lrp5-6 complex in the membrane, avoiding its degradation by inhibiting the activity of ZNRF3 and RNF43 proteins. Furthermore, LGR4 recruits IQGAP1 with an increasing affinity for DVL and recruits MEK, which phosphorylates LRP5/6, leading to the recruitment and inhibition of the β-catenin destruction complex into the Fzd/Lrp5-6 complex receptor.

Figure 3

RANKL-induced LGR4 signaling pathway. (a) RANKL is another molecule that can promote LGR4 activity. Three isoforms of RANKL have been described; the full-length RANKL (RANKL1), a form lacking part of the intra-cytoplasmic domain (RANKL2), and a soluble form (RANKL3). All three isoforms have a TNF ligand family homologous domain in their extracellular part. (b) RANKL interacts with LGR4 and induces the Gαq protein pathway and also inhibits GSK3β. Furthermore, RANKL activates RANK and promotes NF-κB canonical RANK signaling among other pathways, for instance; PI3K-AKT and MAPK. LGR4 competes with RANK to bind RANKL and suppresses canonical RANK signaling. Besides, it has been suggested that RANKL can compete with RSPOs to bind LGR4 and in this way disrupt RSPO-induced Wnt/β-catenin signaling. Osteoprotegerin (OPG) acts as a RANKL endogenous inhibitor.