Leucine-rich repeat-containing G-protein coupled receptor 5/GPR49 activates G12/13-Rho GTPase pathway

The Pivotal Role of Long Noncoding RNA RAB5IF in the Proliferation of Hepatocellular Carcinoma Via LGR5 Mediated β-Catenin and c-Myc Signaling

In the current study, the function of long noncoding RNA (LncRNA) RAB5IF was elucidated in hepatocellular carcinoma (HCCs) in association with LGR5 related signaling. Here TCGA analysis revealed that LncRNA RAB5IF was overexpressed in HCC, and its overexpression level was significantly (p < 0.05) correlated with poor prognosis in patients with HCC. Furthermore, LncRNA RAB5IF depletion suppressed cell proliferation and colony formation, increased sub G1 population, cleavage of poly ADP-ribose polymerase (PARP) and cysteine aspartyl-specific protease (caspase 3) and attenuated the expression of procaspase 3, pro-PARP and B-cell lymphoma 2 (Bcl-2) in HepG2 and Hep3B cells. Furthermore, LncRNA RAB5IF depletion reduced the expression of LGR5 and its downstreams such as β-catenin and c-Myc in HepG2 and Hep3B cells. Notably, LGR5 depletion also attenuated the expression of pro-PARP, pro-caspase3, β-catenin and c-Myc in HepG2 and Hep3B cells. Conversely, LGR5 overexpression upregulated β-catenin and c-Myc in Alpha Mouse Liver 12 (AML-12) normal hepatocytes. Overall, these findings provide novel evidence that LncRNA RAB5IF promotes the growth of hepatocellular carcinoma cells via LGR5 mediated β-catenin and c-Myc signaling as a potent oncogenic target.

Frizzleds as GPCRs - More Conventional Than We Thought!

For more than 30 years, WNT/β-catenin and planar cell polarity signaling has formed the basis for what we understand to be the primary output of the interaction between WNTs and their cognate receptors known as Frizzleds (FZDs). In the shadow of these pathways, evidence for the involvement of heterotrimeric G proteins in WNT signaling has grown substantially over the years - redefining the complexity of the WNT signaling network. Moreover, the distinct characteristics of FZD paralogs are becoming better understood, and we can now apply concepts valid for classical GPCRs to grasp FZDs as molecular machines at the interface of ligand binding and intracellular effects. This review discusses recent developments in the field of WNT/FZD signaling in the context of GPCR pharmacology, and identifies remaining mysteries with an emphasis on structural and kinetic components that support this dogma shift.

Downregulation of Lgr6 inhibits proliferation and invasion and increases apoptosis in human colorectal cancer

The aim of the present study was to analyze the role of leucine‑rich repeat‑containing G‑protein coupled receptor 6 (Lgr6) in the proliferation and invasion of colorectal cancer (CRC) cells, and to investigate its possible mechanisms. The expression of Lgr6 in CRC tissues was observed by real time‑quantitative polymerase chain reaction and western blotting. Then cell viability, apoptosis and cell invasion was measured by MTT, flow cytometry or Matrigel‑Transwell system, respectively in CRC cells after transfected with Lgr6 siRNA or Lgr6 vector. Furthermore, the expression of apoptosis‑associated protein and PI3K/AKT signaling (phosphorylated‑PI3K, phosphorylated‑AKT, t‑PI3K, t‑AKT) were measured by real‑time PCR/or western blot analysis. The results demonstrated that the level of Lgr6 was higher in CRC tissues than that in adjacent tissues, and Lgr6 overexpression increased CRC proliferation, and invasion of CRC cells in vitro. Notably, suppressing the expression of Lgr6 in CRC cells increased the expression of B‑cell lymphoma-2 (Bcl‑2)‑associated X protein and caspase‑3, but decreased the expression of Bcl‑2 at the mRNA and protein levels. Lgr6 also had the ability to regulate the phosphoinositide 3‑kinase/AKT signaling pathway. It was concluded that Lgr6 has a tumor‑promoting role in the development of CRC, and may serve as a potential diagnostic and prognostic biomarker for the disease.

LGR5 receptor promotes cell-cell adhesion in stem cells and colon cancer cells via the IQGAP1-Rac1 pathway

Leucine-rich repeat-containing G protein–coupled receptor 5 (LGR5) is a bona fide marker of adult stem cells in several epithelial tissues, most notably in the intestinal crypts, and is highly up-regulated in many colorectal, hepatocellular, and ovarian cancers. LGR5 activation by R-spondin (RSPO) ligands potentiates Wnt/β-catenin signaling in vitro; however, deletion of LGR5 in stem cells has little or no effect on Wnt/β-catenin signaling or cell proliferation in vivo. Remarkably, modulation of LGR5 expression has a major impact on the actin cytoskeletal structure and cell adhesion in the absence of RSPO stimulation, but the molecular mechanism is unclear. Here, we show that LGR5 interacts with IQ motif-containing GTPase-activating protein 1 (IQGAP1), an effector of Rac1/CDC42 GTPases, in the regulation of actin cytoskeleton dynamics and cell–cell adhesion. Specifically, LGR5 decreased levels of IQGAP1 phosphorylation at Ser-1441/1443, leading to increased binding of Rac1 to IQGAP1 and thus higher levels of cortical F-actin and enhanced cell–cell adhesion. LGR5 ablation in colon cancer cells and crypt stem cells resulted in loss of cortical F-actin, reduced cell–cell adhesion, and disrupted localization of adhesion-associated proteins. No evidence of LGR5 coupling to any of the four major subtypes of heterotrimeric G proteins was found. These findings suggest that LGR5 primarily functions via the IQGAP1–Rac1 pathway to strengthen cell–cell adhesion in normal adult crypt stem cells and colon cancer cells.

Selective novel inverse agonists for human GPR43 augment GLP-1 secretion

GPR43/Free Fatty Acid Receptor 2 (FFAR2) is known to be activated by short-chain fatty acids and be coupled to Gi and Gq family of heterotrimeric G proteins. GPR43 is mainly expressed in neutrophils, adipocytes and enteroendocrine cells, implicated to be involved in inflammation, obesity and type 2 diabetes. However, several groups have reported the contradictory data about the physiological functions of GPR43, so that its roles in vivo remain unclear. Here, we demonstrate that a novel compound of pyrimidinecarboxamide class named as BTI-A-404 is a selective and potent competitive inverse agonist of human GPR43, but not the murine ortholog. Through structure-activity relationship (SAR), we also found active compound named as BTI-A-292. These regulators increased the cyclic AMP level and reduced acetate-induced cytoplasmic Ca(2+) level. Furthermore, we show that they modulated the downstream signaling pathways of GPR43, such as ERK, p38 MAPK, and NF-κB. It was surprising that two compounds augmented the secretion of glucagon-like peptide 1 (GLP-1) in NCI-H716 cell line. Collectively, these novel and specific competitive inhibitors regulate all aspects of GPR43 signaling and the results underscore the therapeutic potential of them.

LGR5 is required for the maintenance of spheroid-derived colon cancer stem cells

Colon cancer stem cells (CCSCs) are involved in colon cancer and promote tumor progression and recurrence. LGR5, a marker for intestinal stem cells (ISCs), is also considered to serve as a marker for CCSCs. However, the precise function of LGR5 in CCSCs is unclear. In this study, we demonstrated that LGR5 was highly expressed in CCSCs-enriched HT29 spheroid cells. Downregulation of LGR5 with small interfering RNA (siRNA) decreased the expression of stem the cell markers CD133 and CD44 in HT29 spheroid cells. In addition, silencing of LGR5 inhibited cell proliferation, secondary tumor sphere formation and induced cell apoptosis, and G0/G1 phase arrest in vitro by modulating Bcl-2, Bcl-xL and Bax. Knockdown of LGR5 enhanced chemosensitivity and reduced the invasive ability of HT29 spheroid cells. Moreover, LGR5-siRNA suppressed tumorigenicity of HT29 spheroid cells in vivo. The findings suggested that LGR5 plays a vital role in the maintenance of CCSCs and is a potential therapeutic target for colon cancer.

Structure and function of LGR5: an enigmatic G-protein coupled receptor marking stem cells

G-protein coupled receptors (GPCRs) are an important class of membrane protein that transmit extracellular signals invoked by sensing molecules such as hormones and neurotransmitters. GPCR dysfunction is implicated in many diseases and hence these proteins are of great interest to academia and the pharmaceutical industry. Leucine-rich repeat-containing GPCRs contain a characteristic extracellular domain that is an important modulator of intracellular signaling. One member of this class is the leucine-rich repeat-containing G-protein-coupled receptor 5 (LGR5), a stem cell marker in intestinal crypts, and mammary glands. LGR5 modulates Wnt signaling in the presence of the ligand R-spondin (RSPO). The mechanism of activation of LGR5 by RSPO is not understood, nor is the intracellular signaling mechanism known. Recently reported structures of the extracellular domain of LGR5 bound to RSPO reveal a horseshoe-shaped architecture made up of consecutive leucine-rich repeats, with RSPO bound on the concave surface. This review discusses the discovery of LGR5 and the impact it is having on our understanding of stem cell and cancer biology of the colon. In addition, it covers functional relationships suggested by sequence homology and structural analyses, as well as some intriguing conundrums with respect to the involvement of LGR5 in Wnt signaling.

Rho, nuclear actin, and actin-binding proteins in the regulation of transcription and gene expression

Actin cytoskeleton is one of the main targets of Rho GTPases, which act as molecular switches on many signaling pathways. During the past decade, actin has emerged as an important regulator of gene expression. Nuclear actin plays a key role in transcription, chromatin remodeling, and pre-mRNA processing. In addition, the "status" of the actin cytoskeleton is used as a signaling intermediate by at least the MKL1-SRF and Hippo-pathways, which culminate in the transcriptional regulation of cytoskeletal and growth-promoting genes, respectively. Rho GTPases may therefore regulate gene expression by controlling either cytoplasmic or nuclear actin dynamics. Although the regulation of nuclear actin polymerization is still poorly understood, many actin-binding proteins, which are downstream effectors of Rho, are found in the nuclear compartment. In this review, we discuss the possible mechanisms and key proteins that may mediate the transcriptional regulation by Rho GTPases through actin.

The stem cell-expressed receptor Lgr5 possesses canonical and functionally active molecular determinants critical to β-arrestin-2 recruitment

Lgr5 is a membrane protein related to G protein-coupled receptors (GPCR)s whose expression identifies stem cells in multiple tissues and is strongly correlated with cancer. Despite the recent identification of endogenous ligands for Lgr5, its mode of signaling remains enigmatic. The ability to couple to G proteins and βarrestins are classical molecular behaviors of GPCRs that have yet to be observed for Lgr5. Therefore, the goal of this study was to determine if Lgr5 can engage a classical GPCR behavior and elucidate the molecular determinants of this process. Structural analysis of Lgr5 revealed several motifs consistent with its ability to recruit βarr2. Among them, a "SSS" serine cluster located at amino acid position 873-875 within the C-terminal tail (C-tail), is in a region consistent with other GPCRs that bind βarr2 with high-affinity. To test its functionality, a ligand-independent βarr2 translocation assay was implemented. We show that Lgr5 recruits βarr2 and that the "SSS" amino acids (873-875) are absolutely critical to this process. We also demonstrate that for full efficacy, this cluster requires other Lgr5 C-tail serines that were previously shown to be important for constitutive and βarr2 independent internalization of Lgr5. These data are proof of principle that a classical GPCR behavior can be manifested by Lgr5. The existence of alternative ligands or missing effectors of Lgr5 that scaffold this classical GPCR behavior and the downstream signaling pathways engaged should be considered. Characterizing Lgr5 signaling will be invaluable for assessing its role in tissue maintenance, repair, and disease.

Regulation of the follistatin gene by RSPO-LGR4 signaling via activation of the WNT/β-catenin pathway in skeletal myogenesis

WNT signaling plays multiple roles in skeletal myogenesis during gestation and postnatal stages. The R-spondin (RSPO) family of secreted proteins and their cognate receptors, members of leucine-rich repeat-containing G protein-coupled receptor (LGR) family, have emerged as new regulatory components of the WNT signaling pathway. We previously showed that RSPO2 promoted myogenic differentiation via activation of WNT/β-catenin signaling in mouse myoblast C2C12 cells in vitro. However, the molecular mechanism by which RSPO2 regulates myogenic differentiation is unknown. Herein, we show that depletion of the LGR4 receptor severely disrupts myogenic differentiation and significantly diminishes the response to RSPO2 in C2C12 cells, showing a requirement of LGR4 in RSPO signaling during myogenic differentiation. We identify the transforming growth factor β (TGF-β) antagonist follistatin (Fst) as a key mediator of RSPO-LGR4 signaling in myogenic differentiation. We further demonstrate that Fst is a direct target of the WNT/β-catenin pathway. Activation and inactivation of β-catenin induced and inhibited Fst expression, respectively, in both C2C12 cells and mouse embryos. Specific TCF/LEF1 binding sites within the promoter and intron 1 region of the Fst gene were required for RSPO2 and WNT/β-catenin-induced Fst expression. This study uncovers a molecular cross talk between WNT/β-catenin and TGF-β signaling pivotal in myogenic differentiation.

Structures of Wnt-antagonist ZNRF3 and its complex with R-spondin 1 and implications for signaling

Zinc RING finger 3 (ZNRF3) and its homolog RING finger 43 (RNF43) antagonize Wnt signaling in adult stem cells by ubiquitinating Frizzled receptors (FZD), which leads to endocytosis of the Wnt receptor. Conversely, binding of ZNRF3/RNF43 to LGR4-6 – R-spondin blocks Frizzled ubiquitination and enhances Wnt signaling. Here, we present crystal structures of the ZNRF3 ectodomain and its complex with R-spondin 1 (RSPO1). ZNRF3 binds RSPO1 and LGR5-RSPO1 with micromolar affinity via RSPO1 furin-like 1 (Fu1) domain. Anonychia-related mutations in RSPO4 support the importance of the observed interface. The ZNRF3-RSPO1 structure resembles that of LGR5-RSPO1-RNF43, though Fu2 of RSPO1 is variably oriented. The ZNRF3-binding site overlaps with trans-interactions observed in 2:2 LGR5-RSPO1 complexes, thus binding of ZNRF3/RNF43 would disrupt such an arrangement. Sequence conservation suggests a single ligand-binding site on ZNRF3, consistent with the proposed competing binding role of ZNRF3/RNF43 in Wnt signaling.