Expression of the new CXCL12 receptor, CXCR7, in gliomas

Relationship between chemokine/chemokine receptor and glioma prognosis and outcomes: Systematic review and meta-analysis

BACKGROUND

Glioma is a primary tumor originating from the central nervous system, and despite ongoing efforts to improve treatment, its overall survival rate remains low. There are a limited number of reports regarding the clinical grading, prognostic impact, and utility of chemokines. Therefore, conducting a meta-analysis is necessary to obtain convincing and conclusive results.

METHODS

A comprehensive literature search was conducted using various databases, including PubMed, Web of Science, The Cochrane Library, Embase, Ovid Medline, CNKI, Wanfang Database, VIP, and CBM. The search encompassed articles published from the inception of the databases until March 2024. The estimated odds ratio (ORs), standard mean difference (SMDs), and hazard ratio (HR) with their corresponding 95% confidence intervals (95% CI) were calculated to assess the predictive value of chemokine and receptor levels in glioma risk. Additionally, heterogeneity tests and bias tests were performed to evaluate the reliability of the findings.

RESULTS

This meta-analysis included a total of 36 studies, involving 2,480 patients diagnosed with glioma. The results revealed a significant association between the expression levels of CXCR4 (n = 8; OR = 22.28; 95 % CI = 11.47-43.30; p = 0.000), CXCL12 (n = 4; OR = 10.69; 95 % CI = 7.03-16.24; p = 0.000), CCL2 (n = 6; SMD = -0.83; 95 % CI = -0.98--0.67; p = 0.000), CXCL8 (n = 3; SMD = 0.75; 95 % CI = 0.47-1.04; p = 0.000), CXCR7 (n = 3; OR = 20.66; 95 % CI = 10.20-41.82; p = 0.000), CXCL10 (n = 2; SMD = 3.27; 95 % CI = 2.91-3.62; p = 0.000) and the risk of glioma. Additionally, a significant correlation was observed between CXCR4 (n = 8; OR = 4.39; 95 % CI = 3.04-6.32; p = 0.000), (n = 6; SMD = 1.37; 95 % CI = 1.09-1.65; p = 0.000), CXCL12 (n = 6; OR = 6.30; 95 % CI = 3.87-10.25; p = 0.000), (n = 5; ES = 2.25; 95 % CI = 1.15-3.34; p = 0.041), CCL2 (n = 3; OR = 9.65; 95 % CI = 4.55-20.45; p = 0.000), (n = 4; SMD = -1.47; 95 % CI = -1.68--1.26; p = 0.000), and CCL18 (n = 3; SMD = 1.62; 95 % CI = 1.30-1.93; p = 0.000) expression levels and high-grade glioma (grades 3-4). Furthermore, CXCR4 (HR = 2.38, 95 % CI = 1.66-3.40; p = 0.000) exhibited a strong correlation with poor overall survival (OS) rates in glioma patients.

CONCLUSION

The findings of this study showed a robust association between elevated levels of CXCR4, CXCL12, CCL2, CXCL8, CXCL10 and CXCR7 with a higher risk of glioma. Furthermore, the WHO grading system was validated by the strong correlation shown between higher expression of CXCR4, CXCL12, CCL2, and CCL18 and WHO high-grade gliomas (grades 3-4). Furthermore, the results of the meta-analysis suggested that CXCR4 might be a helpful biomarker for predicting the worse prognosis of glioma patients.

Atypical chemokine receptors in cancer

Atypical chemokine receptors (ACKRs) are a group of seven-transmembrane spanning serpentine receptors that are structurally homologous to classical G-protein-coupled receptors and bind cognate chemokines with high affinities but do not signal via G-proteins or mediate cell migration. However, ACKRs efficiently modify the availability and function of chemokines in defined microanatomical environments, can signal via intracellular effectors other than G-proteins, and play complex roles in physiology and disease, including in cancer. In this review, we summarize the findings on the diverse contributions of individual ACKRs to cancer development, progression, and tumor-host interactions. We discuss how changes in ACKR expression within tumor affect cancer growth, tumor vascularization, leukocyte infiltration, and metastasis formation, ultimately resulting in differential disease outcomes. Across many studies, ACKR3 expression was shown to support tumor growth and dissemination, whereas ACKR1, ACKR2, and ACKR4 in tumors were more likely to contribute to tumor suppression. With few notable exceptions, the insights on molecular and cellular mechanisms of ACKRs activities in cancer remain sparse, and the intricacies of their involvement are not fully appreciated. This is particularly true for ACKR1, ACKR2 and ACKR4. A better understanding of how ACKR expression and functions impact cancer should pave the way for their future targeting by new and effective therapies.

Endothelial Indoleamine-2,3-Dioxygenase-1 is not Critically Involved in Regulating Antitumor Immunity in the Central Nervous System

The vascular niche of malignant gliomas is a key compartment that shapes the immunosuppressive brain tumor microenvironment (TME). The blood-brain-barrier (BBB) consisting of specialized endothelial cells (ECs) and perivascular cells forms a tight anatomical and functional barrier critically controlling transmigration and effector function of immune cells. During neuroinflammation and tumor progression, the metabolism of the essential amino acid tryptophan (Trp) to metabolites such as kynurenine has long been identified as an important metabolic pathway suppressing immune responses. Previous studies have demonstrated that indoleamine-2,3-dioxygenase-1 (IDO1), a key rate-limiting enzyme in tryptophan catabolism, is expressed within the TME of high-grade gliomas. Here, we investigate the role of endothelial IDO1 (eIDO1) expression for brain tumor immunity. Single-cell RNA sequencing data revealed that in human glioma tissue, IDO1 is predominantly expressed by activated ECs showing a JAK/STAT signaling pathway-related CXCL11⁺ gene expression signature. In a syngeneic experimental glioma model, eIDO1 is induced by low-dose tumor irradiation. However, cell type-specific ablation of eIDO1 in experimental gliomas did not alter frequency and phenotype of tumor-infiltrating T cells nor tumor growth. Taken together these data argue against a dominant role of eIDO1 for brain tumor immunity.

Drug Delivery Systems in the Development of Novel Strategies for Glioblastoma Treatment

Glioblastoma multiforme (GBM) is a grade IV glioma considered the most fatal cancer of the central nervous system (CNS), with less than a 5% survival rate after five years. The tumor heterogeneity, the high infiltrative behavior of its cells, and the blood-brain barrier (BBB) that limits the access of therapeutic drugs to the brain are the main reasons hampering the current standard treatment efficiency. Following the tumor resection, the infiltrative remaining GBM cells, which are resistant to chemotherapy and radiotherapy, can further invade the surrounding brain parenchyma. Consequently, the development of new strategies to treat parenchyma-infiltrating GBM cells, such as vaccines, nanotherapies, and tumor cells traps including drug delivery systems, is required. For example, the chemoattractant CXCL12, by binding to its CXCR4 receptor, activates signaling pathways that play a critical role in tumor progression and invasion, making it an interesting therapeutic target to properly control the direction of GBM cell migration for treatment proposes. Moreover, the interstitial fluid flow (IFF) is also implicated in increasing the GBM cell migration through the activation of the CXCL12-CXCR4 signaling pathway. However, due to its complex and variable nature, the influence of the IFF on the efficiency of drug delivery systems is not well understood yet. Therefore, this review discusses novel drug delivery strategies to overcome the GBM treatment limitations, focusing on chemokines such as CXCL12 as an innovative approach to reverse the migration of infiltrated GBM. Furthermore, recent developments regarding in vitro 3D culture systems aiming to mimic the dynamic peritumoral environment for the optimization of new drug delivery technologies are highlighted.

Patient-Oriented Perspective on Chemokine Receptor Expression and Function in Glioma

Simple Summary

Chemokines and their receptors have been pointed out as key actors in a variety of human cancers, playing pivotal roles in multiples processes and pathways. The present study aims at deciphering the functions of several chemokine receptors in gliomas, starting from publicly available patient-derived transcriptomic data with support from the current literature in the field, and sheds light on the clinical relevance of chemokine receptors in targeted therapeutic approaches for glioma patients.

Abstract

Gliomas are severe brain malignancies, with glioblastoma (GBM) being the most aggressive one. Despite continuous efforts for improvement of existing therapies, overall survival remains poor. Over the last years, the implication of chemokines and their receptors in GBM development and progression has become more evident. Recently, large amounts of clinical data have been made available, prompting us to investigate chemokine receptors in GBM from a still-unexplored patient-oriented perspective. This study aims to highlight and discuss the involvement of chemokine receptors—CCR1, CCR5, CCR6, CCR10, CX3CR1, CXCR2, CXCR4, ACKR1, ACKR2, and ACKR3—most abundantly expressed in glioma patients based on the analysis of publicly available clinical datasets. Given the strong intratumoral heterogeneity characterizing gliomas and especially GBM, receptor expression was investigated by glioma molecular groups, by brain region distribution, emphasizing tissue-specific receptor functions, and by cell type enrichment. Our study constitutes a clinically relevant and patient-oriented guide that recapitulates the expression profile and the complex roles of chemokine receptors within the highly diversified glioma landscape. Additionally, it strengthens the importance of patient-derived material for development and precise amelioration of chemokine receptor-targeting therapies.

Advances in Chemokine Signaling Pathways as Therapeutic Targets in Glioblastoma

With a median patient survival of 15 months, glioblastoma (GBM) is still one of the deadliest malign tumors. Despite immense efforts, therapeutic regimens fail to prolong GBM patient overall survival due to various resistance mechanisms. Chemokine signaling as part of the tumor microenvironment plays a key role in gliomagenesis, proliferation, neovascularization, metastasis and tumor progression. In this review, we aimed to investigate novel therapeutic approaches targeting various chemokine axes, including CXCR2/CXCL2/IL-8, CXCR3/CXCL4/CXCL9/CXCL10, CXCR4/CXCR7/CXCL12, CXCR6/CXCL16, CCR2/CCL2, CCR5/CCL5 and CX3CR1/CX3CL1 in preclinical and clinical studies of GBM. We reviewed targeted therapies as single therapies, in combination with the standard of care, with antiangiogenic treatment as well as immunotherapy. We found that there are many antagonist-, antibody-, cell- and vaccine-based therapeutic approaches in preclinical and clinical studies. Furthermore, targeted therapies exerted their highest efficacy in combination with other established therapeutic applications. The novel chemokine-targeting therapies have mainly been examined in preclinical models. However, clinical applications are auspicious. Thus, it is crucial to broadly investigate the recently developed preclinical approaches. Promising preclinical applications should then be investigated in clinical studies to create new therapeutic regimens and to overcome therapy resistance to GBM treatment.

Novel insights into the interaction between long non-coding RNAs and microRNAs in glioma

Glioma is the most common brain tumor of the central nervous system. Long non-coding RNAs (lncRNAs) and microRNAs (miRNAs) have been identified to play a vital role in the initiation and progression of glioma, including tumor cell proliferation, survival, apoptosis, invasion, and therapy resistance. New documents emerged, which indicated that the interaction between long non-coding RNAs and miRNAs contributes to the tumorigenesis and pathogenesis of glioma. LncRNAs can act as competing for endogenous RNA (ceRNA), and molecular sponge/deregulator in regulating miRNAs. These interactions stimulate different molecular signaling pathways in glioma, including the lncRNAs/miRNAs/Wnt/β-catenin molecular signaling pathway, the lncRNAs/miRNAs/PI3K/AKT/mTOR molecular signaling pathway, the lncRNAs-miRNAs/MAPK kinase molecular signaling pathway, and the lncRNAs/miRNAs/NF-κB molecular signaling pathway. In this paper, the basic roles and molecular interactions of the lncRNAs and miRNAs pathway glioma were summarized to better understand the pathogenesis and tumorigenesis of glioma.

The Role of Selected Chemokines and Their Receptors in the Development of Gliomas

Among heterogeneous primary tumors of the central nervous system (CNS), gliomas are the most frequent type, with glioblastoma multiforme (GBM) characterized with the worst prognosis. In their development, certain chemokine/receptor axes play important roles and promote proliferation, survival, metastasis, and neoangiogenesis. However, little is known about the significance of atypical receptors for chemokines (ACKRs) in these tumors. The objective of the study was to present the role of chemokines and their conventional and atypical receptors in CNS tumors. Therefore, we performed a thorough search for literature concerning our investigation via the PubMed database. We describe biological functions of chemokines/chemokine receptors from various groups and their significance in carcinogenesis, cancer-related inflammation, neo-angiogenesis, tumor growth, and metastasis. Furthermore, we discuss the role of chemokines in glioma development, with particular regard to their function in the transition from low-grade to high-grade tumors and angiogenic switch. We also depict various chemokine/receptor axes, such as CXCL8-CXCR1/2, CXCL12-CXCR4, CXCL16-CXCR6, CX3CL1-CX3CR1, CCL2-CCR2, and CCL5-CCR5 of special importance in gliomas, as well as atypical chemokine receptors ACKR1-4, CCRL2, and PITPMN3. Additionally, the diagnostic significance and usefulness of the measurement of some chemokines and their receptors in the blood and cerebrospinal fluid (CSF) of glioma patients is also presented.

The diverse and complex roles of atypical chemokine receptors in cancer: From molecular biology to clinical relevance and therapy

Chemokines regulate directed cell migration, proliferation and survival and are key components in cancer biology. They exert their functions by interacting with seven-transmembrane domain receptors that signal through G proteins (GPCRs). A subgroup of four chemokine receptors known as the atypical chemokine receptors (ACKRs) has emerged as essential regulators of the chemokine functions. ACKRs play diverse and complex roles in tumor biology from tumor initiation to metastasis, including cancer cell proliferation, adherence to endothelium, epithelial-mesenchymal transition (EMT), extravasation from blood vessels, tumor-associated angiogenesis or protection from immunological responses. This chapter gives an overview on the established and emerging roles that the atypical chemokine receptors ACKR1, ACKR2, ACKR3 and ACKR4 play in the different phases of cancer development and dissemination, their clinical relevance, as well as on the hurdles to overcome in ACKRs targeting as cancer therapy.

The potential role of stromal cell-derived factor-1α/CXCR4/CXCR7 axis in adipose-derived mesenchymal stem cells

To investigate the potential role of stromal cell-derived factor-1α (SDF-1α)/CXCR4/CXCR7 axis in adipose-derived mesenchymal stem cells (ADSCs), quantitative reverse transcription-polymerase chain reaction (qRT-PCR) was employed to screen the effective small interfering RNA against CXCR4 and CXCR7 in ADSCs. The messenger RNA (mRNA) and proteins abundances of AKT (p-AKT), ERK (p-ERK), JNK (p-JNK), and p38 (p-p38) in different groups were identified by qRT-PCR, western blot, and immunofluorescence staining method. Meanwhile, cell migration and cell proliferation with SDF-1 treated were examined by a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and transwell permeable assay, respectively. Moreover, the interaction between CXCR4 and CXCR7 was examined by a GST pull-down assay. CXCR4 small interfering RNA3 (siRNA3) and CXCR7 siRNA3 have been proved to the most effective tools for knockdown CXCR4 and CXCR7 expressions. mRNA abundance of JNK and p38 could be affected by SDF-1α/CXCR4/CXCR7 axis. However, western blot analysis of p-AKT, p-ERK, p-JNK, and p-p38 in CXCR43-treated ADSCs was significantly higher than that in the control group. Moreover, the immunofluorescence staining analysis revealed that the expressions of p-ATK and p-JNK proteins were significantly higher in NC- and SDF-1-treated subgroups than that in the CXCR4 and CXCR7 groups. p-ATK and p-JNK proteins in CXCR4 group were similar to that in CXCR7 group. Cell migration analysis of CXCR4-treated ADSCs suggested that knockdown CXCR4 could effectively promote cell migration (p < .05). Moreover, CXCR4 could interact with CXCR7. The results in this study could provide a better understanding of SDF-1α/CXCR4/CXCR7 axis during ADSCs development.

Atypical Chemokine Receptor 3 (ACKR3): A Comprehensive Overview of its Expression and Potential Roles in the Immune System

Atypical chemokine receptor 3 (ACKR3), previously known as C-X-C chemokine receptor type 7 (CXCR7), has emerged as a key player in several biologic processes, particularly during development. Its CXCL11 and CXCL12 scavenging activity and atypical signaling properties, together with a new array of other nonchemokine ligands, have established ACKR3 as a main regulator of physiologic processes at steady state and during inflammation. Here, we present a comprehensive review of ACKR3 expression in mammalian tissues in search of a possible connection with the receptor function. Besides the reported roles of ACKR3 during development, we discuss the potential contribution of ACKR3 to the function of the immune system, focusing on the myeloid lineage.

The Role of ACKR3 in Breast, Lung, and Brain Cancer

Recent reports regarding the significance of chemokine receptors in disease have put a spotlight on atypical chemokine receptor 3 (ACKR3). This atypical chemokine receptor is overexpressed in numerous cancer types and has been involved in the modulation of tumor cell proliferation and migration, tumor angiogenesis, or resistance to drugs, thus contributing to cancer progression and metastasis occurrence. Here, we focus on the clinical significance and potential mechanisms underlying the pathologic role of ACKR3 in breast, lung, and brain cancer and discuss its possible relevance as a prognostic factor and potential therapeutic target in these contexts.

Chemokine receptor CXCR7 is an independent prognostic biomarker in glioblastoma

BACKGROUND

Glioblastoma (GBM) is the most common and most fatal primary brain cancer in adults. Due to the complex nature of GBM, its pathogenesis still remain unclear. Accumulating evidence suggest that chemokine receptor CXCR7 contribute to the development of various types of tumors.

OBJECTIVE

We aim to examine the prognostic significance of CXCR7 in GBM.

METHODS

CXCR7 were first detected by Immunohistochemistry. The association between CXCR7 and overall survival (OS) were examined. Moreover, multivariate analyses were conducted to evaluate the prognostic factors in GBM.

RESULTS

Of all 146 GBM patients recruited, 77 were in the high-expression subgroup, the rest 69 were in low-expression subgroup. There are no differences between these two subgroups in terms of age, gender, family history of cancer, extent of surgery, chemotherapy, radiotherapy, KPS, MGMT methylation status and tumor size. However, high CXCR7 expression was robustly correlated with poor OS in GBM. Multivariate analysis confirmed age, KPS scores, chemotherapy, IDH1 mutation, MGMT methylation and CXCR7 were independent factors in survival prognosis.

CONCLUSIONS

CXCR7 may involve in the clinical GBM progression, and CXCR7 could be a valuable prognostic marker in the treatment of GBM.

The 5T4 oncofetal glycoprotein does not act as a general organizer of the CXCL12 system in cancer cells

The chemokine, CXCL12, promotes cancer growth and metastasis through interaction with either CXCR4 and/or CXCR7. This tumor-specific organization of the CXCL12 system obscures current therapeutic approaches, aiming at the selective inactivation of CXCL12 receptors. Since it has been previously suggested that the cellular use of CXCR4 or CXCR7 is dictated by the 5T4 oncofetal glycoprotein, we have now tested whether 5T4 would represent a general and reliable marker for the organization of the CXCL12 system in cancer cells. The CXCR4 antagonist, AMD3100, as well as the CXCR7 antagonist, CCX771, demonstrated that the cancer cell lines A549, C33A, DLD-1, MDA-231, and PC-3 use either CXCR7 and/or CXCR4 for mediating CXCL12-induced chemotaxis and cell proliferation. The use of CXCL12 receptors as well as their subcellular localization remained unchanged in most cell lines following siRNA-mediated depletion of 5T4. In distinct cell lines, inhibition of 5T4 expression, however, modulated tumor cell migration and proliferation per se. Collectively our analyses fail to demonstrate general organizational influences of 5T4 of the CXCL12 system in different cancer cell lines, and, hence, dismiss its future use as a diagnostic marker.

Serum amyloid A1 is upregulated in human glioblastoma

Serum amyloid A1 (SAA1) is a sensitive acute phase reactant primarily produced by the liver in response to acute inflammation. We have recently shown that SAA affects proliferation, migration, and invasion of glioblastoma cell lines, which suggest its participation in the malignant process. Consistently, levels of SAA have been used as a non-invasive biomarker for the prognosis of many cancers. In this study, we aimed to investigate SAA serum levels and expression of SAA genes in human astrocytomas tissues. Serum and tissue samples were obtained from patients with astrocytoma grades I to III and glioblastoma (GBM or grade IV). Levels of circulating SAA were significantly higher in the serum of patients with AGII-IV when compared to non-neoplastic samples derived from non-neoplastic patients (NN) (p > 0.0001). Quantitative real time PCR (qRT-PCR) of 148 astrocytomas samples (grades I-IV) showed that SAA1 mRNA was significantly higher in GBM when compared to AGI-III and NN samples (p < 0.0001). Immunohistochemistry analysis revealed cytoplasmic positivity for SAA in GBM. There was no correlation of SAA1 with clinical end-point of overall survival among GBM patients. However, it was found a positive correlation between SAA1 and genes involved in tumor progression, such as: HIF1A (r = 0.50; p < 0.00001), CD163 (r = 0.52; p < 0.00001), CXCR4 (r = 0.42; p < 0.00001) and CXCR7 (r = 0.33; p = 0.002). In conclusions, we show that astrocytoma patients have increased levels of serum SAA and SAA1 is expressed and secreted in GBM, and its co-expression with tumor-related genes supports its involvement in GBM angiogenesis and progression.

Glioma Cell Invasion is Significantly Enhanced in Composite Hydrogel Matrices Composed of Chondroitin 4- and 4,6-Sulfated Glycosaminoglycans

Glioblastoma multiforme (GBM) is the most aggressive form of astrocytoma accounting for a majority of primary malignant brain tumors in the United States. Chondroitin sulfate proteoglycans (CSPGs) and their glycosaminoglycan (GAG) side chains are key constituents of the brain extracellular matrix (ECM) implicated in promoting tumor invasion. However, the mechanisms by which sulfated CS-GAGs promote brain tumor invasion are currently unknown. We hypothesize that glioma cell invasion is triggered by the altered sulfation of CS-GAGs in the tumor extracellular environment, and that this is potentially mediated by independent mechanisms involving CXCL12/CXCR4 and LAR signaling respectively. This was tested in vitro by encapsulating the human glioma cell line U87MG-EGFP into monosulfated (4-sulfated; CS-A), composite (4 and 4,6-sulfated; CS-A/E), unsulfated hyaluronic acid (HA), and unsulfated agarose (AG; polysaccharide) hydrogels within microfluidics-based choice assays. Our results demonstrated the enhanced preferential cell invasion into composite hydrogels, when compared to other hydrogel matrices (p<0.05). Haptotaxis assays demonstrated the significantly (p<0.05) faster migration of U87MG-EGFP cells in CXCL12 containing CS-GAG hydrogels when compared to other hydrogel matrices containing the same chemokine concentration. This is likely due to the significantly (p<0.05) greater affinity of composite CS-GAGs to CXCL12 over other hydrogel matrices. Results from qRT-PCR assays further demonstrated the significant (p<0.05) upregulation of the chemokine receptor CXCR4, and the CSPG receptor LAR in glioma cells within CS-GAG hydrogels compared to control hydrogels. Western blot analysis of cell lysates derived from glioma cells encapsulated in different hydrogel matrices further corroborate qRT-PCR results, and indicate the presence of a potential variant of LAR that is selectively expressed only in glioma cells encapsulated in CS-GAG hydrogels. These results suggest that sulfated CS-GAGs may directly induce enhanced invasion and haptotaxis of glioma cells associated with aggressive brain tumors via distinct mechanisms.

C-X-C motif receptor 7 in gastrointestinal cancer

Chemokine receptors are key mediators of normal physiology and numerous pathological conditions, including inflammation and cancer. This receptor family is an emerging target for anticancer drug development. C-X-C motif receptor 7 (CXCR7) is an atypical chemokine receptor that was first cloned from a canine cDNA library as an orphan receptor and was initially named receptor dog cDNA 1 (RDC1). Shortly after demonstrating that RDC1 binds with its ligand, stromal cell-derived factor-1α and interferon-inducible T-cell α chemoattractant, RDC1 was officially deorphanized and renamed CXCR7, as the seventh receptor in the CXC class of the chemokine receptor family. Recent accumulating evidence has demonstrated that CXCR7 expression is augmented in the majority of tumor cells compared with their normal counterparts and is involved in cell proliferation, survival, migration, invasion and angiogenesis during the initiation and progression of breast, lung and prostate cancer. In the present review, the expression and role of CXCR7, as well as its clinical relevance in cancer of the gastrointestinal system, were investigated. In addition, the potential of this chemokine receptor as a therapeutic target in the treatment of gastrointestinal cancer was discussed.

Circulating biomarker panels for targeted therapy in brain tumors

An important goal of oncology is the development of cancer risk-identifier biomarkers that aid early detection and target therapy. High-throughput profiling represents a major concern for cancer research, including brain tumors. A promising approach for efficacious monitoring of disease progression and therapy could be circulating biomarker panels using molecular proteomic patterns. Tailoring treatment by targeting specific protein-protein interactions and signaling networks, microRNA and cancer stem cell signaling in accordance with tumor phenotype or patient clustering based on biomarker panels represents the future of personalized medicine for brain tumors. Gathering current data regarding biomarker candidates, we address the major challenges surrounding the biomarker field of this devastating tumor type, exploring potential perspectives for the development of more effective predictive biomarker panels.

G protein-coupled receptors as oncogenic signals in glioma: emerging therapeutic avenues

Gliomas are the most common malignant intracranial tumors. Newly developed targeted therapies for these cancers aim to inhibit oncogenic signals, many of which emanate from receptor tyrosine kinases, including the epidermal growth factor receptor (EGFR) and the vascular endothelial growth factor receptor (VEGFR). Unfortunately, the first-generation treatments targeting these oncogenic signals provide little survival benefit in both mouse xenograft models and human patients. The search for new treatment options has uncovered several G protein-coupled receptor (GPCR) candidates and generated a growing interest in this class of proteins as alternative therapeutic targets for the treatment of various cancers, including glioblastoma multiforme (GBM). GPCRs constitute a large family of membrane receptors that influence oncogenic pathways through canonical and non-canonical signaling. Accordingly, evidence indicates that GPCRs display a unique ability to crosstalk with receptor tyrosine kinases, making them important molecular components controlling tumorigenesis. This review summarizes the current research on GPCR functionality in gliomas and explores the potential of modulating these receptors to treat this devastating disease.

Expression and function of CXCR2, CXCR7 of acute leukemic cells in rat

OBJECTIVE

To investigate the expression and function of chemokine receptor CXCR2 and CXCR7 in the rat with acute leukemia.

METHODS

Flow cytometry and RT-PCR were used to detect the CXCR2, CXCR7 expression on the bone marrow cell surface of the acute leukemia group and the control group.

RESULTS

The bone marrow cell surface CXCR2, CXCR7 relative fluorescence intensity of the observation group was significantly higher than the control group (P<0.05). The CXCR7 expression of the extramedullary infiltration group was significantly higher than non-extramedullary infiltration group (P<0.05). The CXCR2, CXCR7mRNA median expression level of the observation group was higher than the control group. The CXCR2 expression and CXCR7 expression of the observation group was positively correlated, and the correlation coefficient was 0.782 (P<0.01).

CONCLUSIONS

The chemokine receptor CXCR2 and CXCR7 are highly expressed in acute leukemia, which may be associated with the occurrence of leukemia.

CXCR7 and CXCR4 Expressions in Infiltrative Astrocytomas and Their Interactions with HIF1α Expression and IDH1 Mutation

The CXCR7, a new receptor for CXCL12 with higher affinity than CXCR4 has raised key issues on glioma cell migration. The aim of this study is to investigate the CXCR7 mRNA expression in diffuse astrocytomas tissues and to evaluate its interactions with CXCR4 and HIF1α expression and IDH1 mutation. CXCR7, CXCR4 and HIF1α mRNA expression were evaluated in 129 frozen samples of astrocytomas. IDH1 mutation status was analyzed with gene expressions, matched with clinicopathological parameters and overall survival time. Protein expression was analyzed by immunohistochemistry in different grades of astrocytoma and in glioma cell line (U87MG) by confocal microscopy. There was significant difference in the expression levels of the genes studied between astrocytomas and non-neoplasic (NN) controls (p < 0.001). AGII showed no significant correlation between CXCR7/HIF1α (p = 0.548); there was significant correlation between CXCR7/CXCR4 (p = 0.042) and CXCR7/IDH1 (p = 0.008). GBM showed significant correlations between CXCR7/CXCR4 (p = 0.002), CXCR7/IDH1 (p < 0.001) and CXCR7/HIF1α (p = 0.008). HIF1α overexpression was associated with higher expressions of CXCR7 (p = 0.01) and CXCR4 (p < 0.0001), while IDH1 mutation was associated with lower CXCR7 (p = 0.009) and CXCR4 (p = 0.0005) mRNA expressions. Protein expression increased with malignancy and in U87MG cell line was mainly localized in the cellular membrane. CXCR7 was overexpressed in astrocytoma and correlates with CXCR4 and IDH1 in AGII and CXCR4, IDH1 and HIF1α in GBM. Overexpression HIF1α was related with higher expressions of CXCR7 and CXCR4, otherwise IDH1 mutation related with lower expression of both genes. No association between CXCR7 and CXCR4 expression and survival data was related.

Inhibition of CXCR7 extends survival following irradiation of brain tumours in mice and rats

Background:

In experimental models of glioblastoma multiforme (GBM), irradiation (IR) induces local expression of the chemokine CXCL12/SDF-1, which promotes tumour recurrence. The role of CXCR7, the high-affinity receptor for CXCL12, in the tumour's response to IR has not been addressed.

Methods:

We tested CXCR7 inhibitors for their effects on tumour growth and/or animal survival post IR in three rodent GBM models. We used immunohistochemistry to determine where CXCR7 protein is expressed in the tumours and in human GBM samples. We used neurosphere formation assays with human GBM xenografts to determine whether CXCR7 is required for cancer stem cell (CSC) activity in vitro.

Results:

CXCR7 was detected on tumour cells and/or tumour-associated vasculature in the rodent models and in human GBM. In human GBM, CXCR7 expression increased with glioma grade and was spatially associated with CXCL12 and CXCL11/I-TAC. In the rodent GBM models, pharmacological inhibition of CXCR7 post IR caused tumour regression, blocked tumour recurrence, and/or substantially prolonged survival. CXCR7 expression levels on human GBM xenograft cells correlated with neurosphere-forming activity, and a CXCR7 inhibitor blocked sphere formation by sorted CSCs.

Conclusions:

These results indicate that CXCR7 inhibitors could block GBM tumour recurrence after IR, perhaps by interfering with CSCs.

Immune regulation by atypical chemokine receptors

Chemokines have fundamental roles in regulating immune and inflammatory responses, primarily through their control of leukocyte migration and localization. The biological functions of chemokines are typically mediated by signalling through G protein-coupled chemokine receptors, but chemokines are also bound by a small family of atypical chemokine receptors (ACKRs), the members of which are unified by their inability to initiate classical signalling pathways after ligand binding. These ACKRs are emerging as crucial regulatory components of chemokine networks in a wide range of developmental, physiological and pathological contexts. In this Review, we discuss the biochemical and immunological properties of ACKRs and the potential unifying themes in this family, and we highlight recent studies that identify novel roles for these molecules in development , homeostasis, inflammatory disease, infection and cancer.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXIX. Update on the extended family of chemokine receptors and introducing a new nomenclature for atypical chemokine receptors

Sixteen years ago, the Nomenclature Committee of the International Union of Pharmacology approved a system for naming human seven-transmembrane (7TM) G protein-coupled chemokine receptors, the large family of leukocyte chemoattractant receptors that regulates immune system development and function, in large part by mediating leukocyte trafficking. This was announced in Pharmacological Reviews in a major overview of the first decade of research in this field [Murphy PM, Baggiolini M, Charo IF, Hébert CA, Horuk R, Matsushima K, Miller LH, Oppenheim JJ, and Power CA (2000) Pharmacol Rev 52:145-176]. Since then, several new receptors have been discovered, and major advances have been made for the others in many areas, including structural biology, signal transduction mechanisms, biology, and pharmacology. New and diverse roles have been identified in infection, immunity, inflammation, development, cancer, and other areas. The first two drugs acting at chemokine receptors have been approved by the U.S. Food and Drug Administration (FDA), maraviroc targeting CCR5 in human immunodeficiency virus (HIV)/AIDS, and plerixafor targeting CXCR4 for stem cell mobilization for transplantation in cancer, and other candidates are now undergoing pivotal clinical trials for diverse disease indications. In addition, a subfamily of atypical chemokine receptors has emerged that may signal through arrestins instead of G proteins to act as chemokine scavengers, and many microbial and invertebrate G protein-coupled chemokine receptors and soluble chemokine-binding proteins have been described. Here, we review this extended family of chemokine receptors and chemokine-binding proteins at the basic, translational, and clinical levels, including an update on drug development. We also introduce a new nomenclature for atypical chemokine receptors with the stem ACKR (atypical chemokine receptor) approved by the Nomenclature Committee of the International Union of Pharmacology and the Human Genome Nomenclature Committee.

CXCR7-mediated progression of osteosarcoma in the lungs

Background:

Osteosarcoma (OS) is the most frequent primary malignant bone tumour in children and adolescents with a high propensity for lung metastasis. Chemokines and chemokine receptors have been described to have an important role in many malignancies including OS. The aim of this study was to investigate the expression of CXCR7 receptor in OS tissues and its role in the progression of the disease in the lungs.

Methods:

Immunohistochemistry was used to study CXCR7 expression in primary tumours and metastatic tissues from patients with OS. Its contribution to tumour expansion in the lungs has been also assessed using animal models and synthetic-specific CXCR7 ligands.

Results:

CXCR7 was expressed on human primary bone tumours and on lung metastases. Its expression was predominantly located on tumour-associated blood vessels. Mice challenged with OS cells and systematically treated with synthetic CXCR7 ligands presented a significant reduction of lung nodules compared with untreated mice.

Conclusion:

This study shows that CXCR7 has a critical role in OS progression in the lungs, where are expressed CXCR7 ligands, especially CXCL12. Moreover, we highlight that synthetic CXCR7 ligands could represent a powerful therapeutic tool to impede lung OS progression.

CXCR7 is induced by hypoxia and mediates glioma cell migration towards SDF-1α

BACKGROUND

Glioblastomas, the most common and malignant brain tumors of the central nervous system, exhibit high invasive capacity, which hinders effective therapy. Therefore, intense efforts aimed at improved therapeutics are ongoing to delineate the molecular mechanisms governing glioma cell migration and invasion.

METHODS

In order to perform the studies, we employed optimal cell culture methods and hypoxic conditions, lentivirus-mediated knockdown of protein expression, Western Blot analysis, migration assays and immunoprecipitation. We determined statistical significance by unpaired t-test.

RESULTS

In this report, we show that U87MG, LN229 and LN308 glioma cells express CXCR7 and that exposure to hypoxia upregulates CXCR7 protein expression in these cell lines. CXCR7-expressing U87MG, LN229 and LN308 glioma cells migrated towards stromal-derived factor (SDF)-1α/CXCL12 in hypoxic conditions in the Boyden chamber assays. While shRNA-mediated knockdown of CXCR7 expression did not affect the migration of any of the three cell lines in normoxic conditions, we observed a reduction in the migration of LN229 and LN308, but not U87MG, glioma cells towards SDF-1α in hypoxic conditions. In addition, knockdown of CXCR7 expression in LN229 and LN308 glioma cells decreased levels of SDF-1α-induced phosphorylation of ERK1/2 and Akt. Inhibiting CXCR4 in LN229 and LN308 glioma cells that were knocked down for CXCR7 did not further reduce migration towards SDF-1α in hypoxic conditions and did not affect the levels of phosphorylated ERK1/2 and Akt. Analysis of immunoprecipitated CXCR4 from LN229 and LN308 glioma cells revealed co-precipitated CXCR7.

CONCLUSIONS

Taken together, our findings indicate that both CXCR4 and CXCR7 mediate glioma cell migration towards SDF-1α in hypoxic conditions and support the development of therapeutic agents targeting these receptors.

The molecular and cell biology of pediatric low-grade gliomas

Pilocytic astrocytoma (PA) is the most common glial cell tumor arising in children. Sporadic cases are associated with KIAA1549:BRAF fusion rearrangements, while 15-20% of children develop PA in the context of the neurofibromatosis 1 (NF1) inherited tumor predisposition syndrome. The unique predilection of these tumors to form within the optic pathway and brainstem (NF1-PA) and cerebellum (sporadic PA) raises the possibility that gliomagenesis requires more than biallelic inactivation of the NF1 tumor suppressor gene or expression of the KIAA1549:BRAF transcript. Several etiologic explanations include differential susceptibilities of preneoplastic neuroglial cell types in different brain regions to these glioma-causing genetic changes, contributions from non-neoplastic cells and signals in the tumor microenvironment, and genomic modifiers that confer glioma risk. As clinically-faithful rodent models of sporadic PA are currently under development, Nf1 genetically-engineered mouse (GEM) models have served as tractable systems to study the role of the cell of origin, deregulated intracellular signaling, non-neoplastic cells in the tumor microenvironment and genomic modifiers in gliomagenesis. In this report, we highlight advances in Nf1-GEM modeling and review new experimental evidence that supports the emerging concept that Nf1- and KIAA1549:BRAF-induced gliomas arise from specific cell types in particular brain locations.

Expression and functional heterogeneity of chemokine receptors CXCR4 and CXCR7 in primary patient-derived glioblastoma cells

Glioblastoma (GBM) is the most common primary brain tumor in adults. The poor prognosis and minimally successful treatments of these tumors indicates a need to identify new therapeutic targets. Therapy resistance of GBMs is attributed to heterogeneity of the glioblastoma due to genetic alterations and functional subpopulations. Chemokine receptors CXCR4 and CXCR7 play important roles in progression of various cancers although the specific functions of the CXCL12-CXCR4-CXCR7 axis in GBM are less characterized. In this study we examined the expression and function of CXCR4 and CXCR7 in four primary patient-derived GBM cell lines of the proliferative subclass, investigating their roles in in vitro growth, migration, sphere and tube formation. CXCR4 and CXCR7 cell surface expression was heterogeneous both between and within each cell line examined, which was not reflected by RT-PCR analysis. Variable percentages of CXCR4+CXCR7- (CXCR4 single positive), CXCR4-CXCR7+ (CXCR7 single positive), CXCR4+CXCR7+ (double positive), and CXCR4-CXCR7- (double negative) subpopulations were evident across the lines examined. A subpopulation of slow cell cycling cells was enriched in CXCR4 and CXCR7. CXCR4+, CXCR7+, and CXCR4+/CXCR7+ subpopulations were able to initiate intracranial tumors in vivo. CXCL12 stimulated in vitro cell growth, migration, sphere formation and tube formation in some lines and, depending on the response, the effects were mediated by either CXCR4 or CXCR7. Collectively, our results indicate a high level of heterogeneity in both the surface expression and functions of CXCR4 and CXCR7 in primary human GBM cells of the proliferative subclass. Should targeting of CXCR4 and CXCR7 provide clinical benefits to GBM patients, a personalized treatment approach should be considered given the differential expression and functions of these receptors in GBM.

CXCL12 enhances human neural progenitor cell survival through a CXCR7- and CXCR4-mediated endocytotic signaling pathway

Chemokine CXCL12 is widely expressed in the central nervous system and essential for the proper functioning of human neural progenitor cells (hNPCs). Although CXCL12 is known to function through its receptor CXCR4, recent data have suggested that CXCL12 binds to chemokine receptor CXCR7 with higher affinity than to CXCR4. However, little is known about the function of CXCR7 in hNPCs. Using a primary hNPC culture system, we demonstrated that CXCL12 promotes hNPC survival in the events of camptothecin-induced apoptosis or growth factor deprivation, and that this effect requires both CXCR7 and CXCR4. Through fluorescence-activated cell sorting analysis and immunocytochemistry, we determined that CXCR7 is mainly localized in the early endosome, while CXCR4 is more broadly expressed at the cell surface and on both early and recycling endosomes. Furthermore, we found that endocytosis is required for the prosurvival function of CXCL12. Using dual-color total internal reflection fluorescence microscopy and immunoprecipitation, we demonstrated that CXCR7 quickly trafficks to plasma membrane in mediating CXCL12 endocytosis and colocalizes with CXCR4 after CXCL12 treatment. Investigating the molecular mechanisms, we found that ERK1/2 endocytotic signaling pathway is essential for hNPC survival upon apoptotic challenges. Consistent with these findings, a significantly higher number of apoptotic NPCs were found in the developing brain of CXCR7 knockout mice. In conclusion, CXCL12 protects hNPCs from apoptotic challenges through CXCR7- and CXCR4-mediated endocytotic signaling. Since survival of hNPCs is important for neurogenesis, CXCR7 may become a new therapeutic target to properly regulate critical processes of brain development.

Chemokine CXCL12 in neurodegenerative diseases: an SOS signal for stem cell-based repair

The dynamic relation between stem cells and their niche governs self-renewal and progenitor cell deployment. The chemokine CXCL12 (C-X-C motif ligand 12) and its signaling receptor CXCR4 (C-X-C motif receptor 4) represent an important pathway that regulates homing and maintenance of stem cells in neural niches. Neural stem cells (NSCs) reside in specific niches where communication with blood vessels is regulated by CXCL12. In neurodegenerative diseases and brain tumors, reactive vasculature forms in response to diseased tissues to create new niches that secrete CXCL12, enhancing the recruitment of neural progenitor cells (NPCs) to lesion sites via long-range migration. These observations suggest that the CXCL12-CXCR4 axis maintains NSCs and serves as an emergent salvage signal for initiating endogenous stem cell-based tissue repair.

CXCL12 mediates trophic interactions between endothelial and tumor cells in glioblastoma

Emerging evidence suggests endothelial cells (EC) play a critical role in promoting Glioblastoma multiforme (GBM) cell proliferation and resistance to therapy. The molecular basis for GBM-EC interactions is incompletely understood. We hypothesized that the chemokine CXCL12 and its receptor CXCR4 could mediate direct interactions between GBM cells and tumor-associated endothelial cells and that disruption of this interaction might be the molecular basis for the anti-tumor effects of CXCR4 antagonists. We investigated this possibility in vivo and in an in vitro co-culture model that incorporated extracellular matrix, primary human brain microvascular ECs (HBMECs) and either an established GBM cell line or primary GBM specimens. Depletion of CXCR4 in U87 GBM cells blocked their growth as intracranial xenografts indicating that tumor cell CXCR4 is required for tumor growth in vivo. In vitro, co-culture of either U87 cells or primary GBM cells with HBMECs resulted in their co-localization and enhanced GBM cell growth. Genetic manipulation of CXCL12 expression and pharmacological inhibition of its receptors CXCR4 and CXCR7 revealed that the localizing and trophic effects of endothelial cells on GBM cells were dependent upon CXCL12 and CXCR4. These findings indicate that the CXCL12/CXCR4 pathway directly mediates endothelial cell trophic function in GBMs and that inhibition of CXCL12-CXCR4 signaling may uniquely target this activity. Therapeutic disruption of endothelial cell trophic functions could complement the structural disruption of anti-angiogenic regimens and, in combination, might also improve the efficacy of radiation and chemotherapy in treating GBMs.

A dialog between glioma and microglia that promotes tumor invasiveness through the CCL2/CCR2/interleukin-6 axis

Glioma cells in situ are surrounded by microglia, suggesting the potential of glioma-microglia interactions to produce various outcomes. As chemokines are important mediators of cell-cell communication, we sought first to identify commonly expressed chemokines in 16 human glioma lines. We found CCL2 (macrophage chemoattractant protein-1) messenger RNA to be expressed by the majority of glioma lines. However, these lines did not express the CCL2 receptor, CCR2, which was found on microglia. Next, we overexpressed CCL2 in the U87 glioma line, which has low basal level of CCL2, to investigate the hypothesis that glioma-secreted CCL2 interacts with microglia to affect glioma growth. Stable clones with 10- to 12-fold elevation of CCL2 have similar growth rate and invasive capacity as vector controls when cultured in isolation. However, in coculture with microglia in a three-dimensional collagen gel matrix, the invasiveness of CCL2-overexpressing clones was increased. Gene array analyses were then undertaken and they revealed that interleukin (IL)-6 was consistently increased in the coculture. Recombinant IL-6 enhanced the invasiveness of glioma cells when these were cultured alone, whereas a neutralizing antibody to IL-6 attenuated the microglia-stimulated glioma invasiveness. Finally, we found that human glioma specimens in situ contained IL-6 immunoreactivity that was expressed on CD68+ cells. This study has uncovered a mechanism by which glioma cells exploit microglia for increased invasiveness. Specifically, glioma-derived CCL2 acts upon CCR2-bearing microglia, which then produces IL-6 to stimulate gliomas. The CCL2/CCR2/IL-6 loop is a potential therapeutic target for the currently incurable malignant gliomas.

The presumed atypical chemokine receptor CXCR7 signals through G(i/o) proteins in primary rodent astrocytes and human glioma cells

SDF-1/CXCL12 binds to the chemokine receptors, CXCR4 and CXCR7, and controls cell proliferation and migration during development, tumorigenesis, and inflammatory processes. It is currently assumed that CXCR7 would represent an atypical or scavenger chemokine receptor which modulates the function of CXCR4. Contrasting this view, we demonstrated recently that CXCR7 actively mediates SDF-1 signaling in primary astrocytes. Here, we provide evidence that CXCR7 affects astrocytic cell signaling and function through pertussis toxin-sensitive G(i/o) proteins. SDF-1-dependent activation of G(i/o) proteins and subsequent increases in intracellular Ca(2+) concentration persisted in primary rodent astrocytes with depleted expression of CXCR4, but were abolished in astrocytes with depleted expression of CXCR7. Moreover, CXCR7-mediated effects of SDF-1 on Erk and Akt signaling as well as on astrocytic proliferation and migration were all sensitive to pertussis toxin. Likewise, pertussis toxin abolished SDF-1-induced activation of Erk and Akt in CXCR7-only expressing human glioma cell lines. Finally, consistent with a ligand-biased function of CXCR7 in astrocytes, the alternate CXCR7 ligand, I-TAC/CXCL11, activated Erk and Akt through β-arrestin. The demonstration that SDF-1-bound CXCR7 activates G(i/o) proteins in astrocytes could help to explain some discrepancies previously observed for the function of CXCR4 and CXCR7 in other cell types.

CXCR7/CXCR4 heterodimer constitutively recruits beta-arrestin to enhance cell migration

G protein-coupled receptor hetero-oligomerization is emerging as an important regulator of ligand-dependent transmembrane signaling, but precisely how receptor heteromers affect receptor pharmacology remains largely unknown. In this study, we have attempted to identify the functional significance of the heteromeric complex between CXCR4 and CXCR7 chemokine receptors. We demonstrate that co-expression of CXCR7 with CXCR4 results in constitutive recruitment of β-arrestin to the CXCR4·CXCR7 complex and simultaneous impairment of G(i)-mediated signaling. CXCR7/CXCR4 co-expression also results in potentiation of CXCL12 (SDF-1)-mediated downstream β-arrestin-dependent cell signaling pathways, including ERK1/2, p38 MAPK, and SAPK as judged from the results of experiments using siRNA knockdown to deplete β-arrestin. Interestingly, CXCR7/CXCR4 co-expression enhances cell migration in response to CXCL12 stimulation. Again, inhibition of β-arrestin using either siRNA knockdown or a dominant negative mutant abrogates the enhanced CXCL12-dependent migration of CXCR4/CXCR7-expressing cells. These results show how CXCR7, which cannot signal directly through G protein-linked pathways, can nevertheless affect cellular signaling networks by forming a heteromeric complex with CXCR4. The CXCR4·CXCR7 heterodimer complex recruits β-arrestin, resulting in preferential activation of β-arrestin-linked signaling pathways over canonical G protein pathways. CXCL12-dependent signaling of CXCR4 and its role in cellular physiology, including cancer metastasis, should be evaluated in the context of potential functional hetero-oligomerization with CXCR7.