Elucidation of CXCR7-mediated signaling events and inhibition of CXCR4-mediated tumor cell transendothelial migration by CXCR7 ligands

The CXCR7 chemokine receptor promotes B-cell retention in the splenic marginal zone and serves as a sink for CXCL12

The splenic marginal zone (MZ) is comprised of specialized populations of B cells, dendritic cells, and macrophages that are uniquely arrayed outside the white pulp follicles to screen the blood for bacterial and other particulate Ags. Mechanisms responsible for MZ B-cell formation, localization, retention, and function are understood to include antigenic specificity, transcription factors, integrins, and surface receptors for soluble ligands such as S1P. Here, we add to this repertoire by demonstrating that the receptor for CXCL12, CXCR7, is expressed on MZ but not on follicular B cells. Treatment of mice with CXCR7 inhibitors led to disruption of MZ architecture, reduced numbers of MZ B cells, and altered granulocyte homeostasis associated with increasing serum levels of CXCL12. CXCR7 thus appears to function as a scavenger receptor for CXCL12 on MZ B cells.

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.

Chemokine-mediated migration of mesencephalic neural crest cells

Clefts of the lip and/or palate are among the most prevalent birth defects affecting approximately 7000 newborns in the United States annually. Disruption of the developmentally programmed migration of neural crest cells (NCCs) into the orofacial region is thought to be one of the major causes of orofacial clefting. Signaling of the chemokine SDF-1 (Stromal Derived Factor-1) through its specific receptor, CXCR4, is required for the migration of many stem cell and progenitor cell populations from their respective sites of emergence to the regions where they differentiate into complex cell types, tissues and organs. In the present study, "transwell" assays of chick embryo mesencephalic (cranial) NCC migration and ex ovo whole embryo "bead implantation" assays were utilized to determine whether SDF-1/CXCR4 signaling mediates mesencephalic NCC migration. Results from this study demonstrate that attenuation of SDF-1 signaling, through the use of specific CXCR4 antagonists (AMD3100 and TN14003), disrupts the migration of mesencephalic NCCs into the orofacial region, suggesting a novel role for SDF-1/CXCR4 signaling in the directed migration of mesencephalic NCCs in the early stage embryo.

In vivo invasion of head and neck squamous cell carcinoma cells does not require macrophages

Invasion of tumor cells into the local stroma is an important component in cancer progression. Here we report studies of the in vivo invasion of head and neck squamous cell carcinoma (HNSCC) cells in response to applied gradients of a growth factor [epidermal growth factor (EGF)] and a chemokine (CXCL12), using orthotopic floor-of-mouth models. Analysis of the invading cells indicated that >75% of them were tumor cells, about 15% macrophages, and <10% were unidentified. Surprisingly, although macrophages invaded together with tumor cells, macrophage contributions were not required for HNSCC invasion. CXCL12-induced in vivo invasion of HNSCC cells was also observed and found to occur via a unidirectional transactivation of epidermal growth factor receptor (EGFR) through CXCR4. Inhibition of tumor necrosis factor-α-converting enzyme using TNF-α protease inhibitor-2 selectively inhibited CXCL12-induced invasion but not EGF-induced invasion, consistent with CXCL12 activation of EGFR via release of EGFR ligands.

The role of CXCR7 on the adhesion, proliferation and angiogenesis of endothelial progenitor cells

Previous studies confirmed that stromal cell-derived factor 1 (SDF-1) was a principal regulator of retention, migration and mobilization of haematopoietic stem cells and endothelial progenitor cells (EPCs) during steady-state homeostasis and injury. CXC chemokine receptor 4 (CXCR4) has been considered as the unique receptor of SDF-1 and as the only mediator of SDF-1-induced biological effects for many years. However, recent studies found that SDF-1 could bind to not only CXCR4 but also CXC chemokine receptor 7 (CXCR7). The evidence that SDF-1 binds to the CXCR7 raises a concern how to distinguish the potential contribution of the SDF-1/CXCR7 pathway from SDF-1/CXCR4 pathway in all the processes that were previously attributed to SDF-1/CXCR4. In this study, the role of CXCR7 in EPCs was investigated in vitro. RT-PCR, Western blot and flow cytometry assay demonstrate that both CXCR4 and CXCR7 were expressed highly in EPCs. The adhesion of EPCs induced by SDF-1 was inhibited by blocking either CXCR4 or CXCR7 with their antibodies or antagonists. SDF-1 regulated the migration of EPCs via CXCR4 but not CXCR7. However, the transendothelial migration of EPCs was inhibited by either blocking of CXCR4 or CXCR7. Both CXCR7 and CXCR4 are essential for the tube formation of EPCs induced by SDF-1. These results suggested that both CXCR7 and CXCR4 are important for EPCs in response to SDF-1, indicating that CXCR7 may be another potential target molecule for angiogenesis-dependent diseases.

Rapid mobilization of hematopoietic progenitors by AMD3100 and catecholamines is mediated by CXCR4-dependent SDF-1 release from bone marrow stromal cells

Steady-state egress of hematopoietic progenitor cells can be rapidly amplified by mobilizing agents such as AMD3100, the mechanism, however, is poorly understood. We report that AMD3100 increased the homeostatic release of the chemokine stromal cell derived factor-1 (SDF-1) to the circulation in mice and non-human primates. Neutralizing antibodies against CXCR4 or SDF-1 inhibited both steady state and AMD3100-induced SDF-1 release and reduced egress of murine progenitor cells over mature leukocytes. Intra-bone injection of biotinylated SDF-1 also enhanced release of this chemokine and murine progenitor cell mobilization. AMD3100 directly induced SDF-1 release from CXCR4(+) human bone marrow osteoblasts and endothelial cells and activated uPA in a CXCR4/JNK-dependent manner. Additionally, ROS inhibition reduced AMD3100-induced SDF-1 release, activation of circulating uPA and mobilization of progenitor cells. Norepinephrine treatment, mimicking acute stress, rapidly increased SDF-1 release and progenitor cell mobilization, whereas β2-adrenergic antagonist inhibited both steady state and AMD3100-induced SDF-1 release and progenitor cell mobilization in mice. In conclusion, this study reveals that SDF-1 release from bone marrow stromal cells to the circulation emerges as a pivotal mechanism essential for steady-state egress and rapid mobilization of hematopoietic progenitor cells, but not mature leukocytes.

CXCR7 is up-regulated in human and murine hepatocellular carcinoma and is specifically expressed by endothelial cells

Development of hepatocellular carcinoma (HCC) is a complex and progressive disease that involves cycles of liver cell death, inflammation, and tissue regeneration/remodelling. Chemokines and chemokine receptors play numerous and integral roles in the disease progression of HCC. Here we investigated the novel chemokine receptor CXCR7/RDC1 in HCC progression, its two known ligands CXCL12 and CXCL11, as well as the other CXCL12 receptor, CXCR4. Our results show that in a cohort of 408 human HCCs, CXCR7 and CXCL11 were significantly higher in tumours compared to normal liver controls (5- and 10-fold, respectively). Immunohistochemical (IHC) staining on human HCC sections confirmed that both CXCL11 and CXCR7 were much higher in cancer tissues. Furthermore, IHC staining revealed that CXCR7 protein was only expressed in endothelial cells whereas CXCL11 exhibited a much broader tissue expression. At the cellular level we observed that in vitro, human microvascular endothelial cells (HMEC-1) up-regulated CXCR7 under hypoxic and acidic pH conditions, which are well known characteristics of the HCC tumour micro-environment. As for its ligand, we observed that IFNγ robustly induced CXCL11 in hepatic stellate cells, hepatocytes, and HMEC-1s. In addition, in the mouse Diethylnitrosamine model of hepatocarcinogenesis we observed a very strong induction of CXCR7 and CXCL11 transcripts, confirming that CXCR7/CXCL11 up-regulation is conserved between human and mice liver cancer. Altogether, our results strongly support the hypothesis that the CXCL11/CXCR7 pathway is involved HCC progression.

DNA methylation regulates the expression of CXCL12 in rheumatoid arthritis synovial fibroblasts

In the search for specific genes regulated by DNA methylation in rheumatoid arthritis (RA), we investigated the expression of CXCL12 in synovial fibroblasts (SFs) and the methylation status of its promoter and determined its contribution to the expression of matrix metalloproteinases (MMPs). DNA was isolated from SFs and methylation was analyzed by bisulfite sequencing and McrBC assay. CXCL12 protein was quantified by enzyme-linked immunosorbent assay before and after treatment with 5-azacytidine. RASFs were transfected with CXCR7-siRNA and stimulated with CXCL12. Expression of MMPs was analyzed by real-time PCR. Basal expression of CXCL12 was higher in RASFs than osteoarthritis (OA) SFs. 5-azacytidine demethylation increased the expression of CXCL12 and reduced the methylation of CpG nucleotides. A lower percentage of CpG methylation was found in the CXCL12 promoter of RASFs compared with OASFs. Overall, we observed a significant correlation in the mRNA expression and the CXCL12 promoter DNA methylation. Stimulation of RASFs with CXCL12 increased the expression of MMPs. CXCR7 but not CXCR4 was expressed and functional in SFs. We show here that RASFs produce more CXCL12 than OASFs due to promoter methylation changes and that stimulation with CXCL12 activates MMPs via CXCR7 in SFs. Thereby we describe an endogenously activated pathway in RASFs, which promotes joint destruction.

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.

The novel chemokine receptor CXCR7 regulates trans-endothelial migration of cancer cells

Background

Migration of metastatic tumor cells from the bloodstream into lymph nodes is thought to be facilitated by expression of the chemokine receptors CCR7, CXCR4 and, for B cell-derived tumors, CXCR5. Expression of their respective chemokine ligands (CCL19, CCL21, CXCL12 and CXCL13) by endothelial cells inside the lymph nodes facilitates the trans-endothelial migration (TEM) of these cells through high endothelial venules into the lymph node parenchyma. It is known that CXCR7, a second CXCL12 receptor, regulates TEM of CXCR4+CXCR7+ tumor cells towards a CXCL12 source. In this study, we set out to assess the potential stimulation by CXCL12 of tumor cell TEM towards other chemokines and whether CXCR7 might be able to regulate such effects.

Methods

The human Burkitt's lymphoma cell line NC-37, which expresses CXCR4, CXCR5, CXCR7 and CCR7, was selected as a model system. TEM of these cells through a human HUVEC endothelial cell monolayer was used as the main model system for these studies. Regulation of their TEM behavior by various concentrations of the various cognate chemokines for the above-mentioned receptors, placed in either the source or target wells of modified Boyden chamber migration plates, was assessed by quantifying the number of cells migrated under each experimental condition.

Results

Exposure of CXCR4⁺CXCR7⁺ cancer cells to CXCL12 greatly potentiated their TEM towards the chemokines CCL19 and CXCL13. This CXCL12-potentiated TEM was inhibited by the second CXCR7 chemokine ligand, CXCL11, as well as CXCR7-specific small molecule antagonists and antibodies. In contrast, the CXCR4 antagonist AMD3100 was less effective at inhibiting CXCL12-potentiated TEM. Thus, CXCR7 antagonists may be effective therapeutic agents for blocking CXCL12-mediated migration of CXCR4⁺CXCR7⁺ tumor cells into lymph nodes, regardless of whether the cancer cells follow a CXCL12 gradient or whether serum CXCL12 stimulates their migration towards CCR7 and CXCR5 chemokines in the lymph nodes.

Differential estrogen-regulation of CXCL12 chemokine receptors, CXCR4 and CXCR7, contributes to the growth effect of estrogens in breast cancer cells

CXCR4 and CXCR7 are the two receptors for the chemokine CXCL12, a key mediator of the growth effect of estrogens (E2) in estrogen receptor (ER)-positive breast cancers. In this study we examined E2-regulation of the CXCL12 axis components and their involvement in the growth of breast cancer cells. CXCR4 and CXCR7 were differentially regulated by E2 which enhanced the expression of both CXCL12 and CXCR4 but repressed the expression of CXCR7. Formaldehyde-associated isolation of regulatory elements (FAIRE) revealed that E2-mediated transcriptional regulation of these genes is linked to the control of the compaction state of chromatin at their promoters. This effect could be accomplished via several distal ER-binding sites in the regions surrounding these genes, all of which are located 20–250 kb from the transcription start site. Furthermore, individual down-regulation of CXCL12, CXCR4 or CXCR7 expression as well as the inhibition of their activity significantly decreases the rate of basal cell growth. In contrast, E2-induced cell growth was differentially affected. Unlike CXCR7, the inhibition of the expression or activity of either CXCL12 or CXCR4 significantly blunted the E2-mediated stimulation of cellular growth. Besides, CXCR7 over-expression increased the basal MCF-7 cell growth rate and decreased the growth effect of E2. These findings indicate that E2 regulation of the CXCL12 signaling axis is important for the E2-mediated growth effect of breast cancer cells. These data also provide support for distinct biological functions of CXCR4 and CXCR7 and suggest that targeting CXCR4 and/or CXCR7 would have distinct molecular effects on ER-positive breast tumors.

Reciprocal regulation of CXCR4 and CXCR7 in intestinal mucosal homeostasis and inflammatory bowel disease

IBDs are characterized by increased influx of immune cells to the mucosa of genetically susceptible persons. Cellular migration to injury sites is mediated by chemokines. CXCL12 is a ubiquitous, constitutive chemokine that participates in stem cell proliferation and migration and mediates T lymphocyte migration to inflamed tissues. We have recently reported that CXCL12 and its receptor, CXCR4, are expressed in normal and more prominently, inflamed human intestinal mucosa. However, the interactions and roles of CXCL12 and its receptors, CXCR4 and the recently discovered CXCR7, in intestinal inflammation have not been defined. In the present study, we further dissected the effects of CXCL12 on lymphocytes in intestinal homeostasis and inflammation and delineated the interplay between CXCL12 and its receptors CXCR4 and CXCR7. To that end, fresh mononuclear cells were isolated from mucosa and PB of healthy or IBD patients. Phenotypical and functional assays were conducted using flow cytometry, Transwell migration chambers, and ELISA. The data show that CXCL12-mediated migration of T cells is CXCR4- but not CXCR7-dependent. T cell activation reciprocally regulates CXCR7 and CXCR4 expression and migratory capacity. IBD PBTs expressed more CXCR7 than normal PBTs. Finally, T cells attracted by CXCL12 are mostly of a memory phenotype. In conclusion, the present study suggests that the interplay between CXCL12 and its receptors affects homeostasis and inflammation in the intestinal mucosa.

CXCR7 protein expression in human adult brain and differentiated neurons

Background

CXCR7 and CXCR4 are receptors for the chemokine CXCL12, which is involved in essential functions of the immune and nervous systems. Although CXCR7 transcripts are widely expressed throughout the central nervous system, little is known about its protein distribution and function in the adult brain. To evaluate its potential involvement in CXCL12/CXCR4 signaling in differentiated neurons, we studied CXCR7 protein expression in human brain and cultured neurons.

Methodology/Principal Findings

Immunohistochemistry and RT-PCR analyses of cortex and hippocampus from control and HIV-positive subjects provided the first evidence of CXCR7 protein expression in human adult neurons, under normal and pathological conditions. Furthermore, confocal microscopy and binding assays in cultured neurons show that CXCR7 protein is mainly located into cytoplasm, while little to no protein expression is found on neuronal plasma membrane. Interestingly, specific CXCR7 ligands that inhibit CXCL12 binding to CXCR7 do not alter CXCR4-activated survival signaling (pERK/pAkt) in rat cortical neurons. Neuronal CXCR7 co-localizes to some extent with the endoplasmic reticulum marker ERp29, but not with early/late endosome markers. Additionally, large areas of overlap are detected in the intracellular pattern of CXCR7 and CXCR4 expression.

Conclusions/Significance

Overall, these results implicate CXCR4 as the main CXCL12 signaling receptor on the surface of differentiated neurons and suggest that CXCR7 may interact with CXCR4 at the intracellular level, possibly affecting CXCR4 trafficking and/or coupling to other proteins.

The chemokine receptor CXCR7 functions to regulate cardiac valve remodeling

CXCR7 (RDC1), a G-protein-coupled receptor with conserved motifs characteristic of chemokine receptors, is enriched in endocardial and cushion mesenchymal cells in developing hearts, but its function is unclear. Cxcr7 germline deletion resulted in perinatal lethality with complete penetrance. Mutant embryos exhibited aortic and pulmonary valve stenosis due to semilunar valve thickening, with occasional ventricular septal defects. Semilunar valve mesenchymal cell proliferation increased in mutants from embryonic day 14 onward, but the cell death rate remained unchanged. Cxcr7 mutant valves had increased levels of phosphorylated Smad1/5/8, indicating increased BMP signaling, which may partly explain the thickened valve leaflets. The hyperproliferative phenotype appeared to involve Cxcr7 function in endocardial cells and their mesenchymal derivatives, as Tie2-Cre Cxcr7(flox/-) mice had semilunar valve stenosis. Thus, CXCR7 is involved in semilunar valve development, possibly by regulating BMP signaling, and may contribute to aortic and pulmonary valve stenosis.

Prognostic impact of CXCR4 and CXCR7 expression in pancreatic adenocarcinoma

BACKGROUND

Chemokines and their receptors are known to play important roles in the tumorigenesis of many malignancies. The aim of this study was to evaluate the prognostic impact of the expression of the chemokine receptors CXCR4 and CXCR7 in patients with pancreatic adenocarcinoma (PAC).

METHODS

Expression of CXCR4 and CXCR7 in specimens from 249 patients with PAC was evaluated by immunohistochemistry on a tissue microarray and matched with clinicopathological parameters and overall survival.

RESULTS

Expression of CXCR4 was detected in 215 patients (86.4%) and CXCR7 in 47 patients (18.9%). No association between CXCR4 and CXCR7 expression was evident, although all the CXCR7 positive tumors were also CXCR4 positive. pT1/2 tumors showed a higher frequency of CXCR7 expression than pT3/4 tumors (P = 0.018), while more dedifferentiated tumors had elevated CXCR7 expression (P = 0.036). Overall and disease-free survival revealed no association with either CXCR4 or CXCR7 expression.

CONCLUSION

CXCR7 is associated with tumor grade and inversely associated with tumor size and may play a potential role in tumor progression and differentiation. In contrast to previously reported data our results revealed no significant association between CXCR4 expression and clinical or pathological data.

Atypical chemokine receptors

Atypical chemokine receptors (ACRs) are cell surface receptors with seven transmembrane domains structurally homologous to chemokine G-protein coupled receptors (GPCRs). However, upon ligation by cognate chemokines, ACRs fail to induce classical signaling and downstream cellular responses characteristic for GPCRs. Despite this, by affecting chemokine availability and function, ACRs impact on a multitude of pathophysiological events and have emerged as important molecular players in health and disease. This review discusses individual characteristics of the currently known ACRs, highlights their similarities and differences and attempts to establish their group identity. It summarizes the progress made in mapping ACR expression, understanding their diverse in vitro and in vivo functions of ACRs and uncovering their contributions to disease pathogeneses.

CXCR4 signaling mediates morphine-induced tactile hyperalgesia

Morphine and related compounds are the first line of therapy in the treatment of moderate to severe pain. Over time, individuals taking opioids can develop an increasing sensitivity to noxious stimuli, even evolving into a painful response to previously non-noxious stimuli (opioid-induced hyperalgesia; OIH). The mechanism underlying OIH is not well understood although complex intracellular neural mechanisms, including opioid receptor desensitization and down-regulation, are believed to be major mechanisms underlying OIH. However, OIH may also be associated with changes in gene expression. A growing body of evidence suggests that cellular exposure to mu agonists upregulate chemokines/receptors and recent work from our laboratory implicates chemokine upregulation in a variety of neuropathic pain behaviors. Here we characterized the degree to which chemokines/receptors signaling is increased in primary afferent neurons of the dorsal root ganglion (DRG) following chronic morphine sulfate treatment and correlated these changes with tactile hyperalgesic behavior in rodents. We demonstrate that mRNA expression of the chemokine, stromal-derived factor-1 (SDF1/CXCL12) is upregulated following morphine treatment in sensory neurons of the rat. The release of SDF1 was found to be constitutive when compared with the activity dependent release of the C-C chemokine, monocyte chemoattractant protein-1 (MCP1/CCL2) in a line of F11 neuroblastoma-sensory neuron hybrid cells. We further determined that there is pronounced CXCR4 expression in satellite glial cells and following morphine treatment, increased functional CXCR4 expression in sensory neurons of the DRG. Moreover, intraperitoneal administration of the specific CXCR4 antagonist, AMD3100, completely reversed OIH in the rat. Taken together; the data suggest that opioid-induced SDF1/CXCR4 signaling is central to the development of long lasting OIH and that receptor antagonists represent a promising novel approach to the management of the side effects associated with the use of opioids for chronic pain management.

CXCL12 (SDF1alpha)-CXCR4/CXCR7 pathway inhibition: an emerging sensitizer for anticancer therapies?

Addition of multiple molecularly targeted agents to the existing armamentarium of chemotherapeutics and radiotherapies represents a significant advance in the management of several advanced cancers. In certain tumor types with no efficacious therapy options, these agents have become the first line of therapy, for example, sorafenib in advanced hepatocellular carcinoma or bevacizumab in recurrent glioblastoma. Unfortunately, in many cases, the survival benefits are modest, lasting only weeks to a few months. Moreover, they may not show benefit in patients with localized disease (i.e., in the adjuvant setting). Recent studies have provided increasing evidence that activation of the chemokine CXCL12 (SDF1α) pathway is a potential mechanism of tumor resistance to both conventional therapies and biological agents via multiple complementary actions: (i) by directly promoting cancer cell survival, invasion, and the cancer stem and/or tumor-initiating cell phenotype; (ii) by recruiting "distal stroma" (i.e., myeloid bone marrow-derived cells) to indirectly facilitate tumor recurrence and metastasis; and (iii) by promoting angiogenesis directly or in a paracrine manner. Here, we discuss recent preclinical and clinical data that support the potential use of anti-CXCL12 agents (e.g., AMD3100, NOX-A12, or CCX2066) as sensitizers to currently available therapies by targeting the CXCL12/CXCR4 and CXCL12/CXCR7 pathways.

Cxcr7 controls neuronal migration by regulating chemokine responsiveness

The chemokine Cxcl12 binds Cxcr4 and Cxcr7 receptors to control cell migration in multiple biological contexts, including brain development, leukocyte trafficking, and tumorigenesis. Both receptors are expressed in the CNS, but how they cooperate during migration has not been elucidated. Here, we used the migration of cortical interneurons as a model to study this process. We found that Cxcr4 and Cxcr7 are coexpressed in migrating interneurons, and that Cxcr7 is essential for chemokine signaling. Intriguingly, this process does not exclusively involve Cxcr7, but most critically the modulation of Cxcr4 function. Thus, Cxcr7 is necessary to regulate Cxcr4 protein levels, thereby adapting chemokine responsiveness in migrating cells. This demonstrates that a chemokine receptor modulates the function of another chemokine receptor by controlling the amount of protein that is made available for signaling at the cell surface.

CXCR7 influences leukocyte entry into the CNS parenchyma by controlling abluminal CXCL12 abundance during autoimmunity

During CNS autoimmunity, brain endothelial cell CXCR7 internalizes CXCL12 from the perivascular space, thereby permitting leukocyte migration into the CNS parenchyma.

Loss of CXCL12, a leukocyte localizing cue, from abluminal surfaces of the blood–brain barrier occurs in multiple sclerosis (MS) lesions. However, the mechanisms and consequences of reduced abluminal CXCL12 abundance remain unclear. Here, we show that activation of CXCR7, which scavenges CXCL12, is essential for leukocyte entry via endothelial barriers into the central nervous system (CNS) parenchyma during experimental autoimmune encephalomyelitis (EAE), a model for MS. CXCR7 expression on endothelial barriers increased during EAE at sites of inflammatory infiltration. Treatment with a CXCR7 antagonist ameliorated EAE, reduced leukocyte infiltration into the CNS parenchyma and parenchymal VCAM-1 expression, and increased abluminal levels of CXCL12. Interleukin 17 and interleukin 1β increased, whereas interferon-γ decreased, CXCR7 expression on and CXCL12 internalization in primary brain endothelial cells in vitro. These findings identify molecular requirements for the transvascular entry of leukocytes into the CNS and suggest that CXCR7 blockade may have therapeutic utility for the treatment of MS.

Activation of CXCR7 receptor promotes oligodendroglial cell maturation

OBJECTIVE

Differentiation of oligodendroglial precursor cells is crucial for central nervous system (re)myelination and is influenced by multiple extrinsic and intrinsic factors. Chemokines, a group of small proteins, are highly conserved among mammals and have been implicated in a variety of biological processes during development, tissue homeostasis, and repair. We investigated whether the chemokine CXCL12 influences oligodendrocytes and what cellular differentiation/maturation processes are controlled by this molecule.

METHODS

Experimental autoimmune encephalomyelitis was induced using myelin oligodendrocyte glycoprotein. Immunostainings and quantitative gene expression analysis were used to study expression of the 2 currently known CXCL12 receptors on oligodendroglial cells. Stimulation of cultured primary oligodendroglial precursor cells was performed to determine the impact of the ligand/receptor interaction on morphological maturation and on myelin expression. Blocking and suppression experiments were conducted to reveal the identity of the transmitting receptor.

RESULTS

This analysis revealed the presence of CXCR4 as well as CXCR7 and that cellular maturation in vivo and in vitro is accompanied by upregulation of CXCR7 and downregulation of CXCR4. Of note, in the diseased demyelinating central nervous system, CXCR7 expression is maintained on oligodendroglial cells, whereas CXCR4 could not be detected. We then demonstrated that CXCL12 stimulation promotes morphological maturation of cultured primary oligodendrocyte precursor cells as well as their myelin expression. Pharmacological inhibition of the CXCR7 receptor was shown to block CXCL12-dependent effects entirely.

INTERPRETATION

Our findings suggest that a specific activation of the CXCR7 receptor could provide a means to promote oligodendroglial differentiation in the diseased or injured central nervous system.

Microfluidic platform for chemotaxis in gradients formed by CXCL12 source-sink cells

Chemokine CXCL12 promotes CXCR4-dependent chemotaxis of cancer cells to characteristic organs and tissues, leading to metastatic disease. This study was designed to investigate how cells expressing CXCR7 regulate chemotaxis of a separate population of CXCR4 cells under physiologic conditions in which cells are exposed to gradients of CXCL12. We recapitulated a cancer-stroma microenvironment by patterning CXCR4-expressing cancer cells in microchannels at spatially defined positions relative to CXCL12-producing cells and CXCR7-expressing cells. CXCR7 scavenges and degrades CXCL12, which has been proposed to facilitate CXCR4-dependent chemotaxis through a source-sink model. Using the microchannel device, we demonstrated that chemotaxis of CXCR4 cells depended critically on the presence and location of CXCR7 cells (sink) relative to chemokine secreting cells (source). Furthermore, inhibiting CXCR4 on migrating cells or CXCR7 on sink cells blocked CXCR4-dependent chemotaxis toward CXCL12, showing that the device can identify new therapeutic agents that block migration by targeting chemoattractant scavenging receptors. Our system enables efficient chemotaxis under much shallower yet more physiological chemoattractant gradients by generating an in vitro microenvironment where combinations of cellular products may be secreted along with formation of a chemoattractant gradient. In addition to elucidating mechanisms of CXCL-12 mediated chemotaxis, this simple and robust method can be broadly useful for engineering multiple microenvironments to investigate intercellular communication.

The peptidomimetic CXCR4 antagonist TC14012 recruits beta-arrestin to CXCR7: roles of receptor domains

CXCR7 is an atypical chemokine receptor that signals through β-arrestin in response to agonists without detectable activation of heterotrimeric G-proteins. Its cognate chemokine ligand CXCL12 also binds CXCR4, a chemokine receptor of considerable clinical interest. Here we report that TC14012, a peptidomimetic inverse agonist of CXCR4, is an agonist on CXCR7. The potency of β-arrestin recruitment to CXCR7 by TC14012 is much higher than that of the previously reported CXCR4 antagonist AMD3100 and differs only by one log from that of the natural ligand CXCL12 (EC(50) 350 nM for TC14012, as compared with 30 nM for CXCL12 and 140 μM for AMD3100). Moreover, like CXCL12, TC14012 leads to Erk 1/2 activation in U373 glioma cells that express only CXCR7, but not CXCR4. Given that with TC14012 and AMD3100 two structurally unrelated CXCR4 antagonists turn out to be agonists on CXCR7, this likely reflects differences in the activation mechanism of the arrestin pathway by both receptors. To identify the receptor domain responsible for these opposed effects, we investigated CXCR4 and CXCR7 C terminus-swapping chimeras. Using quantitative bioluminescence resonance energy transfer, we find that the CXCR7 receptor core formed by the seven-transmembrane domains and the connecting loops determines the agonistic activity of both TC14012 and AMD3100. Moreover, we find that the CXCR7 chimera bearing the CXCR4 C-terminal constitutively associates with arrestin in the absence of ligands. Our data suggest that the CXCR4 and CXCR7 cores share ligand-binding surfaces for the binding of the synthetic ligands, indicating that CXCR4 inhibitors should be tested also on CXCR7.

Molecular basis of anti-inflammatory action of platelet-rich plasma on human chondrocytes: mechanisms of NF-κB inhibition via HGF

Loss of articular cartilage through injury or disease presents major clinical challenges also because cartilage has very poor regenerative capacity, giving rise to the development of biological approaches. As autologous blood product, platelet-rich plasma (PRP) provides a promising alternative to surgery by promoting safe and natural healing. Here we tested the possibility that PRP might be effective as an anti-inflammatory agent, providing an attractive basis for regeneration of articular cartilage, and two principal observations were done. First, activated PRP in chondrocytes reduced the transactivating activity of NF-κB, critical regulator of the inflammatory process, and decreased the expression of COX-2 and CXCR4 target genes. By analyzing a panel of cytokines with different biological significance, in activated PRP we observed increases in hepatocyte growth factor (HGF), interleukin-4 and tumor necrosis factor-α (TNF-α). HGF and TNF-α, by disrupting NF-κB-transactivating activity, were important for the anti-inflammatory function of activated PRP. The key molecular mechanisms involved in PRP-inhibitory effects on NF-κB activity were for HGF the enhanced cellular IkBα expression, that contributed to NF-κB-p65 subunit retention in the cytosol and nucleo-cytoplasmic shuttling, and for TNF-α the p50/50 DNA-binding causing inhibition of target-gene expression. Second, activated PRP in U937-monocytic cells reduced chemotaxis by inhibiting chemokine transactivation and CXCR4-receptor expression, thus possibly controlling local inflammation in cartilage. In conclusion, activated PRP is a promising biological therapeutic agent, as a scaffold in micro-invasive articular cartilage regeneration, not only for its content of proliferative/differentiative growth factors, but also for the presence of anti-inflammatory agents including HGF.

Alterations of CXCR4 function in μ-opioid receptor-deficient glia

The chemokine receptor CXCR4 and the μ-opioid receptor (MOR) are G-protein-coupled receptors that are essential for normal function of the nervous and immune systems. Several studies have suggested that MOR is a key regulator of CXCR4 in the brain; however, the molecular basis of the opioid-chemokine interaction is not fully understood, and it may involve different mechanisms in neuronal and glial cells. Our previous studies demonstrated that MOR stimulation specifically upregulates the protein ferritin heavy chain - an inhibitor of CXCR4 - in neurons, and suggested that additional mechanisms could be operative in glia. In this study, we investigated CXCR4 function in brains and astroglial cultures deprived of MOR. Reduced coupling of CXCR4 to G-proteins was found in brain slices and tissue homogenates of MOR(-/-) mice as compared with wild-type controls. CXCR4-induced signaling was also reduced in glial cultures from MOR(-/-) mice, as shown by analysis of CXCR4 downstream targets (Akt and ERK1/2). Pharmacological studies with δ-opioid receptor (DOR)-specific ligands suggested that DOR-CXCR4 interactions are implicated in the inhibition of CXCR4 in MOR-deficient cells both in vitro and in vivo. Moreover, increased CXCR4/DOR co-immunoprecipitation was found in brain tissue and cultured glia from MOR(-/-) mice. Importantly, CXCR4 function was restored by pretreatment with a DOR antagonist. Overall, these findings indicate that DOR plays a crucial role in the regulation of CXCR4 in glia, probably via silent receptor heterodimers. The data also suggest that the opiate system interferes with normal CXCR4 function in different ways, depending on receptor subtypes.

CXCR7 protein is not expressed on human or mouse leukocytes

Since the discovery that CXCR7 binds to CXCL12/SDF-1α, the role of CXCR7 in CXCL12-mediated biological processes has been under intensive scrutiny. However, there is no consensus in the literature on the expression of CXCR7 protein by peripheral blood cells. In this study we analyzed human and mouse leukocytes and erythrocytes for CXCR7 protein expression, using a competitive CXCL12 binding assay as well as by flow cytometry and immunohistochemistry using multiple CXCR7 Abs. CXCR7(-/-) mice were used as negative controls. Together, these methods indicate that CXCR7 protein is not expressed by human peripheral blood T cells, B cells, NK cells, or monocytes, or by mouse peripheral blood leukocytes. CXCR7 protein is, however, expressed on mouse primitive erythroid cells, which supply oxygen to the embryo during early stages of development. These studies therefore suggest that, whereas CXCR7 protein is expressed by primitive RBCs during murine embryonic development, in adult mammals CXCR7 protein is not expressed by normal peripheral blood cells.

CXCL12 chemokine expression and secretion regulates colorectal carcinoma cell anoikis through Bim-mediated intrinsic apoptosis

Background

Resistance to anoikis, apoptosis triggered by a loss of cellular adhesion to the underlying extracellular matrix, is a hallmark of metastatic cancer. Previously we have shown re-establishment of CXCL12 expression in colorectal carcinoma cells inhibits metastasis by enhancing anoikis sensitivity. The objective of these studies was to define the signaling mechanisms regulating CXCL12-mediated anoikis.

Methodology/Principal Findings

Adhesion, examined by crystal violet staining, immunofluorescence microscopy, and immunoblot analysis indicated decreased focal adhesion signaling corresponding with loss of adhesion in cells constitutively simulated by CXCL12. Loss of adhesion was inhibited by pertussis toxin treatment, indicating CXCL12 regulating anoikis through G_αi-protein coupled receptors. Non-adherent HCT116 and HT29 colorectal carcinoma cells expressing CXCL12 exhibited enhanced anoikis sensitivity by propidium iodide staining, caspase activity assays, and immunoblot compared to GFP control cells. CXCL12 producing carcinomas cultured on poly-HEMA displayed heightened Bim and loss of Mcl-1 and Bcl-2 preceding cytochrome c release, and caspase-9 activation. RNAi knockdown of Bim reversed anoikis sensitivity of CXCL12-expressing cells and fostered increased soft-agar foci formation and hepatic tumors in an orthotopic mouse model of metastasis.

Conclusions/Significance

These data indicate CXCL12 provides a barrier to metastasis by increasing anoikis via activation of a Bim-mediated intrinsic apoptotic pathway. These results underscore the importance of retaining CXCL12 expression to sensitize colorectal carcinomas to anoikis and minimize tumor progression.

CXCR7 protein expression correlates with elevated mmp-3 secretion in breast cancer cells

Metastatic breast cancer is the leading cause of cancer-related death in women worldwide and, despite recent therapeutic advances, the disease remains incurable. A critical step in cancer cell metastasis is the degradation of extracellular matrix components by matrix metalloproteinases (MMPs), which permits malignant cells to separate from the primary tumor and access circulatory conduits for seeding distant organs. This study reports a correlation between the elevated secretion of MMP-3 by breast cancer cells and the expression of CCR7 protein, a recently discovered non-classical chemokine receptor that may play a role in metastasis by regulating tumor cell transendothelial migration. MMP-3 secretion is increased in human mammary tumor cells that overexpress CXCR7, and is reduced in mouse breast cancer cells in which the endogenous CXCR7 expression has been knocked down via RNAi. The correlation between CXCR7 and MMP-3 expression in breast cancer may provide additional therapeutic rationale for targeting CXCR7 in order to prevent metastatic disease.

The blood-brain barrier, chemokines and multiple sclerosis

The infiltration of leukocytes into the central nervous system (CNS) is an essential step in the neuropathogenesis of multiple sclerosis (MS). Leukocyte extravasation from the bloodstream is a multistep process that depends on several factors including fluid dynamics within the vasculature and molecular interactions between circulating leukocytes and the vascular endothelium. An important step in this cascade is the presence of chemokines on the vascular endothelial cell surface. Chemokines displayed along the endothelial lumen bind chemokine receptors on circulating leukocytes, initiating intracellular signaling that culminates in integrin activation, leukocyte arrest, and extravasation. The presence of chemokines at the endothelial lumen can help guide the movement of leukocytes through peripheral tissues during normal immune surveillance, host defense or inflammation. The expression and display of homeostatic or inflammatory chemokines therefore critically determine which leukocyte subsets extravasate and enter the peripheral tissues. Within the CNS, however, infiltrating leukocytes that cross the endothelium face additional boundaries to parenchymal entry, including the abluminal presence of localizing cues that prevent egress from perivascular spaces. This review focuses on the differential display of chemokines along endothelial surfaces and how they impact leukocyte extravasation into parenchymal tissues, especially within the CNS. In particular, the display of chemokines by endothelial cells of the blood brain barrier may be altered during CNS autoimmune disease, promoting leukocyte entry into this immunologically distinct site. Recent advances in microscopic techniques, including two-photon and intravital imaging have provided new insights into the mechanisms of chemokine-mediated capture of leukocytes within the CNS.

CXC chemokines in cancer angiogenesis and metastases

The tumor microenvironment is extremely complex that depends on tumor cell interaction with the responding host cells. Angiogenesis, or new blood vessel growth from preexisting vasculature, is a preeminent feature of successful tumor growth of all solid tumors. While a number of factors produced by both the tumor cells and host responding cells have been discovered that regulate angiogenesis, increasing evidence is growing to support the important role of CXC chemokines in this process. As a family of cytokines, the CXC chemokines are pleiotropic in their ability to regulate tumor-associated angiogenesis, as well as cancer cell metastases. In this chapter, we will discuss the disparate activity that CXC chemokines play in regulating cancer-associated angiogenesis and metastases.

Constitutive and chemokine-dependent internalization and recycling of CXCR7 in breast cancer cells to degrade chemokine ligands

CXCR7 is a receptor for chemokines including CXCL12 (SDF-1), a molecule that promotes tumor growth and metastasis in breast cancer and other malignancies. Building upon the recent observation that CXCR7 sequesters CXCL12, we investigated mechanisms for CXCR7-dependent uptake of chemokines. Breast cancer cells expressing CXCR7 accumulated chemokines CXCL12 and CXC11 present at concentrations < 1 ng/ml, unlike cells expressing CXCR4. CXCR7-dependent accumulation of chemokines was reduced by inhibitors of clathrin-mediated endocytosis. Following CXCR7-mediated internalization, CXCL12 trafficked to lysosomes and was degraded, although levels of CXCR7 remained stable. CXCR7 reduced CXCL12 in the extracellular space, limiting amounts of chemokine available to acutely stimulate signaling through CXCR4. CXCR7 constitutively internalized and recycled to the cell membrane even in the absence of ligand, and addition of chemokines did not significantly enhance receptor internalization. Chemokines at concentrations less than the Kd for ligand-receptor binding did not alter levels of CXCR7 at the cell surface. Higher concentrations of chemokine ligands reduced total cell surface expression of CXCR7 without affecting receptor internalization, indicating that receptor recycling was inhibited. CXCR7-dependent uptake of chemokines and receptor trafficking were regulated by β-arrestin 2. These studies establish mechanisms through which CXCR7 regulates availability of chemokine ligands in the extracellular space.

Chemokine receptor CXCR7 regulates the invasion, angiogenesis and tumor growth of human hepatocellular carcinoma cells

Background

In spite of recent advances in diagnostic and therapeutic measures, the prognosis of hepatocellular carcinoma (HCC) patients remains poor. Therefore, it is crucial to understand what factors are involved in promoting development of HCC. Evidence is accumulating that members of the chemokine receptor family are viewed as promising therapeutic targets in the fight against cancer. More recent studies have revealed that chemokine receptor CXCR7 plays an important role in cancer development. However, little is known about the effect of CXCR7 on the process of HCC cell invasion and angiogenesis. The aim of this study is to investigate the expression of CXCR7 in hepatocellular carcinoma tissues and cell lines and to evaluate the role of CXCR7 in tumor growth, angiogenesis and invasion of HCC cells.

Methods

We constructed CXCR7 expressing shRNA, and CXCR7shRNA was subsequently stably transfected into human HCC cells. We evaluated the effect of CXCR7 inhibition on cell invasion, adhesion, VEGF secretion, tube formation and tumor growth. Immunohistochemistry was done to assess the expression of CXCR7 in human hepatocellular carcinoma tissues and CD31 in tumor of mice. We also evaluated the effect of VEGF stimulation on expression of CXCR7.

Results

CXCR7 was overexpressed in hepatocellular carcinoma tissues. We showed that high invasive potential HCC cell lines express high levels of CXCR7. In vitro, CXCL12 was found to induce invasion, adhesion, tube formation, and VEGF secretion in SMMC-7721 cells. These biological effects were inhibited by silencing of CXCR7 in SMMC-7721 cells. In addition, we also found that VEGF stimulation can up-regulate CXCR7 expression in SMMC-7721 cells and HUVECs. More importantly, enhanced expression of CXCR7 by VEGF was founctional. In vivo, tumor growth and angiogenesis were suppressed by knockdown of CXCR7 in SMMC-7721 cells. However, silencing of CXCR7 did not affect metastasis of tumor in vivo.

Conclusions

Increased CXCR7 expression was found in hepatocellular carcinoma tissues. Knockdown of CXCR7 expression by transfected with CXCR7shRNA significantly inhibits SMMC-7721 cells invasion, adhesion and angiogenesis. Finally, down-regulation of CXCR7 expression lead to a reduction of tumor growth in a xenograft model of HCC. This study provides new insights into the significance of CXCR7 in invasion and angiogenesis of HCC.

Neutralizing endogenous chemokines with small molecules. Principles and potential therapeutic applications

Regulation of cellular responses to external stimuli such as hormones, neurotransmitters, or cytokines is achieved through the control of all steps of the complex cascade starting with synthesis, going through maturation steps, release, distribution, degradation and/or uptake of the signalling molecule interacting with the target protein. One possible way of regulation, referred to as scavenging or neutralization of the ligand, has been increasingly studied, especially for small protein ligands. It shows innovative potential in chemical biology approaches as well as in disease treatment. Neutralization of protein ligands, as for example cytokines or chemokines can lead to the validation of signalling pathways under physiological or pathophysiological conditions, and in certain cases, to the development of therapeutic molecules now used in autoimmune diseases, chronic inflammation and cancer treatment. This review explores the field of ligand neutralization and tries to determine to what extent small chemical molecules could substitute for neutralizing antibodies in therapeutic approaches.

CXCR7 is an active component of SDF-1 signalling in astrocytes and Schwann cells

The alternative SDF-1 (stromal cell derived factor-1) receptor, CXCR7, has been suggested to act as either a scavenger of extracellular SDF-1 or a modulator of the primary SDF-1 receptor, CXCR4. CXCR7, however, also directly affects the function of various tumor-cell types. Here, we demonstrate that CXCR7 is an active component of SDF-1 signalling in astrocytes and Schwann cells. Cultured cortical astrocytes and peripheral nerve Schwann cells exhibit comparable total and cell-surface levels of expression of both SDF-1 receptors. Stimulation of astrocytes with SDF-1 resulted in the temporary activation of Erk1/2, Akt and PKCzeta/lambda, but not p38 and PKCalpha/beta. Schwann cells showed SDF-1-induced activation of Erk1/2, Akt and p38, but not PKCalpha/beta and PKCzeta/lambda. The respective signalling pattern remained fully inducible in astrocytes from CXCR4-deficient mice, but was abrogated following depletion of astrocytic CXCR7 by RNAi. In Schwann cells, RNAi-mediated depletion of either CXCR4 or CXCR7 silenced SDF-1 signalling. The findings of the astrocytic receptor-depletion experiments were reproduced by CXCR7 antagonist CCX754, but not by CXCR4 antagonist AMD3100, both of which abolished astrocytic SDF-1 signalling. Further underlining the functional importance of CXCR7 signalling in glial cells, we show that the mitogenic effects of SDF-1 on both glial cell types are impaired upon depleting CXCR7.

The chemokine receptor CXCR7 is expressed on lymphatic endothelial cells during renal allograft rejection

CXCR7 is an atypical receptor for the chemokines CXCL11 and CXCL12, which were found to be involved in animal models of allograft injury. We studied the expression of CXCR7 and its ligands in human kidneys by first quantifying the mRNA in 53 renal allograft biopsies. Receptor and ligand mRNAs were expressed in renal allografts, with a significant induction of CXCL11 and CXCL12 in biopsies showing borderline lesions and acute rejection. Immunohistochemical analysis for CXCR7 was performed in a series of 64 indication and 24 protocol biopsies. The indication biopsies included 46 acute rejections, 6 with interstitial fibrosis and tubular atrophy, and 12 pretransplant biopsies as controls. In control biopsies, CXCR7 protein was found on smooth muscle and on endothelial cells of a small number of peritubular vessels. The number of CXCR7-positive vessels was increased in acute rejection and, using double immunofluorescence labeling, a subset of these CXCR7-positive endothelial cells were identified as lymphatic vessels. Both CXCR7-positive blood and lymphatic vessels increased during allograft rejection. We found that CXCR7 is present in both blood and lymphatic endothelial cells in human renal allografts. Whether its presence modulates the formation of chemokine gradients and the recruitment of inflammatory cells will require further experimental studies.

Expression and function of CXCL12/CXCR4 in rat urinary bladder with cyclophosphamide-induced cystitis

Chemokines, otherwise known as chemotactic cytokines, are proinflammatory mediators of the immune response and have been implicated in altered sensory processing, hyperalgesia, and central sensitization following tissue injury or inflammation. To address the role of CXCL12/CXCR4 signaling in normal micturition and inflammation-induced bladder hyperreflexia, bladder inflammation in adult female Wistar rats (175-250 g) was induced by injecting cyclophosphamide (CYP) intraperitoneally at acute (150 mg/kg; 4 h), intermediate (150 mg/kg; 48 h), and chronic (75 mg/kg; every 3rd day for 10 days) time points. CXCL12, and its receptor, CXCR4, were examined in the whole urinary bladder of control and CYP-treated rats using enzyme-linked immunosorbent assays (ELISAs), quantitative PCR (qRT-PCR), and immunostaining techniques. ELISAs, qRT-PCR, and immunostaining experiments revealed a significant (P < or = 0.01) increase in CXCL12 and CXCR4 expression in the whole urinary bladder, and particularly in the urothelium, with CYP treatment. The functional role of CXCL12/CXCR4 signaling in micturition was evaluated using conscious cystometry with continuous instillation of saline and CXCR4 receptor antagonist (AMD-3100; 5 microM) administration in control and CYP (48 h)-treated rats. Receptor blockade of CXCR4 using AMD-3100 increased bladder capacity in control (no CYP) rats and reduced CYP-induced bladder hyperexcitability as demonstrated by significant (P < or = 0.01) increases in intercontraction interval, bladder capacity, and void volume. These results suggest a role for CXCL12/CXCR4 signaling in both normal micturition and with bladder hyperreflexia following bladder inflammation.

Beta-arrestin- but not G protein-mediated signaling by the "decoy" receptor CXCR7

Ubiquitously expressed seven-transmembrane receptors (7TMRs) classically signal through heterotrimeric G proteins and are commonly referred to as G protein-coupled receptors. It is now recognized that 7TMRs also signal through beta-arrestins, which act as versatile adapters controlling receptor signaling, desensitization, and trafficking. Most endogenous receptors appear to signal in a balanced fashion using both beta-arrestin and G protein-mediated pathways. Some 7TMRs are thought to be nonsignaling "decoys" because of their inability to activate typical G protein signaling pathways; it has been proposed that these receptors act to scavenge ligands or function as coreceptors. Here we demonstrate that ligand binding to the decoy receptor CXCR7 does not result in activation of signaling pathways typical of G proteins but does activate MAP kinases through beta-arrestins in transiently transfected cells. Furthermore, we observe that vascular smooth muscle cells that endogenously express CXCR7 migrate to its ligand interferon-inducible T-cell alpha chemoattractant (ITAC), an effect that is significantly attenuated by treatment with either a CXCR7 antagonist or beta-arrestin depletion by siRNA. This example of an endogenous "beta-arrestin-biased" 7TMR that signals through beta-arrestin in the absence of G protein activation demonstrates that some 7TMRs encoded in the genome have evolved to signal through beta-arrestin exclusively and suggests that other receptors that are currently thought to be orphans or decoys may also signal through such nonclassical pathways.

An antiestrogen-binding protein in human tissues

Although nonsteroidal antiestrogens of the triphenylethylene type are generally considered to act through the estrogen receptor, some observations suggest that estrogen target tissues may also contain a binding protein specific for these compounds. The data so far reported, however, are also consistent with ligand-induced changes in conformation or in the state of aggregation of the estrogen receptor. The studies reported here demonstrate the existence of a protein in human myometrial cytosol which binds 1-[4-(2-dimethylaminoethoxy)phenyl]1,2-diphenylbut-1(Z)-ene ([3H]tamoxifen) with high affinity (Kd = 2.3 X 10(-9) M). This protein exhibits striking specificity for nonsteroidal antiestrogens. Estradiol competes weakly for bound [3H]tamoxifen, while other estrogens and nonestrogenic steroid hormones do not compete at all. Sedimentation analysis and molecular sieve chromatography indicate that the antiestrogen-binding protein is a larger species than the estrogen receptor and elutes from DEAE-Sephacel at a lower KCl concentration (0.03 M) than the estrogen receptor (0.15 M). Differential thermal stability of the estrogen receptor and the antiestrogen-binding protein was demonstrable in the absence of added ligand. The antiestrogen-binding protein was ubiquitous, being present in many tissues where estrogen receptor was undetectable. These findings support the separate existence of an antiestrogen-binding protein.