Stromal cell-Derived Factor 1 (SDF1) is the natural ligand of CXCR4, the coreceptor of HIV-1 X4 viruses. This study investigated the role of the single nucleotide polymorphism (SNP) rs1801157 (NM_000609.5:c.*519G>A) of the SDF1 gene in the natural history of mother-to-child transmission of HIV-1 and disease progression of HIV-1-infected children. The study was conducted in 428 children born to HIV-1-seropositive mothers, who had not undergone antiretroviral therapy (ART) during pregnancy, and in 120 HIV-1-infected children for whom the end-point was the onset of AIDS or the initiation of ART; 16 children developed early AIDS (<24 months of life), 13 from 24 to 84 months of age, and 14 had late AIDS (>84 months). The rs1801157 SNP was not associated with risk of perinatal infection in any genetic models tested. By contrast, this SNP influenced disease progression in a time-dependent manner. rs1801157 GA heterozygous children had a higher risk of late AIDS (HR = 6.3, 95%CI 1.9–20.7, p = 0.002) than children with the rs1801157 GG genotype. Children were studied for viral coreceptor usage at birth, after 84 months of age and/or at AIDS onset. While R5 viruses using CCR5 coreceptor were predominant at birth (94%) and at early AIDS (85%), viruses using CXCR4 coreceptor emerged during the course of infection and were detected in 49% of children older than 84 months and in 62% of late AIDS. The rs1801157 SNP did not influence the emergence of R5X4 viruses, but children with the rs1801157 GA genotype and R5X4 viruses were at significantly higher risk of late AIDS than children with rs1801157 GG genotype (OR = 8.0, 95% CI 1.2–52.2, p = 0.029). Our results indicate that the rs1801157 SNP does not influence perinatal infection, but impacts disease progression. This effect is time-dependent and linked to the coreceptor-usage of viral variants that undergo evolution during the course of HIV-1 infection.
The CXC-chemokine receptor 4 (CXCR4) is a G protein-coupled receptor for stromal cell-derived factor-1 (SDF-1/CXCL12). SDF-1 induced CXCR4 signaling is indispensable for embryonic development and crucial for immune cell homing and has been implicated in metastasis of numerous types of cancer. CXCR4 also serves as the major coreceptor for cellular entry of T-cell line-tropic (X4) HIV-1 strains. Tyrosine residues in the N-terminal tail of CXCR4, which are post-translationally sulfated, are implicated in the high affinity binding of SDF-1 to CXCR4. However, the specific roles of three potential tyrosine sulfation sites are not well understood. We investigated the pattern and sequence of CXCR4 sulfation by using recombinant human tyrosylprotein sulfotransferases TPST-1 and TPST-2 to modify a peptide that corresponds to amino acids 1-38 of the receptor (CXCR4 1-38). We analyzed the reaction products with a combination of reversed-phase HPLC, proteolytic cleavage, and mass spectrometry. We found that CXCR4 1-38 is sulfated efficiently by both TPST enzymes, leading to a final product with three sulfotyrosine residues. Sulfates were added stepwise to the peptide producing specific intermediates with one or two sulfotyrosines. The pattern of sulfation in these intermediates indicates that with both enzymes Tyr-21 is sulfated first, followed by Tyr-12 or Tyr-7. Using heteronuclear NMR spectroscopy, we demonstrated that the SDF-1 binding affinity of CXCR4 1-38 increases with the number of sulfotyrosines present, which suggests a potential physiological role for sulfation of all three sites in the CXCR4 N-terminus. These results provide a structural basis for understanding the role of post-translational tyrosine sulfation in SDF-1 induced CXCR4 signaling.
The interaction of the chemokine stromal cell-derived factor 1 (SDF-1) with its receptor CXCR4 is vital for cell trafficking during development, is capable of inhibiting human immunodeficiency virus type 1 (HIV-1) utilization of CXCR4 as a coreceptor, and has been implicated in delaying disease progression to AIDS in vivo. Because of the importance of this chemokine-chemokine receptor pair to both development and disease, we investigated the molecular basis of the interaction between CXCR4 and its ligands SDF-1 and HIV-1 envelope. Using CXCR4 chimeras and mutants, we determined that SDF-1 requires the CXCR4 amino terminus for binding and activates downstream signaling pathways by interacting with the second extracellular loop of CXCR4. SDF-1-mediated activation of CXCR4 required the Asp-Arg-Tyr motif in the second intracellular loop of CXCR4, was pertussis toxin sensitive, and did not require the distal C-terminal tail of CXCR4. Several CXCR4 mutants that were not capable of binding SDF-1 or signaling still supported HIV-1 infection, indicating that the ability of CXCR4 to function as a coreceptor is independent of its ability to signal. Direct binding studies using the X4 gp120s HXB, BH8, and MN demonstrated the ability of HIV-1 gp120 to bind directly and specifically to the chemokine receptor CXCR4 in a CD4-dependent manner, using a conformationally complex structure on CXCR4. Several CXCR4 variants that did not support binding of soluble gp120 could still function as viral coreceptors, indicating that detectable binding of monomeric gp120 is not always predictive of coreceptor function.
Stromal cell-derived factor-1 (SDF1) and its receptor CXC chemokine receptor 4 (CXCR4) play a critical role in progenitor cell homing, mobilization and differentiation. It would be interesting to assess the predictive value of SDF-1alpha level for EPC number, and to ascertain whether there is a relationship between SDF1 gene variation, plasma SDF-1alpha level, and the number and function of circulating EPCs. We also tested whether EPC number and function was related to CXCR4 gene variation.
Methodology and Principal Findings
We genotyped a cohort of individuals who participated in the Bruneck Study for single nucleotide polymorphisms (SNPs) in the SDF1 and CXCR4 genes, and measured blood SDF1α level as well as EPC number and function. SDF1α levels were correlated with age, gender, alcohol consumption, circulating reticulocyte numbers, and concentrations of matrix metalloproteinase-9, C-reactive protein, cystatin C, fibrinogen and homocytein. In blood samples taken in 2005, EPC number was inversely associated with SDF1α level (p<0.001). EPC number in 2005 was also inversely associated with SDF1α level in 2000 (p = 0.009), suggesting a predictive value of plasma SDF1α level for EPC number. There was an association between the SDF1 gene rs2297630 SNP A/A genotype, increased SDF1α level (p = 0.002) and lower EPC number (p = 0.006).
Our data indicate that a SDF1 gene variation (rs2297630) has an influence on SDF1α level and circulating EPC number, and that plasma SDF1α level is a predictor of EPC number.
Genetic polymorphisms in chemokine and chemokine receptor genes influence susceptibility to human immunodeficiency virus type 1 (HIV-1) infection and disease progression, but little is known regarding the association between these allelic variations and the ability of the host to transmit virus. In this study, we show that the maternal heterozygous SDF1 genotype (SDF1 3′A/wt) is associated with perinatal transmission of HIV-1 (risk ratio [RR], 1.8; 95% confidence interval [CI], 1.0 to 3.3) and particularly postnatal breastmilk transmission (RR, 3.1; 95% CI, 1.1 to 8.6). In contrast, the infant SDF1 genotype had no effect on mother-to-infant transmission. These data suggest that SDF1, which is a ligand for the T-tropic HIV-1 coreceptor CXCR4, may affect the ability of a mother to transmit the virus to her infant. This suggests that a genetic polymorphism in a gene encoding a chemokine receptor ligand may be associated with increased infectivity of the index case and highlights the importance of considering transmission as well as clinical outcome in designing chemokine-based therapies for HIV-1.
CXCR4 is a chemokine receptor used by some strains of HIV-1 as an entry coreceptor in association with cell surface CD4 on human cells. In human immunodeficiency virus type 1 (HIV-1)-infected individuals, the appearance of viral isolates with a tropism for CXCR4 (T tropic) has been correlated with late disease progression. The presumed natural ligands for CXCR4 are SDF-1α and SDF-1β, which are proposed to play a role in blocking T-tropic HIV-1 cell entry. Here, we demonstrate that addition of an N-terminal methionine residue to SDF-1β (Met-SDF-1β) results in a dramatically enhanced functional activity compared to that of native SDF-1β. Equivalent concentrations of Met-SDF-1β are markedly more inhibitory for T-tropic HIV-1 replication than SDF-1β. A comparison of the biological activities of these two forms of SDF-1β reveals that Met-SDF-1β induces a more pronounced intracellular calcium flux yet binds with slightly lower affinity to CXCR4 than SDF-1β. Down-modulation of CXCR4 is similar after exposure of cells to either chemokine form for 2 h. However, after a 48-h incubation, the surface expression of CXCR4 is much lower for cells treated with Met-SDF-1β. The enhanced blocking of T-tropic HIV-1 by Met-SDF-1β appears to be related to prolonged CXCR4 down-modulation.
An interesting finding in the epidemiology of human immunodeficiency virus (HIV) infection is that certain mutations in genes coding for chemokines, and their receptors and ligands, may confer resistance or susceptibility to HIV-1 infection and acquired immunodeficiency syndrome (AIDS) progression. The mutation most frequently studied is stromal cell-derived factor (SDF)1-3′A, a single nucleotide polymorphism in the 3′ untranslated region at the 801 position of the SDF1 gene, which seems to be associated with susceptibility or resistance to diseases, including AIDS. We examined the frequency of the above polymorphisms in the Tunisian population, and evaluated their contribution to a protective genetic background against HIV infection and progression.
Methods and materials
One hundred forty blood samples from HIV-infected patients from the Cellular Immunology Research Laboratory at the National Blood Transfusion Center were compared with those of 164 random blood donors from the same center. Genotyping was initially performed by polymerase chain reaction (PCR) analysis. SDF1 PCR product genomic regions were further subjected to restriction fragment length polymorphism analysis for genotype determination. Screening for the SDF1 polymorphism in the HIV-infected population yielded 56 heterozygous (40%), 52 mutation homozygous (37.1%), and 32 wild-type homozygous (22.8%) subjects. In contrast, in our healthy population, we found 70/164 heterozygous (42.6%), nine mutation homozygous (5.4%), and 85 wild-type homozygous (51.8%) subjects. The allele frequencies in the HIV-infected and healthy populations were f(SD1 3′A) = 57.1%, f(SDF1) = 42.8%, f(SDF1 3′A) = 26.8%, and f(SDF1) = 73.1%, respectively. The allelic and genotypic frequencies of the SDF1 3′A in our population show significantly higher distribution profiles compared with those observed in other Caucasian, European, and African American populations. Our results were examined by χ2 test and appear to confirm an association between polymorphism and AIDS progression. A higher odds ratio (>1) was found for the SDF1-3′A allele than for the wild-type allele (<1).
This result seems to confirm that the SDF1-3′A allele is associated with acceleration and progression from HIV infection to AIDS in the Tunisian population.
human immunodeficiency virus; SDF1 polymorphism; Tunisia
The Chemokine receptor CXCR4 and its ligand stromal derived factor-1 (SDF-1/CXCL12) are important players involved in cross-talk between leukemia cells and the bone marrow (BM) microenvironment. CXCR4 expression is associated with poor prognosis in AML patients with and without the mutated FLT3 gene.
CXCL12 which is constrictively secreted from the BM stroma and AML cells is critical for the survival and retention of AML cells within the BM. In vitro, CXCR4 antagonists were shown to inhibit the migration of AML cells in response to CXCL12. In addition, such antagonists were shown to inhibit the survival and colony forming potential of AML cells and abrogate the protective effects of stromal cells on chemotherapy-induced apoptosis in AML cells. In vivo, using immune deficient mouse models, CXCR4 antagonists were found to induce the mobilization of AML cells and progenitor cells into the circulation and enhance anti leukemic effects of chemotherapy. The hypothesis that CXCL12/CXCR4 interactions contribute to the resistance of AML cells to signal transduction inhibitor- and chemotherapy-induced apoptosis is currently being tested in a series of Phase I/II studies in humans.
CXCR4; CXCL12; AML; Bone marrow; Microenvironment.
Several members of the chemokine receptor family have been shown to function in association with CD4 to permit human immunodeficiency virus type 1 (HIV-1) entry and infection. The CXC chemokine receptor CXCR4/fusin is a receptor for pre–B cell growth stimulating factor (PBSF)/stromal cell–derived factor 1 (SDF-1) and serves as a coreceptor for the entry of T cell line–tropic HIV-1 strains. Thus, the development of CXCR4 antagonists or agonists may be useful in the treatment of HIV-1 infection. T22 ([Tyr5,12,Lys7]-polyphemusin II) is a synthesized peptide that consists of 18 amino acid residues and an analogue of polyphemusin II isolated from the hemocyte debris of American horseshoe crabs (Limulus polyphemus). T22 was found to specifically inhibit the ability of T cell line–tropic HIV-1 to induce cell fusion and infect the cell lines transfected with CXCR4 and CD4 or peripheral blood mononuclear cells. In addition, T22 inhibited Ca2+ mobilization induced by pre–B cell growth stimulating factor (PBSF)/SDF-1 stimulation through CXCR4. Thus, T22 is a small molecule CXCR4 inhibitor that blocks T cell line–tropic HIV-1 entry into target cells.
The CXC chemokine ligand 12 (CXCL12)/stromal cell-derived factor-1 (SDF-1) and CXC receptor 4 (CXCR4) axis is involved in human colorectal cancer (CRC) carcinogenesis and can promote the progression of CRC. Interaction between CRC cells and endothelium is a key event in tumor progression. The aim of this study was to investigate the effect of SDF-1 on the adhesion of CRC cells.
Human CRC DLD-1 cells were used to study the effect of SDF-1 on intercellular adhesion molecule-1 (ICAM-1) expression and cell adhesion to endothelium.
SDF-1 treatment induced adhesion of DLD-1 cells to the endothelium and increased the expression level of the ICAM-1. Inhibition of ICAM-1 by small interfering RNA (siRNA) and neutralizing antibody inhibited SDF-1-induced cell adhesion. By using specific inhibitors and short hairpin RNA (shRNA), we demonstrated that the activation of ERK, JNK and p38 pathways is critical for SDF-1-induced ICAM-1 expression and cell adhesion. Promoter activity and transcription factor ELISA assays showed that SDF-1 increased Sp1-, C/EBP-β- and NF-κB-DNA binding activities in DLD-1 cells. Inhibition of Sp1, C/EBP-β and NF-κB activations by specific siRNA blocked the SDF-1-induced ICAM-1 promoter activity and expression. The effect of SDF-1 on cell adhesion was mediated by the CXCR4.
Our findings support the hypothesis that ICAM-1 up-regulation stimulated by SDF-1 may play an active role in CRC cell adhesion.
Colorectal cancer; Stromal cell-derived factor-1; Intercellular adhesion molecule-1; Cell adhesion; Transcriptional regulation
The entry of human immunodeficiency virus type 1 (HIV-1) into the cell is initiated by the interaction of the viral surface envelope protein with two cell surface components of the target cell, CD4 and a chemokine coreceptor, usually CXCR4 or CCR5. The natural ligand of CXCR4 is stromal cell-derived factor 1α (SDF-1α). Whereas the overlap between HIV-1 and SDF-1α functional sites on the extracellular domains of CXCR4 has been well documented, it has yet to be determined whether there are sites in the transmembrane (TM) helices of CXCR4 important for HIV-1 and/or SDF-1α functions, and if such sites do exist, whether they are overlapping or distinctive for the separate functions of CXCR4. For this study, by employing alanine-scanning mutagenesis, 125I-SDF-1α competition binding, Ca2+ mobilization, and cell-cell fusion assays, we found that the mutation of many CXCR4 TM residues, including Tyr45, His79, Asp97, Pro163, Trp252, Tyr255, Asp262, Glu288, His294, and Asn298, could selectively decrease HIV-1-mediated cell fusion but not the binding activity of SDF-1α. Phe87 and Phe292, which were involved in SDF-1α binding, did not play a significant role in the coreceptor activity of CXCR4, further demonstrating the disconnection between physiological and pathological activities of CXCR4 TM domains. Our data also show that four mutations of the second extracellular loop, D182A, D187A, F189A, and P191A, could reduce HIV-1 entry without impairing either ligand binding or signaling. Taken together, our first detailed characterization of the different functional roles of CXCR4 TM domains may suggest a mechanistic basis for the discovery of new selective anti-HIV agents.
Ligation of CCR5 by the CC chemokines RANTES, MIP-1α or MIP-1β, and of CXCR4 by the CXC chemokine SDF-1α, profoundly inhibits the replication of HIV strains that use these coreceptors for entry into CD4+ T lymphocytes. The mechanism of entry inhibition is not known. We found a rapid and extensive downregulation of CXCR4 by SDF-1α and of CCR5 by RANTES or the antagonist RANTES(9-68). Confocal laser scanning microscopy showed that CCR5 and CXCR4, after binding to their ligands, are internalized into vesicles that qualify as early endosomes as indicated by colocalization with transferrin receptors. Internalization was not affected by treatment with Bordetella pertussis toxin, showing that it is independent of signaling via Gi-proteins. Removal of SDF-1α led to rapid, but incomplete surface reexpression of CXCR4, a process that was not inhibited by cycloheximide, suggesting that the coreceptor is recycling from the internalization pool. Deletion of the COOH-terminal, cytoplasmic domain of CXCR4 did not affect HIV entry, but prevented SDF-1α–induced receptor downregulation and decreased the potency of SDF-1α as inhibitor of HIV replication. Our results indicate that the ability of the coreceptor to internalize is not required for HIV entry, but contributes to the HIV suppressive effect of CXC and CC chemokines.
Chemokines and their receptors are potential therapeutic targets in rheumatoid arthritis (RA). Among these, several studies suggested the involvement of CXC chemokine 4 (CXCR4) and its ligand CXC ligand 12 (SDF-1) in RA pathogenesis. However, the role of these molecules in T-cell function is not known completely because of embryonic lethality of Cxcr4- and Cxcl12-deficient mice. In this report, we generated T cell-specific Cxcr4-deficient mice and showed that the CXCR4 in T cells is important for the development of collagen-induced arthritis (CIA).
T cell-specific Cxcr4-deficient mice were generated by using the Cre-loxP system. Mice harboring loxP sites flanking exon 2 of the Cxcr4gene (Cxcr4flox/flox) were generated by homologous recombination and crossed with Cre transgenic mice expressing Cre recombinase under the control of Lck promoter (Cxcr4+/+/Lck-Cremice) to generate T cell-specific Cxcr4-deficient mice (Cxcr4flox/flox/Lck-Cre mice). CIA was induced by immunization with chicken type II collagen and Complete Freund's Adjuvant (CFA).
The incidence, but not the severity, of CIA was significantly reduced in Cxcr4flox/flox/Lck-Cre mice compared with Cxcr4+/+/Lck-Cre mice. We found that the expression of CXCR4 was enhanced in activated T cells, and the migration of Cxcr4-deficient T cells toward SDF-1 was severely impaired. However, antibody production, cellular proliferative response, and cytokine production on treatment with type II collagen (IIC) were normal in these knockout mice, suggesting that CXCR4 is not involved in T-helper functions. Interestingly, the proportion of CXCR4-expressing T cells was much increased in affected joints compared with that in draining lymph nodes in CIA-induced mice, and distribution of Cxcr4flox/flox/Lck-Cre mouse-derived T cells into affected joints was suppressed compared with that in Cxcr4+/+/Lck-Cre T cells.
These results indicate that CXCR4 expression in T cells is important for the development of CIA, by recruiting activated T cells toward inflammatory sites, and suggest that CXCR4 is a good target for the treatment of RA in humans.
The WHIM syndrome features susceptibility to human Papillomavirus infection-induced warts and carcinomas, hypogammaglobulinemia, recurrent bacterial infections, B and T-cell lymphopenia, and neutropenia associated with retention of senescent neutrophils in the bone marrow (i.e. myelokathexis). This rare disorder is mostly linked to inherited heterozygous autosomal dominant mutations in the gene encoding CXCR4, a G protein coupled receptor with a unique ligand, the chemokine CXCL12/SDF-1. Some individuals who have full clinical forms of the syndrome carry a wild type CXCR4 gene. In spite of this genetic heterogeneity, leukocytes from WHIM patients share in common dysfunctions of the CXCR4-mediated signaling pathway upon exposure to CXCL12. Dysfunctions are characterized by impaired desensitization and receptor internalization, which are associated with enhanced responses to the chemokine. Our increasing understanding of the mechanisms that account for the aberrant CXCL12/CXCR4-mediated responses is beginning to provide insight into the pathogenesis of the disorder. As a result we can expect to identify markers of the WHIM syndrome, as well as other disorders with WHIM-like features that are associated with dysfunctions of the CXCL12/CXCR4 axis.
The chemokine stromal cell–derived factor (SDF-1; also known as chemokine ligand 12 [CXCL12]) regulates many essential biological processes, including cardiac and neuronal development, stem cell motility, neovascularization, angiogenesis, apoptosis, and tumorigenesis. It is generally believed that SDF-1 mediates these many disparate processes via a single cell surface receptor known as chemokine receptor 4 (CXCR4). This paper characterizes an alternate receptor, CXCR7, which binds with high affinity to SDF-1 and to a second chemokine, interferon-inducible T cell α chemoattractant (I-TAC; also known as CXCL11). Membrane-associated CXCR7 is expressed on many tumor cell lines, on activated endothelial cells, and on fetal liver cells, but on few other cell types. Unlike many other chemokine receptors, ligand activation of CXCR7 does not cause Ca2+ mobilization or cell migration. However, expression of CXCR7 provides cells with a growth and survival advantage and increased adhesion properties. Consistent with a role for CXCR7 in cell survival and adhesion, a specific, high affinity small molecule antagonist to CXCR7 impedes in vivo tumor growth in animal models, validating this new receptor as a target for development of novel cancer therapeutics.
Seven transmembrane (7TM) G-protein-coupled receptor (GPCR) families are important targets for drug discovery, and specific antagonists for GPCR can accelerate research in the field of medicinal chemistry. The chemokine receptor CXCR4 is a GPCR that possesses a unique ligand CXCL12/stromal cell-derived factor-1 (SDF-1). The interaction between CXCL12 and CXCR4 is essential for the migration of progenitor cells during embryonic development of the cardiovascular, hemopoietic and central nervous systems, and also involved in several intractable disease processes, including HIV infection, cancer cell metastasis, progression of acute and chronic leukemias, rheumatoid arthritis and pulmonary fibrosis. Thus, CXCR4 may be an important therapeutic target in all of these diseases, and various CXCR4 antagonists have been proposed as potential drugs. Fourteen-mer peptides, T140 and its analogs, and downsized cyclic pentapeptides have been developed by us as potent CXCR4 antagonists. This article describes the development of a number of specific CXCR4 antagonists in our laboratory, including downsizing.
cancer metastasis; chemokine receptor; CXCR4 antagonist; downsizing; HIV infection; rheumatoid arthritis
Human herpesvirus 7 (HHV-7) is a T-lymphotropic virus which utilizes the CD4 receptor as its main receptor to enter the target cells. Hence, HHV-7 can interfere with human immunodeficiency virus type 1 (HIV-1) infection in CD4+ T cells. It was recently suggested that the CXC chemokine receptor 4 (CXCR4), which was found to be a crucial coreceptor for T-tropic HIV-1 strains, may also play a role in the HHV-7 infection process. However, the results presented here demonstrate that CXCR4 is not involved in HHV-7 infection. The natural ligand of CXCR4, SDF-1α, was not able to inhibit HHV-7 infection in SupT1 cells or in CD8+ T-cell-depleted peripheral blood mononuclear cells. Also, AMD3100, a specific CXCR4 antagonist with potent antiviral activity against T-tropic HIV strains (50% inhibitory concentration [IC50], 1 to 10 ng/ml), completely failed to inhibit HHV-7 infection (IC50, >250 μg/ml). Thus, two different agents known to specifically interact with CXCR4 were not able to inhibit HHV-7 infection. Other T-lymphoid cell lines, expressing both CD4 and CXCR4 (e.g., HUT-78 and MT-4) could not be infected by HHV-7. In addition, the CD4-transfected cell lines HOS.CD4 and U87.CD4 and the CD4/CXCR4 double-transfected cell lines HOS.CD4.CXCR4 and U87.CD4.CXCR4 were not infectable with HHV-7. Also, we found no down-regulation of surface-bound or intracellular CXCR4 in HHV-7-infected CD4+ T cells. As compared to uninfected SupT1 cells, stromal cell-derived factor 1α (SDF-1α)/CXCR4-mediated intracellular calcium flux was unchanged in SupT1 cells that were acutely or persistently infected with HHV-7. All these data argue against CXCR4 as a receptor involved in the HHV-7 infection process.
The chemokine receptor CXCR4 plays an important role as the receptor for the normal physiological function of stromal cell-derived factor 1α (SDF-1α) and the coreceptor for the entry of human immunodeficiency virus type 1 (HIV-1) into the cell. In a recent work (S. Tian et al., J. Virol. 79:12667-12673, 2005), we found that many residues throughout CXCR4 transmembrane (TM) and extracellular loop 2 domains are specifically involved in interaction with HIV-1 gp120, as most of these sites did not play a role in either SDF-1α binding or signaling. These results provided direct experimental evidence for the distinct functional sites on CXCR4 for HIV-1 and the normal ligand SDF-1α. To further understand the CXCR4-ligand interaction and to develop new CXCR4 inhibitors to block HIV-1 entry, we have recently generated a new family of unnatural chemokines, termed synthetically and modularly modified (SMM) chemokines, derived from the native sequence of SDF-1α or viral macrophage inflammatory protein II (vMIP-II). These SMM chemokines contain various de novo-designed sequence replacements and substitutions by d-amino acids and display more enhanced CXCR4 selectivity, binding affinities, and/or anti-HIV activities than natural chemokines. Using these novel CXCR4-targeting SMM chemokines as receptor probes, we conducted ligand binding site mapping experiments on a panel of site-directed mutants of CXCR4. Here, we provide the first experimental evidence demonstrating that SMM chemokines interact with many residues on CXCR4 TM and extracellular domains that are important for HIV-1 entry, but not SDF-1α binding or signaling. The preferential overlapping in the CXCR4 binding residues of SMM chemokines with HIV-1 over SDF-1α illustrates a mechanism for the potent HIV-1 inhibition by these SMM chemokines. The discovery of distinct functional sites or conformational states influenced by these receptor sites mediating different functions of the natural ligand versus the viral or synthetic ligands has important implications for drug discovery, since the sites shared by SMM chemokines and HIV-1 but not by SDF-1α can be targeted for the development of selective HIV-1 inhibitors devoid of interference with normal SDF-1α function.
The chemokine receptor CXCR4 is required, together with CD4, for entry by some isolates of HIV-1, particularly those that emerge late in infection. The use of CXCR4 by these viruses likely has profound effects on viral host range and correlates with the evolution of immunodeficiency. Stromal cell-derived factor-1 (SDF-1), the ligand for CXCR4, can inhibit infection by CXCR4-dependent viruses. To understand the mechanism of this inhibition, we used a monoclonal antibody that is specific for CXCR4 to analyze the effects of phorbol esters and SDF-1 on surface expression of CXCR4. On human T cell lines SupT1 and BC7, CXCR4 undergoes slow constitutive internalization (1.0% of the cell surface pool/min). Addition of phorbol esters increased this endocytosis rate >6-fold and reduced cell surface CXCR4 expression by 60 to 90% over 120 min. CXCR4 was internalized through coated pits and coated vesicles and subsequently localized in endosomal compartments from where it could recycle to the cell surface after removal of the phorbol ester. SDF-1 also induced the rapid down modulation (half time ∼5 min) of CXCR4. Using mink lung epithelial cells expressing CXCR4 and a COOH-terminal deletion mutant of CXCR4, we found that an intact cytoplasmic COOH-terminal domain was required for both PMA and ligand-induced CXCR4 endocytosis. However, experiments using inhibitors of protein kinase C indicated that SDF-1 and phorbol esters trigger down modulation through different cellular mechanisms.
SDF-1 inhibited HIV-1 infection of mink cells expressing CD4 and CXCR4. The inhibition of infection was less efficient for CXCR4 lacking the COOH-terminal domain, suggesting at least in part that SDF-1 inhibition of virus infection was mediated through ligand-induced internalization of CXCR4. Significantly, ligand induced internalization of CXCR4 but not CD4, suggesting that CXCR4 and CD4 do not normally physically interact on the cell surface. Together these studies indicate that endocytosis can regulate the cell-surface expression of CXCR4 and that SDF-1–mediated down regulation of cell-surface coreceptor expression contributes to chemokine-mediated inhibition of HIV infection.
Like human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV), HIV-2 requires a coreceptor in addition to CD4 for entry into cells. HIV and SIV coreceptor molecules belong to a family of seven-transmembrane-domain G-protein-coupled receptors. Here we show that primary HIV-2 isolates can use a broad range of coreceptor molecules, including CCR1, CCR2b, CCR3, CCR4, CCR5, and CXCR4. Despite broad coreceptor use, the chemokine ligand SDF-1 substantially blocked HIV-2 infectivity of peripheral blood mononuclear cells, indicating that its receptor, CXCR4, was the predominant coreceptor for infection of these cells. However, expression of CXCR4 together with CD4 on some cell types did not confer susceptibility to infection by all CXCR4-using virus isolates. These data therefore indicate that another factor(s) influences the ability of HIV-2 to replicate in human cell types that express the appropriate receptors for virus entry.
Stromal cell-derived factor (SDF)-1 (CXC chemokine ligand-12) is a member of the CXC subfamily of chemokines, which, through its cognate receptor (CXC chemokine receptor [CXCR]4), plays an important role in chemotaxis of cancer cells and in tumour metastasis. We conducted the present study to evaluate the effect of SDF-1 on the invasiveness and migration of breast cancer cells, and we analyzed the expression of SDF-1 and its relation to clinicopathological features and clinical outcomes in human breast cancer.
Expression of SDF-1 mRNA in breast cancer, endothelial (HECV) and fibroblast (MRC5) cell lines and in human breast tissues were studied using RT-PCR. MDA-MB-231 cells were transfected with a SDF-1 expression vector, and their invasiveness and migration was tested in vitro. In addition, the expression of SDF-1 was investigated using immunohistochemistry and quantitative RT-PCR in samples of normal human mammary tissue (n = 32) and mammary tumour (n = 120).
SDF-1 expression was identified in MRC5, MDA-MB-435s and MDA-MB-436 cell lines, but CXCR4 expression was detected in all cell lines and breast tissues. An autocrine loop was created following transfection of MDA-MB-231 (which was CXCR4 positive and SDF-1 negative) with a mammalian expression cassette encoding SDF-1 (MDA-MB-231SDF1+/+) or with control plasmid pcDNA4/GFP (MDA-MB-231+/-). MDA-MB-231SDF1+/+ cells exhibited significantly greater invasion and migration potential (in transfected cells versus in wild type and empty MDA-MB-231+/-; P < 0.01). In mammary tissues SDF-1 staining was primarily seen in stromal cells and weakly in mammary epithelial cells. Significantly higher levels of SDF-1 were seen in node-positive than in node-negative tumours (P = 0.05), in tumours that metastasized (P = 0.05), and tumours from patients who died (P = 0.03) than in tumours from patients who were disease free. It was most notable that levels of SDF-1 correlated significantly with overall survival (P = 0.001) and incidence-free survival (P = 0.035).
SDF-1 can increase the invasiveness and migration of breast cancer cells. Its levels correlated with node involvement and long-term survival in patients with breast cancer. SDF-1 may therefore have potential value in assessing clinical outcomes of patients with breast cancer.
The NL4.3 T-cell-line-tropic human immunodeficiency virus type 1 strain is sensitive to the CXC chemokine stromal cell-derived factor 1α (SDF-1α), the natural ligand for CXC chemokine receptor 4 (CXCR4); the 50% inhibitory concentration (IC50) in MT-4 cells is 130 ng/ml. We generated resistant virus through passaging of the virus in the presence of increasing concentrations of SDF-1α. After 24 passages, the virus was no longer sensitive to SDF-1α (SDF-1αres virus) (IC50, >2 μg/ml) and became resistant to SDF-1β (IC50, >2 μg/ml) and to a specific CXCR4 monoclonal antibody (IC50, >20 μg/ml). The SDF-1αres virus was about 10-fold less sensitive than the wild-type virus to the bicyclam AMD3100, a specific CXCR4 antagonist. The SDF-1αres virus contained the following mutations in the gp120 molecule: N106K in the V1 loop; S134N and F145L in the V2 loop; F245I in the C2 loop; K269E, Q278H, I288V, and N293D in the V3 loop; a deletion of 5 amino acids (FNSTW) at positions 364 to 368 in the V4 loop; and R378T in the CD4 binding domain. Replication of the NL4.3 wild-type virus and the SDF-1αres virus was demonstrated in U87 cells that coexpressed CD4 and CXCR4 (U87.CD4.CXCR4) but not in U87.CD4.CCR5 cells. Thus, the resistant virus was not able to switch to the CC chemokine receptor 5 (CCR5) coreceptor (the main coreceptor for macrophage-tropic viruses). The SDF-1αres virus replicated in HOS.CD4 cells expressing CCR1, CCR2b, CCR3, CCR4, CCR5, and CXCR4 but also in HOS.CD4.pBABE cells. However, all HOS transfectant cells expressed a low level of CXCR4. Neither of the two virus strains was able to infect HOS.CXCR4 or HOS.CCR5 transfectants, demonstrating the necessity of the CD4 receptor. The T-cell-line-tropic SDF-1αres virus was thus able to overcome the inhibitory effect of SDF-1α through mutations in gp120 but still needed CXCR4 to enter the cells.
The dissemination of T cell hybridomas to multiple nonhematopoietic tissues is blocked by pertussis toxin, suggesting the involvement of a chemokine. To study whether this chemokine is SDF-1, we employed a strategy proposed previously for gene therapy of AIDS, whereby the SDF-1 receptor CXCR4 (also a coreceptor for HIV) is retained in the endoplasmic reticulum (ER) and fails to reach the cell surface. We transfected SDF-1, carrying an ER retention sequence, into a T cell hybridoma. This altered chemokine is retained in the ER, where it binds CXCR4 and prevents the latter protein from reaching the surface. These cells failed to migrate toward SDF-1 or to invade fibroblast monolayers, although they could still migrate toward thymus and activation-regulated chemokine (TARC) and invade TARC-treated monolayers. Furthermore, the ability of the transfected cells to disseminate to multiple organs upon intravenous injection into mice was abolished. This dissemination reflects the in vivo migration patterns of activated and memory T cells into nonhematopoietic tissues, which is thus likely to depend on CXCR4. Attempts to block CXCR4 function as a therapy for AIDS may affect this migration with consequences for T cell function. Our results also suggest a decisive role for CXCR4 in the dissemination of hematopoietic malignancies expressing this receptor.
Chemokine (C-X-C motif) receptor 4 (CXCR4) is the receptor for chemokine (C-X-C motif) ligand 12 (CXCL12, also known as stromal derived factor-1, Sdf1). CXCR4, a protein consisting 352 amino acids, is known to transduce various signals such as cell differentiation, cell survival, cell proliferation, cell chemotaxis and apoptosis [1, 2]. The expression of CXCR4 is observed in embryonic stem cells, blood cells, haematopoietic stem cells, endothelial cells, angioblasts and smooth muscle cells [3-9]. The CXCL12-CXCR4 signaling pathway has very important roles in the embryonic development. Mutant mice for CXCL12 or CXCR4 genes showed lethality due to defects in neurogenesis, angiogenesis, cardiogenesis, myelopoiesis, lymphopoiesis and germ cell development [10-13]. Recently, we reported that CXCL12-CXCR4 signaling pathway has a crucial role in regional specification of the gut endoderm during early development . Here, we would like to focus on the role of CXCL12-CXCR4 signaling pathway in pancreatic development and summarize recent findings of its role in the induction of the pancreatic progenitor cells.
CXCL12; CXCR4; signaling pathway
The recruitment of selected dendritic cell (DC) subtypes conditions the class of the immune response. Here we show that the migration of human plasmacytoid DCs (pDCs), the blood natural interferon α–producing cells, is induced upon the collective action of inducible and constitutive chemokines. Despite expression of very high levels of CXCR3, pDCs do not respond efficiently to CXCR3 ligands. However, they migrate in response to the constitutive chemokine stromal cell–derived factor 1 (SDF-1)/CXCL12 and CXCR3 ligands synergize with SDF-1/CXCL12 to induce pDC migration. This synergy reflects a sensitizing effect of CXCR3 ligands, which, independently of a gradient and chemoattraction, decrease by 20–50-fold the threshold of sensitivity to SDF-1/CXCL12. Thus, the ability of the constitutive chemokine SDF-1/CXCL12 to induce pDC recruitment might be controlled by CXCR3 ligands released during inflammation such as in virus infection. SDF-1/CXCL12 and the CXCR3 ligands Mig/CXCL9 and ITAC/CXCL1 display adjacent expression both in secondary lymphoid organs and in inflamed epithelium from virus-induced pathologic lesions. Because pDCs express both the lymph node homing molecule l-selectin and the cutaneous homing molecule cutaneous lymphocyte antigen, the cooperation between inducible CXCR3 ligands and constitutive SDF-1/CXCL12 may regulate recruitment of pDCs either in lymph nodes or at peripheral sites of inflammation.
dendritic cells; chemokines; migration; regulation; virus