T134, a small-molecule CXCR4 inhibitor, has no cross-drug resistance with AMD3100, a CXCR4 antagonist with a different structure

Antiviral Effects of Animal Toxins: Is There a Way to Drugs?

Viruses infect all types of organisms, causing viral diseases, which are very common in humans. Since viruses use the metabolic pathways of their host cells to replicate, they are difficult to eradicate without affecting the cells. The most effective measures against viral infections are vaccinations and antiviral drugs, which selectively inhibit the viral replication cycle. Both methods have disadvantages, which requires the development of new approaches to the treatment of viral diseases. In the study of animal venoms, it was found that, in addition to toxicity, venoms exhibit other types of biological activity, including an antiviral one, the first mention of which dates back to middle of the last century, but detailed studies of their antiviral activity have been conducted over the past 15 years. The COVID-19 pandemic has reinforced these studies and several compounds with antiviral activity have been identified in venoms. Some of them are very active and can be considered as the basis for antiviral drugs. This review discusses recent antiviral studies, the found compounds with high antiviral activity, and the possible mechanisms of their action. The prospects for using the animal venom components to create antiviral drugs, and the expected problems and possible solutions are also considered.

Microtiter plate-based antibody-competition assay to determine binding affinities and plasma/blood stability of CXCR4 ligands

C-X-C chemokine receptor type 4 (CXCR4) is involved in several intractable disease processes, including HIV infection, cancer cell metastasis, leukemia cell progression, rheumatoid arthritis, asthma and pulmonary fibrosis. Thus, CXCR4 represents a promising drug target and several CXCR4 antagonizing agents are in preclinical or clinical development. Important parameters in drug lead evaluation are determination of binding affinities to the receptor and assessment of their stability and activity in plasma or blood of animals and humans. Here, we designed a microtiter plate-based CXCR4 antibody competition assay that enables to measure inhibitory concentrations (IC₅₀ values) and affinity constants (K_i values) of CXCR4 targeting drugs. The assay is based on the observation that most if not all CXCR4 antagonists compete with binding of the fluorescence-tagged CXCR4 antibody 12G5 to the receptor. We demonstrate that this antibody-competition assay allows a convenient and cheap determination of binding affinities of various CXCR4 antagonists in living cells within just 3 h. Moreover, the assay can be performed in the presence of high concentrations of physiologically relevant body fluids, and thus is a useful readout to evaluate stability (i.e. half-life) of CXCR4 ligands in serum/plasma, and even whole human and mouse blood ex vivo. Thus, this optimized 12G5 antibody-competition assay allows a robust and convenient determination and calculation of various important pharmacological parameters of CXCR4 receptor-drug interaction and may not only foster future drug development but also animal welfare by reducing the number of experimental animals.

Structural and Biological Characterizations of Novel High-Affinity Fluorescent Probes with Overlapped and Distinctive Binding Regions on CXCR4

CXC-type chemokine receptor 4 (CXCR4) is well known as a co-receptor for cellular entry and infection of human immunodeficiency virus type 1 (HIV-1). As an important member of the G protein-coupled receptor (GPCR) family, CXCR4 also mediates a variety of cellular processes and functions, such as cell chemotaxis, proliferation, and calcium signal transductions. Identification and characterization of molecular ligands or probes of CXCR4 have been an intensive area of investigations as such ligands or probes are of significant clinical values for the studies and treatments of HIV-1 infection and other human diseases mediated by the receptor. The crystal structures of CXCR4 in complex with different ligands have revealed two distinctive binding regions or subpockets. Thus, understanding the interactions of diverse ligands with these distinctive CXCR4 binding regions has become vital for elucidating the relationship between binding modes and biological mechanisms of ligand actions. Peptidic CVX15 is the only ligand that has been validated to bind one of these distinctive binding regions (or so called the major subpocket) of CXCR4. Therefore, in this study, we developed an efficient probe system including two high-affinity peptidic fluorescent probes, designated as FITC-CVX15 and FITC-DV1, with the aim of targeting distinctive CXCR4 subpockets. We conducted rational design and chemical characterization of the two CXCR4-specific probes and examined their application in biological experiments including competitive binding assays, flow cytometry analysis, and confocal imaging. Especially these two probes were applied in parallel CXCR4 competitive binding assays to detect and analyze potential binding modes of diverse CXCR4 ligands, together with molecular docking and simulations. Our results have indicated that these peptidic fluorescent probe systems provide novel ligand detecting tools, as well as present a new approach for analyzing distinctive binding modes of diverse CXCR4 ligands.

Involvement of CXCR4 in Normal and Abnormal Development

CXC motif chemokine receptor type 4 (CXCR4) is associated with normal and abnormal development, including oncogenesis. The ligand of CXCR4 is stromal cell-derived factor (SDF), also known as CXC motif ligand (CXCL) 12. Through the SDF-1/CXCR4 axis, both homing and migration of hematopoietic (stem) cells are regulated through niches in the bone marrow. Outside of the bone marrow, however, SDF-1 can recruit CXCR4-positive cells from the bone marrow. SDF/CXCR4 has been implicated in the maintenance and/or differentiation of stemness, and tissue-derived stem cells can be associated with SDF-1 and CXCR4 activity. CXCR4 plays a role in multiple pathways involved in carcinogenesis and other pathologies. Here, we summarize reports detailing the functions of CXCR4. We address the molecular signature of CXCR4 and how this molecule and cells expressing it are involved in either normal (maintaining stemness or inducing differentiation) or abnormal (developing cancer and other pathologies) events. As a constituent of stem cells, the SDF-1/CXCR4 axis influences downstream signal transduction and the cell microenvironment.

Pathogenic Viruses Commonly Present in the Oral Cavity and Relevant Antiviral Compounds Derived from Natural Products

Many viruses, such as human herpesviruses, may be present in the human oral cavity, but most are usually asymptomatic. However, if individuals become immunocompromised by age, illness, or as a side effect of therapy, these dormant viruses can be activated and produce a variety of pathological changes in the oral mucosa. Unfortunately, available treatments for viral infectious diseases are limited, because (1) there are diseases for which no treatment is available; (2) drug-resistant strains of virus may appear; (3) incomplete eradication of virus may lead to recurrence. Rational design strategies are widely used to optimize the potency and selectivity of drug candidates, but discovery of leads for new antiviral agents, especially leads with novel structures, still relies mostly on large-scale screening programs, and many hits are found among natural products, such as extracts of marine sponges, sea algae, plants, and arthropods. Here, we review representative viruses found in the human oral cavity and their effects, together with relevant antiviral compounds derived from natural products. We also highlight some recent emerging pharmaceutical technologies with potential to deliver antivirals more effectively for disease prevention and therapy.

High expression of CXCR4 and stem cell markers in a monocrotaline and chronic hypoxia-induced rat model of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a severe and fatal clinical syndrome. C-X-C chemokine receptor type 4 (CXCR4) is known to serve a key role in recruiting mesenchymal stem cells (MSCs) from the bone marrow. In the present study, a rat model of PAH induced by 5 weeks of chronic hypoxia and treatment with a single injection of monocrotaline (60 mg/kg) was used to investigate the involvement of CXCR4 in PAH. Successful establishment of the PAH model was confirmed by significant differences between the PAH and control groups in right ventricular systolic pressure, Fulton index, wall thickness, vascular occlusion score determined by immunohistochemical staining and the expression of inflammatory markers measured by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The expression of CXCR4 and other stem cell markers were compared in the PAH and control groups. RT-qPCR showed that the expression of CXCR4, SCF, c-Kit, and CD29, which are expressed in MSCs, was significantly higher in the PAH group. Immunohistochemical staining also showed that the numbers of CXCR4-, c-Kit- and CD90-positive cells were significantly higher in the PAH group. These results suggest that CXCR4 is involved in the pathogenesis of PAH and that stem cells may serve an important role in pulmonary vascular remodeling.

Recent updates for designing CCR5 antagonists as anti-retroviral agents

The healthcare system faces various challenges in human immunodeficiency virus (HIV) therapy due to resistance to Anti-Retroviral Therapy (ART) as a consequence of the evolutionary process. Despite the success of antiretroviral drugs like Zidovudine, Zalcitabine, Raltegravir WHO ranks HIV as one of the deadliest diseases with a mortality of one million lives in 2016. Thus, there emerges an urgency of developing a novel anti-retroviral agent that combat resistant HIV strains. The clinical development of ART from a single drug regimen to current triple drug combination is very slow. The progression in the structural biology of the viral envelope prompted the discovery of novel targets, which can be demonstrated a proficient approach for drug design of anti-retroviral agents. The current review enlightens the recent updates in the structural biology of the viral envelope and focuses on CCR5 as a validated target as well as ways to overcome CCR5 resistance. The article also throws light on the SAR studies and most prevalent mutations in the receptor for designing CCR5 antagonists that can combat HIV-1 infection. To conclude, the paper lists diversified scaffolds that are in pipeline by various pharmaceutical companies that could provide an aid for developing novel CCR5 antagonists.

Chemokines and Their Receptors in Islet Allograft Rejection and as Targets for Tolerance Induction

Graft rejection is a major barrier to successful outcome of transplantation surgery. Islet transplantation introduces insulin secreting tissue into type 1 diabetes mellitus recipients, relieving patients from exogenous insulin injection. However, insulitis of grafted tissue and allograft rejection prevent long-term insulin independence. Leukocyte trafficking is necessary for the launch of successful immune responses to pathogen or allograft. Chemokines, small chemotactic cytokines, direct the migration of leukocytes through their interaction with chemokine receptors found on cell surfaces of immune cells. Unique receptor expression of leukocytes, and the specificity of chemokine secretion during various states of immune response, suggest that the extracellular chemokine milieu specifically homes certain leukocyte subsets. Thus, only those leukocytes required for the current immune task are attracted to the inflammatory site. Chemokine blockade, using antagonists and monoclonal antibodies directed against chemokine receptors, is an emerging and specific immunosuppressive strategy. Importantly, chemokine blockade may potentiate tolerance induction regimens to be used following transplantation surgery, and prevent the need for life-long immunosuppression of islet transplant recipients. Here, the role for chemokine blockade in islet transplant rejection and tolerance is reviewed.

Molecular basis for the antagonistic activity of an anti-CXCR4 antibody

Antagonistic antibodies targeting the G-protein C-X-C chemokine receptor 4 (CXCR4) hold promising therapeutic potential in various diseases. We report for the first time the detailed mechanism of action at a molecular level of a potent anti-CXCR4 antagonistic antibody (MEDI3185). We characterized the MEDI3185 paratope using alanine scanning on all 6 complementary-determining regions (CDRs). We also mapped its epitope using CXCR4 mutagenesis to assess the relative importance of the CXCR4 N-terminal peptide, extracellular loops (ECL) and ligand-binding pocket. We show that the interaction between MEDI3185 and CXCR4 is mediated mostly by CDR3H in MEDI3185 and ECL2 in CXCR4. The MEDI3185 epitope comprises the entire ECL2 sequence, lacks any so-called ‘hot-spot’ and is remarkably resistant to mutations. The structure of MEDI3185 variable domains was modeled, and suggested a β-strand/β-strand interaction between MEDI3185 CDR3H and CXCR4 ECL2, resulting in direct steric hindrance with CXCR4 ligand SDF-1. These findings may have important implications for designing antibody therapies against CXCR4.

Discovery of non-peptide small molecular CXCR4 antagonists as anti-HIV agents: Recent advances and future opportunities

CXCR4 plays vital roles in HIV-1 life cycle for it's essential in mediating the interaction of host and virus and completing the entry process in the lifecycle of HIV-1 infection. Compared with some traditional targets, CXCR4 provides a novel and less mutated drug target in the battle against AIDS. Its antagonists have no cross resistance with other antagonists. Great achievements have been made recent years and a number of small molecular CXCR4 antagonists with diversity scaffolds have been discovered. In this review, recent advances in the discovery of CXCR4 antagonists with special attentions on their evolution and structure-activity relationships of representative CXCR4 antagonists are described. Moreover, some classical medicinal chemistry strategies and novel methodologies are also introduced.

A strategy to discover decoy chemokine ligands with an anti-inflammatory activity

Excessive signaling by chemokines has been associated with chronic inflammation or cancer, thus attracting substantial attention as promising therapeutic targets. Inspired by chemokine-clearing molecules shaped by pathogens to escape the immune system, we designed a generic screening assay to discover chemokine neutralizing molecules (neutraligands) and unambiguously distinguish them from molecules that block the receptor (receptor antagonists). This assay, called TRIC-r, combines time-resolved intracellular calcium recordings with pre-incubation of bioactive compounds either with the chemokine or the receptor-expressing cells. We describe here the identification of high affinity neutraligands of CCL17 and CCL22, two chemokines involved in the Th2-type of lung inflammation. The decoy molecules inhibit in vitro CCL17- or CCL22-induced intracellular calcium responses, CCR4 endocytosis and human T cell migration. In vivo, they inhibit inflammation in a murine model of asthma, in particular the recruitment of eosinophils, dendritic cells and CD4⁺T cells. Altogether, we developed a successful strategy to discover as new class of pharmacological tools to potently control cell chemotaxis in vitro and in vivo.

The current status in hematopoietic stem cell mobilization

Hemotopoietic stem cell mobilization with cytokines alone, has still been widely accepted as the initial attempt for stem cell mobilization. Chemotherapy based mobilization can be preferred as first choice in high risk patients or for remobilization. But mobilization failure still remains to be a problem in one third of patients. Salvage mobilization strategies have been composed to give one more chance to 'poor mobilizers'. Synergistic effect of a reversible inhibitor of CXCR4, plerixafor, with G-CSF has opened a new era for these patients. Preemptive approach in predicted poor mobilizers, immediate salvage approach for patients with suboptimal mobilization or remobilization approach of plerixafor in failed mobilizers have all been demonstrated convincing results in various studies. Alternative CXCR4 inhibitors, VLA4 inhibitors, bortezomib, parathormone have also been emerged as novel agents for mobilization failure.

Tacrolimus promotes hepatocellular carcinoma and enhances CXCR4/SDF‑1α expression in vivo

The aim of our study was to elucidate the effect of tacrolimus (FK506) and of C-X-C chemokine receptor type 4 (CXCR4), which is a receptor specific to the stromal cell-derived factor-1α (SDF-1α), on growth and metastasis of hepatocellular carcinoma (HCC). Following treatment with different concentrations of FK506, AMD3100 or normal saline (NS), the proliferation of Morris rat hepatoma 3924A (MH3924A) cells was measured by the MTT assay, the expression of CXCR4 was analyzed with immunohistochemistry, and the morphological changes and the invasiveness of cells were studied with a transwell assay and under a scanning electron microscope, respectively. In addition, August Copenhagen Irish rat models implanted with tumor were used to examine the pathological changes and invasiveness of tumor in vivo, the expression of CXCR4 in tumor tissues and the expression of SDF-1α in the adjacent tissues to the HCC ones, using immunohistochemistry. In vitro, FK506 (100–1,000 μg/l) significantly promoted the proliferation of MH3924A cells (P<0.01), and increased the expression of CXCR4 in MH3924A cells, albeit with no significance (P>0.05). By contrast, AMD3100 had no effect on the proliferation of MH3924A cells, but significantly reduced the expression of CXCR4 (P<0.05). The invasiveness of MH3924A cells was significantly (P<0.01) enhanced following treatment with FK506, SDF-1α, FK506 + AMD3100, FK506 + SDF-1α or FK506 + AMD3100 + SDF-1α. In vivo, tumor weight (P=0.041), lymph node metastasis (P=0.002), the number of pulmonary nodules (P=0.012), the expression of CXCR4 in tumor tissues (P=0.048) and that of SDF-1α in adjacent tissues (P=0.026) were significantly different between the FK506-treated and the NS group. Our results suggest that FK506 promotes the proliferation of MH3924A cells and the expression of CXCR4 and SDF-1α in vivo. Therefore, inhibiting the formation of the CXCR4/SDF-1α complex may partly reduce the promoting effect of FK506 on HCC.

V3-independent competitive resistance of a dual-X4 HIV-1 to the CXCR4 inhibitor AMD3100

A CXCR4 inhibitor-resistant HIV-1 was isolated from a dual-X4 HIV-1 in vitro. The resistant variant displayed competitive resistance to the CXCR4 inhibitor AMD3100, indicating that the resistant variant had a higher affinity for CXCR4 than that of the wild-type HIV-1. Amino acid sequence analyses revealed that the resistant variant harbored amino acid substitutions in the V2, C2, and C4 regions, but no remarkable changes in the V3 loop. Site-directed mutagenesis confirmed that the changes in the C2 and C4 regions were principally involved in the reduced sensitivity to AMD3100. Furthermore, the change in the C4 region was associated with increased sensitivity to soluble CD4, and profoundly enhanced the entry efficiency of the virus. Therefore, it is likely that the resistant variant acquired the higher affinity for CD4/CXCR4 by the changes in non-V3 regions. Taken together, a CXCR4 inhibitor-resistant HIV-1 can evolve using a non-V3 pathway.

Minimalist hybrid ligand/receptor-based pharmacophore model for CXCR4 applied to a small-library of marine natural products led to the identification of phidianidine a as a new CXCR4 ligand exhibiting antagonist activity

Here, we present a minimal hybrid ligand/receptor-based pharmacophore model (PM) for CXCR4, a chemokine receptor deeply involved in several pathologies, such as HIV infection, rheumatoid arthritis, cancer development/progression, and metastasization. This model, considerably simpler than those thus far proposed for this receptor, has been used to search for new CXCR4 inhibitors in a small marine natural product library available at ICB-CNR Institute (Pozzuoli, NA, Italy), since natural products, with their naturally selected chemical and functional diversity, represent a rich source of bioactive scaffolds; computational approaches allow searching for new scaffolds with a minimal waste of possibly precious natural product samples; and our "stripped-down" model substantially increases the probabilities of identifying potential hits even in small-sized libraries. This search, also validated by a systematic virtual screening of the same library, has led to the identification of a new CXCR4 ligand, phidianidine A (PHIA). Docking studies supported PHIA activity and suggested its possible binding modes to CXCR4. Using the CXCR4-expressing/CXCR7-negative GH4C1 cell line we show that PHIA inhibits CXCL12-induced DNA synthesis, cell migration, and ERK1/2 activation. The specificity of these effects was confirmed by the lack of PHIA activity in GH4C1 cells, in which siRNA highly reduces CXCR4 expression and the lack of cytoxicity of PHIA was also verified. Thus, PHIA represents a promising lead for a new family of CXCR4 modulators with wide margins of improvement in potency and specificity offered by the small and very simple underlying PM.

Mechanism of HIV antiretroviral drugs progress toward drug resistance

The rapid replication rate of HIV-1 RNA and its inherent genetic variation have led to the production of many HIV-1 variants with decreased drug susceptibility. The capacity of HIV to develop drug resistance mutations is a major obstacle to long-term effective anti-HIV therapy. Incomplete suppression of viral replication with an initial drug regimen diminishes the clinical benefit to the patient and may promote the development of broader drug resistance that may cause subsequent treatment regimens to be ineffective. The increased clinical use of combination antiretroviral treatment for HIV-1 infection has led to the selection of viral strains resistant to multiple drugs, including strains resistant to all licensed nucleoside analog RT inhibitors and protease inhibitors. Therefore, it is important to understand the influence of such mutations on viral properties such as replicative fitness, fidelity, and mutation rates. Although research continues to improve our understanding of resistance, leading to refined treatment strategies and, in some cases, improved outcome, resistance to antiretroviral therapy remains a major cause of treatment failure among patients living with HIV-1.

Microenvironmental considerations in the application of human mesenchymal stem cells in regenerative therapies

The therapeutic utilization of stem cells has been ongoing for several decades, principally in the form of bone marrow (BM) transplants to treat various hematological disorders and other immune-related diseases. More recently, stem cells have been examined as a potential therapy for a multitude of other diseases and disorders, many of which are currently untreatable. One consideration that poses a formidable task for the successful clinical application of stem cells in new disease models is the impact of the host tissue microenvironment on the desired therapeutic outcome. In vitro, stem cells exist in surroundings directly controllable by the researcher to produce the desired cellular behavior. In vivo, the transplanted cells are exposed to a dynamic host microenvironment laden with soluble mediators and immunoreactive cells. In this review, we focus on the possible contribution by microenvironmental factors, and how these influences can be overcome in therapies utilizing mesenchymal stem cells (MSCs), such as for graft versus host disease, multiple sclerosis and ischemia among others. Specifically, we examine three ubiquitous microenvironmental factors, IL-1alpha/beta(,) TNFalpha, and SDF-1alpha, and consider how inhibitors and receptor antagonists to these molecules could be applied to increase the efficacy of MSC therapies while minimizing unforeseen harm to the patient.

Role of CXCR4 in HIV infection and its potential as a therapeutic target

The chemokine receptors CCR5 and CXCR4 are the two major coreceptors for HIV entry. Numerous efforts have been made to develop a new class of anti-HIV agents that target these coreceptors as an additional or alternative therapy to standard HAART. Among the CCR5 inhibitors developed so far, maraviroc is the first drug that has been approved by the US FDA for treating patients with R5 HIV-1. Although many CXCR4 inhibitors, some of which are highly active and orally bioavailable, have also been studied, they are still at preclinical stages or have been suspended during development. Importantly, the interaction between CXCR4 and its ligand SDF-1 is involved in various disease conditions, such as cancer cell metastasis, leukemia cell proliferation, rheumatoid arthritis and pulmonary fibrosis. Therefore, CXCR4 inhibitors have potential as novel therapeutics for the treatment of these diseases as well as HIV infection.

Entry inhibitors in the treatment of HIV-1 infection

Infection of target cells by HIV is a complex, multi-stage process involving attachment to host cells and CD4 binding, coreceptor binding, and membrane fusion. Drugs that block HIV entry are collectively known as entry inhibitors, but comprise a complex group of drugs with multiple mechanisms of action depending on the stage of the entry process at which they act. Two entry inhibitors, maraviroc and enfuvirtide, have been approved for the treatment of HIV-1 infection, and a number of agents are in development. This review covers the entry inhibitors and their use in the management of HIV-1 infection. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol 85, issue 1, 2010.

NMR evidence of charge-dependent interaction between various PND V3 and CCR5 N-terminal peptides

The third variable (V3) loop is an important region of glycoprotein 120 (gp120) for many biological processes, as it contains the highly conserved GPGR sequence and it represents the binding site for human immunodeficiency virus 1 (HIV-1) antibodies and for CCR5 and CXCR4 host cell coreceptors. The interaction of the principal neutralizing determinant (PND) V3 with the chemokine receptor CCR5 N-terminal region has been reported to be crucial for HIV-1 infection. The goal of this study is to characterize the solution structures of three HIV-1 gp120 V3 subtype B peptides and their interaction with a nonsulfated N-terminal CCR5 peptide. NMR titration experiments revealed that the CCR5Nt-PND V3 interaction is dependent on the number of the positively charged V3 residues, which is in agreement with the observation that increase in positive charge in the V3 sequence correlates with the augmentation of the interaction. As expected for free peptides in solution, the peptides representing the PND V3 region of gp120 exhibit conformational flexibility, but they also exhibit a large number of NOEs which allowed convergence to a dominant conformation. The PND V3 peptides retain the U-turn conformation observed in the crystal structures of gp120 complexes independently of CCR5 presence. The interaction of different regions of the CCR5Nt peptide is gradually increasing proportionally to the positive charge increase in the V3 peptides. The data demonstrate that the PND V3 and CCR5Nt peptide sequences have propensities for interaction even in the absence of sulfated tyrosines and that their binding and selectivity is determined by simple electrostatic attraction mechanisms.

The novel CXCR4 antagonist KRH-3955 is an orally bioavailable and extremely potent inhibitor of human immunodeficiency virus type 1 infection: comparative studies with AMD3100

The previously reported CXCR4 antagonist KRH-1636 was a potent and selective inhibitor of CXCR4-using (X4) human immunodeficiency virus type 1 (HIV-1) but could not be further developed as an anti-HIV-1 agent because of its poor oral bioavailability. Newly developed KRH-3955 is a KRH-1636 derivative that is bioavailable when administered orally with much more potent anti-HIV-1 activity than AMD3100 and KRH-1636. The compound very potently inhibits the replication of X4 HIV-1, including clinical isolates in activated peripheral blood mononuclear cells from different donors. It is also active against recombinant X4 HIV-1 containing resistance mutations in reverse transcriptase and protease and envelope with enfuvirtide resistance mutations. KRH-3955 inhibits both SDF-1alpha binding to CXCR4 and Ca(2+) signaling through the receptor. KRH-3955 inhibits the binding of anti-CXCR4 monoclonal antibodies that recognize the first, second, or third extracellular loop of CXCR4. The compound shows an oral bioavailability of 25.6% in rats, and its oral administration blocks X4 HIV-1 replication in the human peripheral blood lymphocyte-severe combined immunodeficiency mouse system. Thus, KRH-3955 is a new promising agent for HIV-1 infection and AIDS.

Staphylococcal superantigen-like 10 inhibits CXCL12-induced human tumor cell migration

PURPOSE

Tumor cell migration and metastasis share many similarities with leukocyte trafficking, which is critically regulated by chemokines and their receptors. CXCR4 is the most widely expressed chemokine receptor in many different types of cancer and has been linked to tumor dissemination and poor prognosis. Several CXCR4 antagonists have been synthesized. A totally novel approach to discover chemokine receptor antagonists is the use of bacteria. Bacteria produce chemokine receptor inhibitors to prevent neutrophil extravasation and migration toward the infection site to escape clearance by innate immune cells. The aim of the current study was to find and identify the mechanism of a bacterial protein that specifically targets CXCR4, a chemokine receptor shared by neutrophils and cancer cells.

EXPERIMENTAL DESIGN

Several staphylococcal proteins were screened for their capacity to prevent binding of a function-blocking antibody against CXCR4.

RESULTS

Staphylococcal superantigen-like 10 was found to bind CXCR4 expressed on human T acute lymphoblastic leukemia, lymphoma, and cervical carcinoma cell lines. It potently inhibited CXCL12-induced calcium mobilization and cell migration.

CONCLUSIONS

Staphylococcal superantigen-like 10 is a potential lead in the development of new anticancer compounds preventing metastasis by targeting CXCR4.

CXCR4/CXCL12 hyperexpression plays a pivotal role in the pathogenesis of lupus

Among various surface molecules screened, CXCR4 was significantly up-regulated on monocytes, neutrophils, B cell subsets, and plasma cells in multiple murine models of lupus with active nephritis, including B6.Sle1Yaa, BXSB, and MRL.lpr. TLR-mediated signaling and inflammatory cytokines accounted in part for this increase. Increased CXCR4 expression was associated with functional consequences, including increased migration and enhanced B cell survival. Simultaneously, the ligand for CXCR4, CXCL12, was significantly up-regulated in the nephritic kidneys. Treatment with a peptide antagonist of CXCR4 prolonged survival and reduced serum autoantibodies, splenomegaly, intrarenal leukocyte trafficking, and end organ disease in a murine model of lupus. These findings underscore the pathogenic role of CXCR4/CXCL12 in lymphoproliferative lupus and lupus nephritis and highlight this axis as a promising therapeutic target in this disease.

HIV entry inhibitors and their potential in HIV therapy

This review discusses recent progress in the development of anti-HIV agents targeting the viral entry process. The three main classes (attachment inhibitors, co-receptor binding inhibitors, and fusion inhibitors) are further broken down by specific mechanism of action and structure. Many of these inhibitors are in advanced clinical trials, including the HIV maturation inhibitor bevirimat, from the authors' laboratories. In addition, the CCR5 inhibitor maraviroc has recently been FDA-approved. Possible roles for these agents in anti-HIV therapy, including treatment of virus resistant to current drugs, are also discussed.

Update on the development of HIV entry inhibitors

HIV fusion and entry are two steps in the viral lifecycle that can be targeted by several classes of antiviral drugs. The discovery of chemokines focused the attention on cellular co-receptors used by the virus for entering cells, and on the various steps of such processes that are subject to interactions with small molecules. Intense research has led to a wide range of effective compounds that are able to inhibit these initial steps of viral replication. All steps in the process of HIV entry into the cell may be targeted by specific compounds, grouped into three main classes (attachment inhibitors, co-receptor binding inhibitors and fusion inhibitors), which may be developed as novel antiretrovirals. Thus, several inhibitors of the gp120–CD4 interaction have been discovered (e.g., zintevir and BMS-378806). Small molecule chemokine receptor antagonists acting as HIV entry inhibitors have also been described recently, including those which interact with both the CXCR4 co-receptor (e.g., AMD3100, AMD3465, ALX40-4C, T22, T134 and T140) and CCR5 co-receptor antagonists (TAK-779, TAK-220, E913, AK-602 and NSC 651016 in clinical trials). Recently, a third family of antivirals started to be used clinically (in addition to reverse transcriptase and protease inhibitors), with the advent of enfuvirtide (T20), the first fusion inhibitor to be approved as an anti-HIV agent. Some of these compounds demonstrated in vitro synergism with other classes of antivirals, thus offering the rationale for their combination in therapies for HIV-infected individuals. Many HIV entry and fusion inhibitors are currently being investigated in controlled clinical trials, and a number of them are bioavailable as oral formulations. In 2007, the US FDA approved maraviroc as an anti-HIV agent. Maraviroc is the product of a medicinal chemistry effort initiated following identification of an imidazopyridine CCR5 ligand from a high-throughput screen of the Pfizer compound file. Maraviroc demonstrated potent antiviral activity against all CCR5-tropic HIV-1 viruses tested, including 43 primary isolates from various clades and diverse geographic origin. Maraviroc was active against 200 clinically derived HIV-1 envelope-recombinant pseudoviruses, 100 of which were derived from viruses resistant to existing drug classes. Furthermore, in October 2007, the FDA announced the approval of raltegravir for the treatment of HIV-1 infection as part of combination antiretroviral therapy in treatment-experienced patients with evidence of HIV-1 replication despite optimized background antiretroviral therapy. At present, raltegravir is the only drug in the integrase inhibitor class approved for clinical use. With the approval of raltegravir, oral agents targeting all three constitutive viral enzymes, reverse transcriptase, protease and integrase, are now represented in FDA-approved therapies.

Peptide inhibition of HIV-1: current status and future potential

More than 2 decades of intensive research has focused on defining replication mechanisms of HIV type 1 (HIV-1), the etiologic agent of AIDS. The delineation of strategies for combating this viral infection has yielded many innovative approaches toward this end. HIV-1 is a lentivirus in the family retroviridae that is relatively small with regard to both structure and genome size, having a diploid RNA genome of approximately 9 kb, with only three major genes and several gene products resulting from alternate splicing and translational frameshifting. Most marketed drugs for treating AIDS are inhibitors of HIV-1 reverse transcriptase or protease enzymes, but new targets include the integrase enzyme, cell surface interactions that facilitate viral entry, and also virus particle maturation and assembly. The emergence of drug-resistant variants of HIV-1 has been the main impediment to successful treatment of AIDS. Thus, there is a pressing need to develop novel treatment strategies targeting multiple stages of the virus life-cycle. Research efforts aimed at developing successful means for combating HIV-1 infection have included development of peptide inhibitors of HIV-1. This article summarizes past and current endeavors in the development of peptides that inhibit replication of HIV-1 and the role of peptide inhibitors in the search for new anti-HIV drugs.

CXCR4, inhibitors and mechanisms of action

In this review, the author discusses recent advances in anti-HIV inhibitors, targeting CXCR4, including natural and modified chemokines, peptides and organic compounds, their mechanisms of action, and the molecular process of virus invasion of immune cells. Peptides with strong anti-HIV activity exhibit several common features, such as electrostatic charges, cyclization, beta-turns and dimerization induced by a sulphide bond. Organic compounds, such as cyclams, display a unique metal-mediated mechanism in the binding process to its target CXCR4. Understanding of their mechanisms of action may be useful for the design of more effective drugs. Consecutive interactions of viral glycoprotein gp120 with CD4 and the co-receptor, CXCR4 or another co-receptor CCR5 on the cell surface leads to virus invasion into host cells. The molecular details of the binding between HIV glycoproteins and the co-receptors also provide a basis for anti-HIV therapy.

Small neutralizing molecules to inhibit actions of the chemokine CXCL12

The chemokine CXCL12 and the receptor CXCR4 play pivotal roles in normal vascular and neuronal development, in inflammatory responses, and in infectious diseases and cancer. For instance, CXCL12 has been shown to mediate human immunodeficiency virus-induced neurotoxicity, proliferative retinopathy and chronic inflammation, whereas its receptor CXCR4 is involved in human immunodeficiency virus infection, cancer metastasis and in the rare disease known as the warts, hypogammaglobulinemia, immunodeficiency, and myelokathexis (WHIM) syndrome. As we screened chemical libraries to find inhibitors of the interaction between CXCL12 and the receptor CXCR4, we identified synthetic compounds from the family of chalcones that reduce binding of CXCL12 to CXCR4, inhibit calcium responses mediated by the receptor, and prevent CXCR4 internalization in response to CXCL12. We found that the chemical compounds display an original mechanism of action as they bind to the chemokine but not to CXCR4. The highest affinity molecule blocked chemotaxis of human peripheral blood lymphocytes ex vivo. It was also active in vivo in a mouse model of allergic eosinophilic airway inflammation in which we detected inhibition of the inflammatory infiltrate. The compound showed selectivity for CXCL12 and not for CCL5 and CXCL8 chemokines and blocked CXCL12 binding to its second receptor, CXCR7. By analogy to the effect of neutralizing antibodies, this molecule behaves as a small organic neutralizing compound that may prove to have valuable pharmacological and therapeutic potential.

Rapid and recurrent neutrophil mobilization regulated by T134, a CXCR4 peptide antagonist

The CXCR4/stromal cell-derived factor-1 (SDF-1) axis plays important roles in development, leukocyte trafficking, HIV infection, and tumorigenesis. Its critical function in bone marrow stem cell and hematopoietic progenitor cell retention, homing and release has been well-characterized by genetic and pharmacological analyses. However, its role in neutrophil retention and release is still poorly understood. In this study, we demonstrated that T134, a peptide antagonist of human CXCR4, is also a potent antagonist of mouse CXCR4. Treatment of C57BL/6 mice with T134 resulted in a rapid and time-dependent increase of white blood cells (WBC) and neutrophils, as well as hematopoietic stem and progenitor cells in peripheral blood. Interestingly, recurrent WBC and neutrophil mobilization was achieved by repeated T134 treatment, and the T134-mediated increase and subsequent retreat of WBC and neutrophils correlated with T134 activity in the peripheral blood. Kinetic analysis revealed that T134 binding to CXCR4 did not induce any significant cell-surface receptor downregulation, indicating that T134-induced WBC and neutrophil mobilization is likely due to direct blockage of the CXCR4/SDF-1 interaction. The results from this study support an important role of CXCR4/SDF-1 axis in neutrophil retention and release in the marrow.

Clinical importance and therapeutic implications of the pivotal CXCL12-CXCR4 (chemokine ligand-receptor) interaction in cancer cell migration

Chemokines are small, secreted proteins and are now the largest known cytokine family. They mediate their effects through a family of G-protein-coupled receptors and were initially recognized for their ability to act as chemo-attractants and activators of specific types of leucocytes in a variety of immune and inflammatory responses. However, during the past 5 years there has been a chemokine revolution in cancer and all scientists and clinicians in oncology-related fields are now aware of their crucial role at all stages of neoplastic transformation and progression. The most important chemokine ligand-receptor interaction is that of the CXCL12 (stromal cell-derived factor-1, SDF-1) ligand with its exclusive receptor CXCR4; this interaction has a pivotal role in the directional migration of cancer cells during the metastatic process. This has been demonstrated by in vitro and in vivo experiments in addition to retrospective clinical studies. These findings have exciting implications in the field of cancer therapeutics, with several small molecule CXCR4 antagonists having been developed, which may provide clinical benefit in the therapy of cancer metastasis. Interestingly, it is likely that the effect of the anti-HER2 antibody [trastuzumab (Herceptin] in breast cancer involves downregulation of the CXCR4 receptor. Unfortunately, a major problem is that chemokine receptors are expressed in other cells within the body, particularly those of the immune system and it is not clear what effects long-term CXCR4 antagonism could have on innate and adaptive immunity. However, there is little doubt that the great strides made in elucidating the complex relationship between chemokines and their role in cancer will soon translate into significant survival benefits for patients.

Sensitization of B16 tumor cells with a CXCR4 antagonist increases the efficacy of immunotherapy for established lung metastases

Expression of the chemokine receptor CXCR4 by tumor cells promotes metastasis, possibly by activating prosurvival signals that render cancer cells resistant to immune attack. Inhibition of CXCR4 with a peptide antagonist, T22, blocks metastatic implantation of CXCR4-transduced B16 (CXCR4-luc-B16) melanoma cells in lung, but not the outgrowth of established metastases, raising the question of how T22 can best be used in a clinical setting. Herein, whereas the treatment of CXCR4-luc-B16 cells in vitro with the CXCR4 ligand CXCL12 did not reduce killing induced by cisplatin or cyclophosphamide, CXCL12 markedly reduced Fas-dependent killing by gp100-specific (pmel-1) CD8(+) T cells. T22 pretreatment restored sensitivity of CXCR4-luc-B16 cells to pmel-1 killing, even in the presence of CXCL12. Two immune-augmenting regimens were used in combination with T22 to treat experimental lung metastases. First, low-dose cyclophosphamide treatment (100 mg/kg) on day 5 in combination with T22 (days 4-7) yielded a approximately 70% reduction of B16 metastatic tumor burden in the lungs compared with cyclophosphamide treatment alone (P < 0.001). Furthermore, whereas anti-CTL antigen 4 (CTLA4) monoclonal antibody (mAb; or T22 treatment) alone had little effect on established B16 metastases, pretreatment with T22 (in combination with anti-CTLA4 mAb) resulted in a 50% reduction in lung tumor burden (P = 0.02). Thus, in vitro, CXCR4 antagonism with T22 renders B16 cells susceptible to killing by antigen-specific T cells. In vivo, T22 synergizes with cyclophosphamide or anti-CTLA4 mAb in the treatment of established lung metastases, suggesting a novel strategy for augmenting the efficacy of immunotherapy.

Human melanoma metastases express functional CXCR4

PURPOSE

The chemokine receptor CXCR4 was identified as an independent predictor of poor prognosis in primary melanoma. The aim of the study was to investigate the role of CXCR4 in human melanoma metastases.

EXPERIMENTAL DESIGN

CXCR4 expression was evaluated in melanoma metastases and in metastatic cell lines through immunohistochemistry, immunoblotting, immunofluorescence, and reverse transcription-PCR. The function of CXCR4 was tested in the presence of the ligand, CXCL12, through induction of extracellular signal-regulated kinase-1 and -2 (Erk-1 and -2) phosphorylation, proliferation, apoptosis, and migration capabilities.

RESULTS

CXCR4 expression was detected in 33 out of 63 (52.4%) metastases from cutaneous melanomas. Metastatic melanoma cell lines expressed cell surface CXCR4; PES 43, Alo 40, and COPA cell lines showed the highest levels of CXCR4 (>90% of positive cells); PES 41, Alo 39, PES 47, POAG, and CIMA cell lines showed low to moderate degrees of expression (5-65% of positive cells). Other chemokine receptors, CCR7 and CCR10, were detected on the melanoma cell lines; CXCL12 activated Erk-1 and Erk-2, the whose induction was specifically inhibited by AMD3100 treatment. CXCL12 increased the growth in PES 41, PES 43, and PES 47 cells under suboptimal (1% serum) and serum-free culture conditions; AMD3100 (1 mumol/L) inhibited the spontaneous and CXCL12-induced proliferation. No rescue from apoptosis was shown but PES 41, PES 43, and PES 47 cells migrate toward CXCL12.

CONCLUSIONS

These findings indicate that CXCR4 is expressed and active in human melanoma metastases, suggesting that active inhibitors such as AMD3100 may be experienced in human melanoma.

HIV co-receptor inhibitors as novel class of anti-HIV drugs

Entry inhibitors constitute a new class of drugs to treat infection by human immunodeficiency virus type 1 (HIV-1). The first member of this class, enfuvirtide, previously known as T-20 and targeting gp41, has now been licensed for therapeutic use. Several other entry inhibitors are in various stages of pre-clinical or clinical development. In this review we focus on the chemokine receptor inhibitors targeting CCR5 and CXCR4 that are the main HIV co-receptors for viral entry.

CXCL12 is displayed by rheumatoid endothelial cells through its basic amino-terminal motif on heparan sulfate proteoglycans

The chemokine CXCL12 (also known as stromal cell-derived factor, SDF-1) is constitutively expressed by stromal resident cells and is involved in the homeostatic and inflammatory traffic of leukocytes. Binding of CXCL12 to glycosaminoglycans on endothelial cells (ECs) is supposed to be relevant to the regulation of leukocyte diapedesis and neoangiogenesis during inflammatory responses. To improve our understanding of the relevance of this process to rheumatoid arthritis (RA), we have studied the mechanisms of presentation of exogenous CXCL12 by cultured RA ECs. RA synovial tissues had higher levels of CXCL12 on the endothelium than osteoarthritis (OA) tissues; in both, CXCL12 colocalized to heparan sulfate proteoglycans (HSPGs) and high endothelial venules. In cultured RA ECs, exogenous CXCL12α was able to bind in a CXCR4-independent manner to surface HSPGs. Desulfation of RA EC HSPGs by pretreatment with sodium chlorate, or by replacing in a synthetic CXCL12α the residues Lys24 and Lys27 by Ser (CXCL12α-K2427S), decreased or abrogated the ability of the chemokine to bind to RA ECs. Ex vivo, synovial ECs from patients with either OA or RA displayed a higher CXCL12-binding capacity than human umbilical vein ECs (HUVECs), and in HUVECs the binding of CXCL12 was increased on exposure to tumor necrosis factor-α or lymphotoxin-α₁β₂. Our findings indicate that CXCL12 binds to HSPGs on ECs of RA synovium. The phenomenon relates to the interaction of HSPGs with a CXCL12 domain with net positive surface charge located in the first β strand, which encompasses a canonical BXBB HSPG-binding motif. Furthermore, we show that the attachment of CXCL12 to HSPGs is upregulated by inflammatory cytokines. Both the upregulation of a constitutive chemokine during chronic inflammation and the HSPG-dependent immobilization of CXCL12 in EC surfaces are potential sites for therapeutic intervention.

Mutations in both env and gag genes are required for HIV-1 resistance to the polysulfonic dendrimer SPL2923, as corroborated by chimeric virus technology

A drug-resistant NL4.3/SPL2923 strain has previously been generated by in vitro selection of HIV-1(NL4.3) in the presence of the polysulfonic dendrimer SPL2923 and mutations were reported in its gp120 gene (Witvrouw et al., 2000). Here, we further analysed the (cross) resistance profile of NL4.3/SPL2923. NL4.3/SPL2923 was found to contain additional mutations in gp41 and showed reduced susceptibility to SPL2923, dextran sulfate (DS) and enfuvirtide. To delineate to what extent the mutations in each env gene were accountable for the phenotypic (cross) resistance of NL4.3/SPL2923, the gp120-, gp41- and gp160-sequences derived from this strain were placed into a wild-type background using env chimeric virus technology (CVT). The cross resistance of NL4.3/SPL2923 towards DS was fully reproduced following gp160-recombination, while it was only partially reproduced following gp120- or gp41-recombination. The mutations in gp41 of NL4.3/SPL2923 were sufficient to reproduce the cross resistance to enfuvirtide. Unexpectedly, the reduced sensitivity towards SPL2923 was not fully reproduced after gp160-recombination. The search for mutations in NL4.3/SPL2923 in viral genes other than env revealed several mutations in the gene encoding the HIV p17 matrix protein (MA) and one mutation in the gene encoding the p24 capsid protein (CA). In order to analyse the impact of the gag mutations alone and in combination with the mutations in env on the phenotypic resistance towards SPL2923, we developed a novel p17- and p17/gp160-CVT. Phenotypic analysis of the NL4.3/SPL2923 p17- and p17/gp160-recombined strains indicated that the mutations in both env and gag have to be present to fully reproduce the resistance of NL4.3/SPL2923 towards SPL2923.

Stromal cells attract B-cell progenitors to promote B-cell-B-cell contact and maturation

The in vitro differentiation of B-lineage progenitors into Ig-secreting mature B cells has classically required a co-culture system containing lipopolysaccharide (LPS) and stromal cells. We have previously showed that B-lineage progenitors cultured in round-bottomed wells can mature and secrete immunoglobulin M (IgM) on par with cultures containing stromal cells. This clearly demonstrates that any factors essential for progenitor cell maturation can be found in cultures containing media, serum, LPS and B-cell progenitors. However, stromal cells are important for the maturation observed when cells are cultured in flat-bottomed wells. We hypothesized that stromal cells may attract B-cell progenitors and promote contacts between responsive cells, a phenomenon that is mimicked by the cultures in round-bottomed wells. In this study, we explore how stromal cells accomplish these functions. We show that stromal cells attract B-cell progenitors in a pertussis toxin-sensitive manner. The stromal cell line S17 produces the chemokine CXCL12, which is able to induce the chemotaxis of B-lineage progenitors. Chemotaxis can be blocked by a small peptide inhibitor (T134) of CXCR4, the CXCL12 receptor. Further, disrupting chemotaxis can reduce the supportive role played by S17 when B-lineage progenitors are cultured in flat-bottomed wells.

Ternary complex formation of human immunodeficiency virus type 1 Env, CD4, and chemokine receptor captured as an intermediate of membrane fusion

Human immunodeficiency virus (HIV) Env-induced fusion is highly temperature dependent. When effector and target cells were coincubated at 37 degrees C, there was a kinetic delay before fusion commenced. When effector and target cells were coincubated for varied times at 23 degrees C, a temperature that does not permit fusion, a temperature-arrested stage was created. Raising temperature to 37 degrees C from the 23 degrees C intermediate eliminated the kinetic delay. Inhibitors (T22, AMD3100, and Sch-C) that block fusion by binding chemokine receptors were added after creating the intermediate so as to assess the extent of engagement between gp120 and chemokine receptors at that stage. For both CXCR4 and CCR5 as coreceptors, increasingly long times of coincubation at 23 degrees C reduced the efficacy of the coreceptor-binding inhibitors in blocking fusion. This implies that an increasing number of ternary Env/CD4/coreceptor complexes form over time at 23 degrees C. It also shows that ternary complex formation has a lower temperature threshold than the downstream steps that include Env folding into a six-helix bundle; this provides an experimental means to separate coreceptor binding by gp120 from the subsequent refolding of gp41 into a six-helix bundle structure. As the time of cell coincubation at 23 degrees C was prolonged, more cells quickly fused upon the raising of the temperature to 37 degrees C, and the increase quantitatively correlated with the greater percentage of fusion that was resistant to drugs. Therefore the pronounced kinetic delay in HIV Env-induced fusion is caused predominantly by the time needed for ternary complexes to form.

A Cxcl12-Cxcr4 Chemokine Signaling Pathway Defines the Initial Trajectory of Mammalian Motor Axons

Motor neurons, alone among neurons in the vertebrate CNS, extend axons out of the neural tube to innervate peripheral targets. Two classes of motor neurons, termed vMNs and dMNs, extend axons out of the neural tube via ventral and dorsal exit points, respectively, in accord with their homeodomain transcription factor repertoire. Downstream of these transcriptional codes, the cell surface receptors that shape initial motor axon trajectories have not been identified. We show here that the chemokine receptor Cxcr4 is expressed on the axons of vMNs as they follow their ventral trajectory, whereas its ligand, Cxcl12, is expressed by mesenchymal cells surrounding the ventral neural tube. Genetic studies reveal that Cxcl12-Cxcr4 signaling directs the ventral trajectory of spinal vMNs. In its absence, these neurons adopt a dMN-like trajectory, despite preservation of their vMN transcriptional identity. Thus, the status of Cxcr4 signaling helps to determine the initial axonal trajectory of mammalian motor neurons.

Akt activation, but not extracellular signal-regulated kinase activation, is required for SDF-1alpha/CXCR4-mediated migration of epitheloid carcinoma cells

Emerging evidence shows that the stromal cell-derived factor 1 (SDF-1)/CXCR4 interaction regulates multiple cell signaling pathways and a variety of cellular functions such as cell migration, proliferation, and survival. There is little information linking the cellular functions and individual signaling pathways mediated by SDF-1 and CXCR4 in human cancer cells. In this study, we have shown that human epitheloid carcinoma HeLa cells express functional CXCR4 by reverse transcription-PCR, immunofluorescent staining, and 125I-SDF-1alpha ligand binding analyses. The treatment of HeLa cells with recombinant SDF-1alpha results in time-dependent Akt and extracellular signal-regulated kinase 1/2 (ERK1/2) activations. The SDF-1alpha-induced Akt and ERK1/2 activations are CXCR4 dependent as confirmed by their total inhibition by T134, a CXCR4-specific peptide antagonist. Cell signaling analysis with pathway-specific inhibitors reveals that SDF-1alpha-induced Akt activation is not required for ERK1/2 activation and vice versa, indicating that activations of Akt and ERK1/2 occur independently. Functional analysis shows that SDF-1alpha induces a CXCR4-dependent migration of HeLa cells. The migration can be totally blocked by phosphoinositide 3-kinase inhibitors, wortmannin or LY294002, whereas mitogen-activated protein/ERK kinase inhibitors, PD98059 and U0126, have no significant effect on SDF-1alpha-induced migration, suggesting that Akt activation, but not ERK1/2 activation, is required for SDF-1alpha-induced migration of epitheloid carcinoma cells.

HIV chemokine receptor inhibitors as novel anti-HIV drugs

The chemokine receptors CXCR4 and CCR5 are the main coreceptors used by the T-cell-tropic (CXCR4-using, X4) and macrophage-tropic (CCR5-using, R5) HIV-1 strains, respectively, for entering their CD4+ target cells. In this review, we focus on the function of these chemokine receptors in HIV infection and their role as novel targets for viral inhibition. Besides some modified chemokines with antiviral activity, several low-molecular weight CCR5 and CXCR4 antagonistic compounds have been described with potent antiviral activity. The best CXCR4 antagonists described are the bicyclam derivatives, which consistently block X4 but also R5/X4 viral replication in PBMCs. We believe that chemokine receptor antagonists will become important new antiviral drugs to combat AIDS. Both CXCR4 and CCR5 chemokine receptor inhibitors will be needed in combination and even in combinations of antiviral drugs that also target other aspects of the HIV replication cycle to obtain optimum antiviral therapeutic effects.

The clinical potential of chemokine receptor antagonists

Chemokines belong to a family of chemotactic cytokines that direct the migration of immune cells towards sites of inflammation. They mediate their biological effects by binding to cell surface receptors, which belong to the G protein-coupled receptor superfamily. Since chemokines and their receptors have been implicated in the pathophysiology of a number of autoinflammatory diseases, chemokine receptor antagonists could prove to be useful therapeutics to target these diseases. Here, we review the role of chemokines in autoimmunity, concentrating mainly on the chemokine receptors CCR1 and CCR5, and discuss the potential utility of antagonists that target these 2 receptors as they progress through the clinic.

A role for CXCR4 signaling in survival and migration of neural and oligodendrocyte precursors

Oligodendrocyte development is controlled by a number of survival and migratory factors. The present study shows that signaling of CXCR4 receptor by the chemokine CXCL12 regulates survival and migration of neural precursors (NP) as well as oligodendrocyte progenitors (OP). CXCR4 is expressed by E14 striatal NP and OP generated by neurospheres. In CXCR4-defective mice, the number of NP in neurosphere outgrowth was twofold less than in wild-type (WT) mice; NP radial cell migration was also decreased. In contrast, the addition of CXCL12 to WT NP increased radial migration from the sphere in a dose-dependent manner with a maximal response at 200 nM. When oligodendrocytes differentiated in neurosphere outgrowth, CXCR4 was downregulated. OP isolated from newborn brain coexpressed CXCR4 with platelet-derived growth factor receptor-alpha (PDGFR alpha) or chondroitin sulfate proteoglycan; receptor expression also decreased during differentiation in vitro. Neonatal OP showed a peak migratory response to 20 nM of CXCL12 in chemotactic chambers, a migration inhibited by a CXCR4 antagonist and anti-CXCL12 antibody. In the embryonic spinal cord, the number of OP-expressing PDGFR alpha was reduced more than twofold in CXCR4-defective mice compared with WT and the ratio of ventral to dorsal OP was significantly increased. This indicates a defect in OP survival and their dorsal migration from the ventral cord region, probably because CXCR4(-/-) OP are unable to respond to CXCL12 made by vascular endothelia and the pia mater. We propose that CXCR4 signaling regulate survival and outward chemotactic migration of OP during embryonic and postnatal CNS development.

Elevated serum levels of stromal-derived factor-1alpha are associated with increased osteoclast activity and osteolytic bone disease in multiple myeloma patients

Multiple myeloma (MM) is an incurable plasma cell (PC) malignancy able to mediate massive destruction of the axial and craniofacial skeleton. The aim of this study was to investigate the role of the potent chemokine, stromal-derived factor-1alpha (SDF-1alpha) in the recruitment of osteoclast precursors to the bone marrow. Our studies show that MM PC produce significant levels of SDF-1alpha protein and exhibit elevated plasma levels of SDF-1alpha when compared with normal, age-matched subjects. The level of SDF-1alpha positively correlated with the presence of multiple radiological bone lesions in individuals with MM, suggesting a potential role for SDF-1alpha in osteoclast precursor recruitment and activation. To examine this further, peripheral blood-derived CD14+ osteoclast precursors were cultured in an in vitro osteoclast-potentiating culture system in the presence of recombinant human SDF-1alpha. Although failing to stimulate an increase in TRAP+, multinucleated osteoclast formation, our studies show that SDF-1alpha mediated a dramatic increase in both the number and the size of the resorption lacunae formed. The increased osteoclast motility and activation in response to SDF-1alpha was associated with an increase in the expression of a number of osteoclast activation-related genes, including RANKL, RANK, TRAP, MMP-9, CA-II, and Cathepsin K. Importantly, the small-molecule CXCR4-specific inhibitor, 4F-Benzoyl-TE14011 (T140), effectively blocked osteoclast formation stimulated by the myeloma cell line, RPMI-8226. Based on these findings, we believe that the synthesis of high levels of SDF-1alpha by MM PC may serve to recruit osteoclast precursors to local sites within the bone marrow and enhance their motility and bone-resorbing activity. Therefore, we propose that inhibition of the CXCR4-SDF-1alpha axis may provide an effective means of treatment for MM-induced osteolysis.

HIV-1 entry inhibitors: closing the front door

Highly active antiretroviral therapy (HAART) has led to major declines in morbidity and mortality of HIV-1-infected individuals, but the increasing prevalence of drug-resistant viral isolates, combined with the toxicity and other limitations of current treatments, make the development of new therapies a high priority. As knowledge of viral entry has expanded, this step of the viral life cycle has become a target for novel therapeutic strategies. An emerging group of antiretrovirals, known collectively as entry inhibitors, targets several distinct steps in viral entry including CD4 binding, chemokine receptor engagement and the structural changes in the viral envelope required for fusion between viral and cellular membranes. Many entry inhibitors are in various stages of clinical development, with one already licensed for use. This review will provide an overview of the mechanisms involved in the entry process, highlight promising entry blockers under development and discuss several considerations related to treatment that are unique to this class of antiretroviral drugs.