Targeting neonatal ischemic brain injury with a pentapeptide-based irreversible caspase inhibitor

Brain protection of the newborn remains a challenging priority and represents a totally unmet medical need. Pharmacological inhibition of caspases appears as a promising strategy for neuroprotection. In a translational perspective, we have developed a pentapeptide-based group II caspase inhibitor, TRP601/ORPHA133563, which reaches the brain, and inhibits caspases activation, mitochondrial release of cytochrome c, and apoptosis in vivo. Single administration of TRP601 protects newborn rodent brain against excitotoxicity, hypoxia-ischemia, and perinatal arterial stroke with a 6-h therapeutic time window, and has no adverse effects on physiological parameters. Safety pharmacology investigations, and toxicology studies in rodent and canine neonates, suggest that TRP601 is a lead compound for further drug development to treat ischemic brain damage in human newborns.

Targeted Vpr-derived peptides reach mitochondria to induce apoptosis of alphaVbeta3-expressing endothelial cells

The HIV-1 encoded apoptogenic protein Vpr induces mitochondrial membrane permeabilization (MMP) via interactions with the voltage-dependent anion channel (VDAC) and the adenine nucleotide translocator (ANT). We have designed a peptide, TEAM-VP, composed of two functional domains, one a tumor blood vessel RGD-like 'homing' motif and the other an MMP-inducing sequence derived from Vpr. When added to isolated mitochondria, TEAM-VP interacts with ANT and VDAC, reduces oxygen consumption and overcomes Bcl-2 protection to cause inner and outer MMP. TEAM-VP specifically recognizes cell-surface expressed alpha(V)beta(3) integrins, internalizes, temporarily localizes to lysosomes and progressively co-distributes with the mitochondrial compartment with no sign of lysosomal membrane permeabilization. Finally TEAM-VP reaches mitochondria of angiogenic endothelial cells to induce mitochondrial fission, dissipation of the mitochondrial transmembrane potential (DeltaPsi(m)), cytochrome c release and apoptosis hallmarks. Hence, this chimeric peptide constitutes the first example of a virus-derived mitochondriotoxic compound as a candidate to kill selectively tumor neo-endothelia.

Upstream control of apoptosis by caspase-2 in serum-deprived primary neurons

During development as well as in pathological situations, neurons that fail to find appropriate targets or neurotrophic factors undergo cell death. Using primary cortical neurons subjected to acute serum-deprivation (SD), we have examined caspases activation, mitochondrial dysfunction and cell death parameters. Among a panel of metabolic, signaling and caspases inhibitors only those able to interfere with caspase-2 like activity protect primary neurons against SD-induced cell death. In situ detection and subcellular fractionation demonstrate a very early activation of cytosolic caspase-2, which controls Bax cleavage, relocalization and mitochondrial membrane permeabilization (MMP). Both z-VDVAD-fmk and a siRNA specific for caspase-2 abolish Bax changes, mitochondrial membranes permeabilization, as well as cytochrome c release-dependent activation of caspase-9/caspase-3, nuclear alterations, phosphatidylserine exposure, neurites dismantling and neuronal death. Hence, caspase-2 is an early checkpoint for apoptosis initiation in primary neurons subjected to serum deprivation.

Dynamic analysis of apoptosis in primary cortical neurons by fixed- and real-time cytofluorometry

We describe here a cytofluorometric technology for the characterization of decision, execution, and degradation steps of neuronal apoptosis. Multiparametric flow cytometry was developed and combined to detailed fluorescence microscopy observations to establish the chronology and hierarchy of death-related events: neuron morphological changes, mitochondrial transmembrane potential (DeltaPsi(m)) collapse, caspase-3 and -9 activation, phosphatidyl-serine exposure, nuclear dismantling and final plasma membrane permeabilization. Moreover, we developed a reliable real-time flow cytometric monitoring of DeltaPsi(m) and plasma membrane integrity in response to neurotoxic insults including MPTP treatment. Taking advantage of recently developed specific fluorescent probes and a third generation pan-caspase inhibitor, this integrated approach will be pertinent to study the cell biology of neuronal apoptosis and to characterize new neuro-toxic/protective molecules.

HIV-1 envelope glycoproteins-mediated apoptosis is regulated by CD4 dependent and independent mechanisms

The progressive loss of CD4 T lymphocytes is one of the hallmarks of HIV infection. The reverse correlation observed in vivo, between plasmatic HIV levels and CD4 T lymphocyte counts, supports the concept that direct HIV-mediated cell death contributes to this depletion. Previously, we and others have demonstrated, in vitro, that interactions between membrane-expressed HIV-envelope glycoprotein complexes and CD4 ecto-molecules are critical to cell killing which occurs mainly by apoptosis. Here, by the use of a co-culture model, in which chronically HIV-1 infected cells trigger apoptosis in uninfected CD4+ target cells, we have investigated the role of different CD4 domains in HIV envelope-mediated apoptosis. Target cells were A201 lymphoblastoid cell lines expressing wild-type CD4 or mutant forms of CD4. We show that the cytoplasmic domain of CD4 was not required for apoptosis induction. In contrast, the HIV permissive cell line expressing a CD4/CD8 chimeric molecule which contains only the first 171 amino acids of CD4, appeared to be resistant to HIV-induced apoptosis; thus suggesting that the D3-D4 CD4 module plays somewhat a regulatory role. Pre-treatment of wild-type CD4 expressing target cells by the phorbol ester PMA which leads to down-regulation of CD4, completely abolished apoptosis. Interestingly, in cells expressing CD4 devoid of its cytoplasmic domain, PMA blocked partially cell death without affecting, as expected, the CD4 expression. Taken together, these results demonstrate that although CD4 expression is essential for HIV envelope induced apoptosis, the apoptotic signal could be delivered in the absence of its cytoplasmic domain. Consistent with this, we suggest that other membrane associated molecule(s) are recruited for the signalling to initiate apoptosis.

Combined use of radioimagers and radioactive 3'OH DNA nick end labelling to quantify apoptosis in cell lines and tissue sections: applications to virus-induced apoptosis

DNA fragmentation is a key feature of the degradation phase of apoptosis. In this work we have developed an assay, based on radioimager (beta-IMAGER and micro-IMAGER) quantification of radioactive nick end labelling (RANEL), which is quantitative, rapid and sensitive to study in vitro and in vivo induced apoptosis. To establish the technique, in vitro apoptosis of T cell lines was induced by stimulation of the Fas receptor; cells were labelled using TdT-mediated [alpha-33P] dCTP nick end labelling, after which then radioactivity was quantified using a beta-IMAGER. We have also shown that the RANEL method can be applied to the quantification and visualisation, by micro-IMAGER analysis, of liver tissue sections from mouse Fas-induced fulminant hepatitis or from Dengue-1 virus infected individuals. Finally, this system has also been used to detect apoptosis induced by rabies virus in Jurkat T cells. These data have established a large field of application for the RANEL assay.

Palmitate induces apoptosis via a direct effect on mitochondria

The fatty acid palmitate can induce apoptosis. Here we show that the palmitate-induced dissipation of the mitochondrial transmembrane potential (Delta Psi m), which precedes nuclear apoptosis, is not prevented by inhibitors of mRNA synthesis, protein synthesis, caspases, or pro-apoptotic ceramide signaling. However, the mitochondrial and nuclear effects of palmitate are inhibited by overexpression of anti-apoptotic proto-oncogene product Bcl-2 and exacerbated by 2-bromo-palmitate as well as by carnitine. The cytoprotective actions of Bcl-2, respectively, is not antagonized by etomoxir, an inhibitor of carnitine palmitoyl transferase 1 (CPT1), suggesting that the recently described physical interaction between CPT1 and Bcl-2 is irrelevant to Bcl-2-mediated inhibition of palmitate-induce apoptosis. When added to purified mitochondria, palmitate causes the release of soluble factors capable of stimulating the apoptosis of isolated nuclei in a cell-free system. Mitochondria purified from Bcl-2 over-expressing cells are protected against the palmitate-stimulated release of such factors. These data suggest that palmitate causes apoptosis via a direct effect on mitochondria.

Effect of a preparation of Saccharomyces cerevisiae on microbial profiles and fermentation patterns in the large intestine of horses fed a high fiber or a high starch diet

Eight horses were allotted into pairs consisting of one cecum- and right ventral colon-fistulated animal and one cecum-fistulated animal. They were fed daily at the same level of intake either a high-fiber (HF) or a high-starch (HS) diet without or with 10 g of a Saccharomyces cerevisiae preparation, in a 4 x 4 Latin square design. The HS diet provided a starch overload (i.e., 3.4 g starch x kg(-1) BW x meal(-1)) while maintaining a high amount of fiber intake (i.e., dietary NDF/starch ratio was 1.0). A 21-d period of adaptation to the treatments occurred before cecal and colonic contents were withdrawn 4 h after the morning meal to count total anaerobic, cellulolytic, and lactic acid-utilizing bacteria, lactobacilli, and streptococci. Lactic acid, volatile fatty acids, ammonia concentrations, and pH were measured on cecal and colonic fluid samples collected hourly during the first 12-h postfeeding. When the HS diet was fed, the concentration of total anaerobic and lactic acid-utilizing bacteria increased (P < 0.001), whereas that of cellulolytic bacteria decreased (P < 0.05) in the cecum. The concentration of lactobacilli and streptococci increased (P < 0.001) in the cecal and colonic contents. These alterations of the microbial profiles were associated with decreases (P < 0.001) of pH, (acetate + butyrate)/propionate ratio and with an increase (P < 0.001) of lactic acid concentration. Supplementing the S. cerevisiae preparation increased (P < 0.01) the concentration of viable yeast cells, averaging 4.3 x 10(6) and 4.5 x 10(4) cfu/mL in the cecal and colonic contents, respectively. Yeast supplementation had almost no effect on microbial counts in the cecum and colon. The supplementation of S. cerevisiae appeared to modify (P < 0.05) pH, concentrations of lactic acid and ammonia, molar percentages of acetate and butyrate with the HS diet and [(acetate + butyrate)/propionate] ratio when the HF diet was fed. The effects of the S. cerevisiae preparation were greater in the cecum than in the colon, which coincided with the abundance of yeast cells. When the digestion of starch in the small intestine was saturated, the effect of the addition of a S. cerevisiae preparation appeared to limit the extent of undesirable changes in the intestinal ecosystem of the horse.

The anti-HIV pentameric pseudopeptide HB-19 is preferentially taken up in vivo by lymphoid organs where it forms a complex with nucleolin

The HB-19 pseudopeptide 5[Kpsi(CH(2)N)PR]-TASP, psi(CH(2)N) for reduced peptide bond, is a specific inhibitor of HIV infection in different CD4(+) cell lines and in primary T-lymphocytes and macrophages. It blocks virus-particle attachment to permissive cells by binding and forming a stable complex with nucleolin expressed on the cell surface. Here, we have investigated the tissue distribution of the tritiated HB-19 by using beta-radio imager whole-body mapping in rats. A rapid, selective, and stable distribution and accumulation of the systematically administered HB-19 was demonstrated within the spleen, liver, bone, and kidney as soon as 5 min following its administration. No apparent uptake of HB-19 occurred in the brain and the muscle tissue. Interestingly and despite its rapid clearance from the blood, at 24 h postexposure a significant proportion of HB-19 was still recovered from target organs, of which 16-37% could be accounted for intact pseudopeptide. The elimination of HB-19 mainly occurred by renal glomerular filtration and most of the excreted radioactivity appeared to be HB-19 metabolites. Finally, injection of the biotin-labeled HB-19 pseudopeptide but not its control counterpart allowed the recovery of the HB-19-nucleolin complex from the liver, spleen, thymus, and bone marrow, thus indicating that the in vivo molecular target of HB-19 is surface nucleolin. Our results demonstrate the preferential uptake and stability of HB-19 in lymphoid organs that are the site of HIV propagation.

The adenine nucleotide translocator: a target of nitric oxide, peroxynitrite, and 4-hydroxynonenal

Nitric oxide (NO), peroxynitrite, and 4-hydroxynonenal (HNE) may be involved in the pathological demise of cells via apoptosis. Apoptosis induced by these agents is inhibited by Bcl-2, suggesting the involvement of mitochondria in the death pathway. In vitro, NO, peroxynitrite and HNE can cause direct permeabilization of mitochondrial membranes, and this effect is inhibited by cyclosporin A, indicating involvement of the permeability transition pore complex (PTPC) in the permeabilization event. NO, peroxynitrite and HNE also permeabilize proteoliposomes containing the adenine nucleotide translocator (ANT), one of the key components of the PTPC, yet have no or little effects on protein-free control liposomes. ANT-dependent, NO-, peroxynitrite- or HNE-induced permeabilization is at least partially inhibited by recombinant Bcl-2 protein, as well as the antioxidants trolox and butylated hydroxytoluene. In vitro, none of the tested agents (NO, peroxynitrite, HNE, and tert-butylhydroperoxide) causes preferential carbonylation HNE adduction, or nitrotyrosylation of ANT. However, all these agents induced ANT to undergo thiol oxidation/derivatization. Peroxynitrite and HNE also caused significant lipid peroxidation, which was antagonized by butylated hydroxytoluene but not by recombinant Bcl-2. Transfection-enforced expression of vMIA, a viral apoptosis inhibitor specifically targeted to ANT, largely reduces the mitochondrial and nuclear signs of apoptosis induced by NO, peroxynitrite and HNE in intact cells. Taken together these data suggest that NO, peroxynitrite, and HNE may directly act on ANT to induce mitochondrial membrane permeabilization and apoptosis.

Mitochondrial control of cell death induced by HIV-1-encoded proteins

In most examples of physiological or pathological cell death, mitochondrial membrane permeabilization (MMP) constitutes an early critical event of the lethal process. Signs of MMP that precede nuclear apoptosis include the translocation of cytochrome c and apoptosis-inducing factor (AIF) from mitochondria to an extra-mitochondrial localization, as well as the dissipation of the mitochondrial transmembrane potential. MMP also occurs in HIV-1-induced apoptosis. Different HIV-1 encoded proteins (Env, Vpr, Tat, PR) can directly or indirectly trigger MMP, thereby causing cell death. The gp120/gp41 Env complex constitutes an example for an indirect MMP inducer. Env expressed on the plasma membrane of HIV-1 infected (or Env-transfected) cells mediates cell fusion with CD4/CXCR4-expressing uninfected cells. After a cell type-dependent latency period, syncytia then undergo MMP and apoptosis. Vpr exemplifies a direct MMP inducer. Vpr binds to the adenine nucleotide translocator (ANT), a mitochondrial inner membrane protein which also interacts with apoptosis-regulatory proteins from the Bcl-2/Bax family. Binding of Vpr to ANT favors formation of a non-specific pore leading to MMP. The structural motifs of the Vpr protein involved in MMP are conserved among most pathogenic HIV-1 isolates and determine the cytotoxic effect of Vpr. These data suggest the possibility that viruses employ multiple strategies to regulate host cell apoptosis by targeting mitochondria.

Control of mitochondrial membrane permeabilization by adenine nucleotide translocator interacting with HIV-1 viral protein rR and Bcl-2

Viral protein R (Vpr), an apoptogenic accessory protein encoded by HIV-1, induces mitochondrial membrane permeabilization (MMP) via a specific interaction with the permeability transition pore complex, which comprises the voltage-dependent anion channel (VDAC) in the outer membrane (OM) and the adenine nucleotide translocator (ANT) in the inner membrane. Here, we demonstrate that a synthetic Vpr-derived peptide (Vpr52-96) specifically binds to the intermembrane face of the ANT with an affinity in the nanomolar range. Taking advantage of this specific interaction, we determined the role of ANT in the control of MMP. In planar lipid bilayers, Vpr52-96 and purified ANT cooperatively form large conductance channels. This cooperative channel formation relies on a direct protein-protein interaction since it is abolished by the addition of a peptide corresponding to the Vpr binding site of ANT. When added to isolated mitochondria, Vpr52-96 uncouples the respiratory chain and induces a rapid inner MMP to protons and NADH. This inner MMP precedes outer MMP to cytochrome c. Vpr52-96-induced matrix swelling and inner MMP both are prevented by preincubation of purified mitochondria with recombinant Bcl-2 protein. In contrast to König's polyanion (PA10), a specific inhibitor of the VDAC, Bcl-2 fails to prevent Vpr52-96 from crossing the mitochondrial OM. Rather, Bcl-2 reduces the ANT-Vpr interaction, as determined by affinity purification and plasmon resonance studies. Concomitantly, Bcl-2 suppresses channel formation by the ANT-Vpr complex in synthetic membranes. In conclusion, both Vpr and Bcl-2 modulate MMP through a direct interaction with ANT.

Apoptosis induction by the photosensitizer verteporfin: identification of mitochondrial adenine nucleotide translocator as a critical target

We report that the photosensitizer verteporfin kills lymphoma cells by an apoptotic process involving a dissipation of the mitochondrial inner transmembrane potential (deltapsim). Light-activated verteporfin-induced apoptosis was abolished by transfection with Bcl-2, a procedure reported to inhibit the mitochondrial permeability transition pore complex (PTPC). Verteporfin triggered the deltapsim loss in isolated mitochondria in vitro, and this effect was suppressed by bongrekic acid and cyclosporin A. Verteporfin plus light also permeabilized proteoliposomes containing the semipurified PTPC or the purified PTPC component adenine nucleotide translocator (ANT), yet had no effect on protein-free control liposomes. Verteporfin phototoxicity on ANT proteoliposomes was mediated by reactive oxygen species and was prevented by recombinant Bcl-2 or the adenine nucleotides ATP and ADP. In conclusion, verteporfin belongs to a class of clinically used chemotherapeutic agents acting on PTPC and ANT.

Permeabilization of the mitochondrial inner membrane during apoptosis: impact of the adenine nucleotide translocator

Mitochondrial membrane permeabilization can be a rate limiting step of apoptotic as well as necrotic cell death. Permeabilization of the outer mitochondrial membrane (OM) and/or inner membrane (IM) is, at least in part, mediated by the permeability transition pore complex (PTPC). The PTPC is formed in the IM/OM contact site and contains the two most abundant IM and OM proteins, adenine nucleotide translocator (ANT, in the IM) and voltage-dependent anion channel (VDAC, in the OM), the matrix protein cyclophilin D, which can interact with ANT, as well as apoptosis-regulatory proteins from the Bax/Bcl-2 family. Here we discuss that ANT has two opposite functions. On the one hand, ANT is a vital, specific antiporter which accounts for the exchange of ATP and ADP on IM. On the other hand, ANT can form a non-specific pore, as this has been shown by electrophysiological characterization of purified ANT reconstituted into synthetic lipid bilayers or by measuring the permeabilization of proteoliposomes containing ANT. Pore formation by ANT is induced by a variety of different agents (e.g. Ca(2+), atractyloside, thiol oxidation, the pro-apoptotic HIV-1 protein Vpr, etc.) and is enhanced by Bax and inhibited by Bcl-2, as well as by ADP. In isolated mitochondria, pore formation by ANT leads to an increase in IM permeability to solutes up to 1500 Da, swelling of the mitochondrial matrix, and OM permeabilization, presumably due to physical rupture of OM. Although alternative mechanisms of mitochondrial membrane permeabilization may exist, ANT emerges as a major player in the regulation of cell death. Cell Death and Differentiation (2000) 7, 1146 - 1154

Apoptosis of syncytia induced by the HIV-1-envelope glycoprotein complex: influence of cell shape and size

Cells stably transfected with a lymphotropic HIV-1 Env gene form syncytia when cocultured with CD4(+)CXCR4(+) cells. Heterokaryons then spontaneously undergo apoptosis, while manifesting signs of mitochondrial membrane pemeabilization as well as nuclear chromatin condensation. Modulation of cellular geometry was achieved by growing syncytia on self-assembled monolayers of terminally substituted alkanethiolates designed to control the adhesive properties of the substrates. Spreading of syncytia, induced by culturing them on small circular adhesive islets (diameter 5 microm), placed at a distance that cells can bridge (10 microm), inhibited spontaneous and staurosporin-induced signs of apoptosis, both at the mitochondrial and at the nuclear levels, and allowed for the generation of larger syncytia. Transient cell spreading conferred a memory of apoptosis inhibition which was conserved upon adoption of a conventional cell shape. Limiting syncytium size by culturing them on square-shaped planar adhesive islands of defined size (400 to 2500 microm(2)), separated by nonadhesive regions, enhanced the rate of apoptotic cell death, as indicated by an accelerated permeabilization of the outer mitochondrial membrane, loss of the mitochondrial inner transmembrane potential, and an increased frequency of nuclear apoptosis. In conclusion, external constraints on syncytial size and shape strongly modulate their propensity to undergo apoptosis.

Apoptosis control in syncytia induced by the HIV type 1-envelope glycoprotein complex: role of mitochondria and caspases

Syncytia arising from the fusion of cells expressing a lymphotropic HIV type 1-encoded envelope glycoprotein complex (Env) with cells expressing the CD4/CXC chemokine receptor 4 complex spontaneously undergo cell death. Here we show that this process is accompanied by caspase activation and signs of mitochondrial membrane permeabilization (MMP), including the release of intermembrane proteins such as cytochrome c (Cyt-c) and apoptosis-inducing factor (AIF) from mitochondria. In Env-induced syncytia, caspase inhibition did not suppress AIF- and Cyt-c translocation, yet it prevented all signs of nuclear apoptosis. Translocation of Bax to mitochondria led to MMP, which was inhibited by microinjected Bcl-2 protein or bcl-2 transfection. Bcl-2 also prevented the subsequent nuclear chromatin condensation and DNA fragmentation. The release of AIF occurred before that of Cyt-c and before caspase activation. Microinjection of AIF into syncytia sufficed to trigger rapid, caspase-independent Cyt-c release. Neutralization of endogenous AIF by injection of an antibody prevented all signs of spontaneous apoptosis occurring in syncytia, including the Cyt-c release and nuclear apoptosis. In contrast, Cyt-c neutralization only prevented nuclear apoptosis, and did not affect AIF release. Our results establish that the following molecular sequence governs apoptosis of Env-induced syncytia: Bax-mediated/Bcl-2-inhibited MMP --> AIF release --> Cyt-c release --> caspase activation --> nuclear apoptosis.

Mitochondrion as a novel target of anticancer chemotherapy

Mitochondrial membrane permeabilization is a critical event in the process leading to physiologic or chemotherapy-induced apoptosis (programmed cell death). This permeabilization event is, at least in part, under the control of the permeability transition pore complex (PTPC). Oncoproteins from the Bcl-2 family and tumor suppressor proteins from the Bax family interact with PTPC to inhibit or facilitate membrane permeabilization, respectively. Conventional chemotherapeutic agents elicit mitochondrial permeabilization in an indirect fashion by induction of endogenous effectors that are involved in the physiologic control of apoptosis. However, an increasing number of experimental anticancer drugs, including lonidamine, arsenite, betulinic acid, CD437, and several amphipathic cationic alpha-helical peptides, act directly on mitochondrial membranes and/or on the PTPC. Such agents may induce apoptosis in circumstances in which conventional drugs fail to act because endogenous apoptosis induction pathways, such as those involving p53, death receptors, or apical caspase activation, are disrupted. However, stabilization of the mitochondrial membrane by antiapoptotic Bcl-2-like proteins reduces the cytotoxic potential of most of these drugs. Targeting of specific PTPC components may overcome this Bcl-2-mediated apoptosis inhibition. One strategy involves cross-linking of critical redox-sensitive thiol groups within the PTPC; another involves the use of ligands to the mitochondrial benzodiazepine receptor. Thus, the design of mitochondrion-targeted cytotoxic drugs may constitute a novel strategy for overcoming apoptosis resistance.

Apoptosis and cell cycle: distinct checkpoints with overlapping upstream control

The question as to whether apoptosis (programmed cell death) is controlled by one or few checkpoints is still unresolved. A growing body of evidence suggests that (one of) the decisive event(s) of cell death consists in the permeabilization of mitochondrial membranes. Indeed, multiple pro-apopotic signal transduction pathways converge on the proteins of the Bcl-2/Bax family which, in concert with the so-called permeability transition pore complex (PTPC), regulate mitochondrial membrane barrier function. Mitochondrial permeabilization causes the release of soluble intermembrane proteins, some of which are involved in the activation of apoptotic proteases and nucleases. Thus, the putative checkpoint determining the death/life decision is clearly different from the known checkpoints of cell cycle progression. Prominent oncogenes (e.g., c-Myc, Ras, Raf, Bcl-2) and tumor suppressor genes (e.g., p53, Bax) have been shown to modulate apoptosis via a direct or indirect effect on mitochondrial membranes. All these oncoproteins and tumor suppressor proteins may simultaneously influence the cell cycle and the propensity to undergo apoptosis. Several cell cycle regulatory proteins (e.g., cyclins, cdk, etc.) can induce or inhibit apoptosis via yet unknown pathways.

Mitochondrial membrane permeabilization during the apoptotic process

Apoptosis may be viewed as a triphasic process. During the pre-mitochondrial initiation phase, very different pro-apoptotic signal transduction or damage pathways can be activated. These pathways then converge on the mitochondrion, where they cause the permeabilization of the inner and/or outer membranes with consequent release of soluble intermembrane proteins into the cytosol. The process of mitochondrial membrane permeabilization would constitute the decision/effector phase of the apoptotic process. During the post-mitochondrial degradation phase downstream caspases and nucleases are activated and the cell acquires an apoptotic morphology. Recently, a number of different second messengers (calcium, ceramide derivatives, nitric oxide, reactive oxygen species) and pro-apoptotic proteins (Bax, Bak, Bid, and caspases) have been found to directly compromise the barrier function of mitochondrial membranes, when added to isolated mitochondria. The effects of several among these agents are mediated at least in part via the permeability transition pore complex (PTPC), a composite channel in which members of the Bcl-2 family interact with sessile transmembrane proteins such as the adenine nucleotide translocator. These findings suggest that the PTPC may constitute a pharmacological target for chemotherapy and cytoprotection.

Arsenite induces apoptosis via a direct effect on the mitochondrial permeability transition pore

The molecular mode of action of arsenic, a therapeutic agent employed in the treatment of acute promyelocytic leukemia, has been elusive. Here we provide evidence that arsenic compounds may act on mitochondria to induce apoptosis. Arsenite induces apoptosis accompanied by a loss of the mitochondrial transmembrane potential (Delta Psim). Inhibition of caspases prevents the arsenite-induced nuclear DNA loss, but has no effect on the Delta Psim dissipation and cytolysis induced by arsenite. In contrast, Bcl-2 expression induced by gene transfer prevents all hallmarks of arsenite-induced cell death, including the Delta Psim collapse. PK11195, a ligand of the mitochondrial benzodiazepine receptor, neutralizes this Bcl-2 effect. Mitochondria are required in a cell-free system to mediate arsenite-induced nuclear apoptosis. Arsenite causes the release of an apoptosis-inducing factor (AIF) from the mitochondrial intermembrane space. This effect is prevented by the permeability transition (PT) pore inhibitor cyclosporin A, as well as by Bcl-2, which is known to function as an endogenous PT pore antagonist. Arsenite also opens the purified, reconstituted PT pore in vitro in a cyclosporin A- and Bcl-2-inhibitible fashion. Altogether these data suggest that arsenite can induce apoptosis via a direct effect on the mitochondrial PT pore.

The implication of the chemokine receptor CXCR4 in HIV-1 envelope protein-induced apoptosis is independent of the G protein-mediated signalling

OBJECTIVE

The envelope glycoprotein complex (gp120/gp41)n of HIV-1 is one of the viral products responsible for increased apoptosis in HIV infection. Here the role of the chemokine receptor CXCR4 in HIV-1 envelope protein-induced apoptosis was investigated.

METHODS

Apoptosis occurring in cocultures of chronically HIV-1 IIIB-infected cells with CD4 target cells expressing the CXCR4 receptor was quantified by terminal deoxinucleotidyl transferase dUTP nick end labeling (TUNEL) or propidium iodide staining followed by fluorescent antibody cell sorting, which allows the evaluation of single-cell killing. Moreover global (single cell- and syncytium-associated) apoptosis was quantified by a new radioactive TUNEL-derived assay.

RESULTS

By using these different techniques it was shown that single and syncytium-forming CD4 T cells die by apoptosis upon contact with envelope protein expressing cells independently of viral replication. Moreover, both the CXCR4 agonist SDF-1alpha, and the antagonist AMD3100, showed inhibitory effects on HIV-1 envelope protein-induced apoptosis in the CD4 T-cell subset of peripheral blood mononuclear cells and CD4 cell lines. CXCR4 signalling-induced by HIV-1 envelope proteins in CD4 T cells was not detected. Furthermore, it was shown that envelope protein-induced apoptosis can occur after treating target cells with the Gi-protein inhibitor pertussis toxin.

CONCLUSIONS

Evidence is provided for a role of CXCR4 in the mechanisms of HIV envelope protein-induced pathogenesis, contributing to selective CD4 cell killing. The results suggest that CXCR4 is involved in HIV-1-induced apoptosis; however, this role does not appear to involve G-protein-mediated CXCR4 signalling.

Molecular characterization of mitochondrial apoptosis-inducing factor

Mitochondria play a key part in the regulation of apoptosis (cell death). Their intermembrane space contains several proteins that are liberated through the outer membrane in order to participate in the degradation phase of apoptosis. Here we report the identification and cloning of an apoptosis-inducing factor, AIF, which is sufficient to induce apoptosis of isolated nuclei. AIF is a flavoprotein of relative molecular mass 57,000 which shares homology with the bacterial oxidoreductases; it is normally confined to mitochondria but translocates to the nucleus when apoptosis is induced. Recombinant AIF causes chromatin condensation in isolated nuclei and large-scale fragmentation of DNA. It induces purified mitochondria to release the apoptogenic proteins cytochrome c and caspase-9. Microinjection of AIF into the cytoplasm of intact cells induces condensation of chromatin, dissipation of the mitochondrial transmembrane potential, and exposure of phosphatidylserine in the plasma membrane. None of these effects is prevented by the wide-ranging caspase inhibitor known as Z-VAD.fmk. Overexpression of Bcl-2, which controls the opening of mitochondrial permeability transition pores, prevents the release of AIF from the mitochondrion but does not affect its apoptogenic activity. These results indicate that AIF is a mitochondrial effector of apoptotic cell death.

Dipeptidyl-peptidase IV-beta--further characterization and comparison to dipeptidyl-peptidase IV activity of CD26

Dipeptidyl peptidase IV-beta (DPP IV-beta) is a novel protein which shows a peptidase activity similar to the T-cell-activation antigen CD26. To further characterize this DPP IV-beta and confirm its cell surface expression, we have developed a purification strategy using the CD26- cell line C8166. The purification process includes biotinylation of cell surface proteins before preparation of cell extracts and processing by gel-filtration, ion-exchange and lectin chromatographies. Consistent with the molecular mass of DPP IV-beta estimated by gel-filtration chromatography, the final purified fraction, manifesting a typical DPP IV activity, showed a major biotinylated 75-80-kDa band in SDS/PAGE, thus suggesting the monomeric nature of this enzyme. Kinetic parameters of DPP IV-beta and the sensitivity to a new family of irreversible DPP IV inhibitors, were studied in comparison to CD26. Both enzymes followed a Michaelis kinetics with different Km values for Gly-Pro-NH-Np (NH-Np, para-nitroanilide) hydrolysis (0.28+/-0.05 mM and 0.12+/-0.02 mM). More significant differences were observed in the sensitivity to inhibitors, which exerted a much higher activity on CD26 than on DPP IV-beta. These differences permitted us to study DPP IV-beta expression in CD26-expressing cells, showing the expression of this new enzyme in all lymphoid cells tested, and a rapid enhancement in phytohemagglutinin-stimulated or protein-A-stimulated peripheral blood mononuclear cells. Our results indicate that, although DPP IV-beta and CD26 are coexpressed and manifest a typical DPP IV activity, there are distinct features in their catalytic activities that may confer to each enzyme a complementary role in peptide processing.

Identification of V3 loop-binding proteins as potential receptors implicated in the binding of HIV particles to CD4(+) cells

The binding of human immunodeficiency virus (HIV) type 1 particles to CD4(+) cells could be blocked either by antibodies against the V3 loop domain of the viral external envelope glycoprotein gp120, or by the V3 loop mimicking pseudopeptide 5[Kpsi(CH2N)PR]-TASP, which forms a stable complex with a cell-surface-expressed 95-kDa protein. Here, by using an affinity matrix containing 5[Kpsi(CH2N)PR]-TASP and cytoplasmic extracts from human CEM cells, we purified three V3 loop-binding proteins of 95, 40, and 30 kDa, which after microsequencing were revealed to be as nucleolin, putative HLA class II-associated protein (PHAP) II, and PHAP I, respectively. The 95-kDa cell-surface protein was also isolated and found to be nucleolin. We show that recombinant preparations of gp120 bind the purified preparations containing the V3 loop-binding proteins with a high affinity, comparable to the binding of gp120 to soluble CD4. Such binding is inhibited either by 5[Kpsi(CH2N)PR]-TASP or antibodies against the V3 loop. Moreover, these purified preparations inhibit HIV entry into CD4(+) cells as efficiently as soluble CD4. Taken together, our results suggest that nucleolin, PHAP II, and PHAP I appear to be functional as potential receptors in the HIV binding process by virtue of their capacity to interact with the V3 loop of gp120.

Increased rate of HIV-1 entry and its cytopathic effect in CD4+/CXCR4+ T cells expressing relatively high levels of CD26

The role of the T-cell activation antigen CD26 was evaluated in viral entry and infection of CD4(+)/CXCR4(+) cells by the lymphotropic HIV-1 Lai isolate. For this purpose, CEM T cells, which are permissive to HIV infection and express low levels of CD26, were used to establish by transfection four groups of cell clones expressing either low, high, and very high levels of CD26, or expressing the anti-sense RNA of CD26. Entry was monitored by the detection of proviral DNA synthesis and the kinetics of virus production, whereas the cytopathic effect was demonstrated by the occurrence of apoptosis. HIV entry and infection were consistently accelerated by at least 24 to 48 h in clones expressing high levels of CD26 compared to the parental cells or to the clones expressing low levels of CD26. Interestingly, infection of clones expressing very high levels of CD26 was not accelerated and showed a kinetics of infection similar to that of low CD26 expressing clones. Moreover, HIV infection was significantly reduced in the clones expressing CD26 anti-sense RNA. In the different clones, apoptosis was dependent on the severity of virus infection and occurred after the accumulation of HIV envelope glycoproteins. Our results demonstrate that with equivalently expressed levels of CD4 and CXCR4 in cell lines established from CEM cells, relatively high levels of CD26 contribute to an increased rate of HIV entry, infection, and apoptosis. Furthermore, they point out that overexpression of CD26 in a given cell line may lead to a negative effect on HIV infection. Consequently, CD26 appears to regulate HIV entry and apoptosis, processes which are critical for viral pathogenesis.

Specific and irreversible cyclopeptide inhibitors of dipeptidyl peptidase IV activity of the T-cell activation antigen CD26

The dipeptidyl peptidase IV (DPP IV) activity of CD26 is characterized by its post-proline-cleaving capacity that plays an important but not yet understood role in biological processes. Here we describe a new family of specific and irreversible inhibitors of this enzyme. Taking into account the substrate specificity of DPP IV for P2-P1><-P1' cleavage, we have designed and synthesized cyclopeptides c[(alphaH2N+)-Lys-Pro-Aba-(6-CH2-S+R2)-Glyn] 2TFA- (Aba = 3-aminobenzoic acid, R = alkyl) possessing a proline at the P1 position and a lysine in the P2 position, which allows the closing of the cycle on its side chain. These molecules show a free N-terminus, necessary for binding to the CD26 catalytic site, and a latent quinoniminium methide electrophile, responsible for inactivation. Treatment of c[alphaZ-Lys-Pro-Aba-(6-CH2-OC6H5)-Glyn], obtained by peptide synthesis in solution, with R2S/TFA simutaneously cleaved the Z protecting group and the phenyl ether function and led to a series of cyclopeptide sulfonium salts. These cyclopeptides inhibited rapidly and irreversibly the DPP IV activity of CD26, with IC50 values in the nanomolar range. Further studies were carried out to investigate the effect of the modification of the ring size (n = 2 or 4) and the nature of the sulfur substituents (R = Me, Bu, Oct). Cycle enlargement improved the inhibitory activity of the methylsulfonio cyclopeptide, whereas the increase of the alkyl chain length on the sulfur atom had no apparent effect. Other aminopeptidases were not inhibited, and a much weaker activity was observed on a novel isoform of DPP IV referred to as DPP IV-beta. Thus, this new family of irreversible inhibitors of DPP IV is highly specific to the peptidase activity of CD26.

CD26 as a positive regulator of HIV envelope-glycoprotein induced apoptosis in CD4+ T cells

The membrane-expressed HIV-1 envelope glycoprotein complex, gp120/gp41, has been shown to be responsible for the initiation of cell killing by apoptosis in CD4+ T cells. By using two experimental approaches we demonstrate that CD26, independent of its DPP IV activity, appears to be implicated in this function of the gp120/gp41 complex to initiate apoptosis.

Further characterization of DPP IV-beta, a novel cell surface expressed protein with dipeptidyl peptidase activity

By using a CD26 negative human lymphoblastoid cell line (C8166), here we describe the characterization of a cell-surface protein which manifests CD26-like dipeptidyl peptidase IV (DPP IV) activity. This protein, referred to as DPP IV-beta, shows a higher KM value for Gly-Pro-pNA than CD26 (0.31 mM compared to 0.11 mM, respectively). In addition, DPP IV-beta was found not to bind 125I-labeled adenosine deaminase (a property of human CD26). Gel filtration experiments using extracts from C8166 and MOLT4 (a CD26 positive human T cell line) cells, revealed that the apparent molecular mass of DPP IV-beta is 82 kDa, whereas that of CD26 is 110 kDa. In order to conveniently differentiate both activities, a new family of inhibitors, that selectively blocks peptidase activity associated to CD26, has been developed.

HIV-1 envelope gp120 and viral particles block adenosine deaminase binding to human CD26

CD26, known to be the adenosine deaminase (ADA) binding protein, has been implicated in HIV infection. In human B and T cell lines we show that, irrespective of CD4 expression, 125I-labeled ADA binding to CD26 is inhibited by recombinant soluble HIV-1 envelope glycoprotein gp120 and by HIV-1 infectious particles. Overlapping synthetic peptides covering the entire gp120 sequence were tested to map the region in gp120 responsible for ADA binding inhibition. Only peptides in the C3 region significantly inhibited the binding of ADA to CD26. These results indicate that a specific function of gp120 is the inhibition of ADA binding to CD26 in both CD4+ and CD4- cells. Since the interaction ecto-ADA/CD26 is required for the activation of T cells, it remains possible that HIV particle-mediated blockade of ecto-ADA/CD26 interaction may have significant consequences in the pathogenesis of AIDS disease.

The level of CD26 determines the rate of HIV entry in a CD4+ T-cell line

We have reported that CD26 could serve as a cofactor of CD4 in HIV entry. Recently, more evidence has been provided for the implication of CD26 in HIV entry, replication and cytopathic effect. Along with, we have demonstrated that the level of CD26 may determine the rate of HIV-envelope induced-apoptosis. The role of CD26 in HIV entry was further investigated using CEM T-cell line. Clones were established by transfection, expressing different levels of CD26. Entry, infection and cytopathic effect were monitored in several independent clones, and were found to be delayed in clones CD26-Low and CD26-SuperHigh compared to clones CD26-High. The delay was most significant in clones CD26-AntiSense, without any apparent cytopathic effect. These results demonstrate that relatively enhanced levels of CD26 contribute to an increased virus infection. Furthermore, they illustrate that CD26-SuperHigh clones manifest a phenotype similar to CD26-Low clones. This point out the critical role of CD26 in the rate of HIV entry and its cytopathic effect, two events which are initiated by the interaction of HIV envelope glycoproteins with cell-surface CD4.

Adenosine deaminase binding to human CD26 is inhibited by HIV-1 envelope glycoprotein gp120 and viral particles

CD26, known to be the adenosine deaminase (ADA)-binding protein, has been implicated in HIV infection. Several studies have revealed a correlation between depletion of CD4+/CD26+ T lymphocytes, increased serum levels of ADA, and the evolution of AIDS in infected individuals. We show that in human B and T cell lines, irrespective of CD4 expression, 125I-labeled ADA binding to CD26 is inhibited by recombinant soluble HIV-1 envelope glycoprotein gp120 and by HIV-1 infectious particles. Accordingly, an anti-CD4 mAb, which inhibits the binding of gp120 to CD4 and blocks viral infection, did not affect inhibition of 125I-labeled ADA binding to CD26 by HIV particles. On the other hand, mAbs directed against the V3 loop and the C-terminal region of gp120 abolished completely the inhibitory effect. Overlapping synthetic peptides covering the entire gp120 sequence were tested to map the region in gp120 responsible for ADA binding inhibition. Only peptides in the C3 region significantly inhibited the binding of ADA to CD26. These results provide indirect evidence for the interaction of gp120 with CD26 and indicate that a specific function of gp120 is the inhibition of ADA binding to CD26 in both CD4+ and CD4- cells. Because ADA deficiency leads to severe combined immunodefiency syndrome, it remains possible that HIV particle-mediated blockade of ADA-CD26 interaction may have significant consequences in the pathogenesis of AIDS.

Adenosine deaminase binding to human CD26 is inhibited by HIV-1 envelope glycoprotein gp120 and viral particles

CD26, known to be the adenosine deaminase (ADA)-binding protein, has been implicated in HIV infection. Several studies have revealed a correlation between depletion of CD4+/CD26+ T lymphocytes, increased serum levels of ADA, and the evolution of AIDS in infected individuals. We show that in human B and T cell lines, irrespective of CD4 expression, 125I-labeled ADA binding to CD26 is inhibited by recombinant soluble HIV-1 envelope glycoprotein gp120 and by HIV-1 infectious particles. Accordingly, an anti-CD4 mAb, which inhibits the binding of gp120 to CD4 and blocks viral infection, did not affect inhibition of 125I-labeled ADA binding to CD26 by HIV particles. On the other hand, mAbs directed against the V3 loop and the C-terminal region of gp120 abolished completely the inhibitory effect. Overlapping synthetic peptides covering the entire gp120 sequence were tested to map the region in gp120 responsible for ADA binding inhibition. Only peptides in the C3 region significantly inhibited the binding of ADA to CD26. These results provide indirect evidence for the interaction of gp120 with CD26 and indicate that a specific function of gp120 is the inhibition of ADA binding to CD26 in both CD4+ and CD4- cells. Because ADA deficiency leads to severe combined immunodefiency syndrome, it remains possible that HIV particle-mediated blockade of ADA-CD26 interaction may have significant consequences in the pathogenesis of AIDS.

Pseudopeptide TASP inhibitors of HIV entry bind specifically to a 95-kDa cell surface protein

The template assembled synthetic peptide constructs (TASP), pentavalently presenting the tripeptide KPR or RPK, are potent and specific inhibitors of human immunodeficiency virus (HIV) infection by preventing viral entry into permissive cells. Here the 5[KPsi(CH2N)PR]-TASP construct, Psi(CH2N) for reduced peptide bond, was used in studies to demonstrate its specific binding to a 95-kDa cell surface protein ligand. Compared to its nonreduced 5[KPR]-TASP counterpart, the pseudopeptide 5[KPsi(CH2N)PR]-TASP manifested higher affinity to bind to its cell surface ligand, increased activity to inhibit HIV infection, and resistance to degradation when incubated in serum from an HIV-1 seropositive individual. In ligand blotting experiments, the biotin-labeled 5[KPsi(CH2N)PR]-TASP identified a single 95-kDa protein in crude cell extracts. This 95-kDa protein (p95) is expressed on the cell surface since surface iodination of cells resulted in its labeling, and moreover, following incubation of cells with the biotin-labeled 5[KPsi(CH2N)PR]-TASP, the p95.TASP complex was recovered by affinity chromatography using avidin-agarose. All anti-HIV TASP constructs but not their control derivatives affected the binding of biotin-labeled 5[KPsi(CH2N)PR]-TASP to p95, thus emphasizing the specific nature of this binding. Since 5[KPsi(CH2N)PR]-TASP does not interact with HIV-envelope glycoproteins, our results suggest that TASP inhibitors mediate directly or indirectly a block in HIV-mediated membrane fusion process by binding to the cell surface expressed p95.

HIV envelope glycoprotein-induced cell killing by apoptosis is enhanced with increased expression of CD26 in CD4+ T cells

The membrane-expressed HIV-1 envelope glycoprotein complex, gp120 and gp41, has been shown to be responsible for the initiation of cell killing by apoptosis in CD4+ T cells. By using two experimental approaches we demonstrate here that CD26, also known as dipeptidyl peptidase IV (DPP IV), appears to be implicated in this function of the gp120/gp41 complex to initiate apoptosis. In the first experimental model, we used persistently HIV-1-infected H9/IIIB cells expressing the membrane-associated gp120/gp41 complex as effector cells to induce apoptosis in Jurkat CD4+ T cells: parental or transfected in order to express high levels of recombinant CD26, either wild-type or mutated at its Ser-630 which inactivates the DPP IV activity of CD26. Parental Jurkat cells and transfected control cell clones express low but reproducibly detectable levels of endogenous CD26. In coculturing experiments using H9/IIIB and Jurkat cells, the occurrence of apoptosis was found to be retarded by at least 24 hr in Jurkat cells expressing low levels of endogenous CD26, compared to cell clones expressing high levels of either wild-type or mutated catalytically inactive transfected CD26. In the second experimental model, the different Jurkat cell lines were infected with vaccinia recombinant viruses expressing HIV-1 env gene, either wild-type to generate a functional gp120/gp41 complex or mutated to generate an uncleavable membrane-expressed precursor of the envelope glycoprotein gp160. At 18 hr postinfection with such vaccinia recombinant viruses, apoptosis was observed only in Jurkat cells with enhanced levels of CD26 and expressing the gp120/gp41 complex. Apoptosis was not detected in the different Jurkat cell lines expressing the uncleavable precursor gp160. In both of the experimental models used, no significant differences were observed between the transfected cells expressing either the wild-type or the mutated form of CD26, thus suggesting that the DPP IV activity of CD26 is not essential for its function as a cofactor of CD4 in the mechanism of initiation of apoptosis by the HIV envelope gp120/gp41 complex. Taken together, these results indicate that CD26 in CD4+ T cells may determine the rate of initiation of apoptosis by the mature HIV-1 envelope glycoproteins, i.e., CD26 is being involved as a cofactor of CD4 in the mechanism of triggering apoptosis by the gp120/gp41 complex. As signaling through CD26 could lead to T cell activation, we propose that this latter might be modified following the binding of the gp120/gp41 complex to CD4 and thus leading to apoptosis.

Dipeptidyl-peptidase IV-beta, a novel form of cell-surface-expressed protein with dipeptidyl-peptidase IV activity

The T-cell activation antigen CD26, is a type II membrane glycoprotein with intrinsic dipeptidyl-peptidase IV (DPP IV) activity, characterized by its capacity to cleave off N-terminal dipeptides containing proline as the penultimate residue. Independent of its catalytic activity, CD26 has also been characterized as adenosine deaminase binding protein. By using CD26 negative human C8166 cells, here we describe the existence of another cell-surface protein which manifests CD26-like DPP IV activity. For convenience, this protein will be referred to as DPP IV-beta. Consistent with the cell-surface expression of DPP IV-beta, intact C8166 cells manifested a high level of DPP IV, whereas, they manifested poor activity against substrates of DPP II known to have an intracellular localization. A partially purified preparation of CD26 from human MOLT4 cells, and the DPP IV-beta expressed on intact cells were found to possess similar catalytic activity and pH optimum. In addition, cell-surface CD26 and DPP IV-beta on intact MOLT4 and C8166 cells, respectively, resisted digestion by proteolytic enzymes such as trypsin and proteinase K. However, adenosine deaminase activity was not detectable on the surface of C8166 cells in contrast to CD26 positive MOLT4 cells. In accord with this, 125I-labeled adenosine deaminase which binds CD26 was found not to bind DPP IV-beta. Gel-filtration experiments using 0.5% Triton X-100 extracts from C8166 and MOLT4 cells, revealed that the apparent molecular mass of DPP IV-beta is 82 kDa, whereas that of CD26 is 110 kDa as expected. Taken together, our results suggest that DPP IV-beta is a CD26-like protein which could be characterized by distinct properties.

Specific binding of adenosine deaminase but not HIV-1 transactivator protein Tat to human CD26

Adenosine deaminase (ADA) and the HIV-1 transactivator protein Tat have been reported to bind to human CD26, also known as dipeptidyl peptidase IV (DPP IV). In order to demonstrate the specificity of such binding under native conditions of CD26, i.e., when expressed on the cell surface, we established murine cell lines expressing transfected human CD26, either wild-type or mutated at its serine-630, which inactivates the DPP IV activity. This experimental system is advantageous since murine ADA does not bind human CD26, whereas human and bovine ADA bind. Consequently, murine cell clones expressing either the wild-type or mutated form of human CD26 were found to bind specifically bovine 125I-labeled ADA with a high affinity (KD = 12 +/- 2 nM and 11 +/- 4 nM, respectively). No specific binding of 125I-labeled ADA was observed to murine clones not expressing human CD26. The binding of 125I-labeled ADA to CD26 was further characterized by the use of monoclonal antibodies specific to human CD26. The results obtained were in accord with those reported previously using other experimental models. These observations indicated that the murine cells expressing human CD26 provide a highly suitable model to investigate the potential binding of HIV-1 Tat to CD26. In contrast to previously published results, however, we could not demonstrate a specific interaction between Tat and human CD26. The 125I-labeled ADA-specific binding to human CD26 was not affected by Tat, even at concentrations which induced cell death. Similarly, the binding of several monoclonal antibodies to human CD26 was not modified by the addition of Tat. More significantly, Tat binding to different murine cell clones (human CD26 negative or positive) was found not to be correlated with the expression of human CD26. Finally, the toxic effect of Tat on the growth of different murine cell clones was independent of human CD26 expression. Taken together, these observations further confirm the specific binding of ADA to human CD26 and point out that CD26 is not the target of HIV-1 Tat protein.

Inhibition of HIV infection by pseudopeptides blocking viral envelope glycoprotein-mediated membrane fusion and cell death

The RP dipeptide motif is highly conserved in the third hypervariable region (V3 loop) of the extracellular envelope glycoprotein of different types of HIV isolates. In view of this, we have designed and synthesized a construction referred to as "template assembled synthetic peptide" (TASP), in which a lysine-rich short polypeptide was used as a template to covalently anchor arrays of tripeptides, such as RPR, RPK, or KPR. The pentavalent presentation, 5(RPR)-, 5(RPK)-, or 5(KPR)-TASP, molecules manifested maximum inhibitory activity on HIV infection with a 50% inhibitory concentration value of 1-5 microM, respectively. Structure and inhibitory-activity relationship studies using analogs of 5(KPR)-TASP indicated that the positively charged side chains of the K and R residues in the tripeptide molecules are critical for the optimal inhibitory activity of the pentavalent construct. Interestingly, replacement of L-amino acid residues by D-amino acids or reduction of the peptide bond between the first two amino acids of the tripeptide generated peptide-TASP analogs active at sub-microM, concentrations. The anti-HIV action of the peptide-TASP constructs is specific, since they inhibit infection of several types of CD4-expressing cells by HIV-1 Lai and HIV-2 EHO but not by the simian SIV-mac isolate. Our results suggest that these inhibitors block three post-CD4 binding functions of the HIV envelope glycoproteins, mediation of viral entry, syncytium formation, and triggering cell death by apoptosis. As the peptide-TASP derivatives with unnatural amino acid sequences in the tripeptide moiety retain full inhibitory activity, they should provide potent protease-resistant peptide inhibitors as potential therapeutic agents for treatment of AIDS patients.

T cell activation antigen, CD26, as a cofactor for entry of HIV in CD4+ cells

The CD4 molecule is essential for binding HIV particles, but is not sufficient for efficient viral entry and infection. The cofactor was shown to be dipeptidyl peptidase IV (DPP IV), also known as CD26. This serine protease cleaves its substrates at specific motifs; such motifs area also highly conserved in the V3 loops of HIV-1, HIV-2, and related simian isolates. Entry of HIV-1 or HIV-2 into T lymphoblastoid and monocytoid cell lines was inhibited by a specific monoclonal antibody against DPP IV or specific peptide inhibitors of this protease. Coexpression of human CD4 and CD26 in murine NIH 3T3 cells rendered them permissive to infection by HIV-1 and HIV-2. These observations could provide the basis for developing simple and specific inhibitors of HIV and open a possibility for vaccine development.