Inhibition of cleavage of Moloney murine leukemia virus gag and env coded precursor polyproteins by cerulenin

Induction of cancer cell apoptosis by flavonoids is associated with their ability to inhibit fatty acid synthase activity

The consumption of food products containing high amounts of flavonoids has been reported to lower the risk of various cancers. The mechanisms underlying the cancer-protective effects of these naturally occurring polyphenolic compounds, however, remain elusive. Based on our previous finding that the cytotoxic effect of the flavanol epigallocatechin-3-gallate on prostate cancer cells correlates with its ability to inhibit fatty acid synthase (FAS, a key lipogenic enzyme overexpressed in many human cancers), we examined the anti-lipogenic effects of a panel of 18 naturally occurring polyphenolic compounds. In addition to epigallocatechin-3-gallate, five other flavonoids, more particularly luteolin, quercetin, kaempferol, apigenin, and taxifolin, also markedly inhibited cancer cell lipogenesis. Interestingly, in both prostate and breast cancer cells, a remarkable dose-response parallelism was observed between flavonoid-induced inhibition of fatty acid synthesis, inhibition of cell growth, and induction of apoptosis. In support for a role of fatty acid synthesis in these effects, the addition of exogenous palmitate, the end product of FAS, markedly suppressed the cytotoxic effects of flavonoids. Taken together, these findings indicate that the potential of flavonoids to induce apoptosis in cancer cells is strongly associated with their FAS inhibitory properties, thereby providing a new mechanism by which polyphenolic compounds may exert their cancer-preventive and antineoplastic effects.

Epigallocatechin-3-gallate is a potent natural inhibitor of fatty acid synthase in intact cells and selectively induces apoptosis in prostate cancer cells

Chemical inhibitors of fatty acid synthase (FAS) inhibit growth and induce apoptosis in several cancer cell lines in vitro and in tumor xenografts in vivo. Recently the green tea component epigallocatechin-3-gallate (EGCG) was shown to act as a natural inhibitor of FAS in chicken liver extracts. Here we investigated whether EGCG inhibits FAS activity in cultured prostate cancer cells and how this inhibition affects endogenous lipid synthesis, cell proliferation and cell viability. The high levels of FAS activity in LNCaP cells were dose-dependently inhibited by EGCG and this inhibition was paralleled by decreased endogenous lipid synthesis, inhibition of cell growth and induction of apoptosis. In contrast, epicatechin (EC), another closely related green tea polyphenolic compound, which does not inhibit FAS, had no effect on LNCaP cell growth or viability. Treatment of nonmalignant cells with low levels of FAS activity (fibroblasts) with EGCG led to a decrease in growth rate but not to induction of apoptosis. These data indicate that EGCG inhibits FAS activity as efficiently as presently known synthetic inhibitors and selectively causes apoptosis in LNCaP cells but not in nontumoral fibroblasts. These findings establish EGCG as a potent natural inhibitor of FAS in intact cells and strengthen the molecular basis for the use of EGCG as a chemopreventive and therapeutic antineoplastic agent.

Lipid rafts and HIV-1: from viral entry to assembly of progeny virions

BACKGROUND

Lipid rafts are currently an intensely investigated topic of cell biology. In addition to a demonstrated role in signal transduction of the host cell, lipid rafts serve as entry and exit sites for microbial pathogens and toxins, such as FimH-expressing enterobacteria, influenza virus, measles virus and cholera toxin. Furthermore, caveolae, a specialised form of lipid raft, are required for the conversion of the non-pathogenic prion protein to the pathogenic scrapie isoform.

OBJECTIVES

A number of reports have shown, directly or indirectly, that lipid rafts are important at various stages of the human immunodeficiency virus type-1 (HIV-1) replication cycle. The purpose of this paper is to provide a brief overview of the role of membrane-associated lipid rafts in cell biology, and to evaluate how HIV-1 has hijacked this cellular component to support HIV-1 replication. Special sections are devoted to discussing the role of lipid rafts in (1) the entry of HIV-1, (2) signal transduction regulation in HIV-1-infected cells, (3) the trafficking of HIV-1 proteins via lipid rafts during HIV-1 assembly; and a further section discusses the role of cholesterol in mature HIV-1.

SUMMARY

Like a number of other pathogens, HIV-1 has evolved to rely on the host cell lipid rafts to support its propagation during multiple stages of the HIV-1 replication cycle. This review has highlighted the importance of lipid rafts in HIV-1 replication.

Positive and negative aspects of the human immunodeficiency virus protease: development of inhibitors versus its role in AIDS pathogenesis

In this review we summarize multiple aspects of the human immunodeficiency virus (HIV) protease from both structural and functional viewpoints. After an introductory overview, we provide an up-to-date status report on protease inhibitors (PI). This proceeds from a discussion of PI structural design, to how PI are optimally utilized in highly active antiretroviral triple therapy (one PI along with two reverse transcriptase inhibitors), the emergence of PI resistance, and the natural role of secretory leukocyte PI. Then we switch to another focus: the interaction of HIV protease with other genes in acute and persistent infection, which in turn may have an effect on AIDS pathogenesis. We conclude with a discussion on future directions in HIV treatment, involving multiple-target anti-HIV therapy, vaccine development, and novel reactivation-inhibitory reagents.

DMP 323, a nonpeptide cyclic urea inhibitor of human immunodeficiency virus (HIV) protease, specifically and persistently blocks intracellular processing of HIV gag polyprotein

DMP 323, a C-2-symmetrical cyclic urea, is representative of a new class of inhibitors of human immunodeficiency virus protease. In this study, we correlate the potent antiviral activity of DMP 323 in acute infections with antiprotease activity assessed by monitoring the inhibition of the processing of viral gag precursor polyprotein from chronically infected lymphoid and monocytoid cell lines. Electron microscopic examination confirmed that the inhibition of gag processing was associated with the production of immature viral particles. Reduction of DMP 323 in the environment of unprocessed gag viral particles did not result in the resumption of gag processing for at least 72 h.

Picornavirus inhibitors

Picornaviruses are among the best understood animal viruses in molecular terms. A number of important human and animal pathogens are members of the Picornaviridae family. The genome organization, the different steps of picornavirus growth and numerous compounds that have been reported as inhibitors of picornavirus functions are reviewed. The picornavirus particles and several agents that interact with them have been solved at atomic resolution, leading to computer-assisted drug design. Picornavirus inhibitors are useful in aiding a better understanding of picornavirus biology. In addition, some of them are promising therapeutic agents. Clinical efficacy of agents that bind to picornavirus particles has already been demonstrated.

The HIV-1 protease as a therapeutic target for AIDS

HIV produces a small , dimeric aspartyl protease which specifically cleaves the polyprotein precursors encoding the structural proteins and enzymes of the virus. This proteolytic activity is absolutely required for the production of mature, infectious virions and is therefore an attractive target for therapeutic intervention. This review summarizes the strategies and multidisciplinary efforts that have been applied to date to the identification of specific inhibitors of this critical viral enzyme. These inhibitors include rationally designed peptide substrate analogs, compounds conceived from tertiary structure information on the enzyme and natural products. Future directions in the discovery and development of HIV-1 protease inhibitors are also discussed.

Human immunodeficiency virus type 1 vif, vpr, and vpu mutants can produce persistently infected cells

A series of human immunodeficiency virus type 1 (HIV-1) mutants in vif, vpr, vpu, and nef were constructed from an infectious plasmid (pNL 432) containing the full-length HIV-1 DNA by frameshift mutations. The capacities for replication and cell killing of these mutant viruses were examined in a clonal cell line (M 10) isolated from HTLV-I-transformed MT-4 cells. In all cases, the mutant viruses replicated, expressed HIV-1 antigens, and induced drastic cytopathic effects. However, some M 10 cells survived infection with vif, vpr, and vpu mutant viruses and became persistently HIV-1-infected, whereas no cells survived infection with the nef mutant as well as the wild-type virus. The HIV-1 particles produced from the surviving cells after infection with the vif, vpr, or vpu mutant viruses were fully replicative in M 10 cells without apparent cytopathic effects.

Synthesis of Semliki Forest virus RNA requires continuous lipid synthesis

The involvement of lipid biosynthesis in the replication of Semliki Forest virus (SFV) in HeLa cells has been analyzed by the use of cerulenin, an inhibitor of lipid synthesis. The presence of this agent from the beginning of infection blocks the appearance of viral proteins. However, when the antibiotic is added at later stages of infection it has no effect on protein synthesis, the cleavage of viral proteins and their acylation by palmitic acid. Cerulenin is a powerful inhibitor of viral RNA synthesis, as analyzed by [3H]uridine incorporation, incorporation of [32P]phosphate into viral replication complexes, or Northern blot analysis of viral RNAs hybridized with minus- or plus-stranded riboprobes. Finally, analysis of phospholipids made in SFV-infected cells indicates that viral infection clearly stimulates the synthesis of phosphatidyl choline and modifies the membrane formed as analyzed by sucrose gradient centrifugation. Cerulenin blocks the synthesis of phospholipids and inhibits the formation of new membranes. These results show that, when the synthesis of lipids is blocked by cerulenin, SFV RNA replication is hampered, suggesting that the synthesis of viral RNAs needs continuous lipid synthesis and membrane formation.

Production of infectious particles from defective human immunodeficiency virus type 1 (HIV-1)-producing cell clones by superinfection with infectious HIV-1

A total of 81 cell clones persistently infected with the LAV-1 or HTLV-IIIB strain of human immunodeficiency virus type 1 (HIV-1) was isolated from cells which were obtained by serial passage of some proliferating MT-4 cells after a drastic cytolysis of most cells by HIV-1-infection. These cell clones were classified into 8 types (I to VIII) in terms of the expression of HIV-1 antigens, syncytium formation capacity, and reverse transcriptase activity and infectivity of virus particles in the culture fluid. Type I cell clones were producers of infectious HIV-1 particles, while types II to VIII cell clones did not produce infectious HIV-1 or were producers of uninfectious defective HIV-1 particles. Immunoprecipitation followed by SDS-polyacrylamide gel electrophoresis (PAGE) showed that the gag precursor protein in L-2 cell clone (type IV) was not cleaved to mature gag proteins, while the env precursor protein on L-3 cell clone (type III) was not cleaved to mature env protein. H-7 cell clone (type VIII) did not express any HIV-1 antigen. All these cell clones after the superinfection with infectious HIV-1 synthesized intact gag and env proteins, which were, at least in part, related to the HIV-1 genome persistently present in the cell clones before the superinfection, resulting in production of infectious HIV-1. For example, it was found that L-2 cell clone contained a single copy of the LAV-1 genome per haploid cell and produced doughnut-shaped particles. On the other hand, the cell clone isolated from the L-2 cell clone superinfected with infectious HTLV-IIIB contained the integrated HTLV-IIIB genome in addition to the LAV-1 genome present before the superinfection, and produced intact HIV-1 particles in addition to doughnut-shaped particles from a single cell. These results indicate that complementation and/or genetic recombination events in the superinfected cells may account for the production of infectious intact HIV-1 virions.

Molecular cloning of a novel isolate of feline immunodeficiency virus biologically and genetically different from the original U.S. isolate

The Japanese isolate (TM1 strain) of feline immunodeficiency virus (FIV) which replicates in a feline CD4 (fCD4)-positive lymphoblastoid cell line (MYA-1 cells) was molecularly cloned from extrachromosomal closed circular DNA. The restriction map of the clone, termed pFTM 191 complete genome (CG), showed a considerable difference from that of the U.S. isolate (Petaluma strain) of FIV. The sequence homology in the long terminal repeat between the TM1 and Petaluma strain was 82%. The pFTM 191 CG was biologically active after transfection into Crandell feline kidney cells which were permissive for replication of FIV Petaluma. However, the progeny virions could not reinfect fCD4-negative Crandell feline kidney cells but could infect fCD4-positive MYA-1 cells. When a specific-pathogen-free cat was inoculated with the virus derived from the pFTM 191 CG, the cat seroconverted within 8 weeks postinoculation and FIV was reisolated at 4, 8, and 20 weeks postinoculation. These results indicate the infectivity of the pFTM 191 CG in vivo.

In vitro inhibition of HIV-1 proteinase by cerulenin

Retroviruses encode proteinases necessary for the proteolytic processing of the viral gag and gag-pol precursor proteins. These enzymes have been shown to be structurally and functionally related to aspartyl proteinases such as pepsin and renin. Cerulenin is a naturally occurring antibiotic, commonly used as an inhibitor of fatty acid synthesis. Cerulenin has been observed to inhibit production of Rous sarcoma virus and murine leukaemia virus by infected cells, possibly by interfering with proteolytic processing of viral precursor proteins. We show here that cerulenin inhibits the action of the HIV-1 proteinase in vitro, using 3 substrates: a synthetic heptapeptide (SQNYPIV) which corresponds to the sequence at the HIV-1 gag p17/p24 junction, a bacterially expressed gag precursor, and purified 66 kDa reverse transcriptase. Inhibition of cleavage by HIV-1 proteinase required preincubation with cerulenin. Cerulenin also inactivates endothiapepsin, a well-characterised fungal aspartyl proteinase, suggesting that the action of cerulenin is a function of the common active site structure of the retroviral and aspartic proteinases. Molecular modelling suggests that cerulenin possesses several of the necessary structural features of an inhibitor of aspartyl proteinases and retroviral proteinases. Although cerulenin itself is cytotoxic and inappropriate for clinical use, it may provide leads for the rational design of inhibitors of the HIV proteinase which could have application in the chemotherapy of AIDS.

Synthetic non-peptide inhibitors of HIV protease

We have studied the inhibition of HIV protease by the antifungal antibiotic cerulenin, as well as by several related synthetic, structurally simpler analogs. The effect of these compounds on HIV protease was conveniently studied by monitoring the cleavage of an authentic single peptide bond in a synthetic nonapeptide corresponding to a natural cleavage site in HIV-1 gag precursor polyprotein. The relative inhibitory effects of these compounds have afforded an insight into the structural characteristics which impart antiprotease activity.

Inhibition of bacterially expressed HIV protease activity determined by an in vitro cleavage assay with MuLV Pr65gag

HIV protease is a virally coded enzyme that cleaves gag as well as gag-pol precursor polyproteins into functional products needed for virus assembly. A pUC plasmid containing an HIV insert starting at the 5' end of the pol gene and ending just inside the intergrase coding sequence was expressed in E. coli. It provided an 11 kD gene product (protease) that specifically cleaved the Gazdar MuLV Pr65gag precursor into Pr40gag (p30 + p10) and Pr27gag (p15 + p12) intermediates, as well as lower molecular weight gag-encoded products. These were detected by immunoblotting with either MuLV anti-p30 or p12 sera. Using cleavage of MuLV Pr65gag as an assay system, pepstatin A, fusidic acid, and cerulenin were observed to inhibit HIV protease cleavage by greater than 50% at concentrations of 0.1, 0.2-0.5, and 0.5 mM, respectively.

Processing of the structural proteins of human immunodeficiency virus type 1 in the presence of monensin and cerulenin

The synthesis and processing of structural proteins of human immunodeficiency virus type 1 (HIV-1) were studied in infected cells treated with monensin and cerulenin. In MOLT-3 cells chronically infected with HTLV-IIIB, monensin inhibited the proteolytic cleavage of the env-coded polyprotein gp160 to gp120, leading to the accumulation of the precursor gp160. The formation of syncytia normally observed when CEM cells are cocultivated with HIV-1-infected MOLT-3 cells was significantly inhibited in the presence of monensin. The effect of the ionophore on the culture was reversible, as withdrawal of monensin from the medium restored the ability of the cells to form syncytia with CEM cells and led to the resumption of the processing of gp160 to gp120. Monensin did not affect the synthesis and processing of gag-coded proteins and regulatory proteins. Cerulenin, an inhibitor of de novo fatty acid biosynthesis, inhibited the myristoylation and the proteolytic cleavage of the gag-coded polyprotein Pr53gag to p24 but did not affect the processing of gp160. However, use for monensin and cerulenin as antiviral agents for treatment of HIV-1 infection cannot be foreseen because of the pronounced in vitro toxicity observed.

Defective human immunodeficiency virus (HIV) particles produced by cloned cells of HTLV-I-carrying MT-4 cells persistently infected with HIV

Persistently HIV-infected cell lines were isolated from surviving and proliferating cells after infection of HTLV-I-carrying MT-4 cells with cell-free human immunodeficiency virus (HIV); HTLV-IIIB and LAV. The media of the cloned cell cultures did not cause HIV infection of MT-4, MOLT-4, TALL-1, or HL-60 cells. Most of the constituents of the virus in the media were env proteins and many defective doughnut-shaped particles released from the cells were identified by electron microscopy.

The effect of cerulenin on the synthesis of the precursor gag polyprotein in defective murine leukemia and sarcoma virus producing cell lines

The effect of cerulenin, an inhibitor of de novo fatty acid (and cholesterol) biosynthesis, on the synthesis of the precursor gag polyprotein, Pr65gag in a defective murine leukemia virus (334C) producing murine cell line (3JE) and a defective murine sarcoma virus (Gazdar) producing hamster cell line (HTG-2) was examined. In contrast to Moloney murine leukemia virus (M-MuLV) producing cell lines (MJD-54, clone 2) the amount of the Pr65gag remaining in the presence of cerulenin (20 micrograms/ml) was greatly reduced in both defective virus-infected cells. This effect appears specific for the Pr65gag polyprotein, since the env precursor polyprotein Pr80env was normally synthesized and remained undegraded in cerulenin-treated 3JE-infected cells. Thin-section electron micrographs showed an increased accumulation of virion particles in vesicles of treated HTG-2 cells.