Safety, activity, and pharmacokinetics of GLQ223 in patients with AIDS and AIDS-related complex

A Sixty-Year Research and Development of Trichosanthin, a Ribosome-Inactivating Protein

Tian Hua Fen, a herbal powder extract that contains trichosanthin (TCS), was used as an abortifacient in traditional Chinese medicine. In 1972, TCS was purified to alleviate the side effects. Because of its clinical applications, TCS became one of the most active research areas in the 1960s to the 1980s in China. These include obtaining the sequence information in the 1980s and the crystal structure in 1995. The replication block of TCS on human immunodeficiency virus in lymphocytes and macrophages was found in 1989 and started a new chapter of its development. Clinical studies were subsequently conducted. TCS was also found to have the potential for gastric and colorectal cancer treatment. Studies on its mechanism showed TCS acts as an rRNA N-glycosylase (EC 3.2.2.22) by hydrolyzing and depurinating A-4324 in α-sarcin/ricin loop on 28S rRNA of rat ribosome. Its interaction with acidic ribosomal stalk proteins was revealed in 2007, and its trafficking in mammalian cells was elucidated in the 2000s. The adverse drug reactions, such as inducing immune responses, short plasma half-life, and non-specificity, somehow became the obstacles to its usage. Immunotoxins, sequence modification, or coupling with polyethylene glycerol and dextran were developed to improve the pharmacological properties. TCS has nicely shown the scientific basis of traditional Chinese medicine and how its research and development have expanded the knowledge and applications of ribosome-inactivating proteins.

Natural Products with Inhibitory Activity against Human Immunodeficiency Virus Type 1

Infections caused by human immunodeficiency virus (HIV) are considered one of the main public health problems worldwide. Antiretroviral therapy (ART) is the current modality of treatment for HIV-1 infection. It comprises the combined use of several drugs and can decrease the viral load and increase the CD4⁺ T cell count in patients with HIV-1 infection, thereby proving to be an effective modality. This therapy significantly decreases the rate of morbidity and mortality owing to acquired immunodeficiency syndrome (AIDS) and prolongs and improves the quality of life of infected patients. However, nonadherence to ART may increase viral resistance to antiretroviral drugs and transmission of drug-resistant strains of HIV. Therefore, it is necessary to continue research for compounds with anti-HIV-1 activity, exhibiting a potential for the development of an alternative or complementary therapy to ART with low cost and fewer side effects. Natural products and their derivatives represent an excellent option owing to their therapeutic potential against HIV. Currently, the derivatives of natural products available as anti-HIV-1 agents include zidovudine, an arabinonucleoside derivative of the Caribbean marine sponge (Tectitethya crypta), which inhibits the reverse transcriptase of the virus. This was the first antiviral agent approved for treatment of HIV infection. Additionally, bevirimat (isolated from Syzygium claviflorum) and calanolide A (isolated from Calophyllum sp.) are inhibitors of viral maturation and reverse transcription process, respectively. In the present review, we aimed to describe the wide repertoire of natural compounds exhibiting anti-HIV-1 activity that can be considered for designing new therapeutic strategies to curb the HIV pandemic.

Antiviral Activity of Ribosome-Inactivating Proteins

Ribosome-inactivating proteins (RIPs) are rRNA N-glycosylases from plants (EC 3.2.2.22) that inactivate ribosomes thus inhibiting protein synthesis. The antiviral properties of RIPs have been investigated for more than four decades. However, interest in these proteins is rising due to the emergence of infectious diseases caused by new viruses and the difficulty in treating viral infections. On the other hand, there is a growing need to control crop diseases without resorting to the use of phytosanitary products which are very harmful to the environment and in this respect, RIPs have been shown as a promising tool that can be used to obtain transgenic plants resistant to viruses. The way in which RIPs exert their antiviral effect continues to be the subject of intense research and several mechanisms of action have been proposed. The purpose of this review is to examine the research studies that deal with this matter, placing special emphasis on the most recent findings.

In-silico analysis of ligand-receptor binding patterns of α-MMC, TCS and MAP30 protein to LRP1 receptor

Alpha-momorcharin (α-MMC), trichosanthin (TCS), and momordica anti-HIV protein of 30 kD (MAP30) are potential anti-tumor drug candidates but have cytotoxicity to normal cells. The binding of these proteins to LRP1 receptor and the subsequent endocytosis are essential to their cytotoxicity, but this binding process remains largely unknown. This study, in-silico analysis of the binding patterns, was conducted via the protein-protein docking software, ZDOCK 3.0.2 package, to better understand the binding process. Specifically, α-MMC, TCS and MAP30 were selected and bound to binding subunits CR56 and CR17 of LRP1. After docking, the 10 best docking solutions are retained based on the default ZDOCK scores and used for structural assessment. Our results showed that, α-MMC bound to LRP1 stably at the amino acid residues 1-20, at which 8 residues formed 21 hydrogen bonds with 15 residues of CR56 and 10 residues formed 15 hydrogen bonds with 12 residues of CR17. In contrast, TCS and MAP30 bound mainly to LRP1 at the residues 1-57/79-150 and residues 58-102, respectively, which were functional domains of TCS and MAP30. Since residues 1-20 are outside the functional domain of α-MMC, α-MMC is considered more suitable to attenuate by mutating the receptor binding site. Thus, our analysis lays the foundation for future genetic engineering work on α-MMC, and makes important contributions to its potential clinical use in cancer treatment.

Engineering of Ribosome-inactivating Proteins for Improving Pharmacological Properties

Ribosome-inactivating proteins (RIPs) are N-glycosidases, which depurinate a specific adenine residue in the conserved α-sarcin/ricin loop (α-SRL) of rRNA. This loop is important for anchoring elongation factor (EF-G for prokaryote or eEF2 for eukaryote) in mRNA translocation. Translation is inhibited after the attack. RIPs therefore may have been applied for anti-cancer, and anti-virus and other therapeutic applications. The main obstacles of treatment with RIPs include short plasma half-life, non-selective cytotoxicity and antigenicity. This review focuses on the strategies used to improve the pharmacological properties of RIPs on human immunodeficiency virus (HIV) and cancers. Coupling with polyethylene glycol (PEG) increases plasma time and reduces antigenicity. RIPs conjugated with antibodies to form immunotoxins increase the selective toxicity to target cells. The prospects for future development on the engineering of RIPs for improving their pharmacological properties are also discussed.

Pharmacodynamic material basis of traditional Chinese medicine based on biomacromolecules: a review

Biomacromolecules, the first components of bioactive substances in traditional Chinese medicines (TCM) have wide bioactivity-related efficacy but have not yet been fully appreciated compared to small molecule components. The present review brings a novel and systemic point of view to deepen the understanding of the pharmacodynamic material basis of TCM based on biomacromolecules (polysaccharides, proteins and nucleic acids). Biomacromolecules have been, are and will have considerable roles in the efficacy of Chinese medicine, as evidenced by the number of biological activities related to traditional clinical efficacy. The direct and indirect mechanisms of biomacromolecules are further accounted for in a variety of neurotransmitters, hormones, and immune substances to maintain immune function in both sensitive and stable equilibrium. The biological functions of biomacromolecules have been elaborated on in regard to their roles in the process of plant growth and development to the relationship between primary metabolism and secondary metabolism and to the indispensable role of polysaccharides, proteins, and nucleic acids in the quality formation of TCM. Understanding the functional properties and mechanisms of biological macromolecules will help to demystify the drug properties and health benefits of TCM.

Clinical Use of Toxic Proteins and Peptides from Tian Hua Fen and Scorpion Venom

Traditional Chinese Medicine (TCM) has been practiced in China for thousands of years. As a complementary and alternative treatment, herbal medicines that are frequently used in the TCM are the most accepted in the Western world. However, animal materials, which are equally important in the TCM practice, are not well-known in other countries. On the other hand, the Chinese doctors had documented the toxic profiles of hundreds of animals and plants thousand years ago. Furthermore, they saw the potential benefits of these materials and used their toxic properties to treat a wide variety of diseases, such as heavy pain and cancer. Since the 50s of the last century, efforts of the Chinese government and societies to modernize TCM have achieved tremendous scientific results in both laboratory and clinic. A number of toxic proteins have been isolated and their functions identified. Although most of the literature was written in Chinese, this review provide a summary, in English, regarding our knowledge of the clinical use of the toxic proteins isolated from a plant, Tian Hua Fen, and an animal, scorpion, both of which are famous toxic prescriptions in TCM.

Antifungal Proteins with Antiproliferative Activity on Cancer Cells and HIV-1 Enzyme Inhibitory Activity from Medicinal Plants and Medicinal Fungi

A variety of fungi, plants, and their different tissues are used in Traditional Chinese Medicine to improve health, and some of them are recommended for dietary therapy. Many of these plants and fungi contain antifungal proteins and peptides which suppress spore germination and hyphal growth in phytopathogenic fungi. The aim of this article is to review antifungal proteins produced by medicinal plants and fungi used in Chinese medicine which also possess anticancer and human immunodeficiency virus-1 (HIV-1) enzyme inhibitory activities.

Structural and Functional Investigation and Pharmacological Mechanism of Trichosanthin, a Type 1 Ribosome-Inactivating Protein

Trichosanthin (TCS) is an RNA N-glycosidase that depurinates adenine-4324 in the conserved α-sarcin/ricin loop (α-SRL) of rat 28 S ribosomal RNA (rRNA). TCS has only one chain, and is classified as type 1 ribosome-inactivating protein (RIP). Our structural studies revealed that TCS consists of two domains, with five conserved catalytic residues Tyr70, Tyr111, Glu160, Arg163 and Phe192 at the active cleft formed between them. We also found that the structural requirements of TCS to interact with the ribosomal stalk protein P2 C-terminal tail. The structural analyses suggest TCS attacks ribosomes by first binding to the C-terminal domain of ribosomal P protein. TCS exhibits a broad spectrum of biological and pharmacological activities including anti-tumor, anti-virus, and immune regulatory activities. This review summarizes an updated knowledge in the structural and functional studies and the mechanism of its multiple pharmacological effects.

Which Plant Proteins Are Involved in Antiviral Defense? Review on In Vivo and In Vitro Activities of Selected Plant Proteins against Viruses

Plants have evolved a variety of defense mechanisms to tackle virus attack. Endogenous plant proteins can function as virus suppressors. Different types of proteins mediate defense responses against plant viruses. Pathogenesis-related (PR) proteins are activated upon pathogen infections or in different stress situations and their production is one of many components in plant defense. Ribosome-inactivating proteins (RIPs) suppress translation by enzymatically damaging ribosomes and they have been found to have antiviral activity. RNA-binding proteins (RBPs) bind to target RNAs via specialized RNA-binding domain and can directly or indirectly function in plant defense system against RNA viruses. Proteins involved in silencing machinery, namely Dicer-like (DCL) proteins, Argonaute (AGO) proteins, and RNA-dependent RNA polymerases (RDRs) confer innate antiviral defense in plants as they are able to degrade foreign RNA of viral origin. This review aims to provide a comprehensive and up-to-date picture of plant proteins participating in antiviral defense. As a result we discuss proteins conferring plant antiviral resistance and their potential future applications in different fields of life including agriculture and medicine.

Ribosome-Inactivating Proteins from Plants: A Historical Overview

This review provides a historical overview of the research on plant ribosome-inactivating proteins (RIPs), starting from the first studies at the end of eighteenth century involving the purification of abrin and ricin, as well as the immunological experiments of Paul Erlich. Interest in these plant toxins was revived in 1970 by the observation of their anticancer activity, which has given rise to a large amount of research contributing to the development of various scientific fields. Biochemistry analyses succeeded in identifying the enzymatic activity of RIPs and allowed for a better understanding of the ribosomal machinery. Studies on RIP/cell interactions were able to detail the endocytosis and intracellular routing of ricin, thus increasing our knowledge of how cells handle exogenous proteins. The identification of new RIPs and the finding that most RIPs are single-chain polypeptides, together with their genetic sequencing, has aided in the development of new phylogenetic theories. Overall, the biological properties of these proteins, including their abortifacient, anticancer, antiviral and neurotoxic activities, suggest that RIPs could be utilized in agriculture and in many biomedical fields, including clinical drug development.

Cytotoxicity mechanism of α-MMC in normal liver cells through LRP1 mediated endocytosis and JNK activation

Alpha-momorcharin (α-MMC), a type I ribosome-inactivating protein isolated from Momordica charantia, is a potential drug candidate with strong anti-tumor activity. However, α-MMC has a severe hepatotoxicity when applied in vivo, which may greatly hinders its use in clinic in the future. The biological mechanism of hepatotoxicity induced by α-MMC is largely unknown, especially the mechanism by which α-MMC enters the hepatocytes. In this study, we investigated α-MMC-induced cytotoxicity in normal liver L02 cell line as well as the mechanism underlying it. As expected, α-MMC is more toxic in L02 cells than in various normal cells from other organs. The cytotoxic effect of α-MMC on L02 cells is found to be mediated through cell apoptosis as detected by flow cytometry and fluorescence microscopy. Importantly, α-MMC was shown to bind to a specific receptor on cell membrane, as the density of the cell membrane receptor is closely related to both the amount of α-MMC endocytosed and the cytotoxicity in different cell lines. By using LRP1 competitive inhibitor α2-M or siRNA targeting LRP1, we further identified that LRP1 protein served as the membrane receptor for α-MMC. Both α2-M and siRNA targeting LRP1 can significantly inhibit α-MMC's endocytosis as well as its cytotoxicity in L02 cells. In addition, it was found that α-MMC can activate the JNK signalling pathways via LRP1 in L02 cells. As JNK activation often leads to cell apoptosis, the activation of JNK may play an important role in α-MMC-induced cytotoxicity. To our knowledge, this is the first report showing that LRP1 mediates the cytotoxicity of α-MMC through (1) endocytosis and induced apoptosis and (2) the activation of the JNK pathway. Our findings shed light on the fundamental mechanism of hepatotoxicity of α-MMC and offer reference to understand its mechanism of lymphocytotoxicity and neurotoxicity.

Biological activities of ribosome-inactivating proteins and their possible applications as antimicrobial, anticancer, and anti-pest agents and in neuroscience research

Ribosome-inactivating proteins (RIPs) are enzymes which depurinate ribosomal RNA (rRNA), thus impeding the process of translation resulting in inhibition of protein synthesis. They are produced by various organisms including plants, fungi and bacteria. RIPs from plants are linked to plant defense due to their antiviral, antifungal, antibacterial, and insecticidal activities in which they can be applied in agriculture to combat microbial pathogens and pests. Their anticancer, antiviral, embryotoxic, and abortifacient properties may find medicinal applications. Besides, conjugation of RIPs with antibodies or other carriers to form immunotoxins has been found useful to research in neuroscience and anticancer therapy.

The recombinant maize ribosome-inactivating protein transiently reduces viral load in SHIV89.6 infected Chinese Rhesus Macaques

Ribosome inactivating proteins (RIPs) inhibit protein synthesis by depurinating the large ribosomal RNA and some are found to possess anti-human immunodeficiency virus (HIV) activity. Maize ribosome inactivating protein (RIP) has an internal inactivation loop which is proteolytically removed for full catalytic activity. Here, we showed that the recombinant active maize RIP protected chimeric simian-human immunodeficiency virus (SHIV) 89.6-infected macaque peripheral blood mononuclear cells from lysis ex vivo and transiently reduced plasma viral load in SHIV89.6-infected rhesus macaque model. No evidence of immune dysregulation and other obvious side-effects was found in the treated macaques. Our work demonstrates the potential development of maize RIP as an anti-HIV agent without impeding systemic immune functions.

Alpha-momorcharin (α-MMC) exerts effective anti-human breast tumor activities but has a narrow therapeutic window in vivo

Alpha-momorcharin (α-MMC), a ribosome inactivating protein (RIP) extracted from the seeds of Momordica charantia, exerts anti-tumor, antiviral, and anti-fungal activities. However, α-MMC has an obvious toxicity that limits its clinical application. We examined the effect of α-MMC on the inhibition of human breast cancer and assessed its general toxicity to find the therapeutic window in vivo for its potential clinical use. It was purified using column chromatography, and then injected into the xenograft nude mouse model induced by MDA-MB-231 and MCF-7. The anti-tumor efficacy was evaluated with T/C%. Next, the α-MMC was injected at a series of doses to Balb/C mice to assess its general toxicity. The MTT assay, the apoptosis test, and the cell cycle inhibition of α-MMC in human breast cancer cells were performed. In the xenografted tumors induced by MDA-MB-231 and MCF-7, α-MMC exerted an obvious inhibition effects on tumor growth at the dosage of 1.2mg/kg and 0.8 mg/kg. For in vivo toxicity experiments of α-MMC in Balb/C mice, the minimal toxic dose of α-MMC was 1.2mg/kg. Alpha-MMC induced apoptosis by increasing caspase3 activities, and the cell cycle was arrested at the G0/G1 or G2/M phases. The measurements of IC50 were 15.07 μg/mL, 33.66 μg/mL, 42.94 μg/mL for MDA-MB-231, MCF-7 and MDA-MB-453 respectively. Alpha-MMC exhibits anti-tumor effects in human breast cancer in vivo and in vitro. It inhibits breast cancer cells through the inhibition of tumor growth and induction of cell apoptosis. However, due to its obvious toxicity, α-MMC has a relatively narrow therapeutic window in vivo.

Trichosanthin inhibits breast cancer cell proliferation in both cell lines and nude mice by promotion of apoptosis

Breast cancer ranks as a common and severe neoplasia in women with increasing incidence as well as high risk of metastasis and relapse. Translational and laboratory-based clinical investigations of new/novel drugs are in progress. Medicinal plants are rich sources of biologically active natural products for drug development. The 27-kDa trichosanthin (TCS) is a ribosome inactivating protein purified from tubers of the Chinese herbal plant Trichosanthes kirilowii Maximowicz (common name Tian Hua Fen). In this study, we extended the potential medicinal applications of TCS from HIV, ferticide, hydatidiform moles, invasive moles, to breast cancer. We found that TCS manifested anti-proliferative and apoptosis-inducing activities in both estrogen-dependent human MCF-7 cells and estrogen-independent MDA-MB-231 cells. Flow cytometric analysis disclosed that TCS induced cell cycle arrest. Further studies revealed that TCS-induced tumor cell apoptosis was attributed to activation of both caspase-8 and caspase-9 regulated pathways. The subsequent events including caspase-3 activation, and increased PARP cleavage. With regard to cell morphology, stereotypical apoptotic features were observed. Moreover, in comparison with control, TCS- treated nude mice bearing MDA-MB-231 xenograft tumors exhibited significantly reduced tumor volume and tumor weight, due to the potent effect of TCS on tumor cell apoptosis as determined by the increase of caspase-3 activation, PARP cleavage, and DNA fragmentation using immunohistochemistry. Considering the clinical efficacy and relative safety of TCS on other human diseases, this work opens up new therapeutic avenues for patients with estrogen-dependent and/or estrogen-independent breast cancers.

Ribosome inactivating proteins from plants inhibiting viruses

Many plants contain ribosome inactivating proteins (RIPs) with N-glycosidase activity, which depurinate large ribosomal RNA and arrest protein synthesis. RIPs so far tested inhibit replication of mRNA as well as DNA viruses and these proteins, isolated from plants, are found to be effective against a broad range of viruses such as human immunodeficiency virus (HIV), hepatitis B virus (HBV) and herpes simplex virus (HSV). Most of the research work related to RIPs has been focused on antiviral activity against HIV; however, the exact mechanism of antiviral activity is still not clear. The mechanism of antiviral activity was thought to follow inactivation of the host cell ribosome, leading to inhibition of viral protein translation and host cell death. Enzymatic activity of RIPs is not limited to depurination of the large rRNA, in addition they can depurinate viral DNA as well as RNA. Recently, Phase I/II clinical trials have demonstrated the potential use of RIPs for treating patients with HIV disease. The aim of this review is to focus on various RIPs from plants associated with anti-HIV activity.

A switch-on mechanism to activate maize ribosome-inactivating protein for targeting HIV-infected cells

Maize ribosome-inactivating protein (RIP) is a plant toxin that inactivates eukaryotic ribosomes by depurinating a specific adenine residue at the α-sarcin/ricin loop of 28S rRNA. Maize RIP is first produced as a proenzyme with a 25-amino acid internal inactivation region on the protein surface. During germination, proteolytic removal of this internal inactivation region generates the active heterodimeric maize RIP with full N-glycosidase activity. This naturally occurring switch-on mechanism provides an opportunity for targeting the cytotoxin to pathogen-infected cells. Here, we report the addition of HIV-1 protease recognition sequences to the internal inactivation region and the activation of the maize RIP variants by HIV-1 protease in vitro and in HIV-infected cells. Among the variants generated, two were cleaved efficiently by HIV-1 protease. The HIV-1 protease-activated variants showed enhanced N-glycosidase activity in vivo as compared to their un-activated counterparts. They also possessed potent inhibitory effect on p24 antigen production in human T cells infected by two HIV-1 strains. This switch-on strategy for activating the enzymatic activity of maize RIP in target cells provides a platform for combating pathogens with a specific protease.

The anti-viral protein of trichosanthin penetrates into human immunodeficiency virus type 1

Trichosanthin (TCS) is a type I ribosome-inactivating protein with potent inhibitory activity against human immunodeficiency virus type 1, and has been clinically applied in acquired immunodeficiency syndrome (AIDS) therapy. Previous studies revealed that TCS recognized human immunodeficiency virus type 1 (HIV-1) particles. Here, we investigated the physical relationship between TCS and HIV-1 particles, and demonstrated that TCS penetrates into viral particles, where it is protected from various protease digestion. The penetration of TCS exerts no obvious effect on viral integrity. FYY140-142, D176, and K177 were identified as key amino acid residues for the membranetranslocation process. Moreover, TCS penetrated into HIV-1 virions showed potent anti-viral activity. Overall, the observations suggest that the penetration of TCS into HIV-1 particles may be important for eliminating the virus.

Trichosanthin inhibits integration of human immunodeficiency virus type 1 through depurinating the long-terminal repeats

Trichosanthin (TCS) is a type I ribosome-inactivating protein with potent inhibitory activity against human immunodeficiency virus type 1. However, the anti-viral mechanism remains elusive. By a well-accepted HIV-1 integration assay, we demonstrated that TCS prevents HIV-1 DNA integration in a dose dependent manner in cell culture. At the same condition, TCS fails to induce obvious cytotoxicity and is also unable to interference viral early events such as viral entry, uncoating or reverse transcription. The HIV-1 integrase can integrate HIV-1 long-terminal repeats into cellular chromosome. The interaction of TCS with these viral integration components was also examined, indicating that TCS does not interact with HIV-1 integrase by the GST-pull down assay, but binds to the long terminal repeats in a transient manner. We further revealed that TCS can efficiently depurinate HIV-1 long-terminal repeats, which may be responsible for the inhibitory activity on HIV-1 integration. In conclusion, we elucidated that TCS specifically inhibits HIV-1 integration by depurinating the long-terminal repeats.

Effect of site-directed PEGylation of trichosanthin on its biological activity, immunogenicity, and pharmacokinetics

Trichosanthin (TCS) is a type I ribosome-inactivating protein (RIP) with multiple biological and pharmacological activities. It has been approved effective in the clinical treatment of AIDS and tumor, but its strong immunogenicity and short plasma half-life have limited the clinical administration. To reduce the immunogenicity and prolong the plasma half-life of this compound, three TCS muteins (M(1), M(2) and M(3)) and two PEGylated TCS muteins (PM(1) and PM(2)) were constructed by site-directed mutagenesis and PEGylation, respectively. Compared with the unmodified TCS, both PEGylated TCS showed a 3- to 4-fold decrease in immunogenicity, a 0.5- to 0.8-fold decrease in non-specific toxicity, and a 4.5- to 6-fold increase in plasma half-life. But there is a problem of activity reduction. The increased circulating half-life in vivo may compensate for the reduced activity. Together with the other benefits of PEGylation such as reduced immunogenicity and toxicity, it is worthwhile to further explore the potential application of the PEGylated TCS as a better therapeutic agent for AIDS and tumor.

Mapping the antigenic determinants and reducing the immunogenicity of trichosanthin by site-directed mutagenesis

Trichosanthin (TCS) is a type I ribosome-inactivating protein (RIP) possessing multiple pharmacological properties. One of its interesting properties is to inhibit human immunodeficiency virus (HIV) replication but its strong immunogenicity has limited the repeated clinical administration. To map the antigenic determinants and reduce the immunogenicity of TCS, two potential antigenic sites (YFF81-83 and KR173-174) were identified by computer modeling, and then three TCS mutants namely TCS(YFF81-83ACS), TCS(KR173-174CG), and TCS(YFF-KR) were constructed by site-directed mutagenesis. The RI activity and DNase-like activity of the three constructed TCS mutants were similar to natural TCS but with much lower immunogenicity. Results suggested that the two selected sites are all located at or near the antigenic determinants of TCS. In toxicity studies, the LD(50) of the three TCS mutants was not different from natural TCS. These findings would be useful in designing a better therapeutic agent for AIDS.

Trichosanthin suppresses the elevation of p38 MAPK, and Bcl-2 induced by HSV-1 infection in Vero cells

Trichosanthin (TCS) is a type 1 ribosome-inactivating protein (RIP) effective against HIV-1 and HSV-1 replication. The mechanism of its antiviral activity is not clear. Many believe that it is related to ribosome inactivation. Some RIPs and viral infection affect the phosphorylation of MAPK and Bcl-2 and these proteins may be the common element linking RIP and viral infection. This study investigated the effect of HSV-1 infection on p38 MAPK and Bcl-2 as well as possible interference by TCS. Results showed that HSV-1 infection induced an elevation of phosphorylated p38 and Bcl-2 in Vero cells, which could be partially blocked by TCS. At the same time, both viral replication and host cells viability were lowered. Viral replication, Vero cell viability, p38 MAPK and Bcl-2 were further reduced with the addition of a p38 MAPK inhibitor (SB203580). This suggested that TCS may interfere with MAPK and Bcl-2 signals generated by infection leading to inhibition of viral replication. In summary, our results demonstrated that HSV-1 infection in Vero cells induced an elevation of p38 MAPK and Bcl-2. TCS suppressed this rise and reduced viral replication. The MAPK family may play a role in the antiviral mechanism of TCS.

Enhanced apoptotic action of trichosanthin in HIV-1 infected cells

Trichosanthin (TCS) is a type 1 ribosome-inactivating protein (RIP) effective against HIV-1 replication. The mechanism is not clear. Present results suggested that the antiviral action may be partly mediated through enhanced apoptosis on infected cells. TCS induced apoptosis in normal H9 cells and this action was more potent in those infected with HIV-1. In flow cytometry study, TCS induced larger population of apoptotic H9 cells chronically infected with HIV-1 in a dose-dependent manner. At TCS concentration of 25 microg/ml, 8.4% of normal H9 cells were found to be apoptotic whereas the same concentration induced 24.5% in HIV-1 chronically infected cells. Such difference was not found in the control experiments without TCS treatment. Two other studies supported this action. Cytotoxic study showed that cell viability was always lower in HIV-1 infected cells after TCS treatment, and DNA fragmentation study confirmed more laddering in infected cells. The mechanism of TCS induced apoptosis in normal or infected H9 cells is not clear. Results in this study demonstrated that TCS is more effective in inducing apoptosis in HIV-1 infected cells. This may explain in part the antiviral action of TCS.

Role of blood transfusion in trichosanthin-induced anaphylaxis

Trichosanthin (TCS) is a type 1 ribosome inactivating protein extracted from Chinese medicinal herb. It possesses various biological functions such as abortifacient, anti-tumor and anti-viral activities. Clinical trial of this compound against human immunodeficiency virus (HIV) had been conducted. However, its use is limited by its high immunogenicity that elicits hypersensitivity reaction. This may lead to fatal anaphylactic response. The study described an approach of using blood transfusion to reduce TCS induced anaphylaxis in rats using a cross-circulation model. A TCS-sensitized Sprague Dawley rat was connected to a normal rat via the femoral vessels in a cross-circulation circuit before antigenic challenge. The donor rat served as a blood exchange basin to lower the level of the blood-borne components responsible for the anaphylactic reaction in the sensitized rat. Our results showed that cross-circulation shortened the duration of circulatory hypotension and reduced mortality of TCS induced anaphylaxis. The control group not undergoing cross-circulation had a mortality of 50% at 2h post-TCS challenge and there was no mortality in the cross-circulation group. This demonstrated that prior blood transfusion can be one of the alternatives to reduce anaphylactic response of TCS.

Recent advances in trichosanthin, a ribosome-inactivating protein with multiple pharmacological properties

Trichosanthin (TCS), a ribosome-inactivating protein extracted from the root tuber of Chinese medicinal herb Trichosanthes kirilowii Maximowicz, has multiple pharmacological properties including abortifacient, anti-tumor and anti-HIV. It is traditionally used to induce abortion but its antigenicity and short plasma half-life have limited the repeated clinical administration. In this review, work to locating antigenic sites and prolonging plasma half-life are discussed. Studies on structure-function relationship and mechanism of cell entry are also covered. Recently, TCS has been found to induce apoptosis, enhance the action of chemokines and inhibit HIV-1 integrase. These findings give new insights on the pharmacological properties of TCS and other members of ribosome-inactivating proteins.

Ribosome-inactivating proteins

The main results of the research performed in the last 30 years on ribosome-inactivating proteins (RIPs) are reviewed, with emphasis on the new, controversial and uncertain aspects. The nature, distribution, mechanism of action and properties of these proteins are briefly reported, together with their possible applications. A pattern appears of a still largely unexplored subject, whose role in nature is probably important, and not limited to the biology of plants, since RIPs have been found also in other organisms.

Site-directed PEGylation of trichosanthin retained its anti-HIV activity with reduced potency in vitro

Trichosanthin (TCS) was the first ribosome inactivating protein found to possess anti-HIV-1 activity. Phase I/II clinical trial of this compound had been done. Antigenicity and short plasma half-life were the major side effects preventing further clinical trial. Modification of TCS is therefore necessary to revive the interest to develop this compound as an anti-HIV agent. Three potential antigenic sites (Ser-7, Lys-173, and Gln-219) were identified by computer modeling. Through site-directed mutagenesis, these three antigenic amino acids were mutated to a cysteine residue resulting in 3 TCS mutants, namely S7C, K173C, and Q219C. These mutants were further coupled to polyethylene glycol with a molecular size of 20kDa (PEG) via the cysteine residue. This produced another three TCS derivatives, namely PEG20k-S7C, PEG20k-K173C, and PEG20k-Q219C. PEGylation had been widely used recently to decrease immunogenicity by masking the antigenic sites and prolong plasma half-life by expanding the molecular size. The in vitro anti-HIV-1 activity of these mutants and derivatives was tested. Results showed that the anti-HIV-1 activity of S7C, K173C, and Q219C was decreased by about 1.5- to 5.5-fold with slightly lower cytotoxicity. On the other hand, PEGylation produced larger decrease (20- to 30-fold) in anti-HIV activity. Cytotoxicity was, however, weakened only slightly by about 3-fold. The in vitro study showed that the anti-HIV activity of PEGylated TCS was retained with reduced potency. The in vivo activity is expected to have only slightly changed due to other beneficial effects like prolonged half-life.

Independency of anti-HIV-1 activity from ribosome-inactivating activity of trichosanthin

Trichosanthin (TCS) is a type I ribosome-inactivating (RI) protein possessing multiple biological and pharmacological activities. Its major action is inhibition of human immunodeficiency virus (HIV) replication but the mechanism is still elusive. All evidences showed that this action is related to its RI activity. Previous studies found that TCS mutants with reduced RI activity simultaneously lost some anti-HIV activity. In this study, an exception was demonstrated by two TCS mutants retaining almost all RI activity but were devoid of anti-HIV-1 activity. Five mutants were constructed by using site-directed mutagenesis with either deletion or addition of amino acids to the C-terminal sequence. Results showed that the RI activity of mutants with C-terminal deletion mutants (TCS(C2), TCS(C4), and TCS(C14)) decreased by 1.2-3.3-fold with parallel downshifting of its anti-HIV-1 activity (1.4-4.8-fold). Another two mutants, TCS(C19aa) and TCS(KDEL) having 19 amino acid extension and a KDEL signal sequence added to the C-terminal sequence, retained all RI activity but subsequently lost most of the anti-HIV-1 activity. These findings suggested that ribosome inactivation alone might not be adequate to explain the anti-HIV action of TCS.

Anti-HIV-1 property of trichosanthin correlates with its ribosome inactivating activity

Trichosanthin (TCS) is a type I ribosome inactivating (RI) protein possessing anti-tumor and antiviral activity, including human immunodeficiency virus (HIV). The mechanism of these actions is not entirely clear, but is generally attributed to its RI property. In order to study the relationship between the anti-HIV-1 activity of TCS and its RI activity, three TCS mutants with different RI activities were constructed by using site-directed mutagenesis. The anti-HIV-1 activities of the three mutants were tested in vitro. Results showed that two TCS mutants, namely TCS(M(120-123)), TCS(E160A/E189A), with the greatest decrease in RI activity, lost almost all of the anti-HIV activity and cytopathic effect. Another mutant TCS(R122G), which exhibited a 160-fold decrease in RI activity, retained some anti-HIV activity. The results from this study suggested that RI activity of TCS may have significant contribution to its anti-HIV-1 property.

Relationship between trichosanthin cytotoxicity and its intracellular concentration

Trichosanthin (TCS) is a type I ribosome-inactivating protein with board spectrum of biological activity. Toxicity of this compound differs in different cell lines and this study examined the cause of such difference. It is generally believed that TCS toxicity is mediated through intracellular ribosome inactivation. Therefore, TCS toxicity should be determined by the amount inside cells rather than outside. Three different cell types IC21, JAR and Vero cell lines were chosen with high, medium and low sensitivity to TCS. Intracellular concentrations of fluorescein isothiocyanate labeled TCS were determined by laser scanning confocal microscopy. A good relationship was demonstrated between intracellular TCS concentration and toxicity. Highest intracellular concentration was found in IC21, followed by JAR, and lowest in Vero cells. When the intracellular TCS concentrations in these cells were reduced by using a competitive inhibitor to block cell entry, cytotoxicity was not observed. In conclusion, there is strong evidence to indicate that cytotoxicity of TCS is dependent on its intracellular concentration. Variation of cytotoxicity in different cells may be related to the mechanisms affecting its internalization.

Enhancement of the anti-herpetic effect of trichosanthin by acyclovir and interferon

Trichosanthin (TCS) is a type I ribosome-inactivating protein that has a wide range of pharmacological activities. The present study investigated the effectiveness of TCS on herpes simplex virus (HSV-1). The anti-viral activity and toxicity of TCS on Vero cells were measured. Results showed that the ED(50), TD(50) and the therapeutic indices were 38.5, 416.5 and 10.9 microg/ml, respectively. Anti-viral activity of TCS was substantially potentiated when it was used in conjunction with other anti-viral agents. The ED(50) of TCS was reduced 125-fold by acyclovir at a concentration of 0.001 microg/ml, which was practically devoid of significant anti-viral activity. Similarly, the ED(50) of TCS was reduced 100-fold by interferon-alpha2a at a concentration of 100 IU/ml. In conclusion, TCS is effective against HSV-1 and other anti-viral agents such as acyclovir or interferon can potentiate its action substantially.

The interaction of trichosanthin with supported phospholipid membranes studied by surface plasmon resonance

Trichosanthin (TCS) is a toxic protein isolated from a Chinese herbal medicine, the root tuber of Trichosanthes kirilowii Maximowicz of the Curcurbitaceae family. It is now used in China to terminate early and mid-trimester pregnancies. The ribosome inactivating property is thought to be account for its toxicity; it can inactivate the eukaryotic ribosome through its RNA N-glycosidase activity. The interactions of TCS with biological membrane is thought to be essential for its physiological effect, for it must get across the membrane before it can enter the cytoplasm and exert its RIP function. In the present work, the interaction of TCS with supported phospholipid monolayers is studied by surface plasmon resonance. The results show that electrostatic forces dominate the interaction between TCS and negatively charged phospholipid containing membranes under acid condition and that both the pH value and the ionic strength can influence its binding. It is proposed that, besides electrostatic forces, hydrophobic interaction may also be involved in the binding process.

Reactive oxygen species involved in trichosanthin-induced apoptosis of human choriocarcinoma cells

The type-I ribosome-inactivating protein trichosanthin (TCS) has a broad spectrum of biological and pharmacological activities, including abortifacient, anti-tumour and anti-HIV activities. We have found for the first time that TCS stimulated the production of reactive oxygen species (ROS) in JAR cells (a human choriocarcinoma cell line) in a time- and concentration-dependent manner by using the fluorescent probe 2',7'-dichlorofluorescein diacetate with confocal laser scanning microscopy. ESR spectral studies and the inhibition of ROS formation by the superoxide radical anion (O(2)(-.)) scavenger superoxide dismutase, the H(2)O(2) scavenger catalase and the hydroxyl radical (OH(.)) scavenger mannitol suggested the involvement of O(2)(-.), H(2)O(2) and OH(.). TCS-induced ROS formation was shown to be dependent on the presence of both extracellular and intracellular Ca(2+); moreover, ROS production paralleled the intracellular Ca(2+) elevation induced by TCS, suggesting that ROS production might be a consequence of Ca(2+) signalling. TCS-induced activation of caspase-3 was initiated within 2 h; however, TCS-induced production of ROS was initiated within 5 min, suggesting that the production of ROS preceded the activation of caspase-3. Simultaneous observation of the nuclear morphological changes via two-photon laser scanning microscopy and ROS production via confocal laser scanning microscopy revealed that ROS is involved in the apoptosis of JAR cells. The involvement of ROS was also confirmed by the inhibition of TCS-induced cell death by the antioxidant Trolox and the ROS scavengers catalase and mannitol. Diethylenetriaminepenta-acetic acid, an inhibitor of metal-facilitated OH(.) formation, markedly inhibited TCS-induced cell death, suggesting that TCS induced OH(.) formation via the Fenton reaction. The finding that ROS is involved in the TCS-induced apoptosis of JAR cells might provide new insight into the anti-tumour and anti-HIV mechanism of TCS.

Trichosanthin interacts with acidic ribosomal proteins P0 and P1 and mitotic checkpoint protein MAD2B

Trichosanthin is a ribosome-inactivating protein with multiple pharmacological properties. By a yeast two-hybrid system, ribosomal phosphoproteins P0 and P1 and a putative mitotic checkpoint protein, MAD2B, were found to interact with an active-site mutated trichosanthin (TCS). The interactions were verified by an in vitro binding assay of recombinant wild-type TCS and target proteins. The interaction domain of P0 was mapped to amino acids 220-273, which had been previously reported to be involved in the interaction with P1 and P2 in yeast. Consistent with our previous finding that the last seven residues of TCS are not essential for an active conformation, the same deletion did not affect the interaction with P0. Our present study suggests that TCS may disrupt the binding of elongation factors to the P-complex, in addition to the well-known N-glycosidase activity for ribosome inactivation.

Trichosanthin interacts with and enters cells via LDL receptor family members

The type-I ribosome-inactivating protein trichosanthin displays selective cytotoxicity, suggesting specific mechanisms for entry into cells. Here we show that trichosanthin binds specifically to the endocytic receptors LRP and megalin, and that binding as well as uptake into cells is inhibited by the receptor-associated protein (RAP). The results suggest that the known abortifacient and renotoxic actions of trichosanthin are caused by LRP-mediated uptake in trophoblasts and megalin-mediated uptake in proximal tubule epithelial cells, respectively.

Engineering of a mini-trichosanthin that has lower antigenicity by deleting its C-terminal amino acid residues

Trichosanthin is a ribosome-inactivating protein that possesses antitumor and antiviral activities. Clinical trials of trichosanthin on AIDS patients, however, elicit anaphylactic reactions. To reduce the antigenicity of trichosanthin as a drug while preserving its biological activity, the C-terminal domain (residues 203 to 247), which contains a putative antigenic site, was systemically deleted. We have found that the minimum length of trichosanthin that can fold into an active conformation is residue 1 to 240. The mini-trichosanthin (C7) generated by deleting the last seven C-terminal amino acid residues has 2.7-fold decrease in antigenicity, 10-fold reduction in in vitro ribosome-inactivation activity, and in vivo cytotoxicity toward K562 cells, and 2-fold reduction in abortificient activity. Structural analyses of C7 indicate decrease in the helix content, increased exposure of Trp192, and lower thermodynamic stability. The deletion of the C-terminal residues (Leu241 to Ala247) probably perturbs local structure of the C-terminal antigenic epitope that results in the decrease in antigenicity and activities of C7.

Reducing the immunogenicity and improving the in vivo activity of trichosanthin by site-directed pegylation

PEG modification (PEGylation) has been shown to reduce immunogenicity and prolong circulating half-life of proteins. In the present study, site-directed PEGylation was used to reduce immunogenicity and prolong plasma half-life of trichosanthin (TCS). Four TCS mutants, i.e. S7C, Q219C, K173C and [K173C,Q219C] (KQ), were constructed by site-directed mutagenesis. PEG modifications were done by reacting PEG5k-maleimide or PEG20k-maleimide reagent with the newly introduced cysteine residue of the mutants. The plasma clearance rate of PEGylated TCS mutants decreased up to 100-fold and the decrease was inversely proportional to the effective molecular size. The in vitro activities such as ribosome-inactivating activity and cytotoxicity were also decreased. However, the in vivo abortifacient activity was, slightly decreased, unchanged, or even enhanced in some preparations. PEG5k modification had little effect on immunogenicity. However, PEG20k modification significantly reduced immunogenicity. All PEG20k modified TCS mutants induced lower level IgG and IgE antibodies. In particular, PEG20k-KQ and PEG20k-K173C induced weaker systemic anaphylaxis reaction in guinea pigs. In conclusion, the present results suggest that PEG20k is better than PEG5k for reducing immunogenicity and prolonging plasma half-life. The conjugate can become a better therapeutic agent.

Site-directed polyethylene glycol modification of trichosanthin: effects on its biological activities, pharmacokinetics, and antigenicity

Trichosanthin (TCS), a type I ribosome-inactivating protein (RIP), was modified with polyethylene glycol (PEG) in order to reduce its antigenicity and prolong its half-life. Computer modeling identified three potential antigenic sites namely Q219, K173 and S7. By site-directed mutagenesis, these sites were changed into cysteine through which PEG can be covalently attached. The resulting TCS had a PEG coupled directly above one of its potential antigenic determinants, hence masking the antigenic region and prevent binding of antibodies specific to this site. In general, mutation did not bring about significant changes in ribosome-inactivating activity, cytotoxicity, and abortifacient activity of TCS. However, the in vitro activities of PEG modified (PEGylated) TCS muteins were 3-20 folds lower and the in vivo activity 50% less than that of nTCS. Pharmacokinetics study indicated that all three PEGylated TCS muteins showed 6-fold increase in mean residence time as compared to unmodified muteins. The binding affinity of an IgE monoclonal antibody (TE1) to TCS was greatly reduced after PEG modification (PEGylation) at position Q219, suggesting that TE1 recognized an epitope very near to residue Q219. PEGylated TCS muteins induced similar IgG response but 4-16 fold lower IgE response in mice compared with nTCS.

Lowering of trichosanthin immunogenicity by site-specific coupling to dextran

Trichosanthin is a type I ribosome-inactivating protein possessing a broad spectrum of biological and pharmacological activities. Therapeutic use of this compound is hampered by its immunogenicity. It was shown earlier that coupling of dextran to trichosanthin can increase plasma half-life and reduce antigenicity. However, the site where dextran attaches to trichosanthin cannot be controlled; ideally, it should be at or near the antigenic determinant. The present study attempted to couple dextran to trichosanthin at a potential antigenic site. By site-directed mutagenesis, two sites, R29 and K173, were replaced by cysteine, and dextran was coupled to the newly created cysteine residues. The dextran-trichosanthin complex retained 50% of abortifacient activity and had a mean residence time in rats 27-fold longer than natural trichosanthin. Acute hypersensitivity reaction in guinea pigs was reduced greatly after coupling of K173C (a trichosanthin mutant with lysine-173 replaced by cysteine) to dextran. Compared with natural trichosanthin, dextran-K173C had a decrease in IgG and IgE response, whereas the coupling of R29C (a trichosanthin mutant with arginine-29 replaced by cysteine) to dextran did not show significant reduction of immunogenicity. This suggests that K173 but not R29 is located at or near an antigenic determinant. This study has demonstrated an alternative approach for mapping of antigenic determinants. The information obtained is also useful in producing an improved trichosanthin derivative for therapeutic use.

Purification, characterization and molecular cloning of trichoanguin, a novel type I ribosome-inactivating protein from the seeds of Trichosanthes anguina

The seeds of the plant Trichosanthes anguina contain a type I ribosome-inactivating protein (RIP), designated trichoanguin, which was purified to apparent homogeneity by the combined use of ion-exchange chromatographies, i.e. first with DE-52 cellulose and then with CM-52 cellulose. The protein was found to be a glycoprotein with a molecular mass of 35 kDa and a pI of 9.1. It strongly inhibits the protein synthesis of rabbit reticulocyte lysate, with an IC50 of 0.08 nM, but only weakly that of HeLa cells, with an IC50 of 6 microM. Trichoanguin cleaves at the A4324 site of rat 28 S rRNA by its N-glycosidase activity. The cDNA of trichoanguin consists of 1039 nt and encodes an open reading frame coding for a polypeptide of 294 amino acid residues. The first 19 residues of this polypeptide encode a signal peptide sequence and the last 30 residues comprise an extension at its C-terminus. There are four potential glycosylation sites, located at Asn-51, Asn-65, Asn-201 and Asn-226. A comparison of the amino acid sequence of trichoanguin with those of RIPs such as trichosanthin, alpha-momorcharin, ricin A-chain and abrin A-chain reveals 55%, 48%, 36% and 34% identity respectively. Molecular homology modelling of trichoanguin indicates that its tertiary structure closely resembles those of trichosanthin and alpha-momorcharin. The large structural similarities might account for their common biological effects such as an abortifacient, an anti-tumour agent and anti-HIV-1 activities. Trichoanguin contains two cysteine residues, Cys-32 and Cys-155, with the former being likely to be located on the protein surface, which is directly amenable for conjugation with antibodies to form immunoconjugates. It is therefore conceivable that trichoanguin might be a better type I RIP than any other so far examined for the preparation of immunotoxins, with a great potential for application as an effective chemotherapeutic agent for the treatment of cancer.

A new class of DNA glycosylase/apurinic/apyrimidinic lyases that act on specific adenines in single-stranded DNA

Although the biological function of DNA glycosylases is to protect the genome by removal of potentially cytotoxic or mutagenic bases, this investigation describes the existence of natural DNA glycosylases with activity on undamaged, nonmispaired bases. Gelonin, pokeweed antiviral protein, and ricin, previously described as ribosome-inactivating proteins, are shown to damage single-stranded DNA by removal of a protein-specific set of adenines and cleavage at the resulting abasic sites. Using an oligonucleotide as the substrate reveals that the reaction proceeds via the enzyme-DNA imino intermediate characteristic of DNA glycosylase/AP lyases. The adenine glycosylase activity on single-stranded DNA reported here challenges the concept that a normal base has to be in a mismatch to be specifically removed. By contrast to other glycosylases, these enzymes are expected to damage DNA rather than participate in repair processes. The significance of this DNase activity to the biological function of these plant proteins and to their toxicity to animal cells remains to be determined.

Ribosome-inactivating proteins (RIPs) of wild Oregon cucumber (Marah oreganus)

Two type 1 RIPs, designated as MOR-I and MOR-II, have been isolated from Marah oreganus (manroot) seed extract. They are similar but not identical to trichosanthin, a type 1 RIP in the same family. MOR-I and MOR-II are monomeric proteins with molecular weights of 27989.0 and 27632.8 respectively and have pI values greater than 8.8. MOR-I and MOR-II inhibit cell-free protein synthesis with IC50s of 0.063 and 0.071 nM, respectively, and are relatively stable with respect to temperature and pH variations. They share a conserved N-terminal amino acid sequence (D-SF-LS) and cross-react with goat anti-trichosanthin polyclonal serum.

Rationale for the use of immunotoxins in the treatment of HIV-infected humans

The first step in the replication of human immunodeficiency virus (HIV) is selective binding of the envelope glycoprotein (gp120) to CD4 receptors on T cells or macrophages. After penetration in these cells, the genome of the virus is integrated in the human genome. HIV-infection causes depletion of CD4-positive cells resulting in a severe immunosuppression. It is believed that eliminating HIV-infected cells is crucial in limiting further reduction of CD4-positive cells and thus, preventing disease progression. The most commonly used drugs, such as zidovudine (AZT), appeared to be not completely effective. Therefore many investigators are searching for alternative treatment modalities. The use of immunotoxins (ITs) to eliminate HIV-infected cells is discussed. ITs are chimeric molecules in which cell-binding ligands are coupled to toxins and can specifically eliminate undesired cells. The cell-binding carriers of anti-HIV ITs have been directed against different regions of the HIV envelope glycoprotein (gp120 and gp41) and surface antigens (e.g CD4, CD25). The ITs have been composed of different ribosome-inactivating proteins (RIPs) like pokeweed antiviral protein (PAP), Pseudomonas exotoxin (PE), Diphtheria toxin (DT), or ricin. In in vitro studies, several of these ITs have been shown to be effective and specific in killing acute and persistently HIV-infected cells. The ITs were effective at concentrations (ID50 range from 10(-9) M to 10(-12) M) that were not toxic to uninfected cells or cells without the antigen. The IT CD4(178)PE40, a fusion protein directed against the CD4 binding site of gp120, has been investigated in two in vivo trials. The results were disappointing considering the antiviral activity in vitro. This was thought to be due to the rapid clearance of the IT and the differential resistance of clinical HIV isolates. Use of a panel of ITs is likely to be more effective because multiple approaches cover the intrinsic variability of HIV and the presence of IT-resistant or latently infected cells, as well as the blocking presence of neutralizing anti-HIV antibodies and the immunogenicity of most ITs. It may be possible to control the virus completely with a panel of ITs in combination with other antiviral or immunosuppressive agents such as RT inhibitors (e.g AZT), interferon alpha, or cyclosporine. More research will be necessary to develop such a combined therapy.

The antigenic sites of trichosanthin, a ribosome-inactivating protein with multiple pharmacological properties

Trichosanthin (TCS) fragments were produced by Tn1000 deletion mutagenesis and by cyanogen bromide (CNBr) cleavage and their immunoreactivity was examined by incubating with various antibodies. Twelve C-terminally truncated TCS variants were successfully synthesized under the control of a T7 RNA driven promoter. The smallest antigenic fragment mapped corresponded to the N-terminal 20 amino acids (aa). Six CNBr fragments of TCS were created and identified by electrospray mass spectrometry and N-terminal sequencing. Three antigenic fragments corresponding to aa 1-72, 101-152 and 153-246, respectively were mapped. Fragments corresponding to aa 1-72 and 153-246 were immunoreactive to the same monoclonal antibody showing they are components of a discontinuous epitope. On the other hand, the fragment containing aa 73-100 was not detected by any of the antibodies used.

An additional mechanism of ribosome-inactivating protein cytotoxicity: degradation of extrachromosomal DNA

Inhibition of protein synthesis by cleavage of the N-glycosidic bond of a specific adenine of 28 S rRNA has been accepted as the mechanism by which plant ribosome-inactivating proteins (RIPs) cause cytotoxicity. The cytotoxic action of gelonin on Plasmodium falciparum malaria parasites appears to occur by a different mechanism. Parasite intoxication, which is manifested by mitochondrial dysfunction and lack of nucleic acid synthesis in the erythrocytic cycle following exposure to the toxin, is caused by the elimination of the parasite 6 kb extrachromosomal (mitochondrial) DNA. This is the first report which demonstrates that the DNA-damaging activities of RIPs observed in vitro can contribute to their cytotoxicity.

Structure/function relationship study of Tyr14 and Arg22 in trichosanthin, a ribosome-inactivating protein

Amino acids Tyr14 and Arg22 in trichosanthin are residues on helix A1 close to the active-site cleft. They are invariant in various type-I and type-II ribosome-inactivating proteins. In this study, Tyr14 was changed to Phe and Arg22 to Lys and Leu. Modified proteins were purified, and activities compared by assaying their median inhibitory concentration (ID50) on a rabbit-reticulocyte-lysate protein-synthesis system. While the ID50 of wild-type trichosanthin was 0.02 nM, those for [Phe14], [Lys22], [Leu22] and [Phe14, Leu22]trichosanthin were 0.10, 0.03, 0.25 and 0.15 nM, respectively. Therefore, compared with Tyr14, Arg22 appears to play a more important role in trichosanthin. Structural studies on [Leu22]trichosanthin showed that two water molecules occupy the space left by the side chain of Arg22, and hydrogen bonds exist between these water molecules and nearby residues to retain the conformation. The use of intermolecular rather than intramolecular hydrogen bonds may have an adverse effect on stability or folding of the protein and results in a mild decrease in activity.