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

Phylogeny and domain architecture of plant ribosome inactivating proteins

Ribosome inactivating proteins (RIPs) are rRNA N-glycosylases (EC 3.2.2.22) best known for hydrolyzing an adenine base from the conserved sarcin/ricin loop of ribosomal RNA. Protein translation is inhibited by ribosome depurination; therefore, RIPs are generally considered toxic to cells. The expression of some RIPs is upregulated by biotic and abiotic stress, though the connection between RNA depurination and defense response is not well understood. Despite their prevalence in approximately one-third of flowering plant orders, our knowledge of RIPs stems primarily from biochemical analyses of individuals or genomics-scale analyses of small datasets from a limited number of species. Here, we performed an unbiased search for proteins with RIP domains and identified several-fold more RIPs than previously known - more than 800 from 120 species, many with novel associated domains and physicochemical characteristics. Based on protein domain configuration, we established 15 distinct groups, suggesting diverse functionality. Surprisingly, most of these RIPs lacked a signal peptide, indicating they may be localized to the nucleocytoplasm of cells, raising questions regarding their toxicity against conspecific ribosomes. Our phylogenetic analysis significantly extends previous models for RIP evolution in plants, predicting an original single-domain RIP that later evolved to acquire a signal peptide and different protein domains. We show that RIPs are distributed throughout 21 plant orders with many species maintaining genes for more than one RIP group. Our analyses provide the foundation for further characterization of these new RIP types, to understand how these enzymes function in plants.

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.

Trichosanthin inhibits cervical cancer by regulating oxidative stress-induced apoptosis

Based on many studies, trichosanthin (TCS) has an antiviral effect that regulates immune response, and targets cancer cells to exert broad-spectrum anti-tumor pharmacological activities. It is speculated that TCS may be a potential natural active drug for preventing as well as treating cervical cancer. But the clearer impact along with underlying TCS mechanism on cervical cancer are still unclear. The purpose of this study is to investigate the function and potential mechanism of TCS in cervical cancer. We measured the viability of cervical cancer cell lines (HeLa & caski cells) using CCK-8 analysis, detected cell proliferation efficiency through Ki-67 staining, analyzed cell apoptosis rate via flow cytometry as well as annexin V-FITC/PI double staining, performed apoptosis-related protein expression through western blotting, evaluated cell migration along with invasion by wound as well as transwell assays, carried out MMP via JC-1 and Rh123 fluorescent probes, as well as detected intracellular ATP and ROS levels by flow cytometry, respectively, to evaluate the effects of TCS. We found that TCS inhibited viability along with proliferation, induced apoptosis, as well as inhibited HeLa & caski cell migration along with invasion in a time- and dose-dependent manner. Additionally, TCS also reduced MMP, and the production of adenosine triphosphate, as well as induced the increase of intracellular reactive oxygen species in cancer cell lines. In accordance with the present studies, TCS inhibits HeLa & caski cell proliferation along with migration but promotes their apoptosis, which may be mediated by regulating oxidative stress.

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.

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.

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.

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.

Improvement of the Pharmacological Properties of Maize RIP by Cysteine-Specific PEGylation

To improve the pharmacological properties of maize ribosome-inactivating protein (maize RIP) for targeting HIV-infected cells, the previously engineered TAT-fused active form of maize RIP (MOD) was further engineered for cysteine-directed PEGylation. In this work, two potential antigenic sites, namely Lys-78 and Lys-264, were identified. They were mutated to cysteine residue and conjugated with PEG_5k or PEG_20k. The resultant PEG derivatives of MOD variants were examined for ribosome-inactivating activity, circulating half-life and immunogenicity. Our results showed that MOD-PEG conjugates had two- to five-fold lower biological activity compared to the wild-type. Mutation of the two sites respectively did not decrease the anti-MOD IgG and IgE level in mice, but the conjugation of PEG did dramatically reduce the antigenicity. Furthermore, pharmacokinetics studies demonstrated that attachment of PEG_20k prolonged the plasma half-life by five-fold for MOD-K78C and 17-fold for MOD-K264C, respectively. The site-specific mutation together with PEGylation therefore generated MOD derivatives with improved pharmacological properties.

Toxic proteins in plants

Plants have evolved to synthesize a variety of noxious compounds to cope with unfavorable circumstances, among which a large group of toxic proteins that play a critical role in plant defense against predators and microbes. Up to now, a wide range of harmful proteins have been discovered in different plants, including lectins, ribosome-inactivating proteins, protease inhibitors, ureases, arcelins, antimicrobial peptides and pore-forming toxins. To fulfill their role in plant defense, these proteins exhibit various degrees of toxicity towards animals, insects, bacteria or fungi. Numerous studies have been carried out to investigate the toxic effects and mode of action of these plant proteins in order to explore their possible applications. Indeed, because of their biological activities, toxic plant proteins are also considered as potentially useful tools in crop protection and in biomedical applications, such as cancer treatment. Genes encoding toxic plant proteins have been introduced into crop genomes using genetic engineering technology in order to increase the plant's resistance against pathogens and diseases. Despite the availability of ample information on toxic plant proteins, very few publications have attempted to summarize the research progress made during the last decades. This review focuses on the diversity of toxic plant proteins in view of their toxicity as well as their mode of action. Furthermore, an outlook towards the biological role(s) of these proteins and their potential applications is discussed.

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.

Inhibition of HIV-1 replication by balsamin, a ribosome inactivating protein of Momordica balsamina

Ribosome-inactivating proteins (RIPs) are endowed with several medicinal properties, including antiviral activity. We demonstrate here that the recently identified type I RIP from Momordica balsamina also possesses antiviral activity, as determined by viral growth curve assays and single-round infection experiments. Importantly, this activity is at play even as doses where the RIP has no cytotoxic effect. In addition, balsamin inhibits HIV-1 replication not only in T cell lines but also in human primary CD4⁺ T cells. This antiviral compound exerts its activity at a viral replicative step occurring later than reverse-transcription, most likely on viral protein translation, prior to viral budding and release. Finally, we demonstrate that balsamin antiviral activity is broad since it also impedes influenza virus replication. Altogether our results demonstrate that type I RIP can exert a potent anti-HIV-1 activity which paves the way for new therapeutic avenues for the treatment of viral infections.

Enhanced cytotoxic activity of a bifunctional chimeric protein containing a type 1 ribosome-inactivating protein and a serine protease inhibitor

Both ribosome-inactivating proteins (RIPs) and plant proteinase inhibitors, belong to protein families known to regulate cellular homeostasis and likely involved in plant defense. Nevertheless the interest in these protein classes is due to their potential use for the treatment of several important human diseases such as cancer. Thus, in the present study, type 1 ribosome-inactivating protein and wheat subtilisin/chymotrypsin inhibitor, were engineered into a chimeric protein with cytotoxic action selective for murine tumor cells, while lacking any appreciable toxicity on murine normal cells. This chimeric protein selectively sensitizes to apoptotic death cells derived from Simian-virus-40-transformed mouse fibroblasts (SVT2 cells). The cytotoxicity of this new recombinant product has been detected also on three different human malignant cells. Therefore action on tumor cells of this protein could represent a potentially very attractive novel tool for anticancer drug design.

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.

RNA toxins: mediators of stress adaptation and pathogen defense

RNA toxins are a group of enzymes primarily synthesized by bacteria, fungi, and plants that either cleave or depurinate RNA molecules. These proteins may be divided according to their RNA substrates: ribotoxins are nucleases that cleave ribosomal RNA (rRNA), ribosome inactivating proteins are glycosidases that remove a base from rRNA, messenger RNA (mRNA) interferases are nucleases that cleave mRNAs, and anticodon nucleases cleave transfer RNAs (tRNAs). These modifications to the RNAs may substantially alter gene expression and translation rates. Given that some of these enzymes cause cell death, it has been suggested that they function mainly in defense, either to kill competing cells or to elicit suicide and thereby limit pathogen spread from infected cells. Although good correlations have been drawn between their enzymatic functions and toxicity, recent work has shown that some RNA toxins cause apoptosis in the absence of damage to RNA and that defense against pathogens can be achieved without host cell death. Moreover, a decrease in cellular translation rate, insufficient to cause cell death, allows some organisms to adapt to stress and environmental change. Although ascribing effects observed in vitro to the roles of these toxins in nature has been challenging, recent results have expanded our understanding of their modes of action, and emphasized the importance of these toxins in development, adaptation to stress and defense against pathogens.

The 156KELK159 tetrapeptide of HIV-1 integrase is critical for lentiviral gene integration

HIV-1 integrase (HIV-1 IN), a key element of HIV-1-derived lentiviral vectors, is crucial for the stable maintenance of the vector gene by inserting them into host genome. HIV-1 IN has been found to have functions other than integration, such as involving in virion morphology, viral DNA synthesis and viral DNA nuclear import. In our study, the yeast two-hybrid assay identified a tetrapeptide 156KELK159 in HIV-1 IN that was crucial for HIV-1 IN and Daxx interaction. To investigate the functions of the tetrapeptide 156KELK159 of the HIV-1 IN, both the wild type HIV-1 IN and a mutant without 156KELK159 were used to package the EGFP reporter gene contained lentivirus. p24 based titer assay revealed that deleting the tetrapeptide did not affect virus packaging. The result was verified by quantitative real time PCR with viral specific primers. But the 156KELK159 was crucial for lentiviral gene integration. Deleting the tetrapeptide made the percentage of cells expressing the reporter gene significantly decreased and did not affect the level of DNA entered into the cells or nucleus. Real time reverse transcription PCR and FACS were used to detect the lentiviral report gene expression in infection maintaining cells and revealed 156KELK159 did not affect lentiviral vector gene expression. Our results may shed light on the regulatory mechanism of gene integration of lentivirus.

Histonedeacetylase inhibitor Oxamflatin increase HIV-1 transcription by inducing histone modification in latently infected cells

HIV-1 latency represents a major problem in the eradication of HIV-1 in infected individuals treated with highly active anti-retroviral therapy. Histone deacetylase (HDAC) inhibits HIV-1 gene expression and virus production and may contribute to quiescence of HIV-1 within resting CD4+ T cells. Here, we evaluated the effect of Oxamflatin, a class I HDAC inhibitor, on the epigenetic change at HIV-1 long terminal repeat (LTR) and the induction of the latent viruses in the latency Jurkat T cell line. Flow cytometry assay showed that Oxamflatin activate HIV-1 gene expression in these latently infected cells by 2-17 fold than background levels. Chromatin immunoprecipitation (ChIP) assays further revealed that Oxamflatin increase the acetylation level of histone H3 and histone H4 at the nucleosome 1(nuc-1) site of the HIV-1 LTR compared to mock treatment. We also found that Oxamflatin had a synergization with prostratin, or 5-azacytidine or tumor necrosis factor-α to activate the HIV-1 promoter. Taken together, our results suggest that the histone acetylation plays an important role in regulating HIV-1 LTR gene expression, and Oxamflatin has potential as drug candidates as antilatency therapies.

Peptides derived from the HIV-1 integrase promote HIV-1 infection and multi-integration of viral cDNA in LEDGF/p75-knockdown cells

BACKGROUND

The presence of the cellular Lens Epithelium Derived Growth Factor p75 (LEDGF/p75) protein is essential for integration of the Human immunodeficiency virus type 1 (HIV-1) cDNA and for efficient virus production. In the absence of LEDGF/p75 very little integration and virus production can be detected, as was demonstrated using LEDGF/p75-knockdown cells.

RESULTS

Here we show that the failure to infect LEDGF/p75-knockdown cells has another reason aside from the lack of LEDGF/p75. It is also due to inhibition of the viral integrase (IN) enzymatic activity by an early expressed viral Rev protein. The formation of an inhibitory Rev-IN complex in virus-infected cells can be disrupted by the addition of three IN-derived, cell-permeable peptides, designated INr (IN derived-Rev interacting peptides) and INS (IN derived-integrase stimulatory peptide). The results of the present work confirm previous results showing that HIV-1 fails to infect LEDGF/p75-knockdown cells. However, in the presence of INrs and INS peptides, relatively high levels of viral cDNA integration as well as productive virus infection were obtained following infection by a wild type (WT) HIV-1 of LEDGF/p75-knockdown cells.

CONCLUSIONS

It appears that the lack of integration observed in HIV-1 infected LEDGF/p75-knockdown cells is due mainly to the inhibitory effect of Rev following the formation of a Rev-IN complex. Disruption of this inhibitory complex leads to productive infection in those cells.

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.