Low-density lipoprotein receptor-related protein 1 is an essential receptor for trichosanthin in 2 choriocarcinoma cell lines

LRP1-Mediated Endocytosis May Be the Main Reason for the Difference in Cytotoxicity of Curcin and Curcin C on U2OS Osteosarcoma Cells

Curcin and Curcin C, both of the ribosome-inactivating proteins of Jatropha curcas, have apparent inhibitory effects on the proliferation of osteosarcoma cell line U20S. However, the inhibitory effect of the latter is 13-fold higher than that of Curcin. The mechanism responsible for the difference has not been studied. This work aimed to understand and verify whether there are differences in entry efficiency and pathway between them using specific endocytosis inhibitors, gene silencing, and labeling techniques such as fluorescein isothiocyanate (FITC) labeling. The study found that the internalization efficiency of Curcin C was twice that of Curcin for U2OS cells. More than one entering pathway was adopted by both of them. Curcin C can enter U2OS cells through clathrin-dependent endocytosis and macropinocytosis, but clathrin-dependent endocytosis was not an option for Curcin. The low-density lipoprotein receptor-related protein 1 (LRP1) was found to mediate clathrin-dependent endocytosis of Curcin C. After LRP1 silencing, there was no significant difference in the 50% inhibitory concentration (IC₅₀) and endocytosis efficiency between Curcin and Curcin C on U2OS cells. These results indicate that LRP1-mediated endocytosis is specific to Curcin C, thus leading to higher U2OS endocytosis efficiency and cytotoxicity than Curcin.

Advances on Delivery of Cytotoxic Enzymes as Anticancer Agents

Cancer is one of the most serious human diseases, causing millions of deaths worldwide annually, and, therefore, it is one of the most investigated research disciplines. Developing efficient anticancer tools includes studying the effects of different natural enzymes of plant and microbial origin on tumor cells. The development of various smart delivery systems based on enzyme drugs has been conducted for more than two decades. Some of these delivery systems have been developed to the point that they have reached clinical stages, and a few have even found application in selected cancer treatments. Various biological, chemical, and physical approaches have been utilized to enhance their efficiencies by improving their delivery and targeting. In this paper, we review advanced delivery systems for enzyme drugs for use in cancer therapy. Their structure-based functions, mechanisms of action, fused forms with other peptides in terms of targeting and penetration, and other main results from in vivo and clinical studies of these advanced delivery systems are highlighted.

Genetically-engineered "all-in-one" vaccine platform for cancer immunotherapy

An essential step for cancer vaccination is to break the immunosuppression and elicit a tumor-specific immunity. A major hurdle against cancer therapeutic vaccination is the insufficient immune stimulation of the cancer vaccines and lack of a safe and efficient adjuvant for human use. We discovered a novel cancer immunostimulant, trichosanthin (TCS), that is a clinically used protein drug in China, and developed a well-adaptable protein-engineering method for making recombinant protein vaccines by fusion of an antigenic peptide, TCS, and a cell-penetrating peptide (CPP), termed an “all-in-one” vaccine, for transcutaneous cancer immunization. The TCS adjuvant effect on antigen presentation was investigated and the antitumor immunity of the vaccines was investigated using the different tumor models. The vaccines were prepared via a facile recombinant method. The vaccines induced the maturation of DCs that subsequently primed CD8⁺ T cells. The TCS-based immunostimulation was associated with the STING pathway. The general applicability of this genetic engineering strategy was demonstrated with various tumor antigens (i.e., legumain and TRP2 antigenic peptides) and tumor models (i.e., colon tumor and melanoma). These findings represent a useful protocol for developing cancer vaccines at low cost and time-saving, and demonstrates the adjuvant application of TCS—an old drug for a new application.

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.

Advances on Tumor-Targeting Delivery of Cytotoxic Proteins

Great attention has been paid to cytotoxic proteins (e.g., ribosome-inactivating proteins, RIPs) possessing high anticancer activities; unlike small drugs, cytotoxic proteins can effectively retain inside the cells and avoid drug efflux mediated by multidrug resistance transporters due to the large-size effect. However, the clinical translation of these proteins is severely limited because of various biobarriers that hamper their effective delivery to tumor cells. Hence, in order to overcome these barriers, many smart drug delivery systems (DDS) have been developed. In this review, we will introduce two representative type I RIPs, trichosanthin (TCS) and gelonin (Gel), and overview the major biobarriers for protein-based cancer therapy. Finally, we outline advances on the development of smart DDS for effective delivery of these cytotoxic proteins for various applications in cancer treatment.

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.

Intein-mediated site-specific synthesis of tumor-targeting protein delivery system: Turning PEG dilemma into prodrug-like feature

Poor tumor-targeted and cytoplasmic delivery is a bottleneck for protein toxin-based cancer therapy. Ideally, a protein toxin drug should remain stealthy in circulation for prolonged half-life and reduced side toxicity, but turn activated at tumor. PEGylation is a solution to achieve the first goal, but creates a hurdle for the second because PEG rejects interaction between the drugs and tumor cells therein. Such PEG dilemma is an unsolved problem in protein delivery. Herein proposed is a concept of turning PEG dilemma into prodrug-like feature. A site-selectively PEGylated, gelatinase-triggered cell-penetrating trichosanthin protein delivery system is developed with three specific aims. The first is to develop an intein-based ligation method for achieving site-specific modification of protein toxins. The second is to develop a prodrug feature that renders protein toxins remaining stealthy in blood for reduced side toxicity and improved EPR effect. The third is to develop a gelatinase activatable cell-penetration strategy for enhanced tumor targeting and cytoplasmic delivery. Of note, site-specific modification is a big challenge in protein drug research, especially for such a complicated, multifunctional protein delivery system. We successfully develop a protocol for constructing a macromolecular prodrug system with intein-mediated ligation synthesis. With an on-column process of purification and intein-mediated cleavage, the site-specific PEGylation then can be readily achieved by conjugation with the activated C-terminus, thus constructing a PEG-capped, cell-penetrating trichosanthin system with a gelatinase-cleavable linker that enables tumor-specific activation of cytoplasmic delivery. It provides a promising method to address the PEG dilemma for enhanced protein drug delivery, and importantly, a facile protocol for site-specific modification of such a class of protein drugs for improving their druggability and industrial translation.

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.

The anti-cancerous activity of recombinant trichosanthin on prostate cancer cell PC3

CONTEXT

Trichosanthin produced in the root tube of Trichosanthes kirilowii shows anti-tumor activity on a series of cancer cells including Hela, MCF-7, HL-60. But there is little information about its effect on the carcinogenesis of prostate cancer.

OBJECTIVE

This work was designed to study the role of trichosanthin on prostate cancer cells PC3.

MATERIALS AND METHODS

Trichosanthin was expressed in BL21 strain and purified by affinity chromatography. MTT assay was designed to determine the effect of trichosanthin on growth of PC3 cells at doses of 10, 20, 40, 60, 80, and 120 μg/ml. Then the effect of 50 μg/ml rTCS alone or combined with 2 μM IL-2 on PC3 cell proliferation was analyzed. And the mechanism of rTCS was studied by western blot. After that the in vivo effect of rTCS combined with IL-2 was explored in mice bearing PC3 xenograft tumor.

RESULTS

Trichosanthin was successfully expressed in BL21 and purified by 100 mM imidazole. It was shown to inhibit proliferation of PC3 cells in a dose-dependent manner with IC50 50.6 μg/ml. When combined with cytokine IL-2, a significant synergic effect was obtained. The inhibition rate on PC3 was around 50 % in combination group while only 35.5 % in single rTCS group at 50 μg/ml. Further, the expression of full length caspase-8 and Bcl-2 decreased significantly while cleaved caspase-8 and Bax were up-regulated, which suggest that caspase-8-mediated apoptosis pathway may be activated by rTCS in PC3 cells. Moreover, our data demonstrated that tumor volume and tumor weight were significantly reduced in rTCS-treated or rTCS/IL-2-treated nude mice bearing PC3 xenograft tumor compared with control. And significant difference was also found between rTCS and rTCS/IL-2 group.

CONCLUSIONS

This study demonstrates that rTCS is a potential agent with high in vitro and in vivo anti-tumor activity on PC3 cells. And rTCS combined with IL-2 is a promising strategy in treating patients with prostate cancer in future.

Ribosome-inactivating proteins (RIPs) and their important health promoting property

Ribosome-inactivating proteins (RIPs), widely present in plants, certain fungi and bacteria, can inhibit protein synthesis by removing one or more specific adenine residues from the large subunit of ribosomal RNAs (rRNAs).

Anti-tumor activities and apoptotic mechanism of ribosome-inactivating proteins

Ribosome-inactivating proteins (RIPs) belong to a family of enzymes that attack eukaryotic ribosomes and potently inhibit cellular protein synthesis. RIPs possess several biomedical properties, including anti-viral and anti-tumor activities. Multiple RIPs are known to inhibit tumor cell proliferation through inducing apoptosis in a variety of cancers, such as breast cancer, leukemia/lymphoma, and hepatoma. This review focuses on the anti-tumor activities of RIPs and their apoptotic effects through three closely related pathways: mitochondrial, death receptor, and endoplasmic reticulum pathways.

Regulation of SPRY3 by X chromosome and PAR2-linked promoters in an autism susceptibility region

Sprouty proteins are regulators of cell growth and branching morphogenesis. Unlike mouse Spry3, which is X-linked, human SPRY3 maps to the pseudoautosomal region 2; however, the human Y-linked allele is not expressed due to epigenetic silencing by an unknown mechanism. SPRY3 maps adjacent to X-linked Trimethyllysine hydroxylase epsilon (TMLHE), recently identified as an autism susceptibility gene. We report that Spry3 is highly expressed in central and peripheral nervous system ganglion cells in mouse and human, including cerebellar Purkinje cells and retinal ganglion cells. Transient over-expression or knockdown of Spry3 in cultured mouse superior cervical ganglion cells inhibits and promotes, respectively, neurite growth and branching. A 0.7 kb gene fragment spanning the human SPRY3 transcriptional start site recapitulates the endogenous Spry3-expression pattern in LacZ reporter mice. In the human and mouse the SPRY3 promoter contains an AG-rich repeat and we found co-expression, and promoter binding and/or regulation of SPRY3 expression by transcription factors MAZ, EGR1, ZNF263 and PAX6. We identified eight alleles of the human SPRY3 promoter repeat in Caucasians, and similar allele frequencies in autism families. We characterized multiple SPRY3 transcripts originating at two CpG islands in the X-linked F8A3-TMLHE region, suggesting X chromosome regulation of SPRY3. These findings provide an explanation for differential regulation of X and Y-linked SPRY3 alleles. In addition, the presence of a SPRY3 transcript exon in a previously described X chromosome deletion associated with autism, and the cerebellar interlobular variation in Spry3 expression coincident with the reported pattern of Purkinje cell loss in autism, suggest SPRY3 as a candidate susceptibility locus for autism.

Aralin, a type II ribosome-inactivating protein from Aralia elata, exhibits selective anticancer activity through the processed form of a 110-kDa high-density lipoprotein-binding protein: a promising anticancer drug

Aralin from Aralia elata is a newly identified type II ribosome- inactivating protein, which preferentially induces apoptosis in cancer cells. In this study, we identified that the aralin receptor is a 110-kDa high-density lipoprotein-binding protein (HDLBP), which functions as a HDL receptor. The sensitivities of tumor cell lines to aralin were dependent on the expression levels of the 110-kDa HDLBP and its forced expression in aralin-resistant Huh7 cells conferred aralin sensitivity. HDLBP-knockdown HeLa cells showed a significant aralin resistance in vitro and in vivo. Conversely, ectopic expression of the 150-kDa HDLBP resulted in increased aralin sensitivity in vivo, accompanying enhanced expression of the 110-kDa HDLBP. Thus, these results showed that the 110-kDa HDLBP in lipid rafts acted as an aralin receptor and that its expression levels determined aralin sensitivity, suggesting that aralin could be a promising anticancer drug for HDLBP-overexpressing tumors.

Preferential cytotoxicity of the type I ribosome inactivating protein alpha-momorcharin on human nasopharyngeal carcinoma cells under normoxia and hypoxia

All primary nasopharyngeal carcinoma (NPC) tumors contain hypoxic regions which are implicated in decreased local control and increased distant metastases, as well as resistance to chemotherapy in advanced NPC patients. One of the promising therapeutic approaches for NPC is to use drugs that can target hypoxic factors in tumors. In the present investigation, the type I ribosome inactivating protein α-momorcharin (α-MMC), isolated from seeds of the bitter gourd Momordica charantia, reduced cell viability and inhibited clonogenic formation of human NPC CNE2 and HONE1 cells under normoxia and cobalt chloride-induced hypoxia. By comparison, α-MMC exhibited only slight cytotoxicity on human nasopharyngeal epithelial NP69 cells under normoxia. Interestingly, α-MMC suppressed the expression levels of hypoxia-inducible factor 1-alpha (HIF1α) and vascular endothelial growth factor (VEGF) in hypoxic NPC, as well as the growth of human umbilical vein endothelial cells. Further study disclosed that α-MMC targeted endoplasmic reticulum and down-regulated unfolded protein response (UPR) in NPC cells. Moreover, α-MMC induced apoptosis in NPC cells in a dose- and time-dependent manner. It initiated mitochondrial- and death receptor-mediated apoptotic signaling in CNE2 cells, but there was hardly any effect on HONE1 cells. In addition, α-MMC brought about G0/G1 phase cell cycle arrest in CNE2 cells and S phase arrest in HONE1 cells. Collectively, α-MMC preferentially exhibited inhibitory effect on normoxic and hypoxic NPC cells partly by blocking survival signaling (e.g. HIF1α, VEGF and UPR), and triggering apoptotic pathways mediated by mitochondria or death receptor. These observations indicate the potential utility of α-MMC for prophylaxis and therapy of NPC.

Anti-tumor action of trichosanthin, a type 1 ribosome-inactivating protein, employed in traditional Chinese medicine: a mini review

Trichosanthin (TCS) as a midterm abortifacient medicine has been used clinically in traditional Chinese medicine for centuries. Additionally, TCS manifests a host of pharmacological properties, for instance, anti-HIV and anti-tumor activities. TCS has been reported to inhibit cell growth of a diversity of cancers, including cervical cancer, choriocarcinoma, and leukemia/lymphoma, etc. This article purported to review the various anti-tumor activities of TCS and the mechanism of apoptosis it induced in these tumor cells. These research progresses provide an insight into cancer research and treatment as well as disclose new pharmacological properties of the ancient but popular Chinese medicine.

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.