Discovery of antitumor effects of leczymes.

Sialic-acid binding lectin from bullfrog (Rana catesbeiana) eggs, cSBL, is a cytotoxic ribonuclease (RNase) belonging to the RNase A superfamily. cSBL is cytotoxic to tumor cells, such as malignant pleural mesothelioma by inducing apoptotic cell death caused by the degradation of RNA in tumor cells. In addition, we have reported some data that cSBL could be involved in the endoplasmic reticulum stress pathway, and it was also assumed to cause apoptotic cell death. The most significant property of cSBL is its specificity toward malignant cells. Furthermore, since the antitumor activity of cSBL was confirmed by in vivo experiments using mouse xenograft models, it is expected to be a candidate for clinical chemotherapy. Here, we summarize the history of cSBL, alternatively called "leczyme," with its present and future.

Catfish egg lectin affects influx and efflux rates of sunitinib in human cervical carcinoma HeLa cells.

New treatment protocols are aiming to reduce the dose of the multitargeted tyrosine kinase inhibitor sunitinib, as sunitinib elicits many adverse effects depending on its dosage. Silurus asotus egg lectin (SAL) has been reported to enhance the incorporation of propidium iodide as well as doxorubicin into Burkitt's lymphoma Raji cells through binding to globotriaosylceramide (Gb3) on the cell surface. The objective of this study was to examine whether SAL enhances the cytotoxic effect of sunitinib in Gb3-expressing HeLa cells. Although the treatment with SAL delayed the cell growth and enhanced the propidium iodide uptake, cell death accompanied by membrane collapse was not observed. The viability of sunitinib-treated HeLa cells was significantly reduced when the treatment occurred in combination with SAL compared to their separate usage. Sunitinib uptake significantly increased for 30 min in SAL-treated cells, and this increment was almost completely abolished by the addition of L-rhamnose, a hapten sugar of SAL, but not by D-glucose. After removal of SU from the medium, the intracellular sunitinib level in SAL-treated cells was higher than in untreated cells for 24 h, which was not observed in Gb3-deficient HeLa cells. Furthermore, we observed that SAL promoted the formation of lysosome-like structures, which are LAMP1 positive but not acidic in HeLa cells, which can trap sunitinib. Interestingly, SAL-induced vacuolation in HeLa cells was not observed in another Gb3 positive Raji cells. Our findings suggest that SAL/Gb3 interaction promoted sunitinib uptake and suppressed sunitinib excretion and that sunitinib efficiently exerted cytotoxicity against HeLa cells.

Sialic Acid-Binding Lectin from Bullfrog Eggs Exhibits an Anti-Tumor Effect Against Breast Cancer Cells Including Triple-Negative Phenotype Cells.

Sialic acid-binding lectin from Rana catesbeiana eggs (cSBL) is a multifunctional protein that has lectin and ribonuclease activity. In this study, the anti-tumor activities of cSBL were assessed using a panel of breast cancer cell lines. cSBL suppressed the cell growth of all cancer cell lines tested here at a concentration that is less toxic, or not toxic at all, to normal cells. The growth suppressive effect was attributed to the cancer-selective induction of apoptosis. We assessed the expressions of several key molecules associated with the breast cancer phenotype after cSBL treatment by western blotting. cSBL decreased the expression level of estrogen receptor (ER) α, while it increased the phosphorylation level of p38 mitogen-activated protein kinase (MAPK). cSBL also suppressed the expression of the progesterone receptor (PgR) and human epidermal growth factor receptor type 2 (HER2). Furthermore, it was revealed that cSBL decreases the expression of the epidermal growth factor receptor (EGFR/HER1) in triple-negative breast cancer cells. These results indicate that cSBL induces apoptosis with decreasing ErbB family proteins and may have great potential for breast cancer chemotherapy, particularly in triple-negative phenotype cells.

Catfish rhamnose-binding lectin induces G0/1 cell cycle arrest in Burkitt's lymphoma cells via membrane surface Gb3.

Silurus asotus egg lectin (SAL), an α-galactoside-binding protein isolated from the eggs of catfish, is a member of the rhamnose-binding lectin family that binds to Gb3 glycan (Galα1-4Galß1-4Glc). We have previously demonstrated that SAL reduces the proliferation of Gb3-expressing Burkitt's lymphoma Raji cells and confirm here that it does not reduce their viability, indicating that unlike other lectins, it is not cytotoxic. The aim of this study was to determine the signal transduction mechanism(s) underlying this novel SAL/Gb3 binding-mediated effect profile. SAL/Gb3 interaction arrested the cell cycle through increasing the G0/1 phase population of Raji cells. SAL suppressed the transcription of cell cycle-related factors such as c-MYC, cyclin D3, and cyclin-dependent protein kinase (CDK)-4. Conversely, the CDK inhibitors p21 and p27 were elevated by treatment with SAL. In particular, the production of p27 in response to SAL treatment increased steadily, whereas p21 production was maximal at 12 h and lower at 24 h. Activation of Ras-MEK-ERK pathway led to an increase in expression of p21. Notably, treatment of Raji cells with anti-Gb3 mAb alone did not produce the above effects. Taken together, our findings suggest that Gb3 on the Raji cell surface interacts with SAL to trigger sequential GDP-Ras phosphorylation, Ras-MEK-ERK pathway activation, p21 production, and cell cycle arrest at the G0/1 phase.

Lissoclibadin 1, a Polysulfur Aromatic Alkaloid from the Indonesian Ascidian Lissoclinum cf. badium, Induces Caspase-Dependent Apoptosis in Human Colon Cancer Cells and Suppresses Tumor Growth in Nude Mice.

Lissoclibadins, polysulfur aromatic alkaloids, were isolated from the Indonesian ascidian Lissoclinum cf. badium. Lissoclibadins 1 (1), 3 (2), 4 (3), 7 (4), 8 (5), and 14 (6) inhibited the growth of four human solid cancer cell lines: HCT-15 (colon adenocarcinoma), HeLa-S3 (cervix adenocarcinoma), MCF-7 (breast adenocarcinoma), and NCI-H28 (mesothelioma). Lissoclibadin 1 (1) exerted the most potent cytotoxic effects in vitro and mainly promoted apoptosis through an intrinsic pathway with the activation of a caspase-dependent pathway in HCT-15 cells. In vivo studies demonstrated that 1 suppressed tumor growth in nude mice carrying HCT-15 cells without significant secondary adverse effects. In conclusion, the results obtained in the present study demonstrate that 1 has potential as a chemotherapeutic candidate for preclinical investigations.

Binding profiles and cytokine-inducing effects of fish rhamnose-binding lectins on Burkitt's lymphoma Raji cells.

Rhamnose-binding lectin (RBL) is one of the animal lectin categories which take part in the innate immune responses of fish. Osmerus lanceolatus lectin (OLL) from shishamo smelt eggs is an RBL composed of two tandem-repeated domains, both of which are considered to be a carbohydrate-recognition domain. SAL, catfish (Silurus asotus) egg RBL composed of three domains, binds to Burkitt's lymphoma Raji cells through globotriaosylceramide (Gb3) carbohydrate chain and to reduce cell size and growth by altering membrane composition without causing cell death. In this experiment, we tried to compare the binding effects of these two RBLs on Raji cells. Flow cytometric and fluorescence microscopic analyses revealed that OLL also directly bound to and shrunk Raji cells with ten times less reactivity than SAL but reduced cell growth with decreasing cell viability. Anti-Gb3 antibody completely blocked the binding of SAL to Raji cells but not that of OLL. In addition, the direct bindings of OLL and SAL to Raji cells were comparably inhibited by melibiose, but lactose was more effective inhibitor for the binding of OLL than that of SAL. These results suggest that OLL has slightly different cell-binding property compared with SAL and binds not only to Gb3 but also to the other carbohydrate receptor-bearing ß-galactoside chains. The quantitative RT-PCR analysis revealed that SAL induced the expression of TNF-α but not of IFN-γ, IL-1ß, and IL-10. Thus, SAL-induced cytostatic effect on Raji cells might be partially caused by TNF-α-mediated signaling pathway.

Sialidase NEU3 Contributes to the Invasiveness of Bladder Cancer.

Bladder cancer is the 10th most commonly diagnosed cancer worldwide. The current standard treatment for advanced bladder cancer is neoadjuvant cisplatin (NAC)-based chemotherapy followed by cystectomy. However, the response rate to chemotherapy is only 50%, owing to cisplatin resistance, and there is a need for novel therapies. Because the invasiveness of bladder cancer greatly influences patient prognosis, a mechanistic analysis of the invasive function can lead to therapeutic targets. Sialidases, which remove sialic acid residues from the nonreducing ends of sugar chains and catalyze the initial reaction in the degradation of sugar chains, are predicted to be involved in cell invasion and motility. However, the involvement of sialidases in bladder cancer, especially their relationship with the invasive ability, remains unclear. Here, using patient tissues and multiple bladder cancer cell lines, we show that the sialidase NEU3 is highly expressed in bladder cancer. Analysis of NEU3's function using its siRNA-mediated knockdown revealed that NEU3 contributes to bladder cancer invasiveness. Mechanistic analysis showed that NEU3 activates ERK and PI3K signaling. Our results show that NEU3 is involved in the malignancy of bladder cancer, and its suppression may lead to novel treatments for bladder cancer.

Domain composition of rhamnose-binding lectin from shishamo smelt eggs and its carbohydrate-binding profiles.

Osmerus (Spirinchus) lanceolatus egg lectin (OLL) is a member of the rhamnose-binding lectin (RBL) family which is mainly found in aqueous beings. cDNA of OLL was cloned, and its genomic architecture was revealed. The deduced amino acid (aa) sequence indicated that OLL was composed of 213 aa including 95 aa of domain N and 97 aa of domain C. N and C showed 73 % sequence identity and contained both -ANYGR- and -DPC-KYL-peptide motifs which are conserved in most of the RBL carbohydrate recognition domains. The calculated molecular mass of mature OLL was 20,852, consistent with the result, and 20,677.716, from mass spectrometry. OLL was encoded by eight exons: exons 1 and 2 for a signal peptide; exons 3-5 and 6-8 for N- and C-domains, respectively. Surface plasmon resonance spectrometric analyses revealed that OLL showed comparable affinity for Galα- and ß-linkages, whereas Silurus asotus lectin (SAL), a catfish RBL, bound preferentially to α-linkages of neoglycoproteins. The Kd values of OLL and SAL against globotriaosylceramide (Gb3) were 1.69 × 10⁻5 M for and 2.81 × 10⁻6 M, respectively. Thus, the carbohydrate recognition property of OLL is slightly different from that of SAL. On the other hand, frontal affinity chromatography revealed that both OLL and SAL interacted with only glycolipid-type oligosaccharides such as Gb3 trisaccharides, not with N-linked oligosaccharides. The domain composition of these RBLs and an analytical environment such as the "cluster effect" of a ligand might influence the binding between RBL and sugar chains.

Catfish Egg Lectin Enhances the Cytotoxicity of Sunitinib on Gb3-Expressing Renal Cancer Cells.

Metastatic renal cell carcinoma (RCC) is not sufficiently responsive to anticancer drugs, and thus, developing new drugs for advanced RCC remains vital. We previously reported that the treatment of globotriaosylceramide (Gb3)-expressing cells with catfish (Silurus asotus) egg lectin (SAL) increased the intracellular uptake of propidium iodide (PI) and sunitinib (SU). Herein, we investigated whether SAL pretreatment affects the intracellular uptake and cytotoxic effects of molecular-targeted drugs in RCC cells. We analyzed Gb3 expression in TOS1, TOS3, TOS3LN, and ACHN human RCC cells. Surface Gb3 expression was higher in TOS1 and TOS3 cells than in TOS3LN and ACHN cells. In the PI uptake assay, 41.5% of TOS1 cells and 21.1% of TOS3 cells treated with SAL were positive for PI. TOS1 cell viability decreased to 70% after treatment with 25 µM SU alone and to 48% after pretreatment with SAL (50 µg/mL). Time-series measurements of the intracellular fluorescence of SU revealed significantly enhanced SU uptake in SAL-treated TOS1 cells compared to control cells. SAL treatment did not increase PI uptake in normal renal cells. Our findings suggest that adequate cytotoxic activity may be achieved even when SU is administered at a sufficiently low dose not to cause side effects in combination with SAL.

[Basic Research on Bullfrog Egg-derived Sialic Acid-binding Lectin for Cancer Treatment].

Sialic acid-binding lectin from Rana catesbeiana (cSBL) is a multifunctional protein with both lectin and ribonuclease activity and is, therefore, called a leczyme. It exerts cancer cell-selective antitumor effects on a variety of cancer cells in vitro and in vivo under conditions where no undesired side effects are observed. cSBL elicits antitumor effects by degrading cellular RNA and subsequently inducing apoptosis via a pathway mediated by mitochondria and endoplasmic reticulum stress. Further, it exerts synergistic antitumor effects with other molecules such as tumor necrosis factor-related apoptosis-inducing ligand and pemetrexed. Recent studies have revealed that long-term treatment of cancer cells with cSBL causes significant pleiotropic changes in the expression profiles of several genes, including multiple genes involved in metabolic pathways. Furthermore, cSBL reduces the expression of some cancer-related molecules such as human epidermal growth factor receptors, aldo-keto reductase 1B10, and ATP-binding cassette transporter C2. The information described above is expected to lead to useful applications, such as effective regimens comprising cSBL and other drugs. These findings reveal favorable properties of cSBL as an anticancer drug, which may contribute to the development of new therapeutic strategies for cancer treatment.

Transcriptomic alterations in malignant pleural mesothelioma cells in response to long­term treatment with bullfrog sialic acid­binding lectin.

Malignant pleural mesothelioma (MPM) is a universally lethal type of cancer that is increasing in incidence worldwide; therefore, the development of new drugs for MPM is an urgent task. Bullfrog sialic acid­binding lectin (cSBL) is a multifunctional protein that has carbohydrate­binding and ribonuclease activities. cSBL exerts marked antitumor activity against numerous types of cancer cells, with low toxicity to normal cells. Although in vitro and in vivo studies revealed that cSBL was effective against MPM, the mechanism by which cSBL exerts antitumor effects is not fully understood. To further understand the mechanism of action of cSBL, the present study aimed to identify the key molecules whose expression was affected by cSBL. The present study established cSBL­resistant MPM cells. Microarray analyses revealed that there were significant pleiotropic changes in the expression profiles of several genes, including multiple genes involved in metabolic pathways in cSBL­resistant cells. Furthermore, the expression of some members of the aldo­keto reductase family was revealed to be markedly downregulated in these cells. Among these, it was particularly interesting that cSBL action reduced the level of AKR1B10, which has been reported as a biomarker candidate for MPM prognosis. These findings revealed novel aspects of the effect of cSBL, which may contribute to the development of new therapeutic strategies for MPM.

Binding of Silurus asotus lectin to Gb3 on Raji cells causes disappearance of membrane-bound form of HSP70.

Heat shock proteins (HSPs) are divided into stress-inducible and constitutive types. Generally, HSP70 (stress inducible) and HSC70 (constitutive) are representative of their types, respectively. From the results of immunocytochemical analysis, both HSP70 and HSC70 were constitutively expressed in globotriaosylceramide (Gb3)-expressing Raji cells as well as Gb3-negative K562 cells. Furthermore, the membrane-bound form of HSP70 was present on the surfaces of two cell lines as patch and cap-like structures, and was recovered in the cholesterol rich microdomains (CRM) prepared from them. On the other hand, HSP70 was partially co-localized with Gb3 on the surface of Raji cells. This result suggested that HSP70 was not associated with all of Gb3 molecules but with Gb3 specifically located in the particular environment. The effect of Silurus asotus lectin (SAL), which is one of the rhamnose-binding lectins and specifically binds to Gb3, on the disappearance of membrane-bound HSP70 was dependent on whether Gb3 was present or not. These results suggested that the disappearance of membrane-bound HSP70 was caused by SAL binding to Gb3, that the reduction of membrane-bound HSP70 might result in the decrease in cell volume observed, and that the mechanism of SAL-induced HSP70 expression may differ from that of heat shock in Raji cells.

Bacterial Expression of Rhamnose-Binding Lectin from Catfish Eggs.

SUEL-like lectins, also termed rhamnose-binding lectins (RBL), are unique in animal lectin families because of their tandemly repeated structure that is characteristic of carbohydrate-recognition domains, as well as their α-galactoside-binding capacity. RBLs are known to be expressed in inclusion bodies in Escherichia coli. Here, we describe the methods for the expression and refolding of Silurus asotus lectin (SAL) using E. coli KRX as the host strain. From our results, highly basic and reduced conditions due to arginine and dithiothreitol, respectively, tend to keep SAL recombinants soluble.

Sialic acid-binding lectin from bullfrog eggs inhibits human malignant mesothelioma cell growth in vitro and in vivo.

Malignant mesothelioma is an aggressive cancer that results from exposure to asbestos. The therapeutic options for this type of cancer are limited; therefore, the development of novel therapeutic agents is urgently required. Sialic acid-binding lectin isolated from Rana catesbeiana oocytes (cSBL) is a novel therapeutic candidate for cancer, which exhibits antitumor activity mediated through RNA degradation. In the present study, we evaluated the effect of cSBL in vitro and in vivo. Xenograft-competent H2452 and MSTO human mesothelioma cell lines were treated with cSBL, and the pathway by which cSBL induces apoptosis was analyzed. In vivo studies were performed using nude mice inoculated with one of the two cell lines, and the effects of cSBL and pemetrexed were monitored simultaneously. Furthermore, the pharmacological interactions between the three agents (pemetrexed, cisplatin and cSBL) were statistically assessed. It was demonstrated that cSBL treatments caused morphological and biochemical apoptotic changes in both cell lines. Caspase cascade analysis revealed that an intrinsic pathway mediated cSBL-induced apoptosis. The administration of cSBL significantly inhibited tumor growth in two xenograft models, without any adverse effects. Furthermore, the combination index and dose reduction index values indicated that the cSBL + pemetrexed combination showed the highest synergism, and thus potential for reducing dosage of each drug, compared with the other combinations, including the existing pemetrexed + cisplatin regimen. cSBL exerted prominent antitumor effects on malignant mesothelioma cells in vitro and in vivo, and showed favorable effects when combined with pemetrexed. These results suggest that cSBL has potential as a novel drug for the treatment of malignant mesothelioma.

Synergistic anti-tumor effect of bullfrog sialic acid-binding lectin and pemetrexed in malignant mesothelioma.

Malignant mesothelioma is an aggressive cancer with limited therapeutic options. Sialic acid-binding lectin isolated from Rana catesbeiana oocytes (cSBL) is a multifunctional protein with anti-cancer activity. The effects of pemetrexed, cisplatin, and cSBL were evaluated in mesothelioma and normal mesothelial cell lines. We evaluated cytotoxicity, apoptosis, caspase-3 cleavage and activation, cell proliferation, cell cycle arrest, and levels of cell cycle proteins in H28 cells treated with pemetrexed, cisplatin, and cSBL alone or in combination. Treatment with cSBL alone was cytotoxic to mesothelioma cells. The anti-cancer effect of cSBL was observed in a broader range of cell lines and exhibited greater cancer cell selectivity than pemetrexed or cisplatin. Combination treatment with pemetrexed + cSBL resulted in greater dose-dependent cytotoxicity than pemetrexed + cisplatin, the standard of care in mesothelioma. The synergistic effect of pemetrexed + cSBL was mediated by the cytostatic effect of pemetrexed and the cytotoxic effect of cSBL. It thus appears that cSBL has therapeutic potential for the treatment of mesothelioma.

RNase activity of sialic acid-binding lectin from bullfrog eggs drives antitumor effect via the activation of p38 MAPK to caspase-3/7 signaling pathway in human breast cancer cells.

Sialic acid-binding lectin obtained from bullfrog eggs (SBL) induces cell death in cancer cells but not in normal cells. This antitumor effect is mediated through its ribonuclease (RNase) activity. However, the underlying molecular mechanisms remain unclear. We found that the p38 mitogen-activated protein kinase (MAPK) signaling pathway was activated when SBL induced cell death in three human breast cancer cell lines: SK-BR-3, MCF-7, and MDA­MB231. The suppression of p38 MAPK phosphorylation by a p38 MAPK inhibitor as well as short interference RNA knockdown of p38 MAPK expression significantly decreased cell death and increased the cell viability of SBL-treated MDA­MB231 cells. H103A, an SBL mutant lacking in RNase activity, showed decreased SBL-induced cell death compared with native SBL. However, the loss of RNase activity of SBL had no effect on its internalization into cells. The H103A mutant also displayed decreased phosphorylation of p38 MAPK. Moreover, SBL promoted caspase­3/7 activation followed by a cleavage of poly (ADP-ribose)-polymerase, whereas the SBL mutant, H103A, lost this ability. The SBL-induced caspase­3/7 activation was suppressed by the p38 MAPK inhibitor, SB203580, as well as pan-caspase inhibitor, zVAD-fmk. In the presence of zVAD-fmk, the SBL-induced cell death was decreased. In addition, the cell viability of SBL-treated MDA­MB231 cells recovered by zVAD-fmk treatment. Taken together, our results suggest that the RNase activity of SBL leads to breast cancer cell death through the activation of p38 MAPK followed by the activation of caspase­3/7.

Downregulation of Hsp70 inhibits apoptosis induced by sialic acid-binding lectin (leczyme).

Heat shock proteins (Hsps) are molecular chaperones that maintain homeostasis of organisms. In regards to the Hsps, many studies have investigated the structure, expression, localization and functions of Hsp70 and Hsc70 including expression in the glycosphingolipid-enriched microdomain (GEM) on the cell surface and involvement in cell death. Sialic acid-binding lectin (SBL) isolated from oocytes of Rana catesbeiana is a multifunctional protein which has lectin activity, ribonuclease activity and antitumor activity. SBL has potential as a new type of anticancer drug, since it causes cancer-selective induction of apoptosis by multiple signaling pathways in which RNA is its target; and the participation of the mitochondrial pathway and the endoplasmic reticulum (ER) stress-mediated pathway has been suggested. It has also been suggested that receptor(s) for SBL (SBLR) may exist in the GEM on the cell surface. In the present study, we studied the possible involvement of Hsp70 and Hsc70 in SBL-induced apoptosis. We showed that Hsp70 and Hsc70 were expressed on the P388 cell surface similar to SBLR, and their distribution in cells dramatically changed immediately prior to the execution of apoptosis following stimulation of SBL. Functional study of Hsp70 revealed that decreased expression of Hsp70 diminished the apoptosis induced by SBL. It is suggested that Hsp70 participates in the antitumor effect of SBL.

Leczyme: a new candidate drug for cancer therapy.

Sialic acid-binding lectin (SBL), isolated from oocytes of Rana catesbeiana, is leczyme and has both lectin and ribonuclease (RNase) activities. A remarkable antitumor effect of SBL has also been reported. SBL agglutinates various kinds of tumor cells but not normal cells. SBL agglutination activity is not affected by mono- or oligosaccharides. However, SBL-induced agglutination and antitumor effects are inhibited by sialomucin but not asialomucin. In addition, SBL has very little effect on sialidase-treated cells. SBL causes cancer-selective induction of apoptosis by multiple signaling pathways, which target RNA. Synergistic antitumor effects with other molecules, such as tumor necrosis factor-related apoptosis ligand (TRAIL) and interferon- γ (IFN-γ), have been reported. Thus, SBL may be a novel candidate molecule for anticancer drug development. Sialoglycoconjugates on the tumor cell surface may be associated with lectin activity and antitumor effects of SBL. We review the properties of SBL, particularly its lectin, RNase, and antitumor activities, and comprehensively examine the potential application of SBL for clinical purposes.

Cancer-selective induction of apoptosis by leczyme.

Sialic acid-binding lectin (SBL) is a multi-functional protein that is isolated from oocytes of Rana catesbeiana. It has both lectin and ribonuclease (enzyme) properties, and therefore is called leczyme. We examined the anti-tumor effects of SBL and discovered that SBL has potential as a new type of anti-cancer drug. SBL causes a cancer-selective induction of apoptosis by multiple signaling pathways whereby RNA is its target. It is suggested that the mitochondrial pathway and endoplasmic reticulum stress-mediated pathway participate in SBL-induced signaling. The synergistic anti-tumor effects with other molecules, such as tumor necrosis factor-related apoptosis ligand and interferon γ, have been reported. In this study, we summarize the effects of SBL and focus on its cancer-selective apoptotic properties. In addition, we present a possible explanation for its cancer specificity.