Noninsecticidal parasporal proteins of a Bacillus thuringiensis serovar shandongiensis isolate exhibit a preferential cytotoxicity against human leukemic T cells

The Cytocidal Spectrum of Bacillus thuringiensis Toxins: From Insects to Human Cancer Cells

Bacillus thuringiensis (Bt) is a ubiquitous bacterium in soils, insect cadavers, phylloplane, water, and stored grain, that produces several proteins, each one toxic to different biological targets such as insects, nematodes, mites, protozoa, and mammalian cells. Most Bt toxins identify their particular target through the recognition of specific cell membrane receptors. Cry proteins are the best-known toxins from Bt and a great amount of research has been published. Cry are cytotoxic to insect larvae that affect important crops recognizing specific cell membrane receptors such as cadherin, aminopeptidase-N, and alkaline phosphatase. Furthermore, some Cry toxins such as Cry4A, Cry4B, and Cry11A act synergistically with Cyt toxins against dipteran larvae vectors of human disease. Research developed with Cry proteins revealed that these toxins also could kill human cancer cells through the interaction with specific receptors. Parasporins are a small group of patented toxins that may or may not have insecticidal activity. These proteins could kill a wide variety of mammalian cancer cells by recognizing specific membrane receptors, just like Cry toxins do. Surface layer proteins (SLP), unlike the other proteins produced by Bt, are also produced by most bacteria and archaebacteria. It was recently demonstrated that SLP produced by Bt could interact with membrane receptors of insect and human cancer cells to kill them. Cyt toxins have a structure that is mostly unrelated to Cry toxins; thereby, other mechanisms of action have been reported to them. These toxins affect mainly mosquitoes that are vectors of human diseases like Anopheles spp (malaria), Aedes spp (dengue, zika, and chikungunya), and Culex spp (Nile fever and Rift Valley fever), respectively. In addition to the Cry, Cyt, and parasporins toxins produced during spore formation as inclusion bodies, Bt strains also produce Vip (Vegetative insecticidal toxins) and Sip (Secreted insecticidal proteins) toxins with insecticidal activity during their vegetative growth phase.

Crystal Protein of a Novel Bacillus thuringiensis Strain Inducing Cell Cycle Arrest and Apoptotic Cell Death in Human Leukemic Cells

Parasporal inclusions of a native non haemolytic Bacillus thuringiensis strain KAU 59 was screened for its cytotoxicity against human lymphocytic leukemic cell line jurkat and normal human lymphocytes. The cytotoxicity of proteinase activated and non activated solubilised parasporal inclusions against both cell lines was assessed by Cell Titer 96 Aqueous Non Radioactive Cell Proliferation Assay Kit using MTS. The 50 per cent effective concentration (EC₅₀) values were deduced from log probit analysis at 48 h. Morphological changes associated with cytotoxicity were evaluated and molecular mechanisms of cell death were elucidated by TUNEL assay at 48 h post-inoculation. The fluorescence assisted cell sorting was done in the flow cytometer to assess the stage of cell cycle arrest. Relative quantification of caspase-3 expression in Jurkat cells treated with parasporal inclusion protein of KAU 59 was done by qRTPCR The results indicated that the protein was cytotoxic to jurkat cells at the same time non toxic to normal lymphocytes. Cytotoxicity was evident only after proteolytic activation. Apoptotic cell death was confirmed in the protein treated cells by TUNEL Assay and also up regulated caspase-3 gene expression (P < 0.001). S phase cell cycle arrest was confirmed by and fluorescence associated cell sorting.

Specific Cytotoxic Effects of Parasporal Crystal Proteins Isolated from Native Saudi Arabian Bacillus thuringiensis Strains against Cervical Cancer Cells

Currently, global efforts are being intensified towards the discovery of local Bacillus thuringiensis (Bt) isolates with unique anticancer properties. Parasporins (PS) are a group of Bt non-insecticidal crystal proteins with potential and specific in vitro anticancer activity. However, despite the significant therapeutic potential of PS-producing Bt strains, our current knowledge on the effects of these proteins is limited. Hence, the main objective of this study was to screen Bt-derived parasporal toxins for cytotoxic activities against colon (HT-29) and cervical (HeLa) cancerous cell lines. Nine non-larvicidal and non-hemolytic Bt strains, native to Saudi Arabia, were employed for the isolation of their parasporal toxins. 16S rDNA sequencing revealed a 99.5% similarity with a reference Bt strain. While PCR screening results indicated the absence of selected Cry (Cry4A, Cry4B, Cry10 and Cry11), Cyt (Cyt1 and Cyt2) and PS (PS2, PS3 and PS4) genes, it concluded presence of the PS1 gene. SDS-PAGE analysis revealed that proteolytically-cleavaged PS protein profiles exhibit patterns resembling those observed with PS1Aa1, with major bands at 56 kDa and 17 kDa (Bt7), and 41 kDa and 16 kDa (Bt5). Solubilized and trypsinized PS proteins from all Bt strains exhibited a marked and dose-dependent cytotoxicity against HeLa cancerous cells but not against HT-29 cells. IC₅₀ values ranged from 3.2 (Bt1) to 14.2 (Bt6) with an average of 6.8 µg/mL. The observed cytotoxicity of PS proteins against HeLa cells was specific as it was not evident against normal uterus smooth muscle cells. RT-qPCR analysis revealed the overexpression of caspase 3 and caspase 9 by 3.7, and 4.2 folds, respectively, indicative of the engagement of intrinsic pathway of apoptosis. To the best of our knowledge, this is the first report exploring and exploiting the versatile repertoire of Saudi Arabian environmental niches for the isolation of native and possibly novel Saudi Bt strains with unique and specific anticancer activity. In conclusion, native Saudi Bt-derived PS proteins might have a potential to join the arsenal of natural anticancer drugs.

Anti-cancer Parasporin Toxins are Associated with Different Environments: Discovery of Two Novel Parasporin 5-like Genes

Cry toxins are primarily a family of insecticidal toxins produced by the bacterium Bacillus thuringiensis (Bt). However, some Cry toxins, called parasporins (PSs), are non-insecticidal and have been shown to differentially kill human cancer cells. Based on amino acid homology, there are currently six different classes of parasporins (PS1-6). It is not known what role parasporins play in nature, nor if certain PSs are associated with Bt found in particular environments. Herein, we present ten parasporin-containing isolates of Bt from the Caribbean island of Trinidad. Genes coding for PS1 and PS6 were found in isolates associated mainly with artificial aquatic environments (e.g., barrels with rain water), while Bt possessing two novel PS5-like genes (ps5-1 and ps5-2), were isolated from manure collected directly from the rectum of cattle. The amino acid sequences inferred from the two PS5-like genes were 51 % homologous to each other, while being only 41 or 45 % similar to PS5Aa1/Cry64Aa, the only reported member of the parasporin five class. The low level of amino acid homology between the two PS5-like genes and PS5Aa1 indicate that the two PS5-like genes may represent a new class of parasporins, or greatly expand the level of diversity within the current parasporin 5 class.

Structural insights into Bacillus thuringiensis Cry, Cyt and parasporin toxins

Since the first X-ray structure of Cry3Aa was revealed in 1991, numerous structures of B. thuringiensis toxins have been determined and published. In recent years, functional studies on the mode of action and resistance mechanism have been proposed, which notably promoted the developments of biological insecticides and insect-resistant transgenic crops. With the exploration of known pore-forming toxins (PFTs) structures, similarities between PFTs and B. thuringiensis toxins have provided great insights into receptor binding interactions and conformational changes from water-soluble to membrane pore-forming state of B. thuringiensis toxins. This review mainly focuses on the latest discoveries of the toxin working mechanism, with the emphasis on structural related progress. Based on the structural features, B. thuringiensis Cry, Cyt and parasporin toxins could be divided into three categories: three-domain type α-PFTs, Cyt toxin type β-PFTs and aerolysin type β-PFTs. Structures from each group are elucidated and discussed in relation to the latest data, respectively.

Bacillus thuringiensis: a genomics and proteomics perspective

Bacillus thuringiensis (Bt) is a unique bacterium in that it shares a common place with a number of chemical compounds which are used commercially to control insects important to agriculture and public health. Although other bacteria, including B. popilliae and B. sphaericus, are used as microbial insecticides, their spectrum of insecticidal activity is quite limited compared to Bt. Importantly, Bt is safe for humans and is the most widely used environmentally compatible biopesticide worldwide. Furthermore, insecticidal Bt genes have been incorporated into several major crops, rendering them insect resistant, and thus providing a model for genetic engineering in agriculture.This review highlights what the authors consider the most relevant issues and topics pertaining to the genomics and proteomics of Bt. At least one of the authors (L.A.B.) has spent most of his professional life studying different aspects of this bacterium with the goal in mind of determining the mechanism(s) by which it kills insects. The other authors have a much shorter experience with Bt but their intellect and personal insight have greatly enriched our understanding of what makes Bt distinctive in the microbial world. Obviously, there is personal interest and bias reflected in this article notwithstanding oversight of a number of published studies. This review contains some material not published elsewhere although several ideas and concepts were developed from a broad base of scientific literature up to 2010.

Novel Bacillus thuringiensis δ-endotoxin active against Locusta migratoria manilensis

A novel δ-endotoxin gene was cloned from a Bacillus thuringiensis strain with activity against Locusta migratoria manilensis by PCR-based genome walking. The sequence of the cry gene was 3,432 bp long, and it encoded a Cry protein of 1,144 amino acid residues with a molecular mass of 129,196.5 kDa, which exhibited 62% homology with Cry7Ba1 in the amino acid sequence. The δ-endotoxin with five conserved sequence blocks in the amino-terminal region was designated Cry7Ca1 (GenBank accession no. EF486523). Protein structure analysis suggested that the activated toxin of Cry7Ca1 has three domains: 227 residues forming 7 α-helices (domain I); 213 residues forming three antiparallel β-sheets (domain II); and 134 residues forming a β-sandwich (domain III). The three domains, respectively, exhibited 47, 44, and 34% sequence identity with corresponding domains of known Cry toxins. SDS-PAGE and Western blot analysis showed that Cry7Ca1, encoded by the full-length open reading frame of the cry gene, the activated toxin 1, which included three domains but without the N-terminal 54 amino acid residues and the C terminus, and the activated toxin 2, which included three domains and N-terminal 54 amino acid residues but without the C terminus, could be expressed in Escherichia coli. Bioassay results indicated that the expressed proteins of Cry7Ca1 and the activated toxins (toxins 1 and 2) showed significant activity against 2nd instar locusts, and after 7 days of infection, the estimated 50% lethal concentrations (LC₅₀s) were 8.98 μg/ml for the expressed Cry7Ca1, 0.87 μg/ml for the activated toxin 1, and 4.43 μg/ml for the activated toxin 2. The δ-endotoxin also induced histopathological changes in midgut epithelial cells of adult L. migratoria manilensis.

Solubilization, activation, and insecticidal activity of Bacillus thuringiensis serovar thompsoni HD542 crystal proteins

Cry15Aa protein, produced by Bacillus thuringiensis serovar thompsoni HD542 in a crystal together with a 40-kDa accompanying protein, is one of a small group of nontypical, less well-studied members of the Cry family of insecticidal proteins and may provide an alternative for the more commonly used Cry proteins in insect pest management. In this paper, we describe the characterization of the Cry15Aa and 40-kDa protein's biochemical and insecticidal properties and the mode of action. Both proteins were solubilized above pH 10 in vitro. Incubation of solubilized crystal proteins with trypsin or insect midgut extracts rapidly processed the 40-kDa protein to fragments too small to be detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, whereas the Cry15 protein yielded a stable product of approximately 30 kDa. Protein N-terminal sequencing showed that Cry15 processing occurs exclusively at the C-terminal end. Cry15 protein showed in vitro hemolytic activity, which was greatly enhanced by preincubation with trypsin or insect gut extract. Larvae of the lepidopteran insects Manduca sexta, Cydia pomonella, and Pieris rapae were susceptible to crystals, and presolubilization of the crystals enhanced activity to P. rapae. Activity for all three species was enhanced by preincubation with trypsin. Larvae of Helicoverpa armigera and Spodoptera exigua were relatively insensitive to crystals, and activity against these insects was not enhanced by prior solubilization or trypsin treatment. The 40-kDa crystal protein showed no activity in the insects tested, nor did its addition or coexpression in Escherichia coli increase the activity of Cry15 in insecticidal and hemolytic assays.

Isolation and characterization of a novel Bacillus thuringiensis strain expressing a novel crystal protein with cytocidal activity against human cancer cells

AIMS

To characterize a novel, unusual, Bacillus thuringiensis strain, to clone its Cry gene and determine the spectrum of action of the encoded Cry protein.

METHODS AND RESULTS

The B. thuringiensis strain, referred to as M15, was isolated from dead two-spotted spider mites (Tetranychus urticae Koch; Arthropoda: Arachnida: Tetranychidae). It is an autoagglutination-positive strain and is therefore non-serotypeable. A sporulated culture produces a roughly spherical parasporal inclusion body, the crystal, tightly coupled to the spore. Although the crystal appears to be composed of at least two major polypeptides of 86 and 79 kDa as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, Southern hybridization indicates that the corresponding crystal protein gene is likely present in only one copy. The crystal protein gene was cloned and, based on nucleotide sequence homology with an orthologous cry31Aa1 gene, assigned the name cry31Aa2. Although initially isolated from spider mites, B. thuringiensis M15 is non-toxic to spider mites and it does not produce the wide spectrum beta-exotoxin. Assays on mammalian cells, however, reveal that Cry31Aa2, when cleaved with trypsin, is cytocidal to some human cancer cells but not to normal human cells. No cytocidal activity was induced after protease treatment of Cry31Aa2 with either chymotrypsin or proteinase K. Trypsin, chymotrypsin and proteinase K cleavage sites were determined.

CONCLUSIONS

The B. thuringiensis strain M15 exhibits specific cytocidal activities against some human cancer cells.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study raises questions as to the actual role of this bacterial strain and its crystal protein in the environment. It may be possible to further develop the Cry31Aa2 protein to target specific human cancer cells.

Parasporin-2Ab, a Newly Isolated Cytotoxic Crystal Protein from Bacillus thuringiensis

A novel crystal protein that exhibited potent cytotoxicity against human leukemic T-cells was cloned from the Bacillus thuringiensis TK-E6 strain. The protein, designated as parasporin-2Ab (PS2Ab), was a polypeptide of 304 amino acid residues with a predicted molecular weight of 33,017. The deduced amino acid sequence of PS2Ab showed significant homology (84% identitiy) to parasporin-2Aa (PS2Aa) from the B. thuringiensis A1547 strain. Upon processing of PS2Ab with proteinase K, the active form of 29 kDa was produced. The activated PS2Ab showed potent cytotoxicity against MOLT-4 and Jurkat cells and the EC(50) values were estimated as 0.545 and 0.745 ng/mL, respectively. The cytotoxicity of PS2Ab was significantly higher than that of PS2Aa reported elsewhere. Although both cytotoxins were structurally related, it was thought that the minor differences found were responsible for the different cytotoxicities of PS2Ab and PS2Aa.

Parasporin-1, a Novel Cytotoxic Protein from Bacillus thuringiensis, Induces Ca2+ Influx and a Sustained Elevation of the Cytoplasmic Ca2+ Concentration in Toxin-sensitive Cells

Parasporin-1 is a novel non-insecticidal inclusion protein from Bacillus thuringiensis that is cytotoxic to specific mammalian cells. In this study, we investigated the effects of parasporin-1 on toxin-sensitive cell lines to elucidate the cytotoxic mechanism of parasporin-1. Parasporin-1 is not a membrane pore-forming toxin as evidenced by measurements of lactate dehydrogenase release, propidium iodide penetration, and membrane potential in parasporin-1-treated cells. Parasporin-1 decreased the level of cellular protein and DNA synthesis in parasporin-1-sensitive HeLa cells. The earliest change observed in cells treated with this toxin was a rapid elevation of the intracellular free-Ca(2+) concentration; increases in the intracellular Ca(2+) levels were observed 1-3 min following parasporin-1 treatment. Using four different cell lines, we found that the degree of cellular sensitivity to parasporin-1 was positively correlated with the size of the increase in the intracellular Ca(2+) concentration. The toxin-induced elevation of the intracellular Ca(2+) concentration was markedly decreased in low-Ca(2+) buffer and was not observed in Ca(2+)-free buffer. Accordingly, the cytotoxicity of parasporin-1 decreased in the low-Ca(2+) buffer and was restored by the addition of Ca(2+) to the extracellular medium. Suramin, which inhibits trimeric G-protein signaling, suppressed both the Ca(2+) influx and the cytotoxicity of parasporin-1. In parasporin-1-treated HeLa cells, degradation of pro-caspase-3 and poly(ADP-ribose) polymerase was observed. Furthermore, synthetic caspase inhibitors blocked the cytotoxic activity of parasporin-1. These results indicate that parasporin-1 activates apoptotic signaling in these cells as a result of the increased Ca(2+) level and that the Ca(2+) influx is the first step in the pathway that underlies parasporin-1 toxicity.

Occurrence of parasporin-producing Bacillus thuringiensis in Vietnam

A total of 63 Bacillus thuringiensis isolates were recovered from urban soils of Hanoi, Vietnam. Of these, 34 were identified to 12 H serogroups. None of the isolates showed larvicidal activities against three lepidopterous insects. Three isolates belonging to the two serovars, colmeri (H21) and konkukian (H34), were highly toxic to larvae of the mosquito Aedes aegypti. Parasporal inclusion proteins of four isolates exhibited cytocidal activities against HeLa cells. Immunologically, proteins of four isolates were closely related to parasporin-1 (Cry31Aa), a parasporal protein that preferentially kills human cancer cells. Haemolytic activities were associated with parasporal proteins of the three mosquitocidal isolates but not with those of the four cancer-cell-killing isolates. PCR experiments and nucleotide sequence analysis revealed that the genes of four anti-cancer isolates are closely related to the gene parasporin-1 (cry31Aa) but are dissimilar to those of the three other existing parasporins. Our results suggest that the soil of northern Vietnam is a good reservoir of parasporin-producing B. thuringiensis.

Typical Three-Domain Cry Proteins of Bacillus thuringiensis Strain A1462 Exhibit Cytocidal Activity on Limited Human Cancer Cells

Bacillus thuringiensis strain A1462 produced two parasporal inclusion proteins with a molecular mass of 88 kDa that were converted to 64-kDa toxins when activated by proteinase K digestion. Both toxins exhibited strong cytocidal activity against two human cancer cell lines, HL60 (myeloid leukemia cells) and HepG2 (liver cancer cells), while low or no toxicities were observed against 11 human and three mammalian cell lines, including four non-cancer cell lines. The cytotoxicity of both toxins on susceptible cells was characterized by rapid cell swelling. Gene cloning experiments provided two novel genes encoding 88-kDa Cry proteins, Cry41Aa and Cry41Ab. The amino acid sequences of the two proteins contain five block regions commonly conserved in B. thuringiensis insecticidal Cry proteins. This is the first report of the occurrence of typical three-domain Cry proteins with cytocidal activity preferential for cancer cells.

Efficient solubilization, activation, and purification of recombinant Cry45Aa of Bacillus thuringiensis expressed as inclusion bodies in Escherichia coli

A cytotoxic protein Cry45Aa of Bacillus thuringiensis expressed as inclusion bodies in Escherichia coli was solubilized in 10 mM HCl. Protein concentration of saturated solution of the recombinant Cry45Aa in 10 mM HCl was about 25 times higher than that in the buffer of previous method (in 50 mM sodium carbonate buffer, pH 10.5, containing 1 mM EDTA, and 10 mM dithiothreitol). The Cry45Aa solubilized in the acidic solution was activated by pepsin as an alternative to proteinase K in the previous method. Cytotoxic activity against CACO-2 cells of the pepsin-treated Cry45Aa was almost identical to the proteinase K-treated protein. The pepsin-treated Cry45Aa was purified by cation-exchange chromatography. The concentration of the purified protein was 539 microg/ml, which was 27-fold higher than that of the activated Cry45Aa by the previously method. The cytotoxic activity of the purified protein was stable in broad pH region (pH 2.0-11.0) for 3 days, and 97% cytotoxic activity remained after incubation at 30 degrees C for 360 min.

Bacillus thuringiensis serovar shandongiensis strain 89-T-34-22 produces multiple cytotoxic proteins with similar molecular masses against human cancer cells

AIMS

To prove that Bacillus thuringiensis serovar shandongiensis strain 89-T-34-22 produces several novel cytotoxic proteins against human leukaemic T cells.

METHODS AND RESULTS

Parasporal inclusion protein was solubilized and processed by proteinase K and was separated by anion-exchange chromatography. Cytopathic effects of each fraction against MOLT-4 and Jurkat cells were monitored.

CONCLUSIONS

Existence of at least two novel cytotoxic proteins was suggested and N-terminal sequences of the newly identified proteins were determined to be QSTTDVIREY and X (Y or I) (P or I) NLANELA (X indicates uncertain amino acids). Molecular masses of the two proteins were approx. 27-28 kDa.

SIGNIFICANCE AND IMPACT OF THE STUDY

In this study, we demonstrated that the strain 89-T-34-22 produces at least two novel cytotoxic proteins with similar molecular masses against human cancer cells. This is the first strain of B. thuringiensis which produces multiple cytotoxic proteins against human cancer cells.

A Bacillus thuringiensis Crystal Protein with Selective Cytocidal Action to Human Cells

Bacillus thuringiensis crystal proteins, well known to be toxic to certain insects but not pathogenic to mammals, are used as insecticidal proteins in agriculture and forest management. We here identified a crystal protein that is non-insecticidal and non-hemolytic but has strong cytocidal activity against various human cells with a markedly divergent target specificity, e.g. highly cytotoxic to HepG2 and Jurkat and less cytotoxic to the normal hepatocyte (HC) and HeLa. In slices of liver and colon cancer tissues, the toxin protein preferentially killed the cancer cells, leaving other cells unaffected. The cytocidal effect of the protein is non-apoptotic with swelling and fragmentation of the susceptible cells, although the apoptotic process does occur when the cell damage proceeded slowly. The amino acid sequence deduced from the nucleotide sequence of the cloned gene of the protein has little sequence homology with the insecticidal crystal proteins of B. thuringiensis. These observations raise the presence of a new group of the B. thuringiensis toxin and the possibility of new applications for the protein in the medical field.

The cytotoxicity of Bacillus thuringiensis subsp. coreanensis A1519 strain against the human leukemic T cell

A novel cytotoxic protein was isolated from the crystal produced by Bacillus thuringiensis subsp. coreanensis A1519 strain. Upon treatment of the crystal proteins by proteinase K, the significant cytotoxicity toward the leukemic T cell, MOLT-4, was exhibited. The microscopic observation indicated that the cell death was accompanied by no extensive rupture of the cell membrane. It was, therefore, suggested that the cell death of MOLT-4 was induced through a mechanism other than the colloid-osmotic swelling and cell lysis as caused by hitherto known B. thuringiensis crystal proteins. The 29-kDa polypeptide proved to be an active component of the proteinase K-digested A1519 crystal proteins. EC(50) of the purified 29-kDa polypeptide was 0.078 microg/ml. The N-terminal amino acid sequence of the 29-kDa polypeptide shared no significant homology with all the known proteins, suggesting that this polypeptide belong to a new family of B. thuringiensis crystal proteins. In the ligand blotting analysis, specific binding proteins for the 29-kDa polypeptide were detected from the cell membrane of MOLT-4.

A 28 kDa protein of the Bacillus thuringiensis serovar shandongiensis isolate 89-T-34-22 induces a human leukemic cell-specific cytotoxicity

A 28 kDa protein that exhibits cytocidal activity specific for human leukemic T (MOLT-4) cells was purified from proteinase K-digested parasporal inclusion of a Bacillus thuringiensis serovar shandongiensis isolate. The N-terminal sequence of the protein was identical with that of the 32 kDa protein, regarded as a protoxin, of the inclusion proteins. The median effective concentration of this protein was 0.23 microg/ml against MOLT-4 cells and its specific activity was 7.9 times greater than that of the whole inclusion proteins. The 28 kDa protein induced necrosis-like cytotoxicity against MOLT-4 cells and the cytopathic effect with the passage of time was characterized by cell swelling, nuclear membrane isolation and chromatin condensation.