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

TOXINAS DE Bacillus thuringiensis CON ACTIVIDAD ANTICANCERÍGENA: PARASPORINAS

Las toxinas Cry de Bacillus thuringiensis (Bt) han sido reconocidas por su acción biocontroladora contra insectos plaga. Recientemente se ha descrito que algunas cepas de Bt presentan proteínas que no presentan actividad insecticida, pero al ser enfrentadas a líneas celulares de cáncer de diferentes tipos han demostrado actividad citotóxica. Estas proteínas han sido denominadas parasporinas (PS) y surgen como una potencial alternativa para el tratamiento del cáncer debido a que presentan alta citotoxicidad hacia diferentes líneas celulares cancerígenas, y baja o nula citotoxicidad hacia células normales.

Characterization of parasporin gene harboring Indian isolates of Bacillus thuringiensis

Bacillus thuringiensis (Bt) is popularly known as insecticidal bacterium. However, non-insecticidal Bt strains are more extensively available in natural environment than the insecticidal ones. Parasporin (PS) is a collection of genealogically heterogeneous Cry proteins synthesized in non-insecticidal isolates of Bt. An important character generally related with PS proteins is their strong cytocidal activity preferentially on human cancer cells of various origins. Identification and characterization of novel parasporin protein which are non-hemolytic and non-insecticidal but having selective anticancer activity raise the possibility of a novel application of Bt in medical field. In the present study, seven new indigenous isolates (T6, T37, T68, T98, T165, T186, and T461) of Bt showed variation in colony morphology, crystal characters and protein profiles with each other. Out of the seven new isolates screened for parasporin (ps) and cry genes, two of the new indigenous isolates (T98 and T186) of Bt showed the presence of ps4 gene. Partial ps4 gene was cloned from the two new isolates and the sequence of partial ps4 gene showed high homology with its holotype ps4Aa1. These two isolates were characterized based on the proteolytic processing of the inclusion proteins and the proteolytic products were found to be comparable to the PS4 reference strain A1470. The two isolates of Bt did not show toxicity toward Spodoptera litura and Helicoverpa armigera. Based on the results of this study, it can be concluded that the isolates T98 and T186 are parasporin producers.

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.

Identification of a second cytotoxic protein produced by Bacillus thuringiensis A1470

Bacillus thuringiensis A1470 produces multiple proteins with similar molecular masses (~30 kDa) with cytotoxicity against human cell lines. One that was previously identified, parasporin-4, is a β-pore-forming toxin. The N-terminal sequence of a second cytotoxic protein was identical to a partial sequence of parasporin-2 produced by B. thuringiensis A1547. PCR was performed on total plasmid DNA from A1470 by using primers for parasporin-2 to amplify a gene which was then cloned. The cloned gene differed from A1547 parasporin-2 by 8 bp and the predicted protein differed by four amino acids. The gene was expressed in Escherichia coli, and the cytotoxic activities of the recombinant protein against four human cell lines (MOLT-4, Jurkat, HeLa, and HepG2) were similar to those of A1547 parasporin-2. We then confirmed that the A1470 strain simultaneously produces parasporin-2 and parasporin-4.

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.

Identification of a novel cytotoxic protein, Cry45Aa, from Bacillus thuringiensis A1470 and its selective cytotoxic activity against various mammalian cell lines

Parasporal inclusion proteins produced by Bacillus thuringiensis strain A1470 exhibit strong cytotoxicity against human leukemic T cells when activated by protease treatment. One of the cytotoxic proteins was separated by anion exchange chromatography and gel filtration chromatography and designated Cry45Aa. Its gene was then expressed in recombinant Escherichia coli, in which the Cry45Aa precursor was accumulated in an inclusion body. It was solubilized in sodium carbonate buffer and processed with proteinase K, and cytotoxic activities of the protein against various mammalian cell lines were evaluated using the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H tetrazolium bromide assay. The protein exhibited high cytotoxic activity against CACO-2, Sawano, MOLT-4, TCS, and HL60 cells and moderate activity against U-937 DE-4, PC12, and HepG2 cells. On the other hand, the EC50 values against Jurkat, K562, HeLa, A549, Vero, COS-7, NIH3T3, CHO, and four normal tissue cells (human primary hepatocyte cells, UtSMC, MRC-5, and normal T cells) were >2 microg/mL.