Glycan region of GPI anchored-protein is required for cytocidal oligomerization of an anticancer parasporin-2, Cry46Aa1 protein, from Bacillus thuringiensis strain A1547

Site-Directed Mutants of Parasporin PS2Aa1 with Enhanced Cytotoxic Activity in Colorectal Cancer Cell Lines

Parasporin 2 has cytotoxic effects against numerous colon cancer cell lines, making it a viable alternative to traditional treatments. However, its mechanism of action and receptors remain unknown. In this study, site-directed mutagenesis was used to obtain PS2Aa1 mutants with variation in domain I at positions 256 and 257. Variants 015, 002, 3-3, 3-35, and 3-45 presented G256A, G256E, G257A, G257V, and G257E substitutions, respectively. Cytotoxicity tests were performed for the cell viability of cell lines SW480, SW620, and CaCo-2. Mutants 3-3, 3-35, and 3-45 efficiently killed the cell lines. It was found that the activated forms of caspase-3 and PARP were in higher abundance as well as increased production of γH2AX when 3-35 was used to treat CaCo-2 and SW480. To assess possible membrane-binding receptors involved in the interaction, an APN receptor blocking assay showed reduced activity of some parasporins. Hence, we performed molecular docking and molecular dynamics simulations to analyze the stability of possible interactions and identify the residues that could be involved in the protein–protein interaction of PS2Aa1 and APN. We found that residues 256 and 257 facilitate the interaction. Parasporin 3-35 is promising because it has higher cytotoxicity than PS2Aa1.

Computational study, synthesis and evaluation of active peptides derived from Parasporin-2 and spike protein from Alphacoronavirus against colorectal cancer cells

Parasporin-2Aa1 (PS2Aa1) is a toxic protein of 37 KDa (30 kDa, activated form produced by proteolysis) that was shown to be cytotoxic against specific human cancer cells, although its mechanism of action has not been elucidated yet. In order to study the role of some native peptide fragments of proteins on anticancer activity, here we investigated the cytotoxic effect of peptide fragments from domain-1 of PS2Aa1 and one of the loops present in the binding region of the virus spike protein from Alphacoronavirus (HCoV-229E), the latter according to scientific reports, who showed interaction with the human APN (h-APN) receptor, evidence corroborated through computational simulations, and thus being possible active against colon cancer cells. Peptides namely P264-G274, Loop1-PS2Aa, and Loop2-PS2Aa were synthesized using the Fmoc solid-phase synthesis and characterized by mass spectrometry (MS). Additionally, one region from loop 1 of HCoV-229E, Loop1-HCoV-229E, was also synthesized and characterized. The A4W-GGN5 anticancer peptide and 5-fluorouracil (5-FU) were taken as a control in all experiments. Circular dichroism revealed an α-helix structure for the peptides derived from PS2Aa1 (P264-G274, Loop1-PS2Aa, and Loop2-PS2Aa) and β-laminar structure for the peptide derived from Alphacoronavirus spike protein Loop1-HCoV-229E. Peptides showed a hemolysis percentage of less than 20% at 100 µM concentration. Besides, peptides exhibited stronger anticancer activity against SW480 and SW620 cells after exposure for 48 h. Likewise, these compounds showed significantly lower toxicity against normal cells CHO-K1. The results suggest that native peptide fragments from Ps2Aa1 may be optimized as a novel potential cancer-therapeutic agents.

In vitro analysis of the anticancer activity of Lysinibacillus sphaericus binary toxin in human cancer cell lines

Binary or Bin toxin produced by Lysinibacillus sphaericus is composed of BinA (42 kDa) and BinB (51 kDa) subunits. These work together to exert maximal toxicity against mosquito larvae via pore formation and induction of apoptosis. The C-terminal domains in both subunits are homologous to those of aerolysin-type β pore-forming toxins, including parasporin-2 (PS2). The latter is one of the Bacillus thuringiensis toxins that exhibits specific cytotoxicity against human cancer cells. The present study investigates the possible anticancer activity of Bin toxin using PS2 as a control. We demonstrate that treatment with a high concentration of trypsin-activated Bin inhibits cell proliferation in human cancer cells A549, Caco-2, HepG2, HK-1 and KKU-M055. In the most susceptible cells, HK-1, Bin toxin exposure led to morphological alterations, decreased migration, decreased adhesion activity and apoptosis induction. Although these effects necessitated high concentrations, they suggest that Bin toxin may be optimized as a novel potential cancer-therapeutic agent.

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.

Study of the Bacillus thuringiensis Cry1Ia Protein Oligomerization Promoted by Midgut Brush Border Membrane Vesicles of Lepidopteran and Coleopteran Insects, or Cultured Insect Cells

Bacillus thuringiensis (Bt) produces insecticidal proteins that are either secreted during the vegetative growth phase or accumulated in the crystal inclusions (Cry proteins) in the stationary phase. Cry1I proteins share the three domain (3D) structure typical of crystal proteins but are secreted to the media early in the stationary growth phase. In the generally accepted mode of action of 3D Cry proteins (sequential binding model), the formation of an oligomer (tetramer) has been described as a major step, necessary for pore formation and subsequent toxicity. To know if this could be extended to Cry1I proteins, the formation of Cry1Ia oligomers was studied by Western blot, after the incubation of trypsin activated Cry1Ia with insect brush border membrane vesicles (BBMV) or insect cultured cells, using Cry1Ab as control. Our results showed that Cry1Ia oligomers were observed only after incubation with susceptible coleopteran BBMV, but not following incubation with susceptible lepidopteran BBMV or non-susceptible Sf21 insect cells, while Cry1Ab oligomers were persistently detected after incubation with all insect tissues tested, regardless of its host susceptibility. The data suggested oligomerization may not necessarily be a requirement for the toxicity of Cry1I proteins.

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.

Cell lines as models for the study of Cry toxins from Bacillus thuringiensis

Cell lines have been use extensively for the study of the mode of action of different pore forming toxins produced by different bacterial species. Bacillus thuringiensis Cry toxins are not the exception and their mechanism of action has been analyzed in different cell lines. Here we review the data obtained with different cell lines, including those that are naturally susceptible to the three domain Cry toxins (3d-Cry) and other non-susceptible cell lines that have been transformed with 3d-Cry toxin binding molecules cloned from the susceptible insects. The effects on Cry toxin action after expressing different insect gut proteins, such as glycosyl-phosphatidyl-inositol (GPI) anchored proteins (like alkaline phosphatase (ALP) aminopeptidase (APN)), or trans-membrane proteins (like cadherin (CAD) or ATP-binding cassette subfamily C member 2 (ABCC2) transporter) in cell lines showed that, with few exceptions, expression of GPI-anchored proteins do not correlated with increased susceptibility to the toxin, while the expression of CAD or ABCC2 proteins correlated with induced susceptibility to Cry toxins in the transformed cells lines. Also, that the co-expression of CAD and ABCC2 transporter induced a synergistic effect in the toxicity of 3d-Cry toxins. Overall the data show that in susceptible cell lines, the 3d-Cry toxins induce pore formation that correlates with toxicity. However, the intracellular responses remain controversial since it was shown that the same 3d-Cry toxin in different cell lines activated different responses such as adenylate cyclase-PKA death response or apoptosis. Parasporins are Cry toxins that are toxic to cancer cell lines that have structural similarities with the insecticidal Cry toxins. They belong to the 3d-Cry toxin or to MTX-like Cry toxin families but also show important differences with the insecticidal Cry proteins. Some parasporins are pore-forming toxins, and some activate apoptosis. In this review we summarized the results of the different studies about the Cry toxins mode of action using cultured cell lines and discuss their relation with the studies performed in insect larvae.