title: Selection of anthrax toxin protective antigen variants that discriminate between the cellular receptors TEM8 and CMG2 and achieve targeting of tumor cells.
abstract: Anthrax toxin, a three-component protein toxin secreted by Bacillus anthracis, assembles into toxic complexes at the surface of receptor-bearing eukaryotic cells. The protective antigen (PA) protein binds to receptors, either tumor endothelial cell marker 8 (TEM8) or CMG2 (capillary morphogenesis protein 2), and orchestrates the delivery of the lethal and edema factors into the cytosol. TEM8 is reported to be overexpressed during tumor angiogenesis, whereas CMG2 is more widely expressed in normal tissues. To extend prior work on targeting of tumor with modified anthrax toxins, we used phage display to select PA variants that preferentially bind to TEM8 as compared with CMG2. Substitutions were randomly introduced into residues 605-729 of PA, within the C-terminal domain 4 of PA, which is the principal region that contacts receptor. Candidates were characterized in cellular cytotoxicity assays with Chinese hamster ovary (CHO) cells expressing either TEM8 or CMG2. A PA mutant having the substitutions R659S and M662R had enhanced specificity toward TEM8-overexpressing CHO cells. This PA variant also displayed broad and potent tumoricidal activity to various human tumor cells, especially to HeLa and A549/ATCC cells. By contrast, the substitution N657Q significantly reduced toxicity to TEM8 but not CMG2-overexpressing CHO cells. Our results indicate that certain amino acid substitutions within PA domain 4 create anthrax toxins that selectively kill human tumor cells. The PA R659S/M662R protein may be useful as a therapeutic agent for cancer treatment.
authors.affiliation: Laboratory of Bacterial Diseases, NIAID, National Institutes of Health, Bethesda, Maryland 20892-3202.
authors.forename: Kuang-Hua
authors.initials: KH
authors.lastname: Chen
authors.affiliation: Laboratory of Bacterial Diseases, NIAID, National Institutes of Health, Bethesda, Maryland 20892-3202.
authors.forename: Shihui
authors.initials: S
authors.lastname: Liu
authors.affiliation: Infectious and Inflammatory Disease Center, Burnham Institute for Medical Research, La Jolla, California 92037.
authors.forename: Laurie A
authors.initials: LA
authors.lastname: Bankston
authors.affiliation: Infectious and Inflammatory Disease Center, Burnham Institute for Medical Research, La Jolla, California 92037.
authors.forename: Robert C
authors.initials: RC
authors.lastname: Liddington
authors.affiliation: Laboratory of Bacterial Diseases, NIAID, National Institutes of Health, Bethesda, Maryland 20892-3202. Electronic address: sleppla@niaid.nih.gov.
authors.forename: Stephen H
authors.initials: SH
authors.lastname: Leppla
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution / Q000235:genetics; D000818:Animals; D000940:Antigenic Variation; D000942:Antigens, Bacterial / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D001427:Bacterial Toxins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D016466:CHO Cells; D002460:Cell Line; D045744:Cell Line, Tumor; D006224:Cricetinae; D003412:Cricetulus; D006367:HeLa Cells; D006801:Humans; D008565:Membrane Proteins / Q000378:metabolism*; D008840:Microfilament Proteins; D008969:Molecular Sequence Data; D009363:Neoplasm Proteins / Q000378:metabolism*; D017434:Protein Structure, Tertiary / Q000235:genetics; D011956:Receptors, Cell Surface / Q000378:metabolism*; D018000:Receptors, Peptide
chemical_list: C473287:ANTXR1 protein, human; C447150:ANTXR2 protein, human; D000942:Antigens, Bacterial; D001427:Bacterial Toxins; D008565:Membrane Proteins; D008840:Microfilament Proteins; D009363:Neoplasm Proteins; D011956:Receptors, Cell Surface; D018000:Receptors, Peptide; C030325:anthrax toxin