title: Soapwort saponins trigger clathrin-mediated endocytosis of saporin, a type I ribosome-inactivating protein.
abstract: Saporin, a type I ribosome-inactivating protein (RIP), removes adenine residues from the 28S ribosomal RNA as part of a process that leads to inhibition of protein synthesis. However, as shown in this study, neither saporin nor his-tagged saporin (both 0.6-6 pM) exert toxicity on several human cell lines including H-2171, SK-N-SH, HEP-G2, MOLT-3, THP-1, HL-60 and ECV-304. Saporin and his-tagged saporin became highly cytotoxic when they were used in a combined treatment with Soapwort saponins (SA). When combined with SA (2-4 microg/ml) saporin became as cytotoxic as the highly toxic type II RIP rViscumin reflected by an IC50 of 42.5x10(-12) M for saporin and 21.5x10(-12) M for rViscumin. We demonstrated that saporin was internalized via clathrin-mediated endocytosis, followed by the release into the endosomal transport system. Our results indicate that SA triggers this endocytic event rendering the otherwise cell membrane impermeable type I RIP saporin a potent cytotoxin. This effect was not cell line-specific suggesting that saporin exploits a common SA-dependent mechanism to enter cells.
authors.affiliation: Institute of Pharmacy, Free University Berlin, D-14195 Berlin, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Weng
authors.forename: C
authors.initials: C
authors.lastname: Bachran
authors.forename: H
authors.initials: H
authors.lastname: Fuchs
authors.forename: M F
authors.initials: MF
authors.lastname: Melzig
mesh_terms: D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002463:Cell Membrane Permeability / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D002746:Chlorpromazine / Q000494:pharmacology; D002966:Clathrin / Q000378:metabolism*; D016572:Cyclosporine / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004705:Endocytosis / Q000187:drug effects*; D011992:Endosomes / Q000187:drug effects / Q000378:metabolism; D005372:Filipin / Q000494:pharmacology; D006801:Humans; D007099:Imipramine / Q000494:pharmacology; D018942:Macrolides / Q000494:pharmacology; D008856:Microscopy, Fluorescence; D008870:Microtubules / Q000187:drug effects / Q000378:metabolism; D015739:Nocodazole / Q000494:pharmacology; D019319:Okadaic Acid / Q000494:pharmacology; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000494:pharmacology; D054789:Ribosome Inactivating Proteins, Type 1 / Q000737:chemistry / Q000378:metabolism* / Q000633:toxicity; D054790:Ribosome Inactivating Proteins, Type 2 / Q000494:pharmacology; D012270:Ribosomes / Q000187:drug effects / Q000378:metabolism; D012503:Saponins / Q000494:pharmacology*; D000076985:Saporins; D014118:Toxins, Biological / Q000494:pharmacology
chemical_list: D002966:Clathrin; D018942:Macrolides; D011994:Recombinant Proteins; D054789:Ribosome Inactivating Proteins, Type 1; D054790:Ribosome Inactivating Proteins, Type 2; D012503:Saponins; D014118:Toxins, Biological; C488862:mistletoe lectin I; D019319:Okadaic Acid; D016572:Cyclosporine; D005372:Filipin; C040929:bafilomycin A1; D000076985:Saporins; D007099:Imipramine; D015739:Nocodazole; D002746:Chlorpromazine