title: Depletion of cyclophilins B and C leads to dysregulation of endoplasmic reticulum redox homeostasis.
abstract: Protein folding within the endoplasmic reticulum is assisted by molecular chaperones and folding catalysts that include members of the protein-disulfide isomerase and peptidyl-prolyl isomerase families. In this report, we examined the contributions of the cyclophilin subset of peptidyl-prolyl isomerases to protein folding and identified cyclophilin C as an endoplasmic reticulum (ER) cyclophilin in addition to cyclophilin B. Using albumin and transferrin as models of cis-proline-containing proteins in human hepatoma cells, we found that combined knockdown of cyclophilins B and C delayed transferrin secretion but surprisingly resulted in more efficient oxidative folding and secretion of albumin. Examination of the oxidation status of ER protein-disulfide isomerase family members revealed a shift to a more oxidized state. This was accompanied by a >5-fold elevation in the ratio of oxidized to total glutathione. This "hyperoxidation" phenotype could be duplicated by incubating cells with the cyclophilin inhibitor cyclosporine A, a treatment that triggered efficient ER depletion of cyclophilins B and C by inducing their secretion to the medium. To identify the pathway responsible for ER hyperoxidation, we individually depleted several enzymes that are known or suspected to deliver oxidizing equivalents to the ER: Ero1αβ, VKOR, PRDX4, or QSOX1. Remarkably, none of these enzymes contributed to the elevated oxidized to total glutathione ratio induced by cyclosporine A treatment. These findings establish cyclophilin C as an ER cyclophilin, demonstrate the novel involvement of cyclophilins B and C in ER redox homeostasis, and suggest the existence of an additional ER oxidative pathway that is modulated by ER cyclophilins.
authors.affiliation: Department of Biochemistry, University of Toronto, Toronto M5S 1A8, Canada.
authors.forename: Pawel
authors.initials: P
authors.lastname: Stocki
authors.affiliation: Department of Biochemistry, University of Toronto, Toronto M5S 1A8, Canada.
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Chapman
authors.affiliation: Department of Biochemistry, University of Toronto, Toronto M5S 1A8, Canada.
authors.forename: Lori A
authors.initials: LA
authors.lastname: Beach
authors.affiliation: Department of Biochemistry, University of Toronto, Toronto M5S 1A8, Canada. Electronic address: david.williams@utoronto.ca.
authors.forename: David B
authors.initials: DB
authors.lastname: Williams
mesh_terms: D000072622:Cyclophilin C; D021983:Cyclophilins / Q000235:genetics / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000201:enzymology* / Q000235:genetics; D005978:Glutathione / Q000235:genetics / Q000378:metabolism; D056945:Hep G2 Cells; D006706:Homeostasis / Q000502:physiology*; D006801:Humans; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000235:genetics / Q000378:metabolism; D050862:Oxidoreductases Acting on Sulfur Group Donors / Q000235:genetics / Q000378:metabolism; D054464:Peroxiredoxins / Q000235:genetics / Q000378:metabolism
chemical_list: D008562:Membrane Glycoproteins; C081786:cyclophilin B; C405327:ERO1A protein, human; D010088:Oxidoreductases; C515980:PRDX4 protein, human; D054464:Peroxiredoxins; D050862:Oxidoreductases Acting on Sulfur Group Donors; C117183:QSOX1 protein, human; D021983:Cyclophilins; D000072622:Cyclophilin C; D005978:Glutathione
keywords: Cyclosporine A; Disulfide; Endoplasmic Reticulum (ER); Oxidase; Peptidyl-Prolyl Isomerase; Protein Folding; Protein-disulfide Isomerases; Redox Regulation