title: Targeting the monocytic ubiquitin system with extracellular ubiquitin.
abstract: Recent findings suggest that extracellular ubiquitin has pleiotropic effects on host defence mechanisms, but its cellular mechanism of action is not yet understood. Using fluorescence and in vivo confocal microscopy, we observed uptake of N-terminal fluorescein-labelled ubiquitin into human PBMC and MonoMac 6 cells. Immunoblotting experiments indicated that extracellular ubiquitin is then rapidly conjugated to a multitude of intracellular proteins. LPS and lipoteichoic acid significantly increased uptake and subsequent conjugation to intracellular proteins dose dependently. This mechanism showed saturation kinetics with a K(d) value for ubiquitin in the low nanomolar range (<10 nmol/L) and a B(max) value of 0.14-0.27 micromol ubiquitin/mg protein. These results suggest that the monocytic ubiquitin system can be targeted with physiologically relevant concentrations of extracellular ubiquitin during inflammation. This concept could provide a simple explanation for a multitude of extracellular ubiquitin's actions and open up new strategies to influence ubiquitin-dependent intracellular processes.
authors.affiliation: Division of Trauma and Surgical Critical Care, DeWitt Daughtry Family Department of Surgery, University of Miami Miller School of Medicine, Miami, Florida 33136, USA. mmajetschak@med.miami.edu
authors.forename: Matthias
authors.initials: M
authors.lastname: Majetschak
authors.forename: Norbert
authors.initials: N
authors.lastname: Ponelies
authors.forename: Thomas
authors.initials: T
authors.lastname: Hirsch
mesh_terms: D045744:Cell Line, Tumor; D002478:Cells, Cultured; D006801:Humans; D008070:Lipopolysaccharides / Q000494:pharmacology*; D009000:Monocytes / Q000187:drug effects / Q000378:metabolism*; D013682:Teichoic Acids / Q000494:pharmacology*; D025801:Ubiquitin / Q000378:metabolism*
chemical_list: D008070:Lipopolysaccharides; D013682:Teichoic Acids; D025801:Ubiquitin; C009900:lipoteichoic acid