title: Amyloid-β peptide-induced extracellular S100A9 depletion is associated with decrease of antimicrobial peptide activity in human THP-1 monocytes.
abstract: BACKGROUND
  S100A9 protein (myeloid-related protein MRP14, also referred to as calgranulin B) is a reliable marker of inflammation, an important proinflammatory factor of innate immunity and acts as an additional antimicrobial peptide in the innate immune system. Evidence indicates that S100A9 contributes to Alzheimer's disease (AD) pathology, although the precise mechanisms are not clear.
  
  
  METHODS
  We were interested to study the mechanisms of S100A9 release upon Aβ1-42 stimulation, the potential roles of extracellular S100A9 depletion in Aβ-induced cytotoxicity, and the interaction with innate immune response in THP-1 monocytic cells that have been challenged with mostly Aβ1-42 monomers instead of oligomers. We used protein preparation, Ca(2+) influx fluorescence imaging, MTT assay, siRNA knockdown, colony forming units (CFUs) assay and western blotting techniques to perform our study.
  
  
  RESULTS
  Aβ1-42 monomers elicited a marked decrease of S100A9 release into the cell culture supernatant in a dose-dependent manner in human THP-1 monocytes. This reduction of S100A9 release was accompanied by an increase of intracellular Ca(2+) level. Aβ1-42-mediated decrease of S100A9 release was not associated with Aβ1-42-induced cytotoxicity as measured by MTT reduction assay. This observation was confirmed with the recombinant S100A9, which had little effect on Aβ1-42-induced cytotoxicity. Moreover, depletion of S100A9 with siRNA did not significantly evoke the cell toxicity. On the other hand, Aβ1-42-induced extracellular S100A9 depletion resulted in decreased antimicrobial activity of the culture supernatant after Aβ1-42 stimulation. Immunodepletion of S100A9 with anti-S100A9 also decreased the antimicrobial peptide activity of the vehicle treated culture supernatant. Consistently, the recombinant S100A9 clearly elicited the antimicrobial peptide activity in vitro, confirming the observed antimicrobial activity of S100A9 in the culture supernatant.
  
  
  CONCLUSIONS
  Collectively, our findings suggest that the mostly monomeric form of Aβ1-42 negatively regulates the innate immune system by down-regulating the secretion of S100A9, which is likely a main mediator of antimicrobial activity in the conditioned media of human THP-1 monocytes.
authors.affiliation: Department of Microbiology, School of Medicine, Ewha Medical Research Institute, Ewha Womans University, 911-1, Mok-6-dong, Yangcheonku, Seoul 158-710, Republic of Korea.
authors.forename: Eun Ok
authors.initials: EO
authors.lastname: Lee
authors.forename: Ji Hye
authors.initials: JH
authors.lastname: Yang
authors.forename: Keun-A
authors.initials: KA
authors.lastname: Chang
authors.forename: Yoo-Hun
authors.initials: YH
authors.lastname: Suh
authors.forename: Young Hae
authors.initials: YH
authors.lastname: Chong
mesh_terms: D016229:Amyloid beta-Peptides / Q000633:toxicity*; D023181:Antimicrobial Cationic Peptides / Q000096:biosynthesis*; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism; D040502:Calgranulin B / Q000378:metabolism*; D002478:Cells, Cultured; D003114:Colony-Forming Units Assay; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D006801:Humans; D007113:Immunity, Innate / Q000187:drug effects; D008856:Microscopy, Fluorescence; D009000:Monocytes / Q000187:drug effects / Q000378:metabolism*; D034741:RNA, Small Interfering / Q000235:genetics; D011994:Recombinant Proteins / Q000378:metabolism; D013778:Tetrazolium Salts; D013844:Thiazoles
chemical_list: D016229:Amyloid beta-Peptides; D023181:Antimicrobial Cationic Peptides; D040502:Calgranulin B; D034741:RNA, Small Interfering; D011994:Recombinant Proteins; D013778:Tetrazolium Salts; D013844:Thiazoles; C022616:thiazolyl blue; D002118:Calcium