title: Monocarboxylate transporter 4, associated with the acidification of synovial fluid, is a novel therapeutic target for inflammatory arthritis.
abstract: OBJECTIVE
  Synovial fluid pH is decreased in patients with rheumatoid arthritis (RA); however, the underlying mechanisms are unclear. We undertook this study to examine the mechanism by which synovial fluid pH is regulated and to explore the possibility of a therapeutic strategy by manipulating this mechanism.
  
  
  METHODS
  We determined the pH and lactate concentration in synovial fluid from 16 RA patients. Cultured synovial fibroblasts (SFs) from the inflamed joints of 9 RA patients (RASFs) were examined for the expression of ion transporters that regulate intracellular and extracellular pH. The ion transporter up-regulated in RASF lines was then suppressed in RASFs by small interfering RNA (siRNA), and the effect of transfection on viability and proliferation was investigated. Finally, we examined the therapeutic effect of electrotransfer of monocarboxylate transporter 4 (MCT4)-specific siRNA into the articular synovium of mice with collagen-induced arthritis (CIA).
  
  
  RESULTS
  Synovial fluid pH correlated inversely with both the Disease Activity Score in 28 joints using the C-reactive protein level and the synovial fluid lactate levels. RASFs exhibited up-regulated transcription of MCT4 messenger RNA. MCT4 exported intracellular lactate into the extracellular space. RASFs had significantly higher MCT4 protein levels than did SFs from patients with osteoarthritis. Knockdown of MCT4 induced intrinsic apoptosis of RASFs, thereby inhibiting their proliferation. Moreover, electrotransfer of MCT4-specific siRNA into the articular synovium of mice with CIA significantly reduced the severity of arthritis.
  
  
  CONCLUSIONS
  RA activity correlated with decreased synovial fluid pH. This may be due to increased MCT4 expression in RASFs. Silencing MCT4 induced apoptosis in RASFs and reduced the severity of CIA, suggesting that MCT4 is a potential therapeutic target for inflammatory arthritis.
authors.affiliation: Kyoto Prefectural University of Medicine, Kyoto, Japan.
authors.forename: Wataru
authors.initials: W
authors.lastname: Fujii
authors.affiliation: Kyoto Prefectural University of Medicine, Kyoto, Japan.
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Kawahito
authors.affiliation: Kyoto Prefectural University of Medicine, Kyoto, Japan.
authors.forename: Hidetake
authors.initials: H
authors.lastname: Nagahara
authors.affiliation: Kyoto Prefectural University of Medicine, Kyoto, Japan.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Kukida
authors.affiliation: Kyoto Prefectural University of Medicine, Kyoto, Japan.
authors.forename: Takahiro
authors.initials: T
authors.lastname: Seno
authors.affiliation: Kyoto Prefectural University of Medicine, Kyoto, Japan.
authors.forename: Aihiro
authors.initials: A
authors.lastname: Yamamoto
authors.affiliation: Kyoto Prefectural University of Medicine, Kyoto, Japan.
authors.forename: Masataka
authors.initials: M
authors.lastname: Kohno
authors.affiliation: Kyoto Prefectural University of Medicine, Kyoto, Japan.
authors.forename: Ryo
authors.initials: R
authors.lastname: Oda
authors.affiliation: Kyoto Prefectural University of Medicine, Kyoto, Japan.
authors.forename: Daigo
authors.initials: D
authors.lastname: Taniguchi
authors.affiliation: Kyoto Prefectural University of Medicine, Kyoto, Japan.
authors.forename: Hiroyoshi
authors.initials: H
authors.lastname: Fujiwara
authors.affiliation: Kyoto Prefectural University of Medicine, Kyoto, Japan.
authors.forename: Akika
authors.initials: A
authors.lastname: Ejima
authors.affiliation: Kyoto Prefectural University of Medicine, Kyoto, Japan.
authors.forename: Tsunao
authors.initials: T
authors.lastname: Kishida
authors.affiliation: Kyoto Prefectural University of Medicine, Kyoto, Japan.
authors.forename: Osam
authors.initials: O
authors.lastname: Mazda
authors.affiliation: Kyoto Pharmaceutical University, Kyoto, Japan.
authors.forename: Eishi
authors.initials: E
authors.lastname: Ashihara
mesh_terms: D000368:Aged; D000818:Animals; D017209:Apoptosis / Q000235:genetics; D001169:Arthritis, Experimental / Q000235:genetics / Q000378:metabolism*; D001172:Arthritis, Rheumatoid / Q000188:drug therapy / Q000235:genetics / Q000378:metabolism*; D002358:Cartilage, Articular / Q000378:metabolism; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D019344:Lactic Acid / Q000378:metabolism; D008297:Male; D051379:Mice; D008875:Middle Aged; D027501:Monocarboxylic Acid Transporters / Q000235:genetics / Q000378:metabolism*; D009124:Muscle Proteins / Q000235:genetics / Q000378:metabolism*; D034741:RNA, Small Interfering; D015398:Signal Transduction / Q000235:genetics; D013582:Synovial Fluid / Q000378:metabolism*; D013583:Synovial Membrane / Q000378:metabolism*; D014162:Transfection
chemical_list: D027501:Monocarboxylic Acid Transporters; D009124:Muscle Proteins; D034741:RNA, Small Interfering; C406962:SLC16A4 protein, human; D019344:Lactic Acid