title: Implication of the proprotein convertases in iron homeostasis: proprotein convertase 7 sheds human transferrin receptor 1 and furin activates hepcidin.
abstract: The first seven members of the proprotein convertase (PC) family activate protein precursors by cleavage after basic residues. While PC7 has no known specific substrates, it shows redundancy with other PCs. A genome-wide association study suggested that circulating levels of shed human transferrin receptor 1 (hTfR1) are regulated by PC7. We thus examined whether hTfR1 constitutes a specific substrate for PC7. Coexpression of hTfR1 with PCs in several cell lines indicated that PC7 is the only convertase that sheds this receptor into the medium. Site-directed mutagenesis showed that cleavage occurs at the unusual site KTECER100 ↓LA, in which the P1 Arg100 and P6 Lys95 are critical. Pharmacological treatments revealed that shedding of hTfR1 by PC7 requires endocytosis into acidic clathrin-coated vesicles. A PC7 chimera, in which the transmembrane domain and the cytosolic tail of PC7 were replaced by that of the convertase furin, lost its ability to cleave the receptor, demonstrating the importance of these domains in the regulation of PC7 function. Analysis of primary hepatocytes from mice lacking furin, PC5, PACE4, or PC7 revealed that hepcidin, which limits iron availability in the circulation, is specifically generated by furin and not by PC7. Finally, depletion of iron in the medium of hepatoma cell lines incubated with the iron chelator desferrioxamine resulted in PC7 down-regulation.
  
  
  CONCLUSIONS
  Among the PC family members, only furin activates hepcidin in hepatocytes, and uniquely the full-length membrane-bound PC7 can directly shed hTfR1 by cleavage at Arg100 ↓. Our results support the notion that, when iron is limiting, hTfR1 levels increase at least in part by way of the down-regulation of PC7 expression. (HEPATOLOGY 2013;).
authors: Laboratory of Biochemical Neuroendocrinology, Clinical Research Institute of Montreal (IRCM), Affiliated to the University of Montreal, Montreal, Quebec, Canada.
authors: Johann
authors: J
authors: Guillemot
authors: Maryssa
authors: M
authors: Canuel
authors: Rachid
authors: R
authors: Essalmani
authors: Annik
authors: A
authors: Prat
authors: Nabil G
authors: NG
authors: Seidah
mesh_terms: D000818:Animals; D015703:Antigens, CD / Q000378:metabolism*; D023181:Antimicrobial Cationic Peptides / Q000378:metabolism*; D019556:COS Cells; D002522:Chlorocebus aethiops; D015536:Down-Regulation; D011992:Endosomes / Q000378:metabolism; D045683:Furin / Q000378:metabolism*; D057809:HEK293 Cells; D056945:Hep G2 Cells; D022781:Hepatocytes / Q000378:metabolism*; D064451:Hepcidins; D006801:Humans; D007501:Iron / Q000378:metabolism*; D051379:Mice; D017434:Protein Structure, Tertiary; D011990:Receptors, Transferrin / Q000378:metabolism*; D013381:Subtilisins / Q000378:metabolism*
chemical_list: D015703:Antigens, CD; D023181:Antimicrobial Cationic Peptides; C068126:CD71 antigen; C578702:HAMP protein, human; C579599:Hamp protein, mouse; D064451:Hepcidins; D011990:Receptors, Transferrin; D007501:Iron; D013381:Subtilisins; C107583:proprotein convertase PC7; D045683:Furin