title: Circulating insulin-like growth factors may contribute substantially to insulin receptor isoform A and insulin receptor isoform B signalling.
abstract: BACKGROUND
  Only a fraction of circulating insulin-like activity is due to insulin itself. The aim of this study was to determine total serum insulin-like activity mediated via the insulin receptor isoform A (IR-A) and isoform B (IR-B) by using kinase receptor activation (KIRA) assays specific for the IR-A and IR-B.
  
  
  METHODS
  The IR-A and IR-B KIRA assays use human embryonic kidney cells which have been transfected with the human IR-A or IR-B gene and quantify serum-mediated phosphorylation of the IR.
  
  
  RESULTS
  Both IR KIRA assays were sensitive (detection limit 32 pmol/L) and precise (intra- and inter assay CV: <12% and <15%). The EC₅₀s of insulin, IGF-I and IGF-II were 1.4, 11.2 and 6.7 nmol/L for the IR-A KIRA assay, and 1.3, 31.0 and 15.7 nmol/L for the IR-B KIRA assay. The operational range of both assays allowed for determination of total insulin-like activity in human serum. Analysis of serum samples showed that there was a significant positive correlation between serum insulin-like and immunoreactive insulin concentrations (IR-A: r = 0.56, p = 0.01, IR-B: r = 0.68, p = 0.001). Importantly, addition of IGF-I or IGF-II antibodies to human serum samples could substantially decrease the endpoint signal in both KIRA assays.
  
  
  CONCLUSIONS
  We showed that serum IGF-I and IGF-II may substantially contribute to IR signalling. Since IR isoform specific KIRA assays also take into account the contribution of IGFs present in serum on IR signalling, they may help to gain more insight into the roles of IGF mediated IR-A and IR-B activation in health and disease.
authors.affiliation: Division of Endocrinology, Department of Internal Medicine, Erasmus MC, Rotterdam, The Netherlands.
authors.forename: Aimee J
authors.initials: AJ
authors.lastname: Varewijck
authors.forename: Michael P
authors.initials: MP
authors.lastname: Brugts
authors.forename: Jan
authors.initials: J
authors.lastname: Frystyk
authors.forename: Jeannette A
authors.initials: JA
authors.lastname: Goudzwaard
authors.forename: Pieter
authors.initials: P
authors.lastname: Uitterlinden
authors.forename: Adriana M
authors.initials: AM
authors.lastname: Waaijers
authors.forename: Yang
authors.initials: Y
authors.lastname: Feng
authors.forename: Dimiter S
authors.initials: DS
authors.lastname: Dimitrov
authors.forename: Steven W J
authors.initials: SW
authors.lastname: Lamberts
authors.forename: Leo J
authors.initials: LJ
authors.lastname: Hofland
authors.forename: Joseph A M J L
authors.initials: JA
authors.lastname: Janssen
mesh_terms: D057134:Antibodies, Neutralizing / Q000378:metabolism; D015703:Antigens, CD / Q000235:genetics / Q000378:metabolism*; D015870:Gene Expression; D057809:HEK293 Cells; D006801:Humans; D007150:Immunohistochemistry; D007328:Insulin / Q000097:blood / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000032:analysis / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D007335:Insulin-Like Growth Factor II / Q000032:analysis / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D007668:Kidney / Q000166:cytology / Q000378:metabolism; D009994:Osmolar Concentration; D010766:Phosphorylation; D020033:Protein Isoforms / Q000037:antagonists & inhibitors / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D021122:Protein Subunits / Q000037:antagonists & inhibitors / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D011972:Receptor, Insulin / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D015398:Signal Transduction; D014443:Tyrosine / Q000378:metabolism
chemical_list: D057134:Antibodies, Neutralizing; D015703:Antigens, CD; C035170:IGF2 protein, human; D007328:Insulin; D020033:Protein Isoforms; D021122:Protein Subunits; D012333:RNA, Messenger; D011994:Recombinant Proteins; D014443:Tyrosine; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; C435941:INSR protein, human; D011972:Receptor, Insulin