Insulin-like growth factor (IGF)-I binding to a cell membrane associated IGF binding protein-3 acid-labile subunit complex in human anterior pituitary gland.
The binding characteristics of [(125) I]insulin-like growth factor (IGF)-I were studied in human brain and pituitary gland. Competition binding studies with DES(1-3)IGF-I and R(3) -IGF-I, which display high affinity for the IGF-I receptor and low affinity for IGF binding proteins (IGFBPs), were performed to distinguish [(125) I]IGF-I binding to IGF-I receptors and IGFBPs. Specific [(125) I]IGF-I binding in brain regions and the posterior pituitary was completely displaced by DES(1-3)IGF-I and R(3) -IGF-I, indicating binding to IGF-I receptors. In contrast, [(125) I]IGF-I binding in the anterior pituitary was not displaced by DES(1-3)IGF-I and R(3) -IGF-I, suggesting binding to an IGF-binding site that is different from the IGF-I receptor. Binding affinity of IGF-I to this site was about 10-fold lower than for the IGF-I receptor. Using western immunoblotting we were also unable to detect IGF-I receptors in human anterior pituitary. Instead, western immunoblotting and immunoprecipitation experiments showed a 150-kDa IGFBP-3-acid labile subunit (ALS) complex in the anterior pituitary and not in the posterior pituitary and other brain regions. RT-PCR experiments showed the expression of ALS mRNA in human anterior pituitary indicating that the anterior pituitary synthesizes ALS. In the brain regions and posterior pituitary, IGFBP-3 was easily washed away during pre-incubation procedures as used in the [(125) I]IGF-I binding experiments. In contrast, the IGFBP-3 complex in the anterior pituitary could not be removed by these washing procedures. Our results indicate that the human anterior pituitary contains a not previously described tightly cell membrane-bound 150-kDa IGFBP-3-ALS complex that is absent in brain and posterior pituitary.
Department of Neurology, Academisch Ziekenhuis Groningen, Groningen, The Netherlands. n.wilczak@neuro.azg.nl
Nadine
N
Wilczak
Nicole
N
Kühl
Daniel
D
Chesik
Albert
A
Geerts
Paul
P
Luiten
Jacques
J
De Keyser
D000368:Aged; D001667:Binding, Competitive / Q000187:drug effects / Q000502:physiology; D015153:Blotting, Western; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism*; D005260:Female; D005625:Frontal Lobe / Q000737:chemistry / Q000166:cytology / Q000378:metabolism; D006023:Glycoproteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000031:analogs & derivatives / Q000378:metabolism* / Q000493:pharmacokinetics; D046911:Macromolecular Substances; D008297:Male; D008875:Middle Aged; D010903:Pituitary Gland, Anterior / Q000737:chemistry / Q000166:cytology / Q000378:metabolism*; D011233:Precipitin Tests; D012333:RNA, Messenger / Q000032:analysis / Q000378:metabolism; D017526:Receptor, IGF Type 1 / Q000032:analysis; D020133:Reverse Transcriptase Polymerase Chain Reaction
D002352:Carrier Proteins; D006023:Glycoproteins; D018972:Insulin-Like Growth Factor Binding Protein 3; D046911:Macromolecular Substances; D012333:RNA, Messenger; C075944:insulin-like growth factor binding protein, acid labile subunit; D007334:Insulin-Like Growth Factor I; D017526:Receptor, IGF Type 1