The phosphorylation pattern of insulin-like growth factor-binding protein-1 in normal plasma is different from that in amniotic fluid and changes during pregnancy.
We have determined the phosphorylation pattern of circulating insulin-like growth factor-binding protein-1 (IGFBP-1) in normal subjects and assessed how this changes in pregnancy. Two RIAs employing different monoclonal antibodies (MAbs 6303 or 6305) were used to measure IGFBP-1. In normal subjects, RIA 6303 measured 11-fold higher levels than RIA 6305 (72.8 vs. 6.6 micrograms/L; P < 0.008). However, in amniotic fluid (AF), the two assays gave similar results. Immunoprecipitation of plasma and AF with MAb 6303 and 6305 before nonsodium dodecyl sulfate-electrophoresis and Western ligand blotting revealed different IGFBP-1 isoforms and differential antibody recognition as the cause of this discrepancy. In AF, both MAbs precipitated nonphosphorylated and phosphorylated isoforms, whereas in plasma, only a single highly phosphorylated species, not seen in AF, was observed. This form of IGFBP-1 was precipitated by MAb 6303 only. During pregnancy, the phosphorylation state of IGFBP-1 in the maternal circulation was altered, as nonphosphorylated IGFBP-1 and three lesser phosphoforms were also observed. The appearance of these other variants resulted in a significant increase in IGFBP-1 measured by RIA 6305 (37, 51, and 83 micrograms/L in first, second, and third trimesters, respectively; P < 0.0005 vs. controls). The changes in IGFBP-1 phosphorylation induced by pregnancy may influence the modulatory effects of IGFBP-1 on IGF bioavailability and, hence, fetal growth.
Westwood
M
M
Department of Medicine, University of Manchester, United Kingdom.
Gibson
J M
JM
Davies
A J
AJ
Young
R J
RJ
White
A
A
D000328:Adult; D000653:Amniotic Fluid / Q000378:metabolism*; D000911:Antibodies, Monoclonal; D015153:Blotting, Western; D002352:Carrier Proteins / Q000097:blood* / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D006801:Humans; D007163:Immunosorbent Techniques; D018970:Insulin-Like Growth Factor Binding Protein 1; D008297:Male; D010766:Phosphorylation; D011247:Pregnancy / Q000378:metabolism*; D011863:Radioimmunoassay; D013997:Time Factors
D000911:Antibodies, Monoclonal; D002352:Carrier Proteins; D018970:Insulin-Like Growth Factor Binding Protein 1