Insulin-like growth factor-I (IGF-I) concentration in 150-kilodalton complexes containing human IGF-binding protein-3 (hIGFBP-3) after intravenous injection of adult rats with hIGFBP-3.
After i.v. injection into adult rats, human insulin-like growth factor-binding protein-3 (hIGFBP-3) forms 150-kDa complexes with excess endogenous rat acid-labile subunit (ALS) within 2 min (Lewitt et al., 1993, Endocrinology 133:1797). Because their previous in vitro studies indicated that hIGFBP-3 only bound to ALS in the presence of IGF-I, and because little free IGF-I is present in plasma, the authors postulated that IGF-I had been mobilized to the circulation to saturate the 150-kDa hIGFBP-3 complexes. We examined this hypothesis by determining whether the hIGFBP-3 that appears in 150-kDa complexes 2 min after i.v. injection is accompanied by an increase in IGF-I. Within 2 min, some of the injected hIGFBP-3 (approximately 30% as much as endogenous intact rat IGFBP-3) is present in complexes that are cleared slowly from the circulation and presumed to be 150-kDa complexes. Gel filtration and immunoprecipitation studies performed on blood collected 2 min after injection confirmed that the injected hIGFBP-3 (46-82% as much as rat IGFBP-3) was associated with ALS in 150-kDa complexes. The formation of 150-kDa complexes containing hIGFBP-3 was not accompanied by a corresponding change in the IGF-I content (determined by RIA) of whole serum or 150-kDa serum fractions, suggesting that the hIGFBP-3 had rapidly associated with rALS in vivo without mobilizing IGF-I. Surprisingly, however, hIGFBP-3 was cleared much more rapidly from 150-kDa complexes formed after injection of hIGFBP-3 than after injection of hIGFBP-3:IGF-I complexes, suggesting that the complexes observed after hIGFBP-3 injection might not have formed in vivo. In fact, 150-kDa complexes formed to a similar extent when hIGFBP-3 was added ex vivo to blood collected from rats that had not received hIGFBP-3. We conclude that hIGFBP-3 can rapidly associate with rALS to form 150-kDa complexes in vivo without the mobilization of IGF-I. Because 150-kDa complexes also are formed ex vivo, however, we are unable to resolve whether the complexes that formed in vivo formed as binary or ternary complexes.
Lee
C Y
CY
Molecular and Cellular Endocrinology Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.
Wu
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Suh
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Rechler
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D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism; D002850:Chromatography, Gel; D006023:Glycoproteins / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000737:chemistry / Q000494:pharmacology*; D007334:Insulin-Like Growth Factor I / Q000737:chemistry*; D007700:Kinetics; D008970:Molecular Weight; D011487:Protein Conformation; D011817:Rabbits; D011863:Radioimmunoassay; D051381:Rats; D013002:Somatomedins / Q000378:metabolism
D002352:Carrier Proteins; D006023:Glycoproteins; D018972:Insulin-Like Growth Factor Binding Protein 3; D013002:Somatomedins; C075944:insulin-like growth factor binding protein, acid labile subunit; D007334:Insulin-Like Growth Factor I