Insulin-like growth factor binding protein-3 proteolysis in children with insulin-dependent diabetes mellitus: a possible role for insulin in the regulation of IGFBP-3 protease activity.
Limited proteolysis of serum insulin-like growth factor (IGF) binding protein (IGFBP)-3 has been described in various conditions and may increase the bioavailability of IGFs. The physiological regulators of serum IGFBP-3 protease activity are unknown. To characterize the relationship between insulin and IGFBP-3 protease activity, we have examined serum IGFBP-3 proteolysis in children with untreated insulin-dependent diabetes mellitus (IDDM) and have followed the effect of insulin therapy on serum IGFBP-3 proteolysis at 1 day, 1 week, and 1 month after the initiation of insulin therapy. Ligand blot analysis of sera from untreated children with IDDM showed that intact IGFBP-3 was 50 +/- 9% of the age-matched control pool. After the initiation of insulin treatment, IGFBP-3 did not change significantly at 1 day after treatment but increased dramatically at 1 week (90 +/- 13%) and 1 month after treatment (102 +/- 13%). In contrast, when measured by immunoradiometric assay (which detects both intact and fragments of IGFBP-3), IGFBP-3 levels were 70% of the control pool before insulin therapy and did not increase significantly until 1 month after treatment. Immunoblot analysis demonstrated that intact IGFBP-3 doublet was diminished to 41 +/- 7% of controls, whereas the major IGFBP-3 fragment (30 kDa) was increased in IDDM sera before insulin therapy. After insulin, intact IGFBP-3 increased and the 30-kDa fragment decreased to values comparable to those observed in controls. In vivo IGFBP-3 proteolysis, which implies preassay exposure of serum IGFBP-3 to proteases, was estimated by immunoblot analysis. IGFBP-3 proteolysis was increased before insulin therapy (160 +/- 9%) and decreased to 81 +/- 9% at 1 week and to 71 +/- 11% at 1 month after insulin treatment. Residual serum IGFBP-3 protease activity was estimated by a 125I-IGFBP-3 degradation assay. Serum IGFBP-3 protease activity increased significantly in untreated diabetics, compared with activity in controls (128 +/- 5% vs. 99 +/- 11%). During insulin therapy, serum IGFBP-3 protease activity decreased gradually to 91 +/- 5% of control values at 1 month. Molecular sizes of the IGFBP-3 proteolytic fragments (30 kDa, 24 kDa, and 19 kDa) and inhibition profile of IGFBP-3 protease were similar in IDDM and pregnancy sera, indicating that similar proteases (cation-dependent serine proteases) were active in both conditions. These results suggest an important role of insulin in the regulation of IGFBP-3 protease activity. Increased IGFBP-3 proteolysis in the sera of children with IDDM may serve to counteract the catabolic state induced by insulin deficiency.
Bereket
A
A
Department of Pediatrics, State University of New York at Stony Brook 11794-8111, USA.
Lang
C H
CH
Blethen
S L
SL
Fan
J
J
Frost
R A
RA
Wilson
T A
TA
D000293:Adolescent; D000704:Analysis of Variance; D001786:Blood Glucose / Q000378:metabolism; D015153:Blotting, Western; D002352:Carrier Proteins / Q000097:blood*; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D003922:Diabetes Mellitus, Type 1 / Q000097:blood*; D016883:Diabetic Ketoacidosis / Q000097:blood; D010450:Endopeptidases / Q000378:metabolism*; D005260:Female; D006706:Homeostasis; D006801:Humans; D006854:Hydrocortisone / Q000097:blood; D015592:Immunoradiometric Assay; D007223:Infant; D007328:Insulin / Q000097:blood / Q000502:physiology*; D018969:Insulin-Like Growth Factor Binding Proteins; D008297:Male; D010446:Peptide Fragments / Q000097:blood*; D011480:Protease Inhibitors / Q000494:pharmacology; D011627:Puberty; D011994:Recombinant Proteins / Q000378:metabolism; D012016:Reference Values
D001786:Blood Glucose; D002352:Carrier Proteins; D007328:Insulin; D018969:Insulin-Like Growth Factor Binding Proteins; D010446:Peptide Fragments; D011480:Protease Inhibitors; D011994:Recombinant Proteins; D010450:Endopeptidases; C080473:insulin-like growth factor binding protein-3 protease; D006854:Hydrocortisone