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From the Clinical Research Centers |
Baylor College of Medicine (D.R.P., S.K.D., E.D.B.), Houston, Texas 77030; Genentech, Inc. (J.W.F.), South San Francisco, California 94080; University of Washington (S.L.W.), Seattle, Washington 98108; Columbia Hospital at Medical City (R.J.H.), Dallas, Texas 75230; and Loma Linda University and Jerry L. Pettis Veterans Administration Medical Center (S.M.), Loma Linda, California 92357
Address all correspondence and requests for reprints to: Dr. David R. Powell, Texas Childrens Hospital, Feigin Center, MC# 32482, 6621 Fannin, Houston, Texas 77030. E-mail: dpowell{at}bcm.tmc.edu
| Abstract |
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| Introduction |
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IGFBP-3 levels are high in CRF serum due to an excess of IGFBP-3 fragments with low IGF affinity (7, 8, 22, 23). Before GH treatment, IGFBP-3 levels do not correlate significantly with the height SD score of growth-retarded CRF children (12). However, GH induces a rise in IGFBP-3 levels in CRF serum that correlates significantly and positively with increases in height SD score and in serum levels of IGF-I, IGF-II, and acid-labile subunit (ALS) (12). This suggests that serum IGFBP-3 participates in the GH-induced catch-up growth of these children, perhaps by releasing IGFs to target tissues such as growth plate chondrocytes.
IGFBP-4 is a well known inhibitor of IGF action on cartilage (24) and cultured cells (25, 26, 27). IGFBP-5, identified in medium conditioned by bone cells and in human bone matrix, potentiates IGF actions in osteoblasts (27, 28, 29, 30) and either potentiates (31) or inhibits (32) IGF action in other cell types depending on experimental conditions. These observations suggest that IGFBP-4 and IGFBP-5 may play roles in the growth of CRF children. To evaluate this possibility, we used recently developed antisera to characterize levels and forms of IGFBP-4 and IGFBP-5 in serum from growth-retarded CRF children before and during GH therapy.
| Subjects and Methods |
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Sera were obtained from 44 children with CRF who did not yet require dialysis [glomerular filtration rate (GFR) between 1040 mL/min·1.73 m2] and who were part of an open label, multicenter trial of the effects of recombinant human GH on CRF children. Of the 44 children, 30 were randomized to the GH treatment group, and 14 were randomized to the untreated group. The study design, approved by the institutional review board for research involving human subjects of each participating center, has been described (12). Characteristics of the GH-treated and untreated groups at baseline and during the first year of study, including GFR, anthropometrics, and serum levels of IGF-I, free IGF-I, IGF-II, IGFBP-1, IGFBP-2, IGFBP-3, IGFBP-6, ALS, insulin, GH-binding protein, and PTH have also been described (12, 14). Control sera were obtained from 10 healthy prepubertal children (12, 14). Blood samples were centrifuged, and the sera were frozen (-80 C) until assay.
Serum protein assays
IGFBP-4 RIA. IGFBP-4 was measured by RIA using guinea pig antiserum raised against recombinant human IGFBP-4 (rhIGFBP-4) expressed in Escherichia coli as reported previously (33); this antiserum has no significant cross-reactivity with other IGFBPs (33, 34). The same rhIGFBP-4 served as standard and radioactive trace.
IGFBP-5 RIA. IGFBP-5 was measured by RIA using guinea pig antiserum raised against rhIGFBP-5 expressed in E. coli as reported previously (35); this antiserum has no significant cross-reactivity with other IGFBPs (35, 36). The same rhIGFBP-5 served as standard and radioactive trace.
Immunoblotting
Serum (20 µL) diluted with 30 µL H2O and 50 µL nonreducing dissociation buffer (0.125 mol/L Tris HCl, pH 6.8; 20% glycerol; and 4% SDS) was subjected to SDS-PAGE overnight at 10 mA through a 1020% gradient gel. Proteins were transfered to a 0.45-µm pore size BA-S nitrocellulose membrane (Schleicher & Schuell, Inc., Keene, NH) as previously described (35). Nitrocellulose membranes were blocked for 1 h with 5% nonfat dry milk, incubated for 1 h with antiserum to IGFBP-4 (1:1000 dilution) or IGFBP-5 (1:200 dilution), and then incubated for 1 h with horseradish peroxidase-conjugated rabbit antiguinea pig IgG (1:1000 dilution; Zymed Laboratories, San Francisco, CA). Antigen-antibody reactions were visualized using ECL chemiluminescence reagents as recommended by the manufacturer (Amersham Life Sciences, Arlington Heights, IL).
Statistics
Data, presented as the mean ± SD, were
converted to log10 values for statistical analysis.
Differences within untreated and GH-treated CRF groups at 3 and 12
months, between these two groups at baseline, and between all CRF
children and normal children at baseline were evaluated for
significance by Students t test. Differences between
untreated and GH-treated CRF groups at 3 and 12 months were evaluated
for significance by analysis of covariance using the baseline level as
covariate. For all comparisons, differences were considered significant
when P
0.05. When correlating variables measured at
baseline in all CRF children, Pearson correlation coefficients were
calculated. When correlating variables measured in GH-treated CRF
children during the treatment phase of the study, Pearson correlation
coefficients were calculated using the mean of the 3 and 12 month
values for each variable.
| Results |
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By RIA, the mean serum IGFBP-4 level of 39.4 ± 9.9 nmol/L in
the 44 CRF children was significantly higher than that of 10.6 ±
2.9 nmol/L in the 10 normal children (P < 0.001).
Immunoblots performed with the same IGFBP-4 antiserum suggest that
increases in both intact and fragmented IGFBP-4 contribute to the rise
in IGFBP-4 levels in CRF serum (Fig. 1
).
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Serum IGFBP-5 levels at baseline
By RIA, the mean serum IGFBP-5 level of 8.0 ± 2.6 nmol/L in
the 44 CRF children was not significantly different from that of
9.4 ± 2.5 nmol/L in the 10 normal children. Immunoblots performed
with the same IGFBP-5 antiserum suggest that the majority of IGFBP-5 in
CRF serum is fragmented (Fig. 2
).
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IGFBP-4 levels rose slightly but significantly in the serum of CRF
children after 3 and 12 months of GH treatment (Fig. 4A
). GH did not alter the relative
abundance of intact vs. fragmented IGFBP-4 (data not shown).
The mean serum IGFBP-4 levels measured after 3 and 12 months of GH
treatment in 30 CRF children correlated significantly and negatively
with GFR (r = -0.51; P = 0.006), but did not
correlate with the GH-induced change in height SD score.
During GH treatment, serum IGFBP-4 levels did not correlate
significantly with IGF-I, IGF-II, IGFBP-1, IGFBP-2, IGFBP-3, or ALS
levels.
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GH significantly increased IGFBP-5 levels in the serum of children
with CRF (Fig. 4B
). In the 30 CRF children treated with GH, IGFBP-5
levels rose from 8.3 ± 2.8 nmol/L at 0 months to 11.6 ± 4.2
nmol/L after 3 months of treatment and 12.3 ± 3.2 nmol/L after 12
months of treatment. In contrast, serum IGFBP-5 levels in untreated CRF
children at 0 months (7.5 ± 2.2 nmol/L) and 12 months (7.7
± 1.5 nmol/L) of study were not significantly different. By
immunoblot, GH did not clearly alter the relative amounts of intact
vs. fragmented IGFBP-5 in sera of CRF children (Fig. 2
).
The mean serum IGFBP-5 levels measured after 3 and 12 months of GH
treatment in 30 CRF children correlated significantly and positively
with the improved height SD score of these children after
12 months of GH; positively with the mean of 3 and 12 month levels of
serum IGF-I, IGF-II, IGFBP-3, and ALS; and negatively with the mean
of 3 and 12 month levels of serum IGFBP-1 and IGFBP-2 (Table 1
).
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| Discussion |
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PTH stimulates IGFBP-4 production by bone cells and correlates positively with serum IGFBP-4 levels in normal individuals (33, 38), but the absence of a significant correlation between PTH and IGFBP-4 levels in the present study suggests that PTH is not responsible for high IGFBP-4 levels in CRF sera. The significant negative correlation between serum IGFBP-4 levels and GFR suggests that intact and fragmented IGFBP-4 accumulate in CRF serum due to decreased renal clearance rather than increased production.
At baseline, serum IGFBP-5 levels correlated positively and significantly with serum levels of IGF-I, free IGF-I, IGF-II, IGFBP-3, and ALS, all of which are up-regulated by GH (9, 12, 39, 40, 41). This suggests that IGFBP-5 is a GH-responsive protein, consistent with the 15% increase in IGFBP-5 levels noted in the serum of GH-deficient children after 12 months of GH treatment (36). In the present study, GH increased serum IGFBP-5 levels by 40% and 49% after 3 and 12 months of treatment, respectively, and the increased IGFBP-5 levels correlated significantly with the increased levels of IGFs, IGFBP-3, and ALS measured at 3 and 12 months. These observations provide strong evidence that IGFBP-5 is a GH-responsive protein in vivo.
In serum, IGFBP-3 and IGF levels correlate positively and strongly with ALS levels, probably because ALS greatly prolongs the half-life of IGFs and IGFBP-3 by associating with these proteins in a stable IGFBP-3/IGF/ALS ternary complex (42). The correlation of IGFBP-5 levels with levels of ALS, IGFs, and IGFBP-3 suggests that IGFBP-5, like IGFBP-3, forms a ternary complex with an IGF and an ALS protein. This is consistent with recent data indicating that IGFBP-5 circulates primarily in IGFBP-5/IGF/ALS ternary complexes in human serum (43) and with the fact that IGFBP-3 and -5 share a homologous 18-amino acid basic domain that allows IGFBP-3 to bind ALS (44).
The GH-induced rises in serum IGF-I, IGF-II, IGFBP-3, and ALS levels in CRF children each correlate positively with improved linear growth, suggesting that these proteins may promote growth in CRF children, probably as part of the IGFBP-3/IGF/ALS ternary complex (12, 15). The observations that serum IGFBP-5 levels in GH-treated CRF children correlate positively with improved linear growth; positively with serum levels of IGFBP-3, IGFs, and ALS; and negatively with serum levels of IGFBP-1 and -2, which are likely to be growth inhibitors in CRF (12, 13), all suggest that IGFBP-5 is more likely to stimulate than inhibit the growth of CRF children. IGFBP-3 can stimulate anabolic pathways by releasing IGFs to target tissues after proteolysis lowers IGFBP-3 affinity for IGFs. This may be how IGFBP-3 promotes growth in CRF, as most IGFBP-3 in the ternary complex of CRF serum is a 29-kDa fragment with low IGF affinity (7, 15, 23). IGFBP-5, also found in CRF serum mainly as IGFBP-5 fragments, may promote growth by this same mechanism. Alternatively, IGFBP-5 fragments may directly stimulate cell growth in the absence of IGFs (30).
In CRF children, IGFBP-4 levels rose approximately 26% after 12 months of GH treatment, but did not correlate significantly with levels of IGF-I, IGF-II, IGFBP-3, IGFBP-5, or ALS either before or during GH treatment. This suggests that serum IGFBP-4/IGF/ALS ternary complexes do not form, consistent with the recent data of Twigg and Baxter (43). At present, it is unclear whether this slight rise in serum IGFBP-4 levels participates in the improved growth of GH-treated CRF children. However, the delayed embryonic growth of mice lacking an IGFBP-4 gene suggests that under certain circumstances, IGFBP-4 may be a growth stimulator (45).
Thus, the present data show that IGFBP-4 and IGFBP-5 are GH-responsive proteins. The data also suggest that IGFBP-5 forms a ternary complex in serum and may act as a growth enhancer in CRF children. Future studies will determine what, if any, role IGFBP-4 plays in the linear growth process in vivo.
| Footnotes |
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2 Southwest Pediatric Nephrology Study Group Centers/Participants:
Baylor College of Medicine (Houston, TX): David Powell, M.D., Eileen
Brewer, M.D., Andrea Forbes, R.N., and Evelyn Janoff, R.N.; Arkansas
Childrens Hospital (Little Rock, AR): Eileen Ellis, M.D., Donna
Floyd-Gimons, R.N., and Melissa House, R.N.; Baylor University Medical
Center (Dallas, TX): Ronald Hogg, M.D., and Tammy Fisher, R.N.;
California Pacific Medical Center (San Francisco, CA): Susan Conley,
M.D., and Deborah Acres, R.N.; Cedars-Sinai Medical Center (Los
Angeles, CA): Elaine Kamil, M.D., and Cathy Vogt, R.N.; Childrens
Hospital and Medical Center (Seattle, WA): Sandra L. Watkins, M.D.,
Louise Peck, R.N., and Kelly McCarthy, R.N.; Childrens Hospital
Medical Center (Cincinnati, OH): Fred Strife, M.D.; Childrens
Hospital of Buffalo (Buffalo, NY): Leonard Feld, M.D., and Cathy
Sherin, R.N.; Childrens Memorial Hospital (Chicago, IL): Craig
Langman, M.D., and Katy Schmeissing, R.N, M.S.; Childrens Mercy
Hospital Medical Center (Kansas City, MO): Bradley Warady, M.D., and
Gina Weddle, R.N.; Childrens National Medical Center (Washington DC):
Mary Ellen Turner, M.D.; Cook Childrens Hospital
(Fort Worth, TX): William Allen, M.D., Watson Arnold, M.D., and Peggy
Brigance, R.N.; Crippled Childrens Foundation Research Center
(Memphis, TN): Robert Wyatt, M.D., and Paula Miller, R.N.; Loma Linda
University Medical Center (Loma Linda, CA): Shoba Sahney, M.D., and
Sandi Swiridoff, R.N.; University of Oklahoma (Oklahoma City, OK):
Adolfo Garnica, M.D., and James Wenzl, M.D.; Tulane University Medical
Center (New Orleans, LA): Frank Boineau, M.D., Karen Welling, R.N.,
M.S.N., and Melissa Parenti, R.N.; University of Alabama (Birmingham,
AL): Edward Kohaut, M.D., and Sandra Overstreet, R.N.; University of
California (Los Angeles, CA): Robert Ettenger, M.D., Ora Yadin, M.D.,
and Lila Moulton, R.N.; University of Chicago Childrens Hospital
(Chicago, IL): Sharon Bartosh, M.D., and Eileen Swanson, R.N.;
University of Colorado Health Sciences Center (Denver, CO): Douglas
Ford, M.D., Carol Salbenblatt, R.N., and Terri Bisio, R.N.; University
of Texas Medical Branch (Galveston, TX): Alok Kalia, M.D., Ann Burns,
R.N., and Mary Ann Armendaiz, R.N.; University of Texas Medical School
(Houston, TX): Ronald Portman, M.D., and Patty Brannan, R.N.;
University of Texas Southwestern Medical Center (Dallas, TX): Steven
Alexander, M.D., and Nancy Simonds, R.N.; University of Utah Medical
Center (Salt Lake City, UT): Miriam Turner, M.D., Richard Siegler,
M.D., and Carolyn Wagner-Munford, R.N.; and University of Virginia
(Charlottesville, VA): Robert Chevalier, M.D., Fern Campbell, R.N.
Southwest Pediatric Nephrology Study Group Central Office: Columbia
Hospital at Medical City (Dallas, TX): Ronald J. Hogg, M.D.,
Director; Kaye Green, Administrative Coordinator. ![]()
Received April 29, 1998.
Revised October 27, 1998.
Accepted November 9, 1998.
| References |
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) subunit of the high molecular
weight insulin-like growth factor binding protein complex. J Clin
Endocrinol Metab. 70:13471353.[Abstract]
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