Endocrinology Vol. 142, No. 5 1958-1967
Copyright © 2001 by The Endocrine Society
Phenotypic Manifestations of Insulin-Like Growth Factor-Binding Protein-3 Overexpression in Transgenic Mice1
Tomislav Modric2,
Josef V. Silha,
Zengdun Shi,
Yaoting Gui,
Adisak Suwanichkul,
Susan K. Durham,
David R. Powell and
Liam J. Murphy3
Departments of Internal Medicine (L.J.M.) and Physiology (T.M.,
J.V.S., Z.S., Y.G., L.J.M.), University of Manitoba, Winnipeg,
Manitoba, Canada R3E 0W3; and Department of Pediatrics, Baylor College
of Medicine (A.S., S.K.D., D.R.P.), Houston, Texas 77030
Address all correspondence and requests for reprints to: L. J. Murphy, M.B., Ph.D., Departments of Internal Medicine and Physiology, University of Manitoba, Winnipeg, Manitoba, Canada R3E 0W3. E-mail: ljmurph{at}cc.umanitoba.ca
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Abstract
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In cell culture systems insulin-like growth factor (IGF)-binding
protein-3 (IGFBP-3) can both enhance and inhibit IGF-I action. To
investigate the biological role of IGFBP-3 in vivo,
transgenic (Tg) mice that constitutively overexpress the human IGFBP-3
complementary DNA (cDNA) driven by the mouse phosphoglycerate kinase I
(PGK) and the cytomegalovirus (CMV) promoters were examined. Serum
levels of human IGFBP-3 in CMVBP-3 and PGKBP-3 Tg mice were 4.7 and 5.8
µg/ml, respectively and total IGFBP-3 was increased 4.9- and 7.7-fold
compared with that in wild-type (Wt) mice. In PGKBP-3 Tg mice the
levels of transgene expression were similar in all tissues. Although
CMVBP-3 mice demonstrated similar levels of expression of the transgene
as PGKBP-3 mice in most tissues, markedly elevated expression was
apparent in the kidney and heart. The transgene-derived IGFBP-3
circulated as a 150-kDa ternary complex, and serum IGF-I levels were
elevated 1.9- to 2.8-fold in Tg mice compared with Wt mice. A
significant reduction in birth weight of approximately 10% and a
modest reduction in litter size were apparent in both Tg strains. Early
postnatal growth, as assessed by both body weight and length, was
significantly reduced in Tg mice compared with Wt mice. This was more
marked in PGKBP-3 than in CMVBP-3 mice, who demonstrated a propensity
to adiposity after weaning. The relative organ weights of brain and
kidney were reduced in both Tg strains, whereas liver size and
epididymal fat were significantly increased in CMVBP-3, but not
PGKBP-3, mice. Our data indicate that overexpression of IGFBP-3 is
associated with modest intrauterine and postnatal growth retardation
despite elevated circulating IGF-I levels.
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Introduction
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INSULIN-LIKE growth factor I (IGF-I) and
IGF-II are present in plasma and most biological fluids as a complex
with IGF-binding proteins (IGFBPs). Of the six IGFBPs identified to
date, IGFBP-3 is the most abundant in plasma (1). However,
the function of IGFBP-3 in vivo is not clear. In
vitro experiments examining the effects of IGFBP-3 on various cell
cultures have provided conflicting data, with both enhancement and
inhibition of IGF-I actions observable depending upon cell types and
culture conditions used (2, 3, 4, 5, 6, 7, 8, 9). For example, in human skin
fibroblasts IGFBP-3 either inhibits or potentiates IGF-I induced DNA
synthesis depending upon the timing of addition of IGFBP-3 and IGF-I
(2). In bovine fibroblasts IGFBP-3 increased
IGF-I-stimulated amino acid uptake via enhanced sensitivity of the
protein kinase B/AKT pathway (3).
In addition to these IGF-dependent effects, an increasing amount of
in vitro data suggests that IGFBP-3 exerts effects on cell
proliferation and apoptosis that are IGF independent. For example,
IGFBP-3 has antiproliferative effects on breast cells that are
unresponsive to IGF-I (10) and on mouse fibroblasts that
lack IGF-I receptors (11). In addition, proteolytic
fragments of IGFBP-3 that have markedly reduced affinity for IGF-I
retain antiproliferative effects in vitro
(12).
The ability of IGFBP-3 to bind to a variety of other serum proteins,
including acid-labile subunit (ALS), plasminogen, fibrinogen, and
fibrin, has been reported (13, 14, 15). Cell surface binding
sites for IGFBP-3 have also been identified in some cell types
(16, 17, 18).
These and other reports suggest that IGFBP-3 and other binding proteins
may have biological effects in vivo over and above simply
modulation of IGF action (19) and justify studies to
ascertain the role of IGFBP-3 in vivo. To this end we have
generated Tg mice that overexpress IGFBP-3 at high levels. Here we
report phenotypic characterization of these transgenic (Tg) mice.
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Materials and Methods
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Generation of the Tg mice
The transgene was constructed using a 950-bp fragment of the
human IGFBP-3 cDNA (15). This fragment was inserted
downstream of a 650-bp rabbit ß-globin intron and upstream of a
fragment of the bovine GH gene containing the polyadenylation signal
(20). Either the mouse phosphoglycerate kinase promoter
(PGK-I) (21) or the cytomegalovirus (CMV) promoter
(22) was subcloned upstream of the rabbit ß-globin
intron (23).
Tg mice were generated by pronucleus injection of the linearized
transgene fragment, devoid of plasmid sequences, into fertilized CD-1
zygotes. The microinjected embryos were transferred into CD-1 foster
mice using standard techniques (24). The founders were
bred with wild-type CD-1 mice. CD-1 mice, bred in a similar fashion,
provided wild-type (Wt), nontransgenic control mice of the same genetic
background. All experiments were performed in accordance with protocols
approved by the animal care committee of the Faculty of Medicine,
University of Manitoba.
Southern blot analysis
The presence of the transgene was detected by Southern blot
analysis of tail DNA. Filters were hybridized with a human (h) IGFBP-3
cDNA fragment of the transgene under stringent conditions. For
determination of transgene copy number, serial dilution of tail DNA
from homozygous Tg mice were analyzed by dot-blot hybridization and
quantified densitometrically. The data were compared to a standard
curve of human IGFBP-3 cDNA generated in a similar fashion.
IGFBP-3 and IGF-I assays
IGFBP-3 was measured using an immunoradiometric assay from
Diagnostics Systems Laboratories, Inc. (Webster, TX).
Total plasma IGF-I was measured using a rat IGF-I RIA with an assay kit
from the same source. To detect the presence of IGFBP-3 in the ternary
complex, serum from Tg mice was chromatographed on a Sephacryl S-300HR
column.
RNA extraction and ribonuclease protection assays (RPAs)
RNA was extracted from a variety of tissues from 50- to
60-day-old mice using the single step method described by Chomczynski
and Sacchi (25). The concentration of RNA was determined
spectrophotometrically, and the integrity of the RNA in all samples was
documented by visualization of the 18S and 28S ribosomal bands after
electrophoresis through an 0.8% formaldehyde/agarosis gel. Two
different RPAs were used to measure IGFBP-3 messenger RNA (mRNA)
abundance. One RPA was species specific and measured only the
transgene-derived human IGFBP-3; the other assay measured both mouse
and human IGFBP-3 mRNA. To generate radiolabeled complementary RNA
(cRNA) for the nonspecies-specific RPA, the plasmid (pBluescript SK)
containing a full-length human IGFBP-3 cDNA was digested with
SphI. A 326-bp fragment of coding sequences, nucleotides
647973 was cloned into the pGEM-7Z vector. This sequence has over
90% homology with the mouse IGFBP-3 sequence, and both human and mouse
IGFBP-3 mRNAs were detected using this assay. Radiolabeled cRNA (SA,
0.5 x 109 cpm/µg) was generated by
transcription with T7 polymerase using
[
-32P]CTP or
[
-32P]UTP. An RPA specific for the
transgene-derived IGFBP-3 mRNA was also developed using a 267-bp
fragment containing sequence corresponding to the 3'-end of hIGFBP-3
cDNA and the bovine GH polyadenylase signal region of the
transgene. Total RNA (20 µg) was denatured at 78 C and hybridized
with 106 cpm radiolabeled cRNA in solution at 55
C for 16 h as previously described (25). After
hybridization, T2 RNase was added to digest unbound unprotected RNA.
The protected hybrids were denatured and separated by electrophoresis
through a 5% polyacrylamide/urea gel. Dried gels were exposed to
Kodak XAR film (Eastman Kodak Co., Rochester,
NY) at -70 C for 1224 h. A riboprobe for mouse cyclophilin
(Ambion, Inc., Austin, TX) was used as the internal
standard in some RPA assays. Century RNA markers from Ambion, Inc., were used to determine the size of the protected
fragment.
Western and ligand blotting
Sera (2 µl) from Tg and Wt mice were analyzed on a 10%
SDS-polyacrylamide gel and transferred to nitrocellulose membrane. For
immunodetection, membranes were incubated with biotinylated goat
anti-hIGFBP-3 antibody (Diagnostics Systems Laboratories, Inc.) at 4 C overnight. After washing, the membrane was
incubated with streptavidin-horseradish peroxidase conjugate
(Life Technologies, Inc., Burlington, Canada). Detection
of immune complexes was achieved using an enhanced chemiluminescence
Western blotting kit (Amersham Pharmacia Biotech, Baie
dUrfe, Canada).
For ligand blotting, the membrane was incubated with
[125I]IGF-I, (500,000 cpm; NEN Life Science Products, Boston, MA) at 4 C overnight. The membrane was
subsequently washed four times with Tris-buffered saline (pH 7.6),
0.1% Tween 20, and exposed to Kodak XAR film at -70 C
for 2472 h.
Statistical analysis
Data are expressed as the mean ± SEM.
Students t test was used for single comparisons between Tg
and Wt mice. For determining statistical differences between multiple
groups, an ANOVA followed by Dunnetts t test was used.
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Results
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Two founders were obtained using the PGK promoter transgene
(PGKBP-3), and three founders were obtained with the CMV promoter
transgene (CMVBP-3). All but one of CMVBP-3 founders were successfully
bred to Wt CD1 mice. Two Tg strains were developed for each promoter
construct. These founders were bred with Wt CD-1 mice to generate
homozygous Tg strains. Preliminary growth, allometry, and transgene
expression data were obtained with both of the Tg strains derived from
each promoter. For each Tg strain from a given transgene construct,
these preliminary data were similar. Because of the cost of maintaining
multiple Tg strains, a detailed examination of a single Tg mouse strain
for each of the two promoters was undertaken.
A Southern blot of genomic DNA from Tg mice is shown in Fig. 1
. The DNA was restricted with four
endonucleases. With each of the enzymes used, a different digestion
pattern was apparent with the two DNA samples, indicating a difference
in the transgene insertion sites in the two strains of mice. The
transgene copy numbers were 9 and 21 for PGKBP-3 and CMVBP-3 mice
respectively.

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Figure 1. Southern blot of genomic DNA from homozygous
IGFBP-3 Tg mice. A restriction map of the transgene constructs used to
generate the Tg mice is shown in the upper panel. Tail
DNA (10 µg) was digested with each of the restriction endonucleases.
hIGFBP-3 cDNA was used as a hybridization probe. The positions of the
HindIII DNA size markers are indicated.
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Tg mice derived from both promoter constructs were phenotypically
normal and fertile, and the female IGFBP-3 Tg mice appeared to lactate
normally. Serum levels of human IGFBP-3 were measured in 8-week-old
mice (Fig. 2
). Using a human IGFBP-3
immunoradiometric assay, cross-reaction with endogenous mouse IGFBP-3
was apparent. The IGFBP-3 in levels in Wt mice were about 0.3
µg/ml. In serum from the CMVBP-3 and PGKBP-3 Tg mice the levels of
human IGFBP-3 were similar in male and female mice and were
approximately 17-fold higher than those measured in Wt mice. There was
no significant difference in IGFBP-3 levels between CMVBP-3 and PGKBP-3
mice. Chromatographic analysis of sera from Tg mice indicated that the
majority of the IGFBP-3 was present as a 150-kDa complex (Fig. 3
).

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Figure 2. Serum IGFBP-3 levels in Wt and Tg mice. The
mean ± SEM for 715 mice/group are shown. The
line indicates a significant difference between the Tg
of both strains, PGKBP-3 and CMVBP-3, and their sex-matched Wt mice at
the P < 0.0001 level.
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Figure 3. Chromatographic analysis of serum from PKGBP-3 Tg
mice. Serum was analyzed on a Sephacryl S-300HR column. The
arrow indicates the position of elution of alcohol
dehydrogenase. A similar elution pattern was apparent when serum from
CMVBP-3 Tg mice was analyzed.
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Total serum IGF-I was also measured in the Tg mice (Fig. 4
). IGF-I levels were significantly
increased in both PGKBP-3 and CMVBP-3 Tg mice compared with Wt mice.
There were no significant differences in the IGF-I levels between the
CMVBP-3 and PGKBP-3 strains of Tg mice. Among the individual Tg mice,
there was a significant correlation between IGFBP-3 levels and IGF-I
levels (r2 = 0.62; n = 21; P
< 0.01).

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Figure 4. Serum IGF-I levels in Wt and Tg mice. The
mean ± SEM for 715 mice/group are shown. The
significant differences between the Tg mice and their sex-matched Wt
control mice are indicated: *, P < 0.05; ***,
P < 0.005.
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Sera from 2-month-old homozygous Tg and Wt mice were analyzed by both
Western blotting and ligand blotting. Using the ligand blotting
technique, IGFBP-3 abundance was dramatically increased in both CMVBP-3
and PGKBP-3 Tg mice compared with Wt mice (Fig. 5
, upper panel). When the
autoradiographs were analyzed by densitometry, the abundance of intact
IGFBP-3 in sera from Tg mice was increased 4.9 ± 0.2- and
7.7 ± 0.6-fold in CMVBP-3 and PGKBP-3 mice, respectively,
compared with Wt mice. The additional bands detected by ligand blotting
probably represent IGFBP-2 and IGFBP-4. There was no significant
difference in the abundance of these lower mol wt binding proteins
between Tg and Wt mice. Western blotting with anti-hIGFBP-3 antisera
confirmed the presence of transgene-derive IGFBP-3 in the sera from Tg
mice.

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Figure 5. Western and ligand blot analysis of sera from Wt
and Tg mice. Sera from two male and female Wt mice and two male and
female Tg mice were analyzed. The upper panel shows the
ligand blot obtained using [125I]IGF-I as a probe, and
the lower panel shows the immunoblot obtained using an
antihuman IGFBP-3 antibody.
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An RPA was used to assess the expression of the transgene products in
various tissues collected from 2-month-old male mice (Fig. 6
). Because of the sequence similarity
between human and mouse IGFBP-3 cDNAs, this assay detected
endogenous mouse IGFBP-3 mRNA in addition to the hIGFBP-3 mRNA
derived from the transgene. In the Wt mice endogenous IGFBP-3
expression was highest in the kidney, lung, spleen, and heart, with
lower levels of expression in the liver and brain. In each tissue,
IGFBP-3 mRNA abundance was increased in both PGKBP-3 and CMVBP-3 Tg
mice.

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Figure 6. Tissue distribution of IGFBP-3 mRNA expression in
Wt and Tg mice. An RPA that does not distinguish between mouse and
human IGFBP-3 mRNA was used to quantify mRNA abundance in Wt and Tg
mice. Total RNA (20 µg) from each tissue was analyzed. A
representative result of data from a single male Tg and Wt mouse is
shown in each panel. The arrow indicates the 300-bp
protected fragment.
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Transgene-derived IGFBP-3 mRNA in epididymal fat and testes was
assessed separately using a RPA that was specific for human IGFBP-3
mRNA (Fig. 7
). When the results were
quantified by densitometry, similar levels of IGFBP-3 mRNA were
detected in adipose tissue from CMVBP-3 and PGKBP-3 mice (154.0 ±
9.4 vs. 179.9 ± 12.3 OD area arbitrary units;
P < 0.01). In contrast, hIGFBP-3 mRNA was
significantly more abundant in testicular tissue from CMVBP-3 Tg mice
than in that from PGKBP-3 Tg mice (196.4 ± 4.0 vs.
157.5 ± 9.8 OD area arbitrary units; P <
0.01).

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Figure 7. Transgene expression in epididymal fat and
testicular tissue from Tg mice. A human IGFBP-3 mRNA-specific RPA was
used to quantify mRNA abundance. Total RNA (20 µg) from five male Tg
mice of each strain was analyzed. In addition, tissues from a Wt mouse
were assayed. A cyclophilin cRNA was included in the assay as an
internal control.
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When all litters were considered, the litter size in homozygous
offspring of both PGKBP-3 and CMVBP-3 Tg mice was similar to that in Wt
mice (Fig. 8A
). As there was a tendency
for litter size to increase in female mice with each subsequent litter,
a careful comparison was made between size of the first litter in Tg
and Wt mice. When 2-month-old virgin PGKBP-3 and CMVBP-3 Tg mice were
mated with Wt male mice, a small, but significant, reduction in litter
size was apparent compared with litters from virgin female Wt mice
(Fig. 8B
). Litter size was smaller in PGKBP-3 Tg mice compared with
CMVBP-3 Tg mice. In contrast when male CMVBP-3 and PGKBP-3 Tg mice were
mated with virgin Wt mice there was no significant reduction in litter
size.

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Figure 8. Litter size in IGFBP-3 Tg mice. A, The litter size
for homozygous mice is shown. Included in these data are all litters,
including the first litter and subsequent litters from the same mouse.
B, Data from first litters only were considered. Two-month-old female
mice (identified with an asterisk) were mated with male
Wt mice or CMVBP-3 Tg mice as indicated. The data represent the
mean ± SEM for 514 litters. The significant
differences between the Tg mice and control Wt group are indicated.
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When considered as a group, both homozygous CMVBP-3 and PGKBP-3 Tg pups
were about 10% lighter than Wt pups at birth. This reduction achieved
statistical significance (Fig. 9A
).
PGKBP-3 Tg mice tended to be lighter than CMVBP-3 mice, but the
difference between the two Tg strains was not statistically
significant. A more detailed study was undertaken in litters derived
from crossing Tg mice with Wt mice. In this experimental design, Tg and
Wt mice from the same litter could be directly compared. Both male and
female Tg newborn mice of each strain were significantly lighter than
Wt controls (Fig. 9B
). Furthermore, a similar reduction in birth weight
was apparent when Tg pups were born to Wt female mice, indicating that
the effect on fetal growth was a result of fetal, rather than maternal,
transgene expression. For example, both male and female hemizygous Tg
pups derived from the cross of CMVBP-3 male mice with Wt mice were
significantly lighter than male and female Wt pups, respectively.

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Figure 9. Birth weight in homozygous and hemizygous IGFBP-3
Tg mice. A, The birth weight of Wt and IGFBP-3 Tg pups from the two
different transgene constructs. The data represent the mean ±
SEM for 1121 mice. Both male and female pups have been
included in the analysis. B, Heterozygous pups derived from mating Wt
and IGFBP-3 Tg mice are shown. In each mating the genotype of the
female mice is identified with an asterisk. The
significant differences between the Tg mice and control Wt group are
indicated.
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The growth curves for Tg mice are shown in Fig. 10
. Both male and female mice from each
of the Tg mouse strains were significantly lighter than their Wt
controls during the first 30 days of life. Weight gain in PGKBP-3 mice
of both sexes continued at a rate parallel to that seen in the Wt mice
over the next 2 months, and these Tg mice remained significantly
lighter than their Wt sex-matched controls throughout the study. In
contrast, CMVBP-3 mice of both sexes gained more weight in the period
from 4060 days than their Wt controls. As a result, male CMVBP-3 mice
did not differ significantly in body weight from the Wt mice at 3
months of age. This phenomenon of delayed weight gain was more marked
in female CMVBP-3 mice, which were significantly heavier than their Wt
controls at 3 months (31.9 ± 1.2 vs. 30.5 ±
0.6 g; P < 0.005).

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Figure 10. Body weight gain in Wt and IGFBP-3 Tg mice. The
change in body weight with age in Wt and Tg mice is shown for male and
female mice. The data represent the mean of 922 mice/group. For each
time point the SEM was less than 4% of the mean.
Representative SEMs are shown for each group of mice.
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Linear growth was assessed by measuring the nose to the tip of tail
length as well as tail length in anesthetized animals at 6 and 12 weeks
of age (Table 1
). At 6 and 12 weeks of
age in both male and female PGKBP-3 Tg mice all three parameters were
significantly reduced compared with those in Wt mice. A similar, but
less marked, reduction in linear growth was apparent in CMVBP-3 mice.
However, the differences in these parameters between Wt mice and
CMVBP-3 mice did not achieve statistical significance, with the
exception of tail length in 12-week-old male CMVBP-3 mice.
Organ allometry was performed at 6 and 12 weeks in overnight fasted
animals. Different patterns of organ growth were apparent with the two
different transgene constructs (Fig. 11
). The relative liver weight was
consistently larger in CMVBP-3 Tg mice than Wt mice, whereas it was
similar in female PGKBP-3 and Wt mice, or slightly reduced in the case
of 6-week-old male PGKBP-3 mice. Relative kidney weight was
significantly reduced in CMVBP-3 mice, but was only marginally reduced
in PGKBP-3 mice. In both PGKBP-3 and CMVBP-3 Tg mice there was a
significant reduction in absolute brain size. The reduction in relative
brain weight was more marked in CMVBP-3 mice than in PGKBP-3 mice.
There was a modest reduction in the relative weight of the heart in
6-week-old male and female CMVBP-3 mice compared with Wt mice. The
absolute and relative weights of the spleen tended to be greater in
both CMVBP-3 and PGKBP-3 mice than in Wt mice (Fig. 11
). The most
striking differences between the CMVBP-3 and PGKBP-3 mice were in terms
of epididymal fat pad and testicular weight. Relative weight of the
testes was markedly reduced in CMVBP-3 mice compared with Wt mice,
whereas it was not reduced in PGKBP-3 mice. CMVBP-3 mice had
significantly more epididymal fat than Wt mice, whereas PGKBP-3 mice
and Wt mice did not differ in this regard.

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Figure 11. Relative organ weight in IGFBP-3 Tg mice. The
absolute organ weight was expressed in terms of the body weight
(milligrams per g BW) and then as a percentage of the mean for the sex-
and age-match wild-type mice. The data represent the mean ±
SEM for 726 mice/group. The significant differences
between Tg and Wt mice are indicated: *, P < 0.05;
**, P < 0.005; ***, P <
0.001. EFP, Epididymal fat pad.
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Discussion
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In plasma and in many biological fluids, IGFBP-3 is the most
abundant of the IGFBPs (1). A role for IGFBP-3 in the
transport and modulation of the biological actions of the IGFs has been
proposed. As both inhibition and potentiation of IGF-I actions are
demonstrable in vitro, the exact physiological function of
this and other binding proteins remains unclear. In addition,
IGFBP-3 appears to inhibit cell proliferation and stimulate apoptosis
in an apparent IGF-I-independent fashion in some cell lines (10, 12, 16). Here we report that overexpression of IGFBP-3 in Tg
mice using two different promoter constructs results in a modest growth
retardation, which manifests at birth and continues in the postnatal
period.
The circulating levels of IGFBP-3 in the Tg were comparable to those
seen in human serum and 17-fold higher than those measured in the Wt
mouse circulation. Although there is a high degree of sequence homology
between mouse and human IGFBP-3 proteins, a mouse IGFBP-3 standard was
not used in the human IGFBP-3 immunoradiometric assay. The absolute
level of endogenous IGFBP-3 in mouse circulation is unknown. However,
when sera from Tg and Wt mice were analyzed by ligand blots there was
an approximately 5- to 7-fold increased abundance of intact IGFBP-3.
IGFBP-3 degradation products were also markedly increased in Tg mice.
Thus, in these Tg mice very high levels of expression of the transgene
were achieved with each of the promoters used.
The increase in circulating IGFBP-3 in Tg mice was accompanied by an
increase in plasma IGF-I levels; however on a molar basis the increase
in IGF-I levels was less than the increase in IGFBP-3. This would
suggest that there would be excess unsaturated IGFBP-3 present in the
circulation of the Tg mice and probably reduced free IGF-I levels. The
majority of the human IGFBP-3 detected in the plasma from Tg mice was
present as a 150-kDa complex. This would be consistent with the recent
observation that IGFBP-3/ALS binary complexes can form under
appropriate physiological conditions (27). It also
indicates that human IGFBP-3, which is structurally similar to mouse
IGFBP-3, is able to bind to mouse ALS.
Despite the very marked increase in circulating IGFBP-3 levels in Tg
mice, the effect on growth was modest. The reduction in birth weight in
the IGFBP-3 Tg mice, about 710% depending upon the gender and the
strain, was less than the 816% seen in the IGFBP-1 Tg mice
previously generated in this laboratory (28). As the PGK
promoter was used to generate IGFBP-1 Tg mice, a similar tissue pattern
and timing of transgene expression would be expected for PGKBP-3 mice.
However, the circulating levels of IGFBP-1 in PGKBP-1 Tg mice were much
lower (1060 ng/ml) (28) than the levels of IGFBP-3
observed in PGKBP-3 mice. Postnatal growth retardation was less marked
in the PGKBP-3 mice compared with the PGKBP-1 Tg mice. For example, at
40 days the reductions in body weight in male and female IGFBP-1 Tg
mice compared with that in Wt mice were 5.2 and 8.4 g
(28) compared with 4.7 and 2.4 g, respectively, for
PGKBP-3 mice.
Interestingly, postnatal growth retardation was less marked in CMVBP-3
Tg mice than in PGKBP-3 mice despite similar circulating IGFBP-3 levels
and tissue IGFBP-3 mRNA abundance. CMVBP-3 mice demonstrated a
propensity to adiposity that was clearly demonstrable in the second
month of life. Body weight growth curves for male CMVBP-3 and PGKBP-3
Tg mice were not statistically different until after 40 days of age. In
female Tg mice the difference between the two strains was apparent
slightly earlier.
The growth retardation in the IGFBP-3 Tg mice was apparent in the face
of increased plasma IGF-I levels in both strains of Tg mice. Total
circulating IGF-I levels were increased 2- to 3-fold in the Tg mice. No
attempt was made to assess free IGF-I levels, because it is unclear how
precise or accurate such assays would be in the face of very markedly
increased IGFBP-3 levels. However, our data, indicating growth
retardation in the presence of elevated circulating IGF-I levels,
complement the recent reports of normal growth in the presence of
markedly reduced total plasma IGF-I levels in mice in which conditional
knockout of hepatic IGF-I expression has been achieved (29, 30).
As different promoters were used, the phenotype of the two strains of
transgenic mice differed. However, many features were common to both
strains. These included reduced birth weight, reduced early postnatal
growth, and decreased litter size. The transgene was expressed at high
levels in the kidney, heart, and brain of CMVBP-3 mice. These organs
were relatively smaller in CMVBP-3 than Wt mice and were also smaller
in CMVBP-3 mice than in PGKBP-3 mice, in which transgene expression was
similar in all tissues. However, the major differences between the two
strains of IGFBP-3 Tg mice was the adiposity that developed in the
CMVBP-3 mice after weaning. This was manifested as a marked increase in
the size of the epididymal fat pad in CMVBP-3 mice. Although other fat
masses were not quantified, gross macroscopic examination of the
dissected CMVBP-3 mice suggested that there was a generalized
increase in adiposity. This would also be consistent with the increased
weight gain observed in CMVBP-3 mice in the second and third months of
life.
The mechanism underlying this phenotypic difference in adiposity
between the two strains of IGFBP-3 Tg mice is not immediately apparent.
The transgene was expressed in mature adipose tissue from both CMVBP-3
and PGKBP-3 mice at similar levels. However, it is possible that
different levels of expression could occur in adipoblasts and
preadipocytes. IGF-I is known to be important in the differentiation of
preadipocytes to mature fat cells (30, 31, 32, 33). We
previously reported that PGKBP-1 Tg mice have impaired adipogenesis in
response to caloric excess (34). In the PGKBP-1 there is
both a reduced number of stem cells (adipoblasts) and impaired
differentiation of preadipocytes (34). Although further
detailed investigation is required, PGKBP-3 Tg mice, unlike PGKBP-1
mice, do not appear to have smaller fat deposits than Wt mice.
In Tg mice expressing IGFBP-3 driven by the mMT-1 promoter
previously generated in this laboratory, no growth retardation was
observed (35). However, unlike the PGKBP-3 and CMVBP-3
mice reported here, the level of transgene expression in the
mMT-1/IGFBP-3 Tg mice was very low. It is of interest, however, that
these mice, like the IGFBP-3 Tg mice reported here had splenomegaly.
They also had hepatomegaly (35), which was a consistent
feature of the CMVBP-3 Tg mice.
Although some differences in the phenotypes of CMVBP-3 and PGKBP-3 mice
were observed, the data reported here clearly demonstrate that
overexpression of IGFBP-3 is associated with impaired intrauterine
growth and early postnatal growth. Organs such as brain and kidney,
where IGF-I appears to have an important role in growth (36, 37) were disproportionately growth retarded by IGFBP-3
overexpression. These modest effects on growth are most likely
explicable on the basis of inhibition of IGF action, although it is
impossible to exclude IGF-independent growth inhibitory effects of
IGFBP-3.
The generation of IGFBP-3 Tg mice reported here should provide a useful
model for investigating the in vivo effects of IGFBP-3 in
certain disease processes, such as breast and prostate cancer, as well
as osteoporosis, where epidemiological studies have suggested that
IGFBP-3 may have a role in these conditions in humans.
 |
Footnotes
|
|---|
1 This work was supported by grants from the Medical Research Council
of Canada (to L.J.M.) and Grant R01-DK-38773 (to D.R.P.). 
2 Recipient of Canadian Diabetes Association Postdoctoral Fellowship.
Present address: Animal Health Discovery Research, Pharmacia,
Kalamazoo, Michigan. 
3 Recipient of a Medical Research Council Senior Scientist award and
an endowed Research Professorship in Metabolic Diseases. 
Received October 25, 2000.
 |
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