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Endocrine Research Unit, Mayo Clinic and Mayo Foundation, Rochester, Minnesota 55905
Address all correspondence and requests for reprints to: Sundeep Khosla, M.D., Mayo Clinic and Mayo Foundation, Endocrine Research Unit, West Joseph 5–194, 200 First Street SW, Rochester, Minnesota 55905. E-mail: khosla{at}mayo.edu
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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-dihydrotestosterone
(5DHT), and testosterone, but not dehydroepiandrosterone, increased
IGF-I messenger RNA (mRNA) levels up to 4-fold in a dose
(10-12-10-6 M)- and time (2–72
h)-dependent fashion. These changes were prevented by the specific
androgen receptor antagonist, hydroxyflutamide. In addition, 5-DHT
decreased IGFBP-4 mRNA and protein levels by 2- and 4-fold,
respectively, and increased IGFBP-2 and -3 mRNA and protein levels by
6- and 7-fold (for mRNA) and 3- and 5-fold (for protein), respectively.
hFOB/AR-6 cells expressed the type-I IGF receptor, but this was not
regulated by 5DHT. 5DHT and IGFBP-3 specifically increased
hFOB/AR-6 cell proliferation, and a monoclonal antibody specific for
IGF-I blocked this effect. Thus, androgens increase the expression of
IGF-I, IGFBP-2, and IGFBP-3, but decrease levels of the inhibitory
IGFBP-4 in an androgen-responsive human osteoblastic cell line. Our
data are consistent with the hypothesis that the effects of androgen on
bone cells may be mediated at least in part by increases in IGF-I
production and by differential regulation of IGFBPs. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The presence of androgen receptors (AR) in rodent and human osteosarcoma cell lines, normal human osteoblast-like cells (15, 16, 17), osteoclasts (18, 19), and marrow-derived stromal cells (20) indicates that androgens may directly affect bone cell function. However, many of the effects of androgens on bone cells may be mediated via regulation of the production of growth factors and cytokines, such as insulin-like growth factor I (IGF-I) and IGF-II. IGF-I and IGF-II are the most abundant growth factors stored in the skeleton (21, 22, 23, 24, 25). They are produced and secreted by bone cells (21, 22, 23, 26), and bone cells express both type I and type II IGF receptors (27, 28, 29). Thus, androgens may regulate the IGFs, which may, in turn, act as mitogenic and differentiative factors for bone cells through an autocrine/paracrine mechanism as well as through classical endocrine pathways. Consistent with this hypothesis, it has been reported that in the prostate, a classical androgen target tissue, there are important interactions between androgens and the IGF system (30, 31, 32, 33).
Key determinants of IGF bioactivity and bioavailability are the IGF-binding proteins (IGFBPs), six of which have been characterized to date (34). Bone cells also express IGFBPs (21, 35). IGFBPs can act to enhance or inhibit the effects of IGF-I and IGF-II, depending on the particular IGFBP, cell type, culture conditions, and stage of differentiation (21, 36, 37). In addition, various hormones involved in bone cell function and local skeletal factors regulate IGFBP production (38, 39, 40).
Although the IGF/IGFBP system is clearly an important regulator of bone
cell function, a major limitation of studying the effects of androgen
on this system has been the lack of appropriate model systems. Primary
cultures of bone cells suffer from potential cellular heterogeneity and
the expression of variable levels of ARs. In contrast, although
transformed cell lines circumvent some of these problems, they have a
very different profile of IGFBP expression compared with normal
osteoblasts (41). To overcome these problems, our group developed a
human fetal osteoblast cell line (hFOB) transfected with a
temperature-sensitive mutant form of the simian virus 40 large T
antigen (SV40-LTA) (42). These cells proliferate at 33.5 C, but at 39.5
C the mutant form of SV40-LTA is inactivated, and the cells
differentiate and express a mature osteoblast phenotype (43), including
the expression of a normal profile of IGFBPs (41). However, as these
cells lack functional ARs, we have recently introduced a physiological
level of functional human hARs (
3900 ARs/nucleus) into these cells,
making them androgen responsive (hFOB/AR-6) cells (44). In the present
study we used this androgen-responsive, clonal osteoblast cell line to
determine the effects of androgens on the IGF/IGFBP system.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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-32P]deoxy (d)-CTP was purchased from NEN Life Science Products (Boston, MA). Nitro-cellulose filters were
purchased from Schleicher & Schuell, Inc. (Keene, NH).
Hydroxyflutamide (OHF) was provided by Dr. Rudolph Neri
(Schering-Plough Corp., Kennilworth, NJ). Recombinant
human IGF-I and IGF-II were purchased from Amgen, Inc.
(Thousand Oaks, CA), and R & D Systems (Minneapolis, MN),
respectively. The human ß-actin complementary DNA (cDNA) probe (1.8
kb) and the ExpressHyb hybridization solution were purchased from
CLONTECH Laboratories, Inc. (Palo Alto, CA). The human
IGF-I cDNA probe (244 bp) was obtained from R & D Systems.
Monoclonal antibody against IGF-I (Sm1.2) was obtained from
Upstate Biotechnology, Inc. (Lake Placid, NY). The human
IGFBP-3 cDNA probe was a gift from Dr. D. R. Powell (Baylor
College of Medicine, Houston, TX), and the human IGFBP-2, -4, and -5
cDNAs were provided by Dr. S. Shimasaki (The Whittier Institute, La
Jolla, CA). Human recombinant IGFBP-2 and -3 were gifts from
Sandoz Pharmaceuticals Corp. (Basel, Switzerland) and
Celtrix Pharmaceuticals, Inc. (Santa, Clara, CA),
respectively.
Cell culture
hFOB/AR-6 cells were maintained in a 1:1 mixture of phenol-free
DMEM/Ham’s F-12 medium (DMEM/HF12) containing 10% (vol/vol)
charcoal-stripped (cs) FBS supplemented with either geneticin (300
µg/ml) or hygromycin B (100 µg/ml) at 33.5 C, the permissive
temperature for the expression of the large T antigen gene (39). The
medium was changed every other day using alternately geneticin and
hygromycin to select for hAR-expressing cells. The experiments were
performed at 39.5 C, and hFOB/AR-6 cells used in all the experiments
were between passages 8–12.
RNA extraction, cDNA synthesis, and RT-PCR
The hFOB/AR-6 cells were plated in 12-well microtiter plates at
a density of 2 x 105 cells in DMEM/HF12 containing
10% (vol/vol) csFBS and antibiotic and cultured for 48 h at 33.5
C. The cells were then washed twice in PBS and cultured for 24 h
at 33.5 C in DMEM/HF12 containing 0.1% (wt/vol) BSA to eliminate the
residual androgens present in the serum after charcoal stripping. The
cells were then washed with PBS and cultured in the same medium at 39.5
C for various time intervals in the absence or presence of treatment.
Total cellular RNA was isolated using the RNA-STAT kit. cDNA was
synthesized from 1 µg total RNA in a 20-µl reaction mix containing
1 x incubation buffer for AMV reverse transcriptase; 2.5
µM poly(deoxythymidine); 1 mM each of dATP,
dCTP, dGTP, and dTTP; 20 U ribonuclease inhibitor; and 20 U AMV reverse
transcriptase for 2 h at 42 C. Aliquots of cDNA were amplified in
a 25-µl PCR reaction mixture containing 0.2 µM 5'- and
3'-oligo primers; 1 x expanded high fidelity PCR buffer;
0.1 nM each of dATP, dCTP, dGTP, and dTTP; 0.25 µl
[-32P]dCTP (10 µCi/µl); and 0.35 U expanded high
fidelity Taq DNA polymerase. For each assay performed, each
cDNA sample was run in duplicate. Amplification reactions specific for
the following cDNAs were carried out: IGF-I, IGF-II, and the
housekeeping gene glyceraldehyde-3-phosphate dehydrogenase
(GAPDH) as previously described (45). Amplifications were
performed in a GeneAmp 9600 thermal cycler (Perkin Elmer Corp., Norwalk, CT). The PCR products were analyzed by
electrophoresis of 9-µl samples in 1.5% (wt/vol) agarose gels. The
amplified DNA fragments were visualized by ethidium bromide staining
and quantified by counting the radioactivity in gel slices. The
quantitative differences between cDNA samples were normalized to the
radioactivity present in the GAPDH PCR products.
Northern blot hybridization
Ten to 20 µg total RNA was separated on a 1.5% (wt/vol)
agarose gel containing 2.2 M formaldehyde. RNA was then
transferred to a nylon membrane (Hybond N+, Amersham Pharmacia Biotech, Arlington Heights, IL) by capillary blotting.
Methylene blue staining of the membrane was used to verify equal
loading and efficient transfer. The cDNA inserts (25 ng) were
radiolabeled with 5 µl [-32P]dCTP to a specific
activity of more than 109 cpm/µg DNA using a random
primer DNA labeling kit (46). Hybridization was carried out for 1
h at 68 C in ExpressHyb solution (47). After stringent washing [three
times, 10 min each time, at room temperature in 2 x standard
sodium citrate (SSC) and 0.05% (wt/vol) SDS; twice, 20 min each time,
at 50 C in 0.1 x SSC and 0.1% (wt/vol) SDS] membranes were
subjected to autoradiography at -80 C. Band intensity was quantified
by densitometry (Pharmacia LKB, Piscataway, NJ). Control
hybridization with human ß-actin cDNA verified that equal amounts of
RNA were loaded.
Cell-conditioned medium
hFOB/AR-6 cells were plated in 12-well microtiter plates at a
density of 1 x 105 cells in DMEM/HF12 containing 10%
(vol/vol) csFBS and antibiotic and were cultured for 48 h at 33.5
C. The cells were then washed twice in PBS and cultured for 24 h
at 33.5 C in DMEM/HF12 containing 0.1% (wt/vol) BSA to eliminate the
residual androgens present in the serum after charcoal stripping. The
cells were washed again with PBS and cultured in the same medium at
39.5 C with the test agents or vehicle for various time intervals as
indicated. The conditioned media were collected, centrifuged to
eliminate cell debris, aliquoted, and stored at -20 C.
Western ligand blot analysis
Aliquots of conditioned medium containing the same amount of
total protein (as measured by Bradford method) were analyzed by
SDS-PAGE using a 7.5–15% (wt/vol) gradient under nonreducing
conditions. The separated proteins were electroblotted onto
nitro-cellulose filters using a Bio Trans Unit (Gelman Sciences, Ann
Arbor, MI), and the IGFBPs were identified by incubation with
[125I]IGF-I at 4 C overnight. Filters were then
visualized by autoradiography.
Affinity cross-linking
Cell lysates of hFOB/AR-6 cells treated with and without
5-dihydrotestosterone (5DHT; 10-8 M) for
48 h were used for [125I]IGF-I affinity labeling.
hFOB/AR-6 cells were plated in six-well microtiter plates at a density
of 5 x 105 cells in DMEM/HF12 containing 10%
(vol/vol) csFBS and were cultured for 48 h at 33.5 C. The cells
were then washed twice in PBS and cultured for 24 h at 33.5 C in
DMEM/HF12 containing 0.1% (wt/vol) BSA to eliminate the residual
androgens present in the serum after charcoal stripping. The cells were
cultured in the same medium at 39.5 C in the presence or absence of
5DHT (10-8 M). After 48 h, hFOB/AR-6
cells were washed twice with cold HEPES/BSA binding buffer (120
mM NaCl, 5 mM KCl, 2.4 mM
MgSO4·7H2O, 10 mM dextrose, 15
mM
NaC2H3O2·3H2O, 100
mM HEPES, and 0.5% BSA) and then incubated in the same
buffer with [125I]IGF-I (1 x 106
cpm/well) in the presence or absence of unlabeled 40 mM
IGF-I or 20 µM insulin for 2.5 h at 15 C. The cells
were then washed and incubated with 10 mM disuccinimydil
suberate for 15 min at 15 C. Tris-EDTA Quench buffer (100
mM Tris and 10 mM EDTA, pH 7.4) was added to
the cells to stop the reaction. The cells were then solubilized, and
the protein was analyzed by 10% (vol/vol) SDS-PAGE under reducing
conditions using protein mixture as the standard. The gels were then
dried and autoradiographed at -80 C.
Cell proliferation
hFOB/AR-6 cell proliferation was assessed by
[3H]thymidine incorporation. Cells were plated in 48-well
microtiter plates at a density of 1 x 104 cells/well
in DMEM/HF12 containing 10% (vol/vol) csFBS and antibiotic and
cultured for 48 h at 33.5 C. Cells were then washed twice in PBS
and cultured for 24 h at 39.5 C in DMEM/HF12 containing 0.1%
(wt/vol) BSA to eliminate the residual androgens present in the serum
after charcoal stripping. hFOB/AR-6 cells were cultured in the same
medium at 39.5 C for 2 days with and without 5DHT (10
nM), Sm1.2 (10 nM), IGF-I (10 nM),
and OHF (10 µM), alone or in combination. In a second set
of experiments hFOB/AR-6 cells were incubated with increasing doses of
IGFBP-2 and -3 (10–100 nM) in the absence or presence of
Sm1.2. To assess the synthesis of DNA, 1 µCi
[3H]thymidine was added for the last 24 h of
incubation. Cells were harvested by trypsinization, and
[3H]thymidine was extracted by trichloroacetic acid
precipitation and detected by scintillation counting (48).
Statistical analysis
All values are expressed as the mean ± SEM.
Student’s paired t test was used to evaluate differences
between the stimulated samples and their respective controls. The
significance of dose or time responses was assessed by multiple
measures ANOVA. P < 0.05 was considered statistically
significant.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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DHT on IGF messenger RNA (mRNA) expressionDHT on IGF mRNA levels was first evaluated (Fig. 1
). Although testosterone at a dose of 10
nM was as effective as 5DHT in increasing IGF-I mRNA
levels (by 400%), the adrenal androgen dehydroepiandrosterone
(DHEA) did not affect IGF-I mRNA levels. In addition,
treatment with the specific AR antagonist OHF abolished the
5DHT-induced increase in IGF-I mRNA levels. In contrast to the
effects of androgens on IGF-I mRNA levels, we could not demonstrate
specific effects of 5DHT or testosterone on IGF-II mRNA levels (data
not shown). Thus, we investigated in detail only the effects of
androgens on IGF-I mRNA levels. As shown in Fig. 2A, increasing concentrations of 5DHT
(10-12–10-6 M) increased IGF-I
mRNA levels in a dose-dependent manner, with a maximal increase over
control levels of 381% at 10 nM. Treatment of hFOB/AR-6
cells with 10 nM 5DHT also increased IGF-I mRNA levels
in a time-dependent fashion, with a maximal increase over control
levels of 460% by 48 h posttreatment (Fig. 2B).
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DHT on IGF-I mRNA
levelsDHT for 48 h at 39.5 C increased IGF-1 mRNA (Fig. 3).
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, hFOB/AR-6 cells
constitutively expressed mRNAs for IGFBP-2, -3, and -4, and they did
not express IGFBP-5 (data not shown). Treatment of hFOB/AR-6 cells with
5DHT increased IGFBP-2 (Fig. 4A) and IGFBP-3 (Fig. 4B) mRNA levels
by 6- and 7-fold, respectively, and decreased IGFBP-4 mRNA
steady state levels by 2-fold (Fig. 4B).
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, hFOB/AR-6 cells
secrete mainly approximately 46-, 36-, and 28/24-kDa IGFBPs into the
medium, corresponding to IGFBP-3, -2, and -4, respectively, identified
above by Northern blot analysis. 5DHT treatment resulted in a
decrease in IGFBP-4 and an increase in IGFBP-2 and -3 levels (Fig. 5).
Densitometric analysis revealed that treatment with 5DHT decreased
IGFBP-4 levels in a dose-dependent manner
(10-12–10-6 M), with a maximum
effect of 4.4-fold at 10 nM. In contrast, IGFBP-3 protein
levels were increased, with a maximum effect at 10 nM of
6-fold compared with control values. 5DHT treatment also increased
IGFBP-2 protein levels by 1.7-fold at 1 nM and by 2.1-fold
at 10 nM (Fig. 5). IGFBP-4, -2, and -3 protein levels were
also regulated by 5DHT in a time-dependent manner, with a maximum
decrease at 48 h of 2-fold for IGFBP-4 and a maximum increase at
48 h of 3- and 5-fold for IGFBP-2 and -3, respectively (Fig. 6).
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). The 130-kDa band represents the
-subunit of the type I IGF receptor (49). The specificity of the
receptor-ligand complex was shown by the ability of an excess of
unlabeled IGF-I and insulin to displace the [125I]IGF-I
bound to the 130-kDa band. In addition, a band of 40–50 kDa was
observed. Labeling of this protein was displaceable by unlabeled IGF-I,
but not by insulin, indicating cell-associated IGFBPs. Treatment with
5DHT at a dose of 10 nM for 48 h did not affect the
signal intensity of the type I IGF receptor band (Fig. 7).
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DHT, IGFBP-2, and IGFBP-3 on cell
proliferationDHT significantly increased
[3H]thymidine incorporation by 60% over control values
(Fig. 8). The induction of DNA synthesis
was specific for 5DHT, as OHF blocked the 5DHT-induced increase
in DNA synthesis (Fig. 8). In addition, cotreatment with 5DHT and an
antibody against IGF-I (Sm1.2) blocked the 5DHT-induced increase in
DNA synthesis (Fig. 8). Moreover, IGF-I induced DNA synthesis by 48%,
and cotreatment with IGF-I and Sm1.2 blocked the IGF-I-induced increase
in DNA synthesis (Fig. 8). As it has been reported that IGFBP-2 and
IGFBP-3 can enhance or inhibit IGF action (50, 51, 52, 53) and can directly
affect cell proliferation, we also investigated the possible effects of
IGFBP-2 and IGFBP-3 on hFOB/AR-6 cell proliferation. IGFBP-2 had no
effect on cell proliferation (data not shown). In contrast, IGFBP-3 at
doses between 10–100 nM significantly increased
[3H]thymidine incorporation (by 2-fold over control
values), with a maximal effect at a dose of 100 nM in
hFOB/AR-6 cells cultured at 39.5 C (Fig.
9). When hFOB/AR-6 cells were
cotreated with IGFBP-3 (50 nM) and Sm1.2 (the specific
monoclonal antibody against IGF-I), the effect of IGFBP-3 on DNA
synthesis was blocked (Fig. 9).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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5DHT and testosterone treatment increased, in a dose- and
time-dependent manner, IGF-I mRNA levels. By contrast, the
adrenocortical androgen DHEA did not affect IGF-I mRNA
levels. This is consistent with our previous observations that in
hFOB/AR-6 cells, DHEA treatment, unlike 5DHT, did not
result in transcriptional activation of the hAR (44). As expected, the
AR antagonist OHF blocked the 5DHT-induced increase in IGF-I mRNA
levels. These results indicate that androgens specifically increased
IGF-I levels and that this effect was AR mediated. The time course of
the effects of 5DHT on IGF-I levels, with a maximal effect at
48 h, suggests posttranscriptional regulation of IGF-I by 5DHT,
although our studies did not directly address this issue. Consistent
with our findings in hFOB/AR-6 cells, Maor et al. (54) have
recently shown that androgens increased IGF-I gene expression in the
mandibular condyle of 3.5- to 5.5-week-old mice. In contrast to our
findings in hFOB/AR-6 cells, however, androgens also increased type I
IGF receptor levels in that system. Finally, our group has previously
reported (45) that in hFOB/ER9 cells, which have high levels of
functional human estrogen receptor (55), 17ß-estradiol also increased
levels of IGF-I.
IGFBPs are important modulators of IGF action (21, 36). Despite the
similarity in amino acid sequences of the IGFBPs, their effects on bone
cell function differ. In fact, several studies in different types of
cells have shown that IGFBPs can either stimulate or inhibit IGF action
depending on the stage of cell differentiation and culture conditions
(34, 35, 36, 37). hFOB/AR-6 cells express mainly IGFBP-2, -3, and -4, and
treatment with 5DHT increased IGFBP-2 and -3 and decreased
IGFBP-4 mRNAs. 5DHT treatment induced an equal increase in
IGFBP-2 and -3 and a proportional decrease in IGFBP-4 protein levels in
hFOB/AR-6 medium. Our group has previously reported that estrogen
treatment resulted in an increase in IGFBP-4 levels in hFOB/ER-9 cells,
partially due to posttranslational effects, by decreasing IGFBP-4
proteolysis (39). Thus, androgens and estrogens have opposite effects
on levels of this inhibitory protein, although we were unable to
demonstrate any effect of 5DHT on IGFBP-4 proteolysis (data not
shown).
Our data also indicate that hFOB/AR-6 cells are potential targets for the IGFs because they express the type I IGF receptor, although androgens did not regulate the levels of this receptor.
Finally, we also evaluated the role of the IGF system in mediating the
effects of androgen on these cells. In our previous report, we found
that treatment of hFOB/AR-6 cells with androgens resulted in an
inhibition of cell proliferation (56). However, the previous studies
were performed after a 6-day exposure of the cells to 5DHT at 33.5
C, the permissive temperature for the SV40-LTA, when the cells rapidly
proliferate without differentiating. Because in the present study we
were unable to demonstrate any effects of androgens on the IGF/IGFBP
system at 33.5 C (data not shown), we assessed androgen effects on
hFOB/AR-6 cell proliferation at 39 C after a 48-h exposure, when the
changes in the IGF/IGFBP system were most pronounced. Under these
conditions, androgen treatment of these cells increased DNA synthesis,
as assessed by [3H]thymidine incorporation. As expected,
OHF blocked the 5DHT-induced increase in DNA synthesis. Moreover, in
the presence of the monoclonal antibody against IGF-I (Sm1.2), 5DHT
failed to increase DNA synthesis by hFOB/AR-6 cells, suggesting that
the effects of 5DHT on DNA synthesis were mediated by IGF-I.
IGFBP-2 had no effect on hFOB/AR-6 cell proliferation. Several studies indicate that IGFBP-2 can either inhibit IGF action (57, 58) or act as a mitogenic factor (59). Our group has recently shown that hepatitis C-associated osteosclerosis, a rare disorder characterized by an increase in bone mass in adult life, is associated with elevated levels of IGF-IIE (the IGF-II prohormone) and IGFBP-2 (60). In those studies we demonstrated that IGF-II promoted the binding of IGFBP-2 to the extracellular matrix produced by human osteoblasts and that in the presence of extracellular matrix, the complex of IGF-II and IGFBP-2 could stimulate cell proliferation as well as IGF-II alone. These studies suggested that IGFBP-2 may facilitate the transport of IGF-IIE or IGF-II to skeletal tissue. Thus, the increase in IGFBP-2 observed in hFOB/AR-6 cells after androgen treatment may result in an increase in the bioavailability of IGFs.
To better understand the consequence of the androgen-induced increase in IGFBP-3 production by hFOB/AR-6 cells, we evaluated whether IGFBP-3 could act independently on hFOB/AR-6 cells. Our findings that IGFBP-3 increased hFOB/AR-6 cell proliferation in a dose-dependent manner are in contrast with several studies in osteoblastic cells in which IGFBP-3 had an antiproliferative effect (47, 48, 49). However, Ernst and Rodan (50) reported that in osteoblastic cells, an increase in endogenous IGFBP-3 was correlated with enhanced IGF-I activity, and Slootweg et al. (59) described a mitogenic effect of IGFBP-2 and -3 in rat osteosarcoma cells. Moreover, in the presence of Sm1.2 (the monoclonal antibody against IGF-I), IGFBP-3 failed to increase DNA synthesis by hFOB/AR-6 cells, suggesting that the effects of IGFBP-3 on DNA synthesis were also mediated by IGF-I.
In summary, our data indicate androgens increase IGF-I production and alter IGFBP production by hFOB/AR-6 cells in a manner that would result in a net increase in IGF-I bioactivity in the cellular microenvironment. Thus, levels of the consistently inhibitory IGFBP-4 (21, 35, 36) were decreased, and levels of IGFBP-2 and -3 were increased by androgens. The latter IGFBPs can be either inhibitory or stimulatory to IGF-I action (21, 36, 53), and in our system, IGFBP-3 increased hFOB/AR-6 proliferation, perhaps by enhancing endogenous IGF-I bioactivity, whereas IGFBP-2 had no effect. We recognize, however, that additional studies are needed to better define IGF-I/IGFBP interactions in this system and also to examine effects of the IGF/IGFBP system on other phenotypic parameters of these cells, such as markers of osteoblastic differentiation. Finally, in addition to possible autocrine effects on these cells, the IGFs and/or IGFBPs may, in turn, act in a paracrine manner to increase the proliferation of osteoblast and osteoblastic precursor cells, leading to an increase in the number of osteoblasts on bone surfaces during bone remodeling. In support of this, we have recently found that human marrow stromal cells are also target cells for the IGFs (61). In particular, IGFs have a mitogenic effect on these cells, without a significant effect on differentiation (61). However, although our data indicate that the IGF/IGFBP system is a potential mediator of the effects of androgen on bone, clearly further studies are needed to more directly assess the specific target cell(s) for androgen action on the skeleton in vivo.
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Acknowledgments |
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Footnotes |
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Received May 27, 1999.
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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