(Received for publication, October 18, 1996, and in revised form, January 24, 1997)
From the Department of Pediatrics, University of Pennsylvania,
Philadelphia, Pennsylvania 19104 and Jefferson Cancer
Institute, Thomas Jefferson University,
Philadelphia, Pennsylvania 19103
Insulin-like growth factor (IGF) binding
protein-3 (IGFBP-3) is known to block IGF action and inhibit cell
growth. IGFBP-3 is thought to act by sequestering free IGFs or,
possibly, act via a novel IGF-independent mechanism. Supporting its
role as a primary growth inhibitor, IGFBP-3 production has been shown to be increased by cell growth-inhibitory agents, such as transforming growth factor- (TGF-
), and the tumor suppressor gene p53. In this
paper, we demonstrate, for the first time, a novel function of IGFBP-3
as an apoptosis-inducing agent and show that this action is mediated
through an IGF·IGF receptor-independent pathway. In the p53 negative
prostate cancer cell line, PC-3, the addition of recombinant IGFBP-3
resulted in a dose-dependent induction of apoptosis.
125I-IGFBP-3 bound with high affinity to specific
proteins in PC-3 cell lysates and plasma membrane preparations. These
membrane-associated molecules may serve as receptors that mediate the
direct effect of IGFBP-3 on apoptosis. In addition, in an IGF
receptor-negative mouse fibroblast cell line, treatment with
recombinant IGFBP-3 as well as transfection of the IGFBP-3 gene induced
apoptosis, suggesting that neither IGFs nor IGF receptors are required
for this action. Furthermore, treatment with TGF-
1, a known
apoptosis-inducing agent, resulted in the induction of IGFBP-3
expression 6-12 h before the onset of apoptosis. This effect of
TGF-
1 was prevented by co-treatment with IGFBP-3-neutralizing
antibodies or IGFBP-3-specific antisense thiolated oligonucleotides.
These findings suggest that IGFBP-3 induces apoptosis through a novel
pathway independent of either p53 or the IGF·IGF
receptor-mediated cell survival pathway and that IGFBP-3
mediates TGF-
1 induced apoptosis in PC-3 cells.
The insulin-like growth factor (IGF)1-binding protein-3 (IGFBP-3) belongs to a family of high affinity IGFBPs, which bind to IGFs and modulate their actions. In addition to regulating the availability of free IGFs and, therefore, their mitogenic activity (1-4), IGFBPs also play an important role in directly regulating cell growth. These independent cell growth-regulatory effects of IGFBPs have been shown to be either growth-inducing (5-10), or growth-inhibiting (9, 12-17).
We and others have previously demonstrated the effects of IGFBP-3 as a
negative regulator of cell proliferation in prostatic and other tissues
(8-10, 12, 13). This negative growth regulation by IGFBP-3 has been
proposed to involve a separate cellular signaling pathway (18, 19).
Further, in support of its role as a negative regulator of cell growth
and proliferation, IGFBP-3 gene expression has also been shown to be
induced by other growth-inhibitory (and apoptosis-inducing) agents such
as transforming growth factor-1 (TGF-
1) (20-22), retinoic acid
(21), tumor necrosis factor-
(TNF-
) (23), and the tumor
suppressor gene, p53 (24). However, the direct apoptosis-inducing
ability of IGFBP-3 has not previously been demonstrated. In this study,
we have investigated a novel role of IGFBP-3 as an apoptosis-inducing
agent.
We hypothesized that the growth-inhibitory effect of
IGFBP-3 is mediated not only by regulating the availability
of free IGFs and by inducing growth arrest, but also by inducing
apoptosis. We further considered that this process may involve an
IGF-independent mechanism. To test these hypotheses, we investigated
the ability of IGFBP-3 to induce apoptosis in a prostate carcinoma cell
line (PC-3) and in an IGF receptor-negative (R()) mouse
fibroblast cell line (13, 19, 25). Furthermore, to determine the
importance of IGFBP-3 as a critical cell growth-regulatory factor, we
also investigated its role as a mediator of the apoptosis induced by
TGF-
1.
Tissue culture supplies were purchased from Flow
Laboratories (McLean, VA), Corning (Corning, NY), and Hyclone (Logan,
UT). Recombinant human IGF-I was the kind gift of Fujisawa
Pharmaceuticals (Osaka, Japan). Recombinant human IGF-II was generously
provided by Eli Lilly (Indianapolis, IN). Recombinant DNA-derived,
glycosylated (Chinese hamster ovary) and nonglycosylated
(Escherichia coli) human IGFBP-3 were the generous gifts of
A. Sommer (Celtrix Inc., Santa Clara, CA). The IGFBP-3 phosphorothioate
oligodeoxynucleotides used in these experiments (originally published
by Oh et al.; Ref. 20) were prepared by OLIGOS Etc., Inc.
(Guilford, CT). The IGFBP-3 antisense oligodeoxynucleotides were
complementary to the 20 nucleotides that encode the N terminus of human
IGFBP-3 as described previously (24) and had the sequence 5-CAT GAC GCC TGC AAC CGG GG-3
(positions 2021-2040); the sequence of the IGFBP-3 sense oligodeoxynucleotides was 5
-CCC CGG TTG CAG GCG TCA
TG-3
. IGFBP-3 antibodies were purchased from Diagnostic Systems Laboratories (Webster, TX) and were prepared by affinity purification on an IGFBP-3 column. Control IgG (affinity-purified anti-goat IgG) was
purchased from Vector Laboratories (Burlingame, CA). Valinomycin was
purchased from Sigma. Fluorescein isothiocyanate (FITC)-conjugated
annexin, propidium iodide (PI), and binding buffer were purchased from
R & D systems (Oxon, United Kingdom). The interleukin-1
-converting
enzyme (ICE) inhibitor (ICE-I) (Ac-Tyr-Val-Ala-Asp-aldehyde) was
purchased from Oncogene Research Products (Cambridge, MA).
The human PC-3 cells were purchased from
ATCC (Rockville, MD) and were originally initiated from a grade IV
prostatic adenocarcinoma from a 62-year-old male Caucasian. The PC-3
cells were grown in 75-cm2 flasks according to the
recommended protocol (FK-12 supplemented with 10% fetal bovine serum
and 1% penicillin-streptomycin. For each experiment, cells were
dissociated, centrifuged, and resuspended in serum containing FK-12
media with antibiotics and inoculated at a density of 1 × 105 cells/cm2 in 24-well or 6-well tissue
culture dishes and grown to confluence in a humidified atmosphere of
5% CO2 at 37 °C before treatment. After a quick wash
with serum-free FK-12 media (SFM), the confluent cells were treated
with various concentrations of IGFBP-3, TGF-1, and/or other
specified reagents. SFM with antibiotics was used as the control
treatment.
Fibroblasts from an IGF-I
receptor knockout mouse were generated from 18-day embryos as described
previously (25) and were designated R() cells. The
R(
) cells were maintained in Dulbecco's modified
Eagle's medium containing 10% fetal bovine serum and Geneticin
(G418). For the generation of cell lines stably expressing the human
IGFBP-3 gene, R(
) cells were co-transfected with the
pKGhBP-3 (19) and pLHL4 vectors. Transfections were done in suspension
with 5 µg of plasmid DNA for 1 × 106 cells using
the calcium phosphate suspension method (13, 25). The cells were then
plated at a concentration of 8 × 103
cells/cm2. Cells selected in hygromycin were designated
R(
)/BP-3 and were continuously grown in media
supplemented with hygromycin (50 µg/ml). Several clones were
confirmed for IGFBP-3 mRNA and protein expression and were used for
further biological experiments (19).
DNA fragmentation analyses were performed using cells (1 × 106/ml), which were washed with Tris-buffered saline and resuspended in 1 ml of 0.15 M sodium chloride and 0.015 M sodium citrate (pH 7.0) containing 10 mM EDTA, 1% (w/v) sodium laurylsarkosinate, and 0.5 mg/ml proteinase K. Following digestion with proteinase K, the DNA was precipitated with 2 volumes of cold absolute ethanol, pelleted, resuspended in 10 mM Tris-HCl (pH 8.0) containing 1 mM EDTA, heated to 70 °C, and loaded onto a 1.5% agarose gel containing 0.1 mg/ml ethidium bromide. Electrophoresis was carried out in 40 mM Tris acetate, 1 mM EDTA, pH 8.0, until the marker dye had migrated approximately 5 cm. DNA was visualized under UV light and photographed.
Terminal Deoxynucleotidyltransferase-mediated dUTP Nick End Labeling (TUNEL)In situ detection of apoptosis in
cultured cells was performed with the use of direct immunoperoxidase
detection of biotin-labeled genomic DNA in monolayer cells. In brief,
following treatment with different conditions, the monolayer cultures
were fixed in 3.7% paraformaldehyde solution for 10 min at room
temperature followed by dehydration in 70% ethanol for 5 min at room
temperature. Following this step, the endogenous peroxidase was
quenched by treatment with 2% hydrogen peroxide in methanol for 5 min.
The cells were incubated in the labeling mixture (Biotin dNTP mix; 50 × MgCl2, terminal deoxynucleotidyltransferase, and
labeling buffer) for 60 min at 37 °C. The free 3-OH DNA in the
apoptotic cells was visualized using the streptavidin-horseradish
peroxidase-diaminobenzidine detection system. These apoptotic cells
appeared as dark brown cells.
Cells were harvested from confluent monolayer cultures including the floating cells in the conditioned media and were fixed with 1% paraformaldehyde on ice and treated with 70% ethanol for 10 min before use. Apoptotic cells were detected using the Apoptag system, which utilizes terminal deoxynucleotidyltransferase and digoxigenin-labeled D-UTP (Oncore, Gaithersburg, MD). Following incubation in 70% ethanol, the cells were washed in cold PBS and incubated with fluorescein-labeled anti-digoxigenin. After a 30-min incubation, the cells were rinsed in PBS with 0.05 Triton X-100 and resuspended in 0.5 ml of PBS containing PI (5 µg/ml) and RNase (1 mg/ml). The cells were incubated in the dark on ice for 30 min prior to flow cytometric analysis. Stained cells were evaluated for fluorescence, employing a Coulter EPICS Elite flow cytometer operated at 488 nm and 300 milliwatts output. Individual cells were electronically gated to exclude aggregates from evaluation. Single parameter fluorescence histogram data were generated for both isotypic control antibody-treated cells and apoptosis-specific antibody-treated cells. Saved fluorescence histograms were evaluated by Overton's histogram subtraction statistical model (Immuno 4 software, Coulter Immunology, Hialeah, FL) to determine both the percentage of positive cells and relative fluorescence intensity in mean channel fluorescence units. The K+-selective ionophore valinomycin (1 × 105 M) was used as a positive control (27). SFM-treated conditions were used as negative controls. The percentage of apoptotic cells in both control and experimental conditions were compared.
Comparative Analyses of Apoptosis and NecrosisTo confirm that the population of apoptotic cells observed in the IGFBP-3-treated conditions are truly undergoing programmed cell death and not necrosis, we utilized the previously published novel method and detected both apoptosis and necrosis from the same samples using quantitative FACS analysis. This method utilizes the binding of FITC-labeled annexin V to phosphatidylserine in the cell membrane that surfaces only during the early phase of apoptosis, indicating the loss of cell membrane phospholipid asymmetry (28-30). However, these apoptotic cells with intact cell membranes do not stain with the PI. Utilizing the morphological changes that occur in both apoptotic and necrotic cells, we simultaneously stained the samples with annexin-FITC and PI and subjected the samples to flow cytometric analyses to detect the percentage of apoptotic (FITC-stained cells) and necrotic cells (PI-stained cells) in a given population. A minimum of 6,000 cells was maintained for all samples.
Apoptosis ELISA AssayPhotometric cell death detection
ELISA (Boehringer Mannheim) was performed to quantitate the apoptotic
index by detecting the histone-associated DNA fragments (mono- and
oligonucleosomes) generated by the apoptotic cells. The assay is based
on the quantitative sandwich enzyme immunoassay principle using mouse
monoclonal antibodies directed against DNA and histones, respectively,
for the specific determination of these nucleosomes in the cytoplasmic
fraction of cell lysates. In brief, equal numbers of cells were plated in 24-well culture plates (1 × 104/cm2)
in serum-supplemented FK-12 medium and grown to confluency for 72 h. At the time of sample collection, the confluent cells were washed
with PBS and treated with various concentrations of IGFBP-3, TGF-1,
or other required agents for the designated time period. The cells were
dissociated gently (PBS with 0.1 M EDTA) and pelleted along
with the floating cells (mostly apoptotic cells) collected from the
conditioned media. The cell pellets were used to prepare the cytosol
fractions that contained the smaller fragments of DNA. Equal volumes of
these cytosolic fractions were incubated in anti-histone
antibody-coated wells (96-well plates), and the histones of the DNA
fragments were allowed to bind to the anti-histone antibodies. The
peroxidase-labeled mouse monoclonal DNA antibodies were used to
localize and detect the bound fragmented DNA using photometric
detection with 2,2
-azino-di-(3-ethylbenzathiazoline sulfonate) as the
substrate. Valinomycin (1 × 105 M) was
used as a positive control (27). SFM-treated conditions were used as
negative controls. Each experimental condition was carried out with at
least three samples and was repeated at least three times. The reaction
products in each 96-well plate were read using a Bio-Rad microplate
reader (model 3550-UV). Averages of the values ± S.E. from double
absorbance measurements of the samples were plotted.
Confluent PC-3 cells were
briefly washed with cold PBS and allowed to dissociate in dispersion
buffer (1 mM EDTA in PBS, pH 7.4). Free floating cells were
collected and centrifuged (2000 rpm for 5 min), resuspended in cold
lysis buffer containing 10 mM HEPES, 1.5 mM
EDTA, 0.5 mM dithiothreitol, 1 mM
phenylmethylsulfonyl fluoride, 1 µM aprotinin, and 1 mM pepstatin in PBS (pH 7.4), vortexed, and boiled at
100 °C for 5 min. Aliquots were stored at 70 °C until further
use.
Samples were collected using the method described above for the preparation of cell lysates. The pellets were resuspended in homogenization buffer (10 mM HEPES, 1.5 mM EDTA, 0.5 mM dithiothreitol, and 1 mM phenylmethylsulfonyl fluoride, pH 7.4). Cells were homogenized using a Dounce homogenizer, maintaining the uniformity of the homogenization process. Homogenized cells were centrifuged at 3000 rpm at 4 °C for 5 min, and the nuclear pellet was discarded. The supernatant was recentrifuged at 12,000 rpm at 4 °C for 30 min, and the pellet containing the debris was discarded. The supernatant containing the plasma membrane was then resuspended in solubilization buffer (50 mM HEPES, 0.15 M NaCl2, 2 mM MgSo4, 1 mM phenylmethylsulfonyl fluoride, pH 7.4) and further homogenized using the same technique as mentioned above. Following centrifugation at 33,000 rpm at 4 °C for 1 h using a Sorvall ultracentrifuge (DuPont, OTD B60), the resulting pellet was resuspended in isolation buffer and used for separation on SDS-PAGE.
Western ImmunoblotsThe Western immunoblot analysis was
performed as described previously (31). Serum-free conditioned media
from PC-3 cultures treated with various concentrations of TGF-1 for
different time periods were used. SFM incubated with similar culture
conditions were used as controls. Samples of 100 µl (from 1 × 106 cells) were electrophoresed through 12.5% nonreducing
SDS-PAGE overnight at constant voltage, electroblotted onto
nitrocellulose, blocked with 5% nonfat dry milk in Tris-buffered
saline, probed with specific IGFBP-3 antibodies, and detected using a
peroxidase-linked enhanced chemiluminescence detection system
(Pierce).
PC-3 cell lysates and plasma membrane fractions were electrophoresed through 12.5% nonreducing SDS-PAGE overnight at constant voltage and electroblotted onto nitrocellulose, blocked with 1% bovine serum albumin in Tris-buffered saline, and incubated with 5 × 104 cpm of 125I-IGFBP-3 (DSL Webster, TX) for 12 h. The membranes were exposed to film for 3 days and visualized by autoradiography.
Densitometric and Statistical AnalysisDensitometric
measurement of immunoblots were performed using a Bio-Rad GS-670
Imaging densitometer (Bio-Rad, Melville, NY). Protein levels were
estimated by comparing the optical density of each specific protein
band from control (SFM) conditions with that from the TGF-1-treated
conditions. All experiments were repeated at least three times. When
applicable, means ± S.E. are shown. Student's t tests
were used for statistical analysis.
We detected
IGFBP-3-induced apoptosis in PC-3 cells using both qualitative (DNA
laddering and TUNEL) and quantitative (FACS and ELISA) methods. The
apoptotic DNA cleavage in cells treated with IGFBP-3 (500 ng/ml for
72 h) was visualized as DNA laddering (Fig.
1A). The DNA extracted from cells treated
with SFM showed limited fragmentation (lane 4). However, DNA
extracted from IGFBP-3-treated cells demonstrated significant
fragmentation, with bands varying in size primarily from 100 to 300 base pairs (lane 3). Cells treated with 10% fetal bovine
serum were used as a negative control to demonstrate the absence of any
fragmented DNA (lane 2).
As an alternative method of detection, and to localize the apoptotic cells in situ, we detected the fragmented DNA in monolayer cell cultures treated with SFM or IGFBP-3 using TUNEL (Fig. 1B). The DNA fragments bound to the peroxidase-diaminobenzidine reaction product in apoptotic cells were visualized as dark brown cells. Cells in SFM displayed an insignificant number of apoptotic cells (Fig. 1B, i); however, IGFBP-3 treatment revealed numerous apoptotic cells (Fig. 1B, ii). This method was not used to quantitate the number of apoptotic cells in the SFM- and IGFBP-3-treated conditions, since many of the apoptotic cells, after 72 h of incubation, were found floating in the conditioned media. Loss of cells from the culture plate due to the increased apoptotic index was seen as empty spaces in the IGFBP-3-treated condition. However, the control condition showed confluent cells.
FACS analysis of fragmented DNA antibody staining (Fig. 1C) further demonstrated and quantitated the apoptotic index in SFM- and IGFBP-3-treated conditions. The basal apoptotic index due to serum withdrawal was 1.4% of the total number of cells analyzed. The addition of IGFBP-3 to SFM resulted in the significant increase in the apoptotic index (94.7% of the total) as measured by fragmented DNA antibody staining. Using related FACS approaches (Fig. 1D) but labeling the unpermeablized samples with both FITC-conjugated annexin (which stains only apoptotic cells) and PI (which stains necrotic cells with disrupted membranes), the number of apoptotic cells under the treated condition and the ratio of apoptotic cells to necrotic cells were calculated. Treatment with IGFBP-3 resulted in a 3-fold increase in apoptotic cells (Fig. 1E) when compared with SFM treatment. The percentage of necrotic cells was low in both treatment conditions and was not increased by IGFBP-3.
Effects of IGFs on IGFBP-3-induced ApoptosisSimilar to the
observations obtained using FACS, quantitative analysis with
photometric ELISA (Fig. 2) revealed a basal level of
apoptosis in SFM-treated cells. In contrast, the serum-treated cultures were completely devoid of apoptotic cells (Fig.
2A). In addition, the addition of IGF-I (200 ng/ml) also
prevented the effect of serum starvation on apoptosis. On the other
hand, the addition of IGFBP-3 to the SFM induced a further significant increase (p < 0.001 compared with SFM) in the
apoptotic index above the basal level caused by serum deprivation (Fig.
2A). This induction of apoptosis by IGFBP-3 was as potent as
the apoptosis induced by the ionophore valinomycin, which has
previously been demonstrated to be a potent apoptosis-inducing agent
(27). A dose response study revealed that IGFBP-3 induced apoptosis at concentrations as low as 50 ng/ml and demonstrated a dose response up
to 500 ng/ml (Fig. 2B). This effect was only partially
inhibited by exogenous IGF (Fig. 2C) (p < 0.05 compared with IGFBP-3 treatment) and was not inhibited by the IGF
analogue (long R3-IGF-I) that does not bind to IGFBPs (Fig.
2C), suggestive of an IGF-independent mechanism for this
IGFBP-3 effect. All of these methods confirmed that treatment with
IGFBP-3 for 72 h resulted in a significant increase in the
apoptotic index in PC-3 cells.
Activation of the ICE Pathway by IGFBP-3
Analyses using
the apoptosis ELISA demonstrated that IGFBP-3-induced apoptosis
was inhibited by the reversible ICE-I
(Ac-Tyr-Val-Ala-Asp-aldehyde) (Fig.
3A). The ICE-I completely suppressed
IGFBP-3-induced apoptosis in a dose-dependent fashion at
concentrations ranging from 0.4 to 5 µmol/liter (Fig. 3B).
These concentrations are known to inhibit ICE and ICE-like proteases
and to block apoptosis induced by a variety of stimuli in other cell
types (32-34). These data demonstrate the involvement of ICE or
ICE-like proteases in the IGFBP-3-induced pathway.
Demonstration of IGFBP-3 Association Proteins/Receptors in PC-3 Cells
Detection of IGFBP-3-binding molecules using reverse
Western ligand blots revealed a number of bands varying in size from 18 to 150 kDa that represent proteins with high affinity to
125I-IGFBP-3 (Fig. 4). These molecules were
detected both in whole cell lysates (lane 1) and in the
purified plasma membrane fraction (lane 2). However, the
150-, 68-, and 18-kDa bands were strongly enriched in the membrane
fraction, while some bands (44 and 35 kDa) were seen more prominently
in the cell lysates, suggesting a cytoplasmic or nuclear origin. The
selective localization of some of these molecules in the membrane
fraction suggests the possibility of these proteins serving as IGFBP-3
cell surface receptors that may mediate IGFBP-3 action.
IGF-independent Effects of IGFBP-3
The possibility that
IGFBP-3 acts to induce apoptosis independently of IGFs and IGF
receptors was investigated by testing the ability of IGFBP-3
to induce apoptosis in the IGF receptor-negative (R())
fibroblast cells derived from an IGF-1R knockout mouse (25). These
cells have been shown previously to neither bind nor respond to IGFs.
To test the effect of IGFBP-3 on these cells, we used both treatment
with exogenous IGFBP-3 protein and transfection with the IGFBP-3 gene
(Fig. 5). The R(
) cell line demonstrated a
basal level of apoptosis when cultured in 30% serum. The DNA extracted
from R(
) cells and R(
) cells transfected
with IGFBP-3 (R(
)/BP-3) grown in 30% serum for 72 h
(Fig. 5A) reveals that the DNA fragmentation was far more
prevalent in R(
)/BP-3 (Fig. 5A, lane
3). This observation was also quantitated using photometric ELISA
(Fig. 5B). The transfection of the IGFBP-3 gene resulted in
a substantial increase in the degree of apoptosis (p < 0.001). The addition of exogenous IGFBP-3 (500 ng/ml) also significantly increased the apoptotic index in R(
) cells
(p < 0.001). The addition of IGF-I to
(R(
)) cells did not alter the basal level of apoptosis
observed in cells grown in 30% serum. However, the addition of IGF-I
to R(
)/BP-3 cells under similar conditions partially
inhibited the incidence of apoptosis (p < 0.05).
Similarly, IGFBP-3-specific antibodies also partially inhibited
(p < 0.05) this effect of endogenous IGFBP-3 in
R(
)/BP-3 cells. These results clearly demonstrate a
direct effect of IGFBP-3 on apoptosis that is modulated by IGF binding
to IGFBP-3.
Demonstration of the Role of IGFBP-3 in TGF-
Since TGF-1 is known to induce apoptosis in some
cells and also to up-regulate IGFBP-3 expression in similar cells, we
examined its relation to IGFBP-3-induced apoptosis. At a concentration of 1 ng/ml, TGF-
1 induces apoptosis in PC-3 cells. We recorded the
changes in the apoptotic index after treatment with TGF-
1 both
qualitatively (TUNEL; Fig. 6A) and
quantitatively (ELISA; Fig. 6B). The in situ
localization of apoptotic PC-3 cells using TUNEL revealed an increase
in the number of cells with fragmented DNA (Fig. 6A,
i, arrow) in TGF-
1-treated cells compared with those grown in SFM (Fig. 5A, ii). Quantitative
analyses by ELISA (Fig. 6B) revealed a basal level of
apoptosis in SFM, the suppression of this basal level by addition of
IGF-I, and a significant level of apoptosis induced by TGF-
1. This
induction of apoptosis by TGF-
1 was 95% as potent as the apoptosis
induced by the ionophore, valinomycin. In addition, using the
photometric ELISA, we compared and quantitated the apoptotic index
induced by 1 ng/ml TGF-
1 and 500 ng/ml IGFBP-3 under similar
conditions. When compared with serum-free conditions, both IGFBP-3 and
TGF-
1 demonstrated a significant increase in the apoptotic index
(p < 0.001).
As shown in the inset of Fig. 7,
immunoblotting the conditioned media from TGF-1-treated PC-3 cells
using IGFBP-3-specific antibodies revealed a dramatic (>10-fold)
elevation (p < 0.0001 compared with SFM) of the
40-44-kDa IGFBP-3 protein by 12 h after treatment. However, the
increase in the apoptotic index after TGF-
1 treatment was observed
later, at 18-24 h (data not shown).
To test whether TGF-1-induced apoptosis is mediated through IGFBP-3,
we treated PC-3 cells with TGF-
1 concomitantly with IGFBP-3 sense or
antisense oligonucleotides or with IGFBP-3-specific, affinity-purified,
neutralizing antibodies (Fig. 8). IGFBP-3 and TGF-
1
induced apoptosis as shown above. The IGFBP-3 antisense oligomer
effectively blocked the TGF-
1-induced apoptosis in PC-3 cells
(p < 0.001 compared with TGF-
1 treatment),
suggesting that TGF-
1 induces apoptosis by increasing IGFBP-3
expression. The sense IGFBP-3 oligomer had no such effect. In
addition, specific neutralizing antibodies to IGFBP-3 also blocked
(p < 0.001) TGF-
1- induced apoptosis in these
cells. No blocking effect was observed with the addition of control
IgG. These results suggest that the IGFBP-3 has to be secreted and
presumably bind to its receptor before it induces apoptosis.
The role of IGFBP-3 as a growth-inhibitory protein has been previously demonstrated by us and others in various cell types (9, 12-17). Initially, IGFBP-3 was thought to inhibit growth by binding to IGFs and sequestering them from their receptor. Later, the cell growth-inhibitory effect of IGFBP-3 was suggested to also be IGF-independent and to involve cell growth arrest (14, 15). Recently, this inhibitory effect of IGFBP-3 was suggested to be mediated by interaction with a putative IGFBP-3 receptor. Although the IGF-independent growth-inhibitory role of IGFBP-3 has been recently investigated, an apoptosis-inducing role for IGFBP-3 has not been previously determined. This is the first demonstration of IGFBP-3 as a cell death-promoting agent.
Partial blocking of IGFBP-3-induced apoptosis by IGF suggests two possibilities. First, the presence of IGF may prevent the cells from undergoing apoptotic changes through the IGF receptor-mediated cell survival pathway. Second, some of the IGFBP-3 would not be available to induce apoptosis through its own receptors, since it formed IGF·IGFBP-3 complexes. Furthermore, the inability of IGF to fully block IGFBP-3-induced apoptosis even at a 5-fold higher molar concentrations supports the notion that the pathway of IGFBP-3-induced apoptosis may not always involve IGF and IGF receptor. In addition, since IGF analogues that do not bind IGFBP-3 did not reverse the IGFBP-3 effect at all, this further suggests that IGFBP-3 induces apoptosis via an IGF-independent pathway through an IGFBP-3 receptor. This IGFBP-3 cell surface receptor has been first proposed in Hs578T breast cancer cells by affinity cross-linking of 125I-IGFBP-3 to cell membrane and cell lysate extracts (18). In this study, we have shown that PC-3 cells also bind IGFBP-3 and that several potential IGFBP-3 receptors exists in PC-3 cells.
IGFs have been shown to protect cells from undergoing
apoptosis through an IGF receptor-mediated cell survival pathway
(35-38). Both the effects of decreases in the number of IGF receptors
causing massive apoptosis and the overexpression of IGF receptors
protecting cells from apoptosis have been demonstrated in
vivo (35). The roles of IGFs and the IGF receptors as autocrine
survival factors (36) and as protective agents that prevent apoptosis
induced by other agents such as etoposide have been shown extensively (37). Mutant versions of p53 protein, commonly associated with malignant states, have been shown to derepress the IGF receptor promoter, with ensuing mitogenic activation by locally produced or
circulating IGFs (38). All of the above mentioned studies indicate the
important role of IGFs and IGF receptors in preventing cells from
undergoing apoptosis through a cell survival pathway. We demonstrated
here an alternate pathway for the induction of apoptosis that is
independent of these apoptosis-protecting agents. By demonstrating
IGFBP-3-induced apoptosis in the IGF receptor-negative (R()) murine fibroblast cell line, we proved our
hypothesis that IGFBP-3 may induce apoptosis independently of the IGF
receptor-mediated survival pathway. Therefore, the ratio of free IGFs
and IGFBP-3 will regulate cell growth not only by balancing the rate of
cell proliferation and cell growth arrest, but also by regulating the rate at which the cells might be induced to undergo apoptosis.
The apoptosis-inducing effect of IGFBP-3 in R() cells
provides ample evidence to suggest that similar IGF
receptor-independent pathways are present in PC-3 cells and possibly in
other cell lines. Treatment with IGF-I partially decreased the
incidence of apoptosis in IGFBP-3-overexpressing cells but did not have any effect on R(
) cells, suggesting that the partial
suppression of apoptosis by IGF is through the formation of
IGF·IGFBP-3 complexes. Similar to the results found in PC-3 cells,
IGFBP-3-neutralizing antibodies partially decreased the degree of
apoptosis in IGFBP-3-overexpressing R(
) cells. In PC-3
cells, IGF-I partially blocked IGFBP-3-induced apoptosis, but the IGF
analogue, which binds to the IGF receptor and not to IGFBP-3, was
unable to block IGFBP-3-induced apoptosis. These observations not only
suggest the involvement of an IGF-independent pathway, but they
also demonstrate that IGFBP-3 must be free of IGF to be able to
bind to its receptor and initiate its effect on cell death and that the
activation of the IGF receptor does not protect cells from
IGFBP-3-induced apoptosis.
The expression of the cell growth-inhibitory IGFBP-3 has been shown to
be induced by various apoptosis-inducing agents, such as TGF-1
(20-22), retinoids (21), TNF-
(23), and the tumor suppressor gene
p53 (24). IGFBP-3 has been previously shown to mediate the
growth-inhibitory effect of both retinoic acid and TGF-
1 (20, 21).
However, the mechanism by which the IGFBP-3 reduces the cell number,
under these conditions, is not known. In this work, we have
demonstrated that IGFBP-3 mediates the growth-inhibitory effect of
TGF-
1 by inducing apoptosis. This may apply to other agents that
have not yet been investigated.
The PC-3 cells are p53-negative (39) and have the machinery to express
low levels of IGFBP-3 (8) under serum-free conditions. TGF-1 is a
potent growth inhibitor of epithelial cells and has been shown to
induce apoptosis and down-regulate Bcl-2 expression (40, 41). The
dramatic elevation of the 44-kDa IGFBP-3 protein within 12 h of
TGF-
1 treatment and the significant effect of TGF-
1 on apoptosis
that was observed about 18-24 h after treatment suggest that the
TGF-
1-induced elevation of IGFBP-3 protein in the conditioned media
is the primary signal that activated apoptosis in this cell line.
Blocking TGF-
1-induced apoptosis at the IGFBP-3 transcriptional
level confirmed the role of IGFBP-3 as the mediator of TGF-
1-induced
apoptosis in PC-3 cells. Co-treatment with IGFBP-3 antisense (but not
sense) thiolated oligonucleotide and TGF-
1 verified the role of
IGFBP-3 in the TGF-
1-induced apoptosis. Furthermore, neutralization
of IGFBP-3 action in TGF-
1-treated cells with IGFBP-3-neutralizing
antibodies (but not control IgG) confirmed that IGFBP-3 must be
secreted and allowed to bind to its receptor to initiate apoptosis. The
latter observation also confirms that the TGF-
1-mediated increase in
IGFBP-3 transcription must pass through steps such as IGFBP-3 secretion
and the binding of this protein to its receptor to initiate
apoptosis.
Inappropriate expression of genes involved in cell proliferation has
been shown to alter regulation of apoptosis. Both Bcl-2, which promotes
cell survival, and Bax, which promotes cell death, have been implicated
as major mediators in the control of apoptotic pathways, and it has
been suggested that the ratio of Bcl-2 to Bax controls the relative
susceptibility of cells to death stimuli. TGF-1, retinoic acid,
TNF-
, and p53 are known to induce apoptosis by regulating Bcl-2 and
Bax expression (40-47). Since all of these apoptosis-inducing agents
also induce IGFBP-3 expression, we anticipate that IGFBP-3-induced
apoptosis may also involve regulation of the Bcl-2:Bax ratio. In
addition, the expression of ICE or ICE-like proteases that are final
mediators of the apoptosis pathway is involved in the mechanism of
action of IGFBP-3 as well as the above agents.
The role of IGFBP-3 in mediating p53 effects was proposed when p53 was demonstrated to activate the IGFBP-3 promotor (24). Recently, it has been shown that mutants of p53 that have lost the ability to activate IGFBP-3 and Bax expression but maintained their activation of the cyclin-dependent kinase inhibitor p21 are able to induce cell cycle arrest but are unable to induce apoptosis (48). Furthermore, a p53 mutant that activates Bax expression but only partially activates the IGFBP-3 promotor is only partially effective in inducing apoptosis (49). Thus, a p53-dependent role of IGFBP-3 has been previously demonstrated. By demonstrating IGFBP-3-induced apoptosis in PC-3 cells that lack the p53 gene, we have demonstrated that IGFBP-3 can also induce apoptosis in a p53-independent fashion.
We present a hypothesis based on the results from this study and other
previous reports from this and other groups in the diagrammatic
representation shown in Fig. 9. We propose that
the independent and interdependent effects of IGFs and
IGFBPs on the regulation of cell number involve two pathways
that interact at several levels. IGFs mediate survival via the IGF
receptor. IGFBP-3 is able to block this pathway by sequestering IGFs
away from the IGF receptor. IGFBP-3 mediates apoptosis via its own
receptors, while IGFs can prevent this effect by binding to IGFBP-3.
Thus, IGFBP-3 can mediate cell death by both IGF-dependent
and IGF-independent pathways.
Both normal cell growth (50) and various pathologies associated with
neoplastic cell proliferation, such as breast cancer (20, 26, 51, 52),
prostate cancer, and benign prostatic hyperplasia (11), are also
associated with altered expression of IGFs and IGFBPs. Earlier
observations, however, did not directly demonstrate a role for IGFBP-3
in inducing apoptosis but provided ample evidence to suggest that
IGFBP-3 is important in regulating cell number in such situations. Our
data demonstrate that IGFBP-3 induces apoptosis at physiological
concentrations and that IGFBP-3 may act through an IGF·IGF
receptor-independent pathway. IGFBP-3 mediates the induction of
apoptosis by TGF-1 and may mediate similar actions of other
growth-regulatory factors.