Introduction
The female hormone estrogen has long been recognized as being important for stimulating the growth of a large proportion of breast cancers. Estrogen action is mediated by two receptors; estrogen receptor (ER) alpha and ER beta. Approximately 70% of breast cancers express ERα [
1,
2], and its presence in breast tumors is routinely used to predict a response to endocrine therapy such as tamoxifen - an anti-estrogen that blocks estrogen-stimulated breast cancer cell growth - or aromatase inhibitors (AIs) - agents that suppress estrogen synthesis in the body. These agents are highly effective and are less toxic compared with chemotherapy, and are often offered to ER-positive breast cancer patients to sustain a better quality of life [
3,
4]. Despite the clinical benefits of tamoxifen and AIs, however, a large number of breast cancer patients develop drug resistance. It is estimated that ~40% of patients with early ER-positive breast cancer relapse within 15 years after adjuvant therapy with tamoxifen and 15% of patients treated with an AI relapse within 9 years [
5‐
7]. These resistant tumors are usually more aggressive and are more likely to metastasize, which is often the leading cause of breast cancer-related death. There is strong evidence that endocrine resistance is associated with cross-talk between upstream kinases and ERα, resulting in estrogen-independent activation of the ERα; however, the exact mechanism by which breast cancer cells develop resistance to endocrine therapy is still not fully understood.
Pigment epithelium-derived factor (PEDF) is a 50 kDa glycoprotein that belongs to the non-inhibitory serine protease inhibitor superfamily but it does not inhibit proteases [
8,
9]. PEDF was first discovered as a factor secreted by retinal pigment epithelial cells [
10], but was later found to be expressed in several tissues including the brain, spinal cord, eye, plasma, bone, prostate, pancreas, heart and lung [
11]. PEDF is present in human blood at a concentration of approximately 100 nM (5 μg/ml) or twice the level required to inhibit aberrant blood-vessel growth in the eye [
10]. PEDF possesses potent anti-angiogenic activity, far greater than any other known anti-angiogenic factor [
12], and it has anti-tumor properties including the ability to promote tumor differentiation and initiate apoptosis [
13‐
16]. In endothelial cells, PEDF has been shown to induce apoptosis by activating the Fas/Fas-L caspase-8 apoptotic pathway [
17,
18] and there is evidence that the p38 mitogen-activated protein kinase (MAPK) pathway is involved in the anti-angiogenic activity of PEDF [
19]. More recently, a number of studies have reported that PEDF expression is significantly reduced in several tumor types, including prostate adenocarcinoma [
20], pancreatic adenocarcinoma [
21], glioblastoma [
22], ovarian carcinoma [
23], and breast cancer [
24]. With regards to breast cancer, PEDF expression has been shown to be markedly reduced in breast tumors compared with normal tissue and this reduction is associated with disease progression and poor patient outcome [
24,
25]. At present, however, it is not known whether PEDF plays a role in the development of endocrine resistance.
In this study, we examined the role of PEDF in the development of endocrine resistance using several breast cancer cell lines. Specifically, we evaluated PEDF expression in endocrine-resistant MCF-7:5C, MCF-7:2A, and BT474 breast cancer cells versus endocrine-sensitive MCF-7, T47D, and ZR-75-1 cells and found that PEDF mRNA and protein levels were dramatically reduced in the endocrine-resistant breast cancer cell lines compared with the endocrine-sensitive cell lines. In addition, tissue microarray studies revealed that PEDF protein was significantly reduced in tamoxifen-resistant/recurrence tumors compared with primary tumors. We also found that re-expression of PEDF in endocrine-resistant MCF-7:5C and BT474 cells restored their sensitivity to tamoxifen, whereas siRNA knockdown of PEDF in MCF-7 and T47D cells markedly reduced their sensitivity to tamoxifen. Notably, re-expression of PEDF in endocrine-resistant MCF-7:5C cells resulted in a significant reduction in the level of p-ERα, p-AKT, and rearranged during transfection (RET) proteins, which were constitutively overexpressed in these cells. Lastly, we found that recombinant PEDF (rPEDF) dramatically reduced the tumor growth of MCF-7:5C xenographs in athymic mice and that re-expression of PEDF in MCF-7:5C cells partially restored tamoxifen sensitivity in vivo. Taken together, these findings suggest that PEDF silencing might be a novel mechanism for the development of endocrine resistance in breast cancer.
Materials and methods
Cell lines and culture conditions
The MCF-7 cells used in this study [
26] were cloned from ERα-positive human MCF-7 breast cancer cells originally obtained from the American Type Culture Collection (Manassas, VA, USA). MCF-7 cells were maintained in full serum medium composed of RPMI-1640 medium, 10% fetal bovine serum, 2 mM glutamine, penicillin at 100 U/ml, streptomycin at 100 μg/ml, 1× nonessential amino acids (Invitrogen, Grand Island, NY, USA), and bovine insulin at 6 ng/ml (Sigma-Aldrich, St Louis, MO, USA). ER-positive MCF-7:5C [
27,
28] and MCF-7:2A [
29,
30] breast cancer cells were cloned from MCF-7 cells following long-term (> 12 months) culture in estrogen-free medium composed of phenol red-free RPMI, 10% fetal bovine serum treated three times with dextran-coated charcoal, 2 mM glutamine, bovine insulin at 6 ng/ml, penicillin at 100 U/ml, streptomycin at 100 μg/ml, and 1× nonessential amino acids. MCF-7:5C cells are resistant to AIs (that is, hormone independent) and tamoxifen, but these cells undergo apoptosis in the presence of physiologic concentrations of 17β-estradiol (E2), as previously reported [
28]. MCF-7:2A cells are also resistant to AIs but only partially sensitive to tamoxifen, and these cells undergo apoptosis in the presence of E2 [
29,
31].
The human breast cancer cell line T47D:A18, referred to as T47D in this study, is a hormone-responsive clone of wild-type T47D that has been described previously [
32]. These cells were maintained in phenol red-containing RPMI medium supplemented with 10% fetal bovine serum (FBS), bovine insulin (6 ng/ml), and antibiotics. ER-positive ZR-75-1 and BT474 breast cancer cells were obtained from the American Type Culture Collection and were maintained in phenol red-containing RPMI medium supplemented with 10% FBS, bovine insulin (6 ng/ml), and antibiotics. The BT474 cell line was isolated by Lasfargues and Coutinho from a solid, invasive ductal carcinoma of the breast [
33]. ER-negative MDA-MB-231 breast cancer cells were obtained from the American Type Culture Collection and were cultured in DMEM medium supplemented with 10% FBS and antibiotics.
MCF-7:5C cells stably expressing PEDF (5C-PEDF) were grown in phenol red-free RPMI 1640 medium supplemented with 10% phenol red-free RPMI, 10% fetal bovine serum treated three times with dextran-coated charcoal and 4 μg/ml blasticidin (InvivoGen, San Diego, CA, USA), and BT474 cells stably expressing PEDF (BT474-PEDF) were grown in RPMI medium supplemented with 10% FBS and 4 μg/ml blasticidin (InvivoGen, San Diego, CA, USA).
Cell proliferation assay
This procedure has been described previously [
28,
29,
34]. Briefly, MCF-7 and T47D cells were grown in fully estrogenized medium. Cells were seeded in 24-well plates (30,000/well) and after overnight incubation were transfected with either control (nontarget) or PEDF siRNA. Transfected cells were treated with 10
-6 M 4-hydroxytamoxifen (4OHT) after 48 hours, and then cells were harvested after 72 hours and total DNA was determined using a Fluorescent DNA Quantitation kit (Bio-Rad Laboratories, Hercules, CA, USA), as previously described [
28]. Cell proliferation was also determined by cell counting using the trypan blue exclusion assay. MCF-7 and T47D cells were seeded in six-well plates (1 × 10
5/well) and then treated with 10
-6 M 4OHT for 72 hours. The 4OHT used in the cell proliferation studies was purchased from Sigma-Aldrich.
We also performed proliferation studies using MCF-7:5C, BT474, 5C-PEDF, and BT474-PEDF cells. MCF-7:5C and 5C-PEDF cells were grown in non-estrogenized media, and BT474 and BT474-PEDF cells were grown in fully estrogenized media. For the DNA proliferation assay, cells were seeded at a density of 30,000/well in 24-well plates and after overnight incubation were treated with 10
-12 M to 10
-6 M 4OHT for 7 days with retreatment on alternate days. Cells were then harvested and total DNA quantitated using a Fluorescent DNA kit as described previously [
28]. For cell counting, cells were seeded at 75,000/well in six-well plates and after overnight incubation were treated with 10
-6 M 4OHT for 72 hours. Cells were then harvested and counted using trypan blue exclusion.
Western blot analysis
Immunoblotting was performed using 30 μg protein per well as described previously [
28,
35]. Membranes were probed with primary antibodies against PEDF (Chemicon Inc., Temecula, CA., USA), against ERα and phospho-Ser167-ERα (Santa Cruz Biotechnology, Inc., Santa Cruz, CA, USA), against RET, p-RET (Y1062), mammalian target of rapamycin (mTOR), p-mTOR and AKT, and against pAKT, MAPK, pMAPK and p70S6K (Cell Signaling Technology Inc., Danvers, MA, USA), and against β-actin (Sigma Chemical Co., St Louis, MO, USA). The appropriate secondary antibody conjugated to horseradish peroxidase (Santa Cruz Biotechnology) was used to visualize the stained bands with an enhanced chemiluminescence visualization kit (Amersham, Arlington Heights, IL, USA). Bands were quantitated by densitometry using the Molecular Dynamics Software ImageQuant (GE Healthcare Life Sciences, Piscataway, NJ, USA) and densitometric values were corrected for loading control.
Knockdown of PEDF and RET with small interference RNA
For the iRNA silencing experiments, PEDF, RET, and nontarget control siRNAs were purchased from Dharmacon Inc (Pittsburg, PA, USA). For transfection, 100 nM siRNAs were combined with siRNA transfection reagent according to the manufacturer's instructions. Cells were seeded in 24-well plates at a density of 0.5 × 105 cells/well in antibiotics-free medium 12 hours before the transfection. One and a half microliters of the siRNA (20 μM) were mixed with 1 μl transfection reagent in 50 μl serum-free RPMI-1640 medium and were incubated at room temperature for 25 minutes to form a complex. After washing cells with PBS, the 50 μl transfection mixtures were added to each well with 450 μl RPMI-1640 medium containing 10% FBS at a final concentration of 100 nM siRNA. Twenty-four hours after the transfection, the medium was replaced with fresh 500 μl RPMI-1640 medium containing 10% FBS. Transfected cells were then harvested for western blotting and RT-PCR or subsequently treated with 10-9 M to 10-6 M 4OHT for 3 days to determine cell growth.
RNA isolation and RT-PCR analysis
Total RNA was isolated from cultured cells using the TRIzol reagent (Invitrogen) according to the manufacturer's procedure. First-strand cDNA synthesis was performed from 2.5 μg total RNA using Super-Script Reverse Transcriptase (Invitrogen). cDNA was amplified in a 15-μl PCR mixture containing 1 mm dNTPs, 1× PCR buffer, 2.5 mm MgCl2, and 1 U DNA Taq polymerase (Promega, Madison, WI, USA) with 25 pmol of primers specific for human PEDF (sense, 5'-CATTCACCGGGCTCTCTAC-3'; antisense, 5'-GGCAGCTGGGCAATCTTGCA-3') and human RET (sense, 5'-GGATTTCGGCTTGTCCCGAG-3'; antisense, 5'-CCATGTGGAAGG GAGGGCTC-3'). The conditions in the logarithmic phase of PCR amplification were as follows: 5 minutes initial denaturation at 94°C, 1 minute denaturation at 94°C, 35 seconds annealing at 67°C, and 1.5 minute extension at 72°C for 30 cycles. The number of amplification cycles during which PCR product formation was limited by the template concentration was determined in pilot experiments. PUM1 was used as the internal control (sense, 5'-TCACCGAGGCCCCTCTGAACCCTA-3'; antisense, 5'-GGCAGTAATC TCCTTCTGCATCCT-3').
The reproducibility of the quantitative measurements was evaluated by three independent cDNA syntheses and PCR amplification from each preparation of RNA. Densitometric analysis was performed using Scion Image software (Scion Corp., Frederick, MD, USA), and the relative PEDF or RET mRNA expression levels were determined as the ratio of the signal intensity of PEDF to that of PUM1.
Estrogen response element luciferase assay
To determine ERα transcriptional activity, cells were transfected with an estrogen response element (ERE)-regulated (pERE(5×)TA-ffLuc plus pTA-srLuc) dual-luciferase reporter gene set. pERE(5×)-ffLuc contained five copies of a consensus ERE and a TATA-box driving firefly luciferase; pTATA-srLuc contained a TATA-box element driving renilla luciferase. Cells were grown in the estrogen-free medium containing no exogenous compounds for 2 days before transfection. All transfection experiments were carried-out using LT1 (Mirus Bio LLC, Madison, WI, USA) at a 1:3 ratio of micrograms of plasmid to micoliters of LT1. In the ERE reporter gene experiment, the cells were treated as indicated 24 hours following the transfection. Forty-eight hours following the ERE transfection, the cells were harvested and processed for dual-luciferase reporter activity (Promega, Madison, WI, USA), in which the firefly luciferase activity was normalized by renilla luciferase activity.
Breast cancer tissue microarray and immunohistochemistry
Paraffin-embedded de-identified human breast cancer tissue samples were collected from the Tumor Bank facility at the Fox Chase Cancer Center and the protocols were reviewed and approved by the Institutional Review Board at our institution. The archived tumor samples were obtained from patients who were initially treated with tamoxifen and either responded (n = 150) or responded but then developed recurrence disease (n = 59) with an average time to disease progression of 93 months. Patients provided written informed consent for the use of their tumor samples.
Tissue microarray slides were constructed from 59 matching primary and recurrence tumors using duplicate cores of 0.6 mm per tumor sample. Tissue microarray slides were also created using endocrine-responsive tumors. For PEDF and ERα immunohistochemistry, sections were incubated at room temperature for 20 minutes with anti-PEDF or anti-ERα antibody (Chemicon Inc.) applied at 1:100 dilution in antibody diluent (Dako USA, Carpinteria, CA, USA). A secondary anti-mouse antibody polymer conjugated with horseradish peroxidase (Dako USA) was applied for 30 minutes and 3,3'-diaminobenzidine was used to produce visible, localized staining viewable with light microscopy. Sections without primary antibody served as negative controls. Normal breast tissue from archival specimens was used as positive controls for PEDF and ERα expression. A semi-automated quantitative image analysis system (ACIS II; ChromaVision Medical Systems, Inc., San Juan Capistrano, CA, USA) was used to quantitate the staining of the tissue microarray slides. For immunohistochemical analysis, the brown stain intensity of the chromogen was compared with the blue counterstain used as background. Staining for PEDF was quantified as an intensity score (scale 0 to 255) and staining for ERα was graded as follows: 0, negative (no cells stained); 1, weakly positive (< 10% of cells stained); 2, moderately positive (10 to 50% of cells stained); or 3, strongly positive (> 50% cells stained).
TUNEL staining for apoptosis
Apoptosis was determined by the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) assay using an in situ cell death detection kit (POD; Roche Molecular Biochemicals, Branchburg, NJ, USA), according to the manufacturer's instructions. Briefly, fixed cells were washed, permeabilized, and then incubated with 50 μl terminal deoxynucleotidyl transferase end-labeling cocktail for 60 minutes at 37°C in a humidified atmosphere in the dark. For signal conversion, slides were incubated with 50 μl converter-POD (anti-fluorescein antibody conjugated with horseradish peroxidase) for 30 minutes at 37°C, rinsed with PBS, and then incubated with 50 μl of 3,3'-diaminobenzidine substrate solution for 10 minutes at 25°C. The slides were then rinsed with PBS, mounted under glass coverslips, and analyzed under a light microscope (Inverted Nikon TE300; Melville, NY, USA).
Lentiviral vector design, production, and transduction
For PEDF overexpression, we generated a lentiviral construct encoding the full-length human PEDF cDNA inserted between XbaI and BamHI sites of the prrl.CMV.EGFP.wpre.SIN lentiviral vector. Briefly, PEDF cDNA was amplified by PCR from pCEP4-PEDF plasmid (a gift from Dr Bouck, Northwestern University, Chicago, IL, USA); XbaI and XbaI + EcoRV sites were added to the 5' and 3' ends, respectively, using primers 5'-CTAGTCTAGAGGCCCCAGGATGCAGGC CCTG-3' and 5'-GGCCTCTAGATATCTTAGGGGCCCCTGGGGTCCAG-3'. This fragment was then subcloned into TA cloning vector (Invitrogen, San Diego, CA, USA), digested with EcoRV and XbaI and re-cloned in the prrl.CMV.EGFP.wpre.SIN plasmid digested with XbaI and BamHI. To produce lentiviral stock, 293FT cells (Invitrogen) were plated in 10-cm tissue culture plates. When the cells were 90 to 95% confluent, the complete culture medium was removed and the cells were exposed to 5 ml medium (Opti-MEM I; Invitrogen) with complexes (DNA-Lipofectamine 2000; Invitrogen) containing 9 μg packaging mix (ViralPower; Invitrogen), 3 μg expression plasmid DNA (prrl.CMV.EGFP.wpre.SIN/PEDF), or control plasmid DNA (prrl.CMV.EGFP.wpre.SIN/LacZ) with lipofectamine (Lipofectamine 2000; Invitrogen). Hexadimethrine bromide (Polybrene; Sigma-Aldrich) was added at the final concentration of 10 μg/ml. After incubation for 24 hours, the infection medium was replaced with complete culture medium. Lentivirus-containing supernatants were harvested 72 hours after transfection. The supernatants were centrifuged to remove pellet debris and stored at -80°C.
For lentiviral vector transduction, MCF-7:5C and BT474 cells were plated in six-well plates. When the cells reached 30 to 50% confluence, media were changed to either phenol red-free RPMI medium with 10% charcoal-stripped FBS without antibiotic (MCF-7:5C cells) or complete growth medium without antibiotic (BT474 cells) with the lentiviral stock, and 10 μg/ml hexadimethrine bromide (Polybrene; Sigma-Aldrich) was added to improve lentiviral vector transduction. Lentiviral vector expressing lacZ served as a positive control. After overnight incubation at 37°C in 5% CO2, the media-containing virus was removed and replaced with 2 ml complete culture media. After incubation overnight at 37°C in 5% CO2, media were changed to phenol red-free RPMI medium with 10% charcoal-stripped FBS without antibiotic or respective media with 4 μg/ml blasticidin (InvivoGen). Transduced cell clones were then selected with antibiotic for 2 weeks. PEDF expression was verified by quantitative real-time RT-PCR and western blot analysis in MCF-7:5C and BT474 cells.
Animal studies
The mammary fat pads of 6-week-old to 8-week-old ovariectomized outbred athymic mice (Taconic, Upstate, NY, USA) were bilaterally inoculated with 5 × 10
6 MCF-7:5C cells suspended in 0.1 ml sterile PBS solution as described previously [
28]. When tumors reached a mean cross-sectional area of 0.1 cm
2, the mice were randomized into groups of 10 and were treated with sterile PBS (100 μl) or 4 mg/kg rPEDF that was administered by intraperitoneal injection for a total of 30 days. Mice were injected every 2 days and tumors were measured every 5 days with vernier calipers. The mean cross-sectional tumor area was calculated by multiplying the length (
l) by the width (
w) by π and dividing the product by four (that is,
lwπ/4). The mean cross-sectional tumor area was plotted against time in days to monitor tumor growth. The mice were sacrificed by CO
2 inhalation and cervical dislocation; tumors were excised and immediately fixed in 10% buffered formalin for immunohistochemistry or snap-frozen in liquid nitrogen. Frozen tumor specimens were stored at -80°C for further analysis.
In another experiment, a total of 96 ovariectomized outbred athymic mice, 6 to 8 weeks old, were bilaterally inoculated with 5 × 10
6 MCF-7, BT474, or MCF-7:5C breast cancer cells suspended in 0.1 ml sterile PBS. Mice injected with MCF-7 (
n = 32) or BT474 cells (
n = 32) were simultaneously treated with E2 to stimulate tumor growth. E2 was administered via 0.3 cm long silastic capsules (Innovative Research, Sarasota, FL, USA) that were implanted subcutaneously between the scapules. The capsules remained in place for the duration of the study (3 to 6 weeks). Mice injected with MCF-7:5C cells (
n = 32), however, did not require treatment with E2 because these cells are estrogen independent and are capable of forming tumors in the absence of E2, as reported previously [
28]. When the mean tumor cross-sectional area reached approximately 0.3 cm
2 for MCF-7 and BT474-injected mice and 0.2 cm
2 for MCF-7:5C-injected mice, groups of eight mice were randomly assigned to the following treatments: PBS alone (control), rPEDF, tamoxifen, or tamoxifen plus rPEDF. Tamoxifen was administered orally by gavage at 1.5 mg/day per mouse for 5 days/week for 21 days and rPEDF was administered by intraperitoneal injection at 4 mg/kg every 2 days for 21 days. Tumors were measured weekly with vernier calipers. The mean cross-sectional tumor area was calculated by multiplying the length (
l) by the width (
w) and by π and dividing by 4 (that is,
lwπ/4).
All animal experiments were carried out according to the guidelines of the American Association for Laboratory Animal Science as an approved protocol by the Institutional Animal Care and Use Committee at the Institute for Cancer Research-Fox Chase Cancer Center.
Microvessel density assay
Frozen tissues were cut into 10-μm sections, fixed in acetone at 4°C for 5 minutes, and blocked for endogenous peroxidase. Sections were treated with normal serum for 10 minutes. Tumor sections were incubated with the rat monoclonal antibody against mouse CD34 (BD Pharmingen, San Diego, CA, USA) at 1:100 dilutions at 4°C. After rinsing with PBS, sections were incubated with biotinylated rabbit antigoat immunoglobulins (Dako, Glostrup, Denmark) at 1:1,000 dilutions for 30 minutes at room temperature followed by incubation with horseradish peroxidase-labeled streptavidin-biotin complex for 30 minutes. The peroxidase reaction was visualized using diaminobenzidine. The tumor microvessel density was quantified as tumor vasculature. In negative-control staining, the primary antibodies were omitted.
Statistical analysis
All in vitro experiments were repeated at least twice in either duplicate or triplicate with different cell preparations to ensure consistency of the findings. One-factor analysis of variance was used to demonstrate that there were significant differences between conditions when there were more than two conditions, and paired analyses were performed using either Student's t test or the Mann-Whitney test (GraphPad Software, San Diego, CA, USA) in order to identify the conditions that were significantly different. For in vivo studies, tumor growth curves were analyzed longitudinally using a two-factor analysis of variance comparing tumor cross-sectional areas within treatments in a time-dependent manner. Tumor growth curves represent the mean ± standard error of tumor cross-sectional areas. P < 0.05 was considered statistically significant.
Discussion
Resistance to endocrine therapy presents a major challenge in the management of ERα-positive breast cancer and is an area under intense investigation. While many studies point towards the cross-talk between ERα and growth factor receptor signaling pathways as the key in the development of resistance [
5,
6,
46,
47], the underlying mechanism is still not fully understood and, as a consequence, effective approaches for preventing and overcoming resistance are not yet available. PEDF is a secreted glycoprotein that was first described in the late 1980s after it was identified and isolated from conditioned medium of cultured primary human fetal retinal pigment epithelial cells [
8]. PEDF is ubiquitously expressed in many tissues and possesses potent anti-angiogenic activity, being more than twice as potent as angiostatin and more than seven times as potent as endostatin [
12]. Recent studies indicate that PEDF expression is significantly reduced in a wide range of tumor types and that its re-expression in these tumors delays the onset of primary tumors and decreases metastases [
48]. In the present study, we show that loss of PEDF expression in breast cancer is associated with the development of endocrine resistance and that there is functional crosstalk between PEDF and the ERα signaling pathway. Specifically, we found that PEDF protein and mRNA levels were markedly reduced in tamoxifen-resistant breast tumors and in breast cancer cells that are resistant to AIs and/or tamoxifen. We also found that stable re-expression of PEDF in the resistant cells re-sensitized them to the antiproliferative effects of tamoxifen and that re-expression of PEDF dramatically reduced the expression of the receptor tyrosine kinase RET along with p-AKT and p
Ser167ERα. Furthermore, we found that exogenous administration of rPEDF significantly inhibited the growth of endocrine-resistant breast cancer cells
in vitro and
in vivo but had no effect on the growth of endocrine-sensitive breast cancer cells
in vitro with marginal effect
in vivo. While PEDF is known to exert anti-tumor activity by inhibiting angiogenesis [
49,
50] and inducing apoptosis [
17], the present study is the first to demonstrate a link between loss of PEDF expression and the development of endocrine resistance and to show that PEDF re-expression is capable of reversing tamoxifen resistance in breast cancer.
During the past decade, researchers have prepared various forms of PEDF and demonstrated its beneficial effects in several tumor models. Doll and colleagues reported that exogenous rPEDF protein induced tumor epithelial apoptosis in mouse prostate and pancreas [
13]. Liu and colleagues showed that a short peptide derived from the parent PEDF molecule was able to inhibit osteosarcoma growth [
51]. Hase and colleagues demonstrated that intratumoral injection of a lentivirus vector encoding PEDF resulted in inhibition of human pancreatic cancer in nude mice [
52]. Moreover, Wang and colleagues showed that
in vivo transfer of PEDF mediated by adenoviral vectors exerted a dramatic inhibition of tumor growth in athymic nude mice implanted with the human HCC and in C57BL/6 mice implanted with mouse lung carcinoma [
53]. In the present study we showed that exogenous rPEDF preferentially induced apoptosis in endocrine-resistant MCF-7:5C and BT474 breast cancer cells compared with endocrine-sensitive MCF-7 cells and that rPEDF partially reversed the tamoxifen-resistant phenotype of MCF-7:5C and BT474 cells
in vitro and
in vivo. Interestingly, we found that lentiviral-mediated re-expression of PEDF in the resistant cells also reversed tamoxifen resistance in these cells. Investigation into the mechanism of action of PEDF in the resistant cells indicated that the anti-tumor activity of PEDF in
vivo was due, in part, to its ability to inhibit angiogenesis, as was demonstrated by a reduction in microvessel density and an increase in apoptosis. Interestingly, we found that exogenous PEDF failed to induce apoptosis in MCF-7 breast cancer cells
in vitro; however, it significantly inhibited the growth of MCF-7 tumors in athymic mice, which was due to its anti-angiogenic activity. The anti-tumor activity of PEDF, however, was more pronounced in the endocrine-resistant breast cancer cells compared with the endocrine-sensitive cells. We should note that a similar finding was reported by Konson and coworkers in which they showed that exogenous PEDF preferentially induced apoptosis in endothelial cells compared with MDA-MB-231, HCT116, and U87-MG cancer cells [
54,
55]; however, PEDF efficiently inhibited the growth rate of xenografts generated from these cancer cells. While the reason for this cell-type specific effect of PEDF is not known, there is evidence for multiple PEDF receptors at the cell surface including the recently identified non-integrin 67/37-kDa laminin receptor [
56], extracellular matrix components [
57], and a phospholipase-linked membrane protein [
58]. Differential expression of these receptors on neuronal, endothelial, and cancer cells may provide a partial explanation for the differential effects on these cell populations. Identification of which of these PEDF receptors are present on cancer cells, as well as further elucidation of signaling downstream of PEDF, could lead to the identification of new pharmacologic targets for both anti-cancer and neuronal survival therapies. We are currently trying to determine whether there is a specific PEDF receptor expressed in breast cancer cells and whether the functional activity of the receptor is altered by the endocrine responsiveness of the cells.
Apart from its ability to inhibit to angiogenesis, we also found that PEDF suppressed RET expression in endocrine-resistant breast cancer cells and that this suppression was associated with the reversal of tamoxifen resistance. Specifically, we found that basal RET, p-RET, ERα, and p
Ser167-ERα protein levels were markedly increased in endocrine-resistant MCF-7:5C cells compared with endocrine-sensitive MCF-7 cells and stable expression of PEDF in MCF-7:5C cells or treatment of these cells with rPEDF-suppressed RET, p-RET, and p
Ser167-ERα protein in these cells. Furthermore, we found that suppression of RET expression using siRNA knockdown also reversed tamoxifen resistance in MCF-7:5C cells, which suggests a role for RET in tamoxifen resistance. This finding is important because recent studies have indicated that RET is involved in the biology of ERα-positive breast cancers [
43,
44] and in the response to endocrine treatment [
45]. Two independent studies have identified RET overexpression in a subset of ERα-positive breast cancers [
43,
44], suggesting an important role of RET in this subset. By
in situ hybridization, in a cohort of 245 invasive breast cancers, RET mRNA was detected in 29.7% of the tumors and preferentially expressed in ER-positive cases. Subsequent studies in the same cohort of patient samples corroborated that increased RET mRNA levels correlated with increased RET protein expression. Similar findings were reported for many breast cancer cell lines where RET expression correlated strongly with ERα expression and/or ErbB2/HER2 overexpression [
43].
RET is a receptor tyrosine kinase protein of 150 kDa that is expressed and required during early development for the formation of neural crest-derived lineages, kidney organogenesis, and spermatogenesis [
59]. RET is considered the driving oncogene in various neoplasms of the thyroid, where specific mutations lead to defined tumor types [
60‐
62]. The RET protein spans the cell membrane, so that one end of the protein remains inside the cell and the other end projects from the outer surface of the cell. This positioning of the protein allows it to interact with specific factors outside the cell and to receive signals that help the cell respond to its environment. When molecules that stimulate growth and development such as growth factors attach to the RET protein, a complex cascade of chemical reactions inside the cell is triggered. These reactions instruct the cell to undergo certain changes, such as dividing or maturing to take on specialized functions. RET is the receptor for a family of glial-derived neurotrophic factor (GDNF) ligands, which includes GDNF, artemin, neurturin, and persephin [
60,
63]. These ligands bind RET in conjunction with glycosylphosphatidylinositol-anchored co-receptors of the GDNF receptor alpha family, and the ligand-co-receptor-RET complex formation results in transient RET dimerization and activation of the RET tyrosine kinase domain. RET protein dimerization results in autophosphorylation of several intracellular RET tyrosine residues, and these autophosphorylation sites serve as binding sites for a variety of docking proteins. In particular, the tyrosine Y1062 has been shown to bind Src homology and collagen, insulin receptor substrate1/2, fibroblast growth factor receptor substrate 2, and protein kinase C alpha. These proteins are able to activate multiple signaling pathways, including MAPK, PI3K/AKT, RAS/extracellular signal-regulated kinase and Rac/c-
jun NH kinase, which are mediators of cell motility, proliferation, differentiation, and survival [
64]. While our present study indicates that PEDF is capable of suppressing RET signaling in endocrine-resistant cells, we do not know the exact mechanism by which this occurs. We should note that RET is the receptor for several ligands including GNDF, which is a potent neurotropic factor similar to PEDF. Like other trophic factors, PEDF is thought to exert its biological effects by specifically binding and activating one or more receptors. While PEDF receptors have not yet been fully characterized, there is a possibility that PEDF, like GDNF, is able to bind to RET and thus regulate its expression and activity in breast cancer cells. This possibility is currently being investigated in our laboratory.
RET and other growth factor receptor tyrosine kinases are known to activate ERα through phosphorylation [
36]. The ERα contains two distinct transcription activation domains, AF-1 and AF-2, which can function independently or synergistically. AF-2 is located in the ligand-binding domain region of ERα and its activity is dependent on estrogen binding, whereas AF-1 activity is regulated by phosphorylation that can occur independently of estrogen binding [
5]. The extracellular signal-regulated kinase 1/2 pathway phosphorylates ERα directly and/or via p90RSK, whereas AKT phosphorylates ERα directly and/or via mTOR. In contrast, RET increases ERα phosphorylation at Ser118 and Ser167 through activation of the mTOR/p70S6K pathway [
43,
59,
65], which can be independent of the PI3K/AKT pathway. Notably, p70S6K, mTOR, and p-AKT were also constitutively overexpressed in endocrine-resistant MCF-7:5C cells prior to stable expression of PEDF in these cells. In addition, basal ERα transcriptional activity, as determined by ERE luciferase assay, was significantly elevated in MCF-7:5C cells compared with wild-type MCF-7 cells, and treatment of these cells with rPEDF inhibited phosphorylation of ERα and RET and suppressed the basal ERE activity in these cells. Interestingly, we found that suppression of RET expression using siRNA and inhibition of the mTOR pathway using rapamycin was able to reverse tamoxifen resistance in MCF-7:5C cells; however, inhibition of the PI3K/AKT pathway in these cells did not reverse their tamoxifen-resistant phenotype but it did reduce their hormone-independent growth. Notably, crosstalk between RET and ERα has previously been reported by Plaza-Menacho and coworkers, who showed that activation of RET by its ligand GDNF increased ERα phosphorylation on Ser118 and Ser167 and increased estrogen-independent activation of ERα transcriptional activity [
45]. Further, they identified mTOR as a key component in this downstream signaling pathway and they showed in tamoxifen-resistant (TAM
R-1) MCF-7 cells that targeting RET restored tamoxifen sensitivity.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
RJ performed the in vitro studies. MH constructed the tissue microarrays and performed all of the immunohistochemistry experiments. JL-W conceived the study, participated in the research design and implementation of the study, analyzed and interpreted the data, and drafted the manuscript. All authors read and approved the final manuscript for publication.