Cancer progression by breast tumors with Pit-1-overexpression is blocked by inhibition of metalloproteinase (MMP)-13
© Sendon-Lago et al.; licensee BioMed Central. 2015
Received: 11 December 2013
Accepted: 12 December 2014
Published: 20 December 2014
The POU class 1 homeobox 1 transcription factor (POU1F1, also known as Pit-1) is expressed in the mammary gland and its overexpression induces profound phenotypic changes in proteins involved in cell proliferation, apoptosis, and invasion. Patients with breast cancer and elevated expression of Pit-1 show a positive correlation with the occurrence of distant metastasis. In this study we evaluate the relationship between Pit-1 and two collagenases: matrix metalloproteinase-1 (MMP-1) and matrix metalloproteinase-13 (MMP-13), which have been related to metastasis in breast cancer.
We began by transfecting the MCF-7 and MDA-MB-231 human breast adenocarcinoma cell lines with the Pit-1 overexpression vector (pRSV-hPit-1). Afterward, the mRNA, protein, and transcriptional regulation of both MMP-1 and MMP-13 were evaluated by real-time PCR, Western blot, chromatin immunoprecipitation (ChIP), and luciferase reporter assays. We also evaluated Pit-1 overexpression with MMP-1 and MMP-13 knockdown in a severe combined immunodeficiency (SCID) mouse tumor xenograft model. Finally, by immunohistochemistry we correlated Pit-1 with MMP-1 and MMP-13 protein expression in 110 human breast tumors samples.
Our data show that Pit-1 increases mRNA and protein of both MMP-1 and MMP-13 through direct transcriptional regulation. In SCID mice, knockdown of MMP-13 completely blocked lung metastasis in Pit-1-overexpressing MCF-7 cells injected into the mammary fat pad. In breast cancer patients, expression of Pit-1 was found to be positively correlated with the presence of both MMP-1 and MMP-13.
Our data indicates that Pit-1 regulates MMP-1 and MMP-13, and that inhibition of MMP-13 blocked invasiveness to lung in Pit-1-overexpressed breast cancer cells.
To develop metastasis, breast cancer cells need, among other steps, to break their intercellular adhesion complexes and basement membrane to acquire motility to invade adjacent tissues . Proteolytic enzymes of various classes (metallo, aspartic, cysteine, serine, and threonine) execute the breaking down of matrix elements. However, some components, particularly the interstitial collagens, are very resistant to proteolytic attacks, being degraded only by matrix metalloproteinases (MMPs) . MMPs are synthesized as inactive zymogens, which are then activated predominantly pericellularly by either other MMPs or serine proteases. MMPs’ activity is specifically inhibited by the so-called tissue inhibitors of metalloproteases (TIMPs). Interstitial collagenases are a subfamily of MMPs that cleaves the stromal collagens. This subfamily includes, among others, collagenase 1 (MMP-1), and collagenase 3 (MMP-13). MMP-1 is the most ubiquitously expressed of the interstitial collagenases. It is produced by a wide variety of normal cells, for example, stromal fibroblasts, macrophages, endothelial cells, and epithelial cells, as well as by numerous tumors . MMP-1 is often upregulated in breast cancer, especially in basal-type tumors , and seems to be critically involved in metastatic dissemination [5,6]. Moreover, it has been suggested that MMP-1 is associated with shortened relapse-free survival  and poor outcome in breast cancer . Human collagenase-3 (MMP-13) was first identified in breast carcinoma [8-10]. Nielsen et al.  reported that MMP-13 expression by myofibroblasts was often associated with microinvasive events, and they proposed that MMP-13 may play an essential role during the transition from ductal carcinoma in situ lesions to invasive ductal carcinoma of the breast.
The POU class 1 homeobox 1 transcription factor (POU1F1, also known as Pit-1) was originally described in the pituitary gland, where it regulates cell differentiation during organogenesis and acts as an activator for pituitary gene transcription [12,13]. Pit-1 is also expressed in human breast . Compared to normal breast, Pit-1 expression is higher in breast tumors, increases cell proliferation, and regulates the expression of two breast cancer related hormones, growth hormone (GH) and prolactin (PRL) [14-16], which are also involved in both MMP regulation and breast cancer metastasis [17,18]. In addition, Pit-1 overexpression in a mouse xenograft tumor model promotes tumor growth and metastasis in lung. Furthermore, elevated Pit-1 expression in patients with breast cancer is positively correlated with the occurrence of distant metastasis .
In the present study, we used human mammary cell lines to analyze the regulation of MMP-1 and MMP-13 by Pit-1. In addition, we used immunodeficient mice to evaluate the role of both metalloproteinases in Pit-1-induced cancer invasiveness. Finally, we evaluated Pit-1, MMP-1, and MMP-13 protein expression in 110 human breast invasive ductal carcinomas.
Cell culture and reagents
The human breast adenocarcinoma cell lines MCF-7 and MDA-MB-231 were obtained from the European Collection of Cell Cultures (ECCC, Salisbury, UK). These cell lines were grown in 100-mm Petri dishes in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% fetal bovine serum (FBS), 100 U/ml penicillin, and 100 μg/ml streptomycin in an air-CO2 (95:5) atmosphere at 37°C. Confluent cells were washed twice with phosphate-buffered saline (PBS) and harvested by a brief incubation with trypsin- ethylenediaminetetraacetic acid (EDTA) solution (Sigma-Aldrich, St. Louis, MO, USA). Geneticin (G418), culture medium, and sera were purchased from Invitrogen Life Technologies, Carlsbad, CA, USA. Immobilon-P membranes were from Millipore (Merck Millipore, Billerica, MA, USA). Mytomycin C, MTT, puromycin, and hygromycin B were from Sigma-Aldrich.
Plasmids and transfections
Transient transfection, Pit-1 knockdown, and stable transfection of Pit-1 into MCF-7 cells were performed as previously described [19,20]. The Pit-1-overexpressing MCF-7 cells were then transfected with pBABE-puro-Luc vector and 48 hours later treated with 2.5 μg/μl of puromycin to select clones (MCF-7-hPit-1-luc cells). The shpLKO.1-MMP-1 and shpLKO.1-MMP-13 lentiviral vectors containing two different short hairpin RNA (shRNA) sequences for MMP-1 and MMP-13 were obtained from Thermo Fisher Scientific (Waltham, MA, USA) (see Additional file 1 and Figure S1A in Additional file 2).
The pLKO.1-puro non-target shRNA control containing a shRNA (shControl) was obtained from Sigma-Aldrich. MCF-7-hPit-1-luc-shControl, MCF-7-hPit-1-luc-shMMP-1 and MCF-7-hPit-1-luc-shMMP-13 cells were obtained through infection with shControl, shMMP-1, and shMMP-13 virus particles, respectively. Briefly, cell culture medium was replaced by a medium without FBS and containing 5 μg/ml of polybrene.
After 4 hours, culture medium was again replaced (DMEM plus 10% FBS), and lentiviral particles (Mission pLKO.1-puro non-target shRNA, Mission-MMP-1 shRNA, and Mission MMP-13 shRNA transduction particles, Sigma-Aldrich) were added and incubated at 37°C for 24 hours. Pit-1 knockdown was carried out using two different Pit-1 small interfering RNA (siRNA) (Pit-1 siRNA-1 and Pit-1 siRNA-2), as previously described (19). A scrambled siRNA was employed as control. Sequences of siRNAs are detailed in Additional file 1. The proximal promoter regions of the human MMP-1 and MMP-13 genes were synthesized by PCR and the product subcloned into the Xho I and Hind III site of the pGL2-basic plasmid (see Additional file 1). Site-directed mutagenesis was performed with the QuikChange kit from Stratagene (Agilent Technologies, Santa Clara, CA, USA). The mutagenized oligonucleotide primers were as follows (mutagenized bases on the sense strand are identified by lowercase letters): 5′-GATTGCCTAGTCT- ATgacTAGCTAATCAAG-3′ and 5′-CCAGGACCCCTGtcgaCATCTTGAATGG-3′ for MMP-1 and MMP-13, respectively. The newly constructed mutant plasmids were designated pGL2-B-MMP-1-1633/-1MUT and pGL2-B-MMP-13-145/+2MUT.
For luciferase assays, MCF-7 cells were transfected in 6-well plates containing 6 μl of jetPEI Polyplus transfection reagent (PolyPlusTransfection, Illkirch, France), 2 μg of pRc-RSV or pRSV-hPit-1, 1 μg of each reporter plasmid, and 50 ng of pRL-TK-Renilla (as transfection control) for 48 hours. The cells were lysed in buffer (100 μl lysis buffer, Promega Corporation, Madison, WI, USA) and luciferase activity was then measured in a Mithras LB 940 apparatus (Berthold Technologies, Bad Wildbad, Germany).
RNA isolation and quantitative RT-PCR
Total RNA was isolated from the cell lines using TRIzol (Invitrogen). cDNA was synthesized with Transcriptor First Strand cDNA Synthesis Kit (Roche Diagnostics, Basel, Switzerland), and reactions of quantitative real-time PCR were done using iQ SYBR Green Supermix (Bio-Rad Laboratories, Alcobendas, Spain) on iCycler equipment (7500 PCR Systems, Applied Biosystems, Life Technologies, Carlsbad, CA, USA). The Pit-1, MMP-1, MMP-13 and 18S samples were denatured at 94°C for 10 sec, annealed at 58, 59, 59 and 60°C, respectively, for 10 sec and extended at 72°C for 10 sec, for a total of 33, 35, 35 and 30 cycles, respectively.
The samples were quantified using Sequence Detection Software 1.4 (Applied Biosystems), with 18S as normalization control. The oligonucleotide sequences are described in Additional file 1.
Cell proliferation (MTT) assay
Cell proliferation experiments were carried out using MTT assay. MCF-7-hPit-1-luc cells, MCF-7-hPit-1-luc-shControl cells, MCF-7-hPit-1-luc-shMMP-1 cells, or MCF-7-hPit-1-luc-shMMP-13 cells (2.5 × 104 cells/ml) were seeded in a volume of 0.5 ml in 24-well tissue culture plates. The absorbance of the samples was recorded 48 hours after transfection at 590 nm in a multiwell plate reader (LB 940 Mithras, Berthold Technologies). Results were plotted as the mean ± SD values of quadruplicates from at least two independent experiments.
Western blot analysis
Western blotting was carried out as described elsewhere . Briefly, 60 μg of total protein was subjected to SDS-PAGE electrophoresis. Proteins were transferred to a nitrocellulose membrane, blocked, and immunolabeled overnight at 4°C with a primary antibody (detailed in Additional file 1). Then, the membrane was washed three times with PBS-Tween-20, and incubated with the appropriate secondary antibody for 1 hour. The signal was detected with the Pierce enhanced chemiluminescence (ECL) Western blotting substrate (Thermo Fisher Scientific, Rockford, IL, USA), and visualized by placing the blot in contact with standard X-ray film. Relative protein expression was quantified using the ImageJ software (National Institutes of Health, Bethesda, MD, USA) in at least three different blots, and values correspond to mean values of fold-change in relation to beta-actin values.
Chromatin immunoprecipitation (ChIP) assays were performed using the Upstate protocol as described previously . Diluted soluble chromatin fractions were immunoprecipitated with 1 μg polyclonal anti-Pit-1 antibody (Santa Cruz Biotechnologies, Heidelberg, Germany), or control human immunoglobulin G (IgG) (Sigma-Aldrich). The histone-DNA crosslinks were reversed by 4-hour incubation at 65°C. PCR was used to analyze the DNA fragments from ChIP assays. Primer sequences are detailed in Additional file 1.
Wound-healing and cell invasion assays
To perform the wound-healing assay, cells were seeded in 60 mm plates and allowed to reach confluence. Wounding was created using plastic pipette tip, and the cells were serum starved and treated with mitomycin C for 24 and 48 hours. Images were captured by an Olympus DP72 camera (Olympus, Tokyo, Japan), and the distance between the wound edges was measured. Cell invasion assay was performed in BD BioCoat matrigel invasion chambers according to the manufacturer’s instructions (BD Biosciences, San Agustin de Gualix, Spain), as previously described . Uncoated porous filters (8-μm pore size) were used for estimating cell migration, and matrigel-precoated filters were used for examining cell invasion. Values for cell invasion were expressed as the mean number of cells per field over four fields per filter for triplicate experiments.
All animal studies were approved by the University of Santiago de Compostela Ethics Committee for Animal Experiments. Female mice (age matched, between 6 and 8 weeks) homozygous for the severe combined immune deficiency (SCID) (CB17-Prkdcscid, Parc Research Biomedica, Barcelona, Spain) were used for xenografting studies.
Experimental metastasis assays were done as previously described . Briefly, 1 × 106 MCF-7-hPit-1-luc cells (n = 9, controls), MCF-7-hPit-1-luc-shControl cells (n = 8, controls), MCF-7-hPit-1-luc-shMMP-1cells (n = 8), or MCF-7-hPit-1-luc-shMMP-13 cells (n = 8) in 0.15 ml of PBS and matrigel (50:50, BD Biosciences) were injected into the mammary fat pad. At day 24 for control mice or day 33 for MMP-1 and MMP-13 knockdown mice, orthotopic primary mammary tumors were measured (as described below), and removed under anesthesia. Seventeen days later (day 41 after cell injection for controls, and day 50 for MMP-1 and MMP-13 knockdown mice) mice were sacrificed, and lungs removed and examined for metastasis. Xenografts were visualized by luminescence at days 10 (all groups), 24 (Pit-1-overexpressed mice), and 33 (Pit-1-overexpressed and MMP-1 or MMP-13 knockdown) using the In Vivo Imaging System (IVIS, Caliper Life Sciences, Alameda, CA, USA). An intensity map was obtained using the Living Image software (Caliper Life Sciences). The software uses a color-based scale to represent the intensity of each pixel (ranging from blue for low to red for high). Lung micrometastasis was explored in paraffin sections by hematoxylin and eosin (H&E) staining and cytokeratin 7 (CK7) immunostaining. Immunohistochemistry studies were performed in an Autostainer Link 48 (Dako, Glostrup, Denmark). FLEX Ready-to-Use Primary Antibodies to CK7, CK19, and ki-67 (Dako) were used. For detection we used EnVision FLEX/HRP (Dako). The number of metastatic foci was counted in lung after staining with H&E and immunostaining with CK7, and the size of each foci was evaluated by measuring its diameter (in μm) using the Olympus DP-Soft morphometry program in an Olympus DX51 microscope. For automated ki-67 scoring the ACIS III (Automated Cellular Imaging Systems, Dako-Agilent Technologies, Carpinteria, CA, USA) was used. The ACIS III system scans the slide and is capable of differentiating positive and negative nuclei. Six representative areas were selected in each section and the system generated an average score.
Patients and immunohistochemistry
One hundred and ten patients with invasive breast cancer (without distant metastasis at the time of initial diagnoses) treated at Fundación Hospital de Jove of Gijón (Spain), between 1990 and 2003, were selected based on the availability of clinical history and a minimum 5-year follow-up. The clinicopathological characteristics of patients and their tumors are shown in Table S1 in Additional file 3. Women were treated according to our institutional guidelines. The study adhered to national regulations and was approved by our regional Ethics and Investigation Committee (Comité Ético de Investigación Clínica Regional del Principado de Asturias). Breast carcinoma tissue samples were obtained at the time of surgery. Prior informed consent was obtained from patients. Routinely fixed (overnight in 10% buffered formalin), paraffin-embedded tumor samples stored in our pathology laboratories were used. Histopathologically representative tumor areas without necrosis were defined on H&E-stained sections. Serial 5-μm sections were consecutively cut with a microtome (Leica Microsystems, Barcelona, Spain) and transferred to adhesive-coated slides. Imunohistochemistry was done on these sections using a TechMate TM50 autostainer (Dako) as previously described . A polyclonal anti-Pit-1 (Santa Cruz Biotechnology), monoclonal anti-MMP-1 (NeoMarkers, Fremont, CA, USA) and anti-MMP-13 (Santa Cruz Biotechnology) antibodies were used. To enhance antigen retrieval, tissue sections were treated in a PT-Link™ (Dako) at 97°C for 20 minutes, in citrate buffer pH 6.1 for MMP-1 and in Tris-EDTA buffer pH 9 for MMP-13, and then washed in PBS. Endogenous peroxidase activity was blocked by incubating the slides in peroxidase-blocking solution (Dako) for 5 minutes. The EnVision Detection Kit (Dako) was used as the staining detection system. Sections were counterstained with hematoxylin, dehydrated with ethanol, and permanently coverslipped. For each antibody preparation, the location of immunoreactivity, percentage of reactive area and intensity were determined. All cases were semiquantified for each protein-stained area. An image analysis system with the Olympus BX51 microscope and soft analysis (analySIS™, soft imaging system) were used as follows: tumor sections were stained with antibodies according to the method explained above and counterstained with hematoxylin. There were different optical thresholds for both stains. Each slide was scanned with a 400X power objective and four fields were selected per case to determine protein-reactive areas. The computer program selected and traced a line around antibody-reactive areas (higher optical threshold: red spots), with the remaining, nonstained areas (hematoxylin-stained tissue with lower optical threshold) standing out as a blue background. Area ratios of stained (red) versus nonstained (blue) were determined for all fields. To evaluate immunostaining intensity we used a numeric score from 0 to 3: 0 = no reactivity; 1 = weak reactivity; 2 = moderate reactivity; and 3 = intense reactivity. Using an Excel spreadsheet, the score of one field was obtained by multiplying the intensity score (I) by the percentage of reactivity area (PA) (total score: I × PA). In addition, for each tumor the mean score of the four fields evaluated was calculated. We also evaluated the immunohistochemical staining exclusively in cancerous cells or in stromal cells (mononuclear inflammatory cells, (MICs)- and fibroblast-like cells), and every evaluated field contained at least 10 stromal cells. We considered immunostaining to be positive when at least 10% of cells showed positivity. We distinguished stromal cells from cancer cells because the latter are larger in size, and because fibroblasts are spindle-shaped whereas mononuclear inflammatory cells are rounded. Moreover, while cancer cells are arranged forming either acinar or trabecular patterns, stromal cells are spread.
Values are expressed as mean ± standard deviation (SD). Means were compared using two-tailed Student’s t test or one-way ANOVA, with the Tukey-Kramer multiple comparison test for post hoc comparisons. After analyzing the human tumor distribution of score values by the Kolmogorov-Smirnov test, nonparametric methods were used to analyze the data. Immunostaining score values for each protein were expressed as a median (range). Correlation between score values was calculated by using the Spearman correlation test. Comparison of immunostaining values between groups was done with the Mann-Whitney or Kruskal-Wallis tests. Statistical results were corrected applying Bonferroni’s correction. P values of less than 0.05 were considered statistically significant. The PASW Statistics 18 program was used for all calculations (SPSS Inc, Chicago, IL, USA).
Pit-1 regulates MMP-1 and MMP-13 mRNA and protein levels in MCF-7 and MDA-MB-231 cell lines
Regulation of MMP-1 and MMP-13 by the Pit-1 transcription factor at transcriptional level
Knockdown of MMP-1 and MMP-13 reduces motility and invasion in Pit-1-overexpressing cells
MMP-13 knockdown in a xenograft mice model with Pit-1 overexpression blocked breast cancer invasiveness
After excision of tumors, mice lived until day 41 (Pit-1 and Pit + shControl) and 50 (Pit-1 + shMMP-1 or shMMP-13) after injection of MCF-7 cells. Mice were then sacrificed and lungs were removed, fixed, paraffin-embedded, sectioned, and stained with H&E and specific antibodies for immunohistochemistry analysis. Five out of the nine mice injected with Pit-1-overexpressing cells (controls), six out of the eight mice injected with Pit-1-overexpressing cells + shControl, and four out of the eight mice injected with Pit-1-overexpressing + shMMP-1 transfected cells developed micrometastases in lung, while none of the eight mice injected with the MCF-7 cells transfected with the pTRE2-hPit-1-Luc-shMMP-13 showed micrometastases in lungs (Figure 5D). Micrometastases in lungs showed immunopositivity for CK-7, and CK-19 (Figure 5E). These antibodies react only with human and not with mouse cytokeratins. Neither size (Pit-1: 166.2 ± 174.1 μm, Pit-1 + shControl: 183.7 ± 79.2 μm, and Pit-1 + shMMP-1: 185.5 ± 132.1 μm) nor number of metastases (Table S2 in Additional file 7) were significant among groups of mice.
Expression of both MMP-1 and MMP-13 correlates with Pit-1 expression in human breast tumors
Relationships between MMP-1 and MMP-13 expression by each cell type and Pit-1 global expression in 110 breast carcinomas
Pit-1 median (range)
5.6 (0 – 13.1)
38.3 (0 – 194.5)
21.5 (0 – 89.6)
27.4 (0 – 194.5)
31.8 (0 – 124.5)
36.7 (0 – 194.5)
12.2 (0 – 135.6)
45.1 (0 – 194.5)
32.5 (0 – 194.5)
43.9 (0 – 143.2)
33.8 (0 – 194.5)
45.0 (0 – 132.3)
In addition, we evaluated the potential association between score values from Pit-1, MMP-1 and MMP-13 and relapse-free survival in all patients included in the present study. The determination of optimal cutoff values for Pit-1 and MMP-1 in breast tumors was done for predicting recurrence. P values obtained for each cutoff value are plotted against the value itself (Figure S6A-B in Additional file 9). This statistical analysis showed no significant association between score values of MMP-13 and recurrence (data not shown). However, our data showed that high score values for Pit-1 and MMP-1 were significantly associated with recurrence. This analysis led us to define a score value of 22 for Pit-1 (χ 2 = 4.71, P = 0.03) and of 130 for MMP-1 (χ 2 = 6.52, P = 0.011) as optimal cutoff points. These values identified 73 (66.4%) and 40 (36.4%) patients, respectively, with a high probability of recurrence (Figure S6C-D in Additional file 9). We also determined the relapse-free survival curves for patients with breast carcinomas based on the combination of these optimal cutoff points. Both high Pit-1 and MMP-1 expression identified a patient subgroup (n = 31, 28.2%) with the highest probability of recurrence (P = 0.004) (Figure S6E in Additional file 9). The relationship between Pit-1, MMP-13 expression and prognosis was determined in MICs. Breast tumors containing MICs that were positive for Pit-1 and MMP13 (n = 24, 21.8%) had a high probability of recurrence (P = 0.010) (Figure S6F in Additional file 9).
In this study we found that Pit-1 regulated MMP-1 and MMP-13 in breast cancer cells at transcriptional level. Our data indicated that knockdown of MMP-13 blocked mammary cancer invasiveness to lung in xenografts with Pit-1 overexpression. In addition MMP-1 and MMP-13 positive expression in fibroblasts and tumor cells is positively correlated with high Pit-1 score values in human breast tumors.
The role of Pit-1 in breast carcinogenesis has recently been demonstrated . Pit-1 overexpression in the mammary fat pads of SCID mice is related with metastasis in lung, and elevated levels of Pit-1 in node-positive breast cancer patients are positively correlated with distant metastasis. Interestingly, moderately elevated Pit-1 expression levels were found in 36% of patients with invasive ductal carcinoma of the breast . However, the mechanism of Pit-1 metastasis induction is unknown. Given that MMPs are key proteins involved in the metastatic process, in the present study we evaluated the role of Pit-1 on the expression and biological activity of two collagenases, MMP-1 and MMP-13, whose role in several processes of the metastatic disease is well known . In the MCF-7 and MDA-MB-231 human breast cancer cell lines, we showed that Pit-1 regulates MMP-1 and MMP-13. This regulation is carried out at transcriptional level, as demonstrated by ChIP and luciferase reporter assays. Knockdown of either MMP-1 or MMP-13 significantly reduced motility and invasion capacity of MCF-7 cells with Pit-1 overexpression. These are relevant findings considering that MMP-1 and MMP-13 have exceptionally wide substrate specificity when compared with other MMPs, and because these molecules are implicated in the degradation of the connective stromal tissue and invasion of the basement membranes, which are key actions in the metastatic process. MMP-1 cleaves several components of the extracellular matrix, including collagen type I (the principal component of the connective tissue), II, III, VII, VIII, and IX, aggrecan, as well as serine protease inhibitors, and α2 macroglobulin [3,27]. MMP-13 efficiently degrades the native helix of fibrillar collagens with preferential activity on type II collagen [28,29]. However, MMP-13 is also able to degrade several other extracellular matrix proteins in vitro, including collagens type IV, X, and XIV; fibronectin; tenascin; and fibrillin [30,31]. In addition, it has been shown that MMP-13 plays a central role in the MMP activation cascade, both activating and being activated by other MMPs (MMP-14, 2 or 3) .
In order to explore whether knockdown of each of these MMPs could also modify the metastatic potential of Pit-1-overexpressing MCF-7 cells in vivo, we evaluated the effect of MMP-1 and MMP-13 knockdown in an SCID mice tumor xenograft model. Our data indicated that knockdown of either MMP-1 or MMP-13 reduced cell proliferation, as demonstrated by the low ki-67 expression in tumors compared to controls. Probably as a consequence of this, tumor growth was also decreased in xenografts. This was previously demonstrated with MMP-13 in the human squamous cell carcinoma xenografts , and with MMP-1 in the human breast carcinoma cell line MDA-MB-231 in the mammary fat pad xenograft model , and recently in the MMP-1a knockout mouse, which has significantly decreased lung tumor growth and angiogenesis . On the contrary, overexpression of MMP-1 (in conjunction with other genes) in human breast carcinoma cells increased xenograft growth rates , and facilitated the assembly of new tumor blood vessels, the release of tumor cells into the circulation, and the breaching of lung capillaries by circulating tumor cells to seed pulmonary metastasis . In human patients, it has recently been demonstrated that MMP-1 expression by MICs from sentinel lymph nodes (SLNs) was significantly associated with metastatic spread to non-SLNs, suggesting that the degradation capacity of MMP-1 in the extracellular matrix may be responsible for promoting tumor spread via the lymph nodes .
Our data indicate that MMP-13 knockdown completely blocked cancer cell invasiveness to lung, suggesting that MMP-13 is a necessary mediator of Pit-1 induction of breast metastasis to lung. This experimental finding could be in line with previous clinical data indicating that MMP-1 and MMP-13 seem to be related with different metastatic profiles in breast cancer. Whereas MMP-13 expression (but not MMP-1 expression) was significantly and independently associated with the occurrence of distant metastasis in breast cancer [7,38], MMP-1 expression was strongly associated with the metastatic progression across the axillary lymphatic system . These data seem to support the hypothesis that hematogenous metastasis and regional lymph node metastasis are different processes of tumor spread [39,40], which may require different substrate-specific degradation.
In the present study we also found positive correlations between global expression (score values) of Pit-1 and either MMP-1 or MMP-13 expression in human primary breast carcinomas. Nevertheless, the great complexity of interactions between cancerous cells and stromal cells in the context of these malignancies should be taken into account. Thus, for example, it has been described that MMP-13 is produced by fibroblast-like cells located in the stromal compartment of the breast cancer tissue , whereas other studies have indicated that MMP-13 is synthesized predominantly by epithelial tumor cells [8,42]. In our study, both MMP-1 and MMP-13 expression was observed in all three cell types studied (tumor cells, fibroblasts, and MICs), but we only found a significant correlation between MMP-1 and MMP-13 positive expression and high Pit-1 levels in tumor cells and fibroblasts. It is well known that tumor stroma play a fundamental role in tumor growth, invasion and dissemination, and that fibroblasts are the prevailing component of tumor stroma [7,43-45]. Thus, it is tempting to speculate that high Pit-1 levels in fibroblasts from breast tumors could induce increased MMP-1 and MMP-13 expression, which in turn may increase collagen degradation and facilitate the dissemination of tumor cells to lung. However, only MMP-13 knockdown blocks dissemination of Pit-1-overexpressing tumor cells to lung, suggesting that MMP-13 mediates in this process.
In summary, our data indicates that Pit-1 increases and activates MMP-1 and MMP-13 expression acting at transcriptional level by binding to their promoters. In mice, knockdown of MMP-13 blocks Pit-1-induced breast cancer cell invasiveness induced by Pit-1. Finally, in human breast tumors there is a significant correlation between Pit-1 and MMP-1 and MMP-13 expression in both tumor and fibroblast cells, suggesting a relationship between Pit-1 and MMPs expression in Pit-1-induced metastasis to lung.
Breast cancer is a heterogeneous illness that encompasses several distinct disease entities, often referred to intrinsic subtypes of breast cancer. It has previously been demonstrated that patients with breast cancer and overexpression of the Pit-1 transcription factor are associated with higher occurrence of distant metastasis, but the mechanisms remain unknown. The present study demonstrates that Pit-1 increases MMP-1 and MMP-13 expression at transcriptional level. Given that these MMPs have been related to breast cancer metastasis, we explored the effect of Pit-1 overexpression and MMP-1 or MMP-13 knockdown in an SCID mouse xenograft tumor model. Both Pit-1 overexpression and Pit-1 overexpression together with MMP-1 knockdown induced metastasis in lung. On the other hand, Pit-1 overexpression and MMP-13 knockdown completely blocked breast cancer invasiveness to lung. We further showed that Pit-1 positively correlated with MMP-1 and MMP-13 expression in 110 human breast tumors, and positive Pit-1 expression also correlated with positive expression of MMP-1 and MMP-13 in tumor cells and fibroblasts. Taken together, our data point to MMP-13 as a target in breast tumors with Pit-1 overexpression.
Dulbecco’s modified Eagle’s medium
fetal bovine serum
hematoxylin and eosin
mononuclear inflammatory cell
polymerase chain reaction
POU class 1 homeobox 1
POU class 1 homeobox 1
severe combined immunodeficiency
sodium dodecyl sulfate polyacrylamide gel electrophoresis
small hairpin RNA
small interfering RNA
sentinel lymph node
tissue inhibitors of metalloproteases
This study was supported by grants from Ministerio de Economia y Competividad (SAF2012-38240) and Xunta de Galicia (10PXIB208230PR) to R.P-F, and to F.J.V. from Fondo de Investigación Sanitaria del Instituto de Salud Carlos III (FISPI13/02745). S.S. thanks the Fundacion Cientifica de la Asociacion Española Contra el Cancer for a postdoctoral fellowship.
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