Mena deficiency delays tumor progression and decreases metastasis in polyoma middle-T transgenic mouse mammary tumors
© Roussos et al.; licensee BioMed Central Ltd. 2010
Received: 11 August 2010
Accepted: 25 November 2010
Published: 25 November 2010
The actin binding protein Mammalian enabled (Mena), has been implicated in the metastatic progression of solid tumors in humans. Mena expression level in primary tumors is correlated with metastasis in breast, cervical, colorectal and pancreatic cancers. Cells expressing high Mena levels are part of the tumor microenvironment for metastasis (TMEM), an anatomical structure that is predictive for risk of breast cancer metastasis. Previously we have shown that forced expression of Mena adenocarcinoma cells enhances invasion and metastasis in xenograft mice. Whether Mena is required for tumor progression is still unknown. Here we report the effects of Mena deficiency on tumor progression, metastasis and on normal mammary gland development.
To investigate the role of Mena in tumor progression and metastasis, Mena deficient mice were intercrossed with mice carrying a transgene expressing the polyoma middle T oncoprotein, driven by the mouse mammary tumor virus. The progeny were investigated for the effects of Mena deficiency on tumor progression via staging of primary mammary tumors and by evaluation of morbidity. Stages of metastatic progression were investigated using an in vivo invasion assay, intravital multiphoton microscopy, circulating tumor cell burden, and lung metastases. Mammary gland development was studied in whole mount mammary glands of wild type and Mena deficient mice.
Mena deficiency decreased morbidity and metastatic dissemination. Loss of Mena increased mammary tumor latency but had no affect on mammary tumor burden or histologic progression to carcinoma. Elimination of Mena also significantly decreased epidermal growth factor (EGF) induced in vivo invasion, in vivo motility, intravasation and metastasis. Non-tumor bearing mice deficient for Mena also showed defects in mammary gland terminal end bud formation and branching.
Deficiency of Mena decreases metastasis by slowing tumor progression and reducing tumor cell invasion and intravasation. Mena deficiency during development causes defects in invasive processes involved in mammary gland development. These findings suggest that functional intervention targeting Mena in breast cancer patients may provide a valuable treatment option to delay tumor progression and decrease invasion and metastatic spread leading to an improved prognostic outcome.
Metastasis is the primary cause of death from breast cancer, the most common form of cancer affecting women in the United States, and the second leading cause of cancer related deaths in women around the world . Tumor cells make use of different cellular processes to execute the steps of metastasis: invasion, intravasation, extravasation, tumor cell dissemination, extravasation and growth of distant metastases . Use of multiphoton intravital microscopy enabled the observation of migration of tumor cells across the endothelial barrier into blood vessels at sites containing at least one peri-vascular macrophage . This led to the development of an in vivo invasion assay used to capture live, invasive and metastatic tumor cells from the primary tumor for analysis . Furthermore, the study of invasion, intravasation and metastasis using both methods has shown the involvement of a paracrine loop between macrophages and tumor cells that secrete EGF or colony stimulating factor-1 (CSF1), respectively [3, 4].
Expression profiling of invasive tumor cells captured using the in vivo invasion assay revealed the upregulation of several key cytoskeletal regulatory proteins involved in cell motility [5–10]. Mena was among the proteins that comprise what was termed the Invasion Signature and has since been shown to be upregulated in rat, mouse and human mammary tumors and to correlate with metastatic risk [5, 10–12]. Evaluation of high risk primary and metastatic cervical, colorectal and pancreatic cancers have also shown enhanced expression of Mena as compared to low risk cases [11, 13–15]. Mena has been used in the development of a prognostic marker of hematogenous metastasis called TMEM (Tumor Micro-Environment for Metastasis), shown to be associated with risk of metastasis in breast cancer patients independently of traditional prognostic markers .
Mena (also known as ENAH-enabled homolog) is a member of the Ena/VASP family of actin-regulatory proteins that function in multiple different cell types to regulate cell morphology and motility [16, 17]. Similar to other Ena/VASP proteins, Mena regulates the geometry and assembly of actin filament networks through binding of profilin and both G- and F-actin, the ability to promote filament elongation through monomer delivery and anti-capping activity [18, 19], and the ability to reduce the density of Arp2/3-mediated branching [17, 19–22].
There are at least two alternative splice variants of Mena in tumor cells; the two best characterized isoforms are Mena invasive (INV), found exclusively in invasive tumor cells and Mena11a, found in primary tumor cells but lost in invasive cells . Recent studies in rodent models show an increase in metastasis upon forced expression of Mena and MenaINV without any affect on primary tumor growth . This increase in metastasis was found to arise from increased EGF-induced invasion, tumor cell protrusion and matrix degradation activity by invadopodia .
To determine if Mena is required for tumor progression and metastasis, we used previously generated Mena Null mice  and intercrossed them with transgenic mice carrying the mammary tumor virus (MMT-V)-polyoma middle T antigen (PyMT) . Mammary tumors growing in the PyMT expressing mice go through distinct morphologic stages of tumor progression comparable to progression of human breast disease and develop spontaneous metastases following disease progression . Thus, they provide an excellent model for the investigation of the role of Mena in tumor progression and metastasis.
All Mena isoforms are completely eliminated in Mena homozygous mice and reduced in heterozygote mice . Mena homozygous mutant (Mena Null) mice are fully viable, but slightly smaller than their littermates until adulthood . Mena Null mice exhibit a variety of subtle nervous system defects such as misrouted axon fiber tracts in the cortex including the major brain commissures, the corpus callosum and hippocampus; these phenotypes all become more severe and other structures such as the optic nerve and spinal nerves become disrupted when the other two Ena/VASP paralogs are deleted along with Mena [24, 27–29]. The invertebrate orthologs of Mena have been shown to function downstream of axonal guidance receptors DCC and Robo [30, 31], which have also been implicated in the morphogenesis of the mammary gland . Little, however, is known about the role of Mena in the developing mammary gland. Since tumor cells are thought to acquire the ability to invade surrounding stroma via a re-awakening of developmental processes [33–35], the investigation of Mena in mammary gland development could provide insight into Mena's role in tumor cell invasion and dissemination in mammary tumors.
We provide evidence that, in Mena Null mice, overall tumor burden is unaffected while tumor latency is increased and mortality is decreased. Additionally, we show that in Mena Null mice, invasion, intravasation, motility and metastasis are significantly decreased. Deficiency of Mena also leads to decreased terminal end bud formation and branching during mammary gland development but has no affect on ductal growth. These findings provide important insight into the functions of Mena during tumor progression and metastasis and provide evidence that these malignant processes could represent the revival of Mena's role in processes resembling mammary gland development within the primary tumor. Our study provides evidence that inhibition of Mena activity may be a useful approach to prevention of metastasis.
Materials and methods
Animal models and assays for tumor progression
All studies were carried out using protocols approved by the Albert Einstein College of Medicine Animal Care and Use Committee. GFP labeled macrophages were recreated on the FVB Tg(Csf1r-eGFP)1 background using an identical construct and strategy as published . CFP labeled mammary carcinoma cells were generated on the FVB background by co-injection of Tg(MMTV-iCRE)1jwp together with Tg(PCAG-loxp-CAT-stop-loxp-CFP) plasmids by conventional methods in the Albert Einstein Transgenic Mouse Facility and were similar to previously described mice . These mice were then crossed with previously described Mena heterozygotes . Germline transmission of alleles was verified by PCR using the following primer sequences: MMTV-iCRE-PCAG-loxp-CAT-stop-loxp-CFP: (forward) CAG GGC CTT CTC CAC ACC AGC, (reverse) CTG GCT GTG AAG ACC ATC, cFMS-GFP: (forward) TCA TTC CAG AAC CAG AGC, (reverse) TGC TCA GGT AGT GGT TGT CG. Primers sequences used to identify transmission of the disrupted Mena allele can be provided upon request. All PyMT mouse tumors were palpated beginning at six weeks of age and measured by calipers to monitor tumor formation.
Orthotopic PyMT xenograft tumors were derived from subcutaneous injection of 1 × 106 CFP-PyMT primary tumor cells FAC sorted from PyMT/Mena wild type (WT), PyMT/Mena heterozygote (Het) and PyMT/Mena homozygous (Null) mice into the mammary gland of five-to-seven-week-old female severe combined immune deficient (SCID) mice purchased from the National Cancer Institute . The purpose of this study was to confirm that the PyMT tumor cells injected into SCID mice (all with an identical genotype) that were also wild type for Mena, would grow and metastasize in a manner similar to that identified in their parental mouse strain. Once tumors reached 2 cm3 in size (approximately 8 to 12 weeks after injection) animals were used for the indicated experiments.
Orthotopic MTLn3 xenograft tumors were derived from mammary gland injection of 1 × 106 MTLn3 rat adenocarcinoma cells or MTLn3 cells forced to express Mena, into SCID mice [23, 38]. Once tumors reached 2 cm3 in size (approximately four weeks after injection) animals were used for the indicated experiments. MTLn3-Mena expressing cells expressed the Mena protein four-fold over endogenous levels to mimic Mena over-expression previously identified in invasive tumor cells . For a full description of cell line derivation, clonal selection and culturing of these cell lines see reference .
Staging of mammary tumors from PyMT mice was done according to criteria previously described . Tumors growing in PyMT mice have been shown to progress at different rates, for example at 10 weeks of age it is common to find some tumors that are hyperplastic while other tumors from the same animal have already progressed to invasive carcinoma . Additionally, even within the same tumor there can be regions of that tumor that are normal and regions that show varied histological progression . Therefore, we staged multiple different tumors and different regions within each tumor from each animal evaluated (15 to 20 of each PyMT Mena WT, Het and Null mice) at 10 weeks of age to ensure identification of the most progressed histology. Tumor stage was finally determined and classified based on the most advanced histological stage identified, (that is, a mouse with regions of hyperplasia and invasive carcinoma either within the same tumor or in different tumors, the stage of progression was reported as carcinoma). Evaluation of staging was performed by double blind. To determine the number of tumor cells in circulation, blood was drawn from the right ventricle of the heart in anesthetized mice harboring at least one tumor that was 2 cm3 in size as previously described . Single tumor cells were counted seven days after plating; all cells counted were CFP positive.
Metastasis characterization and Immunohistochemistry (IHC)
Mammary tumors, mammary glands and lungs were fixed in 10% buffered formalin and routinely embedded in paraffin. Spontaneous lung metastases (>2 mm) were evaluated in formalin fixed, paraffin embedded and hematoxylin and eosin stained lung tissue. To quantify lung metastases, eight 10-μm paraffin sections were cut at 50 μm apart and placed on slides for further staining. Lung metastases were counted in all five lung lobes. Metastases spanning multiple sections were only counted in the first section where they were identified. Evaluation of metastases was performed double blind. Fixed mammary tumors matched for age and tumor stage were stained for vasculature using an antibody against CD-34 (1:100) (Invitrogen, Carlsbad, CA, USA), five 200× fields were analyzed in five animals per genotype to determine differences in vascular density. Fixed mammary glands stained with F4/80 at 1:50 (a gift from Dr. Richard Stanley) to identify macrophages as previously described . F4/80 has been shown to specifically identify mononuclear mouse macrophages  and has been used in IHC and FAC sorting to identify macrophages by many different groups [42–44]. Evaluation of the expression of Mena in mammary glands was evaluated in formalin fixed, paraffin embedded tissue from six-week-old PyMT negative Mena WT, Het and Null mice. Primary antibody against Mena was used at (1:1000) , antigen retrieval was done using buffered citrate, and DAB (Invitrogen) was used to visualize staining, hematoxylin was used as a counterstain. All slides were imaged using Zeiss AxioObserver.Z1 5× DIC1, EC Plan-Neofluar 10×/0.3 Ph1, EC Plan-Neofluar 20×/0.5 Ph2 M27, EC Plan-Neofluar 40×/0.75 Ph2 M27, EC Plan-neofluar 63×/1.4 Oil and an AxioCamHR3. All objectives and microscopes were manufactured by Zeiss, Oberkochen, Germany.
Whole mount mammary gland preparation and staining
The evaluation of mammary gland maturation was performed on five of each PyMT negative Mena WT, Het, and Null mice at 6 and 10 weeks of age. Following euthanasia, the fourth abdominal mammary glands were surgically removed and placed on a glass slide and fixed and stained as previously described . Ductal length, terminal end bud counts and branching were determined as previously described . To determine normalized macrophages/TEB area, macrophages were counted around individual TEBs and the area of each of these TEBs was measured. Data represent the number of macrophages/TEB area in pixels and are normalized to a constant area. To determine the average number of macrophages/area, macrophages were counted within an identical fixed area either surrounding TEBs or in stroma (without any TEBs). Data represent raw counts of macrophages within this fixed area. A minimum of 10 fields were counted from five mice per genotype for these experiments.
Intravital multiphoton imaging was performed on 15 of each PyMT Mena WT, Het and Null mice as described previously using a 20× 1.95 NA water immersion objective with correction lens [46, 47]. Time-lapse movies were analyzed for frequency of motility in three dimensions and through time using ImageJ . A cell movement event was defined as a translocation ≥1 cell diameter within a field, a field is defined as 100 μm *512 × 512* minute. To acquire this size volume took two minutes. All movies of cell movements were 30 minutes in duration.
In vivoinvasion assay
The in vivo invasion assay was performed in 5 to 10 mice per condition as previously described . Briefly, needles were held in place by a micromanipulator around a single mammary tumor of an anesthetized mouse. Needles contained a mixture of Matrigel™, 25 nM EGF, buffer and EDTA. After four hours of cell collection the contents of the needles were extruded. Cells were then stained with DAPI and counted using an Olympus IX70 inverted microscope with a 10× NA 0.30 objective.
Statistical significances were determined using unpaired, two-tailed Student's t-tests assuming equal variances and an alpha level of 0.05 for all experiments unless otherwise indicated. For lung metastasis and assays evaluating the number of circulating tumor cells in the blood non-parametric the Mann Whitney Wilcoxon rank sum test was used. For Kaplan-Meier plots Log-rank (Mantel-Cox) t-test was performed.
Deficiency of Mena increases tumor latency and decreases morbidity
Mice were considered 'not moribund' until they either died or had to be euthanized due to illness or immobilization as a result of their tumor burden. Mena Null mice had a later tumor onset than did the Mena WT or Het mice (Figure 1A) and survived longer than either Mena WT or Het mice that reached the tumor size limit or died (P-value = 0.01, 0.03 respectively) (Figure 1B). There were no significant differences in morbidity between Mena WT and Het mice (P-value = 0.91) (Figure 1B).
In those mice with tumors at 10 weeks of age, Mena Null mice had significantly fewer tumors/animal (Figure 1C). Additionally, tumor growth was significantly decreased in Mena Null mice that were growing tumors at both 8 and 10 weeks of age. These results reflect the increased tumor latency in Mena Null mice observed between 60 and 100 days as shown in Figure 1A. Interestingly, as Mena Null mice aged, the number of tumors per animal as well as tumor growth were not significantly different as compared to Mena WT or Het mice (Figure S1 in Additional file 1 Figure 1D).
Deficiency of Mena slows progression to invasive carcinoma
Staging of tumors from PyMT Mena WT, Het and Null mice at 10 weeks of age showed that by 10 weeks 80% of Mena WT and Het mice had at least one tumor or parts of a tumor that had progressed to carcinoma (Figure 2C). Neither Mena WT nor Het mice had tumors that could be classified as hyperplastic due to the presence of more advanced stages of progression within these tumors (Figure 2C). Twenty percent of both genotypes of mice had tumors staged as adenoma (Figure 2C). Interestingly, 21% of Mena Null mice had mammary tumors that were staged as hyperplastic, 43% that were staged as adenoma and only 36% that were staged as carcinoma (Figure 2C). Additionally, we observed the presence of normal tissue in all animals (Mena WT, Het and Null) regardless of tumor stage. Thus, deficiency of Mena delays but does not prevent progression to malignancy (Figure 2C).
Deficiency of Mena dampens EGF-induced in vivo invasion and motility
Another measure of invasive potential is cell migration within and out of the primary tumor into the stroma. Prior work showed increased motility in orthotopic mammary tumors derived from injection of MTLn3 rat adenocarcinoma cells forced to express Mena . We used intravital multiphoton microscopy to investigate the effect of Mena deficiency on tumor cell motility in vivo in PyMT Mena WT, Het and Null mice. The average number of tumor cells moving per field was significantly decreased in Mena Null mice as compared to both Mena WT and Het mice (Figure 3B, C Additional files 2, 3, 4).
Deficiency of Mena decreased intravasation and metastasis to the lung
Previous studies show that the forced expression of Mena in orthotopic mammary tumors derived from tumor cell injection into the mammary gland of SCID mice, significantly increased metastasis to the lung . Thus, we hypothesized that deficiency of Mena would reduce the number of metastases to the lung. Evaluation of the number of lung metastases in PyMT from Mena WT, Het and Null mice, whose tumors were stage matched for carcinoma, showed that Mena Null mice had significantly fewer lung metastases as compared to Mena WT and Het mice (Figure 4B). Additionally, there were no differences observed in appearance (size and location) of lung metastases (Figure 4C).
These findings suggest that deficiency of Mena significantly decreases metastasis by affecting invasion, intravasation and motility within the primary tumor.
Mena Null xenografts show decreased intravasation and lung metastasis
Given that a deficiency of Mena had such a profound effect on intravasation as compared to Mena WT mice (Figures 4A and 5A) we investigated if there were any differences in vascular density. Immunohistochemical staining of blood vessels was performed on primary tumors from PyMT Mena WT and Null mice and we found that Mena Null mice had significantly fewer vessels located throughout the tumors as compared to tumors from Mena WT mice (Figure S4A in Additional file 1). Immunohistochemical staining of blood vessels was also performed on primary tumors from xenograft mice derived from injection of tumor cells isolated from PyMT Mena WT and Null mice and we found that there was no difference in vascular density in Mena Null tumors as compared to Mena WT (Figure S4B in Additional file 1). Lastly, immunohistochemical staining of blood vessels performed on primary tumors from xenograft mice derived from injection of MTLn3 and MTLn3-Mena overexpressing tumor cells revealed no difference in vascular density in Mena overexpressing tumors as compared to MTLn3 control (Figure S4C in Additional file 1). Hence, the large difference in intravasation score observed in these different cases is not explained by differences in microvessel density alone.
Mena deficiency reduces terminal end bud formation and branching during mammary gland development
Through the use of newly developed technologies including, high density mircroarray based expression profiling, intravital imaging and the collection of live invasive tumor cells from primary tumors, transiently expressed genes, such as Mena were identified as modulators of metastasis . Such findings support the theory of metastasis whereby a population of tumor cells exhibits metastatic capabilities very early in tumor progression as a result of transient changes in gene expression. The Invasion Signature is a set of transiently regulated genes in metastatic tumor cells that control their chemotactic and migratory behavior; Mena has been identified as a master regulator of cell motility pathways in the Invasion Signature [8, 23]. The upregulation of Mena in rat, mouse, and human mammary tumors [5, 11, 12], use of Mena in the prognostic marker TMEM , and Mena's significant role in the regulation of metastatic progression [19, 21, 23], render Mena an interesting therapeutic target. Our findings indicate that elimination of Mena activity could be a useful goal for therapy of metastatic disease.
Mena deficiency delays tumor progression and decreases invasion, intravasation, and metastases
Our results demonstrate that loss of Mena in a transgenic mouse model of breast carcinoma decreases metastatic potential due to decreased invasion and intravasation. Importantly, Mena Null mice remain healthy for longer despite their equally large tumor burden as compared to Mena WT and Het mice. While a delay in onset of tumor development is observed, this does not seem to be a major contributing factor affecting the drastic increase in survival given that Mena Null mice still show histological progression to invasive carcinoma. Thus, the dramatic increase in survival of Mena Null mice is most likely associated with the significant decrease in formation of lung metastases. We propose that the increase in tumor latency may be a result of decreased ductal branching and TEB formation identified in Mena Null mammary gland development. Additionally, the decrease in tumor latency seen in Mena Het animals could be attributed to the increase in TEB identified during mammary gland development. This phenomenon will be investigated in future studies.
Given that metastasis is responsible for the majority of cancer related deaths , it is important to determine the mechanisms involved in this process. Previous studies in PyMT mice illustrate the contribution of the microenvironment to metastatic potential and describe a paracrine loop whereby invasion, intravasation and metastasis involve signaling between macrophages and tumor cells where these cells secrete EGF and CSF1 respectively [3, 4]. Our findings illustrate that a deficiency in Mena leads to blunted EGF induced in vivo invasion, intravasation and lung metastases.
Previous studies have shown that the motility of tumor cells correlates with their ability to intravasate and that both processes require the participation of macrophages . Multiphoton intravital microscopy is a powerful tool used to study tumor cell behavior within primary tumors and enables the visualization of differences in motility within the tumor microenvironment . We demonstrate that tumor cells in Mena Null mice move significantly less than tumor cells in Mena WT and Het mice, another contributing factor toward decreased metastatic potential. These findings are consistent with an increase in movement and velocity in MTLn3 rat adenocarcinoma cells forced to express Mena isoforms . Additionally, given that there is no difference in vascular density in xenograft tumors derived from injection of Mena Null and Mena WT cells obtained from PyMT animals or in xenografts derived from MTLn3 Mena overexpressing cells, we conclude that the decrease in intravasation observed in both transgenic PyMT Mena Null mice and in Mena Null xenograft mice is the result of a Mena deficiency in tumor cells leading to a change in their intravasation phenotype and is not due to changes in vascular density. We hypothesize that the decrease in vasculature observed in transgenic PyMT Mena Null mice is related to the deficiency of Mena in endothelial cells and requires further investigation.
Mena deficiency delays ductal branching and terminal end bud formation during mammary gland development
Control of tumor cell invasion has been linked to the reawakening and seizure of developmental programs by malignant cells [33, 35]. Our findings, that loss of Mena decreases ductal branching and TEB formation (both considered to be invasive processes in the developing mammary gland), and blunts invasion during metastatic progression, suggests that Mena's role in developmental invasion is recapitulated during metastatic progression. Interestingly, during nervous system development, Ena/VASP proteins are required for normal response to the axon guidance factors SLIT and Netrin [30, 59]. Both SLIT and Netrin have been shown to play an important role in mammary gland morphogenesis [32, 60, 61] and in regulation of cell invasion across basement membranes . Thus, we speculate that Mena's affects on invasion during development and metastasis may involve SLIT and Netrin. Future studies will evaluate this hypothesis and determine the mechanism behind the observations reported here. Additionally in humans, intraductal spreading can be observed in malignant lesions classified as ductal carcinoma in situ (DCIS) and may be related to the process of ductal branching during development [63, 64]. Since DCIS shows no evidence of invasion into surrounding stroma, a decrease in ductal branching in Mena Null mice could contribute to the delay in histological tumor progression but not metastatic progression. Additionally, intraductal spreading as identified in human cancer was not observed in the PyMT model of invasive breast carcinoma (Rani Sellers, personal communication) therefore, it will be interesting to determine if Mena deficiency affects this phenomenon in a different model.
Our finding that fewer macrophages are recruited to TEBs in the developing mammary gland of Mena Null mice, and our previous findings showing macrophage dependent in vivo invasion, intravasation and motility of Mena expressing cells , suggests that Mena is involved in a signaling loop with macrophages during development as well as during metastatic progression. Additionally, macrophages lack detectable Mena expression, but they do express the related Ena/VASP family members VASP and EVL (Unpublished observations, FBG). Thus tumor cell/macrophage interactions, or lack thereof, as described above, may be due to a Mena deficiency in tumor cells specifically (as supported by xenotransplant studies described in Figure 5). Further studies will be conducted to investigate this hypothesis.
Deficiency of Mena in the PyMT transgenic model of breast carcinoma increases tumor latency and decreases the rate of tumor progression to the histologic stage of carcinoma. Significant reductions in EGF induced in vivo invasion, intravasation and motility are strongly correlated with the decrease in lung metastasis and morbidity seen in Mena Null mice. This striking decrease in metastasis is recapitulated by the implantation of Mena Null tumor cells into a mouse host that is wild type for Mena showing a tumor cell autonomous effect. Additionally, Mena deficiency in non-tumor bearing mice leads to defects in invasive stages of mammary gland development such as terminal end bud formation and ductal branching, raising the intriguing possibility that Mena is part of a developmental program that is re-activated during metastatic progression. Given that loss of Mena is compatible with viability, we propose that inhibition of Mena could be used as a treatment for metastatic disease in breast cancer patients.
colony stimulating factor 1
epidermal growth factor
enhanced green fluorescent protein
fluorescence activated cell sorting
polyoma Middle T antigen
severe combined immunodeficiency
terminal end bud
tumor microenvironment for metastasis
We would like to thank Drs. Jeffery Segall, Sumanta Goswami, Diane Cox, and Antonia Patsialou for stimulating discussion and helpful suggestions. We give many thanks to David Entenberg and Jenny Tadros for their technical support; Einstein histopathology, Flow Cytometry and Analytical Imaging Facilities, and Koch Institute for Microscopy, Histology and Flow Cytometry sorting core facilities for their services. Grant support provided by CA100324 (JBW, JWP), CA113395 (ETR, JSC), UO1CA105490 (JC, JW), P30CA013330 (RSS), GM58801 and funds from the Ludwig center at MIT (FBG), ICBP grant U54 CA112967 (FBG). Data in this paper are from a thesis to be submitted in partial fulfillment of the requirements for the Degree of Doctor of Philosophy in the Graduate Division of Medical Sciences, Albert Einstein College of Medicine, Yeshiva University.
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