Immortalization phenotype dissociated from the preneoplastic phenotype in mouse mammary epithelial outgrowths in vivo

Stem cells and the mammary microenvironment

An entire mammary epithelial outgrowth, capable of full secretory differentiation, may comprise the progeny of a single cellular antecedent. This conclusion is based upon the maintenance of retroviral insertion sites within the somatic DNA of successive transplant generations derived from a single mammary fragment. In addition, dissociation of these clonal dominant glands and implantation of dispersed cells at limiting dilution demonstrated that both duct-limited and lobule-limited outgrowths were developed as well as complete, fully differentiated glands. Thus, transplantation has revealed three distinct mammary epithelial progenitors in the mouse. Recently, using cre-lox conditional activation of reporter genes, the lobule-limited progenitor was lineally marked by lacZ expression. In situ, these cells were shown to regenerate secretory lobules upon successive pregnancies. In transplant studies, they demonstrated the capacity for self- renewal and contributed to the new generation of all of the epithelial cell types among mammary secretory lobules. Using this conditional activation model, cells isolated from other tissues of the WAP-Cre/Rosa26/lacZReporter mice, co-mingled with normal wild type mammary epithelial cells and transplanted into epithelium-divested mammary fat pads, were shown to be amenable to redirection of their cell fate by interaction with the mammary microenvironment in vivo. This suggests the ascendancy of the microenvironment over the intrinsic nature of somatic stem cells.

Premalignant and malignant mammary lesions induced by MMTV and chemical carcinogens

This article reviews the types of mammary premalignant and malignant lesions induced in the mouse mammary gland after exposure to MMTV and chemical carcinogens. There are different morphological types of hyperplastic lesions in the mouse mammary gland. These lesions are designated as alveolar hyperplasia (HAN), ductal hyperplasia (DH), and cystic lesions; both non-keratinized and keratinized. The HAN and DH have been demonstrated to be precursors to invasive lesions. The HAN as a group tend to be ovarian-hormone independent for growth and transformation, mammary fat pad site-dependent for growth, possess unlimited replication potential (i.e., immortal), are at increased risk for progression to cancer, and are genetically stable. Serial transplantation demonstrates that any given stage of hyperplasia can progress to the next stage, sometimes rapidly and sometimes slowly. Multiple growth factor pathways are deregulated early in the development of HAN depending on etiology of the HAN. One general conclusion that emerges from such studies of HAN induced by different etiologies is that the early stages of premalignancy are a result of defects in cell cycle regulation with subsequent alterations playing a role in the acquisition of invasive phenotype. The predominant lesion induced by chemical carcinogens is the ductal hyperplasia (DH). Although DH show many of the essential biological alterations seen in HAN, they also exhibit a higher frequency of hormone-dependence and genetic instability, thus the DH appearing in chemical carcinogen treated mice and in transgenic mice mimic the histological, biological and genetic properties seen in human premalignant lesions more faithfully than do the HAN. The mammary tumors that arise from both general classes of premalignant lesions are very heterogeneous and exhibit different potentials for metastasis. The cell and molecular biology of metastasis represents an understudied area, in part because of the absence of suitable models to study the metastatic process. Newer transgenic mouse models provide a renewed opportunity to engage in the study of the mechanisms and processes underlying mammary metastasis.

Overexpression of Separase induces aneuploidy and mammary tumorigenesis

Separase is an endopeptidase that separates sister chromatids by cleaving cohesin Rad21 during the metaphase-to-anaphase transition. Conditional expression of Separase in tetracycline-inducible diploid FSK3 mouse mammary epithelial cells with both p53 WT and mutant (Ser-233-234) alleles of unknown physiological significance develops aneuploidy within 5 days of Separase induction in vitro. Overexpression of Separase induces premature separation of chromatids, lagging chromosomes, and anaphase bridges. In an in vivo mouse mammary transplant model, induction of Separase expression in the transplanted FSK3 cells for 3-4 weeks results in the formation of aneuploid tumors in the mammary gland. Xenograft studies combined with histological and cytogenetic analysis reveal that Separase-induced tumors are clonal in their genomic complements and have a mesenchymal phenotype suggestive of an epithelial-mesenchymal transition. Induction of Separase resulted in trisomies for chromosomes 8, 15, and 17; monosomy for chromosome 10; and amplification of the distal region of chromosomes 8 and 11. Separase protein is found to be significantly overexpressed in human breast tumors compared with matched normal tissue. These results collectively suggest that Separase is an oncogene, whose overexpression alone in mammary epithelial cells is sufficient to induce aneuploidy and tumorigenesis in a p53 mutant background.

DNA vaccination with CD44 variant isoform reduces mammary tumor local growth and lung metastasis

We have shown recently that cDNA vaccination, using a virtual lymph node, ameliorates experimental allergic encephalomyelitis. Successful cure from mammary tumor requires resolution of local tumor growth and metastases. We have examined whether targeting of CD44 cell surface adhesion molecule by cDNA vaccination plays a role in resolving mammary tumor development. We show here that CD44 cDNA vaccination decreases the tumor mass and metastatic potential in experimental mammary tumor of BALB/c mice. Vaccination of mice, inoculated with the mammary tumors, by cDNA of CD44 variant (CD44v) but not by cDNA of standard CD44, markedly reduced local tumor development and lung metastasis. Concomitantly, transfection of CD44 antisense into a highly metastatic mammary tumor cell line disrupted the CD44 expression of the cells and reduced their ability to establish local tumors as well as metastatic colonies in the lung. Moreover, when CD44v, but not standard CD44 sense cDNA, was transfected into the poorly metastatic cell line, tumor development was markedly enhanced. It is possible therefore that DNA vaccination with a specific CD44v construct could induce an immune resistance to mammary tumor progression.

Re-evaluation of mammary stem cell biology based on in vivo transplantation

Over nearly half a century, transplantation methods have been employed to regenerate the mammary gland in vivo. Recent highly cited reports claim to have demonstrated the regeneration of an entire functional mammary gland from a single mammary epithelial cell. Nevertheless, re-examination of the literature on the transplantation biology of mammary gland regeneration reveals that a complex, combinatorial interaction between variously differentiated mammary epithelial cells and the mammary fat pad stroma is indispensable to this process. In the present article, these issues are reviewed and discussed to provide a greater understanding of the complexity of these multiplex interactions.

The role of human papillomavirus 16 E6 in anchorage-independent and invasive growth of mouse tonsil epithelium

OBJECTIVE

To provide a manipulatable system to study the mechanism of human papillomavirus 16 (HPV16) E6-related transformation of an epithelial cell type affected by HPV16 in humans.

DESIGN

Biochemical and physiological studies of mouse tonsil epithelial cells (MTECs) transformed with HPV16 oncogenes plus H-ras in vitro and in vivo.

SETTING

Basic research laboratory.

PARTICIPANTS

C57BL/6 mice.

INTERVENTIONS

Transduction of the HPV16 oncogenes E6 and E7 in retroviral vectors into MTECs with isolation of multiple individual clones that expressed E6, E7, or both alone or in conjunction with H-ras.

MAIN OUTCOME MEASURES

Growth in culture, anchorage-independent growth, and growth in immune competent, syngeneic mice.

RESULTS

The MTECs that expressed E6 degraded p53 by a mechanism that is inhibited by proteasomal blockade. Although normal MTECs senesced after 20 population doublings, E6 alone or in combination with E7 was sufficient to immortalize MTECs beyond 25 population doublings, lower their population-doubling time, and permit anchorage-independent growth. However, only MTECs that express E6 plus H-ras or E6/E7 plus H-ras formed invasive tumors in immune competent, syngeneic mice at orthotopic intraoral and subdermal sites.

CONCLUSIONS

We found that HPV16 E6 and E7 alone are not sufficient for invasive growth. However, the synergistic activity of H-ras and E6 was sufficient to result in invasive growth.

Deletion of Tip30 leads to rapid immortalization of murine mammary epithelial cells and ductal hyperplasia in the mammary gland

Transformation of mammary epithelial cells (MECs) from the normal to the neoplastic stage requires the dysregulation of tumor suppressor genes and proto-oncogenes. Tip30 is a tumor suppressor that can inhibit estrogen receptor-mediated transcription in MECs, but its role in MEC proliferation remains unknown. Here, we show that deleting the Tip30 gene leads to ductal hyperplasia in mouse mammary glands early in life and extensive mammary hyperplasia with age. Tip30(-/-) mammary glands transplanted into wild-type mammary fat pads also display mammary trees with extensive ductal hyperplasia. Strikingly, Tip30 deletion promotes proliferation of primary MECs and results in rapid immortalization of MECs in vitro relative to wild-type cells. Gene array analysis identified significant increases in the expression of mammary epithelial growth factors Wisp2 and Igf-1 in Tip30(-/-) cells. Knockdown of either Wisp2 or Igf-1 using short interfering RNA dramatically inhibited proliferation of Tip30(-/-) cells. Together, these results suggest that Tip30 is an intrinsic and negative regulator of MEC proliferation partly through the inhibition of Wisp2 and Igf-1 expression, and its absence in the mammary gland may predispose MECs to neoplastic transformation.

Cell cycle genes in a mouse mammary hyperplasia model

Human mammary epithelial cells emerge spontaneously from senescence, exhibiting eroding telomeric sequences, and ultimately enter crisis to generate the type of chromosomal abnormalities seen in early stages of breast cancer. In a mouse mammary tumor model, the spontaneous escape of senescence can be observed as an increase in DNA synthesis that is reflected by alterations in the cell cycle profile and increases in the expression levels and activities of cell cycle molecular components. This review provides an overview of gene alterations in the cell cycle components in mouse mammary hyperplasia.

Mammary epithelial stem cells: transplantation and self-renewal analysis

An entire mammary epithelial outgrowth, capable of full secretory differentiation, may be comprised of the progeny of a single cellular antecedent. This conclusion is based upon the maintenance of retroviral insertion sites within the somatic DNA of successive transplant generations derived from a single mammary fragment. In addition, dissociation of these clonal dominant glands and implantation of dispersed cells at limiting dilution demonstrated that both duct-limited and lobule-limited outgrowths were developed, as well as complete, fully differentiated glands. Thus, transplantation has revealed three distinct mammary epithelial progenitors in the mouse. Similar studies have extended this observation to rat mammary tissue. Recently, using cre-lox conditional activation of reporter genes, a new epithelial progenitor, specific for mammary secretory epithelium in postlactation females has been uncovered. In situ, these cells were shown to regenerate secretory lobules upon successive pregnancies. In transplant studies, they demonstrated the capacity for self-renewal and contributed to the new generation of all of the known epithelial cell types among mammary epithelium. In limiting dilution, the parity-induced progenitors were capable of engendering lobule-limited and duct-limited outgrowths in their entirety, but not completely developed glands. Serial transplant studies indicate that these progenitors have a significant but limited capacity for self-renewal.

Linking sister chromatid cohesion and apoptosis: role of Rad21

Rad21 is one of the major cohesin subunits that holds sister chromatids together until anaphase, when proteolytic cleavage by separase, a caspase-like enzyme, allows chromosomal separation. We show that cleavage of human Rad21 (hRad21) also occurs during apoptosis induced by diverse stimuli. Induction of apoptosis in multiple human cell lines results in the early (4 h after insult) generation of 64- and 60-kDa carboxy-terminal hRad21 cleavage products. We biochemically mapped an apoptotic cleavage site at residue Asp-279 (D(279)) of hRad21. This apoptotic cleavage site is distinct from previously described mitotic cleavage sites. hRad21 is a nuclear protein; however, the cleaved 64-kDa carboxy-terminal product is translocated to the cytoplasm early in apoptosis before chromatin condensation and nuclear fragmentation. Overexpression of the 64-kDa cleavage product results in apoptosis in Molt4, MCF-7, and 293T cells, as determined by TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) and Annexin V staining, assaying of caspase-3 activity, and examination of nuclear morphology. Given the role of hRad21 in chromosome cohesion, the cleaved C-terminal product and its translocation to the cytoplasm may act as a nuclear signal for apoptosis. In summary, we show that cleavage of a cohesion protein and translocation of the C-terminal cleavage product to the cytoplasm are early events in the apoptotic pathway and cause amplification of the cell death signal in a positive-feedback manner.

Biological and genetic properties of the p53 null preneoplastic mammary epithelium

The absence of the tumor suppressor gene p53 confers an increased tumorigenic risk for mammary epithelial cells. In this report, we describe the biological and genetic properties of the p53 null preneoplastic mouse mammary epithelium in a p53 wild-type environment. Mammary epithelium from p53 null mice was transplanted serially into the cleared mammary fat pads of p53 wild-type BALB/c female to develop stable outgrowth lines. The outgrowth lines were transplanted for 10 generations. The outgrowths were ductal in morphology and progressed through ductal hyperplasia and ductal carcinoma in situ before invasive cancer. The preneoplastic outgrowth lines were immortal and exhibited activated telomerase activity. They are estrogen and progesterone receptor-positive, and aneuploid, and had various levels of tumorigenic potential. The biological and genetic properties of these lines are distinct from those found in most hyperplastic alveolar outgrowth lines, the form of mammary preneoplasia occurring in most traditional models of murine mammary tumorigenesis. These results indicate that the preneoplastic cell populations found in this genetically engineered model are similar in biological properties to a subset of precurser lesions found in human breast cancer and provide a unique model to identify secondary events critical for tumorigenicity and invasiveness.

Wnt-1 and int-2 mammary oncogene effects on the beta-catenin pathway in immortalized mouse mammary epithelial cells are not sufficient for tumorigenesis

Development of strategies for prevention of breast cancer development requires an understanding of the effects of mammary oncogenes on mammary cells at early stages in neoplastic transformation. As mammary oncogenes wnt-1 and int-2 affect different signal transduction pathways, we investigated their effects on established mouse mammary epithelial cell lines (MMECLs) reflecting early stages in tumorigenesis. Normal interactions between beta-catenin and E-cadherin were abrogated in all three immortalized MMECLs and the cells lacked beta-catenin-mediated transactivation activity, detectable using a reporter assay, suggesting that alterations in cell adhesion may be very early events in mammary tumorigenesis. Immortalized FSK4 and EL12 cells and hyperplastic TM3 cells were stably transfected with expression vectors encoding wnt-1 or int-2 or the control vector, and drug-selected pooled cells from each line were confirmed by reverse transcription-polymerase chain reaction to express the transfected oncogene; this expression persisted in the cells analysed in vitro and in vivo. Resultant phenotypic changes depended both on the oncogene and the target mammary cell line. In FSK4 cells, expression of wnt-1 or int-2 resulted in proliferative changes in vitro, including reduced contact inhibition, increased beta-catenin expression, and decreased p53 transcriptional activity, but neither oncogene conferred upon those cells the ability to produce tumors in vivo. EL12 cells were highly refractory to the effects of both oncogenes, with the only measurable changes being increased E-cadherin levels induced by both oncogenes and increased proliferation of the int-2-transfected cells in the absence of serum. Parental TM3 cells were phenotypically similar to wnt-1- or int-2-transfected FSK4 cells and displayed an increased rate of proliferation in vitro and markedly increased tumorigenicity in vivo following transfection with int-2 but not with wnt-1. These results suggest that wnt-1 signaling is redundant in the hyperplastic TM3 cells and indicate that wnt-1-induced effects in the immortalized FSK4 and EL12 cells were not sufficient to mediate a tumorigenic phenotype. This study showed that the wnt-1 and int-2 oncogenes have similar but distinguishable effects on immortalized MMECLs and that the genetic background of the mammary cells greatly influences the consequences of oncogene expression at early stages of cell transformation.

Cloning and characterization of ectopically expressed transcripts for the actin-binding protein MIPP in mouse mammary carcinomas

Mipp is a kelch-related, placental-specific gene that is ectopically expressed in many BALB/c mouse mammary carcinomas of various etiologies. The Kelch family encompasses proteins that are emerging as key links between microfilaments and a variety of cellular structures and functions. Mouse mammary tumors express two mipp transcripts (2.2 and 5.6 kb). We cloned the 2.2 kb mipp mRNA and analysed the product of its 1.7 kb ORF. The 584 residue MIPP protein has an N-terminal BTB domain and six C-terminal tandem Kelch repeats. Despite expression of two mipp RNAs, only a single MIPP protein is expressed in mammary tumors. MIPP protein binds to microfilaments in vitro and co-immunoprecipitates with actin. MIPP co-localized with concanavalin A at the endoplasmic reticulum, suggesting that MIPP might mediate interactions between microtubules and actin filaments. Because MIPP expression is widespread in mouse mammary tumors, it might contribute to tumorigenesis. Although MIPP had little effect on the growth rate of human breast cell lines following transfection, it greatly reduced the formation of duct-like structures on reconstituted basement membrane. Our results suggest that MIPP could contribute to malignant progression in the mouse mammary epithelial cells by perverting their response to cues from the extracellular matrix.

The preneoplastic phenotype in murine mammary tumorigenesis

Preneoplastic lesions in murine mammary tumorigenesis have been extensively investigated over the past 50 years. The two general types of lesion that have malignant potential are the alveolar hyperplasias represented by the classical hyperplastic alveolar nodule and the ductal hyperplasias. The former type of lesion is induced by viral, chemical and hormonal agents; the latter by chemical agents and specific genetic alterations. Individual animal models have been utilized to elucidate the basic biological properties of the lesions and some of the basic molecular alterations. The biological phenotype of the two types of lesions include immortalization and epithelial hyperplasia. The ductal hyperplasias are distinguished from the alveolar hyperplasias by their pattern of epithelial hyperplasia and their extent of aneuploidy. The molecular alterations underlying epithelial hyperplasia are numerous and dependent on the particular animal model. An important issue for future studies is how faithfully any of these models mimic human premalignant progression. A minimal set of criteria is proposed that includes morphological progression, hormone dependence and genetic instability. It is likely that hyperplasias from a specific mouse model will represent a subset of the lesions found in human disease. Analogous hyperplasias from several defined genetic models, adequately characterized at the biological and molecular levels, would provide appropriate models for testing chemopreventive agents.

Expression of MAT1/PEA-15 mRNA isoforms during physiological and neoplastic changes in the mouse mammary gland

MAT1 is a novel transforming gene which was cloned from a mouse mammary tumor induced by N-methyl-N-nitrosourea in vitro in the presence of lithium as a mitogen. Later, it was found to be identical to the 3' untranslated region (UTR) of the 2.5 kb isoform of PEA-15 (phosphoprotein enriched in astrocytes-15 kDa). We re-cloned MAT1/PEA-15 cDNAs and showed 2.5, 2.0 and 1.8 kb isoforms and confirmed MAT1 localization as reported. The 2.0 and 1.8 kb isoforms were produced by alternative splicing and alternative polyadenylation at the 3' UTR, respectively. To analyze the role of MAT1/PEA-15, we examined the expression of MAT1/PEA-15 mRNA in normal mammary tissues and in mammary tumors. The mammary gland during pregnancy, lactation and weaning showed weak but stable expression. Compared with normal mammary gland, mammary tumors showed stronger expression. Aberrant expression of MAT1/PEA-15 isoforms was found in mouse mammary epithelial cell lines, FSK7 and TM6, which lost the 2.5/2.0 and 2.5 kb isoforms, respectively. In contrast to other oncogenes like c-myc, MAT1/PEA-15 mRNA was extremely stable after actinomycin D and cycloheximide treatments suggesting that other protein expression is prerequisite for degradation of MAT1/PEA-15 mRNA. It evoked the possibility of the 3' UTR of MAT1/PEA-15 (designated as MAT1-T) as a riboregulator in mammary tumorigenesis and necessity for further analysis of human breast cancers as well as mouse mammary tumors.

Immortal, non-tumourigenic mouse mammary outgrowths express high levels of cyclin B1 and activation of cyclin B1/cdc2 kinase

Neoplastic transformation of mouse mammary epithelial cells is the result of several identifiable phenotypic changes which presumably require sequential genetic alterations. In our model system, mammary cells progress from a mortal state (virgin duct) to several morphologically distinct intermediate states. The intermediate states are distinct cell populations that are phenotypically identified as immortal, non-tumourigenic (i.e. EL11), weakly tumourigenic ductal/alveolar hyperplasia (i.e. EL12) and moderately tumourigenic alveolar hyperplasiaa (i.e. TM12) to invasive tumours (i.e. EL12T/TM12T). We have studied the changes in total cyclin A and B1 levels, cyclin A and B1 complexed to cdc2, cyclin B1cdc2 kinase activity and cyclin D proteins in EL11 and EL12 immortalized outgrowth lines. Results revealed increased levels in total cyclin B1 (> 5-fold), cyclin B1/cdc2 (3-4-fold) and cyclin B1/cdc2 kinase activity (2-3.5-fold) in EL11 and EL12 phenotypes when compared to control mammary gland (virgin). No changes in the levels of total cyclin A or cycln A associated to cdc2 were observed. Cyclin D1, D2 and D3 protein levels were low in the EL11 immortal ductal outgrowth. Exposure to hormones via a pituitary isograft stimulated the synthesis of cyclin D1 and D2 but not D3 associated to cdk4 as well as total cdk4 proteins. Bromodeoxyuridine (BrdUrd) labelling indices showed marked increases in immortal ductal outgrowths (EL11 and EL12) when compared to virgin, suggesting that epithelial cells are cycling in these cell populations. Even in the presence of hormone stimulation, EL11 outgrowths were not tumourigenic, suggesting that other events are necessary to drive the cells to a tumourigenic phenotype. The results suggest that increased levels of cyclin B1 and cyclin B1-cdc2 kinase activities are early events and may be an important marker for the immortalized phenotype.

Experimental mammary epithelial morphogenesis in an in vivo model: evidence for distinct cellular progenitors of the ductal and lobular phenotype

An in vivo transplantation system has been used to evaluate the developmental capacities of specific mouse mammary epithelial cell populations. Specifically, mouse mammary epithelial cells with distinctly limited developmental potentials have been identified using this procedure. Two distinct epithelial cell progenitors have been identified by experiments designed to determine whether basal lobular and ductal phenotypes could develop independently under conditions imposed by a limiting dilution. The prediction that these separate epithelial progenitors must exist was based upon the results from transplantation experiments carried out in epithelium-divested mammary fat pads of syngeneic mice with mammary epithelium from two different transgenic mouse models. The results presented here demonstrate the following points: 1) lobular, i.e. secretory, progenitor cells are present as distinct entities among the mammary epithelial cells found in immature virgin female mice; 2) similarly, ductal epithelial progenitors are present within the same population; 3) lobular progenitors are present in greater numbers, although both cell populations are extremely small; 4) as expected, some inocula produce outgrowths with simultaneous development of both lobular and ductal phenotypes--it is not known whether this indicates cooperative interaction between the two epithelial progenitors or signals the presence of a third progenitor type capable of producing both ductular and lobular committed daughters; 5) these findings have important consequences in the design of experiments aimed at testing the effects of known and putative mammary oncogenes and tumor suppressor genes, using techniques which include cellular transformation in vitro followed by in vivo cultivation and evaluation.

Tumor suppressor p53 mutations and breast cancer: a critical analysis

Alterations in the tumor suppressor gene p53 are the most commonly identified changes in cancer, including neoplasia of the breast. The activity of p53 is regulated post-translationally. Phosphorylation state, subcellular localization, and interaction with any of a number of cellular proteins are likely to influence the function of p53. The exact effect of p53-mediated growth suppression seems to be cell-type specific but appears to be directly related to the ability of p53 to act as a specific transcriptional activator. The role that transcriptional repression plays in the function of WT p53 is less clear. It is also possible that p53 has a more direct activity in DNA replication and repair. Most documented p53 mutations result in single amino acid substitutions which may confer one or more of a spectrum of transforming abilities on the protein. Mutation may lead to nuclear accumulation of p53 protein; however, inactivation of p53 by nuclear exclusion and interaction with the mdm2 protein also appear to be important in tumorigenesis. Used in conjunction with other established factors, accumulation of cellular p53 may be a useful prognostic indicator in breast cancer. A syngeneic mouse model system yielded evidence that p53 mutations are important in the early, preneoplastic stages of mammary tumorigenesis. This murine system may provide the ability to investigate the functions of p53 in the early stages of breast cancer which are technically difficult to examine in the human system.

Gene expression screening for specific genes associated with mouse mammary tumor development

The preneoplastic hyperplastic alveolar nodule is a frequent and well-characterized precursor to mammary tumors in the mouse. Although the biological characteristics of the preneoplastic state have been understood for many years, the molecular alterations associated with preneoplasia and tumorigenicity are unknown. We applied the technique of differential display of mRNA to two closely matched cell populations of mammary preneoplasias that differed only in their tumorigenic potential. Two mRNAs were isolated that were overexpressed only in tumorigenic preneoplasias and in tumors and not in normal pregnant mammary gland or in nontumorigenic preneoplasias. Partial nucleotide sequencing indicated that one of the mRNAs had not yet been described, whereas the second mRNA was highly homologous to a relatively uncharacteristic gene termed pT-2. These results illustrate the utility of the differential display method for isolating and identifying uniquely expressed genes from tissues maintained in the microenvironment where tumors arise naturally.