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We have previously shown that expression of a dominant-negative type II transforming growth factor-beta receptor (DNIIR) in mammary epithelium under control of the MMTV promoter/enhancer causes alveolar hyperplasia and differentiation in virgin mice. Here we show that MMTV-DNIIR female mice have accelerated mammary gland differentiation during early pregnancy with impaired development during late pregnancy and lactation followed by delayed postlactational involution. Mammary tumors, mostly carcinoma in situ, developed spontaneously in the MMTV-DNIIR mice with a long median latency (27.5 months). Crossbreeding to MMTV-transforming growth factor (TGF)-alpha mice to obtain mice expressing both transgenes resulted in mammary tumor formation with a much shorter latency more similar to those expressing only the MMTV-TGF-alpha transgene (<10 months median latency). The major difference in mammary tumors arising in MMTV-TGF-alpha compared to bigenic MMTV-DNIIR/MMTV-TGF-alpha was the marked suppression of tumor invasion by DNIIR transgene expression. Invading carcinoma cells in both MMTV-DNIIR and bigenic animals showed loss of DNIIR transgene expression as determined by in situ hybridization. The data indicate that signaling from endogenous TGF-betas not only plays an important role in normal mammary gland physiology but also can also suppress the early stage of tumor formation and contribute to tumor invasion once carcinomas have developed.
Overexpression of the matrix metalloproteinase matrilysin (matrix metalloproteinase-7) in the mouse mammary gland promotes mammary hyperplasia and accelerates the onset of oncogene-induced mammary tumors. In cell culture models, acute exposure of cells coexpressing Fas and Fas ligand (FasL) to matrilysin induces apoptosis, whereas chronic exposure to matrilysin selects for apoptosis-resistant cells. We now demonstrate that matrilysin promotes resistance to apoptosis in vivo. Matrilysin expression increased apoptosis in the involuting mammary gland of mice that had undergone a single pregnancy and lactation cycle. Premature basement membrane disruption was detected in matrilysin-expressing mice, which could account for the increase in apoptosis. However, multiparous mice, in which the involuting mammary epithelial cells have been repeatedly exposed to matrilysin, show a significant decrease in apoptosis. Mammary tissue from multiparous matrilysin-expressing mice showed decreased FasL expression, suggesting that loss of FasL is at least one mechanism of matrilysin-induced resistance to apoptosis. We propose that matrilysin promotes mammary tumor formation by enhancing the selection of cells that are resistant to apoptosis.
TGF-betas are potent inhibitors of epithelial cell proliferation. However, in established carcinomas, autocrine/paracrine TGF-beta interactions can enhance tumor cell viability and progression. Thus, we studied the effect of a soluble Fc:TGF-beta type II receptor fusion protein (Fc:TbetaRII) on transgenic and transplantable models of breast cancer metastases. Systemic administration of Fc:TbetaRII did not alter primary mammary tumor latency in MMTV-Polyomavirus middle T antigen transgenic mice. However, Fc:TbetaRII increased apoptosis in primary tumors, while reducing tumor cell motility, intravasation, and lung metastases. These effects correlated with inhibition of Akt activity and FKHRL1 phosphorylation. Fc:TbetaRII also inhibited metastases from transplanted 4T1 and EMT-6 mammary tumors in syngeneic BALB/c mice. Tumor microvessel density in a mouse dorsal skin window chamber was unaffected by Fc:TbetaRII. Therefore, blockade of TGF-beta signaling may reduce tumor cell viability and migratory potential and represents a testable therapeutic approach against metastatic carcinomas.
Overexpression of ErbB-2/Neu has been causally associated with mammary epithelial transformation. Here we report that blockade of the epidermal growth factor receptor (EGFR) kinase with AG-1478 markedly delays breast tumor formation in mouse mammary tumor virus (MMTV)/Neu + MMTV/transforming growth factor alpha bigenic mice. This delay was associated with inhibition of EGFR and Neu signaling, reduction of cyclin-dependent kinase 2 (Cdk2) and mitogen-activated protein kinase (MAPK) activities and cyclin D1, and an increase in the levels of the Cdk inhibitor p27(Kip1). In addition, BrdUrd incorporation into tumor cell nuclei was prevented with no signs of tumor cell apoptosis. These observations prompted us to investigate the stability of p27. Recombinant p27 was degraded rapidly in vitro by untreated but not by AG-1478-treated tumor lysates. Proteasome depletion of the tumor lysates, addition of the specific MEK1/2 inhibitor U-0126, or a T187A mutation in recombinant p27 all prevented p27 degradation. Cdk2 and MAPK precipitates from untreated tumor lysates phosphorylated recombinant wild-type p27 but not the T187A mutant in vitro. Cdk2 and MAPK precipitates from AG-1478-treated tumors were unable to phosphorylate p27 in vitro. These data suggest that increased signaling by ErbB receptors up-regulates MAPK activity, which, in turn, phosphorylates and destabilizes p27, thus contributing to dysregulated cell cycle progression.
Age-dependent loss of androgen sensitivity of the rat liver is associated with a marked increase in dehydroepiandrosterone/hydroxysteroid sulfotransferase (rStd) activity. Sulfonated steroid hormones are known to be ineffective in binding receptor proteins. These observations suggest that intracellular androgen sulfonation can physiologically influence androgen action. We have examined the inhibitory effect of rStd on androgen action in the human prostate cancer-derived PC-3 cells transfected with the rat androgen receptor (AR) expression plasmid and two androgen-responsive promoter reporter constructs (murine mammary tumor long-terminal repeat ligated to chloramphenicol acetyltransferase (CAT) gene and rat probasin androgen response element (ARE) ligated to firefly luciferase (LUC) gene). These transfected cells were dependent on 5alpha-dihydrotestosterone (DHT) for the activation of both reporter genes and showed about a 200- and a 800-fold increase of CAT and LUC activity, respectively, at 10(-10) M DHT over the no-hormone control. Expression of the sulfonating enzyme in this cell transfection system via the rStd expression plasmid caused a dose-dependent decline in the reporter activity with approximately 90% inhibition of androgen action at a rStd:AR plasmid ratio of 100. From these results we conclude that irrespective of a high level of AR, changes in the Std expression can markedly alter the androgen sensitivity of target cells.
To examine the role of matrilysin (MAT), an epithelial cell-specific matrix metalloproteinase, in the normal development and function of reproductive tissues, we generated transgenic animals that overexpress MAT in several reproductive organs. Three distinct forms of human MAT (wild-type, active, and inactive) were placed under the control of the murine mammary tumor virus promoter/enhancer. Although wild-type, active, and inactive forms of the human MAT protein could be produced in an in vitro culture system, mutations of the MAT cDNA significantly decreased the efficiency with which the MAT protein was produced in vivo. Therefore, animals carrying the wild-type MAT transgene that expressed high levels of human MAT in vivo were further examined. Mammary glands from female transgenic animals were morphologically normal throughout mammary development, but displayed an increased ability to produce beta-casein protein in virgin animals. In addition, beginning at approximately 8 mo of age, the testes of male transgenic animals became disorganized with apparent disintegration of interstitial tissue that normally surrounds the seminiferous tubules. The disruption of testis morphology was concurrent with the onset of infertility. These results suggest that overexpression of the matrix-degrading enzyme MAT alters the integrity of the extracellular matrix and thereby induces cellular differentiation and cellular destruction in a tissue-specific manner.
The beta-type transforming growth factors (TGF beta) are potent inhibitors of cell proliferation. The mechanisms of TGF beta growth inhibition have been investigated. In skin keratinocytes, TGF beta 1 rapidly suppresses c-myc expression at the level of transcriptional initiation, and expression of c-myc was shown to be necessary for proliferation of these cells. Overexpression of c-myc, using an inducible construct, blocks growth inhibition by TGF beta 1. In 11.5 day p.c. lung bud organ cultures, TGF beta 1 inhibits tracheobronchial epithelial development, including branching morphogenesis. At this stage of development, the tracheobronchial epithelia express N-myc, but not c-myc, TGF beta 1 was shown to markedly inhibit N-myc expression in epithelia of the lung bud organ cultures. N-myc gene knockout experiments by others have shown that N-myc is required for branching morphogenesis of the tracheobronchial tree. The data indicate that suppression of expression of either N-myc or c-myc may play a role in TGF beta growth inhibition. To study the role of TGF beta 1 in normal mammary development and in mammary neoplasia, we have constructed three transgenic mouse lines that express a simian TGF beta 1S223/225 mutated to produce a constitutively active product under the control of the MMTV enhancer/promoter. Expression of the transgene was associated with marked suppression of the normal pattern of mammary ductal tree development in female transgenics from all three lines. However, during pregnancy, alveolar outgrowths developed from the hypoplastic ductal tree, and lactation occurred. Unlike many other transgenic mouse models in which expression of TGF alpha or oncogenes under control of the MMTV promoter leads to mammary epithelial hyperplasia and increased tumor formation, the MMTV-TGF beta 1 transgene causes conditional hypoplasia of the mammary ductal tree. No spontaneous tumors have been detected in the MMTV-TGF beta 1 transgenic animals, indicating that overexpression of TGF beta 1 in mammary epithelia does not enhance, and may actually suppress, early stages of carcinoma development. Other studies have shown that overexpression of TGF beta 1 in carcinoma cells enhances tumorigenicity and metastatic spread. We propose that TGF beta has a bifunctional role in carcinogenesis, retarding carcinoma development but enhancing progression once neoplastic transformation has occurred and the growth inhibitory response to TGF beta has been lost.
The transforming growth factors beta (TGFs-beta) are potent inhibitors of cell proliferation and are usually secreted in a latent form. TGF-beta 1, TGF-beta 2, and TGF-beta 3 are expressed in distinct but overlapping patterns in the developing mouse mammary gland. To study the role of transforming growth factor-beta 1 (TGF-beta 1) in normal mammary development and in mammary neoplasia, we have constructed three transgenic mouse lines that express a simian TGF-beta 1 s223/225 mutated to produce a constitutively active product under the control of the MMTV enhancer/promoter. Expression of the transgene, as confirmed by in situ hybridization, immunohistochemistry, and Northern blot analysis, was associated with marked suppression of the normal pattern of mammary ductal tree development in female transgenics. Reduction in total ductal tree volume was observed at 7 weeks, soon after estrous begins, and was most apparent at 13 weeks, as ductal growth in the normal mammary gland declines. This effect was seen in all three lines. However, during pregnancy, alveolar outgrowths developed from the hypoplastic ductal tree, and lactation occurred, therefore, all transgenic females could feed full litters. Unlike many other transgenic mouse models in which expression of growth factors or oncogenes under control of the MMTV promoter leads to mammary epithelial hyperplasia and increased tumor formation, the MMTV-TGF-beta 1S223/225 transgene causes conditional hypoplasia of the mammary ductal tree and no spontaneous tumors have been detected in the MMTV-TGF-beta 1S223/225 transgenic animals.
In cell culture, type alpha transforming growth factor (TGF-alpha) stimulates epithelial cell growth, whereas TGF-beta 1 overrides this stimulatory effect and is growth inhibitory. Transgenic mice that overexpress TGF-alpha under control of the mouse mammary tumor virus (MMTV) promoter/enhancer exhibit mammary ductal hyperplasia and stochastic development of mammary carcinomas, a process that can be accelerated by administration of the chemical carcinogen 7,12-dimethylbenz[a]anthracene. MMTV-TGF-beta 1 transgenic mice display mammary ductal hypoplasia and do not develop mammary tumors. We report that in crossbreeding experiments involving the production of mice carrying both the MMTV-TGF-beta 1 and MMTV-TGF-alpha transgenes, there is marked suppression of mammary tumor formation and that MMTV-TGF-beta 1 transgenic mice are resistant to 7,12-dimethylbenz[a]anthracene-induced mammary tumor formation. These data demonstrate that overexpression of TGF-beta 1 in vivo can markedly suppress mammary tumor development.
To study the role of transforming growth factor alpha (TGF alpha) in normal mammary development and mammary neoplasia in vivo, we have generated transgenic mice in which a human TGF alpha cDNA is expressed under the control of the MMTV enhancer/promoter. Overexpression of TGF alpha in the mammary epithelium, as confirmed by in situ hybridization and immunohistochemistry, is associated with hyperplasia of alveoli and terminal ducts in virgin female and pregnant transgenic mice. A range of morphologic abnormalities including lobular hyperplasia, cystic hyperplasia, adenoma, and adenocarcinoma is seen in mammary tissue of transgenic females. In contrast, no morphologic abnormalities are seen in transgenic males in spite of TGF alpha overexpression in salivary glands and reproductive organs. TGF alpha can therefore act as an oncogene in vivo and appears to predispose mammary epithelium to neoplasia and carcinoma.