A tale of two proteins: differential roles and regulation of Smad2 and Smad3 in TGF-beta signaling

Transforming growth factor-beta (TGF-beta) is an important growth inhibitor of epithelial cells, and insensitivity to this cytokine results in uncontrolled cell proliferation and can contribute to tumorigenesis. Smad2 and Smad3 are direct mediators of TGF-beta signaling, however little is known about the selective activation of Smad2 versus Smad3. The Smad2 and Smad3 knockout mouse phenotypes and studies comparing Smad2 and Smad3 activation of TGF-beta target genes, suggest that Smad2 and Smad3 have distinct roles in TGF-beta signaling. The observation that TGF-beta inhibits proliferation of Smad3-null mammary gland epithelial cells, whereas Smad3 deficient fibroblasts are only partially growth inhibited, suggests that Smad3 has a different role in epithelial cells and fibroblasts. Herein, the current understanding of Smad2 and Smad3-mediated TGF-beta signaling and their relative roles are discussed, in addition to potential mechanisms for the selective activation of Smad2 versus Smad3. Since alterations in the TGF-beta signaling pathway play an important role in promoting tumorigenesis and cancer progression, methods for therapeutic targeting of the TGF-beta signaling pathway are being pursued. Determining how Smad2 or Smad3 differentially regulate the TGF-beta response may translate into developing more effective strategies for cancer therapy.

Enhanced hepatocyte growth factor signaling by type II transforming growth factor-beta receptor knockout fibroblasts promotes mammary tumorigenesis

Transforming growth factor-beta (TGF-beta) plays complex dual roles as an inhibitor and promoter of tumor progression. Although the influence of the stromal microenvironment on tumor progression is well recognized, little is known about the functions of TGF-beta signaling in the stroma during tumor progression. Using cre-lox technology, expression of the type II TGF-beta receptor was selectively knocked out in fibroblasts (Tgfbr2(FspKO)). In a co-xenograft model, we show that Tgfbr2(FspKO) fibroblasts enhance mammary carcinoma growth and metastasis in mice while increasing hepatocyte growth factor (HGF) expression and c-Met signaling downstream pathways including signal transducers and activators of transcription 3 (Stat3) and p42/44 mitogen-activated protein kinase (MAPK). Treatment of tumor-bearing mice with a pharmacologic inhibitor (EXEL-7592) of c-Met blocks tumor progression and reduces levels of phospho-Stat3 and phospho-p42/44 MAPK. Similarly, small interfering RNA knockdown of c-Met expression in mammary tumor cells reduces metastasis and c-Met signaling caused by Tgfbr2(FspKO) fibroblasts. The results show that TGF-beta signaling in fibroblasts suppresses tumor metastasis by antagonizing HGF/c-Met signaling within tumor epithelial cells. Furthermore, this co-xenograft model represents a unique context to study stromal TGF-beta and HGF signaling in mammary tumorigenesis.

Effects of BRCA1 transgene expression on murine mammary gland development and mutagen-induced mammary neoplasia

To characterize the role of BRCA1 in mammary gland development and tumor suppression, a transgenic mouse model of BRCA1 overexpression was developed. Using the mouse mammary tumor virus (MMTV) promoter/enhancer, transgenic mice expressing human BRCA1 or select mutant controls were generated. Transgenic animals examined during adolescence were shown to express the human transgene in their mammary glands. The mammary glands of 13-week-old virgin homozygous MMTV-BRCA1 mice presented the morphology of moderately increased lobulo-alveolar development. The mammary ductal trees of both hemizygous and homozygous MMTV-BRCA1t340 were similar to those of control non-transgenic littermates. Interestingly, both hemi- and homozygous mice expressing a splice variant of BRCA1 lacking the N-terminal RING finger domain (MMTV-BRCA1sv) exhibited marked mammary lobulo-alveolar development, particularly terminal end bud proliferation. Morphometric analyses of mammary gland whole mount preparations were used to measure epithelial staining indices of ~35% for homozygous MMTV-BRCA1 mice and ~60% for both hemizygous and homozygous MMTV-BRCA1sv mice versus ~25% for non-transgenic mice. Homozygous MMTV-BRCA1 mice showed delayed development of tumors when challenged with 7,12 dimethylbenzanthracene (DMBA), relative to non-transgenic and homozygous BRCA1t340 expressing mice. In contrast, homozygous MMTV-BRCA1sv transgenic animals were sensitized to DMBA treatment and exhibited a very rapid onset of mammary tumor development and accelerated mortality. MMTV-BRCA1 effects on mortality were restricted to DMBA-induced tumors of the mammary gland. These results demonstrate in vivo roles for BRCA1 in both mammary gland development and in tumor suppression against mutagen-induced mammary gland neoplasia.

Inhibition of TGF-beta with neutralizing antibodies prevents radiation-induced acceleration of metastatic cancer progression

We investigated whether TGF-beta induced by anticancer therapies accelerates tumor progression. Using the MMTV/PyVmT transgenic model of metastatic breast cancer, we show that administration of ionizing radiation or doxorubicin caused increased circulating levels of TGF-beta1 as well as increased circulating tumor cells and lung metastases. These effects were abrogated by administration of a neutralizing pan-TGF-beta antibody. Circulating polyomavirus middle T antigen-expressing tumor cells did not grow ex vivo in the presence of the TGF-beta antibody, suggesting autocrine TGF-beta is a survival signal in these cells. Radiation failed to enhance lung metastases in mice bearing tumors that lack the type II TGF-beta receptor, suggesting that the increase in metastases was due, at least in part, to a direct effect of TGF-beta on the cancer cells. These data implicate TGF-beta induced by anticancer therapy as a pro-metastatic signal in tumor cells and provide a rationale for the simultaneous use of these therapies in combination with TGF-beta inhibitors.

Transcriptional Cooperation between the Transforming Growth Factor-β and Wnt Pathways in Mammary and Intestinal Tumorigenesis

Transforming growth factor-beta (TGF-beta) and Wnt ligands function in numerous developmental processes, and alterations of both signaling pathways are associated with common pathologic conditions, including cancer. To obtain insight into the extent of interdependence of the two signaling cascades in regulating biological responses, we used an oligonucleotide microarray approach to identify Wnt and TGF-beta target genes using normal murine mammary gland epithelial cells as a model. Combination treatment of TGF-beta and Wnt revealed a novel transcriptional program that could not have been predicted from single ligand treatments and included a cohort of genes that were cooperatively induced by both pathways. These included both novel and known components or modulators of TGF-beta and Wnt pathways, suggesting that mutual feedback is a feature of the coordinated activities of the ligands. The majority of the cooperative targets display increased expression in tumors derived from either Min (many intestinal neoplasia) or mouse mammary tumor virus (MMTV)-Wnt1 mice, two models of Wnt-induced tumors, with nine of these genes (Ankrd1, Ccnd1, Ctgf, Gpc1, Hs6st2, IL11, Inhba, Mmp14, and Robo1) showing increases in both. Reduction of TGF-beta signaling by expression of a dominant-negative TGF-beta type II receptor in bigenic MMTV-Wnt1/DNIIR mice increased mammary tumor latency and was correlated with a decrease in expression of Gpc1, Inhba, and Robo1, three of the TGF-beta/Wnt cooperative targets. Our results indicate that the TGF-beta and Wnt/beta-catenin pathways are firmly intertwined and generate a unique gene expression pattern that can contribute to tumor progression.

Aggressive pancreatic ductal adenocarcinoma in mice caused by pancreas-specific blockade of transforming growth factor-beta signaling in cooperation with active Kras expression

Pancreatic ductal adenocarcinoma (PDAC) is an almost uniformly lethal disease in humans. Transforming growth factor-beta (TGF-beta) signaling plays an important role in PDAC progression, as indicated by the fact that Smad4, which encodes a central signal mediator downstream from TGF-beta, is deleted or mutated in 55% and the type II TGF-beta receptor (Tgfbr2) gene is altered in a smaller subset of human PDAC. Pancreas-specific Tgfbr2 knockout mice have been generated, alone or in the context of active Kras (Kras(G12D)) expression, using the Cre-loxP system driven by the endogenous Ptf1a (pancreatic transcription factor-1a) locus. Pancreas-selective Tgfbr2 knockout alone gave no discernable phenotype in 1.5 yr. Pancreas-specific Kras(G12D) activation alone essentially generated only intraepithelial neoplasia within 1 yr. In contrast, the Tgfbr2 knockout combined with Kras(G12D) expression developed well-differentiated PDAC with 100% penetrance and a median survival of 59 d. Heterozygous deletion of Tgfbr2 with Kras(G12D) expression also developed PDAC, which indicated a haploinsufficiency of TGF-beta signaling in this genetic context. The clinical and histopathological manifestations of the combined Kras(G12D) expression and Tgfbr2 knockout mice recapitulated human PDAC. The data show that blockade of TGF-beta signaling and activated Ras signaling cooperate to promote PDAC progression.

Transforming growth factor beta receptor type II inactivation induces the malignant transformation of intestinal neoplasms initiated by Apc mutation

The transforming growth factor-beta (TGF-beta) signaling pathway is a tumor-suppressor pathway that is commonly inactivated in colon cancer. TGF-beta is a secreted ligand that mediates its effects through a transmembrane heteromeric receptor complex, which consists of type I (TGFBR1) and type II subunits (TGFBR2). Approximately 30% of colon cancers carry TGFBR2 mutations, demonstrating that it is a common target for mutational inactivation in this cancer. To assess the functional role of TGFBR2 inactivation in the multistep progression sequence of colon cancer, we generated a mouse model that recapitulates two common genetic events observed in human colon cancer by mating Apc(1638N/wt) mice with mice that are null for Tgfbr2 in the intestinal epithelium, Villin-Cre;Tgfbr2(E2flx/E2flx) mice. In this model, we observed a dramatic increase in the number of intestinal adenocarcinomas in the Apc(1638N/wt);Villin-Cre;Tgfbr2(E2flx/E2flx) mice (called Apc(1638N/wt);Tgfbr2(IEKO)) compared with those mice with intact Tgfbr2 (Apc(1638N/wt);Tgfbr2(E2flx/E2flx)). Additionally, in vitro analyses of epithelial tumor cells derived from the Apc(1638N/wt);Tgfbr2(IEKO) mice showed enhanced expression and activity of matrix metalloproteinase MMP-2 and MMP-9, as well as increased TGF-beta1 secretion in the conditioned medium. Similarly, primary tumor tissues from the Apc(1638N/wt);Tgfbr2(IEKO) mice also showed elevated amounts of TGF-beta1 as well as higher MMP-2 activity in comparison with Apc(1638N/wt);Tgfbr2(E2flx/E2flx)-derived tumors. Thus, loss of TGFBR2 in intestinal epithelial cells promotes the invasion and malignant transformation of tumors initiated by Apc mutation, providing evidence that Wnt signaling deregulation and TGF-beta signaling inactivation cooperate to drive the initiation and progression, respectively, of intestinal cancers in vivo.

Tgfbeta signaling is required for atrioventricular cushion mesenchyme remodeling during in vivo cardiac development

The transforming growth factorbeta (Tgfbeta) signaling pathway plays crucial roles in many biological processes. To understand the role(s) of Tgfbeta signaling during cardiogenesis in vivo and to overcome the early lethality of Tgfbr2(-/-) embryos, we applied a Cre/loxp system to specifically inactivate Tgfbr2 in either the myocardium or the endothelium of mouse embryos. Our results show that Tgfbr2 in the myocardium is dispensable for cardiogenesis in most embryos. Contrary to the prediction from results of previous in vitro collagen gel assays, inactivation of Tgfbr2 in the endocardium does not prevent atrioventricular cushion mesenchyme formation, arguing against its essential role in epithelium-mesenchyme transformation in vivo. We further demonstrate that Tgfbeta signaling is required for the proper remodeling of the atrioventricular canal and for cardiac looping, and that perturbation in Tgfbeta signaling causes the double-inlet left ventricle (DILV) defect. Thus, our study provides a unique mouse genetic model for DILV, further characterization of which suggests a potential cellular mechanism for the defect.

Oncogenic function of a novel WD-domain protein, STRAP, in human carcinogenesis

The development and progression of malignancies is a complex multistage process that involves the contribution of a number of genes giving growth advantage to cells when transformed. The role of transforming growth factor-beta (TGF-beta) in carcinogenesis is complex with tumor-suppressor or prooncogenic activities depending on the cell type and the stage of the disease. We have previously reported the identification of a novel WD-domain protein, STRAP, that associates with both TGF-beta receptors and that synergizes with the inhibitory Smad, Smad7, in the negative regulation of TGF-beta-induced transcription. Here, we show that STRAP is ubiquitously expressed and is localized in both cytoplasm and nucleus. STRAP is up-regulated in 60% colon and in 78% lung carcinomas. Stable expression of STRAP results in activation of mitogen-activated protein kinase/extracellular signal-regulated kinase pathway and in down-regulation of the cyclin-dependent kinase inhibitor p21(Cip1), which results in retinoblastoma protein hyperphosphorylation. In addition, we have observed that Smad2/3 phosphorylation, TGF-beta-mediated transcription, and growth inhibition are induced in STRAP-knockout mouse embryonic fibroblasts compared with wild-type cells. Ectopic expression of STRAP in A549 lung adenocarcinoma cell line inhibits TGF-beta-induced growth inhibition and enhances anchorage-independent growth of these cells. Moreover, overexpression of STRAP increases tumorigenicity in athymic nude mice. Knockdown of endogenous STRAP by small interfering RNA increases TGF-beta signaling, reduces ERK activity, increases p21(Cip1) expression, and decreases tumorigenicity. Taken together, these results suggest that up-regulation of STRAP in human cancers may provide growth advantage to tumor cells via TGF-beta-dependent and TGF-beta-independent mechanisms, thus demonstrating the oncogenic function of STRAP.

Tumour microenvironment: TGFbeta: the molecular Jekyll and Hyde of cancer

Transforming growth factor-beta (TGFbeta) signalling regulates cancer through mechanisms that function either within the tumour cell itself or through host-tumour cell interactions. Studies of tumour-cell-autonomous TGFbeta effects show clearly that TGFbeta signalling has a mechanistic role in tumour suppression and tumour promotion. In addition, factors in the tumour microenvironment, such as fibroblasts, immune cells and the extracellular matrix, influence the ability of TGFbeta to promote or suppress carcinoma progression and metastasis. The complex nature of TGFbeta signalling and crosstalk in the tumour microenvironment presents a unique challenge, and an opportunity to develop therapeutic intervention strategies for targeting cancer.

Effect of IGF-I in the chondrogenesis of bone marrow mesenchymal stem cells in the presence or absence of TGF-beta signaling

A novel role for IGF-I in MSC chondrogenesis was determined. IGF-I effects were evaluated in the presence or absence of TGF-beta signaling by conditionally inactivating the TGF-beta type II receptor. We found that IGF-I had potent chondroinductive actions on MSCs. IGF-I effects were independent from and additive to TGF-beta.

INTRODUCTION

Mesenchymal stem cells (MSCs) can be isolated from adult bone marrow (BM), expanded, and differentiated into several cell types, including chondrocytes. The role of IGF-I in the chondrogenic potential of MSCs is poorly understood. TGF-beta induces MSC chondrogenic differentiation, although its actions are not well defined. The aim of our study was to define the biological role of IGF-I on proliferation, chondrogenic condensation, apoptosis, and differentiation of MSCs into chondrocytes, alone or in combination with TGF-beta and in the presence or absence of TGF-beta signaling.

MATERIALS AND METHODS

Mononuclear adherent stem cells were isolated from mouse BM. Chondrogenic differentiation was induced by culturing high-density MSC pellets in serum- and insulin-free defined medium up to 7 days, with or without IGF-I and/or TGF-beta. We measured thymidine incorporation and stained 2-day-old pellets with TUNEL, cleaved caspase-3, peanut-agglutinin, and N-cadherin. Seven-day-old pellets were measured in size, stained for proteoglycan synthesis, and analyzed for the expression of collagen II and Sox-9 by quantitative real time PCR. We obtained MSCs from mice in which green fluorescent protein (GFP) was under the Collagen2 promoter and determined GFP expression by confocal microscopy. We conditionally inactivated the TGF-beta type II receptor (TbetaRII) in MSCs using a cre-lox system, generating TbetaRII knockout MSCs (RIIKO-MSCs).

RESULTS AND CONCLUSIONS

IGF-I modulated MSC chondrogenesis by stimulating proliferation, regulating cell apoptosis, and inducing expression of chondrocyte markers. IGF-I chondroinductive actions were equally potent to TGF-beta1, and the two growth factors had additive effects. Using RIIKO-MSCs, we showed that IGF-I chondrogenic actions are independent from the TGF-beta signaling. We found that the extracellular signal-related kinase 1/2 mitogen-activated protein kinase (Erk1/2 MAPK) pathway mediated the TGF-beta1 mitogenic response and in part the IGF-I proliferative action. Our data, by showing the role of IGF-I and TGF-beta1 in the critical steps of MSC chondrogenesis, provide critical information to optimize the therapeutic use of MSCs in cartilage disorders.

Under pressure: stromal fibroblasts change their ways

In this issue of Cell, Hill et al. (2005) demonstrate in a mouse model of prostate cancer that the tumor cells can initiate and promote expansion of stromal fibroblasts that lack the tumor-suppressor protein p53 through a paracrine mechanism. This results in selection of highly proliferative fibroblasts associated with the carcinoma that further promote tumor progression.

Genomic and proteomic analysis of mammary tumors arising in transgenic mice

Transforming growth factor-beta (TGF-beta) is the prototype of a large family of signaling molecules. TGF-beta signaling profoundly influences tumor development as demonstrated in several engineered mouse models. The present study was designed to identify differences by cDNA microarray and MALDI-TOF MS analyses in mammary carcinomas with and without TGF-beta signaling. The results demonstrate a significant potential for combination of profiling technologies to further understand the molecular mechanisms of breast cancer.

TGF-beta and cancer

TGF-beta signaling regulates tumorigenesis and in human cancer its signaling pathways are often modified during tumor progression. Prior to initiation and early during progression TGF-beta acts upon the epithelium as a tumor suppressor, however at later stages it is often a tumor promoter. Over the years, many studies have focused on the epithelial cell autonomous role for TGF-beta, however, TGF-beta is not strictly limited to this compartment in vivo. Recent studies addressing TGF-beta mediated stromal-epithelial interactions have significantly improved our understanding related to the regulation of cancer. In addition, stromal fibroblast cell autonomous effects have been observed in response to TGF-beta stimulation. According to the current literature and experimental evidence, TGF-beta is a potent ligand that regulates carcinoma initiation, progression and metastasis through a broad and complex spectrum of interdependent interactions.

Proliferation of estrogen receptor-alpha-positive mammary epithelial cells is restrained by transforming growth factor-beta1 in adult mice

Transforming growth factor (TGF)-beta1 is a potent inhibitor of mammary epithelial proliferation. In human breast, estrogen receptor (ER)-alpha cells rarely co-localize with markers of proliferation, but their increased frequency correlates with breast cancer risk. To determine whether TGF-beta1 is necessary for the quiescence of ER-alpha-positive populations, we examined mouse mammary epithelial glands at estrus. Approximately 35% of epithelial cells showed TGF-beta1 activation, which co-localized with nuclear receptor-phosphorylated Smad 2/3, indicating that TGF-beta signaling is autocrine. Nuclear Smad co-localized with nuclear ER-alpha. To test whether TGF-beta inhibits proliferation, we examined genetically engineered mice with different levels of TGF-beta1. ER-alpha co-localization with markers of proliferation (ie, Ki-67 or bromodeoxyuridine) at estrus was significantly increased in the mammary glands of Tgf beta1 C57/bl/129SV heterozygote mice. This relationship was maintained after pregnancy but was absent at puberty. Conversely, mammary epithelial expression of constitutively active TGF-beta1 via the MMTV promoter suppressed proliferation of ER-alpha-positive cells. Thus, TGF-beta1 activation functionally restrains ER-alpha-positive cells from proliferating in adult mammary gland. Accordingly, we propose that TGF-beta1 dysregulation may promote proliferation of ER-alpha-positive cells associated with breast cancer risk in humans.

Loss of TGF-beta type II receptor in fibroblasts promotes mammary carcinoma growth and invasion through upregulation of TGF-alpha-, MSP- and HGF-mediated signaling networks

Stromal fibroblasts regulate epithelial cell behavior through direct and indirect cell-cell interactions. To clarify the role of TGF-beta signaling in stromal fibroblasts during mammary development and tumorigenesis, we conditionally knocked out the TGF-beta type II receptor gene in mouse mammary fibroblasts (Tgfbr2(fspKO)). Tgfbr2(fspKO) mice exhibit defective mammary ductal development, characterized in part by increased ductal epithelial cell turnover associated with an increase in stromal fibroblast abundance. Tgfbr2(fspKO) mammary fibroblasts transplanted with mammary carcinoma cells promote growth and invasion, which is associated with increased activating phosphorylation of the receptors: erbB1, erbB2, RON, and c-Met. Furthermore, the increased receptor phosphorylation correlates with increased secretion of the cognate ligands by Tgfbr2(fspKO) fibroblasts. Treatment of tumor cells with fibroblast-conditioned medium leads to increased tumor cell proliferation and motility, which are blocked by addition of pharmacologic inhibitors of TGF-alpha signaling or neutralizing antibodies to macrophage-stimulating protein (MSP), HGF, or c-Met. These studies characterize a significant role for stromal TGF-beta signaling in mammary tissue homeostasis and mammary tumor progression via regulation of TGF-alpha, MSP, and HGF signaling pathways.

Signaling cross-talk between IGF-binding protein-3 and transforming growth factor-(beta) in mesenchymal chondroprogenitor cell growth

Cartilage formation is driven by mesenchymal chondroprogenitor cells (MCCs) that proliferate and differentiate into chondrocytes. The molecular mechanisms by which growth factors regulate MCC fate are not well defined. Insulin-like growth factor binding protein-3 (IGFBP-3) has intrinsic bioactivity that is independent of IGF binding. We previously reported that IGFBP-3 has IGF-independent antiproliferative and apoptotic effects in MCCs, and requires STAT-1 activation to mediate its apoptotic effect. Transforming growth factor-beta (TGF-beta) is a key chondroinductive growth factor. The objective of the study is to define the interactions between IGFBP-3 and TGF-beta in MCC growth and their intracellular signaling pathways. We used the RCJ3*1C5*18 mesenchymal chondrogenic cells that without biochemical or oncogenic transformation progress in culture from MCCs to differentiated chondrocytes. Cell proliferation was assessed in MCCs treated with IGFBP-3 or transfected with IGFBP-3, in the presence or absence of TGF-beta. To demonstrate that IGFBP-3 effects were IGF-independent an IGFBP-3 analog that lacks IGF binding was used (GGG-IGFBP-3). To determine the functional roles of the TGF-beta-mediated signaling and the STAT-1 pathway, cells were either stably transfected with a dominant negative TGF-beta type II receptor (MCC-DNTbetaRII) or treated with a STAT-1 morpholino antisense oligonucleotide. We found that in MCCs, TGF-beta antagonized the antiproliferative effect of IGFBP-3. IGFBP-3 increased the cyclin-dependent kinase inhibitor p21 expression and this effect was abolished by TGF-beta. Furthermore, TGF-beta inhibited STAT-1 phosphorylation induced by IGFBP-3. Similarly to TGF-beta, STAT-1 antisense oligonucleotide inhibited the IGFBP-3 antiproliferative action. Although TGF-beta in MCC-DNTbetaRII lacked Smad-mediated signaling, it persistently antagonized the IGFBP-3 antiproliferative action. However, TGF-beta even in MCC-DNTbetaRII cells induced ERK1/2 phosphorylation, and treatment with MEK inhibitor, UO126, inhibited the antagonistic effects of TGF-beta on IGFBP-3. Furthermore, UO126 blocked the TGF-beta inhibition of STAT-1 phosphorylation induced by IGFBP-3. Collectively, these results demonstrate cross-talk between the IGFBP-3-dependent STAT-1 signaling and the TGF-beta-dependent ERK pathway that regulates MCC proliferation.

Activin receptor-like kinase 2 and Smad6 regulate epithelial-mesenchymal transformation during cardiac valve formation

Epithelial-mesenchymal transformation (EMT) occurs during both development and tumorigenesis. Transforming growth factor beta (TGFbeta) ligands signal EMT in the atrioventricular (AV) cushion of the developing heart, a critical step in valve formation. TGFbeta signals through a complex of type I and type II receptors. Several type I receptors exist although activin receptor-like kinase (ALK) 5 mediates the majority of TGFbeta signaling. Here, we demonstrate that ALK2 is sufficient to induce EMT in the heart. Both ALK2 and ALK5 are expressed throughout the heart with ALK2 expressed abundantly in endocardial cells of the outflow tract (OFT), ventricle, and AV cushion. Misexpression of constitutively active (ca) ALK2 in non-transforming ventricular endocardial cells induced EMT, while caALK5 did not, thus demonstrating that ALK2 activity alone is sufficient to stimulate EMT. Smad6, an inhibitor of Smad signaling downstream of ALK2, but not ALK5, inhibited EMT in AV cushion endocardial cells. These data suggest that ALK2 activation may stimulate EMT in the AV cushion and that Smad6 may act downstream of ALK2 to negatively regulate EMT.

Tumor-stroma interactions

The importance of stromal cells and the factors that they express during cancer initiation and progression has been highlighted by recent literature. The cellular components of the stroma of epithelial tissues are well-recognized as having a supportive role in carcinogenesis, where the initiating mutations of a tumor originate in the epithelial cells. The use of mouse models and xenografts suggests that mutations in the stromal fibroblasts can also initiate epithelial tumors. Many of these tumors result from the alteration of paracrine growth factor pathways that act on the epithelia. However, the tissue specificity of the responses to the growth factors is a mystery not yet solved.

Inactivation of TGF-beta signaling in hepatocytes results in an increased proliferative response after partial hepatectomy

The transforming growth factor beta (TGF-beta) signaling pathway, which is activated by the TGF-beta receptor complex consisting of type I and type II TGF-beta receptors (TGFBR1 and TGFBR2), regulates cell growth and death. TGF-beta and components of its signaling pathway, particularly TGFBR2, have been implicated as tumor suppressor genes and important antimitogenic factors in the gastrointestinal tract and liver. An in vivo approach to study these effects has been hindered by the embryonic lethality of Tgfbr2(-/-) mice and poor viability of the Tgfb1(-/-) mice. Consequently, we have developed a hepatocyte-specific Tgfbr2 knockout mouse, the Alb-cre Tgfbr2(flx/flx) mouse, to study the physiologically relevant effects of TGF-beta signaling on epithelial cell proliferation in vivo. After 70% hepatectomy, we observed increased proliferation and an increased liver mass : body weight ratio in the Alb-cre Tgfbr2(flx/flx) mice compared to Tgfbr2(flx/flx) mice. We also observed decreased expression and increased phosphorylation of p130 in the livers from the Alb-cre Tgfbr2(flx/flx) mice as well as increased expression of cyclin E, which is transcriptionally regulated, in part, by p130:E2F4. Consistent with these results, in a hepatocyte cell line derived from the Tgfbr2(flx/flx) mice, we found that TGF-beta increases the nuclear localization of E2F4, and presumably the transcriptional repression of the p130:E2F4 complex. Thus, we have demonstrated that TGF-beta signaling in vivo regulates the mitogenic response in the regenerating liver, affecting the liver mass : body weight ratio after partial hepatectomy, and that these mitogenic responses are accompanied by alterations in p130 expression and phosphorylation, implicating p130 as one of the proteins regulated in vivo by TGF-beta during liver regeneration.

Activation of the Erk pathway is required for TGF-beta1-induced EMT in vitro

Transforming growth factor-beta1 (TGF-beta1) can be tumor-suppressive through the activation of the Smad-mediated signaling pathway. TGF-beta1 can also enhance tumor progression by stimulating epithelial-to-mesenchymal transition (EMT) through additional pathways. EMT is characterized by the acquisition of a fibroblast-like cell morphology, dissolution of tight junctions, disruption of adherence junctions, and formation of actin stress fibers. There is evidence linking the activation of mitogen-activated protein kinase pathways to the induction of TGF-beta1-mediated EMT. However, the role of Erk in the induction of TGF-beta1-mediated EMT remains unclear. TGF-beta1 treatment of normal murine mammary gland (NMuMG) epithelial cells resulted in increased gene expression of Ras, Raf, MEK1/2, and Erk1/2, as shown by microarray analysis and real-time polymerase chain reaction. Upon 24 and 48 hours of treatment with TGF-beta1, NMuMG and mouse cortical tubule (MCT) epithelial cells underwent EMT as shown by changes in cell morphology, delocalization of zonula occludens-1 and E-cadherin from cell-cell junctions, and formation of actin stress fibers. TGF-beta1 treatment also resulted in increased levels of phosphorylated Erk and Erk kinase activity. Treatment with an MEK inhibitor, U0126, inhibited increased Erk phosphorylation and kinase activity, and blocked TGF-beta1-induced EMT in both cell lines. These data show that TGF-beta1 induces the activation of the Erk signaling pathway, which is required for TGF-beta1-mediated EMT in vitro.

Effect of conditional knockout of the type II TGF-beta receptor gene in mammary epithelia on mammary gland development and polyomavirus middle T antigen induced tumor formation and metastasis

Transforming growth factor-beta (TGF-beta) isoforms are growth factors that function physiologically to regulate development, cellular proliferation, and immune responses. The role of TGF-beta signaling in mammary tumorigenesis is complex, as TGF-beta has been reported to function as both a tumor suppressor and tumor promoter. To elucidate the role of TGF-beta signaling in mammary gland development, tumorigenesis, and metastasis, the gene encoding type II TGF-beta receptor, Tgfbr2, was conditionally deleted in the mammary epithelium (Tgfbr2MGKO). Loss of Tgfbr2 in the mammary epithelium results in lobular-alveolar hyperplasia in the developing mammary gland and increased apoptosis. Tgfbr2MGKO mice were mated to the mouse mammary tumor virus-polyomavirus middle T antigen (PyVmT) transgenic mouse model of metastatic breast cancer. Loss of Tgfbr2 in the context of PyVmT expression results in a shortened median tumor latency and an increased formation of pulmonary metastases. Thus, our studies support a tumor-suppressive role for epithelial TGF-beta signaling in mammary gland tumorigenesis and show that pulmonary metastases can occur and are even enhanced in the absence of TGF-beta signaling in the carcinoma cells.

Conditional deletion of the TGF-beta type II receptor in Col2a expressing cells results in defects in the axial skeleton without alterations in chondrocyte differentiation or embryonic development of long bones

Members of the TGF-beta superfamily are secreted signaling proteins that regulate many aspects of development including growth and differentiation in skeletal tissue. There are three isoforms of TGF-beta that act through the same heteromeric receptor complex. To address the question of the role of TGF-beta signaling in skeletal development, we generated mice with a conditional deletion of the TGF-beta type II receptor gene (Tgfbr2) specifically in Col2a expressing cells using the Cre/lox recombinase system. Alizarin red-/Alcian blue-stained skeletons were prepared from embryos at 17.5, 15.5, and 13.5 days of gestation. Col2acre+/-;TgfbrloxP/loxP and Col2acre-/-;Tgfbr2+/loxP skeletons were compared. Multiple defects were observed in the base of the skull and in the vertebrae. Specifically, the size and spacing of the vertebrae were altered, and defects were detected in the closure of the neural arches. In addition, alterations in transverse processes, costal joints, and zygapophyses were detected. While the vertebral bodies were only moderately affected, the intervertebral discs (IVDs) were either missing or incomplete. Alterations in the vertebrae could be detected as early as E13.5 days. Surprisingly, alterations in length and mineralization of long bones were not detected at E17.5 days. In addition, the expression patterns of markers for chondrocyte differentiation were not altered in vertebrae or long bones suggesting that loss of responsiveness to TGF-beta in chondrocytes does not affect embryonic endochondral bone formation. In contrast, mice that survived postnatally demonstrated alterations in the length of specific bones. Skeletons from Col2acre+/-;Tgfbr2loxP/loxP mice were compared to those from mice null for the TGF-beta2 ligand. The differences observed between these models allow distinctions to be made between the roles of the various isoforms of TGF-beta and the signaling in specific cell types. The data provide information regarding mechanisms of skeletal development and suggest that TGF-beta signaling is a critical component.

Keratin 19 gene drives Cre recombinase expression throughout the early postimplantation mouse embryo

The development of Cre-lox technology has created new opportunities for studying the tissue-specific functions of genes in vivo during development and disease. We analyzed the spatial and temporal activity of Cre recombinase whose coding sequence was inserted into the endogenous locus for keratin 19. Rather than providing epithelial-specific recombination during organogenesis, this K19cre allele allows unexpected recombination in early embryonic development, resulting in recombination of a loxP-flanked allele throughout all tissues of the mouse, but with sparing of the extraembryonic endoderm, including the anterior visceral endoderm.