Smad3 mutant mice develop metastatic colorectal cancer

Overexpression of a dominant-negative type II TGFbeta receptor tagged with green fluorescent protein inhibits the effects of TGFbeta on cell growth and gene expression of mouse adrenal tumor cell line Y-1 and enhances cell tumorigenicity

Transforming growth factor beta (TGFbeta) has been reported to be a potent growth inhibitor of epithelial cells. The purpose of the present work was to study in vitro and in vivo the effects of overexpression of a dominant-negative type II TGFbeta receptor on the proliferation and differentiation of Y-1 cells. Stable transfections were performed with a mutant TbetaRII (TbetaRII-KR) fused with the Enhanced Fluorescent Green Protein (EGFP). The expression of this fusion protein and its overexpression were demonstrated by northern blot and immunoblot with EGFP and TbetaRII probes and antibodies respectively. The membrane localization of this fusion protein was confirmed by confocal microscopy. The functionality of this fusion protein was demonstrated by its blocking effects on TGFbeta action on DNA synthesis and on Y-1 expression of steroidogenic acute regulatory protein (StAR) and 3beta-hydroxysteroid dehydrogenase (3beta-HSD). Moreover, in nude mice the tumorigenicity of cells stably transfected with the fusion protein was higher than that of cells stably transfected with EGFP alone. Taken together, the present results show that TbetaRII-KR/EGFP blocks the effects of TGFbeta1 on Y-1 cells and acts as a potent dominant-negative receptor preventing TGFbeta signaling.

Mouse tumor model for neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder characterized by increased incidence of benign and malignant tumors of neural crest origin. Mutations that activate the protooncogene ras, such as loss of Nf1, cooperate with inactivating mutations at the p53 tumor suppressor gene during malignant transformation. One hundred percent of mice harboring null Nf1 and p53 alleles in cis synergize to develop soft tissue sarcomas between 3 and 7 months of age. These sarcomas exhibit loss of heterozygosity at both gene loci and express phenotypic traits characteristic of neural crest derivatives and human NF1 malignancies.

Genetic pathways in colorectal and other cancers

Cells from cancers show aberrant behaviour such as unrestrained growth, invasion into adjacent tissue and metastasis. All these features of cancer cell behaviour can be explained in terms of genetic changes and the functional impact of these changes. In this review, colorectal cancer (CRC) is examined as a classical example of multistep carcinogenesis. First there is an overview which shows that cancers develop by a process of somatic evolution. This gives rise to preferred genetic pathways of tumorigenesis. The factors which may influence the development and ultimate choice of genetic pathways are then examined. Next, CRC is studied as a specific disease and the putative genetic pathways are described. The mutations that comprise these pathways and the possible functional sequelae of these are explored. The review concludes with a look at those avenues which may further elucidate the natural history of CRC and lead to improved therapy.

TGF-beta and cancer

The relationships between transforming growth factor-beta (TGF-beta) and cancer are varied and complex. The paradigm that is emerging from the experimental evidence accumulated over the past decade or so is that TGF-beta can play two different and opposite roles with respect to the process of malignant progression. During early stages of carcinogenesis, TGF-beta acts predominantly as a potent tumor suppressor and may mediate the actions of chemopreventive agents such as retinoids and nonsteroidal anti-estrogens. However, at some point during the development and progression of malignant neoplasms, bioactive TGF-betas make their appearance in the tumor microenvironment and the tumor cells escape from TGF-beta-dependent growth arrest. In many cases, this resistance to TGF-beta is the consequence of loss or mutational inactivation of the genes that encode signaling intermediates. These include the types I and II TGF-beta receptors, as well as receptor-associated and common-mediator Smads. The stage of tumor development or progression at which TGF-beta-resistant clones come to dominate the tumor cell population in different types of neoplasm remains to be defined. The phenotypic switch from TGF-beta-sensitivity to TGF-beta-resistance that occurs during carcinogenesis has several important implications for cancer prevention and treatment.

Generation of Polymeric Immunoglobulin Receptor-Deficient Mouse with Marked Reduction of Secretory IgA

We generated mouse lacking exon 2 of polymeric Ig receptor (pIgR) gene by a gene-targeting strategy (pIgR-deficient mouse; pIgR−/− mouse) to define the physiological role of pIgR in the transcytosis of Igs. pIgR−/− mice were born at the expected ratio from a cross between pIgR+/− mice, indicating that disruption of the pIgR gene in mice is not lethal. pIgR and secretory component proteins were not detected in pIgR−/− mice by Western blot analysis. Moreover, immunohistochemical analysis showed that pIgR protein is not expressed in jejunal and colonic epithelial cells of pIgR−/− mice, whereas IgA+ cells are present in the intestinal mucosa of pIgR−/− mice as well as wild-type littermates. Disruption of the pIgR gene caused a remarkable increase in serum IgA concentration and a slight increment of serum IgG and IgE levels, leaving serum IgM level unaltered. In contrast, IgA was much reduced but not negligible in the bile, feces, and intestinal contents of pIgR−/− mice. Additionally, IgA with a molecular mass of 280 kDa preferentially accumulated in the serum of pIgR−/− mice, suggesting that transepithelial transport of dIgA is severely blocked in pIgR−/− mice. These results demonstrate that dIgA is mainly transported by pIgR on the epithelial cells of intestine and hepatocytes, but a small quantity of IgA may be secreted via other pathways.

Smad3 inhibits transforming growth factor-beta and activin signaling by competing with Smad4 for FAST-2 binding

Transcriptional regulation by transforming growth factor-beta and activin is mediated by interaction of Smad2 and Smad3 with specific transcription factors and/or DNA elements. However, Smad3 behaves differently from Smad2 in regulating transcription by a winged-helix transcription factor, FAST-2, on an activin-responsive element (ARE) in the Xenopus Mix.2 promoter. Smad3 alone was able to stimulate the ARE through FAST-2, but inhibited the ARE transactivation mediated by Smad2/Smad4 following receptor activation. We characterized the functional domains that are involved in these two activities of Smad3. Deletion of the MH1 domain as well as mutations of four lysine residues in the MH1 domain abrogated the inhibitory activity of Smad3, but did not compromise the self-stimulatory function. In contrast, deletion of the MH2 domain or a point mutation of glycine 379 within this domain obliterated the self-stimulatory activity of Smad3, but not the inhibitory activity. In an electrophoretic mobility shift assay, we found that Smad3 was able to associate with the FAST-2.ARE complex and that this association was dependent on FAST-2. In addition, Smad3 was not able to directly bind the ARE in a DNase I protection assay, in which FAST-2 binds the ARE around a motif (TGTGTATT) previously characterized to associate with the human FAST-1 protein. Interestingly, Smad4 was also able to directly associate with the FAST-2.ARE complex through binding with FAST-2. In a gel shift assay, the association of FAST-2 with Smad4 was mutually exclusive from the association with Smad3. Taken together, these data indicate that Smad3 exerts the inhibitory activity by competitive association with FAST-2.

A Deletion in the Gene for Transforming Growth Factor β Type I Receptor Abolishes Growth Regulation by Transforming Growth Factor β in a Cutaneous T-Cell Lymphoma

Spontaneous regression of skin lesions is characteristic of lymphomatoid papulosis (LyP), a clonal cutaneous lymphoproliferative disorder. A minority of LyP patients progress to anaplastic large cell lymphoma (ALCL) in which skin lesions no longer regress and extracutaneous dissemination often occurs. In 1 such case, we developed a tumor cell line, JK cells, and show that these cells are resistant to the growth inhibitory effects of transforming growth factor β (TGF-β) due to the loss of cell surface expression of the TGF-β type I receptor (TβR-I). Reverse transcriptase-polymerase chain reaction (RT-PCR) and sequencing of JK cell TβR-I cDNA clones identified a deletion that spanned the last 178 bp of exon 1, including the initiating methionine. Hybridization of a radiolabeled fragment internal to the deletion was detected in the genomes of TGF-β–responsive cells, but not in JK cells, indicating that they contain no wild-type TβR-I gene. PCR primers that flanked the deleted TβR-I region amplified a single band from JK cell genomic DNA that lacked the last 178 bp of exon 1 and all of the ≈ 5 kb of intron 1. This JK cell-specific genomic TβR-I PCR product was distinct from products amplified from TGF-β–responsive cells and was also readily detected in tumor biopsies obtained before the establishment of the JK cell line. Our results identify the first inactivating mutation in TβR-I gene in a human lymphoma that renders it insensitive to growth inhibition by TGF-β.

Interaction of the Ski oncoprotein with Smad3 regulates TGF-beta signaling

TGF-beta treatment of cells induces a variety of physiologic responses, including growth inhibition, differentiation, and induction of apoptosis. TGF-beta induces phosphorylation and nuclear translocation of Smad3. We describe here the association of Smad3 with the nuclear protooncogene protein Ski in response to the activation of TGF-beta signaling. Association with Ski represses transcriptional activation by Smad3, and overexpression of Ski renders cells resistant to the growth-inhibitory effects of TGF-beta. The transcriptional repression as well as the growth resistance to TGF-beta by overexpression of Ski can be overcome by overexpression of Smad3. These results demonstrate that Ski is a novel component of the TGF-beta signaling pathway and shed light on the mechanism of action of the Ski oncoprotein.

Mice lacking Smad3 show accelerated wound healing and an impaired local inflammatory response

The generation of animals lacking SMAD proteins, which transduce signals from transforming growth factor-beta (TGF-beta), has made it possible to explore the contribution of the SMAD proteins to TGF-beta activity in vivo. Here we report that, in contrast to predictions made on the basis of the ability of exogenous TGF-beta to improve wound healing, Smad3-null (Smad3ex8/ex8) mice paradoxically show accelerated cutaneous wound healing compared with wild-type mice, characterized by an increased rate of re-epithelialization and significantly reduced local infiltration of monocytes. Smad3ex8/ex8 keratinocytes show altered patterns of growth and migration, and Smad3ex8/ex8 monocytes exhibit a selectively blunted chemotactic response to TGF-beta. These data are, to our knowledge, the first to implicate Smad3 in specific pathways of tissue repair and in the modulation of keratinocyte and monocyte function in vivo.

Remarkable versatility of Smad proteins in the nucleus of transforming growth factor-beta activated cells

Smad proteins were identified three years ago as intracellular mediators of signaling by Transforming Growth Factor-beta (TGF-beta) family members. Two subclasses of the Smad proteins, the receptor-regulated Smads and common mediator Smads, transduce signals from the cell surface to the nucleus, where they participate in the regulation of gene expression. Meanwhile, it has become evident that Smads should be envisaged as very versatile proteins, which integrate multiple signaling pathways and can directly affect target gene expression in many ways. Indeed, their direct binding to DNA and their interaction in the nucleus with non-Smad proteins, many of which are DNA-binding activators or repressors of transcription uncover a unique but complex mode of action. We summarize some of the most recent data with regard to this aspect in this rapidly advancing field.

Evidence that Smad2 is a tumor suppressor implicated in the control of cellular invasion

The Smad2 protein plays an essential role in the transforming growth factor-beta (TGF-beta) signaling pathway. This pathway mediates growth inhibitory signals from the cell surface to the nucleus. Although Smad2 protein is significantly mutated in human cancers, there is no definitive evidence implicating Smad2 as a tumor-suppressor gene. Here we show that overexpression of the tumor-derived missense mutation Smad2.D450E, an unphosphorylable form of Smad2 found in colorectal and lung cancers, did not abolish the TGF-beta-mediated growth arrest, suggesting that resistance to the growth-inhibiting effects of TGF-beta exhibited by human tumors cannot be linked to the inactivation of Smad2 protein. In contrast, overexpression of Smad2.D450E induces cellular invasion, and this effect was enhanced by TGF-beta. A similar invasive phenotype was obtained in cells expressing another inactivating mutation in Smad2 (Smad2.P445H) found in colorectal cancer. These findings indicate that genetic defects in Smad2 are sufficient to confer the invasion-promoting effect of TGF-beta and reveal that TGF-beta acts through Smad2 to induce cellular invasion by a novel mechanism that is independent of Smad2 phosphorylation by the activated TGF-beta type I receptor.

Genetic and epigenetic contributions to colorectal cancer

Both genetic and epigenetic factors contribute to the development of colorectal cancer. Specific genetic changes in proto-oncogenes, tumor suppressor genes, and DNA mismatch repair genes have led to a genetic model of colorectal tumorigenesis. Recent data highlight the importance of the TGF-beta signaling pathway in regulating the progression of colorectal cancer. The loss of the tumor suppressor activity of this pathway as well as the potentially cooperative genetic aberrations involving APC, K-ras, and p53 are reviewed in the context of the multi-step adenoma-carcinoma sequence that characterizes the development of colorectal tumorigenesis. In addition, contributing epigenetic factors including age, diet, angiogenesis, and immune response are also discussed. Combining our knowledge of the genetic and epigenetic events implicated in this disease may allow a broader understanding of the pathogenesis of colorectal cancer and hence the design of better anti-tumor interventions.

Cryptorchidism in mice mutant for Insl3

Impaired testicular descent (cryptorchidism) is one of the most frequent congenital abnormalities in humans, involving 2% of male births. Cryptorchidism can result in infertility and increases risk for development of germ-cell tumours. Testicular descent from abdomen to scrotum occurs in two distinct phases: the trans-abdominal phase and the inguino-scrotal phase. Currently, little is known about the factors that regulate the trans-abdominal phase of testicular descent. Leydig insulin-like hormone (Insl3) is a member of the insulin hormone superfamily expressed in the developing testis. We show here that mice mutant for Insl3 are viable, but exhibit bilateral cryptorchidism due to developmental abnormalities of the gubernaculum, resulting in abnormal spermatogenesis and infertility. Female homozygotes have impaired fertility associated with deregulation of the oestrus cycle. These findings reveal roles for Insl3 in the development of the urogenital tract and in female fertility. Insl3 may act as a hormone to regulate the growth and differentiation of the gubernaculum, thereby mediating intra-abdominal testicular descent.

Localization of Smads, the TGF-beta family intracellular signaling components during endochondral ossification

Members of the transforming growth factor-beta (TGF-beta) family transduce signals from the cell membrane to the nucleus via specific type I and type II receptors and Smad proteins. Smad1 and Smad5 mediate intracellular signaling of bone morphogenetic protein (BMP), whereas Smad2 and Smad3 transduce TGF-beta signaling. Smad4 is a common mediator required for both pathways. Smad6 and Smad7 inhibit signaling by members of the TGF-beta superfamily. Here, we examined the expression of Smad1 to Smad7 proteins during endochondral ossification of epiphyseal plate of growing rats using immunohistochemical techniques. The expression of Smad proteins was correlated with the expression of TGF-beta1 and its receptors, and BMP-2/4 and BMP receptors. The results show that TGF-beta1 and BMP-2/4 were actively expressed in chondrocytes that are undergoing proliferation and maturation, which overlaps with expression of their corresponding type I and type II receptors. The Smads, however, exhibited a distinct expression pattern, respectively. For example, Smad1 and Smad5 were highly expressed in proliferating chondrocytes and in those chondrocytes that are undergoing maturation. The TGF-beta/activin-restricted Smads were also expressed in a nearly complementary fashion; Smad2 was strongly expressed in proliferating chondrocytes, whereas Smad3 was strongly observed in maturing chondrocytes. Smad4 was broadly expressed in all zones of epiphyseal plate. Inhibitory Smads, Smad6 and Smad7, were strongly expressed in the zone of cartilage that contained mature chondrocytes. Our findings show a colocalization of the pathway-restricted and inhibitory Smads with activating ligands or ligands whose action they antagonize and their receptors in various zones of epiphyseal growth plate, suggesting that TGF-beta superfamily Smad signaling pathways plays a morphogenic role during endochondral bone formation.

Evidence for control of nitric oxide synthesis by intracellular transforming growth factor-beta1 in tumor cells. Implications for tumor development

Transforming growth factor-beta1 (TGF-beta1) has been shown to down-regulate NO synthesis in a variety of normal cells. In the present study, we investigated the influence of TGF-beta1 upon NO production in tumor cells and its consequences for tumor development. During the growth of PROb colon carcinoma cells intraperitoneally injected in syngeneic BDIX rats, intratumoral concentration of TGF-beta1 increases while NO concentration stays very low. Tumor regression induced by intraperitoneal injections of a lipid A is associated with a decrease in TGF-beta1 and an increase in NO intratumoral concentration. In these tumors, PROb tumor cells are the NO- and TGF-beta1-secreting cells. Using PROb cells transfected with an expression vector coding for TGF-beta1 antisense mRNA, we demonstrate in vitro that there is an inverse correlation between the amount of TGF-beta1 secreted and the ability of PROb cells to secrete NO. As the same results were obtained in the presence of an anti-TGF-beta type II receptor neutralizing antibody, and as exogenous TGF-beta1 is without any effect on NO secretion by PROb cells, TGF-beta1 apparently down-regulates NO synthesis in PROb cells by an intracellular mechanism. These results suggest that endogenous TGF-beta1 constitutes a potential target in a search for new antitumoral agents.

Altered phenotype of HT29 colonic adenocarcinoma cells following expression of the DCC gene

On 18q, frequently deleted in late stage colorectal cancers, a gene, Deleted in Colon Cancer (DCC), has been identified and postulated to play a role as a tumor suppressor gene. DCC is retained in the majority of mucinous tumors, which produce high levels of mucins, and seems to be preferentially expressed in intestinal goblet cells. To investigate whether DCC is related to mucin expression and can modulate the transformed phenotype, we introduced a full-length DCC cDNA into HT29 cells, which can be induced in vitro to express MUC2, the gene that encodes the major colonic mucin. Expression of DCC did not modulate constitutive or induced expression of MUC2, nor did DCC induce a mature goblet cell phenotype. However, HT29 clones expressing high and low levels of DCC protein showed a significant decrease in cell proliferation and tumorigenicity. Furthermore, increased shedding and an elevated rate of spontaneous apoptosis were associated with higher levels of expression of DCC. In summary, while restoration of DCC expression in a human colon carcinoma cell line did not influence expression of differentiation markers, DCC expression did affect the growth and tumorigenic properties of the cells suggesting that DCC can modulate the malignant phenotype of colon cancer.

Angiogenesis defects and mesenchymal apoptosis in mice lacking SMAD5

The transforming growth factor-beta (TGF-beta) signals are mediated by a family of at least nine SMAD proteins, of which SMAD5 is thought to relay signals of the bone morphogenetic protein (BMP) pathway. To investigate the role of SMAD5 during vertebrate development and tumorigenesis, we disrupted the Smad5 gene by homologous recombination. We showed that Smad5 was expressed predominantly in mesenchyme and somites during embryogenesis, and in many tissues of the adult. Mice homozygous for the mutation died between days 10.5 and 11.5 of gestation due to defects in angiogenesis. The mutant yolk sacs lacked normal vasculature and had irregularly distributed blood cells, although they contained hematopoietic precursors capable of erythroid differentiation. Smad5 mutant embryos had enlarged blood vessels surrounded by decreased numbers of vascular smooth muscle cells, suffered massive apoptosis of mesenchymal cells, and were unable to direct angiogenesis in vitro. These data suggest that SMAD5 may regulate endothelium-mesenchyme interactions during angiogenesis and that it is essential for mesenchymal survival.

Smad5 knockout mice die at mid-gestation due to multiple embryonic and extraembryonic defects

Smad5 has been implicated as a downstream signal mediator for several bone morphogenetic proteins (BMPs). To understand the in vivo function of Smad5, we generated mice deficient in Smad5 using embryonic stem (ES) cell technology. Homozygous mutant embryos die between E9.5 and E11.5, and display variable phenotypes. Morphological defects are first detected at E8.0 in the developing amnion, gut and heart (the latter defect being similar to BMP-2 knockout mice). At later stages, mutant embryos fail to undergo proper turning, have craniofacial and neural tube abnormalities, and are edematous. In addition, several extraembryonic lesions are observed. After E9.0, the yolk sacs of the mutants contain red blood cells but lack a well-organized vasculature, which is reminiscent of BMP-4, TGF-beta1 and TGF-beta type II receptor knockout mice. In addition, the allantois of many Smad5 mutants is fused to the chorion, but is not well-elongated. A unique feature of the Smad5 mutant embryos is that ectopic vasculogenesis and hematopoiesis is observed in the amnion, likely due to mislocation of allantois tissue. Despite the expression of Smad5 from gastrulation onwards, and in contrast to knockouts of Smad2 and Smad4, Smad5 only becomes essential later in extraembryonic and embryonic development.

Inherited susceptibility to colorectal adenomas and carcinomas: evidence for a new predisposition gene on 15q14-q22

BACKGROUND & AIMS

The aim of this study was to evaluate the role of known colorectal adenoma and carcinoma susceptibility genes and to locate a novel susceptibility gene in an Ashkenazi family (SM1311) with dominantly inherited predisposition to colorectal adenomas and carcinomas.

METHODS

Clinicopathologic and family history data were collected. Genetic linkage and mutational analyses were used to investigate the genetic basis of the family's disease.

RESULTS

Affected members of SM1311 develop multiple tubular, villous, tubulovillous, and/or serrated colorectal adenomas throughout the large bowel, and some develop colon carcinoma. There are no extracolonic features clearly associated with disease in SM1311. We have shown that the family's phenotype does not result from APC mutations (including the I1307K variant) or from genetic changes in the other known genes that predispose to colon cancer. Using genetic linkage analysis, supplemented by allele loss in tumors, we have provided evidence for a new colorectal cancer susceptibility gene, CRAC1 (colorectal adenoma and carcinoma), mapping to chromosome 15q14-q22.

CONCLUSIONS

We provide evidence for a novel colorectal adenoma and carcinoma susceptibility gene on chromosome 15q14-q22. Further studies are needed to confirm this localization and to evaluate the contribution of CRAC1 to this disease.

Targeted disruption of Smad3 reveals an essential role in transforming growth factor beta-mediated signal transduction

The Smads are a family of nine related proteins which function as signaling intermediates for the transforming growth factor beta (TGF-beta) superfamily of ligands. To discern the in vivo functions of one of these Smads, Smad3, we generated mice harboring a targeted disruption of this gene. Smad3 null mice, although smaller than wild-type littermates, are viable, survive to adulthood, and exhibit an early phenotype of forelimb malformation. To study the cellular functions of Smad3, we generated Smad3 null mouse embryonic fibroblasts (MEFs) and dermal fibroblasts. We demonstrate that null MEFs have lost the ability to form Smad-containing DNA binding complexes and are unable to induce transcription from the TGF-beta-responsive promoter construct, p3TP-lux. Using the primary dermal fibroblasts, we also demonstrate that Smad3 is integral for induction of endogenous plasminogen activator inhibitor 1. We subsequently demonstrate that Smad3 null MEFs are partially resistant to TGF-beta's antiproliferative effect, thus firmly establishing a role for Smad3 in TGF-beta-mediated growth inhibition. We next examined cells in which Smad3 is most highly expressed, specifically cells of immune origin. Although no specific developmental defect was detected in the immune system of the Smad3 null mice, a functional defect was observed in the ability of TGF-beta to inhibit the proliferation of splenocytes activated by specific stimuli. In addition, primary splenocytes display defects in TGF-beta-mediated repression of cytokine production. These data, taken together, establish a role for Smad3 in mediating the antiproliferative effects of TGF-beta and implicate Smad3 as a potential effector for TGF-beta in modulating immune system function.

A mechanism of repression of TGFbeta/ Smad signaling by oncogenic Ras

TGFbeta can override the proliferative effects of EGF and other Ras-activating mitogens in normal epithelial cells. However, epithelial cells harboring oncogenic Ras mutations often show a loss of TGFbeta antimitogenic responses. Here we report that oncogenic Ras inhibits TGFbeta signaling in mammary and lung epithelial cells by negatively regulating the TGFbeta mediators Smad2 and Smad3. Oncogenically activated Ras inhibits the TGFbeta-induced nuclear accumulation of Smad2 and Smad3 and Smad-dependent transcription. Ras acting via Erk MAP kinases causes phosphorylation of Smad2 and Smad3 at specific sites in the region linking the DNA-binding domain and the transcriptional activation domain. These sites are separate from the TGFbeta receptor phosphorylation sites that activate Smad nuclear translocation. Mutation of these MAP kinase sites in Smad3 yields a Ras-resistant form that can rescue the growth inhibitory response to TGFbeta in Ras-transformed cells. EGF, which is weaker than oncogenic mutations at activating Ras, induces a less extensive phosphorylation and cytoplasmic retention of Smad2 and Smad3. Our results suggest a mechanism for the counterbalanced regulation of Smad2/Smad3 by TGFbeta and Ras signals in normal cells, and for the silencing of antimitogenic TGFbeta functions by hyperactive Ras in cancer cells.

Targeted disruption of SMAD3 results in impaired mucosal immunity and diminished T cell responsiveness to TGF-beta

SMAD3 is one of the intracellular mediators that transduces signals from transforming growth factor-beta (TGF-beta) and activin receptors. We show that SMAD3 mutant mice generated by gene targeting die between 1 and 8 months due to a primary defect in immune function. Symptomatic mice exhibit thymic involution, enlarged lymph nodes, and formation of bacterial abscesses adjacent to mucosal surfaces. Mutant T cells exhibit an activated phenotype in vivo, and are not inhibited by TGF-beta1 in vitro. Mutant neutrophils are also impaired in their chemotactic response toward TGF-beta. Chronic intestinal inflammation is infrequently associated with colonic adenocarcinoma in mice older than 6 months of age. These data suggest that SMAD3 has an important role in TGF-beta-mediated regulation of T cell activation and mucosal immunity, and that the loss of these functions is responsible for chronic infection and the lethality of Smad3-null mice.

Genetic pathways in colorectal and other cancers

Cells from cancers show aberrant behaviour such as unrestrained growth, invasion into adjacent tissue and metastasis. All these features of cancer cell behaviour can be explained in terms of genetic changes and the functional impact of these changes. In this review, colorectal cancer (CRC) is examined as a classical example of multistep carcinogenesis. First there is an overview which shows that cancers develop by a process of somatic evolution. This gives rise to preferred genetic pathways of tumorigenesis. The factors which may influence the development and ultimate choice of genetic pathways are then examined. Next, CRC is studied as a specific disease and the putative genetic pathways are described. The mutations that comprise these pathways and the possible functional sequelae of these are explored. The review concludes with a look at those avenues which may further elucidate the natural history of CRC and lead to improved therapy.

An antisense transcript to SMAD5 expressed in fetal and tumor tissues

SMAD5, a transducer of TGF-beta/BMP inhibitory signals and a tumor suppressor candidate, localizes to the region of invariant loss in human myeloid neoplasms, on chromosome 5q31.1. Recent evidence indicates a gene-dosage effect along the TGF-beta/BMP signaling pathways. We have identified a novel transcript designated DAMS, whose 3' exonic sequences contain in part an alternate 5' exon of SMAD5, in the antisense orientation. Expressed sequenced tags (ESTs) for DAMS are found in fetal tissues (heart, adrenal glands, and total fetus) and pancreatic tumor cDNA libraries. In contrast to SMAD5, DAMS expression is not readily detectable in adult and fetal tissues. Semiquantitative PCR suggests that the stoichiometry between SMAD5 and DAMS transcripts ranges between 15 and 120 in normal and malignant hematopoietic cells. The findings raise the possibility that DAMS may be a fail-safe mechanism for precise regulation of SMAD5 transcript levels that may be critical in maintaining normal homeostasis.

Mutation of Pten/Mmac1 in mice causes neoplasia in multiple organ systems

Pten/Mmac1+/- heterozygous mice exhibited neoplasms in multiple organs including the endometrium, liver, prostate, gastrointestinal tract, thyroid, and thymus. Loss of the wild-type allele was detected in neoplasms of the thymus and liver. Surprisingly, tumors of the gastrointestinal epithelium developed in association with gut lymphoid tissue. Tumors of the endometrium, thyroid, prostate, and liver were not associated with lymphoid tissue and appeared to be highly mitotic. In addition, these mice have nonneoplastic hyperplasia of lymph nodes that was caused by an inherited defect in apoptosis detected in B cells and macrophages. Examination of peripheral lymphoid tissue including lymphoid aggregates associated with polyps revealed that the normal organization of B and T cells was disrupted in heterozygous animals. Taken together, these data suggest that PTEN is a regulator of apoptosis and proliferation that behaves as a "landscaper" tumor suppressor in the gut and a "gatekeeper" tumor suppressor in other organs.

Smad6 suppresses TGF-beta-induced growth inhibition in COLO-357 pancreatic cancer cells and is overexpressed in pancreatic cancer

Transforming growth factor (TGF)-beta signaling is initiated by heterodimerization of TGF-beta receptor type I (TbetaRI) and type II (TbetaRII). Subsequently, the signal is transduced via Smad proteins, which upon phosphorylation and heterodimerization translocate to the nucleus and regulate gene transcription. Smad6 functions as an intracellular antagonist of TGF-beta signaling. In the present study we demonstrate that Smad6 is overexpressed in vivo in human pancreatic cancer cells. We also show that stable transfection of a full-length Smad6 construct into COLO-357 pancreatic cancer cells abrogates TGF-beta1 induced growth inhibition, and leads to enhanced anchorage-independent growth. Thus, enhanced expression of the TGF-beta signaling inhibitor Smad6 in pancreatic cancer may present a novel mechanism of TGF-beta resistance, which might have the potential to enhance the transformed phenotype of human cancer cells.

Growth factors in inflammatory bowel disease

The pathogenesis of both ulcerative colitis and Crohn's disease is unknown but these forms of inflammatory bowel disease (IBD) may be associated with an inability of the intestinal mucosa to protect itself from luminal challenges and/or inappropriate repair following intestinal injury. Numerous cell populations regulate these broad processes through the expression of a complex array of peptides and other agents. Growth factors can be distinguished by their actions regulating cell proliferation. These factors also mediate processes such as extracellular matrix formation, cell migration and differentiation, immune regulation, and tissue remodeling. Several families of growth factors may play an important role in IBD including: epidermal growth factor family (EGF) [transforming growth factor alpha (TGF alpha), EGF itself, and others], the transforming growth factor beta (TGF beta) super family, insulin-like growth factors (IGF), fibroblast growth factors (FGF), hepatocyte growth factor (HGF), trefoil factors, platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF) and others. Collectively these families may determine susceptibility of IBD mucosa to injury and facilitate tissue repair.

Growth factors in inflammatory bowel disease

The pathogenesis of both ulcerative colitis and Crohn's disease is unknown but these forms of inflammatory bowel disease (IBD) may be associated with an inability of the intestinal mucosa to protect itself from luminal challenges and/or inappropriate repair following intestinal injury. Numerous cell populations regulate these broad processes through the expression of a complex array of peptides and other agents. Growth factors can be distinguished by their actions regulating cell proliferation. These factors also mediate processes such as extracellular matrix formation, cell migration and differentiation, immune regulation, and tissue remodeling. Several families of growth factors may play an important role in IBD including: epidermal growth factor family (EGF) [transforming growth factor alpha (TGF alpha), EGF itself, and others], the transforming growth factor beta (TGF beta) super family, insulin-like growth factors (IGF), fibroblast growth factors (FGF), hepatocyte growth factor (HGF), trefoil factors, platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF) and others. Collectively these families may determine susceptibility of IBD mucosa to injury and facilitate tissue repair.

TGF-beta signaling blockade inhibits PTHrP secretion by breast cancer cells and bone metastases development

Breast cancer frequently metastasizes to the skeleton, and the associated bone destruction is mediated by the osteoclast. Growth factors, including transforming growth factor-beta (TGF-beta), released from bone matrix by the action of osteoclasts, may foster metastatic growth. Because TGF-beta inhibits growth of epithelial cells, and carcinoma cells are often defective in TGF-beta responses, any role of TGF-beta in metastasis is likely to be mediated by effects on the surrounding normal tissue. However, we present evidence that TGF-beta promotes breast cancer metastasis by acting directly on the tumor cells. Expression of a dominant-negative mutant (TbetaRIIDeltacyt) of the TGF-beta type II receptor rendered the human breast cancer cell line MDA-MB-231 unresponsive to TGF-beta. In a murine model of bone metastases, expression of TbetaRIIDeltacyt by MDA-MB-231 resulted in less bone destruction, less tumor with fewer associated osteoclasts, and prolonged survival compared with controls. Reversal of the dominant-negative signaling blockade by expression of a constitutively active TGF-beta type I receptor in the breast cancer cells increased tumor production of parathyroid hormone-related protein (PTHrP), enhanced osteolytic bone metastasis, and decreased survival. Transfection of MDA-MB-231 cells that expressed the dominant-negative TbetaRIIDeltacyt with the cDNA for PTHrP resulted in constitutive tumor PTHrP production and accelerated bone metastases. These data demonstrate an important role for TGF-beta in the development of breast cancer metastasis to bone, via the TGF-beta receptor-mediated signaling pathway in tumor cells, and suggest that the bone destruction is mediated by PTHrP.

Genetic events and the role of TGF beta in epithelial tumour progression

The mouse skin model of chemical carcinogenesis has been very well characterized with respect to epigenetic changes, which occur during tumour cell initiation, promotion and progression. The use of transgenic and gene knock-out mice has contributed greatly to knowledge in this area. The H-ras genetic locus has been shown to undergo multiple genetic changes, including mutagenic activation, amplification of the mutant gene, and loss of the normal allele. These different genetic events lead to thresholds of ras activity which contribute to different stages along the pathway to neoplasia. The genetic and epigenetic events which lead to tumour invasion and metastasis have been less well characterized than studies on tumour initiation and promotion, despite the fact that it is metastases which ultimately kill the animal/patient. In the mouse skin model, loss of p53 contributes to malignant conversion. Gene deletion of the INK4 locus is associated with transformation to a highly invasive spindle cell tumor phenotype. This spindle cell transformation can also be induced in vitro or in vivo by TGF beta 1, possible by synergizing with mutant H-ras. TGF beta can have both positive and negative effects on tumourigenesis, acting early as a tumour suppresser, but later as a stimulator of tumour invasion. It is this latter effect which may be clinically more significant, since many human tumours overexpress TGF beta, yet the majority still retain the intracellular signaling systems necessary for the cell to respond to this growth factor.

p53-independent role of MDM2 in TGF-beta1 resistance

Transforming growth factor-beta (TGF-beta) inhibits cell proliferation, and acquisition of TGF-beta resistance has been linked to tumorigenesis. A genetic screen was performed to identify complementary DNAs that abrogated TGF-beta sensitivity in mink lung epithelial cells. Ectopic expression of murine double minute 2 rescued TGF-beta-induced growth arrest in a p53-independent manner by interference with retinoblastoma susceptibility gene product (Rb)/E2F function. In human breast tumor cells, increased MDM2 expression levels correlated with TGF-beta resistance. Thus, MDM2 may confer TGF-beta resistance in a subset of tumors and may promote tumorigenesis by interference with two independent tumor suppressors, p53 and Rb.