The viral oncoprotein E1A blocks transforming growth factor beta-mediated induction of p21/WAF1/Cip1 and p15/INK4B

TGF-β signaling in health, disease, and therapeutics

Transforming growth factor (TGF)-β is a multifunctional cytokine expressed by almost every tissue and cell type. The signal transduction of TGF-β can stimulate diverse cellular responses and is particularly critical to embryonic development, wound healing, tissue homeostasis, and immune homeostasis in health. The dysfunction of TGF-β can play key roles in many diseases, and numerous targeted therapies have been developed to rectify its pathogenic activity. In the past decades, a large number of studies on TGF-β signaling have been carried out, covering a broad spectrum of topics in health, disease, and therapeutics. Thus, a comprehensive overview of TGF-β signaling is required for a general picture of the studies in this field. In this review, we retrace the research history of TGF-β and introduce the molecular mechanisms regarding its biosynthesis, activation, and signal transduction. We also provide deep insights into the functions of TGF-β signaling in physiological conditions as well as in pathological processes. TGF-β-targeting therapies which have brought fresh hope to the treatment of relevant diseases are highlighted. Through the summary of previous knowledge and recent updates, this review aims to provide a systematic understanding of TGF-β signaling and to attract more attention and interest to this research area.

WDR74 functions as a novel coactivator in TGF-β signaling

Smads are critical intracellular signal transducers for transforming growth factor-β (TGF-β) in mammalian cells. In this study, we have identified WD repeat-containing protein 74 (WDR74) as a novel transcriptional coactivator for Smads in the canonical TGF-β signaling pathway. Through direct interactions with Smad proteins, WDR74 enhances TGF-β-mediated phosphorylation and nuclear accumulation of Smad2 and Smad3. Consequently, WDR74 enables stronger transcriptional responses and more robust TGF-β-induced physiological responses. Our findings have elucidated a critical role of WDR74 in regulating TGF-β signaling.

Viruses as key modulators of the TGF-β pathway; a double-edged sword involved in cancer

Transforming growth factor-β (TGF-β) signaling pathway is a key network in cell signaling that controls vital processes such as proliferation, differentiation, apoptosis, epithelial-mesenchymal transition, and migration, thus acting as a double-edged sword in normal development and diseases, in particular organ fibrosis, vascular disorders, and cancer. Early in tumorigenesis, the pathway exerts anti-tumor effects through suppressing cell cycle and inducing apoptosis, while during late stages, it functions as a tumor promoter by enhancing tumor invasiveness and metastasis. This signaling pathway can be perturbed by environmental and genetic factors such as microbial interference and mutation, respectively. In this way, the present review describes the modulation of the TGF-β pathway by oncogenic human viral pathogens and other viruses. The main mechanisms by which viruses interferes with TGF-β signaling seems to be through (1) the alteration of either TGF-β protein expression or activation, (2) the modulation of the TGF-β receptors or SMADs factors (by interfering with their levels and functions), (3) the alteration of none-SMAD pathways, and (4) indirect interaction with the pathway by the modulation of transcriptional co-activator/repressor and regulators of the pathway. Given the axial role of this pathway in tumorigenesis, it can be regarded as an attractive target for cancer therapy. Hence, further investigations on this subject may represent molecular targets among either TGF-β signaling molecules or viral factors for the treatment and management of viral infection consequences such as cancer.

Phosphorylation: Implications in Cancer

Post translational modifications (PTMs) are involved in variety of cellular activities and phosphorylation is one of the most extensively studied PTM, which regulates a number of cellular functions like cell growth, differentiation, apoptosis and cell signaling in healthy condition. However, alterations in phosphorylation pathways result in serious outcomes in the form of diseases, especially cancer. Many signalling pathways including Tyrosine kinase, MAP kinase, Cadherin-catenin complex, Cyclin-dependent kinase etc. are major players of the cell cycle and deregulation in their phosphorylation-dephosphorylation cascade has been shown to be manifested in the form of various types of cancers. Tyrosine kinase family encompasses the greatest number of oncoproteins. MAPK cascade has an importance role in cancer growth and progression. Bcl-2 family proteins serve either proapoptotic or antiapoptotic function. Cadherin-catenin complex regulates cell adhesion properties and cyclins are the key regulators of cell cycle. Altered phosphorylations in any of the above pathways are strongly associated with cancer, at the same time they serve as the potential tergets for drug development against cancer. Drugs targeting tyrosine kinase are potent anticancer drugs. Inhibitors of MEK, PI3K and ERK signalling pathways are undergoing clinical trials. Thus, drugs targeting phosphorylation pathways represent a promising area for cancer therapy.

Platelet activating factor-induced expression of p21 is correlated with histone acetylation

Ultraviolet (UV)-irradiated keratinocytes secrete the lipid mediator of inflammation, platelet-activating factor (PAF). PAF plays an essential role in UV-induced immune suppression and skin cancer induction. Dermal mast cell migration from the skin to the draining lymph nodes plays a prominent role in activating systemic immune suppression. UV-induced PAF activates mast cell migration by up-regulating mast cell CXCR4 surface expression. Recent findings indicate that PAF up-regulates CXCR4 expression via histone acetylation. UV-induced PAF also activates cell cycle arrest and disrupts DNA repair, in part by increasing p21 expression. Do epigenetic alterations play a role in p21 up-regulation? Here we show that PAF increases Acetyl-CREB-binding protein (CBP/p300) histone acetyltransferase expression in a time and dose-dependent fashion. Partial deletion of the HAT domain in the CBP gene, blocked these effects. Chromatin immunoprecipitation assays indicated that PAF-treatment activated the acetylation of the p21 promoter. PAF-treatment had no effect on other acetylating enzymes (GCN5L2, PCAF) indicating it is not a global activator of histone acetylation. This study provides further evidence that PAF activates epigenetic mechanisms to affect important cellular processes, and we suggest this bioactive lipid can serve as a link between the environment and the epigenome.

Zinc finger protein 451 is a novel Smad corepressor in transforming growth factor-β signaling

ZNF451 is a transcriptional cofactor localized to promyelocytic leukemia bodies. Here, we present evidence demonstrating that ZNF451 physically interacts with Smad3/4 and functionally inhibits TGF-β signaling. Increased expression of ZNF451 attenuates TGF-β-induced growth inhibitory and gene transcriptional responses, whereas depletion of ZNF451 enhances TGF-β responses. Mechanistically, ZNF451 blocks the ability of Smad3/4 to recruit p300 in response to TGF-β, which causes reduction of histone H3K9 acetylation on the promoters of TGF-β target genes. Taken together, ZNF451 acts as a transcriptional corepressor for Smad3/4 and negatively regulates TGF-β signaling.

The transcriptome of the adenovirus infected cell

Alternations of cellular gene expression following an adenovirus type 2 infection of human primary cells were studied by using superior sensitive cDNA sequencing. In total, 3791 cellular genes were identified as differentially expressed more than 2-fold. Genes involved in DNA replication, RNA transcription and cell cycle regulation were very abundant among the up-regulated genes. On the other hand, genes involved in various signaling pathways including TGF-β, Rho, G-protein, Map kinase, STAT and NF-κB stood out among the down-regulated genes. Binding sites for E2F, ATF/CREB and AP2 were prevalent in the up-regulated genes, whereas binding sites for SRF and NF-κB were dominant among the down-regulated genes. It is evident that the adenovirus has gained a control of the host cell cycle, growth, immune response and apoptosis at 24 h after infection. However, efforts from host cell to block the cell cycle progression and activate an antiviral response were also observed.

Adenovirus E1A and E1B-19K proteins protect human hepatoma cells from transforming growth factor beta1-induced apoptosis

Primary and some transformed hepatocytes undergo apoptosis in response to transforming growth factor beta1 (TGFbeta). We report that infection with species C human adenovirus conferred resistance to TGFbeta-induced apoptosis in human hepatocellular carcinoma cells (Huh-7). Protection against TGFbeta-mediated cell death in adenovirus-infected cells correlated with the maintenance of normal nuclear morphology, lack of pro-caspases 8 and 3 processing, maintenance of the mitochondrial membrane potential, and lack of cellular DNA degradation. The TGFbeta pro-apoptotic signaling pathway was blocked upstream of mitochondria in adenovirus-infected cells. Both the N-terminal sequences of the E1A proteins and the E1B-19K protein were necessary to protect infected cells against TGFbeta-induced apoptosis.

Association of v-ErbA with Smad4 disrupts TGF-beta signaling

Disruption of the transforming growth factor-beta (TGF-beta) pathway is observed in the majority of cancers. To further understand TGF-beta pathway inactivation in cancer, we stably expressed the v-ErbA oncoprotein in TGF-beta responsive cells. v-ErbA participates in erythroleukemic transformation of cells induced by the avian erythroblastosis virus (AEV). Here we demonstrate that expression of v-ErbA was sufficient to antagonize TGF-beta-induced cell growth inhibition and that dysregulation of TGF-beta signaling required that v-ErbA associate with the Smad4 which sequesters Smad4 in the cytoplasm. We also show that AEV-transformed erythroleukemia cells were resistant to TGF-beta-induced growth inhibition and that TGF-beta sensitivity could be recovered by reducing v-ErbA expression. Our results reveal a novel mechanism for oncogenic disruption of TGF-beta signaling and provide a mechanistic explanation of v-ErbA activity in AEV-induced erythroleukemia.

E1A physically interacts with RUNX3 and inhibits its transactivation activity

The adenoviral gene, termed early region 1A (E1A), is crucial for transformation and has been used very effectively as a tool to determine the molecular mechanisms that underlie the basis of cellular transformation. pRb, p107, p130, p300/CBP, p400, TRRAP, and CtBP were identified to be E1A-binding proteins and their roles in cellular transformation have been established. Although the major function of E1A is considered to be the regulation of gene expression that is critical for differentiation and cell cycle exit, one of the most significant questions relating to E1A transformation is how E1A mediates this regulation. RUNX3 is a transcription factor that was first described as a gastric cancer tumor suppressor but is now known to be involved in many different cancers. Exogenous expression of RUNX3 strongly inhibits the growth of cells. Here, we show that the adenovirus oncoprotein E1A interacts with RUNX3 in vitro and in vivo. RUNX3 interacts with the N-terminus (amino acids 2-29) of E1A, which is known to interact with p300/CBP, p400, and TRRAP. E1A interacts directly with the Runt domain of RUNX3 but does not interfere with CBFbeta-RUNX3 interactions. In addition, E1A inhibits the transactivation activity of RUNX3 on the p21(WAF1/CIP1) promoter. Consistent with these observations, the growth inhibition induced by RUNX3 is reduced by E1A. These results demonstrate that E1A specifically binds to RUNX3 and inactivates its transactivation activity. We propose that one of the mechanisms for the oncogenic activity of E1A is the inhibition of RUNX3, similar to that of RB and p300/CBP.

Differential activation of ERK and Rac mediates the proliferative and anti-proliferative effects of hyaluronan and CD44

Hyaluronan, a widely distributed component of the extracellular matrix, exists in a high molecular weight (native) form and lower molecular weight form (HMW- and LMW-HA, respectively). These different forms of hyaluronan bind to CD44 but elicit distinct effects on cellular function. A striking example is the opposing effects of HMW- and LMW-HA on the proliferation of vascular smooth muscle cells; the binding of HMW-HA to CD44 inhibits cell cycle progression, whereas the binding of LMW-HA to CD44 stimulates cell cycle progression. We now report that cyclin D1 is the primary target of LMW-HA in human vascular smooth muscle cells, as it is for HMW-HA, and that the opposing cell cycle effects of these CD44 ligands result from differential regulation of signaling pathways to cyclin D1. HMW-HA binding to CD44 selectively inhibits the GTP loading of Rac and Rac-dependent signaling to the cyclin D1 gene, whereas LMW-HA binding to CD44 selectively stimulates ERK activation and ERK-dependent cyclin D1 gene expression. These data describe a novel mechanism of growth control in which a ligand-receptor system generates opposing effects on mitogenesis by differentially regulating signaling pathways to a common cell cycle target. They also emphasize how a seemingly subtle change in matrix composition can have a profound effect on cell proliferation.

Molecular Biology for the Radiation Oncologist: the 5Rs of Radiobiology meet the Hallmarks of Cancer

Recent advances in our understanding of the biology of cancer have provided enormous opportunities for the development of novel therapies against specific molecular targets. It is likely that most of these targeted therapies will have only modest single agent activities but may have the potential to accentuate the therapeutic effects of ionising radiation. In this introductory review, the 5Rs of classical radiobiology are interpreted in terms of their relationship to the hallmarks of cancer. Future articles will focus on the specific hallmarks of cancer and will highlight the opportunities that exist for designing new combination treatment regimens.

Retinoblastoma family proteins as key targets of the small DNA virus oncoproteins

RB, the most investigated tumor suppressor gene, is the founder of the RB family of growth/tumor suppressors, which comprises also p107 (RBL1) and Rb2/p130 (RBL2). The protein products of these genes, pRb, p107 and pRb2/p130, respectively, are also known as 'pocket proteins', because they share a 'pocket' domain responsible for most of the functional interactions characterizing the activity of this family of cellular factors. The interest in these genes and proteins springs essentially from their ability to regulate negatively cell cycle processes and for their ability to slow down or abrogate neoplastic growth. The pocket domain of the RB family proteins is dramatically hampered in its functions by the interference of a number of proteins produced by the small DNA viruses. In the last two decades, the 'viral hypothesis' of cancer has received a considerable renewed impulse from the notion that small DNA viruses, such as Adenovirus, Human papillomavirus (HPV) and Polyomavirus, produce factors that can physically interact with major cellular regulators and alter their function. These viral proteins (oncoproteins) act as multifaceted molecular devices that have evolved to perform very specific tasks. Owing to these features, viral oncoproteins have been widely employed as invaluable experimental tools for the identification of several key families of regulators, particularly of the cell cycle homeostasis. Adenovirus early-region 1A (E1A) is the most widely investigated small DNA tumor virus oncoprotein, but relevant interest in human oncology is raised by the E1A-related E7 protein from transforming HPV strains and by Polyomavirus oncoproteins, particularly large and small T antigens from Simian virus 40, JC virus and BK virus.

Actions of TGF-beta as tumor suppressor and pro-metastatic factor in human cancer

Transforming growth factor-beta (TGF-beta) is a secreted polypeptide that signals via receptor serine/threonine kinases and intracellular Smad effectors. TGF-beta inhibits proliferation and induces apoptosis in various cell types, and accumulation of loss-of-function mutations in the TGF-beta receptor or Smad genes classify the pathway as a tumor suppressor in humans. In addition, various oncogenic pathways directly inactivate the TGF-beta receptor-Smad pathway, thus favoring tumor growth. On the other hand, all human tumors overproduce TGF-beta whose autocrine and paracrine actions promote tumor cell invasiveness and metastasis. Accordingly, TGF-beta induces epithelial-mesenchymal transition, a differentiation switch that is required for transitory invasiveness of carcinoma cells. Tumor-derived TGF-beta acting on stromal fibroblasts remodels the tumor matrix and induces expression of mitogenic signals towards the carcinoma cells, and upon acting on endothelial cells and pericytes, TGF-beta regulates angiogenesis. Finally, TGF-beta suppresses proliferation and differentiation of lymphocytes including cytolytic T cells, natural killer cells and macrophages, thus preventing immune surveillance of the developing tumor. Current clinical approaches aim at establishing novel cancer drugs whose mechanisms target the TGF-beta pathway. In conclusion, TGF-beta signaling is intimately implicated in tumor development and contributes to all cardinal features of tumor cell biology.

Cloning, expression and characterization of a novel human CAP10-like gene hCLP46 from CD34(+) stem/progenitor cells

A novel human gene, named as human CAP10-like protein 46 kDa (hCLP46), was isolated and identified from human acute myeloid leukemia transformed from myelodysplastic syndrome (MDS-AML) CD34(+) cells. hCLP46 (3q13.33) contains 11 exons encoding a putative protein of 392 amino acids, with a highly conserved CAP10 domain, a hydrophobic signal peptide at its N-terminus, and an endoplasmic reticulum (ER) retention signal motif KTEL at the C-terminus. The homologs of hCLP46 exist in different organisms from plants to animal kingdoms. Subcellular localization analysis showed that hCLP46 is an ER-resident protein. hCLP46 expressed in most human adult tissues at different intensities, with lengths of 3.5 kb and 1.9 kb. Transcript of hCLP46 was not detectable in colon, thymus, and small intestine, but was abundant in liver, indicating that hCLP46 may be involved in important physiological functions in the liver. hCLP46 over-expressed U937 cells had higher growth rate than the cells without exogenic hCLP46 protein expression, suggesting that hCLP46 protein possess the ability of promoting cell proliferation.

Impact of the interaction between adenovirus E1A and CtBP on host cell gene expression

In cell lines harbouring inducible adenovirus E1A genes, the cytotoxicity of wild type E1A was manifested by poor and subsiding expression of the E1A protein during prolonged induction. In contrast, cells expressing E1A deleted in the C-terminal binding protein (CtBP)-interaction domain (E1ADeltaCID) demonstrated high levels of expression for extended time. Microarray analyses of host cell gene expression demonstrated that approximately 70% of the regulated genes were increased upon E1A induction and that the majority of E1A-regulated genes were similarly regulated by wild type E1A and E1ADeltaCID. However, for 29 genes, regulation by wild type E1A and E1ADeltaCID were different. Consistent with the altered transforming capacity of E1A unable to bind CtBP, genes involved in tumour cell progression and growth suppression were found among the differently regulated genes. Moreover, promoter sequences of genes up regulated by wild type E1A and/or repressed by E1ADeltaCID demonstrated a higher prevalence of potential binding sites for the CtBP-targeted transcription factors Ets, Ikaros and/or partial differentialEF1/ZEB, suggesting that the failure to block CtBP-repression contributed to the "hyper-transforming" phenotype of E1ADeltaCID. Since E1ADeltaCID also specifically activated host cell gene expression, we find it likely that additional, possibly CtBP-independent, mechanisms contribute to the altered phenotype of E1ADeltaCID-expressing cells.

p21WAF1/CIP1 selectively controls the transcriptional activity of estrogen receptor alpha

Estrogen receptors (ER) are ligand-dependent transcription factors that regulate growth, differentiation, and maintenance of cellular functions in a wide variety of tissues. We report here that p21WAF1/CIP1, a cyclin-dependent kinase (Cdk) inhibitor, cooperates with CBP to regulate the ERalpha-mediated transcription of endogenous target genes in a promoter-specific manner. The estrogen-induced expression of the progesterone receptor and WISP-2 mRNA transcripts in MCF-7 cells was enhanced by p21WAF1/CIP1, whereas that of the cyclin D1 mRNA was reduced and the pS2 mRNA was not affected. Chromatin immunoprecipitation assays revealed that p21WAF1/CIP1 was recruited simultaneously with ERalpha and CBP to the endogenous progesterone receptor gene promoter in an estrogen-dependent manner. Experiments in which the p21WAF1/CIP1 protein was knocked down by RNA interference showed that the induction of the expression of the gene encoding the progesterone receptor required p21WAF1/CIP1, in contrast with that of the cyclin D1 and pS2 genes. p21WAF1/CIP1 induced not only cell cycle arrest in breast cancer cells but also milk fat globule protein and lipid droplets, indicators of the differentiated phenotype, as well as cell flattening and increase of the volume of the cytoplasm. These results indicate that p21WAF1/CIP1, in addition to its Cdk-regulatory role, behaves as a transcriptional coactivator in a gene-specific manner implicated in cell differentiation.

Role of transforming growth factor Beta in human cancer

Transforming growth factor beta (TGF-beta) is a ubiquitous and essential regulator of cellular and physiologic processes including proliferation, differentiation, migration, cell survival, angiogenesis, and immunosurveillance. Alterations in the TGF-beta signaling pathway, including mutation or deletion of members of the signaling pathway and resistance to TGF-beta-mediated inhibition of proliferation are frequently observed in human cancers. Although these alterations define a tumor suppressor role for the TGF-beta pathway in human cancer, TGF-beta also mediates tumor-promoting effects, either through differential effects on tumor and stromal cells or through a fundamental alteration in the TGF-beta responsiveness of the tumor cells themselves. TGF-beta and members of the TGF-beta signaling pathway are being evaluated as prognostic or predictive markers for cancer patients. Ongoing advances in understanding the TGF-beta signaling pathway will enable targeting of this pathway for the chemoprevention and treatment of human cancers.

The Kaposi's sarcoma-associated herpesvirus K-bZIP protein represses transforming growth factor beta signaling through interaction with CREB-binding protein

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is involved in the pathogenesis of KS, primary effusion lymphoma, and multicentric Castleman's disease. K-bZIP, the protein encoded by the open reading frame K8 of KSHV, is a member of the basic region-leucine zipper family of transcription factors. We studied the mechanisms that underlie KSHV-induced oncogenesis by investigating whether K-bZIP perturbs signaling through transforming growth factor beta (TGF-beta), which inhibits proliferation of a wide range of cell types. K-bZIP repressed TGF-beta-induced, Smad-mediated transcriptional activity and antagonized the growth-inhibitory effects of TGF-beta. Since both K-bZIP and Smad are known to interact with CREB-binding protein (CBP), the effect of CBP on inhibition of Smad-mediated transcriptional activation by K-bZIP was examined. K-bZIP mutants, which lacked the CBP-binding site, could not repress TGF-beta-induced or Smad3-mediated transcriptional activity. Overexpression of CBP restored K-bZIP-induced inhibition of Smad3-mediated transcriptional activity. Competitive interaction studies showed that K-bZIP inhibited the interaction of Smad3 with CBP. These results suggest that K-bZIP, through its binding to CBP, disrupts TGF-beta signaling by interfering with the recruitment of CBP into transcription initiation complexes on TGF-beta-responsive elements. We propose a possibility that K-bZIP may contribute to oncogenesis through its ability to promote cell survival by repressing TGF-beta signaling.

Epstein-Barr virus nuclear antigen 3C regulates cyclin A/p27 complexes and enhances cyclin A-dependent kinase activity

Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA3C) is essential for primary B-cell transformation. In this report we show that cyclin A, an activator of S phase progression, bound tightly to EBNA3C. EBNA3C interacted with cyclin A in vitro and associated with cyclin A complexes in EBV-transformed lymphoblastoid cell lines. Importantly, EBNA3C stimulated cyclin A-dependent kinase activity and rescued p27-mediated inhibition of cyclin A/Cdk2 kinase activity by decreasing the molecular association between cyclin A and p27 in cells. Additionally, phosphorylation of the retinoblastoma protein, a major regulator of cell cycle progression, was enhanced both in vitro and in vivo in the presence of EBNA3C. Cyclin A interacted with a region of the carboxy terminus of EBNA3C, shown to be important both for stimulation of cyclin A-dependent kinase activity and for cell cycle progression. This provides the first evidence of an essential EBV latent antigen's directly targeting a cell cycle regulatory protein and suggests a novel mechanism by which EBV deregulates the mammalian cell cycle, which is of critical importance in B-cell transformation.

Role of the TAB2-related protein TAB3 in IL-1 and TNF signaling

The cytokines IL-1 and TNF induce expression of a series of genes that regulate inflammation through activation of NF-kappaB signal transduction pathways. TAK1, a MAPKKK, is critical for both IL-1- and TNF-induced activation of the NF-kappaB pathway. TAB2, a TAK1-binding protein, is involved in IL-1-induced NF-kappaB activation by physically linking TAK1 to TRAF6. However, IL-1-induced activation of NF-kappaB is not impaired in TAB2-deficient embryonic fibroblasts. Here we report the identification and characterization of a novel protein designated TAB3, a TAB2-like molecule that associates with TAK1 and can activate NF-kappaB similar to TAB2. Endogenous TAB3 interacts with TRAF6 and TRAF2 in an IL-1- and a TNF-dependent manner, respectively. Further more, IL-1 signaling leads to the ubiquitination of TAB2 and TAB3 through TRAF6. Cotransfection of siRNAs directed against both TAB2 and TAB3 inhibit both IL-1- and TNF-induced activation of TAK1 and NF-kappaB. These results suggest that TAB2 and TAB3 function redundantly as mediators of TAK1 activation in IL-1 and TNF signal transduction.

Transforming growth factor beta1 receptor II is downregulated by E1A in adenovirus-infected cells

Transforming growth factor beta1 (TGF-beta1) signaling is compromised in many tumors, thereby allowing the tumor to escape the growth-inhibitory and proapoptotic activities of the cytokine. Human adenoviruses interfere with a number of cellular pathways involved in cell cycle regulation and apoptosis, initially placing the cell in a "tumor-like" state by forcing quiescent cells into the cell cycle and also inhibiting apoptosis. We report that adenovirus-infected cells resemble tumor cells in that TGF-beta1 signaling is inhibited. The levels of TGF-beta1 receptor II (TbetaRII) in adenovirus-infected cells were decreased, and this decrease was mapped, by using virus mutants, to the E1A gene and to amino acids 2 to 36 and the C-terminal binding protein binding site in the E1A protein. The decrease in the TbetaRII protein was accompanied by a decrease in TbetaRII mRNA. The decrease in TbetaRII protein levels in adenovirus-infected cells was greater than the decrease in TbetaRII mRNA, suggesting that downregulation of the TbetaRII protein may occur through more than one mechanism. Surprisingly in this context, the half-lives of the TbetaRII protein in infected and uninfected cells were similar. TGF-beta1 signaling was compromised in cells infected with wild-type adenovirus, as measured with 3TP-lux, a TGF-beta-sensitive reporter plasmid expressing luciferase. Adenovirus mutants deficient in TbetaRII downregulation did not inhibit TGF-beta1 signaling. TGF-beta1 pretreatment reduced the relative abundance of adenovirus structural proteins in infected cells, an effect that was potentiated when cells were infected with mutants incapable of modulating the TGF-beta signaling pathway. These results raise the possibility that inhibition of TGF-beta signaling by E1A is a means by which adenovirus counters the antiviral defenses of the host.

Genetic and epigenetic alterations in colon cancer

Colorectal cancer affected approximately 135,000 people in the United States in 2001, resulting in 57,000 deaths. Colorectal cancer develops as the result of the progressive accumulation of genetic and epigenetic alterations that lead to the transformation of normal colonic epithelium to colon adenocarcinoma. The loss of genomic stability is a key molecular and pathophysiologic step in this process and serves to create a permissive environment for the occurrence of alterations in tumor suppressor genes and oncogenes. Alterations in these genes, which include APC, CTNNB1, K-RAS, MADH4/SMAD4, and TGFBR2, appear to promote colon tumorigenesis by perturbing the function of signaling pathways, such as the TGF-ss signaling pathway, or by affecting genes that regulate genomic stability, such as the mutation mismatch repair genes.

Human T-cell lymphotropic virus oncoprotein Tax represses TGF-beta 1 signaling in human T cells via c-Jun activation: a potential mechanism of HTLV-I leukemogenesis

Human T-cell leukemia virus I is the etiologic agent of adult T-cell leukemia (ATL), an aggressive T-cell malignancy. The viral oncoprotein Tax, through the activation of nuclear factorkappaB (NF-kappaB), CCAAT-enhancer binding protein (CREB), and activated protein-1 (AP-1) pathways, is a transcriptional regulator of critical genes for T-cell homeostasis. In ATL cells, activated AP-1 complexes induce the production of transforming growth factor beta1 (TGF-beta1). TGF-beta1 is an inhibitor of T-cell proliferation and cytotoxicity. Here we show that, in contrast to normal peripheral T cells, ATL cells are resistant to TGF-beta1-induced growth inhibition. The retroviral transduction of the Tax protein in peripheral T cells resulted in the loss of TGF-beta1 sensitivity. Transient transfection of Tax in HepG2 cells specifically inhibited Smad/TGF-beta1 signaling in a dose-dependent manner. In the presence of Tax transfection, increasing amounts of Smad3 restored TGF-beta1 signaling. Tax mutants unable to activate NF-kappaB or CREB pathways were also able to repress Smad3 transcriptional activity. Next we have demonstrated that Tax inhibits TGF-beta1 signaling by reducing the Smad3 DNA binding activity. However, Tax did not decrease the expression and the nuclear translocation of Smad3 nor did it interact physically with Smad3. Rather, Tax induced c-Jun N-terminal kinase (JNK) activity and c-Jun phosphorylation, leading to the formation of Smad3/c-Jun complexes. Whereas c-Jun alone abrogates Smad3 DNA binding, cotransfection of Tax and of a dominant-negative form of JNK or a c-Jun antisense-restored Smad3 DNA binding activity and TGF-beta1 responsiveness. In ATL and in normal T cells transduced by Tax, c-Jun was constitutively phosphorylated. Thus, we describe a new function of Tax, as a repressor of TGF-beta1 signaling through JNK/c-Jun constitutive activation, which may play a critical role in ATL leukemogenesis.

Antagonism of glucocorticoid receptor transactivity and cell growth inhibition by transforming growth factor-beta through AP-1-mediated transcriptional repression

We have examined the interaction of the glucocorticoid receptor (GR) and transforming growth factor-beta (TGF-beta) signal pathways because of their mutual involvement in the regulation of cell growth, development and differentiation. Most studies of this cross-talk event have focused on the effects of glucocorticoids (GCs) on TGF-beta responses. In this work, we show that TGF-beta can antagonize dexamethasone (Dex)-mediated growth suppression in mouse fibrosarcoma L929 cells. TGF-beta also repressed GR-mediated reporter (pMMTV-CAT) gene expression in a concentration-dependent manner, with an IC(50) of 5 ng/ml of TGF-beta. Maximal inhibition (76%) was observed at 10 ng/ml of TGF-beta. Conversely, Dex inhibited TGF-beta-mediated promoter (p3TP-Lux) activity in these same cells. As TGF-beta inhibition of GR-mediated gene expression occurred after Dex-mediated nuclear translocation of GR, we conclude that TGF-beta inhibition of GR signaling occurs at the level of GR-mediated transcription activity. However, TGF-beta did not repress GR-mediated gene expression using the pGRE(2)E1B-CAT minimal promoter construct, suggesting that TGF-beta did not inhibit intrinsic GR activity but, rather, required DNA-binding factor(s) distinct from GR. As the MMTV promoter contains several putative AP-1 binding sites, we hypothesized that AP-1, a transcription factor composed of c-jun and c-fos proteins, might be involved in the TGF-beta inhibition of GR functions. Curcumin, a potent inhibitor of AP-1 expression, completely abolished the inhibitory effect of TGF-beta on GR-mediated gene expression without affecting GR activity in the absence of TGF-beta, and this drug blocked TGF-beta-induced binding of AP-1 to a response element derived from the MMTV sequence. Furthermore, curcumin abolished TGF-beta inhibition of Dex-induced growth suppression. Taken as a whole, our data suggest that TGF-beta can antagonize the growth inhibitory properties of GR by blocking GR transactivity at complex promoters through a mechanism involving transcriptional repression by DNA-bound AP-1.

Transforming growth factor-beta1-modulated cerebral gene expression

Transforming growth factor-beta1 (TGF-beta1) plays a central role in the response of the brain to different types of injury. Increased TGF-beta1 has been found in the central nervous system of patients with acute or chronic disorders such as stroke or Alzheimer disease. To further define the molecular targets of TGF-beta1 in cerebral tissues, a selection of high-density cDNA arrays was used to characterize the mRNA expression profile of 7,000 genes in transgenic mice overexpressing TGF-beta1 from astrocytes as compared with the wild-type line. Selected findings were further evaluated by reverse transcription-polymerase chain reactions from independent transgenic and wild-type mice. Furthermore, the expression pattern of seven selected genes such as Delta-1, CRADD, PRSC-1, PAI-1, Apo-1/Fas, CTS-B, and TbetaR-II were confirmed in either cultured cortical neurons or astrocytes following TGF-beta1 treatment. The authors' observations enlarge the repertoire of known TGF-beta1-modulated genes and their possible involvement in neurodegenerative processes.

Adenovirus-5 E1A: paradox and paradigm

The adenovirus early region 1A (E1A) proteins were described originally as immortalizing oncoproteins that altered transcription in rodent cells. Surprisingly, the 243-amino-acid form of adenovirus-5 E1A was found subsequently to reverse-transform many human tumour cells. Tumour suppression apparently results from the ability of E1A to re-programme transcription in tumour cells, and the molecular basis of this intriguing effect is now beginning to emerge. These discoveries have provided a tool with which to study the regulation of fundamental cellular processes.

Transforming growth factor beta 1 dysregulation in a human oral carcinoma tumour progression model

A human oral tumour progression model was established that consists of normal epithelial cells and three cell lines representing stages from dysplastic to metastatic cells. To investigate the impact of exogenous transforming growth factor-beta 1 on this model system, we analysed the responsiveness of those cells to transforming growth factor-beta 1 and explored the potential mechanism underlying the transforming growth factor-beta 1 activity. We found that the growth of all cell types, regardless of their stage of tumour progression, is inhibited by transforming growth factor-beta 1, although to different degrees. Transforming growth factor-beta 1 induced the expression of cyclin-dependent kinase inhibitors p15(INK4B), p21WAF1/(CIP1) and p27(KIP1). In contrast, transforming growth factor-beta 1 was found to stimulate the invasive potential of one cell type that represents the most advanced stage of tumour phenotype, suggesting that the impact of transforming growth factor-beta 1 on functional features of tumour cells other than cellular proliferation may play a significant role in the process of oral tumour progression.

Etiology of osteosarcoma

Although the prognosis and quality of life of patients with osteosarcoma were improved significantly during the past decades, the pathogenesis and etiology of this disease remain obscure. Significant interest and effort in this cancer led to the identification of numerous etiologic agents. Several chemical agents such as beryllium, viruses such as FBJ, subsequently found to contain the src-oncogene, and radiation were shown to be potent inducers of osteosarcoma. Paget's disease, electrical burn, or trauma all are thought to be other factors that may contribute to the pathogenesis. More recently, patients with hereditary diseases such as Rothmund-Thomson syndrome, Bloom syndrome, and Li-Fraumeni syndrome were found to have an increased risk of having osteosarcoma develop. During the past few years, the molecular analysis brought a wealth of new information with numerous genes that were associated with osteosarcoma and its clinical disease progression. They can be categorized into self-sufficiency in growth signals, insensitivity to growth inhibitory signals, evasion of apoptosis, limitless replicative potential, sustained angiogenesis, and tissue evasion and metastasis. Although the understanding of these processes in osteosarcoma still is incomplete, it may have the potential to significantly affect the patient care in the future.

The cytotoxic effect of E1B 55-kDa mutant adenovirus on human hepatocellular carcinoma cell lines

It has been suggested the E1B 55 kDa mutant adenovirus dl1520 can selectively kill p53-deficient human tumor cells. In this study, we examined the cytotoxic effect of dl1520 on nine human hepatocellular carcinoma (HCC) cell lines with different p53 genetic and functional status. The results showed that HCC cell lines with deleted or mutant p53 gene and reduced p53 transcriptional activities were more susceptible to dl1520-induced cytolysis. Hep3B (p53-null) and HepG2 (p53-wt) cells were arrested at G2/M phase when cytolysis occurred. Cyclin-dependent kinase inhibitor (CDKI) p21(Waf-1/Cip-1) was downregulated 24 hours after dl1520 infection in HepG2 cells and increased when cytolysis occurred. No p21 expression was detected in Hep3B cells. DNA fragmentation was found in both Hep3B and HepG2 cells after dl1520 infection. Bax expression increased in dl1520-infected HepG2 cells but not in Hep3B cells. Notably, three Bax-like proteins, molecular mass around 40 to 80 kDa, accumulated 48 hours after adenovirus infection in Hep3B cells but not in HepG2 cells. These results suggest that the susceptibility of HCC cells to dl1520-induced cytolysis is related to both p53 genotype and functional status, and is mediated by both cell cycle disturbance and apoptosis.

Repression of p15INK4b expression by Myc through association with Miz-1

Deregulated expression of c-myc can induce cell proliferation in established cell lines and in primary mouse embryonic fibroblasts (MEFs), through a combination of both transcriptional activation and repression by Myc. Here we show that a Myc-associated transcription factor, Miz-1, arrests cells in G1 phase and inhibits cyclin D-associated kinase activity. Miz-1 upregulates expression of the cyclin-dependent kinases (CDK) inhibitor p15INK4b by binding to the initiator element of the p15INK4b promoter. Myc and Max form a complex with Miz-1 at the p15 initiator and inhibit transcriptional activation by Miz-1. Expression of Myc in primary cells inhibits the accumulation of p15INK4b that is associated with cellular senescence; conversely, deletion of c-myc in an established cell line activates p15INK4b expression. Alleles of c-myc that are unable to bind to Miz-1 fail to inhibit accumulation of p15INK4b messenger RNA in primary cells and are, as a consequence, deficient in immortalization.

Role of transforming growth factor beta in cancer

Transforming growth factor beta (TGF-beta) is an effective and ubiquitous mediator of cell growth. The significance of this cytokine in cancer susceptibility, cancer development and progression has become apparent over the past few years. TGF-beta plays various roles in the process of malignant progression. It is a potent inhibitor of normal stromal, hematopoietic, and epithelial cell growth. However, at some point during cancer development the majority of transformed cells become either partly or completely resistant to TGF-beta growth inhibition. There is growing evidence that in the later stages of cancer development TGF-beta is actively secreted by tumor cells and not merely acts as a bystander but rather contributes to cell growth, invasion, and metastasis and decreases host-tumor immune responses. Subtle alteration of TGF-beta signaling may also contribute to the development of cancer. These various effects are tissue and tumor dependent. Identifying and understanding TGF-beta signaling pathway abnormalities in various malignancies is a promising avenue of study that may yield new modalities to both prevent and treat cancer. The nature, prevalence, and significance of TGF-beta signaling pathway alterations in various forms of human cancer as well as potential preventive and therapeutic interventions are discussed in this review.

Role of transforming growth factor beta in cancer

Transforming growth factor beta (TGF-beta) is an effective and ubiquitous mediator of cell growth. The significance of this cytokine in cancer susceptibility, cancer development and progression has become apparent over the past few years. TGF-beta plays various roles in the process of malignant progression. It is a potent inhibitor of normal stromal, hematopoietic, and epithelial cell growth. However, at some point during cancer development the majority of transformed cells become either partly or completely resistant to TGF-beta growth inhibition. There is growing evidence that in the later stages of cancer development TGF-beta is actively secreted by tumor cells and not merely acts as a bystander but rather contributes to cell growth, invasion, and metastasis and decreases host-tumor immune responses. Subtle alteration of TGF-beta signaling may also contribute to the development of cancer. These various effects are tissue and tumor dependent. Identifying and understanding TGF-beta signaling pathway abnormalities in various malignancies is a promising avenue of study that may yield new modalities to both prevent and treat cancer. The nature, prevalence, and significance of TGF-beta signaling pathway alterations in various forms of human cancer as well as potential preventive and therapeutic interventions are discussed in this review.

Smad2, Smad3 and Smad4 cooperate with Sp1 to induce p15(Ink4B) transcription in response to TGF-beta

Transforming growth factor-beta (TGF-beta) arrests growth of epithelial cells by inducing the transcription of p15(Ink4B), a cyclin-dependent kinase inhibitor. In this study, we demonstrate that p15(Ink4B) induction was mediated by a TGF-beta-induced complex of Smad2, Smad3, Smad4 and Sp1. Mutations in the Sp1- or Smad-binding sequences decreased or abolished the TGF-beta responsiveness of the p15(Ink4B) promoter. Interference with, or deficiency in, Smad2, Smad3 or Smad4 functions also reduced or abolished the TGF-beta-dependent p15(Ink4B) induction, whereas the absence of Sp1 reduced the basal and TGF-beta-induced p15(Ink4B) transcription. In the nucleoprotein complex, Smad2 interacted through its C-domain with Sp1 and enhanced the DNA binding and transcriptional activity of Sp1. Smad3 interacted indirectly with Sp1 through its association with Smad2 and/or Smad4, and bound directly to the p15(Ink4B) promoter. Finally, Smad4 interacted through its N-domain with Sp1. Our data demonstrate the physical interactions and functional cooperativity of Sp1 with a complex of Smad2, Smad3 and Smad4 in the induction of the p15(Ink4B) gene. These findings explain the tumor suppressor roles of Smad2 and Smad4 in growth arrest signaling by TGF-beta.

TGF-beta-mediated cell cycle arrest of HPV16-immortalized human ectocervical cells correlates with decreased E6/E7 mRNA and increased p53 and p21(WAF-1) expression

Transforming growth factor beta (TGF-beta) suppresses proliferation and potentiates apoptosis of HPV16-immortalized human cervical epithelial cells (ECE16-1). Exposure of ECE16-1 to TGF-beta1 increased expression of p53 and induced cell cycle arrest. We examined, by Western blotting, expression of p53 and related cell cycle regulatory proteins after treatment. p53 levels increased as a function of time and dose. Increased p53 appeared to be active, since TGF-beta1 treatment increased the activity of a p53 transcriptional response element in a luciferase reporter plasmid. Additionally, the proteins of the p53-regulated genes, p21(WAF1), mdm2, and Bax, were increased with similar time and dose responses. We did not observe consistent changes in protein levels of cyclin D, cyclin E, CDK4, CDK6, CDK2, p27(Kip1), p16(INK4a), or RNA levels of p15(INK4b). Activity of CDK4 or 6, measured by phosphorylation of an Rb fragment, remained constant during the response period; however, activity of CDK2 (phosphorylation of histone H1) decreased. Concordantly, increased levels of p21(WAF1) were immunoprecipitated with anti-CDK2 antibodies. During treatment, the phosphorylation state of Rb shifted to a hypophosphorylated form. mRNA for the HPV E6/E7 genes decreased; however, significant changes in the E7 protein were not observed, while increased levels of Rb immunoprecipitated with anti-E7 antibodies were observed. These data are consistent with the following model. In ECE16-1 cells, there exists a fine balance between inhibitory levels of p53 and Rb and the antagonists, E6 and E7. TGF-beta1 treatment decreases steady-state levels of E6/E7 mRNA, which results in a shifted balance (lowered activity of E6) in favor of increased p53 expression, resulting in activation of the cell cycle inhibitory gene, p21(WAF1). This protein binds the cyclin E/CDK2 complex that maintains Rb in a phosphorylated state. Rb shifts to a hypophosphorylated state, resulting in G1 arrest, presumably by binding E2F transcription factors.

A role for transcriptional repression of p21CIP1 by c-Myc in overcoming transforming growth factor beta -induced cell-cycle arrest

c-Myc plays a vital role in cell-cycle progression. Deregulated expression of c-Myc can overcome cell-cycle arrest in order to promote cellular proliferation. Transforming growth factor beta (TGFbeta) treatment of immortalized human keratinocyte cells inhibits cell-cycle progression and is characterized by down-regulation of c-Myc followed by up-regulation of p21(CIP1). A direct role of c-Myc in this pathway was demonstrated by the observation that ectopic expression of c-Myc overcame the cell-cycle block induced by TGFbeta treatment. The induction of p21(CIP1) transcription by TGFbeta was blocked in human keratinocyte cells stably expressing c-Myc. Furthermore, overexpression of c-Myc in NIH 3T3 cells repressed the basal levels of p21(CIP1) mRNA. Repression of p21(CIP1) transcription by c-Myc occurred at the promoter level in a region near the start site of transcriptional initiation and was independent of histone deacetylase activity. These data suggest that the down-regulation of c-Myc after TGFbeta signaling is important for subsequent regulation of p21(CIP1) and cell-cycle inhibition. Thus, repression of the cell-cycle inhibitory gene p21(CIP1) plays a role in c-Myc-dependent cell-cycle progression.

Smad-dependent stimulation of type I collagen gene expression in human skin fibroblasts by TGF-beta involves functional cooperation with p300/CBP transcriptional coactivators

Transforming growth factor-beta (TGF-beta) stimulation of Type I collagen gene (COL1A2) transcription involves the Smad signal transduction pathway, but the mechanisms of Smad-mediated transcriptional activation are not fully understood. We now demonstrate that the ubiquitous transcriptional coactivators p300 and CREB-binding protein (CBP) enhanced basal as well as TGF-beta- or Smad3-induced COL1A2 promoter activity, and stimulated the expression of endogenous Type I collagen. The adenoviral E1A oncoprotein abrogated stimulation of COL1A2 activity in transfected fibroblasts, and reduced the basal level of collagen gene expression. This effect was due to specific interaction of E1A with cellular p300/CBP because (a) a mutant form of E1A defective in p300 binding failed to abrogate stimulation, and (b) forced expression of p300/CBP restored the ability of TGF-beta to stimulate COL1A2 promoter activity in the presence of E1A. The effect of p300 on COL1A2 transcription appeared to be due, in part, to its intrinsic acetyltransferase activity, as stimulation induced by a histone acetyltransferase-deficient mutant p300 was substantially reduced. Transactivation of COL1A2 by p300 involved the Smad signaling pathway, as Smad4-deficient cells failed to respond to p300, and stimulation was rescued by overexpression of Smad4. Furthermore, minimal constructs containing only the Smad-binding CAGACA element of COL1A2 were transactivated by p300 in the presence of TGF-beta. These results indicate, for the first time, that the multifunctional p300/CBP coactivators play a major role in Smad-dependent TGF-beta stimulation of collagen gene expression in fibroblasts. Oncogene (2000) 19, 3546 - 3555

Growth control of human biliary epithelial cells by interleukin 6, hepatocyte growth factor, transforming growth factor beta1, and activin A: comparison of a cholangiocarcinoma cell line with primary cultures of non-neoplastic biliary epithelial cells

A well characterized human cholangiocarcinoma (CC) cell line, SG231, was compared with primary cultures of normal human biliary epithelial cells (BECs) for alterations in interleukin 6 (IL-6) and hepatocyte growth factor (HGF)-mediated stimulation and transforming growth factor beta1 (TGF-beta1) and activin A-mediated inhibition of growth. Results were compared with immunolabeling of the original tumor and after injection of SG231 into the liver of BALB/cByJ-scid mice. In vitro, both BECs and CCs expressed met, gp80, and gp130 messenger RNA (mRNA) and protein, but the levels of expression were higher in the CCs than in the BECs. In both the CCs and BECs, exogenous HGF or IL-6 induced phosphorylation of met or gp130, respectively, and a concentration-dependent increase in DNA synthesis. However, the CCs but not BECs, continued to grow in basal serum-free medium (SFM) and spontaneously produced both IL-6 and HGF under these conditions, which resulted in auto-phosphorylation of gp130 and met, respectively; and neutralizing anti-HGF or anti-IL-6 alone inhibited CC growth, indicative of autocrine growth control circuits. Conversely, activin A inhibits the growth of both BECs and CCs, but does not significantly increase apoptosis. Activin-A-induced growth inhibition of both CCs and BECs can be reversed by 100 ng/mL exogenous IL-6, but not by 10 to 100 ng/mL HGF. TGF-beta1 inhibited the growth of BECs but had no mitoinhibitory or proapoptotic effects on CCs. Immunolabeling of the original tumor and after inoculation into scid mice showed positive staining for met, gp130, gp80, and IL-6. This study contributes to a further understanding of BEC growth control and derangements that can occur during cholangiocarcinogenesis.

Selective transcriptional regulations in the human liver cell by hepatitis B viral X protein

The hepatitis B viral X protein (HBx) is known as a transcription factor and potential oncogene. To gain a better view of the effect of HBx on the transcriptional regulation in the human liver cell, we constructed a HepG2 cell line stably expressing HBx (HepG2-HBx), and performed cDNA microarray analysis on 588 cellular cDNAs comparing with untransformed control cells. Two genes (IGFR-2, RhoA) of oncogenes, one gene (p55CDC) of cell cycle regulators, three genes (thrombin receptor, MLK-3, MacMARCKS) of intracellular transducers, one gene (HSP27) of stress response proteins, two genes (FAST kinase, Bak) of apoptosis response proteins, one gene (p21(WAF)) of transcription factors were highly up-regulated; one gene (transcription elongation factor SII) of transcription factors and two genes (monocyte chemotactic protein 1, T-lymphocyte-secreted protein I-309) of growth factors were highly down-regulated. These results showed selective transcriptional regulation by HBx in the human liver cell.

Transforming growth factor-beta and breast cancer: Cell cycle arrest by transforming growth factor-beta and its disruption in cancer

Altered responsiveness to extracellular signals and cell cycle dysregulation are hallmarks of cancer. The cell cycle is governed by cyclin-dependent kinases (cdks) that integrate mitogenic and growth inhibitory signals. Transforming growth factor (TGF)-beta mediates G1 cell cycle arrest by inducing or activating cdk inhibitors, and by inhibiting factors required for cdk activation. Mechanisms that lead to cell cycle arrest by TGF-beta are reviewed. Loss of growth inhibition by TGF-beta occurs early in breast cell transformation, and may contribute to breast cancer progression. Dysregulation of cell cycle effectors at many different levels may contribute to loss of G1 arrest by TGF-beta. Elucidation of these pathways in breast cancer may ultimately lead to novel and more effective treatments for this disease.

The role of the Smad3 protein in phorbol ester-induced promoter expression

The Sp1 transcription factor plays an important role in mediating the p53-independent activation of the p21(WAF1) (WAF1) promoter by phorbol 12-myristate13-acetate (PMA) in hematopoietic cells. Using GAL4-Sp1 fusion proteins and a luciferase reporter, PMA is shown to activate the transcriptional activity of Sp1 independent of the WAF1 promoter. This activation does not require the Ser/Thr-rich region of Sp1 and can be mediated by 41 amino acids (152-193) of Sp1 that are important for the interaction with human TAF130. Because transforming growth factor-beta enhances WAF1 promoter activity through both Sp1 and Smad proteins, the role of Smads in PMA transcriptional activation was examined. PMA addition to hematopoietic cells was found to activate a GAL4/Smad-dependent promoter and the transforming growth factor-beta-responsive promoter, p3TP-lux. Immunofluorescence data demonstrate that PMA addition to hematopoietic cells induces the translocation of Smad3 to the nucleus. However, Smad3 does not stimulate the WAF1 promoter, but rather slightly inhibits the PMA-mediated induction of transcription from this upstream region. Additionally, transfection of Smad3 did not enhance the activation of GAL4/Sp1 by PMA. These results demonstrate that, while PMA can activate Smad-mediated transcription, Smad proteins do not appear to play a major role in the PMA induction of the WAF1 promoter.

E1A inhibits transforming growth factor-beta signaling through binding to Smad proteins

Smads form a recently identified family of proteins that mediate intracellular signaling of the transforming growth factor (TGF)-beta superfamily. Smads bind to DNA and act as transcriptional regulators. Smads interact with a variety of transcription factors, and the interaction is likely to determine the target specificity of gene induction. Smads also associate with transcriptional coactivators such as p300 and CBP. E1A, an adenoviral oncoprotein, inhibits TGF-beta-induced transactivation, and the ability of E1A to bind p300/CBP is required for the inhibition. Here we determined the Smad interaction domain (SID) in p300 and found that two adjacent regions are required for the interaction. One of the regions is the C/H3 domain conserved between p300 and CBP, and the other is a nonconserved region. p300 mutants containing SID inhibit transactivation by TGF-beta in a dose-dependent manner. E1A inhibits the interaction of Smad3 with a p300 mutant that contains SID but lacks the E1A binding domain. We found that E1A interacts specifically with receptor-regulated Smads, suggesting a novel mechanism whereby E1A antagonizes TGF-beta signaling.