Transforming growth factor-beta responsiveness in DPC4/SMAD4-null cancer cells

TGF-β signaling and its role in the regulation of hematopoietic stem cells

Transforming growth factor-betas (TGF-βs) and their family members that include bone morphogenic proteins and activins have been implicated in the regulation of proliferation, hibernation, quiescence and differentiation of hematopoietic stem cells (HSCs). Increasing evidence suggests that the superfamily of TGF-βs play an integral role in the intercellular cross-talk between the stem cells and their microenvironment as well as within the cells at an intracellular level. Active sites of hematopoiesis, such as fetal liver and bone marrow are known to have abundant presence of TGF-β indicating their importance in the maintenance and regulation of hematopoiesis. One of the striking features of TGF-β superfamily is the variety of effects they evoke, contingent on the developing history of the responding cells. In the present review, we discuss the Smad-dependent and Smad-independent TGF-β signaling pathways in order to understand and underscore their role in the regulation of HSCs.

SMAD4 loss triggers the phenotypic changes of pancreatic ductal adenocarcinoma cells

BACKGROUND

SMAD4 is a gastrointestinal malignancy-specific tumor suppressor gene found mutated in one third of colorectal cancer specimens and half of pancreatic tumors. SMAD4 inactivation by allelic deletion or intragenic mutation mainly occurs in the late stage of human pancreatic ductal adenocarcinoma (PDAC). Various studies have proposed potential SMAD4-mediated anti-tumor effects in human malignancy; however, the relevance of SMAD4 in the PDAC molecular phenotype has not yet been fully characterized.

METHODS

The AsPC-1, CFPAC-1 and PANC-1 human PDAC cell lines were used. The restoration or knockdown of SMAD4 expression in PDAC cells were confirmed by western blotting, luciferase reporter and immunofluorescence assays. In vitro cell proliferation, xenograft, wound healing, quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR), Western blotting, and immunohistochemistry analysis were conducted using PDAC cells in which SMAD4 was either overexpressed or knocked down.

RESULTS

Here, we report that re-expression of SMAD4 in SMAD4-null PDAC cells does not affect tumor cell growth in vitro or in vivo, but significantly enhances cells migration in vitro. SMAD4 restoration transcriptionally activates the TGF-β1/Nestin pathway and induces expression of several transcriptional factors. In contrast, SMAD4 loss in PDAC leads to increased expression of E-cadherin, vascular endothelial growth factor (VEGF), epidermal growth factor receptor (EGFR) and CD133. Furthermore, SMAD4 loss causes alterations to multiple kinase pathways (particularly the phosphorylated ERK/p38/Akt pathways), and increases chemoresistance in vitro. Finally, PDAC cells with intact SMAD4 are more sensitive to TGF-β1 inhibitor treatment to reduced cell migration; PDAC cells lacking SMAD4 showed decreased cell motility in response to EGFR inhibitor treatment.

CONCLUSIONS

This study revealed the molecular basis for SMAD4-dependent differences in PDAC with the aim of identifying the subset of patients likely to respond to therapies targeting the TGF-β or EGFR signaling pathways and of identifying potential therapeutic interventions for PDAC patients with SMAD4 defects.

Alterations in the Smad pathway in human cancers

Members of the TGF-beta superfamily exhibit various biological activities, and perturbations of their signaling are linked to certain clinical disorders including cancer. The role of TGF-beta signaling as a tumor suppressor pathway is best illustrated by the presence of inactivating mutations in genes encoding TGF-beta receptors and Smads in human carcinomas. This perspective is further supported by studies of tumor development in mouse models after modulation of receptors and Smads. TGF-beta also controls processes such as cell invasion, immune regulation, and microenvironment alterations that cancer cells may exploit to their advantage for their progression. Consequently, the output of a TGF-beta response is highly situation dependent, across different tissues, and also in cancer in general. Understanding the mechanisms of TGF-beta superfamily signaling is thus important for the development of new ways to treat various types of cancer. This review focuses on recent advances in understanding the Smad dependent TGF-beta pathway as it relates to human carcinogenesis.

The Notch ligand Delta-like 1 integrates inputs from TGFbeta/Activin and Wnt pathways

Unlike the well-characterized nuclear function of the Notch intracellular domain, it has been difficult to identify a nuclear role for the ligands of Notch. Here we provide evidence for the nuclear function of the Notch ligand Delta-like 1 in colon cancer (CC) cells exposed to butyrate. We demonstrate that the intracellular domain of Delta-like 1 (Dll1icd) augments the activity of Wnt signaling-dependent reporters and that of the promoter of the connective tissue growth factor (CTGF) gene. Data suggest that Dll1icd upregulates CTGF promoter activity through both direct and indirect mechanisms. The direct mechanism is supported by co-immunoprecipitation of endogenous Smad2/3 proteins and Dll1 and by chromatin immunoprecipitation analyses that revealed the occupancy of Dll1icd on CTGF promoter sequences containing a Smad binding element. The indirect upregulation of CTGF expression by Dll1 is likely due to the ability of Dll1icd to increase Wnt signaling, a pathway that targets CTGF. CTGF expression is induced in butyrate-treated CC cells and results from clonal growth assays support a role for CTGF in the cell growth-suppressive role of butyrate. In conclusion, integration of the Notch, Wnt, and TGFbeta/Activin signaling pathways is in part mediated by the interactions of Dll1 with Smad2/3 and Tcf4.

BMP suppresses PTEN expression via RAS/ERK signaling

Bone morphogenetic protein (BMP), a member of the transforming growth factor beta family, classically utilizes the SMAD signaling pathway for its growth suppressive effects,and loss of this signaling cascade may accelerate cell growth. In the colon cancer predisposition syndrome Juvenile Polyposis, as well as in the late progression stages of nonsyndromic colorectal cancers, SMAD4 function is typically abrogated. Here, we utilized the SMAD4-null SW480 colon cancer cell line to examine BMPs effect on a potential target gene, PTEN, and how its expression might be regulated. Initial treatment of the SMAD4-null cells with BMP resulted in mild growth suppression, but with prolonged exposure to BMP, the cells become growth stimulatory, which coincided with observed decreases in transcription and translation of PTEN, and with corresponding increases in phospho-AKT protein levels. BMP-induced PTEN suppression was mediated via the RAS/ERK pathway, as pharmacologic inhibition of RAS/ERK, or interference with protein function in the cytosol by DN-RAS prevented BMP-induced growth promotion and changes in PTEN levels, as did treatment with noggin, a BMP ligand inhibitor. Thus, BMP downregulates PTEN via RAS/ERK in a SMAD4-null environment that contributes to cell growth, and constitutes a SMAD4-independent but BMP-responsive signaling pathway.

On the role of transforming growth factor-beta in the growth inhibitory effects of retinoic acid in human pancreatic cancer cells

Background

Retinoids are potent growth inhibitory and differentiating agents in a variety of cancer cell types. We have shown that retinoids induce growth arrest in all pancreatic cancer cell lines studied, regardless of their p53 and differentiation status. However, the mechanism of growth inhibition is not known. Since TGF-β2 is markedly induced by retinoids in other cancers and mediates MUC4 expression in pancreatic cancer cells, we investigated the role of TGF-β in retinoic acid-mediated growth inhibition in pancreatic cancer cells.

Results

Retinoic acid markedly inhibited proliferation of two cell lines (Capan-2 and Hs766T) in a concentration and time-dependent manner. Retinoic acid increased TGF-β2 mRNA content and secretion of the active and latent forms of TGF-β2 (measured by ELISA and bioassay). The concentrations of active and TGF-β2 secreted in response to 0.1 – 10 μM retinoic acid were between 1–5 pM. TGF-β2 concentrations within this range also inhibited proliferation. A TGF-β neutralizing antibody blocked the growth inhibitory effects of retinoic acid in Capan-2 cells and partially inhibitory the effects in Hs766T cells.

Conclusion

These findings indicate that TGF-β can cause growth inhibition of pancreatic cancer cells, in a p53-independent manner. Furthermore, it demonstrates the fundamental role of TGF-β in growth inhibition in response to retinoic acid treatment is preserved in vitro.

Ski promotes tumor growth through abrogation of transforming growth factor-beta signaling in pancreatic cancer

OBJECTIVE

We hypothesized that human pancreatic cancer resists TGF-beta signaling and cell death through increased Ski expression.

SUMMARY BACKGROUND DATA

Ski is an oncogenic protein that acts as a TGF-beta repressor and prevents related gene transcription. Previous work suggests that Ski acts as an oncoprotein in melanoma and esophageal cancer. Ski expression and function have not been determined in human pancreatic cancer.

METHODS

Immunohistochemistry and immunoblots assessed Ski expression in human pancreatic cancer. Panc-1 cells were treated with or without Ski siRNA, and Ski and Smad protein expression, transcriptional reporter activation, and growth assays were determined. Panc-1 cells were inoculated in the flank of nude mice and tumor volume and histology assessed after administration of Ski siRNA or control vector.

RESULTS

Ski was abundantly expressed in human pancreatic cancer specimens assessed by immunohistochemistry (91%) and immunoblot analysis (67%). Panc-1 cells exhibited nascent Ski expression that was maximally inhibited 48 hours after transfection with Ski siRNA. TGF-beta transcriptional activity was increased 2.5-fold in Ski siRNA-treated cells compared with control (P < 0.05). Ski siRNA increased TGF-beta-induced Smad2 phosphorylation and p21 expression. Panc-1 growth in culture was decreased 2-fold at 72 hours. A Ski siRNA expression vector injected into nude mice resulted in a 5-fold decrease in growth.

CONCLUSION

Inhibition of Ski through RNA interference restored TGF-beta signaling and growth inhibition in vitro, and decreased tumor growth in vivo.

Transforming Growth Factor β Is a Critical Regulator of Adult Human Islet Plasticity

Tissue plasticity is well documented in the context of pancreatic regeneration and carcinogenesis, with recent reports implicating dedifferentiated islet cells both as endocrine progenitors and as the cell(s) of origin in pancreatic adenocarcinoma. Accordingly, it is noteworthy that accumulating evidence suggests that TGFβ signaling is essential to pancreatic endocrine development and maintenance, whereas its loss is associated with the progression to pancreatic adenocarcinoma. The aim of this study was to examine the role of TGFβ in an in vitro model of islet morphogenetic plasticity. Human islets were embedded in a collagen gel and cultured under conditions that induced transformation into duct-like epithelial structures (DLS). Addition of TGFβ caused a dose-dependent decrease in DLS formation. Although it was demonstrated that collagen-embedded islets secrete low levels of TGFβ, antibody-mediated neutralization of this endogenously released TGFβ improved DLS formation rates, suggesting local TGFβ concentrations may in fact be higher. Time course studies indicated that TGFβ signaling was associated with an increase in ERK and p38 MAPK phosphorylation, although inhibitor-based studies were consistent with an islet endocrine-stabilizing effect mediated by p38 alone. Localization of TGFβ signaling molecules suggested that the action of TGFβ is directly on the β-cell to inhibit apoptosis and thus stabilize endocrine phenotype.

Targeting signal transduction in pancreatic cancer treatment

Pancreatic cancer is a lethal disease with a 5-year survival rate of 4%. The only opportunity for improved survival continues to be complete surgical resection for those with localized disease. Although chemotherapeutic options are limited for the few patients with resectable disease, this problem is even more magnified in the majority (85%) of patients with unresectable or metastastic disease. Therefore, there is an urgent need for improved therapeutic options. The recent success of inhibitors of signal transduction for the treatment of other cancers supports the need to identify and validate aberrant signaling pathways important for pancreatic tumor growth. This review focuses on the validation of specific signaling networks and the present status of inhibitors of these pathways as therapeutic approaches for pancreatic cancer treatment.

Effect of the different phosphorylated Smad2 protein localizations on the invasive breast carcinoma phenotype

Smad2 participates in the TGF-beta signaling pathway, where it cooperates with transcription factors to regulate expression of defined genes. The purpose of this study was to investigate the expression pattern of phosphorylated Smad2 (pSmad2) in association with clinicopathological parameters and biological markers of proliferation and invasion. Immunohistochemistry was applied on paraffin-embedded sections from 164 patients with invasive breast carcinomas to detect the expression of the proteins pSmad2, ER, PR, Ki67, topoisomerase IIa, ERK2, catenin-p120, MMP-14 and TIMP-2. pSmad2 protein was detected in the nuclei of the malignant cells (68.1%) and in the tumor fibroblasts (55.2%). Nuclear pSmad2 was inversely correlated with histological grade and LN (p=0.047 and p=0.05) as well as with Ki67 and topoIIa (p=0.003 and p=0.021, respectively). There was also an inverse relation between nuclear pSmad2 and normal immunoexpression of the adhesion molecule catenin-p120 (p=0.028). Both nuclear and stromal pSmad2 were positively correlated with ERK2 of tumor fibroblasts (p=0.008 and p=0.0001, respectively), while stromal pSmad2 was furthermore related to stromal MMP-14 and tumor TIMP-2 (p=0.006 and p=0.022, respectively). Patients with high expression of cancerous pSmad2 tended to have a better prognosis, although statistic significance was never reached. pSmad2 was found to play a dual role, according to its distribution. Nuclear localization was thus found to be related to a less aggressive tumor phenotype, whereas stromal location was associated with an invasive phenotype.

Smad4 is dispensable for normal pancreas development yet critical in progression and tumor biology of pancreas cancer

SMAD4 is inactivated in the majority of pancreatic ductal adenocarcinomas (PDAC) with concurrent mutational inactivation of the INK4A/ARF tumor suppressor locus and activation of the KRAS oncogene. Here, using genetically engineered mice, we determined the impact of SMAD4 deficiency on the development of the pancreas and on the initiation and/or progression of PDAC-alone or in combination with PDAC--relevant mutations. Selective SMAD4 deletion in the pancreatic epithelium had no discernable impact on pancreatic development or physiology. However, when combined with the activated KRAS(G12D) allele, SMAD4 deficiency enabled rapid progression of KRAS(G12D)-initiated neoplasms. While KRAS(G12D) alone elicited premalignant pancreatic intraepithelial neoplasia (PanIN) that progressed slowly to carcinoma, the combination of KRAS(G12D) and SMAD4 deficiency resulted in the rapid development of tumors resembling intraductal papillary mucinous neoplasia (IPMN), a precursor to PDAC in humans. SMAD4 deficiency also accelerated PDAC development of KRAS(G12D) INK4A/ARF heterozygous mice and altered the tumor phenotype; while tumors with intact SMAD4 frequently exhibited epithelial-to-mesenchymal transition (EMT), PDAC null for SMAD4 retained a differentiated histopathology with increased expression of epithelial markers. SMAD4 status in PDAC cell lines was associated with differential responses to transforming growth factor-beta (TGF-beta) in vitro with a subset of SMAD4 wild-type lines showing prominent TGF-beta-induced proliferation and migration. These results provide genetic confirmation that SMAD4 is a PDAC tumor suppressor, functioning to block the progression of KRAS(G12D)-initiated neoplasms, whereas in a subset of advanced tumors, intact SMAD4 facilitates EMT and TGF-beta-dependent growth.

Expression of TGF-beta1, TbetaRII and Smad4 in colorectal carcinoma

BACKGROUND

Many colorectal carcinomas are resistant to the growth inhibitory response of transforming growth factor-beta (TGF-beta) due to alterations of components along the TGF-beta signaling pathway. The aim of this study was to examine the expression of TGF-beta1, TbetaRII and Smad4 in human colorectal carcinoma and their relationships with cancer growth.

METHODS

Immunohistochemistry and in situ hybridization were performed in 38 cases of colorectal carcinoma.

RESULTS

Intense signal for TGF-beta1 protein and TGF-beta1 mRNA were found in 71.1% (27/38) and 77.8% (21/27) of colorectal carcinoma, respectively. Intensive TbetaRII mRNA were detected only in 40% (11/27) cancer tissues (p<0.05). 65.8% (25/38) of colorectal carcinoma displayed decreased expression in TbetaRII immunoreactivity staining (p<0.05). Smad4 protein and Smad4 mRNA were reduced in 63.2% (24/38) and 63% (17/27) of tumors, respectively. Smad4 expression was related to tumor differentiation and Duke's stage (p<0.05). Furthermore, TGF-beta1-positive tumors with lymph node metastasis preferentially had significant reduced Smad4 expression (p<0.05).

CONCLUSIONS

Down-regulation of TbetaRII as well as the over-expression of TGF-beta1 play a possible role for the escape of colorectal carcinoma from TGF-beta-mediated growth inhibition. Reduced Smad4 is associated with malignancy and progression of colorectal carcinoma.

The inherited genetics of pancreatic cancer and prospects for secondary screening

It is estimated that pancreatic cancer has a familial component in approximately 5-10% of cases. Some of these cases are part of a defined cancer syndrome with a known gene mutation but in the remaining the causative gene remains unknown. In recent years, a better understanding of the molecular events that occur in the progression model of pancreatic cancer has lead to the development of secondary screening programmes with the aim of identifying early precursor lesions or pre-invasive cancer at a stage amenable to curative resection. High-risk groups who have an inherited predisposition for pancreatic cancer form the ideal group to study in developing a robust screening programme. Multimodality screening using computed tomography and endoluminal ultrasound in combination with molecular analysis of pancreatic juice are proving promising as diagnostics tools or at least serving as predictors of risk over a defined period.

Restoration of Smad4 in BxPC3 pancreatic cancer cells attenuates proliferation without altering angiogenesis

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive human malignancy in which the transforming growth factor beta (TGF-beta) signal transducer, Smad4, is commonly mutated or deleted. BxPC3 human pancreatic cancer cells exhibit a homozygous deletion of the Smad4 gene, yet are growth inhibited by TGF-beta1. In the present study, we sought to determine whether reintroduction of Smad4 into BxPC3 cells alters their behavior in vitro and in vivo. Sham transfected and Smad4 expressing BxPC3 cells exhibited similar responses to TGF-beta1 with respect to p21 upregulation, hypophosphorylation of the RB protein, Smad2 phosphorylation, and Smad2/3 nuclear translocation. TGF-beta1 did not alter p27 expression, and silencing of p21 with an appropriate siRNA markedly attenuated TGF-beta1-mediated growth inhibition. Nonetheless, the presence of Smad4 was associated in vitro with a more prolonged doubling time, enhanced sensitivity to the growth inhibitory actions of exogenous TGF-beta1, and a more flattened cellular morphology. In vivo, Smad4 expression resulted in delayed tumor growth and decreased cellular proliferation, without effects on either apoptosis or angiogenesis. These findings indicate that, in spite of the absence of Smad4, growth inhibition in BxPC3 cells by TGF-beta1 is dependent on p21 upregulation and maintenance of RB in a hypophosphorylated, active state. Moreover, the presence of a functional Smad4 attenuates the capacity of BxPC3 cells to proliferate in vivo. However, this effect is transient, indicating that Smad4 growth inhibitory actions are circumvented in the later stages of pancreatic tumorigenicity.

Smad4 dependency defines two classes of transforming growth factor {beta} (TGF-{beta}) target genes and distinguishes TGF-{beta}-induced epithelial-mesenchymal transition from its antiproliferative and migratory responses

In response to transforming growth factor beta (TGF-beta), Smad4 forms complexes with activated Smad2 and Smad3, which accumulate in the nucleus, where they both positively and negatively regulate TGF-beta target genes. Mutation or deletion of Smad4 is found in about 50% of pancreatic tumors and in about 15% of colorectal tumors. As Smad4 is a central component of the TGF-beta/Smad pathway, we have determined whether Smad4 is absolutely required for all TGF-beta responses, to evaluate the effect of its loss during human tumor development. We have generated cell lines from the immortalized human keratinocyte cell line HaCaT or the pancreatic tumor cell line Colo-357, which stably express a tetracyline-inducible small interfering RNA targeted against Smad4. In response to tetracycline, Smad4 expression is effectively silenced. Large-scale microarray analysis identifies two populations of TGF-beta target genes that are distinguished by their dependency on Smad4. Some genes absolutely require Smad4 for their regulation, while others do not. Functional analysis also indicates a differential Smad4 requirement for TGF-beta-induced functions; TGF-beta-induced cell cycle arrest and migration, but not epithelial-mesenchymal transition, are abolished after silencing of Smad4. Altogether our results suggest that loss of Smad4 might promote TGF-beta-mediated tumorigenesis by abolishing tumor-suppressive functions of TGF-beta while maintaining some tumor-promoting TGF-beta responses.

Smad4 dependency defines two classes of transforming growth factor {beta} (TGF-{beta}) target genes and distinguishes TGF-{beta}-induced epithelial-mesenchymal transition from its antiproliferative and migratory responses

In response to transforming growth factor beta (TGF-beta), Smad4 forms complexes with activated Smad2 and Smad3, which accumulate in the nucleus, where they both positively and negatively regulate TGF-beta target genes. Mutation or deletion of Smad4 is found in about 50% of pancreatic tumors and in about 15% of colorectal tumors. As Smad4 is a central component of the TGF-beta/Smad pathway, we have determined whether Smad4 is absolutely required for all TGF-beta responses, to evaluate the effect of its loss during human tumor development. We have generated cell lines from the immortalized human keratinocyte cell line HaCaT or the pancreatic tumor cell line Colo-357, which stably express a tetracyline-inducible small interfering RNA targeted against Smad4. In response to tetracycline, Smad4 expression is effectively silenced. Large-scale microarray analysis identifies two populations of TGF-beta target genes that are distinguished by their dependency on Smad4. Some genes absolutely require Smad4 for their regulation, while others do not. Functional analysis also indicates a differential Smad4 requirement for TGF-beta-induced functions; TGF-beta-induced cell cycle arrest and migration, but not epithelial-mesenchymal transition, are abolished after silencing of Smad4. Altogether our results suggest that loss of Smad4 might promote TGF-beta-mediated tumorigenesis by abolishing tumor-suppressive functions of TGF-beta while maintaining some tumor-promoting TGF-beta responses.

Transforming growth factor-beta1 to the bone

TGF-beta1 is a ubiquitous growth factor that is implicated in the control of proliferation, migration, differentiation, and survival of many different cell types. It influences such diverse processes as embryogenesis, angiogenesis, inflammation, and wound healing. In skeletal tissue, TGF-beta1 plays a major role in development and maintenance, affecting both cartilage and bone metabolism, the latter being the subject of this review. Because it affects both cells of the osteoblast and osteoclast lineage, TGF-beta1 is one of the most important factors in the bone environment, helping to retain the balance between the dynamic processes of bone resorption and bone formation. Many seemingly contradictory reports have been published on the exact functioning of TGF-beta1 in the bone milieu. This review provides an overall picture of the bone-specific actions of TGF-beta1 and reconciles experimental discrepancies that have been reported for this multifunctional cytokine.

Regulation of GDF-8 signaling by the p38 MAPK

Growth differentiation factor-8 (GDF-8), a member of the TGF-beta superfamily, is a negative regulator of skeletal muscle growth, which functions through activation of the Smad proteins. We found that GDF-8 can activate the p38 mitogen-activated protein kinase (MAPK) through the TGF-beta-activated kinase 1 (TAK1), and this appeared to be independent of Smad signaling. GDF-8-induced transcriptional activation was inhibited by expression of dominant negative MKK6 or treatment with the p38 inhibitor SB203580, while overexpression of wild-type forms of either MKK6 or p38 augmented GDF-8-induced transcriptional activity. In addition, ATF-2, a known transcription factor target of p38, was found to be phosphorylated on GDF-8 stimulation and was detected in a complex with Smad3/Smad4 upon GDF-8 treatment. Furthermore, we found that the p38 MAPK played an important role in GDF-8-induced inhibition of proliferation and upregulation of the cyclin kinase inhibitor p21. Together, these results highlight a functional link between the p38 MAPK and GDF-8-activated Smad pathways, and identify a critical role for the p38 MAPK in GDF-8's function as a negative regulator of muscle growth.

Pancreatic differentiation

Over the last decade, with the advent of new techniques and technologies in modern molecular biology, our understanding of the underlying mechanisms responsible for organ differentiation has developed rapidly. Despite this, our knowledge of these signaling pathways is still far from complete. Some of these advances, such as the creation of transgenic mouse models, have given us new tools to help us understand the interactions of the various transcription factors that are responsible for the creation of various cell types from a single cell type during embryogenesis. This knowledge then gives rise to the concept of creating new ways to manipulate stem cells in order to correct the deficiencies present in various disease processes. Here, we present work that focuses specifically on pancreatic development. The ultimate goal of our research in studying the mechanisms of the basic differentiation of pancreatic precursor cells is to gain the knowledge necessary to be able to engineer stem cells specifically into beta-cells in the treatment of diabetes.

Activin A suppresses neuroblastoma xenograft tumor growth via antimitotic and antiangiogenic mechanisms

The tumor suppressor function of activin A, together with our findings that activin A is an inhibitor of angiogenesis, which is down-regulated by the N-MYC oncogene, prompted us to investigate in more detail its role in the malignant transformation process of neuroblastomas. Indeed, neuroblastoma cells with restored activin A expression exhibited a diminished proliferation rate and formed smaller xenograft tumors with reduced vascularity, whereas lung metastasis rate remained unchanged. In agreement with the decreased vascularity of the xenograft tumors, activin A inhibited several crucial angiogenic responses of cultured endothelial cells, such as proteolytic activity, migration, and proliferation. Endothelial cell proliferation, activin A, or its constitutively active activin receptor-like kinase 4 receptor (ALK4T206D), increased the expression of CDKN1A (p21), CDKN2B (p15), and CDKN1B (p27) CDK inhibitors and down-regulated the expression of vascular endothelial growth factor receptor-2, the receptor of a key angiogenic factor in cancer. The constitutively active forms of SMAD2 and SMAD3 were both capable of inhibiting endothelial cell proliferation, whereas the dominant-negative forms of SMAD3 and SMAD4 released the inhibitory effect of activin A on endothelial cell proliferation by only 20%. Thus, the effects of activin A on endothelial cell proliferation seem to be conveyed via the ALK4/SMAD2-SMAD3 pathways, however, non-SMAD cascades may also contribute. These results provide novel information regarding the role of activin A in the malignant transformation process of neuroblastomas and the molecular mechanisms involved in regulating angiogenesis thereof.

Over-expression of TGF-beta1 in Smad4-deficient human oral carcinoma cells causes tumour regression in vivo by mechanisms that sensitize cells to apoptosis

We have shown previously that transforming growth factor-beta (TGF-beta) is a potent tumour suppressor in Smad4-deficient human malignant oral keratinocytes but the mechanism by which this occurs is unknown. In the present study, we show that over-expression of TGF-beta1 causes regression of tumours derived from Smad4-deficient oral keratinocytes transplanted orthotopically to athymic mice. Further, tumour regression is associated with the induction of apoptosis without changes in cell proliferation. In vitro, TGF-beta1 did not induce apoptosis directly in these cells but sensitized cells to cisplatin, but not Fas, -induced cell death. The data suggest that TGF-beta1 induces tumour regression in vivo by Smad4-independent pathways that sensitize keratinocytes to mitochondrial-mediated apoptosis.

Targeting endogenous transforming growth factor beta receptor signaling in SMAD4-deficient human pancreatic carcinoma cells inhibits their invasive phenotype1

Transforming growth factor-beta (TGF-beta) suppresses tumor formation by blocking cell cycle progression and maintaining tissue homeostasis. In pancreatic carcinomas, this tumor suppressive activity is often lost by inactivation of the TGF-beta-signaling mediator, Smad4. We found that human pancreatic carcinoma cell lines that have undergone deletion of MADH4 constitutively expressed high endogenous levels of phosphorylated receptor-associated Smad proteins (pR-Smad2 and pR-Smad3), whereas Smad4-positive lines did not. These elevated pR-Smad levels could not be attributed to a decreased dephosphorylation rate nor to increased expression of TGF-beta type I (TbetaR-I) or type II (TbetaR-II) receptors. Although minimal amounts of free bioactive TGF-beta1 and TGF-beta2 were detected in conditioned medium, treatment with a pan-specific (but not a TGF-beta3 specific) TGF-beta-neutralizing antibody and with anti-alpha(V)beta(6) integrin antibody decreased steady-state pSmad2 levels and activation of a TGF-beta-inducible reporter gene in neighboring cells, respectively. Thus, activation of TGF-beta at the cell surface was responsible for the increased autocrine endogenous and paracrine signaling. Blocking TbetaR-I activity using a selective kinase inhibitor (SD-093) strongly decreased the in vitro motility and invasiveness of the pancreatic carcinoma cells without affecting their growth characteristics, morphology, or the subcellular distribution of E-cadherin and F-actin. Moreover, exogenous TGF-beta strongly stimulated in vitro invasiveness of BxPC-3 cells, an effect that could also be blocked by SD-093. Thus, the motile and invasive properties of Smad4-deficient pancreatic cancer cells are at least partly driven by activation of endogenous TGF-beta signaling. Therefore, targeting the TbetaR-I kinase represents a potentially powerful novel therapeutic approach for the treatment of this disease.

Evidence of MKK4 pro-oncogenic activity in breast and pancreatic tumors

MKK4, located in close proximity to p53 gene, is thought to be a tumor suppressor and a metastasis suppressor gene. A low-rate MKK4 gene alteration has been found in a few tumor types, including breast and pancreatic. A suppressor activity for prostate and ovarian tumor metastasis has also been suggested. To understand the pathobiologic roles of MKK4 in tumorigenesis, we examined the phenotypic changes in response to perturbation of the MKK4 expression in breast and pancreatic cancer cell lines. Ectopic expression of MKK4 by adenoviral delivery in MKK4-negative cancer lines stimulated the cell proliferation and invasion, whereas knockdown of MKK4 expression by small interference RNA in an MKK4-positive breast cancer cell line, MDA-MB-231, resulted in decreased anchorage-independent growth, suppressed tumor growth in mouse xenograft model, and increased cell susceptibility to apoptosis brought by stress signals such as serum deprivation. These results argue that MKK4 functions as a pro-oncogenic molecule instead of a suppressor in breast and pancreatic tumors.

Differential Activation of Smads in HeLa and SiHa Cells That Differ in Their Response to Transforming Growth Factor-β

We assessed the responsiveness of six human cervical cancer cell lines to transforming growth factor (TGF)-beta with p3TP-lux reporter assay and found that HeLa and SiHa cells were highly responsive to TGF-beta. However, when pSBE4-BV/Luc reporter with four Smad binding elements was used, only the SiHa, not the HeLa, cells showed Smad activation. Smad DNA binding activity was relatively more in SiHa than in HeLa cells upon TGF-beta treatment, and the active complex contained Smad 2 and Smad 4. In 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays, HeLa cells treated with 5 ng/ml of TGF-beta for 24 h showed proliferation, whereas SiHa cells showed growth inhibition under the same conditions. TGF-beta treatment resulted in G(0)/G(1) arrest with a reduction in S-phase only in SiHa cells. A chemical inhibitor of Smad activation (SB203580) blocked the growth inhibitory effect of TGF-beta in SiHa, whereas the proliferative response in HeLa was unaffected. TGF-beta-induced translocation of phospho-Smad 2 was relatively less in HeLa than in SiHa cells. MAPK activation occurred within 5 min and persisted up to 15 min upon TGF-beta treatment in HeLa but was negligible in SiHa cells. TGF-beta activated JNK in HeLa, but SiHa cells showed a down-regulation of its activity. When an inhibitor of MAPK (U0126) was used, the TGF-beta-mediated proliferative response in HeLa cells was completely abolished. SB203580 did not affect MAPK activation induced by TGF-beta in HeLa cells. We report for the first time an activation, presumably independent of Smad activation, of TGF-beta-dependent MAPK within 5 min of treatment that resulted in cell cycle progression in a cervical adenocarcinoma cell line, HeLa.

Molecular and functional consequences of Smad4 C-terminal missense mutations in colorectal tumour cells

Smad4 is an essential signal transducer of the transforming growth factor beta (TGF-beta) signalling pathway and has been identified as a tumour suppressor, being mutated in approx. 50% of pancreatic cancers and approx. 15% of colorectal cancers. Two missense mutations in the C-terminal domain of Smad4, D351H (Asp351-->His) and D537Y (Asp537-->Tyr), have been described recently in the human colorectal cancer cell lines CACO-2 and SW948 respectively [Woodford-Richens, Rowan, Gorman, Halford, Bicknell, Wasan, Roylance, Bodmer and Tomlinson (2001) Proc. Natl. Acad. Sci. U.S.A. 98, 9719-9723]. Previous work in vitro suggested that only Asp-351 was required for interaction with Smad2 [Wu, Fairman, Penry and Shi (2001) J. Biol. Chem. 276, 20688-20694]. In the present study, we investigate the functional consequences of these point mutations in vivo. We demonstrate that neither of these colorectal cancer cells undergo growth arrest in response to TGF-beta, which can be explained, at least in part, by their inability to up-regulate cyclin-dependent kinase inhibitors p21 (CIP1 ) or p15 ( INK4b) after TGF-beta stimulation. Although the point-mutated Smad4s are expressed at normal levels in these colorectal cancer cells, they cannot interact with either TGF-beta-induced phosphorylated Smad2 or Smad3. As a result, these Smad4 mutants do not accumulate in the nucleus after TGF-beta stimulation, are not recruited to DNA by relevant Smad-binding transcription factors and cannot generate transcriptionally active DNA-bound complexes. Therefore both these colorectal tumour cells completely lack functional Smad4 activity owing to the missense mutations. Given the location of these mutations in the three-dimensional structure of the Smad4 C-terminal domain, the results also give us significant insights into Smad complex formation.

Role of transforming growth factor β in breast carcinogenesis

Transforming growth factor (TGF) beta is a pre-eminent negative growth regulator that has antiproliferative effects on a range of epithelial cells. This ability has evoked interest in this growth factor as a tumour suppressor with potential clinical significance. In the early stages of breast carcinogenesis, a growth-inhibitory response to TGFbeta is maintained, which depends on an intact TGFbeta signalling pathway. Tumour development and progression of cells along a neoplastic continuum is accompanied by loss of this growth-inhibitory response to TGFbeta, which might instead promote tumour growth indirectly through a combination of permissive effects on stromal tissue, angiogenesis, and the immune system. This review discusses the complexity of functional pleiotropy and the continually changing roles of TGFbeta as a tumour evolves, along with competing therapeutic strategies. The boosting of local endogenous amounts of TGFbeta in conjunction with enhancement of cellular responsiveness might be appropriate in early malignant disease, and anti-TGFbeta approaches could yield a therapeutic gain in metastatic states.

The role of chromosome 18 abnormalities in the progression of pancreatic adenocarcinoma

To date, the events that mediate tumor progression in pancreatic cancer are still poorly understood. Cytogenetic, allelotype, and somatic cell hybrid studies in human pancreatic adenocarcinoma have suggested that chromosome 18 may carry tumor suppressor genes (TSGs), including SMAD4. We previously identified that LOH of 18q at the SMAD4 locus, along with LOHs on 17p and 12q, positively associated with poor prognoses of pancreatic cancer patients. However, restoration of the SMAD4 gene did not suppress in vitro proliferation of pancreatic cancer cells that harbored homozygous deletion of this gene. An intraductal papillary mucinous neoplasm (IPMN ) is thought to be one of the premalignant lesions of the pancreas that progresses to carcinoma. Although there were frequent LOH (7/14, 50%) at the SMAD4 locus in IPMN samples, SMAD4 protein was observed immunohistochemically in tumor cells, and no mutations of the SMAD4 gene were observed, suggesting that it is the existence of a TSG in 18q, other than SMAD4, that suppresses cell growth. To functionally assess the activity of chromosome 18 in pancreatic cancer, we transferred a normal copy of the chromosome into pancreatic ductal carcinoma cells with and without completely inactivated SMAD4. In this study, in vitro growth of the hybrid cells was significantly suppressed compared with the parental cells, regardless of the initial SMAD4 status. To estimate the metastatic ability of the hybrids, we used a lung colonization model. At the end of the experiment, there was significant suppression of the number of surface metastases developing in mice injected with hybrids in comparison with those injected with parental cells. To identify and characterize genes that are involved in the progression of pancreatic cancer, we used micro-array expression analysis employing a 20k oligo-array system. It was revealed that there was increased expression of 4 genes relating to apoptosis in the 18 chromosome hybrids cells compared with the parental cells. We are now analyzing the function of these genes.

Smad4-independent regulation of p21/WAF1 by transforming growth factor-beta

The transforming growth factor-beta (TGF-beta)-Smad signaling pathway inhibits the growth of human epithelial cells and plays a role in tumor suppression. The Smad4 gene is mutated or deleted in 50% of pancreatic cancers. In this study, the Smad4-null pancreatic cancer cell line BxPC-3 was transfected with either the Smad4 expression vector or the empty vector and incubated in the presence or absence of TGF-beta. The cells were analysed using a cDNA microarray, which included 2280 named genes to screen for target genes regulated by TGF-beta in either a Smad4-dependent or -independent manner. The microarray and subsequent quantitative RT-PCR analysis demonstrated that the Smad4-independent and -dependent signaling pathways driven by TGF-beta upregulated only one of the 2280 genes, respectively, suggesting that Smad4-independent signaling downstream of TGF-beta might be as widespread as Smad4-dependent signaling. In this study, we demonstrated that the cyclin-dependent kinase inhibitor p21/WAF1, which has been considered the major effector of the Smad-dependent growth inhibitory signal of TGF-beta, is upregulated in a Smad4-independent manner. The upregulation occurs through Smad2/3-dependent transcriptional activation of the p21/WAF1 promoter region. These results suggest a novel mechanism of gene regulation, that is, a novel signal mediator other than Smad4.

Extracellular control of pancreatic differentiation

Our understanding of basic mechanisms of differentiation has evolved rapidly in the last two decades. Spurred by advances in molecular biology and other research technologies, these advances have become of heightened importance with the recent advent of the possibility of engineering different types of stem cells into needed cell and tissue sources. As pediatric surgeons, we have the potential to play a key role in interfacing between the basic science necessary to understand differentiation processes, and its application at the bedside. In this brief article, we outline our in-depth analysis of mechanisms of basic differentiation of pancreatic precursor cells in an effort to better understand ways in which we can engineer a stem cell pool to form mature pancreatic cells.

Restoration of SMAD4 by gene therapy reverses the invasive phenotype in pancreatic adenocarcinoma cells

SMAD4 is a critical cofactor in signal transduction pathways activated in response to transforming growth factor-beta (TGF-beta)-related ligands, regulating cell growth and differentiation. The roles played by SMAD4 inactivation in tumours highlighted it as a tumour-suppressor gene. However, restoration of the TGF-beta antiproliferative pathway following SMAD4 gene transfer in null-tumour cell lines is controversial. Herein, we report the inhibitory effects of SMAD4 on pancreatic tumour invasion and angiogenesis. Adenoviral transfer of this gene in a panel of SMAD4 homozygous-deleted human pancreatic tumour cell lines restored SMAD4 protein expression and function. Although it did not affect proliferation significantly in vitro, SMAD4 inhibited in vivo tumour growth in immunodeficient mice. In this xenograft setting, differential suppression of tumour growth in vivo was mediated, at least in part, through downregulation of vascular endothelial growth factor and expression of gelatinases. We documented the reduced invasion and angiogenesis histologically and by intravital microscopy, and gained mechanistic insight at the messenger and protein level. Finally, we found a negative reciprocal regulation between SMAD4 and ETS-1. ETS-1 is considered a marker for tumour invasion. Upon SMAD4 deletion, we detected high expression levels of ETS-1 in pancreatic tumour cells, suggesting the shift of the pancreatic tumour toward an invasive phenotype.

Loss of Smad4 Protein Expression Occurs Infrequently in Endometrial Carcinomas

Smad4 is a member of the Smad proteins, which are needed for mediating signals of transforming growth factor beta from the cell surface to the nucleus. Smad4 is also a tumor suppressor gene for cancers of the pancreas, colon, and lung. The aim of this study was to investigate the expression and prognostic significance of this gene product in endometrial cancer. Immunohistochemical staining for Smad4 was performed on formalin-fixed, paraffin-embedded specimens of endometrial tumors with an anti-Smad4 monoclonal antibody (clone B8): 97 primary endometrial carcinomas, 20 cases of endometrial hyperplasia, and 26 cases of metastases from endometrial carcinoma. The immunoreactivity of each tumor was correlated with the clinical and histopathologic parameters of the patients. Diffusely positive expression of Smad4 protein was detected in all 20 cases of endometrial hyperplasia and in most of the primary and metastatic endometrial cancers. The frequency of positive expression decreased progressively with tumor grade. Clinically, however, it was not associated with tumor progression, nor did it predict patient outcome. Although loss of heterozygosity at chromosome 18q21 (the location of the Smad4 gene) is frequent in endometrial carcinomas, the authors show in this immunohistochemical study that inactivation of this gene occurs infrequently in this tumor.

Attenuation of the TGF-beta-Smad signaling pathway in pancreatic tumor cells confers resistance to TGF-beta-induced growth arrest

We have investigated the mechanism whereby tumor cells become resistant to the antiproliferative effects of transforming growth factor (TGF)-beta, while maintaining other responses that can lead to increased malignancy and invasiveness. TGF-beta signaling results in nuclear accumulation of active Smad complexes which regulate transcription of target genes. Here we show that in two pancreatic carcinoma cell lines, PT45 and Panc-1, that are resistant to TGF-beta-induced growth arrest, the TGF-beta-Smad signaling pathway is attenuated compared with epithelial cells that are sensitive to the antiproliferative effects of TGF-beta (HaCaT and Colo-357). In PT45 and Panc-1 cells, active Smad complexes remain nuclear for only 1-2 h compared with more than 6 h in HaCaT and Colo-357 cells. The attenuated pathway in PT45 and Panc-1 cells correlates with low levels of TGF-beta type I receptor and results in an altered expression profile of TGF-beta-inducible genes required for cell cycle arrest. Most significantly, expression of the CDK inhibitor, p21(Cip1/WAF1), which is required for TGF-beta-induced growth arrest in these cells, is not maintained. Moreover, we show that artificially attenuating the TGF-beta-Smad signaling pathway in HaCaT cells is sufficient to prevent TGF-beta-induced growth arrest. Our results demonstrate that the duration of TGF-beta-Smad signaling is a critical determinant of the specificity of the TGF-beta response.

Smads 2 and 3 are differentially activated by transforming growth factor-beta (TGF-beta ) in quiescent and activated hepatic stellate cells. Constitutive nuclear localization of Smads in activated cells is TGF-beta-independent

Hepatic stellate cells are the primary cell type responsible for matrix deposition in liver fibrosis, undergoing a process of transdifferentiation into fibrogenic myofibroblasts. These cells, which undergo a similar transdifferentiation process when cultured in vitro, are a major target of the profibrogenic agent transforming growth factor-beta (TGF-beta). We have studied activation of the TGF-beta downstream signaling molecules Smads 2, 3, and 4 in hepatic stellate cells (HSC) cultured in vitro for 1, 4, and 7 days, with quiescent, intermediate, and fully transdifferentiated phenotypes, respectively. Total levels of Smad4, common to multiple TGF-beta superfamily signaling pathways, do not change as HSC transdifferentiate, and the protein is found in both nucleus and cytoplasm, independent of treatment with TGF-beta or the nuclear export inhibitor leptomycin B. TGF-beta mediates activation of Smad2 primarily in early cultured cells and that of Smad3 primarily in transdifferentiated cells. The linker protein SARA, which is required for Smad2 signaling, disappears with transdifferentiation. Additionally, day 7 cells demonstrate constitutive phosphorylation and nuclear localization of Smad 2, which is not affected by pretreatment with TGF-beta-neutralizing antibodies, a type I TGF-beta receptor kinase inhibitor, or activin-neutralizing antibodies. These results demonstrate essential differences between TGF-beta-mediated signaling pathways in quiescent and in vitro transdifferentiated hepatic stellate cells.

TGF-beta-induced nuclear localization of Smad2 and Smad3 in Smad4 null cancer cell lines

Smad4 is a tumor suppressor gene that is commonly lost or mutated in colorectal and pancreatic cancers. The activated transforming growth factor-beta (TGF-beta) receptor phosphorylates Smad2 and Smad3, which then complex with Smad4 and translocate to the nucleus. Smad4 mutations when detected as present in some human cancers have been considered sufficient to inactivate TGF-beta signaling. In this work, we describe a colon cancer cell line, VACO-9M, that is Smad4 null when analysed by multiple assays. To study the role of Smad4 in TGF-beta-induced translocation of the receptor-activated Smads to the nucleus, we analysed by immunofluorescence the cellular localization of endogenous Smad2 and Smad3 after TGF-beta treatment of VACO-9M, plus four additional Smad4 null cell lines of breast (MDA-MB-468), or pancreatic (BxPC3, Hs766T, CFPAC-1) origin. In each cell line, TGF-beta treatment resulted in both Smad2 and Smad3 moving to the nucleus in a Smad4-independent fashion. Nuclear translocation of Smad2 and Smad3 was, however, not sufficient to activate reporters for TGF-beta-induced transcriptional responses, which were however restored by transient transfection of wild-type Smad4. We conclude that Smad4 is not required for nuclear translocation of Smad2 and Smad3, but is needed for activation of at least certain transcriptional responses.

Expression of Smad4 in the FaDu cell line partially restores TGF-beta growth inhibition but is not sufficient to regulate fibronectin expression or suppress tumorigenicity

Mutations of the Smad4 gene, a member of a group of TGF-beta signal transduction components, occur in several types of cancer suggesting that its inactivation significantly affects TGF-beta responsiveness in these tumors. To further investigate the role of Smad4 with respect to TGF-beta signaling and carcinogenesis, we re-expressed the Smad4 gene in the Smad4-deficient cancer cell line FaDu by microcell-mediated chromosome transfer (MMCT) and retroviral infection to closely approximate physiological protein levels. The Smad4-expressing FaDu clones were then evaluated for TGF-beta responsiveness to assess the role of Smad4 in TGF-beta-induced growth inhibition and target gene regulation. We found that the re-expression of the Smad4 gene by either method partially restored TGF-beta responsiveness in FaDu cells with respect to both growth inhibition and expression of p21WAF1/CIP1 and p15INK4B. However, only the microcell hybrids showed growth retardation in organotypic raft culture and an enhanced ability to upregulate fibronectin. In contrast, the re-expression of Smad4 by either method failed to suppress tumorigenicity. These results suggest that in addition to a homozygous deletion of Smad4, FaDu cells contain additional defects within the TGF-beta signaling pathway, thereby limiting the extent of TGF-beta responsiveness upon Smad4 re-expression and perhaps accounting for the inability to induce p15INK4B to a high level. They also demonstrate the advantages of providing a physiological extracellular environment, when assessing TGFbeta responsiveness.

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.

Pancreatic cancer biology and genetics

Pancreatic ductal adenocarcinoma is an aggressive and devastating disease, which is characterized by invasiveness, rapid progression and profound resistance to treatment. Advances in pathological classification and cancer genetics have improved our descriptive understanding of this disease; however, important aspects of pancreatic cancer biology remain poorly understood. What is the pathogenic role of specific gene mutations? What is the cell of origin? And how does the stroma contribute to tumorigenesis? A better understanding of pancreatic cancer biology should lead the way to more effective treatments.

Smad4/DPC4-dependent regulation of biglycan gene expression by transforming growth factor-beta in pancreatic tumor cells

Overexpression of the small leucine-rich proteoglycan biglycan (BGN) in fibrosis and desmoplasia results from enhanced activity of transforming growth factor-beta (TGF-beta). In pancreatic adenocarcinoma, the tumor cells themselves may contribute to BGN synthesis in vivo, since 8 of 18 different pancreatic carcinoma cell lines constitutively expressed BGN mRNA, as shown by reverse transcription-PCR analysis. In PANC-1 cells, TGF-beta1 dramatically stimulated BGN mRNA accumulation through a BGN transcription-independent, cycloheximide-sensitive mechanism and strongly increased the synthesis and release of the proteoglycan form of BGN. The ability of TGF-beta1 to induce BGN mRNA was critically dependent on Smad signaling, since 1) the up-regulation of BGN mRNA was preceded by a marked increase in Smad2 phosphorylation in TGF-beta1-treated PANC-1 cells, 2) TGF-beta1 was unable to induce BGN mRNA in pancreatic carcinoma cell lines that carry homozygous deletions of the Smad4/DPC4 gene, 3) inhibition of the Smad pathway in PANC-1 cells by transfection with a dominant negative Smad4/DPC4 mutant significantly reduced TGF-beta1-induced BGN mRNA expression, 4) stable reintroduction of wild type Smad4/DPC4 into Smad4-null CFPAC-1 cells restored the TGF-beta1 effect, and 5) overexpression of Smad2 and Smad3 in PANC-1 cells augmented TGF-beta1 induction of BGN mRNA, whereas forced expression of Smad7, an inhibitory Smad, effectively blocked it. These results clearly show that a functional Smad pathway is crucial for TGF-beta regulation of BGN mRNA expression. Since BGN has been shown to inhibit growth of pancreatic cancer cells, the Smad4/DPC4 mediation of the TGF-beta effect may represent a novel tumor suppressor function for Smad4/DPC4: antiproliferation via expression of autoinhibitory BGN.

Smad4 induces the tumor suppressor E-cadherin and P-cadherin in colon carcinoma cells

Smad4 is an intracellular transmitter of TGF-beta signals and its tumor suppressor function is presumed to reside in its capacity to mediate TGF-beta-induced growth inhibition. However, there is accumulating evidence that this hypothesis may be too simple. The roles of TGF-beta in carcinogenesis are complex and also comprise tumor promoting functions particularly in late stage carcinogenesis. Importantly, functional inactivation of Smad4 in colon carcinomas frequently occurs at late stages when tumors acquire invasive and metastatic capabilities. We have previously reported that stable re-expression of Smad4 in SW480 human colon carcinoma cells was adequate to suppress tumor growth in nude mice. However, it did not affect cell growth in vitro nor did it restore TGF-beta responsiveness. Here, we report that Smad4 transcriptionally induced classical cadherins including the invasion suppressor E-cadherin, presumably re-establishing epithelial morphology. Smad4-induced cadherins were able to recruit catenins to the plasma membrane and were functionally active in cell-cell adhesion. These results indicate a novel pathway of Smad4-mediated tumor suppression and suggest that Smad4 in colon cells may be involved in the maintenance of epithelial traits.

Signal transductions induced by bone morphogenetic protein-2 and transforming growth factor-beta in normal human osteoblastic cells

Transforming growth factor beta (TGF-beta) activates Ras/MAPK signaling in many cell types. Because TGF-beta and BMP-2 exert similar effects, we examined if this signaling is stimulated by both factors and analyzed the relationship between this signaling and the Smads in osteoblasts. BMP-2 and TGF-beta stimulated Ras, MAPK, and AP-1 activities. The DNA binding activities of c-Fos, FosB/Delta FosB, Fra-1, Fra-2, and JunB were up-regulated whereas JunD activity was decreased. c-Fos, FosB/Delta FosB, and JunB were associated with Smad4. The stimulation of AP-1 by BMP-2 and TGF-beta was dependent on Smad signaling, and anti-Smad4 antibody interfered with AP-1 activity. Thus, BMP-2 and TGF-beta activate both Ras/MAPK/AP-1 and Smad signaling in osteoblasts with Smads modulating AP-1 activity. To determine the roles of MAPK in BMP-2 and TGF-beta function, we analyzed the effect of ERK and p38 inhibitors on the regulation of bone matrix protein expression and JunB and JunD levels by these two factors. ERK and p38 mediated TGF-beta suppression of osteocalcin and JunD as well as stimulation of JunB. p38 was essential in BMP-2 up-regulation of type I collagen, fibronectin, osteopontin, osteocalcin, and alkaline phosphatase activity whereas ERK mediated BMP-2 stimulation of fibronectin and osteopontin. Thus, ERK and p38 differentially mediate TGF-beta and BMP-2 function in osteoblasts.

Transforming growth factor beta 1 induces proliferation in colon carcinoma cells by Ras-dependent, smad-independent down-regulation of p21cip1

Transforming growth factor beta1 (TGFbeta1) can act as a tumor suppressor or a tumor promoter depending on the characteristics of the malignant cell. We recently demonstrated that colon carcinoma cells transfected with oncogenic cellular K-rasV12, but not oncogenic cellular H-rasV12, switched from TGFbeta1-insensitive to TGFbeta1-growth-stimulated and also became more invasive (Yan, Z., Deng, X., and Friedman, E. (2001) J. Biol. Chem. 276, 1555-1563). We now demonstrate that TGFbeta1 growth stimulation of colon carcinoma cells is Ras-dependent and smad-independent. In U9 colon carcinoma cells, which are responsive to TGFbeta1 by growth stimulation, a truncating mutation at Gln-311 was found in the smad4 gene. Very little smad4 protein was detected in these cells. Loss of smad4 protein was confirmed by functional studies. In U9 cells co-transfected wild-type smad4, but not mutant smad4, mediated response of the 3TP-lux and pSBE promoter reporter constructs to TGFbeta1. Proliferation initiated by TGFbeta1 in U9 cells required Ras-mediated down-regulation of p21cip1 protein. Less p21cip1 was associated with cdk2 small middle dotcyclin complexes in TGFbeta1-treated U9 cells, and the cdk2 complexes had increased kinase activity. Elevation of p21cip1 levels diminished proliferative response to TGFbeta1. U9 cells expressing DN-N17ras neither proliferated in response to TGFbeta1 nor down-regulated the cdk inhibitor p21cip1, and TGFbeta1 activation of 3TP-lux in U9 cells was inhibited by DN-N17ras in a dose-dependent manner. TGFbeta1 also decreased p21cip1 levels and stimulated proliferation in SW480 cells, which express mutant K-Ras but no smad4 protein. TGFbeta1 did not activate or inhibit the p21cip1 promoter construct in U9 cells even in the presence of co-transfected smad4, or alter p21cip1 mRNA levels. Thus the decrease in p21cip1 levels was mediated by a TGFbeta-initiated Ras-dependent, but smad-independent post-transcriptional mechanism.

Suppressor and oncogenic roles of transforming growth factor-beta and its signaling pathways in tumorigenesis

Transforming growth factor-beta (TGF-beta) has been implicated in oncogenesis since the time of its discovery almost 20 years ago. The complex, multifunctional activities of TGF-beta endow it with both tumor suppressor and tumor promoting activities, depending on the stage of carcinogenesis and the responsivity of the tumor cell. Dysregulation or alteration of TGF-beta signaling in tumorigenesis can occur at many different levels, including activation of the ligand, mutation or transcriptional suppression of the receptors, or alteration of downstream signal transduction pathways resulting from mutation or changes in expression patterns of signaling intermediates or from changes in expression of other proteins which modulate signaling. New insights into signaling from the TGF-beta receptors, including the identification of Smad signaling pathways and their interaction with mitogen-activated protein (MAP) kinase pathways, are providing an understanding of the changes involved in the change from tumor suppressor to tumor promoting activities of TGF-beta. It is now appreciated that loss of sensitivity to inhibition of growth by TGF-beta by most tumor cells is not synonymous with complete loss of TGF-beta signaling but rather suggests that tumor cells gain advantage by selective inactivation of the tumor suppressor activities of TGF-beta with retention of its tumor promoting activities, especially those dependent on cross talk with MAP kinase pathways and AP-1.

TGF-beta1 acts as a tumor suppressor of human malignant keratinocytes independently of Smad 4 expression and ligand-induced G(1) arrest

This study examined the role of TGF-beta1 in human keratinocyte malignancy. Two carcinoma-derived human oral keratinocyte cell lines, BICR 31 and H314, were selected on the basis of their known resistance to TGF-beta1-induced G(1) arrest, the presence of wild type TGF-beta cell surface receptors and normal Ras. Smad 4 protein was undetectable in both cell lines, but Smad 2 and Smad 3 were expressed at levels comparable with a fully TGF-beta responsive cell line, and treatment of the cells with TGF-beta1 resulted in the phosphorylation of Smad 2. Treatment with exogenous TGF-beta1 resulted in a failure to induce transcription from an artificial Smad-dependent promoter and a failure to down-regulate c-myc, but resulted in an up-regulation of AP-1 associated genes (Fra-1, JunB and fibronectin). Transient transfection of Smad 4 into BICR 31 restored TGF-beta1-induced growth inhibition and Smad-dependent transcriptional activation. Protracted treatment of cells with exogenous TGF-beta1 resulted in the attenuation of cell growth in vitro. To over-express TGF-beta1, both cell lines were transfected with latent TGF-beta1 cDNA; neutralization studies of conditioned media demonstrated that whilst the majority of the peptide was in the latent form, a small proportion was present as the active peptide. Cells that over-expressed endogenous TGF-beta1 grew more slowly in vitro compared to both the vector-only controls and cells that did not over-express the peptide. Orthotopic transplantation of cells that over-expressed endogenous TGF-beta1 to the floor of the mouth in athymic mice resulted in marked inhibition of primary tumor formation compared to controls. Expression of a dominant-negative TGF-beta type II receptor in cells that over-expressed endogenous TGF-beta1 resulted in enhanced cell growth in vitro and diminished the tumor suppressor effect of the ligand in vivo, indicating that the endogenous TGF-beta1 was acting in an autocrine capacity. The results demonstrate that over-expression of endogenous TGF-beta1 in human malignant oral keratinocytes leads to growth inhibition in vivo and tumor suppression in vitro by mechanisms that are independent of Smad 4 expression and TGF-beta1-induced G(1) arrest.

TGF-beta signaling in tumor suppression and cancer progression

Epithelial and hematopoietic cells have a high turnover and their progenitor cells divide continuously, making them prime targets for genetic and epigenetic changes that lead to cell transformation and tumorigenesis. The consequent changes in cell behavior and responsiveness result not only from genetic alterations such as activation of oncogenes or inactivation of tumor suppressor genes, but also from altered production of, or responsiveness to, stimulatory or inhibitory growth and differentiation factors. Among these, transforming growth factor beta (TGF-beta) and its signaling effectors act as key determinants of carcinoma cell behavior. The autocrine and paracrine effects of TGF-beta on tumor cells and the tumor micro-environment exert both positive and negative influences on cancer development. Accordingly, the TGF-beta signaling pathway has been considered as both a tumor suppressor pathway and a promoter of tumor progression and invasion. Here we evaluate the role of TGF-beta in tumor development and attempt to reconcile the positive and negative effects of TGF-beta in carcinogenesis.

TGFbeta-mediated signaling and transcriptional regulation in pancreatic development and cancer

Transforming growth factor-beta (TGFbeta) plays a critical role in pancreatic development and cell proliferation. Binding of TGFbeta to its membrane receptor kinases activates the Smad signaling proteins, allowing them to translocate to the nucleus and participate in the transcriptional control of TGFbeta target genes. In addition, there is an increasing number of cellular mechanisms affecting the final response of a cell to TGFbeta. This includes crosstalk with other signaling pathways and the induction of TGFbeta early response genes, such as the TGFbeta-inducible early response gene (TIEG) family of transcription factors. Like the Smads, TIEGs behave as downstream effector proteins in TGFbeta-mediated pancreatic growth control. The discovery of the Smads and TIEGs has provided new insights into TGFbeta-regulated functions. Their significance in pancreatic development and cancer is discussed in this review.

The molecular genetics of pancreatic ductal carcinoma: a review

In the last several years, numerous advances in the field of molecular genetics have been applied to pancreatic ductal carcinoma- the 5th leading cause of cancer death in the United States. This review summaries the current knowledge about adenocarcinoma of the pancreas.

The genetics of pancreatic adenocarcinoma: a roadmap for a mouse model

Pancreatic cancer is among the leading causes of cancer death. Although a genetic profile for pancreatic cancer is emerging, many biological aspects of this disease are poorly understood. Indeed, fundamental questions regarding progenitor cell lineages, host stromal milieu, and the role of specific genetic alterations in tumor progression remain unresolved. A mouse model engineered with signature mutations would provide a powerful ally in the study of pancreatic cancer biology and may guide improved prognostic assessment and treatment for the human disease. In this review, we discuss the molecular basis for normal pancreatic development and the genetics of human pancreatic adenocarcinoma in the hope of charting a course for the development of a faithful mouse model for this lethal cancer.

Adenovirus-mediated gene transfer of dominant-negative Smad4 blocks TGF-beta signaling in pancreatic acinar cells

Transforming growth factor-beta (TGF-beta) is a potent inhibitor of pancreatic acinar cell growth. Smad4 is a central mediator in the TGF-beta signaling pathway. To study the effect of Smad4 on pancreatic growth, cell cycle protein expression, and the expression of a TGF-beta-responsive promoter in vitro, we constructed an adenovirus containing dominant-negative COOH terminal truncated Smad4 (AddnSmad4) downstream of the rat elastase promoter. Acinar cells expressed dominant-negative Smad4 within 8 h after infection, and expression persisted for 72 h. Mouse pancreatic acini were infected with either AddnSmad4 or control adenovirus expressing green fluorescent protein, and TGF-beta was added 8 h after infection. Acinar cells were then incubated for 1, 2, or 3 days, and [(3)H]thymidine incorporation was determined. AddnSmad4 significantly reduced TGF-beta inhibition of [(3)H]thymidine incorporation, with maximal effects on day 3. AddnSmad4 also completely blocked TGF-beta-mediated growth inhibition in the presence of basic fibroblast growth factor. We next examined the effects of AddnSmad4 on TGF-beta-induced expression of the cell cycle regulatory proteins p21(Cip1) and p27(Kip1). TGF-beta induced upregulation of p21(Cip1), which was completely blocked by AddnSmad4. AddnSmad4 also inhibited TGF-beta-induced expression of the TGF-beta-responsive luciferase reporter 3TP-Lux. These results show that Smad4 is essential in TGF-beta-mediated signaling in pancreatic acinar cells.