TGF-beta signaling in tumor suppression and cancer progression

Repression of TGF-beta signaling by the oncogenic protein SKI in human melanomas: consequences for proliferation, survival, and metastasis

Transforming growth factor-beta (TGF-beta ) has dual and paradoxical functions as a tumor suppressor and promoter of tumor progression and metastasis. TGF-Ji-mediated growth inhibition is gradually lost during melanoma tumor progression, but there are no measurable defects at the receptor level. Furthermore, melanoma cells release high levels of TGF-beta to the microenvironment, which upon activation induces matrix deposition, angiogenesis, survival, and transition to more aggressive phenotypes. The SKI and SnoN protein family associate with and repress the activity of Smad2, Smad3, and Smad4, three members of the TGF-fl signaling pathway. SKI also facilitates cell-cycle progression by targeting the RB pathway by at least two ways: it directly associates with RB and represses its activity when expressed at high levels, and indirectly, it represses Smad-mediated induction of p21(Waf-1) This results in increased CDK2 activity, RB phosphorylation,and inactivation. Therefore, high levels of SKI result in lesions to the RB pathway in a manner similar to p16 (INK4a) loss. SKI mRNA and protein levels dramatically increase during human melanoma tumor progression. In addition,the SKI protein shifts from nuclear localization in intraepidermal melanoma cells to nuclear and cytoplasmic in invasive and metastatic melanomas. Here, I discuss the basis for repression of intracellular TGF-beta signaling by SKI, some additional activities of this protein, and propose that by disrupting multiple tumor suppressor pathways, SKI functions as a melanoma oncogene.

Spectrum of molecular changes during hepatocarcinogenesis induced by DEN and other chemicals in Fisher 344 male rats [Mechanisms of Ageing and Development 123 (2002) 1665-1680]

Unlike other tissues such as breast, colon and renal cell carcinoma, it is not an easy task to single out any representative oncogene or tumor suppressor genes in the development of hepatocellular carcinoma (HCC), which play a pivotal role. To investigate putatively altered main pathways in HCC, F344 male rats were treated with a single injection of N-nitrosodiethylamine (DEN), followed by either twice/week injections of nodularin for 10 weeks or thioacetamide (TAA) in drinking water for 39 weeks. p53 expression was dramatic in both hepatocytes and mesenchymal cells after a single injection of DEN, however, PCR-SSCP assay could not detect any p53 mutation during the development of hepatocellular adenoma (HCA). The data indicate that wtp53 response was mostly for removal of damaged cells during the initiation of carcinogenesis. When treated with DEN-TAA, induction of gankyrin expression during hepatic fibrosis preceded the loss of pRB protein, accompanied with significant expressions of G1phase cyclins and CDKs. Moreover, p16(INK4A) exon 1 was hypermethylated during the development of poorly differentiated HCCs. These changes would result in complete inactivation of the pRB regulatory pathway during hepatocarcinogenesis. Induction of TGF-beta1 expression with loss of its receptor expression occurred rapidly in the altered hepatocytes by DEN-nodularin treatment.

CONCLUSION

Therefore, escape from TGF-beta1 induced apoptosis and severe degradation of pRB protein during the early stage of carcinogenesis can perform a symphony to proliferate and to transform the altered hepatocytes to tumor cells. Inactivation of p16(INK4A) and p53 genes at the later stage of carcinogenesis would endow HCC with malignancy, which is highly resistant to any therapeutic trials.

TGF-beta1 suppresses apoptosis via differential regulation of MAP kinases and ceramide production

Serum deprivation induces apoptosis in NIH3T3 cells, which is associated with increased intracellular ceramide generation and with the activation of p38 mitogen-activated protein (MAP) kinase. Treatment of cells with transforming growth factor-beta1 (TGF-beta1) activated the extracellular signal regulated kinases 1 and 2 (ERK1/ERK2), inhibited the serum deprivation-induced p38 activation and the increase in intracellular ceramide formation, leading to the stimulation of cell proliferation and the suppression of apoptosis. Inhibition of p38 MAP kinase by SB203580 significantly reduced the serum-deprivation-induced apoptosis. Overexpression of p38 increased the cell apoptosis and reduced the antiapoptotic effect of TGF-beta1. Inhibition of ERK1/ERK2 by PD98059 completely inhibited the TGF-beta1-stimulated proliferation and partially inhibited the antiapoptotic effects of TGF-beta1. Neither SB203580 nor PD98059 has obvious effect on TGF-beta1-mediated inhibition of the increased ceramide generation. Serum-deprivation-induced apoptosis in NIH3T3 cells can also be blocked by broad-spectrum caspase inhibitor. TGF-beta1 treatment has an inhibitory effect on caspase activities. Our results indicate that ceramide, p38, and ERK1/ERK2 play critical but differential roles in cell proliferation and stress-induced apoptosis. TGF-beta1 suppresses the serum-deprivation-induced apoptosis via its distinct effects on complex signaling events involving the activation of ERK1/ERK2 and the inhibition of p38 activation and increased ceramide generation.

The loss of TGF-beta signaling promotes prostate cancer metastasis

In breast and colon cancers, transforming growth factor (TGF)-beta signaling initially has an antineoplastic effect, inhibiting tumor growth, but eventually exerts a proneoplastic effect, increasing motility and cancer spread. In prostate cancer, studies using human samples have correlated the loss of the TGF-beta type II receptor (T beta R II) with higher tumor grade. To determine the effect of an inhibited TGF-beta pathway on prostate cancer, we bred transgenic mice expressing the tumorigenic SV40 large T antigen in the prostate with transgenic mice expressing a dominant negative T beta R II mutant (DN II R) in the prostate. Transgene(s) and TGF-beta 1 expression were identified in the prostate and decreased protein levels of plasminogen activator inhibitor type I, as a marker for TGF-beta signaling, correlated with expression of the DN II R. Although the sizes of the neoplastic prostates were not enlarged, increased amounts of metastasis were observed in mice expressing both transgenes compared to age-matched control mice expressing only the large T antigen transgene. Our study demonstrates for the first time that a disruption of TGF-beta signaling in prostate cancer plays a causal role in promoting tumor metastasis.

Biochemical staging of prostate cancer

PSA continues to be one of the most effective and widely used cancer screening tools available. Its popularity in prostate cancer screening, however, has eroded its usefulness in the staging of this disease. As more men are screened every year on a routine basis with DRE and PSA, the average PSA at diagnosis has drifted down to well below 10 ng/mL in many centers, including ours. This trend is likely to accelerate, as a PSA cut off for prompting biopsy of the prostate of 2.5 ng/mL gains more widespread acceptance. The recent realization that, at these levels, serum PSA is more reflective of the presence of BPH than of the extent of cancer and, therefore, does not provide additional staging information, has renewed the search for new biochemical markers that are capable of predicting prostate cancer stage and prognosis. Because of the heterogeneity of this disease, it is unlikely that a single biochemical marker that is capable of accurately staging all prostate cancer patients will be found. For this reason, nomograms that are capable of integrating various parameters to predict stage and prognosis will remain indispensable. As new biochemical markers that provide independent predictive information about stage or prognosis are identified, they can be incorporated into currently available nomograms. Of the biochemical markers discussed in this article, IL-6sR and TGF-beta1 are the most promising. By incorporating them into a preoperative nomogram designed to predict PSA recurrence, we found that they improved the ability to predict biochemical recurrence by a statistically and clinically significant margin. The ability to stage prostate cancer and predict response to therapy has improved dramatically over the last 3 decades. Nevertheless, there is still a need for new biochemical markers that will improve the ability to predict an individual patient's stage and response to therapy. Incorporating these new markers into nomograms will enhance the ability to provide optimal care for each prostate cancer patient.

Cloning, expression, and characterization of chicken transforming growth factor beta 4

Transforming growth factor beta 4 (TGF-beta 4) is unique to avian species, though its roles in vivo have not yet been well established. In this paper we describe the expression and partial characterization of recombinant chicken TGF-beta 4. By using a GC-rich PCR system in a modified 5'RACE methodology we generated the 5'-end of cDNA sequence encoding the TGF-beta 4 precursor, which was in-frame cloned into pcDNA3.1/V5-His-TOPO and transfected into the Chinese hamster ovary cell line (CHO-K1). A cell line stably expressing TGF-beta 4 precursor protein was established from CHO-K1 cells. Acid-activated mature TGF-beta 4 inhibited the growth of mink lung epithelial (Mv1Lu) cell line. TGF-beta 4 also stimulated the expression of type I procollagen and enhanced heat shock protein 47 (Hsp47) expression in chicken tendon fibroblasts. Hsp47 expression by TGF beta 4 is likely regulated through activation of heat shock transcription factor 1 (HSF1). Because the presence of TGF-beta 1 has not been documented in avian cells and our data show that TGF-beta 4 elicits biological activities in chicken tendon cells, which closely parallel that of TGF-beta 1, we propose that TGF-beta 4 plays roles in avian species similar to what TGF-beta 1 plays in mammalian species.

Regulation of biglycan gene expression by transforming growth factor-beta requires MKK6-p38 mitogen-activated protein Kinase signaling downstream of Smad signaling

Several signaling pathways have been implicated in mediating TGF-beta1-induced extracellular matrix production and fibrosis. We have shown recently that induction of biglycan (BGN) expression by TGF-beta1 depended on a functional Smad pathway (Chen, W.-B., Lenschow, W., Tiede, K., Fischer, J. W., Kalthoff, H., and Ungefroren, H. (2002) J. Biol. Chem. 277, 36118-36128). Here, we present evidence that the ability of TGF-beta 1 to induce BGN mRNA, in addition to Smads, requires p38 MAPK signaling, because 1) pharmacological inhibitors of p38 dose-dependently inhibited the TGF-beta effect without significantly affecting the transcriptional activity of a constitutively active mutant of the TGF-beta type I receptor or Smad2 phosphorylation at concentrations up to 10 microm, 2) the up-regulation of BGN mRNA was preceded by a delayed increase in the phosphorylation of p38 and its upstream activator MKK6 in TGF-beta 1-treated PANC-1 cells, 3) inhibition of the p38 pathway by stable retroviral transduction with a dominant negative mutant of either p38 or MKK6 reduced TGF-beta 1-induced BGN mRNA expression, and 4) overexpression of wild-type p38 or MKK6, but not MKK3, augmented the TGF-beta 1 effect on BGN mRNA. We further demonstrate that the (delayed) p38 activation by TGF-beta 1 is downstream of Smads and requires a functional Smad pathway, because blocking TGF-beta-induced p38 activity with SB202190 had no effect on Smad2 phosphorylation, but blocking Smad signaling by forced expression of Smad7 abolished TGF-beta1 induction of p38 activation and, as shown earlier, BGN mRNA expression; finally, re-expression of Smad4 in Smad4-null CFPAC-1 cells restored TGF-beta-induced p38 phosphorylation and, as demonstrated previously, BGN mRNA accumulation. These results clearly show that TGF-beta induction of BGN expression in pancreatic cells requires activation of MKK6-p38 MAPK signaling downstream of Smad signaling and provide a mechanistic clue to the up-regulation of BGN seen in inflammatory response-related fibrosis and desmoplasia.

Both Max and TFE3 cooperate with Smad proteins to bind the plasminogen activator inhibitor-1 promoter, but they have opposite effects on transcriptional activity

Transforming growth factor (TGF)-beta regulates gene expression in large part through combinatorial interactions between members of the Smad family and other transcription factors. The basic helix-loop-helix leucine zipper (bHLHZIP) protein TFE3 and Smad3 synergistically activate transcription of the plasminogen activator inhibitor-1 (PAI-1) as well as other genes. We investigated interactions among different bHLHZIP and Smad family proteins. TFE3, TFEB, and Max associated with Smad3 and Smad4 in the absence of DNA and at the PE2.1 element of the PAI-1 promoter. These interactions were mediated by the leucine zipper and MH1 regions of the respective proteins. No interactions were observed with the E47 bHLH family protein. Chimeric proteins, in which leucine zippers from bHLHZIP or bZIP proteins were fused to heterologous bHLH domains, associated with Smad proteins both in the absence of DNA and at the PE2.1 element. The kinetics of bHLHZIP and Smad protein binding at the PE2.1 element were examined using surface plasmon resonance analysis. TFE3 exhibited cooperative DNA binding with Smad proteins, whereas no cooperativity was observed between E47 and Smads. Max inhibited transcription activation by Smad3 and TGF-beta at the PAI-1 promoter, whereas TFE3 and TFEB stimulated transcription activation. These results suggest that Smad family proteins can interact with several bHLHZIP proteins, resulting in different transcriptional outcomes.

Interfering with TGFbeta-induced Smad3 nuclear accumulation differentially affects TGFbeta-dependent gene expression

BACKGROUND

Transforming growth factor-beta (TGFbeta) plays an important role in late-stage carcinogenesis by stimulating invasive behavior of cancer cells, promoting neo-angiogenesis and by helping cancer cells to escape surveillance by the immune system. It also supports colonization of the bone by metastatic breast cancer cells by increasing expression of osteolytic parathyroid hormone-related protein (PTHrP). Interfering with TGFbeta signalling may thus weaken the malignant properties of cancer cells. We investigated to what extent two inhibitors, SB-202190 and SB-203580, interfere with TGFbeta-signalling in invasive MDA-MB-231 breast cancer cells. These compounds, formerly used as p38-MAPK-specific inhibitors, were recently also demonstrated to inhibit TGFbeta type I receptor kinase.

RESULTS

Our results show that these inhibitors delay the onset of TGFbeta-induced nuclear accumulation of Smad3 and reduces its amplitude. This effect was accompanied by a strong reduction in TGFbeta-responsivess of the slow-responder genes pthrp, pai-1 and upa, while the reactivity of the fast-responder gene smad7 to TGFbeta remained almost unchanged. Neither was the TGFbeta response of the fast-responder ese-1/esx gene, whose expression we found to be strongly downregulated by TGFbeta, affected by the inhibitors.

CONCLUSION

The data show that SB-202190 and SB-203580 suppress TGFbeta-dependent activation of genes that are important for the acquisition of invasive behavior, while having no effect on the expression of the natural TGFbeta inhibitor Smad7. This suggests that these compounds are potent inhibitors of malignant behavior of cancer cells.

In situ detection of TGF betas, TGF beta receptor II mRNA and telomerase activity in rat cholangiocarcinogenesis

AIM: Initial report on the in situ examination of the mRNA expression of transforming growth factor betas (TGFβs), TGFβ type II receptor (TβRII) and telomerase activity in the experimental rat liver tissue during cholangiocarcinogenesis.

METHODS: Rat liver cholangiocarcinogenesis was induced by 3’-methyl 4-dimethylazobenzene (3’Me-DAB). In situ hybridization was used to examine the TGFβs) and TGFβ type II receptor (TβRII) mRNA, in situ TRAP was used to check the telomerase activity in the tissue samples.

RESULTS: There was no TGFβs, TβRII mRNA expression or telomerase activity in the control rat cholangiocytes. The expression of TGFβ1, TβRII was increased in regenerative, hyperplastic, dysplastic cholangiocytes and cholangiocarcinoma (CC) cells. The expression of TGFβ2 mRNA was observed in only a part of hyperplastic, dysplastic cholangiocytes. TGFβ3 expression was very weak, only in hyperplastic lesion. There was positive telomerase activity in the regenerative, hyperplastic, dysplastic cholangiocytes, and CC cells. Stroma fibroblasts of these lesions also showed positive TGFβs, TβRII mRNA expression and telomerase activity.

CONCLUSION: There were TGFβs, TβRII expression and telomerase activity in hyperplastic, dysplastic cholangiocytes, cholangiocarcinoma cells as well as in stroma fibroblasts during cholangiocarcinogenesis. Their expression or activity is important in cholangiocarcinogenesis andstroma formation.

Regulation of TGF-beta signaling and its roles in progression of tumors

Transforming growth factor-beta (TGF-beta) is a potent growth inhibitor of most types of cells; therefore, perturbations of TGF-beta signaling are believed to result in progression of various tumors. On the other hand, TGF-beta has been shown to act as an oncogenic cytokine through induction of extracellular matrices, angiogenesis, and immune suppression. A wide variety of effects of TGF-beta are mediated by physical interaction of signal transducer Smad proteins with various transcription factors. Among these, Runx3 plays a pivotal role in prevention of gastric cancer. TGF-beta signaling is regulated by various mechanisms in the cytoplasm and nucleus. Inhibitory Smads (I-Smads) repress TGF-beta signaling mainly by interacting with activated TGF-beta receptors. Smad ubiquitin regulatory factors (Smurfs) play important roles in facilitating the inhibitory signals induced by I-Smads. In addition, the transcriptional co-repressors c-Ski and SnoN interact with Smads, and repress transcription induced by TGF-beta. Abnormalities of these regulators of TGF-beta signaling may thus participate in the progression of various tumors.

Transcriptional repression by Myc

The Myc oncoprotein is a transcription factor that can both activate and repress genes. Transcriptional activation by Myc is well understood, but, by contrast, the mechanisms through which Myc represses transcription have remained elusive. Recent evidence suggests that complex formation by Myc with a zinc-finger transcription factor, Miz-1, plays an important role in mediating repression by Myc. The findings might explain how Myc interferes with cell-cycle arrest in response to TGF-beta, APC and DNA damage.

Characterization of the transforming growth factor-beta 1-induced apoptotic transcriptome in FaO hepatoma cells

We have previously shown that transforming growth factor-beta(1) (TGF-beta(1))-induced apoptosis in FaO hepatoma cells is mediated by cytochrome c release, apoptosome formation, and caspase activation. Although TGF-beta(1) acts via the SMAD signaling pathway to initiate de novo gene transcription, little is known about the downstream gene targets that are involved in the regulation of apoptosis. Therefore, in this study, we used in-house microarrays (approximately 5500 genes) to identify pathway-specific gene clustering in TGF-beta(1)-treated cells. A total of 142 genes showed time-dependent changes in expression during TGF-beta(1)-induced apoptosis. The polycaspase inhibitor benzyloxycarbonyl-VAD-fluoromethyl ketone, which, on its own, had no effect on gene transcription, blocked TGF-beta(1)-induced cell death and significantly altered the expression of 261 genes, including 185 down-regulated genes. Cluster analysis identified up-regulation of early response genes (0-4 h) encoding for the extracellular matrix and cytoskeleton, including the pro-apoptotic CTGF gene, and delayed response genes (8-16 h), including pro-apoptotic genes. A second delayed response cluster (44 genes) was also observed when TGF-beta(1)-induced caspase activation was blocked by benzyloxycarbonyl-VAD-fluoromethyl ketone. This cluster included genes encoding stress-related proteins (e.g. Jun, ATF3, TAB1, and TANK), suggesting that their up-regulation may be in response to secondary necrosis. Finally, we identified an early response set of nine down-regulated genes that are involved in antioxidant defense. We propose that the regulation of these genes by TGF-beta(1) could provide a molecular mechanism for the observed elevation in reactive oxygen species after TGF-beta(1) treatment and may represent the primary mechanism through which TGF-beta(1) initiates apoptosis.

ERK signaling pathway is involved in p15INK4b/p16INK4a expression and HepG2 growth inhibition triggered by TPA and Saikosaponin a

The signal pathway mediating induction of p15(INK4b) and p16(INK4a) during HepG2 growth inhibition triggered by the phorbol ester tumor promoter TPA (12-O-tetradecanoylphorbol 13-acetate) and the Chinese herb Saikosaponin a was investigated. Western blot of three activated forms of mitogen-activated protein kinase (MAPK) (p-ERK, p-JNK and p-p38) demonstrated that phosphorylation of ERK is dramatically induced (11.6-fold ) by TPA during 15 min to 1 h and significantly induced (2.5-fold) by Saikosaponin alpha at 30 min, whereas phosphorylation of JNK was induced only by TPA during 30 min to 1 h. Phosphorylation of p38 was not induced by either drug. During this period, phosphorylation of one of the downstream transcriptional factors of MAPK cascade, ATF2, was 3.2- and 2.0-fold induced by TPA and Saikosaponin a, respectively, whereas that of another transcriptional factor, c-jun, was induced by TPA only. On the other hand, expressions of proto-oncogene c-jun, junB and c-fos were induced by TPA and Saikosaponin a during 30 min to 6 h of treatment. Pretreatment of 20 microg/ml PD98059, an inhibitor of MEK which is the upstream kinase of ERK, prevents the TPA- and Saikosaponin a-triggered HepG2 growth inhibition by 50 and 30%, respectively, accompanied by a 50 - 85% decrease of the p15(INK4b)/p16(INK4a) RNAs and proteins induced by both drugs. Inductions of c-fos RNA by both drugs and c-jun phosphorylation by TPA were also significantly reduced by PD98059 pretreatment. In addition, AP-1 DNA-binding assay using nonisotopic capillary electrophoresis and laser-induced fluorescence (CE/LIF) demonstrated that the AP-1-related DNA-binding activity was significantly induced by TPA and Saikosaponin a, which can be reduced by PD98059 pretreatment. These results suggested that activation of ERK together with its downstream transcriptional machinery mediated p15(INK4b) and p16(INK4a) expression that led to HepG2 growth inhibition.

New perspectives on growth factor-sex steroid interaction in the prostate

Many organs respond to both sex steroids and growth factors. Regulation of these pathways is integral to cell-cell communications during development and aberrant changes cause disease pathogenesis. Traditionally, paracrine and endocrine actions of growth factors and steroid hormones are considered independently. Recently, new data indicated that activin/TGFbeta and sex steroid signalling are linked; explicitly, that the pathways cross-talk intracellularly. Here we present new perspectives on these interactions, using examples predominantly from the prostate, as it is a well-characterised organ in this context. While this information provides insight to the potential mechanisms behind these interactions, it also presents a new challenge; the action of any of these factors cannot be considered exclusively without considering the impact on the other biological pathways.

RUNX transcription factors as key targets of TGF-beta superfamily signaling

The Runt domain transcription factors, RUNX1, RUNX2 and RUNX3, are integral components of signaling cascades mediated by both TGF-beta and bone morphogenetic proteins (BMPs) in several important biological systems. RUNX2 functions synergistically with Smad1 and Smad5 to regulate bone-specific genes when BMP induces osteogenesis. RUNX3, which has been mapped to locus 1p36, is a major tumor suppressor of gastric cancer and appears to be an important component of the TGF-beta-induced tumor suppressor pathway. A possible relationship between the TGF-beta-induced tumor-suppressor pathway and a postulated tumor suppressor gene on 1p36 must be examined.

The not-so innocent bystander: the microenvironment as a therapeutic target in cancer

The microenvironment in which cancer arises is often regarded as a bystander to the clonal expansion and acquisition of malignant characteristics of the tumour. However, a major function of the microenvironment is to suppress cancer, and its disruption is required for the establishment of cancer. In addition, tumour cells can further distort the microenvironment to promote growth, recruit non-malignant cells that provide physiological resources, and facilitate invasion. In this review, the authors discuss the contribution of the microenvironment, i.e., the stroma and its resident vasculature, inflammatory cells, growth factors and the extracellular matrix (ECM), in the development of cancer, and focus on two components as potential therapeutic targets in breast cancer. First, the ECM, which imparts crucial signalling via integrins and other receptors, is a first-line barrier to invasion, modulates aggressive behaviour and may be manipulated to provide novel impediments to tumour growth. Second, the authors discuss the involvement of TGF-beta1 as an example of one of many growth factors that can regulate ECM composition and degradation and that play complex roles in cancer. Compared to the variable routes taken by cells to become cancers, the response of tissues to cancer is relatively consistent. Therefore, controlling and eliminating cancer may be more readily achieved indirectly via the tissue microenvironment.

Epithelial to mesenchymal transition in Madin-Darby canine kidney cells is accompanied by down-regulation of Smad3 expression, leading to resistance to transforming growth factor-beta-induced growth arrest

In normal epithelial cells, transforming growth factor-beta (TGF-beta) typically causes growth arrest in the G(1) phase of the cell cycle and may eventually lead to apoptosis. However, transformed cells lose these inhibitory responses and often instead show an increase in malignant character following TGF-beta treatment. In the canine kidney-derived epithelial cell line, MDCK, synergism between activation of the Raf/MAPK pathway and the resulting autocrine production of TGF-beta triggers transition from an epithelial to a mesenchymal phenotype. During this process, these cells become refractive to TGF-beta-induced cell cycle arrest and apoptosis. TGF-beta signals are primarily transduced to the nucleus through complexes of receptor-regulated Smads, Smad2 and Smad3 with the common mediator Smad, Smad4. Here we show that the transition from an epithelial to mesenchymal phenotype is accompanied by gradual down-regulation of expression of Smad3. Restoration of Smad3 to previous levels of expression restores the cell cycle arrest induced by TGF-beta without reverting the cells to an epithelial phenotype or impacting on the MAPK pathway. Regulation of apoptosis is not affected by Smad3 levels. These data attribute to Smad3 a critical role in the control of cell proliferation by TGF-beta, which is lost following an epithelial to mesenchymal transition.

Disruption of transforming growth factor-beta signaling in ELF beta-spectrin-deficient mice

Disruption of the adaptor protein ELF, a beta-spectrin, leads to disruption of transforming growth factor-beta (TGF-beta) signaling by Smad proteins in mice. Elf-/- mice exhibit a phenotype similar to smad2+/-/smad3+/- mutant mice of midgestational death due to gastrointestinal, liver, neural, and heart defects. We show that TGF-beta triggers phosphorylation and association of ELF with Smad3 and Smad4, followed by nuclear translocation. ELF deficiency results in mislocalization of Smad3 and Smad4 and loss of the TGF-beta-dependent transcriptional response, which could be rescued by overexpression of the COOH-terminal region of ELF. This study reveals an unexpected molecular link between a major dynamic scaffolding protein and a key signaling pathway.

Growth regulatory factors and signalling proteins in testicular germ cell tumours

The molecular basis of testicular germ cell tumourigenesis are not well elucidated. Growth factors regulate cell growth, differentiation and apoptosis. Major families of growth factors are present in the male gonad from early fetal development to adult life. They are involved in germ cell proliferation and differentiation. Growth signalling pathways suffer deregulation in many human malignancies. Given the importance of growth signals in normal testicular development and their acquired deregulation in most human cancers, growth factors and signalling molecules that have been implicated in the genesis of testicular germ cell tumours, are reviewed. We detected a somatic mutation of SMAD4 gene, responsible for loss of protein function in seminomas. This mutational inactivation may affect the activity of several members of TGFbeta superfamily (TGFbeta, activin, inhibin, BMP). VEGF expression has been shown to predict metastasis in seminomas. A significant association of HST-1 expression, a member of fibroblast growth factors, with the nonseminomatous phenotype and with tumour stage has been described. In contrast, C-KIT is expressed by seminomas only, from the preinvasive stage. Despite intense expression in almost all seminomas, activating mutation of C-KIT gene is seldom reported. Recently, the first animal model of classical testicular seminoma has been identified in transgenic mouse overexpressing GDNF. RET (GDNF receptor) expression is demonstrated in human seminomas, and not in nonseminomatous tumours. However, the exact molecular alterations of GDNF/RET/GFRalpha1 complex in germ cell tumours are not known. Finally, beside growth factors, other signalling molecules such as peptide hormones may be involved in testicular carcinogenesis. We have demonstrated a specific pattern of somatostatin receptors expression in each type of testicular germ cell tumours, with a loss of sst3 and sst4 in seminomas and loss of sst4 and expression of sst1 in nonseminomas only. These data suggest an antiproliferative action of somatostatin in testicular cancers. In summary, many growth factors and signalling molecules seem to represent specific markers for different histological types of germ cell tumours (seminomas versus nonseminomas) and may play a role in the differentiation of germ cell tumours. Despite a complex signalling pathway involved in the physiological functions of male gonad, little is known about the implication of this signalling network in testicular malignancies. From a practical stand-point, further studies on the role of growth factors in human germ cell tumours may offer a new therapeutical perspective with the development of specific pharmacological signalling modulators that could be used as therapeutic agents.

Rapid up-regulation of alpha4 integrin-mediated leukocyte adhesion by transforming growth factor-beta1

The alpha4 integrins (alpha4beta1 and alpha4beta7) are cell surface heterodimers expressed mostly on leukocytes that mediate cell-cell and cell-extracellular matrix adhesion. A characteristic feature of alpha4 integrins is that their adhesive activity can be subjected to rapid modulation during the process of cell migration. Herein, we show that transforming growth factor-beta1 (TGF-beta1) rapidly (0.5-5 min) and transiently up-regulated alpha4 integrin-dependent adhesion of different human leukocyte cell lines and human peripheral blood lymphocytes (PBLs) to their ligands vascular cell adhesion molecule-1 (VCAM-1) and connecting segment-1/fibronectin. In addition, TGF-beta1 enhanced the alpha4 integrin-mediated adhesion of PBLs to tumor necrosis factor-alpha-treated human umbilical vein endothelial cells, indicating the stimulation of alpha4beta1/VCAM-1 interaction. Although TGF-beta1 rapidly activated the small GTPase RhoA and the p38 mitogen-activated protein kinase, enhanced adhesion did not require activation of both signaling molecules. Instead, polymerization of actin cytoskeleton triggered by TGF-beta1 was necessary for alpha4 integrin-dependent up-regulated adhesion, and elevation of intracellular cAMP opposed this up-regulation. Moreover, TGF-beta1 further increased cell adhesion mediated by alpha4 integrins in response to the chemokine stromal cell-derived factor-1alpha. These data suggest that TGF-beta1 can potentially contribute to cell migration by dynamically regulating cell adhesion mediated by alpha4 integrins.

Advances in breast cancer treatment and prevention: preclinical studies on aromatase inhibitors and new selective estrogen receptor modulators (SERMs)

Intensive basic and clinical research over the past 20 years has yielded crucial molecular understanding into how estrogen and the estrogen receptor act to regulate breast cancer and has led to the development of more effective, less toxic, and safer hormonal therapy agents for breast cancer management and prevention. Selective potent aromatase inhibitors are now challenging the hitherto gold standard of hormonal therapy, the selective estrogen-receptor modulator tamoxifen. Furthermore, new selective estrogen-receptor modulators such as arzoxifene, currently under clinical development, offer the possibility of selecting one with a more ideal pharmacological profile for treatment and prevention of breast cancer. Two recent studies in preclinical model systems that evaluate mechanisms of action of these new drugs and suggestions about their optimal clinical use are discussed.

Interference with TGF-beta1 and -beta3 in tumor stroma lowers tumor interstitial fluid pressure independently of growth in experimental carcinoma

A high tumor interstitial fluid pressure (TIFP) is a pathologic characteristic distinguishing the stroma of carcinomas from normal interstitial loose connective tissues. The role of TGF-beta1 and -beta3 in generating a high TIFP was investigated in xenografted experimental anaplastic thyroid carcinoma (ATC) derived from the human ATC cell line KAT-4. A single intravenous injection of a soluble recombinant TGF-beta receptor type II-murine Fc:IgG(2A) chimeric protein that specifically inhibits TGF-beta1 and -beta3, significantly lowered TIFP in a time and concentration dependent manner but did not change total tissue water content in the tumors. Tumor growth rate was higher in tumors treated with the TGF-beta1 and -beta3 inhibitor compared to control tumors during the first 10 days after administration of the inhibitor. The apoptotic index of carcinoma cells, and expression of the cell cycle inhibitor p27(Kip1), were, however, increased in TGF-beta1 and -beta3 inhibitor-treated tumors. Prolonged treatment periods and administration of a second dose of the inhibitor decreased tumor growth rate. The TGF-beta1 and -beta3 inhibitor did not affect proliferation or expression of phosphorylated Smad2 protein in KAT-4 cells cultured in vitro. Our results indicate that members of the TGF-beta family are potential targets for novel anti-cancer treatment directed to the stroma. First by controlling TIFP and by that potentially the uptake of anticancer drugs into tumors and second by their suggested role in maintaining a supportive tumor stroma.

Transcriptional activation of collagenase-3 by transforming growth factor-beta1 is via MAPK and Smad pathways in human breast cancer cells

Transforming growth factor (TGF)-beta1, a crucial molecule in metastatic bone cancer, stimulates collagenase-3 expression in the human breast cancer cell line, MDA-MB231. Cycloheximide inhibited this stimulation, indicating that de novo protein synthesis was essential for this response. We examined whether mitogen-activated protein kinase (MAPK) and/or Smad pathways are involved in TGF-beta1-stimulated collagenase-3 expression in MDA-MB231 cells. Biochemical blockade of extracellular regulated kinase-1/2 and p38 MAPK pathways partially abolished TGF-beta1-stimulated collagenase-3 mRNA expression; whereas overexpression of a dominant negative form of Smad3 completely blocked the TGF-beta1-response. These data indicate that TGF-beta1-induced MAPK and Smad pathways are involved in TGF-beta1-stimulated collagenase-3 expression in MDA-MB231 cells.

Smad-dependent GADD45beta expression mediates delayed activation of p38 MAP kinase by TGF-beta

Transforming growth factor-beta (TGF-beta), when bound to its specific receptor, activates the transcription factor Smad by phosphorylation. TGF-beta also activates the p38 MAPK pathway, but there seem to be disparate mechanisms for the early p38 activation and delayed p38 activation. In this report, we demonstrate that Smad-dependent expression of GADD45beta is responsible for the delayed activation of p38 by TGF-beta. The GADD45beta protein binds and activates MTK1 (= MEKK4), which is a member of the MAPKKK family kinases and an upstream activator of the p38 MAPK cascade. Both TGF-beta-induced GADD45beta expression and the delayed p38 activation require functional Smad proteins. Antisense inhibition of GADD45beta expression suppresses the TGF-beta-induced delayed p38 activation, whereas overexpression of GADD45beta activates the p38 MAPK via MTK1. Expression of the angiogenesis inhibitor thrombospondin-1 (TSP-1) is induced by TGF-beta via Smad-dependent p38 activation. Thus TGF-beta-induced p38 activation, mediated by GADD45beta expression, may play an important role in the biological effects of TGF-beta.

A novel transforming growth factor-beta receptor-interacting protein that is also a light chain of the motor protein dynein

The phosphorylated, activated cytoplasmic domains of the transforming growth factor-beta (TGFbeta) receptors were used as probes to screen an expression library that was prepared from a highly TGFbeta-responsive intestinal epithelial cell line. One of the TGFbeta receptor-interacting proteins isolated was identified to be the mammalian homologue of the LC7 family (mLC7) of dynein light chains (DLCs). This 11-kDa cytoplasmic protein interacts with the TGFbeta receptor complex intracellularly and is phosphorylated on serine residues after ligand-receptor engagement. Forced expression of mLC7-1 induces specific TGFbeta responses, including an activation of Jun N-terminal kinase (JNK), a phosphorylation of c-Jun, and an inhibition of cell growth. Furthermore, TGFbeta induces the recruitment of mLC7-1 to the intermediate chain of dynein. A kinase-deficient form of TGFbeta RII prevents both mLC7-1 phosphorylation and interaction with the dynein intermediate chain (DIC). This is the first demonstration of a link between cytoplasmic dynein and a natural growth inhibitory cytokine. Furthermore, our results suggest that TGFbeta pathway components may use a motor protein light chain as a receptor for the recruitment and transport of specific cargo along microtublules.

The tumor microenvironment: a potential arbitrator of the tumor suppressive and promoting actions of TGFbeta

Transforming growth factor beta (TGFbeta) members are secreted in biologically inactive complexes that must be activated in order to enable binding to their cell surface receptors. Interestingly, many of the proteins that can activate TGFbeta have been implicated in either suppressing or promoting tumorigenesis. Included among these are matrix proteins (thrombospondin-1), receptors (integrins alphanubeta6 and alphanubeta8) and proteases (matrix metalloproteases and plasmin). These proteins cannot only activate TGFbeta, but can also modulate cell responsiveness to TGFbeta. In this section, we review data highlighting the complexity and bidirectionality of TGFbeta matrix interactions within the tumor microenvironment, and propose that these dynamic interactions are a critical spatial and temporal determinant of the effects of TGFbeta on tumorigenesis.

Role of transforming growth factor beta in ovarian surface epithelium biology and ovarian cancer

Ovarian cancers arise out of the ovarian surface epithelium (OSE), which is the single layer of epithelial cells covering the ovary. These cells go through repeated cycles of proliferation with the growth and rupture of ovarian follicles. One growth factor involved in the regulation of OSE is transforming growth factor beta (TGFbeta). The different isoforms of TGFbeta (TGFbeta1, TGFbeta2 and TGFbeta3) and its receptor are all present in both OSE and the underlying ovarian surface stroma. The levels of the TGFbeta isoforms and receptors are regulated independently of each other in these different ovarian tissues. Observations suggest the existence of multiple autocrine/paracrine TGFbeta signalling loops. TGFbeta acts to inhibit proliferation of normal OSE and early stage ovarian carcinomas. Conversely, in later stage ovarian cancer the inhibitory actions of TGFbeta on epithelial proliferation have been overcome, while TGFbeta is able to promote malignant neoplastic behaviours. The regulation of TGFbeta signalling by ovarian steroid hormones may be one mechanism by which the OSE responds to cyclic changes in the underlying follicles.

Spectrum of molecular changes during hepatocarcinogenesis induced by DEN and other chemicals in Fischer 344 male rats

Unlike other tissues such as breast, colon and renal cell carcinoma, it is not an easy task to single out any representative oncogene or tumor suppressor genes in the development of hepatocellular carcinoma (HCC), which play a pivotal role. To investigate putatively altered main pathways in HCC, F344 male rats were treated with a single injection of N-nitrosodiethylamine (DEN), followed by either twice/week injections of nodularin for 10 weeks or thioacetamide (TAA) in drinking water for 39 weeks. p53 expression was dramatic in both hepatocytes and mesenchymal cells after a single injection of DEN, however, PCR-SSCP assay could not detect any p53 mutation during the development of hepatocellular adenoma. The data indicate that wtp53 response was mostly for removal of damaged cells during the initiation of carcinogenesis. When treated with DEN-TAA, induction of gankyrin expression during hepatic fibrosis preceded the loss of pRB protein, accompanied with significant expressions of G1 phase cyclins and CDKs. Moreover, p16(INK4A) exon 1 was hypermethylated during the development of poorly differentiated HCCs. These changes would result in complete inactivation of the pRB regulatory pathway during hepatocarcinogenesis. Induction of TGF-beta1 expression with loss of its receptor expression occurred rapidly in the altered hepatocytes by DEN-nodularin treatment.

CONCLUSION

Therefore, escape from TGF-beta1 induced apoptosis and severe degradation of pRB protein during the early stage of carcinogenesis can perform a symphony to proliferate and to transform the altered hepatocytes to tumor cells. Inactivation of p16(INK4A) and p53 genes at the later stage of carcinogenesis would endow HCC with malignancy, which is highly resistant to any therapeutic trials.

Both the Smad and p38 MAPK pathways play a crucial role in Runx2 expression following induction by transforming growth factor-beta and bone morphogenetic protein

The Runx family of transcription factors plays pivotal roles during normal development and in neoplasias. In mammals, Runx family genes are composed of Runx1 (Pebp2alphaB/Cbfa2/Aml1), Runx2 (Pebp2alphaA/Cbfa1/Aml3) and Runx3 (Pebp2alphaC/Cbfa3/Aml2). Runx1 and Runx3 are known to be involved in leukemogenesis and gastric carcinogenesis, respectively. Runx2, on the other hand, is a common target of transforming growth factor-beta1 (TGF-beta1) and bone morphogenetic protein-2 (BMP-2) and plays an essential role in osteoblast differentiation. Runx2 is induced by the receptor-activated Smad; Runx2 mediates the blockage of myogenic differentiation and induces osteoblast differentiation in C2C12 pluripotent mesenchymal precursor cells. However, Smad does not directly induce Runx2 expression; an additional step of de novo protein synthesis is required. Here we report that Smad-induced junB functions as an upstream activator of Runx2 expression. Furthermore, not only the Smad pathway but also the mitogen-activated protein kinase (MAPK) cascades are involved in the induction of Runx2 by TGF-beta1 and BMP-2. Our results demonstrate that following TGF-beta and BMP induction, both the Smad and p38 MAPK pathways converge at the Runx2 gene to control mesenchymal precursor cell differentiation.

Binding of TGF-beta1 latency-associated peptide (LAP) to alpha(v)beta6 integrin modulates behaviour of squamous carcinoma cells

The integrin alpha(v)beta6 is not detectable on normal keratinocytes in vivo but expression is increased significantly in oral squamous cell carcinoma where this heterodimer has been shown to play a role in cell migration, invasion and protease expression. Although regarded initially as a fibronectin receptor, alpha(v)beta6 may bind to arginine-glycine-aspartic acid sequences in other matrix molecules including tenascin and vitronectin. Interestingly, alpha(v)beta6 has also been shown to have high affinity for the TGF-beta1 latency associated peptide and to participate in the activation of the TGF-beta1 latent complex. Since TGF-beta1 is present in squamous carcinomas, it is possible that latency associated peptide may modulate malignant keratinocyte behaviour independently from the classical TGF-beta signalling pathways through its interaction with integrins. We show here that when latency associated peptide is immobilised onto a surface, it acts as an alpha(v)beta6-specific ligand for oral squamous carcinoma cells promoting adhesion and haptotactic migration in addition to alpha(v)beta6-dependent increase in pro-MMP-9 expression. In contrast, even very low concentrations of soluble latency associated peptide (0.1 microg ml(-1)) inhibited alpha(v)beta6-dependent adhesion, migration and invasion. Thus alpha(v)beta6-dependent processes of oral squamous cell carcinoma, is likely to be modulated, not only by the local concentration of latency associated peptide in the stroma, but also whether it is immobilised in the matrix or released as a soluble protein.

Mechanisms in epithelial plasticity and metastasis: insights from 3D cultures and expression profiling

Most human tumors are of epithelial origin (carcinomas) and metastases from such tumors lead to >80% of all cancer deaths. In contrast to aberrant control of proliferation, cell cycle, apoptosis, angiogenesis, and lifespan, mechanisms involved in local invasion and metastasis are still insufficiently understood. We will review a set of (often conflicting) in vitro/in vivo data that suggest the existence of several types of epithelial cell plasticity changes towards a fibroblastoid, invasive phenotype, which increasingly emerge as crucial events during metastasis. New cellular models were identified, which form organotypic structures under near-physiological 3D-culture conditions in vitro as well as tumors/metastases in vivo. In these models, key proteins and signaling pathways were identified (e.g., TGFbeta, ERK/MAPK, PI3K, and PDGF), which specify distinct types of epithelial plasticity correlated with steps in cancer progression and metastasis. The existence of several distinct epithelial plasticity phenotypes is also strongly suggested by expression profiling of polysome-bound mRNA, yielding a better representation of the proteome than conventional expression profiling.

TGF-beta signaling in renal disease

Since discovery over a decade ago of a role for the cytokine TGF-beta as key mediator of glomerular and tubulointerstitial pathobiology in chronic kidney diseases, studies of TGF-beta signaling in the kidney have focused on the molecular biology of fibrogenesis. In recent years, glomerular and tubular epithelial cell apoptosis and cellular transdifferentiation have been proposed as putative primary pathomechanisms that may underlie progression of renal disease. This review describes evidence in support of nonlinear models and functional roles of TGF-beta signaling in mediating apoptosis and epithelial-to-mesenchymal transdifferentiation (EMT) in chronic progressive renal disease. Emphasis is placed on cell context-dependent models of TGF-beta signaling providing a conceptual framework to consolidate seemingly distinct pathomechanisms of progression of glomerular and tubulointerstitial disease.

Activins, inhibins, and follistatins: from endocrinology to signaling. A paradigm for the new millennium

It has been 70 years since the name inhibin was used to describe a gonadal factor that negatively regulated pituitary hormone secretion. The majority of this period was required to achieve purification and definitive characterization of inhibin, an event closely followed by identification and characterization of activin and follistatin (FS). In contrast, the last 15-20 years saw a virtual explosion of information regarding the biochemistry, physiology, and biosynthesis of these proteins, as well as identification of activin receptors, and a unique mechanism for FS action-the nearly irreversible binding and neutralization of activin. Many of these discoveries have been previously summarized; therefore, this review will cover the period from the mid 1990s to present, with particular emphasis on emerging themes and recent advances. As the field has matured, recent efforts have focused more on human studies, so the endocrinology of inhibin, activin, and FS in the human is summarized first. Another area receiving significant recent attention is local actions of activin and its regulation by both FS and inhibin. Because activin and FS are produced in many tissues, we chose to focus on a few particular examples with the most extensive experimental support, the pituitary and the developing follicle, although nonreproductive actions of activin and FS are also discussed. At the cellular level, it now seems that activin acts largely as an autocrine and/or paracrine growth factor, similar to other members of the transforming growh factor beta superfamily. As we discuss in the next section, its actions are regulated extracellularly by both inhibin and FS. In the final section, intracellular mediators and modulators of activin signaling are reviewed in detail. Many of these are shared with other transforming growh factor beta superfamily members as well as unrelated molecules, and in a number of cases, their physiological relevance to activin signal propagation remains to be elucidated. Nevertheless, taken together, recent findings suggest that it may be more appropriate to consider a new paradigm for inhibin, activin, and FS in which activin signaling is regulated extracellularly by both inhibin and FS whereas a number of intracellular proteins act to modulate cellular responses to these activin signals. It is therefore the balance between activin and all of its modulators, rather than the actions of any one component, that determines the final biological outcome. As technology and model systems become more sophisticated in the next few years, it should become possible to test this concept directly to more clearly define the role of activin, inhibin, and FS in reproductive physiology.

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.

Proteasomal activity modulates TGF-ss signaling in a gene-specific manner

Transforming growth factor-beta (TGF-beta) signaling relies on Smad-signaling pathway controlled in part by the proteasome. Here we demonstrate that inhibition of the proteasome function in mink epithelial cells accumulates both positive and negative modulators of TGF-beta signaling, phospho-Smad2 and SnoN. Inhibition of the proteasome led to abrogation of TGF-beta target gene regulation in a gene-specific manner. While regulation of p15Ink4b and myc by TGF-beta are lost, PAI-1 induction, previously shown to occur in a Smad3-dependent manner, was not affected by treatment of the cells with the proteasomal inhibitor MG132. The results suggest that proteasomal activity is required for TGF-beta signaling in a gene-specific manner.

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.

Disruption of the gene encoding the latent transforming growth factor-beta binding protein 4 (LTBP-4) causes abnormal lung development, cardiomyopathy, and colorectal cancer

Transforming growth factor-betas (TGF-betas) are multifunctional growth factors that are secreted as inactive (latent) precursors in large protein complexes. These complexes include the latency-associated propeptide (LAP) and a latent transforming growth factor-beta binding protein (LTBP). Four isoforms of LTBPs (LTBP-1-LTBP-4) have been cloned and are believed to be structural components of connective tissue microfibrils and local regulators of TGF-beta tissue deposition and signaling. By using a gene trap strategy that selects for integrations into genes induced transiently during early mouse development, we have disrupted the mouse homolog of the human LTBP-4 gene. Mice homozygous for the disrupted allele develop severe pulmonary emphysema, cardiomyopathy, and colorectal cancer. These highly tissue-specific abnormalities are associated with profound defects in the elastic fiber structure and with a reduced deposition of TGF-beta in the extracellular space. As a consequence, epithelial cells have reduced levels of phosphorylated Smad2 proteins, overexpress c-myc, and undergo uncontrolled proliferation. This phenotype supports the predicted dual role of LTBP-4 as a structural component of the extracellular matrix and as a local regulator of TGF-beta tissue deposition and signaling.

c-Jun associates with the oncoprotein Ski and suppresses Smad2 transcriptional activity

The Smad proteins are key intracellular effectors of transforming growth factor-beta (TGF-beta) cytokines. The ability of Smads to modulate transcription results from a functional cooperativity with the coactivators p300/cAMP-response element-binding protein-binding protein (CBP), or the corepressors TGIF and Ski. The c-Jun N-terminal kinase (JNK) pathway, another downstream target activated by TGF-beta receptors, has also been suggested to inhibit TGF-beta signaling through interaction of c-Jun with Smad2 and Smad3. Here we show that c-Jun directly interacts with Ski to enhance the association of Ski with Smad2 in the basal state. Interestingly, TGF-beta signaling induces dissociation of c-Jun from Ski, thereby relieving active repression by c-Jun. Moreover, activation of JNK pathway suppressed the ability of TGF-beta to induce dissociation of c-Jun from ski. Thus, the formation of a c-Jun/Ski complex maintains the repressed state of Smad2-responsive genes in the absence of ligand and participates in negative feedback regulation of TGF-beta signaling by the JNK cascade.

Transforming growth factor-beta 2 is a transcriptional target for Akt/protein kinase B via forkhead transcription factor

Tumors evade cell death by constitutively activating cell survival pathways and suppressing intrinsic death machinery. Activation of cell survival pathways leads to transcriptional repression of genes associated with cell death and activation of ones promoting anti-apoptosis. Akt/protein kinase B phosphorylates forkhead transcription factors and prevents their nuclear localization, leading to repression of genes involved in apoptosis, such as Fas ligand (FasL). Using bioinformatic approaches, we have identified three consensus sequences for forkhead transcription factor binding in transforming growth factor beta2 (TGF-beta2) promoter. TGF-beta inhibits cell proliferation and induces apoptosis in many cell types, and acquisition of TGF-beta resistance is linked to tumorigenesis. In this study, we show that activated Akt down-regulates TGF-beta2 promoter, and sequences within the promoter that are related to consensus forkhead binding sites are necessary for repression. Forkhead factor FKHRL1 binds in vitro to the three consensus sequences and can activate TGF-beta2 promoter in normal and Akt-transformed cell lines. In human breast and pancreatic tumors, activated Akt expression correlated with down-regulation of TGF-beta 2 mRNA levels. A number of tumor cells expressing activated Akt were responsive to TGF-beta addition, indicating the presence of an intact TGF-beta-signaling pathway. These results suggest that repression of TGF-beta 2 promoter activity in cells expressing activated Akt may play a role in promoting tumorigenesis and escape from the growth-inhibitory and/or apoptotic effects of TGF-beta.

Effect of transforming growth factor-beta1, insulin-like growth factor-I, and hepatocyte growth factor on proteoglycan production and regulation in canine melanoma cell lines

OBJECTIVE

To identify extracellular proteoglycans produced by canine melanoma cell lines and analyze the effect of transforming growth factor-beta1 (TGF-beta1), insulin-like growth factor-I (IGF-I), and hepatocyte growth factor (HGF) on these proteoglycans.

SAMPLE POPULATION

3 canine melanoma cell lines (ie, CML-1, CML-6M, and CML-10c2).

PROCEDURE

Extracellular proteoglycans were analyzed by use of metabolic labeling and western immunoblot analysis. The effect of TGF-beta1 on cell proliferation was determined by incorporation of 5-bromo-2'-deoxyuridine.

RESULTS

The CML-1 and CML-6M melanoma cell lines produced 2 main extracellular proteoglycans. One of them was identified as versican, a proteoglycan found in undifferentiated human melanoma cell lines. The CML-10c2 cells produced a small amount of extracellular proteoglycans. Addition of TGF-beta1 (1.25 to 6.25 ng/ml) increased the release of sulfated proteoglycans into the medium. The TGF-beta1 had mainly a posttranslational effect, because it increased the molecular mass of the sulfated bands. Addition of IGF-I (50 ng/ml) slightly increased production of proteoglycans in the CML-6M cell line, whereas HGF (50 ng/ml) did not have any effect on proteoglycan production.

CONCLUSIONS AND CLINICAL RELEVANCE

The proteoglycan content and response toTGF-beta1 treatment for CML-1 and CML-6M canine melanoma cell lines are similar to that for undifferentiated human melanoma cell lines. In contrast, CML-10c2 cells produced a low amount of proteoglycans with high molecular weight. Because these extracellular proteoglycans are involved in the control of cell adhesion, proliferation, and migration, they may play an important role in the progression of melanomas in dogs.

Identification and characterization of a novel rat ov-serpin family member, trespin

Serpins are responsible for regulating a variety of proteolytic processes through a unique irreversible suicide substrate mechanism. To discover novel genes regulated by transforming growth factor-beta1 (TGF-beta 1), we performed differential display reverse transcriptase-PCR analysis of NRP-152 rat prostatic epithelial cells and cloned a novel rat serpin that is transcriptionally down-regulated by TGF-beta and hence named trespin (TGF-beta-repressible serine proteinase inhibitor (trespin). Trespin is a 397-amino acid member of the ov-serpin clade with a calculated molecular mass of 45.2 kDa and 72% amino acid sequence homology to human bomapin; however, trespin exhibits different tissue expression, cellular localization, and proteinase specificity compared with bomapin. Trespin mRNA is expressed in many tissues, including brain, heart, kidney, liver, lung, prostate, skin, spleen, and stomach. FLAG-trespin expressed in HEK293 cells is localized predominantly in the cytoplasm and is not constitutively secreted. The presence of an arginine at the P1 position of trespin's reactive site loop suggests that trespin inhibits trypsin-like proteinases. Accordingly, in vitro transcribed and translated trespin forms detergent-stable and thermostable complexes with plasmin and elastase but not subtilisin A, trypsin, chymotrypsin, thrombin, or papain. Trespin interacts with plasmin at a near 1:1 stoichiometry, and immunopurified mammal-expressed trespin inhibits plasmin in a dose-dependent manner. These data suggest that trespin is a novel and functional member of the rat ov-serpin family.

TGF-beta receptor-activated p38 MAP kinase mediates Smad-independent TGF-beta responses

Through the action of its membrane-bound type I receptors, transforming growth factor-beta (TGF-beta) elicits a wide range of cellular responses that regulate cell proliferation, differentiation and apoptosis. Many of the signaling responses induced by TGF-beta are mediated by Smad proteins, but certain evidence has suggested that TGF-beta can also signal independently of Smads. We found in mouse mammary epithelial (NMuMG) cells, which respond to TGF-beta treatment in multiple ways, that TGF-beta-induced activation of p38 MAP kinase is required for TGF-beta-induced apoptosis, epithelial-to-mesenchymal transition (EMT), but not growth arrest. We further demonstrated that activation of p38 is independent of Smads using a mutant type I receptor, which is incapable of activating Smads but still retains the kinase activity. This mutant receptor is sufficient to activate p38 and cause NMuMG cells to undergo apoptosis. However, it is not sufficient to induce EMT. These results indicate that TGF-beta receptor signals through multiple intracellular pathways and provide first-hand biochemical evidence for the existence of Smad-independent TGF-beta receptor signaling.

E2F4/5 and p107 as Smad cofactors linking the TGFbeta receptor to c-myc repression

Smad3 is a direct mediator of transcriptional activation by the TGFbeta receptor. Its target genes in epithelial cells include cyclin-dependent kinase inhibitors that generate a cytostatic reponse. We defined how, in the same context, Smad3 can also mediate transcriptional repression of the growth-promoting gene c-myc. A complex containing Smad3, the transcription factors E2F4/5 and DP1, and the corepressor p107 preexists in the cytoplasm. In response to TGFbeta, this complex moves into the nucleus and associates with Smad4, recognizing a composite Smad-E2F site on c-myc for repression. Previously known as the ultimate recipients of cdk regulatory signals, E2F4/5 and p107 act here as transducers of TGFbeta receptor signals upstream of cdk. Smad proteins therefore mediate transcriptional activation or repression depending on their associated partners.

Metastasis is driven by sequential elevation of H-ras and Smad2 levels

Metastasis is a multistep process that involves local tumour invasion followed by dissemination to, and re-establishment at, distant sites. Here we show that during multistage tumorigenesis, discrete expression thresholds of activated Smad2 and H-ras are sequentially surpassed, driving tumour progression through distinct phases from a differentiated squamous carcinoma to a motile invasive stage, followed by an overt change from epithelial to mesenchymal cell type, finally culminating in metastatic tumour spread. Smad2 activation alone induces migration of tumour cells. Elevated H-ras levels, however, are required for nuclear accumulation of Smad2, both of which are essential for the epithelial mesenchymal transition (EMT). Having undergone EMT, fibroblastoid carcinoma cells with elevated levels of activated Smad2, gain the capability to spread to a wide variety of tissues by a further increase in Smad2 expression. These findings have far-reaching implications for the prevention of tumour growth, invasion and metastasis.

Chemoprevention: an essential approach to controlling cancer

Mortality that results from the common forms of cancer is still unacceptably high. Despite immense advances in the understanding of the mechanisms of carcinogenesis, in bringing potent new drugs to the clinic and in treating several relatively rare forms of cancer, overall mortality statistics are unlikely to change in a fundamental way until there has been a re-orientation of emphasis in cancer research that will direct greater resources towards prevention of new disease, rather than treatment of end-stage disease.