Loss of type III transforming growth factor β receptor expression increases motility and invasiveness associated with epithelial to mesenchymal transition during pancreatic cancer progression

Carcinogenesis, Feb 2008

Epithelial to mesenchymal transitions (EMTs) contribute to increases in cellular motility and invasiveness during embryonic development and tumorigenesis. The transforming growth factor β (TGF-β) signaling pathway is a key regulator of EMT. The TGF-β superfamily coreceptor, the type III TGF-β receptor (TβRIII or betaglycan), is required for EMT during embryonic heart development and palate fusion. Here, we establish that in a pancreatic cancer model of EMT, TβRIII expression is specifically lost during EMT at the mRNA and protein levels, whereas levels of the TGF-β type I and type II receptors are maintained at the mRNA level. Loss of TβRIII expression at the protein level precedes the loss of E-cadherin and cytoskeletal reorganization during early stages of EMT. However, maintaining TβRIII expression does not block these aspects of EMT, but instead suppresses the increased motility and invasiveness associated with EMT. Reciprocally, shRNA-mediated knockdown of endogenous TβRIII increases cellular motility without affecting Snail or E-cadherin levels. The ability of TβRIII to suppress motility and invasiveness does not depend on its cytoplasmic domain or its coreceptor function. Instead, this suppression of invasion is partially mediated by ectodomain shedding of TβRIII, generating soluble TβRIII (sTβRIII). In human pancreatic cancer specimens, TβRIII expression decreases at both the mRNA and protein levels, with the degree of loss correlating with worsening tumor grade. Taken together, these studies support a role for loss of TβRIII expression during the EMT of pancreatic cancer progression, with a specific role for sTβRIII in suppressing EMT-associated increases in motility and invasion.

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Loss of type III transforming growth factor β receptor expression increases motility and invasiveness associated with epithelial to mesenchymal transition during pancreatic cancer progression

Kelly J.Gordon 2 Mei Dong 1 Elizabeth M.Chislock 2 Timothy A.Fields 0 Gerard C.Blobe 1 2 To whom correspondence should be addressed. Tel: 2 Fax: 2 Email: blobe @mc.duke.edu 0 Department of Pathology, Duke University Medical Center , Durham, NC 27710, USA 1 Department of Medicine 2 Department of Pharmacology and Cancer Biology Epithelial to mesenchymal transitions (EMTs) contribute to increases in cellular motility and invasiveness during embryonic development and tumorigenesis. The transforming growth factor b (TGF-b) signaling pathway is a key regulator of EMT. The TGF-b superfamily coreceptor, the type III TGF-b receptor (TbRIII or betaglycan), is required for EMT during embryonic heart development and palate fusion. Here, we establish that in a pancreatic cancer model of EMT, TbRIII expression is specifically lost during EMT at the mRNA and protein levels, whereas levels of the TGF-b type I and type II receptors are maintained at the mRNA level. Loss of TbRIII expression at the protein level precedes the loss of E-cadherin and cytoskeletal reorganization during early stages of EMT. However, maintaining TbRIII expression does not block these aspects of EMT, but instead suppresses the increased motility and invasiveness associated with EMT. Reciprocally, shRNA-mediated knockdown of endogenous TbRIII increases cellular motility without affecting Snail or E-cadherin levels. The ability of TbRIII to suppress motility and invasiveness does not depend on its cytoplasmic domain or its coreceptor function. Instead, this suppression of invasion is partially mediated by ectodomain shedding of TbRIII, generating soluble TbRIII (sTbRIII). In human pancreatic cancer specimens, TbRIII expression decreases at both the mRNA and protein levels, with the degree of loss correlating with worsening tumor grade. Taken together, these studies support a role for loss of TbRIII expression during the EMT of pancreatic cancer progression, with a specific role for sTbRIII in suppressing EMTassociated increases in motility and invasion. Introduction Pancreatic cancer is the fourth leading cause of cancer death in the USA, with a death rate closely matching the incidence rate, a median survival of 46 months and a 5-year survival rate of ,5% (1). Several factors account for this poor prognosis, including delayed diagnosis, as patients seldom exhibit symptoms until the cancer has become locally invasive or metastatic, lack of effective screening tests and largely ineffective treatment options (1). Clearly, greater understanding of the processes underlying pancreatic invasiveness and metastasis is required to develop effective treatment strategies for this deadly disease. An important contributor to the metastatic process is epithelial to mesenchymal transition (EMT), in which adherent and non-motile epithelial cells acquire motility and invasiveness (2,3). EMT is associated with the induction of transcription factors including Snail and Slug, the loss of epithelial markers including E-cadherin and cytokerAbbreviations: CM, conditioned media; EMT, epithelial to mesenchymal transition; NTC, non-targeting control; sTbRIII, soluble TbRIII; TGF-b, transforming growth factor b. atins, which are important in the maintenance of epithelial cell junctions, and gain of mesenchymal markers including vimentin and N-cadherin. EMT facilitates metastasis, as epithelial-derived cancer cells within the primary tumor undergo this transition to acquire motility and invasiveness, enabling them to penetrate the basement membrane and access the blood stream and then extravasate to form distant metastasis. As metastatic tumors are the cause of death for .95% of cancer patients, defining mechanisms regulating this metastatic cascade remains a priority for cancer therapy (2,3). The transforming growth factor b (TGF-b) pathway is an important regulator of EMT, as TGF-b initiates and maintains both developmental and carcinogenic EMT in different systems in vitro and in vivo (4). Cancer cells may utilize these developmental TGF-b-mediated EMT pathways to promote their metastatic capability. In addition, TGF-b potently inhibits the growth of epithelial cells yet promotes the growth of mesenchymal cells (5). Therefore, carcinogenic EMT may provide the mechanism by which TGF-b switches from a tumor suppressor in early stages of tumorigenesis to a tumor promoter at later stages. TGF-b signals through three cell surface receptors, the type III (TbRIII, or betaglycan), type II (TbRII) and type I (TbRI) receptors. TbRIII is a TGF-b superfamily coreceptor that binds all TGF-b isoforms and presents them to TbRII (68). Once ligand bound, TbRII recruits and phosphorylates TbRI to activate its kinase activity. TbRI then phosphorylates and activates Smads2/3, which bind to Smad4, and the complex accumulates in the nucleus and interacts with other transcription factors to regulate the expression of a multitude of target genes (68). Recent studies have demonstrated that TbRIII may have additional functions independent of ligand presentation. Indeed, the TbRIII knockout mouse is embryonic lethal due to extensive heart and liver defects (9). In addition, TbRIII is required for the EMT that occurs during cardiac development to form the valves and septa (10) and is required for the EMT responsible for palate fusion (11,12). We have previously demonstrated that alterations in TbRIII expression critically modulate TGF-b signaling; the cytoplasmic domain of TbRIII interacts with the PDZ domain-containing protein GIPC to stabilize TbRIII on the cell surface and increase TGF-b signaling (13) and with the scaffolding protein b-arrestin2 to down-regulate TbRIII and TbRII from the cell surface and decrease TGF-b signaling (14). Most recently, we have demonstrated that TbRIII expression is dramatically lost in breast (15), prostate (16) and ovarian cancers (17). These studies suggest that loss of TbRIII expression has important consequences during carcinogenesis. TGF-b is a critical regulator of pancreatic cancer homeostasis (18), and components of the TGF-b pathway are often the target of disruption during pancreatic carcinogenesis. In a majority of human pancreatic cancers and cell lines, TbRI, TbRII and TbRIII expression levels are altered at the protein and/or mRNA level (1921), and the downstream effector Smad4 is mutated in 50% of all human pancreatic cancers (2224). These mutations suggest that alterations in TGFb signaling contribute to pancreatic carcinogenesis. Here, we investigate the role of the TGF-b signaling pathway in regulating EMT, specifically motility and invasiveness in a pancreatic cancer model. Materials and methods PANC-1 and COS-7 cells obtained from American Type Culture Collection (Manassas, VA) were maintained in Dulbeccos modified eagles media supplemented with 10% fetal bovine serum. EMT was induced by treating with 0400 pM of TGF-b1 (R&D Systems, Minneapolis, MN) for up to 48 h as described previously (25). Adenoviral constructs All adenoviral constructs were made using the Becton Dickinson Adeno-X expression system (Becton, Dickinson and Company, Franklin Lakes, NJ) purified using the Adeno-X Virus Purification Kit and titered using the Adeno-X Rapid Titer Kit. The TbRIII and non-targeting control shRNAs were generated by Dharmacon (Lafayette, CO). For E-cadherin staining, cells were fixed with a 1:1 solution of methanol and acetone at 20 C. Blocking was performed with 1% normal rabbit serum. Cells were incubated with a 1:200 dilution of E-cadherin antibody (BD Biosciences, San Jose, CA) for 45 min, followed by incubation with an anti-mouse antibody conjugated to Alexa 488 (Molecular Probes, Carlsbad, CA) for 45 min. For F-actin staining, cells were fixed in 3.7% formaldehyde and permeabilized with 0.1% Triton X. Blocking was performed with 1% bovine serum albumin and then cells were incubated with a 1:40 dilution of Phalloidin conjugated to Texas Red (Molecular Probes) for 20 min. Nuclei were stained with DAPI and mounted onto glass slides. Immunofluorescence images were obtained using a Nikon inverted microscope. All assays were performed at least three times and results from one experiment are provided. TGF-b binding and cross-linking Cells were incubated with Krebs-Ringer-HEPES buffer (50 mM HEPES, pH 7.5, 130 mM NaCl, 5 mM MgSO4, 1 mM CaCl2 and 5 mM KCl) and 0.5% bovine serum albumin for 30 min at 37 C, and then with 100 pM [125I]-TGFb1 for 3 h at 4 C. For the shedding experiment, PANC-1 cells were serum starved for 24 h, the media was collected and incubated with 25 pM [125I]TGF-b1 for 3 h at 4 C. [125I]-TGF-b1 was cross-linked with 0.5 mg/ml disuccinimidyl suberate for 15 min and quenched with 20 mM glycine. Cells were rinsed in Krebs-Ringer-HEPES and half of the samples were lysed with radioimmunoprecipitation assay buffer and half were lysed with hot 2 Laemmli buffer. Those samples lysed with radioimmunoprecipitation assay were subjected to overnight immunoprecipitation at 4 C using antibodies directed toward the cytoplasmic domain of TbRIII, TbRII or TbRI bound to protein A sepharose beads. For the soluble TbRIII (sTbRIII) experiments, media was subjected to overnight immunoprecipitation at 4 C using a polyclonal antibody directed toward the extracellular domain of TbRIII (R&D Systems) bound to protein G sepharose beads. All the samples were analyzed by sodium dodecyl sulfatepolyacrylamide gel electrophoresis and phosphorimaging analysis of dried gels. All assays were performed at least three times and results from one experiment are provided. Reverse transcription and real-time PCR RNA was isolated from cells using the RNAEasy kit (Qiagen, Germantown, MD), reverse transcribed using an oligodT primer (Invitrogen, Carlsbad, CA) and then PCR was performed using primers specific for GAPDH (27), Ecadherin (28), cytokeratin 8 (29) and cytokeratin 19 (30), vimentin (31) and N-cadherin (forward 5#-GACAATGCCCCTCAAGTGTT-3#, reverse 5#CCATTAAGCCGAGTGATGGT-3#). Products were analyzed on a 2% agarose gel and images were acquired with a Bio-Rad Gel Doc. Real-time PCR was performed using SYBR Green Supermix (Bio-Rad, Hercules, CA) and primers for TbRI, TbRII and TbRIII (32,33). All assays were performed at least three times and results from one experiment are provided. Matrigel invasion and motility assays Invasion assays were performed using 24-well Matrigel-coated transwells (BD Biosciences). Motility assays were performed using 24-well fibronectin (Calbiochem, San Diego, CA)-coated transwells (Corning Costar, Lowell, MA). A total of 100 000 cells were placed in the upper chamber in serum-free media and media containing 10% fetal bovine serum was placed in the lower chamber. After 48 h of incubation in the Matrigel chambers and 18 h in the fibronectin transwells, the cells on the upper surface of the filter were gently scrubbed with a cotton swab. The cells on the underside of the filter were fixed and stained with the Three-Step Stain Set (VWR). The filters were removed and mounted onto glass slides. Each filter was examined using a Nikon inverted microscope at 10 magnification ( 20 for shRNA motility experiment) and the number of cells in the center field was counted. For the sTbRIII invasion assay, conditioned media (CM) was collected from COS-7 cells transiently transfected (Lipofectamine 2000) with 2 lg of a pcDNA3.1 or sTbRIII vector and this media was used in the upper chamber of the transwell. All assays were performed at least three times. Invasion and motility were normalized to untreated PANC-1 in each experiment. Results represent the mean SEM from three independent experiments. Immunohistochemistry of human pancreatic cancer tissue specimens Studies were performed as previously published (15), using a pancreatic cancer tissue array from Protein Biotechnologies (San Diego, CA). The intensity of staining in each specimen was blindly scored on a scale of 15 by a certified pathologist (T.A.F.). TbRIII gene expression analysis on cDNA filter array Studies were performed as previously published (15). TGF-b1-induced Smad2 and p38 phosphorylation Cells were treated with 400 pM of TGF-b1 for 48 h to induce EMT. Cells were then serum starved for 2 h and treated with either 20 or 200 pM of TGF-b1 for 40 min. Cells were lysed in 2 Laemmli buffer and lysates were subjected to sodium dodecyl sulfatepolyacrylamide gel electrophoresis. Western blotting was performed using Smad2, phospho-specific Smad2 and phospho-specific p38 antibodies (Cell-Signaling Technology, Danvers, MA). Western blotting for HA-tagged TbRIII expression was performed using an aHA antibody (Roche Applied Science, Basel Switzerland) and an ab-actin antibody (Sigma, St Louis, MO) to control for protein loading. All assays were performed at least three times and results from one experiment are illustrated. Oncomine microarray data Gene profiling studies publicly available on the Oncomine Cancer Profiling Database were used to investigate TbRIII mRNA levels in pancreatic cancer (34,35). Statistical analysis Significance of results was assessed using the one-tailed Students t-test on the VassarStats Web site (36); P , 0.05, P , 0.025, P , 0.01. The P-value for the Oncomine study was generated by the Oncomine database (34). Error bars, were indicated, represent the SEM (n 5 3). Results TbRIII expression decreases during EMT in a pancreatic cancer model An important contributor to the metastatic process is EMT (2,3). Based on the defined role for the TGF-b signaling pathway in both EMT and pancreatic cancer, we investigated alterations in the TGF-b signaling pathway in a pancreatic cancer model of EMT. The human pancreatic cancer cell line PANC-1 has been reported to undergo EMT in response to TGF-b1, with maximal EMT occurring after 48 h of treatment with 400 pM of TGF-b1 (25). To characterize the TGF-b signaling pathway during EMT, we treated PANC-1 cells with TGF-b1 for up to 48 h and assessed EMT induction by following the loss of cellular junction E-cadherin immunofluorescence staining, a wellestablished EMT marker. E-cadherin staining at cellcell junctions and total E-cadherin levels began to decrease at 12 h and continued to decrease until 48 h after TGF-b1 treatment (Figure 1A). Loss of E-cadherin expression also occurred in a dose-dependent manner, with loss observed after 48 h of treatment with 100 pM of TGF-b1 and maximal loss occurring with 400 pM of TGF-b1 (Figure 1B). These dose-dependent and time-dependent reductions in E-cadherin levels also occurred when PANC-1 cells were treated with TGF-b2 (data not shown). EMT was further confirmed by cytoskeletal reorganization as demonstrated by a shift in cortical to diffuse F-actin staining (Figure 1C), as well as by the concomitant decrease in the mRNA levels of the epithelial markers E-cadherin, cytokeratin 8 and cytokeratin 19, and increase in the mRNA levels of the mesenchymal markers vimentin and N-cadherin (Figure 1D). Having rigorously defined TGF-b-induced EMT in the PANC-1 model, we investigated TGF-b receptor expression during EMT. As endogenous TGF-b receptors could not be detected in PANC-1 cells by western blot or by immunoprecipitation followed by western blot using commercially available antibodies (data not shown), we analyzed the steady-state cell surface levels of the TGF-b receptors expression by [125I]-TGF-b1 binding and cross-linking, which examines both expression and binding affinity. Before EMT, PANC-1 cells express all three TGF-b1 receptors, TbRIII, TbRII and TbRI (Figure 2A). Treatment of PANC-1 cells with TGF-b1 resulted in a rapid decrease in [125I]-TGF-b1-bound and cross-linked TGF-b receptors, with decreases first detectable at 2 h (Figure 2A). While TbRII expression increased by 48 h, TbRI and TbRIII binding remained low (supplementary Figure 1, available at Carcinogenesis Online). As TbRIII presents and increases ligand binding to TbRI and TbRII, loss of TbRIII expression may contribute to the decreases in binding observed for TbRI and TbRII. TbRIII levels continued to fall to nearly undetectable levels after 12 h of treatment (Figure 2A), when the earliest sign of EMT, loss of E-cadherin peripheral staining, could first be detected (Figure 1A). Loss of TbRIII expression was maintained after 12 h until 72 h after TGF-b1 treatment (Figure 2A, data not shown), similar to loss of E-cadherin expression (Figure 1A). As TbRIII is a proteoglycan that runs as a broad high molecular weight smear between 150 and 200 kDa, loss of TbRIII expression was confirmed by immunoprecipitating TbRIII (Figure 2A). Loss of TbRIII expression during EMT was also dose dependent (Figure 2B), with loss of TbRIII initially observed after 48 h with 100 pM of TGF-b1 and maximal at 400 pM, similar to loss of E-cadherin expression (Figure 1B). We compared the loss of TbRIII and E-cadherin protein during the EMT time course (Figure 2C), and determined that loss of TbRIII protein expression preceded loss of E-cadherin protein expression. Taken together, these data support time- and dose-dependent loss of TbRIII expression during TGF-b1induced EMT in this pancreatic cancer model, with loss of TbRIII expression preceding loss of E-cadherin expression during EMT. Interestingly, during TGF-b treatment, cell surface expression of an 120 kDa TGF-b-binding protein increased in both a time- and dosedependent manner (Figure 2A and B). As this was the approximate size of the TbRIII core and the increase in expression mirrored the loss of expression of full-length TbRIII, we investigated whether this was the TbRIII core. Using antibodies to either the extracellular or the cytoplasmic domain of TbRIII, we were unable to immunoprecipitate this TGF-b-binding protein (data not shown). Therefore, the identity of this potentially novel TGF-b-binding protein remains to be elucidated. Regulation of TbRIII expression at the protein and mRNA level during EMT Although there was an acute loss of cell surface TbRIII expression, this loss was maintained .3 days after treatment with a single dose of TGF-b1 (Figure 2A, data not shown), suggesting that TbRIII expression might be regulated at several levels, with loss at the protein level accounting for the initial and rapid loss of TbRIII expression (within hours), and loss at the mRNA level accounting for the sustained loss of TbRIII expression (for days). To investigate regulation of TbRIII expression at the protein level, we first assessed whether receptor down-regulation was responsible for the acute loss of TbRIII expression. Cell surface receptors are down-regulated after internalization through either lysosomal- or proteasomal-mediated degradation. Accordingly, we assessed the effects of lysosome and proteasome inhibitors on loss of TbRIII expression during TGF-b1-induced EMT. These inhibitors did not significantly affect the loss of TbRIII induced by TGF-b1 during EMT (data not shown), suggesting that receptor down-regulation is not responsible for the dramatic loss of TbRIII cell surface expression during EMT in PANC-1 cells. As with other coreceptors, TbRIII undergoes ectodomain shedding, with proteolytic cleavage in the extracellular domain near the transmembrane segment releasing the soluble extracellular domain (sTbRIII) into the extracellular space (37). When examining sTbRIII expression by immunoprecipitating [125I]-TGF-b1-bound and crosslinked sTbRIII with a TbRIII-specific antibody, sTbRIII was detected in the CM of untreated PANC-1 cells (Figure 2D, lane 3) suggesting that TbRIII is constitutively shed from the surface. However, in TGFb1-treated PANC-1 cells undergoing EMT, there was a significant increase in the amount of sTbRIII in the media (Figure 2D, lane 4). These data suggest that TbRIII shedding increases during EMT in PANC-1 cells, resulting in a rapid decrease in cell surface levels of TbRIII. To investigate regulation of TbRIII expression at the mRNA level, we assessed steady-state mRNA levels for TbRIII, as well as for TbRII and TbRI, using reverse transcriptionreal-time PCR. There was significant loss of TbRIII expression at the mRNA level during EMT (Figure 2E, P , 0.01), with loss of expression beginning at 2 h and persisting out to 48 h (data not shown). In contrast, steady-state mRNA levels for TbRI and TbRII remained similar before and after EMT (Figure 2E). Taken together, these results demonstrate that loss of TbRIII expression during EMT occurs through regulation at both the mRNA and protein levels. They also support specific regulation of TbRIII during EMT among TGF-b receptors, as mRNA levels of TbRII and TbRI are maintained. Loss of TbRIII expression is required for the increased motility and invasiveness associated with EMT The specific loss of TbRIII expression at both the mRNA and protein levels during EMT suggested that loss of TbRIII expression might have functional significance. As the loss of TbRIII expression occurred prior to the onset of E-cadherin loss (Figure 2C), the first detectable sign of EMT, we investigated whether loss of TbRIII expression was required for E-cadherin loss to occur in this pancreatic model. To prevent loss of TbRIII expression, we transiently expressed HA-tagged TbRIII from an exogenous promoter in PANC-1 prior to the induction of EMT. This effectively blocked the EMT-induced loss of TbRIII expression, with the TbRIII adenovirally infected TGF-b treated post-EMT PANC-1 cells expressing more TbRIII than the treated non-infected PANC-1 cells (Figure 5A, lanes 3 and 4). Under these conditions, the PANC-1 cells still underwent EMT as judged by loss of cellcell junction E-cadherin staining (Figure 3A), suggesting that loss of TbRIII was not required for induction of EMT. To directly investigate a role for loss of TbRIII expression in EMT, we assessed the effects of shRNA-mediated silencing of TbRIII expression on EMT markers in the PANC-1 model. TbRIII shRNA specifically decreased TbRIII expression at the mRNA (Figure 3B) and protein levels (Figure 3E), whereas a non-targeting control shRNA had no effect on TbRIII expression. There were no effects of reducing TbRIII expression on the EMT markers E-cadherin or Snail, or on the TGF-b-mediated reduction in E-cadherin mRNA expression or induction of the mesenchymal marker Snail mRNA expression during EMT (Figure 3B). These studies suggest that loss of TbRIII expression was not sufficient or required for induction of EMT in PANC-1 cells. EMT is associated not only with altered morphology and altered expression of epithelial and mesenchymal markers but also with altered function, including both increased motility and invasiveness. As expected, TGF-b-induced EMT resulted in increases in both motility (Figure 3C, top panels) and invasiveness (Figure 3D, top panels) in the PANC-1 model. When we assessed the effect of preventing loss of TbRIII expression on TGF-b-mediated increases in motility, the TGF-b-mediated increases in motility were suppressed (Figure 3C, bottom right panel, P , 0.05). In addition, when we prevented loss of TbRIII expression in PANC-1 cells, the TGF-b-mediated increases in invasiveness were also suppressed (Figure 3D, bottom right panel, P , 0.01). Expression of TbRIII was able to significantly decrease the basal invasiveness of PANC-1, even in the absence of TGF-b1 treatment (Figure 3D, bottom left panel, P , 0.01). To further investigate the role of TbRIII in regulating motility, we used shRNA to specifically silence TbRIII expression in PANC-1 cells and assessed effects on motility before and after EMT. As expected, decreasing TbRIII protein levels by 70% in PANC-1 prior to initiating EMT (Figure 3E, top panel) increased their motility (Figure 3E, bottom panel, P , 0.05). Taken together, these studies suggest that loss of TbRIII expression is not required for induction of EMT but is required for the increase in motility and invasiveness associated with EMT during pancreatic cancer progression. The cytoplasmic domain of TbRIII and ligand-presenting function are not required for TbRIII-mediated suppression of motility and invasion TbRIII serves as a TGF-b coreceptor to enhance TGF-b ligand binding to TbRII and increase TGF-b signaling (26). To carry out this coreceptor function, TbRIII must be able to bind ligand through its extracellular domain and interact with TbRII through its cytoplasmic domain (26,38,39). To investigate the mechanism by which TbRIII expression decreased basal invasiveness and suppressed the TGF-bmediated increases in motility and invasiveness associated with EMT in the PANC-1 cell line, we explored the role of the cytoplasmic domain in mediating the effects of TbRIII on suppressing motility and invasiveness. Surprisingly, TbRIII lacking its cytoplasmic domain, TbRIIIDcyto, was just as effective as TbRIII in decreasing motility and invasiveness (Figure 4A and B, data not shown). As a TGF-b coreceptor, TbRIII is thought to enhance TGF-b-mediated Smad-dependent signaling. To further investigate whether TbRIII was functioning as a TGF-b coreceptor, we examined the effects of TbRIII expression on TGF-b signaling by assessing Smad2 phosphorylation before and after EMT in the PANC-1 model. Prior to EMT, TGF-b potently induced Smad2 phosphorylation (Figure 4C, lanes 13). As TGF-b treatment was used to induce EMT, after EMT induction, the cells were washed and incubated in serum-free media for 2 h prior to restimulation with TGF-b. Intriguingly, after EMT, the ability of TGF-b to induce Smad2 phosphorylation was almost completely abrogated (Figure 4C, lanes 46). To determine whether this decrease in TGF-b responsiveness was due to loss of TbRIII expression, we assessed the effect of preventing loss of TbRIII expression on TGF-bmediated Smad phosphorylation. Preventing loss of TbRIII expression in PANC-1 cells did not rescue the decrease in TGF-b-induced Smad phosphorylation after EMT (Figure 4C, lanes 1012). Instead, TbRIII decreased Smad phosphorylation before and after EMT. TbRIII also suppressed TGF-b-mediated p38 phosphorylation (Figure 4C, lanes 712). Taken together, these results suggest that the TGF-b coreceptor function of TbRIII did not mediate the effects of TbRIII on motility and invasiveness. TbRIII suppresses EMT-associated increases in invasiveness, in part, through generation of sTbRIII As we had demonstrated that the ectodomain shedding of TbRIII to produce sTbRIII increased during EMT (Figure 2D), we explored whether sTbRIII was mediating the effects of TbRIII. Initially, we investigated the effects of overexpressing full-length TbRIII in PANC-1 cells on sTbRIII levels and established that there were higher levels of sTbRIII in the media of PANC-1 cells overexpressing full-length TbRIII compared with control cells (Figure 5A, lanes 7 and 8). To determine whether sTbRIII could affect EMT-associated invasiveness, we used CM from COS-7 cells transiently transfected with either empty vector or a sTbRIII-expressing vector (Figure 5B) in Matrigel invasion assays with untreated and TGF-b treated PANC-1 cells. Both basal and TGF-b-mediated increases in invasion were attenuated (P , 0.025) when PANC-1 cells were incubated in CMcontaining sTbRIII (Figure 5C). These data suggest that TbRIII functions to suppress EMT-associated increases in invasiveness, in part, through the generation of sTbRIII. As sTbRIII did not fully recapitulate the effect of full-length TbRIII, there are probably additional mechanisms that TbRIII utilizes to suppress motility and invasiveness associated with EMT. TbRIII expression is lost in human pancreatic cancer The dramatic loss of TbRIII expression during EMT in this human pancreatic cancer cell model suggested that TbRIII expression might be altered during human pancreatic cancer progression. To assess TbRIII expression and investigate its association with tumor grade, we performed immunohistochemical analysis of TbRIII expression on a pancreatic cancer tissue array containing 71 pancreatic cancer specimens and 9 matched normal pancreatic tissue specimens. Pancreatic adenocarcinoma is thought to arise from pancreatic ductal cells (40). Immunohistochemical staining of normal human pancreatic specimens revealed that TbRIII is highly expressed in pancreatic ductal cells (Figure 6A, left two panels, arrows). However, in pancreatic cancer specimens, TbRIII expression is significantly reduced or absent (Figure 6A, right two panels, arrows indicate remnant ductal cells). The proportion with abundant TbRIII expression decreased from 67% in normal pancreatic cancer specimens to 14% in Grade 1 specimens, to 5% in Grades 2 and 3 specimens. At the same time, the proportion with low or no TbRIII expression increased from 11% in normal pancreatic specimens to 14% in Grade 1 specimens, to 40% in Grade 2 specimens and to 50% in Grade 3 specimens (Figure 6A). Importantly, the normal pancreatic specimen with low TbRIII expression represents a specimen with no pancreatic ducts present to assess. The progressive loss of TbRIII expression with increasing pancreatic tumor grade suggests that loss of TbRIII expression may be an important contributor to pancreatic carcinogenesis. To evaluate whether there was progressive loss of TbRIII expression in matched patient specimens, we compared the expression of TbRIII in all nine matched tumor and normal specimens. In six of these nine (67%) cases, there was reduced TbRIII expression in the tumor relative to matched normal tissue (supplementary Figure 2A, Fig. 5. TbRIII suppresses EMT-associated increases in invasiveness partially through generation of sTbRIII. (A) PANC-1 cells adenovirally infected with GFP and PANC-1 cells adenovirally infected with TbRIII were treated with 400 pM of TGF-b1 for 48 h. Cell surface and media levels of TbRIII were assessed by [125I]-TGF-b1 binding and cross-linking followed by immunoprecipitation with an aTbRIII extracellular antibody. Sodium dodecyl sulfatepolyacrylamide gel electrophoresis and phosphorimaging of dried gels were performed. (B) COS-7 cells were transiently transfected with either empty vector or a sTbRIII construct. The amount of sTbRIII present in CM from these cells was assessed by [125I]-TGF-b1 binding and crosslinking. (C) PANC-1 cells adenovirally infected with GFP were treated with 400 pM of TGF-b1 for 48 h. These and untreated PANC-1 GFP-expressing cells were incubated in COS-7 control CM or CM-containing sTbRIII in the top chamber of the Matrigel transwell, and invasiveness was assessed after 48 h. PANC-1 cells adenovirally overexpressing TbRIII were treated with 400 pM of TGF-b1 for 48 h and then incubated in the control CM to assess their invasiveness in parallel. All assays were performed at least three times and invasion and motility were normalized to untreated PANC-1 in each experiment. Data represent the mean SEM (n 5 3), P , 0.025. available at Carcinogenesis Online). One additional case exhibiting no change in TbRIII expression already exhibited little to no TbRIII expression in the normal pancreatic specimen, as there were no ducts present to assess (supplementary Figure 2A, available at Carcinogenesis Online). In the remaining two cases, TbRIII expression in tumor and normal tissue remained the same (supplementary Figure 2A, available at Carcinogenesis Online). Overall, these data suggest that there is loss of TbRIII protein expression during pancreatic carcinogenesis. To investigate whether there was also loss of TbRIII expression at the mRNA level, we analyzed a cDNA array containing seven human pancreatic cancer samples with matched normal controls (supplementary Figure 2B, available at Carcinogenesis Online). TbRIII mRNA levels were reduced in pancreatic cancer tissue, an average of 2.64 0.49-fold relative to matched normal control tissue, with six out of seven (86%) of the pancreatic adenocarcinoma specimens exhibiting loss of expression. To confirm and extend these results, we analyzed the previously published gene profiling studies, publicly available through the Oncomine Gene Profiling Database (34,35), for relative TbRIII expression in normal and tumor specimens. This analysis confirmed that TbRIII mRNA levels were decreased in pancreatic tumors, as the median expression value for TbRIII mRNA was significantly lower (P , 0.05) in pancreatic adenocarcinomas compared with normal pancreatic tissue (supplementary Figure 2C, available at Carcinogenesis Online). Taken together, our studies support the loss of TbRIII expression during pancreatic cancer progression, with specific effects on EMT-associated motility and invasiveness. Discussion Here we demonstrate, for the first time, that loss of TbRIII expression occurs during pancreatic carcinogenesis and during TGF-b-induced EMT in a pancreatic cancer model. Loss of TbRIII expression does not appear to be essential for induction of EMT, but instead appears to be required for the increased motility and invasiveness associated with EMT (Figure 3). Before EMT, PANC-1 cells have high cell surface TbRIII expression (Figure 2) and produce abundant amounts of sTbRIII (Figures 2 and 5). Although sTbRIII has been best characterized as an antagonist of TGF-b signaling, this does not appear to be the mechanism by which sTbRIII partially contributes to decreased motility and invasiveness, as PANC-1 cells lose TGF-b responsiveness independently of loss of TbRIII/sTbRIII expression (Figure 4). sTbRIII could potentially suppress EMT-associated invasiveness through sequestration of other ligands, as most coreceptors are promiscuous and TbRIII has been demonstrated to bind inhibin (41) and basic fibroblast growth factor (42) (Figure 6B). Early during TGF-binduced EMT (i.e. hours), shedding of TbRIII increases, which reduces TbRIII cell surface levels and transiently increases sTbRIII levels (Figure 2D). Reduction of TbRIII and ultimately sTbRIII expression is then maintained (over days) by repression of TbRIII mRNA expression by TGF-b (Figure 2A and E). After EMT, in the absence of cell surface TbRIII or sTbRIII, the PANC-1 cells exhibit increased motility and invasiveness (Figure 3). This increase in motility and invasiveness can be suppressed by restoring expression of TbRIII without suppressing TGF-b-induced EMT (Figure 3). In a reciprocal manner, shRNA-mediated decreases in TbRIII expression increase motility, without altering EMT (Figure 3). Importantly, these studies dissociate some of the markers of the EMT phenotype (i.e. induction of Snail and loss of E-cadherin) from the functional consequences of EMT, namely increased motility and invasiveness (Figure 6B). Indeed, we have observed that TbRIII can also suppress the migration of L3.6p pancreatic cancer cells, which do not undergo EMT (data not shown), and that TbRIII can suppress the migration and invasiveness of breast (15) and prostate cancer cells (16) in the absence of EMT. Although sTbRIII does partially mediate the effects of TbRIII on motility and invasiveness, the mechanisms by which TbRIII and sTbRIII inhibit migration and invasion in these diverse cancer models are currently being investigated. TbRIII has previously been implicated as an important EMT regulator during developmental processes. During chick-heart development, TbRIII is required for the EMT necessary for formation of the valves and septa, and exogenous TbRIII expression is sufficient to induce TGF-b-responsive EMT in non-competent cardiac ventricular cells (10). Conversely, specifically blocking TGF-b binding to TbRIII prevents the TGF-b-induced EMT in cardiac endothelial cells (10). In addition, TbRIII is required for the EMT during palatal fusion, as siRNA-mediated reduction of TbRIII levels in palatal shelve cultures abrogates their ability to undergo EMT and fuse (11,12). Several studies have suggested a role for TbRIII in regulating carcinogenic EMT, with down-regulation of TbRIII mRNA expression occurring during EMT in breast and skin cancer cell line models (43,44). The current study provides the first evidence for a functional role of TbRIII in carcinogenic EMT; specifically regulation of EMT-associated motility and invasiveness, without altering loss of E-cadherin expression and localization. Interestingly, a recent study demonstrated that shRNA-mediated decreases in TbRIII expression in the normal murine mammary epithelial cell line, NMuMG, induced loss of Ecadherin and increased motility and invasiveness (45). Whether TbRIII also specifically regulates motility and invasion in breast and skin cancer models of EMT, as well as in developmental models of EMT is currently under investigation. TGF-b has dichotomous effects on epithelial and mesenchymal cells, inhibiting the proliferation of epithelial cells while stimulating the proliferation of mesenchymal cells. TGF-b also has dichotomous tumor suppressor and tumor promoter effects in most human cancers, and altered responsiveness during carcinogenic EMT has been suggested as a potential mechanism for this paradoxical role (4). Here, we demonstrate that there are opposing effects of TGF-b on Smad2 and p38 phosphorylation before and after EMT in the PANC-1 pancreatic cancer model, with PANC-1 cells being TGF-b responsive prior to EMT, but TGF-b unresponsive after EMT (Figure 4C). In addition, when examining TGF-b-mediated inhibition of proliferation, we observed a similar phenomenon: PANC-1 cells were TGF-b responsive prior to EMT, but TGF-b unresponsive after EMT (data not shown). Both before and after EMT, expression of exogenous TbRIII resulted in further suppression of Smad2 and p38 phosphorylation, an effect which could be due to increased sTbRIII production (Figure 5A). However, as restoring TbRIII expression was unable to restore TGF-b responsiveness in either case, the effects of TbRIII on motility and invasiveness appear to be independent of effects on TGF-b signaling. These studies support a fundamental alteration in TGF-b responsiveness during carcinogenic EMT, which may help explain the dichotomous roles of TGF-b during cancer progression. The mechanism of altered TGF-b responsiveness during EMT in the PANC-1 model is currently under investigation. In the current study, we demonstrate that sTbRIII can partially suppress motility and invasion, indicating that ectodomain shedding is a critical regulator of TbRIII function. We have previously demonstrated that sTbRIII is important in suppressing the tumorigenic effects of TbRIII during breast cancer progression (15). As sTbRIII exerts important anti-motility and -invasive effects, insight into mechanisms regulating the ectodomain shedding and generation of sTbRIII is needed, including identification of the proteases responsible for TbRIII shedding. Overexpression of membrane tethered matrix metalloproteinase 1 is able to increase shedding of TbRIII (46). As expression of matrix metalloproteinases increase during EMT (3), matrix metalloproteinases are prime candidates for mediating TbRIII shedding. We are currently investigating the mechanisms regulating the ectodomain shedding of TbRIII. TbRIII expression has been examined in both pancreatic cancer cell lines and human adenocarcinomas at the mRNA level with conflicting results. Friess et al. (47) initially reported no loss of TbRIII expression in human pancreatic cancer specimens as assessed by northern blot analysis. In contrast, Venkatasubbarao et al. (48) analyzed mRNA levels by RTPCR and found trace or no TbRIII expression in four of six pancreatic cancer cell lines, including PANC-1, and in 10 of 26 (38%) pancreatic adenocarcinomas. In neither study were comparisons made between matched normal pancreas and pancreatic tumor to evaluate whether there was a decrease in expression and neither study examined protein expression. In the present study, we provide evidence that there is loss of TbRIII expression in pancreatic adenocarcinoma specimens compared with their normal matched counterparts at both the mRNA and protein levels. We also demonstrate that loss of TbRIII increases as the tumor grade increases, suggesting loss of TbRIII expression as a critical component of pancreatic carcinogenesis. In addition, we demonstrate that TbRIII is highly expressed in normal pancreatic ductal cells, where pancreatic adenocarcinoma is thought to arise (40). Our studies suggest that one function of TbRIII in these ductal cells may be to maintain their epithelial character and suppress motility and invasion, with loss of TbRIII expression during pancreatic cancer progression facilitating the increased invasion and metastasis observed in human pancreatic cancer. As 90% of pancreatic cancer patients die from metastases, manipulating TbRIII or sTbRIII levels in pancreatic cancer patients may provide an important therapeutic tool to suppress metastases in these patients. The current study suggests that TbRIII may function as a tumor suppressor in pancreatic cancer. Consistent with this role as a tumor suppressor, the genomic locus for TbRIII on chromosome 1p is deleted in 49% of human pancreatic cancer specimens (4952). Our laboratory has previously determined that TbRIII exerts a tumorsuppressing role in breast, prostate, and ovarian cancer (1517). In addition, Copland et al. (53) demonstrated significant loss of TbRIII expression in renal cell carcinoma. These studies, in addition to the present study, suggest a broad role for TbRIII as a tumor suppressor in epithelial-derived malignancies. Whether the effects of TbRIII on EMT-induced motility and invasion are operational in these other cancers warrants further investigation. Supplementary material Supplementary Figures 1 and 2 and the colour version of Figures 3, 4 and 6 can be found at http://carcin.oxfordjournals.org/. National Institutes of Health/National Cancer Institute (R01CA106307) to G.C.B. Acknowledgements The authors would like to thank Tam How for her technical assistance in generating the TbRIII adenovirus constructs, Dr P.J.Caseys laboratory for generously providing the Green Fluorescent Protein adenovirus construct and Dr F.Lopez-Casillas for generously providing the sTbRIII vector. Conflict of Interest Statement: None declared.


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Kelly J. Gordon, Mei Dong, Elizabeth M. Chislock, Timothy A. Fields, Gerard C. Blobe. Loss of type III transforming growth factor β receptor expression increases motility and invasiveness associated with epithelial to mesenchymal transition during pancreatic cancer progression, Carcinogenesis, 2008, 252-262, DOI: 10.1093/carcin/bgm249