The use of cystatin C to inhibit epithelial–mesenchymal transition and morphological transformation stimulated by transforming growth factor-β
© Sokol et al.; licensee BioMed Central Ltd. 2005
Received: 31 August 2004
Accepted: 26 July 2005
Published: 23 August 2005
Transforming growth factor-β (TGF-β) is a potent suppressor of mammary epithelial cell (MEC) proliferation and is thus an inhibitor of mammary tumor formation. Malignant MECs typically evolve resistance to TGF-β-mediated growth arrest, enhancing their proliferation, invasion, and metastasis when stimulated by TGF-β. Recent findings suggest that therapeutics designed to antagonize TGF-β signaling may alleviate breast cancer progression, thereby improving the prognosis and treatment of breast cancer patients. We identified the cysteine protease inhibitor cystatin C (CystC) as a novel TGF-β type II receptor antagonist that inhibits TGF-β binding and signaling in normal and cancer cells. We hypothesized that the oncogenic activities of TGF-β, particularly its stimulation of mammary epithelial–mesenchymal transition (EMT), can be prevented by CystC.
Retroviral infection was used to constitutively express CystC or a CystC mutant impaired in its ability to inhibit cathepsin protease activity (namely Δ14CystC) in murine NMuMG MECs and in normal rat kidney (NRK) fibroblasts. The effect of recombinant CystC administration or CystC expression on TGF-β stimulation of NMuMG cell EMT in vitro was determined with immunofluorescence to monitor rearrangements of actin cytoskeletal architecture and E-cadherin expression. Soft-agar growth assays were performed to determine the effectiveness of CystC in preventing TGF-β stimulation of morphological transformation and anchorage-independent growth in NRK fibroblasts. Matrigel invasion assays were performed to determine the ability of CystC to inhibit NMuMG and NRK motility stimulated by TGF-β.
CystC and Δ14CystC both inhibited NMuMG cell EMT and invasion stimulated by TGF-β by preventing actin cytoskeletal rearrangements and E-cadherin downregulation. Moreover, both CystC molecules completely antagonized TGF-β-mediated morphological transformation and anchorage-independent growth of NRK cells, and inhibited their invasion through synthetic basement membranes. Both CystC and Δ14CystC also inhibited TGF-β signaling in two tumorigenic human breast cancer cell lines.
Our findings show that TGF-β stimulation of initiating metastatic events, including decreased cell polarization, reduced cell–cell contact, and elevated cell invasion and migration, are prevented by CystC treatment. Our findings also suggest that the future development of CystC or its peptide mimetics hold the potential to improve the therapeutic response of human breast cancers regulated by TGF-β.
Oncogenic epithelial–mesenchymal transitions (EMTs) comprise a complex array of gene expression and repression that elicits tumor metastasis in localized carcinomas [1, 2]. The acquisition of metastatic phenotypes by dedifferentiated tumors is the most lethal facet of cancer and is the leading cause of cancer-related death [3, 4]. Transforming growth factor-β (TGF-β) is a powerful tumor suppressor that normally represses these processes by prohibiting epithelial cell proliferation, and by creating a cell microenvironment that inhibits epithelial cell motility, invasion, and metastasis [5, 6]. Carcinogenesis often subverts the tumor-suppressing function of TGF-β, thereby endowing TGF-β with oncogenic activities that promote the growth and spread of developing tumors, including the initiation and stabilization of tumor EMT [1, 2, 5–7]. The duality of TGF-β in both suppressing and promoting cancer development was observed originally with transgenic TGF-β1 expression in mouse keratinocytes, which initially suppressed benign skin tumor formation before promoting malignant conversion and spindle cell carcinoma generation . More recently, TGF-β signaling was shown to inhibit the tumorigenicity of normal, premalignant, and malignant breast epithelial cells, while stimulating that of highly invasive and metastatic breast cancer cells . Fundamental gaps exist in our knowledge of how malignant cells overcome the cytostatic actions of TGF-β and of how TGF-β stimulates the progression of developing tumors. Indeed, these knowledge gaps have prevented science and medicine from developing treatments effective in antagonizing TGF-β oncogenicity in progressing cancers, particularly those of the breast.
The ability of TGF-β to induce cancer growth and metastasis suggests that developing therapeutics to antagonize and/or circumvent TGF-β signaling may prove effective in treating metastatic malignancies, perhaps by preventing the stimulation of EMT by TGF-β. Cystatin C (CystC) is a small, ubiquitously expressed cysteine protease inhibitor found in nearly all bodily fluids [10, 11]. By inactivating cathepsin protease activities, CystC reduces bone resorption, neutrophil chemotaxis, and tissue inflammation, and also inhibits cancer cell invasion . We showed recently that CystC not only inhibits cathepsin-mediated invasion but also antagonizes TGF-β signaling in normal and cancer cells by interacting physically with the TGF-β type II receptor (TβR-II), thereby preventing TGF-β binding . Thus, CystC is a novel TβR-II antagonist that may prove useful in blocking the oncogenic activities of TGF-β, particularly its ability to stimulate EMT. In the present study we tested this hypothesis by measuring the ability of CystC to antagonize the oncogenic activities of TGF-β in two established in vitro models of cancer progression: first, EMT of normal murine NMuMG mammary epithelial cells (MECs), and second, morphological transformation and anchorage-independent growth of normal rat kidney (NRK) fibroblasts. We show that CystC significantly decreased TGF-β stimulation of EMT and morphological transformation in mammary epithelial cells and kidney fibroblasts, respectively. We show further that CystC significantly antagonized TGF-β signaling in two tumorigenic human breast cancer cell lines. Thus, by antagonizing TGF-β signaling and preventing its stimulation of EMT, CystC may represent a novel TGF-β chemopreventive agent effective in neutralizing the tumor-promoting activities of TGF-β and its stimulation of tumor metastasis.
Materials and methods
Recombinant CystC expression and purification
The synthesis of bacterial expression vectors encoding human CystC or Δ14CystC fused to the carboxy terminus of glutathione S-transferase (GST), and their purification from transformed Escherichia coli, were described previously .
Retroviral CystC expression
The creation of bicistronic retroviral vectors (namely pMSCV-IRES–GFP, where GFP stands for green fluorescent protein) encoding human CystC or Δ14CystC was described previously . Mouse NMuMG, rat NRK kidney fibroblasts, and human MDA-MB-231 and MCF10A-CA1a breast cancer cells were infected overnight with control (pMSCV-IRES–GFP), CystC, or Δ14CystC retroviral supernatants produced by EcoPac2 retroviral packaging cells (Clontech) as described . Cells expressing GFP were isolated and collected 48 hours later on a MoFlo cell sorter (Cytomation), and were subsequently expanded to yield stable polyclonal populations of control, CystC-expressing, or Δ14CystC-expressing cells. The expression and secretion of recombinant CystC proteins by individual infected cell lines were monitored by immunoblotting conditioned medium with anti-CystC antibodies as described .
The ability of TGF-β to alter actin cytoskeletal architecture and E-cadherin expression was monitored essentially as described . In brief, control, CystC-expressing, or Δ14CystC-expressing NMuMG cells were allowed to adhere overnight to glass coverslips in 24-well plates (50,000 cells per well). The cells were stimulated the following day with TGF-β1 (5 ng/ml) for 0 to 36 hours at 37°C. In some experiments, control NMuMG cells were stimulated with TGF-β1 in the absence or presence of 10 μg/ml recombinant GST, GST–CystC, or GST–Δ14CystC. On completion of agonist stimulation, the cells were washed in ice-cold PBS and immediately fixed in 3.7% formaldehyde. After extensive washing in PBS, the cells were blocked in PBS supplemented with 1.5% FBS, followed by incubation with rhodamine–phalloidin (0.25 μM). Alternatively, the cells were blocked in goat γ-globulin (200 μg/ml; Jackson Immunoresearch) before the detection of E-cadherin by sequential incubations with monoclonal anti-E-cadherin (1:50 dilution; BD Bioscience), followed by biotinylated goat anti-mouse antibody (5 μg/ml; Jackson Immunoresearch), and finally by Alexa-streptavidin (1.2 μg/ml; Molecular Probes). Images were captured on a Nikon Diaphot microscope.
The ability of TGF-β to alter E-cadherin expression was also monitored by immunoprecipitating E-cadherin from buffer H/Triton (1.5 mM EGTA, 50 mM β-glycerophosphate, 1 mM DTT, 0.2 mM Na3VO4, 1 mM benzamidine, 10 μg/ml Leupeptin, 10 μg/ml aprotinin, and 1% Triton X-100; pH 7.3) . NMuMG whole cell extracts (250 μg per tube), followed by immunoblotting with anti-E-cadherin antibodies (1:1000 dilution). In other experiments, an aliquot of NMuMG whole cell extract was removed before immunoprecipitation of E-cadherin and used to control for differences in protein loading by immunoblotting for extracellular signal-related kinase 1/2. Both assay protocols yielded similar results.
Cell biological assays
The effect CystC or Δ14CystC on various MECs (such as NMuMG, MCF10A-CA1a, or MDA-MB-231) or NRK cell activities was determined as follows: first, cell proliferation with a [3H]thymidine incorporation assay as described [14, 15]; second, cell invasion with a modified Boyden-chamber assay and Matrigel matrices as described [12, 15]; and third, gene expression with pSBE-luciferase and p3TP-luciferase reporter gene assays as described [12, 14].
In addition, the effect of recombinant GST, CystC, or Δ14CystC on Smad2 phosphorylation stimulated by TGF-β was determined by allowing MDA-MB-231 cells (100,000 cells per well) to adhere overnight to 24-well plates. The following morning, the cells were washed twice in ice-cold PBS and incubated in serum-free medium supplemented with 25 μg/ml recombinant GST, CystC, or Δ14CystC for 2 hours at 37°C. Afterward, the cells were stimulated with TGF-β1 (1 ng/ml) for 30 min at 37°C, and subsequently were subjected to phospho-Smad2 immunoblot analyses as described .
Soft agar assay
The growth of NRK cells in soft agar was performed as described . In brief, duplicate cultures of control, CystC-expressing, or Δ14CystC-expressing NRK cells (10,000 cells per plate) were grown in 0.3% agar on a cushion of 0.6% agar in 35 mm plates. NRK cell growth in the absence or presence of 5 ng/ml TGF-β1 was allowed to proceed for 7 days, whereupon the number of colonies formed was quantified under a light microscope.
CystC prevents TGF-β stimulation of EMT in NMuMG cells
The importance of EMT in promoting cancer progression and tumor metastasis is becoming increasingly apparent [1, 2]. Although the formation and growth of early-stage tumors is normally suppressed by TGF-β, cancer progression typically enables TGF-β to stimulate the growth and metastasis of late-stage tumors, in part through its induction of EMT . We recently identified the cysteine protease inhibitor CystC as a novel TβR-II antagonist . Indeed, the physical association of CystC with TβR-II not only prevented TGF-β binding and, consequently, TGF-β signaling in normal and cancer cells, but it also inhibited their invasion through synthetic basement membranes . Collectively, these findings led us to propose CystC as a novel chemopreventive agent capable of antagonizing the tumor-promoting activities of TGF-β in late-stage tumors, particularly the ability of TGF-β to induce EMT.
In contrast to the situation in developing tissues, inappropriate induction of EMT by TGF-β in adult tissues enhances tumorigenesis [2, 19]. TGF-β stimulation of EMT is studied routinely in murine NMuMG MECs, which readily undergo EMT when treated with TGF-β [13, 20, 21]. Unstimulated NMuMG cells exhibited typical epithelial cuboidal morphology characterized by strong cortical and diffuse cytoplasmic actin staining (Fig. 1b). In response to TGF-β, NMuMG cells undergo transition into fibroblasts and exhibit distinct actin stress fibers emanating from focal adhesions (Fig. 1b). This NMuMG cell response to TGF-β was prevented by the overexpression of either CystC or Δ14CystC (Fig. 1b). Similarly, treatment of NMuMG cells with recombinant CystC or Δ14CystC prevented TGF-β stimulation of actin cytoskeletal reorganization at 8 hours, and significantly reduced this response at 24 hours (Fig. 1c).
CystC inhibits TGF-β signaling in human breast cancer cells
CystC prevents stimulation of morphological transformation in NRK cells by TGF-β
TGF-β is widely expressed during development to regulate the interactions between epithelial and mesenchymal cells, particularly those in the lung, kidney, and mammary gland. Inappropriate reactivation of EMT during tumorigenesis is now recognized as an important process necessary for the acquisition of invasive and metastatic phenotypes by tumors [1, 2]. By cooperating with oncogenes and growth factors, TGF-β potently induces EMT and serves to stabilize this transition by means of autocrine signaling. Moreover, these events seem to underlie the oncogenic activities of TGF-β and its ability to promote cancer progression [20, 24–26]. A comprehensive understanding of how TGF-β both suppresses and promotes tumorigenesis remains an unknown and fundamental question that directly affects our ability to effectively target the TGF-β signaling system during the treatment of human malignancies, particularly those of the breast. Indeed, solving this paradox remains the most important aspect of the biological and pathological actions of this multifunctional cytokine.
Despite recent advances in understanding the molecular mechanisms underlying EMT, the question of how to prevent this process effectively in response to TGF-β remains unanswered. We recently discovered that CystC antagonizes TGF-β signaling in normal and cancer cells by interacting physically with TβR-II, thereby preventing TGF-β binding . Importantly, we demonstrated the effectiveness of CystC in inhibiting the invasion of cancer cells and the TGF-β-stimulated invasion of fibroblasts . Because EMT is necessary for the acquisition of invasive and metastatic phenotypes by cancer cells, and because CystC inhibited TGF-β-stimulated invasion, we proposed CystC as a potential antagonist of EMT stimulated by TGF-β.
Accordingly, in this study we show that CystC and Δ14CystC both prevent EMT and its associated increase in MEC motility (Figs 1, 2, 3), as well as antagonizing TGF-β signaling in human breast cancer cells (Figs 4 and 5). We further show for the first time that these CystC molecules inhibit TGF-β signaling in NRK fibroblasts, thus preventing their morphological transformation and invasion through synthetic basement membranes (Fig. 6). Although our understanding of CystC function in regulating TGF-β signaling is in its infancy, our findings suggest that this protease inhibitor might provide an innovative model for the development of novel TβR-II antagonists designed to combat the stimulation of tumor progression and EMT by TGF-β. We further propose that the chemopreventive effectiveness of CystC will be potentiated by its inhibition of cathepsin B-mediated invasion and metastasis [27–30] and its inhibition of the cathepsin B-mediated activation of latent TGF-β [31–33], which co-localizes with cathepsin B to the invading face of malignant tumors [34–37]. Moreover, CystC-mediated cathepsin B inactivation will reduce the activity of the urokinase plasminogen system, which enhances tumor cell extracellular matrix degradation, as well as growth factor and latent TGF-β activation . Cumulatively, the chemopreventive activities of CystC will antagonize cancer cell responses to TGF-β by inhibiting TGF-β binding  and by reducing TGF-β bioavailability within tumor microenvironments, thereby alleviating the stimulation of EMT and tumor metastasis in late-stage tumors by TGF-β.
Molecular dissection of TGF-β signaling systems necessary for its induction of EMT has clearly established a role for Smad2/3 in mediating EMT, particularly when coupled with signals emanating from oncogenic Ras [13, 39, 40]. However, Smad2/3-independent signaling has also been implicated in TGF-β stimulation of EMT. For instance, TGF-β stimulates EMT in cancers of the breast and other tissues by activating phosphoinositide 3-kinase, Akt, RhoA, p160(ROCK), and p38 mitogen-activated protein (MAP) kinase [40–44]. In addition, EMT in TGF-β-treated MECs is abrogated by measures that inhibit β1 integrin activity , thus establishing the necessity of β1 integrin expression for EMT stimulated by TGF-β. Finally, by repressing Id2 and Id3 expression , inducing Snail and SIP1 expression , and stimulating nuclear factor-κB activity , TGF-β regulates transcription factor activity operant in mediating the transition from epithelial to mesenchymal cell markers. We show that CystC inhibits the stimulation of Smad2 phosphorylation by TGF-β and the subsequent induction of reporter gene expression in normal and cancer MECs. Thus, reduced Smad2/3 signaling mediated by CystC probably underlies part of its ability to inhibit EMT stimulated by TGF-β. Future studies need to address the role of CystC in regulating Smad2/3-independent signaling stimulated by TGF-β, as well as determining their contribution in preventing the oncogenic activities of TGF-β in human breast cancer cells.
Finally, we were quite surprised to find that CystC, despite its ability to inhibit Smad2/3 signaling, failed to alter the growth-suppressing activities of TGF-β. Although the molecular mechanism(s) underlying this unexpected CystC activity remains to be elucidated, our findings suggest that CystC does not function to abrogate all TGF-β signaling, but may instead specifically alter and/or modulate certain aspects of TGF-β signaling when complexed to TβR-II. In support of this supposition, we find that activation of MAP kinases and Akt by TGF-β requires high cytokine concentrations, whereas that of Smad2/3 requires markedly lower cytokine concentrations (more than 10-fold lower; data not shown). Mechanistically, we propose that maximal stimulation of Smad2/3 by TGF-β requires minimal receptor occupancy (that is, large receptor reserves), whereas maximal stimulation of MAP kinases and AKT requires maximal receptor occupancy (that is, no receptor reserves). Thus, manipulations designed to antagonize the binding of TGF-β to its receptors might elicit disproportionate inhibition of TGF-β signaling systems, resulting in greater inhibition of Smad2/3-independent versus Smad2/3-dependent pathways. Future studies need to address this important question and determine which TGF-β signaling systems are preferentially inhibited by the formation of CystC–TβR-II complexes in normal and cancerous MECs.
Here we have shown the effectiveness of CystC in inhibiting MEC EMT and fibroblast morphological transformation stimulated by TGF-β, and in antagonizing TGF-β signaling in human breast cancer cells. The ability of CystC to inhibit TGF-β signaling in MECs occurs independently of its inactivation of cathepsin protease activity, presumably by means of CystC–TβR-II complex formation and the prevention of TGF-β binding . We suggest that CystC or its peptide mimetics ultimately hold the potential to improve the therapeutic response of human malignancies regulated by TGF-β, particularly cancers of the breast. Experiments designed to test this hypothesis in animal models of TGF-β oncogenicity and to identify CystC determinants necessary in mediating TGF-β antagonism are currently underway.
green fluorescent protein
mammary epithelial cell
normal rat kidney
transforming growth factor-β
TGF-β type II receptor.
TGF-β1 was generously provided by R&D Systems Inc. Members of the Schiemann Laboratory are thanked for critical reading of the manuscript. We also thank William Townend, Shirley Sobus, and Joshua Loomis for expertise and help provided on studies performed in the Cytometry Core Facility at the National Jewish Medical and Research Center. Support was provided in part by the National Institutes of Health (CA095519 and CA114039), the Elsa U. Pardee Foundation, and the Cancer League of Colorado to WPS.
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