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Cell, Tumor, and Stem Cell Biology |
1 Renal Laboratory, Imperial College London, Hammersmith Campus; 2 Department of Urology, Guy's Hospital, London Bridge, London, United Kingdom; 3 Servicio de Immunología, Hospital de la Princesa, Universidad Autónoma de Madrid, Madrid, Spain; and 4 Henry Wellcome Building of Genomic Medicine, Oxford, United Kingdom
Requests for reprints: Patrick H. Maxwell, Renal Laboratory, Imperial College London, Hammersmith Campus, Du Cane Road, London W12 0NN, United Kingdom. Phone: 44-20-8383-8594; Fax: 44-20-8383-2062; E-mail: p.maxwell{at}imperial.ac.uk.
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Abstract |
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(HIF-) subunits. We show that HIF activation is necessary and sufficient to suppress E-cadherin in renal cancer cells. Given the fundamental role of E-cadherin in controlling epithelial behavior, our findings give insight into how VHL inactivation/HIF activation may lead to kidney cancer and also indicate a mechanism by which reduced oxygenation could alter E-cadherin expression in other cancers and influence normal homeostasis in other epithelia. (Cancer Res 2006; 66(7): 3567-75) |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionReferences |
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70% lifetime risk of clear cell renal cell carcinoma (CCRCC), multiple visceral cysts (affecting the kidney and also other organs), hemangioblastomas of the central nervous system and retina, and pheochromocytomas (1). In accordance with Knudson's two-hit model for tumor suppressor action, disease manifestations involve somatic mutation or inactivation of the remaining normal allele. The great majority of sporadic CCRCC, which is the commonest form of kidney cancer, also involve biallelic inactivation of VHL. The VHL gene is situated at 3p25 and encodes two protein products of apparent molecular weight of 19 and 30 kDa (pVHL19 and pVHL30, referred to collectively as pVHL), both of which suppress tumor growth of VHL-defective CCRCC lines in xenograft experiments (1).
The best characterized function of pVHL is its role in regulating cellular responses to oxygen, through its role in the oxygen-dependent inactivation of the transcription factor hypoxia-inducible factor (HIF; ref. 2). In normal cells, HIF plays a central role in coordinating changes in gene expression in response to alterations in oxygen supply (3). In most normal tissues in vivo and in vitro under standard cell culture conditions, activation of the HIF pathway is usually minimal. However, when oxygen tension is reduced or when VHL is inactivated, HIF becomes stabilized and activated, inducing the transcription of a wide range of target genes. In general, HIF activation promotes adaptation of the cell, organ, or organism to hypoxia, by modulating a range of processes that include increases in glucose transport, glycolysis, and angiogenesis (1–3). Oxygen-responsiveness of the HIF transcriptional complex is mediated through the HIF- subunits (the best characterized of which are HIF-1 and HIF-2), which are continuously destroyed in the presence of oxygen (4, 5). This is achieved by oxygen-dependent enzymatic hydroxylation of specific prolyl residues in the central part of the HIF- molecule, which then allows ubiquitination by a cognate E3 ligase complex in which pVHL acts as the recognition component (1–4). The enzymes responsible for HIF prolyl hydroxylation are a family of three dioxygenases termed PHD1, PHD2, and PHD3 (for prolyl hydroxylase domain-containing proteins; also known as EGLN 1-3 and HPH 1-3; ref. 4).
As a consequence of pVHL inactivation, the HIF pathway is constitutively active in both hereditary and sporadic pVHL-defective CCRCC. Expression of pVHL in otherwise defective CCRCC-derived cell lines restores normal degradation of HIF- subunits and also suppresses tumor growth in xenograft models (6). Importantly, overexpression of a modified HIF-2 molecule, which cannot be recognized by pVHL, is sufficient to override the tumor suppressor effects of VHL (6). Furthermore, knock down of HIF-2 in pVHL-negative CCRCC cell lines abrogates their tumorigenic potential (6). These observations show that suppression of HIF-2 is necessary and sufficient for pVHL's action as a tumor suppressor in fully transformed CCRCC cells. Current challenges are to understand (a) the key downstream targets of HIF involved in tumorigenicity, (b) the extent to which these HIF-mediated changes in fully transformed cancer cells reflect tumor initiation and progression in the normal renal epithelium, and (c) the role of a number of other reported functions of pVHL that are not related to HIF (1, 7–12).
Intercellular adhesion in the context of epithelia is mediated by specialized structures termed intercellular junctions, which include desmosomes and hemidesmosomes, "gap" or communicative junctions, and the adherens junction and tight junction (13–15). The adherens junction is mainly composed of transmembrane calcium-dependent glycoproteins called cadherins (15). E-cadherin is the classic epithelial cadherin, and by dimerizing with other E-cadherin molecules on adjacent cells, it seals the basolateral intercellular space, helping to maintain tissue integrity and architecture (15, 16). Besides this structural role, E-cadherin also exerts important effects on cell signaling, through the interaction of its cytoplasmic domain with the catenin family of proteins (15–17). Although -catenin connects E-cadherin to the actin cytoskeleton, 
-catenin and particularly ß-catenin have the ability to interact with the T-cell factor family of transcription factors, whose transcriptional activity they increase (17). By controlling the amount of ß-catenin available in the cytoplasm, E-cadherin can regulate the expression of critical genes involved in cell death/proliferation (e.g., cyclin D1 and c-myc). Taken together, it is thus not surprising that loss of E-cadherin expression is a hallmark feature of cancers of epithelial origin (13, 18, 19).
In this report, we show, by analysing very early microscopic lesions of pVHL inactivation in the kidneys of patients with VHL disease, that pVHL inactivation strikingly reduces E-cadherin expression in human kidney tubular cells in vivo. In vitro studies using pVHL-defective cell lines obtained from sporadic CCRCC tumors established that reexpression of VHL restores E-cadherin expression in fully transformed CCRCC cells. To define the underlying mechanism, we used several approaches that show that the effect of pVHL loss on E-cadherin expression is mediated via activation of HIF.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionReferences |
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Desferrioxamine and cobalt chloride (CoCl2) were purchased from Sigma (St. Louis, MO); dimethyl-oxalylglycine was purchased from Frontier Scientific (Logan, UT). For hypoxic experiments, cells were exposed to 1% oxygen for the indicated times using either a hypoxic workstation (INVIVO2 100, Ruskinn, Leeds, United Kingdom) or a hypoxic incubator Galaxy R (Biotech, Palo Alto, CA).
The antibodies used were E-cadherin (HECD1; Cancer Research UK), HIF-1 (clone 54; Transduction Labs, Lexington, KY), HIF-2 (p190b; Cancer Research UK), Glut1 (DAKO, Carpinteria, CA), -tubulin (Sigma), AQP2 (Neomarkers, Fremont, CA), GP200 (Sigma), and THP (Biogenesis, Kingston, NH). M75 antibody to carbonic anhydrase IX (CAIX) was generously provided by S. Pastorekova (Center of Molecular Medicine, Institute of Virology, Slovak Academy of Sciences, Bratislava, Slovak Republic); AntiVHL was a gift from Dr. K. Padgett (Novus Biologicals, Littleton, CO).
Clinical material and immunohistochemistry. Surgical samples, including six nephrectomy specimens from five subjects with VHL disease, were analyzed (Table 1 ). Specimens were formalin fixed and embedded in paraffin wax; 3-µm serial sections mounted on Snowcoat X-tra slides (Surgipath, Richmond, IL) were dewaxed in xylene and rehydrated using graded ethanol washes. For antigen retrieval, sections were immersed in preheated DAKO target retrieval solution (DAKO) and treated for 90 seconds in a pressure cooker (E-cadherin, AQP-2, and THP) or incubated for 5 minutes at room temperature with Proteinase K (DAKO; GP 200). Sections analyzed contained both tumor and adjacent normal renal parenchyma acting as an internal control; in addition, substitution of the primary antibody with antibody diluent was used as a negative control. Antigen/antibody complexes were detected using the Envision system (DAKO) according to the manufacturer's instructions. Sections were counterstained with hematoxylin for 30 seconds, dehydrated in graded ethanol washes, and mounted in DPX (Lamb, London, United Kingdom).
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Immunofluorescence microscopy. Cells were grown to confluence on coverslips in 24-well dishes, fixed for 10 minutes at –20°C with methanol, and washed with TBS. Alexa 488 conjugates (Molecular Probes, Eugene, OR) were used for visualization; coverslips were mounted using 4',6-diamidino-2-phenylindole containing Vectashield (Vector Laboratories, Burlingame, CA). Samples were analyzed using an immunofluorescence microscope (Olympus BX41) equipped with IMAGE PRO software.
Real-time reverse transcription-PCR. Cells were grown to confluence, and total cellular RNA was isolated using RNA Bee (Biogenesis). Total RNA (2 µg/20 µL reaction) was retrotranscribed using an avian myeloblastosis virus retrotranscription kit (Roche, Indianapolis, IN). PCR was carried out using an Opticon 2 machine (MJ Research, Waltham, MA). Analysis of each experimental sample was in duplicate or triplicate. All real-time reverse transcription-PCR (RT-PCR) data are given as a value normalized to the level of ß-actin expression in the same retrotranscription. ß-Actin expression was not significantly altered by hypoxia, dimethyl-oxalylglycine, desferrioxamine, or CoCl2, nor did it differ between VHL-negative RCC and RCC/VHL cell lines.
Snail mRNA was measured using an "On Demand" Taqman assay from Applied Biosystems (Foster City, CA).
ß-Actin, E-cadherin, PHD3, and vascular endothelial growth factor (VEGF) mRNA were measured using SYBR Green (AB gene) and the following primers: ß-actin, 5'-CCCAGAGCAAGAGAGG-3' (forward) and 5'-GTCCAGACGCAGGATG-3' (reverse); E-cadherin, 5'-GACAACAAGCCCGAATT-3' (forward) and 5'-GGAAACTCTCTCGGTCCA-3' (reverse); PHD3, 5'-GATGCTGAAGAAAGGGC-3' (forward) and 5'-CTGGCAAAGAGAGTATCTG-3' (reverse); VEGF, 5'-TGCCAAGTGGTCCCAG-3' (forward) and 5'-GTGAGGTTTGATCCGC-3' (reverse).
RNase protection assay. 32P-labeled riboprobes were generated for E-cadherin (nucleotides 3411-3551; accession no. NM_004360) and U6 small nuclear (U6sn) RNA (nucleotides 1-107; accession no. X01366). Radiolabeled riboprobes were protected from RNase digestion by parallel hybridization to 25 µg of total RNA for E-cadherin and 125 ng total RNA for U6sn at 60°C. These were combined after RNase digestion and analyzed on 8% polyacrylamide gels.
Small interfering RNA transfections. The concentration of small interfering RNA (siRNA) oligos was 100 nmol/L; for the combined HIF-1 and HIF-2 transfections, 50 nmol/L was used for each. Transfections were done in p60 culture dishes using LipofectAMINE 2000 (Invitrogen, San Diego, CA). Cells were transfected as a pool, and after 15 to 20 hours, were divided onto 24- or 6-well dishes or plated on coverslips. Cells were analyzed 3 to 5 days after transfection.
siRNA oligo sequences were HIF-1 (21), 5'-CUGAUGACCAGCAACUUGAdTdT-3' (sense) and 5'-UCAAGUUGCUGGUCAUCAGdTdT-3' (antisense); HIF-2 (21), 5'-CAGCAUCUUUGAUAGCAGUdTdT-3' (sense) and 5'-ACUGCUAUCAAAGAUGCUGdTdT-3' (antisense); Firefly luciferase (http://www.rockefeller.edu/labheads/tuschl/), 5'-CGUACGCGGAAUACUUCGAdTdT-3' (sense) and 5'-AAGCUAAAGGUACACAAUUdTdT-3' (antisense).
Infection of HIF retroviral vectors. Viral supernatants were prepared by transfecting the Phoenix packaging cell line (Orbigen, San Diego, CA) using LipofectAMINE 2000. After the initial transfection, Phoenix cells were grown at 32°C. The supernatant was collected and filtered (0.45 µm), then supplemented with 0.25 volume fresh medium with 7.5 µg/mL Polybrene (Sigma), and added to target cells which had been plated the day before on p100 dishes at 30% to 40% confluence. After 20 hours, cells were washed, and fresh media were added for 20 hours before performing a second round of infection.
Constitutively active forms of HIF-1 (carrying the following substitutions: P402A and P564A) or HIF-2 (substitutions: P405A and P531A) were cloned into pBMN-I-EGFP (22). Infection efficiency (as checked by flow cytometry) was 90% in each experiment, with comparable efficiency for empty vector and those encoding HIF.
pVHL retroviral vectors. pCMVR-Neo empty or expressing pVHL30, were a kind gift from Wilhelm Krek (Institute of Cell Biology, Zurich, Switzerland) and Alexander Hergovich (Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland; ref. 12). pCMVR-Neo containing pVHL19, pVHL V84L or pVHL L188V, were obtained by transferring inserts from pcDNA3 plasmids described previously (12, 23). Infections were done as above; cells were selected with G418.
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Results and Discussion |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionReferences |
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We first verified the reported distribution of E-cadherin in normal renal parenchyma. As expected, E-cadherin was observed exclusively in the distal part of the renal tubule, including the cortical thick ascending limb, the distal convoluted tubule, and the collecting duct (Fig. 1A ). Next, we examined E-cadherin and CAIX expression in serial sections of kidney material from patients with VHL disease (for clinical details and the number and characteristics of the early foci, see Table 1). Strikingly, the portions of the distal tubules that expressed CAIX showed a substantial decrease in E-cadherin (Fig. 1B, four lesions of different complexity are shown). Moreover, the CCRCC tumors found in these kidneys from VHL patients showed very little or no labeling for E-cadherin (Fig. 1C), which was also the case in samples obtained from sporadic CCRCC (Fig. 1C, the dotted line marks the limit between non affected renal parenchyma and the tumor). These experiments therefore show that in renal tubular epithelium loss of function of pVHL is associated with a marked decrease in the expression of E-cadherin.
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-subunits of HIF in the presence of oxygen (2). To examine whether the observed suppression of E-cadherin was mediated by HIF, we used several approaches. First, we studied the effects of expressing selected missense mutations of pVHL (pVHL V84L and pVHL L188V). These mutations are associated with isolated familial pheocromocytoma, referred to as type 2C VHL disease. We and others have shown that these mutant pVHL molecules regulate HIF normally (23, 26, 27). Introduction of these molecules provides a method for restoring HIF regulation without correcting all consequences of pVHL loss of function. RCC4 and RCC10 cells were infected with retrovirus coding for pVHL19, pVHL30, pVHL V84L, and L188V or empty vector. pVHL19, pVHL30, pVHL V84L and pVHL L188V down-regulated both HIF-1 and HIF-2 protein levels and increased E-cadherin expression in both RCC4 and RCC10 cells (Fig. 3A
). Restoration of E-cadherin expression was also verified by immunofluorescence microscopy and real-time RT-PCR analysis in RCC10-infected cells (Fig. 3B and C). The level of mRNA of PHD3 and VEGF was analyzed in parallel and showed the expected pattern for genes whose expression is increased by HIF activation.
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or HIF-2. These engineered HIF molecules contained mutations of the two prolyl residues, which are targets for oxygen-dependent hydroxylation and pVHL capture. Both HIF-1 and HIF-2 reduced expression of E-cadherin protein and mRNA. BNIP3 and PHD3, which are relatively selective targets for HIF-1 and HIF-2 (22, 28), were analyzed in parallel (Fig. 5B
). The effect of active HIF-2 on E-cadherin mRNA was somewhat more potent than the effect of active HIF-1 (Fig. 5B).
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or HIF-2; siRNA for firefly luciferase was used as a control. The efficacy and subunit selectivity of these duplexes was verified by Western blotting (Fig. 6A
). As we described in another recent study (22), inhibition of HIF-2 was associated with a consistent increment in HIF-1. Combined inhibition of both HIF-1 and HIF-2 was less efficient in down-regulating HIF-1 than HIF-2 (Fig. 6A). Inhibition of HIF-1 alone did not result in E-cadherin expression. Inhibition of HIF-2 resulted in detectable but rather variable expression of E-cadherin. Combined inhibition of HIF-1 and HIF-2 siRNA robustly increased the expression of E-cadherin as assessed by Western blotting (Fig. 6A), immunofluorescence (Fig. 6B), and real-time RT-PCR (Fig. 6C). Together, with the effects of expressing active HIF- molecules, these results would suggest that both HIF-1 and HIF-2 contribute to suppression of E-cadherin in pVHL-negative CCRCC.
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Several aspects of our study are of interest. First, the observation that E-cadherin is expressed by renal carcinoma cell lines when complemented with pVHL supports our recent data, suggesting that CCRCC may originate from the distal rather than the proximal tubule (24). Although a proximal tubular origin has generally been accepted (29), there is other evidence consistent with the tumors arising from the distal tubule (30). Identifying the correct cell type of origin is a prerequisite for any meaningful comparison of CCRCC with normal epithelium, because the functional specialization and the genes that are expressed are quite different in the distal and proximal tubule. A second significant aspect of our study is that it provides a potential mechanism by which loss of pVHL function could contribute to tumor initiation, evolution, and phenotype, based on the extensive evidence that down-regulation of E-cadherin promotes progression and invasion in multiple cancers. To date, E-cadherin has received little attention in clear cell renal cell carcinoma, at least in part because this molecule is not expressed in the normal proximal tubule. Interestingly, E-cadherin has been shown to be subject to loss of heterozygosity and methylation in some renal carcinoma cell lines (including 786-O cells; ref. 31), which would be consistent with expression in the parental cell type in the distal tubule and selection for loss of E-cadherin function during cancer evolution, both through pVHL inactivation/HIF activation and other additional genetic or epigenetic modifications. It is, however, noteworthy that pVHL suppresses tumor formation by 786-O cells (6), although 786-O/VHL cells do not express E-cadherin in cell culture (data not shown; ref. 32). This establishes, at least in this cell model, that restoration of E-cadherin expression is not essential for suppression of xenograft growth by pVHL in a fully transformed CCRCC cell line.
A remaining question is how HIF reduces E-cadherin expression. Inactivation of E-cadherin in cancer is generally due to either gene methylation or transcriptional silencing, the latter being achieved through binding of transcriptional repressors to specific DNA sequences (termed E-boxes) in the proximal E-cadherin promoter (13). This raises the possibility that HIF is inducing the expression of a repressor, which in turn decreases the transcription of E-cadherin mRNA. Multiple transcriptional repressors of E-cadherin have been described (13, 33–35), among which SNAI1 (SNAIL) is the best characterized. Using real-time RT-PCR, we examined the expression of SNAIL in VHL-negative RCC4, RCC10, and 786-O cell lines and their corresponding stable transfectants with pVHL. Interestingly, we found that SNAIL mRNA expression was increased by the absence of pVHL in all three CCRCC cell lines (Supplementary Fig. S1). This suggests that increased expression of SNAIL mRNA may be involved in down regulating E-cadherin in CCRCC, a possibility that we are currently investigating.
In summary, our study shows that pVHL inactivation leads to down-regulation of E-cadherin both in vivo and in vitro, and that these effects are at least in part mediated via HIF. Given the fact that the HIF response to hypoxia is seen in all cell types examined to date, our study raises the interesting possibility that E-cadherin expression in other settings besides renal epithelium and kidney cancer may also be influenced by HIF. If so, this possibility could offer insight into why mutations in E-cadherin are infrequent in cancers in general, because HIF activation is very common in solid tumors and may thus provide a reliable epigenetic route to down-regulating E-cadherin efficiently. It would also explain why expression of E-cadherin in tumors is frequently patchy (as it commonly happens with areas of hypoxia) rather than homogeneous. In this context, we note a recent study reporting that hypoxia reduces E-cadherin expression in ovarian cancer cells in vivo and in vitro (36). Finally, it is tempting to speculate that oxygen gradients could also be a general mechanism to modulate E-cadherin expression (e.g., during embryonic development and normal epithelial differentiation).
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Acknowledgments |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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Footnotes |
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M.A. Esteban and M.G.B. Tran contributed equally to this work.
Received 8/ 1/05. Revised 12/12/05. Accepted 1/16/06.
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References |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionReferences |
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is sufficient to suppress pVHL-defective tumor growth. PLoS Biol 2003;1:E83.[Medline] and HIF-2 under normoxic conditions in renal carcinoma cells by von Hippel-Lindau tumor suppressor gene loss of function. Oncogene 2000;19:5435–43.[CrossRef][Medline] versus Hif-2 in regulation of the transcriptional response to hypoxia. Cancer Res 2003;63:6130–4. regulation by disease-causing pVHL mutations correlate with patterns of tumourigenesis in von Hippel-Lindau disease. Hum Mol Genet 2001;10:1029–38.This article has been cited by other articles:
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 S. K. Harten, D. Shukla, R. Barod, A. Hergovich, M. S. Balda, K. Matter, M. A. Esteban, and P. H. Maxwell Regulation of Renal Epithelial Tight Junctions by the von Hippel-Lindau Tumor Suppressor Gene Involves Occludin and Claudin 1 and Is Independent of E-Cadherin Mol. Biol. Cell, February 1, 2009; 20(3): 1089 - 1101. [Abstract] [Full Text] [PDF]
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