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1 Cancer Research UK London Research Institute, London, United Kingdom; 2 Center for Molecular Medicine Cologne, University of Cologne, Institute of Pathology, Cologne, Germany; and Departments of 3 Dermatology and 4 Pathology, Columbia University, College of Physicians and Surgeons, New York, New York
Requests for reprints: Fiona M. Watt, Cancer Research UK Cambridge Research Institute, Li Ka Shing Centre, Robinson Way, Cambridge CB2 ORE, United Kingdom. Phone: 44-12-2340-4400; Fax: 44-12-2340-4199; E-mail: fiona.watt{at}cancer.org.uk.
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Abstract |
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NLef1 transgenic mice, which express NH2-terminally truncated Lef1 in the epidermal basal layer. Transgenic mice developed more tumors, more rapidly than littermate controls, even without exposure to tumor promoter. They developed sebaceous tumors, whereas controls developed squamous cell carcinomas. K14NLef1 epidermis failed to up-regulate p53 and p21 proteins during tumorigenesis or in response to UV irradiation, and this correlated with impaired p14ARF induction. We propose that LEF1 NH2-terminal mutations play a dual role in skin cancer, specifying tumor type by inhibiting Wnt signaling and acting as a tumor promoter by preventing induction of p53. [Cancer Res 2007;67(7):2916–21] |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Surprisingly, an association between tumors and inhibition of Wnt signaling has also been reported. Mutation or deletion of the NH2 terminus of Lef1 prevents ß-catenin binding. When N32Lef1 is expressed in the basal layer of the epidermis via the K14 promoter, mice (K14NLef1 transgenics) develop sebaceous tumors at high frequency (5). A high proportion of human sebaceous adenomas and sebeomas have double nucleotide substitutions in exon 1 of the LEF1 gene (6). These result in E45K and S61P amino acid substitutions in the NH2 terminus, which impair binding to ß-catenin and inhibit ß-catenin–dependent transcription.
At present, it is unclear how Lef1 mutation contributes to tumor formation. One possibility is that its role is solely to specify the differentiated characteristics of the tumor because ß-catenin signaling levels control lineage selection in normal epidermis (7–13). In K14NLef1 transgenics, hair follicles convert into epidermal cysts with sebocyte and interfollicular differentiation (5, 13). Another possibility is that, in addition to directing the differentiated characteristics of a tumor, Lef1 mutations increase epidermal susceptibility to tumor development. NLef1 and E45K + S61P LEF1, although unable to bind ß-catenin, retain other properties, such as the ability to bind Groucho corepressors and to increase expression of Indian hedgehog (6, 12), and these might positively contribute to tumorigenesis (e.g., by stimulating proliferation of sebocyte progenitors; ref. 12). In culture, NH2-terminally truncated Tcf4 blocks induction of p14ARF by ß-catenin (14). Thus, NLef1 could potentially prevent accumulation of the p53 tumor suppressor protein by preventing induction of ARF.
In the present report, we set out to test the hypothesis that deletion of the NH2 terminus of Lef1 both directs tumor type and stimulates tumor formation.
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Materials and Methods |
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NLef1 transgenic mice (founder line L; ref. 5) were maintained on a CBA x C57Bl/6 F1 background.
Tumor experiments. Seven-week-old female K14NLef1 transgenic mice and wild-type littermate mice were used (15). Typically, in each experiment, 20 wild-type and 20 transgenic mice received a subthreshold dose of carcinogen in 200 µL acetone [100 nmol 7,12-dimethylbenz(a)anthracene (DMBA) or 1.6 µmol benzo[a]pyrene (B[a]P); Sigma]. Wild-type mice received 12-O-tetradecanoylphorbol-13-acetate (TPA; 6 nmol in 200 µL acetone) thrice per week for 25 weeks, whereas transgenic mice were treated twice per week for 10 weeks because of their high sensitivity to TPA. Control groups of 20 mice were treated with DMBA or B[a]P and acetone, acetone and TPA, or acetone alone. Experiments were done twice and results obtained were similar. Scoring of tumors was carried out once a week for up to 50 weeks after tumor initiation. Bromodeoxyuridine (BrdUrd) labeling and tissue harvesting were done as previously described (15).
UV irradiation. Dorsal skin (4 cm2) of five K14NLef1 transgenic and five wild-type littermate controls were exposed to one dose of UVB (0.36 J/m2) or UVB and UVA (1 and 10 J/m2, respectively) using a UV 801 irradiator (Waldmann GmbH, Villingen-Schwenningen, Germany). UV-exposed and nonexposed skin was harvested 24 h later.
Immunohistochemistry. For Ki67, p53, p21, and ARF immunostaining, formalin-fixed tumor sections were deparaffinized and microwaved in Citra Plus antigen retrieval solution (Bio Genex) for 7 min and incubated for 15 min in the retrieval solution. Sections were blocked using 0.2% fish skin gelatin (Sigma) and probed with antibodies to Ki67 (Visionbiosystems; 1:100), mouse p53 (CM5, Novacastra; 1:500), ARF (5C3, Abcam, Cambridge, United Kingdom), or p21 (clone SX118, Becton Dickinson; 1:500). Staining was visualized using the ABC staining kit (Vector Laboratories). BrdUrd incorporation was detected as previously described (15).
DNA sequencing. Exon 1 and exon 2 of the Ha-Ras gene and exons 4 to 9 of p53 were amplified by PCR using the primer pairs described in Supplementary data.
Reverse transcription-PCR. RNA was isolated from skin and tumors using Tri-reagent (Helena BioSciences Ltd., Gateshead, United Kingdom) and transcribed into cDNA using Ready-to-go You-prime First-strand beads and oligo-d(T) primer (Amersham Biosciences, GE Healthcare). Primers are described in Supplementary data.
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Results |
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NLef1 increases sensitivity to chemical carcinogenesis and is a tumor promoter. K14NLef1 transgenic and wild-type littermate control mice were subjected to classic two-stage carcinogenesis protocols (Fig. 1
). The skin received one application of DMBA or B[a]P to induce Ha-Ras mutations (15) and repeated TPA treatments to stimulate tumor promotion.
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NLef1 transgenic mice treated with DMBA and TPA developed seven times as many tumors as their wild-type littermates (K14NLef1: 22 tumors per mouse; wild-type: 3 tumors per mouse; P < 0.001, Student's t test; Fig. 1A). Transgenic animals treated with B[a]P and TPA developed 10 times more tumors than control mice (K14NLef1: 16 tumors per mouse; wild-type: 1.6 tumors per mouse; P < 0.001; Fig. 1B). Of K14NLef1 transgenic mice, 100% developed tumors with either protocol, whereas the proportion of tumor-bearing wild-type mice was 90% in response to DMBA (Fig. 1C) and 65% in response to B[a]P (Fig. 1D). In addition, tumors developed much faster in K14NLef1 mice than in littermate controls (Fig. 1C and D). We conclude that K14NLef1 transgenic mice were much more sensitive to two-stage carcinogenesis than transgene-negative mice.
The classic controls in chemical carcinogenesis experiments are to treat animals with DMBA or TPA alone (16). As predicted, wild-type mice subjected to DMBA or TPA alone did not develop tumors (data not shown; ref. 15). TPA treatment did not affect the spontaneous tumor incidence (<1 tumor per mouse; 27% of mice >3 months old develop tumors; ref. 5) in K14NLef1 transgenics (data not shown; ref. 5). However, in contrast to wild-type mice, K14NLef1 transgenics treated with DMBA or B[a]P alone developed skin tumors; furthermore, the frequency and kinetics were similar whether transgenics received carcinogen alone or in combination with TPA (P = 0.388 for DMBA and TPA versus DMBA alone; P = 0.552 for B[a]P and TPA versus B[a]P alone; Fig. 1). We conclude that NLef1 functions as a tumor promoter, cooperating with Ha-Ras to induce tumor formation.
Mutations in Ha-Ras codon 61 (CAA
CTA), the signature DMBA induced lesion (16), were detected in 10 of 10 tumors of K14NLef1 mice treated with DMBA or DMBA and TPA. In contrast, no mutations in Ha-Ras exons 1 and 2 were found in five of five spontaneous K14NLef1 tumors. Thus, although NLef1 promotes development of tumors with Ha-Ras mutations, Ras mutations are not the underlying cause of the spontaneous tumors in K14NLef1 mice.
NLef1 determines the differentiated characteristics of tumors with Ha-Ras mutations. In response to DMBA and TPA, wild-type mice first develop benign papillomas, some of which progress (1% in Fig. 1) to malignant squamous cell carcinomas (15, 16). Papillomas and squamous cell carcinomas have characteristics of interfollicular epidermal differentiation, including accumulation of cornified layers (Fig. 2A
). In contrast, all of the tumors induced in K14NLef1 transgenic mice with DMBA or B[a]P ± TPA exhibited a high degree of sebocyte differentiation, resembling the spontaneous tumors that lack Ras mutations (Fig. 2B and C). Macroscopically, some of the K14NLef1 tumors consisted of a "head" on a stalk ("raised"), resembling wild-type papillomas, whereas others were flattened ("flat"). However, raised tumors were not precursors of flat tumors, and histologically they were indistinguishable (Fig. 2B and C). We conclude that NLef1 overrides the genetic program for squamous differentiation in tumors bearing Ha-Ras mutations.
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NLef1 tumors are distinguished by reduced p53 and p21 protein levels. We did not detect any significant difference in the number of S-phase (BrdUrd-positive) undifferentiated cells in wild-type and K14NLef1 tumors (Fig. 3A
; Supplementary Table S1). This led us to consider other potential mechanisms for the tumor-promoting effect of NLef1. In cell culture, activation of ß-catenin induces accumulation of transcriptionally active p53 (14, 17), which in turn could protect against neoplastic conversion (18). Because NLef1 blocks ß-catenin activation (5), we investigated whether it prevented up-regulation of p53 in skin tumors.
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Nß-cateninER transgenics (3) also displayed p53 accumulation (n = 2; Fig. 3B; Supplementary Table S1). In contrast, the sebaceous tumors (n = 13) that developed in K14NLef1 mice were either negative for p53 (n = 8) or had only a small number of positive cells (n = 5; Fig. 3B; Supplementary Table S1), regardless of whether they were spontaneous or induced by DMBA or B[a]P ± TPA. Sequencing exons 4 to 9 of p53 in six of six DMBA/TPA–induced K14NLef1 tumors revealed p53 to be wild-type.
Immunohistochemical staining revealed a strong correlation between expression of p53 and expression of the p53-responsive gene p21 (18). Wild-type papillomas and squamous cell carcinomas (n = 3) and K14Nß-cateninER hair follicle tumors (n = 2) stained positive for p21 protein (Fig. 3C; Supplementary Table S1). In contrast, K14NLef1 sebaceous tumors (n = 13) were negative for p21 (Fig. 3C; Supplementary Table S1). Our observations suggest that NLef1 prevents accumulation of transcriptionally active p53 (14, 17).
To determine whether lack of p53 protein reflected transcriptional or posttranslational regulation, we isolated RNA from tumors and unaffected skin of wild-type and K14NLef1 transgenic mice. We carried out reverse transcription-PCR (RT-PCR) with primers for p53, p21, and ARF, a protein that increases p53 protein stability (18). p53 and ARF mRNAs were undetectable in skin from wild-type and transgenic animals (Fig. 3D). p53 mRNA was readily detected in all tumors analyzed (Fig. 3D). However, wild-type and K14NLef1 tumors differed in ARF mRNA levels (Fig. 3D), with wild-type tumors expressing more (Fig. 3D). In wild-type tumors the ARF band was more intense than the p53 band, whereas in transgenic tumors the p53 band was more intense. p21 levels were higher in tumors than in unaffected skin, but there were no differences between wild-type and transgenic tumors (Fig. 3D).
The reduction in ARF expression was confirmed by immunohistochemistry. Two of two wild-type squamous cell carcinomas and one of one wild-type papilloma contained a large number of cells with ARF-positive nuclei (Fig. 3D). In contrast, ARF was undetectable in three of three spontaneous tumors and 13 of 16 transgenic tumors induced by DMBA or B[a]P (Fig. 3D), and in 3 of 16 transgenic tumors there were fewer than 10 ARF-positive nuclei per section (data not shown). Taken together, the results suggest that the lack of detectable p53 protein in K14NLef1 tumors reflects rapid degradation of the protein as a result of an impaired ability to up-regulate ARF.
Differential responsiveness of wild-type and NLef1 epidermis to UV radiation. Up-regulation of p53 is a well-documented response of mammalian epidermis to UV radiation (19, 20). If induction of p53 protein is indeed impaired by expression of N32Lef1, then the epidermis of transgenic mice should show increased susceptibility to UV irradiation. To test this, we exposed transgenic and wild-type mouse back skin to a combination of UVA and UVB (data not shown) or UVB alone and compared exposed and unexposed skin 24 h later (Fig. 4A–C
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NLef1 mice was dramatically altered, with regions of epidermal detachment from the dermis (Fig. 4A, arrows). UV irradiation increased apoptosis and proliferation in both wild-type and transgenic skin (Fig. 4B; data not shown). The fold increase in Ki67-positive cells was variable but generally higher in transgenic (mean, 2.6; range, 1.08–4.57) than in wild-type (mean, 1.3; range, 0.82–1.82) epidermis (five independent experiments).
UV irradiation resulted in strong nuclear accumulation of p53 in wild-type skin (Fig. 4C). In contrast, there were very few cells with nuclear p53 in UV-treated K14NLef1 skin (Fig. 4C). The percentage of p53-positive nuclei was >70% lower in K14NLef1 compared with wild-type epidermis (2.5% versus 9.0%; five independent experiments; Fig. 4D). Thus, the sensitivity of K14NLef1 epidermis to UV-induced damage correlates with a failure to up-regulate p53. Nuclear accumulation of ARF could not have been detected in skin from wild-type and K14NLef1 mice with or without UV treatment (Fig. 4C and data not shown).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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NLef1 transgenics, we now show that NH2-terminal deletion of Lef1 contributes to tumorigenesis in two ways. It specifies the type of tumor formed as a result of chemically induced Ras mutations and it acts as a tumor promoter.
Chemically induced tumors bearing Ras mutations in wild-type epidermis usually express markers of the interfollicular epidermal differentiation pathway (16). In contrast, chemically induced tumors in K14NLef1 mice contained differentiated sebocytes. N32Lef1 thus specifies tumor type independent of the presence (chemically induced) or absence (spontaneous) of Ras mutations. Our experiments do not distinguish between the alternative possibilities that tumor type depends on the target cell (sebocyte or interfollicular epidermal progenitor) or that a common target cell with multilineage differentiation potential (putative stem cell) forms different types of tumor in response to the specific signals it receives (16). However, it is striking that the types of tumor directed by N32Lef1 and stabilized ß-catenin reflect the differentiated lineages selected by normal epidermis in response to different levels of ß-catenin activation (8, 16).
UV-induced mutations in p53, resulting in elevated levels of the protein, are detected at high frequency in phenotypically normal epidermis and are common in skin tumors (20). Human sebaceous tumors express high levels of p53, consistent with sunlight-induced mutations, and these could potentially drive tumor formation, with mutant LEF1 solely specifying tumor type (6). However, our experiments show conclusively that NLef1 plays a positive role in tumorigenesis, in particular acting as a strong tumor promoter in mice treated with DMBA alone. Sequencing of K14NLef1 tumors did not reveal any p53 mutations and K14NLef1 tumors lacked detectable p53 protein. The discrepancy between the p53 status of human and mouse sebaceous tumors may be explained by the different time scales involved: the mouse tumors developed within weeks (Fig. 1), whereas the average age of humans with LEF1 mutant tumors was 70 years, allowing time for additional oncogenic changes such as accumulation of p53 mutations and failure of DNA mismatch repair (6).
Activation of p53 by oncoproteins occurs mainly via ARF, which binds to murine double minute-2 and thereby suppresses p53 ubiquitination and degradation (18). We found that K14NLef1 tumors had, like wild-type tumors, elevated p53 mRNA; however, the levels of ARF mRNA and protein were reduced. This provides in vivo validation of cell culture experiments showing that ARF transcription is blocked by NTcf4 (14, 17). We therefore propose that NLef1 specifies tumor type by preventing ß-catenin–dependent induction of hair follicle genes and acts as a tumor promoter by preventing accumulation of the tumor suppressor p53 (Fig. 4D).
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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.
We thank S. Broad, R. Rudling, J. Groeninger, B. Cross, A. Mowbray, and the Cancer Research UK Histopathology Unit for expert technical assistance; X. Lu and P. Jordan for advice; C. Lo Celso for skin samples; G.P. Marcuzzi for help with UV irradiation; and Manon Zweers for statistical analysis.
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Footnotes |
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Current address for F.M. Watt: CR-UK Cambridge Research Institute, Robinson Way, Cambridge CB2 0RE, United Kingdom.
Received 9/18/06. Revised 2/ 9/07. Accepted 2/15/07.
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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NLef1 in mouse epidermis results in differentiation of hair follicles into squamous epidermal cysts and formation of skin tumours. Development 2002;129:95–109.
3ß1, but not 2ß1, suppresses malignant conversion. Cancer Res 2001;61:5248–54.This article has been cited by other articles:
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 F.M. Watt and C.A. Collins Role of {beta}-catenin in Epidermal Stem Cell Expansion, Lineage Selection, and Cancer Cold Spring Harb Symp Quant Biol, November 6, 2008; (2008) sqb.2008.73.011v1. [Abstract] [PDF]
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