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Advances in Brief |
1 The Wistar Institute, Philadelphia, Pennsylvania; 2 Department of Oncology-Pathology, Cancer Center Karolinska, Karolinska Institute, Stockholm, Sweden; and 3 Department of Pathology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
| ABSTRACT |
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| Introduction |
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We have shown recently that the expression of another melanoma-associated receptor tyrosine kinase ligand, basic fibroblast growth factor (bFGF), along with UVB irradiation induced pigmented lesions in human skin (7) . Increased protein expression of bFGF was achieved by injection of an adenoviral vector (Ad5) containing the bFGF gene into human skin grafted onto severe combined immunodeficiency disease mice. The increased production of bFGF by fibroblasts in the dermis stimulated the melanocytes in the epidermis. Whereas bFGF alone induced hyperpigmentation and proliferation of the melanocytes, only the combination of bFGF expression and UVB irradiation of the skin led to early stage transformation of melanocytes (7) .
Here we tested whether a combined cutaneous expression of synergistic growth factors for melanocytes plays an etiologic role in melanomagenesis. We chose bFGF, the most important autocrine growth factor of melanoma, as well as stem cell factor (SCF) and endothelin-3 (ET-3). SCF and ET-3 are both essential for normal melanocyte development during embryogenesis, and support survival and growth of melanocytes synergistically (8, 9, 10) . Along with the expression of the three growth factors, the human skin grafts were exposed to UVB irradiation at an erythemal dose three times weekly.
| Materials and Methods |
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vß3 vitronectin receptor, Mel-CAM, melanotransferrin, chondroitin sulfate proteoglycan, and acetyl-GD3 (13)
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Adenoviral Vectors for bFGF, Green Fluorescent Protein, LacZ, ET-3, and SCF.
The adenoviral vector bFGF/Ad5 carrying the gene for the Mr 18,000 form of the bFGF protein has been described (12)
. The control adenoviral vectors LacZ/Ad5 and green fluorescent protein/Ad5 (Vector Core, University of Pennsylvania, Philadelphia, PA) induce expression of the reporter gene ß-galactosidase from Escheria coli and green fluorescent protein, respectively.
For construction of ET3/Ad5 and SCF/Ad5, total RNA from logarithmically growing human foreskin fibroblasts were isolated. Reverse transcription was performed with oligodeoxythymidylic acid primer and SuperScript II RnaseH- Reverse Transcriptase (Life Technologies, Inc., Rockville, MD). Primers for PCR were designed specifically for SCF and ET-3 as follows: SCF (forward) 5'-GATCGCAGCGCTGCCTTTCCTTATGAA-3'; SCF (reverse) 5'-TGCAACAGGGGGTAACATAAATGG-3'; ET-3 (forward) 5'-GCTCCGGCGCCTGATCTAGGTTCATGG-3'; ET-3 (reverse) 5'-CCTAAGGGGCTCCTTCCTGAAAGAGG-3'. The amplified PCR fragments were cloned into pCR 2.1 (Invitrogen, Carlsbad, CA) and sequence verified using automated fluorescence sequencing (ABI, Foster City, CA). The cDNA fragments were then cloned into pShuttle-CMV, orientations of which were checked by restriction mapping and recombined with pADEasy-1 in E. coli strain BJ5183 essentially as described (14) . The correct recombined clones were identified by restriction mapping, and the DNA was transfected into 293 cells. All of the vectors were serially amplified, purified through double cesium chloride centrifugation as described before (15) , and titered to 15 x 1010 plaque-forming units/ml.
Human skin grafts were injected intradermally with the adenoviral vectors at a concentration of 5 x 108 plaque-forming units in a total volume of 100 µl sterile PBS. Injections were performed once per week by the same person (R. T.).
Human Skin Grafting.
Human foreskins from newborns, and abdominal, breast, and facial skin from adult donors who underwent plastic surgery (Cooperative Human Tissue Network, Philadelphia, PA) were prepared and grafted within 48 h of excision as described (7
, 11)
. Grafts were well healed after 46 weeks and used for the experiments. The Wistar Institutional Animal Care and Use Committee approved all of the protocols.
UV Irradiation.
UV light was provided by two Westinghouse FS72T12/UVB lamps (UV Resources International, Lakewood, OH) with a peak output at 313 nm and a range of 280370 nm. The light was filtered through cellulose triacetate Kodacel TA 407 sheets (Eastman Kodak, Rochester, NY) to exclude wavelengths below 295 nm. The UV dose was continuously monitored with a PMA 2100 radiometer (Solar Light, Philadelphia, PA) and ranged between 30 and 50 mJ/cm2 for UVB, and 0.1 and 0.2 J/cm2 for UVA. During irradiation, mice were separated from each other and allowed to move freely in the cage. Irradiation was performed three times weekly for
10 min each time over a period of up to 4 weeks.
Histology, Immunohistochemistry, and Immunofluorescence.
Preparation, fixation, and embedding of tissues were performed as described (7
, 11)
. The DNA-binding fluorochrome Hoechst 33258 (Sigma, St. Louis, MO) was used to distinguish human from murine cells.
Immunohistochemistry and immunofluorescence techniques have been described previously (7 , 11) . Primary monoclonal and polyclonal antibodies used in this study were bFGF-8 (12) , ET-3 (DPC Biermann, Bad Neuheim, Germany), FGFR1 (QED Bioscience Inc., San Diego, CA), SCF (IBL, Gunma, Japan), S100, CD117/c-kit (Dako, Carpinteria, CA), Ki-67 (clone MIB-1; Immunotech, Westbrook, ME), HMB45 (Biogenex, San Ramon, CA), Melan-A (clone A103; Novocastra, Newcastle upon Tyne, United Kingdom), NKIC3 (Monosan, Uden, the Netherlands), and Phospho-p44/42 mitogen-activated protein kinase (Thr202/Tyr204; Cell Signaling, Beverly, MA). A mouse IgG1 isotype antibody (P3) was used as negative control for each staining.
Mutation Analysis for N-RAS and BRAF.
Serial sections were prepared as published previously (16)
. The sections were microdissected with a PixCell laser-capture microscope (Arcturus Engineering, Mountain View, CA), and DNA extracts were used for PCR. BRAF exon 15 was amplified by using primers as described previously (17)
and for seminested PCR by using primer 5'-AATCAGTGGAAAAATAGCCTCA-3'. N-RAS exon 2 was amplified by using primers and conditions as described previously (16)
. Single-strand conformation polymorphism for BRAF analysis was done in the absence of glycerol at 5°C and 30 W. For each DNA extract, two or three independent PCR reactions were performed to confirm reproducibility. Each experiment was repeated three times. The sequences of the shifted bands were analyzed using outer primers in both directions as published previously (16)
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Statistics.
All of the data are expressed as mean ± SD of the mean of observations. Individual groups were compared with Students unpaired t test. P < 0.05 was considered significant.
| Results and Discussion |
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When the weekly induced expression of all three of the growth factors bFGF, ET-3, and SCF was combined with 3050 mJ/cm2 UVB irradiation three times weekly, severe pigmented lesions developed (Fig. 2A)
. Histologically these lesions were composed of cytologically atypical melanocytes and represented in situ melanomas (6 of 17; 35%) and invasive melanomas (11 of 17; 65%; Fig. 2, BE
) similar to those seen in patients. Of the 79 human skin grafts that received the combination of growth factors and UVB irradiation, pigmented lesions were clinically observed in 89%, and melanomas fitting the standard histopathologic criteria were observed in 34% (Table 1)
. Immunohistochemical analyses of the lesions revealed expression of the melanoma markers S100 (Fig. 2E)
, HMB45 (Fig. 2D)
, Melan-A, and NKIC3 (data not shown). They also expressed the growth factor receptors FGFR1 and c-kit (data not shown). Melanocytic cells isolated and cultured from these lesions formed colonies in soft agar (Fig. 2F)
comparable with established melanoma cell lines (Fig. 2G)
, whereas normal human melanocytes isolated from human skin did not (data not shown). Flow cytometry analysis for melanoma cell surface markers associated with invasion and metastasis revealed expression of ß3 integrin, melanoma cell adhesion molecule, melanotransferrin, chondroitin sulfate proteoglycan, and acetyl-GD3 in all of three cultures tested (data not shown). Whereas E-cadherin was also detected in these cultures, expression of N-cadherin was only found in melanocytic cell fractions isolated from the deeper parts of the skin, i.e., dermis and basement membrane, and in one of three melanocytic cell fractions isolated from the epidermis. This implies that a change from E- to N-cadherin expression occurred in the cells invading the skin, which is characteristic of invasive melanoma (18)
. These data provide new evidence that human melanoma can be induced by UVB, when melanocytes are activated through their growth factor receptors leading to uncontrolled growth and increased susceptibility to the carcinogenic effects of UV radiation. Because melanoma lesions with nest formation, pagetoid growth, and invasion could not be induced by each growth factor alone, the data demonstrate that bFGF, ET-3, and SCF must have exerted synergistic effects on the melanocytes that cannot be explained by growth stimulation only. Possibly, induction of migration as shown for SCF in melanocytes in vitro was another pathogenic effect. Possible physiological causes for an increased receptor stimulation by activated growth factors include body surface expansion during adolescence or burns and wounds (19)
. Because the studies were done in skin specimens from newborns, we then investigated whether the observed effects on melanocytes were age-dependent. Adult skin grafts from the abdomen, breast, and face of 11 different donors were exposed to a combination of bFGF, ET-3, SCF, and UVB (n = 9); combination of bFGF, ET-3, and SCF without UVB (n = 11); UVB alone (n = 10); or were observed only (n = 8). After 4 weeks, in situ melanomas were found in 4556% (n = 5, respectively) of the grafts exposed to the three growth factors independent from the exposure to UVB (Fig. 3, A and B)
, whereas there were no melanocytic lesions by UVB only or observation only. Compared with the results in newborn skin, melanomas in adult skin were less severe and noninvasive, but could be induced without UVB. These data indicates that young skin is more susceptible to the transforming effects of external growth factor receptor activation and UVB radiation, but that possibly pre-existing acquired mutations in adult skin predispose to melanoma development triggered by receptor tyrosine kinase activation only. The detection of phosphorylation of extracellular signal-regulated kinase 1 in all of the analyzed invasive melanoma lesions (5 of 5) by immunohistochemistry (Fig. 3C)
indicates that the RAS-RAF-extracellular signal-regulated kinase-mitogen-activated protein kinase pathway was activated in the transformed cells, mostly likely through the external growth factor stimulation. Activating mutations of the N-RAS proto-oncogene and, more recently, of the BRAF gene leading to constitutive activation of the RAS-RAF-extracellular signal-regulated kinase-mitogen-activated protein kinase pathway have been described in human melanoma (16
, 17)
. We analyzed the adult skin and newborn skin lesions for these mutations. There were no mutations in N-RAS codon 61 in melanoma cells of microdissected adult skin nor in newborn skin lesions. BRAF was not mutated in 10 of 10 analyzed adult skin lesions except for a silence mutation in 1 sample (TCT601TCC). However, in 2 of 5 analyzed newborn skin lesions, an activating V599E BRAF mutation in exon 15 was found in a small proportion of the cells (Fig. 3D)
. Because the BRAF mutation does not display a UV signature mutation site, it may have pre-existed or it may have been induced by yet unidentified mechanisms. Genetic alterations induced by UVB may only occur at time points later than 4 weeks (20)
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We conclude that a melanoma phenotype can be induced in human skin by a combination of three growth factors and UVB within a very short period of time, but additional factors or an extended time of growth stimulation and UVB irradiation are needed for the acquisition of stable genetic alterations that are required for autonomous growth and unlimited life span.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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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.
Requests for reprints: Meenhard Herlyn, The Wistar Institute, 3601 Spruce Street, Philadelphia, PA 19104. Phone: (215) 898-3950; Fax: (215) 898-0980; E-mail: herlynm{at}wistar.upenn.edu
Received 11/ 3/03. Revised 12/ 8/03. Accepted 12/ 8/03.
| REFERENCES |
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