Changes in Expression of Putative Antigens Encoded by Pigment
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Proc. Natl. Acad. Sci. USA Vol. 92, pp. 10152-10156, October 1995 Medical Sciences Changes in expression of putative antigens encoded by pigment genes in mouse melanomas at different stages of malignant progression (albino locus/brown locus/slaty locus/silver locus) SETH J. ORLOW*, VINCENT J. HEARINGt, CHIE SAKAIt, KAZUNORI URABEt, BAO-KANG ZHOU*, WILLYS K. SILVERSt, AND BEATRICE MINTZO§ *Ronald 0. Perelman Department of Dermatology and Department of Cell Biology, New York University School of Medicine, New York, NY 10016; tLaboratory of Cell Biology, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892; and tInstitute for Cancer Research, Fox Chase Cancer Center, Philadelphia, PA 19111 Contributed by Beatrice Mintz, July 25, 1995 ABSTRACT Cutaneous melanomas of Tyr-SV40E trans- This problem might be overcome by identifying changes in genic mice (mice whose transgene consists of the tyrosinase the antigenic profile associated with melanoma progression promotor fused to the coding regions of simian virus 40 early such that the design of a treatment protocol could encompass genes) strikingly resemble human melanomas in their devel- all the melanoma cells. Such a characterization in the case of opment and progression. Unlike human melanomas, the human melanoma would be complicated by genetic differences mouse tumors all arise in genetically identical individuals, among individuals, especially as the genetic background, as thereby better enabling expression of specific genes to be well as allelic differences at pigment gene loci themselves, can characterized in relation to advancing malignancy. The prod- influence the expression and interactions of these genes (11, ucts of pigment genes are of particular interest because 12). The melanomas of Tyr-SV40E transgenic mice [mice peptides derived from these proteins have been reported to whose transgene consists of the tyrosinase promoter fused to function as autoantigens with immunotherapeutic potential in the coding regions of simian virus 40 (SV40) early genes], on some melanoma patients. However, the diminished pigmen- the other hand, afford an opportunity to investigate the tation characteristic of advanced melanomas raises the products of pigment genes in tumors arising in genetically many identical (C57BL/6 inbred strain) animals and representing all possibility that some of the relevant products may no longer stages of tumor progression (13-16). Unlike long-established be expressed in the most malignant cells. We have therefore cell culture lines of animal or human melanomas that have investigated the contributions of several pigment genes in been experimentally selected for a particular pigmentary or melanotic vs. relatively amelanotic components of primary other phenotype, the phenotypes of the transgenic mouse and metastatic mouse melanomas. The analyses reveal melanomas reflect the progression actually taking place in marked differences within and among tumors in levels of vivo. mRNAs and proteins encoded by the wild-type alleles at the The Tyr-SV40E transgene confers melanoma susceptibility albino, brown, slaty, and silver loci. Tyrosinase (the protein by activation of SV40 transforming sequences under the encoded by the albino locus) was most often either absent or transcriptional control of the mouse tyrosinase promoter, undetectable as melanization declined. The protein encoded which is encoded at the albino locus and is specifically ex- by the slaty locus (tyrosinase-related protein 2) was the only pressed in pigment cells (13). The transgene acts as an one of those tested that was clearly present in all the tumor initiating stimulus in melanocytes, only rarely giving rise samples. These results suggest that sole reliance on targeting spontaneously to cutaneous melanomas. If the cells are further tyrosinase-based antigens might selectively favor survival of subjected to growth-promoting influences in vitro (17, 18) or more malignant cells, whereas targeting the ensemble of the in vivo (19), they may become tumorigenic. Cutaneous mela- antigens tested might contribute toward a more inclusive and nomas are experimentally inducible by taking advantage of the effective antimelanoma strategy. existence of separate transgenic lines of mice, each descended from a single egg injected with transgene DNA and having a Immunotherapy based on recognition by cytotoxic T lympho- characteristic level of transgene expression (13). By grafting cytes of antigenic peptides on melanoma cells has shown skin from mice of a line with high expression, and therefore promise in the treatment of cutaneous melanoma (1-9). high melanoma susceptibility, to hosts of a low-susceptibility Among the melanoma antigens thus far identified are proteins line, the donor skin may be exposed to growth factors and or encoded genes cytokines associated with wound healing. Melanomas fre- peptides by specific pigment genes-i.e., quently develop in the grafts and metastasize in the hosts (15), involved in melanin production. A major problem is that whose risk of eye melanomas, especially in the retinal pigment melanomas tend to become amelanotic as the malignancy epithelium (20), is far below that of the donors. The his- progresses, and can therefore be expected to lose expression of topathogenesis of these mouse skin melanomas (16) strikingly some of these genes. The gradual emergence of increasing resembles that of human cutaneous melanomas (21). numbers of hypomelanotic or amelanotic cells in the primary We examine the expression in the mouse melanomas of four tumors or their metastases increases the possibility that such pigment genes whose protein homologues have been impli- cells will selectively escape immune destruction. The superior cated as autoantigens in human melanoma. The products of all mitotic activity of the amelanotic cells (10) may in fact four genes become localized in melanosomes, the organelles in exacerbate the course of the disease after the tumor burden which melanin is synthesized. Three of them are known to has been temporarily reduced by the death of melanotic cells. catalyze steps in melanin biosynthesis: they are tyrosinase (EC The publication costs of this article were defrayed in part by page charge Abbreviations: SV40, simian virus 40; TRP-1 and -2, tyrosinase- payment. This article must therefore be hereby marked "advertisement" in related protein 1 and 2, respectively. accordance with 18 U.S.C. §1734 solely to indicate this fact. §To whom reprint requests should be addressed. 10152 Downloaded by guest on September 27, 2021 Medical Sciences: Orlow et al. Proc. Natl. Acad. Sci. USA 92 (1995) 10153 1.14.18.1), the rate-limiting enzyme and the product of the bridized with other probes as necessary. TYRS-J, a probe mouse albino locus (22-24); tyrosinase-related protein 1 specific for tyrosinase, was kindly provided by H. Yamamoto (TRP-1), which functions as 5,6-dihydroxyindole-2-carboxylic and T. Takeuchi (Sendai, Japan); pMT4, which is specific for acid oxidase and is encoded at the mouse brown locus (25); and TRP-1, was provided by S. Shibahara (Sendai, Japan); and tyrosinase-related protein 2 (TRP-2), which has 3,4-dihy- TRP-2a, which is specific for TRP-2, was a gift from I. Jackson droxyphenylalanine (DOPA)chrome tautomerase (EC (Edinburgh). 5.3.3.12) function and is encoded at the mouse slaty locus (26, Melanogenic Assays. Tissues were homogenized in 1% 27). The fourth gene product, initially designated Pmel 17, is Nonidet P-40/0.01% SDS/0.1 M Tris-HCl, pH 7.2/1 ,ug of the product of the silver locus and is incorporated into the aprotinin per ml/100 ,tM phenylmethylsulfonyl fluoride on ice melanosomal matrix (28, 29); its enzymatic function, if any, by using a Potter-Elvehjem glass homogenizer, incubated for remains unknown. Comparisons of melanotic and relatively 1 h at 4°C, and then stored at -70°C until used. Following amelanotic components of the mouse melanomas analyzed centrifugation at 12,000 x g for 3 min, the supernatants were here reveal marked disparities within and among tumors, used for melanogenic assays. Four assays for melanogenic including cases of absent or borderline expression of some of catalytic activities, as described below, were carried out at pH the proteins. The exception was TRP-2, which was found in all 6.8 and 37°C for 60 min. the tumor examples analyzed. (i) Tyrosine hydroxylase activity was measured by using the [3H]tyrosine assay (31, 32). This method specifically measures the tritiated water produced during the hydroxylation of MATERIALS AND METHODS tyrosine to DOPA. (ii) DOPA oxidase activity was measured Melanomas. The melanomas in this study all arose in skin on the basis of incorporation of [3-14C]DOPA into acid- from Tyr-SV40E hemizygous donors of line 8, which are highly insoluble melanin, as described (30). (iii) DOPAchrome tau- susceptible to melanoma, after discs of skin were grafted to tomerase activity was measured by HPLC (33, 34) as the low-susceptibility hemizygous hosts of line 12 (15). Four disappearance of DOPAchrome substrate and the production primary melanomas (designated P) and two metastases (des- of 5,6-dihydroxyindole-2-carboxylic acid rather than 5,6- ignated M) were analyzed. These primary melanomas were dihydroxyindole; data were converted to pmol of products by chosen because they were zonal tumors in which a deeply comparison with known standards. (iv) Melanin production melanotic and an apparently amelanotic zone were clearly was measured by incorporation of [14C]tyrosine into acid- evident