Hyperpigmentation and melanocytic hyperplasia in transgenic mice expressing the human T24 Ha-ras gene regulated by a mouse tyrosinase promoter

Uveal melanoma modeling in mice and zebrafish

Despite extensive research and refined therapeutic options, the survival for metastasized uveal melanoma (UM) patients has not improved significantly. UM, a malignant tumor originating from melanocytes in the uveal tract, can be asymptomatic and small tumors may be detected only during routine ophthalmic exams; making early detection and treatment difficult. UM is the result of a number of characteristic somatic alterations which are associated with prognosis. Although UM morphology and biology have been extensively studied, there are significant gaps in our understanding of the early stages of UM tumor evolution and effective treatment to prevent metastatic disease remain elusive. A better understanding of the mechanisms that enable UM cells to thrive and successfully metastasize is crucial to improve treatment efficacy and survival rates. For more than forty years, animal models have been used to investigate the biology of UM. This has led to a number of essential mechanisms and pathways involved in UM aetiology. These models have also been used to evaluate the effectiveness of various drugs and treatment protocols. Here, we provide an overview of the molecular mechanisms and pharmacological studies using mouse and zebrafish UM models. Finally, we highlight promising therapeutics and discuss future considerations using UM models such as optimal inoculation sites, use of BAP1mut-cell lines and the rise of zebrafish models.

Derivation and Use of Cell Lines from Mouse Models of Melanoma

The establishment of consistent genetically modified mouse melanoma models and cell lines is of paramount importance for prevention and treatment. In this study, we review the different mouse melanoma cell lines that have been established. After careful molecular characterization of the established mouse melanoma cell lines, modification of the genome, microenvironment, or even the environment using appropriate in cellulo and in vivo assays may reveal novel genetic and nongenetic changes. These murine melanoma cell lines with defined genetic mutations allow the testing of innovative therapies based on chemistry, physics, and biology using alternative methods. In addition to the fundamental aspects, these results are important for humans because of the relevance of these murine melanoma cell lines to human disease.

Insulin-like Growth Factor 1 Signaling in Mammalian Hearing

Insulin-like growth factor 1 (IGF-1) is a peptide hormone belonging to the insulin family of proteins. Almost all of the biological effects of IGF-1 are mediated through binding to its high-affinity tyrosine kinase receptor (IGF1R), a transmembrane receptor belonging to the insulin receptor family. Factors, receptors and IGF-binding proteins form the IGF system, which has multiple roles in mammalian development, adult tissue homeostasis, and aging. Consequently, mutations in genes of the IGF system, including downstream intracellular targets, underlie multiple common pathologies and are associated with multiple rare human diseases. Here we review the contribution of the IGF system to our understanding of the molecular and genetic basis of human hearing loss by describing, (i) the expression patterns of the IGF system in the mammalian inner ear; (ii) downstream signaling of IGF-1 in the hearing organ; (iii) mouse mutations in the IGF system, including upstream regulators and downstream targets of IGF-1 that inform cochlear pathophysiology; and (iv) human mutations in these genes causing hearing loss.

NF1-Dependent Transcriptome Regulation in the Melanocyte Lineage and in Melanoma

The precise role played by the tumor suppressor gene NF1 in melanocyte biology and during the transformation into melanoma is not completely understood. In particular, understanding the interaction during melanocyte development between NF1 and key signaling pathways, which are known to be reactivated in advanced melanoma, is still under investigation. Here, we used RNAseq datasets from either situation to better understand the transcriptomic regulation mediated by an NF1 partial loss of function. We found that NF1 mutations had a differential impact on pluripotency and on melanoblast differentiation. In addition, major signaling pathways such as VEGF, senescence/secretome, endothelin, and cAMP/PKA are likely to be upregulated upon NF1 loss of function in both melanoblasts and metastatic melanoma. In sum, these data bring new light on the transcriptome regulation of the NF1-mutated melanoma subgroup and will help improve the possibilities for specific treatment.

Model Systems for the Study of Malignant Melanoma

Since the first resection of melanoma by Hunter in 1787, efforts to treat patients with this deadly malignancy have been ongoing. Initial work to understand melanoma biology for therapeutics development began with the employment of isolated cancer cells grown in cell cultures. However, these models lack in vivo interactions with the tumor microenvironment. Melanoma cell line transplantation into suitable animals such as mice has been informative and useful for testing therapeutics as a preclinical model. Injection of freshly isolated patient melanomas into immunodeficient animals has shown the capacity to retain the genetic heterogeneity of the tumors, which is lost during the long-term culture of melanoma cells. Upon advancement of technology, genetically engineered animals have been generated to study the spontaneous development of melanomas in light of newly discovered genetic aberrations associated with melanoma formation. Culturing melanoma cells in a matrix generate tumor spheroids, providing an in vitro environment that promotes the heterogeneity commonplace with human melanoma and displaces the need for animal care facilities. Advanced 3D cultures have been created simulating the structure and cellularity of human skin to permit in vitro testing of therapeutics on melanomas expressing the same phenotype as demonstrated in vivo. This review will discuss these models and their relevance to the study of melanomagenesis, growth, metastasis, and therapy.

Genetically engineered mouse models of melanoma

Melanoma is a complex disease that exhibits highly heterogeneous etiological, histopathological, and genetic features, as well as therapeutic responses. Genetically engineered mouse (GEM) models provide powerful tools to unravel the molecular mechanisms critical for melanoma development and drug resistance. Here, we expound briefly the basis of the mouse modeling design, the available technology for genetic engineering, and the aspects influencing the use of GEMs to model melanoma. Furthermore, we describe in detail the currently available GEM models of melanoma. Cancer 2017;123:2089-103. © 2017 American Cancer Society.

Multiple roles of NF1 in the melanocyte lineage

NF1 is a tumour suppressor gene, germline mutations of which lead to neurofibromatosis type 1 syndrome. Patients develop benign tumours from several types of cells including neural crest-derived cells. NF1 somatic mutations also occur in 15% of sporadic melanoma, a cancer originating from melanocytes. Evidence now suggests the involvement of NF1 mutations in melanoma resistance to targeted therapies. Although NF1 is ubiquitously expressed, genetic links between NF1 and genes involved in melanocyte biology have been described, implying the lineage-specific mechanisms. In this review, we summarize and discuss the latest advances related to the roles of NF1 in melanocyte biology and in cutaneous melanoma.

Reconstructing skin cancers using animal models

The American Cancer Society estimates that skin cancer is the most prevalent of all cancers with over 2 million cases of nonmelanoma skin cancer each year and 75,000 melanoma cases in 2012. Representative animal cancer models are important for understanding the underlying molecular pathogenesis of these cancers and the development of novel targeted anticancer therapeutics. In this review, we will discuss some of the important animal models that have been useful to identify important pathways involved in basal cell carcinoma, squamous cell carcinoma, and melanoma.

Primary melanoma of the CNS in children is driven by congenital expression of oncogenic NRAS in melanocytes

NRAS mutations are common in human melanoma. To produce a mouse model of NRAS-driven melanoma, we expressed oncogenic NRAS (NRAS(G12D)) in mouse melanocytes. When NRAS(G12D) was expressed in the melanocytes of developing embryos, it induced melanocyte proliferation and congenital melanocytic lesions reminiscent of human blue nevi but did not induce cutaneous melanoma. Unexpectedly, however, it did induce early-onset primary melanoma of the central nervous system (CNS). The tumors were rapidly proliferating and caused neurologic symptoms, rapid health deterioration, and death. NRAS is not a common driver oncogene of primary melanoma of the CNS in adults, but we report two cases of primary melanoma of the CNS in children, both of which carried oncogenic mutations in NRAS. We conclude that acquisition of somatic mutations in NRAS in CNS melanocytes is a predisposing risk factor for primary melanoma of the CNS in children, and we present a mouse model of this disease.

SIGNIFICANCE

We show that the acquisition of NRAS mutations in melanocytes during embryogenesis is a risk factor for early-onset melanoma of the CNS. We have developed a powerful mouse model to study this rare but devastating childhood disease, and to develop therapeutic approaches for its treatment.

Whole recombinant yeast vaccine induces antitumor immunity and improves survival in a genetically engineered mouse model of melanoma

Malignant melanoma is one of the deadliest forms of skin cancer and its incidence is expected to rise over the next two decades. At present, there are no effective therapies for advanced melanoma. We have previously shown that administration of whole recombinant yeast expressing human MART-1 (hMART-IT) induces protective antimelanoma immunity in a B16F10 transplantable mouse model. In this study, we examine the effectiveness of the hMART-IT vaccine in a congenic strain of genetically engineered mouse model of melanoma, which recapitulates both the underlying genetics and the proper tumor microenvironment of naturally occurring melanoma. Subcutaneous administration of hMART-IT induced cytotoxicity against melanoma cells and antigen-specific production of Th1-specific cytokines by splenocytes. Weekly administration of hMART-IT significantly delayed the development of melanoma and prolonged the survival of mice compared with controls. Although histological analysis demonstrated diffuse infiltration of CD4(+) T cells and CD8(+) T cells, no reduction of regulatory T cells was observed, suggesting that hMART-IT cannot prevent immunotolerance in the tumor microenvironment. This study provides a proof of concept that genetically engineered mouse models lend valuable insights into immunotherapeutics being tested in the preclinical setting.

p53 prevents progression of nevi to melanoma predominantly through cell cycle regulation

p53 is the central member of a critical tumor suppressor pathway in virtually all tumor types, where it is silenced mainly by missense mutations. In melanoma, p53 predominantly remains wild type, thus its role has been neglected. To study the effect of p53 on melanocyte function and melanomagenesis, we crossed the ‘high-p53’Mdm4+/− mouse to the well-established TP-ras0/+ murine melanoma progression model. After treatment with the carcinogen dimethylbenzanthracene (DMBA), TP-ras0/+ mice on the Mdm4+/− background developed fewer tumors with a delay in the age of onset of melanomas compared to TP-ras0/+ mice. Furthermore, we observed a dramatic decrease in tumor growth, lack of metastasis with increased survival of TP-ras0/+: Mdm4+/− mice. Thus, p53 effectively prevented the conversion of small benign tumors to malignant and metastatic melanoma. p53 activation in cultured primary melanocyte and melanoma cell lines using Nutlin-3, a specific Mdm2 antagonist, supported these findings. Moreover, global gene expression and network analysis of Nutlin-3-treated primary human melanocytes indicated that cell cycle regulation through the p21WAF1/CIP1 signaling network may be the key anti-melanomagenic activity of p53.

A mouse model of melanoma driven by oncogenic KRAS

The small G-protein NRAS is mutated in 22% of human melanomas, whereas the related proteins KRAS and HRAS are mutated in only 2% and 1% of melanomas, respectively. We have developed a mouse model of melanoma in which Cre recombinase/LoxP technology is used to drive inducible expression of (G12V)KRAS in the melanocytic lineage. The mice develop skin hyperpigmentation, nevi, and tumors that bear many of the cardinal histopathology features and molecular characteristics of human melanoma. These tumors invade and destroy the underlying muscles and cells derived from them can grow as subcutaneous tumors and colonize the lungs of nude mice. These data establish that oncogenic KRAS can be a founder event in melanomagenesis.

Roles of endothelin signaling in melanocyte development and melanoma

Endothelin (Edn) signaling via the G-coupled, Edn receptor type B (Ednrb) is essential for the development of melanocytes from the neural crest (NC) and has been associated with melanoma progression. Edn3 plays varying roles during melanocyte development, promoting the proliferation and self-renewal of NC-derived multi- and bi-potential precursors as well as the survival, proliferation, differentiation and migration of committed melanocyte precursors. Melanocyte differentiation is achieved via the interaction of Ednrb and Kit signaling, with Ednrb being specifically required in the final differentiation step, rather than in the initial specification of melanocytic fate. Ednrb has also been implicated in the de-differentiation of mature melanocytes, a process that takes place during the malignant transformation of these cells. Ednrb was found to be upregulated in melanoma metastases and was shown to alter tumor-host interactions leading to melanoma progression. Antagonists to this receptor were shown to inhibit melanoma cell growth and increase the apoptotic rate of these cells, and to lead to disease stabilization in melanoma patients. Thus, Edn signaling inhibition may prove useful in the treatment of certain types of melanoma.

Oncogenic Braf induces melanocyte senescence and melanoma in mice

We show here that inducible expression of Braf(V600E) off the endogenous Braf gene in mouse melanocytes stimulates skin hyperpigmentation and the appearance of nevi harboring senescent melanocytes. Additionally, approximately 70% of Braf(V600E) mice develop melanomas that reproduce many of the cardinal histological and molecular features of human melanoma and whose cells can colonize the lungs of nude mice. We show that the tumor suppressor p16(INK4a) is not required to induce melanocyte senescence and that its loss is not required for tumor progression, although it does regulate tumor penetrance and latency. Thus, we have developed a mouse model of melanoma driven by Braf(V600E) expressed at physiological levels that reflects the genetics and pathology of the human disease.

Integrin A6 Cleavage in Mouse Skin Tumors

We have previously identified a structural variant of the α6 integrin (Laminin receptor) called α6p. The α6p variant is a 70 kDa form of the full-length α6 integrin (140 kDa) that remains paired with either the β1 or β4 subunit on the cell surface. α6p is produced by urokinase-type plasminogen activator (uPA), which removes the extracellular β-barrel domain while the receptor is on the cell surface. The α6p integrin was present in human prostate cancer tissue but not in normal tissue and the cleavage of the α6 integrin extracellular domain promotes tumor cell invasion and migration on laminin. The objective of the present study was to determine whether the α6p integrin is observed in other models of carcinogenesis. Our results indicate detectable low levels of α6p in normal mouse skin, and comparatively elevated levels in mouse papillomas and squamous cell carcinomas induced by DMBA, TPA and MNNG treatments. Furthermore, we have found that α6p was present at high levels in skin melanomas of transgenic mice that over express activated Ha-ras under the control of the tyrosinase promoter. Finally, subcutaneous injection into athymic nude mice of a malignant mouse keratinocyte derived cell line (6M90) that is α6p negative, results in the development of tumors that contain α6p integrin. The latter results indicate that α6p is induced in vivo suggesting that the tumor microenvironment plays a major role in the production of α6p. Taken together, these data suggest that the cell surface cleavage of the α6 integrin may be a novel mechanism of integrin regulation and might be an important step during skin tissue remodeling and during carcinogenesis.

Two cases of uveal amelanotic melanoma in transgenic Tyr-HRAS+ Ink4a/Arf heterozygous mice

Uveal melanoma (UM) is uncommon among wild type mice. Efforts to develop transgenic mice to study this disease have resulted in pigmented tumors derived from the retinal pigment epithelium (RPE) or mixed tumors of RPE and UM complicating the study of UM specifically. Reported here are two early stage intraocular amelanotic melanomas discovered in 2 Tyr-HRAS+ Ink4a/Arf heterozygous (1 normal CKDN2A allele) transgenic FVB/n mice. These tumors were morphologically and immunohistochemically similar to spontaneous UM recently reported in the Ink4a/Arf homozygous (CKDN2A knockout) parent strain. The tumors originated in the posterior uveal tract. The neoplasms were comprised of bundles of spindle-shaped melanocytes admixed with some epithelioid cells. Tumors were immunohistochemically positive for neuron-specific enolase, S-100, pan-ras, but negative for cytokeratin and Melan-A. The development of early lenticular opacity and bilateral cataracts is a consistent phenotype of transgenic mice in which the retinoblastoma signaling pathway has been disrupted. Lenticular opacity and cataracts are rarely observed clinically in Tyr-HRAS+ Ink4a/Arf heterozygotes, rendering this strain suitable for ophthalmoscopy. Consequently, Tyr-HRAS+ Ink4a/Arf heterozygotes provide practical advantages, compared to the cataract-prone CKDN2A knockout strains, for real-time ophthalomoscopic detection and monitoring of UM while developing chemotherapeutic regimens and other research to understand the biology of UM.

Modeling the behavior of uveal melanoma in the liver

PURPOSE

To model the behavior of uveal melanoma in the liver.

METHODS

A 15-muL suspension of metastatic MUM2B or either primary OCM1 or M619 uveal melanoma cells was injected into the liver parenchyma of 105 CB17 SCID mice through a 1-cm abdominal incision. Animals were killed at 2, 4, 6, or 8 weeks after injection. Before euthanatization, 3% FITC-BSA buffer was injected into the retro-orbital plexus of one eye of three mice. Liver tissues were examined by light and fluorescence microscopy, and were stained with human anti-laminin. Vasculogenic mimicry patterns were reconstructed from serial laser scanning confocal microscopic stacks.

RESULTS

OCM1a cells formed microscopic nodules in the mouse liver within 2 weeks after injection and metastasized to the lung 6 weeks later. By contrast, M619 and MUM2B cells formed expansile nodules in the liver within 2 weeks and gave rise to pulmonary metastases within 4 weeks after injection. Vasculogenic mimicry patterns, composed of human laminin and identical with those in human primary and metastatic uveal melanomas, were detected in the animal model. The detection of human rather than mouse laminin in the vasculogenic mimicry patterns in this model demonstrates that these patterns were of tumor cell origin and were not co-opted from the mouse liver microenvironment.

CONCLUSIONS

There are currently no effective treatments for metastatic uveal melanoma. This direct-injection model focuses on critical interactions between the tumor cell and the liver. It provides for translationally relevant approaches to the development of new modalities to detect small tumor burdens in patients, to study the biology of clinical dormancy of metastatic disease in uveal melanoma, to design and test novel treatments to prevent the emergence of clinically manifest liver metastases after dormancy, and to treat established uveal melanoma metastases.

Inhibition of phosphatidylinositol-3-kinase and mitogen-activated protein kinase kinase 1/2 prevents melanoma development and promotes melanoma regression in the transgenic TPRas mouse model

A number of human melanomas show hyperactivation of the Ras pathway due to mutations of the molecule or alteration of upstream or downstream effectors. In this study, we evaluated the effect of blocking the two Ras downstream pathways phosphatidylinositol-3-kinase/Akt and Raf/mitogen-activated protein kinase kinase/extracellular signal-regulated kinase on melanoma development and regression in the TPRas mouse model. The inhibition of these two signaling cascades by topically applied Ly294002 and U0126 significantly delayed melanoma development and significantly decreased the tumor incidence, particularly when the drugs were applied in combination. Treatment with the inhibitors of established melanomas resulted in complete remission in 33% of mice and partial regression in 46% of mice when drugs were delivered in combination. These responses correlated with increased apoptosis and decreased proliferation both in vitro and in vivo and reduced tumor angiogenesis. In conclusion, this study strongly supports the role of the phosphatidylinositol-3-kinase/Akt and Raf/mitogen-activated protein kinase kinase/extracellular signal-regulated kinase pathways in the development and maintenance of Ras-dependent melanomas and supports the notion that specific inhibition of these effectors may represent a very promising avenue for the treatment and prevention of the disease.

Toward a Molecular Classification of Melanoma

The incidence of melanoma is increasing at one of the highest rates of any form of cancer in the United States, with the current lifetime risk being one in 68. At present, there are limited systemic therapies to treat advanced stages of melanoma, and the key to improved survival remains early detection. Recent discoveries have allowed for a clearer picture of the molecular events leading to melanoma development and progression. Since identifying prevalent activating mutations of the BRAF kinase in melanomas, there has been a flood of additional molecular studies to further clarify the role of this pathway and others in melanomagenesis. In particular, recent genetic studies have demonstrated specific genotype-phenotype correlations that provide the first major insights into the molecular subclassification of melanoma and the heterogeneous nature of this malignancy. In this article, we review the most up-to-date molecular discoveries in melanoma biology and provide a framework for understanding their significance in melanoma development and progression. We also provide details on the development of novel therapies based on these recent molecular discoveries and insight into current and planned clinical trials. It is expected that these latest studies in melanoma will help define the critical molecular events involved in disease onset and progression and allow us to move rapidly toward a true molecular classification. We eagerly anticipate rationally designed melanoma therapies based on such a classification scheme and the associated improvements in patient outcomes.

Protective effect of hyperpigmented skin on UV-mediated cutaneous cancer development

Recently, we crossed an original haired RET-transgenic mouse of line 242 with a hairless mouse and established a hairless RET-(HL/RET)-transgenic mouse line (242-hr/hr) with hyperpigmented skin but no tumors. In this study, we examined the effect of hyperpigmented skin in HL/RET-transgenic mice on UV irradiation-mediated cutaneous cancer development. UV irradiation to this mouse line never induced melanoma despite the presence of melanoma-inducible transgenic RET oncogenes. On the contrary, the hyperpigmented skin efficiently protected UV-mediated squamous carcinoma development in the skin. Probably underlying this result, hyperpigmentation protected the skin from damage and blocked the accompanying signal transduction for tyrosine phosphorylation of multiple cellular proteins and activation/phosphorylation of extracellular signal-regulated, c-Jun N-terminal, and p38 kinases. Thus, we demonstrated hyperpigmentation-mediated in vivo protection against UV irradiation-induced skin cancer.

Rapid growth of invasive metastatic melanoma in carcinogen-treated hepatocyte growth factor/scatter factor-transgenic mice carrying an oncogenic CDK4 mutation

Currently, novel mouse models of melanoma are being generated that recapitulate the histopathology and molecular pathogenesis observed in human disease. Impaired cell-cycle control, which is a hallmark of both familial and sporadic melanoma, promotes slowly growing carcinogen-induced melanomas in the skin of mice carrying a mutated cyclin-dependent kinase 4 (CDK4(R24C)). Deregulated receptor tyrosine kinase signaling, which is another important feature of human melanoma, leads to spontaneous development of metastatic melanoma after a long latency period in mice overexpressing hepatocyte growth factor/scatter factor (HGF/SF mice). Here we report that treatment with 7,12-dimethylbenz[a]anthracene and 12-O-tetradecanoylphorbol-13-acetate induced metastatic melanomas in all HGF/SF mice on the C57BL/6 background, which histologically resemble human melanoma. Importantly, mutant CDK4 dramatically increased the number and the growth kinetics of carcinogen-induced primary melanomas in the skin and promoted the growth of spontaneous metastases in lymph nodes and lungs in all HGF/SF mice within the first 3 months of life. Apart from very few skin papillomas, we did not observe tumors of other histology in carcinogen-treated HGF/SF x CDK4(R24C) mice. This new experimental mouse model can now be exploited to study further the biology of melanoma and evaluate new treatment modalities.

Metastasizing melanoma formation caused by expression of activated N-RasQ61K on an INK4a-deficient background

In human cutaneous malignant melanoma, a predominance of activated mutations in the N-ras gene has been documented. To obtain a mouse model most closely mimicking the human disease, a transgenic mouse line was generated by targeting expression of dominant-active human N-ras (N-RasQ61K) to the melanocyte lineage by tyrosinase regulatory sequences (Tyr::N-RasQ61K). Transgenic mice show hyperpigmented skin and develop cutaneous metastasizing melanoma. Consistent with the tumor suppressor function of the INK4a locus that encodes p16INK4A and p19(ARF), >90% of Tyr::N-RasQ61K INK4a-/- transgenic mice develop melanoma at 6 months. Primary melanoma tumors are melanotic, multifocal, microinvade the epidermis or epithelium of hair follicles, and disseminate as metastases to lymph nodes, lung, and liver. Primary melanoma can be transplanted s.c. in nude mice, and if injected i.v. into NOD/SCID mice colonize the lung. In addition, primary melanomas and metastases contain cells expressing the stem cell marker nestin suggesting a hierarchical structure of the tumors comprised of primitive nestin-expressing precursors and differentiated cells. In conclusion, a novel mouse model with melanotic and metastasizing melanoma was obtained by recapitulating genetic lesions frequently found in human melanoma.

Effects on proliferation and melanogenesis by inhibition of mutant BRAF and expression of wild-type INK4A in melanoma cells

Activating BRAF mutations and loss of wild-type INK4A expression both occur at high frequencies in melanomas. Here, we present evidence that BRAF and INK4A have different effects on melanogenesis, a marker of melanocytic differentiation. Human melanoma cell line 624Mel harbors mutations in both BRAF and INK4A. The in vitro and in vivo growth of these cells was inhibited by either reduced expression of mutant BRAF using stable retroviral RNA interference (RNAi) or retrovirus-mediated stable expression of wild-type INK4A cDNA. Consistent with the observed growth inhibition, phosphorylation of S780 and S795 in pRB, both CDK4/6 targets, was suppressed in cells expressing either mutant BRAF RNAi or wild-type INK4A. Interestingly, melanoma cells expressing mutant BRAF RNAi had increased pigmentation, produced more mature melanosomes and melanin and expressed higher levels of tyrosinase and tyrosinase-related protein-1, whereas melanogenesis was not induced by wild-type INK4A. We found that the melanocyte lineage-specific master control protein microphthalmia-associated transcription factor was upregulated by inhibition of mutant BRAF, which may be the cause for the melanogenic effect of BRAF RNAi. The results suggest that, although both BRAF and INK4A lesions promote cell growth and tumor formation, mutant BRAF may also induce dedifferentiation in melanoma cells.

Involvement of metabotropic glutamate receptor�1, a G protein coupled receptor, in melanoma development

Melanoma is the aberrant proliferation of melanocytes, the cells in the skin responsible for pigment production. In the United States the current lifetime risk of melanoma development is 1 in 57 in males and 1 in 81 in females. In its early stages melanoma can be surgically removed with great success; however, advanced stages of melanoma have a high mortality rate due to the lack of responsiveness to currently available therapies. The development of animal models to be used in the studies of melanoma will provide the means for developing improved and targeted treatments for this disease. This review focuses on the recent report of a mouse melanoma model, TG-3, which has implicated the ectopic expression of the metabotropic glutamate receptor 1 (Grm1), a G protein coupled receptor (GPCR), in melanomagenesis and metastasis. The involvement of other GPCRs in cellular transformation, particularly GPCRs in melanoma biology, and signaling of Grm1 are also discussed.

Topical Treatment with Inhibitors of the Phosphatidylinositol 3′-Kinase/Akt and Raf/Mitogen-Activated Protein Kinase Kinase/Extracellular Signal-Regulated Kinase Pathways Reduces Melanoma Development in Severe Combined Immunodeficient Mice

Topical treatment with inhibitors of the phosphatidylinositol 3'-kinase/Akt and Raf/mitogen-activated protein kinase kinase/extracellular signal-regulated kinase pathways inhibited the growth of TPras transgenic melanomas in severe combined immunodeficient mice, blocked invasive behavior, and reduced angiogenesis. The inhibitor Ly294002, which is specific for phosphatidylinositol 3'-kinase, effectively reduced melanoma cell growth both in vitro and in vivo. Both Ly294002 and U0126, a mitogen-activated protein kinase kinase 1/2 inhibitor, reduced invasion, which correlated with reduction of the metalloproteinase matrix metalloproteinase 2. Tumor angiogenesis was disrupted through inhibition of vascular endothelial growth factor production from the tumor cells and antiangiogenic effects on endothelial cells. Observations with TPras melanoma cells that express dominant negative Deltap85 or kinase-inactive Raf(301) supported the specificity of the phenomena observed with the chemical inhibitors. These studies demonstrate that topical treatment targeting Ras effectors is efficacious, without systemic toxicities, and may prove to be useful in treating and preventing the progression of cutaneous melanoma.

The genetics of malignant melanoma: lessons from mouse and man

Therapeutic resistance and proclivity for metastasis are hallmarks of malignant melanoma. Genetic, epidemiological and genomic investigations are uncovering the spectrum of stereotypical mutations that are associated with melanoma and how these mutations relate to risk factors such as ultraviolet exposure. The ability to validate the pathogenetic relevance of these mutations in the mouse, coupled with advances in rational drug design, has generated optimism for the development of effective prevention programmes, diagnostic measures and targeted therapeutics in the near future.

A pivotal role for ERK in the oncogenic behaviour of malignant melanoma?

During the process of oncogenic transformation, melanoma cells escape from normal growth-control mechanisms and acquire the ability to invade surrounding tissues and organs. The Ras/Raf/MEK/ERK pathway is a major pathway involved in the control of growth signals, cell survival and invasion. Melanomas are known to harbour activating mutations of both Ras and BRAF, suggesting that the downstream effector ERK may be playing a major role in the oncogenic behaviour of these tumours. The past few years have seen a growth in the understanding of the role of ERK and the MAP kinase pathway in melanoma. The aim of the current review is to assess the role of ERK in melanoma behaviour and to determine whether modulation of these kinases could offer new therapeutic opportunities.

Pathways to melanoma development: lessons from the mouse

Because of subtle differences between mouse and human skin, mice have traditionally not been an ideal model to study melanoma development. Understanding of the molecular mechanisms of melanoma predisposition, however, has been greatly improved by modeling various pathway defects in the mouse. This review analyzes the latest developments in mouse models of melanoma, and summarizes what these may indicate about the development of this neoplasm in humans. Mutations of genes involved in human melanoma have been recapitulated with some unexpected results, particularly with respect to the role of the two transcripts (Ink4a and Arf) encoded by the Cdkn2a locus. Both the Ink4a/pRb and Arf/p53 pathways are involved in melanoma development in mice, and possible mechanisms of cross-talk between the two pathways are discussed. We also know from mouse models that Ras/mitogen-activated protein kinase pathway activation is very important in melanoma development, either through direct activation of Ras (e.g., Hras G12V), or via activation of Ras-effector pathways by other oncogenes (e.g., Ret, Hgf/Sf). Ras can cooperate with the Arf/p53 pathway, and probably the Ink4a/Rb pathway, to induce melanoma. These three growth regulation pathways (Ink4a/pRb, Arf/p53, and Ras/mitogen-activated protein kinase) seem to represent three major "axes" of melanoma development in mice. Finally, we summarize experiments using genetically modified mice that have given indications of the intensity and timing of ultraviolet radiation exposure that may be most responsible for melanoma development.

Tyr-TGFalpha transgenic mice develop ocular melanocytic lesions

Transforming growth factor-alpha (TGFalpha) has been implicated in melanocyte transformation, as it is expressed in melanocytic lesions and in melanoma cells. We investigated its role in melanoma development using a transgenic mouse model. The mice were generated by microinjection of a transgene with 270 bp of the mouse tyrosinase promoter and the cDNA for human TGFalpha. No significant skin abnormalities were found, but individuals from three transgenic lines developed ocular melanocytoses (seven out of 10 transgenics), usually after a long latency period. In particular, the melanocyte component of the choroid was thicker than in non-transgenic controls, consistent with hyperplasia. The retinal pigment epithelium was unaffected. Melanocytic lesions were also present in the posterior eye, and abnormal distributions of melanocytes were found in neural tissue of the brain, skeletal muscle of the head and the Harderian glands, indicating migration from the choroid. It was concluded that mice engineered to express the normal growth factor TGFalpha from a tyrosinase promoter spontaneously developed melanocytic lesions in the eye but not the skin.

Mutant laboratory mice with abnormalities in pigmentation: annotated tables

Mammalian pigment cell research has recently entered a phase of significantly increased activity due largely to the exploitation of the many mutant mouse stocks that are coming on stream. Numerous transgenic, targeted mutagenesis (so-called 'knockouts'), conditional (so-called 'gene switch') and spontaneous mutant mice develop abnormal coat color phenotypes. The number of mice that exhibit such abnormalities is increasing exponentially as genetic engineering methods become routine. Since defined abnormalities in such mutant mice provide important clues to the as yet often poorly understood functional roles of many gene products, this overview includes a corresponding, annotated table of mutant mice with pigmentation alterations. These range from early developmental defects via a large array of coat color abnormalities to a melanoma metastasis model. This overview should provide helpful pointers to investigators who are looking for mouse models to explore or to compare functional activities of genes of interest and for comparing coat color phenotypes of spontaneous or genetically engineered mouse mutants with novel ones. Secondly, this review includes a table of mouse models of specific human diseases with genetically defined pigmentation abnormalities. In summary, this annotated table should serve as a useful reference for anyone interested in the molecular controls of pigmentation.

Regulation of epidermal growth factor receptor expression in human melanocytes

The epidermal growth factor receptor (EGFR) and its ligand, transforming growth factor alpha (TGFalpha), are reportedly involved in autocrine growth of melanoma cells. The signal pathway has also been implicated in early events of transformation, suggesting a function for EGFR in normal cells. This study reports the presence of EGFR in cultured melanocytes and examines some cellular responses to TGFalpha. Western analysis revealed 170 kDa bands in extracts of cultured neonatal human melanocytes, corresponding to the receptor Mr. Protein expression was more pronounced in cells during active growth. EGFR were less evident in cultures populated predominantly by melanized cells, indicating that receptor expression became reduced in differentiating cells. Immunocytochemistry confirmed these observations and also showed that EGFR reactivity was predominantly localized in the cell body but absent from dendrites. Addition of TGFalpha to early cultures induced a rapid increase in phosphotyrosine signal of the 170 kDa protein. Longer treatment (24-48 h) increased the intensity of the EGFR signal, suggesting that receptors had been upregulated. However, inclusion of TGFalpha in cultures did not result in an increase in cell numbers when compared to controls. The observations provide evidence of the existence of a receptor-mediated pathway in melanocytes which has transforming potential in vivo.

The state-of-the-art in chemoprevention of skin cancer

The incidence of skin cancer (both melanoma and non-melanoma) continues to grow at an alarming rate. Our chemoprevention strategies include the development of novel agents evaluated by (1) preclinical mechanistic studies in models of ultraviolet (UV) radiation-induced skin carcinogenesis; (2) clinical studies of immunohistochemical surrogate endpoint biomarkers in high-risk patients; and (3) randomised, placebo-controlled phase I, II and III clinical chemoprevention trials. Recent clinical results validate this development model. Molecular targets of chemopreventive strategies for melanoma and non-melanoma skin cancers include the ras and activator protein-1 (AP-1) signal transduction pathways. A transgenic murine melanoma model has been developed for evaluating potential agents in vivo. Agents at various stages of study include the green tea catechin epigallocatechin gallate (EGCG), the limonene derivative perillyl alcohol, the ornithine decarboxylase inhibitor alpha-difluoromethylornithine (DFMO), selenium, retinoids and salicylates. New chemopreventive agents that can be used to complement sunscreens may result in decreased incidence, morbidity and mortality of skin cancer.

Fas-induced apoptosis in human malignant melanoma cell lines is associated with the activation of the p34(cdc2)-related PITSLRE protein kinases

The Cdc2L locus encoding the PITSLRE protein kinases maps to chromosome band 1p36 and consists of two duplicated and tandemly linked genes. The purpose of the present study was to determine whether diminution of PITSLRE kinases leads to deregulation of apoptosis. The human melanoma cell lines A375 (Cdc2L wild-type alleles) and UACC 1227 (mutant Cdc2L alleles) were tested with agonist anti-Fas monoclonal antibody. We found that exposure of these cells to anti-Fas for 24, 48, or 72 h resulted in differential sensitivity to Fas-induced apoptosis. In A375, cell death started at 24-48 h post-treatment, and it was maximal by 72 h. Conversely, UACC 1227 cells were resistant to Fas-mediated apoptosis. Induction of PITSLRE histone H1 kinase activity was observed in A375 anti-Fas treated but not in UACC 1227 cells. Also, the PITSLRE protein kinase activity in A375 anti-Fas-treated cells preceded maximal levels of apoptosis. Finally, fluorescence confocal microscopy revealed a nuclear localization of PITSLRE proteins in normal melanocytes and A375 cells but a cytoplasmic localization in UACC 1227 cells. The differences in PITSLRE protein and cellular localization between A375 and UACC 1227 cells appear to account for the differences in sensitivity of the two cells lines to anti-Fas and staurosporine. These observations suggest that alterations in PITSLRE gene expression and protein localization may result in the loss of apoptotic signaling.

Transgenic mouse models for tumors of melanocytes and retinal pigment epithelium

Cutaneous and ocular melanomas are due to malignant transformation of neural crest-derived melanocytes. The rising incidence of this tumor in humans has stimulated experiments to devise suitable mouse models. In the past years, transgenic mouse lines have been generated using different oncogenes - Ha-ras, SV40 T antigen (Tag), ret - which develop benign lesions of melanocytes, melanoma, and/or eye tumors. Pigment cell tumors in humans, although rather rare, can also develop from the retinal pigment epithelium (RPE), a cell layer of neuroectodermal origin. We, therefore, established transgenic models for this ocular tumor. Regulated by the promoter of tyrosinase-related protein-1 (TRP-1), two oncogenes, ret and SV40 Tag, were targeted to the developing RPE in transgenic mice. The TRP-1/ret transgenic mice displayed microphthalmia and benign tumors of the RPE. Expression of SV40 T antigen (TRP-1/Tag) led to malignant tumors, which were invasive and metastasized to inguinal lymph node and spleen.

Abnormalities in the p34cdc2-related PITSLRE protein kinase gene complex (CDC2L) on chromosome band 1p36 in melanoma

The two genes encoding the PITSLRE protein kinase isoforms, CDC2L1 and CDC2L2, are localized to human chromosome band 1p36. The PITSLRE protein kinases are a part of the p34cdc2 supergene family. Several protein products of the CDC2L locus may be effector(s) in apoptotic signaling. The larger PITSLRE p110 isoforms appear to regulate some aspect of RNA splicing/transcription during the cell cycle. One or more of these genes may function as tumor suppressor genes in melanoma. Using fluorescence in situ hybridization, one allele of the CDC2L gene complex on chromosome 1 was either deleted or translocated in 8 of 14 different melanoma cell lines. We also observed mutations in the 5' promoter region of the CDC2L1 gene in four different cell lines relative to normal melanocytes using PCR-SSCP analysis and direct DNA sequencing. Western blot analysis revealed decreased level of PITSLRE protein expression in several cell lines, as well as in four surgical malignant melanoma specimens relative to normal melanocytes. Thus, the decreased PITSLRE protein expression appears to result from deletion of the CDC2L alleles and possibly by mutations within the 5' promoter region. We propose that aberrations in the CDC2L genes may contribute to the pathogenesis or progression of melanoma.

Pigmented uveal tumours in a transgenic mouse model

AIMS/BACKGROUND

The authors have developed transgenic mouse strains at the Arizona Cancer Center using a tyrosinase promoter to target expression of the mutated T24 Ha-ras gene in melanin producing cells. Histopathology and electron microscopy (EM) were performed to characterise the intraocular tumours observed phenotypically.

METHODS

Transgenic TPras mice (n = 8) and normal, age matched control mice (n = 6) were sacrificed at 3 weeks, 6 weeks, 7 weeks, 4 months, 5 months, 9 months, and 11 months. Six were processed in formalin for light microscopic examination and eight in a glutaraldehyde/formalin solution for electron microscopic examination.

RESULTS

Six of the TPras mice were found to have bilateral pigmented melanocytic/RPE proliferations of the uveal tract. The cytological characteristics of the tumours included low nuclear to cytoplasmic ratios (N:C ratios), bland nuclei, and abundant intracytoplasmic melanin. By EM two populations of cells were identified, including spindle-shaped cells with round to oval melanin granules and cuboidal cells with apically located, cigar-shaped, melanin granules, and basement membrane formation. A 3 week and an 11 month old TPras mouse had a higher grade, bilateral, melanocytic proliferation of the uveal tract which, although not metastatic, was morphologically melanoma. Cytological features included increased N:C ratios, nuclear pleomorphism, and prominent nucleoli. The uveal tract was normal in both eyes in all of the control animals.

CONCLUSION

Pigmented intraocular tumours developed in transgenic strains of mice that express a mutated Ha-ras gene in melanin producing cells. The morphology was most consistent with a melanoma in two of the mice and a benign melanocytic/RPE proliferation in the remaining mice.

Tumor suppressor mutations in mice: the next generation

Mouse strains carrying tumor suppressor mutations genetically mimic familial forms of human cancer. New tumor suppressors have and will be identified and mutated in the mouse; however, it is clear that future investigation will focus on a new generation of experiments aimed at improving existing models, and using them to delineate the molecular pathways to tumorigenesis and to test the value of rationally designed drug therapies.

New evidence for presence of tyrosinase in substantia nigra, forebrain and midbrain

Tyrosinase and tyrosinase-related proteins (TRP-1 and TRP-2) are essential for melanin synthesis and are expressed in neural crest-derived melanocytes and in the pigment epithelium of the retina. Recent results suggest expression of all three proteins within the central nervous system. We performed a transgenic assay using beta-galactosidase as reporter gene to monitor tyrosinase promoter activity in vivo. During embryogenesis, we found expression in several locations of developing forebrain and midbrain. Tyrosinase, TRP-1 and TRP-2 had been equally found in extracts of adult mouse brain. In adult brain, we detected tyrosinase promoter activity in cortex, olfactory system, hippocampus, epithalamus and substantia nigra, areas corresponding to positive staining during embryogenesis. Thus, tyrosinase promoter is active throughout murine brain development, and tyrosinase could be implicated in neuromelanin formation in the substantia nigra, and in neurodegenerative disorders like Parkinson's disease.

Cooperative effects of INK4a and ras in melanoma susceptibility in vivo

The familial melanoma gene (INK4a/MTS1/CDKN2) encodes potent tumor suppressor activity. Although mice null for the ink4a homolog develop a cancer-prone condition, a pathogenetic link to melanoma susceptibility has yet to be established. Here we report that mice with melanocyte-specific expression of activated H-rasG12V on an ink4a-deficient background develop spontaneous cutaneous melanomas after a short latency and with high penetrance. Consistent loss of the wild-type ink4a allele was observed in tumors arising in ink4a heterozygous transgenic mice. No homozygous deletion of the neighboring ink4b gene was detected. Moreover, as in human melanomas, the p53 gene remained in a wild-type configuration with no observed mutation or allelic loss. These results show that loss of ink4a and activation of Ras can cooperate to accelerate the development of melanoma and provide the first in vivo experimental evidence for a causal relationship between ink4a deficiency and the pathogenesis of melanoma. In addition, this mouse model affords a system in which to identify and analyze pathways involved in tumor progression against the backdrop of genetic alterations encountered in human melanomas.

Chromosomal and genetic alterations of 7,12-dimethylbenz[a]anthracene-induced melanoma from TP-ras transgenic mice

The TP-ras transgenic mouse line expresses an activated human T24 Ha-ras gene with a mutation in codon 12, regulated by a mouse tyrosinase promoter. The transgene is expressed in melanocytes of the skin, eyes, and brain. The mice develop cutaneous melanoma when treated with 7,12-dimethylbenz[a]anthracene. Cell lines have been generated from the cutaneous tumors and metastatic lesions. By using fluorescence in situ hybridization with mouse whole chromosome paints, the cell lines were characterized for chromosomal abnormalities. Key findings in the tumor cells included translocations of chromosome 4 and alterations in chromosome 6. One tumor cell line contained a double translocation involving chromosomes 3 and 6. To extend the results of the chromosome 4 painting, Southern analysis of the p15INK4B, p16INK4A, and p19INK4D genes was performed. Our data indicated that there were homozygous and partial allelic deletions and polymorphisms in the region of chromosome 4 containing these genes, resulting in the absence or reduced expression of the p16 product. These findings are similar to those reported for human melanoma, and the TP-ras transgenic mouse may therefore be a valuable model for studying novel strategies for melanoma prevention and treatment.

Regulation of the tyrosinase promoter in transgenic mice: expression of a tyrosinase-lacZ fusion gene in embryonic and adult brain

The enzyme tyrosinase is indispensable for pigmentation and the gene is expressed mainly in pigment cells. Regulatory elements, at -12 to -15 kb (enhancer) and within the 270 bp directly upstream of the transcription start site, have been described recently and their importance demonstrated in transgenic experiments. We were interested in tyrosinase promoter activity during development and used beta-galactosidase as reporter gene. Transgenic mice were generated carrying a tyrosinase-lacZ fusion gene, containing 6.1 kb of tyrosinase 5' sequences. In transgenic embryos, beta-galactosidase activity was detected along the entire neural tube, with the most prominent expression in the developing telencephalon, and also in the adult brain. Equivalent expression was observed in the developing retina. Tyrosinase protein was identified in embryonic and adult brain, but no DOPAoxidase or tyrosine hydroxylase activity was detected. From our results we conclude that 1) tyrosinase protein is present in embryonic and adult mouse brain and 2) the tyrosinase promoter can direct expression of a reporter gene to pigment cells and neural tissues.