Cooperative effects of INK4a and ras in melanoma susceptibility in vivo

Oncogenic NRAS cooperates with p53 loss to generate melanoma in zebrafish

NRAS mutations are a common oncogenic event in skin cancer, occurring frequently in congenital nevi and malignant melanoma. To study the role of NRAS in zebrafish, a transgenic approach was applied to generate fish that express human oncogenic NRAS(Q61K) under the control of the melanocyte-restricted mitfa promoter. By screening the progeny of the injected animals, two strains stably expressing the NRAS transgene were identified: Tg(mitfa:EGFP:NRAS(Q61K))(1) and Tg(mitfa:EGFP:NRAS(Q61K))(2). Stable expression of this transgene results in hyperpigmented fish displaying a complete ablation of the normal pigment pattern. Although oncogenic NRAS expression alone was found to be insufficient to promote tumor formation, loss of functional p53 was found to collaborate with NRAS expression in the genesis of melanoma. The tumors derived from these animals are variably pigmented and closely resemble human melanoma. Underscoring the pathological similarities between these tumors and human disease and suggesting that common pathways are similar in these models and human disease, gene set enrichment analysis performed on microarray data found that the upregulated genes from zebrafish melanomas are highly enriched in human tumor samples. This work characterizes two zebrafish melanoma models that will be useful tools for the study of melanoma pathogenesis.

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.

Understanding melanoma signaling networks as the basis for molecular targeted therapy

Despite years of research, there has been little improvement in survival for patients with disseminated melanoma. Recent work has identified mutations in BRAF and NRAS, leading to constitutive mitogen-activated protein kinase (MAPK) pathway as well as constitutive activity in the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) pathway, as being critical events in melanoma growth and progression. In the current review, we discuss how these complex mutational and signaling profiles can be understood using a network biology approach, and suggest how an understanding of the key signaling nodes involved in progression and survival will lead to improvements in melanoma therapy.

Drug efficacy testing in mice

The traditional path of drug development passes from in vitro screening and response assessment to validation of drug efficacy in cell line xenografts. While xenografts have their merits, historically, more often than not, they have not served as an accurate predictor of drug efficacy in humans. The refinement and increased availability of genetically engineered mouse models (GEMMs) of cancer has made GEMMs an attractive avenue for the preclinical testing of therapeutic agents. The histopathologic and genetic resemblance of GEMMs to human cancer are an important measure to evaluate their suitability for pre-clinical studies and a number of studies using kinase inhibitors have now been performed in GEMMs. We have highlighted several of the salient advantages and challenges associated with GEMM studies. Well-characterized GEM models of human cancer should aide in the prioritization of both established and novel therapeutics.

Melanoma-initiating cells: a compass needed

Most tumours contain a heterogeneous population of cancer cells, which harbour a range of genetic mutations and have probably undergone deregulated differentiation programmes that allow them to adapt to tumour microenvironments. Another explanation for tumour heterogeneity might be that the cells within a tumour are derived from tumour-initiating cells through diverse differentiation programmes. Tumour-initiating cells are thought to constitute one or more distinct subpopulations within a tumour and to drive tumour initiation, development and metastasis, as well as to be responsible for their recurrence after therapy. Recent studies have raised crucial questions about the nature, frequency and importance of melanoma-initiating cells. Here, we discuss our current understanding of melanoma-initiating cells and outline several approaches that the scientific community might consider to resolve the controversies surrounding these cells.

Melanoma genetics: an update on risk-associated genes

The past 15 years have seen rapid advances in both our understanding of hereditary melanoma genetics and the technologies that enable scientists to make discoveries. Despite great efforts by many groups worldwide, other high-risk melanoma loci besides CDKN2A still remain elusive. A panel of polymorphisms that appears to confer low-to-moderate risk for melanoma has been assembled through functional and genome-wide association studies. The goal of personalized melanoma risk prediction is within our reach, although true clinical use has yet to be established.

Detection of two novel mutations and relatively high incidence of H-RAS mutations in Vietnamese oral cancer

Oral squamous cell carcinoma is the sixth most common cancer in the world and the seventh most common cancer in Vietnam. The RAS and PI3K-AKT signaling pathways play an important role in oral carcinogenesis. Our previous study on PI3K signaling pathway showed the absence of PIK3CA and PTEN gene mutations in Vietnamese oral cancer. We thus hypothesized that the RAS could be more likely activated as an upstream effector. However, the status of RAS mutations in Vietnamese oral cancer had not been studied. In the present study, Fifty six primary tumor DNA samples were screened for mutations of hot spots in exons 1 and 2 of H-RAS and a part of the samples for exon 7 of ERK2 gene in which we previously reported a mutation in an OSCC cell line. The H-RAS mutations were detected in 10 of 56 tumors (18%). Two novel mutations were found, one was an insertion of three nucleotides (GGC) between codons 10 and 11 resulting in in-frame insertion of glycine (10(Gly)11) and the other was a missense mutation in codon 62 (GAG>GGG). We also found T81C single nucleotide polymorphism in 12 of 56 tumors (22%) and there was no mutation in exon 7 of ERK2 gene. The H-RAS mutation incidence showed significant association with advanced stages of the tumor and also with well-differentiated tumor grade. Our study is the first to report H-RAS mutation from Vietnamese ethnicity, with two novel mutations and relatively high incidence of H-RAS mutations. The results suggest that RAS is an important member in the PI3K-AKT signaling and could play an important role in the tumorigenesis of oral carcinoma.

Molecular pathogenesis of cutaneous melanocytic neoplasms

Melanoma is the deadliest form of skin cancer without an effective treatment. An understanding of the genetic basis of melanoma has recently shed light on some of the mechanisms of melanomagenesis. This review explores the major genes involved in familial and sporadic cutaneous melanoma with an emphasis on CDKN2A, CDK4, MC1R, and MAPK pathway targets (e.g., RAS and BRAF), apoptosis regulators (e.g., BCL-2, AKT, and APAF-1), and the tumor-suppressor genes TP53 and PTEN. New directions for therapeutics based on our current knowledge of the genes implicated in melanoma are also discussed.

Dissecting the roles of Raf- and PI3K-signalling pathways in melanoma formation and progression in a zebrafish model

Deregulated Ras signalling is implicated in most human neoplasia, exemplified by melanoma. Whereas Raf activation occurs almost ubiquitously in benign and malignant melanocytic neoplasms, implying an involvement in tumour initiation, phosphoinositide 3-kinase (PI3K) activation occurs predominantly in malignant neoplasms, implying an involvement in malignant progression. Here, we dissect the contributions of these two pathways to tumourigenesis in vivo, by modulating their activities in zebrafish melanocytes. Misexpression of oncogenic Ras (V12RAS) in founder fish induced frequent melanoma, beginning at larval stages, with concomitant activation of Raf-Mek-Erk and PI3K-Akt signalling. Misexpression of effector-domain mutants of V12RAS, or of various downstream effectors, confirmed a selective role for the Raf-Mek-Erk pathway in initiating neoplasia, but highlighted the requirement for additional Ras effector pathways for malignancy. The phenotype of animals with germ-line transmission of V12RAS resembled familial atypical mole and melanoma (FAMM) syndrome: melanocytes displayed hyperplasia, dysplasia, altered terminal differentiation and spontaneously progressed to invasive melanoma. Co-expressing a dominant-interfering form of PI3K abolished V12RAS-induced malignancy, demonstrating a direct role for PI3K signalling in the malignant progression of melanoma in vivo, and highlighting PI3K as a promising target for melanoma therapy.

Melanocytic nevus-like hyperplasia and melanoma in transgenic BRAFV600E mice

BRAF, a cellular oncogene and effector of RAS-mediated signaling, is activated by mutation in approximately 60% of melanomas. Most of these mutations consist of a V600E substitution resulting in constitutive kinase activation. Mutant BRAF thus represents an important therapeutic target in melanoma. In an effort to produce a pre-clinical model of mutant BRAF function in melanoma, we have generated a mouse expressing BRAF V600E targeted to melanocytes. We show that in these transgenic mice, widespread benign melanocytic hyperplasia with histological features of nevi occurs, with biochemical evidence of senescence. Melanocytic hyperplasia progresses to overt melanoma with an incidence dependent on BRAF expression levels. Melanomas show CDKN2A loss, and genetic disruption of the CDKN2A locus greatly enhances melanoma formation, consistent with collaboration between BRAF activation and CDKN2A loss suggested from studies of human melanoma. The development of melanoma also involves activation of the Mapk and Akt signaling pathways and loss of senescence, findings that faithfully recapitulate those seen in human melanomas. This murine model of mutant BRAF-induced melanoma formation thus provides an important tool for identifying further genetic alterations that cooperates with BRAF and that may be useful in enhancing susceptibility to BRAF-targeted therapeutics in melanoma.

Melanoma and genetics

As the incidence of malignant melanoma continues to increase and with the completion of the sequencing of the human genome, there have been increasing efforts to identify the "melanoma gene(s)." Although some patients and families have significantly increased risks due to genetic predisposition, most melanoma cases are sporadic and likely result from low to moderate risk genetic factors. This review focuses on the genes that cover the greatest risk of developing melanoma. It is important to remember that many--if not most--cases of melanoma are the result of undiscovered variants. The strongest genetic risk for the development of melanoma results from heritable alterations in cyclin-dependent kinase inhibitor 2A (CDKN2A) gene, which encodes two separate but related proteins, p16/INK4a and p14/ARF. These proteins help regulate cell division and apoptosis, both of which are necessary to maintain cellular homeostasis. Other important genes include CDK4/6 and retinoblastoma (RB1), which encode downstream proteins in the same pathway as p16/INK4a and p14/ARF. Finally, we discuss the relative importance of the melanocortin 1 receptor (MC1R) gene as a moderate risk factor for melanoma. Although great advances have been made in understanding the molecular basis and genetic predisposition of melanoma, many questions still remain to be answered. Someday soon, it will be possible to predict a patient's risk of melanoma by DNA analysis; however, it is important to reconcile our tremendous technologic capabilities with documented clinical utility.

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.

Uroplakins in urothelial biology, function, and disease

Urothelium covers the inner surfaces of the renal pelvis, ureter, bladder, and prostatic urethra. Although morphologically similar, the urothelia in these anatomic locations differ in their embryonic origin and lineages of cellular differentiation, as reflected in their different uroplakin content, expandability during micturition, and susceptibility to chemical carcinogens. Previously thought to be an inert tissue forming a passive barrier between the urine and blood, urothelia have recently been shown to have a secretory activity that actively modifies urine composition. Urothelial cells express a number of ion channels, receptors, and ligands, enabling them to receive and send signals and communicate with adjoining cells and their broader environment. The urothelial surface bears specific receptors that not only allow uropathogenic E. coli to attach to and invade the bladder mucosa, but also provide a route by which the bacteria ascend through the ureters to the kidney to cause pyelonephritis. Genetic ablation of one or more uroplakin genes in mice causes severe retrograde vesicoureteral reflux, hydronephrosis, and renal failure, conditions that mirror certain human congenital diseases. Clearly, abnormalities of the lower urinary tract can impact the upper tract, and vice versa, through the urothelial connection. In this review, we highlight recent advances in the field of urothelial biology by focusing on the uroplakins, a group of urothelium-specific and differentiation-dependent integral membrane proteins. We discuss these proteins' biochemistry, structure, assembly, intracellular trafficking, and their emerging roles in urothelial biology, function, and pathological processes. We also call attention to important areas where greater investigative efforts are warranted.

Melanoma biology and the promise of zebrafish

Advantageous organismal and technical attributes of the zebrafish are being increasingly applied to study cancer biology. Along with other tumor models, zebrafish that develop melanomas have been generated. In both genetics and phenotype, zebrafish melanomas are strikingly similar to their human counterparts. For this reason, studies in the zebrafish are poised to make significant contributions to melanoma biology. In this review, we summarize important features of human melanoma and discuss how the zebrafish can be used to address many questions that remain unanswered about this devastating disease.

Oncogene-induced senescence does not require the p16(INK4a) or p14ARF melanoma tumor suppressors

Oncogene-induced senescence is considered to act as a potent barrier to cell transformation, and has been seen in vivo during the early stages of tumor development. Human nevus cells frequently express oncogenic N-RAS or B-RAF, and are thought to be permanently growth arrested. Many studies have suggested that the p16(INK4a) and, to a lesser extent, the p14ARF tumor suppressor proteins act as critical triggers of oncogene-induced senescence in nevi, and thus these proteins represent major inhibitors of progression to melanoma. There have also been reports, however, showing that p16(INK4a) and/or p14ARF is not sufficient to execute the oncogene-induced senescence program. In this study, we examined the impact of melanoma-associated N-RAS(Q61K) on melanocyte senescence and utilized RNA-interference vectors to directly assess the individual contribution of human p14ARF and p16(INK4a) genes to the N-RAS-induced senescence program. We formally show that cultured human melanocytes can initiate an effective oncogene-mediated senescence program in the absence of INK4a/ARF-encoded proteins. Our data are consistent with observations showing that senescent nevus cells do not always express p16(INK4a), and highlight the need to thoroughly explore INK4a/ARF-independent molecular pathways of senescence in human melanocytes.

Modelling oncogenic Ras/Raf signalling in the mouse

The Ras/Raf/MEK/ERK (or MAPK) signalling pathway relays extracellular stimuli to the nucleus, thereby regulating diverse cellular responses such as proliferation, growth, differentiation and apoptosis. Perturbation of these processes by aberrant MAPK signalling often leads to malignant transformation as indicated by the frequent occurrence in human cancers of genetic alterations affecting this pathway. In recent years, genetically modified mouse models have proven instrumental in unravelling how deregulated MAPK signalling leads to disease. Indeed, conditional activation of oncogenic K-Ras or B-Raf in mice resulted in neoplasms that closely resemble the human disease. Such tractable mouse models will enable the pursuit of basic biological mechanisms and translational applications regarding the MAPK pathway.

Bypassing melanocyte senescence by beta-catenin: a novel way to promote melanoma

The Wnt/beta-catenin signaling pathway plays a key role in several cellular functions during embryonic development and adult homeostasis. The deregulation of this pathway may lead to the development of cancer, including melanoma. Deregulation of the Wnt/beta-catenin pathway occurs through either the induction/repression of, or specific mutations in, various members of this signaling pathway; this results in the stabilization of beta-catenin and its translocation from the cytoplasm to the nucleus, where it regulates transcription. Although nuclear beta-catenin is clearly involved in malignant transformation, the mechanism by which it exerts its effects remains elusive. This review focuses on the molecular and cellular mechanisms that are driven by beta-catenin and lead to melanocyte transformation. In particular, we describe how beta-catenin induces melanocyte immortalization, a novel activity of this multifunction protein. Finally, we discuss how beta-catenin-induced immortalization can cooperate with MAPKinase pathways to produce melanoma.

Melanoma genetics and therapeutic approaches in the 21st century: moving from the benchside to the bedside

Metastatic melanoma is notoriously one of the most difficult cancers to treat. Although many therapeutic regimens have been tested, very few achieve response rates greater than 25%. Given the rising incidence of melanoma and the paucity of effective treatments, there is much hope and excitement in leveraging recent genetic and molecular insights for therapeutic advantage. Over the past 30 years, elegant studies by many groups have helped decipher the complex genetic networks involved in melanoma proliferation, progression and survival, as well as several genes involved in melanocyte development and survival. Many of these oncogenic loci and pathways have become crucial targets for pharmacological development. In this article we review: (1) our current understanding of melanoma genetics within the context of signaling networks; (2) targeted therapies, including an extensive discussion of promising agents that act in the Bcl-2 signaling network; (3) future areas of research.

Pten deficiency in melanocytes results in resistance to hair graying and susceptibility to carcinogen-induced melanomagenesis

Phosphate and tensin homologue deleted on chromosome 10 (PTEN) is a tumor suppressor gene inactivated in numerous sporadic cancers, including melanomas. To analyze Pten functions in melanocytes, we used the Cre-loxP system to delete Pten specifically in murine pigment-producing cells and generated DctCrePten(flox/flox) mice. Half of DctCrePten(flox/flox) mice died shortly after birth with enlargements of the cerebral cortex and hippocampus. Melanocytes were increased in the dermis of perinatal DctCrePten(flox/flox) mice. When the mutants were subjected to repeated depilations, melanocyte stem cells in the bulge of the hair follicle resisted exhaustion and the mice were protected against hair graying. Although spontaneous melanomas did not form in DctCrePten(flox/flox) mice, large nevi and melanomas developed after carcinogen exposure. DctCrePten(flox/flox) melanocytes were increased in size and exhibited heightened activation of Akt and extracellular signal-regulated kinases, increased expression of Bcl-2, and decreased expression of p27(Kip1). Our results show that Pten is important for the maintenance of melanocyte stem cells and the suppression of melanomagenesis.

G-protein-coupled receptors and melanoma

G-protein-coupled receptors (GPCR) are the largest family of receptors with over 500 members. Evaluation of GPCR gene expression in primary human tumors identified over-expression of GPCR in several tumor types. Analysis of cancer samples in different disease stages also suggests that some GPCR may be involved in early tumor progression and others may play a critical role in tumor invasion and metastasis. Currently, >50% of drug targets to various human diseases are based on GPCR. In this review, the relationships between several GPCR and melanoma development and/or progression will be discussed. Finally, the possibility of using one or more of these GPCR as therapeutic targets in melanoma will be summarized.

Engineering and selection of shuffled AAV genomes: a new strategy for producing targeted biological nanoparticles

We report a DNA shuffling-based approach for developing cell type-specific vectors through directed evolution. Capsid genomes of adeno-associated virus (AAV) serotypes 1-9 were randomly fragmented and reassembled using PCR to generate a chimeric capsid library. A single infectious clone (chimeric-1829) containing genome fragments from AAV1, 2, 8, and 9 was isolated from an integrin minus hamster melanoma cell line previously shown to have low permissiveness to AAV. Molecular modeling studies suggest that AAV2 contributes to surface loops at the icosahedral threefold axis of symmetry, while AAV1 and 9 contribute to two- and fivefold symmetry interactions, respectively. The C-terminal domain (AAV9) was identified as a critical structural determinant of melanoma tropism through rational mutagenesis. Chimeric-1829 utilizes heparan sulfate as a primary receptor and transduces melanoma cells more efficiently than all serotypes. Further, chimeric-1829 demonstrates altered tropism in rodent skeletal muscle, liver, and brain including nonhuman primates. We determined a unique immunological profile based on neutralizing antibody (NAb) titer and crossreactivity studies strongly supporting isolation of a synthetic laboratory-derived capsid variant. Application of this technology to alternative cell/tissue types using AAV or other viral capsid sequences is likely to yield a new class of biological nanoparticles as vectors for human gene transfer.

B16 as a mouse model for human melanoma

This unit details protocols for in vivo models of subcutaneous growth and pulmonary metastases of B16 melanoma. Therapeutic approaches include the use of B16.GM-CSF and rVVmTRP-1 to induce autoimmune vitiligo and tumor protection. The induction and use of gp 100-specific therapeutic cytotoxic T lymphocytes (CTL) are discussed. Methods are also included for CTL induction, isolation and testing, CTL maintenance, and adoptive transfer. Support protocols detail the testing of mouse sera for presence of MDA-specific antibodies by immunoblotting and ELISA, respectively. Additional sections, including growing B16 melanoma, enumerating pulmonary metastases, and use of recombinant viruses for vaccination, are discussed together with safety concerns.

Metabotropic glutamate receptors (mGlus) and cellular transformation

Although the glutamatergic system usually functions in the CNS, expression has been observed in non-neuronal tissues and a subset of cancers. Metabotropic glutamate receptors (mGlus) are highly "druggable" GPCRs and thus a priority for validation as therapeutic targets. We have previously reported that the aberrant expression of mGlu1 is sufficient to induce spontaneous melanoma development in vivo. We isolated and characterized several stable mGlu1-mouse melanocytic clones and demonstrated that these clones are transformed and tumorigenic. We hypothesize that expression of mGlus may not be uncommon in the pathogenesis of tumors other than melanoma, and that activity of an otherwise normal glutamate receptor in an ectopic cellular environment involves signaling pathways which dysregulate cell growth, ultimately leading to tumorigenesis. As most human cancers are of epithelial origin (carcinomas), in this review, the possibility that mGlu1 could function as a complete oncogene and transform epithelial cells is also discussed.

The MAPK pathway in melanoma

PURPOSE OF REVIEW

As understanding of molecular and genetic processes in cancer evolves, so does appreciation of tumor heterogeneity. Tumor profiling has expanded knowledge of relevant pathways, and their interplay. Similar to the revolution in breast cancer with the discovery and successful therapeutic targeting of HER2/neu, the melanoma field is rapidly evolving. The MAPK pathway is dysregulated in most melanomas. Several therapeutic agents directed against this pathway are in development. This review summarizes current understanding of the MAPK pathway in melanoma biology and therapeutic strategies.

RECENT FINDINGS

Recent data support the concept of distinct groups of molecular and genetic abnormalities in melanomas, related to type of sun exposure and body site. MAPK abnormalities, specifically BRAF or NRAS mutations, are most prevalent. The efficacy of sorafenib, a multitargeted kinase inhibitor, in melanoma is still under evaluation. While ineffective as a single agent, efficacy in combination with chemotherapy or targeted agents is being assessed. More specific inhibitors of BRAF, or other MAPK members, may prove more effective.

SUMMARY

Tumor profiling has led to exciting advances. The MAPK pathway is one of several potentially targetable pathways in melanoma. Ultimately, combinatorial therapeutics against relevant disrupted pathways in specific tumors likely will prove most successful.

The RTK/RAS/BRAF/PI3K pathways in melanoma: biology, small molecule inhibitors, and potential applications

The discovery of mutations in the BRAF signaling molecule in a large proportion of cutaneous melanomas immediately suggested the prospect of effective therapies for this disease. The most appealing initial target has been BRAF itself, as most mutations involve a single residue in the kinase domain of the protein. But the identification of the high mutation rate in this signaling intermediate also suggests that other molecules up- and downstream of BRAF might be productively targeted. Indeed, several receptor tyrosine kinases, as well as RAS, are mutated in a small number of melanoma cases. Moreover, genetic alterations in the phosphotidylinositol-3-kinase (PI3K) pathway, especially in PTEN, suggest that this route also poses opportunities for therapeutic exploitation. We will review here the genetic evidence suggesting the utility of targets on these pathways. We will also summarize the recent clinical data that have accumulated from initial trials designed to test BRAF inhibition and targeting of other molecules. Finally, we provide an overview of molecules entering the clinic and soon to be tested in clinical studies, as well as strategies for their employment as monotherapy and in combinations.

Preclinical studies of celastrol and acetyl isogambogic acid in melanoma

PURPOSE

Sensitize melanomas to apoptosis and inhibit their growth and metastatic potential by compounds that mimic the activities of activating transcription factor 2 (ATF2)-driven peptides.

EXPERIMENTAL DESIGN

Small-molecule chemical library consisting of 3,280 compounds was screened to identify compounds that elicit properties identified for ATF2 peptide, including (a) sensitization of melanoma cells to apoptosis, (b) inhibition of ATF2 transcriptional activity, (c) activation of c-Jun NH(2)-terminal kinase (JNK) and c-Jun transcriptional activity, and (d) inhibition of melanoma growth and metastasis in mouse models.

RESULTS

Two compounds, celastrol (CSL) and acetyl isogambogic acid, could, within a low micromolar range, efficiently elicit cell death in melanoma cells. Both compounds efficiently inhibit ATF2 transcriptional activities, activate JNK, and increase c-Jun transcriptional activities. Similar to the ATF2 peptide, both compounds require JNK activity for their ability to inhibit melanoma cell viability. Derivatives of CSL were identified as potent inducers of cell death in mouse and human melanomas. CSL and a derivative (CA19) could also efficiently inhibit growth of human and mouse melanoma tumors and reduce the number of lung metastases in syngeneic and xenograft mouse models.

CONCLUSIONS

These studies show for the first time the effect of CSL and acetyl isogambogic acid on melanoma. These compounds elicit activities that resemble the well-characterized ATF2 peptide and may therefore offer new approaches for the treatment of this tumor type.

Genetic mutations involved in melanoma: a summary of our current understanding

The biomolecular understanding of melanoma is in flux. The importance of high-penetrance genes involved in familial melanoma includes a significant number of mutations that directly lead to impairment of the checkpoints of the normal cell cycle. Furthermore, a greater understanding of the interaction between genetic factors and environmental factors, such as MC1R, CDKN2A, BRAF, and ultraviolet light, is emerging from landmark research. Although currently and with rare exception most clinicians still confine genetic testing to the realm of research, even in familial melanoma, continued and major advances in this arena may lead to development of new and revolutionary means of diagnosis and treatment, patterned on improved understanding of melanoma-related genetic mutations and resultant aberrations in cellular pathways.

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.

Human cutaneous melanoma; a review of NRAS and BRAF mutation frequencies in relation to histogenetic subclass and body site

A majority of cutaneous melanomas show activating mutations in the NRAS or BRAF proto-oncogenes, components of the Ras-Raf-Mek-Erk signal transduction pathway. Consistent data demonstrate the early appearance, in a mutually exclusive manner, of these mutations. The purpose of this paper is to summarize the literature on NRAS and BRAF activating mutations in melanoma tumors with respect to available data on histogenetic classification as well as body site and presumed UV-exposure. Common alterations of the signal transducing network seem to represent molecular hallmarks of cutaneous melanomas and therefore should continue to strongly stimulate design and testing of targeted molecular interventions.

Beta-catenin induces immortalization of melanocytes by suppressing p16INK4a expression and cooperates with N-Ras in melanoma development

Tumor progression is a multistep process in which proproliferation mutations must be accompanied by suppression of senescence. In melanoma, proproliferative signals are provided by activating mutations in NRAS and BRAF, whereas senescence is bypassed by inactivation of the p16(Ink4a) gene. Melanomas also frequently exhibit constitutive activation of the Wnt/beta-catenin pathway that is presumed to induce proliferation, as it does in carcinomas. We show here that, contrary to expectations, stabilized beta-catenin reduces the number of melanoblasts in vivo and immortalizes primary skin melanocytes by silencing the p16(Ink4a) promoter. Significantly, in a novel mouse model for melanoma, stabilized beta-catenin bypasses the requirement for p16(Ink4a) mutations and, together with an activated N-Ras oncogene, leads to melanoma with high penetrance and short latency. The results reveal that synergy between the Wnt and mitogen-activated protein (MAP) kinase pathways may represent an important mechanism underpinning the genesis of melanoma, a highly aggressive and increasingly common disease.

Predominant formation of heavily pigmented dermal melanocytomas resembling 'animal-type' melanomas in hepatocyte growth factor (C57BL/6 x C3H)F1 mice following neonatal UV irradiation

BACKGROUND

Transgenic mice expressing hepatocyte growth factor (HGF) develop cutaneous melanocytic tumors following neonatal UV exposure. Here, we examined the histologic spectrum of UV-induced melanocytic tumors in HGF mice on a pigmented (C57BL/6 x C3H/HeN)F(1) background.

METHODS

Neonatally irradiated (4000 J/m(2)) mice were monitored for 43 weeks, and 31/34 (91%) animals developed a total of 163 melanocytic tumors.

RESULTS

Of 54 primary tumors analyzed, most (49/54, 91%) demonstrated exclusively dermal collections of epithelioid cells with voluminous densely pigmented cytoplasm. Seven of these also demonstrated a population of spindled cells with mitoses. Several (3/54, 6%) tumors exhibited a junctional component with melanocytes present in the epidermis. Staining with PEP8 confirmed the presence of interfollicular melanocytes at the dermal-epidermal junction in neonatal skin.

CONCLUSIONS

In contrast to HGF animals on an albino (FVB) background, HGF animals on the pigmented (C57BL/6 x C3H/HeN)F(1) background do not develop classic radial growth phase melanoma but rather predominantly develop dermal melanocytomas resembling the 'animal-type' melanoma occasionally seen in humans. These results demonstrate the influence of genetic background on histologic pattern of UV-induced melanomas in mice.

Generation of a prophylactic melanoma vaccine using whole recombinant yeast expressing MART-1

Malignant melanoma is a potentially deadly form of skin cancer and people at high-risk of developing melanoma will benefit from effective preventive intervention. Yeast can be used as an efficient vehicle of antigen loading and immunostimulation. Saccharomyces cerevisiae is not pathogenic to humans and can be easily engineered to express specific antigens. In this study, we have developed a melanoma vaccine using a yeast-based platform expressing a full-length melanocyte/melanoma protein to investigate its utility as a prophylactic melanoma vaccine in a transplantable mouse melanoma model. Yeast was engineered and expanded in vitro without technical difficulties, administered easily with subcutaneous injection, and did not show adverse effects, indicating its practical applicability and favourable safety profile. Despite the lack of knowledge of dominant epitopes of the protein recognized by mouse MHC-class I, the vaccine protected mice from tumor development and induced efficient immune responses, suggesting that the precise knowledge of epitopic sequences and the matched HLA type is not required when delivering a full-length protein using the yeast platform. In addition, the vaccine stimulated both CD4(+) T cells and CD8(+) T cells simultaneously. This study provides a 'proof of principle' that recombinant yeast can be utilized as an effective prophylactic vaccine to target patients at high-risk for melanoma.

Loss of xeroderma pigmentosum C (Xpc) enhances melanoma photocarcinogenesis in Ink4a-Arf-deficient mice

Despite an extensive body of evidence linking UV radiation and melanoma tumorigenesis, a clear mechanistic understanding of this process is still lacking. Because heritable mutations in both INK4a and the nucleotide excision repair (NER) pathway predispose individuals to melanoma development, we set out to test the hypothesis that abrogation of NER, by deletion of the xeroderma pigmentosum C (Xpc) gene, will heighten melanoma photocarcinogenesis in an Ink4a-Arf-deficient background. Experimentally, we generated a strain of mice doubly deficient in Xpc and Ink4a-Arf and subjected wild-type, Xpc-/-Ink4a-Arf+/+, Xpc-/-Ink4a-Arf-/-, and Xpc+/+Ink4a-Arf-/- mice to a single neonatal (day P3) dose of UVB without additional chemical promotion. Indeed, there was a significant increase in the development of dermal spindle/epithelioid cell melanomas in Xpc-/-Ink4a-Arf-/- mice when compared with Xpc+/+Ink4a-Arf-/- mice (P = 0.005); wild-type and Xpc-/-Ink4a-Arf+/+ mice failed to develop tumors. These neoplasms bore a striking histologic resemblance to melanomas that arise in the Tyr-vHRAS/Ink4a-Arf-/- context and often expressed melanocyte differentiation marker Tyrp1, thus supporting their melanocytic origination. All strains, except wild-type mice, developed pigmented and non-pigmented epidermal-derived keratinocytic cysts, whereas Xpc+/+Ink4a-Arf-/- mice exhibited the greatest propensity for squamous cell carcinoma development. We then screened for NRas, HRas, Kras, and BRaf mutations in tumor tissue and detected a higher frequency of rare Kras(Q61) alterations in tumors from Xpc-/-Ink4a-Arf-/- mice compared with Xpc+/+Ink4a-Arf-/- mice (50% versus 7%, P = 0.033). Taken together, results from this novel UV-inducible melanoma model suggest that NER loss, in conjunction with Ink4a-Arf inactivation, can drive melanoma photocarcinogenesis possibly through signature Kras mutagenesis.

Models and mechanisms in malignant melanoma

Human melanoma represents the fastest growing malignancy in the US. The etiology of melanoma is highly debated as is the role of ultraviolet (UV) radiation in the initiation and progression of melanoma. This article discusses data from UV exposure and its relationship to the development of melanoma from various models of melanoma as well as various genetic alterations seen in oncogenic transformation of melanocytes. Genetic alterations such as the p16(INK4a) deletion, melanocortin 1 receptor (MC1R), RAS, and v-raf murine sarcoma viral oncogene homolog B1 (BRAF) may be indicative of a predisposition to melanoma development. Historical research as well as current data on the significance of the hot spot mutation in BRAF is discussed in its relative potential to the activating mutation in RAS.

Broad spectrum identification of SUMO substrates in melanoma cells

Like phosphorylation, protein sumoylation likely represents a dynamic PTM to alter protein function in support of cell regulatory systems. The broad-spectrum impact of transient or chronic engagement of signal transduction cascades on protein sumoylation has not been explored. Here, we find that epidermal growth factor (EGF) stimulation evokes a rapid alteration in small ubiquitin modifier (SUMO) target selection, while oncogene expression alters steady-state SUMO-protein profiles. A proteomic SUMO target analysis in melanoma cells identified proteins involved in cellular signaling, growth control, and neural differentiation.

BRAF(E600) in benign and malignant human tumours

Of the RAF family of protein kinases, BRAF is the only member to be frequently activated by mutation in cancer. A single amino acid substitution (V600E) accounts for the vast majority and results in constitutive activation of BRAF kinase function. Its expression is required to maintain the proliferative and oncogenic characteristics of BRAF(E600)-expressing human tumour cells. Although BRAF(E600) acts as an oncogene in the context of additional genetic lesions, in primary cells it appears to be associated rather with transient stimulation of proliferation. Eventually, BRAF(E600) signalling triggers cell cycle arrest with the hallmarks of cellular senescence, as is illustrated by several recent studies in cultured cells, animal models and benign human lesions. In this review, we will discuss recent advances in our understanding of the role of BRAF(E600) in benign and malignant human tumours and the implications for therapeutic intervention.

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.

Systemic targeting inhibitor of kappaB kinase inhibits melanoma tumor growth

Constitutive activation of nuclear factor-kappaB (NF-kappaB) has been directly implicated in tumorigenesis of various cancer types, including melanoma. Inhibitor of kappaB kinase (IKK) functions as a major mediator of NF-kappaB activation. Thus, development of an IKK-specific inhibitor has been a high priority, although it remains unclear whether systemic inhibition of IKK will provide therapeutic benefit. In this study, we show that inhibition of NF-kappaB activity in melanocytes that are persistently expressing an active H-Ras(V12) gene and are deficient in the tumor suppressors inhibitor A of cyclin-dependent kinase 4/alternative reading frame results in reduction of melanoma tumor growth in vivo. This effect is, at least in part, via regulation of NF-kappaB nuclear activation and RelA phosphorylation. Based on this result, we developed a double hammerhead ribozyme long-term expression system to silence either IKKalpha or IKKbeta. The ribozymes were placed in an EBV construct and delivered i.v. to nude mice bearing melanoma lesions, which developed after i.v. injection of H-Ras-transformed melanoma cells. Our in vivo data show that knockdown of endogenous IKKbeta significantly reduces the growth of the melanoma lesions and knockdown of either IKKalpha or IKKbeta prolongs the life span of immunocompetent mice.

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.