Cooperative effects of INK4a and ras in melanoma susceptibility in vivo

Differential oncogenic potential of activated RAS isoforms in melanocytes

RAS genes are mutated in approximately 30% of all human cancers. Interestingly, there exists a strong bias in favor of mutation of only one of the three major RAS genes in tumors of different cellular origins. NRAS mutations occur in approximately 20% of human melanomas, whereas HRAS and KRAS mutations are rare in this disease. To define the mechanism(s) responsible for this preference in melanocytes, we compared the transformation efficiencies of mutant NRAS and KRAS in immortal, non-transformed Ink4a/Arf-deficient melanocytes. NRAS mutation leads to increased cellular proliferation and is potently tumorigenic. In contrast, KRAS mutation does not enhance melanocyte proliferation and is only weakly tumorigenic on its own. Although both NRAS and KRAS activate mitogen-activated protein kinase signaling, only NRAS enhances MYC activity in these cells. Our data suggest that the activity of specific RAS isoforms is context-dependent and provide a possible explanation for the prevalence of NRAS mutations in melanoma. In addition, understanding this mechanism will have important implications for cancer therapies targeting RAS pathways.

Hyperactivation of Ha-ras oncogene, but not Ink4a/Arf deficiency, triggers bladder tumorigenesis

Although ras is a potent mitogenic oncogene, its tumorigenicity depends on cellular context and cooperative events. Here we show that low-level expression of a constitutively active Ha-ras in mouse urothelium induces simple urothelial hyperplasia that is resistant to progression to full-fledged bladder tumors even in the absence of Ink4a/Arf. In stark contrast, doubling of the gene dosage of the activated Ha-ras triggered early-onset, rapidly growing, and 100% penetrant tumors throughout the urinary tract. Tumor initiation required superseding a rate-limiting step between simple and nodular hyperplasia, the latter of which is marked by the emergence of mesenchymal components and the coactivation of AKT and STAT pathways as well as PTEN inactivation. These results indicate that overactivation of Ha-ras is both necessary and sufficient to induce bladder tumors along a low-grade, noninvasive papillary pathway, and they shed light on the recent findings that ras activation, via point mutation, overexpression, or intensified signaling from FGF receptor 3, occurs in 70%-90% of these tumors in humans. Our results highlight the critical importance of the dosage/strength of Ha-ras activation in dictating its tumorigenicity--a mechanism of oncogene activation not fully appreciated to date. Finally, our results have clinical implications, as inhibiting ras and/or its downstream effectors, such as AKT and STAT3/5, could provide alternative means to treat low-grade, superficial papillary bladder tumors, the most common tumor in the urinary system.

Growth factors and oncogenes as targets in melanoma: lost in translation?

If untreated at early stages, melanoma becomes a highly aggressive cancer with rapid metastasis to distant sites. Although cell biologic analyses have uncovered a multitude of signaling pathways involved in melanoma genesis and progression – including the MAPK, PI3K, and FAK pathways – efficacious therapies that target these cellular components have remained elusive. Genome-wide technologies such as microarray chips and array comparative genomic hybridization have generated genetic information that can identify cellular mechanisms critical for the induction and maintainence of the malignant phenotype. Thus, such data can guide the choice of a biologically relevant drug. However, these techniques have also identified melanoma as a genetically and biologically highly heterogeneous disease that likely requires individually tailored therapies based on the patient¹s individual genetic and biologic alterations. In addition, these techniques have generated a large body of data on candidate melanoma genes that await extensive functional validation to separate so called “driver” from “passenger” events. In this review, we cover several advances in melanoma therapeutics and their current limitations as well as emerging genomic, proteomic, and epigenetic strategies for the identification of critical cellular dependencies that may be tractable to therapeutic targeting.

Physical and functional interaction of the p14ARF tumor suppressor with ribosomes

Alterations in the p14(ARF) tumor suppressor are frequent in many human cancers and are associated with susceptibility to melanoma, pancreatic cancer, and nervous system tumors. In addition to its p53-regulatory functions, p14(ARF) has been shown to influence ribosome biogenesis and to regulate the endoribonuclease B23, but there remains considerable controversy about its nucleolar role. We sought to clarify the activities of p14(ARF) by studying its interaction with ribosomes. We show that p14(ARF) and B23 interact within the nucleolar 60 S preribosomal particle and that this interaction does not require rRNA. In contrast to previous reports, we found that expression of p14(ARF) does not significantly alter ribosome biogenesis but inhibits polysome formation and protein translation in vivo. These results suggest a ribosome-dependent p14(ARF) pathway that regulates cell growth and thus complements p53-dependent p14(ARF) functions.

Therapeutic targets in melanoma: MAPKinase pathway

Recent progress in our understanding of the genetic alterations that occur in the pathogenesis of melanoma provides exciting opportunities for therapy. The most important signaling pathways in melanoma lie downstream of NRAS: the RAS-BRAF-MAPK pathway. A great deal of attention has been focused on the high mutation rate in the BRAF oncogene, which approaches 60%, because BRAF itself is an appealing drug substrate and because of the central contribution of BRAF function to melanoma development that the mutation rate signifies. Agents that specifically target BRAF, such as sorafenib, as well as new molecules that function both upstream and downstream of BRAF, are being actively investigated.

Malignant melanoma: genetics and therapeutics in the genomic era

Cell for cell, probably no human cancer is as aggressive as melanoma. It is among a handful of cancers whose dimensions are reported in millimeters. Tumor thickness approaching 4 mm presents a high risk of metastasis, and a diagnosis of metastatic melanoma carries with it an abysmal median survival of 6-9 mo. What features of this malignancy account for such aggressive behavior? Is it the migratory history of its cell of origin or the programmed adaptation of its differentiated progeny to environmental stress, particularly ultraviolet radiation? While the answers to these questions are far from complete, major strides have been made in our understanding of the cellular, molecular, and genetic underpinnings of melanoma. More importantly, these discoveries carry profound implications for the development of therapies focused directly at the molecular engines driving melanoma, suggesting that we may have reached the brink of an unprecedented opportunity to translate basic science into clinical advances. In this review, we attempt to summarize our current understanding of the genetics and biology of this disease, drawing from expanding genomic information and lessons from development and genetically engineered mouse models. In addition, we look forward toward how these new insights will impact on therapeutic options for metastatic melanoma in the near future.

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.

The mighty mouse: genetically engineered mouse models in cancer drug development

Deficiencies in the standard preclinical methods for evaluating potential anticancer drugs,such as xenograft mouse models, have been highlighted as a key obstacle in the translation of the major advances in basic cancer research into meaningful clinical benefits. In this article, we discuss the established uses and limitations of xenograft mouse models for cancer drug development, and then describe the opportunities and challenges in the application of novel genetically engineered mouse models that more faithfully mimic the genetic and biological evolution of human cancers. Greater use of such models in target validation, assessment of tumour response, investigation of pharmacodynamic markers of drug action, modelling resistance and understanding toxicity has the potential to markedly improve the success of cancer drug development.

The Oscar-worthy role of Myc in apoptosis

The discovery that the Myc oncoprotein could drive cells to undergo apoptosis in addition to its well-established role in cellular proliferation came in the early 1990s, at the beginning of a period of explosive research on cell death. Experimental evidence revealed that Myc sensitises cells to a wide range of death stimuli and abrogating this biological activity plays a profound role in tumorigenesis. Our understanding of the molecular mechanism and genetic programme of Myc-induced apoptosis remains shrouded in mystery and the focus of much attention. In this review, we will discuss established data, recent advances and future objectives regarding the regulatory processes and the functional cooperators that effect and abrogate apoptosis induced by Myc.

Characterization of melanocyte-specific inducible Cre recombinase transgenic mice

Conditional Cre-mediated recombination has emerged as a robust method of introducing somatic genetic alterations in an organ-specific manner in the mouse. Here, we generated and characterized mice harboring a 4-hydroxytamoxifen (OHT)-inducible Cre recombinase-estrogen receptor fusion transgene under the control of the melanocyte-specific tyrosinase promoter, designated Tyr::CreER(T2). Cre-mediated recombination was induced in melanocytes in a spatially and temporally controlled manner upon administration of OHT and was documented in embryonic melanoblasts, follicular bulb melanocytes, dermal dendritic melanocytes, epidermal melanocytes of tail skin, and in putative melanocyte stem cells located within the follicular bulge. Functional evidence suggestive of recombination in follicular melanocyte stem cells included the presence of Cre-mediated recombination in follicular bulb melanocytes 1 year after topical OHT administration, by which time several hair cycles have elapsed and the melanocytes residing in this location have undergone multiple rounds of apoptosis and replenishment. These Tyr:: CreER(T2) transgenic mice represent a useful resource for the evaluation of melanocyte developmental genetics, the characterization of melanocyte stem cell function and dynamics, and the construction of refined mouse models of malignant melanoma.

Genetics and biology of pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is the fourth leading cause of cancer death in the United States with a median survival of <6 mo and a dismal 5-yr survival rate of 3%-5%. The cancer's lethal nature stems from its propensity to rapidly disseminate to the lymphatic system and distant organs. This aggressive biology and resistance to conventional and targeted therapeutic agents leads to a typical clinical presentation of incurable disease at the time of diagnosis. The well-defined serial histopathologic picture and accompanying molecular profiles of PDAC and its precursor lesions have provided the framework for emerging basic and translational research. Recent advances include insights into the cancer's cellular origins, high-resolution genomic profiles pointing to potential new therapeutic targets, and refined mouse models reflecting both the genetics and histopathologic evolution of human PDAC. This confluence of developments offers the opportunity for accelerated discovery and the future promise of improved treatment.

Genetic alterations in signaling pathways in melanoma

Alterations in the RAS signaling cascade are almost uniformly present in melanoma. RAS itself is only infrequently mutated in melanoma although downstream of RAS lie BRAF on the mitogen-activated protein kinase pathway and PTEN on the protein kinase B/Akt pathway. These genes are often altered in melanomas; indeed, the most frequent target of mutation in melanomas is BRAF, which is mutated in approximately 60% to 70% of superficial spreading melanomas. These mutations occur in a background that is not normal, with the CDKN2A locus also typically being mutated. We review herein the data that suggest that the distribution of the signaling mutations is important. In general, melanomas carry a mutated NRAS, a mutated BRAF, or concurrent BRAF and PTEN mutations. These data support the hypothesis that the biochemical functions of RAS are portioned by mutations in the pathways lying downstream. Moreover, these mutations have no apparent relationship to the patterns of alteration of CDKN2A and its downstream effectors. Thus, the data also suggest that successful exploitation of mutations in melanoma will be dependent on understanding not only mutations and their frequency but their genetic context as well.

Pocket protein function in melanocyte homeostasis and neoplasia

Melanoma is the most lethal of human skin cancers and its incidence is increasing worldwide [L.K. Dennis (1999). Arch. Dermatol. 135, 275; C. Garbe et al. (2000). Cancer 89, 1269]. Melanomas often metastasize early during the course of the disease and are then highly intractable to current therapeutic regimens [M.F. Demierre and G. Merlino (2004). Curr. Oncol. Rep. 6, 406]. Consequently, understanding the factors that maintain melanocyte homeostasis and prevent their neoplastic transformation into melanoma is of utmost interest from the perspective of therapeutic interdiction. This review will focus on the role of the pocket proteins (PPs), Rb1 (retinoblastoma protein), retinoblastoma-like 1 (Rbl1 also known as p107) and retinoblastoma-like 2 (Rbl2 also known as p130), in melanocyte homeostasis, with particular emphasis on their functions in the cell cycle and the DNA damage repair response. The potential mechanisms of PP deregulation in melanoma and the possibility of PP-independent pathways to melanoma development will also be considered. Finally, the role of the PP family in ultraviolet radiation (UVR)-induced melanoma and the precise contribution that each PP family member makes to melanocyte homeostasis will be discussed in the context of a number of genetically engineered mouse models.

An inducible mouse model of melanoma expressing a defined tumor antigen

Cancer immunotherapy based on vaccination with defined tumor antigens has not yet shown strong clinical efficacy, despite promising results in preclinical models. This discrepancy might result from the fact that available preclinical models rely on transplantable tumors, which do not recapitulate the long-term host-tumor interplay that occurs in patients during progressive tumor development and results in tumor tolerance. To create a faithful preclinical model for cancer immunotherapy, we generated a transgenic mouse strain developing autologous melanomas expressing a defined tumor antigen recognized by T cells. We chose the antigen encoded by P1A, a well-characterized murine cancer germ line gene. To transform melanocytes, we aimed at simultaneously activating the Ras pathway and inactivating tumor suppressor Ink4a/Arf, thereby reproducing two genetic events frequently observed in human melanoma. The melanomas are induced by s.c. injection of 4-OH-tamoxifen (OHT). By activating a CreER recombinase expressed from a melanocyte-specific promoter, this treatment induces the loss of the conditional Ink4a/Arf gene in melanocytes. Because the CreER gene itself is also flanked by loxP sites, the activation of CreER also induces the deletion of its own coding sequence and thereby allows melanocyte-specific expression of genes H-ras and P1A, which are located downstream on the same transgene. All melanomas induced in those mice with OHT show activation of the Ras pathway and deletion of gene Ink4a/Arf. In addition, these melanomas express P1A and are recognized by P1A-specific T lymphocytes. This model will allow to characterize the interactions between the immune system and naturally occurring tumors and thereby to optimize immunotherapy approaches targeting a defined tumor antigen.

Rodent models of brain metastasis in melanoma

Metastasis of melanoma to the central nervous system (CNS) remains one of the major barriers to successful treatment of this disease. Available treatment modalities are of limited clinical efficacy. This problem is compounded by the presence of the blood-brain barrier (BBB), an important consideration in the development of new therapeutic agents. Only in animal models can the dual properties of experimental tumours and the BBB be explored in one system. A variety of rodent models have been developed, utilizing both murine and human melanoma cell lines. These models have highlighted the complex biology of cerebral metastasis, involving apparent disease progression through the selection of subclones at each stage, eventually leading to disease in the brain. As demonstrated in a number of animal studies, different subpopulations of metastatic melanoma cells are likely to be responsible for parenchymal and leptomeningeal CNS disease. In addition, these animal systems have been used to demonstrate the potential efficacy of new chemotherapeutic drugs, radiation treatments and immunotherapeutic approaches for the treatment of melanoma brain metastasis. Key biological questions remain to be answered. In particular, the molecular and cellular mechanisms responsible for establishing cerebral melanoma must be clearly delineated. Several molecules, including vascular endothelial growth factor (VEGF) and integrins, appear to play important, but not definitive, roles. Other, as yet undefined, molecules appear to be critical. The identification of these factors in experimental models, with confirmatory studies in humans, will expand our understanding of cerebral melanoma and provide valuable new therapeutic targets for intervention in this difficult clinical problem.

Induction of Nevi and Skin Tumors in Ink4a/Arf Xpa Knockout Mice by Neonatal, Intermittent, or Chronic UVB Exposures

Nevi and melanomas correlate to childhood and intermittent solar UV exposure, xeroderma pigmentosum patients run increased risk, and p16(Ink4a) expression is often lost in malignant progression. To ascertain the effect of these risk factors, pigmented hairless Ink4a/Arf-, Xpa- knockout mice were subjected to various combinations of neonatal [7,12-dimethylbenz(a)anthracene (DMBA) or UVB exposure] and adult treatments (12-O-tetradecanoylphorbol-13-acetate or subacute daily UVB exposure or intermittent overexposure). Nevi occurred earliest, grew largest, and were most numerous in mice exposed to DMBA followed by intermittent UVB overexposure [effect of six minimal edemal doses (MED), 1 x /2 weeks > 4 MED 1 x /wk]. Neonatal UV exposure enhanced nevus induction but lost its effect after 200 days. The Xpa(-/-) mice proved exquisitely sensitive to UV-driven nevus induction, indicating the involvement of pyrimidine dimer DNA lesions, but Xpa(+/+) mice developed many more nevi (>40 per mouse) at high UV dosages not tolerated by Xpa(-/-) mice. Ink4a/Arf(-/-) mice developed most skin tumors faster, but surprisingly developed nevi slower than their heterozygous counterparts especially after neonatal UV exposure. Despite raising >1,600 nevi, only six melanomas arose in our experiments with Ink4a/Arf knockout mice (five of which in Xpa(+/+) mice at high UV dosages). In contrast to human nevi, these nevi lacked hotspot mutations in Braf or Ras genes, possibly explaining the lack of malignant progression in the Ink4a/Arf(-/-) mice. Hence, although our experiments did not effectively emulate human melanoma, they provided clear evidence that intermittent UV overexposure strongly stimulates and the Ink4a/Arf(-/-) genotype may actually impair nevus development.

Adenovirus-mediated TA-p73β gene transfer increases chemosensitivity of human malignant melanomas

Malignant melanoma is the most aggressive form of skin cancer and has proven to be highly resistant to conventional chemotherapy. Intriguingly, the p53 tumor suppressor, a main mediator of chemoresistance in other tumor types, is rarely mutated in melanoma. However, we have previously shown that anti-apoptotic isoforms of p73 (deltaTA-p73), another member of the p53 family, are overexpressed in metastatic melanomas. DeltaTA-p73 can oppose the pro-apoptotic functions of p53 and full length p73, and thus it could contribute to melanoma chemoresistance. In this study, we use an efficient adenoviral-based gene transfer approach to introduce a transcriptionally active form of p73 (TA-p73beta) in melanoma cells, with the objective of overcoming drug resistance. Interestingly, TA-p73beta significantly sensitized 5 out of 7 aggressive melanoma cell lines to the standard therapeutic agents adriamycin and cisplatin. More importantly, TA-p73beta displayed a synergistic effect in vivo allowing adriamycin or cisplatin to block melanoma cell growth in mouse xenograft models (p < 0.05). In summary, our data show that Ad-mediated TA-p73beta gene expression can markedly sensitize a subset of melanoma cell lines to adriamycin and cisplatin in vitro and in vivo, suggesting a new chemosensitization strategy for malignant melanomas.

The hypoxic microenvironment of the skin contributes to Akt-mediated melanocyte transformation

Constitutive activation of Akt characterizes a high percentage of human melanomas and represents a poor prognostic factor of the disease. We show that Akt transforms melanocytes only in a hypoxic environment, which is found in normal skin. The synergy between Akt and hypoxia is HIF1alpha mediated. Inhibition of HIF1alpha decreases Akt transformation capacity in hypoxia and tumor growth in vivo, while overexpression of HIF1alpha allows anchorage-independent growth in normoxia and development of more aggressive tumors. Finally, we show that mTOR activity is necessary to maintain the transformed phenotype by sustaining HIF1alpha activity. Taken together, these findings demonstrate that Akt hyperactivation and HIF1alpha induction by normally occurring hypoxia in the skin significantly contribute to melanoma development.

Recurrent Patterns of Dual RB and p53 Pathway Inactivation in Melanoma

Two growth inhibitory hurdles that must be overcome by the evolving cancer cell include pathways regulated by RB and p53. In human melanoma cells, inactivation of a single locus, CDKN2A, can lead to abrogation of both RB and p53 functionality through loss of the two CDKN2A cognate transcripts-p16 and p14ARF, respectively. We thus set out to assess how recurrent patterns of genetic injury at three critical human melanoma loci-CDKN2A, TP53, and CDK4-cooperate to disrupt both RB and p53 pathways. Overall, 77.8% of the melanoma cell lines analyzed showed genetic evidence of dual RB and p53 pathway compromise; this percentage is even higher if protein expression loss is considered. Although homozygous deletion of all three critical CDKN2A exons (exons 1 beta, 1 alpha, and 2) represent the most common mechanism, concurrent loss of CDKN2A(Exon1 alpha) and CDKN2A(Exon1 beta) and simultaneous point mutagenesis of CDK4 and TP53 reflect alternative cassettes of dual inactivation. In cell lines with isolated CDKN2A(Exon2) mutations, coincident alterations in TP53 or deletion of CDKN2A(Exon1 beta) suggest that p16 transcript may be preferentially targeted over the p14ARF transcript as additional p53 pathway lesions are recruited. Moreover, predictive modeling of CDKN2A(Exon2) missense mutations further suggests that the amino acid substitutions in this region negatively impact p16 to a greater extent than p14ARF. Taken together, our data point to a clear need in human melanoma cell lines, as in its murine counterpart, to disrupt both RB and p53 pathways and recurrent mechanisms may play into the unique genetic vulnerabilities of this tumor type.

Biallelic deletions in INK4 in cutaneous melanoma are common and associated with decreased survival

PURPOSE

Both the retinoblastoma and p53 pathways are often genetically altered in human cancers and their complex regulation is in part mediated by the three gene products p16, p14(ARF), and p15 of the INK4 locus on chromosome 9p21. Partial or complete biallelic deletions of the INK4 locus have been recognized in a variety of malignant tumors, including malignant melanoma. We have in the present study measured the frequency of INK4 deletions in a large number of melanoma metastases and determined their association with clinicopathologic variables and survival data.

EXPERIMENTAL DESIGN

Quantitative real-time PCR, as well as fluorescence-based fragment analysis, has been used to perform measurements of the relative allelic concentrations of the INK4 genes in 112 human melanoma tumor samples from 86 patients.

RESULTS

Thirty-eight of 86 melanoma patients (44%) had metastases with biallelic losses in INK4. Ten of 20 patients with multiple metastases showed similar deletion patterns in all analyzed tumors. There was no significant association between any of the clinicopathologic variables and loss of INK4. However, loss of INK4 had an adverse effect on median survival from time of diagnosis. Patients with tumors with diploid INK4 had a median survival of 142 months, whereas those with monoallelic or biallelic loss in INK4 had a median survival of only 47 months (P = 0.006).

CONCLUSIONS

Our results point to homozygous deletions in the INK4 region as being one of the most common genetic alterations in malignant cutaneous melanoma. INK4 deletions are associated with an adverse prognosis.

INK4a/ARF: a multifunctional tumor suppressor locus

The INK4a/ARF locus encodes two physically linked tumor suppressor proteins, p16(INK4a) and ARF, which regulate the RB and p53 pathways, respectively. The unusual genomic relationship of the open reading frames of these proteins initially fueled speculation that only one of the two was the true tumor suppressor, and loss of the other merely coincidental in cancer. Recent human and mouse genetic data, however, have firmly established that both proteins possess significant in vivo tumor suppressor activity, although there appear to be species- and cell-type specific differences between the two. For example, ARF plays a clear role in preventing Myc-induced lymphomagenesis in mice, whereas the role for p16(INK4a) is human carcinomas is more firmly established. In this review, I discuss the evolutionary history of the locus, the relative importance of these tumor suppressor genes in human cancer, and recent information suggesting novel biochemical and physiologic functions of these proteins in vivo.

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.

Melanoma genetics and the development of rational therapeutics

Melanoma is a cancer of the neural crest-derived cells that provide pigmentation to skin and other tissues. Over the past 4 decades, the incidence of melanoma has increased more rapidly than that of any other malignancy in the United States. No current treatments substantially enhance patient survival once metastasis has occurred. This review focuses on recent insights into melanoma genetics and new therapeutic approaches being developed based on these advances.

Role of epidermal growth factor receptor signaling in RAS-driven melanoma

The identification of essential genetic elements in pathways governing the maintenance of fully established tumors is critical to the development of effective antioncologic agents. Previous studies revealed an essential role for H-RAS(V12G) in melanoma maintenance in an inducible transgenic model. Here, we sought to define the molecular basis for RAS-dependent tumor maintenance through determination of the H-RAS(V12G)-directed transcriptional program and subsequent functional validation of potential signaling surrogates. The extinction of H-RAS(V12G) expression in established tumors was associated with alterations in the expression of proliferative, antiapoptotic, and angiogenic genes, a profile consistent with the observed phenotype of tumor cell proliferative arrest and death and endothelial cell apoptosis during tumor regression. In particular, these melanomas displayed a prominent RAS-dependent regulation of the epidermal growth factor (EGF) family, leading to establishment of an EGF receptor signaling loop. Genetic complementation and interference studies demonstrated that this signaling loop is essential to H-RAS(V12G)-directed tumorigenesis. Thus, this inducible tumor model system permits the identification and validation of alternative points of therapeutic intervention without neutralization of the primary genetic lesion.

Cell type-specific tumor suppression by Ink4a and Arf in Kras-induced mouse gliomagenesis

Homozygous deletion of the INK4a-ARF locus is one of the most frequent mutations found in human glioblastoma. We have previously shown that combined Ink4a-Arf loss can increase tumor incidence in both glial progenitor cells and astrocytes during mouse gliomagenesis. Here we have investigated the separate contribution of loss of each of the tumor suppressor genes in glial progenitor cells and astrocytes in Akt + Kras-induced gliomagenesis. We show that Arf is the major tumor suppressor gene in both cell types. Arf loss generated glioblastomas from both nestin-expressing glial progenitor cells and glial fibrillary acidic protein-expressing astrocytes, with a significantly higher incidence in astrocytes. Ink4a loss, on the other hand, could only significantly contribute to gliomagenesis from glial progenitor cells and the induced tumors were of lower malignancy than those seen in Arf-deficient mice. Thus, Ink4a and Arf have independent and differential tumor suppressor functions in vivo in the glial cell compartment.

Tbx2 Is Overexpressed and Plays an Important Role in Maintaining Proliferation and Suppression of Senescence in Melanomas

The INK4a and ARF genes found at the CDKN2A locus are key effectors of cellular senescence that is believed to act as a powerful anticancer mechanism. Accordingly, mutations in these genes are present in a wide variety of spontaneous human cancers and CDKN2A germ line mutations are found in familial melanoma. The TBX2 gene encoding a key developmental transcription factor is amplified in pancreatic cancer cell lines and preferentially amplified and overexpressed in BRCA1 and BRCA2 mutated breast tumors. Overexpression of Tbx2 and the related factor Tbx3, which is also overexpressed in breast cancer and melanomas, can suppress senescence in defined experimental systems through repression of ARF expression. However, it is not known how Tbx2 mediates its repressive effect nor whether endogenous Tbx2 or Tbx3 perform a similar antisenescence function in transformed cells. This is a particularly important question because the loss of CDKN2A in many human cancers would, in principle, bypass the requirement for Tbx2/3-mediated repression of ARF in suppressing senescence. We show here that Tbx2 is overexpressed in melanoma cell lines and that Tbx2 targets histone deacetylase 1 to the p21Cip1 (CDKN1A) initiator. Strikingly, expression of an inducible dominant-negative Tbx2 (dnTbx2) leads to displacement of histone deacetylase 1, up-regulation of p21(Cip1) expression, and the induction of replicative senescence in CDKN2A-null B16 melanoma cells. In human melanoma cells, expression of dnTbx2 leads to severely reduced growth and induction of senescence-associated heterochromatin foci. The results suggest that the activity of endogenous Tbx2 is critically required to maintain proliferation and suppress senescence in melanomas.

Towards a Better Understanding of the Molecular Mechanisms Involved in Sunlight-Induced Melanoma

Although much less prevalent than its nonmelanoma skin cancer counterparts, cutaneous malignant melanoma (CMM) is the most lethal human skin cancer. Epidemiological and biological studies have established a strong link between lifetime exposure to ultraviolet (UV) light, particularly sunburn in childhood, and the development of melanoma. However, the specific molecular targets of this environmental carcinogen are not known. Data obtained from genetic and molecular studies over the last few years have identified the INK4a/ARF locus as the "gatekeeper" melanoma suppressor, encoding two tumour suppressor proteins in human, p16 $^{\text{INK4a}}$ and p $14^{\text{ARF}}$ . Recent developments in molecular biotechnology and research using laboratory animals have made a significant gene breakthrough identifying the components of the p16 $^{\text{INK4a}}$ /Rb pathway as the principal and rate-limiting targets of UV radiation actions in melanoma formation. This review summarizes the current knowledge of the molecular mechanisms involved in melanoma development and its relationship to sunlight UV radiation.

The role of B-RAF in melanoma

Melanoma is a form of skin cancer that has a poor prognosis and which is on the rise in Western populations. If detected early, it is easily treated by surgical excision. However, once melanoma metastasises it is notoriously resistant to existing therapies and for many patients the outlook is dismal. Thus a full description of melanoma etiology and a full understanding of the genetic lesions that underlie this disease is required to allow us to develop new and effective therapeutic strategies for its treatment. RAF proteins are a family of serine/threonine-specific protein kinases that form part of a signalling module that regulates cell proliferation, differentiation and survival. In mammals there are three isoforms, A-RAF, B-RAF and C-RAF, and recently it was shown that the B-RAF isoform is mutated in a high proportion of melanomas. In light of these exciting findings, we review what we have learned about B-RAF and its role in cutaneous melanoma.

Towards a Better Understanding of the Molecular Mechanisms Involved in Sunlight-Induced Melanoma

Although much less prevalent than its nonmelanoma skin cancer counterparts, cutaneous malignant melanoma (CMM) is the most lethal human skin cancer. Epidemiological and biological studies have established a strong link between lifetime exposure to ultraviolet (UV) light, particularly sunburn in childhood, and the development of melanoma. However, the specific molecular targets of this environmental carcinogen are not known. Data obtained from genetic and molecular studies over the last few years have identified the INK4a/ARF locus as the "gatekeeper" melanoma suppressor, encoding two tumour suppressor proteins in human, p16 $^{\text{INK4a}}$ and p $14^{\text{ARF}}$ . Recent developments in molecular biotechnology and research using laboratory animals have made a significant gene breakthrough identifying the components of the p16 $^{\text{INK4a}}$ /Rb pathway as the principal and rate-limiting targets of UV radiation actions in melanoma formation. This review summarizes the current knowledge of the molecular mechanisms involved in melanoma development and its relationship to sunlight UV radiation.

Cooperative regulation of the cell division cycle by the protein kinases RAF and AKT

The RAS-activated RAF-->MEK-->extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3'-kinase (PI3'-kinase)-->PDK1-->AKT signaling pathways are believed to cooperate to promote the proliferation of normal cells and the aberrant proliferation of cancer cells. To explore the mechanisms that underlie such cooperation, we have derived cells harboring conditionally active, steroid hormone-regulated forms of RAF and AKT. These cells permit the assessment of the biological and biochemical effects of activation of these protein kinases either alone or in combination with one another. Under conditions where activation of neither RAF nor AKT alone promoted S-phase progression, coactivation of both kinases elicited a robust proliferative response. Moreover, under conditions where high-level activation of RAF induced G(1) cell cycle arrest, activation of AKT bypassed the arrest and promoted S-phase progression. At the level of the cell cycle machinery, RAF and AKT cooperated to induce cyclin D1 and repress p27(Kip1) expression. Repression of p27(Kip1) was accompanied by a dramatic reduction in KIP1 mRNA and was observed in primary mouse embryo fibroblasts derived from mice either lacking SKP2 or expressing a T187A mutated form of p27(Kip1). Consistent with these observations, pharmacological inhibition of MEK or PI3'-kinase inhibited the effects of activated RAS on the expression of p27(Kip1) in NIH 3T3 fibroblasts and in a panel of bona fide human pancreatic cancer cell lines. Furthermore, we demonstrated that AKT activation led to sustained activation of cyclin/cdk2 complexes that occurred concomitantly with the removal of RAF-induced p21(Cip1) from cyclin E/cdk2 complexes. Cumulatively, these data strongly suggest that the RAF-->MEK-->ERK and PI3'K-->PDK-->AKT signaling pathways can cooperate to promote G(0)-->G(1)-->S-phase cell cycle progression in both normal and cancer cells.

Ink4a/Arf links senescence and aging

The mammalian INK4a/ARF locus encodes two linked tumor suppressor proteins, p16INK4a and ARF, which respectively regulate the retinoblastoma (RB) and p53 pathways. Genetic data have firmly established that both proteins possess significant in vivo tumor suppressor activity. In addition to their non-overlapping roles in preventing cancer, one or both proteins are induced under certain circumstances in most cultured murine and human cell types, and thereby are critical effectors of senescence. Likewise, data from murine models have suggested that this anti-cancer growth inhibitory activity of the locus can similarly affect permanent growth arrest in vivo. When such in vivo senescence occurs in a cell possessing self-renewal potential (e.g. a tissue stem cell), there is an attendant decline in the regenerative capabilities of the organ maintained by that stem cell. In turn, the concomitant decline of this stem cell reserve is a cardinal feature of mammalian aging. Expression of the INK4a/ARF locus, therefore, appears not only to be a major suppressor of cancer, but also an effector of mammalian aging.

UV light from 290 to 325 nm, but not broad-band UVA or visible light, augments the formation of melanocytic nevi in a guinea-pig model for human nevi

We have previously described a guinea-pig model where pigmented nevi similar to human nevi can be produced by application of low-dose topical 7,12-dimethylbenzanthracene (DMBA) followed by solar-simulated light. Five groups of guinea-pigs were used to test the effect of various spectral bands of solar-simulated light on low-dose DMBA-induced melanocytic nevi. Animals were irradiated with either UVB to near UVA2 (290-325 nm), UVA, visible light, full solar spectrum or no irradiation three times per wk for 12 mo to determine the broad-band effect of nevi-inducing irradiation. There was a significant increase in nevi/animal in the UVB-treated group (mean 1.53) compared with all groups (versus UVA 0.3, p<0.001; versus visible light 0.24, p<0.001; versus full spectrum (UVB+UVA+visible) 0.68, p=0.02; versus control (nil irradiation) 0.37, p=0.01). No differences in skin thickness were found between any group (p=0.11). In conclusion, we present a report of the active waveband of melanocytic nevi induction; where UVB to near UVA2 is the likely responsible waveband. Furthermore, because there was a significant decrease in nevi/animal receiving the full solar spectrum compared with the UVB group, it is possible that broad-band UVA and or visible light may be inhibitory wavebands for nevi induction.

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.

Recent advances in melanoma biology

The incidence and mortality rates of melanoma have increased at annual rates of 2%-3% for the last 30 years. Disseminated disease is largely refractory to cytotoxic chemotherapy and is almost universally fatal. Several recent advances in melanoma biology offer new strategies for potentially treating this aggressive malignancy. This review focuses on three significant advances involving tumor initiation, etiology, and progression. New experimental models reveal a direct role for UV-B light in initiating melanomas in human skin. Studies on E- and N-cadherin elucidate the importance of local homeostatic mechanisms in regulating tumor progression. Finally, several discoveries concerning apoptotic mechanisms in melanoma suggest strategies for future treatments.

The Inhibitor of Cyclin-Dependent Kinase 4a/Alternative Reading Frame (INK4a/ARF) Locus Encoded Proteins p16INK4a and p19ARF Repress Cyclin D1 Transcription through Distinct cis Elements

The Ink4a/Arf locus encodes two structurally unrelated tumor suppressor proteins, p16(INK4a) and p14(ARF) (murine p19(ARF)). Invariant inactivation of either the p16(INK4a)-cyclin D/CDK-pRb pathway and/or p53-p14(ARF) pathway occurs in most human tumors. Cyclin D1 is frequently overexpressed in breast cancer cells contributing an alternate mechanism inactivating the p16(INK4a)/pRb pathway. Targeted overexpression of cyclin D1 to the mammary gland is sufficient for tumorigenesis, and cyclin D1-/- mice are resistant to Ras-induced mammary tumors. Recent studies suggest cyclin D1 and p16(INK4a) expression are reciprocal in human breast cancers. Herein, reciprocal regulation of cyclin D1 and p16(INK4a) was observed in tissues of mice mutant for the Ink4a/Arf locus. p16(INK4a) and p19(ARF) inhibited DNA synthesis in MCF7 cells. p16(INK4a) repressed cyclin D1 expression and transcription. Repression of cyclin D1 by p16(INK4a) occurred independently of the p16(INK4a)-cdk4-binding function and required a cAMP-response element/activating transcription factor-2-binding site. p19(ARF) repressed cyclin D1 through a novel distal cis-element at -1137, which bound p53 in chromatin-immunoprecipitation assays. Transcriptional repression of the cyclin D1 gene through distinct DNA sequences may contribute to the tumor suppressor function of the Ink4a/Arf locus.

Different requirements for the cytostatic and apoptotic effects of type I interferons. Induction of apoptosis requires ARF but not p53 in osteosarcoma cell lines

The regulation of cell growth is one of the most important effects of type I interferons (IFNs). This response may involve a cytostatic effect or the induction of apoptosis depending on the cell context. Often the growth-inhibitory response of type I IFNs is studied in tumor cell lines carrying mutations of tumor suppressor genes, and therefore, the growth-inhibitory effect can be influenced by inactivation of these important regulators of cell proliferation. In this report, we explored the role of the ARF-p53 pathway in the growth-inhibitory effect of type I IFNs. We found that p53 is only induced in cells that express p14(ARF) (p19(ARF) in mouse cells). Surprisingly, mouse embryonal fibroblasts that are null for p19(ARF) or P53, even after transformation with oncogenic RAS, respond as well as wild type to the growth-inhibitory effect of type I IFNs. Similarly, human ARF(-/-) U2OS and P53(-/-) SAOS-2 cells show a significant decrease in cell proliferation. However, only SAOS-2 or U2OS reconstituted with inducible p14(ARF) undergo apoptosis in response to IFN beta treatment, and this effect was not inhibited by expression of dominant negative p53. These data suggest that (i) at least in specific cell types, the induction of apoptosis by type I IFNs requires an ARF pathway that is p53-independent and (ii) the cytostatic and pro-apoptotic effects of type I IFNs employ different pathways.

V599EB-RAF is an oncogene in melanocytes

The oncogenic version of B-RAF, (V599E)B-RAF, is found in approximately 70% of human melanomas. However, the role that this oncogene plays in melanoma is unclear because (V559E)B-RAF is also found in approximately 80% of benign nevi. We have examined the role of oncogenic B-RAF in the early stages of melanoma by expressing (V599E)B-RAF in cultured melanocytes. In these cells, (V599E)B-RAF induced constitutive mitogen activated ERK-activating kinase (MEK) and extracellular signal-regulated kinase (ERK) signaling, 12-O-tetradecanoylphorbol-13-acetate-independent growth, and tumorigenicity in nude mice. Intriguingly, in RAS-transformed melanocytes, B-RAF depletion did not block MEK-ERK signaling or cell cycle progression. Similarly, B-RAF depletion blocked MEK-ERK signaling in human melanoma cells harboring oncogenic B-RAF, but not in melanoma cells harboring oncogenic RAS. Thus, although B-RAF can act as a potent oncogene in the early stages of melanoma by signaling through MEK and ERK, it is not required for this signaling in RAS-transformed melanocytes due to innate redundancy within the pathway. These findings have important implications for future therapeutic strategies.

The Brn-2 transcription factor links activated BRAF to melanoma proliferation

Malignant melanoma, an aggressive and increasingly common cancer, is characterized by a strikingly high rate (70%) of mutations in BRAF, a key component of the mitogen-activated protein (MAP) kinase signaling pathway. How signaling events downstream from BRAF affect the underlying program of gene expression is poorly understood. We show that the Brn-2 POU domain transcription factor is highly expressed in melanoma cell lines but not in melanocytes or melanoblasts and that overexpression of Brn-2 in melanocytes results in increased proliferation. Expression of Brn-2 is strongly upregulated by Ras and MAP kinase signaling. Importantly, the Brn-2 promoter is stimulated by kinase-activating BRAF mutants and endogenous Brn-2 expression is inhibited by RNA interference-mediated downregulation of BRAF. Moreover, silent interfering RNA-mediated depletion of Brn-2 in melanoma cells expressing activated BRAF leads to decreased proliferation. The results suggest that the high levels of Brn-2 expression observed in melanomas link BRAF signaling to increased proliferation.

Advances in molecular carcinogenesis: current and future use of mouse models to screen and validate molecularly targeted anticancer drugs

Survival of patients with advanced solid tumors has not significantly improved over the past 30 years. Although molecularly targeted anticancer drugs offer promise, few drugs make it through the end of the Food and Drug Administration approval process. Animal models that more closely resemble human carcinogenesis may bridge the gap between preclinical success and benefits for patients. We discuss pros and cons of several mouse models, including genetically engineered mice that each represent different aspects of human cancer, and the screening of targeted drugs in these models.

Genomic alterations in spontaneous and carcinogen-induced murine melanoma cell lines

We have conducted an analysis of genetic alterations in spontaneous murine melanoma cell line B16F0 and its two metastatic clones, B16F1 and B16F10 and the carcinogen-induced murine melanoma cell lines CM519, CM3205, and K1735. We found that unlike human melanomas, the murine melanoma cell lines did not have activating mutations in the Braf oncogene at exon 11 or 15. However, there were distinct patterns of alterations in the ras, Ink4a/Arf, and p53 genes in the two melanoma groups. In the spontaneous B16 melanoma cell lines, expression of p16Ink4a and p19Arf tumor suppressor proteins was lost as a consequence of a large deletion spanning Ink4a/Arf exons 1alpha, 1beta, and 2. In contrast, the carcinogen-induced melanoma cell lines expressed p16Ink4a but had inactivating mutations in either p19Arf (K1735) or p53 (CM519 and CM3205). Inactivation of p19Arf or p53 in carcinogen-induced melanomas was accompanied by constitutive activation of mitogen-activated protein kinases (MAPKs) and/or mutation-associated activation of N-ras. These results indicate that genetic alterations in p16Ink4a/p19Arf, p53 and ras-MAPK pathways can cooperate in the development of murine melanoma.

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.

Targets for molecular therapy of skin cancer

Cancers of the skin encompass the first and second most common neoplasms in the United States, epidermal basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), respectively, as well as the melanocytic malignancy, malignant melanoma (MM). Recently identified alterations in the function of specific genes in these cancers provide new potential therapeutic targets. These alterations affect conserved regulators of cellular proliferation and viability, including the Sonic Hedgehog, Ras/Raf, ARF/p53, p16(INK4A)/CDK4/Rb and NF-kappaB pathways. New modalities designed to target these specific proteins may represent promising approaches to therapy of human skin cancers.

Activated Kras and Ink4a/Arf deficiency cooperate to produce metastatic pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma ranks among the most lethal of human malignancies. Here, we assess the cooperative interactions of two signature mutations in mice engineered to sustain pancreas-specific Cre-mediated activation of a mutant Kras allele (KrasG12D) and deletion of a conditional Ink4a/Arf tumor suppressor allele. The phenotypic impact of KrasG12D alone was limited primarily to the development of focal premalignant ductal lesions, termed pancreatic intraepithelial neoplasias (PanINs), whereas the sole inactivation of Ink4a/Arf failed to produce any neoplastic lesions in the pancreas. In combination, KrasG12D expression and Ink4a/Arf deficiency resulted in an earlier appearance of PanIN lesions and these neoplasms progressed rapidly to highly invasive and metastatic cancers, resulting in death in all cases by 11 weeks. The evolution of these tumors bears striking resemblance to the human disease, possessing a proliferative stromal component and ductal lesions with a propensity to advance to a poorly differentiated state. These findings in the mouse provide experimental support for the widely accepted model of human pancreatic adenocarcinoma in which activated KRAS serves to initiate PanIN lesions, and the INK4A/ARF tumor suppressors function to constrain the malignant conversion of these PanIN lesions into lethal ductal adenocarcinoma. This faithful mouse model may permit the systematic analysis of genetic lesions implicated in the human disease and serve as a platform for the identification of early disease markers and for the efficient testing of novel therapies.

Pharmacogenetic candidate genes for melanoma

The incidence of melanoma is rising at an alarming rate and has become an important public health concern. If detected early, melanoma carries an excellent prognosis after appropriate surgical resection. Unfortunately, advanced melanoma has a poor prognosis and is notoriously resistant to radiation and chemotherapy. The relative resistance of melanoma to a wide-range of chemotherapeutic agents and high toxicity of current therapies has prompted a search for effective alternative treatments that would improve prognosis and limit side effects. Advances in molecular genetics are revealing in increasing detail the mechanisms responsible for the development of melanoma. Hopefully, elucidation of these pathways will provide a means of screening high-risk individuals and allow new drug development for prevention and treatment by identification of specific pharmacological targets. This review will summarize the genetics of melanoma with the goal of providing insights into potential pharmacogenetic candidate genes.