Animal models of melanoma: recent advances and future prospects

Nanotechnology Addressing Cutaneous Melanoma: The Italian Landscape

Cutaneous melanoma is one of the most aggressive solid tumors, with a low survival for the metastatic stage. Currently, clinical melanoma treatments include surgery, chemotherapy, targeted therapy, immunotherapy and radiotherapy. Of note, innovative therapeutic regimens concern the administration of multitarget drugs in tandem, in order to improve therapeutic efficacy. However, also, if this drug combination is clinically relevant, the patient's response is not yet optimal. In this scenario, nanotechnology-based delivery systems can play a crucial role in the clinical treatment of advanced melanoma. In fact, their nano-features enable targeted drug delivery at a cellular level by overcoming biological barriers. Various nanomedicines have been proposed for the treatment of cutaneous melanoma, and a relevant number of them are undergoing clinical trials. In Italy, researchers are focusing on the pharmaceutical development of nanoformulations for malignant melanoma therapy. The present review reports an overview of the main melanoma-addressed nanomedicines currently under study in Italy, alongside the state of the art of melanoma therapy. Moreover, the latest Italian advances concerning the pre-clinical evaluation of nanomedicines for melanoma are described.

Transplantable Melanomas in Hamsters and Gerbils as Models for Human Melanoma. Sensitization in Melanoma Radiotherapy-From Animal Models to Clinical Trials

The focus of the present review is to investigate the role of melanin in the radioprotection of melanoma and attempts to sensitize tumors to radiation by inhibiting melanogenesis. Early studies showed radical scavenging, oxygen consumption and adsorption as mechanisms of melanin radioprotection. Experimental models of melanoma in hamsters and in gerbils are described as well as their use in biochemical and radiobiological studies, including a spontaneously metastasizing ocular model. Some results from in vitro studies on the inhibition of melanogenesis are presented as well as radio-chelation therapy in experimental and clinical settings. In contrast to cutaneous melanoma, uveal melanoma is very successfully treated with radiation, both using photon and proton beams. We point out that the presence or lack of melanin pigmentation should be considered, when choosing therapeutic options, and that both the experimental and clinical data suggest that melanin could be a target for radiosensitizing melanoma cells to increase efficacy of radiotherapy against melanoma.

Activation of Grm1 expression by mutated BRaf (V600E) in vitro and in vivo

Our laboratory previously showed that ectopic expression of Grm1 is sufficient to induce spontaneous melanoma formation with 100% penetrance in transgenic mouse model, TG-3, which harbors wild-type BRaf. Studies identified Grm1 expression in human melanoma cell lines and primary to secondary metastatic melanoma biopsies having wild-type or mutated BRaf, but not in normal melanocytes or benign nevi. Grm1 expression was detected in tissues from mice genetically engineered with inducible melanocyte-specific BRaf^V600E. Additionally, stable clones derived from introduction of exogenous BRaf^V600E in mouse melanocytes also showed Grm1 expression, which was not detected in the parental or empty vector-derived cells, suggesting that expression of BRaf^V600E could activate Grm1 expression. Despite aberrant Grm1 expression in the inducible, melanocyte-specific BRaf^V600E mice, no tumors formed. However, in older mice, the melanocytes underwent senescence, as demonstrated previously by others. It was proposed that upregulated p15 and TGFβ contributed to the senescence phenotype. In contrast, in older TG-3 mice the levels of p15 and TGFβ remained the same or lower. Taken together, these results suggest the temporal regulation on the expression of "oncogenes" such as Grm1 or BRaf^V600E is critical in the future fate of the cells. If BRaf^V600E is turned on first, Grm1 expression can be induced, but this is not sufficient to result in development of melanoma; the cells undergo senescence. In contrast, if ectopic expression of Grm1 is turned on first, then regardless of wild-type or mutated BRaf in the melanocytes melanoma development is the consequence.

Current State of Animal (Mouse) Modeling in Melanoma Research

Despite the considerable progress in understanding the biology of human cancer and technological advancement in drug discovery, treatment failure remains an inevitable outcome for most cancer patients with advanced diseases, including melanoma. Despite FDA-approved BRAF-targeted therapies for advanced stage melanoma showed a great deal of promise, development of rapid resistance limits the success. Hence, the overall success rate of melanoma therapy still remains to be one of the worst compared to other malignancies. Advancement of next-generation sequencing technology allowed better identification of alterations that trigger melanoma development. As development of successful therapies strongly depends on clinically relevant preclinical models, together with the new findings, more advanced melanoma models have been generated. In this article, besides traditional mouse models of melanoma, we will discuss recent ones, such as patient-derived tumor xenografts, topically inducible BRAF mouse model and RCAS/TVA-based model, and their advantages as well as limitations. Although mouse models of melanoma are often criticized as poor predictors of whether an experimental drug would be an effective treatment, development of new and more relevant models could circumvent this problem in the near future.

Modeling Melanoma In Vitro and In Vivo

The behavior of melanoma cells has traditionally been studied in vitro in two-dimensional cell culture with cells adhering to plastic dishes. However, in order to mimic the three-dimensional architecture of a melanoma, as well as its interactions with the tumor microenvironment, there has been the need for more physiologically relevant models. This has been achieved by designing 3D in vitro models of melanoma, such as melanoma spheroids embedded in extracellular matrix or organotypic skin reconstructs. In vivo melanoma models have typically relied on the growth of tumor xenografts in immunocompromised mice. Several genetically engineered mouse models have now been developed which allow the generation of spontaneous melanoma. Melanoma models have also been established in other species such as zebrafish, which are more conducive to imaging and high throughput studies. We will discuss these models as well as novel techniques that are relevant to the study of the molecular mechanisms underlying melanoma progression.

p63 is an alternative p53 repressor in melanoma that confers chemoresistance and a poor prognosis

p63 is up-regulated in melanoma and prevents nuclear accumulation of p53.

The role of apoptosis in melanoma pathogenesis and chemoresistance is poorly characterized. Mutations in TP53 occur infrequently, yet the TP53 apoptotic pathway is often abrogated. This may result from alterations in TP53 family members, including the TP53 homologue TP63. Here we demonstrate that TP63 has an antiapoptotic role in melanoma and is responsible for mediating chemoresistance. Although p63 was not expressed in primary melanocytes, up-regulation of p63 mRNA and protein was observed in melanoma cell lines and clinical samples, providing the first evidence of significant p63 expression in this lineage. Upon genotoxic stress, endogenous p63 isoforms were stabilized in both nuclear and mitochondrial subcellular compartments. Our data provide evidence of a physiological interaction between p63 with p53 whereby translocation of p63 to the mitochondria occurred through a codependent process with p53, whereas accumulation of p53 in the nucleus was prevented by p63. Using RNA interference technology, both isoforms of p63 (TA and ΔNp63) were demonstrated to confer chemoresistance, revealing a novel oncogenic role for p63 in melanoma cells. Furthermore, expression of p63 in both primary and metastatic melanoma clinical samples significantly correlated with melanoma-specific deaths in these patients. Ultimately, these observations provide a possible explanation for abrogation of the p53-mediated apoptotic pathway in melanoma, implicating novel approaches aimed at sensitizing melanoma to therapeutic agents.

Animal models of skin disease for drug discovery

INTRODUCTION

Discovery of novel drugs, treatments, and testing of consumer products in the field of dermatology is a multi-billion dollar business. Due to the distressing nature of many dermatological diseases, and the enormous consumer demand for products to reverse the effects of skin photodamage, aging, and hair loss, this is a very active field.

AREAS COVERED

In this paper, we will cover the use of animal models that have been reported to recapitulate to a greater or lesser extent the features of human dermatological disease. There has been a remarkable increase in the number and variety of transgenic mouse models in recent years, and the basic strategy for constructing them is outlined.

EXPERT OPINION

Inflammatory and autoimmune skin diseases are all represented by a range of mouse models both transgenic and normal. Skin cancer is mainly studied in mice and fish. Wound healing is studied in a wider range of animal species, and skin infections such as acne and leprosy also have been studied in animal models. Moving to the more consumer-oriented area of dermatology, there are models for studying the harmful effect of sunlight on the skin, and testing of sunscreens, and several different animal models of hair loss or alopecia.

Animal models of melanoma: a somatic cell gene delivery mouse model allows rapid evaluation of genes implicated in human melanoma

The increasing incidence and mortality associated with advanced stages of melanoma are cause for concern. Few treatment options are available for advanced melanoma and the 5-year survival rate is less than 15%. Targeted therapies may revolutionize melanoma treatment by providing less toxic and more effective strategies. However, maximizing effectiveness requires further understanding of the molecular alterations that drive tumor formation, progression, and maintenance, as well as elucidating the mechanisms of resistance. Several different genetic alterations identified in human melanoma have been recapitulated in mice. This review outlines recent progress made in the development of mouse models of melanoma and summarizes what these findings reveal about the human disease. We begin with a discussion of traditional models and conclude with the recently developed RCAS/TVA somatic cell gene delivery mouse model of melanoma.

The role of ultraviolet radiation in melanomagenesis

The role of ultraviolet radiation (UV) in the pathogenesis has been discussed controversially for many decades. Studies in mice (SCID, HGF/SF, SV40T) which develop malignant melanoma, show a role of UVB in melanomagenesis. In contrast to this, the role of UVA is less clear. We will review the recent in vitro and in vivo data in support of the hypothesis that UVA is also involved in the development of malignant melanoma. The role of UVA in p53 activation, apoptosis, cell cycle arrest and photoproduct formation is discussed.

Promises and Limitations of Murine Models in the Development of Anticancer T-Cell Vaccines

Murine models have been instrumental in defining the basic mechanisms of antitumor immunity. Most of these mechanisms have since been shown to operate in humans as well. Based on these similarities, active vaccination strategies aimed at eliciting antitumor T-cell responses have been elaborated and successfully implemented in various mouse models. However, the results of human antitumor vaccination trials have been rather disappointing thus far. This review summarizes the different experimental approaches used in mice to induce antitumor T-cell responses and identifies some critical parameters that should be considered when evaluating results from murine models.

Braf(V600E) cooperates with Pten loss to induce metastatic melanoma

Mutational activation of BRAF is the earliest and most common genetic alteration in human melanoma. Hence, to build a model of human melanoma, we generated mice with conditional melanocyte-specific expression of BRaf^V600E. Upon induction of BRaf^V600E expression, mice developed benign melanocytic hyperplasias that failed to progress to melanoma over 15-20 months. By contrast, expression of BRaf^V600E combined with Pten tumor suppressor gene silencing elicited development of melanoma with 100% penetrance, short latency and with metastases observed in lymph nodes and lungs. Melanoma was prevented by inhibitors of mTorc1 (Rapamycin) or MEK1/2 (PD325901) but, upon cessation of drug administration, mice developed melanoma indicating the presence of long-lived melanoma-initiating cells in this system. Importantly, combined treatment with Rapamycin and PD325901 led to shrinkage of established melanomas. These mice, engineered with a common genetic profile to human melanoma, provide an excellent system to study melanoma’s cardinal feature of metastasis and for pre-clinical evaluation of agents designed to prevent or treat metastatic disease.

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.

Wavelength dependence of cellular responses in human melanocytes and melanoma cells following exposure to ultraviolet radiation

PURPOSE

To examine the wavelength dependence of cellular responses in human melanocytes and human melanoma cells exposed to ultraviolet radiation (UVR).

MATERIALS AND METHODS

Primary human melanocytes and G361 human melanoma cells were exposed to ultraviolet-C (UVC), ultraviolet-B (UVB), or ultraviolet-A (UVA) radiation. Dose-response relationships for clonal cell survival were assessed, and flow cytometry was used to monitor cell cycle distributions for up to one week post-irradiation. Chromosomal aberrations were scored in exposed and unexposed melanoma cells.

RESULTS

G361 melanoma cells were more sensitive than melanocytes to killing by UVB and UVC radiation. This difference in sensitivity between cell types was much less marked following UVA irradiation. The melanoma cells showed a sustained, dose-dependent G2/M block following exposure with all wavelengths; in addition, transit through S phase was slowed following UVA irradiation. There was no apparent block to G1 cells entering S phase at any wavelength. Melanocytes, on the other hand, showed a marked G1 arrest, particularly following UVA irradiation. Cytogenetic results showed a dose-dependent increase in chromatid-type aberrations, mostly gaps, breaks and exchanges, in exposed melanoma cells.

CONCLUSION

These results show that G361 malignant melanoma cells have lost the ability to regulate the cell cycle at the G1/S checkpoint and are more sensitive than melanocytes to cell killing by UVC and UVB but not UVA radiation. Similarly, exposure of these melanoma cells to UVC and UVB, and to a much lesser extent UVA, induced chromatid aberrations. UVA nevertheless induced strong cell cycle delays in both cell types, indicating that UVA exposure can significantly affect genome metabolism.

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.

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.

Neoplasia in fishes

Similar to higher vertebrates, neoplasia is not an uncommon disease in fishes, which are the largest group of vertebrates. However,neoplasia in fishes is generally a benign condition with relatively few exceptions of malignant disease. The objective of this discussion is to provide an overview of neoplasia and the various neoplastic disease conditions in fishes according to organ system,including the few neoplasms of species that are familiar to the aquatic animal or exotic animal practitioner. The discussion also considers the various nonneoplastic lesions in fishes that may be confused with neoplasms, and treatment of neoplastic disease in fishes that is generally restricted to surgical intervention.

The antitumor effects of interferon-alpha are maintained in mice challenged with a STAT1-deficient murine melanoma cell line

INTRODUCTION

Interferon-alpha (IFN-alpha) is currently administered to patients with metastatic malignant melanoma and those who are at risk for recurrence following surgery for high-risk lesions. Signal transducer and activator of transcription 1 (STAT1) is a transcription factor that is activated by IFN-alpha and is thought to mediate the majority of its antitumor effects. Loss of STAT1 has been found in IFN-resistant melanoma cells. We developed a murine melanoma cell line in a STAT1-deficient mouse. We also transfected B16 melanoma cells with a wild-type form of STAT1 to induce its overexpression. Using the resulting cell lines and STAT1-deficient mice, we tested whether IFN-alpha could exert an antitumor effect on melanoma cells in the absence of STAT1-mediated signal transduction.

MATERIALS AND METHODS

A melanoma tumor was induced in STAT1-deficient mice via the application of DMBA (tumor initiator) followed by croton oil (tumor promoter). Immunohistochemical analysis confirmed that the resulting tumor was a malignant melanoma. Immunoblot analysis, intracellular flow cytometry, and gel-shift analysis were used to confirm the lack of STAT1 in the derivative cell line (AGS-1). In addition, the STAT1 protein was overexpressed in B16 melanoma cells by stable transfection with a plasmid construct encoding wild-type STAT1. The effects of IFN-alpha on these cell lines were studied in vitro and in vivo.

RESULTS

STAT1 was not expressed in the AGS-1 murine melanoma cell line. Treatment with IFN-alpha did not lead to activation of STAT1. Cell proliferation assays revealed that while IFN-alpha did not exert an antiproliferative effect on this cell line, it was capable of prolonging the survival of STAT1-competent C57BL/6 mice bearing 1 x 10(6) AGS-1 tumor cells in the intraperitoneal position (n = 20, P < 0.05), as compared to PBS-treated controls. Also, the survival of IFN-alpha-treated mice (as compared to PBS-treated controls) was not affected by the overexpression of STAT1 in B16 tumor cells.

CONCLUSIONS

This data suggests that IFN-alpha can enhance survival in an animal model where STAT1-mediated signal transduction and gene regulation is absent within the tumor but is present within the host. This data also indicates that the overexpression of STAT1 within the tumor does not significantly enhance the effects of exogenously administered IFN-alpha in this model. These findings indicate that the bulk of the antitumor actions of IFN-alpha may be derived from its effects on host tissues.

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.

Transplantable melanomas in gerbils (Meriones unguiculatus). II: melanogenesis

We characterized the melanogenic apparatus in a family of transplantable gerbil melanomas (melanotic and amelanotic) using a combination of biophysical, ultrastructural and biochemical methods. Melanotic melanomas produced pure eumelanin but in vesiculo-globular melanosomes ('pheomelanosomes'); the eumelanosomes, characteristically ellipsoidal in shape with fibrillar or fibrillo-lamelar matrix, were never noticed. Melanotic melanomas also had significant tyrosinase activity and Zn, Pb/S, Ca and P content; all higher than in the amelanotic variants. The amelanotic variant, which was devoid of melanin pigment and melanosomes, had clearly detectable tyrosinase activity (albeit at 20% of that in the melanotic variant). Thus, with these multidirectional approaches we demonstrate that pure eumelanin can be synthesized in organelles ultrastructurally defined as pheomelanosomes, but a defect in the formation of melanosomes can prevent in vivo melanin synthesis despite the presence of detectable tyrosinase activity. We conclude that this melanoma system provides an excellent experimental model for the study of molecular components determining pheo- and/or eumelanogenesis. The information generated can be used for defining the roles of melanogenesis and of tyrosinase expression in the regulation of melanoma behavior and the effect of their modification on the course of the disease.

Ultraviolet radiation and cutaneous malignant melanoma

Recent years have seen a steady rise in the incidence of cutaneous malignant melanoma worldwide. Although it is now appreciated that the key to understanding the process by which melanocytes are transformed into malignant melanoma lies in the interplay between genetic factors and the ultraviolet (UV) spectrum of sunlight, the nature of this relation has remained obscure. Recently, prospects for elucidating the molecular mechanisms underlying such gene-environment interactions have brightened considerably through the development of UV-responsive experimental animal models of melanoma. Genetically engineered mice and human skin xenografts constitute novel platforms upon which to build studies designed to elucidate the pathogenesis of UV-induced melanomagenesis. The future refinement of these in vivo models should provide a wealth of information on the cellular and genetic targets of UV, the pathways responsible for the repair of UV-induced DNA damage, and the molecular interactions between melanocytes and other skin cells in response to UV. It is anticipated that exploitation of these model systems will contribute significantly toward the development of effective approaches to the prevention and treatment of melanoma.

Melanoma chemoprevention: a role for statins or fibrates?

Although numerous second-generation isoprenylation inhibitors are proposed or under investigation for the treatment and/or prevention of cancer (eg, R115777, SCH 66336, L-778,123, BMS-214662), the chemotherapeutic and chemopreventive potential of commonly prescribed first-generation isoprenylation inhibitors, the statins, and other classes of lipid-lowering medications, the fibrates, has yet to be seriously explored. Two lipid-lowering medications, lovastatin and gemfibrozil, have been associated with a decreased incidence of melanoma in large, prospective, randomized, double-blind, placebo-controlled clinical cardiology trials. This article reviews melanoma biology and the clinical evidence for the use of lipid-lowering medications for melanoma chemoprevention and/or adjuvant chemotherapy.

Animal models of melanoma: an HGF/SF transgenic mouse model may facilitate experimental access to UV initiating events

Cutaneous malignant melanoma, the most lethal of the skin cancers, known for its intractability to current therapies, continues to increase in incidence, providing a significant public health challenge. There is a consensus that skin cancer is initiated by sunlight exposure. For non-melanoma skin cancer there is substantial evidence that chronic exposure to the ultraviolet B radiation (UVB) (280-320 nm) portion of the sunlight spectrum is responsible. Experimentally, UVB is mutagenic and chronic UVB exposure can cause non-melanoma skin cancer in laboratory animals. Non-melanoma tumors in animals and in humans show characteristic UVB signature lesions in the tumor suppressor p53 and/or in the patched (PTCH) gene. An action spectrum or wavelength dependence for squamous cell carcinoma in the mouse shows a major peak of efficacy in the UVB. For malignant melanoma, however, the situation is unclear and the critical direct target(s) of sunlight in initiating melanoma and even the wavelengths responsible are as yet unidentified. This lack of information is in major part a result of a paucity of animal models for melanoma which recapitulate the role of sunlight in initiating this disease. The epidemiology of melanoma differs significantly from non-melanoma skin cancer. Intense sporadic sunlight exposure in childhood, probably exacerbated by additional adult exposure, is associated with elevated melanoma risk. Melanoma is also a disease of gene-environment interactions with underlying genetic factors playing a significant role. These major differences indicate that extrapolation from information for non-melanoma skin cancer to melanoma is unlikely to be useful. We summarize in this review the experimental information available on the role of UV radiation in melanoma and give an overview of animal melanoma models. A new model derived by neonatal UV irradiation of hepatocyte growth factor/scatter factor (HGF/SF) transgenic mice is described which recapitulates the etiology, the histopathology and molecular pathogenesis of human disease. It is anticipated that the HGF/SF transgenic model will provide a means to access the mechanism(s) by which sunlight initiates this lethal disease and provide an appropriate vehicle for derivation of appropriate therapeutic and preventive strategies.

Melanoma development and progression: a conspiracy between tumor and host

While a genome-centric paradigm in human cancer development was useful for the understanding of some malignancies such as leukemias, causative molecular defects intrinsic to melanocytes have not been defined in the majority of human melanomas. Recent work, however, has shown that regulatory signals governing melanocytic cell growth and differentiation may originate from the surrounding host cells either directly through physical contact or indirectly through soluble factors and extracellular matrix molecules. In this review, we present experimental systems useful for dissecting melanoma-host interactions and highlight evidence that the tumor microenvironment contributes to the oncogenic process. Thus, melanomagenesis is not merely an act of a single outlaw but a conspiracy orchestrated by multiple partners in the neighborhood who come into play in a precise spatiotemporal order. Defining intercellular molecular dialogues in human skin promises to provide key information for the development of novel treatment strategies that target the functional unit of stroma and tumor.

Chemoprevention of human skin cancer

The incidence of skin cancer has been rising in recent years with significant effects on public health. Primary prevention has proven inadequate in impacting the incidence of skin cancer, thus stimulating the development of chemopreventive strategies. The majority of skin cancer chemoprevention studies focus on occurrence of new nonmelanoma skin cancers (NMSC) in individuals with a previous NMSC, or on reduction in the number of premalignant skin lesions such as actinic keratoses (AK). Dysplastic nevi, a likely precursor of melanoma, are also potential targets for chemoprevention strategies. Premalignant lesions are especially attractive as endpoints since they are more common than frank cancer, resulting in reduced sample size, length, and cost of clinical trials. Development of new agents that affect the pathogenesis of skin cancer will be discussed, from elucidation of molecular targets to implementation of trials designed to determine the effects of chemopreventive interventions on human skin cancer.

Emerging concepts and technologies in melanoma research

Melanoma research has made great advances in recent years. Particularly in the field of immunology, melanoma researchers have opened new avenues for basic and translational cancer research overall. Emerging research areas such as molecular epidemiology promise to develop a similar leadership role. On the other hand, research in biology, genetics or experimental therapy of melanoma has remained confined to few laboratories and entire research areas are not covered due to lack of researchers and resources. New developments in defining stem cells in skin and bone marrow enable us to develop new concepts for melanoma development and progression. New technologies allow rapid progress but they require close cooperation between laboratories. The field has to better bridge experimental with clinical research and increase communication. Corroboration with advocacy groups should activate the public for increased awareness and funding.

Invasive melanoma in Cdk4-targeted mice

Many human tumors harbor mutations that result in deregulation of Cdk4 activity. Most of these mutations involve overexpression of D-type cyclins and inactivation of INK4 inhibitors. In addition, a mutation in the Cdk4 protein has been described in patients with familial melanoma (Wolfel, T., Hauer, M., Schneider, J., Serrano, M., Wolfel, C., et al. (1995) Science 269, 1281-1284; Zuo, L., Weger, J., Yang, Q., Goldstein, A. M., Tucker, M. A., et al. (1996) Nat. Genet. 12, 97-99). This mutation, R24C, renders the Cdk4 protein insensitive to inhibition by INK4 proteins including p16(INK4a), a major candidate for the melanoma susceptibility locus. Here we show that knock-in mice expressing a Cdk4 R24C allele are highly susceptible to melanoma development after specific carcinogenic treatments. These tumors do not have mutations in the p19(ARF)/p53 pathway, suggesting a specific involvement of the p16(INK4a)/Cdk4/Rb pathway in melanoma development. Moreover, by using targeted mice deficient for other INK4 inhibitors, we show that deletion of p18(INK4c) but not of p15(INK4b) confers proliferative advantage to melanocytic tumor growth. These results provide an experimental scenario to study the role of Cdk4 regulation in melanoma and to develop novel therapeutic approaches to control melanoma progression.

Sunscreen effects on UV-induced immune suppression

In order to protect the public against the adverse effects of sunlight, the scientific, medical, and particularly the dermatologic community has promoted "safe sun exposure." This strategy includes sun avoidance whenever possible, wearing hats and other protective clothing and/or devices, such as sunglasses, and extensive use of sunscreens. Sunscreen efficacy is determined by measuring the ability of the sunscreen to block ultraviolet (UV)-induced erythema (sun protection factor or SPF), and most sunscreen formulations on the market, if used properly, are very good at preventing erythema and sunburn. How well sunscreens protect against the other adverse effects of sunlight, such as immune suppression, is not as clear. The purpose of this paper is to review and discuss the literature in this area, concentrating on some of the complications of determining how well sunscreens protect against UV-induced immune suppression.