Activation of STAT5 triggers proliferation and contributes to anti-apoptotic signalling mediated by the oncogenic Xmrk kinase

Segregation between an ornamental and a disease driver gene provides insights into pigment cell regulation

Genetic interactions are adaptive within a species. Hybridization can disrupt such species-specific genetic interactions and creates novel interactions that alter the hybrid progeny overall fitness. Hybrid incompatibility, which refers to degenerative genetic interactions that decrease the overall hybrid survival, is one of the results from combining two diverged genomes in hybrids. The discovery of spontaneous lethal tumorigenesis and underlying genetic interactions in select hybrids between diverged Xiphophorus species showed that lethal pathological process can result from degenerative genetic interactions. Such genetic interactions leading to lethal phenotype are thought to shield gene flow between diverged species. However, hybrids between certain Xiphophorus species do not develop such tumors. Here we report the identification of a locus residing in the genome of one Xiphophorus species that represses an oncogene from a different species. Our finding provides insights into normal and pathological pigment cell development, regulation and molecular mechanism in hybrid incompatibility.

Significance

The Dobzhansky-Muller model states epistatic interactions occurred between genes in diverged species underlies hybrid incompatibility. There are a few vertebrate interspecies hybrid cases that support the Dobzhansky-Muller model. This study reports a fish hybrid system where incompatible genetic interactions are involved in neuronal regulation of pigment cell biology, and also identified a novel point of regulation for pigment cells.

Transcriptional background effects on a tumor driver gene in different pigment cell types of medaka

The Xiphophorus melanoma receptor kinase gene, xmrk, is a bona fide oncogene driving melanocyte tumorigenesis of Xiphophorus fish. When ectopically expressed in medaka, it not only induces development of several pigment cell tumor types in different strains of medaka but also induces different tumor types within the same animal, suggesting its oncogenic activity has a transcriptomic background effect. Although the central pathways that xmrk utilizes to lead to melanomagenesis are well documented, genes and genetic pathways that modulate the oncogenic effect and alter the course of disease have not been studied so far. To understand how the genetic networks between different histocytes of xmrk-driven tumors are composed, we isolated two types of tumors, melanoma and xanthoerythrophoroma, from the same xmrk transgenic medaka individuals, established the transcriptional profiles of both xmrk-driven tumors, and compared (1) genes that are co-expressed with xmrk in both tumor types, and (2) differentially expressed genes and their associated molecular functions, between the two tumor types. Transcriptomic comparisons between the two tumor types show melanoma and xanthoerythrophoroma are characterized by transcriptional features representing varied functions, indicating distinct molecular interactions between the driving oncogene and the cell-type-specific transcriptomes. Melanoma tumors exhibit gene signatures that are relevant to proliferation and invasion, while xanthoerythrophoroma tumors are characterized by expression profiles related to metabolism and DNA repair. We conclude the transcriptomic backgrounds, exemplified by cell-type-specific genes that are downstream of xmrk effected signaling pathways, contribute the potential to change the course of tumor development and may affect overall tumor outcomes.

Validity of Xiphophorus fish as models for human disease

Platyfish and swordtails of the genus Xiphophorus provide a well-established model for melanoma research and have become well known for this feature. Recently, modelling approaches for other human diseases in Xiphophorus have been developed or are emerging. This Review provides a comprehensive summary of these models and discusses how findings from basic biological and molecular studies and their translation to medical research demonstrate that Xiphophorus models have face, construct and predictive validity for studying a broad array of human diseases. These models can thus improve our understanding of disease mechanisms to benefit patients.

The human and animals' malignant melanoma: comparative tumor models and the role of microbiome in dogs and humans

Currently, the most progressively occurring incident cancer is melanoma. The mouse is the most popular model in human melanoma research given its various benefits as a laboratory animal. Nevertheless, unlike humans, mice do not develop melanoma spontaneously, so they need to be genetically manipulated. In opposition, there are several reports of other animals, ranging from wild to domesticated animals, that spontaneously develop melanoma and that have cancer pathways that are similar to those of humans. The influence of the gut microbiome on health and disease is being the aim of many recent studies. It has been proven that the microbiome is a determinant of the host's immune status and disease prevention. In human medicine, there is increasing evidence that changes in the microbiome influences malignant melanoma progression and response to therapy. There are several similarities between some animals and human melanoma, especially between canine and human oral malignant melanoma as well as between the gut microbiome of both species. However, microbiome studies are scarce in veterinary medicine, especially in the oncology field. Future studies need to address the relevance of gut and tissue microbiome for canine malignant melanoma development, which results will certainly benefit both species in the context of translational medicine.

Lessons from microRNA biology: Top key cellular drivers of Autosomal Dominant Polycystic Kidney Disease

BACKGROUND

Numerous microRNAs (miRs), small RNAs that target several pathways, have been implicated in the development of Autosomal Dominant Polycystic Kidney Disease (ADPKD), which is the most common genetic cause of kidney failure. The hallmark of ADPKD is tissue overgrowth and hyperproliferation, eventually leading to kidney failure.

SCOPE OF THE REVIEW

Many miRs are dysregulated in disease, yet the intracellular pathways regulated by miRs are less well described in ADPKD. Here, I summarise all the differentially expressed miRs in ADPKD and highlight the top miR-regulated cellular driver of disease.

MAJOR CONCLUSIONS

Literature review has identified 53 abnormally expressed miRs in ADPKD. By performing bioinformatics analysis of their target genes I present 10 key intracellular pathways that drive ADPKD progression. The top key drivers are divided into three main areas: (i) hyperproliferation and the role of JAK/STAT and PI3K pathways (ii) DNA damage and (iii) inflammation and NFκB.

GENERAL SIGNIFICANCE

The description of the 10 top cellular drivers of ADPKD, derived by analysis of miR signatures, is of paramount importance in better understanding the key processes resulting in pathophysiological changes that underlie disease.

Hematopoietic versus leukemic stem cell quiescence: Challenges and therapeutic opportunities

Hematopoietic stem cells (HSC) are responsible for the production of mature blood cells. To ensure that the HSC pool does not get exhausted over the lifetime of an individual, most HSCs are in a state of quiescence with only a small proportion of HSCs dividing at any one time. HSC quiescence is carefully controlled by both intrinsic and extrinsic, niche-driven mechanisms. In acute myeloid leukemia (AML), the leukemic cells overtake the hematopoietic bone marrow niche where they acquire a quiescent state. These dormant AML cells are resistant to chemotherapeutics. Because they can re-establish the disease after therapy, they are often termed as quiescent leukemic stem cells (LSC) or leukemia-initiating cells. While advancements are being made to target particular driver mutations in AML, there is less focus on how to tackle the drug resistance of quiescent LSCs. This review summarises the current knowledge on the biochemical characteristics of quiescent HSCs and LSCs, the intracellular signaling pathways and the niche-driven mechanisms that control quiescence and the key differences between HSC- and LSC-quiescence that may be exploited for therapy.

Xmrks the Spot: Fish Models for Investigating Epidermal Growth Factor Receptor Signaling in Cancer Research

Studies conducted in several fish species, e.g., Xiphophorus hellerii (green swordtail) and Xiphophorus maculatus (southern platyfish) crosses, Oryzias latipes (medaka), and Danio rerio (zebrafish), have identified an oncogenic role for the receptor tyrosine kinase, Xmrk, a gene product closely related to the human epidermal growth factor receptor (EGFR), which is associated with a wide variety of pathological conditions, including cancer. Comparative analyses of Xmrk and EGFR signal transduction in melanoma have shown that both utilize STAT5 signaling to regulate apoptosis and cell proliferation, PI3K to modulate apoptosis, FAK to control migration, and the Ras/Raf/MEK/MAPK pathway to regulate cell survival, proliferation, and differentiation. Further, Xmrk and EGFR may also modulate similar chemokine, extracellular matrix, oxidative stress, and microRNA signaling pathways in melanoma. In hepatocellular carcinoma (HCC), Xmrk and EGFR signaling utilize STAT5 to regulate cell proliferation, and Xmrk may signal through PI3K and FasR to modulate apoptosis. At the same time, both activate the Ras/Raf/MEK/MAPK pathway to regulate cell proliferation and E-cadherin signaling. Xmrk models of melanoma have shown that inhibitors of PI3K and MEK have an anti-cancer effect, and in HCC, that the steroidal drug, adrenosterone, can prevent metastasis and recover E-cadherin expression, suggesting that fish Xmrk models can exploit similarities with EGFR signal transduction to identify and study new chemotherapeutic drugs.

Advantages of targeting the tumor immune microenvironment over blocking immune checkpoint in cancer immunotherapy

Despite great success in cancer immunotherapy, immune checkpoint-targeting drugs are not the most popular weapon in the armory of cancer therapy. Accumulating evidence suggests that the tumor immune microenvironment plays a critical role in anti-cancer immunity, which may result in immune checkpoint blockade therapy being ineffective, in addition to other novel immunotherapies in cancer patients. In the present review, we discuss the deficiencies of current cancer immunotherapies. More importantly, we highlight the critical role of tumor immune microenvironment regulators in tumor immune surveillance, immunological evasion, and the potential for their further translation into clinical practice. Based on their general targetability in clinical therapy, we believe that tumor immune microenvironment regulators are promising cancer immunotherapeutic targets. Targeting the tumor immune microenvironment, alone or in combination with immune checkpoint-targeting drugs, might benefit cancer patients in the future.

Spontaneously occurring melanoma in animals and their relevance to human melanoma

In contrast to other cancer types, melanoma incidence has been increasing over the last 50 years, and while it still represents less than 5% of all cutaneous malignancies, melanoma accounts for the majority of skin cancer deaths, due to its propensity to metastasise. Whilst melanoma most commonly affects the skin, it can also arise in mucosal surfaces, the eye, and the brain. For new therapies to be developed, a better understanding of the genetic landscape, signalling pathways, and tumour–microenvironmental interactions is needed. This is where animal models are of critical importance. The mouse is the foremost used model of human melanoma. Arguably this is due to its plethora of benefits as a laboratory animal; however, it is important to note that unlike humans, melanocytes are not present at the dermal–epidermal junction in mice and mice do not develop melanoma without genetic manipulation. In contrast, there are numerous reports of animals that spontaneously develop melanoma, ranging from sharks and parrots to hippos and monkeys. In addition, several domesticated and laboratory‐bred animals spontaneously develop melanoma or UV‐induced melanoma, specifically, fish, opossums, pigs, horses, cats, and dogs. In this review, we look at spontaneously occurring animal ‘models’ of melanoma and discuss their relevance to the different types of melanoma found in humans. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland..

Balancing STAT Activity as a Therapeutic Strategy

Driven by dysregulated IL-6 family member cytokine signaling in the tumor microenvironment (TME), aberrant signal transducer and activator of transcription (STAT3) and (STAT5) activation have been identified as key contributors to tumorigenesis. Following transformation, persistent STAT3 activation drives the emergence of mesenchymal/cancer-stem cell (CSC) properties, important determinants of metastatic potential and therapy failure. Moreover, STAT3 signaling within tumor-associated macrophages and neutrophils drives secretion of factors that facilitate metastasis and suppress immune cell function. Persistent STAT5 activation is responsible for cancer cell maintenance through suppression of apoptosis and tumor suppressor signaling. Furthermore, STAT5-mediated CD4+/CD25+ regulatory T cells (T_regs) have been implicated in suppression of immunosurveillance. We discuss these roles for STAT3 and STAT5, and weigh the attractiveness of different modes of targeting each cancer therapy. Moreover, we discuss how anti-tumorigenic STATs, including STAT1 and STAT2, may be leveraged to suppress the pro-tumorigenic functions of STAT3/STAT5 signaling.

STAT5 expression correlates with recurrence and survival in melanoma patients treated with interferon-α

Interferons (IFN) have a direct growth-inhibiting effect on tumor cells through Janus kinase-dependent activation of the transcription factor signal transducer and activator of transcription (STAT1). In vitro, signaling through STAT5 has been demonstrated to counteract this effect and lead to IFN resistance of melanoma cell lines. In 32 patients treated with IFN-α in an adjuvant setting, we investigated paraffin-embedded tumor tissue from primary melanomas and melanoma metastases for expression of STAT3 and STAT5, by immunohistochemistry, and for expression of phosphorylated signaling transduction activating transcription factor (pSTAT)3 and pSTAT5, by immunofluorescence. Tumor cell expression levels of these proteins were correlated with patient characteristics and clinical outcomes. The patient cohort consisted of 12 (37.5%) patients at AJCC stage I/II (primary melanoma) and 20 (62.5%) at stage III/IV (metastatic melanoma). Recurrence was observed for 25 (78.1%) either during or after IFN-α therapy. χ Correlation of staining intensities with clinical data revealed association of pSTAT3 and STAT5 expression with sex (P=0.003 and 0.016, respectively) and of STAT3 with tumor stage (P=0.019). Recurrence of melanoma was found to be associated with high STAT5 expression (P=0.017). Multivariable regression analysis revealed STAT5 expression as an independent factor for predicting progression-free survival (P<0.0001) and overall survival (P=0.022). In summary, high expression of STAT5 correlated with melanoma recurrence and survival of patients treated with IFN-α in the adjuvant setting. Recently, it has been suggested that mutations of Janus kinases are involved in resistance to immune checkpoint blocker treatments implying a possible role of STAT5 for immune checkpoint resistance.

Application of the Transcriptional Disease Signature (TDSs) to Screen Melanoma-Effective Compounds in a Small Fish Model

Cell culture and protein target-based compound screening strategies, though broadly utilized in selecting candidate compounds, often fail to eliminate candidate compounds with non-target effects and/or safety concerns until late in the drug developmental process. Phenotype screening using intact research animals is attractive because it can help identify small molecule candidate compounds that have a high probability of proceeding to clinical use. Most FDA approved, first-in-class small molecules were identified from phenotypic screening. However, phenotypic screening using rodent models is labor intensive, low-throughput, and very expensive. As a novel alternative for small molecule screening, we have been developing gene expression disease profiles, termed the Transcriptional Disease Signature (TDS), as readout of small molecule screens for therapeutic molecules. In this concept, compounds that can reverse, or otherwise affect known disease-associated gene expression patterns in whole animals may be rapidly identified for more detailed downstream direct testing of their efficacy and mode of action. To establish proof of concept for this screening strategy, we employed a transgenic strain of a small aquarium fish, medaka (Oryzias latipes), that overexpresses the malignant melanoma driver gene xmrk, a mutant egfr gene, that is driven by a pigment cell-specific mitf promoter. In this model, melanoma develops with 100% penetrance. Using the transgenic medaka malignant melanoma model, we established a screening system that employs the NanoString nCounter platform to quantify gene expression within custom sets of TDS gene targets that we had previously shown to exhibit differential transcription among xmrk-transgenic and wild-type medaka. Compound-modulated gene expression was identified using an internet-accessible custom-built data processing pipeline. The effect of a given drug on the entire TDS profile was estimated by comparing compound-modulated genes in the TDS using an activation Z-score and Kolmogorov-Smirnov statistics. TDS gene probes were designed that target common signaling pathways that include proliferation, development, toxicity, immune function, metabolism and detoxification. These pathways may be utilized to evaluate candidate compounds for potential favorable, or unfavorable, effects on melanoma-associated gene expression. Here we present the logistics of using medaka to screen compounds, as well as, the development of a user-friendly NanoString data analysis pipeline to support feasibility of this novel TDS drug-screening strategy.

The Power of Fish Models to Elucidate Skin Cancer Pathogenesis and Impact the Discovery of New Therapeutic Opportunities

Animal models play important roles in investigating the pathobiology of cancer, identifying relevant pathways, and developing novel therapeutic tools. Despite rapid progress in the understanding of disease mechanisms and technological advancement in drug discovery, negative trial outcomes are the most frequent incidences during a Phase III trial. Skin cancer is a potential life-threatening disease in humans and might be medically futile when tumors metastasize. This explains the low success rate of melanoma therapy amongst other malignancies. In the past decades, a number of skin cancer models in fish that showed a parallel development to the disease in humans have provided important insights into the fundamental biology of skin cancer and future treatment methods. With the diversity and breadth of advanced molecular genetic tools available in fish biology, fish skin cancer models will continue to be refined and expanded to keep pace with the rapid development of skin cancer research. This review begins with a brief introduction of molecular characteristics of skin cancers, followed by an overview of teleost models that have been used in the last decades in melanoma research. Next, we will detail the importance of the zebrafish (Danio rerio) animal model and other emerging fish models including platyfish (Xiphophorus sp.), and medaka (Oryzias latipes) in future cutaneous malignancy studies. The last part of this review provides the recent development and genome editing applications of skin cancer models in zebrafish and the progress in small molecule screening.

Comparison of Xiphophorus and human melanoma transcriptomes reveals conserved pathway interactions

Comparative analysis of human and animal model melanomas can uncover conserved pathways and genetic changes that are relevant for the biology of cancer cells. Spontaneous melanoma in Xiphophorus interspecies backcross hybrid progeny may be informative in identifying genes and functional pathways that are similarly related to melanoma development in all vertebrates, including humans. To assess functional pathways involved in the Xiphophorus melanoma, we performed gene expression profiling of the melanomas produced in interspecies BC1 and successive backcross generations (i.e., BC5 ) of the cross: X. hellerii × [X. maculatus Jp 163 A × X. hellerii]. Using RNA-Seq, we identified genes that are transcriptionally co-expressed with the driver oncogene, xmrk. We determined functional pathways in the fish melanoma that are also present in human melanoma cohorts that may be related to dedifferentiation based on the expression levels of pigmentation genes. Shared pathways between human and Xiphophorus melanomas are related to inflammation, cell migration, cell proliferation, pigmentation, cancer development, and metastasis. Our results suggest xmrk co-expressed genes are associated with dedifferentiation and highlight these signaling pathways as playing important roles in melanomagenesis.

Xiphophorus and Medaka Cancer Models

Besides recently developed zebrafish cancer models, other fish species have been employed for many years as cancer models in laboratory studies. Two models, namely in Xiphophorus and medaka have proven useful in providing important clues to cancer etiology. Medaka is a complementary model to zebrafish in many areas of research since it offers similar resources and experimental tools. Xiphophorus provides the advantages of a natural ("evolutionary mutant") model with established genetics. Xiphophorus hybrids can develop spontaneous and radiation or carcinogen induced cancers. This chapter describes the tumor models in both species, which mainly focus on melanoma, and summarizes the main findings and future research directions.

Whole Body Melanoma Transcriptome Response in Medaka

The incidence of malignant melanoma continues to increase each year with poor prognosis for survival in many relapse cases. To reverse this trend, whole body response measures are needed to discover collaborative paths to primary and secondary malignancy. Several species of fish provide excellent melanoma models because fish and human melanocytes both appear in the epidermis, and fish and human pigment cell tumors share conserved gene expression signatures. For the first time, we have examined the whole body transcriptome response to invasive melanoma as a prelude to using transcriptome profiling to screen for drugs in a medaka (Oryzias latipes) model. We generated RNA-seq data from whole body RNA isolates for controls and melanoma fish. After testing for differential expression, 396 genes had significantly different expression (adjusted p-value <0.02) in the whole body transcriptome between melanoma and control fish; 379 of these genes were matched to human orthologs with 233 having annotated human gene symbols and 14 matched genes that contain putative deleterious variants in human melanoma at varying levels of recurrence. A detailed canonical pathway evaluation for significant enrichment showed the top scoring pathway to be antigen presentation but also included the expected melanocyte development and pigmentation signaling pathway. Results revealed a profound down-regulation of genes involved in the immune response, especially the innate immune system. We hypothesize that the developing melanoma actively suppresses the immune system responses of the body in reacting to the invasive malignancy, and that this mal-adaptive response contributes to disease progression, a result that suggests our whole-body transcriptomic approach merits further use. In these findings, we also observed novel genes not yet identified in human melanoma expression studies and uncovered known and new candidate drug targets for further testing in this malignant melanoma medaka model.

Transcriptional control analyses of the Xiphophorus melanoma oncogene

Melanoma development in interspecific hybrids of Xiphophorus is induced by the overexpression of the mutationally activated receptor tyrosine kinase Xmrk in pigment cells. Based on the melanocyte specificity of the transcriptional upregulation, a pigment cell-specific promoter region was postulated for xmrk, the activity of which is controlled in healthy purebred fish by the molecularly still unidentified regulator locus R. However, as yet the xmrk promoter region is still poorly characterized. In order to contribute to a better understanding of xmrk expression regulation, we performed a functional analysis of the entire putative gene regulatory region of the oncogene using conventional plasmid-based reporter systems as well as a newly established method employing BAC-derived luciferase reporter constructs in melanoma and non-melanoma cell lines. Using the melanocyte-specific mitfa promoter as control, we could demonstrate that our in vitro system is able to reliably monitor regulation of transcription through cell type-specific regulatory sequences. We found that sequences within 200kb flanking the xmrk oncogene do not lead to any specific transcriptional activation in melanoma compared to control cells. Hence, xmrk reporter constructs fail to faithfully reproduce the endogenous transcriptional regulation of the oncogene. Our data therefore strongly indicate that the melanocyte-specific transcription of xmrk is not the consequence of pigment cell-specific cis-regulatory elements in the promoter region. This hints at additional regulatory mechanisms involved in transcriptional control of the oncogene, thereby suggesting a key role for epigenetic mechanisms in oncogenic xmrk overexpression and thereby in tumor development in Xiphophorus.

Xmrk-induced melanoma progression is affected by Sdf1 signals through Cxcr7

Chemokine signals mediated by Sdf1/Cxcl12 through the chemokine receptor Cxcr4 are thought to play an instructive role in tumor migration and organ-specific metastasis. We have used a small aquarium fish model to contribute to a better understanding of how the course of melanoma development is influenced by Sdf1 signals in vivo. We studied oncogene-induced skin tumor appearance and progression in the transgenic medaka (Oryzias latipes) melanoma model. Similar to humans, invasive medaka melanomas show increased levels of sdf1, cxcr4, and cxcr7 gene expression. Stable transgenic fish lines overexpressing sdf1 exclusively in pigment cells showed a reduction in melanoma appearance and progression. Remarkably, diminished levels of functional Cxcr7, but not of Cxcr4b, resulted in strongly reduced melanoma invasiveness and a repression of melanoma. Our results thereby indicate that Sdf1 signals via Cxcr7 are able to constrain melanoma growth in vivo and that these signals influence tumor outcome.

Beyond the zebrafish: diverse fish species for modeling human disease

In recent years, zebrafish, and to a lesser extent medaka, have become widely used small animal models for human diseases. These organisms have convincingly demonstrated the usefulness of fish for improving our understanding of the molecular and cellular mechanisms leading to pathological conditions, and for the development of new diagnostic and therapeutic tools. Despite the usefulness of zebrafish and medaka in the investigation of a wide spectrum of traits, there is evidence to suggest that other fish species could be better suited for more targeted questions. With the emergence of new, improved sequencing technologies that enable genomic resources to be generated with increasing efficiency and speed, the potential of non-mainstream fish species as disease models can now be explored. A key feature of these fish species is that the pathological condition that they model is often related to specific evolutionary adaptations. By exploring these adaptations, new disease-causing and disease-modifier genes might be identified; thus, diverse fish species could be exploited to better understand the complexity of disease processes. In addition, non-mainstream fish models could allow us to study the impact of environmental factors, as well as genetic variation, on complex disease phenotypes. This Review will discuss the opportunities that such fish models offer for current and future biomedical research.

Tuning of alternative splicing--switch from proto-oncogene to tumor suppressor

STAT5B, a specific member of the STAT family, is intimately associated with prostate tumor progression. While the full form of STAT5B is thought to promote tumor progression, a naturally occurring truncated isoform acts as a tumor suppressor. We previously demonstrated that truncated STAT5 is generated by insertion of an alternatively spliced exon and results in the introduction of an early termination codon. Present approaches targeting STAT proteins based on inhibition of functional domains of STAT's, such as DNA-binding, cooperative binding (protein-protein interaction), dimerization and phosphorylation will halt the action of the entire gene, both the proto-oncogenic and tumor suppressor functions of Stat5B.

In this report we develop a new approach aimed at inhibiting the expression of full-length STAT5B (a proto-oncogene) while simultaneously enhancing the expression of STAT5∆B (a tumor suppressor). We have demonstrated the feasibility of using steric-blocking splice-switching oligonucleotides (SSOs) with a complimentary sequence to the targeted exon-intron boundary to enhance alternative intron/exon retention (up to 10%). The functional effect of the intron/exon proportional tuning was validated by cell proliferation and clonogenic assays. The new scheme applies specific steric-blocking splice-switching oligonucleotides and opens an opportunity for anti-tumor treatment as well as for the alteration of functional abilities of other STAT proteins.

JunD/AP-1-mediated gene expression promotes lymphocyte growth dependent on interleukin-7 signal transduction

Interleukin-7 (IL-7) is an essential cytokine for lymphocyte growth that has the potential for promoting immune reconstitution. This feature makes IL-7 an ideal candidate for therapeutic development. As with other cytokines, signaling through the IL-7 receptor induces the JAK/STAT pathway. However, the broad scope of IL-7 regulatory targets likely necessitates the use of other signaling components whose identities remain poorly defined. To this end, we used an IL-7 dependent T-cell line to examine how expression of the glycolytic enzyme, Hexokinase II (HXKII) was regulated by IL-7 in a STAT5-independent manner. Our studies revealed that IL-7 promoted the activity of JNK (Jun N-terminal Kinase), and that JNK, in turn, drove the expression of JunD, a component of the Activating Protein 1 (AP-1) transcription factors. Gel shifts showed that the AP-1 complex induced by IL-7 contained JunD but not c-Fos or c-Jun. Inhibition of JNK/JunD blocked glucose uptake and HXKII gene expression, indicating that this pathway was responsible for promoting HXKII expression. Because others had shown that JunD was a negative regulator of cell growth, we performed a bioinformatics analysis to uncover possible JunD-regulated gene targets. Our search revealed that JunD could control the expression of proteins involved in signal transduction, cell survival and metabolism. One of these growth promoters was the oncogene, Pim-1. Pim-1 is an IL-7-induced protein that was inhibited when the activities of JNK or JunD were blocked, showing that in IL-7 dependent T-cells JunD can promote positive signals transduced through Pim-1. This was confirmed when the IL-7-induced proliferation of CD8 T-cells was impaired upon JunD inhibition. These results show that engagement of the IL-7 receptor drives a signal that is more complex than the JAK/STAT pathway, activating JNK and JunD to induce rapid growth stimulation through the expression of metabolic and signaling factors like HXKII and Pim-1.

Signal transducers and activators of transcription-3/pim1 axis plays a critical role in the pathogenesis of human pulmonary arterial hypertension

BACKGROUND

Pulmonary artery hypertension (PAH) is a proliferative disorder associated with enhanced pulmonary artery smooth muscle cell proliferation and suppressed apoptosis. The sustainability of this phenotype required the activation of a prosurvival transcription factor like signal transducers and activators of transcription-3 (STAT3) and nuclear factor of activated T cell (NFAT). Because these factors are implicated in several physiological processes, their inhibition in PAH patients could be associated with detrimental effects. Therefore, a better understanding of the mechanism accounting for their expression/activation in PAH pulmonary artery smooth muscle cells is of great therapeutic interest.

METHODS AND RESULTS

Using multidisciplinary and translational approaches, we demonstrated that STAT3 activation in both human and experimental models of PAH accounts for the expression of both NFATc2 and the oncoprotein kinase Pim1, which trigger NFATc2 activation. Because Pim1 expression correlates with the severity of PAH in humans and is confined to the PAH pulmonary artery smooth muscle cell, Pim1 was identified as an attractive therapeutic target for PAH. Indeed, specific Pim1 inhibition in vitro decreases pulmonary artery smooth muscle cell proliferation and promotes apoptosis, all of which are sustained by NFATc2 inhibition. In vivo, tissue-specific inhibition of Pim1 by nebulized siRNA reverses monocrotaline-induced PAH in rats, whereas Pim1 knockout mice are resistant to PAH development.

CONCLUSION

We demonstrated for the first time that inhibition of the inappropriate activation of STAT3/Pim1 axis is a novel, specific, and attractive therapeutic strategy to reverse PAH.

Generating and analyzing fish models of melanoma

Melanoma is the most deadly form of skin cancer and incidence continues to rise rapidly (Gray-Schopfer et al., 2007). Melanoma develops from melanocytes, the pigmented cells that color our skin, hair, and eyes. Fish also have melanocytes, among other pigment cell types, and the fish and human developmental programme are highly conserved (Kelsh, 2004). The first fish models of melanoma were established in Xiphophorus, and more recently, transgenic melanoma models in zebrafish and medaka have been developed (Meierjohann and Schartl, 2006; Patton et al., 2010; Schartl et al., 2010). In this Chapter, we describe the basic techniques to generate genetic, environmental, and transgenic models of melanoma, discuss diagnoses, and describe standard molecular analysis techniques.

Gene expression analysis after receptor tyrosine kinase activation reveals new potential melanoma proteins

BACKGROUND

Melanoma is an aggressive tumor with increasing incidence. To develop accurate prognostic markers and targeted therapies, changes leading to malignant transformation of melanocytes need to be understood. In the Xiphophorus melanoma model system, a mutated version of the EGF receptor Xmrk (Xiphophorus melanoma receptor kinase) triggers melanomagenesis. Cellular events downstream of Xmrk, such as the activation of Akt, Ras, B-Raf or Stat5, were also shown to play a role in human melanomagenesis. This makes the elucidation of Xmrk downstream targets a useful method for identifying processes involved in melanoma formation.

METHODS

Here, we analyzed Xmrk-induced gene expression using a microarray approach. Several highly expressed genes were confirmed by realtime PCR, and pathways responsible for their induction were revealed using small molecule inhibitors. The expression of these genes was also monitored in human melanoma cell lines, and the target gene FOSL1 was knocked down by siRNA. Proliferation and migration of siRNA-treated melanoma cell lines were then investigated.

RESULTS

Genes with the strongest upregulation after receptor activation were FOS-like antigen 1 (Fosl1), early growth response 1 (Egr1), osteopontin (Opn), insulin-like growth factor binding protein 3 (Igfbp3), dual-specificity phosphatase 4 (Dusp4), and tumor-associated antigen L6 (Taal6). Interestingly, most genes were blocked in presence of a SRC kinase inhibitor. Importantly, we found that FOSL1, OPN, IGFBP3, DUSP4, and TAAL6 also exhibited increased expression levels in human melanoma cell lines compared to human melanocytes. Knockdown of FOSL1 in human melanoma cell lines reduced their proliferation and migration.

CONCLUSION

Altogether, the data show that the receptor tyrosine kinase Xmrk is a useful tool in the identification of target genes that are commonly expressed in Xmrk-transgenic melanocytes and melanoma cell lines. The identified molecules constitute new possible molecular players in melanoma development. Specifically, a role of FOSL1 in melanomagenic processes is demonstrated. These data are the basis for future detailed analyses of the investigated target genes.

Genetic and environmental melanoma models in fish

Experimental animal models are extremely valuable for the study of human diseases, especially those with underlying genetic components. The exploitation of various animal models, from fruitflies to mice, has led to major advances in our understanding of the etiologies of many diseases, including cancer. Cutaneous malignant melanoma is a form of cancer for which both environmental insult (i.e., UV) and hereditary predisposition are major causative factors. Fish melanoma models have been used in studies of both spontaneous and induced melanoma formation. Genetic hybrids between platyfish and swordtails, different species of the genus Xiphophorus, have been studied since the 1920s to identify genetic determinants of pigmentation and melanoma formation. Recently, transgenesis has been used to develop zebrafish and medaka models for melanoma research. This review will provide a historical perspective on the use of fish models in melanoma research, and an updated summary of current and prospective studies using these unique experimental systems.

A mutated EGFR is sufficient to induce malignant melanoma with genetic background-dependent histopathologies

Melanoma is a tumor with a very low cure rate once metastasized. Although many genes important for melanoma induction, transformation, and metastasis have been identified, the process of melanomagenesis is only partly understood. Melanoma mediators are easiest to investigate in cell culture models, but animal models are required to evaluate their importance in the context of the whole organism. Here, we describe a transgenic melanoma model in medaka. The oncogenic receptor tyrosine kinase, Xmrk, responsible for melanoma formation in Xiphophorus, was stably expressed under the control of a pigment cell-specific promoter. The transgenic fish developed pigment cell tumors with a penetrance of 100%. The model was used for monitoring the in vivo relevance of several apoptosis and differentiation genes, and for induction of melanoma-relevant signal transduction pathways. We found that Stat5 activation, and Mitf and Bcl-2 levels correlated with a more aggressive stage of the malignancy. Interestingly, different types of pigment cell tumors occurred depending on the genetic background, namely invasive melanoma, uveal melanoma, or exophytic and less aggressive pigment cell tumors called xanthoerythrophoroma. Furthermore, on p53 mutant background, the expression of xmrk led to the appearance of giant focal pigment cell tumors, whereas tumor onset was unchanged compared with wild-type medaka.

In vivo identification of novel STAT5 target genes

STAT5A and STAT5B proteins belong to the family of signal transducers and activators of transcription. They are encoded by two separate genes with 91% identity in their amino acid sequences. Despite their high degree of conservation, STAT5A and STAT5B exert non-redundant functions, resulting at least in part from differences in target gene activation. To better characterize the differential contribution of STAT5A and STAT5B in gene regulation, we performed single or double knockdown of STAT5A and STAT5B using small interfering RNA. Subsequent gene expression profiling and RT-qPCR analyses of IL-3-stimulated Ba/F3-β cells led to the identification of putative novel STAT5 target genes. Chromatin immunoprecipitation assays analyzing the corresponding gene loci identified unusual STAT5 binding sites compared to conventional STAT5 responsive elements. Some of the STAT5 targets identified are upregulated in several human cancers, suggesting that they might represent potential oncogenes in STAT5-associated malignancies.

A structural model of the extracellular domain of the oncogenic EGFR variant Xmrk

The epidermal growth factor receptor (EGFR) variant Xmrk of the Mexican fish Xiphophorus is one of the first oncogenes described. Its overexpression in pigment cells of the skin occurs after certain repeated cross-breedings between the platyfish Xiphophorus maculatus and the swordtail Xiphophorus hellerii. Xmrk overexpression results in melanoma with a high malignant potential. The Xiphophorus melanoma model provides a valuable tool for the understanding of melanoma development in general. The xmrk gene is the duplicated version of the proto-oncogene egfr-b. Compared to this, it contains 14 nonsynonymous mutations. Two of these mutations lead to a permanent protein dimerization which results in constitutive signaling. For a better understanding of the structural conditions responsible for this effect, the authors modeled the extracellular domain of Egfr-b, based on the crystal structure of the closely related human EGFR, and introduced the mutations G336R or C555S. The resulting protein models show that in either mutation the formation of an intramolecular disulfide bridge is very likely prevented, leading to an intermolecular disulfide bridge between released cysteine residues. In the G336R model, the distance between the membrane-proximal domains from both chains is smaller than in the C555S model, confirming the previous observation than Egfr-b-G336R is more tumorigenic than Egfr-b-C555S in vivo.

Stat5 as a diagnostic marker for leukemia

The Jak-Stat-Socs pathway is an important component of cytokine receptor signaling. Not surprisingly, perturbation of this pathway is implicated in diseases of hematopoietic and immune origin, including leukemia, lymphoma and immune deficiencies. This review examines the role of a key component of this pathway, Stat5. This has been shown to be activated in a variety of leukemias and myeloproliferative disorders, including downstream of a range of key oncogenes where it has been shown to play an important role in mediating their effects. Therefore, Stat5 represents a useful pan-leukemia/myeloproliferative disorder diagnostic marker and key therapeutic end point, as well as representing an attractive therapeutic target for these disorders.

Stat5 constitutive activation rescues defects in spinal muscular atrophy

Proximal spinal muscular atrophy (SMA) is a motor neuron degeneration disorder for which there is currently no effective treatment. Here, we report three compounds (sodium vanadate, trichostatin A and aclarubicin) that effectively enhance SMN2 expression by inducing Stat5 activation in SMA-like mouse embryonic fibroblasts and human SMN2-transfected NSC34 cells. We found that Stat5 activation enhanced SMN2 promoter activity with increase in both full-length and deletion exon 7 SMN transcripts in SMN2-NSC34 cells. Knockdown of Stat5 expression disrupted the effects of sodium vanadate on SMN2 activation but did not influence SMN2 splicing, suggesting that Stat5 signaling is involved in SMN2 transcriptional regulation. In addition, constitutive activation of Stat5 mutant (Stat5A1*6) profoundly increased the number of nuclear gems in SMA-patient lymphocytes and reduced SMA-like motor neuron axon outgrowth defects. These results demonstrate that Stat5 signaling could be a possible pharmacological target for treating SMA.

Melanoma susceptibility and cell cycle genes inXiphophorus hybrids

Xiphophorus interspecies hybrids provide genetically defined models of both spontaneous and inducible melanomagenesis. In both models, backcrossing F(1) hybrids of different strains of X. maculatus and X. helleri to a X. helleri parental fish results in segregation of melanoma susceptibility, fitting a Mendelian two-gene inheritance model. The sex-linked Xmrk oncogene is required for melanoma development in both crosses. The Xiphophorus CDKN2A/B gene, which is homologous to mammalian CDKN2A/B cyclin-dependent kinase inhibitors (p16 and p15), is a candidate melanoma susceptibility gene. In this model, tumor susceptibility segregates with homozgyosity for CDKN2A/B from the recurrent X. helleri parent in backcross hybrids. We found that both CDKN2A/B mRNA and protein are highly overexpressed in melanoma. Because the p13 protein product of CDKN2A/B is a putative regulator of the G1 checkpoint, we investigated expression of other components of Xiphophorus G1 checkpoint control. By real-time PCR analysis, retinoblastoma gene (RB) is consistently expressed twofold higher in both tumors and melanized skin than in normal tissue, indicating that RB is not downregulated by the overexpression of CDKN2A/B in Xiphophorus melanoma. We also found a significant correlation between the quantitative level of CDKN2A/B and Xmrk RNA in tumors, suggesting a functional relationship between Xmrk and CDKN2A/B expression. Although X. helleri CDKN2A/B protein contains a non-conservative substitution, the biochemical function appears to show little overt defect. These studies indicate that in Xiphophorus melanoma, CDKN2A/B is functionally insufficient to mediate cell-cycle arrest in the presence of Xmrk.

Convergence of TCR and cytokine signaling leads to FOXO3a phosphorylation and drives the survival of CD4+ central memory T cells

The molecular events involved in the establishment and maintenance of CD4⁺ central memory and effector memory T cells (T_CM and T_EM, respectively) are poorly understood. In this study, we demonstrate that ex vivo isolated T_CM are more resistant to both spontaneous and Fas-induced apoptosis than T_EM and have an increased capacity to proliferate and persist in vitro. Using global gene expression profiling, single cell proteomics, and functional assays, we show that the survival of CD4⁺ T_CM depends, at least in part, on the activation and phosphorylation of signal transducer and activator of transcription 5a (STAT5a) and forkhead box O3a (FOXO3a). T_CM showed a significant increase in the levels of phosphorylation of STAT5a compared with T_EM in response to both IL-2 (P < 0.04) and IL-7 (P < 0.002); the latter is well known for its capacity to enhance T cell survival. Moreover, ex vivo T_CM express higher levels of the transcriptionally inactive phosphorylated forms of FOXO3a and concomitantly lower levels of the proapoptotic FOXO3a target, Bim. Experiments aimed at blocking FOXO3a phosphorylation confirmed the role of this phosphoprotein in protecting T_CM from apoptosis. Our results provide, for the first time in humans, an insight into molecular mechanisms that could be responsible for the longevity and persistence of CD4⁺ T_CM.

Multiple pathways contribute to the hyperproliferative responses from truncated granulocyte colony-stimulating factor receptors

Mutations in the granulocyte colony-stimulating factor receptor (G-CSF-R) gene leading to a truncated protein have been identified in a cohort of neutropenia patients highly predisposed to acute myeloid leukemia. Such mutations act in a dominant manner resulting in hyperproliferation but impaired differentiation in response to G-CSF. This is due, at least in part, to defective internalization and loss of binding sites for several negative regulators, leading to sustained receptor activation. However, those signaling pathways responsible for mediating the hyperproliferative function have remained unclear. In this study, analysis of an additional G-CSF-R mutant confirmed the importance of residues downstream of Box 2 as important contributors to the sustained proliferation. However, maximal proliferation correlated with the ability to robustly activate signal transducer and activator of transcription (STAT) 5 in a sustained manner, whereas co-expression of dominant-negative STAT5, but not dominant-negative STAT3, was able to inhibit G-CSF-stimulated proliferation from a truncated receptor. Furthermore, a Janus kinase (JAK) inhibitor also strongly reduced the proliferative response, whereas inhibitors of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) or phosphatidylinositol (PI) 3-kinase reduced proliferation to a lesser degree. These data suggest that sustained JAK2/STAT5 activation is a major contributor to the hyperproliferative function of truncated G-CSF receptors, with pathways involving MEK and PI 3-kinase playing a reduced role.

STAT5 Phosphorylation in Malignant Melanoma Is Important for Survival and Is Mediated Through SRC and JAK1 Kinases

Altered signaling pathways are key regulators of cellular functions in tumor cells. Constitutive activation of signal transducer and activator of transcription (STAT)3 and -5 may be involved in tumor formation and progression. We have investigated the role of STAT5 in cutaneous melanoma metastases using various RNA and protein techniques. In melanoma specimens, Stat5b transcripts were upregulated approximately 3.8-fold. In 13 of 21 (62%) human melanoma metastases, STAT5 was phosphorylated in comparison to normal human melanocytes and benign nevi. The STAT5 target gene Bcl-2 was frequently upregulated. The investigation of the underlying mechanism revealed specific STAT5 activation by recombinant human epidermal growth factor (rEGF). rEGF-induced activation of STAT5 occurred in vitro through the non-receptor tyrosine kinases transforming gene (src) of Rous Sarcoma virus and Janus kinase 1. Inhibition of Stat5b expression by small interfering RNA strongly reduced the expression of Bcl-2 and led to decreased cell viability and increased apoptosis in the melanoma cell lines A375 and BLM. Transfection with dominant-negative Stat5b caused enhanced cell death and G1 arrest in A375 cells. Our study identifies phosphorylated STAT5 in melanoma and shows regulation through rEGF; STAT5 may thus act as a survival factor for growth of human melanoma and may represent a potential target for molecular therapy.

STAT5 contributes to interferon resistance of melanoma cells

BACKGROUND

Malignant melanoma is a highly aggressive neoplastic disease whose incidence is increasing rapidly. In recent years, the use of interferon alpha (IFNalpha) has become the most established adjuvant immunotherapy for melanoma of advanced stage. IFNalpha is a potent inhibitor of melanoma cell proliferation, and the signal transducer and activator of transcription STAT1 is crucial for its antiproliferative action. Although advanced melanomas clinically resistant to IFNalpha are frequently characterized by inefficient STAT1 signaling, the mechanisms underlying advanced-stage interferon resistance are poorly understood.

RESULTS

Here, we demonstrate that IFNalpha activates STAT5 in melanoma cells and that in IFNalpha-resistant cells STAT5 is overexpressed. Significantly, the knockdown of STAT5 in interferon-resistant melanoma cells restored the growth-inhibitory response to IFNalpha. When STAT5 was overexpressed in IFNalpha-sensitive cells, it counteracted interferon-induced growth inhibition. The overexpressed STAT5 diminished IFNalpha-triggered STAT1 activation, most evidently through upregulation of the inhibitor of cytokine-signaling CIS.

CONCLUSIONS

Our data demonstrate that overexpression and activation of STAT5 enable melanoma cells to overcome cytokine-mediated antiproliferative signaling. Thus, overexpression of STAT5 can counteract IFNalpha signaling in melanoma cells, and this finally can result in cytokine-resistant and progressively growing tumor cells. These findings have significant implications for the clinical failure of IFNalpha therapy of advanced melanoma because they demonstrate that IFNalpha induces the activation of STAT5 in melanoma cells, and in STAT5-overexpressing cells, this contributes to IFNalpha resistance.

Cell biology of IL-7, a key lymphotrophin

IL-7 is essential for the development and survival of T lymphocytes. This review is primarily from the perspective of the cell biology of the responding T cell. Beginning with IL-7 receptor structure and regulation, the major signaling pathways appear to be via PI3K and Stat5, although the requirement for either has yet to be verified by published knockout experiments. The proliferation pathway induced by IL-7 differs from conventional growth factors and is primarily through posttranslational regulation of p27, a Cdk inhibitor, and Cdc25a, a Cdk-activating phosphatase. The survival function of IL-7 is largely through maintaining a favorable balance of bcl-2 family members including Bcl-2 itself and Mcl-1 on the positive side, and Bax, Bad and Bim on the negative side. There are also some remarkable metabolic effects of IL-7 withdrawal. Studies of IL-7 receptor signaling have yet to turn up unique pathways, despite the unique requirement for IL-7 in T cell biology. There remain significant questions regarding IL-7 production and the major producing cells have yet to be fully characterized.

STATs as critical mediators of signal transduction and transcription: lessons learned from STAT5

Signal transducers and activators of transcription (Stats) comprise a family of seven transcription factors that are activated by a variety of cytokines, hormones and growth factors. Stats are activated through tyrosine phosphorylation, mainly by Jak kinases, that lead to their dimerization, nuclear translocation and regulation of target gene expression. Stat5 was originally identified as a transcription factor that regulates the beta-casein gene in response to prolactin (PRL), but Stat5 is activated also by several other cytokines and growth factors. The molecular mechanisms that underlie Stat5-mediated transcription involve interactions and cooperation with sequence specific transcription factors and transcriptional coregulators. Our studies identified p100 protein as a coactivator for Stat5, and suggest the existence of a positive regulatory loop in PRL-induced transcription, where PRL stabilizes p100 protein, which in turn can cooperate with Stat5 in transcriptional activation. Suppressors of cytokine signaling (SOCS) proteins are important negative regulators of Stats. A target gene for Stat5, the serine/threonine kinase Pim-1, was found to cooperate with SOCS-1 and SOCS-3 to inhibit Stat5 activity suggesting that Pim-1 together with SOCS-1 and SOCS-3 are components of a negative feedback mechanism that allows Stat5 to regulate its own activation.

Genetic, biochemical and evolutionary facets of Xmrk-induced melanoma formation in the fish Xiphophorus

Certain interspecific hybrids of the fish Xiphophorus spontaneously develop melanoma induced by the derepression of the Xmrk oncogene. Xmrk is a recent duplicate of an orthologue of the mammalian epidermal growth factor receptor gene Egfr. In addition to a specific overexpression in melanoma, amino-acid substitutions in the extracellular domain leading to ligand-independent dimerisation and constitutive autophosphorylation are responsible for the tumorigenic potential of Xmrk. The Xmrk receptor induces several signal transduction pathways mediating cell proliferation and resistance to apoptosis and initiating dedifferentiation. Moreover, Xmrk upregulates the expression of the secreted protein osteopontin, inducing an autocrine loop possibly allowing invasion and survival in the dermis as a first step in malignancy. Hence, Xmrk is able to induce pathways essential for a transformed phenotype. Some of these events are equivalent to those found downstream of the mammalian Egfr, but others have clearly evolved differently or are specific for pigment cells. Xmrk is potentially hazardous, nonessential and located in a very unstable genomic region. Nevertheless, Xmrk has been maintained under purifying selection in divergent Xiphophorus species. Hence, Xmrk has probably a beneficial function under certain conditions. The analysis of this function is a major challenge for future research in the Xiphophorus model.

Identification of transcription factor binding sites upstream of human genes regulated by the phosphatidylinositol 3-kinase and MEK/ERK signaling pathways

We have taken an integrated approach in which expression profiling has been combined with the use of small molecule inhibitors and computational analysis of transcription factor binding sites to characterize regulatory sequences of genes that are targets of specific signaling pathways in growth factor-stimulated human cells. T98G cells were stimulated with platelet-derived growth factor (PDGF) and analyzed by DNA microarrays, which identified 74 immediate-early gene transcripts. Cells were then treated with inhibitors to identify subsets of genes that are targets of the phosphatidylinositol 3-kinase (PI3K) and MEK/ERK signaling pathways. Four groups of PDGF-induced genes were defined: independent of PI3K and MEK/ERK signaling, dependent on PI3K signaling, dependent on MEK/ERK signaling, and dependent on both pathways. The upstream regions of all genes in the four groups were scanned using TRANSFAC for putative cis-elements as compared with a background set of non-induced genes. Binding sites for 18 computationally predicted transcription factors were over-represented in the four groups of co-expressed genes compared with the background sequences (p < 0.01). Many of the cis-elements identified were conserved in orthologous mouse genes, and many of the predicted elements and their cognate transcription factors were consistent with previous experimental data. In addition, chromatin immunoprecipitation assays experimentally verified nine predicted SRF binding sites in T98G cells, including a previously unknown SRF site upstream of DUSP5. These results indicate that groups of human genes regulated by discrete intracellular signaling pathways share common cis-regulatory elements.

Rab7 Prevents Growth Factor-Independent Survival by Inhibiting Cell-Autonomous Nutrient Transporter Expression

Growth factor withdrawal results in the endocytosis and degradation of transporter proteins for glucose and amino acids. Here, we show that this process is under the active control of the small GTPase Rab7. In the presence of growth factor, Rab7 inhibition had no effect on nutrient transporter expression. In growth factor-deprived cells, however, blocking Rab7 function prevented the clearance of glucose and amino acid transporter proteins from the cell surface. When Rab7 was inhibited, growth factor deprived cells maintained their mitochondrial membrane potential and displayed prolonged, growth factor-independent, nutrient-dependent cell survival. Thus, Rab7 functions as a proapoptotic protein by limiting cell-autonomous nutrient uptake. Consistent with this, dominant-negative Rab7 cooperated with E1A to promote the transformation of p53(-/-) mouse embryonic fibroblasts (MEFs). These results suggest that proteins that limit nutrient transporter expression function to prevent cell-autonomous growth and survival.

Melanoma development and pigment cell transformation in xiphophorus

As early as 1927, it was recognised that hybridisation of platyfish (Xiphophorus maculatus) and swordtails (Xiphophorus helleri) results in offspring that develop tumours according to Mendelian laws. Most obviously, the primary event, namely the cell lineage-specific overexpression of a structurally altered receptor tyrosine kinase, finds its parallel in many tumours of birds and mammals. Once expressed at high levels, this receptor, the Xiphophorus melanoma inducing receptor kinase Xmrk, shows constitutive activation. By using different pathways, Xmrk induces both proliferative as well as anti-apoptotic signalling in pigment cells finally leading to cell transformation, tumour induction, and progression. Analyses of the different signalling cascades induced by the Xmrk-receptor led to the identification of the src-kinase Fyn, the MAP kinases ERK1 and ERK2, the "Signal Transducer and Activator of Transcription" STAT5, and the PI3-kinase as its major downstream substrates. This review describes some of the genetic findings, as well as the results from the recent molecular analyses of the factors involved in the initiation and manifestation of pigment cell transformation and melanoma development in Xiphophorus.