Cell-surface proteolysis, growth factor activation and intercellular communication in the progression of melanoma

The pleiotropic role of galectin-3 in melanoma progression: Unraveling the enigma

Melanoma is a highly aggressive skin cancer with poor outcomes associated with distant metastasis. Intrinsic properties of melanoma cells alongside the crosstalk between melanoma cells and surrounding microenvironment determine the tumor behavior. Galectin-3 (Gal-3), a ß-galactoside-binding lectin, has emerged as a major effector in cancer progression, including melanoma behavior. Data from melanoma models and patient studies reveal that Gal-3 expression is dysregulated, both intracellularly and extracellularly, throughout the stages of melanoma progression. This review summarizes the most recent data and hypotheses on Gal-3 and its tumor-modulating functions, highlighting its role in driving melanoma growth, invasion, and metastatic colonization. It also provides insight into potential Gal-3-targeted strategies for melanoma diagnosis and treatment.

Connexins in melanoma: Potential role of Cx46 in its aggressiveness

Melanoma is the most aggressive skin cancer, and in metastatic advanced states, it is completely refractory to chemotherapy. Therefore, it is relevant to understand the molecular bases that rule their aggressiveness. Connexins (Cxs) are proteins that under normal physiological conditions participate in intercellular communication, via the exchange of signaling molecules between the cytoplasm and extracellular milieu and the exchange of ions/second messengers between the cytoplasm of contacting cells. These proteins have shown important roles in cancer progression, chemo- and radiotherapy resistance, and metastasis. Accordingly, Cx26 and Cx43 seem to play important roles in melanoma progression and metastasis. On the other hand, Cx46 is typically expressed in the eye lens, where it seems to be associated with oxidative stress protection in fiber lens cells. However, in the last decade, Cx46 expression has been associated with breast and brain cancers, due to its role in potentiation of both extracellular vesicle release and cancer stem cell-like properties. In this review, we analyzed a potential role of Cx46 as a new biomarker and therapeutic target in melanoma.

Epithelial-to-mesenchymal-like transition events in melanoma

Epithelial-to-mesenchymal transition (EMT), a process through which epithelial tumor cells acquire mesenchymal phenotypic properties, contributes to both metastatic dissemination and therapy resistance in cancer. Accumulating evidence indicates that nonepithelial tumors, including melanoma, can also gain mesenchymal-like properties that increase their metastatic propensity and decrease their sensitivity to therapy. In this review, we discuss recent findings, illustrating the striking similarities-but also knowledge gaps-between the biology of mesenchymal-like state(s) in melanoma and mesenchymal state(s) from epithelial cancers. Based on this comparative analysis, we suggest hypothesis-driven experimental approaches to further deepen our understanding of the EMT-like process in melanoma and how such investigations may pave the way towards the identification of clinically relevant biomarkers for prognosis and new therapeutic strategies.

BRN2, a POUerful driver of melanoma phenotype switching and metastasis

The POU domain family of transcription factors play a central role in embryogenesis and are highly expressed in neural crest cells and the developing brain. BRN2 is a class III POU domain protein that is a key mediator of neuroendocrine and melanocytic development and differentiation. While BRN2 is a central regulator in numerous developmental programs, it has also emerged as a major player in the biology of tumourigenesis. In melanoma, BRN2 has been implicated as one of the master regulators of the acquisition of invasive behaviour within the phenotype switching model of progression. As a mediator of melanoma cell phenotype switching, it coordinates the transition to a dedifferentiated, slow cycling and highly motile cell type. Its inverse expression relationship with MITF is believed to mediate tumour progression and metastasis within this model. Recent evidence has now outlined a potential epigenetic switching mechanism in melanoma cells driven by BRN2 expression that induces melanoma cell invasion. We summarize the role of BRN2 in tumour cell dissemination and metastasis in melanoma, while also examining it as a potential metastatic regulator in other tumour models.

Spatiotemporal Labeling of Melanocytes in Mice

Melanocytes are pigment producing cells in the skin that give rise to cutaneous malignant melanoma, which is a highly aggressive and the deadliest form of skin cancer. Studying melanocytes in vivo is often difficult due to their small proportion in the skin and the lack of specific cell surface markers. Several genetically-engineered mouse models (GEMMs) have been created to specifically label the melanocyte compartment. These models give both spatial and temporal control over the expression of a cellular ‘beacon’ that has an added benefit of inducible expression that can be activated on demand. Two powerful models that are discussed in this review include the melanocyte-specific, tetracycline-inducible green fluorescent protein expression system (iDct-GFP), and the fluorescent ubiquitination-based cell cycle indicator (FUCCI) model that allows for the monitoring of the cell-cycle. These two systems are powerful tools in studying melanocyte and melanoma biology. We discuss their current uses and how they could be employed to help answer unresolved questions in the fields of melanocyte and melanoma biology.

sFRP2 in the aged microenvironment drives melanoma metastasis and therapy resistance

Cancer is a disease of ageing. Clinically, aged cancer patients tend to have a poorer prognosis than young. This may be due to accumulated cellular damage, decreases in adaptive immunity, and chronic inflammation. However, the effects of the aged microenvironment on tumour progression have been largely unexplored. Since dermal fibroblasts can have profound impacts on melanoma progression, we examined whether age-related changes in dermal fibroblasts could drive melanoma metastasis and response to targeted therapy. Here we find that aged fibroblasts secrete a Wnt antagonist, sFRP2, which activates a multi-step signalling cascade in melanoma cells that results in a decrease in β-catenin and microphthalmia-associated transcription factor (MITF), and ultimately the loss of a key redox effector, APE1. Loss of APE1 attenuates the response of melanoma cells to DNA damage induced by reactive oxygen species, rendering the cells more resistant to targeted therapy (vemurafenib). Age-related increases in sFRP2 also augment both angiogenesis and metastasis of melanoma cells. These data provide an integrated view of how fibroblasts in the aged microenvironment contribute to tumour progression, offering new possibilities for the design of therapy for the elderly.

Cathepsin L targeting in cancer treatment

Proteolytic enzymes may serve as promising targets for novel therapeutic treatment strategies seeking to impede cancer progression and metastasis. One such enzyme is cathepsin L (CTSL), a lysosomal cysteine protease. CTSL upregulation, a common occurrence in a variety of human cancers, has been widely correlated with metastatic aggressiveness and poor patient prognosis. In addition, CTSL has been implicated to contribute to cancer-associated osteolysis, a debilitating morbidity affecting both life expectancy and the quality of life. In this review, we highlight the mechanisms by which CTSL contributes to tumor progression and dissemination and discuss the therapeutic utility of CTSL intervention strategies aimed at impeding metastatic progression and bone resorption.

Modulation of matrix elasticity with PEG hydrogels to study melanoma drug responsiveness

Metastatic melanoma is highly resistant to drug treatment, and the underlying mechanisms of this resistance remain unclear. Increased tissue stiffness is correlated with tumor progression, but whether increased tissue stiffness contributes to treatment resistance in melanoma is not known. To investigate the effect of substrate stiffness on melanoma cell treatment responsiveness, PEG hydrogels were utilized as a cell culture system to precisely vary matrix elasticity and investigate melanoma cell responses to a commercially available pharmacological inhibitor (PLX4032). The tensile moduli were varied between 0.6 and 13.1 kPa (E) and the effects of PLX4032 on metabolic activity, apoptosis, and proliferation were evaluated on human cell lines derived from radial growth phase (WM35) and metastatic melanoma (A375). The A375 cells were found to be stiffness-independent; matrix elasticity did not alter cell morphology or apoptosis with PLX4032 treatment. The WM35 cells, however, were more dependent on substrate modulus, displaying increased apoptosis and smaller focal adhesions on compliant substrates. Culturing melanoma cells on PEG hydrogels revealed stage-dependent responses to PLX4032 that would have otherwise been masked if cultured strictly on TCPS. These findings demonstrate the utility of PEG hydrogels as a versatile in vitro culture platform with which to investigate the molecular mechanisms of melanoma biology and treatment responsiveness.

Heparan sulfate proteoglycans and heparin regulate melanoma cell functions

BACKGROUND

The solid melanoma tumor consists of transformed melanoma cells, and the associated stromal cells including fibroblasts, endothelial cells, immune cells, as well as, soluble macro- and micro-molecules of the extracellular matrix (ECM) forming the complex network of the tumor microenvironment. Heparan sulfate proteoglycans (HSPGs) are an important component of the melanoma tumor ECM. Importantly, there appears to be both a quantitative and a qualitative shift in the content of HSPGs, in parallel to the nevi-radial growth phase-vertical growth phase melanoma progression. Moreover, these changes in HSPG expression are correlated to modulations of key melanoma cell functions.

SCOPE OF REVIEW

This review will critically discuss the roles of HSPGs/heparin in melanoma development and progression.

MAJOR CONCLUSIONS

We have correlated HSPGs' expression and distribution with melanoma cell signaling and functions as well as angiogenesis.

GENERAL SIGNIFICANCE

The current knowledge of HSPGs/heparin biology in melanoma provides a foundation we can utilize in the ongoing search for new approaches in designing anti-tumor therapy. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

Predicting and preventing melanoma invasiveness: advances in clarifying E2F1 function

Malignant melanoma of the skin is one of the most aggressive human cancers with increasing incidence, despite efforts to improve primary prevention. In particular, the prognosis of patients at late stages of the disease has not significantly improved in the last three decades, because systemic therapies have proven disappointing. Thus, metastatic melanoma continues to be a daunting clinical problem. The increasingly high rates of lethal outcome associated with advanced melanoma rely on the acquisition of invasiveness, early metastatic dissemination of tumor cells from their primary sites, and generation of chemoresistance as a consequence of alteration of key molecules involved in the regulation of cell survival. Thus far, extensive studies have been conducted to understand the molecular mechanisms that drive tumor progression, but the specific requirements underlying the aggressive behavior are still widely unknown. Understanding the determinants of this process is key to unveiling its dynamics, especially those that promote invasiveness, and may open new routes for the development of therapeutic strategies that control metastatic spread, and eventually the prevention of life-threatening metastases. Here, we review recent advances on molecular aspects, particularly of E2F1 transcription factor function, in the context of patient data, and discuss the implications for targeting melanoma cells when they begin to invade and metastasize.

Heparin plays a key regulatory role via a p53/FAK-dependent signaling in melanoma cell adhesion and migration

Heparin and its various derivatives affect cancer progression in humans. In this study, we show that heparin uptaken intracellularly by melanoma cells activated a signaling cascade, which in turn inhibited melanoma cell adhesion and migration. The reduced ability of M5 cells to adhere onto the fibronectin (FN) substrate was directly correlated to a decrease in the expression of focal adhesion kinase (FAK), which is a key regulator of melanoma motility. Cell treatment with heparin caused a marked downregulation in FAK expression (P ≤ 0.01). This is followed by an analogous inhibition of both constitutive and FN-induced FAK Y397-phosphorylation (P ≤ 0.01). Moreover, heparin stimulated the p53 expression (P ≤ 0.001) of M5 cells and its increased accumulation in the nucleus. This favors a decrease in FAK promoter activation and explains the reduced FAK transcript and protein levels. In conclusion, the results of this study clearly demonstrate that the action of heparin in the regulation of melanoma cell adhesion and migration involves a p53/FAK/signaling pathway, which may be of importance in molecular targeted therapy of the disease.

Stress as a possible mechanism in melanoma progression

The incidence of melanoma, the most aggressive type of cutaneous malignant tumor, is currently on the rise. Treatment in advanced stages is still unsuccessful compared with other malignant tumors, thus it is important to indentify the key mechanisms responsible for melanoma progression and metastasis. Genetic and molecular components, in particular, that are up- or downregulated in melanoma cells, affect the invasive potential of melanoma. Another possible important cofactor highlighted by recent studies is chronic stress, involving environmental and psychological factors, which can be an important cofactor in not only cancer progression in general but also in melanoma spreading. The negative effects of chronic stress have been evaluated epidemiologically in patients with breast and prostate cancer. In particular, the effects of stress mediators, namely, catecholamines have been studied on various human malignancies, including melanoma and have highlighted a significant increase of progression-related molecules. As such, this could be the starting point for a new approach in the treatment of advanced melanoma, in which the negative effects of stress are reduced or blocked.

Targeting fibroblast activation protein inhibits tumor stromagenesis and growth in mice

Membrane-bound proteases have recently emerged as critical mediators of tumorigenesis, angiogenesis, and metastasis. However, the mechanisms by which they regulate these processes remain unknown. As the cell surface serine protease fibroblast activation protein (FAP) is selectively expressed on tumor-associated fibroblasts and pericytes in epithelial tumors, we set out to investigate the role of FAP in mouse models of epithelial-derived solid tumors. In this study, we demonstrate that genetic deletion and pharmacologic inhibition of FAP inhibited tumor growth in both an endogenous mouse model of lung cancer driven by the K-rasG12D mutant and a mouse model of colon cancer, in which CT26 mouse colon cancer cells were transplanted into immune competent syngeneic mice. Interestingly, growth of only the K-rasG12D-driven lung tumors was also attenuated by inhibition of the closely related protease dipeptidyl peptidase IV (DPPIV). Our results indicate that FAP depletion inhibits tumor cell proliferation indirectly, increases accumulation of collagen, decreases myofibroblast content, and decreases blood vessel density in tumors. These data provide proof of principle that targeting stromal cell-mediated modifications of the tumor microenvironment may be an effective approach to treating epithelial-derived solid tumors.

A mediator of Rho-dependent invasion moonlights as a methionine salvage enzyme

RhoA controls changes in cell morphology and invasion associated with cancer phenotypes. Cell lines derived from melanoma tumors at varying stages revealed that RhoA is selectively activated in cells of metastatic origin. We describe a functional proteomics strategy to identify proteins regulated by RhoA and report a previously uncharacterized human protein, named “mediator of RhoA-dependent invasion (MRDI),” that is induced in metastatic cells by constitutive RhoA activation and promotes cell invasion. In human melanomas, MRDI localization correlated with stage, showing nuclear localization in nevi and early stage tumors and cytoplasmic localization with plasma membrane accentuation in late stage tumors. Consistent with its role in promoting cell invasion, MRDI localized to cell protrusions and leading edge membranes in cultured cells and was required for cell motility, tyrosine phosphorylation of focal adhesion kinase, and modulation of actin stress fibers. Unexpectedly MRDI had enzymatic function as an isomerase that converts the S-adenosylmethionine catabolite 5-methylribose 1-phosphate into 5-methylribulose 1-phosphate. The enzymatic function of MRDI was required for methionine salvage from S-adenosylmethionine but distinct from its function in cell invasion. Thus, mechanisms used by signal transduction pathways to control cell movement have evolved from proteins with ancient function in amino acid metabolism.

Plexin B1 is repressed by oncogenic B-Raf signaling and functions as a tumor suppressor in melanoma cells

Human melanomas show oncogenic B-Raf mutations which activate the B-Raf/MKK/ERK cascade. We screened microarrays to identify cellular targets of this pathway, and found that genes upregulated by B-Raf/MKK/ERK showed highest association with cell cycle regulators, whereas genes downregulated were most highly associated with axon guidance genes, including plexin-semaphorin family members. Plexin B1 was strongly inhibited by MAP kinase signaling in melanoma cells and melanocytes. In primary melanoma cells, plexin B1 blocked tumorigenesis as measured by growth of colonies in soft agar, spheroids in extracellular matrix, and xenograft tumors. Tumor suppression depended on residues in the C-terminal domain of plexin B1 which mediate receptor GAP activity, and also correlated with AKT inhibition. Interestingly, the inhibitory response to plexin B1 was reduced or absent in cells from a matched metastatic tumor, suggesting that changes occur in metastatic cells which bypass the tumor suppressor mechanisms. Plexin B1 also inhibited cell migration, but this was seen in metastatic cells and not in matched primary cells. Thus, plexin B1 has tumor suppressor function in early-stage cells, while suppressing migration in late-stage cells. Our findings suggest that B-Raf/MKK/ERK provides a permissive environment for melanoma genesis by modulating plexin B1.

Three-dimensional context regulation of metastasis

Tumor progression ensues within a three-dimensional microenvironment that consists of cellular and non-cellular components. The extracellular matrix (ECM) and hypoxia are two non-cellular components that potently influence metastasis. ECM remodeling and collagen cross-linking stiffen the tissue stroma to promote transformation, tumor growth, motility and invasion, enhance cancer cell survival, enable metastatic dissemination, and facilitate the establishment of tumor cells at distant sites. Matrix degradation can additionally promote malignant progression and metastasis. Tumor hypoxia is functionally linked to altered stromal-epithelial interactions. Hypoxia additionally induces the expression of pro-migratory, survival and invasion genes, and up-regulates expression of ECM components and modifying enzymes, to enhance tumor progression and metastasis. Synergistic interactions between matrix remodeling and tumor hypoxia influence common mechanisms that maximize tumor progression and cooperate to drive metastasis. Thus, clarifying the molecular pathways by which ECM remodeling and tumor hypoxia intersect to promote tumor progression should identify novel therapeutic targets.

Melanoma invasion - current knowledge and future directions

The acquisition of invasive behaviour is the key transition in the progression of benign melanocyte hyperplasia to life threatening melanoma. Understanding this transition and the mechanisms of invasion are the key to understanding why malignant melanoma is such a devastating disease and will aid treatment strategies. Underlying the invasive behaviour is increased cell motility caused by changes in cytoskeletal organization and altered contacts with the extra-cellular matrix (ECM). In addition, changes in the interactions of melanoma cells with keratinocytes and fibroblasts enable them to survive and proliferate outside their normal epidermal location. Proteomic and genomic initiatives are greatly increasing our knowledge of which gene products are deregulated in invasive and metastatic melanoma; however, the next challenge is to understand how these genes promote the invasion of melanoma cells. In recent years new models have been developed that more closely recapitulate the conditions of melanoma invasion in vivo. It is hoped that these models will give us a better understanding of how the genes implicated in melanoma progression affect the motility of melanoma cells and their interactions with the ECM, stromal cells and blood vessels. This review will summarise our current understanding of melanoma invasion and focus on the new model systems that can be used to study melanoma.

Proteolytic profiling of the extracellular matrix degradome

The profiling of protein function is one of the most challenging scientific tasks in the postgenomic age. Traditional protein expression methodologies have focused only on the quantification of proteins under varying conditions or pathologies. Determining the functional differences between protein populations allows for a more accurate view of the outcomes in normal vs diseased proteomes. Because the presence or absence of a protein's function can affect its complex surroundings (consisting of multiple other proteins and substrates), the study of proteome functionality yields information on protein-protein interactions, amplification cascades, signaling pathways, and posttranslational modifications. Of significant interest are proteinases, as proteolysis is responsible for tight regulation of various cellular and tissue processes. Proteinase activities, or lack there of, alter the proteome makeup by regulating other proteins or by generating cleavage products. This chapter describes current proteolytic profiling technologies using activity or target-based formats. In particular, the analysis of collagenolytic matrix metalloproteinase activity using fluorogenic triple-helical substrates is discussed.

Functional implication of truncated P-cadherin expression in malignant melanoma

Cadherins comprise Ca(2+)-dependent homophilic cell-cell adhesion molecules which are responsible for correct location of cells and for tissue integrity. They are crucial factors for the development and maintenance of epithelial architecture. Aberrantly expressed cadherins are known to be involved in malignant transformation of different types of tissues. In a previous study, we determined the expression of a short truncated 50 kDa form of P-cadherin only consisting of the N-terminal part in malignant melanoma. Further analysis revealed that this short 50 kDa form of P-cadherin representing the N-terminal, extracellular region, is secreted by melanoma cells in contrast to the membrane bound form in melanocytes. In order to define the functional relevance of expression of the 50 kDa P-cadherin variant in malignant melanoma, antisense P-cadherin cell clones were generated. The clones in which P-cadherin expression is reduced show no changes in proliferation or in attachment-independent growth when compared to controls. However, a strong reduction of migratory and invasive properties was observed in these cells, suggesting that truncated P-cadherin promotes melanoma cell invasion and migration and therefore has an important role in the progression of malignant melanoma. Functionally, the secreted form of P-cadherin could play a role as regulator of the homophilic interaction between P-cadherin molecules by antagonizing their biological role acting as a dominant negative form to interrupt cell-cell attachment.

Expression of a metastatic phenotype in IFNs-primed/TNFalpha-activated B16 murine melanoma cells: role of JAK1/PKCdelta signal transduction factors

In previous studies, we found that IFNgamma and TNFalpha generated by activated macrophages stimulate the metastatic potential in F10-M3 cells, a clone isolated from B16-F10 murine melanoma line. In this phenomenon, TNFalpha promoted the expression of a metastatic phenotype in tumor cells previously primed with IFNgamma. Here, we demonstrate that IFNalpha or IFNbeta may replace IFNgamma in priming tumor cells. We also noticed that an enhancement of the expression of p55TNFalpha receptor was associated with the preconditioning of tumor cells with IFNgamma and IFNbeta. By the use of an appropriate inhibitor, we observed that JAK1 signal transduction pathway was involved in the expression of a metastatic phenotype and of p55TNFalpha receptor shown in IFNgamma- and IFNbeta-primed melanoma cells stimulated with TNFalpha. Furthermore, the activity of the protein kinase C (PKC) was required for IFNgamma-primed melanoma cells to express a metastatic phenotype after stimulation with TNFalpha. In conclusion, our study shows that a metastatic phenotype was expressed in B16 murine melanoma cells stimulated with TNFalpha regardless of whether the cells were primed with IFNgamma IFNalpha or IFNbeta. The molecular events leading to the expression of a metastatic phenotype in F10-M3 melanoma cells are represented by: (a) an enhanced expression of p55TNFalpha receptor in IFNs-primed tumor cells dependent on JAK1 signal transduction pathway; and (b) an intact PKC activity during TNFalpha stimulation.

Tumor-derived fibronectin is involved in melanoma cell invasion and regulated by V600E B-Raf signaling pathway

Melanomas are malignant tumors of melanocytes that, if not detected early, are highly aggressive and poorly treatable. Activation of extracellular signal-regulated (ERK)/mitogen-activated protein (MAP) kinase signaling is commonly found in melanomas mainly through oncogenic mutations of B-Raf. We previously reported that activation of ERK/MAP kinase stimulates synthesis of fibronectin by upregulating the transcription factor early growth response-1 (Egr-1). To further analyze the link between ERK/MAP kinase pathway and fibronectin in melanoma, we have studied the regulation and role of fibronectin produced by melanoma cells bearing oncogenic B-Raf mutation. We show that fibronectin is expressed in situ during tumor progression and that high fibronectin and Egr-1 levels are found in cells expressing this mutation. Expression of active mutants of B-Raf induces fibronectin, whereas endogenous fibronectin is inhibited by small interfering RNA (siRNA)-mediated depletion of B-Raf or Egr-1. In contrast, stimulation of ERK pathway is insufficient to promote fibronectin upregulation in normal melanocytes. Finally, we show that suppression of fibronectin by siRNA leads to decreased melanoma cell invasiveness in vitro. These results reveal a tumor-specific regulation of fibronectin by constitutive ERK/MAP kinase signaling and indicate that self-production of fibronectin may play a role in melanoma tumorigenesis, by promoting tumor cell invasion.

mda-7/IL-24: multifunctional cancer-specific apoptosis-inducing cytokine

"Differentiation therapy" provides a unique and potentially effective, less toxic treatment paradigm for cancer. Moreover, combining "differentiation therapy" with molecular approaches presents an unparalleled opportunity to identify and clone genes mediating cancer growth control, differentiation, senescence, and programmed cell death (apoptosis). Subtraction hybridization applied to human melanoma cells induced to terminally differentiate by treatment with fibroblast interferon (IFN-beta) plus mezerein (MEZ) permitted cloning of melanoma differentiation associated (mda) genes. Founded on its novel properties, one particular mda gene, mda-7, now classified as a member of the interleukin (IL)-10 gene family (IL-24) because of conserved structure, chromosomal location, and cytokine-like properties has become the focus of attention of multiple laboratories. When administered by transfection or adenovirus-transduction into a spectrum of tumor cell types, melanoma differentiation associated gene-7/interleukin-24 (mda-7/IL-24) induces apoptosis, whereas no toxicity is apparent in normal cells. mda-7/IL-24 displays potent "bystander antitumor" activity and also has the capacity to enhance radiation lethality, to induce immune-regulatory activities, and to inhibit tumor angiogenesis. Based on these remarkable attributes and effective antitumor therapy in animal models, this cytokine has taken the important step of entering the clinic. In a Phase I clinical trial, intratumoral injections of adenovirus-administered mda-7/IL-24 (Ad.mda-7) was safe, elicited tumor-regulatory and immune-activating processes, and provided clinically significant activity. This review highlights our current understanding of the diverse activities and properties of this novel cytokine, with potential to become a prominent gene therapy for cancer.

Melanocyte transformation associated with substrate adhesion impediment

Exclude experimental models of malignant transformation employ chemical and physical carcinogens or genetic manipulations to study tumor progression. In this work, different melanoma cell lines were established after submitting a nontumorigenic melanocyte lineage (melan-a) to sequential cycles of forced anchorage impediment. The great majority of these cells underwent anoikis when maintained in suspension. After one deadhesion cycle, phenotypic alterations were noticeable in the few surviving cells, which became more numerous and showed progressive alterations after each adhesion impediment step. No significant differences in cell surface expression of integrins were detected, but a clear electrophoretic migration shift, compatible with an altered glycosylation pattern, was observed for beta1 chain in transformed cell lines. In parallel, a progressive enrichment of tri- and tetra-antennary N-glycans was apparent, suggesting increased N-acetylglucosaminyltransferase V activity. Alterations both in proteoglycan glycosylation pattern and core protein expression were detected during the transformation process. In conclusion, this model corroborates the role of adhesion state as a promoting agent in transformation process and demonstrates that cell adhesion disturbances may act as carcinogenic stimuli, at least for a nontumorigenic immortalized melanocyte lineage. These findings have intriguing implications for in vivo carcinogenesis, suggesting that anchorage independence may precede, and contribute to, neoplastic conversion.

The EphB4 receptor-tyrosine kinase promotes the migration of melanoma cells through Rho-mediated actin cytoskeleton reorganization

Several studies have reported the up-regulation of EphB receptor-tyrosine kinases and ephrin-B ligands in a variety of tumors, suggesting a functional relation between EphB/ephrin-B signaling and tumor progression. The ability of the EphB receptors to regulate cell migration and promote angiogenesis likely contributes to tumor progression and metastasis. Here we show that EphB receptors, and especially EphB4, regulate the migration of murine melanoma cells. Highly malignant melanoma cells express the highest levels of EphB4 receptor and migrate faster than less malignant melanoma cells. Furthermore, inhibition of EphB receptor forward signaling by overexpression of a form of EphB4 lacking the cytoplasmic portion or by treatment with competitively acting soluble EphB2-Fc results in slower melanoma cell migration. In contrast, overexpression of active EphB4 significantly enhances cell migration. The effects of EphB4 receptor on cell migration and cell morphology require its kinase activity because the inhibition of EphB4 kinase activity by overexpression of kinase dead EphB4 inhibits cell migration and affects the organization of actin cytoskeleton. Activation of EphB4 receptor with its ligand ephrin-B2-Fc enhances the migratory ability of melanoma cells and increases RhoA activity, whereas inhibiting EphB receptor forward signaling decreases RhoA activity. Moreover, expression of dominant negative RhoA blocks the effects of active EphB4 on cell migration and actin organization. These data suggest that EphB4 forward signaling contributes to the high migratory ability of invasive melanoma cells by influencing RhoA-mediated actin cytoskeleton reorganization.

Prognostic relevance of P-cadherin expression in melanocytic skin tumours analysed by high-throughput tissue microarrays

AIM

To investigate whether protein expression or cellular localisation of P-cadherin is associated with clinicopathological characteristics in benign and malignant melanocytic skin tumours.

EXPERIMENTAL DESIGN

P-cadherin expression and the Ki-67 labelling index were analysed immunohistochemically by using tissue microarrays (TMAs). Membranous and cytoplasmic expression was scored semiquantitatively (0 to 2+).

RESULTS

P-cadherin protein expression of any intensity (1+ to 2+) was detected in the membrane in 41.5% (132/318) and in the cytoplasm in 64.2% (204/318) of patients. In general, P-cadherin expression was significantly reduced in malignant melanomas (p<0.001) and melanoma metastases (p<0.001), compared with benign nevi. Additionally, loss of membranous P-cadherin was associated with Clark level (p = 0.011) and tumour thickness (p<0.001). Interestingly, a significantly lower P-cadherin expression was shown by dermal nevi than by compound and junctional nevi (p = 0.005; p = 0.025). In primary melanomas, a Ki-67 labelling index <5% was not associated with P-cadherin protein expression, suggesting that loss of P-cadherin expression was not associated with proliferation. None of the other clinical and histological factors analysed was significantly related to P-cadherin expression. Low cytoplasmic P-cadherin expression was associated with tumour recurrence (p = 0.03) in all the patients who were analysed. After testing various multivariate Cox regression models, loss of cytoplasmic P-cadherin expression remained a highly significant adverse risk factor for tumour recurrence in patients with tumours <2 mm.

CONCLUSIONS

Loss of cytoplasmic P-cadherin expression is common in advanced melanomas and can be a prognostic marker of progression in patients with melanoma, most useful in patients with primary tumours <2 mm in thickness.

Focal adhesion kinase promotes the aggressive melanoma phenotype

Malignant melanoma continues to remain a significant health threat, with death often occurring as a result of metastasis. The metastatic phenotype typically is characterized by augmented tumor cell invasion and migration in addition to tumor cell plasticity as shown by vasculogenic mimicry. Therefore, understanding the molecular mechanisms that promote an aggressive phenotype is essential to predicting the likelihood of metastasis at a stage when intervention may be possible. This study focuses on the role of focal adhesion kinase (FAK), a cytoplasmic tyrosine kinase important for many cellular processes, including cell survival, invasion, and migration. We found FAK to be phosphorylated on its key tyrosine residues, Tyr397 and Tyr576, in only aggressive uveal and cutaneous melanoma cells, which correlates with their increased invasion, migration, and vasculogenic mimicry plasticity. Additionally, we confirmed the presence of FAK phosphorylated on Tyr397 and Tyr576 in both cutaneous and uveal melanoma tumors in situ. Examination of a functional role for FAK in aggressive melanoma revealed that disruption of FAK-mediated signal transduction pathways, through the expression of FAK-related nonkinase (FRNK), results in a decrease in melanoma cell invasion, migration, and inhibition of vasculogenic mimicry. Moreover, we found that FRNK expression resulted in a down-regulation of Erk1/2 phosphorylation resulting in a decrease in urokinase activity. Collectively, these data suggest a new mechanism involved in promoting the aggressive melanoma phenotype through FAK-mediated signal transduction pathways, thus providing new insights into possible therapeutic intervention strategies.

CEACAM1 functionally interacts with filamin A and exerts a dual role in the regulation of cell migration

The carcinoembryonic antigen-related cell adhesion molecule CEACAM1 (CD66a) and the scaffolding protein filamin A have both been implicated in tumor cell migration. In the present study we identified filamin A as a novel binding partner for the CEACAM1-L cytoplasmic domain in a yeast two-hybrid screen. Direct binding was shown by surface plasmon resonance analysis and by affinity precipitation assays. The association was shown for human and rodent CEACAM1-L in endogenous CEACAM1-L expressing cells. To address functional aspects of the interaction, we used a well-established melanoma cell system. We found in different migration studies that the interaction of CEACAM1-L and filamin A drastically reduced migration and cell scattering, whereas each of these proteins when expressed alone, acted promigratory. CEACAM1-L binding to filamin A reduced the interaction of the latter with RalA, a member of the Ras-family of GTPases. Furthermore, co-expression of CEACAM1-L and filamin A led to a reduced focal adhesion turnover. Independent of the presence of filamin A, the expression of CEACAM1-L led to an increased phosphorylation of focal adhesions and to altered cytoskeletal rearrangements during monolayer wound healing assays. Together, our data demonstrate a novel mechanism for how CEACAM1-L regulates cell migration via its interaction with filamin A.

Membrane type-matrix metalloproteinases and tumor progression

Matrix metalloproteinases (MMPs) are a family of zinc endopeptidases that process growth factors, growth factor binding proteins, cell surface proteins, degrade extracellular matrix (ECM) components and thereby play a central role in tissue remodeling and tumor progression. Membrane-type matrix metalloproteinases (MT-MMPs) are a recently discovered subgroup of intrinsic plasma membrane proteins. Their functions have been extended from pericellular proteolysis and control of cell migration to cell signaling, control of cell proliferation and regulation of multiple stages of tumor progression including growth and angiogenesis. This review sheds light on the new functions of MT-MMPs and their inhibitors in tumor development and angiogenesis, and presents recent investigations that document their influence on various cell functions.

Integrin signaling in malignant melanoma

Cell adhesion and migration are essential for embryonic development, tissue regeneration, but also for tumor development. The physical link between the extracellular matrix (ECM) and the actin cytoskeleton is mainly mediated by receptors of the integrin family. Through signals transduced upon integrin ligation to ECM proteins, this family of proteins plays key roles in regulating tumor growth and metastasis as well as tumor angiogenesis. During melanoma development, changes in integrin expression, intracellular control of integrin functions and signals perceived from integrin ligand binding impact upon the ability of tumor cells to interact with their environment and enable melanoma cells to convert from a sessile, stationary to a migratory and invasive phenotype. Antagonists of several integrins are now under evaluation in clinical trials to determine their potential as therapeutics for malignant melanoma and other kinds of cancer.

A secreted form of P-cadherin is expressed in malignant melanoma

Cadherins are Ca-dependent homophilic cell-cell adhesion molecules which are responsible for correct location of cells and tissue integrity. They are critical for development and maintenance of epithelial architecture. Aberrantly expressed cadherins are known to be involved in malignant transformation of different types of tissues. In this study, we show the expression of a short truncated 50 kDa form of the N-terminal part of P-cadherin in seven melanoma cell lines compared to melanocytes and keratinocytes. In vitro protein analysis on cell culture supernatant as well as immunohistochemistry of primary and metastatic melanoma tissue revealed the expression of this short form of P-cadherin. Furthermore, analysis showed that this short 50 kDa form of P-cadherin is secreted by melanoma cells in contrast to the membrane bound form in melanocytes. Analysis on mRNA level detected only exon 1 to 10 of P-cadherin resulting in the 50 kDa form missing the transmembrane and cytoplasmatic region. Genomic sequence analysis did not show any mutations in melanoma cells neither in the exons nor in the exon-intron boundaries. Furthermore, there was no loss of exons 11-16 on the genomic level. Functionally, the secreted form of P-cadherin could play a role as regulator of the homophilic interaction between P-cadherin molecules by antagonizing their biological role acting as a dominant negative form to interrupt cell-cell attachment.

Integrin expression regulates neuroblastoma attachment and migration

Neuroblastoma (NBL) is the most common malignant disease of infancy, and children with bone metastasis have a mortality rate greater than 90%. Two major classes of proteins, integrins and growth factors, regulate the metastatic process. We have previously shown that tumorigenic NBL cells express higher levels of the type I insulin-like growth factor receptor (IGF-IR) and that beta1 integrin expression is inversely proportional to tumorigenic potential in NBL. In the current study, we analyze the effect of beta1 integrin and IGF-IR on NBL cell attachment and migration. Nontumorigenic S-cells express high levels of beta1 integrin, whereas tumorigenic N-cells express little beta1 integrin. Alterations in beta1 integrin are due to regulation at the protein level, as translation is decreased in N-type cells. Moreover, inhibition of protein synthesis shows that beta1 integrin is degraded more slowly in S-type cells (SHEP) than in N-type cells (SH-SY5Y and IMR32). Inhibition of alpha5beta1 integrin prevents SHEP (but not SH-SY5Y or IMR32) cell attachment to fibronectin and increases SHEP cell migration. Increases in IGF-IR decrease beta1 integrin expression, and enhance SHEP cell migration, potentially through increased expression of alphavbeta3. These data suggest that specific classes of integrins in concert with IGF-IR regulate NBL attachment and migration.

Identification and cloning of genes displaying elevated expression as a consequence of metastatic progression in human melanoma cells by rapid subtraction hybridization

Although extensively investigated, the complete repertoire of genes associated with and causative of metastasis remain largely unknown. We developed an efficient approach for identifying differentially expressed genes that involves rapid subtraction hybridization (RaSH) of cDNA clones prepared from two cell populations, a driver and a tester. This RaSH approach has previously documented high sensitivity and effectiveness in identifying genes that are differentially expressed as a function of induction of terminal differentiation in human melanoma cells, resistance or sensitivity to human immunodeficiency virus-1 (HIV-1) infection of human T cells and perturbation in gene expression in normal human fetal astrocytes infected with HIV-1 or treated with HIV-1 gp120 viral envelope glycoprotein or tumor necrosis factor-alpha (TNF-alpha). In the present study, RaSH has been applied to a metastatic melanoma model, which mimics the early events of metastasis in humans, comprising weakly metastatic vs. immunosuppressed newborn rat-selected highly metastatic variants. This has now resulted in the identification of eight genes displaying elevated expression in the high metastatic variants vs. normal immortal melanocytes or weakly metastatic parental clones. These include six known genes, 67-kDa laminin receptor (67LR), endothelin receptor B (ENDRB), Na+/K+-ATPase, Ku antigen, interleukin-receptor-associated kinase-1 (IRAK-1) and ribosomal protein RPLA, which may contribute to the complex process of melanoma metastasis. Additionally, two unknown genes (not reported in current databases) that may also impact on the metastatic phenotype have also been identified. These studies provide additional support of the use of the RaSH approach, in this application in the context of closely related variant cell lines with different metastatic potential, for effective differential gene identification and elucidate eight previously unrecognized genes whose role in melanoma progression to metastatic competence can now be scrutinized.

Melanoma chondroitin sulfate proteoglycan enhances FAK and ERK activation by distinct mechanisms

Melanoma chondroitin sulfate proteoglycan (MCSP) is an early cell surface melanoma progression marker implicated in stimulating tumor cell proliferation, migration, and invasion. Focal adhesion kinase (FAK) plays a pivotal role in integrating growth factor and adhesion-related signaling pathways, facilitating cell spreading and migration. Extracellular signal–regulated kinase (ERK) 1 and 2, implicated in tumor growth and survival, has also been linked to clinical melanoma progression. We have cloned the MCSP core protein and expressed it in the MCSP-negative melanoma cell line WM1552C. Expression of MCSP enhances integrin-mediated cell spreading, FAK phosphorylation, and activation of ERK1/2. MCSP transfectants exhibit extensive MCSP-rich microspikes on adherent cells, where it also colocalizes with α4 integrin. Enhanced activation of FAK and ERK1/2 by MCSP appears to involve independent mechanisms because inhibition of FAK activation had no effect on ERK1/2 phosphorylation. These results indicate that MCSP may facilitate primary melanoma progression by enhancing the activation of key signaling pathways important for tumor invasion and growth.

The RacGEF Tiam1 inhibits migration and invasion of metastatic melanoma via a novel adhesive mechanism

Rho-like GTPases such as RhoA, Rac1 and Cdc42 are key regulators of actin-dependent cell functions including cell morphology, adhesion and migration. Tiam1 (T lymphoma invasion and metastasis 1), a guanine nucleotide exchange factor that activates Rac, is an important regulator of cell shape and invasiveness in epithelial cells and fibroblasts. Overexpression of Tiam1 in metastatic melanoma cells converted the constitutive mesenchymal phenotype into an epithelial-like phenotype. This included the induction of stringent cell-cell contacts mediated by the Ig-like receptor ALCAM (activated leukocyte cell adhesion molecule) and actin redistribution to cell-cell junctions. This phenotypic switch was dependent on increased Rac but not Rho activity, and on the redistribution and adhesive function of ALCAM, whereas cadherins were not involved. Although cell proliferation was significantly enhanced, the gain of cell-cell junctions strongly counteracted cell motility and invasion as shown for two- and three-dimensional collagen assays as well as invasion into human skin reconstructs. The reverse transition from mesenchymal invasive to a resident epithelial-like phenotype implicates a role for Tiam1/Rac signaling in the control of cell-cell contacts through a novel ALCAM-mediated mechanism.

Differential modulation of human melanoma cell metalloproteinase expression by alpha2beta1 integrin and CD44 triple-helical ligands derived from type IV collagen

Tumor cell binding to components of the basement membrane is well known to trigger intracellular signaling pathways. Signaling ultimately results in the modulation of gene expression, facilitating metastasis. Type IV collagen is the major structural component of the basement membrane and is known to be a polyvalent ligand, possessing sequences bound by the alpha1beta1, alpha2beta1, and alpha3beta1 integrins, as well as cell surface proteoglycan receptors, such as CD44/chondroitin sulfate proteoglycan (CSPG). The role of alpha2beta1 integrin and CD44/CSPG receptor binding on human melanoma cell activation has been evaluated herein using triple-helical peptide ligands incorporating the alpha1(IV)382-393 and alpha1(IV)1263-1277 sequences, respectively. Gene expression and protein production of matrix metalloproteinases-1 (MMP-1), -2, -3, -13, and -14 were modulated with the alpha2beta1-specific sequence, whereas the CD44-specific sequence yielded significant stimulation of MMP-8 and lower levels of modulation of MMP-1, -2, -13, and -14. Analysis of enzyme activity confirmed different melanoma cell proteolytic potentials based on engagement of either the alpha2beta1 integrin or CD44/CSPG. These results are indicative of specific activation events that tumor cells undergo upon binding to select regions of basement membrane collagen. Based on the present study, triple-helical peptide ligands provide a general approach for monitoring the regulation of proteolysis in cellular systems.

ADP-ribosylation factor 6 regulates tumor cell invasion through the activation of the MEK/ERK signaling pathway

Tumor cell invasion through the extracellular matrix is accompanied by the formation of invadopodia, which are actin-rich protrusions at the adherent surface of cells at sites of extracellular matrix degradation. Using the invasive human melanoma cell line LOX as a model system, we demonstrate that the ADP-ribosylation factor 6 (ARF6) GTPase is an important regulator of invadopodia formation and cell invasion. We show that ARF6 localizes to invadopodia of LOX cells. Sustained activation of ARF6 significantly enhances the invasive capacity of melanoma as well as breast tumor cell lines, whereas dominant negative ARF6 abolishes basal cell invasive capacity as well as invasion induced by growth factors. Furthermore, using biochemical assays, we show that enhanced invasive capacity is accompanied by the activation of endogenous ARF6. Finally, we provide evidence that ARF6-enhanced melanoma cell invasion depends on the activation of the extracellular signal-regulated kinase (ERK), and that the ARF6 GTPase cycle regulates ERK activation. This study describes a vital role for ARF6 in melanoma cell invasion and documents a link between ARF6-mediated signaling and ERK activation.

Membrane associated proteases and their inhibitors in tumour angiogenesis

Cell surface proteolysis is an important mechanism for generating biologically active proteins that mediate a range of cellular functions and contribute to biological processes such as angiogenesis. Although most studies have focused on the plasminogen system and matrix metalloproteinases (MMPs), recently there has been an increase in the identification of membrane associated proteases, including serine proteases, ADAMs, and membrane-type MMPs (MT-MMPs). Normally, protease activity is tightly controlled by tissue inhibitors of MMPs (TIMPs) and plasminogen activator inhibitors (PAIs). The balance between active proteases and inhibitors is thought to determine the occurrence of proteolysis in vivo. High concentrations of proteolytic system components correlate with poor prognosis in many cancers. Paradoxically, high (not low) PAI-1 or TIMP concentrations predict poor survival in patients with various cancers. Recent observations indicate a much more complex role for protease inhibitors in tumour progression and angiogenesis than initially expected. As knowledge in the field of protease biology has improved, the unforeseen complexities of cell associated enzymes and their interaction with physiological inhibitors have emerged, often revealing unexpected mechanisms of action.

Axis of evil: molecular mechanisms of cancer metastasis

Although the genetic basis of tumorigenesis may vary greatly between different cancer types, the cellular and molecular steps required for metastasis are similar for all cancer cells. Not surprisingly, the molecular mechanisms that propel invasive growth and metastasis are also found in embryonic development, and to a less perpetual extent, in adult tissue repair processes. It is increasingly apparent that the stromal microenvironment, in which neoplastic cells develop, profoundly influences many steps of cancer progression, including the ability of tumor cells to metastasize. In carcinomas, the influences of the microenvironment are mediated, in large part, by bidirectional interactions (adhesion, survival, proteolysis, migration, immune escape mechanisms lymph-/angiogenesis, and homing on target organs) between epithelial tumor cells and neighboring stromal cells, such as fibroblasts as well as endothelial and immune cells. In this review, we summarize recent advances in understanding the molecular mechanisms that govern this frequently lethal metastatic progression along an axis from primary tumor to regional lymph nodes to distant organ sites. Affected proteins include growth factor signaling molecules, chemokines, cell-cell adhesion molecules (cadherins, integrins) as well as extracellular proteases (matrix metalloproteinases). We then discuss promising new therapeutic approaches targeting the microenvironment. We note, however, that there is still too little knowledge of how the many events are coordinated and integrated by the cancer cell, with conspiratorial help by the stromal component of the host. Before drug development can proceed with a legitimate chance of success, significant gaps in basic knowledge need to be filled.