Brain-metastatic Melanoma Cells Research Articles

MET receptor serves as a promising target in melanoma brain metastases

The development of brain metastases hallmarks disease progression in 20–40% of melanoma patients and is a serious obstacle to therapy. Understanding the processes involved in the development and maintenance of melanoma brain metastases (MBM) is critical for the discovery of novel therapeutic strategies. Here, we generated transcriptome and methylome profiles of MBM showing high or low abundance of infiltrated Iba1high tumor-associated microglia and macrophages (TAMs). Our survey identified potential prognostic markers of favorable disease course and response to immune checkpoint inhibitor (ICi) therapy, among them APBB1IP and the interferon-responsive gene ITGB7. In MBM with high ITGB7/APBB1IP levels, the accumulation of TAMs correlated significantly with the immune score. Signature-based deconvolution of MBM via single sample GSEA revealed enrichment of interferon-response and immune signatures and revealed inflammation, stress and MET receptor signaling. MET receptor phosphorylation/activation maybe elicited by inflammatory processes in brain metastatic melanoma cells via stroma cell-released HGF. We found phospho-METY1234/1235 in a subset of MBM and observed a marked response of brain metastasis-derived cell lines (BMCs) that lacked druggable BRAF mutations or developed resistance to BRAF inhibitors (BRAFi) in vivo to MET inhibitors PHA-665752 and ARQ197 (tivantinib). In summary, the activation of MET receptor in brain colonizing melanoma cells by stromal cell-released HGF may promote tumor self-maintenance and expansion and might counteract ICi therapy. Therefore, therapeutic targeting of MET possibly serves as a promising strategy to control intracranial progressive disease and improve patient survival.

The Brain Microenvironment Induces DNMT1 Suppression and Indolence of Metastatic Cancer Cells.

SummaryBrain metastasis is an ineffective process, and many cancer cells enter into an indolent state following extravasation in the brain. Single cell RNA sequencing of melanoma brain metastases reveals that non-proliferating brain metastatic melanoma cells exhibit a pattern of gene expression associated with inhibition of DNA methyltransferase 1 (DNMT1). The brain microenvironment, specifically the combination of reactive astrocytes and mechanically soft surroundings, suppressed DNMT1 expression in various cancer types and caused cell cycle delay. Somewhat unexpectedly, we find that DNMT1 suppression not only induces cell cycle delay but also activates pro-survival signals in brain metastatic cancer cells, including L1CAM and CRYAB. Our results demonstrate that transcriptional changes triggered by DNMT1 suppression is a key step for cancer cells to survive in the brain microenvironment and that they also restrict cancer cell proliferation. The dual consequences of DNMT1 suppression can explain the persistence of indolent cancer cells in the brain microenvironment.

Cluster of Differentiation 46 Is the Major Receptor in Human Blood-Brain Barrier Endothelial Cells for Uptake of Exosomes Derived from Brain-Metastatic Melanoma Cells (SK-Mel-28).

Brain metastasis is a frequent complication of cancer and may be mediated, at least in part, by the internalization of cancer-cell-derived exosomes into brain capillary endothelial cells. Clarifying the mechanism(s) of this internalization is of interest because it could help us to develop ways to block brain metastasis, as well as affording a potential new route for drug delivery into the brain. Therefore, the purpose of the present study was to address this issue by identifying the receptors involved in the internalization of exosomes derived from a brain-metastatic cancer cell line (SK-Mel-28) into human blood-brain barrier endothelial cells (hCMEC/D3 cells). The combination of sulfo-SBED-based cross-linking and comprehensive proteomics yielded 20 proteins as exosome receptor candidates in hCMEC/D3 cells. The uptake of PKH67-labeled exosomes by hCMEC/D3 cells measured at 37 °C was significantly reduced by 95.6% at 4 °C and by 15.3% in the presence of 1 mM RGD peptide, an integrin ligand. Therefore, we focused on the identified RGD receptors, integrin α5 and integrin αV, and CD46, which is reported to act as an adenovirus receptor, together with integrin αV. A mixture of neutralizing antibodies against integrin α5 and integrin αV significantly decreased the exosome uptake by 11.8%, while application of CD46 siRNA reduced it by 39.0%. Immunohistochemical analysis confirmed the presence of CD46 in human brain capillary endothelial cells. These results suggest that CD46 is a major receptor for the uptake of SK-Mel-28-derived exosomes by human blood-brain barrier endothelial cells (hCMEC/D3 cells).

The metastatic microenvironment: Melanoma-microglia cross-talk promotes the malignant phenotype of melanoma cells.

Melanoma has the highest propensity to metastasize to the brain compared to other cancers, as brain metastases are found frequently high in patients who have prolonged survival with visceral metastasis. Once disseminated in the brain, melanoma cells communicate with brain resident cells that include astrocytes and microglia. Microglia cells are the resident macrophages of the brain and are the main immunological cells in the CNS involved in neuroinflammation. Data on the interactions between brain metastatic melanoma cells and microglia and on the role of microglia-mediated neuroinflammation in facilitating melanoma brain metastasis are lacking. To elucidate the role of microglia in melanoma brain metastasis progression, we examined the bidirectional interactions between microglia and melanoma cells in the tumor microenvironment. We identified the molecular and functional modifications occurring in brain-metastasizing melanoma cells and microglia cells after the treatment of each cell type with supernatants of the counter cell type. Both cells induced alteration in gene expression programs, cell signaling, and cytokine secretion in the counter cell type. Moreover, melanoma cells exerted significant morphological changes on microglia cells, enhanced proliferation, induced matrix metalloproteinase-2 (MMP-2) activation, and cell migration. Microglia cells induced phenotypic changes in melanoma cells increasing their malignant phenotype: increased melanoma proliferation, MMP-2 activity, cell migration, brain endothelial penetration, and tumor cells ability to grow as spheroids in 3D cultures. Our work provides a novel insight into the bidirectional interactions between melanoma and micoglia cells, suggesting the contribution of microglia to melanoma brain metastasis formation.

CD74 regulates complexity of tumor cell HLA class II peptidome in brain metastasis and is a positive prognostic marker for patient survival

Despite multidisciplinary local and systemic therapeutic approaches, the prognosis for most patients with brain metastases is still dismal. The role of adaptive and innate anti-tumor response including the Human Leukocyte Antigen (HLA) machinery of antigen presentation is still unclear. We present data on the HLA class II-chaperone molecule CD74 in brain metastases and its impact on the HLA peptidome complexity.We analyzed CD74 and HLA class II expression on tumor cells in a subset of 236 human brain metastases, primary tumors and peripheral metastases of different entities in association with clinical data including overall survival. Additionally, we assessed whole DNA methylome profiles including CD74 promoter methylation and differential methylation in 21 brain metastases. We analyzed the effects of a siRNA mediated CD74 knockdown on HLA-expression and HLA peptidome composition in a brain metastatic melanoma cell line.We observed that CD74 expression on tumor cells is a strong positive prognostic marker in brain metastasis patients and positively associated with tumor-infiltrating T-lymphocytes (TILs). Whole DNA methylome analysis suggested that CD74 tumor cell expression might be regulated epigenetically via CD74 promoter methylation. CD74high and TILhigh tumors displayed a differential DNA methylation pattern with highest enrichment scores for antigen processing and presentation. Furthermore, CD74 knockdown in vitro lead to a reduction of HLA class II peptidome complexity, while HLA class I peptidome remained unaffected.In summary, our results demonstrate that a functional HLA class II processing machinery in brain metastatic tumor cells, reflected by a high expression of CD74 and a complex tumor cell HLA peptidome, seems to be crucial for better patient prognosis.

Effect of overexpression of epidermal growth factor receptor on the growth and migration of a brain-metastatic H1 human melanoma cell line

Objective To investigate the role of epidermal growth factor receptor (EGFR) in the growth and migration of melanoma.Methods A brain-metastatic human melanoma cell line H1,which had been developed in our laboratory,was used in this study.Some H1 melanoma cells were stably transfected with wild-type EGFR via lentiviral vectors to overexpress EGFR (H1EGFRwt),and some H1 cells transfected with green fluorescent protein (GFP) served as the control group (H1GFP).After transfection,the cells were sorted and purified.Then,scratch wound healing assay was performed to estimate the migratory activity of H1 cells; colony-forming assay was conducted to investigate the effects of EGFR on the survival ability and colony-forming ability of H1 cells,and resazurin reduction test was used to evaluate colony formation results; Western blot was carried out to measure the expressions of signaling pathways activated by EGFR.Statistical analysis was done by repeated measures analysis of variance (ANOVA) for the comparison of wound-area healing rate,and by paired t test for the comparison of metabolic activity,between the two groups of H1 cells by using SPSS software version 20.0.Results Flow cytometry showed that H1 cells were successfully transfected with EGFR and GFP respectively,and purified H1EGFRwt and H1GFP cells were obtained after cell sorting.The wound-area healing rate at 6,18,24 and 48 hours after epidermal growth factor (EGF) stimulation was 0.145 ± 0.066,0.479 ± 0.096,0.571 ± 0.198 and 0.672 ± 0.199 respectively for H1EGFRwt cells,0.051 ± 0.036,0.254 ± 0.038,0.303 ± 0.077 and 0.498 ± 0.111 respectively for H1GFP cells.The degree of wound healing in H1EGFRwt cells was significantly higher than that in H1GFP cells (F =68.49,df=5,P ＜ 0.05).Colony-forming assay revealed that the average metabolic activity score of H1EGFRwt cells was significantly higher than that of H1GFP cells (92 225.2 ± 6 632.1 vs.62 935.7 ± 8 159.2,t =2.26,df=9,P ＜ 0.01).Western blot showed that EGF might induce the phosphorylation of downstream signaling proteins such as phosphatidylinositol-specific phospholipase C-γ (PLCγ),signal transducer and activator of transcription 5 (STAT5) and 3 (STAT3) in H1EGFRwt cells,but not in H1GFP cells.In addition,the expressions of phosphorylated serine/threonine protein kinase (pAKT) and mitogen-activated protein kinase (pMAPK) were significantly higher in H1EGFRwt cells than in H1GFP cells after EGF stimulation.Conclusions EGFR may play an important role in the metastasis of H1 melanoma cells,and might serve as a target in the treatment of metastatic melanoma. Key words: Melanoma;  Receptor, epidermal growth factor;  Epidermal growth factor

The PI3K inhibitor BKM120 has potent antitumor activity in melanoma brain metastases in vitro and in vivo.

e20050 Background: In melanoma, the RAF-MEK-ERK and PI3K-AKT signaling pathways play a major role in melanoma progression and drug resistance. On the basis of significant improvement in overall survival, the BRAF inhibitor vemurafenib gained FDA approval for the treatment of patients with metastatic BRAFV600E mutated melanoma. However, vemurafenib appears to be less effective in melanoma brain metastases, and brain metastases are the most common cause of death in patients with metastatic melanoma. In our previous study we reported that the AKT survival pathway is hyperactivated in melanoma brain metastases. Methods: The current study aims to investigate the mechanisms of AKT hyperactivation and the antitumor activity of the PI3K inhibitor BKM120 in melanoma brain metastases in vitro and in vivo. Results: To simulate the tumor environment of brain metastases and extracerebral metastases, brain and matched extracerebral metastatic melanoma cells were stimulated by astrocyte- and fibroblast-conditioned medium, respectively. Both brain and extracerebral metastatic melanoma cells stimulated by astrocyte-conditioned medium showed higher AKT activation and invasiveness in a transwell matrigel invasion assay than cells stimulated by fibroblast-conditioned medium. The PI3K inhibitor BKM120 inhibited the phosphorylation of AKT and the growth of >10 newly isolated cell lines derived from melanoma brain metastases achieving growth inhibition rates of up to 80%. These effects did not depend on BRAF, NRAS or KIT mutation status. Furthermore, BKM120 potently induced apoptosis in brain metastatic melanoma cells and significantly inhibited the tumor growth of human brain metastatic melanoma cells in the brain of nude mice as shown by MRI scans. Conclusions: Brain-derived factors induce hyperactivation of the AKT survival pathway and promote invasiveness and drug resistance of melanoma cells in the brain. The PI3K inhibitor BKM120 inhibits activation of the AKT survival pathway and demonstrates potent antitumor activity in melanoma brain metastases in vitro and in vivo.

Abstract 5229: Single, pre-labeled metastatic melanoma cells can be detected in the mouse brain using MRI

Abstract In metastatic melanoma, brain metastases are clinically diagnosed in 40-60% of patients during the disease, but can be found in up to 90% at autopsy. The prognosis for these patients is poor, due to ineffective treatments and development of multiple brain metastasis. There is a limited knowledge about the biological mechanisms behind this development of multiple metastasis in the brain, which necessitates the development of clinically more relevant animal models. The purpose of this study was to develop a multiple brain metastasis animal model which closely reflects the clinical setting, and specifically to determine the number of single melanoma cells able to arrest in the animal brain. The H1 brain metastatic melanoma cell line was established from a patient melanoma brain metastases obtained during surgery. The H1 cells were pre-labeled with superparamagnetic iron oxide particles (Fedex; Babic et al, Bioconjugate Chem 2008) at a concentration of 10 μg/ml for 24 hrs. The viability of labeled cells was studied in vitro using cell proliferation assays, and Scanning/Transmission Electron Microscopy (SEM/TEM). One million pre-labeled tumor cells were then injected into the left ventricle of the heart in nod scid mice. 24 hrs after injections, T1 weighted (T1w), T2 weighted (T2w) and T2* weighted (T2*w) MR imaging were performed. In addition, T1w and T2w MR images were obtained 5 weeks after injections. We also transfected the H1 melanoma cells with luciferase, and monitored the animals for up to 6 weeks using bioluminescence imaging (OI). Pre-labeling of melanoma cells with Fedex did not affect cell proliferation. The SEM and TEM studies showed iron oxide particle attachment to the tumor cell membrane, as well as localisation of the labelling agents within vesicles in the cytoplasm. By encapsulating pre-labeled tumor cells at various concentrations in soft agar, we were able to detect less than 50 cell/μl using T2 mapping MRI. By pre-labeling the melanoma cells with Fedex, we were able to detect single tumour cells in the animal brains. Imaging studies (OI, MR) 4-6 weeks after injections confirmed multiple metastatic spread to the brain. In conclusion, we have developed a melanoma metastasis model in immunodeficient mice, which reflects the metastatic tumor spread seen in patients. By using pre-labeling with Fedex, we were able to detect single tumour cells in the brain. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5229. doi:10.1158/1538-7445.AM2011-5229

Modulation of GEF‐H1 Induced Signaling by Heparanase in Brain Metastatic Melanoma Cells

Mechanisms of brain metastatic melanoma (BMM) remain largely unknown. Understanding the modulation of signaling pathways that alter BMM cell invasion and metastasis is critical to develop new therapies for BMM. Heparanase has been widely implicated in cancer and is the dominant mammalian endoglycosidase which degrades heparan sulfate chains of proteoglycans (HSPG) including syndecans (SDCs). Recent findings also indicate that heparanase possesses non-enzymatic functions in its latent form. We hypothesized that extracellular heparanase modulates BMM cell signaling by involving SDC1/4 carboxy terminal-associated proteins and downstream targets. We digested BMM cell surface HS with human recombinant active or latent heparanase to delineate their effects on cytoskeletal dynamics and cell invasiveness. We identified the small GTPase guanine nucleotide exchange factor-H1 (GEF-H1) as a new component of a SDC signaling complex that is differentially expressed in BMM cells compared to corresponding non-metastatic counterparts. Second, knockdown of GEF-H1, SDC1, or SDC4 decreased BMM cell invasiveness and GEF-H1 modulated small GTPase activity of Rac1 and RhoA in conjunction with heparanase treatment. Third, both active and latent forms of heparanase affected Rac1 and RhoA activity; notably increasing RhoA activity. Both forms of heparanase were found to mediate the expression and subcellular localization of GEF-H1, and treatment of BMM with latent heparanase modulated SDC1/4 gene expression. Finally, treatment with exogenous heparanase downregulated BMM cell invasion. These studies indicate the relevance of heparanase signaling pathways in BMM progression, and provide insights into the molecular mechanisms regulating HSPG signaling in response to exogenous heparanase.

Molecular Basis for the Critical Role of Suppressor of Cytokine Signaling-1 in Melanoma Brain Metastasis

Our recent study found that activation of signal transducer and activator of transcription 3 (Stat3) is up-regulated in human brain metastatic cells and contributes to brain metastasis of melanoma. However, the molecular mechanisms underlying this increased Stat3 activation and effect on brain metastasis are unknown. In this report, we showed that the expression of Janus-activated kinase 2 (JAK2), a Stat3 activator, was increased, whereas the expression of a negative regulator of Stat3, suppressor of cytokine signaling-1 (SOCS-1), was reduced in the brain metastatic melanoma cell line A375Br, relative to that in the parental A375P cell line. Consistently, SOCS-1 expression was also lower in the human brain metastatic tissues than in the primary melanoma tissues. Mechanistically, increased JAK2 expression in the A375Br cells was due to, at least in part, its decreased degradation, which was directly correlated with low expression of SOCS-1. Moreover, restoration of SOCS-1 expression resulted in the inhibition of Stat3 activation, whereas depletion of SOCS-1 up-regulated Stat3 activation. These clinical, experimental, and mechanistic findings strongly suggest that increased activation of Stat3 in brain metastatic melanoma cells might be due to decreased SOCS-1 expression. Furthermore, restoration of SOCS-1 expression in brain metastatic A375Br cells significantly inhibited brain metastasis in animal models (P<0.001). Additionally, alterations of SOCS-1 expression profoundly affected the expression of matrix metalloproteinase-2 (MMP-2), basic fibroblast growth factor (bFGF), and vascular endothelial growth factor (VEGF) and the melanoma cell invasion and angiogenesis. Collectively, these data suggest that the loss of SOCS-1 expression is a critical event, leading to elevated Stat3 signaling and overexpression of MMP-2, bFGF, and VEGF, as well as enhanced invasion and angiogenesis of melanoma cells, consequently promoting brain metastasis.

Activation of Stat3 in Human Melanoma Promotes Brain Metastasis

Brain metastasis is a major cause of morbidity and mortality in patients with melanoma. The molecular changes that lead to brain metastasis remain poorly understood. In this study, we developed a model to study human melanoma brain metastasis and found that Stat3 activity was increased in human brain metastatic melanoma cells when compared with that in cutaneous melanoma cells. The expression of activated Stat3 is also increased in human brain metastasis specimens when compared with that in the primary melanoma specimens. Increased Stat3 activation by transfection with a constitutively activated Stat3 enhanced brain metastasis, whereas blockade of Stat3 activation by transfection with a dominant-negative Stat3 suppressed brain metastasis of human melanoma cells in animal models. Furthermore, altered Stat3 activity profoundly affected melanoma angiogenesis in vivo and melanoma cell invasion in vitro and significantly affected the expression of basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF), and matrix metalloproteinase-2 (MMP-2) in vivo and in vitro. Finally, Stat3 activity transcriptionally regulated the promoter activity of bFGF in addition to VEGF and MMP-2 in human melanoma cells. These results indicated that Stat3 activation plays an important role in dysregulated expression of bFGF, VEGF, and MMP-2 as well as angiogenesis and invasion of melanoma cells and contributes to brain metastasis of melanoma. Therefore, Stat3 activation might be a new potential target for therapy of human melanoma brain metastases.

Heparanase mechanisms of melanoma metastasis to the brain: Development and use of a brain slice model

Heparanase (HPSE-1) is an endo-beta-D-glucuronidase that cleaves heparan sulfate (HS) chains of proteoglycans (HSPG), and its expression has been associated with increased cell growth, invasion, and angiogenesis of tumors as well as with embryogenesis and tissue development. Since metastatic cancer cells express HPSE-1, we have developed an orthotopic brain slice model to study HPSE-1 involvement in brain-metastatic melanoma. This model allows for the characterization of tumor cell invasion at both quantitative and qualitative levels. Brain-metastatic melanoma cells (B16B15b) showed augmenting levels of HPSE-1 protein expression in a time-dependent manner. Secondly, B16B15b cells pre-treated with HPSE-1 showed a significant increase in the number of cells that invaded into the brain tissue. Finally, HPSE-1 exposure-augmented invasion depth in brain sections by brain-metastatic melanoma cells. We concluded that applying this brain slice model can be beneficial to investigate HPSE-1- related in vivo modalities in brain-metastatic melanoma and brain invasion in general. These results also further emphasize the potential relevance of using this model to design therapies for controlling this type of cancer by blocking HPSE-1 functionality.

Dominant-negative CREB inhibits heparanase functionality and melanoma cell invasion.

Heparanase (HPSE-1) is an endo-beta-D-glucuronidase involved in the degradation of cell-surface/extracellular matrix heparan sulfate (HS) in normal and neoplastic tissues. HPSE-1 represents the first example of purification and cloning of a mammalian HS-degradative enzyme. Elevated HPSE-1 levels are known to be associated with metastatic cancers, directly implicating HPSE-1 in metastatic events. The purpose of this study was to determine the role of cAMP response element-binding protein (CREB) in modulating HPSE-1-mediated effects on human melanoma cell invasion. Highly invasive, brain-metastatic melanoma cells (70W) were transfected with the dominant-negative CREB (KCREB) and subsequently analyzed for changes in their HPSE-1 content, functionality, and cell invasive properties. KCREB-transfected cells showed a decrease in HPSE-1 mRNA expression and activity. This correlated with a significantly decreased invasion of these cells through Matrigel-coated filters. Furthermore, adenoviral vectors containing the full-length human HPSE-1 cDNA in sense orientation (Ad-S/hep) were constructed to investigate CREB effects on HPSE-1. Restoration of HPSE-1 expression and functionality following Ad-S/hep infection of KCREB-transfected 70W cells recovered melanoma cell invasiveness. These results demonstrate that KCREB inhibits HPSE-1 and suggest that one of the roles CREB plays in the acquisition of melanoma cells metastatic phenotype is affecting HPSE-1 activity.

Brain metastases in melanoma: roles of neurotrophins.

Brain metastasis, which occurs in 20% to 40% of all cancer patients, is an important cause of neoplastic morbidity and mortality. Successful invasion into the brain by tumor cells must include attachment to microvessel endothelial cells, penetration through the blood-brain barrier, and, of relevance, a response to brain survival and growth factors. Neurotrophins (NTs) are important in brain-invasive steps. Human melanoma cell lines express low-affinity NT receptor p75NTR in relation to their brain-metastatic propensity with their invasive properties being regulated by NGF, or nerve growth factor, the prototypic NT. They also express functional TrkC, the putative receptor for the invasion-promoting NT-3. In brain-metastatic melanoma cells, NTs promote invasion by enhancing the production of extracellular matrix (ECM)-degradative enzymes such as heparanase, an enzyme capable of locally destroying both ECM and the basement membrane of the blood-brain barrier. Heparanase is an endo-beta-d-glucuronidase that cleaves heparan sulfate (HS) chains of ECM HS proteoglycans, and it is a unique metastatic determinant because it is the dominant mammalian HS degradative enzyme. Brain-metastatic melanoma cells also produce autocrine/paracrine factors that influence their growth, invasion, and survival in the brain. Synthesis of these factors may serve to regulate NT production by brain cells adjacent to the neoplastic invasion front, such as astrocytes. Increased NT levels have been observed in tumor-adjacent tissues at the invasion front of human brain melanoma. Additionally, astrocytes may contribute to the brain-metastatic specificity of melanoma cells by producing NT-regulated heparanase. Trophic, autocrine, and paracrine growth factors may therefore determine whether metastatic cells can successfully invade, colonize, and grow in the CNS.

Brain-metastatic melanoma: a neurotrophic perspective.

The brain is a unique microenvironment enclosed by the skull and maintaining a highly regulated vascular transport barrier. To metastasize to the brain, malignant tumor cells must attach to microvessel endothelial cells, invade the blood-brain barrier (BBB), and respond to brain survival and growth factors. Neurotrophins (NT) are important in brain invasion because they stimulate this process. In brain-metastatic melanoma cells, NT can promote invasion by enhancing the production of extracellular matrixdegradative enzymes such as heparanase, an enzyme capable of locally destroying both the extracellular matrix and the basement membrane of the BBB. We have examined human and murine melanoma cell lines exhibiting varying abilities to form brain metastases, and have found that they express low-affinity neurotrophin receptor p75NTR in relation to their brain-metastatic potentials. They do not, however, express trkA, the gene encoding the tyrosine kinase receptor TrkA, the high-affinity receptor for nerve growth factor (NGF), the prototypic NT. Presence of functional TrkC, the putative receptor for the invasion-promoting neurotrophin NT-3, was also expressed in these cells. Brain-metastatic melanoma cells can also produce autocrine factors and inhibitors that influence their growth, invasion, and survival in the brain. Synthesis of these factors may influence NT production by brain cells adjacent to the neoplastic invasion front, such as oligodendrocytes and astrocytes. In brain biopsies, we observed increased amounts of NGF and NT-3 in tumor-adjacent tissues at the invasion front of human melanoma tumors. Additionally, we found that astrocytes contribute to the brain-metastatic specificity of melanoma cells by producing NT-regulated heparanase. Trophic, autocrine, and paracrine growth factors may therefore determine whether metastatic cells can successfully invade, colonize, and grow in the central nervous system (CNS).

P75 neurotrophin receptor functions as a survival receptor in brain-metastatic melanoma cells.

The p75 neurotrophin receptor (p75(NTR)), a common receptor for members of the neurotrophins (NT) family, was previously identified as a molecular determinant of brain metastasis. We have also reported that NT treatment of murine and human brain-metastatic melanoma cells affects their invasive capacities and increases the production of heparanase, an important and unique extracellular matrix (ECM) degradative enzyme. Neurotrophism can be a survival-support mechanism for brain-metastatic cells and a survival assay was devised to mimic the growth limiting conditions of rapidly expanding metastatic tumors prior to neoangiogenesis. We report that p75(NTR) promoted the survival of brain-metastatic melanoma cells but not melanocytes in stress cultures conditions. Secondly, melanoma cells fluorescently sorted for high p75(NTR) expression (p75(NTR-H) cells) had an up to a 15-fold greater survival than those sorted for low p75(NTR) expression (p75(NTR-L) cells). Thirdly, cells overexpressing p75(NTR) associated with the growth fraction and provided these cells with an inherent growth advantage. Finally, we observed an increased survival of sorted p75(NTR-L) cells, dependent upon treatment of NT members whose functional receptors are present on these cells. Together, these results delineate that p75(NTR)-mediated trophic support profoundly affects competitive melanoma-cell survival when the tumor cell microenvironment becomes growth limiting.

Human melanoma TrkC: its association with a purine-analog-sensitive kinase activity.

The various members of the Trk tyrosine kinase family and p75 neurotrophin receptor (p75(NTR)) have been identified as signaling receptors for the structurally related members of the neurotrophins (NT) family. We have previously reported that NT treatment of murine and human brain-metastatic melanoma cells affects their invasive capacities and increases the production of extracellular-matrix degradative enzymes. These cells express aberrant levels of functional p75(NTR) and TrkC, the putative high-affinity receptor for the neurotrophin NT-3. Here we demonstrate that, by using sensitive immune-complex kinase assays in human brain-metastatic (70W) melanoma cells, TrkC receptors associate with a kinase activity exhibiting a dose-dependent susceptibility to inhibition by the purine-analogs 6-thioguanine and 2-aminopurine. The activity of this purine-analog-sensitive kinase (PASK) was induced by NT-3 in a time-dependent fashion, phosphorylating exogenous myelin basic protein (MBP) but not denatured enolase. It is similar to the one reported to relate with p75(NTR) and TrkA receptors and stimulated by the prototypic NT, nerve growth factor. Thus, PASKs may represent unique signaling components common to NT receptors that could engage joint downstream signaling effectors in brain-metastatic melanoma.

Inhibition of heparanase activity and heparanase-induced angiogenesis by suramin analogues.

Heparanase, a heparan sulfate-specific endo-beta-D-glucuronidase, plays an important role in tumor cell metastasis through the degradation of extracellular matrix heparan sulfate proteoglycans (ECM HSPG). Heparanase activity correlates with the metastatic propensity of tumor cells. Suramin, a polysulfonated naphthylurea, is an inhibitor of heparanase with suramin analogues shown to possess antiangiogenic and antiproliferative properties. We investigated the effects of selected suramin analogues (NF 127, NF 145 and NF 171) on heparanase activity and heparanase-driven angiogenesis. Studies of the ability of cellular extracts and purified heparanase from human, highly invasive and brain-metastatic melanoma (70W) cells revealed that heparanase expressed by these cells was effectively inhibited by suramin analogues in a dose-dependent manner. These analogues possessed more potent heparanase inhibitory activities than suramin: The concentrations required for 50% heparanase inhibition (IC(50)) were 20-30 microM, or at least 2 times lower than that for suramin. One hundred percent inhibition was observed at concentrations of 100 microM and higher. Of relevance, these compounds significantly decreased (i) the invasive capacity of human 70W cells by chemoinvasion assays performed with filters coated with purified HSPG or Matrigel trade mark, and (ii) blood vessel formation by in vivo angiogenic assays, thus linking their antiangiogenic properties with impedance of heparanase-induced angiogenesis. Specifically, inhibition of invasion by NF 127, NF 145 and NF 171 was found at 10 microM concentrations of compounds with a significant decrease of invasive values at concentrations as low as 1.5 microM. In addition, NF 127, NF 145 and NF 171 promoted nearly complete inhibition of heparanase-induced angiogenesis at values ranging from 236 microM (for NF 145) to 362 microM (for NF 127). These results further emphasize the importance of heparanase in invasive and angiogenic mechanisms and the potential clinical application of heparanase inhibitors such as suramin analogues in cancers and angiogenesis-dependent diseases.

Correlation of overexpression of the low-affinity p75 neurotrophin receptor with augmented invasion and heparanase production in human malignant melanoma cells.

The role of growth factor receptors in regulating the progression of human melanocytes toward tumorigenicity and ultimately a malignant phenotype is poorly understood. In particular, the autocrine and paracrine influences that modulate cellular invasion and extracellular matrix (ECM)-degradative enzymes in melanoma cells remain undefined at the molecular level. The low-affinity p75 neurotrophin receptor (p75NTR), a cysteine-rich transmembrane glycoprotein, is frequently expressed in advanced stages of human melanoma, but the biological consequences of this expression are unknown. p75NTR can enhance the invasive potential of brain-metastatic melanoma cells in vitro. We have extended here these results and related the level of p75NTR in human metastatic melanoma cells to their invasive potential to target organs other than brain. Fluorescence activated cell sorting (FACS) analysis showed that 3 melanoma cell lines (SK-MEL-146, SK-MEL-119, 70W) had differential p75NTR contents, whereas SK-MEL-147 cells had elevated amounts of p75NTR. Two other melanoma cell lines (SK-MEL-94, SK-MEL-110) with point mutations in the p75NTR transmembrane domain had reduced (SK-MEL-94) or absent (SK-MEL-110) p75NTR. We also examined these cell lines for presence of TrkA receptor, the high-affinity receptor for nerve growth factor (NGF), the prototypic neurotrophin. No TrkA receptor expression was detected in any of the cell lines. The extent of p75NTR expression correlated with the capability of NGF to promote cellular invasion and with production of heparanase, an important ECM-degradative enzyme. Melanoma cells sorted for high p75NTR expression (p75NTR-H cells) had markedly greater (9- to 13-fold increase) invasive capabilities in response to NGF exposure than those sorted for low p75NTR expression (p75NTR-L cells). Additionally, NGF induced a 8- to 10-fold increase of heparanase activity in p75NTR-H cells. Thus, we propose that p75NTR-mediated trophic support profoundly affects melanoma cell invasion to neurotrophin-rich organs.

Stimulation of the protein tyrosine kinase c-Yes but not c-Src by neurotrophins in human brain-metastatic melanoma cells.

The c-Yes proto-oncogene (pp62c-Yes) encodes a non-receptor-type protein tyrosine kinase (NRPTK) of the Src family. c-Yes activities and protein levels are elevated in human melanoma and melanocyte cell lines. Because the neurotrophins (NT) are important in the progression of melanoma to the brain-metastatic phenotype, we determined whether NT stimulate c-Yes activity in human MeWo melanoma cells and two variant sublines with opposite metastatic capabilities, 3 S 5 and 70W. The highly brain-metastatic 70W subline had an intrinsically higher c-Yes activity than parental MeWo or poorly metastatic 3 S 5 cells. c-Yes kinase was further induced by the prototypic human NT, nerve growth factor (NGF) in a dose and time-dependent manner. In contrast, c-Src activity (pp60-Src) was similar in all these cells and unaffected by NGF exposure. Additionally, human NGF and neurotrophin-3 stimulated c-Yes in brain-metastatic 70W cells. The magnitude of c-Yes activation correlated with the degree of invasion of 70 W cells following incubation of these neurotrophins. To further examine NT stimulation of c-Yes in melanoma cells, three additional cell lines were examined. Metastatic TXM-13 and TXM-18 increased c-Yes activity in response to NGF. In contrast, no increase was observed in low-metastatic TXM-40 cells. Together, these data suggest that altered c-Yes expression may play a role in the malignant progression of the human melanocyte towards the brain-metastatic phenotype and that NT enhance the activity of c-Yes in signaling penetration into the matrix of NT-rich stromal microenvironments such as the brain.