Metabotropic glutamate receptor 1 and glutamate signaling in human melanoma

Glutamate signaling in benign and malignant disorders: current status, future perspectives, and therapeutic implications

Glutamate, a nonessential amino acid, is the major excitatory neurotransmitter in the central nervous system. As such, glutamate has been shown to play a role in not only neural processes, such as learning and memory, but also in bioenergetics, biosynthetic and metabolic oncogenic pathways. Glutamate has been the target of intense investigation for its involvement not only in the pathogenesis of benign neurodegenerative diseases (NDDs) such as Parkinson's disease, Alzheimer's disease, schizophrenia, multiple sclerosis, and amyotropic lateral sclerosis (ALS), but also in carcinogenesis and progression of malignant diseases. In addition to its intracellular activities, glutamate in secreted form is a phylogenetically conserved cell signaling molecule. Glutamate binding activates multiple major receptor families including the metabotropic glutamate receptors (mGluRs) and ionotropic glutamate receptors (iGluRs), both of which have been implicated in various signaling pathways in cancer. Inhibition of extracellular glutamate release or glutamate receptor activation via competitive or non-competitive antagonists decreases growth, migration and invasion and induces apoptosis in breast cancer, melanoma, glioma and prostate cancer cells. In this review, we discuss the current state of glutamate signaling research as it relates to benign and malignant diseases. In addition, we provide a synopsis of clinical trials using glutamate antagonists for the treatment of NDD and malignant diseases. We conclude that in addition to its potential role as a metabolic biomarker, glutamate receptors and glutamate-initiated signaling pathways may provide novel therapeutic opportunities for cancer.

Metabotropic glutamate receptor 1 expression and its polymorphic variants associate with breast cancer phenotypes

Several epidemiological studies have suggested a link between melanoma and breast cancer. Metabotropic glutamate receptor 1 (GRM1), which is involved in many cellular processes including proliferation and differentiation, has been implicated in melanomagenesis, with ectopic expression of GRM1 causing malignant transformation of melanocytes. This study was undertaken to evaluate GRM1 expression and polymorphic variants in GRM1 for associations with breast cancer phenotypes. Three single nucleotide polymorphisms (SNPs) in GRM1 were evaluated for associations with breast cancer clinicopathologic variables. GRM1 expression was evaluated in human normal and cancerous breast tissue and for in vitro response to hormonal manipulation. Genotyping was performed on genomic DNA from over 1,000 breast cancer patients. Rs6923492 and rs362962 genotypes associated with age at diagnosis that was highly dependent upon the breast cancer molecular phenotype. The rs362962 TT genotype also associated with risk of estrogen receptor or progesterone receptor positive breast cancer. In vitro analysis showed increased GRM1 expression in breast cancer cells treated with estrogen or the combination of estrogen and progesterone, but reduced GRM1 expression with tamoxifen treatment. Evaluation of GRM1 expression in human breast tumor specimens demonstrated significant correlations between GRM1 staining with tissue type and molecular features. Furthermore, analysis of gene expression data from primary breast tumors showed that high GRM1 expression correlated with a shorter distant metastasis-free survival as compared to low GRM1 expression in tamoxifen-treated patients. Additionally, induced knockdown of GRM1 in an estrogen receptor positive breast cancer cell line correlated with reduced cell proliferation. Taken together, these findings suggest a functional role for GRM1 in breast cancer.

Octahydropyrrolo[3,4-c]pyrrole negative allosteric modulators of mGlu1

Development of SAR in an octahydropyrrolo[3,4-c]pyrrole series of negative allosteric modulators of mGlu1 using a functional cell-based assay is described in this Letter. The octahydropyrrolo[3,4-c]pyrrole scaffold was chosen as an isosteric replacement for the piperazine ring found in the initial hit compound. Characterization of selected compounds in protein binding assays was used to identify the most promising analogs, which were then profiled in P450 inhibition assays in order to further assess the potential for drug-likeness within this series of compounds.

Comprehensive genomic characterization of cutaneous malignant melanoma cell lines derived from metastatic lesions by whole-exome sequencing and SNP array profiling

Cutaneous malignant melanoma is the most fatal skin cancer and although improved comprehension of its pathogenic pathways allowed to realize some effective molecular targeted therapies, novel targets and drugs are still needed. Aiming to add genetic information potentially useful for novel targets discovery, we performed an extensive genomic characterization by whole-exome sequencing and SNP array profiling of six cutaneous melanoma cell lines derived from metastatic patients. We obtained a total of 3,325 novel coding single nucleotide variants, including 2,172 non-synonymous variants. We catalogued the coding mutations according to Sanger COSMIC database and to a manually curated list including genes involved in melanoma pathways identified by mining recent literature. Besides confirming the presence of known melanoma driver mutations (BRAF(V600E), NRAS(Q61R) ), we identified novel mutated genes involved in signalling pathways crucial for melanoma pathogenesis and already addressed by current targeted therapies (such as MAPK and glutamate pathways). We also identified mutations in four genes (MUC19, PAICS, RBMXL1, KIF23) never reported in melanoma, which might deserve further investigations. All data are available to the entire research community in our Melanoma Exome Database (at https://155.253.6.64/MExDB/). In summary, these cell lines are valuable biological tools to improve the genetic comprehension of this complex cancer disease and to study functional relevance of individual mutational events, and these findings could provide insights potentially useful for identification of novel therapeutic targets for cutaneous malignant melanoma.

N-Acyl-N'-arylpiperazines as negative allosteric modulators of mGlu1: identification of VU0469650, a potent and selective tool compound with CNS exposure in rats

Development of SAR in an N-acyl-N'-arylpiperazine series of negative allosteric modulators of mGlu1 using a functional cell-based assay is described in this Letter. Characterization of selected compounds in protein binding assays was used to aid in selecting VU0469650 for further profiling in ancillary pharmacology assays and pharmacokinetic studies. VU0469650 demonstrated an excellent selectivity profile and good exposure in both plasma and brain samples following intraperitoneal dosing in rats.

Blocking glutamate-mediated signalling inhibits human melanoma growth and migration

Glutamate is an excitatory neurotransmitter that has been shown to regulate the proliferation, migration and survival of neuronal progenitors in the central nervous system through its action on metabotropic and ionotropic glutamate receptors (GluRs). Antagonists of ionotropic GluRs have been shown to cause a rapid and reversible change in melanocyte dendritic morphology, which is associated with the disorganization of actin and tubulin microfilaments in the cytoskeleton. Intracellular expression of microtubule-associated protein (MAP) 2a affects the assembly, stabilization and bundling of microtubules in melanoma cells; stimulates the development of dendrites; and suppresses melanoma cell migration and invasion. In this study, we investigated the relationship between glutamate-mediated signalling and microtubules, cell dendritic morphology and melanoma cell motility. We found that metabotropic GluR1 and N-methyl-d-aspartate receptor antagonists increased dendritic branching and inhibited the motility, migration and proliferation of melanoma cells. We also demonstrated that the invasion and motility of melanoma cells are significantly inhibited by the combination of increased expression of MAP2a and either metabotropic GluR1 or N-methyl-d-aspartate receptor antagonists. Moreover, the blockade of glutamate receptors inhibited melanoma growth in vivo. Collectively, these results demonstrate the importance of glutamate signalling in human melanoma and suggest that the blockade of glutamate receptors is a promising novel therapy for treating melanoma.

Two newly identified exons in human GRM1 express a novel splice variant of metabotropic glutamate 1 receptor

To date, five human metabotropic glutamate (mGlu) 1 receptor splice variants (1a, 1b, 1d, 1f, and 1g) have been described, all of which involve alternative C-terminal splicing. mGlu1a receptor contains a long C-terminal domain (341 amino acids), which has been shown to scaffold with several proteins and contribute to the structure of the post-synaptic density. However, several shorter mGlu1 receptor splice variants lack the sequence required for these interactions, and no major functional differences between these short splice variants have been described. By using RT-PCR we have shown that two human melanoma cell lines express both mGlu1a and mGlu1b receptors. In addition, using 3'RACE, we identified three previously unknown mGlu1 receptor mRNAs. Two differ in the length of their 3' untranslated region (UTR), and encode the same predicted protein as mGlu1g receptor-the shortest of all mGlu1 receptor splice variants. The third mRNA, named mGlu1h, encodes a predicted C-terminal splice variant of 10 additional amino acids. mGlu1h mRNA was observed in two different melanoma cell lines and is overexpressed, compared with melanoma precursor cells, melanocytes. Most importantly, this new splice variant, mGlu1h receptor, is encoded by two previously unidentified exons located within the human GRM1 gene. Additionally, these new exons are found exclusively within the GRM1 genes of higher primates and are highly conserved. Therefore, we hypothesize that mGlu1h receptors play a distinct role in primate glutamatergic signaling.

Genetics of melanoma

Genomic variation is a trend observed in various human diseases including cancer. Genetic studies have set out to understand how and why these variations result in cancer, why some populations are pre-disposed to the disease, and also how genetics affect drug responses. The melanoma incidence has been increasing at an alarming rate worldwide. The burden posed by melanoma has made it a necessity to understand the fundamental signaling pathways involved in this deadly disease. Signaling cascades such as mitogen-activated protein kinase and PI3K/AKT have been shown to be crucial in the regulation of processes that are commonly dysregulated during cancer development such as aberrant proliferation, loss of cell cycle control, impaired apoptosis, and altered drug metabolism. Understanding how these and other oncogenic pathways are regulated has been integral in our challenge to develop potent anti-melanoma drugs. With advances in technology and especially in next generation sequencing, we have been able to explore melanoma genomes and exomes leading to the identification of previously unknown genes with functions in melanomagenesis such as GRIN2A and PREX2. The therapeutic potential of these novel candidate genes is actively being pursued with some presenting as druggable targets while others serve as indicators of therapeutic responses. In addition, the analysis of the mutational signatures of melanoma tumors continues to cement the causative role of UV exposure in melanoma pathogenesis. It has become distinctly clear that melanomas from sun-exposed skin areas have distinct mutational signatures including C to T transitions indicative of UV-induced damage. It is thus necessary to continue spreading awareness on how to decrease the risk factors of developing the disease while at the same time working for a cure. Given the large amount of information gained from these sequencing studies, it is likely that in the future, treatment of melanoma will follow a highly personalized route that takes into account the differential mutational signatures of each individual’s cancer.

Targeting Wnt pathways in disease

Wnt-mediated signal transduction pathways have long been recognized for their roles in regulating embryonic development, and have more recently been linked to cancer, neurologic diseases, inflammatory diseases, and disorders of endocrine function and bone metabolism in adults. Although therapies targeting Wnt signaling are attractive in theory, in practice it has been difficult to obtain specific therapeutics because many components of Wnt signaling pathways are also involved in other cellular processes, thereby reducing the specificity of candidate therapeutics. New technologies, and advances in understanding the mechanisms of Wnt signaling, have improved our understanding of the nuances of Wnt signaling and are leading to promising new strategies to target Wnt signaling pathways.

Metabotropic glutamate receptor 1 (Grm1) is an oncogene in epithelial cells

Non-neuronal expression of components of the glutamatergic system has been increasingly observed, and our laboratory previously had demonstrated the etiological role of ectopically expressed metabotropic glutamate receptor 1 (Grm1/mGluR1) in mouse models of melanoma. We hypothesize that inappropriate glutamatergic signaling in other cell types can dysregulate growth leading to transformation and tumorigenesis. As most cancers are carcinomas, we selected an immortalized primary baby mouse kidney (iBMK) cell model to assess whether Grm1 can transform epithelial cells. These iBMK cells, engineered to be immortal yet non-tumorigenic and retaining normal epithelial characteristics, were used as recipients for exogenous Grm1 cDNA. Several stable Grm1 expressing clones were isolated and the Grm1-receptors were shown to be functional, as evidenced by the accumulation of second messengers in response to Grm1 agonist. Additionally activated by agonist were MAPK and AKT signaling cascades, major intracellular pathways shown by many investigators to be critical in melanomagenesis and other neoplasms. These Grm1-iBMK cells exhibited enhanced cell proliferation in in vitro MTT assays and significant tumorigenicity in in vivo allografts. Persistent Grm1 expression was required for the maintenance of the in vivo tumorigenic phenotype as demonstrated by an inducible Grm1-silencing RNA. These are the first results that indicate Grm1 can be an oncogene in epithelial cells. Additionally, relevance to human disease in the corresponding tumor type of renal cell carcinoma (RCC) may be suggested by observed expression of GRM1/mGluR1 in a number of RCC tumor biopsy samples and cell lines, and the effects of GRM1 modulation on tumorigenicity therein. Moreover RCC cell lines exhibited elevated levels of extracellular glutamate, and some lines responded to drugs which modulate the glutamatergic system. These findings imply a possible role for glutamate signaling apparatus in RCC cell growth, and that the glutamatergic system may be a therapeutic target in renal cell carcinoma.

Serum glutamate levels correlate with Gleason score and glutamate blockade decreases proliferation, migration, and invasion and induces apoptosis in prostate cancer cells

PURPOSE

During glutaminolysis, glutamine is catabolized to glutamate and incorporated into citric acid cycle and lipogenesis. Serum glutamate levels were measured in patients with primary prostate cancer or metastatic castrate-resistant prostate cancer (mCRPCa) to establish clinical relevance. The effect of glutamate deprivation or blockade by metabotropic glutamate receptor 1 (GRM1) antagonists was investigated on prostate cancer cells' growth, migration, and invasion to establish biologic relevance.

EXPERIMENTAL DESIGN

Serum glutamate levels were measured in normal men (n = 60) and patients with primary prostate cancer (n = 197) or mCRPCa (n = 109). GRM1 expression in prostatic tissues was examined using immunohistochemistry (IHC). Cell growth, migration, and invasion were determined using cell cytotoxicity and modified Boyden chamber assays, respectively. Apoptosis was detected using immunoblotting against cleaved caspases, PARP, and γ-H2AX.

RESULTS

Univariate and multivariate analyses showed significantly higher serum glutamate levels in Gleason score ≥ 8 than in the Gleason score ≤ 7 and in African Americans than in the Caucasian Americans. African Americans with mCRPCa had significantly higher serum glutamate levels than those with primary prostate cancer or benign prostate. However, in Caucasian Americans, serum glutamate levels were similar in normal research subjects and patients with mCRPC. IHC showed weak or no expression of GRM1 in luminal acinar epithelial cells of normal or hyperplastic glands but high expression in primary or metastatic prostate cancer tissues. Glutamate deprivation or blockade decreased prostate cancer cells' proliferation, migration, and invasion and led to apoptotic cell death.

CONCLUSIONS

Glutamate expression is mechanistically associated with and may provide a biomarker of prostate cancer aggressiveness.

Non-canonical Smads phosphorylation induced by the glutamate release inhibitor, riluzole, through GSK3 activation in melanoma

Riluzole, an inhibitor of glutamate release, has shown the ability to inhibit melanoma cell xenograft growth. A phase 0 clinical trial of riluzole as a single agent in patients with melanoma resulted in involution of tumors associated with inhibition of both the mitogen-activated protein kinase (MAPK) and phophoinositide-3-kinase/AKT (PI3K/AKT) pathways in 34% of patients. In the present study, we demonstrate that riluzole inhibits AKT-mediated glycogen synthase kinase 3 (GSK3) phosphorylation in melanoma cell lines. Because we have demonstrated that GSK3 is involved in the phosphorylation of two downstream effectors of transforming growth factor beta (TGFβ), Smad2 and Smad3, at their linker domain, our aim was to determine whether riluzole could induce GSK3β-mediated linker phosphorylation of Smad2 and Smad3. We present evidence that riluzole increases Smad2 and Smad3 linker phosphorylation at the cluster of serines 245/250/255 and serine 204 respectively. Using GSK3 inhibitors and siRNA knock-down, we demonstrate that the mechanism of riluzole-induced Smad phosphorylation involved GSK3β. In addition, GSK3β could phosphorylate the same linker sites in vitro. The riluzole-induced Smad linker phosphorylation is mechanistically different from the Smad linker phosphorylation induced by TGFβ. We also demonstrate that riluzole-induced Smad linker phosphorylation is independent of the expression of the metabotropic glutamate receptor 1 (GRM1), which is one of the glutamate receptors whose involvement in human melanoma has been documented. We further show that riluzole upregulates the expression of INHBB and PLAU, two genes associated with the TGFβ signaling pathway. The non-canonical increase in Smad linker phosphorylation induced by riluzole could contribute to the modulation of the pro-oncogenic functions of Smads in late stage melanomas.

Riluzole prodrugs for melanoma and ALS: design, synthesis, and in vitro metabolic profiling

Riluzole (1) is an approved therapeutic for the treatment of ALS and has also demonstrated anti-melanoma activity in metabotropic glutamate GRM1 positive cell lines, a mouse xenograft assay and human clinical trials. Highly variable drug exposure following oral administration among patients, likely due to variable first pass effects from heterogeneous CYP1A2 expression, hinders its clinical use. In an effort to mitigate effects of this clearance pathway and uniformly administer riluzole at efficacious exposure levels, several classes of prodrugs of riluzole were designed, synthesized, and evaluated in multiple in vitro stability assays to predict in vivo drug levels. The optimal prodrug would possess the following profile: stability while transiting the digestive system, stability towards first pass metabolism, and metabolic lability in the plasma releasing riluzole. (S)-O-Benzyl serine derivative 9 was identified as the most promising therapeutically acceptable prodrug.

Molecular pathways: dysregulated glutamatergic signaling pathways in cancer

The neurotransmitter glutamate interacts with glutamate receptor proteins, leading to the activation of multiple signaling pathways. Dysfunction in the glutamatergic signaling pathway is well established as a frequent player in diseases such as schizophrenia, Alzheimer disease, and brain tumors (gliomas). Recently, aberrant functioning of this pathway has also been shown in melanoma. In both glioma and melanoma, glutamate secretion stimulates tumor growth, proliferation, and survival through activation of the mitogen-activated protein kinase and phosphoinositide 3-kinase/Akt pathways. In the future, extracellular glutamate levels and glutamatergic signaling may serve as biological markers for tumorigenicity and facilitate targeted therapy for melanoma. .

Highly pigmented Tg(Grm1) mouse melanoma develops non-pigmented melanoma cells in distant metastases

Murine model systems are critically required tools for the investigation of unknown mechanisms of melanoma development and metastasis and for developing more efficient therapies. The Tg(Grm1)EPv melanoma mouse model is characterized by spontaneous development of pigmented cutaneous melanomas at hairless skin regions, with a short latency and 100% penetrance. Local metastasis was described in initial analyses; however, melanoma cells were not observed in distant organs. Here, we demonstrate that the established Tg(Grm1)EPv melanoma mouse model exhibits more extensive metastasis into distant organs than previously described. Disseminated cells undergo phenotypic changes, as we observed high numbers of non-pigmented Grm1-expressing melanoma cells within distant organs. As such changes during metastasis are common in human melanoma, our findings demonstrate that this mouse model represents an even more useful tool to study unknown mechanisms of metastasis in human melanoma than previously assumed.

Functional effects of GRM1 suppression in human melanoma cells

Ectopic expression of a neuronal receptor, metabotropic glutamate receptor 1 (Grm1), in melanocytes has been implicated in melanoma development in mouse models. The human relevance of this receptor's involvement in melanoma pathogenesis was shown by detecting GRM1 expression in subsets of human melanomas, an observation lacking in benign nevi or normal melanocytes. Grm1-transformed mouse melanocytes and a conditional Grm1 transgenic mouse model confirmed a requirement for sustained expression of Grm1 for the maintenance of transformed phenotypes in vitro and tumorigenicity in vivo. Here, we investigate if continued GRM1 expression is also required in human melanoma cell lines by using two inducible, silencing RNA systems: the ecdysone/Ponasterone A and tetracycline on/off approaches to regulate GRM1 expression in the presence of each inducer. Various in vitro assays were conducted to assess the consequences of a reduction in GRM1 expression on cell proliferation, apoptosis, downstream targeted signaling pathways, and in vivo tumorigenesis. We showed that suppression of GRM1 expression in several human melanoma cell lines resulted in a reduction in the number of viable cells and a decrease in stimulated mitogen-activated protein kinase (MAPK) and PI3K/AKT and suppressed tumor progression in vivo. These results reinforce earlier observations where a reduction in cell growth in vitro and tumorigenesis in vivo were correlated with decreased GRM1 activities by pharmacologic inhibitors of the receptor, supporting the notion that GRM1 plays a role in the maintenance of transformed phenotypes in human melanoma cells in vitro and in vivo and could be a potential therapeutic target for the treatment of melanoma.

Regulation of mGluR1 expression in human melanocytes and melanoma cells

We demonstrated that ectopic expression of metabotropic glutamate receptor 1 (mGluR1/Grm1) in mouse melanocytes was sufficient to induce melanoma development in vivo with 100% penetrance. We also showed that about 60% of human melanoma biopsies and cell lines, but not benign nevi or normal human melanocytes expressed mGluR1, suggesting that GRM1 may be involved in melanomagenesis. mGluR1 is expressed primarily in neurons. In various non-neuronal cells, mGluR1 expression is regulated via binding of Neuron-Restrictive-Silencer-Factor (NRSF) to a Neuron-Restrictive-Silencer-Element (NRSE). Here, we report on the possibility that aberrant mGluR1 expression in melanoma is due to alterations in NRSF and/or NRSE. We show that in human melanocytes, binding of NRSF to NRSE in the GRM1 promoter region is necessary for the suppression of mGluR1 expression. We also show that inhibiting the expression of the transcription factor Sp1 or interference with its ability to bind DNA can result in increased mGluR1 expression perhaps via its function as a negative regulator. In addition, we also provide evidence that demethylation within the promoter region of GRM1 may also be a mechanism for the derepression of mGluR1 expression in melanocytes that progress to cell transformation and tumor formation.

The suppressive effect of metabotropic glutamate receptor 5 (mGlu5) inhibition on hepatocarcinogenesis

Metabotropic glutamate receptors (mGlus) are G-protein-coupled receptors playing an important role in the central nervous system (CNS). Recently, mGlus have been identified in peripheral tissues, and aberrant expression or inhibition of the receptors functions in the development of certain cancers. However, the correlation of mGlu activity with hepatocellular carcinoma (HCC) remains unknown. In this study, we analyzed the effects of inhibiting mGlu5 activity in hepatocarcinoma cell lines and a xenograft model. Inactivation of mGlu5 with 2-Methyl-6-(phenylethyl)-pyridine (MPEP), a specific antagonist of the receptor, caused inhibition of cell growth, migration, and invasion of HepG2 and Bel-7402 cells, assessed by MTT assay, ATP production, wound healing, and Boyden chamber assay, respectively. Moreover, inhibition of tumor growth and the potential metastasis of hepatocellular carcinoma were also found in nude mice. Furthermore, mGlu5-mediated extracellular signal-regulated kinase (ERK) phosphorylation has been found to be partially involved in cell growth and migration, as detected by stimulation of (S)-3,5-Dihydroxyphenylglycine (DHPG), an agonist of the receptor, and blockage of MPEP and U0126, an inhibitor of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (MEK). These data indicate that inhibiting the activity of mGlu5 has the molecular potential to suppress oncogenic actions by blocking downstream effector molecules. The study suggests that mGlu5 activity may contribute to understanding the development of HCC.

Delving into somatic variation in sporadic melanoma

Melanoma, the most aggressive form of skin cancer, has increased in incidence more rapidly than any other cancer. The completion of the human genome project and advancements in genomics technologies has allowed us to investigate genetic alterations of melanoma at a scale and depth that is unprecedented. Here, we survey the history of the different approaches taken to understand the genomics of melanoma - from early candidate genes, to gene families, to genome-wide studies. The new era of whole-exome and whole-genome sequencing has paved the way for an in-depth understanding of melanoma biology, identification of new therapeutic targets, and development of novel personalized therapies for melanoma.

Effect of (S)-3,5-DHPG on microRNA expression in mouse brain

MicroRNAs are small non-coding RNAs that regulate post-transcriptional gene expression. In the short time since the discovery of microRNAs, the literature has burgeoned with studies focused on the biosynthesis of microRNAs, target prediction and binding, and mechanisms of translational repression by microRNAs. Given the prominent role of microRNAs in all areas of cell biology, it is not surprising that microRNAs are also linked to human diseases, including those of the nervous system. One of the least-studied areas of microRNA research is how their expression is regulated outside of development and cancer. Thus, we examined a role for regulation of microRNAs by neurotransmitter receptor activation in mouse brain. We focused on the group I metabotropic glutamate receptors by using intracerebroventricular injection of the selective agonist, (S)-3,5-dihydroxyphenylglycine (DHPG) in mouse brain. We then examined the expression of microRNAs in the cerebral cortex by Ambion and Invitrogen microarrays, and the expression of mature microRNA sequences by SABiosciences qPCR arrays, at 4, 8 and 24 h after DHPG injection. These studies revealed that the largest number of significantly regulated microRNAs was detected 8h after DHPG injection in the microarrays and qPCR arrays. We then used RNA blots to quantify microRNA expression, and in situ hybridization to examine cellular distribution of the microRNAs regulated by DHPG. Bioinformatic analysis of the microRNAs regulated 8 h after DHPG in all three arrays revealed KEGG pathways that are known to correlate with group I mGluR effects, as well as recently described and novel pathways. These studies are the first to show that DHGP regulates the expression of microRNAs in mouse cerebral cortex, and support the hypothesis that group I mGluRs may regulate microRNA expression in mouse brain.

The Akt signaling pathway: an emerging therapeutic target in malignant melanoma

Studies using cultured melanoma cells and patient tumor biopsies have demonstrated deregulated PI3 kinase-Akt3 pathway activity in ~70% of melanomas. Furthermore, targeting Akt3 and downstream PRAS40 has been shown to inhibit melanoma tumor development in mice. Although these preclinical studies and several other reports using small interfering RNAs and pharmacological agents targeting key members of this pathway have been shown to retard melanoma development, analysis of early Phase I and Phase II clinical trials using pharmacological agents to target this pathway demonstrate the need for (1) selection of patients whose tumors have PI3 kinase-Akt pathway deregulation, (2) further optimization of therapeutic agents for increased potency and reduced toxicity, (3) the identification of additional targets in the same pathway or in other signaling cascades that synergistically inhibit the growth and progression of melanoma, and (4) better methods for targeted delivery of pharmaceutical agents inhibiting this pathway. In this review we discuss key potential targets in PI3K-Akt3 signaling, the status of pharmacological agents targeting these proteins, drugs under clinical development, and strategies to improve the efficacy of therapeutic agents targeting this pathway.

mGlu Receptors and Cancerous Growth

G-protein coupled receptors (GPCR) represent a class of therapeutic targets that have been widely exploited for drug designs and development. Metabotropic glutamate receptors (mGluRs) belong to Class C GPCRs and are predominantly involved in maintaining cellular homeostasis in the central nervous system (CNS). The surprising accumulating evidence suggesting other functional roles of mGluRs in human malignancies in addition to synaptic transmission has presented intriguing possibilities to make mGluRs putative novel targets for human cancers. Since our group first described the aberrant expression of mGluR1 as the driving force in melanomagenesis in transgenic mouse models, other subtypes of mGluRs have been implicated in the pathogenesis of various cancer types such as malignant gliomas and medulloblastomas. As such, increased efforts have been generated to elucidate the mechanisms by which mGluRs confer oncogenic potentials. Current knowledge on the participation of various mGluRs in several human cancers suggests that mGluRs are "druggable" members of the GPCR superfamily and their oncogenic implications in cancer, so further understanding on anti-mGluR strategies will be beneficial.

Expression of the metabotropic glutamate receptor 5 (mGluR5) induces melanoma in transgenic mice

Glutamate is the major excitatory neurotransmitter in the mammalian CNS and mediates fast synaptic transmission upon activation of glutamate-gated ion channels. In addition, glutamate modulates a variety of other synaptic responses and intracellular signaling by activating metabotropic glutamate receptors (mGluRs), which are G protein-coupled receptors. The mGluRs are also expressed in nonneuronal tissues and are implicated in a variety of normal biological functions as well as diseases. To study mGluR-activated calcium signaling in neurons, we generated mGluR5 transgenic animals using a Thy1 promoter to drive expression in the forebrain, and one founder unexpectedly developed melanoma. To directly investigate the role of mGluR5 in melanoma formation, we generated mGluR5 transgenic lines under a melanocyte-specific promoter, tyrosinase-related protein 1. A majority of the founders showed a severe phenotype with early onset. Hyperpigmentation of the pinnae and tail could be detected as early as 3-5 d after birth for most of the mGluR5 transgene-positive mice. There was 100% penetrance in the progeny from the tyrosinase-related protein 1-mGluR5 lines generated from founders that developed melanoma. Expression of mGluR5 was detected in melanoma samples by RT-PCR, immunoblotting, and immunohistochemistry. We evaluated the expression of several cancer-related proteins in tumor samples and observed a dramatic increase in the phosphorylation of ERK, implicating ERK as a downstream effector of mGluR5 signaling in tumors. Our findings show that mGluR5-mediated glutamatergic signaling can trigger melanoma in vivo. The aggressive growth and severe phenotype make these mouse lines unique and a potentially powerful tool for therapeutic studies.

Glutamatergic pathway targeting in melanoma: single-agent and combinatorial therapies

PURPOSE

Melanoma is a heterogeneous disease where monotherapies are likely to fail due to variations in genomic signatures. B-RAF inhibitors have been clinically inadequate but response might be augmented with combination therapies targeting multiple signaling pathways. We investigate the preclinical efficacy of combining the multikinase inhibitor sorafenib or the mutated B-RAF inhibitor PLX4720 with riluzole, an inhibitor of glutamate release that antagonizes metabotropic glutamate receptor 1 (GRM1) signaling in melanoma cells.

EXPERIMENTAL DESIGN

Melanoma cell lines that express GRM1 and either wild-type B-RAF or mutated B-RAF were treated with riluzole, sorafenib, PLX4720, or the combination of riluzole either with sorafenib or with PLX4720. Extracellular glutamate levels were determined by glutamate release assays. MTT assays and cell-cycle analysis show effects of the compounds on proliferation, viability, and cell-cycle profiles. Western immunoblotting and immunohistochemical staining showed apoptotic markers. Consequences on mitogen-activated protein kinase pathway were assessed by Western immunoblotting. Xenograft tumor models were used to determine the efficacy of the compounds in vivo.

RESULTS

The combination of riluzole with sorafenib exhibited enhanced antitumor activities in GRM1-expressing melanoma cells harboring either wild-type or mutated B-RAF. The combination of riluzole with PLX4720 showed lessened efficacy compared with the combination of riluzole and sorafenib in suppressing the growth of GRM1-expressing cells harboring the B-RAF(V600E) mutation.

CONCLUSIONS

The combination of riluzole with sorafenib seems potent in suppressing tumor proliferation in vitro and in vivo in GRM1-expressing melanoma cells regardless of B-RAF genotype and may be a viable therapeutic clinical combination.

Comprehensive genomic analysis of a BRCA2 deficient human pancreatic cancer

Capan-1 is a well-characterised BRCA2-deficient human cell line isolated from a liver metastasis of a pancreatic adenocarcinoma. Here we report a genome-wide assessment of structural variations and high-depth exome characterization of single nucleotide variants and small insertion/deletions in Capan-1. To identify potential somatic and tumour-associated variations in the absence of a matched-normal cell line, we devised a novel method based on the analysis of HapMap samples. We demonstrate that Capan-1 has one of the most rearranged genomes sequenced to date. Furthermore, small insertions and deletions are detected more frequently in the context of short sequence repeats than in other genomes. We also identify a number of novel mutations that may represent genetic changes that have contributed to tumour progression. These data provide insight into the genomic effects of loss of BRCA2 function.

Metabotropic glutamate receptor-1: a potential therapeutic target for the treatment of breast cancer

Metabotropic glutamate receptors are G-protein-coupled receptors normally expressed in the central nervous system where they mediate neuronal excitability, synaptic plasticity, and feedback inhibition of neurotransmitter release. However, recent data suggest that these receptors are also expressed and functional in some cancers, most notably melanoma. We detected the expression of metabotropic glutamate receptor-1 (gene: GRM1; protein: mGluR1) in triple negative breast cancer cells and evaluated its role in regulating the pro-proliferative phenotype of these cells. mGluR1 inhibitors (Riluzole or BAY36-7620) inhibited the proliferation of triple negative breast cancer cells in a time- and dose-dependent manner and this inhibition correlated with increased apoptosis as demonstrated by increase in PARP cleavage products and Annexin V staining. mGluR1 knockdown using Lentiviral constructs expressing shRNA targeting GRM1 also inhibited proliferation compared to non-silencing controls. In addition, treatment of mice bearing MDA-MB-231 xenografts with Riluzole or BAY36-7620, by intraperitoneal injection, resulted in a significant reduction in tumor volume of up to 80%. Moreover, Riluzole was effective against triple negative breast cancer xenografts in mice at doses equivalent to those currently being used in humans for the treatment of amyotrophic lateral sclerosis. Our observations implicate mGluR1 and glutamate signaling as a promising new molecular target for the treatment of breast cancer. Even more promising, Riluzole, because it is an oral drug that can be administered with low toxicity, represents a promising approach in the treatment of triple negative breast cancer.

Network-guided analysis of genes with altered somatic copy number and gene expression reveals pathways commonly perturbed in metastatic melanoma

Cancer genomes frequently contain somatic copy number alterations (SCNA) that can significantly perturb the expression level of affected genes and thus disrupt pathways controlling normal growth. In melanoma, many studies have focussed on the copy number and gene expression levels of the BRAF, PTEN and MITF genes, but little has been done to identify new genes using these parameters at the genome-wide scale. Using karyotyping, SNP and CGH arrays, and RNA-seq, we have identified SCNA affecting gene expression ('SCNA-genes') in seven human metastatic melanoma cell lines. We showed that the combination of these techniques is useful to identify candidate genes potentially involved in tumorigenesis. Since few of these alterations were recurrent across our samples, we used a protein network-guided approach to determine whether any pathways were enriched in SCNA-genes in one or more samples. From this unbiased genome-wide analysis, we identified 28 significantly enriched pathway modules. Comparison with two large, independent melanoma SCNA datasets showed less than 10% overlap at the individual gene level, but network-guided analysis revealed 66% shared pathways, including all but three of the pathways identified in our data. Frequently altered pathways included WNT, cadherin signalling, angiogenesis and melanogenesis. Additionally, our results emphasize the potential of the EPHA3 and FRS2 gene products, involved in angiogenesis and migration, as possible therapeutic targets in melanoma. Our study demonstrates the utility of network-guided approaches, for both large and small datasets, to identify pathways recurrently perturbed in cancer.

Chemical-genetic screen identifies riluzole as an enhancer of Wnt/β-catenin signaling in melanoma

To identify new protein and pharmacological regulators of Wnt/β-catenin signaling, we used a cell-based reporter assay to screen a collection of 1857 human-experienced compounds for their ability to enhance activation of the β-catenin reporter by a low concentration of WNT3A. This identified 44 unique compounds, including the FDA-approved drug riluzole, which is presently in clinical trials for treating melanoma. We found that treating melanoma cells with riluzole in vitro enhances the ability of WNT3A to regulate gene expression, to promote pigmentation, and to decrease cell proliferation. Furthermore riluzole, like WNT3A, decreases metastases in a mouse melanoma model. Interestingly, siRNAs targeting the metabotropic glutamate receptor, GRM1, a reported indirect target of riluzole, enhance β-catenin signaling. The unexpected regulation of β-catenin signaling by both riluzole and GRM1 has implications for the future uses of this drug.

In silico repositioning of approved drugs for rare and neglected diseases

One approach to speed up drug discovery is to examine new uses for existing approved drugs, so-called 'drug repositioning' or 'drug repurposing', which has become increasingly popular in recent years. Analysis of the literature reveals many examples of US Food and Drug Administration-approved drugs that are active against multiple targets (also termed promiscuity) that can also be used to therapeutic advantage for repositioning for other neglected and rare diseases. Using proof-of-principle examples, we suggest here that with current in silico technologies and databases of the structures and biological activities of chemical compounds (drugs) and related data, as well as close integration with in vitro screening data, improved opportunities for drug repurposing will emerge for neglected or rare/orphan diseases.

Riluzole enhances ionizing radiation-induced cytotoxicity in human melanoma cells that ectopically express metabotropic glutamate receptor 1 in vitro and in vivo

PURPOSE

Brain metastases are a common preterminal event in patients with metastatic melanoma and require radiation therapy. Our group has previously shown that human GRM1 (hGRM1) expressing melanoma cells release excess extracellular glutamate and are growth inhibited by riluzole, an inhibitor of glutamate release. Riluzole-treated cells accumulate in G(2)/M phase of the cell cycle at 24 hours, and then undergo apoptotic cell death. We evaluated whether riluzole enhanced radiosensitivity in melanoma cells.

EXPERIMENTAL DESIGN

Clonogenic assays were performed to evaluate clonogenic survival after treatment in hGRM1 expressing and nonexpressing melanoma cells. Western immunoblots were performed to confirm apoptotic cell death. A xenograft mouse model was used to validate the in vitro experiments. Tumors harvested from the xenografts were fixed and stained for apoptosis and DNA damage markers.

RESULTS

In the hGRM1-positive cell lines C8161 and UACC903, riluzole enhanced the lethal effects of ionizing radiation; no difference was seen in the hGRM1-negative UACC930 cell line. C8161 cells treated with riluzole plus irradiation also showed the highest levels of the cleaved forms of PARP and caspase-3; excised C8161 xenografts showed the greatest number of apoptotic cells by immunohistochemistry (P < 0.001). On cell cycle analysis, a sequence-dependent enrichment in the G(2)/M phase was shown with the combination of riluzole and irradiation. Xenografts treated with riluzole and weekly radiation fractions showed significant growth inhibition and revealed markedly increased DNA damage.

CONCLUSIONS

We have shown, in vitro and in vivo, that the combination of riluzole and ionizing radiation leads to greater cytotoxicity. These results have clinical implications for patients with brain metastases receiving whole brain radiation therapy.

Peripheral glutamate signaling in head and neck areas

The major excitatory neurotransmitter glutamate is also found in the periphery in an increasing number of nonexcitable cells. In line with this it became apparent that glutamate can regulate a broad array of peripheral biological responses, as well. Of particular interest is the discovery that glutamate receptor reactive reagents can influence tumor biology. However, the knowledge of glutamate signaling in peripheral tissues is still incomplete and, in the case of head and neck areas, is almost lacking. The roles of glutamate signaling pathways in these regions are manifold and include orofacial pain, periodontal bone production, skin and airway inflammation, as well as salivation. Furthermore, the interrelations between glutamate and cancers in the oral cavity, thyroid gland, and other regions are discussed. In summary, this review shall strengthen the view that glutamate receptor reagents may also be promising targets for novel therapeutic concepts suitable for a number of diseases in peripheral tissues. The contents of this review cover the following sections: Introduction; The "Glutamate System"; The Taste of Glutamate; Glutamate Signaling in Dental Regions; Glutamate Signaling in Head and Neck Areas; Glutamate Signaling in Head and Neck Cancer; A Brief Overview of Glutamate Signaling in Other Cancers; and Conclusion.

Exciting times beyond the brain: metabotropic glutamate receptors in peripheral and non-neural tissues

Metabotropic glutamate (mGlu) receptors are G-protein-coupled receptors expressed primarily on neurons and glial cells, where they are located in the proximity of the synaptic cleft. In the central nervous system (CNS), mGlu receptors modulate the effects of l-glutamate neurotransmission in addition to that of a variety of other neurotransmitters. However, mGlu receptors also have a widespread distribution outside the CNS that has been somewhat neglected to date. Based on this expression, diverse roles of mGlu receptors have been suggested in a variety of processes in health and disease including controlling hormone production in the adrenal gland and pancreas, regulating mineralization in the developing cartilage, modulating lymphocyte cytokine production, directing the state of differentiation in embryonic stem cells, and modulating gastrointestinal secretory function. Understanding the role of mGlu receptors in the periphery will also provide a better insight into potential side effects of drugs currently being developed for neurological and psychiatric conditions. This review summarizes the new potential roles of mGlu receptors and raises the possibility of novel pharmacological targets for various disorders.

Discovery and preclinical validation of salivary transcriptomic and proteomic biomarkers for the non-invasive detection of breast cancer

Background

A sensitive assay to identify biomarkers using non-invasively collected clinical specimens is ideal for breast cancer detection. While there are other studies showing disease biomarkers in saliva for breast cancer, our study tests the hypothesis that there are breast cancer discriminatory biomarkers in saliva using de novo discovery and validation approaches. This is the first study of this kind and no other study has engaged a de novo biomarker discovery approach in saliva for breast cancer detection. In this study, a case-control discovery and independent preclinical validations were conducted to evaluate the performance and translational utilities of salivary transcriptomic and proteomic biomarkers for breast cancer detection.

Methodology/Principal Findings

Salivary transcriptomes and proteomes of 10 breast cancer patients and 10 matched controls were profiled using Affymetrix HG-U133-Plus-2.0 Array and two-dimensional difference gel electrophoresis (2D-DIGE), respectively. Preclinical validations were performed to evaluate the discovered biomarkers in an independent sample cohort of 30 breast cancer patients and 63 controls using RT-qPCR (transcriptomic biomarkers) and quantitative protein immunoblot (proteomic biomarkers). Transcriptomic and proteomic profiling revealed significant variations in salivary molecular biomarkers between breast cancer patients and matched controls. Eight mRNA biomarkers and one protein biomarker, which were not affected by the confounding factors, were pre-validated, yielding an accuracy of 92% (83% sensitive, 97% specific) on the preclinical validation sample set.

Conclusions

Our findings support that transcriptomic and proteomic signatures in saliva can serve as biomarkers for the non-invasive detection of breast cancer. The salivary biomarkers possess discriminatory power for the detection of breast cancer, with high specificity and sensitivity, which paves the way for prediction model validation study followed by pivotal clinical validation.

Integrated analysis of mutations, miRNA and mRNA expression in glioblastoma

BACKGROUND

Glioblastoma arises from complex interactions between a variety of genetic alterations and environmental perturbations. Little attention has been paid to understanding how genetic variations, altered gene expression and microRNA (miRNA) expression are integrated into networks which act together to alter regulation and finally lead to the emergence of complex phenotypes and glioblastoma.

RESULTS

We identified association of somatic mutations in 14 genes with glioblastoma, of which 8 genes are newly identified, and association of loss of heterozygosity (LOH) is identified in 11 genes with glioblastoma, of which 9 genes are newly discovered. By gene coexpression network analysis, we identified 15 genes essential to the function of the network, most of which are cancer related genes. We also constructed miRNA coexpression networks and found 19 important miRNAs of which 3 were significantly related to glioblastoma patients' survival. We identified 3,953 predicted miRNA-mRNA pairs, of which 14 were previously verified by experiments in other groups. Using pathway enrichment analysis we also found that the genes in the target network of the top 19 important miRNAs were mainly involved in cancer related signaling pathways, synaptic transmission and nervous systems processes. Finally, we developed new methods to decipher the pathway connecting mutations, expression information and glioblastoma. We identified 4 cis-expression quantitative trait locus (eQTL): TP53, EGFR, NF1 and PIK3C2G; 262 trans eQTL and 26 trans miRNA eQTL for somatic mutation; 2 cis-eQTL: NRAP and EGFR; 409 trans- eQTL and 27 trans- miRNA eQTL for lost of heterozygosity (LOH) mutation.

CONCLUSIONS

Our results demonstrate that integrated analysis of multi-dimensional data has the potential to unravel the mechanism of tumor initiation and progression.

Metabotropic glutamate receptors: from the workbench to the bedside

Metabotropic glutamate (mGlu) receptors were discovered in the mid 1980s and originally described as glutamate receptors coupled to polyphosphoinositide hydrolysis. Almost 6500 articles have been published since then, and subtype-selective mGlu receptor ligands are now under clinical development for the treatment of a variety of disorders such as Fragile-X syndrome, schizophrenia, Parkinson's disease and L-DOPA-induced dyskinesias, generalized anxiety disorder, chronic pain, and gastroesophageal reflux disorder. Prof. Erminio Costa was linked to the early times of the mGlu receptor history, when a few research groups challenged the general belief that glutamate could only activate ionotropic receptors and all metabolic responses to glutamate were secondary to calcium entry. This review moves from those nostalgic times to the most recent advances in the physiology and pharmacology of mGlu receptors, and highlights the role of individual mGlu receptor subtypes in the pathophysiology of human disorders. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Pharmacogenomics of metabotropic glutamate receptor subtype 1 and in vivo malignant melanoma formation

We have previously shown that ectopic expression of metabotropic glutamate receptor subtype 1 in melanocytes is essential for both development and in vivo growth of melanoma using newly developed transgenic mice which conditionally express metabotropic glutamate receptor subtype 1 (mGluR1). In this study, we developed conditional transgenic mice, which harbor melanocytes not only in the dermis and hair follicles but also in the epidermis using stem cell factor transgenic mice. Pigmented plaques on the backs, tails, ears or groins of the transgenic mice began to appear 13 weeks after activation of the mGluR1 transgene, and the transgenic mice produced melanomas at a frequency of 100% 36 weeks after transgene activation. Although this transgenic mouse harbors melanocytes in the epidermis, proliferation of melanoma cells took place in the dermis. To elucidate the signals involved in development and growth of melanoma, inhibitors to phospholipase C, protein kinase C and mitogen-activated protein kinase kinase 1/2, and antagonists to Ca(2+) and calmodulin were administrated to transgenic mice. Each signal inhibitor to phospholipase, protein kinase C, Ca(2+) release, calmodulin and mitogen-activated protein kinase kinase 1/2 inhibited melanoma development. However, once melanoma was developed, the growth of melanoma was dramatically inhibited only by the inhibitor to mitogen-activated protein kinase kinase 1/2 with partial inhibition by inhibitors to protein kinase C and phospholipase C. This inhibition of melanoma growth was well correlated with the expression of phosphorylated extracellular signal-regulated kinase 1/2 and Ki-67. These results indicate that for development of melanoma, activation of every signaling pathway from mGluR1 is required. However, for growth of melanoma, the extracellular signal-regulated kinase pathway plays a key role.

Role of the G Protein-Coupled Receptor, mGlu1, in Melanoma Development

Melanoma remains one of the cancers for which a decline in morbidity has not been achieved with current scientific and medical advances. Mono-therapies targeting melanoma have been largely ineffective, increasing the need for identification of new drugable targets. Multiple tumor suppressors and oncogenes that impart genetic predisposition to melanoma have been identified and are being studied in an attempt to provide insight on the development of anti-melanoma therapies. Metabotropic Glutamate Receptor I (GRM1) has recently been implicated as a novel oncogene involved in melanomagenesis. GRM1 (mGlu₁, protein) belongs to the G protein coupled receptor (GPCR) super family and is normally functional in the central nervous system. Our group showed in a transgenic mouse model system that ectopic expression of Grm1 in melanocytes is sufficient to induce spontaneous melanoma development in vivo. GPCRs are some of the most important therapeutic drug targets discovered to date and they make up a significant proportion of existing therapies. This super family of transmembrane receptors has wide spread expression and interacts with a diverse array of ligands. Diverse physiological responses can be induced by stimulator(s) or suppressor(s) of GPCRs, which contributes to their attractiveness in existing and emerging therapies. GPCR targeting therapies are employed against a variety of human disorders including those of the central nervous system, cardiovascular, metabolic, urogenital and respiratory systems. In the current review, we will discuss how the identification of the oncogenic properties of GRM1 opens up new strategies for the design of potential novel therapies for the treatment of melanoma.

The glutamate release inhibitor Riluzole decreases migration, invasion, and proliferation of melanoma cells

The goal of this study was to examine the effects of metabotropic glutamate receptor-1 (GRM1) blockade on melanoma anchorage-independent growth and invasion. We performed colony and invasion assays using GRM1-expressing melanoma lines and the GRM1-negative UACC930 line. Using the glutamate-release inhibitor Riluzole or the non-competitive GRM1 antagonist BAY 36-7620 we were able to induce considerable inhibition of colony formation and invasion in GRM1-expressing melanoma lines. Neither pharmacological agent induced significant reduction in colony formation or invasion in the GRM1-negative melanoma line, UACC930. Additionally we assessed the efficacy of these inhibitors to inhibit the growth of fresh melanoma tumor samples cultured on a 74-mum nylon mesh. Both Riluzole and BAY 36-7620 significantly inhibited tumor cell growth into the interstitial spaces of the mesh. When repeated with normal mole samples both inhibitors were much less effective in preventing the outgrowth of cells. These experiments show that a specific antagonist of GRM1 (BAY 36-7620) or an inhibitor of glutamate release (Riluzole) can significantly suppress melanoma migration, invasion and colony formation as well as inhibit the proliferation of fresh melanoma cells. These findings, added to our previous work, strengthen the case that GRM1 is a valid therapeutic target in patients with melanoma.

AKT2 is a downstream target of metabotropic glutamate receptor 1 (Grm1)

We reported earlier on the oncogenic properties of Grm1 by demonstrating that stable Grm1-mouse-melanocytic clones proliferate in the absence of growth supplement and anchorage in vitro. In addition, these clones also exhibit aggressive tumorigenic phenotypes in vivo with short latency in tumor formation in both immunodeficient and syngeneic mice. We also detected strong activation of AKT in allograft tumors specifically AKT2 as the predominant isoform involved. In parallel, we assessed several human melanoma biopsy samples and found again that AKT2 was the predominantly activated AKT in these human melanoma biopsies. In cultured stable Grm1-mouse-melanocytic clones, as well as an metabotropic glutamate receptor 1 (Grm1) expressing human melanoma cell line, C8161, stimulation of Grm1 by its agonist led to the activation of AKT, while preincubation with Grm1-antagonist abolished Grm1-agonist-induced AKT activation. In addition, a reduction in tumor volume of Grm1-mouse-melanocytic-allografts was detected in the presence of small interfering AKT2 RNA (siAKT2). Taken together, these results showed that, in addition to the MAPK pathway previously reported being a downstream target of stimulated Grm1, AKT2 is another downstream target in Grm1 mediated melanocyte transformation.

A phase 0 trial of riluzole in patients with resectable stage III and IV melanoma

PURPOSE

Ectopic expression of GRM1 in murine melanocytes results in transformation into a form of melanoma, and more than 60% of human melanoma samples tested ectopically express GRM1. Stimulation of this receptor in vitro results in up-regulation of activated extracellular signal-regulated kinase (ERK). Furthermore, a xenograft model of melanoma treated with riluzole, an oral GRM1 blocking agent, showed decreased tumor growth compared with the untreated controls. We have now completed a phase 0 trial of riluzole in patients with melanoma.

EXPERIMENTAL DESIGN

Patients enrolled on this trial underwent a pretreatment biopsy, took 200 mg of oral riluzole per day for 14 days, and then underwent resection of their remaining tumor. We compared the levels of pERK and pAKT in the pretreatment and post-treatment samples and assessed the metabolic activity of pretreatment and post-treatment tumors using fluorodeoxyglucose positron emission tomography (FDG-PET) scanning.

RESULTS

We accrued 12 patients and all expressed GRM1. We found a significant decrease in pAKT and/or pERK in post-treatment tumor samples as compared with pretreatment samples in 4 (34%) patients. These four patients had a significant decrease in FDG-PET intensity post-treatment as well. Two other patients had a clinical response with no corresponding metabolic response; five patients had similar pretreatment and post-treatment FDG-PET scan findings; and one patient had progressive disease.

CONCLUSIONS

Our data show that glutamate blockade with riluzole can inhibit signaling through the mitogen-activated protein kinase and phosphatidylinositol 3-kinase/AKT pathways and suppress the metabolic activity of melanoma. The ectopic expression of metabotropic glutamate receptors may be important in the pathogenesis of human melanoma, and targeting this pathway may be an effective therapy.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Enhanced endogenous type I interferon cell-driven survival and inhibition of spontaneous apoptosis by Riluzole

Highly active antiretroviral therapy (HAART), although effective in improving the survival of HIV-1-infected individuals, has not been able to reconstitute the adaptive immune response. We have described the use of novel chemical agents to restore T-cell survival/proliferation by inducing cytokine production. Due to its cationic amphiphilic structure, these molecules appear to enhance immune restoration. In this study, we investigated the action of Riluzole (2-amino-6-trifuromethoxybenzothiazole) in HIV-1 infection. Riluzole is able to increase (effective dose from 1 to 1000 nM) the cell-survival of T cells from HIV-1-infected patients and inhibit spontaneous apoptosis. The immunomodulatory effect of riluzole-sensitized cells was ascribed to endogenous type I interferon (IFN) derived from monocytes. Riluzole might be used for restoring the cell survival of immunocompromised patients and eliminating latent infected cells upon HIV-1 reactivation.

Metabotropic glutamate receptor subtype-1 is essential for in vivo growth of melanoma

Ectopic expression of metabotropic glutamate receptor subtype 1 (mGluR1) in mouse melanocytes induces melanoma formation. Although requirement of mGluR1 for development of melanoma in the initial stage has been demonstrated, its role in melanoma growth in vivo remains unclear. In this study, we developed novel transgenic mice that conditionally express mGluR1 in melanocytes, using a tetracycline regulatory system. Pigmented lesions on the ears and tails of the transgenic mice began to appear 29 weeks after activation of the mGluR1 transgene, and the transgenic mice produced melanomas at a frequency of 100% 52 weeks after transgene activation. Subsequent inactivation of the mGluR1 transgene in melanoma-bearing mice inhibited melanoma growth with reduction of immunoreactivity to phosphorylated ERK1/2, whereas mice with persistent expression of mGluR1 developed larger melanoma burdens. mGluR1 expression is thus required not only for melanoma development but also for melanoma growth in vivo. These findings suggest that growth of melanoma can be inhibited in vivo by eliminating only one of the multiple genetic anomalies involved in tumorigenesis.

G-protein-coupled receptors and melanoma

G-protein-coupled receptors (GPCR) are the largest family of receptors with over 500 members. Evaluation of GPCR gene expression in primary human tumors identified over-expression of GPCR in several tumor types. Analysis of cancer samples in different disease stages also suggests that some GPCR may be involved in early tumor progression and others may play a critical role in tumor invasion and metastasis. Currently, >50% of drug targets to various human diseases are based on GPCR. In this review, the relationships between several GPCR and melanoma development and/or progression will be discussed. Finally, the possibility of using one or more of these GPCR as therapeutic targets in melanoma will be summarized.

Metabotropic glutamate receptors (mGlus) and cellular transformation

Although the glutamatergic system usually functions in the CNS, expression has been observed in non-neuronal tissues and a subset of cancers. Metabotropic glutamate receptors (mGlus) are highly "druggable" GPCRs and thus a priority for validation as therapeutic targets. We have previously reported that the aberrant expression of mGlu1 is sufficient to induce spontaneous melanoma development in vivo. We isolated and characterized several stable mGlu1-mouse melanocytic clones and demonstrated that these clones are transformed and tumorigenic. We hypothesize that expression of mGlus may not be uncommon in the pathogenesis of tumors other than melanoma, and that activity of an otherwise normal glutamate receptor in an ectopic cellular environment involves signaling pathways which dysregulate cell growth, ultimately leading to tumorigenesis. As most human cancers are of epithelial origin (carcinomas), in this review, the possibility that mGlu1 could function as a complete oncogene and transform epithelial cells is also discussed.