Lack of Extracellular Signal-Regulated Kinase Mitogen-Activated Protein Kinase Signaling Shows a New Type of Melanoma

Piperine-Chlorogenic Acid Hybrid Inhibits the Proliferation of the SK-MEL-147 Melanoma Cells by Modulating Mitotic Kinases

Melanoma is considered the most aggressive form of skin cancer, showing high metastatic potential and persistent high mortality rates despite the introduction of immunotherapy and targeted therapies. Thus, it is important to identify new drug candidates for melanoma. The design of hybrid molecules, with different pharmacophore fragments combined in the same scaffold, is an interesting strategy for obtaining new multi-target and more effective anticancer drugs. We designed nine hybrid compounds bearing piperine and chlorogenic acid pharmacophoric groups and evaluated their antitumoral potential on melanoma cells with distinct mutational profiles SK-MEL-147, CHL-1 and WM1366. We identified the compound named PQM-277 (3a) to be the most cytotoxic one, inhibiting mitosis progression and promoting an accumulation of cells in pro-metaphase and metaphase by altering the expression of genes that govern G2/M transition and mitosis onset. Compound 3a downregulated FOXM1, CCNB1, CDK1, AURKA, AURKB, and PLK1, and upregulated CDKN1A. Molecular docking showed that 3a could interact with the CUL1-RBX1 complex, which activity is necessary to trigger molecular events essential for FOXM1 transactivation and, in turn, G2/M gene expression. In addition, compound 3a effectively induced apoptosis by increasing BAX/BCL2 ratio. Our findings demonstrate that 3a is an important antitumor candidate prototype and support further investigations to evaluate its potential for melanoma treatment, especially for refractory cases to BRAF/MEK inhibitors.

Associations between exercise capacity, p16INK4a expression and inflammation among adult survivors of childhood cancer

BACKGROUND

Over 50% of childhood cancer survivors are exercise intolerant, with maximal aerobic capacities comparable to individuals decades older, suggesting early physiologic ageing. In addition, 36% of survivors are obese. Optimal exercise capacity provides a foundation to support daily function and healthy body habitus and is associated with benefits to cognition, cardiovascular health, and longevity. Cellular senescence and inflammation are key mechanisms that drive age-related disease, quantifiable as biomarkers in peripheral blood.

AIMS

This study aimed to evaluate associations between p16INKa, a biomarker of cellular senescence, and inflammation and exercise capacity among adult survivors of childhood cancer.

MATERIALS AND METHODS

Eligible survivors were recruited from the St. Jude Lifetime (SJLIFE) Cohort Study. Exercise capacity was assessed by maximal oxygen uptake (VO2, ml/kg/min) obtained via cardiopulmonary exercise testing using a modified Bruce protocol. Body fat (%) was determined from dual energy x-ray absorptiometry (DEXA). Peripheral blood samples were used to evaluate log2 p16INK4a mRNA expression, a biomarker of cellular senescence, and inflammation with high sensitivity C-reactive protein (hs-CRP) levels. Multivariable regression evaluated associations between p16INK4a, hs-CRP, body fat, and exercise capacity.

RESULTS

Participants included 185 five-year childhood cancer survivors (mean age 36.6 [range 20.1 - 55.7] years, 44% male, 77% non-Hispanic white, 53% leukemia/lymphoma). Compared to males, females had lower peak VO2 (mean ± SD, 22.5 ± 8.2 vs. 28.8 ± 7.7 ml/kg/min, p<0.01), higher p16INK4a expression (9.6 ± 1.2 vs. 9.2 ± 1.2 fold, p=0.02), and hs-CRP concentration (5.9 ± 8.4 vs. 3.3 ± 3.9 mg/L, p=0.01). Among females (n=103), hs-CRP concentration (β -0.2, 95% CI -0.34 to -0.05, p=0.01) and p16INK4a expression (β-5.32, 95% CI 10.42 to -0.22, p=0.04) were inversely associated and statistically significant with peak exercise capacity, with a significant interaction between p16INK4a expression and body fat (β 0.15, 95% CI 0.02 to 0.28, p=0.03). Among males (n=82), p16INK4a expression (β -1.01, 95% CI -2.14 to 0.12, p=0.08), and body fat (β -0.54, 95% CI -0.70 to -0.38, p<0.01) were inversely associated with peak exercise capacity.

CONCLUSION

Inflammation and p16INK4a expression, a biomarker of cellular senescence, are associated with lower exercise capacity in childhood cancer survivors, suggesting potential targets or outcome measures for interventions designed to prevent or remediate accelerated physiologic ageing in this population.

Circadian clock protein BMAL1 regulates melanogenesis through MITF in melanoma cells

Solar ultraviolet B radiation (UVB) is one of the leading causes of various skin conditions, including photoaging, sunburn erythema, and melanoma. As a protective response, the skin has inbuilt defense mechanisms, including DNA repair, cell cycle, apoptosis, and melanin synthesis. Though DNA repair, cell cycle, and apoptosis are clock controlled, the circadian mechanisms associated with melanin synthesis are not well understood. Using human melanocytes and melanoma cells under synchronized clock conditions, we observed that the microphthalmia-associated transcription factor (MITF), a rate-limiting protein in melanin synthesis, is expressed rhythmically with 24-hr periodicity in the presence of circadian clock protein, BMAL1. Furthermore, we demonstrated that BMAL1 binds to the promoter region of MITF and transcriptionally regulates its expression, which positively influences melanin synthesis. Finally, we report that an increase in melanin levels due to BMAL1 overexpression protects human melanoma cells from UVB. In conclusion, our studies provide novel insights into the mechanistic role of the circadian clock in melanin synthesis and protection against UVB-mediated DNA damage and genomic instability.

CRN2 binds to TIMP4 and MMP14 and promotes perivascular invasion of glioblastoma cells

CRN2 is an actin filament binding protein involved in the regulation of various cellular processes including cell migration and invasion. CRN2 has been implicated in the malignant progression of different types of human cancer. We used CRN2 knock-out mice for analyses as well as for crossbreeding with a Tp53/Pten knock-out glioblastoma mouse model. CRN2 knock-out mice were subjected to a phenotyping screen at the German Mouse Clinic. Murine glioblastoma tissue specimens as well as cultured murine brain slices and glioblastoma cell lines were investigated by immunohistochemistry, immunofluorescence, and cell biological experiments. Protein interactions were studied by immunoprecipitation, pull-down, and enzyme activity assays. CRN2 knock-out mice displayed neurological and behavioural alterations, e.g. reduced hearing sensitivity, reduced acoustic startle response, hypoactivity, and less frequent urination. While glioblastoma mice with or without the additional CRN2 knock-out allele exhibited no significant difference in their survival rates, the increased levels of CRN2 in transplanted glioblastoma cells caused a higher tumour cell encasement of murine brain slice capillaries. We identified two important factors of the tumour microenvironment, the tissue inhibitor of matrix metalloproteinase 4 (TIMP4) and the matrix metalloproteinase 14 (MMP14, synonym: MT1-MMP), as novel binding partners of CRN2. All three proteins mutually interacted and co-localised at the front of lamellipodia, and CRN2 was newly detected in exosomes. On the functional level, we demonstrate that CRN2 increased the secretion of TIMP4 as well as the catalytic activity of MMP14. Our results imply that CRN2 represents a pro-invasive effector within the tumour cell microenvironment of glioblastoma multiforme.

ERK/MAPK Signaling Drives Overexpression of the Rac-GEF, PREX1, in BRAF- and NRAS-Mutant Melanoma

Recently, we identified that PREX1 overexpression is critical for metastatic but not tumorigenic growth in a mouse model of NRAS-driven melanoma. In addition, a PREX1 gene signature correlated with and was dependent on ERK MAPK activation in human melanoma cell lines. In the current study, the underlying mechanism of PREX1 overexpression in human melanoma was assessed. PREX1 protein levels were increased in melanoma tumor tissues and cell lines compared with benign nevi and normal melanocytes, respectively. Suppression of PREX1 by siRNA impaired invasion but not proliferation in vitro PREX1-dependent invasion was attributable to PREX1-mediated activation of the small GTPase RAC1 but not the related small GTPase CDC42. Pharmacologic inhibition of ERK signaling reduced PREX1 gene transcription and additionally regulated PREX1 protein stability. This ERK-dependent upregulation of PREX1 in melanoma, due to both increased gene transcription and protein stability, contrasts with the mechanisms identified in breast and prostate cancers, in which PREX1 overexpression was driven by gene amplification and HDAC-mediated gene transcription, respectively. Thus, although PREX1 expression is aberrantly upregulated and regulates RAC1 activity and invasion in these three different tumor types, the mechanisms of its upregulation are distinct and context dependent.

IMPLICATIONS

This study identifies an ERK-dependent mechanism that drives PREX1 upregulation and subsequent RAC1-dependent invasion in BRAF- and NRAS-mutant melanoma. Mol Cancer Res; 14(10); 1009-18. ©2016 AACR.

Protein Kinase CK2α Maintains Extracellular Signal-regulated Kinase (ERK) Activity in a CK2α Kinase-independent Manner to Promote Resistance to Inhibitors of RAF and MEK but Not ERK in BRAF Mutant Melanoma

The protein kinase casein kinase 2 (CK2) is a pleiotropic and constitutively active kinase that plays crucial roles in cellular proliferation and survival. Overexpression of CK2, particularly the α catalytic subunit (CK2α, CSNK2A1), has been implicated in a wide variety of cancers and is associated with poorer survival and resistance to both conventional and targeted anticancer therapies. Here, we found that CK2α protein is elevated in melanoma cell lines compared with normal human melanocytes. We then tested the involvement of CK2α in drug resistance to Food and Drug Administration-approved single agent targeted therapies for melanoma. In BRAF mutant melanoma cells, ectopic CK2α decreased sensitivity to vemurafenib (BRAF inhibitor), dabrafenib (BRAF inhibitor), and trametinib (MEK inhibitor) by a mechanism distinct from that of mutant NRAS. Conversely, knockdown of CK2α sensitized cells to inhibitor treatment. CK2α-mediated RAF-MEK kinase inhibitor resistance was tightly linked to its maintenance of ERK phosphorylation. We found that CK2α post-translationally regulates the ERK-specific phosphatase dual specificity phosphatase 6 (DUSP6) in a kinase dependent-manner, decreasing its abundance. However, we unexpectedly showed, by using a kinase-inactive mutant of CK2α, that RAF-MEK inhibitor resistance did not rely on CK2α kinase catalytic function, and both wild-type and kinase-inactive CK2α maintained ERK phosphorylation upon inhibition of BRAF or MEK. That both wild-type and kinase-inactive CK2α bound equally well to the RAF-MEK-ERK scaffold kinase suppressor of Ras 1 (KSR1) suggested that CK2α increases KSR facilitation of ERK phosphorylation. Accordingly, CK2α did not cause resistance to direct inhibition of ERK by the ERK1/2-selective inhibitor SCH772984. Our findings support a kinase-independent scaffolding function of CK2α that promotes resistance to RAF- and MEK-targeted therapies.

Genetic Testing in the Multidisciplinary Management of Melanoma

Melanoma is increasing in incidence and represents an aggressive type of cancer. Efforts have focused on identifying genetic factors in melanoma carcinogenesis to guide prevention, screening, early detection, and targeted therapy. This article reviews the hereditary risk factors associated with melanoma and the known molecular pathways and genetic mutations associated with this disease. This article also explores the controversies associated with genetic testing and the latest advances in identifying genetic targets in melanoma, which offer promise for future application in the multidisciplinary management of melanoma.

Genomic Classification of Cutaneous Melanoma

We describe the landscape of genomic alterations in cutaneous melanomas through DNA, RNA, and protein-based analysis of 333 primary and/or metastatic melanomas from 331 patients. We establish a framework for genomic classification into one of four subtypes based on the pattern of the most prevalent significantly mutated genes: mutant BRAF, mutant RAS, mutant NF1, and Triple-WT (wild-type). Integrative analysis reveals enrichment of KIT mutations and focal amplifications and complex structural rearrangements as a feature of the Triple-WT subtype. We found no significant outcome correlation with genomic classification, but samples assigned a transcriptomic subclass enriched for immune gene expression associated with lymphocyte infiltrate on pathology review and high LCK protein expression, a T cell marker, were associated with improved patient survival. This clinicopathological and multi-dimensional analysis suggests that the prognosis of melanoma patients with regional metastases is influenced by tumor stroma immunobiology, offering insights to further personalize therapeutic decision-making.

Approaches of targeting Rho GTPases in cancer drug discovery

INTRODUCTION

Rho GTPases are master regulators of actomyosin structure and dynamics and play pivotal roles in a variety of cellular processes including cell morphology, gene transcription, cell cycle progression, and cell adhesion. Because aberrant Rho GTPase signaling activities are widely associated with human cancer, key components of Rho GTPase signaling pathways have attracted increasing interest as potential therapeutic targets. Similar to Ras, Rho GTPases themselves were, until recently, deemed "undruggable" because of structure-function considerations. Several approaches to interfere with Rho GTPase signaling have been explored and show promise as new ways for tackling cancer cells.

AREAS COVERED

This review focuses on the recent progress in targeting the signaling activities of three prototypical Rho GTPases, that is, RhoA, Rac1, and Cdc42. The authors describe the involvement of these Rho GTPases, their key regulators and effectors in cancer. Furthermore, the authors discuss the current approaches for rationally targeting aberrant Rho GTPases along their signaling cascades, upstream and downstream of Rho GTPases, and posttranslational modifications at a molecular level.

EXPERT OPINION

To date, while no clinically effective drugs targeting Rho GTPase signaling for cancer treatment are available, tool compounds and lead drugs that pharmacologically inhibit Rho GTPase pathways have shown promise. Small-molecule inhibitors targeting Rho GTPase signaling may add new treatment options for future precision cancer therapy, particularly in combination with other anti-cancer agents.

Identification of a novel MTOR activator and discovery of a competing endogenous RNA regulating autophagy in vascular endothelial cells

MTOR, a central regulator of autophagy, is involved in cancer and cardiovascular and neurological diseases. Modulating the MTOR signaling balance could be of great significance for numerous diseases. No chemical activators of MTOR have been found, and the urgent challenge is to find novel MTOR downstream components. In previous studies, we found a chemical small molecule, 3-benzyl-5-((2-nitrophenoxy) methyl)-dihydrofuran-2(3H)-one (3BDO), that inhibited autophagy in human umbilical vein endothelial cells (HUVECs) and neuronal cells. Here, we found that 3BDO activated MTOR by targeting FKBP1A (FK506-binding protein 1A, 12 kDa). We next used 3BDO to detect novel factors downstream of the MTOR signaling pathway. Activation of MTOR by 3BDO increased the phosphorylation of TIA1 (TIA1 cytotoxic granule-associated RNA binding protein/T-cell-restricted intracellular antigen-1). Finally, we used gene microarray, RNA interference, RNA-ChIP assay, bioinformatics, luciferase reporter assay, and other assays and found that 3BDO greatly decreased the level of a long noncoding RNA (lncRNA) derived from the 3' untranslated region (3'UTR) of TGFB2, known as FLJ11812. TIA1 was responsible for processing FLJ11812. Further experiments results showed that FLJ11812 could bind with MIR4459 targeting ATG13 (autophagy-related 13), and ATG13 protein level was decreased along with 3BDO-decreased FLJ11812 level. Here, we provide a new activator of MTOR, and our findings highlight the role of the lncRNA in autophagy.

AC-93253 triggers the downregulation of melanoma progression markers and the inhibition of melanoma cell proliferation

A major challenge in anti-melanoma therapy is to develop treatments that are effective for advanced melanoma patients. Unfortunately, the currently used regimens are not efficient and have unsatisfactory effects on disease progression, thus increasing the pressure to develop new, profitable drugs and to identify new molecular targets. Here, we show for the first time that AC-93253, a SIRT2 inhibitor, exerts a negative effect on the expression of a set of genes involved in the progression and chemoresistance (e.g., oncogenes, apoptosis-related genes, ABC transporter genes, and cell cycle control genes) of melanoma cells. Furthermore, melanoma cells exposed to AC-93253 and doxorubicin displayed altered biological responses, including apoptosis and proliferation, compared to cells exposed to single treatments. Taken together, we conclude that the usage of AC-93253 in combined therapy could be a promising strategy for melanoma patients.

Mutation-specific RAS oncogenicity explains NRAS codon 61 selection in melanoma

NRAS mutation at codons 12, 13, or 61 is associated with transformation; yet, in melanoma, such alterations are nearly exclusive to codon 61. Here, we compared the melanoma susceptibility of an NrasQ61R knock-in allele to similarly designed KrasG12D and NrasG12D alleles. With concomitant p16INK4a inactivation, KrasG12D or NrasQ61R expression efficiently promoted melanoma in vivo, whereas NrasG12D did not. In addition, NrasQ61R mutation potently cooperated with Lkb1/Stk11 loss to induce highly metastatic disease. Functional comparisons of NrasQ61R and NrasG12D revealed little difference in the ability of these proteins to engage PI3K or RAF. Instead, NrasQ61R showed enhanced nucleotide binding, decreased intrinsic GTPase activity, and increased stability when compared with NrasG12D. This work identifies a faithful model of human NRAS-mutant melanoma, and suggests that the increased melanomagenecity of NrasQ61R over NrasG12D is due to heightened abundance of the active, GTP-bound form rather than differences in the engagement of downstream effector pathways.

SIGNIFICANCE

This work explains the curious predominance in human melanoma of mutations of codon 61 of NRAS over other oncogenic NRAS mutations. Using conditional "knock-in" mouse models, we show that physiologic expression of NRASQ61R, but not NRASG12D, drives melanoma formation.

Loss of keratinocytic RXRα combined with activated CDK4 or oncogenic NRAS generates UVB-induced melanomas via loss of p53 and PTEN in the tumor microenvironment

Understanding the molecular mechanisms behind formation of melanoma, the deadliest form of skin cancer, is crucial for improved diagnosis and treatment. One key is to better understand the cross-talk between epidermal keratinocytes and pigment-producing melanocytes. Here, using a bigenic mouse model system combining mutant oncogenic NRAS(Q61K) (constitutively active RAS) or mutant activated CDK4(R24C/R24C) (prevents binding of CDK4 by kinase inhibitor p16(INK4A)) with an epidermis-specific knockout of the nuclear retinoid X receptor alpha (RXRα(ep-/-)) results in increased melanoma formation after chronic ultraviolet-B (UVB) irradiation compared with control mice with functional RXRα. Melanomas from both groups of bigenic RXRα(ep-/-) mice are larger in size with higher proliferative capacity, and exhibit enhanced angiogenic properties and increased expression of malignant melanoma markers. Analysis of tumor adjacent normal skin from these mice revealed altered expression of several biomarkers indicative of enhanced melanoma susceptibility, including reduced expression of tumor suppressor p53 and loss of PTEN, with concomitant increase in activated AKT. Loss of epidermal RXRα in combination with UVB significantly enhances invasion of melanocytic cells to draining lymph nodes in bigenic mice expressing oncogenic NRAS(Q61K) compared with controls with functional RXRα. These results suggest a crucial role of keratinocytic RXRα to suppress formation of UVB-induced melanomas and their progression to malignant cancers in the context of driver mutations such as activated CDK4(R24C/R24C) or oncogenic NRAS(Q61K).

IMPLICATIONS

These findings suggest that RXRα may serve as a clinical diagnostic marker and therapeutic target in melanoma progression and metastasis.

Targeted next generation sequencing identifies clinically actionable mutations in patients with melanoma

Somatic sequencing of cancers has produced new insight into tumorigenesis, tumor heterogeneity, and disease progression, but the vast majority of genetic events identified are of indeterminate clinical significance. Here, we describe a NextGen sequencing approach to fully analyzing 248 genes, including all those of known clinical significance in melanoma. This strategy features solution capture of DNA followed by multiplexed, high-throughput sequencing and was evaluated in 31 melanoma cell lines and 18 tumor tissues from patients with metastatic melanoma. Mutations in melanoma cell lines correlated with their sensitivity to corresponding small molecule inhibitors, confirming, for example, lapatinib sensitivity in ERBB4 mutant lines and identifying a novel activating mutation of BRAF. The latter event would not have been identified by clinical sequencing and was associated with responsiveness to a BRAF kinase inhibitor. This approach identified focal copy number changes of PTEN not found by standard methods, such as comparative genomic hybridization (CGH). Actionable mutations were found in 89% of the tumor tissues analyzed, 56% of which would not be identified by standard-of-care approaches. This work shows that targeted sequencing is an attractive approach for clinical use in melanoma.

Mechanisms of chromosomal instability in melanoma

A systems biology approach was applied to investigate the mechanisms of chromosomal instability in melanoma cell lines. Chromosomal instability was quantified using array comparative genomic hybridization to identify somatic copy number alterations (deletions and duplications). Primary human melanocytes displayed an average of 8.5 alterations per cell primarily representing known polymorphisms. Melanoma cell lines displayed 25 to 131 alterations per cell, with an average of 68, indicative of chromosomal instability. Copy number alterations included approximately equal numbers of deletions and duplications with greater numbers of hemizygous (-1,+1) alterations than homozygous (-2,+2). Melanoma oncogenes, such as BRAF and MITF, and tumor suppressor genes, such as CDKN2A/B and PTEN, were included in these alterations. Duplications and deletions were functional as there were significant correlations between DNA copy number and mRNA expression for these genes. Spectral karyotype analysis of three lines confirmed extensive chromosomal instability with polyploidy, aneuploidy, deletions, duplications, and chromosome rearrangements. Bioinformatic analysis identified a signature of gene expression that was correlated with chromosomal instability but this signature provided no clues to the mechanisms of instability. The signature failed to generate a significant (P = 0.105) prediction of melanoma progression in a separate dataset. Chromosomal instability was not correlated with elements of DNA damage response (DDR) such as radiosensitivity, nucleotide excision repair, expression of the DDR biomarkers γH2AX and P-CHEK2, nor G1 or G2 checkpoint function. Chromosomal instability in melanoma cell lines appears to influence gene function but it is not simply explained by alterations in the system of DDR.

Genomewide RNAi screen identifies protein kinase Cb and new members of mitogen-activated protein kinase pathway as regulators of melanoma cell growth and metastasis

A large-scale RNAi screen was performed for eight different melanoma cell lines using a pooled whole-genome lentiviral shRNA library. shRNAs affecting proliferation of transduced melanoma cells were negatively selected during 10 days of culture. Overall, 617 shRNAs were identified by microarray hybridization. Pathway analyses identified mitogen-activated protein kinase (MAPK) pathway members such as ERK1/2, JNK1/2 and MAP3K7 and protein kinase C β (PKCβ) as candidate genes. Knockdown of PKCβ most consistently reduced cellular proliferation, colony formation and migratory capacity of melanoma cells and was selected for further validation. PKCβ showed enhanced expression in human primary melanomas and distant metastases as compared with benign melanocytic nevi. Moreover, treatment of melanoma cells with PKCβ-specific inhibitor enzastaurin reduced melanoma cell growth but had only small effects on benign fibroblasts. Finally, PKCβ-shRNA significantly reduced lung colonization capacity of stably transduced melanoma cells in mice. Taken together, this study identified new candidate genes for melanoma cell growth and proliferation. PKCβ seems to play an important role in these processes and might serve as a new target for the treatment of metastatic melanoma.

Genetics and genomics of melanoma

The rapidly increasing incidence of melanoma, coupled with its highly aggressive metastatic nature, is of urgent concern. In order to design rational therapies, it is of critical importance to identify the genetic determinants that drive melanoma formation and progression. To date, signaling cascades emanating from the EGF receptor, c-MET and other receptors are known to be altered in melanoma. Important mutations in signaling molecules, such as BRAF and N-RAS, have been identified. In this review, some of the major genetic alterations and signaling pathways involved in melanoma will be discussed. Given the great deal of genetic heterogeneity observed in melanoma, it is likely that many more genetic determinants exist. Through the use of powerful genomic technologies, it is now possible to identify these additional genetic alterations in melanoma. A critical step in this analysis will be culling bystanders from functionally important drivers, as this will highlight genetic elements that will be promising therapeutic targets. Such technologies and the important points to consider in understanding the genetics of melanoma will be reviewed.

Impact of MAPK Pathway Activation in BRAF(V600) Melanoma on T Cell and Dendritic Cell Function

Constitutive upregulation of the MAPK pathway by a BRAF^V600 mutation occurs in about half of melanomas. This leads to increased oncogenic properties such as tumor cell invasion, metastatic potential, and resistance to apoptosis. Blockade of the MAPK pathway with highly specific kinase inhibitors induces unprecedented tumor response rates in patients with advanced BRAF^V600 mutant melanoma. Immune checkpoint blockade with monoclonal antibodies targeting cytotoxic T-lymphocyte antigen 4 and programed death-1/PD-L1 has also demonstrated striking anti-tumor activity in patients with advanced melanoma. Tumor responses are likely limited by multiple additional layers of immune suppression in the tumor microenvironment. There is emerging preclinical and clinical evidence suggesting that MAPK inhibition has a beneficial effect on the immunosuppressive tumor microenvironment, providing a strong rationale for combined immunotherapy and MAPK pathway inhibition in melanoma. The T cell response has been the main focus in the studies reported to date. Since dendritic cells (DCs) are important in the induction of tumor-specific T cell responses, the impact of MAPK pathway activation in melanoma on DC function is critical for the melanoma directed immune response. BRAF^V600E melanoma cells modulate DCs through the MAPK pathway because its blockade in melanoma cells can reverse suppression of DC function. As both MEK/BRAF inhibition and immune checkpoint blockade have recently taken center stage in the treatment of melanoma, a deeper understanding of how MAPK pathway inhibition affects the tumor immune response is needed.

Molecular pathways: P-Rex in cancer

P-Rex proteins are Rho/Rac guanine nucleotide exchange factors that participate in the regulation of several cancer-related cellular functions such as proliferation, motility, and invasion. Expectedly, a significant portion of these actions of P-Rex proteins must be related to their Rac regulatory properties. In addition, P-Rex proteins control signaling by the phosphoinositide 3-kinase (PI3K) route by interacting with PTEN and mTOR. The interaction with PTEN inhibits its phosphatase activity, leading to AKT activation. The interaction with mTOR may be important in nutrient-stimulated Rac activation and migration. In humans, several studies have implicated P-Rex proteins in the pathophysiology of various neoplasias. Thus, overexpression of P-Rex proteins has been linked to poor patient outcome in breast cancer and may facilitate metastatic dissemination of prostate cancer cells. In addition, whole-genome sequencing described P-Rex2 as a significantly mutated gene in melanoma. Furthermore, expression in melanocytes of mutated forms of P-Rex2 found in patients with melanoma showed the protumorigenic role of these P-Rex mutations in melanoma genesis. These findings open interesting opportunities for P-Rex targeting in cancer. Moreover, the implication of P-Rex partner proteins such as Rac, mTOR, or PTEN in cancer has opened the possibility of acting on P-Rex to restrict protumorigenic signaling through these pathways.

A prognostic signature of G(2) checkpoint function in melanoma cell lines

As DNA damage checkpoints are barriers to carcinogenesis, G(2) checkpoint function was quantified to test for override of this checkpoint during melanomagenesis. Primary melanocytes displayed an effective G(2) checkpoint response to ionizing radiation (IR)-induced DNA damage. Thirty-seven percent of melanoma cell lines displayed a significant defect in G(2) checkpoint function. Checkpoint function was melanoma subtype-specific with "epithelial-like" melanoma lines, with wild type NRAS and BRAF displaying an effective checkpoint, while lines with mutant NRAS and BRAF displayed defective checkpoint function. Expression of oncogenic B-Raf in a checkpoint-effective melanoma attenuated G(2) checkpoint function significantly but modestly. Other alterations must be needed to produce the severe attenuation of G(2) checkpoint function seen in some BRAF-mutant melanoma lines. Quantitative trait analysis tools identified mRNA species whose expression was correlated with G(2) checkpoint function in the melanoma lines. A 165 gene signature was identified with a high correlation with checkpoint function (p < 0.004) and low false discovery rate (≤ 0.077). The G(2) checkpoint gene signature predicted G(2) checkpoint function with 77-94% accuracy. The signature was enriched in lysosomal genes and contained numerous genes that are associated with regulation of chromatin structure and cell cycle progression. The core machinery of the cell cycle was not altered in checkpoint-defective lines but rather numerous mediators of core machinery function were. When applied to an independent series of primary melanomas, the predictive G(2) checkpoint signature was prognostic of distant metastasis-free survival. These results emphasize the value of expression profiling of primary melanomas for understanding melanoma biology and disease prognosis.

Tumor profiling using protein biomarker panels in malignant melanoma: application of tissue microarrays and beyond

Despite advances in our knowledge of the disease, malignant melanoma remains an unpredictable entity. The revolution in molecular biological techniques, such as DNA sequencing and gene-expression profiling, has uncovered many potential protein targets and biomarkers relevant to melanoma progression. Successful clinical application would be aided significantly by downstream proteomic validation of those candidate markers using a combination of immunohistochemistry and tissue microarrays. Yet, research in this context seems to lag behind the output of genomic data relating to melanoma. In this article, we look at the strengths and pitfalls of tissue microarrays in malignant melanoma. We will show how tissue microarrays have become a vital step in the transition from molecular techniques to useful clinical assays and interventions and look at likely future developments for advances in this field.

TWIST1 is an ERK1/2 effector that promotes invasion and regulates MMP-1 expression in human melanoma cells

Tumor cells often use developmental processes to progress toward advanced disease. The E-box transcription factor TWIST1 is essential to epithelial-mesenchymal transition (EMT) and cell migration in the developing neural crest. In melanoma, which derives from the neural crest cell lineage, enhanced TWIST1 expression has been linked to worse clinical prognosis. However, mechanisms underlying TWIST1 expression and whether aberrant TWIST1 levels promote steps in melanoma progression remain unknown. Here, we report that elevated TWIST1 mRNA/protein expression is dependent on extracellular signal-regulated kinase (ERK)1/2 signaling, which is hyperactive in the majority of melanomas. We show that TWIST1 protein levels are especially high in melanoma cell lines generated from invasive, premetastatic stage tumors. Furthermore, TWIST1 expression is required and sufficient to promote invasion through Matrigel and spheroid outgrowth in three-dimensional dermal-mimetic conditions. Alterations to spheroid outgrowth were not as a result of altered cell death, cell-cycle profile, or paradigm EMT protein changes. Importantly, we identify matrix metalloproteinase-1 (MMP-1) as a novel downstream target of TWIST1. We have determined that TWIST1 acts, in a dose-dependent manner, as a mediator between hyperactive ERK1/2 signaling and regulation of MMP-1 transcription. Together, these studies mechanistically show a previously unrecognized interplay between ERK1/2, TWIST1, and MMP-1 that is likely significant in the progression of melanoma toward metastasis.

LKB1/STK11 inactivation leads to expansion of a prometastatic tumor subpopulation in melanoma

Germline mutations in LKB1 (STK11) are associated with the Peutz-Jeghers syndrome (PJS), which includes aberrant mucocutaneous pigmentation, and somatic LKB1 mutations occur in 10% of cutaneous melanoma. By somatically inactivating Lkb1 with K-Ras activation (±p53 loss) in murine melanocytes, we observed variably pigmented and highly metastatic melanoma with 100% penetrance. LKB1 deficiency resulted in increased phosphorylation of the SRC family kinase (SFK) YES, increased expression of WNT target genes, and expansion of a CD24(+) cell population, which showed increased metastatic behavior in vitro and in vivo relative to isogenic CD24(-) cells. These results suggest that LKB1 inactivation in the context of RAS activation facilitates metastasis by inducing an SFK-dependent expansion of a prometastatic, CD24(+) tumor subpopulation.

A prognostic signature of defective p53-dependent G1 checkpoint function in melanoma cell lines

Melanoma cell lines and normal human melanocytes (NHM) were assayed for p53-dependent G1 checkpoint response to ionizing radiation (IR)-induced DNA damage. Sixty-six percent of melanoma cell lines displayed a defective G1 checkpoint. Checkpoint function was correlated with sensitivity to IR with checkpoint-defective lines being radio-resistant. Microarray analysis identified 316 probes whose expression was correlated with G1 checkpoint function in melanoma lines (P≤0.007) including p53 transactivation targets CDKN1A, DDB2, and RRM2B. The 316 probe list predicted G1 checkpoint function of the melanoma lines with 86% accuracy using a binary analysis and 91% accuracy using a continuous analysis. When applied to microarray data from primary melanomas, the 316 probe list was prognostic of 4-yr distant metastasis-free survival. Thus, p53 function, radio-sensitivity, and metastatic spread may be estimated in melanomas from a signature of gene expression.

Increased levels of DUSP6 phosphatase stimulate tumourigenesis in a molecularly distinct melanoma subtype

The mitogen-activated protein kinase (MAPK) pathway is important in melanoma. In this pathway, DUSP6 phosphatase negatively controls the activation of extracellular signal-regulated (ERK) kinase. Through comparison of melanoma signalling pathways between immortal mouse melanocytes and their tumourigenic derivatives, retrieved from mouse xenografts, we identified a molecularly distinct subtype of melanoma, characterized by reduced ERK activity and increased DUSP6 expression. Overexpression of DUSP6 enhanced anchorage-independent growth and invasive ability of immortal mouse melanocytes, suggesting that increased DUSP6 expression contributes to melanoma formation in the mouse xenografts. In contrast, reduced tumourigenicity was observed after DUSP6 overexpression in human melanoma cells. A minority of thick human primary melanomas had high DUSP6 expression and the same poor melanoma-specific survival as the majority of thick primaries with low DUSP6 levels. We have demonstrated that DUSP6 is important in melanoma and that it plays a different role in our distinct subtype of mouse melanoma compared with that in classic human melanoma.

P-Rex1 is required for efficient melanoblast migration and melanoma metastasis

Metastases are the major cause of death from melanoma, a skin cancer that has the fastest rising incidence of any malignancy in the Western world. Molecular pathways that drive melanoblast migration in development are believed to underpin the movement and ultimately the metastasis of melanoma. Here we show that mice lacking P-Rex1, a Rac-specific Rho GTPase guanine nucleotide exchange factor, have a melanoblast migration defect during development evidenced by a white belly. Moreover, these P-Rex1(-/-) mice are resistant to metastasis when crossed to a murine model of melanoma. Mechanistically, this is associated with P-Rex1 driving invasion in a Rac-dependent manner. P-Rex1 is elevated in the majority of human melanoma cell lines and tumour tissue. We conclude that P-Rex1 has an important role in melanoblast migration and cancer progression to metastasis in mice and humans.

Transcriptional up-regulation of RhoE by hypoxia-inducible factor (HIF)-1 promotes epithelial to mesenchymal transition of gastric cancer cells during hypoxia

Epithelial-mesenchymal transition (EMT) is a key process that drives cancer invasion. Recently, hypoxia has been reported to induce EMT, accompanied by cytoskeleton remodeling. As RhoE is a key regulator in cytoskeleton formation, we hypothesized that RhoE may play a role in hypoxia-induced EMT. For the first time, we report that RhoE protein levels increase in gastric cancer cells under hypoxic conditions. Rigorous analysis revealed that RhoE up-regulation is at the transcriptional levels and requires hypoxia-inducible factor (HIF)-1α induction, and that HIF-1α binds a hypoxia-responsive element (HRE) on the RhoE promoter. Additionally, we discovered that hypoxia or overexpression of RhoE in normoxia up-regulates the mesenchymal marker Vimentin, down-regulates the epithelial marker E-cadherin, and significantly increases cell invasion in vitro. Silencing of HIF-1α or RhoE by specific siRNAs rescued these hypoxia-induced effects. Ectopic expression of RhoE also induced up-regulation of MMP2/MMP-9 in gastric cancer cells. This study identifies RhoE as a direct target for HIF-1 in gastric cancer cells. In addition, RhoE up-regulation represents a pivotal cellular adaptive response to hypoxia with implications in gastric cancer cell EMT and invasion. We propose that RhoE-targeted therapy might inhibit the high invasive potential of gastric cancer cells in hypoxic regions.

EZH2-dependent suppression of a cellular senescence phenotype in melanoma cells by inhibition of p21/CDKN1A expression

Polycomb group (PcG) proteins such as Enhancer of zeste homolog 2 (EZH2) are epigenetic transcriptional repressors that function through recognition and modification of histone methylation and chromatin structure. Targets of PcG include cell cycle regulatory proteins which govern cell cycle progression and cellular senescence. Senescence is a characteristic of melanocytic nevi, benign melanocytic proliferations that can be precursors of malignant melanoma. In this study, we report that EZH2, which we find absent in melanocytic nevi but expressed in many or most metastatic melanoma cells, functionally suppresses the senescent state in human melanoma cells. EZH2 depletion in melanoma cells inhibits cell proliferation, restores features of a cellular senescence phenotype, and inhibits growth of melanoma xenografts in vivo. p21/CDKN1A is activated upon EZH2 knockdown in a p53-independent manner and contributes substantially to cell cycle arrest and induction of a senescence phenotype. EZH2 depletion removes histone deacetylase 1 (HDAC1) from the CDKN1A transcriptional start site and downstream region, enhancing histone 3 acetylation globally and at CDKN1A. This results in recruitment of RNA polymerase II, leading to p21/CDKN1A activation. Depletion of EZH2 synergistically activates p21/CDKN1A expression in combination with the HDAC inhibitor trichostatin A. Since melanomas often retain wild-type p53 function activating p21, our findings describe a novel mechanism whereby EZH2 activation during tumor progression represses p21, leading to suppression of cellular senescence and enhanced tumorigenicity.

Melanoma cells show a heterogeneous range of sensitivity to ionizing radiation and are radiosensitized by inhibition of B-RAF with PLX-4032

PURPOSE

To assess the relative radiosensitivities of a large collection of melanoma cell lines and to determine whether pharmacologic inhibition of mutant B-RAF with PLX-4032 can radiosensitize B-Raf+ melanoma cells.

MATERIALS AND METHODS

A large collection of melanoma cell lines (n=37) were treated with 0-8Gy IR and clonogenic survival assays used to generate survival curves to rank relative radiosensitivities among the cell lines. The ability of a B-RAF inhibitor, PLX-4032, to radiosensitize highly radioresistant B-Raf+ cells was also assessed by clonogenic cell survival and spheroid invasion assays and the effects of treatment on the cell cycle assessed by FACS.

RESULTS

Melanoma cell lines displayed a very large, heterogeneous range of SF2 values (1.002-0.053) with a mean of 0.51. Cell lines with surviving fractions of 0.29 or less at SF2 and SF4 were observed at a high frequency of 18.9% and 70.2%, respectively. Treatment of B-Raf+ cells with the B-RAF inhibitor PLX-4032 in combination with radiation provided enhanced inhibition of both colony formation and invasion, and radiosensitized cells through an increase in G(1) arrest.

CONCLUSIONS

Our data suggest that melanomas are not uniformly radioresistant with a significant subset displaying inherent radiosensitivity. Pharmacologic inhibition of B-RAF with PLX-4032 effectively radiosensitized B-Raf+ melanoma cells suggesting that this combination approach could provide improved radiotherapeutic response in B-Raf+ melanoma patients.

Metastasis in an orthotopic murine model of melanoma is independent of RAS/RAF mutation

Melanoma is the most lethal skin tumor in large part because of a propensity for early metastasis. Good models of this most clinically relevant feature of melanoma are lacking. Here, we report the development of an in-vivo model of metastasis that relies on orthotopic injection of green fluorescent protein-tagged lines in immunodeficient mice, serial intravital imaging of tumor progression, and quantification of distant spread by two-photon laser scanning microscopy, immunohistochemistry, and real-time PCR analysis. Using this system, we report an assessment of the in-vivo growth and metastatic properties of 11 well-characterized human melanoma cell lines. A subset of lines showed rapid in-vivo growth with invasion of host vasculature and distant seeding of viscera in this system. The ability to form metastasis in vivo did not correlate with three-dimensional collagen invasion in vitro. Surprisingly, similar lines in terms of molecular genetic events differed markedly in their propensity to metastasize to distant organs such as brain and lung. In particular, two lines harboring B-RAF mutation and high levels of phosphorylated ERK and AKT were reproducibly unable to form tumors after orthotopic injection. Similarly, two previously identified RAS/RAF wildtype 'epithelial like' lines that do not have elevated phosphorylated ERK and AKT or express TWIST1 mRNA still showed a pronounced ability for orthotopic growth and metastatic spread. All the metastatic cell lines in this model showed increased NEDD9 expression, but NEDD9 lentiviral overexpression did not convey a metastatic phenotype on nonmetastatic cells. These data suggest that melanoma metastasis is a molecularly heterogeneous process that may not require epithelial-to-mesenchymal transition or ERK activation, although both may facilitate the process.

Pharmacodynamic characterization of the efficacy signals due to selective BRAF inhibition with PLX4032 in malignant melanoma

PURPOSE

About 65% to 70% of melanomas harbor a mutation in v-raf murine sarcoma viral oncogene homolog B1 (BRAF) that causes the steady-state activation of extracellular signal-regulated kinase (ERK). We sought to investigate the efficacy of PLX4032 (BRAF inhibitor) to identify patterns/predictors of response/resistance and to study the effects of BRAF in melanoma.

EXPERIMENTAL DESIGN

Well-characterized melanoma cell lines, including several with acquired drug resistance, were exposed to PLX4032. Growth inhibition, phosphosignaling, cell cycle, apoptosis, and gene expression analyses were performed before and after exposure to drug.

RESULTS

Using a growth-adjusted inhibitory concentration of 50% cutoff of 1 microM, 13 of 35 cell lines were sensitive to PLX4032, 16 resistant, and 6 intermediate (37%, 46%, and 17% respectively). PLX4032 caused growth inhibition, G(0)/G(1) arrest, and restored apoptosis in the sensitive cell lines. A BRAF mutation predicted for but did not guarantee a response, whereas a neuroblastoma RAS viral oncogene homolog mutation or wild-type BRAF conferred resistance. Cells with concurrent BRAF mutations and melanocortin 1 receptor germ line variants and/or a more differentiated melanocyte genotype had a preferential response. Acquired PLX4032 resistance reestablishes ERK signaling, promotes a nonmelanocytic genotype, and is associated with an increase in the gene expression of certain metallothioneins and mediators of angiogenesis.

CONCLUSIONS

PLX4032 has robust activity in BRAF mutated melanoma. The preclinical use of this molecule identifies criteria for its proper clinical application, describes patterns of and reasons for response/resistance, and affords insight into the role of a BRAF mutation in melanoma.

Expression of linear and novel circular forms of an INK4/ARF-associated non-coding RNA correlates with atherosclerosis risk

Human genome-wide association studies have linked single nucleotide polymorphisms (SNPs) on chromosome 9p21.3 near the INK4/ARF (CDKN2a/b) locus with susceptibility to atherosclerotic vascular disease (ASVD). Although this locus encodes three well-characterized tumor suppressors, p16^INK4a, p15^INK4b, and ARF, the SNPs most strongly associated with ASVD are ∼120 kb from the nearest coding gene within a long non-coding RNA (ncRNA) known as ANRIL (CDKN2BAS). While individuals homozygous for the atherosclerotic risk allele show decreased expression of ANRIL and the coding INK4/ARF transcripts, the mechanism by which such distant genetic variants influence INK4/ARF expression is unknown. Here, using rapid amplification of cDNA ends (RACE) and analysis of next-generation RNA sequencing datasets, we determined the structure and abundance of multiple ANRIL species. Each of these species was present at very low copy numbers in primary and cultured cells; however, only the expression of ANRIL isoforms containing exons proximal to the INK4/ARF locus correlated with the ASVD risk alleles. Surprisingly, RACE also identified transcripts containing non-colinear ANRIL exonic sequences, whose expression also correlated with genotype and INK4/ARF expression. These non-polyadenylated RNAs resisted RNAse R digestion and could be PCR amplified using outward-facing primers, suggesting they represent circular RNA structures that could arise from by-products of mRNA splicing. Next-generation DNA sequencing and splice prediction algorithms identified polymorphisms within the ASVD risk interval that may regulate ANRIL splicing and circular ANRIL (cANRIL) production. These results identify novel circular RNA products emanating from the ANRIL locus and suggest causal variants at 9p21.3 regulate INK4/ARF expression and ASVD risk by modulating ANRIL expression and/or structure.

Unbiased studies of the human genome have identified strong genetic determinants of atherosclerotic vascular disease (ASVD) on chromosome 9p21.3. This region of the genome does not encode genes previously linked to ASVD, but does contain the INK4/ARF tumor suppressor locus. Products of the INK4/ARF locus regulate cell division, a process thought to be important in ASVD pathology. We and others have suggested that genetic variants in 9p21.3 influence INK4/ARF gene expression; however, the mechanisms by which these distant polymorphisms (>100,000 bp away) influence transcription of the locus is unknown. The ASVD–associated genetic variants lie within the predicted structure of a non-coding RNA (ncRNA) called ANRIL. Based upon recent work suggesting that other ncRNAs can repress nearby coding genes, we considered the possibility that ANRIL structure may regulate INK4/ARF gene expression. Coupling molecular analysis with state-of-the-art sequencing technologies in a wide variety of cell types from normal human donors and cancer cells, we found that ANRIL encodes a heterogeneous species of rare RNA transcripts. Moreover, we identified novel, circular ANRIL isoforms (cANRIL) whose expression correlated with INK4/ARF transcription and ASVD risk. These studies suggest a new model wherein ANRIL structure influences INK4/ARF expression and susceptibility to atherosclerosis.

Ras superfamily GEFs and GAPs: validated and tractable targets for cancer therapy?

There is now considerable and increasing evidence for a causal role for aberrant activity of the Ras superfamily of small GTPases in human cancers. These GTPases function as GDP-GTP-regulated binary switches that control many fundamental cellular processes. A common mechanism of GTPase deregulation in cancer is the deregulated expression and/or activity of their regulatory proteins, guanine nucleotide exchange factors (GEFs) that promote formation of the active GTP-bound state and GTPase-activating proteins (GAPs) that return the GTPase to its GDP-bound inactive state. In this Review, we assess the association of GEFs and GAPs with cancer and their druggability for cancer therapeutics.

Somatic p16(INK4a) loss accelerates melanomagenesis

Loss of p16^INK4a–RB and ARF–p53 tumor suppressor pathways, as well as activation of RAS–RAF signaling, is seen in a majority of human melanomas. Although heterozygous germline mutations of p16^INK4a are associated with familial melanoma, most melanomas result from somatic genetic events: often p16^INK4a loss and N-RAS or B-RAF mutational activation, with a minority possessing alternative genetic alterations such as activating mutations in K-RAS and/or p53 inactivation. To generate a murine model of melanoma featuring some of these somatic genetic events, we engineered a novel conditional p16^INK4a-null allele and combined this allele with a melanocyte-specific, inducible CRE recombinase strain, a conditional p53-null allele and a loxP-stop-loxP activatable oncogenic K-Ras allele. We found potent synergy between melanocyte-specific activation of K-Ras and loss of p16^INK4a and/or p53 in melanomagenesis. Mice harboring melanocyte-specific activated K-Ras and loss of p16^INK4a and/or p53 developed invasive, unpigmented and nonmetastatic melanomas with short latency and high penetrance. In addition, the capacity of these somatic genetic events to rapidly induce melanomas in adult mice suggests that melanocytes remain susceptible to transformation throughout adulthood.

Mitigation of hematologic radiation toxicity in mice through pharmacological quiescence induced by CDK4/6 inhibition

Total body irradiation (TBI) can induce lethal myelosuppression, due to the sensitivity of proliferating hematopoietic stem/progenitor cells (HSPCs) to ionizing radiation (IR). No effective therapy exists to mitigate the hematologic toxicities of TBI. Here, using selective and structurally distinct small molecule inhibitors of cyclin-dependent kinase 4 (CDK4) and CDK6, we have demonstrated that selective cellular quiescence increases radioresistance of human cell lines in vitro and mice in vivo. Cell lines dependent on CDK4/6 were resistant to IR and other DNA-damaging agents when treated with CDK4/6 inhibitors. In contrast, CDK4/6 inhibitors did not protect cell lines that proliferated independently of CDK4/6 activity. Treatment of wild-type mice with CDK4/6 inhibitors induced reversible pharmacological quiescence (PQ) of early HSPCs but not most other cycling cells in the bone marrow or other tissues. Selective PQ of HSPCs decreased the hematopoietic toxicity of TBI, even when the CDK4/6 inhibitor was administered several hours after TBI. Moreover, PQ at the time of administration of therapeutic IR to mice harboring autochthonous cancers reduced treatment toxicity without compromising the therapeutic tumor response. These results demonstrate an effective method to mitigate the hematopoietic toxicity of IR in mammals, which may be potentially useful after radiological disaster or as an adjuvant to anticancer therapy.

Similar nucleotide excision repair capacity in melanocytes and melanoma cells

Sunlight UV exposure produces DNA photoproducts in skin that are repaired solely by nucleotide excision repair in humans. A significant fraction of melanomas are thought to result from UV-induced DNA damage that escapes repair; however, little evidence is available about the functional capacity of normal human melanocytes, malignant melanoma cells, and metastatic melanoma cells to repair UV-induced photoproducts in DNA. In this study, we measured nucleotide excision repair in both normal melanocytes and a panel of melanoma cell lines. Our results show that in 11 of 12 melanoma cell lines tested, UV photoproduct repair occurred as efficiently as in primary melanocytes. Importantly, repair capacity was not affected by mutation in the N-RAS or B-RAF oncogenes, nor was a difference observed between a highly metastatic melanoma cell line (A375SM) or its parental line (A375P). Lastly, we found that although p53 status contributed to photoproduct removal efficiency, its role did not seem to be mediated by enhanced expression or activity of DNA binding protein DDB2. We concluded that melanoma cells retain capacity for nucleotide excision repair, the loss of which probably does not commonly contribute to melanoma progression.

Growth-inhibitory and antiangiogenic activity of the MEK inhibitor PD0325901 in malignant melanoma with or without BRAF mutations

The Raf/MEK/ERK pathway is an important mediator of tumor cell proliferation and angiogenesis. Here, we investigated the growth-inhibitory and antiangiogenic properties of PD0325901, a novel MEK inhibitor, in human melanoma cells. PD0325901 effects were determined in a panel of melanoma cell lines with different genetic aberrations. PD0325901 markedly inhibited ERK phosphorylation and growth of both BRAF mutant and wild-type melanoma cell lines, with IC(50) in the nanomolar range even in the least responsive models. Growth inhibition was observed both in vitro and in vivo in xenograft models, regardless of BRAF mutation status, and was due to G(1)-phase cell cycle arrest and subsequent induction of apoptosis. Cell cycle (cyclin D1, c-Myc, and p27(KIP1)) and apoptosis (Bcl-2 and survivin) regulators were modulated by PD0325901 at the protein level. Gene expression profiling revealed profound modulation of several genes involved in the negative control of MAPK signaling and melanoma cell differentiation, suggesting alternative, potentially relevant mechanisms of action. Finally, PD0325901 inhibited the production of the proangiogenic factors vascular endothelial growth factor and interleukin 8 at a transcriptional level. In conclusion, PD0325901 exerts potent growth-inhibitory, proapoptotic, and antiangiogenic activity in melanoma lines, regardless of their BRAF mutation status. Deeper understanding of the molecular mechanisms of action of MEK inhibitors will likely translate into more effective treatment strategies for patients experiencing malignant melanoma.

Expression of p16(INK4a) in peripheral blood T-cells is a biomarker of human aging

Expression of the p16(INK4a) tumor suppressor sharply increases with age in most mammalian tissues, and contributes to an age-induced functional decline of certain self-renewing compartments. These observations have suggested that p16(INK4a) expression could be a biomarker of mammalian aging. To translate this notion to human use, we determined p16(INK4a) expression in cellular fractions of human whole blood, and found highest expression in peripheral blood T-lymphocytes (PBTL). We then measured INK4/ARF transcript expression in PBTL from two independent cohorts of healthy humans (170 donors total), and analyzed their relationship with donor characteristics. Expression of p16(INK4a), but not other INK4/ARF transcripts, appeared to exponentially increase with donor chronologic age. Importantly, p16(INK4a) expression did not independently correlate with gender or body-mass index, but was significantly associated with tobacco use and physical inactivity. In addition, p16(INK4a) expression was associated with plasma interleukin-6 concentration, a marker of human frailty. These data suggest that p16(INK4a) expression in PBTL is an easily measured, peripheral blood biomarker of molecular age.

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

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

Distinct patterns of DNA copy number alterations associate with BRAF mutations in melanomas and melanoma-derived cell lines

A majority of malignant melanomas harbor an oncogenic mutation in either BRAF or NRAS. If BRAF and NRAS transform melanoma cells by a similar mechanism, then additional genetic aberrations would be similar (or random). Alternatively, distinct mutation-associated changes would suggest the existence of unique cooperating requirements for each mutation group. We first analyzed a panel of 52 melanoma cell lines (n = 35, 11, 6 for BRAF*, NRAS*, and BRAF/NRAS(wt/wt), respectively) by array-based comparative genomic hybridization for unique alterations that associate with each mutation subgroup. Subsequently, those DNA copy number changes that correlated with a mutation subgroup were used to predict the mutation status of an independent panel of 43 tumors (n = 17, 13, 13 for BRAF*, NRAS*, and BRAF/NRAS(wt/wt), respectively). BRAF mutant tumors were classified with a high rate of success (74.4%, P = 0.002), whereas NRAS mutants were not significantly distinguished from wild types (26/43, P = 0.12). Copy number gains of 7q32.1-36.3, 5p15.31, 8q21.11, and 8q24.11 were most strongly associated with BRAF* tumors and cell lines, as were losses of 11q24.2-24.3. BRAF* melanomas appear to be associated with a specific profile of DNA copy number aberrations that is distinct from those found in NRAS* and BRAF/NRAS(wt/wt) tumors. These findings suggest that although both BRAF and NRAS appear to function along the same signal transduction pathway, each may have different requirements for cooperating oncogenic events. The genetic loci that make up this profile may harbor therapeutic targets specific for tumors with BRAF mutations.

The role of BRAF mutation and p53 inactivation during transformation of a subpopulation of primary human melanocytes

Melanocytic nevi frequently harbor oncogenic BRAF mutations, but only a minority progress to melanoma. In human melanocytes, persistent BRAF(V600E) expression triggers oncogene-induced senescence, which implies that bypass of oncogene-induced senescence is necessary for malignant transformation of melanocytes. We show that a subpopulation of primary human melanocytes with persistent expression of BRAF(V600E) do not enter oncogene-induced senescence, but instead survive despite heightened MAPK activity. Disruption of the p53 pathway using short-hairpin RNA initiated rapid growth of these V600E(+) melanocytes in vitro. The resultant V600E(+)/p53(sh) melanocytes grew anchorage-independently in soft agar, formed pigmented lesions reminiscent of in situ melanoma in artificial skin reconstructs, and were weakly tumorigenic in vivo. Array comparative genomic hybridization analysis demonstrated that the transformed melanocytes acquired a substantial deletion in chromosome 13, which encodes the Rb1 tumor suppressor gene. Gene expression profiling study of nevi and melanomas showed that p53 target genes were differentially expressed in melanomas compared with nevi, suggesting a dysfunctional p53 pathway in melanoma in vivo. In summary, these data demonstrate that a subpopulation of melanocytes possesses the ability to survive BRAF(V600E)-induced senescence, and suggest that p53 inactivation may promote malignant transformation of these cells.

INK4/ARF transcript expression is associated with chromosome 9p21 variants linked to atherosclerosis

BACKGROUND

Genome-wide association studies (GWAS) have linked common single nucleotide polymorphisms (SNPs) on chromosome 9p21 near the INK4/ARF (CDKN2A/B) tumor suppressor locus with risk of atherosclerotic diseases and type 2 diabetes mellitus. To explore the mechanism of this association, we investigated whether expression of proximate transcripts (p16(INK4a), p15(INK4b), ARF, ANRIL and MTAP) correlate with genotype of representative 9p21 SNPs.

METHODOLOGY/PRINCIPAL FINDINGS

We analyzed expression of 9p21 transcripts in purified peripheral blood T-cells (PBTL) from 170 healthy donors. Samples were genotyped for six selected disease-related SNPs spanning the INK4/ARF locus. Correlations among these variables were determined by univariate and multivariate analysis. Significantly reduced expression of all INK4/ARF transcripts (p15(INK4b), p16(INK4a), ARF and ANRIL) was found in PBTL of individuals harboring a common SNP (rs10757278) associated with increased risk of coronary artery disease, stroke and aortic aneurysm. Expression of MTAP was not influenced by rs10757278 genotype. No association of any these transcripts was noted with five other tested 9p21 SNPs.

CONCLUSIONS/SIGNIFICANCE

Genotypes of rs10757278 linked to increased risk of atherosclerotic diseases are also associated with decreased expression in PBTL of the INK4/ARF locus, which encodes three related anti-proliferative transcripts of known importance in tumor suppression and aging.

Rnd3 regulation of the actin cytoskeleton promotes melanoma migration and invasive outgrowth in three dimensions

The depth of cell invasion into the dermis is a clinical determinant for poor prognosis in cutaneous melanoma. The signaling events that promote the switch from a noninvasive to invasive tumor phenotype remain obscure. Activating mutations in the serine/threonine kinase B-RAF are prevalent in melanoma. Mutant B-RAF is required for melanoma cell invasion. The expression of Rnd3, a Rho family GTPase, is regulated by mutant B-RAF, although its role in melanoma progression is unknown. In this study, we determined the functional contribution of Rnd3 to invasive melanoma. Endogenous Rnd3 was targeted for knockdown using a doxycycline-inducible short hairpin RNA system in invasive human melanoma cells. Depletion of Rnd3 promoted prominent actin stress fibers and enlarged focal adhesions. Mechanistically, stress fiber formation induced by Rnd3 knockdown required the specific involvement of RhoA and ROCK1/2 activity but not RhoB or RhoC. Rnd3 expression in human melanoma cell lines was strongly associated with elevated extracellular signal-regulated kinase phosphorylation and invasive behavior in a three-dimensional dermal-like environment. A functional role for Rnd3 was shown in the invasive outgrowth of melanoma tumor spheroids. Knockdown of Rnd3 reduced the invasive outgrowth of spheroids embedded in collagen gels. Additionally, Rnd3 depletion inhibited collective and border cell movement out from spheroids in a ROCK1/2-dependent manner. Collectively, these findings implicate Rnd3 as a major suppressor of RhoA-mediated actin cytoskeletal organization and in the acquisition of an invasive melanoma phenotype.

Down-regulation of Rap1GAP via promoter hypermethylation promotes melanoma cell proliferation, survival, and migration

Melanoma is the most serious, highly aggressive form of skin cancer with recent dramatic increases in incidence. Current therapies are relatively ineffective, highlighting the need for a better understanding of the molecular mechanisms contributing to the disease. We have previously shown that activation of Rap1 promotes melanoma cell proliferation and migration through the mitogen-activated protein kinase pathway and integrin activation. In the present study, we show that expression of Rap1GAP, a specific negative regulator of Rap1, is decreased in human melanoma tumors and cell lines. Overexpression of Rap1GAP in melanoma cells blocks Rap1 activation and extracellular signal-regulated kinase (ERK) phosphorylation and inhibits melanoma cell proliferation and survival. In addition, overexpression of Rap1GAP also inhibits focal adhesion formation and decreases melanoma cell migration. Rap1GAP down-regulation is due to its promoter methylation, a mechanism of gene silencing in tumors. Furthermore, treatment of melanoma cells with the demethylating agent 5-aza-2'-deoxycytidine reinduces Rap1GAP expression, followed by decreased Rap1 activity, ERK phosphorylation, and cell proliferation and survival-changes that are significantly blunted in cells transfected by small interfering RNA-mediated Rap1GAP knockdown. Taken together, our findings indicate that down-regulation of Rap1GAP via promoter hypermethylation promotes melanoma cell proliferation, survival, and migration.

CD200 expression may help in differential diagnosis between mantle cell lymphoma and B-cell chronic lymphocytic leukemia

Chronic lymphocytic leukemia (B-CLL) and mantle cell lymphoma (MCL) share many features and their differential diagnosis may be challenging, especially when a leukemic picture alone is present. Monoclonal antibody panels are often useful, with CD23 being the most reliable. However, MCL diagnosis should be confirmed by immunohistochemical cyclin D1 detection, sometimes with equivocal or even negative results. Other cytofluorimetric, cytogenetics or molecular techniques are reliable but not widely available. B-CLL leukemic cells express CD200, a membrane glycoprotein belonging to the immunoglobulin superfamily. We investigated its expression on fresh neoplastic cells of 93 patients with a CD5+ lymphoproliferative disease (79 selected B-CLL and 14 MCL in leukemic phase). Although these data cannot be generalized, all B-CLL samples we examined were positive, with CD200 present on the vast majority of the cells while, in MCL patients, CD200 was expressed by a small minority of CD5+ cells in three subjects and totally absent in the remaining 11. We then examined CD200 expression on paraffin-embedded lymphoid tissues and bone marrow (BM) trephine biopsies from 23 B-CLL and 44 MCL patients. Again, all B-CLL cells were CD200+ both in lymph nodes and in BM while all MCL cells were negative. Adding CD200 in routine panels could be of diagnostic utility in excluding MCL diagnosis.

The role of mammalian coronins in development and disease

Coronins have maintained a high degree of conservation over the roughly 800 million years of eukaryotic evolution.1,2 From its origins as a single gene in simpler eukaryotes, the mammalian Coronin gene family has expanded to include at least six members (see Chapter 4). Increasing evidence indicates that Coronins play critical roles as regulators of actin dependent processes such as cell motility and vesicle trafficking3,4 (see Chapters 6-9). Considering the importance of these processes, it is not surprising that recent findings have implicated the involvement of Coronins in multiple diseases. This review primarily focuses on Coronin 1C (HGNC symbol: CORO1C, also known as Coronin 3) which is a transcriptionally dynamic gene that is up-regulated in multiple types of clinically aggressive cancer. In addition to reviewing the molecular signals and events that lead to Coronin 1C transcription, we summarize the results of several studies describing the possible functional roles of Coronin 1C in development as well as disease progression. Here, the main focus is on brain development and on the progression of melanoma and glioma. Finally, we will also review the role of other mammalian Coronin genes in clinically relevant processes such as neural regeneration and pathogenic bacterial infections (see Chapter 10).