Adhesion control of cyclin D1 and p27Kip1 levels is deregulated in melanoma cells through BRAF-MEK-ERK signaling

NRAS mutant melanoma: biological behavior and future strategies for therapeutic management

The recent years have seen a significant shift in the expectations for the therapeutic management of disseminated melanoma. The clinical success of BRAF targeted therapy suggests that long-term disease control may one day be a reality for genetically defined subgroups of melanoma patients. Despite this progress, few advances have been made in developing targeted therapeutic strategies for the 50% of patients whose melanomas are BRAF wild-type. The most well-characterized subgroup of BRAF wild-type tumors is the 15-20% of all melanomas that harbor activating NRAS (Neuroblastoma Rat Sarcoma Virus) mutations. Emerging preclinical and clinical evidence suggests that NRAS mutant melanomas have patterns of signal transduction and biological behavior that is distinct from BRAF mutant melanomas. This overview will discuss the unique clinical and prognostic behavior of NRAS mutant melanoma and will summarize the emerging data on how NRAS-driven signaling networks can be translated into novel therapeutic strategies.

Annexin A10 in human oral cancer: biomarker for tumoral growth via G1/S transition by targeting MAPK signaling pathways

Background

Annexins are calcium and phospholipid binding proteins that form an evolutionary conserved multigene family. Considerable evidence indicates that annexin A10 (ANXA10) is involved in tumoral progression, although little is known about its role in human oral carcinogenesis. In this study, we investigated the involvement of ANXA10 in oral squamous cell carcinoma (OSCC).

Methodology/Principal Findings

ANXA10 mRNA and protein expressions were assessed by quantitative reverse transcriptase polymerase chain reaction and immunoblotting, and we conducted a proliferation assay and cell-cycle analysis in ANXA10 knockdown cells in vitro. We evaluated the correlation between the ANXA10 expression status in 100 primary OSCCs and the clinicopathological features by immunohistochemistry. ANXA10 mRNA and protein expression levels were up-regulated in all cellular lines examined (n = 7, p<0.05). ANXA10 knockdown cells showed that cellular proliferation decreased by inactivation of extracellular regulated kinase (ERK) (p<0.05), and cell-cycle arrest at the G1 phase resulted from up-regulation of cyclin-dependent kinase inhibitors. ANXA10 protein expression in primary OSCCs was also significantly greater than in normal counterparts (p<0.05), and higher expression was correlated with tumoral size (p = 0.027).

Conclusions/Significance

Our results proposed for the first time that ANXA10 is an indicator of cellular proliferation in OSCCs. Our results suggested that ANXA10 expression might indicate cellular proliferation and ANXA10 might be a potential therapeutic target for the development of new treatments for OSCCs.

BH3-only protein silencing contributes to acquired resistance to PLX4720 in human melanoma

B-RAF is mutated to a constitutively active form in 8% of human cancers including 50% of melanomas. In clinical trials, the RAF inhibitor, PLX4032 (vemurafenib), caused partial or complete responses in 48-81% of mutant B-RAF harboring melanoma patients. However, the average duration of response was 6-7 months before tumor regrowth, indicating the acquisition of resistance to PLX4032. To understand the mechanisms of resistance, we developed mutant B-RAF melanoma cells that displayed resistance to RAF inhibition through continuous culture with PLX4720 (the tool compound for PLX4032). Resistance was associated with a partial reactivation of extracellular signal-regulated kinase 1/2 (ERK1/2) signaling, recovery of G1/S cell-cycle events, and suppression of the pro-apoptotic B-cell leukemia/lymphoma 2 (Bcl-2) homology domain 3 (BH3)-only proteins, Bcl-2-interacting mediator of cell death-extra large (Bim-EL) and Bcl-2 modifying factor (Bmf). Preventing ERK1/2 reactivation with MEK (mitogen-activated protein/extracellular signal-regulated kinase kinase) inhibitors blocked G1-S cell-cycle progression but failed to induce apoptosis or upregulate Bim-EL and Bmf. Treatment with the histone deacetylase (HDAC) inhibitor, suberoylanilide hydroxamic acid, led to de-repression of Bim-EL and enhanced cell death in the presence of PLX4720 or AZD6244 in resistant cells. These data indicate that acquired resistance to PLX4032/4720 likely involves ERK1/2 pathway reactivation as well as ERK1/2-independent silencing of BH3-only proteins. Furthermore, combined treatment of HDAC inhibitors and MEK inhibitors may contribute to overcoming PLX4032 resistance.

The current state of targeted therapy in melanoma: this time it's personal

Treatment of metastatic melanoma has long been a challenge. Over the past 8 years significant advances have been made in understanding the genetic changes that drive melanoma development and progression. These studies have shown melanoma to be a heterogeneous group of tumors, driven by a diverse array of oncogenic mutations. There is now good evidence that activating mutations in the serine/threonine kinase BRAF and the receptor tyrosine kinase KIT constitute good therapeutic targets for restricted subgroups of melanoma. In this article, we discuss the genetics and etiology of cutaneous and noncutaneous melanoma and review some of the latest preclinical and clinical data on the new targeted therapy agents that are beginning to make an impact on the lives of melanoma patients.

Berberine inhibited the growth of thyroid cancer cell lines 8505C and TPC1

PURPOSE

Thyroid cancer is the most common malignancy in Korean females and can be treated with good prognosis. However, drugs to treat aggressive types of thyroid cancer such as poorly differentiated or anaplastic thyroid cancer have not yet been established. To that end, we analyzed the effects of berberine on human thyroid cancer cell lines to determine whether this compound is useful in the treatment of aggressive thyroid cancer.

MATERIALS AND METHODS

The two thyroid cancer cell lines 8505C and TPC1, under adherent culture conditions, were treated with berberine and analyzed for changes in cell growth, cell cycle duration, and degree of apoptosis.

RESULTS

Following berberine treatment, both cell lines showed a dose-dependent reduction in growth rate. 8505C cells showed significantly increased levels of apoptosis following berberine treatment, whereas TPC1 cells showed cell cycle arrest at the G0/G1 phase. Immunobloting of p-27 expression following berberine treatment showed that berberine induced a little up-regulation of p-27 in 8505c cells but relatively high up-regulation of p-27 in TPC1 cells.

CONCLUSION

These results suggest that berberine treatment of thyroid cancer can inhibit proliferation through apoptosis and/or cell cycle arrest. Thus, berberine may be a novel anticancer drug for the treatment of poorly differentiated or anaplastic thyroid cancer.

The MEK1/2 inhibitor, selumetinib (AZD6244; ARRY-142886), enhances anti-tumour efficacy when combined with conventional chemotherapeutic agents in human tumour xenograft models

Background:

The Ras/RAF/MEK/ERK pathway is frequently deregulated in cancer and a number of inhibitors that target this pathway are currently in clinical development. It is likely that clinical testing of these agents will be in combination with standard therapies to harness the apoptotic potential of both the agents. To support this strategy, it has been widely observed that a number of chemotherapeutics stimulate the activation of several intracellular signalling cascades including Ras/RAF/MEK/ERK. The MEK1/2 inhibitor selumetinib has been shown to have anti-tumour activity and induce apoptotic cell death as a monotherapy.

Methods:

The aim of this study was to identify agents, which would be likely to offer clinical benefit when combined with selumetinib. Here, we used human tumour xenograft models and assessed the effects combining standard chemotherapeutic agents with selumetinib on tumour growth. In addition, we analysed tumour tissue to determine the mechanistic effects of these combinations.

Results:

Combining selumetinib with the DNA-alkylating agent, temozolomide (TMZ), resulted in enhanced tumour growth inhibition compared with monotherapies. Biomarker studies highlighted an increase in γH2A.X suggesting that selumetinib is able to enhance the DNA damage induced by TMZ alone. In several models we observed that continuous exposure to selumetinib in combination with docetaxel results in tumour regression. Scheduling of docetaxel before selumetinib was more beneficial than when selumetinib was dosed before docetaxel and demonstrated a pro-apoptotic phenotype. Similar results were seen when selumetinib was combined with the Aurora B inhibitor barasertib.

Conclusion:

The data presented suggests that MEK inhibition in combination with several standard chemotherapeutics or an Aurora B kinase inhibitor is a promising clinical strategy.

Resistance to selective BRAF inhibition can be mediated by modest upstream pathway activation

A high percentage of patients with BRAF(V600E) mutant melanomas respond to the selective RAF inhibitor vemurafenib (RG7204, PLX4032) but resistance eventually emerges. To better understand the mechanisms of resistance, we used chronic selection to establish BRAF(V600E) melanoma clones with acquired resistance to vemurafenib. These clones retained the V600E mutation and no second-site mutations were identified in the BRAF coding sequence. Further characterization showed that vemurafenib was not able to inhibit extracellular signal-regulated kinase phosphorylation, suggesting pathway reactivation. Importantly, resistance also correlated with increased levels of RAS-GTP, and sequencing of RAS genes revealed a rare activating mutation in KRAS, resulting in a K117N change in the KRAS protein. Elevated levels of CRAF and phosphorylated AKT were also observed. In addition, combination treatment with vemurafenib and either a MAP/ERK kinase (MEK) inhibitor or an AKT inhibitor synergistically inhibited proliferation of resistant cells. These findings suggest that resistance to BRAF(V600E) inhibition could occur through several mechanisms, including elevated RAS-GTP levels and increased levels of AKT phosphorylation. Together, our data implicate reactivation of the RAS/RAF pathway by upstream signaling activation as a key mechanism of acquired resistance to vemurafenib, in support of clinical studies in which combination therapy with other targeted agents are being strategized to combat resistance.

Using quantitative proteomic analysis to understand genotype specific intrinsic drug resistance in melanoma

The discovery of activating BRAF V600E mutations in 50% of all melanoma patients and the development of small molecule BRAF inhibitors looks set to revolutionize the therapy of disseminated melanoma. However, in the recent clinical trial of the BRAF inhibitor, vemurafenib (PLX4032), a significant percentage of BRAF V600E mutant melanoma patients did not meet the RECIST criteria for a response. Recent work from our lab identified loss of the tumor suppressor phosphatase and tensin homolog (PTEN) as being a possible mediator of intrinsic BRAF inhibitor resistance. In this commentary, we describe the development of a novel mass spectrometry based proteomic screen of Bcl-2 family proteins that was used to delineate the PTEN-dependent differences in apoptosis signaling observed when BRAF was inhibited. We further discuss how use of these sensitive quantitative proteomic methods gives unique insights into the signaling of cancer cells that are not captured through routine biochemical techniques and how this may lead to the development of combination therapy strategies for overcoming intrinsic BRAF inhibitor resistance.

BRAFV600E: implications for carcinogenesis and molecular therapy

The mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathway is frequently mutated in human cancer. This pathway consists of a small GTP protein of the RAS family that is activated in response to extracellular signaling to recruit a member of the RAF kinase family to the cell membrane. Active RAF signals through MAP/ERK kinase to activate ERK and its downstream effectors to regulate a wide range of biological activities including cell differentiation, proliferation, senescence, and survival. Mutations in the v-raf murine sarcoma viral oncogenes homolog B1 (BRAF) isoform of the RAF kinase or KRAS isoform of the RAS protein are found as activating mutations in approximately 30% of all human cancers. The BRAF pathway has become a target of interest for molecular therapy, with promising results emerging from clinical trials. Here, the role of the most common BRAF mutation BRAF(V600E) in human carcinogenesis is investigated through a review of the literature, with specific focus on its role in melanoma, colorectal, and thyroid cancers and its potential as a therapeutic target.

BRAF mutations in hairy-cell leukemia

BACKGROUND

Hairy-cell leukemia (HCL) is a well-defined clinicopathological entity whose underlying genetic lesion is still obscure.

METHODS

We searched for HCL-associated mutations by performing massively parallel sequencing of the whole exome of leukemic and matched normal cells purified from the peripheral blood of an index patient with HCL. Findings were validated by Sanger sequencing in 47 additional patients with HCL.

RESULTS

Whole-exome sequencing identified five missense somatic clonal mutations that were confirmed on Sanger sequencing, including a heterozygous mutation in BRAF that results in the BRAF V600E variant protein. Since BRAF V600E is oncogenic in other tumors, further analyses were focused on this genetic lesion. The same BRAF mutation was noted in all the other 47 patients with HCL who were evaluated by means of Sanger sequencing. None of the 195 patients with other peripheral B-cell lymphomas or leukemias who were evaluated carried the BRAF V600E variant, including 38 patients with splenic marginal-zone lymphomas or unclassifiable splenic lymphomas or leukemias. In immunohistologic and Western blot studies, HCL cells expressed phosphorylated MEK and ERK (the downstream targets of the BRAF kinase), indicating a constitutive activation of the RAF-MEK-ERK mitogen-activated protein kinase pathway in HCL. In vitro incubation of BRAF-mutated primary leukemic hairy cells from 5 patients with PLX-4720, a specific inhibitor of active BRAF, led to a marked decrease in phosphorylated ERK and MEK. CONCLUSIONS; The BRAF V600E mutation was present in all patients with HCL who were evaluated. This finding may have implications for the pathogenesis, diagnosis, and targeted therapy of HCL. (Funded by Associazione Italiana per la Ricerca sul Cancro and others.).

Human melanoma cells express FGFR/Src/Rho signaling that entails an adhesion-independent caveolin-1 membrane association

Caveolae have been indicated as a center of cytoskeleton regulation for Src kinase/Rho GTPase signaling. In addition, Src recruitment on intact cortical actin cytoskeleton appears to be required for bFGF/FGFR signal activation. Recently, we established a relationship between caveolin-1 (Cav-1) expression and cell migration in human malignant melanoma, constitutively activated by a bFGF autoregulatory loop. This work intends to investigate whether caveolae's asset, through bFGF/FGFR/c-Src/Rho signaling, could be related to melanoma cell anchorage. Accordingly, we revealed the existence of a FGFR/Src kinase pathway in Cav-1 enriched detergent-resistant membranes (DRMs) of Me665/1 metastatic melanoma cells, as confirmed by FGFR silencing. Moreover, we determined the expression and phosphorylation levels of Cav-1/Src/Erk signal pathway as a function of FGFR activation and cell density. A sucrose density gradient ultracentrifugation was employed to monitor Cav-1 membrane association and buoyancy in Me665/1 cells treated for actin fragmentation or for altered phosphorylation signals. As a result, melanoma cells show remarkable resistance to Cav-1 disassembly, together with persisting cell signal activity, being Src and Cav-1 crucial modulators of Rho GTPases. In conclusion, our study primarily highlights, in a metastatic melanoma cell line expressing caveolin, the circumstances whereby caveola structural and functional endurance enables the FGFR/Src/Rho GTPases pathway to keep on cell progression.

Mechanisms of resistance to RAF inhibitors in melanoma

The recent RAF inhibitor trial with PLX4032/RG7204 in late-stage mutant B-RAF melanoma patients has been lauded as a success story for personalized cancer therapy since short-term clinical responses were observed in the majority of patients. However, initial responses were followed by subsequent tumor re-growth, and a subset of patients showed intrinsic resistance. Bi-directional translational efforts are now essential to determine the mechanisms underlying acquired/secondary and intrinsic resistance to RAF inhibitors.

Cytoplasmic Skp2 expression is increased in human melanoma and correlated with patient survival

Background

S-phase kinase protein 2 (Skp2), an F-box protein, targets cell cycle regulators via ubiquitin-mediated degradation. Skp2 is frequently overexpressed in a variety of cancers and associated with patient survival. In melanoma, however, the prognostic significance of subcellular Skp2 expression remains controversial.

Methods

To investigate the role of Skp2 in melanoma development, we constructed tissue microarrays and examined Skp2 expression in melanocytic lesions at different stages, including 30 normal nevi, 61 dysplastic nevi, 290 primary melanomas and 146 metastatic melanomas. The TMA was assessed for cytoplasmic and nuclear Skp2 expression by immunohistochemistry. The Kaplan-Meier method was used to evaluate the patient survival. The univariate and multivariate Cox regression models were performed to estimate the harzard ratios (HR) at five-year follow-up.

Results

Cytoplasmic but not nuclear Skp2 expression was gradually increased from normal nevi, dysplastic nevi, primary melanomas to metastatic melanomas. Cytoplasmic Skp2 expression correlated with AJCC stages (I vs II–IV, P<0.001), tumor thickness (≤2.00 vs >2.00 mm, P<0.001) and ulceration (P = 0.005). Increased cytoplasmic Skp2 expression was associated with a poor five-year disease-specific survival of patients with primary melanoma (P = 0.018) but not metastatic melanoma (P>0.05).

Conclusion

This study demonstrates that cytoplasmic Skp2 plays an important role in melanoma pathogenesis and its expression correlates with patient survival. Our data indicate that cytoplasmic Skp2 may serve as a potential biomarker for melanoma progression and a therapeutic target for this disease.

Tumor suppressor p53 status does not determine the differentiation-associated G₁ cell cycle arrest induced in leukemia cells by 1,25-dihydroxyvitamin D₃ and antioxidants

Vitamin D derivatives can induce differentiation of human acute myeloid leukemia (AML) cells. Here, we investigated if the G₁ cell cycle block associated with monocytic differentiation is modulated by the p53 status of the cells treated with 1,25D, alone or with plant antioxidants carnosic acid (C) or silibinin (S), and a p38 MAPK inhibitor SB202190 (SB), a combination (D-C/S-SB) previously shown to enhance differentiation of AML p53null cells. D-C/S-SB enhanced differentiation of OCI-AML3 (p53wt) and as expected HL60 (p53 null) cells, but not of MOLM-13 (p53wt) cells. Conversely, MOLM-13 (p53wt) cells treated with 1,25D and/or D-C/S-SB, resembled HL60 (p53 null) cells in rapid G₁ block, while OCI-AML3 (p53wt) cells showed a delayed G₁ block when treated in a similar way, indicating that there is no relationship between the p53 status and G₁ block. Western blot analysis revealed that 1,25D and D-C/S-SB increased the inhibitory phosphorylation levels MEK-1 (P-Thr286), but decreased the levels of activated ERK1/2 (Thr202/Tyr204;Thr185/Tyr187), again without any apparent relationship to the p53 status. Interestingly, the increased levels of p21(Waf1/Cip1) were insufficient to promote a G₁ block in this system, as only cell lines with increased levels of p27(Kip1) and p35Nck5a, an activator of Cdk5, showed a rapid G₁ block. Overall, our data show that the p53-p21 axis is unlikely to have a role in differentiation-associated G₁ block in AML cells with wt p53, and that this block is achieved by several, possibly co-operating but redundant pathways, that include inhibition of MEK-1 by p35Nck5a-activated Cdk5.

Resistance to transforming growth factor β-mediated tumor suppression in melanoma: are multiple mechanisms in place?

Resistance to transforming growth factor (TGF) β-mediated tumor suppression in melanoma appears to be a crucial step in tumor aggressiveness since it is usually coupled with the ability of TGFβ to drive the oncogenic process via autocrine and paracrine effects. In this review, we will focus mainly on the mechanisms of escape from TGFβ-induced cell cycle arrest because the mechanisms of resistance to TGFβ-mediated apoptosis are still essentially speculative. As expected, some of these mechanisms can directly affect the function of the main downstream effectors of TGFβ, Smad2 and Smad3, resulting in compromised Smad-mediated antiproliferative activity. Other mechanisms can counteract or overcome TGFβ-mediated cell cycle arrest independently of the Smads. In melanoma, some models of resistance to TGFβ have been suggested and will be described. In addition, we propose additional models of resistance taking into consideration the information available on the dysregulation of fundamental cellular effectors and signaling pathways in melanoma.

Akt3-mediated resistance to apoptosis in B-RAF-targeted melanoma cells

Melanoma cells are highly resistant to anoikis, a form of apoptosis induced in nonadherent/inappropriate adhesion conditions. Depleting B-RAF or the prosurvival Bcl-2 family protein Mcl-1 renders mutant B-RAF melanoma cells susceptible to anoikis. In this study, we examined the effect of targeting B-RAF on the survival of primary stage melanoma cells cultured in three-dimensional type I collagen gels, which partially mimics the dermal microenvironment. Depletion/inhibition of B-RAF with small interfering RNA or the mutant B-RAF inhibitor, PLX4720, induced apoptosis of mutant B-RAF melanoma cells in three-dimensional collagen. Apoptosis was dependent on two upregulated BH3-only proteins, Bim-EL and Bmf, and was inhibited by ectopic Mcl-1 expression. Akt3 activation has been associated with the survival of melanoma cells. Mutant B-RAF melanoma cells ectopically expressing a constitutively activated form of Akt3 or endogenously expressing mutant Akt3 were protected from apoptosis induced by B-RAF knockdown or PLX4720 treatment. Furthermore, intrinsically resistant metastatic melanoma cells displayed elevated Akt phosphorylation in three-dimensional collagen and were rendered susceptible to PLX4720 by Akt3 knockdown. Importantly, myristylated Akt3 prevented B-RAF targeting-induced upregulation of Bim-EL and Bmf in three-dimensional collagen and partially protected Mcl-1-depleted cells from apoptosis. These findings delineate how mutant B-RAF protects melanoma cells from apoptosis and provide insight into possible resistance mechanisms to B-RAF inhibitors.

Recovery of phospho-ERK activity allows melanoma cells to escape from BRAF inhibitor therapy

Background:

Resistance to BRAF inhibitors is an emerging problem in the melanoma field. Strategies to prevent and overcome resistance are urgently required.

Methods:

The dynamics of cell signalling, BrdU incorporation and cell-cycle entry after BRAF inhibition was measured using flow cytometry and western blot. The ability of combined BRAF/MEK inhibition to prevent the emergence of resistance was demonstrated by apoptosis and colony formation assays and in 3D organotypic cell culture.

Results:

BRAF inhibition led to a rapid recovery of phospho-ERK (pERK) signalling. Although most of the cells remained growth arrested in the presence of drug, a minor population of cells retained their proliferative potential and escaped from BRAF inhibitor therapy. A function for the rebound pERK signalling in therapy escape was demonstrated by the ability of combined BRAF/MEK inhibition to enhance the levels of apoptosis and abrogate the onset of resistance.

Conclusion:

Combined BRAF/MEK inhibition may be one strategy to prevent the emergence of drug resistance in BRAF-V600E-mutated melanomas.

BRAF mutations, microsatellite instability status and cyclin D1 expression predict metastatic colorectal patients' outcome

Background:

The significance of BRAF mutations, microsatelite instability (MSI) status and cyclin D1 expression in patients with metastatic colorectal cancer (mCRC) was evaluated.

Methods:

Primary tumours from 144 patients treated for mCRC were assessed for BRAF (V600E) mutation, MSI status and cyclin D1. The data were correlated with progression-free survival (PFS) and overall survival (OS).

Results:

BRAF mutations were detected in 10 (out of 22, 45%) patients with MSI-H tumours compared with 2 (out of 122, 1.6%) in those with microsatellite stable tumours (P<0.001). The presence of BRAF mutations was correlated with cyclin D1 overexpression (7 out of 26 patients, 58% vs 5 out of 118 patients, 14% P=0.001). Patients with BRAF-mutated primary tumours had a significantly decreased PFS (2.7 vs 9.8 months; P<0.001) and median OS (14 vs 30 months; P<0.001) than patients with wild-type (wt) tumours. Patients with MSI-H and BRAF-mutated tumours experienced significantly lower PFS (3.1 vs 11.4 months; P=0.008) and OS (14.5 vs 35.5 months; P=0.004) than patients with MSI-H and BRAF wt tumours. Similarly, BRAF mutations and cyclin D1 overexpression were correlated with decreased PFS (3.1 vs 8.6 months; P=0.03) and OS (17.8 vs 39.2 months; P=0.01).

Conclusion:

BRAF V600E mutations are associated with MSI-H status and cyclin D1 overexpression and characterize a subgroup of patients with poor prognosis.

alphaB-crystallin is mutant B-RAF regulated and contributes to cyclin D1 turnover in melanocytic cells

The serine/threonine kinase, B-RAF, is frequently mutated in melanoma and is required for cell proliferation. Proteasomal turnover of cyclins and cyclin-dependent kinase inhibitors via E3 ubiquitin ligases regulates cell cycle progression. We previously showed that B-RAF regulates Cks1, a co-factor for the F-box protein Skp2. Recently, a second F-box protein cofactor was identified, alphaB-crystallin, that binds Fbx4 and promotes cyclin D1 degradation. Here, we demonstrate that alphaB-crystallin is down-regulated in mutant B-RAF melanoma cells compared to melanocytes in a B-RAF and MEK-dependent manner. In a subset of lines, MEK inhibition was sufficient to up-regulate alphaB-crystallin protein levels; whereas in other lines combined MEK and proteasome inhibition was required. alphaB-crystallin knockdown partially stabilized cyclin D1 in melanocytes. Expression of alphaB-crystallin in mutant B-RAF melanoma cells did not promote cyclin D1 turnover under normal conditions, but did enhance turnover following etoposide-induced DNA damage. Together, these data show that alphaB-crystallin is highly expressed in melanocytes contributing, in part, to cyclin D1 turnover. Furthermore, alphaB-crystallin is down-regulated in a B-RAF-dependent manner in melanoma cells and its re-expression regulates cyclin D1 turnover after DNA damage.

BRAF as therapeutic target in melanoma

BRAF is a member of the RAF kinase family, which acts in the ERK/MAP kinase pathway, a signalling cascade that regulates cellular proliferation, differentiation and survival. Single point mutations can turn BRAF into an oncogene, but there appears to be a cell type/tumour specific relevance for BRAF kinase-activating mutations, since they are found predominantly in cutaneous melanoma. With the success of targeting other oncogenic kinases such as BCR-ABL, KIT or members of the epidermal-growth factor receptor (EGFR) family in other cancers, the expectations were high when the first RAF kinase-targeting drug (sorafenib) reached clinical trials. However, disappointingly the first studies using sorafenib in melanoma patients did not show the anticipated single agent efficacy. More recently, the resolution of the BRAF crystal structure has led to the development of better, more specific BRAF inhibitors such as the Plexxikon compound, PLX4032, which induced a dramatic response rate in phase I trials, validating BRAF as a clinically relevant target. In addition, our understanding of melanoma biology and the role BRAF is playing therein has improved significantly. The complexity in the ERK/MAP kinase pathway including important feedback mechanisms has been dissected, and the relevance of cross-talks with other signalling pathways has been revealed, suggesting strategies for the design of improved, more efficient combinatorial therapies. This review highlights the relevance of BRAF and the ERK/MAP kinase pathway for melanoma cell biology and discusses some of the recent advances in both, the understanding of BRAF function in melanoma and the development of improved BRAF targeting inhibitors.

FOXD3 is a mutant B-RAF-regulated inhibitor of G(1)-S progression in melanoma cells

The forkhead box transcription factor FOXD3 is a stemness factor that prevents the production of melanocyte progenitors from the developing neural crest; however, its role in human cancers is not known. Transformation of melanocytes gives rise to melanoma. In two thirds of melanomas, the serine/threonine kinase B-RAF is mutated to a constitutively active form. Here, we show that FOXD3 levels are upregulated following attenuation of B-RAF and mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase (MEK) signaling in mutant B-RAF harboring human melanoma cells. This effect was selective because FOXD3 was not upregulated following MEK inhibition in wild-type B-RAF melanoma cells and mutant B-RAF thyroid carcinoma cells. Ectopic FOXD3 expression potently inhibited melanoma cell growth without altering mutant B-RAF activation of ERK1/2. Inhibition of cell growth was due to a potent G(1) cell cycle arrest and was associated with p53-dependent upregulation of p21(Cip1). FOXD3-induced cell cycle arrest was prevented by p53 depletion and, to a lesser extent, p21(Cip1) depletion. These studies show that FOXD3 is suppressed by B-RAF, uncover a novel role and mechanism for FOXD3 as a negative cell cycle regulator, and have implications for the repression of melanocytic lineage cells.

Understanding melanoma signaling networks as the basis for molecular targeted therapy

Despite years of research, there has been little improvement in survival for patients with disseminated melanoma. Recent work has identified mutations in BRAF and NRAS, leading to constitutive mitogen-activated protein kinase (MAPK) pathway as well as constitutive activity in the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) pathway, as being critical events in melanoma growth and progression. In the current review, we discuss how these complex mutational and signaling profiles can be understood using a network biology approach, and suggest how an understanding of the key signaling nodes involved in progression and survival will lead to improvements in melanoma therapy.

Melanoma biomarkers: current status and utility in diagnosis, prognosis, and response to therapy

Melanoma is the most devastating form of skin cancer and represents a leading cause of cancer death, particularly in young adults. As even relatively small melanomas can readily metastasize, accurate staging of progression is critical. Diagnosis is typically made on the basis of histopathologic criteria; with tumor thickness (Breslow), invasion level (Clark), ulceration, and the extent of lymph node involvement being important prognostic indicators. However, histologic criteria alone cannot diagnose all melanomas and there are often problems in distinguishing subsets of benign nevi from melanoma. There also exists a group of patients with thin primary melanomas for whom surgery should be curative but who ultimately go on to develop metastases. Therefore, there is an urgent need to develop molecular biomarkers that identify melanoma patients with high-risk primary lesions to facilitate greater surveillance and possible adjuvant therapy. The advent of large-scale genomic profiling of melanoma is revealing considerable heterogeneity, suggesting that melanomas could be subgrouped according to their patterns of oncogenic mutation and gene expression. It is hoped that this subgrouping will allow for the personalization of melanoma therapy using novel molecularly targeted agents. Much effort is now geared toward defining the genetic markers that may predict response to targeted therapy agents as well as identifying pharmacodynamic markers of therapy response. In this review, we discuss the utility of melanoma biomarkers for diagnosis and prognosis and suggest how novel molecular signatures can help guide both melanoma diagnosis and therapy selection.

Measurement of constitutive MAPK and PI3K/AKT signaling activity in human cancer cell lines

The growth and survival of cancer cells are often driven by constitutive activity in the mitogen-activated protein kinase (MAPK) and phospho-inositide 3-kinase (PI3K)/AKT signaling pathways. Activity in these signal transduction cascades is known to contribute to the uncontrolled growth and resistance to apoptosis that characterizes tumor progression. There is now a great deal of interest in therapeutically targeting these pathways in cancer using small molecule inhibitors. In this chapter, we describe methods to measure constitutive MAPK and AKT activity in melanoma cell lines, with a focus upon Western blotting, phospho-flow cytometry, and immunofluorescence staining techniques.

Proliferation and survival molecules implicated in the inhibition of BRAF pathway in thyroid cancer cells harbouring different genetic mutations

Background

Thyroid carcinomas show a high prevalence of mutations in the oncogene BRAF which are inversely associated with RAS or RET/PTC oncogenic activation. The possibility of using inhibitors on the BRAF pathway as became an interesting therapeutic approach. In thyroid cancer cells the target molecules, implicated on the cellular effects, mediated by inhibition of BRAF are not well established. In order to fill this lack of knowledge we studied the proliferation and survival pathways and associated molecules induced by BRAF inhibition in thyroid carcinoma cell lines harbouring distinct genetic backgrounds.

Methods

Suppression of BRAF pathway in thyroid cancer cell lines (8505C, TPC1 and C643) was achieved using RNA interference (RNAi) for BRAF and the kinase inhibitor, sorafenib. Proliferation analysis was performed by BrdU incorporation and apoptosis was accessed by TUNEL assay. Levels of protein expression were analysed by western-blot.

Results

Both BRAF RNAi and sorafenib inhibited proliferation in all the cell lines independently of the genetic background, mostly in cells with BRAF^V600E mutation. In BRAF^V600E mutated cells inhibition of BRAF pathway lead to a decrease in ERK1/2 phosphorylation and cyclin D1 levels and an increase in p27^Kip1. Specific inhibition of BRAF by RNAi in cells with BRAF^V600E mutation had no effect on apoptosis. In the case of sorafenib treatment, cells harbouring BRAF^V600E mutation showed increase levels of apoptosis due to a balance of the anti-apoptotic proteins Mcl-1 and Bcl-2.

Conclusion

Our results in thyroid cancer cells, namely those harbouring BRAF^V600Emutation showed that BRAF signalling pathway provides important proliferation signals. We have shown that in thyroid cancer cells sorafenib induces apoptosis by affecting Mcl-1 and Bcl-2 in BRAF^V600E mutated cells which was independent of BRAF. These results suggest that sorafenib may prove useful in the treatment of thyroid carcinomas, particularly those refractory to conventional treatment and harbouring BRAF mutations.

Reduced p16 and increased cyclin D1 and pRb expression are correlated with progression in cutaneous melanocytic tumors

The immunohistochemical expression of cell cycle proteins p16, cyclin D1, and pRb was assessed in 112 benign and malignant melanocytic tumors and correlated with tumor progression, prognosis, and outcome. Comparing benign and malignant tumors, there were significant differences in the median score for all 3 proteins, with decreased p16 (P = .000001), increased cyclin D1 (P = .01), and increased pRb in melanomas (P = .01). There was a progressive loss of expression of p16 with progression from benign naevi to primary melanomas and to metastases. p16 was significantly decreased in primary tumors from melanoma patients who developed recurrent disease (P = .0000013). Cyclin D1 and pRb showed a progressive increase in expression from benign to malignant tumors but with relative decreases in the more advanced tumors (thick primaries and metastatic melanomas). Alterations in cell cycle proteins involved in G1/S transition are implicated in melanocytic tumor progression and have a potential role in diagnosis and prognostication.

Functional proteomics identifies targets of phosphorylation by B-Raf signaling in melanoma

Melanoma and other cancers harbor oncogenic mutations in the protein kinase B-Raf, which leads to constitutive activation and dysregulation of MAP kinase signaling. In order to elucidate molecular determinants responsible for B-Raf control of cancer phenotypes, we present a method for phosphoprotein profiling, using negative ionization mass spectrometry to detect phosphopeptides based on their fragment ion signature caused by release of PO(3)(-). The method provides an alternative strategy for phosphoproteomics, circumventing affinity enrichment of phosphopeptides and isotopic labeling of samples. Ninety phosphorylation events were regulated by oncogenic B-Raf signaling, based on their responses to treating melanoma cells with MKK1/2 inhibitor. Regulated phosphoproteins included known signaling effectors and cytoskeletal regulators. We investigated MINERVA/FAM129B, a target belonging to a protein family with unknown category and function, and established the importance of this protein and its MAP kinase-dependent phosphorylation in controlling melanoma cell invasion into three-dimensional collagen matrix.

The future of targeted therapy approaches in melanoma

BACKGROUND

The past 30 years have seen little improvement in the survival of patients with stage IV melanoma. Following the discovery of activating BRAF mutations in most melanomas, a wealth of preclinical experimentation has validated the BRAF/MAPK pathway as an excellent therapeutic target in melanoma. Despite these encouraging results, early clinical trials on BRAF/MAPK inhibition have been disappointing.

OBJECTIVE

In the current review, we discuss how differences between the preclinical and clinical settings may influence the response of melanoma cells to BRAF/MEK inhibition. As the BRAF/MEK signaling pathway is not solely responsible for the growth and survival of melanoma cells, we further discuss the therapeutic utility of inhibiting the PI3K/AKT and mTOR pathways both alone and in combination with BRAF/MEK.

CONCLUSION

In looking ahead to the future, it is likely that new advances in melanoma biology, such as the identification of melanoma stem cells and a greater understanding of intratumoral heterogeneity, may play a role in the design of any future melanoma targeted therapy.

Increased cyclin D1 expression can mediate BRAF inhibitor resistance in BRAF V600E-mutated melanomas

Recent studies have shown that there is a considerable heterogeneity in the response of melanoma cell lines to MEK and BRAF inhibitors. In the current study, we address whether dysregulation of cyclin-dependent kinase 4 (CDK4) and/or cyclin D1 contribute to the BRAF inhibitor resistance of melanoma cells. Mutational screening identified a panel of melanoma cell lines that harbored both a BRAF V600E mutation and a CDK4 mutation: K22Q (1205Lu), R24C (WM39, WM46, and SK-Mel-28), and R24L (WM902B). Pharmacologic studies showed that the presence of a CDK4 mutation did not alter the sensitivity of these cell lines to the BRAF inhibitor. The only cell line with significant BRAF inhibitor resistance was found to harbor both a CDK4 mutation and a CCND1 amplification. Array comparative genomic hybridization analysis showed that CCND1 was amplified in 17% of BRAF V600E-mutated human metastatic melanoma samples, indicating the clinical relevance of this finding. As the levels of CCND1 amplification in cell lines are lower than those seen in clinical specimens, we overexpressed cyclin D1 alone and in the presence of CDK4 in a drug-sensitive melanoma line. Cyclin D1 overexpression alone increased resistance and this was enhanced when cyclin D1 and CDK4 were concurrently overexpressed. In conclusion, increased levels of cyclin D1, resulting from genomic amplification, may contribute to the BRAF inhibitor resistance of BRAF V600E-mutated melanomas, particularly when found in the context of a CDK4 mutation/overexpression.

Identification of a novel subgroup of melanomas with KIT/cyclin-dependent kinase-4 overexpression

Although many melanomas harbor either activating mutations in BRAF or NRAS, there remains a substantial, yet little known, group of tumors without either mutation. Here, we used a genomic strategy to define a novel group of melanoma cell lines with co-overexpression of cyclin-dependent kinase 4 (CDK4) and KIT. Although this subgroup lacked any known KIT mutations, they had high phospho-KIT receptor expression, indicating receptor activity. Quantitative PCR confirmed the existence of a similar KIT/CDK4 subgroup in human melanoma samples. Pharmacologic studies showed the KIT/CDK4-overexpressing subgroup to be resistant to BRAF inhibitors but sensitive to imatinib in both in vitro and in vivo melanoma models. Mechanistically, imatinib treatment led to increased apoptosis and G(1) phase cell cycle arrest associated with the inhibition of phospho-ERK and increased expression of p27(KIP). Other melanoma cell lines, which retained some KIT expression but lacked phospho-KIT, were not sensitive to imatinib, suggesting that KIT expression alone is not predictive of response. We suggest that co-overexpression of KIT/CDK4 is a potential mechanism of oncogenic transformation in some BRAF/NRAS wild-type melanomas. This group of melanomas may be a subpopulation for which imatinib or other KIT inhibitors may constitute optimal therapy.

The mitogen-activated protein/extracellular signal-regulated kinase kinase inhibitor AZD6244 (ARRY-142886) induces growth arrest in melanoma cells and tumor regression when combined with docetaxel

PURPOSE

Disseminated melanoma is highly therapy resistant. The finding that 66% of melanomas harbor the activating BRAF(V600E) mutation has raised expectations for targeting the Ras/RAF/mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK) kinase (MEK)/ERK pathway in melanoma. This study addresses the anti-melanoma activity of the MEK inhibitor AZD6244 (ARRY-142886).

EXPERIMENTAL DESIGN

We recently have shown that growing melanoma cells as three-dimensional collagen-implanted spheroids enhances resistance to the MEK inhibitor U0126. Here, we investigated the anti-melanoma activity of AZD6244 in two-dimensional cell culture, the three-dimensional spheroid model, and an in vivo model.

RESULTS

In two-dimensional cell culture, AZD6244 was cytostatic and reduced the growth of melanoma cells in a concentration-dependent fashion through the induction of G(1)-phase cell cycle arrest. In our three-dimensional spheroid model, the effects of AZD6244 were largely cytostatic and reversible, with drug washout leading to spheroid regrowth. Finally, 1205Lu cells were grown as tumor xenografts in severe combined immunodeficient mice. After tumor establishment, mice were dosed twice daily with 0, 10, or 30 mg/kg AZD6244 p.o. AZD6244 treatment decreased phospho-ERK in the tumors and significantly suppressed tumor growth. The original tumors remained viable, suggesting that AZD6244 monotherapy was largely cytostatic, and not proapoptotic in this model. Further studies showed that co-administration of AZD6244 (30 mg/kg) with docetaxel (15 mg/kg) led to tumor regression, indicating the potential for MEK inhibitor/chemotherapy drug combinations.

CONCLUSIONS

Inhibition of MEK is cytostatic as a monotherapy in melanoma, but cytotoxic when combined with docetaxel.

BRAF provides proliferation and survival signals in MSI colorectal carcinoma cells displaying BRAF(V600E) but not KRAS mutations

BRAF kinase is a downstream target of KRAS and activates the MAPK pathway. These two molecules are prone to mutations in sporadic microsatellite unstable (MSI) colorectal carcinomas (CRC) and BRAF V600E mutations are inversely associated with oncogenic KRAS mutations. The biological significance of BRAF V600E oncogenic activation is not well established in this type of tumour. We aimed to study proliferation and survival effects induced by BRAF inhibition in MSI CRC cell lines harbouring distinct genetic backgrounds (BRAF V600E or KRAS G13D). Suppression of BRAF in BRAF V600E MSI CRC cell lines by RNA interference significantly inhibited proliferation and induced apoptosis, as demonstrated by BrdU incorporation and TUNEL assay, respectively. No significant differences were seen in proliferation and apoptosis, in cell lines harbouring KRAS G13D, after BRAF inhibition. We further analysed proliferation-associated molecules (pERK1/2, cyclin D1, p27 Kip1) and apoptosis-associated molecules (Bcl-2, Bax, pAkt, pBad, XIAP) in all cell lines. After BRAF down-regulation, we found a more pronounced decrease in ERK1/2 phosphorylation and cyclin D1 expression levels in BRAF-mutated cell lines in comparison to KRAS mutated cells. Upon BRAF inhibition, we also found an increase in p27(Kip1) levels and a more pronounced decrease in the levels of anti-apoptotic protein Bcl-2, specifically in cell lines with BRAF V600E. In conclusion, we have shown that MSI KRAS and BRAF mutant CRC cell lines respond differently to BRAF knockdown. This report provides evidence supporting BRAF as a good target for therapeutic intervention in patients with sporadic MSI CRC harbouring activating mutations in BRAF but not in KRAS.

Preclinical studies of celastrol and acetyl isogambogic acid in melanoma

PURPOSE

Sensitize melanomas to apoptosis and inhibit their growth and metastatic potential by compounds that mimic the activities of activating transcription factor 2 (ATF2)-driven peptides.

EXPERIMENTAL DESIGN

Small-molecule chemical library consisting of 3,280 compounds was screened to identify compounds that elicit properties identified for ATF2 peptide, including (a) sensitization of melanoma cells to apoptosis, (b) inhibition of ATF2 transcriptional activity, (c) activation of c-Jun NH(2)-terminal kinase (JNK) and c-Jun transcriptional activity, and (d) inhibition of melanoma growth and metastasis in mouse models.

RESULTS

Two compounds, celastrol (CSL) and acetyl isogambogic acid, could, within a low micromolar range, efficiently elicit cell death in melanoma cells. Both compounds efficiently inhibit ATF2 transcriptional activities, activate JNK, and increase c-Jun transcriptional activities. Similar to the ATF2 peptide, both compounds require JNK activity for their ability to inhibit melanoma cell viability. Derivatives of CSL were identified as potent inducers of cell death in mouse and human melanomas. CSL and a derivative (CA19) could also efficiently inhibit growth of human and mouse melanoma tumors and reduce the number of lung metastases in syngeneic and xenograft mouse models.

CONCLUSIONS

These studies show for the first time the effect of CSL and acetyl isogambogic acid on melanoma. These compounds elicit activities that resemble the well-characterized ATF2 peptide and may therefore offer new approaches for the treatment of this tumor type.

B-RAF regulation of Rnd3 participates in actin cytoskeletal and focal adhesion organization

The actin cytoskeleton controls multiple cellular functions, including cell morphology, movement, and growth. Accumulating evidence indicates that oncogenic activation of the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase 1/2 (MEK/ERK1/2) pathway is accompanied by actin cytoskeletal reorganization. However, the signaling events contributing to actin cytoskeleton remodeling mediated by aberrant ERK1/2 activation are largely unknown. Mutant B-RAF is found in a variety of cancers, including melanoma, and it enhances activation of the MEK/ERK1/2 pathway. We show that targeted knockdown of B-RAF with small interfering RNA or pharmacological inhibition of MEK increased actin stress fiber formation and stabilized focal adhesion dynamics in human melanoma cells. These effects were due to stimulation of the Rho/Rho kinase (ROCK)/LIM kinase-2 signaling pathway, cumulating in the inactivation of the actin depolymerizing/severing protein cofilin. The expression of Rnd3, a Rho antagonist, was attenuated after B-RAF knockdown or MEK inhibition, but it was enhanced in melanocytes expressing active B-RAF. Constitutive expression of Rnd3 suppressed the actin cytoskeletal and focal adhesion effects mediated by B-RAF knockdown. Depletion of Rnd3 elevated cofilin phosphorylation and stress fiber formation and reduced cell invasion. Together, our results identify Rnd3 as a regulator of cross talk between the RAF/MEK/ERK and Rho/ROCK signaling pathways, and a key contributor to oncogene-mediated reorganization of the actin cytoskeleton and focal adhesions.

New insights into purinergic receptor signaling in neuronal differentiation, neuroprotection, and brain disorders

Ionotropic P2X and metabotropic P2Y purinergic receptors are expressed in the central nervous system and participate in the synaptic process particularly associated with acetylcholine, GABA, and glutamate neurotransmission. As a result of activation, the P2 receptors promote the elevation of free intracellular calcium concentration as the main signaling pathway. Purinergic signaling is present in early stages of embryogenesis and is involved in processes of cell proliferation, migration, and differentiation. The use of new techniques such as knockout animals, in vitro models of neuronal differentiation, antisense oligonucleotides to induce downregulation of purinergic receptor gene expression, and the development of selective inhibitors for purinergic receptor subtypes contribute to the comprehension of the role of purinergic signaling during neurogenesis. In this review, we shall discuss the participation of purinergic receptors in developmental processes and in brain physiology, including neuron-glia interactions and pathophysiology.