Overcoming intrinsic multidrug resistance in melanoma by blocking the mitochondrial respiratory chain of slow-cycling JARID1B(high) cells

Phosphorylation-mediated EZH2 inactivation promotes drug resistance in multiple myeloma

Alterations in chromatin modifications, such as histone methylation, have been suggested as mediating chemotherapy resistance in several cancer types; therefore, elucidation of the epigenetic mechanisms that underlie drug resistance may greatly contribute to the advancement of cancer therapies. In the present study, we identified histone H3-lysine 27 (H3K27) as a critical residue for epigenetic modification in multiple myeloma. We determined that abrogation of drug-induced H3K27 hypermethylation is associated with cell adhesion-mediated drug resistance (CAM-DR), which is the most important form of drug resistance, using a coculture system to evaluate stroma cell adhesion-dependent alterations in multiple myeloma cells. Cell adhesion counteracted anticancer drug-induced hypermethylation of H3K27 via inactivating phosphorylation of the transcription regulator EZH2 at serine 21, leading to the sustained expression of antiapoptotic genes, including IGF1, B cell CLL/lymphoma 2 (BCL2), and hypoxia inducible factor 1, α subunit (HIF1A). Pharmacological and genetic inhibition of the IGF-1R/PI3K/AKT pathway reversed CAM-DR by promoting EZH2 dephosphorylation and H3K27 hypermethylation both in vitro and in refractory murine myeloma models. Together, our findings identify and characterize an epigenetic mechanism that underlies CAM-DR and suggest that kinase inhibitors to counteract EZH2 phosphorylation should be included in combination chemotherapy to increase therapeutic index.

Therapeutic strategies targeting cancer stem cells

Cancer stem cells (CSCs) are undifferentiated cancer cells with a high tumorigenic activity, the ability to undergo self-renewal, and a multilineage differentiation potential. Cancer stem cells are responsible for the development of tumor cell heterogeneity, a key feature for resistance to anticancer treatments including conventional chemotherapy, radiation therapy, and molecularly targeted therapy. Furthermore, minimal residual disease, the major cause of cancer recurrence and metastasis, is enriched in CSCs. Cancer stem cells also possess the property of "robustness", which encompasses several characteristics including a slow cell cycle, the ability to detoxify or mediate the efflux of cytotoxic agents, resistance to oxidative stress, and a rapid response to DNA damage, all of which contribute to the development of therapeutic resistance. The identification of mechanisms underlying such characteristics and the development of novel approaches to target them will be required for the therapeutic elimination of CSCs and the complete eradication of tumors. In this review, we focus on two prospective therapeutic approaches that target CSCs with the aim of disrupting their quiescence or redox defense capability.

Targeting mitochondrial complex I using BAY 87-2243 reduces melanoma tumor growth

Background

Numerous studies have demonstrated that functional mitochondria are required for tumorigenesis, suggesting that mitochondrial oxidative phosphorylation (OXPHOS) might be a potential target for cancer therapy. In this study, we investigated the effects of BAY 87-2243, a small molecule that inhibits the first OXPHOS enzyme (complex I), in melanoma in vitro and in vivo.

Results

BAY 87-2243 decreased mitochondrial oxygen consumption and induced partial depolarization of the mitochondrial membrane potential. This was associated with increased reactive oxygen species (ROS) levels, lowering of total cellular ATP levels, activation of AMP-activated protein kinase (AMPK), and reduced cell viability. The latter was rescued by the antioxidant vitamin E and high extracellular glucose levels (25 mM), indicating the involvement of ROS-induced cell death and a dependence on glycolysis for cell survival upon BAY 87-2243 treatment. BAY 87-2243 significantly reduced tumor growth in various BRAF mutant melanoma mouse xenografts and patient-derived melanoma mouse models. Furthermore, we provide evidence that inhibition of mutated BRAF using the specific small molecule inhibitor vemurafenib increased the OXPHOS dependency of BRAF mutant melanoma cells. As a consequence, the combination of both inhibitors augmented the anti-tumor effect of BAY 87-2243 in a BRAF mutant melanoma mouse xenograft model.

Conclusions

Taken together, our results suggest that complex I inhibition has potential clinical applications as a single agent in melanoma and also might be efficacious in combination with BRAF inhibitors in the treatment of patients with BRAF mutant melanoma.

Electronic supplementary material

The online version of this article (doi:10.1186/s40170-015-0138-0) contains supplementary material, which is available to authorized users.

Mitochondrial oxidative stress as a novel therapeutic target to overcome intrinsic drug resistance in melanoma cell subpopulations

Despite recent success in melanoma therapy, most patients with metastatic disease still undergo deadly progression. We have identified a novel mechanism of multidrug resistance allowing a small subpopulation of slow-cycling melanoma cells to survive based on elevated oxidative bioenergy metabolism. In this study, we asked whether such slow-cycling cells could be eliminated by co-treatment with the copper-chelator elesclomol. Elesclomol-copper complexes can cause oxidative stress by disruption of the mitochondrial respiration chain or by indirect non-mitochondrial induction of reactive oxygen species. We have found that elesclomol effectively kills the slow-cycling subpopulation and prevents the selective enrichment for slow-cycling cells, which usually results after monotreatment. We hypothesize that elesclomol could overcome the multidrug resistance of slow-cycling melanoma cells and prevent tumor repopulation in melanoma patients in future.

Sugar? No Thank You, Just a Deep Breath of Oxygen for Cancer Stem Cells

Tumors are metabolically heterogeneous, and subpopulations of tumorigenic cells have been recently described to rely more on mitochondrial respiration than glycolysis for energy production. In this issue, Sancho et al. (2015) demonstrate that MYC is a master switch regulating metabolic programs in different subpopulations of pancreatic tumor cells.

Emerging targets for combination therapy in melanomas

Cutaneous melanomas are often difficult to treat when diagnosed in advanced stages. Melanoma cells adapt to survive in extreme environmental conditions and are among the tumors with larger genomic instability. Here we discuss some intrinsic and extrinsic mechanisms of resistance of melanoma cells to both conventional and target therapies, such as autophagy, adaptation to endoplasmic reticulum stress, metabolic reprogramming, mechanisms of tumor repopulation and the role of extracellular vesicles in this later phenomenon. These biological processes are potentially targetable and thus provide a platform for research and discovery of new drugs for combination therapy to manage melanoma patient treatment.

Metabolic reprogramming: the emerging concept and associated therapeutic strategies

Tumor tissue is composed of cancer cells and surrounding stromal cells with diverse genetic/epigenetic backgrounds, a situation known as intra-tumoral heterogeneity. Cancer cells are surrounded by a totally different microenvironment than that of normal cells; consequently, tumor cells must exhibit rapidly adaptive responses to hypoxia and hypo-nutrient conditions. This phenomenon of changes of tumor cellular bioenergetics, called “metabolic reprogramming”, has been recognized as one of 10 hallmarks of cancer. Metabolic reprogramming is required for both malignant transformation and tumor development, including invasion and metastasis. Although the Warburg effect has been widely accepted as a common feature of metabolic reprogramming, accumulating evidence has revealed that tumor cells depend on mitochondrial metabolism as well as aerobic glycolysis. Remarkably, cancer-associated fibroblasts in tumor stroma tend to activate both glycolysis and autophagy in contrast to neighboring cancer cells, which leads to a reverse Warburg effect. Heterogeneity of monocarboxylate transporter expression reflects cellular metabolic heterogeneity with respect to the production and uptake of lactate. In tumor tissue, metabolic heterogeneity induces metabolic symbiosis, which is responsible for adaptation to drastic changes in the nutrient microenvironment resulting from chemotherapy. In addition, metabolic heterogeneity is responsible for the failure to induce the same therapeutic effect against cancer cells as a whole. In particular, cancer stem cells exhibit several biological features responsible for resistance to conventional anti-tumor therapies. Consequently, cancer stem cells tend to form minimal residual disease after chemotherapy and exhibit metastatic potential with additional metabolic reprogramming. This type of altered metabolic reprogramming leads to adaptive/acquired resistance to anti-tumor therapy. Collectively, complex and dynamic metabolic reprogramming should be regarded as a reflection of the “robustness” of tumor cells against unfavorable conditions. This review focuses on the concept of metabolic reprogramming in heterogeneous tumor tissue, and further emphasizes the importance of developing novel therapeutic strategies based on drug repositioning.

Successful strategies in the discovery of small-molecule epigenetic modulators with anticancer potential

As a class, epigenetic enzymes have been identified as clear targets for cancer therapeutics based on their broad hyperactivity in solid and hematological malignancies. The search for effective inhibitors of histone writers and of histone erasers has been a focus of drug discovery efforts both in academic and pharmaceutical laboratories and has led to the identification of some promising leads. This review focuses on the discovery strategies and preclinical evaluation studies of a subset of the more advanced compounds that target histone writers or histone erasers. The specificity and anticancer potential of these small molecules is discussed within the context of their development pipeline.

Melanoma: oncogenic drivers and the immune system

Advances and in-depth understanding of the biology of melanoma over the past 30 years have contributed to a change in the consideration of melanoma as one of the most therapy-resistant malignancies. The finding that oncogenic BRAF mutations drive tumor growth in up to 50% of melanomas led to a molecular therapy revolution for unresectable and metastatic disease. Moving beyond BRAF, inactivation of immune regulatory checkpoints that limit T cell responses to melanoma has provided targets for cancer immunotherapy. In this review, we discuss the molecular biology of melanoma and we focus on the recent advances of molecularly targeted and immunotherapeutic approaches.

Histone Modifications, Modifiers and Readers in Melanoma Resistance to Targeted and Immune Therapy

The treatment of melanoma has been revolutionized by new therapies targeting MAPK signaling or the immune system. Unfortunately these therapies are hindered by either primary resistance or the development of acquired resistance. Resistance mechanisms involving somatic mutations in genes associated with resistance have been identified in some cases of melanoma, however, the cause of resistance remains largely unexplained in other cases. The importance of epigenetic factors targeting histones and histone modifiers in driving the behavior of melanoma is only starting to be unraveled and provides significant opportunity to combat the problems of therapy resistance. There is also an increasing ability to target these epigenetic changes with new drugs that inhibit these modifications to either prevent or overcome resistance to both MAPK inhibitors and immunotherapy. This review focuses on changes in histones, histone reader proteins and histone positioning, which can mediate resistance to new therapeutics and that can be targeted for future therapies.

Structure-activity studies of Wnt/β-catenin inhibition in the Niclosamide chemotype: Identification of derivatives with improved drug exposure

The Wnt signaling pathway plays a key role in regulation of organ development and tissue homeostasis. Dysregulated Wnt activity is one of the major underlying mechanisms responsible for many diseases including cancer. We previously reported the FDA-approved anthelmintic drug Niclosamide inhibits Wnt/β-catenin signaling and suppresses colon cancer cell growth in vitro and in vivo. Niclosamide is a multi-functional drug that possesses important biological activity in addition to inhibition of Wnt/β-catenin signaling. Here, we studied the SAR of Wnt signaling inhibition in the anilide and salicylamide region of Niclosamide. We found that the 4'-nitro substituent can be effectively replaced by trifluoromethyl or chlorine and that the potency of inhibition was dependent on the substitution pattern in the anilide ring. Non-anilide, N-methyl amides and reverse amide derivatives lost significant potency, while acylated salicylamide derivatives inhibited signaling with potency similar to non-acyl derivatives. Niclosamide's low systemic exposure when dosed orally may hinder its use to treat systemic disease. To overcome this limitation we identified an acyl derivative of Niclosamide, DK-520 (compound 32), that significantly increased both the plasma concentration and the duration of exposure of Niclosamide when dosed orally. The studies herein provide a medicinal chemical foundation to improve the pharmacokinetic exposure of Niclosamide and Wnt-signaling inhibitors based on the Niclosamide chemotype. The identification of novel derivatives of Niclosamide that metabolize to Niclosamide and increase its drug exposure may provide important research tools for in vivo studies and provide drug candidates for treating cancers with dysregulated Wnt signaling including drug-resistant cancers. Moreover, since Niclosamide is a multi-functional drug, new research tools such as DK520 could directly result in novel treatments against bacterial and viral infection, lupus, and metabolic diseases such as type II diabetes, NASH and NAFLD.

Survivin promotes oxidative phosphorylation, subcellular mitochondrial repositioning, and tumor cell invasion

Survivin promotes cell division and suppresses apoptosis in many human cancers, and increased abundance correlates with metastasis and poor prognosis. We showed that a pool of survivin that localized to the mitochondria of certain tumor cell lines enhanced the stability of oxidative phosphorylation complex II, which promoted cellular respiration. Survivin also supported the subcellular trafficking of mitochondria to the cortical cytoskeleton of tumor cells, which was associated with increased membrane ruffling, increased focal adhesion complex turnover, and increased tumor cell migration and invasion in cultured cells, and enhanced metastatic dissemination in vivo. Therefore, we found that mitochondrial respiration enhanced by survivin contributes to cancer metabolism, and relocalized mitochondria may provide a "regional" energy source to fuel tumor cell invasion and metastasis.

Multiple Molecular Pathways in Melanomagenesis: Characterization of Therapeutic Targets

Molecular mechanisms involved in pathogenesis of malignant melanoma have been widely studied and novel therapeutic treatments developed in recent past years. Molecular targets for therapy have mostly been recognized in the RAS–RAF–MEK–ERK and PI3K–AKT signaling pathways; small-molecule inhibitors were drawn to specifically target key kinases. Unfortunately, these targeted drugs may display intrinsic or acquired resistance and various evidences suggest that inhibition of a single effector of the signal transduction cascades involved in melanoma pathogenesis may be ineffective in blocking the tumor growth. In this sense, a wider comprehension of the multiple molecular alterations accounting for either response or resistance to treatments with targeted inhibitors may be helpful in assessing, which is the most effective combination of such therapies. In the present review, we summarize the known molecular mechanisms underlying either intrinsic and acquired drug resistance either alternative roads to melanoma pathogenesis, which may become targets for innovative anticancer approaches.

Overcoming resistance to BRAF inhibition in BRAF-mutated metastatic melanoma

Almost 50% of metastatic melanoma patients harbor a BRAF(V600) mutation and the introduction of BRAF inhibitors has improved their treatment options. BRAF inhibitors vemurafenib and dabrafenib achieved improved overall survival over chemotherapy and have been approved for the treatment of BRAF-mutated metastatic melanoma. However, most patients develop mechanisms of acquired resistance and about 15% of them do not achieve tumor regression at all, due to intrinsic resistance to therapy. Moreover, early adaptive responses limit the initial efficacy of BRAF inhibition, leading mostly to incomplete responses that may favor the selection of a sub-population of resistant clones and the acquisition of alterations that cause tumor regrowth and progressive disease. The purpose of this paper is to review the mechanisms of resistance to therapy with BRAF inhibitors and to discuss the strategies to overcome them based on pre-clinical and clinical evidences.

Loss of microRNA-27b contributes to breast cancer stem cell generation by activating ENPP1

Cancer stem cells (CSCs) have been identified in various types of cancer; however, the mechanisms by which cells acquire CSC properties such as drug resistance and tumour seeding ability are not fully understood. Here, we identified microRNA-27b (miR-27b) as a key regulator for the generation of a side-population in breast cancer cells that showed CSC properties, and also found that the anti-type II diabetes (T2D) drug metformin reduced this side-population via miR-27b-mediated repression of ectonucleotide pyrophosphatase/phosphodiesterase family member 1 (ENPP1), which is involved in T2D development. ENPP1 induced the generation of the side-population via upregulation of the ABCG2 transporter. ENPP1 was also identified as a substrate of the 26S proteasome, the activity of which is downregulated in CSCs. Overall, these results demonstrate that a T2D-associated gene plays an important role in tumour development and that its expression is strictly controlled at the mRNA and protein levels.

MicroRNAs have a role in the acquisition of stem cell-like properties of cancer cells. Here the authors show that microRNA-27b mediates generation of a side-population of breast cancer stem cells, in part by regulating the protein ENPP1, which has been previously linked to the development of diabetes.

Thrombospondin 1 promotes an aggressive phenotype through epithelial-to-mesenchymal transition in human melanoma

Epithelial-to-mesenchymal transition (EMT), in which epithelial cells loose their polarity and become motile mesenchymal cells, is a determinant of melanoma metastasis. We compared gene expression signatures of mesenchymal-like melanoma cells with those of epithelial-like melanoma cells, and identified Thrombospondin 1 (THBS1) as highly up-regulated in the mesenchymal phenotype. This study investigated whether THBS1, a major physiological activator of transforming growth factor (TGF)-beta, is involved in melanoma EMT-like process. We sought to examine expression patterns in distinct melanoma phenotypes including invasive, de-differentiated, label-retaining and drug resistant populations that are putatively associated with an EMT-like process.

Here we show that THBS1 expression and secretion was elevated in melanoma cells exhibiting invasive, drug resistant, label retaining and mesenchymal phenotypes and correlated with reduced expression of genes involved in pigmentation. Elevated THBS1 levels were detected in Vemurafenib resistant melanoma cells and inhibition of THBS1 led to significantly reduced chemoresistance in melanoma cells. Notably, siRNA-mediated silencing of THBS1 and neutralizing antibody to THBS1 reduced invasion in mesenchymal-like melanoma cells, while ectopic THBS1 expression in epithelial-like melanoma cells enhanced invasion. Furthermore, the loss of THBS1 inhibited in vivo motility of melanoma cells within the embryonic chicken neural tube. In addition, we found aberrant THBS1 protein expression in metastatic melanoma tumor biopsies. These results implicate a role for THBS1 in EMT, and hence THBS1 may serve as a novel target for strategies aimed at the treatment of melanoma invasion and drug resistance.

Melanoma

Melanoma is a common cancer in the Western world with an increasing incidence. Sun exposure is still considered to be the major risk factor for melanoma. The prognosis of patients with malignant (advanced-stage) melanoma differs widely between countries, but public campaigns advocating early detection have led to significant reductions in mortality rates. As well as sun exposure, distinct genetic alterations have been identified as associated with melanoma. For example, families with melanoma who have germline mutations in CDKN2A are well known, whereas the vast majority of sporadic melanomas have mutations in the mitogen-activated protein kinase cascade, which is the pathway with the highest oncogenic and therapeutic relevance for this disease. BRAF and NRAS mutations are typically found in cutaneous melanomas, whereas KIT mutations are predominantly observed in mucosal and acral melanomas. GNAQ and GNA11 mutations prevail in uveal melanomas. Additionally, the PI3K-AKT-PTEN pathway and the immune checkpoint pathways are important. The finding that programmed cell death protein 1 ligand 1 (PDL1) and PDL2 are expressed by melanoma cells, T cells, B cells and natural killer cells led to the recent development of programmed cell death protein 1 (PD1)-specific antibodies (for example, nivolumab and pembrolizumab). Alongside other new drugs - namely, BRAF inhibitors (vemurafenib and dabrafenib) and MEK inhibitors (trametinib and cobimetinib) - these agents are very promising and have been shown to significantly improve prognosis for patients with advanced-stage metastatic disease. Early signs are apparent that these new treatment modalities are also improving long-term clinical benefit and the quality of life of patients. This Primer summarizes the current understanding of melanoma, from mechanistic insights to clinical progress. For an illustrated summary of this Primer, visit: http://go.nature.com/vX2N9s.

Deconstructing the role of the ECM microenvironment on drug efficacy targeting MAPK signaling in a pre-clinical platform for cutaneous melanoma

Therapeutics targeting the BRAF kinase in cutaneous melanoma have significantly improved patient survival. However, durable responses in the face of metastatic disease are rarely realized where the problem of brain metastases is generally growing in magnitude. Tumor and stromal cells dynamically remodel the extracellular matrix (ECM) during the establishment of a metastatic lesion. We reasoned that ECM composition strongly determines drug efficacy on cell motility, adhesion and viability rendering one drug more potent and another less so. To test this hypothesis, we constructed platforms recreating the ECM composition due to the stroma and tumor cells, mimicking the brain's perivascular niche and hyaluronic acid (HA) rich parenchyma. Using human melanoma cell lines, we observed that cell adhesion was minimally affected by BRAF inhibition but ablated by ERK inhibition. Cell motility was impaired for both drugs. We determined that the composition and architecture of the ECM niche modulated drug efficacy. In one series, potency of BRAF inhibition was blunted in 3D Fibronectin-HA hydrogels whereas Laminin-HA hydrogels protected against ERK inhibition. In the other series, Laminin blunted drug efficacy, despite both series sharing the same BRAF mutation. These data reinforce the importance of contextual drug assessment in designing future therapeutics.

Cancer stem cells as therapeutic targets of hepato-biliary-pancreatic cancers

Heterogeneity is one of the essential hallmarks of malignancies. Within bulk cancer cells, a striking variability differs in biological characteristics including the proliferation rate, cell-cell interaction, metastatic tendency and even sensitivity to anticancer therapies. Such diversity makes the investigation and treatment of the cancers complicated. Increasing evidence suggests this plasticity of cancers is a result of self-renewing and differentiation of a small subpopulation of cancer cells with stem-like properties, called cancer stem cells (CSCs). More importantly, CSCs are believed to be responsible for the resistance to conventional therapies and metastatic abilities in clinical practice. This review summarizes the molecular pathogenesis of hepato-biliary-pancreatic CSCs on the basis of the recent studies, and promising strategy of novel therapy targeting the signal transduction pathways or potentially epigenetic addictions of CSCs.

Intratumoral Heterogeneity: From Diversity Comes Resistance

Tumors consist of a heterogeneous mixture of functionally distinct cancer cells. These functional differences can be caused by varying levels of receptor activity, differentiation, and distinct metabolic and epigenetic states. Intratumoral heterogeneity can lead to interdependence among different subpopulations of cells for sustained tumor growth. In addition, subpopulations can vary widely in their responses to therapeutic agents. As such, it is believed that intratumoral heterogeneity may underlie incomplete treatment responses, acquired and innate resistance, and disease relapse observed in the clinic in response to conventional chemotherapy and targeted agents.

Circulating Melanoma Cell Subpopulations: Their Heterogeneity and Differential Responses to Treatment

Metastatic melanoma is a highly heterogeneous tumor; thus, methods to analyze tumor-derived cells circulating in blood should address this diversity. Taking this into account, we analyzed, using multiparametric flow cytometry, the co-expression of the melanoma markers melanoma cell adhesion molecule and melanoma-associated chondroitin sulphate proteoglycan and the tumor-initiating markers ATP-binding cassette sub-family B member 5 (ABCB5), CD271, and receptor activator of NF-κβ (RANK) in individual circulating tumor cells (CTCs) from 40 late-stage (III-IV) and 16 early-stage (I-II) melanoma patients. CTCs were heterogeneous within and between patients, with limited co-expression between the five markers analyzed. Analysis of patient matched blood and metastatic tumors revealed that ABCB5 and RANK subpopulations are more common among CTCs than in the solid tumors, suggesting a preferential selection for these cells in circulation. Pairwise comparison of CTC subpopulations longitudinally before and 6-13 weeks after treatment initiation showed that the percentage of RANK(+) CTCs significantly increased in the patients undergoing targeted therapy (N=16, P<0.01). Moreover, the presence of ⩾5 RANK(+) CTCs in the blood of patients undergoing targeted therapies was prognostic of shorter progression-free survival (hazards ratio 8.73, 95% confidence interval 1.82-41.75, P<0.01). Taken together, our results provide evidence of the heterogeneity among CTC subpopulations in melanoma and the differential response of these subpopulations to targeted therapy.

The KDM5 family of histone demethylases as targets in oncology drug discovery

There is growing evidence for a causal role of the KDM5 family of histone demethylases in human cancer. In particular, KDM5A (JARID1A/RBP2) and KDM5B (JARID1B/PLU1) contribute to cancer cell proliferation, reduce the expression of tumor suppressor genes, promote the development of drug tolerance and maintain tumor-initiating cells. KDM5 enzymes remove tri- and di-methylations of lysine 4 of histone H3 - modifications that occur at the start site of transcription in actively transcribed genes. However, the importance of the histone demethylase activity of KDM5 proteins for cancer cells has not been resolved so far. The currently available approaches suppress or remove the targeted proteins and thereby affect their putative functions as structural components and recruitment factors for other chromatin-associated proteins. Therefore, the development of specific enzymatic inhibitors for KDM5 will promote our understanding of the biological role of their catalytic activity and yield potential novel anticancer therapeutics.

Immune regulation of therapy-resistant niches: emerging targets for improving anticancer drug responses

Emerging evidence has unveiled a critical role for immunological parameters in predicting tumor prognosis and clinical responses to anticancer therapeutics. On the other hand, responsiveness to anticancer drugs greatly modifies the repertoires, phenotypes, and immunogenicity of tumor-infiltrating immune cells, serving as a critical factor to regulate tumorigenic activities and the emergence of therapy-resistant phenotypes. Tumor-associated immune functions are influenced by distinct or overlapping sets of therapeutic modalities, such as cytotoxic chemotherapy, radiotherapy, or molecular-targeted therapy, and various anticancer modalities have unique properties to influence the mode of cross-talk between tumor cells and immune cells in tumor microenvironments. Thus, it is critical to understand precise molecular machineries whereby each anticancer strategy has a distinct or overlapping role in regulating the dynamism of reciprocal communication between tumor and immune cells in tumor microenvironments. Such an understanding will open new therapeutic opportunities by harnessing the immune system to overcome resistance to conventional anticancer drugs.

Altered Splicing of JARID1B in Development of Human Cutaneous Melanoma?

Upregulated expression of histone H3K4 demethylase JARID1B has been found in several types of human cancer, but the expression pattern of this protein in benign naevi and human cutaneous melanomas seems to differ from that described for other tumors. We demonstrate that the apparent contradiction may be because of the fact that the malignant transformation of melanocytes is associated not so much with a general enhancement of a total expression of JARID1B but rather with a change in relative expression levels of individual splicing variants of the protein. Our data indicate that parallel immunohistochemical assays of the expression levels of all the isoforms and of the RBP2-H1 variant of JARID1B may be an efficient technique of differentiating between benign naevi and melanomas.

4th international conference on tumor progression and therapeutic resistance: meeting report

The fourth international conference on tumor progression and therapeutic resistance organized in association with GTCbio was held in Boston, MA from March 9 to 11, 2014. The meeting attracted a diverse group of experts in the field of cancer biology, therapeutics and medical oncology from academia and industry. The meeting addressed the current challenges in the treatment of cancer including tumor heterogeneity, therapy resistance and metastasis along with the need for improved biomarkers of tumor progression and clinical trial design. Keynote speakers included Clifton Leaf, Editor at Fortune Magazine, Dr. Mina Bissell from the Lawrence Berkeley National Laboratory and Dr. Levi Garraway from the Dana Farber Cancer Institute. The meeting featured cutting edge tools, preclinical models and the latest basic, translational and clinical research findings in the field.

Overexpression of JARID1B is associated with poor prognosis and chemotherapy resistance in epithelial ovarian cancer

JARID1B, a histone demethylase, has been reported to be highly expressed in various human cancers. In the present study, we investigated the association of JARID1B level with epithelial ovarian cancer (EOC) and prognosis of patients with EOC. We analyzed JARID1B expression in 20 normal ovaries, 20 benign ovarian tumor (BOT) samples, and 45 epithelial ovarian carcinoma specimens by quantitative PCR (qRT-PCR) and western blotting analyses. JARID1B was further examined in 120 EOC specimens from patients with different histological stages via immunohistochemistry. Possible correlations between JARID1B levels and prognosis as well as chemotherapy resistance of EOC patients were determined by univariate and multivariate analyses. JARID1B level was significantly increased in EOC, as compared to normal ovaries and BOT. Among 120 EOC cases examined, the 5-year progression-free survival (PFS) rates were 17 and 85 % in patients with high and low JARID1B expression, respectively (hazard ratio = 17.85, 95 % confidence interval (CI) 6.31–50.51, P < 0.001). Similarly, the 5-year overall survival (OS) rates for patients with high and low JARID1B expression were 28 and 92 % respectively (hazard ratio = 21.8, 95 % CI 5.92–71.81, P < 0.001). Positive correlation between JARID1B level and chemotherapy resistance was observed in patients with EOC (odds ratio (OR) 36.81, 95 % CI 4.84–280.11, P < 0.001). JARID1B could serve as an important biomarker for prognosis and chemotherapy resistance of EOC patients.

Counteracting oxidative phosphorylation-mediated resistance of melanomas to MAPK pathway inhibition

Mitochondrial oxidative phosphorylation (OxPhos) induces resistance to MAPK pathway inhibitors in melanoma. However, therapeutic targeting of mitochondria is challenging. In a recent study, we showed that inhibition of mTOR kinase activity resensitized resistant melanomas by indirectly inhibiting OxPhos via a novel mechanism. Here, we discuss the implications of these findings.

Impairment of preimplantation porcine embryo development by histone demethylase KDM5B knockdown through disturbance of bivalent H3K4me3-H3K27me3 modifications

KDM5B (JARID1B/PLU1) is a H3K4me2/3 histone demethylase that is implicated in cancer development and proliferation and is also indispensable for embryonic stem cell self-renewal, cell fate, and murine embryonic development. However, little is known about the role of KDM5B during preimplantation embryo development. Here we show that KDM5B is critical to porcine preimplantation development. KDM5B was found to be expressed in a stage-specific manner, consistent with demethylation of H3K4me3, with the highest expression being observed from the 4-cell to the blastocyst stages. Knockdown of KDM5B by morpholino antisense oligonucleotides injection impaired porcine embryo development to the blastocyst stage. The impairment of embryo development might be caused by increased expression of H3K4me3 at the 4-cell and blastocyst stages, which disturbs the balance of bivalent H3K4me3-H3K27me3 modifications at the blastocyst stage. Decreased abundance of H3K27me3 at blastocyst stage activates multiple members of homeobox genes (HOX), which need to be silenced for faithful embryo development. Additionally, the histone demethylase KDM6A was found to be upregulated by knockdown of KDM5B, which indicated it was responsible for the decreased abundance of H3K27me3 at the blastocyst stage. The transcriptional levels of Ten-Eleven Translocation gene family members (TET1, TET2, and TET3) are found to be increased by knockdown of KDM5B, which indicates cross talk between histone modifications and DNA methylation. The studies above indicate that KDM5B is required for porcine embryo development through regulating the balance of bivalent H3K4me3-H3K27me3 modifications.

Ferroxitosis: a cell death from modulation of oxidative phosphorylation and PKM2-dependent glycolysis in melanoma

Reliance on glycolysis is a characteristic of malignancy, yet the development of resistance to BRAF inhibitors in melanoma is associated with gain of mitochondrial function. Concurrent attenuation of oxidative phosphorylation and HIF-1α/PKM2-dependent glycolysis promotes a non-apoptotic, iron- and oxygen-dependent cell death that we term ferroxitosis. The redox cycling agent menadione causes a robust increase in oxygen consumption, accompanied by significant loss of intracellular ATP and rapid cell death. Conversely, either hypoxic adaptation or iron chelation prevents menadione-induced ferroxitosis. Ectopic expression of K213Q HIF-1α mutant blunts the effects of menadione. However, knockdown of HIF-1α or PKM2 restores menadione-induced cytotoxicity in hypoxia. Similarly, exposure of melanoma cells to shikonin, a menadione analog and a potential PKM2 inhibitor, is sufficient to induce ferroxitosis under hypoxic conditions. Collectively, our findings reveal that ferroxitosis curtails metabolic plasticity in melanoma.

Tyrosine kinase inhibition in leukemia induces an altered metabolic state sensitive to mitochondrial perturbations

PURPOSE

Although tyrosine kinase inhibitors (TKI) can be effective therapies for leukemia, they fail to fully eliminate leukemic cells and achieve durable remissions for many patients with advanced BCR-ABL(+) leukemias or acute myelogenous leukemia (AML). Through a large-scale synthetic lethal RNAi screen, we identified pyruvate dehydrogenase, the limiting enzyme for pyruvate entry into the mitochondrial tricarboxylic acid cycle, as critical for the survival of chronic myelogenous leukemia (CML) cells upon BCR-ABL inhibition. Here, we examined the role of mitochondrial metabolism in the survival of Ph(+) leukemia and AML upon TK inhibition.

EXPERIMENTAL DESIGN

Ph(+) cancer cell lines, AML cell lines, leukemia xenografts, cord blood, and patient samples were examined.

RESULTS

We showed that the mitochondrial ATP-synthase inhibitor oligomycin-A greatly sensitized leukemia cells to TKI in vitro. Surprisingly, oligomycin-A sensitized leukemia cells to BCR-ABL inhibition at concentrations of 100- to 1,000-fold below those required for inhibition of respiration. Oligomycin-A treatment rapidly led to mitochondrial membrane depolarization and reduced ATP levels, and promoted superoxide production and leukemia cell apoptosis when combined with TKI. Importantly, oligomycin-A enhanced elimination of BCR-ABL(+) leukemia cells by TKI in a mouse model and in primary blast crisis CML samples. Moreover, oligomycin-A also greatly potentiated the elimination of FLT3-dependent AML cells when combined with an FLT3 TKI, both in vitro and in vivo.

CONCLUSIONS

TKI therapy in leukemia cells creates a novel metabolic state that is highly sensitive to particular mitochondrial perturbations. Targeting mitochondrial metabolism as an adjuvant therapy could therefore improve therapeutic responses to TKI for patients with BCR-ABL(+) and FLT3(ITD) leukemias.

Co-acting gene networks predict TRAIL responsiveness of tumour cells with high accuracy

Background

Identification of differentially expressed genes from transcriptomic studies is one of the most common mechanisms to identify tumor biomarkers. This approach however is not well suited to identify interaction between genes whose protein products potentially influence each other, which limits its power to identify molecular wiring of tumour cells dictating response to a drug. Due to the fact that signal transduction pathways are not linear and highly interlinked, the biological response they drive may be better described by the relative amount of their components and their functional relationships than by their individual, absolute expression.

Results

Gene expression microarray data for 109 tumor cell lines with known sensitivity to the death ligand cytokine tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) was used to identify genes with potential functional relationships determining responsiveness to TRAIL-induced apoptosis. The machine learning technique Random Forest in the statistical environment “R” with backward elimination was used to identify the key predictors of TRAIL sensitivity and differentially expressed genes were identified using the software GeneSpring. Gene co-regulation and statistical interaction was assessed with q-order partial correlation analysis and non-rejection rate. Biological (functional) interactions amongst the co-acting genes were studied with Ingenuity network analysis. Prediction accuracy was assessed by calculating the area under the receiver operator curve using an independent dataset. We show that the gene panel identified could predict TRAIL-sensitivity with a very high degree of sensitivity and specificity (AUC = 0 · 84). The genes in the panel are co-regulated and at least 40% of them functionally interact in signal transduction pathways that regulate cell death and cell survival, cellular differentiation and morphogenesis. Importantly, only 12% of the TRAIL-predictor genes were differentially expressed highlighting the importance of functional interactions in predicting the biological response.

Conclusions

The advantage of co-acting gene clusters is that this analysis does not depend on differential expression and is able to incorporate direct- and indirect gene interactions as well as tissue- and cell-specific characteristics. This approach (1) identified a descriptor of TRAIL sensitivity which performs significantly better as a predictor of TRAIL sensitivity than any previously reported gene signatures, (2) identified potential novel regulators of TRAIL-responsiveness and (3) provided a systematic view highlighting fundamental differences between the molecular wiring of sensitive and resistant cell types.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1144) contains supplementary material, which is available to authorized users.

Effects of epithelial to mesenchymal transition on T cell targeting of melanoma cells

Melanoma cells can switch phenotype in a manner similar to epithelial to mesenchymal transition (EMT). In this perspective article, we address the effects of such phenotype switching on T cell targeting of tumor cells. During the EMT-like switch in phenotype, a concomitant change in expression of multiple tumor antigens occurs. Melanoma cells undergoing EMT escape from killing by T cells specific for antigens whose expression is downregulated by this process. We discuss melanoma antigens whose expression is influenced by EMT. We assess the effect of changes in the expressed tumor antigen repertoire on T-cell mediated tumor recognition and killing. In addition to escape from T cell immunity via changes in antigen expression, mesenchymal-like melanoma cells are generally more resistant to classical chemotherapy and radiotherapy. However, we demonstrate that when targeting antigens whose expression is unaltered during EMT, the capacity of T cells to kill melanoma cell lines in vitro is not influenced by their phenotype. When considering immune therapies such as cancer vaccination, these data suggest escape from T cell killing due to phenotype switching in melanoma could potentially be avoided by careful selection of target antigen.

Is reliance on mitochondrial respiration a "chink in the armor" of therapy-resistant cancer?

A series of recent reports has suggested PGC1α-driven upregulation of mitochondrial oxidative phosphorylation as a selective vulnerability of drug-resistant cancers. Accordingly, chemical inhibitors of respiration led to selective eradication of such cancer cells due to their preferential sensitivity to mitochondrial production of reactive oxygen species. These insights create a timely opportunity for a biomarker guided application of already existing and newly emerging mitochondrial inhibitors in recurrent drug-resistant cancer, including lymphomas, melanomas, and other malignant diseases marked by increased mitochondrial respiration.

A stress-induced early innate response causes multidrug tolerance in melanoma

Acquired drug resistance constitutes a major challenge for effective cancer therapies with melanoma being no exception. The dynamics leading to permanent resistance are poorly understood but are important to design better treatments. Here we show that drug exposure, hypoxia or nutrient starvation leads to an early innate cell response in melanoma cells resulting in multidrug resistance, termed induced drug-tolerant cells (IDTCs). Transition into the IDTC state seems to be an inherent stress reaction for survival toward unfavorable environmental conditions or drug exposure. The response comprises chromatin remodeling, activation of signaling cascades and markers implicated in cancer stemness with higher angiogenic potential and tumorigenicity. These changes are characterized by a common increase in CD271 expression concomitantly with loss of differentiation markers such as melan-A and tyrosinase, enhanced aldehyde dehydrogenase (ALDH) activity and upregulation of histone demethylases. Accordingly, IDTCs show a loss of H3K4me3, H3K27me3 and gain of H3K9me3 suggesting activation and repression of differential genes. Drug holidays at the IDTC state allow for reversion into parental cells re-sensitizing them to the drug they were primarily exposed to. However, upon continuous drug exposure IDTCs eventually transform into permanent and irreversible drug-resistant cells. Knockdown of CD271 or KDM5B decreases transition into the IDTC state substantially but does not prevent it. Targeting IDTCs would be crucial for sustainable disease management and prevention of acquired drug resistance.

Involvement of superoxide and nitric oxide in BRAF(V600E) inhibitor PLX4032-induced growth inhibition of melanoma cells

The BRAF(V600E) inhibitor PLX4032 (Vemurafenib) is an FDA-approved new drug for the treatment of metastatic melanomas, which specifically inhibits the RAS/MEK/ERK signaling pathway to control cell proliferation and adhesion. However, no study has been carried out to investigate the role of intracellular oxidative balance in PLX4032-induced tumor growth inhibition. Herein, for the first time, superoxide (O2˙(-)) and nitric oxide (NO) generated from PLX4032-challenged melanoma cells were monitored using electrochemical sensors and conventional fluorescein staining techniques. Impacts of superoxide dismutase (SOD) and NG-monomethyl-L-arginine monoacetate (L-NMMA), a nitric oxide synthase inhibitor, were also examined to demonstrate the specificity of ROS/NO generation and its biological consequences. PLX4032 specifically triggers production of O2˙(-) and NO from BRAF(V600E) mutant A375 cells. SOD and L-NMMA could abolish the PLX4032-induced increase in intracellular O2˙(-) and NO production, thereby rescuing cell growth in BRAF mutant A375 cells (A375(BRAFV600E)). In addition, PLX4032 treatment could decrease the mitochondrial membrane potential in A375(BRAFV600E) cells. The results suggest that PLX4032 can selectively cause ROS production and depolarization of mitochondrial membranes, potentially initiating apoptosis and growth inhibition of PLX4032-sensitive cells. This work not only proposes a new mechanism for PLX4032-induced melanoma cell inhibition, but also highlights potential applications of electrochemical biosensors in cell biology and drug screening.

Metabolic and transcriptional profiling reveals pyruvate dehydrogenase kinase 4 as a mediator of epithelial-mesenchymal transition and drug resistance in tumor cells

Background

Accumulating preclinical and clinical evidence implicates epithelial-mesenchymal transition (EMT) in acquired resistance to anticancer drugs; however, mechanisms by which the mesenchymal state determines drug resistance remain unknown.

Results

To explore a potential role for altered cellular metabolism in EMT and associated drug resistance, we analyzed the metabolome and transcriptome of three lung cancer cell lines that were rendered drug resistant following experimental induction of EMT. This analysis revealed evidence of metabolic rewiring during EMT that diverts glucose to the TCA cycle. Such rewiring was at least partially mediated by the reduced expression of pyruvate dehydrogenase kinase 4 (PDK4), which serves as a gatekeeper of the TCA cycle by inactivating pyruvate dehydrogenase (PDH). Overexpression of PDK4 partially blocked TGFβ-induced EMT; conversely, PDK4 inhibition via RNAi-mediated knockdown was sufficient to drive EMT and promoted erlotinib resistance in EGFR mutant lung cancer cells. We identified a novel interaction between PDK4 and apoptosis-inducing factor (AIF), an inner mitochondrial protein that appears to play a role in mediating this resistance. In addition, analysis of human tumor samples revealed PDK4-low as a predictor of poor prognosis in lung cancer and that PDK4 expression is dramatically downregulated in most tumor types.

Conclusions

Together, these findings implicate PDK4 as a critical metabolic regulator of EMT and associated drug resistance.

Electronic supplementary material

The online version of this article (doi:10.1186/2049-3002-2-20) contains supplementary material, which is available to authorized users.

Combining a BCL2 inhibitor with the retinoid derivative fenretinide targets melanoma cells including melanoma initiating cells

Investigations from multiple laboratories support the existence of melanoma initiating cells (MICs) that potentially contribute to melanoma's drug resistance. ABT-737, a small molecule BCL-2/BCL-XL/BCL-W inhibitor, is promising in cancer treatments, but not very effective against melanoma, with the anti-apoptotic protein MCL-1 as the main contributor to resistance. The synthetic retinoid fenretinide (4-HPR) has shown promise for treating breast cancers. Here, we tested whether the combination of ABT-737 with 4-HPR is effective in killing both the bulk of melanoma cells and MICs. The combination synergistically decreased cell viability and caused cell death in multiple melanoma cells lines (carrying either BRAF or NRAS mutations), but not in normal melanocytes. The combination increased the NOXA expression and caspase-dependent MCL-1 degradation. Knocking-down NOXA protected cells from combination-induced apoptosis, implicating the role of NOXA in the drug synergy. The combination treatment also disrupted primary spheres (a functional assay for MICs) and decreased the percentage of ALDH^high cells (a marker of MICs) in melanoma cell lines. Moreover, the combination inhibited the self-renewal capacity of MICs, measured by secondary sphere forming assays. In vivo, the combination inhibited tumor growth. Thus, this combination is a promising treatment strategy for melanoma, regardless of mutation status of BRAF or NRAS.

KDM5 interacts with Foxo to modulate cellular levels of oxidative stress

Increased cellular levels of oxidative stress are implicated in a large number of human diseases. Here we describe the transcription co-factor KDM5 (also known as Lid) as a new critical regulator of cellular redox state. Moreover, this occurs through a novel KDM5 activity whereby it alters the ability of the transcription factor Foxo to bind to DNA. Our microarray analyses of kdm5 mutants revealed a striking enrichment for genes required to regulate cellular levels of oxidative stress. Consistent with this, loss of kdm5 results in increased sensitivity to treatment with oxidizers, elevated levels of oxidized proteins, and increased mutation load. KDM5 activates oxidative stress resistance genes by interacting with Foxo to facilitate its recruitment to KDM5-Foxo co-regulated genes. Significantly, this occurs independently of KDM5's well-characterized demethylase activity. Instead, KDM5 interacts with the lysine deacetylase HDAC4 to promote Foxo deacetylation, which affects Foxo DNA binding.

Mitochondrial oxidative stress is the Achille's heel of melanoma cells resistant to Braf-mutant inhibitor

Vemurafenib/PLX4032, a selective inhibitor of mutant BRAFV600E, constitutes a paradigm shift in melanoma therapy. Unfortunately, acquired resistance, which unavoidably occurs, represents one major limitation to clinical responses. Recent studies have highlighted that vemurafenib activated oxidative metabolism in BRAFV600E melanomas expressing PGC1α. However, the oxidative state of melanoma resistant to BRAF inhibitors is unknown. We established representative in vitro and in vivo models of human melanoma resistant to vemurafenib including primary specimens derived from melanoma patients. Firstly, our study reveals that vemurafenib increased mitochondrial respiration and ROS production in BRAFV600E melanoma cell lines regardless the expression of PGC1α. Secondly, melanoma cells that have acquired resistance to vemurafenib displayed intrinsically high rates of mitochondrial respiration associated with elevated mitochondrial oxidative stress irrespective of the presence of vemurafenib. Thirdly, the elevated ROS level rendered vemurafenib-resistant melanoma cells prone to cell death induced by pro-oxidants including the clinical trial drug, elesclomol. Based on these observations, we propose that the mitochondrial oxidative signature of resistant melanoma constitutes a novel opportunity to overcome resistance to BRAF inhibition.

Human mitochondrial NAD(P)(+)-dependent malic enzyme participates in cutaneous melanoma progression and invasion

Cutaneous melanoma is the most life-threatening neoplasm of the skin, accounting for most of the skin cancer deaths. Accumulating evidence suggests that targeting metabolism is an appealing strategy for melanoma therapy. Mitochondrial NAD(P)(+)-dependent malic enzyme (ME2), an oxidative decarboxylase, was evaluated for its biological significance in cutaneous melanoma progression. ME2 mRNA and protein expression significantly increased during melanoma progression, as evidenced by Gene Expression Omnibus analysis and immunohistochemistry on clinically annotated tissue microarrays, respectively. In addition, ME2 knockdown attenuated melanoma cell proliferation in vitro. ME2 ablation resulted in reduced cellular ATP levels and elevated cellular reactive oxygen species production, which activated the AMP-activated protein kinase pathway and inhibited acetyl-CoA carboxylase. Furthermore, ME2 expression was associated with cell migration and invasion. ME2 knockdown decreased anchorage-independent growth in vitro and tumor cell growth in vivo. These results suggested that ME2 might be an important factor in melanoma progression and a novel biomarker of invasion.

Single-cell imaging tools for brain energy metabolism: a review

Neurophotonics comes to light at a time in which advances in microscopy and improved calcium reporters are paving the way toward high-resolution functional mapping of the brain. This review relates to a parallel revolution in metabolism. We argue that metabolism needs to be approached both in vitro and in vivo, and that it does not just exist as a low-level platform but is also a relevant player in information processing. In recent years, genetically encoded fluorescent nanosensors have been introduced to measure glucose, glutamate, ATP, NADH, lactate, and pyruvate in mammalian cells. Reporting relative metabolite levels, absolute concentrations, and metabolic fluxes, these sensors are instrumental for the discovery of new molecular mechanisms. Sensors continue to be developed, which together with a continued improvement in protein expression strategies and new imaging technologies, herald an exciting era of high-resolution characterization of metabolism in the brain and other organs.

Is it time to test biguanide metformin in the treatment of melanoma?

Metformin is the most widely used antidiabetic drug that belongs to the biguanide class. It is very well tolerated and has the major clinical advantage of not inducing hypoglycemia. Metformin decreases hepatic glucose production via a mechanism requiring liver kinase B1, which controls the metabolic checkpoint, AMP-activated protein kinase-mammalian target of rapamycin and neoglucogenic genes. The effects of metformin on this pathway results in reduced protein synthesis and cell proliferation. These observations have given the impetus for many investigations on the role of metformin in the regulation of tumor cell proliferation, cell-cycle regulation, apoptosis, and autophagy. Encouraging results from these studies have shown that metformin could potentially be used as an efficient anticancer drug in various neoplasms such as prostate, breast, lung, pancreas cancers, and melanoma. These findings are strengthened by retrospective epidemiological studies that have found a decrease in cancer risk in diabetic patients treated with metformin. In this review, we have focused our discussion on recent molecular mechanisms of metformin that have been described in various solid tumors in general and in melanoma in particular.

JARID1B is a luminal lineage-driving oncogene in breast cancer

Recurrent mutations in histone-modifying enzymes imply key roles in tumorigenesis, yet their functional relevance is largely unknown. Here, we show that JARID1B, encoding a histone H3 lysine 4 (H3K4) demethylase, is frequently amplified and overexpressed in luminal breast tumors and a somatic mutation in a basal-like breast cancer results in the gain of unique chromatin binding and luminal expression and splicing patterns. Downregulation of JARID1B in luminal cells induces basal genes expression and growth arrest, which is rescued by TGFβ pathway inhibitors. Integrated JARID1B chromatin binding, H3K4 methylation, and expression profiles suggest a key function for JARID1B in luminal cell-specific expression programs. High luminal JARID1B activity is associated with poor outcome in patients with hormone receptor-positive breast tumors.

Adaptive resistance to RAF inhibitors in melanoma

The discovery of activating mutations in BRAF at high frequency in cutaneous melanoma opened the door to new treatment options, which have resulted in significantly better patient outcomes. Treatments such as the FDA-approved RAF inhibitor vemurafenib and the more recently approved dabrafenib and trametinib combination therapy are designed to target the ERK1/2 pathway. Initial success in targeting this pathway is evidenced by the high percentage of melanoma patients who undergo tumor remission. However, the beneficial effects of these targeted therapies are usually short-lived due to the development of resistance, which leads to disease progression. As a result, studies have focused on the acquired forms of resistance that develop following continued exposure to therapy. Conversely, far fewer studies have investigated the adaptive forms of resistance, which activate rapidly, promote cell survival, and may underlie the development of acquired resistance by providing melanoma cells the time to develop additional mutations. We provide a detailed review of the known mechanisms of adaptive resistance in melanoma and relate them to similar responses to targeted therapies in other tumor types.