Adaptive Responses as Mechanisms of Resistance to BRAF Inhibitors in Melanoma

SIRT7 promotes mitochondrial biogenesis to render the adaptive resistance to MAPK inhibition in melanoma

Melanoma, arising from the malignant transformation of melanocytes, stands as the most lethal type of skin cancer. While significant strides have been made in targeted therapy and immunotherapy, substantially enhancing therapeutic efficacy, the prognosis for melanoma patients remains unoptimistic. SIRT7, a nuclear-localized deacetylase, plays a pivotal role in maintaining cellular homeostasis and adapting to external stressors in melanoma, with its activity closely tied to intracellular nicotinamide adenine dinucleotide (NAD+). However, its involvement in adaptive resistance to targeted therapy remains unclear. Herein, we unveil that up-regulated SIRT7 promotes mitochondrial biogenesis to render the adaptive resistance to MAPK inhibition in melanoma. Initially, we observed a significant increase of SIRT7 expression in publicly available datasets following targeted therapy within a short duration. In consistent, we found elevated SIRT7 expression in melanoma cells subjected to BRAF or MEK inhibitors in vitro. The up-regulation of SIRT7 expression was also confirmed in xenograft tumors in mice after targeted therapy in vivo. Furthermore, we proved that SIRT7 deficiency led to decreased cell viability upon prolonged exposure to BRAF or MEK inhibitors, accompanied by an increase in cell apoptosis. Mechanistically, SIRT7 deficiency restrained the upregulation of genes associated with mitochondrial biogenesis and intracellular ATP levels in response to targeted therapy treatment in melanoma cells. Ultimately, we proved that SIRT7 deficieny could sensitize BRAF-mutant melanoma cells to MAPK inhibition targeted therapy in vivo. In conclusion, our findings underscore the role of SIRT7 in fostering adaptive resistance to targeted therapy through the facilitation of mitochondrial biogenesis. Targeting SIRT7 emerges as a promising strategy to overcome MAPK inhibitor adaptive resistance in melanoma.

Targeted therapy or immunotherapy in BRAF-mutated metastatic melanoma: a Spanish center's decade of experience

Background

Targeted therapies and immunotherapy are currently considered the mainstay first-line treatment for advanced BRAF-mutated melanoma. However, the impact of treatment (targeted therapy and immunotherapy) and the prognostic factors are still not clear.

Material and methods

Medical records of 140 patients diagnosed with advanced melanoma between 2011 and 2021 were retrospectively reviewed to extract demographic, BRAF status, treatment, performance status, and survival data. ORR, PFS, and OS were compared between patients diagnosed with advanced melanoma and treated with first-line IT or BRAF/MEKi. The prognostic factors were assessed using Cox regression models.

Results

In all patients and those treated with immunotherapy, we did not find any effect of BRAF status on ORR, PFS, or OS. In patients with BRAF-mutated melanoma, ORR was 43.8% vs. 70% (P=0.04), PFS was 19.2 vs. 11.5 months (p=0.22), and OS was 33.4 vs. 16.4 months for the immunotherapy and targeted therapy groups, respectively (P=0.04). ECOG, presence of brain metastases, and high LDH level from initiation of first-line treatment were all associated with differences in PFS and OS.

Conclusion

Patients with advanced BRAF-mutated melanoma treated with first-line immunotherapy had a significantly longer PFS and OS than those treated with first-line BRAF/MEKi; however, first-line BRAF/MEKi treatment had a significantly higher ORR than first-line immunotherapy.

Targeting the RAS/RAF/MAPK pathway for cancer therapy: from mechanism to clinical studies

Metastatic dissemination of solid tumors, a leading cause of cancer-related mortality, underscores the urgent need for enhanced insights into the molecular and cellular mechanisms underlying metastasis, chemoresistance, and the mechanistic backgrounds of individuals whose cancers are prone to migration. The most prevalent signaling cascade governed by multi-kinase inhibitors is the mitogen-activated protein kinase (MAPK) pathway, encompassing the RAS-RAF-MAPK kinase (MEK)-extracellular signal-related kinase (ERK) pathway. RAF kinase is a primary mediator of the MAPK pathway, responsible for the sequential activation of downstream targets, such as MEK and the transcription factor ERK, which control numerous cellular and physiological processes, including organism development, cell cycle control, cell proliferation and differentiation, cell survival, and death. Defects in this signaling cascade are associated with diseases such as cancer. RAF inhibitors (RAFi) combined with MEK blockers represent an FDA-approved therapeutic strategy for numerous RAF-mutant cancers, including melanoma, non-small cell lung carcinoma, and thyroid cancer. However, the development of therapy resistance by cancer cells remains an important barrier. Autophagy, an intracellular lysosome-dependent catabolic recycling process, plays a critical role in the development of RAFi resistance in cancer. Thus, targeting RAF and autophagy could be novel treatment strategies for RAF-mutant cancers. In this review, we delve deeper into the mechanistic insights surrounding RAF kinase signaling in tumorigenesis and RAFi-resistance. Furthermore, we explore and discuss the ongoing development of next-generation RAF inhibitors with enhanced therapeutic profiles. Additionally, this review sheds light on the functional interplay between RAF-targeted therapies and autophagy in cancer.

BRAF Mutations in Melanoma: Biological Aspects, Therapeutic Implications, and Circulating Biomarkers

Melanoma is an aggressive form of skin cancer resulting from the malignant transformation of melanocytes. Recent therapeutic approaches, including targeted therapy and immunotherapy, have improved the prognosis and outcome of melanoma patients. BRAF is one of the most frequently mutated oncogenes recognised in melanoma. The most frequent oncogenic BRAF mutations consist of a single point mutation at codon 600 (mostly V600E) that leads to constitutive activation of the BRAF/MEK/ERK (MAPK) signalling pathway. Therefore, mutated BRAF has become a useful target for molecular therapy and the use of BRAF kinase inhibitors has shown promising results. However, several resistance mechanisms invariably develop leading to therapeutic failure. The aim of this manuscript is to review the role of BRAF mutational status in the pathogenesis of melanoma and its impact on differentiation and inflammation. Moreover, this review focuses on the mechanisms responsible for resistance to targeted therapies in BRAF-mutated melanoma and provides an overview of circulating biomarkers including circulating tumour cells, circulating tumour DNA, and non-coding RNAs.

Challenges and Opportunities in the Crusade of BRAF Inhibitors: From 2002 to 2022

Serine/threonine-protein kinase B-Raf (BRAF; RAF = rapidly accelerated fibrosarcoma) plays an important role in the mitogen-activated protein kinase (MAPK) signaling cascade. Somatic mutations in the BRAF gene were first discovered in 2002 by Davies et al., which was a major breakthrough in cancer research. Subsequently, three different classes of BRAF mutants have been discovered. This class includes class I monomeric mutants (BRAFV600), class II BRAF homodimer mutants (non-V600), and class III BRAF heterodimers (non-V600). Cancers caused by these include melanoma, thyroid cancer, ovarian cancer, colorectal cancer, nonsmall cell lung cancer, and others. In this study, we have highlighted the major binding pockets in BRAF protein, their active and inactive conformations with inhibitors, and BRAF dimerization and its importance in paradoxical activation and BRAF mutation. We have discussed the first-, second-, and third-generation drugs approved by the Food and Drug Administration and drugs under clinical trials with all four different binding approaches with DFG-IN/OUT and αC-IN/OUT for BRAF protein. We have investigated particular aspects and difficulties with all three generations of inhibitors. Finally, this study has also covered recent developments in synthetic BRAF inhibitors (from their discovery in 2002 to 2022), their unique properties, and importance in inhibiting BRAF mutants.

An updated literature on BRAF inhibitors (2018-2023)

BRAF is the most common serine-threonine protein kinase and regulates signal transduction from RAS to MEK inside the cell. The BRAF is a highly active isoform of RAF kinase. BRAF has two domains such as regulatory and kinase domains. The BRAF inhibitors bind in the c-terminus of the kinase domain and inhibit the downstream pathways. The mutation occurs mainly in the A-loop of the kinase domain. The mutation occurs due to a conversion of valine to glutamate/lysine/arginine/aspartic acid at 600th position. Among the diverse mutations, BRAF^V600E is the most common and responsible for numerous cancer such as melanoma, colorectal, ovarian, and thyroid cancer. Due to mutations in RAC1, loss of PTEN, NF1, CCND1, USP28-FBW7 complex, COT overexpression, and CCND1 amplification, the BRAF kinase enzyme developed resistance over the commercially available BRAF inhibitors. There is still unmute urgence for the development of BRAF inhibitors to overcome the persistent limitation such as resistance, mutation, and adverse effects of drugs. In the current study, we described the structure, activation, downstream signaling pathway, and mutation of BRAF. Our group also provided a detailed review of BRAF inhibitors from the last five years (2018-2023) highlighting the structure-activity relationship, mechanistic study, and molecular docking studies. We hope that the current analysis will be a useful resource for researchers and provide chemists a glimpse into the future as design and development of more effective and secure BRAF kinase inhibitors. The development of BRAF inhibitors to overcome the persistent limitation such as resistance, mutation, and adverse effects of drugs. In depth description about different heterocyclic scaffolds (quinoline, imidazole, pyridine, triazole, pyrrole etc.) as BRAF inhibitors from the last five years (2018-2023) highlighting the structure-activity relationship, mechanistic study, and molecular docking studies.

A Network of MicroRNAs and mRNAs Involved in Melanosome Maturation and Trafficking Defines the Lower Response of Pigmentable Melanoma Cells to Targeted Therapy

BACKGROUND

The ability to increase their degree of pigmentation is an adaptive response that confers pigmentable melanoma cells higher resistance to BRAF inhibitors (BRAFi) compared to non-pigmentable melanoma cells.

METHODS

Here, we compared the miRNome and the transcriptome profile of pigmentable 501Mel and SK-Mel-5 melanoma cells vs. non-pigmentable A375 melanoma cells, following treatment with the BRAFi vemurafenib (vem). In depth bioinformatic analyses (clusterProfiler, WGCNA and SWIMmeR) allowed us to identify the miRNAs, mRNAs and biological processes (BPs) that specifically characterize the response of pigmentable melanoma cells to the drug. Such BPs were studied using appropriate assays in vitro and in vivo (xenograft in zebrafish embryos).

RESULTS

Upon vem treatment, miR-192-5p, miR-211-5p, miR-374a-5p, miR-486-5p, miR-582-5p, miR-1260a and miR-7977, as well as GPR143, OCA2, RAB27A, RAB32 and TYRP1 mRNAs, are differentially expressed only in pigmentable cells. These miRNAs and mRNAs belong to BPs related to pigmentation, specifically melanosome maturation and trafficking. In fact, an increase in the number of intracellular melanosomes-due to increased maturation and/or trafficking-confers resistance to vem.

CONCLUSION

We demonstrated that the ability of pigmentable cells to increase the number of intracellular melanosomes fully accounts for their higher resistance to vem compared to non-pigmentable cells. In addition, we identified a network of miRNAs and mRNAs that are involved in melanosome maturation and/or trafficking. Finally, we provide the rationale for testing BRAFi in combination with inhibitors of these biological processes, so that pigmentable melanoma cells can be turned into more sensitive non-pigmentable cells.

DUSP4 protects BRAF- and NRAS-mutant melanoma from oncogene overdose through modulation of MITF

MAPK inhibitors (MAPKi) remain an important component of the standard of care for metastatic melanoma. However, acquired resistance to these drugs limits their therapeutic benefit. Tumor cells can become refractory to MAPKi by reactivation of ERK. When this happens, tumors often become sensitive to drug withdrawal. This drug addiction phenotype results from the hyperactivation of the oncogenic pathway, a phenomenon commonly referred to as oncogene overdose. Several feedback mechanisms are involved in regulating ERK signaling. However, the genes that serve as gatekeepers of oncogene overdose in mutant melanoma remain unknown. Here, we demonstrate that depletion of the ERK phosphatase, DUSP4, leads to toxic levels of MAPK activation in both drug-naive and drug-resistant mutant melanoma cells. Importantly, ERK hyperactivation is associated with down-regulation of lineage-defining genes including MITF Our results offer an alternative therapeutic strategy to treat mutant melanoma patients with acquired MAPKi resistance and those unable to tolerate MAPKi.

Inhibition of Axl Promotes the Therapeutic Effect of Targeted Inhibition of the PI3K/Akt Pathway in NRAS Mutant Melanoma Cells

Melanoma is a malignant tumor produced by highly aggressive and metastatic melanocytes. NRAS mutation is a relatively common mutation in melanoma cells. Mitogen-activated protein kinase (MAPK) signaling pathway and the PI3K/Akt pathway in melanoma cells are relatively common signaling pathways. In this study, we investigated the effect of inhibition of Axl expression on the targeted inhibition of the PI3K/Akt pathway in NRAS-mutant melanoma cells. In this study, immunohistochemistry and western blot methods were used to detect the expression of Axl and Akt proteins in melanoma cells. Axl inhibitor was added, and it detected the inhibitory efficiency of Akt inhibitor in melanoma cells. Finally, a melanoma mouse model was established, and it detected the proliferation and apoptosis of mouse tumor cells induced by Axl inhibitor and Akt inhibitor. The results showed that Axl and Akt were highly expressed in NRAS-mutant melanoma cells, and stimulation of Axl expression could reduce the inhibitory effect of Akt inhibitor on melanoma cells. The addition of Axl inhibitor can synergistically promote the effect of Akt inhibitor, slow down the proliferation of tumor cells, and induce cell apoptosis. According to the experiment in this study, Axl inhibitor combined with Akt inhibitor has a stronger therapeutic effect on melanoma than Akt inhibitor alone.

Increase in the sensitivity to PLX4720 through inhibition of transcription factor EB-dependent autophagy in BRAF inhibitor-resistant cells

Long-term treatment with oncogenic BRAF inhibitors confers resistance to BRAF inhibitor monotherapy. In this study, a combination treatment strategy with autophagy inhibitors was proposed to increase the sensitivity of BRAF mutant containing A375P melanoma cells that have developed resistance to BRAF inhibitors. We found that the A375P/Multi-drug resistance (A375P/Mdr) cells, which are resistant to both BRAF inhibitors and MEK inhibitors, exhibited a higher basal autophagic flux compared to their parental A375P cells, as determined by tandem mRFP-GFP-tagged LC3 imaging assay and LC3 conversion. In addition, transcription factor EB (TFEB), which acts as a transcription factor regulating the transcription of autophagy-related genes, was much more localized in the nucleus in A375P/Mdr cells than in A375P cells, indicating that the increase in basal autophagic flux was TFEB-dependent. In particular, the overexpression of an activated form of TFEB (TFEBAA) caused a modest increase in PLX4720 resistance in A375P/Mdr cells. Interestingly, treatment with early stage autophagy inhibitors reversed BRAF inhibitor-induced resistance, whereas late autophagy inhibition did not. In contrast, inhibition of ER stress by 4-phenylbutyric acid suppressed basal autophagic flux. Moreover, ER stress inhibition significantly remarkably inhibited the nuclear localization of TFEB, resulting in an increase in the sensitivity of A375P/Mdr cells to PLX4720. Taken together, these results suggest that autophagy may be an important mechanism of acquired resistance to BRAF inhibitors. Thus, targeting autophagy may be suitable for the treatment of tumors resistant to BRAF inhibitor.

The role of PI3'-lipid signalling in melanoma initiation, progression and maintenance

Phosphatidylinositol-3'-kinases (PI3Ks) are a family of lipid kinases that phosphorylate the 3' hydroxyl (OH) of the inositol ring of phosphatidylinositides (PI). Through their downstream effectors, PI3K generated lipids (PI3K-lipids hereafter) such as PI(3,4,5)P3 and PI(3,4)P2 regulate myriad biochemical and biological processes in both normal and cancer cells including responses to growth hormones and cytokines; the cell division cycle; cell death; cellular growth; angiogenesis; membrane dynamics; and autophagy and many aspects of cellular metabolism. Engagement of receptor tyrosine kinase by their cognate ligands leads to activation of members of the Class I family of PI3'-kinases (PI3Kα, β, δ & γ) leading to accumulation of PI3K-lipids. Importantly, PI3K-lipid accumulation is antagonized by the hydrolytic action of a number of PI3K-lipid phosphatases, most notably the melanoma suppressor PTEN (lipid phosphatase and tensin homologue). Downstream of PI3K-lipid production, the protein kinases AKT1-3 are believed to be key effectors of PI3'-kinase signalling in cells. Indeed, in preclinical models, activation of the PI3K→AKT signalling axis cooperates with alterations such as expression of the BRAFV600E oncoprotein kinase to promote melanoma progression and metastasis. In this review, we describe the different classes of PI3K-lipid effectors, and how they may promote melanomagenesis, influence the tumour microenvironment, melanoma maintenance and progression to metastatic disease. We also provide an update on both FDA-approved or experimental inhibitors of the PI3K→AKT pathway that are currently being evaluated for the treatment of melanoma either in preclinical models or in clinical trials.

A Theranostic Cellulose Nanocrystal-Based Drug Delivery System with Enhanced Retention in Pulmonary Metastasis of Melanoma

Metastatic melanoma can be difficult to detect until at the advanced state that decreases the survival rate of patients. Several FDA-approved BRAF inhibitors have been used for treatment of metastatic melanoma, but overall therapeutic efficacy has been limited. Lutetium-177 (177 Lu) enables simultaneous tracking of tracer accumulation with single-photon emission computed tomography and radiotherapy. Therefore, the codelivery of 177 Lu alongside chemotherapeutic agents using nanoparticles (NPs) might improve the therapeutic outcome in metastatic melanoma. Cellulose nanocrystals (CNC NPs) can particularly deliver payloads to lung capillaries in vivo. Herein, 177 Lu-labeled CNC NPs loaded with vemurafenib ([177 Lu]Lu-CNC-V NPs) is developed and the therapeutic effect in BRAF V600E mutation-harboring YUMM1.G1 murine model of lung metastatic melanoma is investigated. The [177 Lu]Lu-CNC-V NPs demonstrate favorable radiolabel stability, drug release profile, cellular uptake, and cell growth inhibition in vitro. In vivo biodistribution reveals significant retention of the [177 Lu]Lu-CNC-V NPs in the lung, liver, and spleen. Ultimately, the median survival time of animals is doubly increased after treatment with [177 Lu]Lu-CNC-V NPs compared to control groups. The enhanced therapeutic efficacy of [177 Lu]Lu-CNC-V NPs in the lung metastatic melanoma animal model provides convincing evidence for the potential of clinical translation for theranostic CNC NP-based drug delivery systems after intravenous administration.

Mechanisms of Resistance to PI3K Inhibitors in Cancer: Adaptive Responses, Drug Tolerance and Cellular Plasticity

The phosphatidylinositol-3-kinase (PI3K) pathway plays a central role in the regulation of several signalling cascades which regulate biological processes such as cellular growth, survival, proliferation, motility and angiogenesis. The hyperactivation of this pathway is linked to tumour progression and is one of the most common events in human cancers. Additionally, aberrant activation of the PI3K pathway has been demonstrated to limit the effectiveness of a number of anti-tumour agents paving the way for the development and implementation of PI3K inhibitors in the clinic. However, the overall effectiveness of these compounds has been greatly limited by inadequate target engagement due to reactivation of the pathway by compensatory mechanisms. Herein, we review the common adaptive responses that lead to reactivation of the PI3K pathway, therapy resistance and potential strategies to overcome these mechanisms of resistance. Furthermore, we highlight the potential role in changes in cellular plasticity and PI3K inhibitor resistance.

Pathogenic germline variants are associated with poor survival in stage III/IV melanoma patients

Patients with late stage resected cutaneous melanoma have poor overall survival (OS) and experience irreversible adverse events from systemic therapy. There is a clinical need to identify biomarkers to predict outcome. Performing germline/tumour whole-exome sequencing of 44 stage III/IV melanoma patients we identified pathogenic germline mutations in CDKN2A, CDK4, ATM, POLH, MRE11A, RECQL4 and XPC, affecting 7/44 patients. These mutations were associated with poor OS (p = 0.0082). We confirmed our findings in The Cancer Genome Atlas (TCGA) human skin cutaneous melanoma cohort where we identified pathogenic variants in 40/455 patients (p = 0.0203). Combining these cohorts (n = 499) further strengthened these findings showing germline carriers had worse OS (p = 0.0009). Additionally, we determined whether tumour mutation burden (TMB) or BRAF status were prognostic markers of survival. Low TMB rate (< 20 Mut/Mb; p = 0.0034) and BRAF p.V600 mutation (p = 0.0355) were associated with worse progression-free survival. Combining these biomarkers indicated that V600 mutant patients had significantly lower TMB (p = 0.0155). This was confirmed in the TCGA (n = 443, p = 0.0007). Integrative analysis showed germline mutation status conferred the highest risk (HR 5.2, 95% CI 1.72–15.7). Stage IV (HR 2.5, 0.74–8.6) and low TMB (HR 2.3, 0.57–9.4) were similar, whereas BRAF V600 status was the weakest prognostic biomarker (HR 1.5, 95% CI 0.44–5.2).

Tumor Microenvironment: Implications in Melanoma Resistance to Targeted Therapy and Immunotherapy

Simple Summary

The response to pharmacological treatments is deeply influenced by the tight interactions between the tumor cells and the microenvironment. In this review we describe, for melanoma, the most important mechanisms of resistance to targeted therapy and immunotherapy mediated by the components of the microenvironment. In addition, we briefly describe the most recent therapeutic advances for this pathology. The knowledge of molecular mechanisms, which are underlying of drug resistance, is fundamental for the development of new therapeutic approaches for the treatment of melanoma patients.

Abstract

Antitumor therapies have made great strides in recent decades. Chemotherapy, aggressive and unable to discriminate cancer from healthy cells, has given way to personalized treatments that, recognizing and blocking specific molecular targets, have paved the way for targeted and effective therapies. Melanoma was one of the first tumor types to benefit from this new care frontier by introducing specific inhibitors for v-Raf murine sarcoma viral oncogene homolog B (BRAF), mitogen-activated protein kinase kinase (MEK), v-kit Hardy–Zuckerman 4 feline sarcoma viral oncogene homolog (KIT), and, recently, immunotherapy. However, despite the progress made in the melanoma treatment, primary and/or acquired drug resistance remains an unresolved problem. The molecular dynamics that promote this phenomenon are very complex but several studies have shown that the tumor microenvironment (TME) plays, certainly, a key role. In this review, we will describe the new melanoma treatment approaches and we will analyze the mechanisms by which TME promotes resistance to targeted therapy and immunotherapy.

A Review of the Molecular Pathways Involved in Resistance to BRAF Inhibitors in Patients with Advanced-Stage Melanoma

Melanoma is an aggressive malignancy of melanocytes and most commonly arises in the skin. In 2002, BRAF gene mutations were identified in melanoma, and this finding resulted in the development of several small-molecule molecular inhibitors that specifically targeted the BRAF V600E mutation. The development of targeted therapies for advanced-stage melanoma, including tyrosine kinase inhibitors (TKIs) of the BRAF (V600E) kinase, vemurafenib and dabrafenib, have been approved for the treatment of advanced melanoma leading to improved clinical outcomes. However, the development of BRAF inhibitor (BRAFi) resistance has significantly reduced the therapeutic efficacy after prolonged treatment. Recent studies have identified the molecular mechanisms for BRAFi resistance. This review aims to describe the impact of BRAFi resistance on the pathogenesis of melanoma, the current status of molecular pathways involved in BRAFi resistance, including intrinsic resistance, adaptive resistance, and acquired resistance. This review will discuss how an understanding of the mechanisms associated with BRAFi resistance may aid the identification of useful strategies for overcoming the resistance to BRAF-targeted therapy in patients with advanced-stage melanoma.

Dissecting Mechanisms of Melanoma Resistance to BRAF and MEK Inhibitors Revealed Genetic and Non-Genetic Patient- and Drug-Specific Alterations and Remarkable Phenotypic Plasticity

The clinical benefit of MAPK pathway inhibition in BRAF-mutant melanoma patients is limited by the development of acquired resistance. Using drug-naïve cell lines derived from tumor specimens, we established a preclinical model of melanoma resistance to vemurafenib or trametinib to provide insight into resistance mechanisms. Dissecting the mechanisms accompanying the development of resistance, we have shown that (i) most of genetic and non-genetic alterations are triggered in a cell line- and/or drug-specific manner; (ii) several changes previously assigned to the development of resistance are induced as the immediate response to the extent measurable at the bulk levels; (iii) reprogramming observed in cross-resistance experiments and growth factor-dependence restricted by the drug presence indicate that phenotypic plasticity of melanoma cells largely contributes to the sustained resistance. Whole-exome sequencing revealed novel genetic alterations, including a frameshift variant of RBMX found exclusively in phospho-AKT^high resistant cell lines. There was no similar pattern of phenotypic alterations among eleven resistant cell lines, including expression/activity of crucial regulators, such as MITF, AXL, SOX, and NGFR, which suggests that patient-to-patient variability is richer and more nuanced than previously described. This diversity should be considered during the development of new strategies to circumvent the acquired resistance to targeted therapies.

SOX2 upregulates side population cells and enhances their chemoresistant ability by transactivating ABCC1 expression contributing to intrinsic resistance to paclitaxel in melanoma

Paclitaxel is the last choice for the treatment of advanced melanoma as a second-line chemotherapeutic agent, but there are still many cases of intrinsic resistance to paclitaxel in melanoma and the reasons that cause paclitaxel resistance remain unclear. Here, we identified that high expression of SRY-box transcription factor 2 (SOX2) and high ratio of side population (SP) cells reduced the sensitivity to paclitaxel in melanoma cells. The knockout and the induction of SOX2 completely depleted and significantly upregulated the ratios of melanoma SP cells, respectively. These data suggest that SOX2, a pluripotent transcription factor for inducing cancer stem cells in melanoma, is also sufficient and necessary for the induction of melanoma SP cells. ATP-binding cassette (ABC) subfamily C member 1 (ABCC1) is one of ABC transporters which causes SP cells to be resistance to chemotherapeutic agents by efficiently pumping drugs out of cells. The knockout and the induction of ABCC1 significantly increased and decreased the sensitivity of melanoma cells to paclitaxel. High expression of ABCC1 was identified in melanoma cell lines with high expression of SOX2 and in their SP cells. SOX2 was identified to induce ABCC1 transcription. Taken together, SOX2 upregulates SP cells and enhances their chemoresistant ability by increasing ABCC1 expression, which contributes to intrinsic resistance to paclitaxel in melanoma. Our findings will lead to new insights into melanoma biology and therapy resistance, and eventually to new therapeutic targets.