Vemurafenib and trametinib reduce expression of CTGF and IL-8 in (V600E)BRAF melanoma cells

Extracellular vesicles promote migration despite BRAF inhibitor treatment in malignant melanoma cells

Extracellular vesicles (EVs) constitute a vital component of intercellular communication, exerting significant influence on metastasis formation and drug resistance mechanisms. Malignant melanoma (MM) is one of the deadliest forms of skin cancers, because of its high metastatic potential and often acquired resistance to oncotherapies. The prevalence of BRAF mutations in MM underscores the importance of BRAF-targeted therapies, such as vemurafenib and dabrafenib, alone or in combination with the MEK inhibitor, trametinib. This study aimed to elucidate the involvement of EVs in MM progression and ascertain whether EV-mediated metastasis promotion persists during single agent BRAF (vemurafenib, dabrafenib), or MEK (trametinib) and combined BRAF/MEK (dabrafenib/trametinib) inhibition.Using five pairs of syngeneic melanoma cell lines, we assessed the impact of EVs - isolated from their respective supernatants - on melanoma cell proliferation and migration. Cell viability and spheroid growth assays were employed to evaluate proliferation, while migration was analyzed through mean squared displacement (MSD) and total traveled distance (TTD) measurements derived from video microscopy and single-cell tracking.Our results indicate that while EV treatments had remarkable promoting effect on cell migration, they exerted only a modest effect on cell proliferation and spheroid growth. Notably, EVs demonstrated the ability to mitigate the inhibitory effects of BRAF inhibitors, albeit they were ineffective against a MEK inhibitor and the combination of BRAF/MEK inhibitors. In summary, our findings contribute to the understanding of the intricate role played by EVs in tumor progression, metastasis, and drug resistance in MM.

RIPK4 downregulation impairs Wnt3A-stimulated invasiveness via Wnt/β-catenin signaling in melanoma cells and tumor growth in vivo

PURPOSE

The role of Wnt signaling in oncogenesis and drug resistance is well known. Receptor-interacting protein kinase (RIPK4) contributing to the increased activity of many signaling pathways, including Wnt/β-catenin, may be an important target for designing new drugs for metastatic melanoma, but its role in melanoma is not fully understood.

METHODS

We tested the effect of genetic manipulation of RIPK4 (CRISPR/Cas9) on xenograft growth. In addition, immunohistochemistry was used to detect active β-catenin, Ki67 and necrosis in xenografts. Wnt signaling pathway activity was examined using Western blot and Top-Flash. The effect of RIPK4 knockout on melanoma cells in vitro stimulated Wnt3A on wound overgrowth, migration and invasion ability was then evaluated.

RESULTS

Our study showed that CRISPR/Cas9-mediated RIPK4 knockout (KO) significantly reduced tumor growth in a mouse model of melanoma, particularly of WM266.4 cells. RIPK4 KO tumors exhibited lower percentages of Ki67+ cells as well as reduced necrotic area and decreased levels of active β-catenin. In addition, we observed that RIPK4 knockout impaired Wnt3A-induced activation of LRP6 and β-catenin, as manifested by a decrease in the transcriptional activity of β-catenin in Top-Flash in both tested melanoma cell lines, A375 and WM266.4. Prolonged incubation (48 h) with Wnt3A showed reduced level of MMP9, C-myc, and increased SOX10, proteins whose transcription is also dependent on β-catenin activity. Moreover, RIPK4 knockout led to the inhibition of scratch overgrowth, migration and invasion of these cells compared to their controls.

CONCLUSION

RIPK4 knockdown inhibits melanoma tumor growth and Wnt3A stimulated migration and invasion indicating that RIPK4 might be a potential target for melanoma therapy.

Pro-Apoptotic Activity of MCL-1 Inhibitor in Trametinib-Resistant Melanoma Cells Depends on Their Phenotypes and Is Modulated by Reversible Alterations Induced by Trametinib Withdrawal

BACKGROUND

Although BRAFV600/MEK inhibitors improved the treatment of melanoma patients, resistance is acquired almost inevitably.

METHODS

Trametinib withdrawal/rechallenge and MCL-1 inhibition in trametinib-resistance models displaying distinct p-ERK1/2 levels were investigated.

RESULTS

Trametinib withdrawal/rechallenge caused reversible changes in ERK1/2 activity impacting the balance between pro-survival and pro-apoptotic proteins. Reversible alterations were found in MCL-1 levels and MCL-1 inhibitors, BIM and NOXA. Taking advantage of melanoma cell dependency on MCL-1 for survival, we used S63845. While it was designed to inhibit MCL-1 activity, we showed that it also significantly reduced NOXA levels. S63845-induced apoptosis was detected as the enhancement of Annexin V-positivity, caspase-3/7 activation and histone H2AX phosphorylation. Percentages of Annexin V-positive cells were increased most efficiently in trametinib-resistant melanoma cells displaying the p-ERK1/2low/MCL-1low/BIMhigh/NOXAlow phenotype with EC50 values at concentrations as low as 0.1 μM. Higher ERK1/2 activity associated with increased MCL-1 level and reduced BIM level limited pro-apoptotic activity of S63845 further influenced by a NOXA level.

CONCLUSIONS

Our study supports the notion that the efficiency of an agent designed to target a single protein can largely depend on the phenotype of cancer cells. Thus, it is important to define appropriate phenotype determinants to stratify the patients for the novel therapy.

Trametinib-Resistant Melanoma Cells Displaying MITFhigh/NGFRlow/IL-8low Phenotype Are Highly Responsive to Alternating Periods of Drug Withdrawal and Drug Rechallenge

Despite significant advances in targeted therapies against the hyperactivated BRAF^V600/MEK pathway for patients with unresectable metastatic melanoma, acquired resistance remains an unsolved clinical problem. In this study, we focused on melanoma cells resistant to trametinib, an agent broadly used in combination therapies. Molecular and cellular changes were assessed during alternating periods of trametinib withdrawal and rechallenge in trametinib-resistant cell lines displaying either a differentiation phenotype (MITF^high/NGFR^low) or neural crest stem-like dedifferentiation phenotype (NGFR^high/MITF^low). Neither drug withdrawal nor drug rechallenge induced cell death, and instead of loss of fitness, trametinib-resistant melanoma cells adapted to altered conditions by phenotype switching. In resistant cells displaying a differentiation phenotype, trametinib withdrawal markedly decreased MITF level and activity, which was associated with reduced cell proliferation capacity, and induced stemness assessed as NGFR-positive cells and senescence features, including IL-8 expression and secretion. All these changes could be reversed by trametinib re-exposure, which emphasizes melanoma cell plasticity. Trametinib-resistant cells displaying a dedifferentiation phenotype were less responsive presumably due to the already low level of MITF, a master regulator of the melanoma phenotype. Considering new directions of the development of anti-melanoma treatment, our study suggests that the phenotype of melanomas resistant to targeted therapy might be a crucial determinant of the selection of second-line therapy for melanoma patients.

Structural and Biofunctional Insights into the Cyclo(Pro-Pro-Phe-Phe-) Scaffold from Experimental and In Silico Studies: Melanoma and Beyond

Short peptides have great potential as safe and effective anticancer drug leads. Herein, the influence of short cyclic peptides containing the Pro-Pro-Phe-Phe sequence on patient-derived melanoma cells was investigated. Cyclic peptides such as cyclo(Leu-Ile-Ile-Leu-Val-Pro-Pro-Phe-Phe-), called CLA, and cyclo(Pro-homoPro-β3homoPhe-Phe-), called P11, exert the cytotoxic and the cytostatic effects in melanoma cells, respectively. CLA was the most active peptide as it reduced the viability of melanoma cells to 50% of control at about 10 µM, whereas P11 at about 40 µM after 48 h incubation. Interestingly, a linear derivative of P11 did not induce any effect in melanoma cells confirming previous studies showing that cyclic peptides exert better biological activity compared to their linear counterparts. According to in silico predictions, cyclic tetrapeptides show a better pharmacokinetic and toxic profile to humans than CLA. Notably, the spatial structure of those peptides containing synthetic amino acids has not been explored yet. In the Cambridge Structural Database, there is only one such cyclic tetrapeptide, cyclo((R)-β2homoPhe-D-Pro-Lys-Phe-), while in the Protein Data Bank—none. Therefore, we report the first crystal structure of cyclo(Pro-Pro-β3homoPhe-Phe-), denoted as 4B8M, a close analog of P11, which is crucial for drug discovery. Comparative molecular and supramolecular analysis of both structures was performed. The DFT findings revealed that 4B8M is well interpreted in the water solution. The results of complex Hirshfeld surface investigations on the cooperativity of interatomic contacts in terms of electrostatic and energetic features are provided. In short, the enrichment ratio revealed O…H/H…O and C…H/H…C as privileged intercontacts in the crystals in relation to basic and large supramolecular H-bonding synthon patterns. Furthermore, the ability of self-assemble 4B8M leading to a nanotubular structure is also discussed.

Interleukin-8 in Melanoma Pathogenesis, Prognosis and Therapy-An Integrated View into Other Neoplasms and Chemokine Networks

Cutaneous melanoma accounts for only about 7% of skin cancers but is causing almost 90% of deaths. Melanoma cells have a distinct repertoire of mutations from other cancers, a high plasticity and degree of mimicry toward vascular phenotype, stemness markers, versatility in evading and suppress host immune control. They exert a significant influence on immune, endothelial and various stromal cells which form tumor microenvironment. The metastatic stage, the leading cause of mortality in this neoplasm, is the outcome of a complex, still poorly understood, cross-talk between tumor and other cell phenotypes. There is accumulating evidence that Interleukin-8 (IL-8) is emblematic for advanced melanomas. This work aimed to present an updated status of IL-8 in melanoma tumor cellular complexity, through a comprehensive analysis including data from other chemokines and neoplasms. The multiple processes and mechanisms surveyed here demonstrate that IL-8 operates following orchestrated programs within signaling webs in melanoma, stromal and vascular cells. Importantly, the yet unknown molecularity regulating IL-8 impact on cells of the immune system could be exploited to overturn tumor fate. The molecular and cellular targets of IL-8 should be brought into the attention of even more intense scientific exploration and valorization in the therapeutical management of melanoma.

Targeting Genome Stability in Melanoma-A New Approach to an Old Field

Despite recent groundbreaking advances in the treatment of cutaneous melanoma, it remains one of the most treatment-resistant malignancies. Due to resistance to conventional chemotherapy, the therapeutic focus has shifted away from aiming at melanoma genome stability in favor of molecularly targeted therapies. Inhibitors of the RAS/RAF/MEK/ERK (MAPK) pathway significantly slow disease progression. However, long-term clinical benefit is rare due to rapid development of drug resistance. In contrast, immune checkpoint inhibitors provide exceptionally durable responses, but only in a limited number of patients. It has been increasingly recognized that melanoma cells rely on efficient DNA repair for survival upon drug treatment, and that genome instability increases the efficacy of both MAPK inhibitors and immunotherapy. In this review, we discuss recent developments in the field of melanoma research which indicate that targeting genome stability of melanoma cells may serve as a powerful strategy to maximize the efficacy of currently available therapeutics.

BH3 mimetics potentiate pro-apoptotic activity of encorafenib in BRAFV600E melanoma cells

BRAF^V600- and MEK1/2-targeting therapies rarely produce durable response in melanoma patients. We investigated five BRAF^V600E melanoma cell lines derived from drug-naïve tumor specimens to assess cell death response to encorafenib (Braftovi), a recently FDA-approved BRAF^V600 inhibitor. Drug-naïve cell lines (i) did not harbor damaging alterations in genes encoding core apoptotic machinery, but they differed in (ii) mitochondrial priming as demonstrated by whole-cell BH3 profiling, and (iii) levels of selected anti-apoptotic proteins. Encorafenib modulated the balance between apoptosis-regulating proteins as it upregulated BIM and BMF, and attenuated NOXA, but did not affect the levels of pro-survival proteins except for MCL-1 and BCL-X_L in selected cell lines. Induction of apoptosis could be predicted using Dynamic BH3 profiling. The extent of apoptosis was dependent on both (i) cell-intrinsic proximity to the apoptotic threshold (initial mitochondrial priming) and (ii) the abundance of encorafenib-induced BIM (iBIM; drug-induced change in priming). While co-inhibition of MCL-1 and BCL-X_L/BCL-2 was indispensable for apoptosis in drug-naïve cells, encorafenib altered cell dependence to MCL-1, and reliance on BCL-X_L/BCL-2 was additionally found in cell lines that were highly primed to apoptosis by encorafenib. This translated into robust apoptosis when encorafenib was combined with selective BH3 mimetics. Our study provides a mechanistic insight into the role of proteins from the BCL-2 family in melanoma cell response to targeted therapy, and presents preclinical evidence that (i) MCL-1 is a druggable target to potentiate encorafenib activity, whereas (ii) pharmacological inhibition of BCL-X_L/BCL-2 might be relevant but only for a narrow group of encorafenib-treated patients.

Parthenolide as Cooperating Agent for Anti-Cancer Treatment of Various Malignancies

Primary and acquired resistance of cancer to therapy is often associated with activation of nuclear factor kappa B (NF-κB). Parthenolide (PN) has been shown to inhibit NF-κB signaling and other pro-survival signaling pathways, induce apoptosis and reduce a subpopulation of cancer stem-like cells in several cancers. Multimodal therapies that include PN or its derivatives seem to be promising approaches enhancing sensitivity of cancer cells to therapy and diminishing development of resistance. A number of studies have demonstrated that several drugs with various targets and mechanisms of action can cooperate with PN to eliminate cancer cells or inhibit their proliferation. This review summarizes the current state of knowledge on PN activity and its potential utility as complementary therapy against different cancers.

Insulin Reduces the Efficacy of Vemurafenib and Trametinib in Melanoma Cells

BACKGROUND

Despite the progress made in the clinical management of metastatic melanoma, a patient's response to treatment cannot be fully predicted, and intrinsic or acquired resistance that is developed in most melanoma patients warrants further research efforts. In addition to genetic factors, microenvironmental input should be considered to explain the diversity of response to treatment among melanoma patients. In this study, we evaluated the impact of insulin on patient-derived BRAFV600E melanoma cells, either untreated or treated with vemurafenib or trametinib, inhibitors of BRAFV600 and MEK1/2, respectively.

METHODS

Cells were cultured in serum-free conditions, either with or without insulin. The activity of the MAPK/ERK and PI3K/AKT pathways was assessed by Western blotting, cell viability, and percentages of Ki-67- and NGFR-positive cells by flow cytometry. Transcript levels were analyzed using qRT-PCR, and γ-H2AX levels by immunoblotting and confocal microscopy. A luminescence-based assay was used to measure glutathione content.

RESULTS

While insulin did not influence the MAPK/ERK pathway activity, it had a strong influence on melanoma cells, in which this pathway was suppressed by either vemurafenib or trametinib. In the presence of insulin, both drugs were much less efficient in 1) inhibiting proliferation and reducing the percentage of Ki-67-positive cells, and 2) inducing apoptosis and phosphorylation of histone H2AX in melanoma cells. Changes induced by vemurafenib and trametinib in glutathione homeostasis and DNA repair gene expression were also attenuated by insulin. Moreover, insulin impaired the combined effects of targeted drugs and doxorubicin in melanoma cells. In addition to insulin-induced PI3K/AKT activity, which was either transient or sustainable depending on the cell line, an insulin-triggered increase in the percentage of cells expressing NGFR, a marker of neural crest stem-like cells, may contribute to the reduced drug efficacy.

CONCLUSION

Our results demonstrate the role of insulin in reducing the efficacy of vemurafenib and trametinib. This needs clinical assessment.

17-Aminogeldanamycin Inhibits Constitutive Nuclear Factor-Kappa B (NF-κB) Activity in Patient-Derived Melanoma Cell Lines

Melanoma remains incurable skin cancer, and targeting heat shock protein 90 (HSP90) is a promising therapeutic approach. In this study, we investigate the effect of 17-aminogeldanamycin, a potent HSP90 inhibitor, on nuclear factor-kappa B (NF-κB) activity in BRAF^V600E and NRAS^Q61R patient-derived melanoma cell lines. We performed time-lapse microscopy and flow cytometry to monitor changes in cell confluence and viability. The NF-κB activity was determined by immunodetection of phospho-p65 and assessment of expression of NF-κB-dependent genes by quantitative real-time polymerase chain reaction (qRT-PCR), Western blotting, and enzyme-linked immunosorbent assay (ELISA). Constitutive activity of p65/NF-κB was evident in all melanoma cell lines. Differences in its level might be associated with genetic alterations in CHUK, IL1B, MAP3K14, NFKBIE, RIPK1, and TLR4, while differences in transcript levels of NF-κB-inducible genes revealed by PCR array might result from the contribution of other regulatory mechanisms. 17-Aminogeldanamycin markedly diminished the level of phospho-p65, but the total p65 protein level was unaltered, indicating that 17-aminogeldanamycin inhibited activation of p65/NF-κB. This conclusion was supported by significantly reduced expression of selected NF-κB-dependent genes: cyclin D1 (CCND1), C-X-C motif chemokine ligand 8 (CXCL8), and vascular endothelial growth factor (VEGF), as shown at transcript and protein levels, as well as secretion of IL-8 and VEGF. Our study indicates that 17-aminogeldanamycin can be used for efficient inhibition of NF-κB activity and the simultaneous diminution of IL-8 and VEGF levels in the extracellular milieu of melanoma.

BRAF inhibition curtails IFN-gamma-inducible PD-L1 expression and upregulates the immunoregulatory protein galectin-1 in melanoma cells

Although melanoma is considered one of the most immunogenic malignancies, spontaneous T‐cell responses to melanoma antigens are ineffective due to tumor cell‐intrinsic or microenvironment‐driven immune evasion mechanisms. For example, oncogenic BRAF V600E mutation in melanoma cells fosters tumor immune escape by modulating cell immunogenicity and microenvironment composition. BRAF inhibition has been shown to increase melanoma cell immunogenicity, but these effects are transient and long‐term responses are uncommon. For these reasons, we aimed to further characterize the role of BRAF‐V600E mutation in the modulation of PD‐L1, a known immunoregulatory molecule, and galectin‐1 (Gal‐1), a potent immunoregulatory lectin involved in melanoma immune privilege. We report herein that vemurafenib downregulates IFN‐γ‐induced PD‐L1 expression by interfering with STAT1 activity and by decreasing PD‐L1 protein translation. Surprisingly, melanoma cells exposed to vemurafenib expressed higher levels of Gal‐1. In coculture experiments, A375 melanoma cells pretreated with vemurafenib induced apoptosis of interacting Jurkat T cells, whereas genetic inhibition of Gal‐1 in these cells restored the viability of cocultured T lymphocytes, indicating that Gal‐1 contributes to tumor immune escape. Importantly, Gal‐1 plasma concentration increased in patients progressing on BRAF/MEK inhibitor treatment, but remained stable in responding patients. Taken together, these results suggest a two‐faceted nature of BRAF inhibition‐associated immunomodulatory effects: an early immunostimulatory activity, mediated at least in part by decreased PD‐L1 expression, and a delayed immunosuppressive effect associated with Gal‐1 induction. Importantly, our observations suggest that Gal‐1 might be utilized as a potential biomarker and a putative therapeutic target in melanoma patients.

Anti-Inflammatory Properties of Chemical Probes in Human Whole Blood: Focus on Prostaglandin E2 Production

We screened 57 chemical probes, high-quality tool compounds, and relevant clinically used drugs to investigate their effect on pro-inflammatory prostaglandin E2 (PGE2) production and interleukin-8 (IL-8) secretion in human whole blood. Freshly drawn blood from healthy volunteers and patients with systemic lupus erythematosus (SLE) or dermatomyositis was incubated with compounds at 0.1 or 1 µM and treated with lipopolysaccharide (LPS, 10 µg/ml) to induce a pro-inflammatory condition. Plasma was collected after 24 h for lipid profiling using liquid chromatography tandem mass spectrometry (LC-MS/MS) and IL-8 quantification using enzyme-linked immunosorbent assay (ELISA). Each compound was tested in at least four donors at one concentration based on prior knowledge of binding affinities and in vitro activity. Our screening suggested that PD0325901 (MEK-1/2 inhibitor), trametinib (MEK-1/2 inhibitor), and selumetinib (MEK-1 inhibitor) decreased while tofacitinib (JAK inhibitor) increased PGE2 production. These findings were validated by concentration-response experiment in two donors. Moreover, the tested MEK inhibitors decreased thromboxane B2 (TXB2) production and IL-8 secretion. We also investigated the lysophophatidylcholine (LPC) profile in plasma from treated whole blood as these lipids are potentially important mediators in inflammation, and we did not observe any changes in LPC profiles. Collectively, we deployed a semi-high throughput and robust methodology to investigate anti-inflammatory properties of new chemical probes.

Non-Apoptotic Cell Death Signaling Pathways in Melanoma

Resisting cell death is a hallmark of cancer. Disturbances in the execution of cell death programs promote carcinogenesis and survival of cancer cells under unfavorable conditions, including exposition to anti-cancer therapies. Specific modalities of regulated cell death (RCD) have been classified based on different criteria, including morphological features, biochemical alterations and immunological consequences. Although melanoma cells are broadly equipped with the anti-apoptotic machinery and recurrent genetic alterations in the components of the RAS/RAF/MEK/ERK signaling markedly contribute to the pro-survival phenotype of melanoma, the roles of autophagy-dependent cell death, necroptosis, ferroptosis, pyroptosis, and parthanatos have recently gained great interest. These signaling cascades are involved in melanoma cell response and resistance to the therapeutics used in the clinic, including inhibitors of BRAFmut and MEK1/2, and immunotherapy. In addition, the relationships between sensitivity to non-apoptotic cell death routes and specific cell phenotypes have been demonstrated, suggesting that plasticity of melanoma cells can be exploited to modulate response of these cells to different cell death stimuli. In this review, the current knowledge on the non-apoptotic cell death signaling pathways in melanoma cell biology and response to anti-cancer drugs has been discussed.

ERK3/MAPK6 controls IL-8 production and chemotaxis

ERK3 is a ubiquitously expressed member of the atypical mitogen activated protein kinases (MAPKs) and the physiological significance of its short half-life remains unclear. By employing gastrointestinal 3D organoids, we detect that ERK3 protein levels steadily decrease during epithelial differentiation. ERK3 is not required for 3D growth of human gastric epithelium. However, ERK3 is stabilized and activated in tumorigenic cells, but deteriorates over time in primary cells in response to lipopolysaccharide (LPS). ERK3 is necessary for production of several cellular factors including interleukin-8 (IL-8), in both, normal and tumorigenic cells. Particularly, ERK3 is critical for AP-1 signaling through its interaction and regulation of c-Jun protein. The secretome of ERK3-deficient cells is defective in chemotaxis of neutrophils and monocytes both in vitro and in vivo. Further, knockdown of ERK3 reduces metastatic potential of invasive breast cancer cells. We unveil an ERK3-mediated regulation of IL-8 and epithelial secretome for chemotaxis.

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.

TYRP1 mRNA level is stable and MITF-M-independent in drug-naïve, vemurafenib- and trametinib-resistant BRAFV600E melanoma cells

TYRP1 mRNA is of interest due to its potential non-coding role as a sponge sequestering tumor-suppressive miRs in melanoma. To our knowledge, there is no report on changes in TYRP1 expression in melanomas after development of resistance to targeted therapies. We used patient-derived drug-naïve RAS^Q61R and BRAF^V600E melanoma cell lines. In BRAF^V600E melanoma cells, resistance to vemurafenib and trametinib was developed. A time-lapse fluorescence microscope was used to rate proliferation, qRT-PCR and Western blotting were used to assess TYRP1 expression and MITF-M level and activity. A high TYRP1 protein level in RAS^Q61R cells corresponded with high TYRP1 mRNA level, whereas undetectable TYRP1 protein in BRAF^V600E cells was accompanied by medium mRNA level, also in cells carrying NF1^R135W variant in addition. TYRP1 expression was MITF-M-independent, since similar transcript status was found in MITF-M^high and MITF-M^low cells. For the first time, we showed that TYRP1 expression remained unaltered in melanoma cells that became resistant to vemurafenib or trametinib, including those cells losing MITF-M. Also drug discontinuation in resistant cells did not substantially affect TYRP1 expression. To verify in vitro results, publicly available microarray data were analyzed. TYRP1 transcript levels stay unaltered in the majority of paired melanoma samples from patients before treatment and after relapse caused by resistance to targeted therapies. As TYRP1 mRNA level remains unaltered in melanoma cells during development of resistance to vemurafenib or trametinib, therapies developed to terminate a sponge activity of TYRP1 transcript may be extended to patients that relapse with resistant disease.

Physiologically Relevant Oxygen Concentration (6% O2) as an Important Component of the Microenvironment Impacting Melanoma Phenotype and Melanoma Response to Targeted Therapeutics In Vitro

Cancer cell phenotype largely depends on oxygen availability. The atmospheric oxygen concentration (21%) used in in vitro studies is much higher than in any human tissue. Using well-characterized patient-derived melanoma cell lines, we compared: (i) activities of several signaling pathways, and (ii) the effects of vemurafenib and trametinib in hyperoxia (21% O₂), normoxia (6% O₂) and hypoxia (1% O₂). A high plasticity of melanoma cells in response to changes in oxygen supplementation and drug treatment was observed, and the transcriptional reprograming and phenotypic changes varied between cell lines. Normoxia enhanced the expression of vascular endothelial growth factor (VEGF), glucose metabolism/transport-related genes, and changed percentages of NGFR- and MITF-positive cells in cell line-dependent manner. Increased protein stability might be responsible for high PGC1α level in MITF^low melanoma cells. Vemurafenib and trametinib while targeting the activity of MAPK/ERK pathway irrespective of oxygen concentration, were less effective in normoxia than hyperoxia in reducing levels of VEGF, PGC1α, SLC7A11 and Ki-67-positive cells in cell line-dependent manner. In conclusion, in vitro studies performed in atmospheric oxygen concentration provide different information on melanoma cell phenotype and response to drugs than performed in normoxia, which might partially explain the discrepancies between results obtained in vitro and in clinical settings.

Plasticity of Drug-Naïve and Vemurafenib- or Trametinib-Resistant Melanoma Cells in Execution of Differentiation/Pigmentation Program

Melanoma plasticity creates a plethora of opportunities for cancer cells to escape treatment. Thus, therapies must target all cancer cell subpopulations bearing the potential to contribute to disease. The role of the differentiation/pigmentation program in intrinsic and acquired drug resistance is largely uncharacterized. MITF level and expression of MITF-dependent pigmentation-related genes, MLANA, PMEL, TYR, and DCT, in drug-naïve and vemurafenib- or trametinib-treated patient-derived melanoma cell lines and their drug-resistant counterparts were analysed and referred to genomic alterations. Variability in execution of pigmentation/differentiation program was detected in patient-derived melanoma cell lines. Acute treatment with vemurafenib or trametinib enhanced expression of pigmentation-related genes in MITF-M^high melanoma cells, partially as the consequence of transcriptional reprograming. During development of resistance, changes in pigmentation program were not unidirectional, but also not universal as expression of different pigmentation-related genes was diversely affected. In selected resistant cell lines, differentiation/pigmentation was promoted and might be considered as one of drug-tolerant phenotypes. In other resistant lines, dedifferentiation was induced. Upon drug withdrawal ("drug holiday"), the dedifferentiation process in resistant cells either was enhanced but reversed by drug reexposure suggesting involvement of epigenetic mechanisms or was irreversible. The irreversible dedifferentiation might be connected with homozygous loss-of-function mutation in MC1R, as MC1R^{R151C  +/+} variant was found exclusively in drug-naïve MITF-M^low dedifferentiated cells and drug-resistant cells derived from MITF^high/MC1R^WT cells undergoing irreversible dedifferentiation. MC1R^{R151C  +/+} variant might be further investigated as a parameter potentially impacting melanoma patient stratification and aiding in treatment decision.

3D Coculture Models Underline Metastatic Melanoma Cell Sensitivity to Vemurafenib

IMPACT STATEMENT

Three dimensional in vitro cell culture systems better reflect the native structural architecture of tissues and are attractive to investigate cancer cell sensitivity to drugs. We have developed and compared several metastatic melanoma (MM) models cultured as a monolayer (2D) and cocultured on three dimensional (3D) dermal equivalents with fibroblasts to better unravel factors modulating cell sensitivity to vemurafenib, a BRAF inhibitor. The heterotypic 3D melanoma model we have established summarizes paracrine signalization by stromal cells and type I collagen matrix, mimicking the natural microenvironment of cutaneous MM, and allows for the identification of potent sensitive melanoma cells to the drug. This model could be a powerful tool for predicting drug efficiency.

Acquired resistance to BRAFi reverses senescence-like phenotype in mutant BRAF melanoma

Targeting MAPK pathway in mutant BRAF melanoma with the specific BRAF inhibitor vemurafenib showed robust initial responses in the majority of patients followed by relapses due to acquired resistance to the drug. In ^V600EBRAF melanoma cell lines, senescence-associated β-galactosidase activity is often encountered in a constitutive manner or induced after MAPK inhibition. However, the link between the senescence-like phenotype and the resistance to BRAF inhibition is not fully understood yet. Our data validate a senescence-like phenotype (low cell proliferation, high cell volume, and high β-Gal activity) in mutant BRAF cells. Vemurafenib increased β-Gal activity in 4 out of 5 sensitive lines and in 2 out of 5 lines with intrinsic resistance to the drug. Interestingly, the 3 lines with acquired resistance to vemurafenib became depending on the drug for proliferation. In absence of drug, these lines showed a lower cell proliferation rate together with a substantial increase of β-Gal activity both in vitro and in vivo. In all settings, the senescence-like phenotype was significantly associated with an inhibition of pRB and cyclin D1, explaining the inhibition of cell proliferation. In conclusion, β-Gal activity is increased by ^V600EBRAF inhibition in the majority of sensitive and intrinsically resistant melanoma cells. Acquired resistance to vemurafenib is associated with a dependence to the drug for cell proliferation and tumor growth, and, in this case, drug removal stimulate β-Gal activity suggesting that the senescence-like phenotype could contribute to the acquired resistance to BRAF inhibition.

Exogenous growth factors bFGF, EGF and HGF do not influence viability and phenotype of V600EBRAF melanoma cells and their response to vemurafenib and trametinib in vitro

It has been shown that the response of ^V600EBRAF melanoma cells to targeted therapeutics is affected by growth factors. We have investigated the influence of three different growth factors, bFGF, EGF and HGF used either alone or in combination, on the response of ^V600EBRAF melanoma cell populations established from surgical specimens to vemurafenib and trametinib, targeting ^V600EBRAF and MEK1/2, respectively.

We report that proliferation and phenotype of ^V600EBRAF melanoma cell populations were not detectably influenced by exogenous growth factors. Neither cell distribution in cell cycle and CCND1 expression nor activity of signaling pathways crucial for melanoma development and maintenance, including the RAF/MEK/ERK pathway, WNT/β-catenin pathway and NF-κB signaling, were affected by the presence of different growth factors. We furthermore show that vemurafenib and trametinib abrogated the activity of ERK1/2, arrested cells in G₀/G₁ cell cycle phase, triggered apoptosis, induced changes in the expression of CXCL8, CCND1 and CTGF and the frequency of Ki-67^high and CD271^high cells. These effects were, however, similar in the presence of different growth factors. Interestingly, comparable results were also obtained for melanoma cells grown without exogenous growth factors bFGF, EGF and HGF for a period as long as 4 months prior the drug treatment.

We conclude that the composition or lack of exogenous growth factors bFGF, EGF and HGF do not markedly influence viability and phenotype of ^V600EBRAF melanoma cells and their response to vemurafenib and trametinib in vitro. Our results question the necessity of these growth factors in the medium that is used for culturing ^V600EBRAF melanoma cells.

Synthetic lethal short hairpin RNA screening reveals that ring finger protein 183 confers resistance to trametinib in colorectal cancer cells

Background

The mitogen-activated extracellular signal-regulated kinase 1/2 (MEK1/2) inhibitor trametinib has shown promising therapeutic effects on melanoma, but its efficacy on colorectal cancer (CRC) is limited. Synthetic lethality arises with a combination of two or more separate gene mutations that causes cell death, whereas individual mutations keep cells alive. This study aimed to identify the genes responsible for resistance to trametinib in CRC cells, using a synthetic lethal short hairpin RNA (shRNA) screening approach.

Methods

We infected HT29 cells with a pooled lentiviral shRNA library and applied next-generation sequencing to identify shRNAs with reduced abundance after 8-day treatment of 20 nmol/L trametinib. HCT116 and HT29 cells were used in validation studies. Stable ring finger protein 183 (RNF183)-overexpressing cell lines were generated by pcDNA4-myc/his-RNF183 transfection. Stable RNF183-knockdown cell lines were generated by infection of lentiviruses that express RNF183 shRNA, and small interference RNA (siRNA) was used to knock down RNF183 transiently. Quantitative real-time PCR was used to determine the mRNA expression. Western blotting, immunohistochemical analysis, and enzyme-linked immunosorbent assay (ELISA) were used to evaluate the protein abundance. MTT assay, colony formation assay, and subcutaneous xenograft tumor growth model were used to evaluate cell proliferation.

Results

In the primary screening, we found that the abundance of RNF183 shRNA was markedly reduced after treatment with trametinib. Trametinib induced the expression of RNF183, which conferred resistance to drug-induced cell growth repression and apoptotic and non-apoptotic cell deaths. Moreover, interleukin-8 (IL-8) was a downstream gene of RNF183 and was required for the function of RNF183 in facilitating cell growth. Additionally, elevated RNF183 expression partly reduced the inhibitory effect of trametinib on IL-8 expression. Finally, xenograft tumor model showed the synergism of RNF183 knockdown and trametinib in repressing the growth of CRC cells in vivo.

Conclusion

The RNF183-IL-8 axis is responsible for the resistance of CRC cells to the MEK1/2 inhibitor trametinib and may serve as a candidate target for combined therapy for CRC.

Electronic supplementary material

The online version of this article (doi:10.1186/s40880-017-0228-1) contains supplementary material, which is available to authorized users.