- Open Access
Vemurafenib resistance reprograms melanoma cells towards glutamine dependence
© Hernandez-Davies et al. 2015
Received: 20 March 2015
Accepted: 24 June 2015
Published: 3 July 2015
V600 BRAF mutations drive approximately 50% of metastatic melanoma which can be therapeutically targeted by BRAF inhibitors (BRAFi) and, based on resistance mechanisms, the combination of BRAF and MEK inhibitors (BRAFi + MEKi). Although the combination therapy has been shown to provide superior clinical benefits, acquired resistance is still prevalent and limits the overall survival benefits. Recent work has shown that oncogenic changes can lead to alterations in tumor cell metabolism rendering cells addicted to nutrients, such as the amino acid glutamine. Here, we evaluated whether melanoma cells with acquired resistance display glutamine dependence and whether glutamine metabolism can be a potential molecular target to treat resistant cells.
Isogenic BRAFi sensitive parental V600 BRAF mutant melanoma cell lines and resistant (derived by chronic treatment with vemurafenib) sub-lines were used to assess differences in the glutamine uptake and sensitivity to glutamine deprivation. To evaluate a broader range of resistance mechanisms, isogenic pairs where the sub-lines were resistant to BRAFi + MEKi were also studied. Since resistant cells demonstrated increased sensitivity to glutamine deficiency, we used glutaminase inhibitors BPTES [bis-2-(5 phenylacetamido-1, 2, 4-thiadiazol-2-yl) ethyl sulfide] and L–L-DON (6-Diazo-5-oxo-l-norleucine) to treat MAPK pathway inhibitor (MAPKi) resistant cell populations both in vitro and in vivo.
We demonstrated that MAPKi-acquired resistant cells uptook greater amounts of glutamine and have increased sensitivity to glutamine deprivation than their MAPKi-sensitive counterparts. In addition, it was found that both BPTES and L-DON were more effective at decreasing cell survival of MAPKi-resistant sub-lines than parental cell populations in vitro. We also showed that mutant NRAS was critical for glutamine addiction in mutant NRAS driven resistance. When tested in vivo, we found that xenografts derived from resistant cells were more sensitive to BPTES or L-DON treatment than those derived from parental cells.
Our study is a proof-of-concept for the potential of targeting glutamine metabolism as an alternative strategy to suppress acquired MAPKi-resistance in melanoma.
Melanoma is one of the most aggressive forms of skin cancer affecting an estimated 76,100 individuals per year and accounting for approximately 9,710 deaths in 2014 [1, 2]. According to the American Cancer Society, incidents of melanoma have been increasing steadily for the past 30 years . Oncogenic mutations in the BRAF gene, encoding a serine threonine kinase that is an essential part of the RAS–RAF–MEK–ERK signaling cascade have been found in approximately 50–70% of metastatic melanoma [1, 3]. The mutation in BRAF is frequently found at residue 600 with valine to glutamic acid (V600E BRAF) and leads to a hyperactive BRAF kinase which results in uncontrolled cell proliferation and oncogene addiction [1, 4, 5].
Single agent inhibition of the BRAF kinase with small molecule inhibitors such as vemurafenib (PLX4032) and dabrafenib, or double-drug combinations of a BRAF inhibitor with an inhibitor of MEK1/2 such as cobimetinib and trametinib have been successively shown to improve patient survival [6–11]. However, even with the superior efficacy of the double-drug combination, disease control is often cut short by the development of acquired resistance. Genetic resistance mechanisms most commonly result in reactivation of the MAPK pathway through NRAS or KRAS mutations, V600E/K BRAF amplification or alternative splicing [5, 12, 13]. In contrast non-genetic resistance mechanisms often result in MAPK pathway-redundant survival with up-regulated expression of receptor tyrosine kinases such as PDGFRβ [5, 12–14].
It has recently been shown that tumor cell metabolism can be exploited to treat cancer . In the 1920s, Otto Warburg found that cancer cells consume very high rates of glucose and secrete large amounts of lactate in the presence of oxygen, deemed the “Warburg Effect” . This inefficient consumption was designed to meet the biosynthetic and energy production requirements that are frequently seen in tumor cells . It has been shown that in addition to glucose, some cancer cells exhibit “glutamine addiction” to support the anabolic processes that stimulate cell proliferation . Glutamine has been shown to be an essential provider of nitrogen for nucleotide and protein synthesis and affect a critical regulator of protein translation, the mammalian target of rapamycin complex (mTORC)1 . Studies have also pointed at oncogenic changes that allow for regulation of glutamine metabolism in cancer cells. For example oncogenic c-myc has been implicated in the transcriptional regulation of high affinity glutamine transporters to promote glutaminolysis . Pancreatic ductal adenocarcinoma (PDAC) cells have also been shown to be strongly dependent on glutamine and this reprogramming of glutamine metabolism was found to be driven by transcriptional up-regulation of key metabolic enzymes mediated by oncogenic KRAS . In melanoma, it has been shown that glutamine transporter ASCT2 was upregulated in V600E BRAF mutant melanoma and played a critical role in glutamine uptake and cell proliferation . Therefore, it is highly plausible that disruption of glutamine metabolism can be utilized as a therapeutic approach to treat tumors.
The findings that cancer cells are addicted to glutamine led to therapeutic approaches aimed at impairing glutamine metabolism. Recent work on inhibitors that target glutaminase, the enzyme that catalyzes the conversion of l-glutamine to l-glutamate and ammonia, suggests significant therapeutic potential for cancer treatment. For example, 6-diazo-5-oxo-1-norleucine (L-DON), targets glutaminase on its active site to inhibit tumor growth [20–22]. Another glutaminase inhibitor, bis-2-[5-(phenylacetamido)-1,3,4-thiadiazol-2-yl]ethyl sulfide (BPTES), and its analogs significantly diminish growth of tumor xenografts in vivo and proliferation of cancer cells in vitro for several tumor types, including lymphomas, breast cancers, and gliomas [23–27].
In this study, we demonstrate that melanoma resistant cells uptake glutamine at a higher rate and are more sensitive to glutamine starvation than their vemurafenib sensitive counterparts. Moreover, we show that glutaminase inhibitors BPTES and L-DON can be used to effectively treat resistant cells in vitro and can be used to treat tumors in vivo. We propose targeting glutamine metabolism can be used as an alternative treatment strategy to target tumors resistant to vemurafenib.
Human melanoma parental (vemurafenib sensitive) lines were generated as previously described . Briefly, cells were established directly from patient biopsies and cultured in RPMI 1640 medium with l-glutamine, 10% fetal bovine serum and 1% penicillin, streptomycin, and amphotericin . M229 parental was previously characterized as BRAFV600E homozygous and M249 parental was described as V600EBRAF heterozygous and both equally sensitive to vemurafenib-mediated growth inhibition in vitro and in vivo . Cells were maintained in Dulbecco’s modified Eagle medium (DMEM) with 10% fetal bovine serum (Omega Scientific, Inc) and 4 mM l-glutamine (Omega Scientific, Inc).
Vemurafenib (PLX 4032) single drug resistant (SDR) sub-lines M249 and M229 with V600E BRAF positive mutations were generated in vitro by chronic vemurafenib exposure . Briefly, M229 parental line was treated with PLX4032 at 1 mM every 3 days for 4–6 weeks to obtain clonal colonies . PDGFRβ RNA upregulation was found to contribute to M229 resistance . M249 resistant sub-line was derived by successively titrating PLX4032 up to 10 mM . M249 resistant sub-line was shown to harbor a NRAS(Q61K) activating mutation not present in the parental M249 cell line that was shown to contribute to resistance . M249 and M229 resistant cells were maintained in DMEM with 10% fetal bovine serum (Omega Scientific, Inc), 4 Mm l-glutamine (Omega Scientific, Inc), and with 1 μM vemurafenib (PLX4032) (Plexxikon).
Double BRAF inhibitor (vemurafenib) and MEK inhibitor (selumetinib) resistant cell lines were generated as previously described . Briefly, the M249 DDR5 double drug resistant cell line (DDR) was generated by treating M249 parental lines with increments of vemurafenib and selumetinib and harbored both the mutant V600E BRAF amplification and F129L MEK1 mutation . M249 double resistant cells were cultured in the above medium maintained with both 1 μM vemurafenib (Plexxikon) and 1 μM selumetinib (Selleck chemicals).
Parental and single drug resistant cells were seeded at 2 × 105 and 1 × 105 cells/well, respectively in 6 well plates in triplicate and allowed to incubate overnight. Media only was also plated as a control for parental and resistant cells. At 24 and 48 h upon cells reaching 60% confluence, cell medium was collected and transferred to micro-centrifuge tubes and placed in the Nova Bioprofiler 100plus Analyzer (Nova Biomedical) for measurement of nutrient uptake. In addition, cells were counted using the Biorad TC20 automated cell counter. Upon measurement, medium only control values were subtracted from readings and values/cell were calculated.
DIMSCAN cell in vitro cytotoxicity assays
M249 and M229 parental, single drug resistant (SDR) cells, and M249 DDR5 double drug resistant cells (DDR) were seeded at 3,000 cells/well and 2,000 cells/well (respectively) in 96 well plates. Cells were allowed to settle overnight prior to treatment. After overnight incubation, cells were washed with 1× PBS to remove traces of medium. Parental cells were treated with either media with or without l-glutamine or d-glucose, 10 μM BPTES [bis-2-(5 phenylacetamido-1, 2, 4-thiadiazol-2-yl) ethyl sulfide] (LT Pharma, Inc.) or 10 μM L-DON (6-Diazo-5-oxo-l-norleucine) (Sigma-Aldrich) alone. Single drug resistant cells were treated in combination with 1 μM vemurafenib. Double drug resistant cells were treated in combination with both 1 μM vemurafenib and 1 μM selumetinib. BPTES stock solution was dissolved in DMSO to a final working concentration of 10 mM and stored at −20°C. L-DON was dissolved in water to a final concentration of 100 mM and also stored at −20°C. DMSO and water only controls were added to each well. Cells were also treated with media without glutamine or glucose containing dialyzed fetal bovine serum (Gemini Scientific). Treated cells were incubated for 24, 48, and 72 h. To prepare for DIMSCAN analysis, 0.5% Eosin Y was added to spaces between wells on a 96 well plate. A solution of fluorescein diacetate (FDA) at 10 mg/ml was added to 0.5% Eosin Y solution to make a working concentration of 40μg/ml to add to cells. After a 20 min incubation, cells were scanned for fluorescence using DIMSCAN, a fluorescence-based digital image microscopy system . Results were analyzed to obtain survival percentage using the DIMSCAN data analyzer by DTT (Children’s Hospital Los Angeles).
M249 and M229 parental and single drug resistant cells were seeded in 6 cm plates overnight to obtain 60% confluence the following day. Cells were washed with 1× PBS. Medium with or without glutamine was added to cells. After 24 h, cells were lysed with RIPA buffer and protease inhibitors and lysates were run on SDS page gels. Gels were transferred to a nitrocellulose membrane and probed with anti cleaved-PARP (Cell Signaling Technology) to assess apoptosis.
M249 and M229 cells were plated at 5 × 104 (parental) and 4 × 104 cells (resistant) in 12 well plates. Double drug resistant lines were plated at 4 × 104 cells in 12 well plates. After overnight incubation and reaching 60% confluence, cells were stained with flourochrome-conjugated Annexin V and propidium iodide using eBioscience reagents and Annexin V staining protocol. Cell acquisition was completed using the 9-color CyAn ADP from Beckman Coulter (Miami, FL, USA). Research reported in this publication included work performed in the Analytical Cytometry Core supported by the National Cancer Institute of the National Institutes of Health under award number P30CA33572. Cells were analyzed using FlowJo data analysis software (Ashland, OR, USA).
shRNA lentiviral particles (Sigma) were used to infect M249 single drug resistant cells at 50% confluence using polybrene (Hexadimethrine bromide) (Sigma). Cells were selected for puromycin resistance (Sigma) after 48 h. Bulk cell populations were utilized for experiments. Knockdown efficiency was assessed using quantitative PCR and determination of relative mRNA expression of NRAS.
In vivo xenograft model
All animal studies were performed according to approved IACUC protocols at the City of Hope Cancer Center. Nod Scid Gamma (NSG) mice were injected with 5 × 105 of M249 parental or single drug resistant cells subcutaneously on the right flank. When tumor size reached an average of 100 mm3 tumor cell volume, mice were treated with 15 mg/kg of BPTES or vehicle control (DMSO) every other day through intraperitoneal injection. Measurements were taken for tumor length and width. Tumor volume (mm3) was calculated by multiplying (length × width × width)/2. NCr nude mice (Taconic) were injected with 2 × 106 of M249 single drug resistant cells subcutaneously on the right flank. When tumor size reached an average of 100 mm3, they were treated bi-weekly with 20 mg/kg of L-DON or vehicle control (water).
All in vitro experiments were performed in triplicate and repeated a minimum of three times. In the figures, representative experiments are shown. Paired t tests were done to calculate p values for representative experiments using Graphpad software (San Diego, CA, USA). Values under p < 0.05 were considered significant.
Vemurafenib resistant cells uptake and use glutamine at a higher rate than vemurafenib sensitive cells
Vemurafenib resistant cells are more sensitive to glutamine deprivation
Vemurafenib resistant cells are more sensitive to glutaminase inhibitors
Knock down of NRAS in vemurafenib resistant cells reduces sensitivity to glutaminase inhibitor
Vemurafenib resistant melanoma tumors are sensitive to glutaminase inhibitor treatment in vivo
Here, we demonstrated that M249 and M229 vemurafenib resistant cells have been reprogrammed to become increasingly dependent on glutamine when compared to their vemurafenib-sensitive counterparts. In addition, we show that acquisition of resistance increased sensitivity of these cells to glutamine deprivation. We also demonstrated that, in addition to single drug resistant lines, double drug resistant lines harboring both the BRAF amplification and MEK1 mutations became sensitive to glutamine deprivation. We were successfully able to exploit this sensitivity to glutamine to kill these single and double drug resistant cells with a combination of glutaminase inhibitors BPTES or L-DON and vemurafenib. In both single and double drug resistant cases, decreased cell survival was observed. In addition, knocking down NRAS in the M249 resistant cells decreased the sensitivity of these cells to both glutaminase inhibitors, indicating the possibility that resistance acquired mutations may influence this increased dependence on glutamine. These results were also obtained in vivo as treating mice injected with M249 single drug resistant cells with BPTES or L-DON resulted in a significant decrease of tumor volume.
Single agent treatment with BRAF inhibitors such as vemurafenib and dabrafenib have demonstrated improved survival for patients with V600E BRAF mutant melanoma and are currently approved by the US Food and Drug Administration for treatment . In addition, combination therapy of BRAF inhibitors with allosteric MEK1 and MEK2 inhibitors are also in clinical trials and have been approved for treatment for BRAF mutant melanomas [7, 13, 28, 30, 31]. However, as with single agent therapies, combination therapy with BRAF and MEK inhibitors have also led to the development of mechanisms of resistance by further amplifying existing resistance mutations or by reactivating the MAPK pathway [9, 13, 28]. These resistance mechanisms have led to the need for alternative treatment options . Recently, blocking the immune-regulatory checkpoints that limit T cell responses using antagonistic antibodies against the programmed death 1 pathway (PD-1) and one of its ligands, programmed death ligand 1 (PD-L1), and blockade of CTLA-4/B7 (cytotoxic T lymphocyte-associated antigen-4) interaction with anti-CTLA-4 antagonistic monoclonal antibodies (mAbs) have demonstrated high clinical benefits in melanoma patients [32, 33]. Besides targeting immuno-responses, whether targeting altered metabolism to treat resistant melanoma cells has not yet been explored. Our study suggests that inhibition of glutamine metabolism could be a promising way to treat single and double resistant tumor types.
As our data suggested that different resistance models could all lead to “glutamine addiction”, it is critical to identify how these cells were able to reprogram to a preferential glutamine dependent metabolism. It will be important in the future to look at glutamine transporters or metabolic enzymes involved in the glutamine metabolic pathway to assess how mutations are directly involved in modifying the intrinsic metabolism of these cells. This will allow for the development of more targeted therapies that can be used to target tumors that do become glutamine dependent.
Overall, these data suggest that both vemurafenib single drug resistant and vemurafenib/selumetinib (MEK inhibitor) double drug resistant lines are sensitive to glutamine and to glutaminase inhibitors. Therefore targeting glutamine metabolism may be a useful tool in the future to treat vemurafenib resistant melanoma.
Currently, therapy used for the treatment of vemurafenib resistant melanoma involves combination BRAF and MEK inhibitors to target inhibition of the MAPK pathway. Due to the aggressiveness of melanoma and its unique ability to develop resistant mutations even after treatment with combination of BRAF and MEK inhibitors, it will become necessary to develop alternative forms of therapy to evade resistance. Our study demonstrated that targeting glutamine metabolism can be a way to potentially treat vemurafenib resistant melanoma.
JHD carried out nutrient uptake, DIMSCAN proliferation assays, knockdown studies and in vivo xenograft experiments. TT, MR, XL, MP and YY participated in in vivo xenograft experiments. KR and MR participated in nutrient uptake assays. KR assisted with AnnexinV staining and development of shNRAS knockdown cell lines. JW is involved in DIMSCAN analysis. RL and GM generated 249 and 229 parental and resistant lines and 249 DDR5 double drug resistant line. RL, JHD and MK helped to draft the manuscript and data interpretation. All authors read and approved the final manuscript.
We thank David Huw Davies for helpful comments on the manuscript. We thank Andrea Zuniga for assisting in experiments. Research is supported by NIH/NCI 1R01CA183989-01A1 and V Foundation. MK is the Pew Scholar in the Biomedical Sciences. MR is supported by the Ralph M. Parson’s Foundation.
Compliance with ethical guidelines
Competing interests The authors declare that they have no competing interests.
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