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Trametinib downregulates survivin expression in RB1-positive KRAS-mutant lung adenocarcinoma cells
Biochemical and Biophysical Research Communications xxx (2018) 1e6
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Trametinib downregulates survivin expression in RB1-positive KRASmutant lung adenocarcinoma cells Toshiyuki Sumi a, b, Sachie Hirai a, Miki Yamaguchi a, Yusuke Tanaka a, b, Makoto Tada a, c, Toshiro Niki d, Hiroki Takahashi b, Yuji Sakuma a, * a
Department of Molecular Medicine, Research Institute for Frontier Medicine, Sapporo Medical University School of Medicine, Sapporo, Japan Department of Respiratory Medicine and Allergology, Sapporo Medical University School of Medicine, Sapporo, Japan Department of Thoracic Surgery, Sapporo Medical University School of Medicine, Sapporo, Japan d Division of Integrative Pathology, Jichi Medical University, 3311-1 Yakushiji, Shimotsuke, Tochigi, 329-0498, Japan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 April 2018 Accepted 30 April 2018 Available online xxx
High expression levels of survivin in KRAS-mutant lung adenocarcinomas are linked with unfavorable patient outcomes, suggesting that survivin is a promising target for tumor treatment. We found that trametinib, a MEK inhibitor, downregulates survivin expression in the RB1-positive KRAS-mutant lung adenocarcinoma cell lines H358 and H441. In these cell lines, trametinib treatment induced p21 expression and dephosphorylated RB1, leading to sustained suppression of survivin. Knockdown of p21 or RB1 restored survivin expression in trametinib-treated cells, at least partially, which supports the contribution of these molecules to trametinib-mediated survivin suppression. In RB1-negative KRASmutant lung adenocarcinoma H2009 cells, survivin downregulation by trametinib was only slight and transient, and trametinib-resistant (TR) cells developed within 1 month of treatment. H2009 TR cells depended much more on survivin for survival than its parental cells, as evidenced by apoptosis induction when survivin was depleted. These findings collectively suggest that trametinib is effective for the treatment of RB1-positive KRAS-mutant lung adenocarcinomas through sustained survivin suppression, but not for RB1-negative lung adenocarcinomas. Thus, the RB1 status could be a biomarker for trametinib application in KRAS-mutant lung adenocarcinomas. © 2018 Elsevier Inc. All rights reserved.
Keywords: KRAS-mutant lung adenocarcinoma Survivin p21 RB1 Trametinib
1. Introduction Lung cancer is one of the most severe forms of cancer, and adenocarcinoma is the most predominant subtype, accounting for ~50% of all lung cancers. Patients with particular lung adenocarcinoma subtypes benefit from molecular targeted drugs, but these are still unavailable for KRAS-mutant lung adenocarcinomas [1]. We recently reported that higher expression levels of survivin, encoded by the BIRC5 gene, in KRAS-mutant lung adenocarcinomas are significantly associated with poorer patient outcomes, and that survivin could be a promising target for tumor treatment [2]. During that study, we observed that the MEK inhibitor trametinib downregulates survivin expression levels in the KRAS-mutant lung
* Corresponding author. Department of Molecular Medicine, Research Institute for Frontier Medicine, Sapporo Medical University School of Medicine, South 1, West 17, Chuo-ku, Sapporo, 060-8556, Japan. E-mail address: [email protected] (Y. Sakuma).
adenocarcinoma cell lines H358 and H441. Although trametinib is currently used in the clinic for the treatment of BRAF-mutant non-small cell lung cancer when combined with the BRAF inhibitor dabrafenib [3], MEK inhibition by trametinib considerably declines within a few days because of feedback from ERK, which is downstream of MEK [4,5]. Trametinib was initially discovered as a drug that elevated p21, also termed cyclin dependent kinase inhibitor 1A (CDKN1A), and activated retinoblastoma transcriptional corepressor 1 (RB1) [6]. RB1 is one of the most important tumor suppressor genes, and controls the cell cycle. Unphosphorylated RB1 binds E2F transcription factors to block transactivation domains so that E2F-reponsive genes are repressed; however, RB1 loses this ability when phosphorylated by the cyclin D-cyclin dependent kinase (CDK) 4/6 or cyclin EeCDK2 complexes [7e9]. p21 greatly affects cell survival and cellular senescence by controlling the activity of multiple cyclineCDK complexes [10]. p21 is usually elevated in senescent cells or in damaged cells by cytotoxic agents, and suppresses CDK2 and CDK4/
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Please cite this article in press as: T. Sumi, et al., Trametinib downregulates survivin expression in RB1-positive KRAS-mutant lung adenocarcinoma cells, Biochemical and Biophysical Research Communications (2018), https://doi.org/10.1016/j.bbrc.2018.04.230
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6 kinase activity and dephosphorylates RB1, resulting in cell cycle arrest. Dephosphorylated RB1 is then immediately degraded by the ubiquitineproteasome pathway, allowing the cell cycle to proceed [11,12]. To our knowledge, no reports have demonstrated that trametinib downregulates survivin expression in KRAS-mutant lung adenocarcinoma cells. In this study, we reveal the mechanism underlying trametinib-induced survivin suppression by analyzing three KRAS-mutant lung adenocarcinoma cell lines, including two RB1-positive (H358 and H441) and an RB1-negative one (H2009) [13].
2. Materials and methods 2.1. Cell culture and drugs Three KRAS-mutant lung adenocarcinoma cell lines, NCIeH358 (G12C), NCIeH441 (G12V), and NCIeH2009 (G12A), were obtained from the American Type Culture Collection (Manassas, VA, USA) and maintained at 37 C in a humidified incubator with 5% CO2. Cells were cultured in RPMI-1640 medium (Nacalai Tesque, Kyoto, Japan) with 10% fetal bovine serum and antibiotics. The MEK inhibitor trametinib was purchased from AdooQ BioScience (Irvine, CA, USA).
2.2. Generation of H2009 trametinib-resistant (TR) cells H2009 cells were grown to subconfluence in 94-mm tissue culture dishes, and treated with 20 nM trametinib. Fresh media containing the drug were replaced every 3 days. The H2009 cells treated decreased in number by about 50% within 7 days, but resumed proliferation in the presence of the drug 2 weeks after the initiation of treatment. Cells that proliferated to subconfluence in a monolayer culture dish were trypsinized and resuspended as single cells in fresh media containing trametinib. The cells that were seeded again on conventional culture plastic plates were able to thrive in media containing trametinib, and we confirmed that they survived 20 passages in the presence of trametinib. The polyclonal cells were termed H2009 TR cells.
2.3. RNA interference assay Cells (3 106) were plated in 94-mm culture dishes and transfected with negative control (NC) small interfering (si)RNA duplexes (1027281; Qiagen, Valencia, CA, USA) or Silencer Select siRNA duplexes targeting CDKN1A (Ambion #s417; ThermoFisher Scientific Japan, Yokohama, Japan), RB1 (Ambion #s523 and #s522, termed RB1 siRNA #1 and #2, respectively, in this study; ThermoFisher Scientific), and BIRC5 (Ambion #s1457 and #s1458, termed BIRC5 siRNA #1 and #2, respectively, in this study; ThermoFisher Scientific) using Lipofectamine RNAiMAX Reagent and OPTI-MEM I (ThermoFisher Scientific), as described previously [2]. The final concentration of siRNA used in each experiment was 10 nM. Downregulation of the expression of targeted genes was verified using western blotting as described below.
2.4. Assessment of cell viability The number of viable cells was estimated using a CellTiter Glo 3D Cell Viability Assay (Promega, Madison, WI, USA) according to the manufacturer's instructions. All results are shown as the mean ± SD.
2.5. Crystal violet staining Crystal violet staining for viable cells was performed as described previously [2]. Briefly, NC siRNA- or BIRC5 siRNAtransfected H2009 parental cells and TR cells were cultured for 48 h. Cells were then plated at a density of 5 105 in wells of a 6 well-plate, then left untreated or treated with trametinib (20 nM) for another 48 h. Viable cells attached to the wells were fixed with methanol and stained with 0.5% crystal violet solution. 2.6. Western blotting Western blotting for the expression of survivin, total ERK1/2, phospho-ERK1/2, p21, p27, and b-actin was performed as described previously [2,14e16]. Additional primary antibodies used in the present study were RB1 (#9309; 4H1; 1:2000; Cell Signaling Technology Japan, Tokyo, Japan) and cleaved caspase 3 (#9661; 1:1000; Cell Signaling Technology). Band intensity levels on X-ray films were normalized to b-actin using Image J software (National Institutes of Health, Bethesda, MD, USA). 2.7. RNA isolation and quantitative reverse transcription (RT)ePCR Total RNA extraction and quantitative RT-PCR were carried out as previously described [14e16]. The following PCR primers were purchased from Qiagen (QuantiTect Primer Assay): Hs_ACTB_1_SG, amplifying ACTB which encodes b-actin, and Hs_BIRC5_1_SG, amplifying BIRC5 which encodes survivin. b-actin was used to standardize the quantity of target mRNAs. Relative mRNA expression levels were calculated using the comparative DDCT method and are presented as the averages of triplicate experiments. 2.8. Statistical analysis Differences in cell viability between untreated and treated cells were evaluated by Dunnett's test, a multiple comparison test, or by paired t-tests. One-way analysis of variance (ANOVA) followed by the TukeyeKramer multiple comparison test was performed to evaluate the therapeutic effects of trametinib on H2009 parental cells and TR cells. p-values less than 0.05 were considered statistically significant. Statistical analyses were performed with JMP software (JMP for Windows version 7, SAS Institute Japan, Tokyo, Japan). 3. Results 3.1. Trametinib downregulates survivin expression by inducing p21 in RB1-positive KRAS-mutant lung adenocarcinoma cells To elucidate whether and to what extent p21 and/or RB1 contribute to trametinib-mediated downregulation of survivin, we examined two RB1-positive KRAS-mutant lung adenocarcinoma cell lines, H358 and H441 cells, and an RB1-negative line, H2009 cells (Fig. 1A). When treated with trametinib, all cell lines decreased in number in a concentration-dependent manner (Fig. 1B). Of note, H358 and H441 cells were sensitive to trametinib, while H2009 cells were relatively resistant (Fig. 1B). In H358 and H441 cells treated with trametinib, survivin expression declined whereas p21 expression was induced (Fig. 1C). We also found that expression levels of total RB1 were clearly reduced in trametinib-treated H358 and H441 cells, while the dephosphorylated form increased in a dose-dependent manner (Fig. 1C). This suggests that induced dephosphorylated RB1, which is known to inhibit E2Fs [17], functions in downregulating survivin expression.
Please cite this article in press as: T. Sumi, et al., Trametinib downregulates survivin expression in RB1-positive KRAS-mutant lung adenocarcinoma cells, Biochemical and Biophysical Research Communications (2018), https://doi.org/10.1016/j.bbrc.2018.04.230
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Fig. 1. p21 plays a key role in trametinib-induced survivin downregulation in RB1-positive KRAS-mutant lung adenocarcinoma cells. A). Western blots of untreated H2009, H358, and H441 cells. B). Effects of trametinib on cell viability in H2009, H358, and H441 cells. Cells were untreated or treated with trametinib for 48 h. Results are shown as the mean ± SD. *p < 0.05, **p < 0.01. NS, not significant. C). Western blots examining the effects of trametinib on H2009, H358, and H441 cells. Cells were untreated or treated with trametinib for 48 h. P-RB1, phosphorylated form of RB1; UnP-RB1, unphosphorylated form of RB1. D).Western blots examining the effects of trametinib on H2009, H358, and H441 cells. Cells were treated with trametinib (20 nM) for the indicated time. E). Quantitative RT-PCR for the expression of BIRC5 mRNA in H2009, H358, and H441 cells treated with trametinib (20 nM) for the indicated time. mRNAs were measured in triplicate. Results are expressed as the mean ± SD. **p < 0.01. F). Western blots examining the effects of p21 knockdown on trametinib-treated H358 and H441 cells. Cells were transfected with NC siRNA or CDKN1A siRNA and then cultured for 48 h. Cells were subsequently treated with trametinib (20 nM) for another 48 h.
ERK phosphorylation (active ERK signaling) was observed as early as day 2 of trametinib treatment, although survivin expression was still markedly suppressed in trametinib-treated H358 cells and H441 cells at day 28 (Fig. 1D). The marked decrease in survivin expression was also confirmed at the RNA level (Fig. 1E). As expected, both H358 and H441 cells could not proliferate actively in the presence of trametinib even at day 28, showing a swollen to flattened morphology suggestive of cells in senescence (data not shown). Surprisingly, RB1 expression, the dephosphorylated form of which is supposed to inhibit survivin expression, was barely detected in trametinib-treated H358 and H441 cells at day 28 (Fig. 1D). This marked reduction in RB1 expression likely results from cell cycle arrest caused by sustained p21 expression, because RB1 is susceptible to degradation through the ubiquitineproteasome pathway when the cell cycle is arrested [8,9]. In H2009 cells treated with trametinib, p21 induction and a slight decrease in survivin expression were also observed at day 2 as in H358 and 441 cells (Fig. 1D and E). However, the expression levels of survivin in trametinib-treated H2009 cells quickly recovered the following day, almost coinciding with the reactivation of ERK in trametinib-treated H2009 cells (Fig. 1D). This suggests that the transient decline of survivin was probably a
result of cell cycle arrest caused by trametinib-mediated MEK inhibition [18]. Interestingly, H2009 cells acquired the ability to thrive in the presence of trametinib (20 nM) by day 28, when p21 expression was no longer detectable while survivin was expressed (Fig. 1D). We next suppressed the expression of p21 (encoded by CDKN1A) using RNA interference (RNAi) in H358 and H441 cells to clarify the role of p21 in survivin expression (Fig. 1F). Comparing NC siRNAtransfected cells and CDKN1A siRNA-transfected cells in the presence of trametinib, p21 depletion clearly increased the expression of survivin, while RB1, including its phosphorylated form, was also upregulated (Fig. 1F). Taken together, this indicates that p21 plays a key role in trametinib-induced survivin suppression in RB1positive H358 and H441 cells. Moreover, both cell lines appeared to be in senescence even on day 28 because of their sustained expression of p21 and persistent suppression of survivin (Fig. 1D). In RB1-negative H2009 cells, although survivin was transiently lowered by trametinib treatment, the cells acquired resistance to trametinib by day 28, when they lost p21 expression and maintained survivin expression. These findings collectively raised the possibility that the RB1 status critically affects long-term sensitivity to trametinib.
Please cite this article in press as: T. Sumi, et al., Trametinib downregulates survivin expression in RB1-positive KRAS-mutant lung adenocarcinoma cells, Biochemical and Biophysical Research Communications (2018), https://doi.org/10.1016/j.bbrc.2018.04.230
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3.2. RB1 depletion attenuates the trametinib-induced survivin decline in RB1-positive KRAS-mutant lung adenocarcinoma cells We subsequently suppressed RB1 expression in H358 and H441 cells using RNAi to elucidate whether RB1 directly affected survivin expression levels. RB1 knockdown significantly increased the number of viable cells in the presence (up to 20 nM) or absence of trametinib in these cells (Fig. 2A). RB1 knockdown also significantly attenuated trametinib-mediated survivin downregulation at both the RNA and protein levels (Fig. 2B and C). These findings indicate that RB1 plays a role in trametinib-induced survivin suppression at an early phase (within 48 h) in RB1-positive KRASmutated H358 and H441 cells. 3.3. Survivin plays a critical role in acquired resistance to trametinib in H2009 cells H358 and H441 cells showed very little growth in the presence of trametinib even at day 28 of treatment, although H2009 trametinibresistant (TR) cells were obtained following long-term culture (Fig. 3A). Using H2009 TR cells, we explored the extent to which survivin contributed to acquired trametinib resistance. Trametinib treatment (20 nM) significantly decreased the number of H2009 parental cells but had little effect on TR cells (Fig. 3B). Trametinibtreated H2009 parental cells, but not TR cells, also underwent apoptosis as shown by the expression of cleaved caspase 3 (Fig. 3C). The depletion of survivin significantly reduced the viability of H2009 TR cells as well as its parental cells (Fig. 3D). Additionally, NC siRNAtransfected TR cells resisted trametinib treatment (up to 20 nM) while survivin-depleted TR cells responded to trametinib, resulting in dose-dependent and significantly decreased viability (Fig. 3E). Survivin-depleted TR cells and parental cells underwent apoptosis in response to trametinib treatment as evidenced by the expression of cleaved caspase 3 (Fig. 3F and G). Of note, cleaved caspase 3 was
more readily detectable in survivin-suppressed, trametinib-untreated TR cells than parental cells (Fig. 3F), suggesting that H2009 TR cells depend heavily on survivin for survival. Taken together, these findings suggest that survivin contributes to acquired resistance to trametinib in H2009 cells. 4. Discussion We report here for the first time that trametinib consistently suppressed survivin expression for at least 28 days in RB1-positive KRAS-mutant lung adenocarcinoma, H358 cells, and H441 cells. This appears to be caused by the induction of p21, a senescenceassociated protein, and the dephosphorylation of RB1. While these mechanisms of action of trametinib were appreciated at the time of its development, they have been less recognized recently [6]. Not only MEK inhibition but also p21 induction serve as indispensable mechanisms of trametinib against tumor growth. In RB1-negative KRAS-mutant lung adenocarcinoma H2009 cells, trametinib-mediated survivin suppression was transient. We readily obtained H2009 TR cells, which critically depended on survivin for survival. We also demonstrated that dephosphorylated RB1 contributed, at least in part, to trametinib-induced survivin suppression at an early phase (within 48 h of treatment) in RB1-positive H358 and H441 cells (Fig. 4). Paradoxically, in trametinib-treated H358 or H441 cells at day 28 (late phase), the expression levels of RB1 were too low to be detected while survivin remained almost completely suppressed. This is probably because the sustained expression of p21 caused these cells to senesce with RB1 prone to degradation as a negative feedback mechanism. Expression levels of survivin are usually low in senescent cells [18], which could reflect the loss of E2Fs after their detachment from RB1 and their subsequent degradation through the ubiquitineproteasome pathway [19]. Thus, survivin expression was almost completely suppressed in trametinib-treated H358 or
Fig. 2. RB1 knockdown attenuates trametinib-induced survivin suppression in RB1-positive KRAS-mutant lung adenocarcinoma cells. A). Effects of RB1 knockdown on cell viability of H358 and H441 cells untreated or treated with trametinib. NC siRNA or RB1 siRNA-transfected cells were cultured for 48 h, then left untreated or treated with trametinib at the indicated concentration for another 48 h. Results are shown as the mean ± SD. NS, not significant. *p < 0.05, **p < 0.01, ***p < 0.001 (paired t-test). B). Quantitative RT-PCR for the expression of BIRC5 mRNA. NC siRNA or RB1 siRNA-transfected cells were cultured for 48 h, then left untreated or treated with trametinib (20 nM) for another 48 h. mRNAs were measured in triplicate. Results are expressed as the mean ± SD. **p < 0.01. C). Upper panel: western blots examining the effects of RB1 depletion on H358 and H441 cells untreated or treated with trametinib. Cells were treated in the same way as described in (B). Lower panel: quantification of survivin protein expression by western blot. Columns, mean (n ¼ 3); bars, ± SD. **p < 0.01.
Please cite this article in press as: T. Sumi, et al., Trametinib downregulates survivin expression in RB1-positive KRAS-mutant lung adenocarcinoma cells, Biochemical and Biophysical Research Communications (2018), https://doi.org/10.1016/j.bbrc.2018.04.230
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Fig. 3. Survivin contributes to trametinib resistance in H2009 cells. A). Representative phase-contrast images of H2009 parental cells and trametinib-resistant (TR) cells. Scale bars, 50 mm. B). Effects of trametinib on cell viability of H2009 parental cells and TR cells. Cells were untreated or treated with trametinib (20 nM) for 48 h. Results are shown as the mean ± SD. NS, not significant. **P < 0.01 (ANOVA followed by the TukeyeKramer multiple comparison test). C). Western blots examining the effects of trametinib on H2009 parental cells and TR cells. Cells were untreated or treated with trametinib (20 nM) for 48 h. D). Effects of BIRC5 knockdown on cell viability of H2009 parental cells and TR cells. Cells were transfected with NC siRNA or BIRC5 siRNA, then cultured for 96 h. Results are shown as the mean ± SD. **P < 0.01. E). Effects of BIRC5 knockdown on cell viability of H2009 parental cells and TR cells untreated or treated with trametinib. NC siRNA or BIRC5 siRNAtransfected cells were cultured for 48 h, then untreated or treated with trametinib (up to 20 nM) for another 48 h. Results are shown as the mean ± SD. *P < 0.01, **P < 0.01. F). Western blots examining the effects of BIRC5 knockdown on H2009 parental cells and TR cells untreated or treated with trametinib. NC siRNA or BIRC5 siRNA-transfected cells were cultured for 48 h, then untreated or treated with trametinib (20 nM) for another 48 h. G). Crystal violet staining for viable cells. NC siRNA or BIRC5 siRNA-transfected cells were cultured for 48 h. Transfected cells (5 105 cells each) were then seeded into 6-well-plates and grown in the absence or presence of trametinib (20 nM) for another 48 h. Viable cells were fixed and stained with crystal violet.
Fig. 4. Schema of mechanisms of action of trametinib underlying survivin suppression in RB1-positive KRAS-mutant lung adenocarcinoma cells. Trametinib treatment induces p21 in RB1-positive KRAS-mutant lung adenocarcinoma cells. (Early phase) Induced p21 inhibits the activity of cyclineCDK complexes, leading to RB1 dephosphorylation. Dephosphorylated RB1 then inhibits the transcriptional activity of E2Fs, resulting in decreased survivin expression. (Late phase) Sustained expression of p21 induces cellular senescence, where survivin is almost completely suppressed.
H441 cells at a late phase despite loss of RB1 expression (Fig. 4). Trametinib treatment was recently shown to inhibit the expression of survivin in RB1-positive BRAF or NRAS-mutant melanoma cells, but not in RB1-negative ones [20]. Additionally, dephosphorylated RB1 restricts the functions of E2Fs in RB1-positive
melanoma cells, resulting in the downregulation of survivin [20]. Our findings are in line with these observations. RB1 deficiencies are frequently observed in non-small cell lung carcinomas (20e30%) [21]. Our findings suggest that RB1-deficient KRAS-mutant lung adenocarcinomas readily acquire resistance to trametinib, raising the
Please cite this article in press as: T. Sumi, et al., Trametinib downregulates survivin expression in RB1-positive KRAS-mutant lung adenocarcinoma cells, Biochemical and Biophysical Research Communications (2018), https://doi.org/10.1016/j.bbrc.2018.04.230
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possibility that the RB1 status in KRAS-mutant lung cancer could be a biomarker for determining the use of trametinib. The present study has several limitations. First, RB1 knockdown cannot completely restore trametinib-induced survivin suppression in RB1-positive KRAS-mutant lung adenocarcinoma cells. This is because several transcription factors other than E2Fs, such as NFkB, STAT3, and KLF5, contribute to BIRC5 (survivin) expression [22e25]. In cells treated with MEK inhibitors, KLF5 appears to be downregulated although NF-kB and STAT3 are usually phosphorylated (activated) [22e25]. Indeed, we detected phosphorylated NF-kB and STAT3 in trametinib-treated H358 cells (data not shown). This suggests that not only E2Fs but also NF-kB and/or STAT3 contribute to survivin expression in RB1-positive KRASmutant lung adenocarcinoma cells. Second, the molecular mechanisms underlying p21 induction remain to be solved. As mentioned above, in RB1-positive, trametinib-sensitive cell lines, p21 expression was durable in the presence of trametinib but only transient in the RB1-negative cell line. Considering that the three KRAS-mutant lung adenocarcinoma cell lines analyzed here are all p53 mutant cells (https://portals.broadinstitute.org/ccle), it is interesting to note that expression patterns of p21, one of the most important p53 target genes, vary widely among cells [10]. In conclusion, trametinib consistently suppressed survivin expression in RB1-positive KRAS-mutant lung adenocarcinoma H358 and H441 cells through p21 induction and RB1 dephosphorylation. However, trametinib-mediated survivin suppression was only transient in RB1-negative KRAS-mutant lung adenocarcinoma H2009 cells, leading to the development of H2009 trametinibresistant cells that depend critically upon survivin for survival. These findings collectively suggest that survivin suppression rather than MEK inhibition determines the efficacy of trametinib on KRASmutant lung adenocarcinomas.
[2]
[3]
[4] [5]
[6]
[7]
[8] [9] [10]
[11]
[12] [13]
[14]
[15]
Conflicts of interest
[16]
None of the authors of the present study have a conflict of interest to declare.
[17]
Funding This work was supported in part by Grants-in-Aid for Young Scientists from JSPS (Grant Number 16K19459, given to TS), and funded by support from the Ono Cancer Research Fund (given to YS).
[18] [19]
[20]
[21] [22]
Acknowledgments
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We thank Dr. Sarah Williams, PhD, from Edanz Group (www. edanzediting.com) for editing a draft of this manuscript. [24]
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Trametinib downregulates survivin expression in RB1-positive KRASmutant lung adenocarcinoma cells Toshiyuki Sumi a, b, Sachie Hirai a, Miki Yamaguchi a, Yusuke Tanaka a, b, Makoto Tada a, c, Toshiro Niki d, Hiroki Takahashi b, Yuji Sakuma a, * a
Department of Molecular Medicine, Research Institute for Frontier Medicine, Sapporo Medical University School of Medicine, Sapporo, Japan Department of Respiratory Medicine and Allergology, Sapporo Medical University School of Medicine, Sapporo, Japan Department of Thoracic Surgery, Sapporo Medical University School of Medicine, Sapporo, Japan d Division of Integrative Pathology, Jichi Medical University, 3311-1 Yakushiji, Shimotsuke, Tochigi, 329-0498, Japan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 April 2018 Accepted 30 April 2018 Available online xxx
High expression levels of survivin in KRAS-mutant lung adenocarcinomas are linked with unfavorable patient outcomes, suggesting that survivin is a promising target for tumor treatment. We found that trametinib, a MEK inhibitor, downregulates survivin expression in the RB1-positive KRAS-mutant lung adenocarcinoma cell lines H358 and H441. In these cell lines, trametinib treatment induced p21 expression and dephosphorylated RB1, leading to sustained suppression of survivin. Knockdown of p21 or RB1 restored survivin expression in trametinib-treated cells, at least partially, which supports the contribution of these molecules to trametinib-mediated survivin suppression. In RB1-negative KRASmutant lung adenocarcinoma H2009 cells, survivin downregulation by trametinib was only slight and transient, and trametinib-resistant (TR) cells developed within 1 month of treatment. H2009 TR cells depended much more on survivin for survival than its parental cells, as evidenced by apoptosis induction when survivin was depleted. These findings collectively suggest that trametinib is effective for the treatment of RB1-positive KRAS-mutant lung adenocarcinomas through sustained survivin suppression, but not for RB1-negative lung adenocarcinomas. Thus, the RB1 status could be a biomarker for trametinib application in KRAS-mutant lung adenocarcinomas. © 2018 Elsevier Inc. All rights reserved.
Keywords: KRAS-mutant lung adenocarcinoma Survivin p21 RB1 Trametinib
1. Introduction Lung cancer is one of the most severe forms of cancer, and adenocarcinoma is the most predominant subtype, accounting for ~50% of all lung cancers. Patients with particular lung adenocarcinoma subtypes benefit from molecular targeted drugs, but these are still unavailable for KRAS-mutant lung adenocarcinomas [1]. We recently reported that higher expression levels of survivin, encoded by the BIRC5 gene, in KRAS-mutant lung adenocarcinomas are significantly associated with poorer patient outcomes, and that survivin could be a promising target for tumor treatment [2]. During that study, we observed that the MEK inhibitor trametinib downregulates survivin expression levels in the KRAS-mutant lung
* Corresponding author. Department of Molecular Medicine, Research Institute for Frontier Medicine, Sapporo Medical University School of Medicine, South 1, West 17, Chuo-ku, Sapporo, 060-8556, Japan. E-mail address: [email protected] (Y. Sakuma).
adenocarcinoma cell lines H358 and H441. Although trametinib is currently used in the clinic for the treatment of BRAF-mutant non-small cell lung cancer when combined with the BRAF inhibitor dabrafenib [3], MEK inhibition by trametinib considerably declines within a few days because of feedback from ERK, which is downstream of MEK [4,5]. Trametinib was initially discovered as a drug that elevated p21, also termed cyclin dependent kinase inhibitor 1A (CDKN1A), and activated retinoblastoma transcriptional corepressor 1 (RB1) [6]. RB1 is one of the most important tumor suppressor genes, and controls the cell cycle. Unphosphorylated RB1 binds E2F transcription factors to block transactivation domains so that E2F-reponsive genes are repressed; however, RB1 loses this ability when phosphorylated by the cyclin D-cyclin dependent kinase (CDK) 4/6 or cyclin EeCDK2 complexes [7e9]. p21 greatly affects cell survival and cellular senescence by controlling the activity of multiple cyclineCDK complexes [10]. p21 is usually elevated in senescent cells or in damaged cells by cytotoxic agents, and suppresses CDK2 and CDK4/
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Please cite this article in press as: T. Sumi, et al., Trametinib downregulates survivin expression in RB1-positive KRAS-mutant lung adenocarcinoma cells, Biochemical and Biophysical Research Communications (2018), https://doi.org/10.1016/j.bbrc.2018.04.230
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6 kinase activity and dephosphorylates RB1, resulting in cell cycle arrest. Dephosphorylated RB1 is then immediately degraded by the ubiquitineproteasome pathway, allowing the cell cycle to proceed [11,12]. To our knowledge, no reports have demonstrated that trametinib downregulates survivin expression in KRAS-mutant lung adenocarcinoma cells. In this study, we reveal the mechanism underlying trametinib-induced survivin suppression by analyzing three KRAS-mutant lung adenocarcinoma cell lines, including two RB1-positive (H358 and H441) and an RB1-negative one (H2009) [13].
2. Materials and methods 2.1. Cell culture and drugs Three KRAS-mutant lung adenocarcinoma cell lines, NCIeH358 (G12C), NCIeH441 (G12V), and NCIeH2009 (G12A), were obtained from the American Type Culture Collection (Manassas, VA, USA) and maintained at 37 C in a humidified incubator with 5% CO2. Cells were cultured in RPMI-1640 medium (Nacalai Tesque, Kyoto, Japan) with 10% fetal bovine serum and antibiotics. The MEK inhibitor trametinib was purchased from AdooQ BioScience (Irvine, CA, USA).
2.2. Generation of H2009 trametinib-resistant (TR) cells H2009 cells were grown to subconfluence in 94-mm tissue culture dishes, and treated with 20 nM trametinib. Fresh media containing the drug were replaced every 3 days. The H2009 cells treated decreased in number by about 50% within 7 days, but resumed proliferation in the presence of the drug 2 weeks after the initiation of treatment. Cells that proliferated to subconfluence in a monolayer culture dish were trypsinized and resuspended as single cells in fresh media containing trametinib. The cells that were seeded again on conventional culture plastic plates were able to thrive in media containing trametinib, and we confirmed that they survived 20 passages in the presence of trametinib. The polyclonal cells were termed H2009 TR cells.
2.3. RNA interference assay Cells (3 106) were plated in 94-mm culture dishes and transfected with negative control (NC) small interfering (si)RNA duplexes (1027281; Qiagen, Valencia, CA, USA) or Silencer Select siRNA duplexes targeting CDKN1A (Ambion #s417; ThermoFisher Scientific Japan, Yokohama, Japan), RB1 (Ambion #s523 and #s522, termed RB1 siRNA #1 and #2, respectively, in this study; ThermoFisher Scientific), and BIRC5 (Ambion #s1457 and #s1458, termed BIRC5 siRNA #1 and #2, respectively, in this study; ThermoFisher Scientific) using Lipofectamine RNAiMAX Reagent and OPTI-MEM I (ThermoFisher Scientific), as described previously [2]. The final concentration of siRNA used in each experiment was 10 nM. Downregulation of the expression of targeted genes was verified using western blotting as described below.
2.4. Assessment of cell viability The number of viable cells was estimated using a CellTiter Glo 3D Cell Viability Assay (Promega, Madison, WI, USA) according to the manufacturer's instructions. All results are shown as the mean ± SD.
2.5. Crystal violet staining Crystal violet staining for viable cells was performed as described previously [2]. Briefly, NC siRNA- or BIRC5 siRNAtransfected H2009 parental cells and TR cells were cultured for 48 h. Cells were then plated at a density of 5 105 in wells of a 6 well-plate, then left untreated or treated with trametinib (20 nM) for another 48 h. Viable cells attached to the wells were fixed with methanol and stained with 0.5% crystal violet solution. 2.6. Western blotting Western blotting for the expression of survivin, total ERK1/2, phospho-ERK1/2, p21, p27, and b-actin was performed as described previously [2,14e16]. Additional primary antibodies used in the present study were RB1 (#9309; 4H1; 1:2000; Cell Signaling Technology Japan, Tokyo, Japan) and cleaved caspase 3 (#9661; 1:1000; Cell Signaling Technology). Band intensity levels on X-ray films were normalized to b-actin using Image J software (National Institutes of Health, Bethesda, MD, USA). 2.7. RNA isolation and quantitative reverse transcription (RT)ePCR Total RNA extraction and quantitative RT-PCR were carried out as previously described [14e16]. The following PCR primers were purchased from Qiagen (QuantiTect Primer Assay): Hs_ACTB_1_SG, amplifying ACTB which encodes b-actin, and Hs_BIRC5_1_SG, amplifying BIRC5 which encodes survivin. b-actin was used to standardize the quantity of target mRNAs. Relative mRNA expression levels were calculated using the comparative DDCT method and are presented as the averages of triplicate experiments. 2.8. Statistical analysis Differences in cell viability between untreated and treated cells were evaluated by Dunnett's test, a multiple comparison test, or by paired t-tests. One-way analysis of variance (ANOVA) followed by the TukeyeKramer multiple comparison test was performed to evaluate the therapeutic effects of trametinib on H2009 parental cells and TR cells. p-values less than 0.05 were considered statistically significant. Statistical analyses were performed with JMP software (JMP for Windows version 7, SAS Institute Japan, Tokyo, Japan). 3. Results 3.1. Trametinib downregulates survivin expression by inducing p21 in RB1-positive KRAS-mutant lung adenocarcinoma cells To elucidate whether and to what extent p21 and/or RB1 contribute to trametinib-mediated downregulation of survivin, we examined two RB1-positive KRAS-mutant lung adenocarcinoma cell lines, H358 and H441 cells, and an RB1-negative line, H2009 cells (Fig. 1A). When treated with trametinib, all cell lines decreased in number in a concentration-dependent manner (Fig. 1B). Of note, H358 and H441 cells were sensitive to trametinib, while H2009 cells were relatively resistant (Fig. 1B). In H358 and H441 cells treated with trametinib, survivin expression declined whereas p21 expression was induced (Fig. 1C). We also found that expression levels of total RB1 were clearly reduced in trametinib-treated H358 and H441 cells, while the dephosphorylated form increased in a dose-dependent manner (Fig. 1C). This suggests that induced dephosphorylated RB1, which is known to inhibit E2Fs [17], functions in downregulating survivin expression.
Please cite this article in press as: T. Sumi, et al., Trametinib downregulates survivin expression in RB1-positive KRAS-mutant lung adenocarcinoma cells, Biochemical and Biophysical Research Communications (2018), https://doi.org/10.1016/j.bbrc.2018.04.230
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Fig. 1. p21 plays a key role in trametinib-induced survivin downregulation in RB1-positive KRAS-mutant lung adenocarcinoma cells. A). Western blots of untreated H2009, H358, and H441 cells. B). Effects of trametinib on cell viability in H2009, H358, and H441 cells. Cells were untreated or treated with trametinib for 48 h. Results are shown as the mean ± SD. *p < 0.05, **p < 0.01. NS, not significant. C). Western blots examining the effects of trametinib on H2009, H358, and H441 cells. Cells were untreated or treated with trametinib for 48 h. P-RB1, phosphorylated form of RB1; UnP-RB1, unphosphorylated form of RB1. D).Western blots examining the effects of trametinib on H2009, H358, and H441 cells. Cells were treated with trametinib (20 nM) for the indicated time. E). Quantitative RT-PCR for the expression of BIRC5 mRNA in H2009, H358, and H441 cells treated with trametinib (20 nM) for the indicated time. mRNAs were measured in triplicate. Results are expressed as the mean ± SD. **p < 0.01. F). Western blots examining the effects of p21 knockdown on trametinib-treated H358 and H441 cells. Cells were transfected with NC siRNA or CDKN1A siRNA and then cultured for 48 h. Cells were subsequently treated with trametinib (20 nM) for another 48 h.
ERK phosphorylation (active ERK signaling) was observed as early as day 2 of trametinib treatment, although survivin expression was still markedly suppressed in trametinib-treated H358 cells and H441 cells at day 28 (Fig. 1D). The marked decrease in survivin expression was also confirmed at the RNA level (Fig. 1E). As expected, both H358 and H441 cells could not proliferate actively in the presence of trametinib even at day 28, showing a swollen to flattened morphology suggestive of cells in senescence (data not shown). Surprisingly, RB1 expression, the dephosphorylated form of which is supposed to inhibit survivin expression, was barely detected in trametinib-treated H358 and H441 cells at day 28 (Fig. 1D). This marked reduction in RB1 expression likely results from cell cycle arrest caused by sustained p21 expression, because RB1 is susceptible to degradation through the ubiquitineproteasome pathway when the cell cycle is arrested [8,9]. In H2009 cells treated with trametinib, p21 induction and a slight decrease in survivin expression were also observed at day 2 as in H358 and 441 cells (Fig. 1D and E). However, the expression levels of survivin in trametinib-treated H2009 cells quickly recovered the following day, almost coinciding with the reactivation of ERK in trametinib-treated H2009 cells (Fig. 1D). This suggests that the transient decline of survivin was probably a
result of cell cycle arrest caused by trametinib-mediated MEK inhibition [18]. Interestingly, H2009 cells acquired the ability to thrive in the presence of trametinib (20 nM) by day 28, when p21 expression was no longer detectable while survivin was expressed (Fig. 1D). We next suppressed the expression of p21 (encoded by CDKN1A) using RNA interference (RNAi) in H358 and H441 cells to clarify the role of p21 in survivin expression (Fig. 1F). Comparing NC siRNAtransfected cells and CDKN1A siRNA-transfected cells in the presence of trametinib, p21 depletion clearly increased the expression of survivin, while RB1, including its phosphorylated form, was also upregulated (Fig. 1F). Taken together, this indicates that p21 plays a key role in trametinib-induced survivin suppression in RB1positive H358 and H441 cells. Moreover, both cell lines appeared to be in senescence even on day 28 because of their sustained expression of p21 and persistent suppression of survivin (Fig. 1D). In RB1-negative H2009 cells, although survivin was transiently lowered by trametinib treatment, the cells acquired resistance to trametinib by day 28, when they lost p21 expression and maintained survivin expression. These findings collectively raised the possibility that the RB1 status critically affects long-term sensitivity to trametinib.
Please cite this article in press as: T. Sumi, et al., Trametinib downregulates survivin expression in RB1-positive KRAS-mutant lung adenocarcinoma cells, Biochemical and Biophysical Research Communications (2018), https://doi.org/10.1016/j.bbrc.2018.04.230
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3.2. RB1 depletion attenuates the trametinib-induced survivin decline in RB1-positive KRAS-mutant lung adenocarcinoma cells We subsequently suppressed RB1 expression in H358 and H441 cells using RNAi to elucidate whether RB1 directly affected survivin expression levels. RB1 knockdown significantly increased the number of viable cells in the presence (up to 20 nM) or absence of trametinib in these cells (Fig. 2A). RB1 knockdown also significantly attenuated trametinib-mediated survivin downregulation at both the RNA and protein levels (Fig. 2B and C). These findings indicate that RB1 plays a role in trametinib-induced survivin suppression at an early phase (within 48 h) in RB1-positive KRASmutated H358 and H441 cells. 3.3. Survivin plays a critical role in acquired resistance to trametinib in H2009 cells H358 and H441 cells showed very little growth in the presence of trametinib even at day 28 of treatment, although H2009 trametinibresistant (TR) cells were obtained following long-term culture (Fig. 3A). Using H2009 TR cells, we explored the extent to which survivin contributed to acquired trametinib resistance. Trametinib treatment (20 nM) significantly decreased the number of H2009 parental cells but had little effect on TR cells (Fig. 3B). Trametinibtreated H2009 parental cells, but not TR cells, also underwent apoptosis as shown by the expression of cleaved caspase 3 (Fig. 3C). The depletion of survivin significantly reduced the viability of H2009 TR cells as well as its parental cells (Fig. 3D). Additionally, NC siRNAtransfected TR cells resisted trametinib treatment (up to 20 nM) while survivin-depleted TR cells responded to trametinib, resulting in dose-dependent and significantly decreased viability (Fig. 3E). Survivin-depleted TR cells and parental cells underwent apoptosis in response to trametinib treatment as evidenced by the expression of cleaved caspase 3 (Fig. 3F and G). Of note, cleaved caspase 3 was
more readily detectable in survivin-suppressed, trametinib-untreated TR cells than parental cells (Fig. 3F), suggesting that H2009 TR cells depend heavily on survivin for survival. Taken together, these findings suggest that survivin contributes to acquired resistance to trametinib in H2009 cells. 4. Discussion We report here for the first time that trametinib consistently suppressed survivin expression for at least 28 days in RB1-positive KRAS-mutant lung adenocarcinoma, H358 cells, and H441 cells. This appears to be caused by the induction of p21, a senescenceassociated protein, and the dephosphorylation of RB1. While these mechanisms of action of trametinib were appreciated at the time of its development, they have been less recognized recently [6]. Not only MEK inhibition but also p21 induction serve as indispensable mechanisms of trametinib against tumor growth. In RB1-negative KRAS-mutant lung adenocarcinoma H2009 cells, trametinib-mediated survivin suppression was transient. We readily obtained H2009 TR cells, which critically depended on survivin for survival. We also demonstrated that dephosphorylated RB1 contributed, at least in part, to trametinib-induced survivin suppression at an early phase (within 48 h of treatment) in RB1-positive H358 and H441 cells (Fig. 4). Paradoxically, in trametinib-treated H358 or H441 cells at day 28 (late phase), the expression levels of RB1 were too low to be detected while survivin remained almost completely suppressed. This is probably because the sustained expression of p21 caused these cells to senesce with RB1 prone to degradation as a negative feedback mechanism. Expression levels of survivin are usually low in senescent cells [18], which could reflect the loss of E2Fs after their detachment from RB1 and their subsequent degradation through the ubiquitineproteasome pathway [19]. Thus, survivin expression was almost completely suppressed in trametinib-treated H358 or
Fig. 2. RB1 knockdown attenuates trametinib-induced survivin suppression in RB1-positive KRAS-mutant lung adenocarcinoma cells. A). Effects of RB1 knockdown on cell viability of H358 and H441 cells untreated or treated with trametinib. NC siRNA or RB1 siRNA-transfected cells were cultured for 48 h, then left untreated or treated with trametinib at the indicated concentration for another 48 h. Results are shown as the mean ± SD. NS, not significant. *p < 0.05, **p < 0.01, ***p < 0.001 (paired t-test). B). Quantitative RT-PCR for the expression of BIRC5 mRNA. NC siRNA or RB1 siRNA-transfected cells were cultured for 48 h, then left untreated or treated with trametinib (20 nM) for another 48 h. mRNAs were measured in triplicate. Results are expressed as the mean ± SD. **p < 0.01. C). Upper panel: western blots examining the effects of RB1 depletion on H358 and H441 cells untreated or treated with trametinib. Cells were treated in the same way as described in (B). Lower panel: quantification of survivin protein expression by western blot. Columns, mean (n ¼ 3); bars, ± SD. **p < 0.01.
Please cite this article in press as: T. Sumi, et al., Trametinib downregulates survivin expression in RB1-positive KRAS-mutant lung adenocarcinoma cells, Biochemical and Biophysical Research Communications (2018), https://doi.org/10.1016/j.bbrc.2018.04.230
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Fig. 3. Survivin contributes to trametinib resistance in H2009 cells. A). Representative phase-contrast images of H2009 parental cells and trametinib-resistant (TR) cells. Scale bars, 50 mm. B). Effects of trametinib on cell viability of H2009 parental cells and TR cells. Cells were untreated or treated with trametinib (20 nM) for 48 h. Results are shown as the mean ± SD. NS, not significant. **P < 0.01 (ANOVA followed by the TukeyeKramer multiple comparison test). C). Western blots examining the effects of trametinib on H2009 parental cells and TR cells. Cells were untreated or treated with trametinib (20 nM) for 48 h. D). Effects of BIRC5 knockdown on cell viability of H2009 parental cells and TR cells. Cells were transfected with NC siRNA or BIRC5 siRNA, then cultured for 96 h. Results are shown as the mean ± SD. **P < 0.01. E). Effects of BIRC5 knockdown on cell viability of H2009 parental cells and TR cells untreated or treated with trametinib. NC siRNA or BIRC5 siRNAtransfected cells were cultured for 48 h, then untreated or treated with trametinib (up to 20 nM) for another 48 h. Results are shown as the mean ± SD. *P < 0.01, **P < 0.01. F). Western blots examining the effects of BIRC5 knockdown on H2009 parental cells and TR cells untreated or treated with trametinib. NC siRNA or BIRC5 siRNA-transfected cells were cultured for 48 h, then untreated or treated with trametinib (20 nM) for another 48 h. G). Crystal violet staining for viable cells. NC siRNA or BIRC5 siRNA-transfected cells were cultured for 48 h. Transfected cells (5 105 cells each) were then seeded into 6-well-plates and grown in the absence or presence of trametinib (20 nM) for another 48 h. Viable cells were fixed and stained with crystal violet.
Fig. 4. Schema of mechanisms of action of trametinib underlying survivin suppression in RB1-positive KRAS-mutant lung adenocarcinoma cells. Trametinib treatment induces p21 in RB1-positive KRAS-mutant lung adenocarcinoma cells. (Early phase) Induced p21 inhibits the activity of cyclineCDK complexes, leading to RB1 dephosphorylation. Dephosphorylated RB1 then inhibits the transcriptional activity of E2Fs, resulting in decreased survivin expression. (Late phase) Sustained expression of p21 induces cellular senescence, where survivin is almost completely suppressed.
H441 cells at a late phase despite loss of RB1 expression (Fig. 4). Trametinib treatment was recently shown to inhibit the expression of survivin in RB1-positive BRAF or NRAS-mutant melanoma cells, but not in RB1-negative ones [20]. Additionally, dephosphorylated RB1 restricts the functions of E2Fs in RB1-positive
melanoma cells, resulting in the downregulation of survivin [20]. Our findings are in line with these observations. RB1 deficiencies are frequently observed in non-small cell lung carcinomas (20e30%) [21]. Our findings suggest that RB1-deficient KRAS-mutant lung adenocarcinomas readily acquire resistance to trametinib, raising the
Please cite this article in press as: T. Sumi, et al., Trametinib downregulates survivin expression in RB1-positive KRAS-mutant lung adenocarcinoma cells, Biochemical and Biophysical Research Communications (2018), https://doi.org/10.1016/j.bbrc.2018.04.230
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possibility that the RB1 status in KRAS-mutant lung cancer could be a biomarker for determining the use of trametinib. The present study has several limitations. First, RB1 knockdown cannot completely restore trametinib-induced survivin suppression in RB1-positive KRAS-mutant lung adenocarcinoma cells. This is because several transcription factors other than E2Fs, such as NFkB, STAT3, and KLF5, contribute to BIRC5 (survivin) expression [22e25]. In cells treated with MEK inhibitors, KLF5 appears to be downregulated although NF-kB and STAT3 are usually phosphorylated (activated) [22e25]. Indeed, we detected phosphorylated NF-kB and STAT3 in trametinib-treated H358 cells (data not shown). This suggests that not only E2Fs but also NF-kB and/or STAT3 contribute to survivin expression in RB1-positive KRASmutant lung adenocarcinoma cells. Second, the molecular mechanisms underlying p21 induction remain to be solved. As mentioned above, in RB1-positive, trametinib-sensitive cell lines, p21 expression was durable in the presence of trametinib but only transient in the RB1-negative cell line. Considering that the three KRAS-mutant lung adenocarcinoma cell lines analyzed here are all p53 mutant cells (https://portals.broadinstitute.org/ccle), it is interesting to note that expression patterns of p21, one of the most important p53 target genes, vary widely among cells [10]. In conclusion, trametinib consistently suppressed survivin expression in RB1-positive KRAS-mutant lung adenocarcinoma H358 and H441 cells through p21 induction and RB1 dephosphorylation. However, trametinib-mediated survivin suppression was only transient in RB1-negative KRAS-mutant lung adenocarcinoma H2009 cells, leading to the development of H2009 trametinibresistant cells that depend critically upon survivin for survival. These findings collectively suggest that survivin suppression rather than MEK inhibition determines the efficacy of trametinib on KRASmutant lung adenocarcinomas.
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[4] [5]
[6]
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[8] [9] [10]
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[12] [13]
[14]
[15]
Conflicts of interest
[16]
None of the authors of the present study have a conflict of interest to declare.
[17]
Funding This work was supported in part by Grants-in-Aid for Young Scientists from JSPS (Grant Number 16K19459, given to TS), and funded by support from the Ono Cancer Research Fund (given to YS).
[18] [19]
[20]
[21] [22]
Acknowledgments
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We thank Dr. Sarah Williams, PhD, from Edanz Group (www. edanzediting.com) for editing a draft of this manuscript. [24]
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Please cite this article in press as: T. Sumi, et al., Trametinib downregulates survivin expression in RB1-positive KRAS-mutant lung adenocarcinoma cells, Biochemical and Biophysical Research Communications (2018), https://doi.org/10.1016/j.bbrc.2018.04.230