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Mutations in the epidermal growth factor receptor (EGFR) kinase domain such as EGFR-L858R and EGFR-G719S have been reported to activate the kinase and also sensitize a subset of patients with non-small cell lung cancer to EGFR kinase inhibitor treatment. Nevertheless, for other common point mutations such as EGFR-L861Q, it is unclear whether and to what extent they sensitize toward gefitinib and erlotinib. Thus far, there is no reliable cellular assay to compare in a ligand-independent manner intrinsic kinase activity and drug sensitivity of the unmutated (wild type) and mutated EGFR kinase domain.
To overcome this obstacle, we introduced L858R, G719S, and L861Q into the backbone of EGFRvIII. EGFRvIII has a wild type-kinase domain but is activated in a ligand-independent manner through a deletion in the extracellular domain.
Using this tool, we show that the L861Q mutation displays enhanced kinase activity and transforming potential compared with L858R, G719S, and also to the wild type-EGFR kinase domain. Interestingly, L861Q does not increase drug sensitivity toward clinically used EGFR kinase inhibitors in contrast to the L858R and G719S mutation. In addition, we demonstrate that EGFR-L861Q could be effectively inhibited with the irreversible second-generation EGFR inhibitor WZ-4002.
Thus, in the common EGFR-L861Q mutation, activation of the kinase domain is uncoupled from a sensitizing effect toward clinically approved kinase inhibitors. Therefore, patients with EGFR-L861Q may not have the same clinical benefit from gefitinib/erlotinib treatment as patients with EGFR-L858R and EGFR-G719S mutations. Treatment with irreversible second-generation kinase inhibitors such as WZ-4002 may be an attractive option in the future for patients with EGFR-L861Q.
Because EGFRvIII contains a wt-kinase domain and does not require ligand for activation, it can be used as test backbone to study the effect of mutations on kinase activity and inhibitor sensitivity in cells in a ligand-independent manner.
Point mutations were introduced into MiGR1-EGFRvIII using the Quickchange Site-Directed Mutagenesis kit (Fermentas, St. Leon-Rot, Germany) and confirmed by sequencing. HEK293 cells were cultured in Dulbecco's Modified Eagle Medium (PAA, Pasching, Austria) supplemented with 10% fetal calf serum (PAA). Ba/F3 cells were cultured in Roswell Park Memorial Institute 1640 (Life Technologies) supplemented with interleukin-3 (IL-3; R&D, Wiesbaden, Germany). Stable Ba/F3 cell lines were established by retroviral infection with MiGR1-EGFRvIII constructs followed by IL-3 withdrawal. Gefitinib was provided by AstraZeneca, and AEE 788 was a kind gift from Novartis Pharma. Erlotinib was purchased from pharmacy. WZ-4002 was purchased from Axon Medchem BV. Each compound was dissolved in dimethyl sulfoxide to make an initial stock solution of 10 mmol/liter (gefitinib, AEE 788 and WZ-4002) and 2.5 mmol/liter (erlotinib).
Proliferation Assay, Soft Agar Assay, and Western Blotting
For proliferation analysis, Ba/F3-EGFRvIII cells (1 × 104) were plated into 96-well plates, and inhibitors were added at indicated concentrations. Cell proliferation was measured at 48 hours using CellTiter96 Proliferation Assay (Promega, Madison, WI).
To test anchorage-independent growth in soft agar, 1 ml of 0.75% agar (Difco, Lawrence, KS) solution in Iscove's Modified Dulbecco's Medium (Life Technologies) was first placed as a bottom layer in six-well plates; 2.5 × 104 cells were seeded in 3 ml containing 0.3% agar. Colonies were stained with 3-(4,5-dimethyl-2-thiazolyl)-2.5-diphenyl-2H-tetrazolium bromide (Sigma, Taufkirchen, Germany) and counted.
HEK293 cells were transiently transfected with wild-type or mutant MiGR1/EGFRvIII using Lipofectamine 2000 reagent. Cell lysates were prepared for biochemical analysis. Cell lysates of stable Ba/F3 cell lines expressing wild-type or mutant EGFRvIII were prepared after treatment with inhibitors at indicated concentrations for 30 minutes. Cells lysis, sodium dodecyl sulfate polyacrylamide gel electrophoresis, and Western blotting were performed as described previously.
Following antibodies were used for analysis: phosphorylated EGFR-Tyr1068 (Cell Signaling, Danvers, MA), p-EGFR-Tyr845 (Santa Cruz Biotechnoloy, Heidelberg, Germany), EGFR (Santa Cruz Biotechnology), pStat5-Tyr694 (Cell Signaling), Stat5 (Santa Cruz Biotechnology), pAkt-Ser473 (Cell Signaling), and Akt1/2 (Santa Cruz Biotechnology). Blots were scanned and subjected to analysis using ImageJ software.
RESULTS AND DISCUSSION
The most common point mutations reported in patients with NSCLC, EGFR-L858R, EGFR-G719S, and EGFR-L861Q, were selected for this study. Our aim was to determine kinase activity and transforming potential of these mutants compared with the wt-EGFR kinase domain. Using the wt EGFR as reference has several limitations: ligand stimulation and the required serum starvation of cells may alter cellular responses. In addition, the wt receptor cannot be used as comparison for the transforming potential of certain EGFR mutants. The use of EGFRvIII as reference abrogates several of these limitations. EGFRvIII contains a wt-kinase domain and, thus, can be used to study the impact of mutations on the kinase activity in an unaltered cellular setting.
Therefore, we first cloned all three point mutations into the EGFRvIII backbone. For analysis of kinase activity and signaling, we chose HEK293 cells, which lack endogenous EGFR.
Both EGFRvIII-G719S and EGFRvIII-L858R showed a two- to four-fold increased autophosphorylation compared with EGFRvIII containing a wt-kinase domain (Figures 1A, B). EGFRvIII-L861Q showed the strongest autophosphorylation, which was more than 10-fold higher than EGFRvIII (Figures 1A, B). This data are in agreement with studies using the wt-EGFR receptor with ligand stimulation.
EGFRvIII-L861Q was also the strongest activator of Stat5 indicating that not only autophosphorylation but also substrate phosphorylation is enhanced by this mutation. None of the EGFRvIII constructs increased Akt activity above levels observed under normal serum conditions (Figure 1A).
To compare the transforming abilities of these mutants, we performed a competitive growth assay, in which the outgrowth of oncogene-transduced cells under growth factor withdrawal in a mixed population is measured. To this end, newly transduced (transduction efficiency ∼20%), unselected stable Ba/F3 cell lines expressing EGFRvIII mutants together with enhanced green fluorescent protein were deprived of IL-3. Outgrowth of enhanced green fluorescent protein-positive cells was measured by fluorescence activated cell sorting analysis over time. EGFRvIII-L861Q expressing cells showed the strongest proliferation advantage in this competition assay (Figure 1C). EGFRvIII-G719S and EGFRvIII-L858R expressing cells were also selected more efficiently than cells expressing unmutated EGFRvIII (Figure 1C). After prolonged IL-3 deprivation, however, all EGFRvIII constructs finally conferred IL-3 independent growth. Thus, mutation L861Q leads to the strongest gain in kinase activity and most rapid induction of cytokine independent growth compared with the wt-kinase domain. To test the transforming potential of EGFRvIII-based mutants in an independent assay, NIH/3T3 cells were transduced with wild-type or mutant EGFRvIII kinase, and stable cell lines were used for a soft agar assay. EGFRvIII with wild-type kinase induced anchorage- and growth factor-independent colony formation, whereas no colonies were detected in untransduced or vector-transduced cells (Figures 1D, E). Moreover, cells expressing EGFRvIII + G719S, EGFRvIII + L858R, and EGFRvIII + L861Q formed higher numbers of colonies compared with cells expressing unmutated EGFRvIII kinase (Figures 1D, E).
We then determined the effect of oncogenic EGFR kinase domain mutations on drug sensitivity. We used stably transduced Ba/F3 cell lines expressing wild-type or mutant EGFRvIII and the EGFR kinase inhibitors gefitinib, erlotinib, and AEE788. Gefitinib and erlotinib are selective EGFR inhibitors already approved in the clinic. AEE788 is a compound in development inhibiting both EGFR and Her2. EGFRvIII-L858R expressing cells were very sensitive to all drugs tested with an half maximal inhibitory concentration (IC50) value of less than 50 nM (Figures 2A–C). EGFRvIII-G719S expressing cells displayed an intermediate sensitivity. Both mutations led to lower IC50 values compared with wild-type EGFRvIII, thus sensitizing cells to EGFR kinase inhibitors (Table 1). In contrast, EGFRvIII-L861Q expressing cells displayed IC50 values identical to wt EGFRvIII (Figures 2A–C). Although this mutation induced the strongest proliferation advantage and highest increase in kinase activity among all mutations tested, it did not lead to enhanced kinase inhibitor sensitivity (Figure 2D). Western blot analysis after treatment of Ba/F3 cells expressing EGFRvIII constructs with gefitinib or erlotinib showed that EGFRvIII-G719S and EGFRvIII-L858R but not EGFRvIII-L861Q displayed enhanced inhibition of downstream Stat5 signaling compared with the wild-type EGFRvIII (Figure 2E). Thus, in this common EGFR mutation, activation of the kinase domain seems to be uncoupled from sensitizing effects toward kinase inhibitors. In a recent report, L858R and G719S mutants were shown to have a reduced binding affinity toward adenosine triphosphate compared with wild-type EGFR, making them more accessible for ATP competitive EGFR inhibitors.
This may explain the lack of enhanced sensitivity toward reversible inhibitors of this mutant. Thus, these results suggest that NSCLC patients with the EGFR-L861Q mutation may not benefit as much from EGFR inhibitor treatment with gefitinib or erlotinib as patients with the EGFR-L858R, EGFR-G719S, or EGFR exon 19 deletions.
TABLE 1IC50 Values (nM) of Wild-Type and Mutant EGFRvIII Kinases against Gefitinib, Erlotinib, AEE788, and WZ-4002 Calculated from Figures 2 and 3
Variation in drug response toward different activating mutations in oncogenic tyrosine kinases has been reported in various cancers, and accumulating evidence indicates that this may have impact on the clinical outcome on inhibitor treatment.
Therefore, it may be beneficial to test alternative EGFR inhibitors toward less sensitive activating EGFR mutations such as EGFR-L861Q. WZ-4002 is a novel irreversible inhibitor of EGFR kinase that was recently shown to have significant activity both in vitro and in vivo.
Both EGFRvIII-L858R and EGFRvIII-G719S showed lower IC50 values toward WZ-4002 treatment compared with wild-type EGFRvIII (Figure 3, Table 1). Interestingly, EGFRvIII-L861Q also showed significantly more sensitivity against WZ-4002 compared with wild-type EGFRvIII (Figure 3, Table 1). WZ-4002 is an irreversible inhibitor and binds to the active conformation of the EGFR kinase.
Because all mutations tested in this study are activating, they may be more sensitive to WZ-4002. Therefore, irreversible second-generation kinase inhibitors such as WZ-4002 may offer a more potent alternative treatment for patients with the EGFR-L861Q mutation in the future.