Background
Despite advances in modern chemotherapy, the prognosis of patients with acute myeloid leukemia (AML) has remained poor and little progress has been made to improve long-term outcome of these patients. The American Cancer Society estimates that 21,380 new AML cases were diagnosed and approximately 10,590 deaths from this disease occurred in 2017 [
1]. The long term, disease-free survival of AML patients under age 60 remains approximately 40% [
2]. Therefore, new approaches are needed if further improvement in the outcome for AML patients is to be achieved.
EPS8 (epidermal growth factor receptor (EGFR) pathway substrate no.8) was first known as a vital substrate for EGFR kinase [
3]. EPS8 is efficiently phosphorylated by various tyrosine kinases, both of the receptor (RTK) and non-receptor type [
4] and is a typical signaling protein of 97 kDa, containing a phosphotyrosine binding protein (PTB) domain, a Src homology 3 (SH3) domain and a sterile alpha-pointed (SAM-PNT) domain [
4]. Further studies of EPS8 have revealed the existence of two additional functional regions. A C terminal effector region, extending from amino acids (aa) 641 to 822, is thought to interact with Sos-1 and subsequently activate Rac specific GEF activity [
5]. The other region, encompassing amino acids 298 to 362, provides a binding surface for the JXM region of EGFR (JMB) [
6]. Importantly, a nuclear localization signal (NLS) is also in this region. Elevated EPS8 expression levels have been found in various solid tumors [
7‐
10] and several hematological malignancies [
11]. Studies have shown that EPS8 is critical in tumorigenesis, proliferation, invasion and metastasis [
12‐
15]. Our previous review has provided a comprehensive picture of the role of EPS8 in different tumor biological behaviors [
16]. Therefore, EPS8 might represent a novel potential target for cancer therapy.
The studies of EPS8 in hematological malignancies are limited. Elevated EPS8 expression was correlated with worse outcome in infant acute lymphoblastic leukemia (ALL) based on gene expression profiles (From a Children’s Oncology Group study) [
11]. We have indicated that EPS8 may be a valuable clinical biomarker for assessing the outcome of ALL patients [
17]. Our previous work showed that EPS8 was overexpressed in AML patients, and the expression level of EPS8 was correlated to the complete remission rate of AML patients treated with chemotherapy [
18]. The 298–362 aa domain of EPS8 contains a nuclear localization signal. The release of EPS8 from tyrosine kinases makes the nuclear targeting signal available to the intracellular molecular machinery responsible for nuclear translocation. R Carbone et al. observed that a fraction of EPS8 is indeed translocated to the nucleus, resulting in increased EPS8 expression [
6]. Ectopic EPS8 expression enhances mitogenic signals, eventually resulting in carcinogenesis.
This study consists of two major parts. First, we found that the MAPK/Erk pathway and PI3K/Akt pathways may play critical roles in EPS8-mediated induction of AML cell proliferation, anti-apoptosis and chemosensitivity in vitro and in vivo. Second, to overcome the limitations of currently available inhibitors for AML treatment, we developed an effective anti-AML peptide (CP-EPS8-NLS) derived from the 298–362 aa domain that specifically mimics the NLS of EPS8. Our findings demonstrate the efficacy of CP-EPS8-NLS in potently repressing AML cell lines both in vitro and in vivo, suggesting a novel therapeutic strategy for inhibition of the NLS of EPS8 in AML therapy.
Methods
Cell lines and culture conditions
The acute promyelocytic leukemia cell lines HL-60 and NB4, acute monocytic leukemia cell line THP-1, acute myelomonocytic leukemia cell line U937, acute erythrocytic leukemia cell line TF1α and acute myelogenous leukemia cell line KG1α were cryopreserved in the Hematological Laboratory of Zhujiang Hospital (Guangzhou, China). HL-60, NB4, TF1α and THP-1 cell lines were purchased from the cell bank of Sun Yet-san University (Guangzhou, China); the source of these cells was ATCC. U937 cell line was purchased from ATCC. The KG1α and HL-60/ADR cell lines were kindly provided by Tianjin Institute of Hematology (Tianjin, China). Normal PBMCs were obtained from 5 unrelated healthy donors at Southern Medical University (Guangzhou, China). All cell lines and the PBMCs were incubated in RPMI 1640 medium (Invitrogen, Carlsbad, CA) supplemented with 10% fetal bovine serum at 37 °C with 5% CO2.
EPS8 GEO expression profile in AML cells
The expression profiles of AML patients from the GSE13159 dataset, containing bone marrow samples from 501 AML patients at diagnosis and 72 healthy volunteers, were generated on Affymetrix Gene Chip HG-U133A arrays (Affymetrix, Santa Clara, CA, USA), and were extracted from CEL files using RMA normalization procedure and custom CDF annotation package. AML samples from GSE12417 and The Cancer Genome Atlas (TCGA) were used to test association between EPS8 expression and AML patient outcomes.
Creation and characterization of stable EPS8 knockdown cell lines
EPS8 expression was stably knocked down in U937 and KG1α cells via RNA interference. The annealed oligonucleotide fragments encoding short hairpin transcripts corresponding to EPS8 were as follows: TAGTGATTCAGGAGTGGAA and AACTTCTAATCGCCATATA. The non-targeting empty plasmid was used as the control shRNA plasmid. According to the manufacturer’s instructions, U937 and KG1α cells (2 × 105/well in six-well plates) were separately transfected separately with control shRNA plasmid or the EPS8 shRNA plasmid using Lipofectamine 2000 reagent (Invitrogen). After the dilution culture was limited under selection with puromycin, several clones in each transfection group were selected for further experiments and designated as U937/NC, U937/sh1, U937/sh2, KG1α/NC and KG1α/sh1.
RT2profiler™ PCR assay in KG1α/sh1 and KG1α/NC cells
An RT
2profiler™ PCR assay (SuperArray, SABiosciences, a QIAGEN Company) was used to profile the expression of 84 EGF/PDGF signaling-specific genes plus 5 housekeeping genes according to the manufacturer’s protocol. The process in detail was described in previous work [
19].
Peptide synthesis
CP-EPS8-NLS, mutated CP-EPS8-NLS and penetratin were synthesized by the Chinese Peptide Company (Hangzhou, China). Peptides purity was greater than 95%. The peptides were dissolved in deionized water at a final concentration of 10 mg/ml and stored at − 20 °C until further use.
Cell viability assays
AML cell lines and normal PBMCs were plated in a 96-well plate (5 × 103 cells/well) and incubated for 24 h before treatment. All cells were then incubated with different concentrations of CP-EPS8-NLS (0, 35, 70, 105, 140 or 175 μM) for 24 h. U937 cells were also treated with mutated CP-EPS8-NLS and penetratin as controls. After treatment, 10 μl of CCK-8 reagent (Dojindo Laboratories, Japan) was added to each well, and cells were incubated for 3 h at 37 °C and 5% CO2. The optical density (OD) was analyzed at 450 nm. The data obtained are presented as percentage viability in best-fit (linear) dose response curves.
Soft agarose cloning assay
Low-melting agarose was dissolved in pure water at 1.2 and 0.7%, sterilized using an autoclave, and then warmed at 42 °C in a water bath. Then, 1 ml of 2× RPMI 1640 was transferred to each well of a 6-well plate, and 1 ml of 1.2% agarose was added. After these two solutions were mixed, 500 μl of 0.7% agarose and 500 μl of RPMI 1640 containing 500 cells were pipetted into each well and treated with CP-EPS8-NLS (0, 35, 70, 175 μM) on the following day. Two weeks later, the colony number was determined.
Cellular distribution of CP-EPS8-NLS in U937 cells
To examine the membrane penetration ability and the distribution of CP-EPS8-NLS, mutated CP-EPS8-NLS and penetratin in AML cells. Fluorescein isothiocyanate (FITC) was conjugated to the N-terminus of these peptides to form FITC-conjugated peptides. U937 cells (2 × 105 cells per plate) were placed in confocal microscope observation wells that had been pretreated with polylysine. Then, cells were treated with FITC-conjugated peptides (40 μM) in 1 ml medium of culture for 4 h and the cells were stained with propidium iodide (PI) for 20 mins to exclude the possibility that peptides penetrate dying cells and then washed twice with PBS. The cells were fixed for 30 mins and stained with DAPI (which produces blue fluorescence after binding to dsDNA). Cells were rinsed three times with PBS, and the fluorescence distribution was analyzed with a confocal laser scanning microscope (LSM 880 with Airyscan).
Analysis of apoptosis and cell cycle
U937, KG1α, HL-60, THP-1 and TF1α cells were seeded at 1 × 105 cells/well in 6-well plates in serum-containing media; cells were cultured for 12 h before treatment. CP-EPS8-NLS was added at concentrations ranging from 0 to 175 μM and incubated at 37 °C and 5% CO2 for 24 h and 48 h respectively in KG1α. U937, KG1α, HL-60, THP-1 and TF1α cells were added at 0 or 70 μM CP-EPS8-NLS for 24 h. CP-EPS8-NLS treated AML cells were collected and washed by PBS, suspended in binding buffer according to the manufacturer’s protocol (BD, Annexin-V-APC & PI Apoptosis Detection Kit). Cells were analyzed with CellQuest software and each measurement was repeated three times to ensure reproducibility. For cell cycle analysis, AML cells were cultured for 12 h before treatment. CP-EPS8-NLS was added to a final concentration of 0 or 70 μM and incubated at 37 °C and 5% CO2 for 24 h. Cells were collected, washed with PBS, and suspended in RNase A and PI for 1 h in the dark. Samples were analyzed on a FACSCalibur Flow Cytometer (Becton Dickinson, New Jersey, USA).
Chemicals
Daunorubicin (DNR), cytarabine (Ara-c), adriamycin (ADR) and perifosine were purchased from Selleckchem, dissolved in RPMI 1640 medium at a final concentration of 10 mg/ml and stored at − 20 °C.
Determination of combination index values and Chou-Talalay analysis
KG1α and U937 cells were seeded at 5 × 10
3 cells/well in 96-well plates for 24 h before treatment. Cells were treated with CP-EPS8-NLS and/or chemotherapeutic agents (DNR, Ara-c or ADR) at 37 °C and 5% CO
2 for 24 h. Then, cell viability was measured using a CCK-8 assay. The assessment of synergy was performed using CompuSyn software. The combination index (CI) theorem of Chou-Talalay offers a quantitative definition for additive effect (CI = 1), synergism (CI < 1) or antagonism(CI > 1) in drug combinations [
20]. Isobolograms were also used to better investigate the combination effects.
Western blot analysis
All prepared cells were homogenized in protein lysate buffer, and debris was removed by centrifugation at 12,000 g for 10 min at 4 °C. The protein concentrations were determined using a Bradford protein assay kit (Beyotime, China). After addition of loading buffer, protein samples were electrophoresed, transferred to PVDF membranes (0.2 μm; Millipore, Bedford, MA), and subsequent blocked. The membranes were immunoblotted with rabbit anti-human primary antibody overnight at 4 °C. Antibodies to EPS8, Erk, p-Erk, Akt, p-Akt (473), p-Akt (450), p-Akt (308), p-Stat3, mTOR, p-mTOR, p38 MAPK, p-GSK3β, p-cRaf, Cyclin E, bcl-2 and GAPDH were obtained from Cell Signaling Technology. After three washes with TBST, the blots were incubated with horseradishperoxidase (HRP)-conjugated secondary antibodies at room temperature for 1 h, and the HRP signal was detected using enhanced chemiluminescence (Pierce Biotechnology, Rockford, IL, USA).
In vivo study
All animal experiments complied with Southern Medical University’s Policy on the Care and Use of Laboratory Animals. Five-week-old athymic BALB/c nu/nu female mice (14–16 g) purchased from the experimental animal center of Southern Medical University (Guangzhou, China) were used for in vivo experiments. Animals were housed at a constant room temperature with a 12 h light/12 h dark cycle and fed a standard rodent diet and water. U937 cells were harvested and injected subcutaneously (5 × 10
6 cells in 100 μl of PBS) into mice. U937-injected mice were treated with CP-EPS8-NLS at the dose of 50 mg/kg body weight or with PBS as a control via intraperitoneal (i.p.) injection every other day. In addition, KG1α cells were harvested and injected subcutaneously (1 × 10
7 cells in 100 μl of PBS) into mice. KG1α-injected mice were treated with CP-EPS8-NLS (50 mg/kg) and/or DNR (20 mg/kg) every other day. Mutated CP-EPS8-NLS and PBS were injected as controls. The maximum tumor volume was not allowed to exceed 3000 mm
3. At the end of the experiment, the animals were sacrificed, and the tumors were removed. The tumor volumes were determined by measuring tumor length (L) and width (W) and calculating the volume (V = 0.5 × L × W
2) [
21].
Statistical analysis
Statistical significance was evaluated using SPSS 11.0 software. P < 0.05 was considered statistically significant. * Represents P < 0.05, ** represents P < 0.01, and *** represents P < 0.001.
Discussion
EPS8 is important in regulating the development and progression of many human cancers. However, the role of EPS8 in hematological malignancies has not been clarified in detail. Elevated EPS8 expression is correlated with worse outcome in ALL patients. In our previous work, using real-time quantitative PCR to detect EPS8 RNA in bone marrow samples from AML patients, we found that EPS8 may have a clinical significance in monitoring (minimal residue disease) MRD that may lead to relapse. EPS8 is an important signaling molecule that integrates multiple pathways, and it controls the Ras-Raf-MEK-Erk signaling cascade, which plays a crucial role in regulating cellular processes including differentiation, proliferation, survival and apoptosis [
25‐
27]. Moreover, formation of the EPS8–Abi1–Sos1 complex recruits to the activated RTKs, leading to Ras activation and integration with PI3K. PIP3, product of PI3K, is thus recruited and activates the PI3K/Akt pathway [
28]. Accumulating evidence has shown the role of the PI3K/Akt pathway in the mechanisms of EPS8 associated tumor proliferation, survival and drug resistance by activating downstream targets, such as FOXM1, mTOR, MMP-9 and caspase-9 [
15,
29]. Many studies have confirmed that aberrant PI3K/Akt/mTOR signaling is significantly related to progression of various solid tumors and hematological malignancies [
30‐
32], and we found that EPS8 expression level were downregulated after the Akt signals were blocked (Additional file
1). We then used a lentivirus-based RNAi system to knock down EPS8 expression in AML cell lines. EPS8 attenuation suppresses the growth ability of AML cells and increases the sensitivity of AML cells to chemotherapy drugs. Our data indicated that EPS8 expression is critical for AML cell growth and proliferation in vitro and in vivo (Fig.
2). EPS8 is possibly involved in the progression and chemosensitivity of AML patients. In this work, we observed that U937/sh2 cells were more sensitive to chemotherapeutic agents (Fig.
2c). We hypothesized that inhibiting EPS8 overexpression or interfering with EPS8 associated signal transduction could eventually inhibit carcinogenesis of AML cells.
Previous studies have suggested that targeted inhibitors interacting with the NLS residues of tumor associated protein can block the nuclear localization ability of proteins in cancer cells, which is crucial for cell cycle progression and has a cellular inhibitory effect [
33]. Here, we show that a novel, synthetic, cell-penetrating peptide derived from nuclear localization sequence of EPS8 induces apoptosis in a broad range of AML types, including acute promyelocytic leukemia (HL-60 and NB4 cells), acute monocytic leukemia (THP-1 cells), acute myelomonocytic leukemia (U937 cells), acute erythrocytic leukemia (TF1α cells) and acute myelogenous leukemia (KG1α cells). We observed the synergistic killing effect of CP-EPS8-NLS combined with therapeutic agents (DNR, Ara-c and ADR). Moreover, CP-EPS8-NLS successfully localized in the nucleus and downregulated the overexpression level of EPS8 in AML cells (directly or indirectly). Western blot assays showed downregulation of p-Akt and p-mTOR after CP-EPS8-NLS treatment. These results were compatible with an previous study demonstrating that activation of PI3K/Akt pathway could induce drug resistance of AML blasts in a PI3K/Akt/mTOR dependent manner [
34]. Phosphorylation of Erk was also down-regulated while the total Erk levels were unchanged. These results verify the inhibitory effects of EPS8 associated AML signal transduction after CP-EPS8-NLS treatment. The in vivo studies provide primary evidence that CP-EPS8-NLS can selectively inhibit the AML cells growth in xenograft nude mouse models. EPS8 overexpression levels were also downregulated in tumor samples from nude mice. Another attractive feature of CP-EPS8-NLS is that it does not significantly suppress the viability of PBMCs from healthy donors. With respect to specificity, the penetratin peptide and a peptide in which key basic residues (lysines) in CP-EPS8-NLS were mutated showed no or little cytotoxic activity in viability studies, and neither peptides downregulated EPS8 expression. Mutated CP-EPS8-NLS also served as a control group in vivo. The fact that EPS8, a tumor associated protein, is overexpressed in AML cells, may indicate cause a state of cellular EPS8 dependency in the sense of oncogene addiction. The NLS of EPS8 may be responsible for nuclear translocation and further activate mitogenic signaling, which induces EPS8 overexpression. Thus, the selective response of AML cells to CP-EPS8-NLS treatment might be a consequence of a CP-EPS8-NLS mediated loss of the nuclear localization function of EPS8 and subsequent downregulation of EPS8.
Acknowledgements
The authors are grateful to Yanjie He (Department of hematology, Zhujiang Hospital, Guangzhou, China) and Jingwen Du (Department of hematology, Zhujiang Hospital, Guangzhou, China) for providing flow cytometry assistance.