Introduction

Acute leukemia comprises a group of hematological neoplasms characterized by the expansion of blasts in the bone marrow and peripheral blood, mainly due to processes that impair normal hematopoiesis, such as dysregulation of cell proliferation, resistance to apoptosis, and loss of the ability to differentiate functional terminal cells [1]. Leukemia blasts can originate from a myeloid or lymphoid lineage, resulting in acute myeloid leukemia (AML) or acute lymphoblastic leukemia (ALL), respectively [2]. The mortality of adult acute leukemia patients is very high, making the identification of new molecular markers and signaling pathways imperative for defining prognosis, therapeutic response, and the development of new treatment options [1, 3, 4].

Recently, large-scale genomics studies have led to advances in the understanding of the molecular basis of acute leukemia, but very little of this new knowledge has been translated into novel therapies [5, 6]. In this respect, previous studies have shown that Stathmin 1 (STMN1) is highly expressed in hematological neoplasms, including AML and ALL [7, 8]. Further evidence supports a contribution of STMN1 to the malignant phenotype by the triggering of three important oncogenes involved in leukemogenesis: JAK2V617F, PML-RARα, and BCR-ABL1 [9,10,11]. The genetic inhibition of STMN1 by shRNA or antisense RNA has been shown to reduce cell proliferation and clonogenicity of AML and ALL cell lines [7, 12]. Notably, STMN1 expression is not detected in normal leukocytes [7, 8]. Taken together, these data suggest that STMN1 may be a selective target for therapy against acute leukemia.

STMN1 is a phosphoprotein that acts by sequestering dimers of α/β tubulin, either by reducing the polymerization of microtubules or by stimulating already formed microtubules to undergo catastrophe by promoting their destabilization [13]. These molecular events contribute to a malignant phenotype characterized by cell cycle progression and high proliferation rates [14]. STMN1 activity is regulated by phosphorylation at specific sites, with serine 16 and 63 being inhibitory sites [13].

Pharmacological inhibition of STMN1 remains a challenge, with only one potential selective inhibitor described thus far in the literature [15]. This compound, GDP366 (1-[4-(4-Aminoethylene[2,3-d]pyrimidine-5-yl)phenyl]-3-(3-methylphenyl)urea), has been described as a dual inhibitor of both STMN1 and BIRC5 (survivin), which are both interesting targets from the point of view of antineoplastic therapy [15]. For example, treatment of HCT116 colon cancer cells with GDP366 inhibited the gene and protein expression of both STMN1 and BIRC5, reduced clonogenicity, led to cell cycle arrest in the G2/M phase, promoted aneuploidy, and increased senescence [15]. The aim of the present study was to identify structural GDP366 analogs and the cellular and molecular mechanisms underlying their suppressive effects in acute leukemia cellular models.

Materials and methods

Gene expression data

STMN1 (probes 200783_s_at and 217714_s_at) mRNA expression data from samples of healthy donors (n = 17, lymphocytes [n = 12] and bone marrow cells [n = 5]), AML patients (favorable risk [n = 120], intermediate risk [n = 342], and adverse risk [n = 66]) [16], B-ALL patients (n = 82), and T-ALL patients (n = 54) were derived from a public access data portal of the Amazonia! database 2008 (http://amazonia.transcriptome.eu) [17]. Gene expression values were measured using Affymetrix HGU133 plus 2.0 arrays, and data sets were cross-referenced using tumor-specific identification numbers.

Identification of GDP366 structural analogs and chemicals

Cheminformatics analyses were based on the identification of compounds with a chemical structure similar to GDP366 using SMILES codes for compounds from the eMolecules database (www.emolecules.com). The protonation state was verified at pH 7.4 using fixpka software v. 1.7.0.2 (QUACPAC: OpenEye Scientific Software, Santa Fe, NM, USA). Conformations were calculated using the standard with OMEGA v. 3.0.0 [18], while the Tanimoto index was applied with the reference compound GDP366 structure based on the three-dimensional shape and color similarity scores in ROCS v. 3.2.2.2 (OpenEye Scientific Software, Santa Fe, NM, USA) [19]. GDP366 (CAS number 501698-03-9) was obtained from MCE (MedicalChemExpress, Monmouth Junction, NJ, USA). AD80 (CAS number 1384071-99-1) was obtained from TargetMol (Target Molecule Corp., Boston, MA, USA). GW768505A (CAS number 1337531-89-1) was obtained from AOBIOUS (AOBIOUS INC, Gloucester, MA, USA). GSK2606414 (CAS number 501693-25-0) was obtained from ApexBio (ApexBio Technology, Houston, TX, USA). All compounds were prepared as 25 mM stock solutions in dimethyl sulfoxide (Me2SO4; DMSO).

Cell culture

Jurkat, Namalwa, and U937 cells were obtained from ATCC (Philadelphia, PA, USA). NB4 cells were provided by Prof. Eduardo Magalhães Rego (Medical school, University of São Paulo). Cells were cultured in RPMI-1640 medium containing 10 % fetal bovine serum (FBS), glutamine, and penicillin/streptomycin, and maintained at 37 °C, 5 % CO2. Cells were authenticated by Short Tandem Repeat (STR) matching analysis using the PowerPlex® 16 HS system (Promega, Madison, WI, USA) and the ABI 3500 Sequence Detector System (Life Technologies, Foster City, CA, USA).

Cell viability assay

Cell viability was evaluated by the methylthiazoletetrazolium (MTT) assay. Briefly, 2 × 104 cells were seeded in a 96-well plate in RPMI-1640 medium with 10 % FBS in the presence of vehicle (Ø) or different concentrations of GDP366, AD80, GW768505A or GSK2606414 (0.0032, 0.016, 0.08, 0.4, 2, 10, and 50 µM) for 24, 48, and/or 72 h. Next, 10 µL MTT solution (5 mg/mL) was added and incubated at 37 °C, 5 % CO2 for 4 h. The reaction was stopped using 100 µL 0.1N HCl in anhydrous isopropanol. Cell viability was evaluated by measuring the absorbance at 570 nm. IC50 values were calculated using nonlinear regression analysis in GraphPad Prism 5 (GraphPad Software, Inc., San Diego, CA, USA).

Apoptosis assessment by annexin V staining

A total of 1 × 105 cells per well were seeded in a 24-well plate in RPMI-1640 medium with 10 % FBS in the presence of vehicle (Ø) or different concentrations of GDP366 (2, 10, and 50 µM) or AD80 (0.4, 2, and 10 µM) for 24 h. The cells were then washed with ice-cold phosphate buffered saline (PBS) and resuspended in a binding buffer containing 1 µg/mL 7AAD and 1 µg/mL fluorescein isothiocyanate (FITC)-labeled annexin V. All specimens were analyzed by flow cytometry (FACSCalibur; Becton Dickinson) after incubation for 15 min at room temperature in a light-protected area. Ten thousand events were acquired for each sample.

Colony formation assay

Colony formation assays were carried out in semisolid methylcellulose medium (2 × 103/mL; MethoCult 4230; StemCell Technologies Inc., Vancouver, BC, Canada) in the presence of vehicle (Ø), GDP366 (2, 10, and 50 µM), or AD80 (0.4, 2, and 10 µM). Colonies were detected after 10 days of culture by adding 150 µL (5 mg/mL) MTT reagent and scored using Image J quantification software (U.S. National Institutes of Health, Bethesda, MD, USA).

Cell cycle analysis

A total of 6 × 105 cells per well were seeded in 6-well plates in the presence of vehicle (Ø), GDP366 (2, 10, and 50 µM) or AD80 (0.4, 2, and 10 µM) for 24 h, fixed with 70 % ethanol and stored at 4 °C for at least 2 h. Fixed cells were stained with 20 µg/mL propidium iodide (PI) containing 10 µg/mL RNase A for 30 min at room temperature in a light protected area. DNA content distribution was acquired using cytometry (FACSCalibur) and analyzed using FlowJo software (Treestar, Inc.).

Proliferation assay using Ki-67 staining

A total of 2 × 105 cells/mL were treated vehicle (Ø), GDP366 (2, 10, and 50 µM) or AD80 (0.4, 2, and 10 µM) for 24 h. The cells were then resuspended in 70 % ethanol and stored at -20 °C. Cells were stained with anti-Ki-67 according to the manufacturer’s instructions (Ki-67 FITC clone B56; BD Bioscience, San Jose, CA, USA) and the mean fluorescence intensity was measured using flow cytometry (FACSCalibur). The IgG isotype was used as a negative control. Ten thousand events were acquired for each sample.

Western blot analysis

Jurkat and NB4 cells were treated with vehicle (Ø), GDP366 (2 and 10 µM), or AD80 (0.4 and 2 µM) for 24 h and submitted for total protein extraction using a buffer containing 100 mM Tris (pH 7.6), 1 % Triton X-100, 150 mM NaCl, 2 mM phenylmethylsulfonyl fluoride (PMSF), 10 mM Na3VO4, 100 mM NaF, 10 mM Na4P2O7, and 4 mM ethylenediaminetetraacetic acid (EDTA). Equal amounts of protein (30 µg) from total extracts were subjected to SDS-PAGE and Western blot analysis with the indicated antibodies and a SuperSignalTM West Dura Extended Duration Substrate System (Thermo Fisher Scientific, San Jose, CA, USA) and a G:BOX Chemi XX6 gel doc system (Syngene, Cambridge, UK). Antibodies directed against phosphorylated Stathmin1S16 (sc-12,948-R), Stathmin 1 (OP18) (sc-55,531), PARP1 (sc-56,197), and α-tubulin (sc-5286) were purchased from Santa Cruz Biotechnology (Santa Cruz, CA, USA). Antibodies directed against γH2AX (#9718), phosphorylated S6 ribosomal protein (RP)S235/236 (#4858), S6RP (#2217), and Survivin (BIRC5; #2808) were obtained from Cell Signaling Technology (Danvers, MA, USA).

PCR array

Total RNA from Jurkat cells treated with vehicle (Ø), GDP366 (10 µM), or AD80 (2 µM) for 24 h was obtained using TRIzol reagent (Thermo Fisher Scientific) from two independent experiments. The cDNA was synthesized from 1 µg RNA using the RT2 First Strand Kit (Qiagen Sciences Inc., Germantown, MD, USA). PCR array analysis was performed using a Human Cytoskeleton Regulators RT2 Profiler PCR Array kit (#PAHS-088ZC; Qiagen Sciences Inc.), according to the manufacturer’s instructions. Amplification was performed using a QuantStudio 3 Real-Time PCR System (Thermo Fisher Scientific). The mRNA levels were normalized to those in untreated cells, and genes that presented a ≥ 1.5-fold change in expression in any direction were included in the heatmap prepared using multiple experiment viewer (MeV) 4.9.0 software. Networks for modulated genes were constructed using the GeneMANIA database (https://genemania.org/) and the main biological interactions and processes were indicated.

Statistical analyses

Statistical analyses were performed using GraphPad Instat 5 (GraphPad Software, Inc., San. Diego, CA, USA). For comparisons, ANOVA and the Bonferroni post-test, Kruskal-Wallis test and Dunn’s post-hoc test, or Student t-test were used. A p-value < 0.05 was considered statistically significant.

Results

Stathmin 1 is highly expressed in a large cohort of acute leukemia patients

Previous studies by our research group [7, 8] and others [20, 21] reported high STMN1 expression in AML and ALL patients. The high STMN1 expression may confer selectivity for compounds that inhibit the signaling pathway mediated by this protein against leukemia cells; therefore, the expression of STMN1 was investigated in a large cohort of patients with acute leukemia (n = 664). STMN1 mRNA was higher in samples from AML patients, independently of risk stratification, compared to healthy donor samples (all p < 0.001). Similarly, B-ALL and T-ALL showed aberrant STMN1 expression compared to healthy donor samples (all p < 0.0001, Fig. 1a). According to the molecular phenotype, AML patients with normal karyotype, other karyotype alteration, t(15,17), t(8,21), inv(16), and − 5/-7q had higher STMN1 levels compared to the healthy donor group (p < 0.05). AML patients with t(15,17) or t(8,21) showed greater STMN1 expression compared to the normal karyotype, other karyotype alteration, complex karyotype, or + 8 AML patients (p < 0.05). All B-ALL subtypes displayed high STMN1 expression (all p < 0.05) compared to healthy donor samples. B-ALL with t(12,21) patients present higher STMN1 levels compared to B-ALL patients with MLL rearrangements (p < 0.05, Supplementary Fig. 1).

Fig. 1
figure 1

STMN1 is highly expressed in AML and ALL patient and cheminformatics analysis identifies GDP366 structural analogs. a STMN1 (200783_s_at and 217714_s_at) mRNA levels in samples from healthy donors (normal leukocytes, LN) and patients with acute myeloid leukemia (AML) (stratified into favorable, intermediate, and adverse groups according to European Leukemia Net [16]), B-acute lymphoblastic leukemia (ALL), and T-ALL. The “y” axis represents gene expression data obtained from the Amazonia! database 2008 (http://amazonia.transcriptome.eu), which were measured using Affymetrix HGU133 plus 2.0 arrays. Data sets were cross-referenced using tumor-specific identification numbers. The numbers of subjects for each group are indicated. ***p < 0.0001; Kruskal-Wallis test and Dunn’s post-hoc test. b The cheminformatics analysis based on the ligand-based similarity search to GDP366 identified three compounds: AD80 GSK2606414, and GW768505A. Chemical structure and Chemical Abstracts Service (CAS) numbers and ROCS setup for GDP366 are indicated. The superposition of GDP366 and AD80 are illustrated. c Dose- and time-response cytotoxicity analyzed using a methylthiazoletetrazolium (MTT) assay for ALL cell lines (Jurkat and Namalwa) and AML cell lines (NB4 and U937) treated with increased concentrations of GDP366, AD80, GSK2606414, or GW768505A (0.0032, 0.016, 0.08, 0.4, 2, 10, and 50 µM) for 72 h. IC50 values were calculated using nonlinear regression analysis. Values are expressed as the percentage of viable cells for each condition relative to vehicle-treated controls. IC50 values were calculated using nonlinear regression analysis

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The cheminformatics analysis and cell‐based assays identify bioactive GDP366 structural analogs

Potential inhibitors of STMN1-mediated signaling were identified by cheminformatics analysis searching for compounds with structural similarity to the reference substance GDP366. One initial step was the identification of compounds similar to GDP366, based on its structural similarity using the similarity coefficient of 0.8 in the SciFinder® database (Chemical Abstract Services). The selected compounds from SciFinder® were analyzed together with the whole eMolecules database using the conformations calculated using the OMEGA classic setup. The lack of a three-dimensional structure for GDP366 complexed with a macromolecule led us to use the ligand-based drug design approach. Some models were obtained after the analysis of the similarity results from ROCS using one conformation of GDP366 as a reference to determine the shape (gray contour) and the features (shown as spheres) in Fig. 1b. GDP366 is similar to other known kinase inhibitors, so this information was then used to determine the weightings for the physicochemical properties (features) in the final model. In the actual model used, the maintenance of aromatic rings containing (green spheres indicating 3× and 5×), and a hydrogen bond donor (blue sphere, 5×) and an acceptor (red sphere, 5×) indicated in GDP366 (Fig. 1b) were overweighted in relation to the other features. The 100 compounds with the best ranking according to the combined Tanimoto index (shape and color) were visually analyzed to identify the key features and overall superposition to the reference substance (exemplified for AD80 in Fig. 1b). AD80 is a multikinase inhibitor with high similarity to GDP366, and this indirectly corroborates the initial hypothesis made for GDP366 as a kinase inhibitor [22]. Ultimately, in addition to GDP366, three other chemical structures were identified: AD80 (CAS number 1384071-99-1), GSK2606414 (337531-89-1), and GW768505A (CAS number 501693-25-0) (Fig. 1b).

The IC50 for each compound was determined using cell viability assays in acute leukemia cellular models: Jurkat (T-ALL), Namalwa (B-ALL), NB4 (AML), and U937 (AML). Among the tested compounds, AD80 was more potent and effective (IC50 ranged 1.1 to 6.3 µM) compared to GW768505A (IC50 ranged from 16 to > 50 µM), and GSK2606414 (IC50 ranged from 36 to > 50 µM). The IC50 for the reference compound, GDP366, ranged from 5.1 to > 50 µM. Based on these findings, we further investigated AD80 and GDP366 (Fig. 1c).

We also found that GDP366 and AD80 reduced cell viability in a dose- and time-dependent manner in all the leukemia cells evaluated, with AD80 more potent and effective than GDP366 (all p < 0.05; Fig. 2a-b).

Fig. 2
figure 2

GDP366 and AD80 reduce cell viability and induce apoptosis in a dose-dependent manner in acute leukemia cellular models. Dose- and time-response cytotoxicity analyzed using a methylthiazoletetrazolium (MTT) assay for Jurkat, Namalwa, NB4, and U937 cells treated with increased concentrations of (a) GDP366 (0.0032, 0.016, 0.08, 0.4, 2, 10, and 50 µM) or (b) AD80 (0.0032, 0.016, 0.08, 0.4, 2, 10, and 50 µM) for 24, 48, and 72 h. Values are expressed as the percentage of viable cells for each condition relative to vehicle-treated cells. Results are shown as mean ± SD of at least four independent experiments. The p values and cell lines are indicated in the graphs; * p < 0.05, ** p < 0.01, *** p < 0.0001; ANOVA and Bonferroni post-test. Apoptosis detected by flow cytometry in Jurkat, Namalwa, NB4, and U937 cells treated with (c) GDP366 (2, 10, and 50 µM) or (d) AD80 (0.4, 2, and 10 µM) for 24 h using an annexin V/7AAD staining method. Representative dot plots are shown for each condition. The upper and lower right quadrants (Q2 plus Q3) cumulatively contain the apoptotic population (annexin V + cells). Bar graphs representing the mean ± SD of at least four independent experiments quantifying apoptotic cell death. The p values and cell lines are indicated in the graphs; * p < 0.05, ** p < 0.01, *** p < 0.0001; ANOVA and Bonferroni post-test

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GDP366 and AD80 induce apoptosis and reduce clonogenicity and cell cycle progression in acute leukemia cells

The cellular events involved in reduced cell viability triggered by GDP366 and AD80 were examined by investigating apoptosis, autonomous clonal growth, and cell cycle progression. GDP366 and AD80 caused time- and dose-dependent inductions of apoptosis (all p < 0.05, Fig. 2c-d) and reductions in autonomous clonal growth in Jurkat, Namalwa, NB4, and U937 cells (all p < 0.05, Fig. 3a-b), with Jurkat and NB4 cells more sensitive for both compounds. DNA content analysis corroborated the cytotoxic effects of GDP366 and AD80 at high concentrations, as observed by the accumulation of cells in the subG1 phase. Conversely, at lower concentrations of GDP366 (2 µM) and AD80 (0.4 µM), an increase was observed in cells in the G2/M phase, especially for the Jurkat, NB4, and U937 cells (Fig. 3c-f).

Fig. 3
figure 3

Autonomous clonal growth and cell cycle progression are inhibited by GDP366 and AD80 in acute leukemia cells. Colonies containing viable cells were detected by MTT assays after 10 days of culture of ALL or AML cells treated with (a) GDP366 (2, 10, and 50 µM) or (b) AD80 (0.4, 2, and 10 µM) for 10 days (long-term exposure). Colony images are shown for one experiment and the bar graphs show the mean ± SD of at least four independent experiments. * p < 0.05, ** p < 0.01, *** p < 0.0001; ANOVA and Bonferroni post-test. Cell cycle progression was determined in Jurkat, Namalwa, NB4, or U937 cells treated with (c-d) GDP366 (2, 10, and 50 µM) or (e-f) AD80 (0.4, 2, and 10 µM) for 24 h using a propidium iodide (PI) staining method. A representative histogram for each condition is shown. Bar graphs represent the mean ± SD of the percentage of cells in the subG1, G0/G1, S, and G2/M phases upon GDP366 or AD80 treatment of at least four independent experiments. The p values and cell lines are indicated in the graphs; * p < 0.05, ** p < 0.01, *** p < 0.001; ANOVA and Bonferroni post-test

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GDP366 and AD80 inhibit the PI3K/STMN1 axis and modulate genes involved in cytoskeleton regulation

A more detailed analysis of the molecular scenario involved in the phenotypes observed after treatment with GDP366 and AD80 was conducted using Jurkat and NB4 cells. Flow cytometry analysis revealed that GDP366 and AD80 treatment resulted in a dose-dependent reduction of expression of Ki-67, a proliferation marker (all p < 0.05, Fig. 4a-b). In Jurkat and NB4 cells, GDP366 induced STMN1 phosphorylation at serine 16 (an inhibitory site), survivin expression, PARP1 cleavage (an apoptosis marker), and γH2AX (a DNA damage marker), while reducing phosphorylation of S6 ribosomal protein (RP) (a PI3K/AKT/mTOR effector) at serine 235/236 (Fig. 4c). AD80 also induced PARP1 cleavage and γH2AX, and reduced S6RP phosphorylation at serine 235/236, but it downregulated STMN1 and survivin expression (Fig. 4d).

Fig. 4
figure 4

Molecular characterization identifies the PI3K/STMN1 axis as a potential target of GDP366 and AD80 in Jurkat and NB4 cells. Ki-67 mean fluorescence intensity (M.F.I.) was determined by flow cytometry after incubation ALL (Jurkat) or AML (NB4) cellular models treated with (a) GDP366 (Ø, 2, 10, and 50 µM) or (b) AD80 (Ø, 0.4, 2, and 10 µM) for 24 h; histogram traces are illustrated. The bar graphs represent the Ki-67 M.F.I normalized to the respective vehicle-treated cells, and results are shown as mean ± SD of at least four independent experiments. The p values and cell lines are indicated in the graphs; ** p < 0.01, *** p < 0.0001; ANOVA and Bonferroni post-test. Western blot analysis for p-Stathmin1S16, Stathmin 1, Survivin, p-S6RPS235/236, S6RP, PARP1, and p-histone H2A.XS139 (γ-H2A.X) in total cell extracts from Jurkat and NB4 cells treated with increased concentrations of (c) GDP366 (vehicle, 2 and 10 µM) or (d) AD80 (vehicle, 0.4 and 2 µM) for 24 h. The membranes were re-probed with the antibody for the detection of the respective total proteins or α-tubulin and developed using a SuperSignal™ West Dura Extended Duration Substrate system and a G:BOX Chemi XX6 gel doc system. (e) Gene expression heatmap from qPCR array analysis of Jurkat cells treated with vehicle, GDP366 (10 µM), or AD80 (2 µM). The mRNA levels were normalized to those of vehicle-treated Jurkat cells and calculated as fold changes in expression; genes demonstrating ≥ 1.5-fold in either direction compared to vehicle-treated cells upon the treatment with GDP366 or AD80 are included in the heatmap. Two independent experiments for each condition were used for the analysis; green indicates repressed mRNA levels and red elevated mRNA levels. (f) Network analysis for genes modulated by GDP366 or AD80 constructed using the GeneMANIA database (https://genemania.org/). The upregulated and downregulated genes in the PCR array are illustrated as strikethrough circles, and the interacting genes included by the software modeling are indicated by strikethrough strands. The main interactions between genes are indicated by colored lines and the five main cellular processes are described in the Figure

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A PCR array analysis was also conducted as an exploratory strategy for examining the potential effects of GDP366 and AD80 in cytoskeleton-related processes. A total of 18 of the 84 genes investigated were modulated after treatment, 5 were modulated by GPD366 (PHLDB2, CDC42EP2, FSCN2, ARHGAP6, and MAPT), 7 by AD80 (MAPK13, ARHGEF11, AURKB, PPP3CB, CDK5, CDC42EP3, and CTTN) and 6 by both drugs (AURKC, MAPRE2, BAIAP2, WASF1, CDC42BPA, and GSN) (Fig. 4e and Supplementary Table 1). Analysis using the GeneMANIA tool and the genes modulated in each treatment revealed involvement of the processes related to regulation and organization of the actin cytoskeleton for GDP366 and the processes related to the regulation of cytokinesis, organization of the actin cytoskeleton, and morphogenesis for AD80 (Fig. 4f).

Discussion

The potential pharmacological inhibitors for STMN1, an oncoprotein related to the malignant phenotype of ALL and AML [14], were investigated in the present study. Increased STMN1 expression has been confirmed in a large cohort of AML and ALL patients; however, the causes of this elevated expression in acute leukemia remains a topic of investigation. Previous studies have shown that STMN1 expression is a marker of proliferation in hematopoietic cells [7]. Two important leukemia-related oncogenes, PML-RARα and BCR-ABL1, were also able to induce STMN1 expression [10, 11]. Taken together, these findings suggest that the malignant transformation mediated by activation of signaling pathways that act under aberrant cell proliferation are capable of inducing STMN1 expression and, in turn, accelerating cell cycle progression and contributing to the leukemia phenotype.

Cheminformatics analysis was applied to identify compounds with structural similarity with GDP366, the only STMN1 inhibitor reported to date [15]. GDP366 has been described as capable of reducing STMN1 and BIRC5 mRNA levels, and both of these targets are of interest from point of view of antineoplastic therapy. In colon cancer models, GDP366 treatment induces mitotic catastrophe and senescence and reduces xenograft tumor growth [15]. In the present study, three compounds with structural similarities to GDP366 were selected for analysis in biological assays: AD80, GSK2606414, and GW768505A.

AD80 has been reported as a selective multikinase inhibitor for RET, RAF, SRC, and S6K [23]. In models of endocrine neoplasia, treatment with AD80 increases apoptosis [23]. In CCRF-CEM (T-ALL) and MOLT3 (AML) cells, both deficient for the PTEN protein and showing S6K1 hyperactivation, AD80 treatment reduces cell viability. AD80 treatment also reduces tumor formation and increases survival in animals with PTEN-/- lymphomas [24]. A recent report showed a decrease in G2/M-related genes following AD80 treatment of HUH7 cells [25]. These findings corroborate the antineoplastic effects observed in ALL and AML cellular models used in the present study.

GSK2606414 was first identified in a panel of 294 kinases as a potent inhibitor of KIT and PERK, and recent studies have demonstrated its ability to inhibit RIPK1, a kinase involved in TNF-α-mediated cell death [26]. KIT is a type III tyrosine kinase receptor expressed in several cell types, including hematopoietic progenitor cells, and is activated by stem cell factor (SCF) to trigger growth and development, cell survival, metabolism, and differentiation, by activating multiple signal transduction pathways [27]. PERK has been implicated in tumor development and resistance to chemotherapy, making it a target for anti-cancer treatments [26, 28]. Despite the relevant targets described for GSK2606414, its effects on cell viability in ALL and AML cellular models were less than desired for an antineoplastic drug. GW768505A has been reported as a potent inhibitor of the autophosphorylation of the TRKA, TRKB, and TRKC kinases in KM12 cells [29].

GDP366 and AD80 treatment decreased the levels of phosphorylated S6 ribosomal protein, an effector of the PI3K/AKT/mTOR signaling. A relationship between STMN1 expression and activation of this signaling cascade has been reported: PI3K inhibition by LY294002 or AKT inhibition by MK2206 reduces the expression of STMN1 [30, 31]. Similarly, the PI3K inhibitor, BKM120, induces STMN1 phosphorylation in ALL cellular models [32]. Our findings further reinforce the relationship between PI3K/AKT/mTOR and STMN1 in acute leukemia cells.

GDP366 and AD80 treatments appear to have different actions on STMN1: GDP366 increases the levels of STMN1 in its phosphorylated form (serine 16, an inhibitory site), while AD80 promotes a decrease in the expression of STMN1. From a functional point of view, both compounds would potentially inhibit the biological functions mediated by STMN1 [13].

An increase in the survivin expression, PARP1 cleavage, and γH2AX was observed after GDP366 treatment, whereas AD80 treatment decreased the expression of survivin and increased PARP1 cleavage and γH2AX expression. The survivin protein plays an important role in cell survival by inhibiting apoptosis, and its high expression has been associated with an aggressive cancer phenotype and poor clinical outcomes, while its expression is practically undetectable in normal tissues [33, 34]. In HCT116 cells, reduced survivin expression has been described following GDP366 treatment [15], but this response was not observed in the ALL and AML cell lines. This finding corroborates the lower potency and effectiveness of GDP366 in acute leukemia models.

The modulation of genes related to the regulation of the actin cytoskeleton, cytokinesis, and morphogenesis was highlighted after treatment with GDP366 and AD80 by PCR array and GeneMANIA analysis of processes related to the development and progression of cancer [35]. The CDK5 gene expression was reduced by AD80 treatment, whereas GSN gene expression was increased by both compounds in Jurkat cells. The CDK5 gene acts in the progression of the cell cycle and promotes proliferation [36], while high levels of the GSN gene have been associated with reduced proliferation, migration, and invasion in solid tumors [37, 38].

In summary, the therapeutic value of pharmacological inhibition of STMN1-mediated signaling has been investigated in AML and ALL cells. Our results indicated that GDP366 and AD80 reduce the activity of the PI3K/STMN1 axis. These compounds also show cytotoxic effects in acute leukemia cell models, with AD80 more potent and effective than GDP366 in the evaluated cellular and molecular events. Our findings further highlight STMN1-mediated signaling as a putative anticancer target for acute leukemia.