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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Cancer Res. Author manuscript; available in PMC 2017 December 15.
Published in final edited form as:
PMCID: PMC5161538
NIHMSID: NIHMS811725

FOXD1-ALDH1A3 signaling is a determinant for the self-renewal and tumorigenicity of mesenchymal glioma stem cells

Abstract

Glioma stem-like cells (GSC) with tumor initiating activity orchestrate the cellular hierarchy in glioblastoma (GBM) and engender therapeutic resistance. Recent work has divided GSC into two subtypes with a mesenchymal (MES) GSC population as the more malignant subtype. In this study, we identify the FOXD1-ALDH1A3 signaling axis as a determinant of the MES GSC phenotype. The transcription factor FOXD1 is expressed predominantly in patient-derived cultures enriched with MES, but not with the proneural (PN) GSC subtype. shRNA-mediated attenuation of FOXD1 in MES GSC ablates their clonogenicity in vitro and in vivo. Mechanistically, FOXD1 regulates the transcriptional activity of ALDH1A3, an established functional marker for MES GSC. Indeed, the functional roles of FOXD1 and ALDH1A3 are likely evolutionally conserved, insofar as RNAi-mediated attenuation of their orthologous genes in Drosophila blocks formation of brain tumors engineered in that species. In clinical specimens of high-grade glioma, the levels of expression of both FOXD1 and ALDH1A3 are inversely correlated with patient prognosis. Lastly, a novel small molecule inhibitor of ALDH we developed, termed GA11, displays potent in vivo efficacy when administered systemically in a murine GSC-derived xenograft model of GBM. Collectively, our findings define a FOXD1-ALDH1A3 pathway in controlling the clonogenic and tumorigenic potential of MES GSC in GBM tumors.

Keywords: glioblastoma, cancer stem cell, stem cell marker, brain cancer, cancer initiating cell, forkhead transcription factors

Introduction

Glioblastoma (GBM) is the most common and fatal primary brain tumor in adults. GBM tumors are resistant to conventional radiation therapy and chemotherapies, making the current available treatments ineffective (1). Intra-tumoral cellular heterogeneity in GBM contributes to tumor aggressiveness and therapy resistance, with glioma stem-like cells (GSCs) at the apex of the hierarchy (1,2). GSCs are poorly differentiated tumor cells with stem cell properties including self-renewal, and are responsible for tumor initiation (35). Recently, we and others established patient-derived GSC lines from GBM patients, which contain two distinct and mutually-exclusive GSC subtypes termed Proneural (PN) and Mesenchymal (MES) (4,6). MES GSCs, the more aggressive and radio-resistant subtype, express higher levels of ALDH1A3 than PN GSCs in vitro (6), suggesting that ALDH1A3 is a potential marker of MES GSCs. Therefore, understanding the regulatory pathway(s) controlling ALDH1A3 expression in this cell type would be expected to identify new and relevant therapeutic targets.

The Forkhead family of transcription factors (TFs) regulate a wide variety of cellular functions during development and many are implicated in cancers (7,8). As a member of this family, FOXD1 is preferentially expressed in human embryonic tissues, including kidney and testis, but not in adult tissues (9). Moreover, FOXD1 regulates organogenesis (1012), especially the commitment to a mesenchymal lineage during organogenesis (1315). FOXD1’s regulatory role in stemness is further demonstrated by facilitating the reprogramming of mouse embryonic fibroblasts into induced pluripotent stem cells (16). Recent studies have suggested parallels between reprogramming and tumorigenesis, and highlighted the shared transcription factors involved. Similarly, the deregulation of FOXD1 is implicated in the tumorigenesis of cancers of prostate, breast, and clear cell sarcoma of the kidney (1719). Nonetheless, the physiological roles of FOXD1 in brain cancers and GSCs remain unknown.

In this study, we demonstrate that FOXD1 is critical for the maintenance of MES GSCs, and thus, the tumorigenicity of this subtype of GBM tumors. By exploring our genome-wide expression profiling data, we identified FOXD1 as the most up-regulated Forkhead TF in the patient-derived MES GSC enriched cultures. We found that FOXD1 promotes clonogenicity and tumorigenicity of MES GSCs both in vitro and in vivo by the direct transcriptional regulation of the key molecule ALDH1A3. Subsequently, we verified that the tumorigenicity of FOXD1 is mediated by ALDH1A3. We also proved that FOXD1 and ALDH1A3 are prognostic factors in glioma clinical samples. Finally, we developed novel anti-ALDH small molecule inhibitors and demonstrated effectiveness in vitro and in vivo.

Materials and Methods

Ethics

This study was performed under the supervision of the respective Institutional Animal Care and Use Committees, and the Human Subjects Research protocols were approved by the Institutional Review Board at the University of Alabama at Birmingham (UAB), MD Anderson Cancer Center (MDA) and/or Ohio State University (OSU) as described previously (6,20).

GSC cultures

The glioma (neuro)spheres used in this study were generated at OSU and MDA. Neurosphere cultures from clinical samples were established and characterized as described previously (Supplementary Table S1) (6,20). The detailed source, year of receipt, and culture methods are described in Supplementary information. The unique identities of each glioma neurosphere line were confirmed by short tandem repeat (STR) analysis as described in Supplementary Table S2 (20).

Mouse intracranial xenograft tumor models

The GSC suspension (1×104 cells for MES83 and 2.5×105 cells for MES267) was injected into the brains of nude mice (6-week-old) as previously described (20,21). When neurological symptoms observed, mice were sacrificed and mouse brains were collected for analysis.

Lentivirus transduction

Two independent lentiviral shRNA constructs for knocking down FOXD1 were purchased from Sigma (TRCN0000013970 and TRCN0000230322). For overexpression, cDNAs of FOXD1 (RC220504, Origene), and FOXG1 (RC207964, Origene) were subcloned into a lentiviral vector (Origene, PS100064) according to the manufacturer’s protocol. Lentiviruses were packaged in 293FT cells (from Invitrogen at 2013). The lentivirus transduction was performed as previously described (20).

Neurosphere formation assay

MES83 and MES28 GSCs infected with lentivirus were seeded into 96 well plates at 1, 10, 20, 30, 40, and 50 cells per well. After 7 days for MES83 GSCs and 10 days for MES28 GSCs, the numbers of spheres with diameters greater than 60μm were counted. Data were analyzed as described previously (http://bioinf.wehi.edu.au/software/elda/) (22).

Immunohistochemistry scoring

German immunohistochemical score (GIS) was used to evaluate the expression of FOXD1 and ALDH1A3 (23,24). The detailed procedure is provided in the supplementary information.

Western blot

The detailed procedure is provided in the supplementary information. Original film scans of western blots were shown in Supplementary Fig. S1.

GEO accession numbers

The accession numbers of GEO datasets used in this study are GSE67089, GSE4290, GSE4536, and GSE2223.

Drosophila stock

The following transgenic Drosophila flies were used: repoGAL4 UAS-GFP/TM3, Sb, UAS-ptenRNAi, UAS-RasV12/Cyo, UAS-aldhRNAi/Cyo, ptcGAL4 UAS-GFP/Cyo, UAS-fd59ARNAi, and UAS-fd59A. The Drosophilas for all experiments were incubated at 25°C.

Immunofluorescence, imaging, and quantification in Drosophila samples

Immunofluorescence was performed as described (25) and the images were captured using the Olympus Fluoview 1000 confocal microscope.

Statistical analysis

Data are presented as mean ± SD. The number of replicates for each experiment is stated in the figure legend. Statistical differences between and among groups were determined by two tailed t-test and one-way analysis of variance (ANOVA) followed by Dunnett’s post-test, respectively. The statistical significance of Kaplan–Meier survival plot was determined by log-rank analysis. Statistical analysis was performed by Microsoft Excel 2013 and Graphpad Prism 6.0, unless mentioned otherwise in the figure legend. P<0.05 was considered as statistically significant.

Additional details about the materials and methods are available in the supplementary information.

Results

FOXD1 and FOXG1 exhibit inverse expression pattern in GSCs

To identify the TFs critical for the GSC phenotype, we explored our dataset (GSE67089 (6)) with 30 patient-derived glioma sphere cultures and 3 human fetal brain-derived sphere cultures (normal spheres) for the expression levels of all TFs (1,988 genes (26)). Twenty eight TFs were found up-regulated in MES glioma spheres by more than 10 fold compared to normal neural progenitor cells. Ten TFs were at higher levels in MES glioma spheres by more than 11 fold than the levels of these TFs in PN glioma spheres. Among them, seven TFs were overlapped, including FOSL1, FOXD1, PLAGL1, STAT6, ARNTL2, BNC1, and HTAIP2 (Fig. 1A).

Figure 1
FOXD1 is a key MES GSC transcription factor

Forkhead TFs are involved in deciding cell fates during development and tumorigenesis (7,16). Our previous study demonstrated the requirement of one Forkhead TF, FOXM1, for the survival and proliferation of oncogenic but not normal neural stem cells (2729). Therefore, we focused on the Forkhead TFs. FOXD1 was the most up-regulated MES-associated gene in the Forkhead family, whereas FOXG1 was the most up-regulated gene in PN glioma spheres (Fig. 1B and Supplementary Fig. S2A, B). This was validated by RT-qPCR (Fig. 1C and D) and western blot (Fig. 1E) with six patient-derived GSC-enriched cultures. In western blot assays, ALDH1A3, CD44, and AXL were included as MES markers while OLIG2 as a PN marker. Similarly, immunocytochemistry exhibited that glioma sphere line MES83 has a higher expression of nuclear FOXD1 than PN84 and PN157 (Fig. 1F). We then investigated whether the stem cell fraction in MES and PN glioma spheres express FOXD1 and FOXG1, respectively. Our previous studies identified CD133 as a PN GSC marker while ALDH1A3 as MES GSC marker (4,6,27). As expected, FOXD1 was almost exclusively expressed in the ALDHhigh subfraction of MES83 cells, but not in ALDHlow subpopulations (Fig. 1G and Supplementary Fig. S2C). In contrast, the CD133high subpopulation had significantly higher expression of FOXG1 than CD133low subpopulations in the PN157 glioma spheres (Fig. 1H and Supplementary Fig. S2D). To extend this observation, we performed bioinformatics analysis using another clinical data (IVY GBM Atlas Project database) (http://glioblastoma.alleninstitute.org/). GBM is known to display intra-tumoral cellular heterogeneity. We investigated RNAseq data that was collected from different regions of GBM tumor tissues that exhibit distinct cellular subtypes including PN and MES tumor cells. This analysis showed an inverse pattern of the expression levels of FOXG1 and FOXD1 in different regions (Supplementary Fig. S2E and F). In contrast, there were no significant differences in either FOXD1 or FOXG1 among the GBM subtypes in the TCGA dataset (Supplementary Fig. S2G). The TCGA dataset measures the average expression levels of genes. This maybe the possible reason why the expression level of either FOXG1 or FOXD1 failed to show a significant difference among the four subtypes of GBM. Collectively, these data suggest that FOXD1 and FOXG1 exhibit inverse expression profiles in vitro: FOXD1 is elevated in MES, while FOXG1 is elevated in PN GSCs.

FOXD1 expression is associated with poorer prognosis in glioma patients

We then examined whether FOXD1 expression is associated with the histopathological grade of glioma and patient prognosis. With immunohistochemistry, 44 glioma tumor tissues with varying grades and nine adjacent normal brain tissues were analyzed. Intense immunostaining, indicating FOXD1 expression, was observed in the nuclei of high-grade glioma tissues (Grade II: 2 of 4; Grade III: 12 of 16; Grade IV: 10 of 24) but rarely in normal brain tissues (1 of 9) (Fig. 2A and B). The survival periods of patients with high levels of FOXD1 were significantly shorter than those with FOXD1 intermediate expression (Fig. 2C, P=0.0039). Inversely, patients with low FOXD1 expression exhibited better prognosis than those with high expression (Fig. 2C, P=0.0324). Consistently, GBM-derived spheres expressed higher FOXD1 than normal brain-derived spheres (GSE4536, Fig. 2D (30)). Similar results were obtained from two other datasets (GSE2223, Fig. 2E (31); and GSE4290, Fig. 2F (32)), and from the Rembrandt database (Fig. 2G). The elevated expression of FOXD1 was also associated with poorer survival in Rembrandt database (Fig. 2H). Altogether, these data suggest that the expression of FOXD1 is elevated in high-grade glioma and is a clinically-relevant target in GBM.

Figure 2
FOXD1 expression is clinically relevant in high-grade gliomas

FOXD1 is required for the clonogenicity of MES GBM in vitro and in vivo

To investigate the physiological role of FOXD1 in MES GSCs, we knocked down the expression of FOXD1 using two different lentiviral shRNA vectors in MES glioma sphere lines (MES83 and MES28) (4,6); a non-targeting shRNA (shNT) was used as a negative control. Western blotting showed that both shRNA clones were capable of knocking down FOXD1, with clone #1 having higher efficiency (Fig. 3A). As a result, MES glioma spheres with FOXD1 knocked-down (KD) grew significantly slower (33) than control cells (Fig. 3B). Moreover, the sphere forming capacity of these cells was dramatically decreased (Fig. 3C and D). Of note, the extent to which cell growth and clonogenicity were inhibited was more prominent with shFOXD1#1 than shFOXD1#2, which was consistent with their effects on FOXD1 KD. Our previous study demonstrated that MES, not PN GSCs depended more on glycolysis (6). As expected, lactic acid levels as a measure of glycolysis were largely reduced in MES83 cells transduced with shFOXD1#1, indicating that FOXD1 may be involved in the regulation of glycolysis of MES GSCs (Supplementary Fig. S3). To determine whether FOXD1 is essential for tumorigenicity in MES glioma spheres in vivo, we used orthotopic xenografts into mouse brains. Injection of MES83 spheres into the striatum of immunocompromised mouse brains resulted in lethal tumors within 30 days (6,34). Although the cells transduced with the lentiviral shNT construct did not show altered tumorigenesis or subsequent mouse survival, xenografting of the FOXD1 silenced MES glioma spheres diminished the proportion of tumor formation (3 of 5 mice) and prolonged survival periods (Fig. 3E). Histologically, the tumors in the control mice were highly vascularized GBM-like brain tumors with central necrosis, and developed by day 18 after transplantation (Fig. 3F and G).

Figure 3
FOXD1 regulates MES GSC growth both in vitro and in vivo

FOXD1 regulates ALDH1A3 transcription in MES glioma spheres

Because FOXD1 is almost exclusively expressed in ALDHhigh MES GBM cells, not in ALDHlow cells (Fig 1G), we examined whether FOXD1 transcriptionally regulates the ALDH1A3 gene, thereby orchestrating the stem cell properties in MES GSCs. Indeed, FOXD1 silencing in the MES83 glioma spheres significantly decreased the ALDH1A3 mRNA and protein levels (Fig. 4A and B). Since there is a common Forkhead TF binding motif in the promoter region of the ALDH1A3 gene (Supplementary Fig. S4A), we hypothesized that FOXD1 activated ALDH1A3 transcription. Thus, we performed a luciferase assay to measure ALDH1A3 promoter activity upon exogenous expression of FOXD1 in 293T cells and MES83 spheres (Fig. 4C and D). As expected, ectopically expressed FOXD1 increased ALDH1A3 reporter activity. Since FOXD1 and FOXG1 exhibit mutually-exclusive expression patterns (Fig. 1B–D), we assumed that FOXG1 may counteract FOXD1 activity in MES GSCs. Intriguingly, co-expression of FOXG1 and FOXD1 in MES83 spheres counter-acted ALDH1A3 reporter activities driven by FOXD1 alone (Fig. 4C and D). Further, the transcriptional regulation of ALDH1A3 by FOXD1 was confirmed by chromatin immunoprecipitation PCR, which indicated that FOXD1 directly binds to the promoter region of the ALDH1A3 gene in MES28 glioma spheres (Fig. 4E). In addition, the overexpression of FOXD1 in MES83 increased the expression of ALDH1A3 (Fig. 4F). To investigate the physiological role of the FOXD1-ALDH1A3 axis in MES GSCs, we evaluated whether ALDH1A3 overexpression rescues the phenotypes of MES glioma spheres induced by FOXD1 KD. The reduced in vitro cell growth and neurosphere formation by FOXD1 KD were partially, yet not completely, restored by the overexpression of ALDH1A3, but not by the control vector (Fig. 4G, H and Supplementary Fig. S4B). On the other hand, FOXD1 overexpression alone did not induce any noticeable changes in the PN spheres, at least in the expressions of representative markers (AXL, CD44, and Olig2) (Supplementary Fig. S4C). We also analyzed dataset (GSE67089) in our previous publication (6) for the expression levels of FOXD1 with all the members of ALDH family. We consistently found that FOXD1 is co-expressed with ALDH1A3, but not other ALDH members, in MES but not in PN subtype spheres (Supplementary Fig. S4D). Collectively, these data suggest that FOXD1 is required, but not sufficient, for the establishment of the MES phenotype in GSCs.

Figure 4
ALDH1A3 is a functional MES GSC marker and is transcriptionally regulated by FOXD1

FOXD1 and ALDH1A3 are evolutionarily conserved genes that contribute to glial neoplasms

Previously, we reported that ALDHhigh MES glioma sphere cells have higher clonogenic potential in vitro than ALDHlow cells in MES GBM (6). To compare the in vivo tumorigenic abilities of ALDHhigh and ALDHlow cells in MES glioma spheres, we injected ALDHhigh or ALDHlow MES83 cells into mouse brains at two different cell numbers (100 and 1,000 cells). When 100 ALDHlow or ALDHhigh cells were injected into mouse brains, only one of six mice injected with ALDHlow cells died by 24th day after transplantation. In contrast, four of six mice in the ALDHhigh group died due to tumor burden. Similar results were observed when 1,000 tumor cells were injected (Fig. 5A), indicating that the tumor initiating cells reside in the minor subset of MES GBM cells with elevated ALDH1 activity (approximately 7% of the MES83 cells).

Figure 5
RNA interference-mediated silencing of FD59A and ALDH attenuates growth of Drosophila glial neoplasia

Next, we examined the functional role of FOXD1 and ALDH1A3 in Drosophila. In flies, the orthologs of FOXD1and ALDH1A3 are fd59A (35) and DmALDH (36), respectively. fd59A is highly expressed in the Drosophila larval (embryonic) Central Nervous System (CNS) (35). We used recently established Drosophila glioma model involving co-activation of oncogenic Ras and PI3K pathways by the GAL4 UAS system (repoGAL4 UASPTENRNAi UASRasV12), which causes the formation of invasive glial neoplasms that mimic human gliomas (37). In the larval glial neoplasms derived from the repoGAL4 UASPTENRNAi; UASRasV12 larvae, Fd59A and ALDH levels were substantially upregulated compared to the normal CNS (repoGAL4 UASGFP) (Fig. 5B and C). To investigate if the formation of the glial neoplasms in Drosophila requires ALDH, we transduced UASALDHRNAi into Drosophila glioma tumors. Elimination of ALDH resulted in reduction in glioma growth by 1.7-fold (Fig. 5D and E, P=0.008). This anti-tumor effect of ALDH silencing was also observed in the mosaic models of Drosophila eye cancer (Supplementary Fig. S5). Next, we investigated the effects of downregulation of Fd59A in these cancers. As shown in Fig. 5D and E, downregulation of Fd59A resulted in a reduction in glioma growth (P=0.007) with a concomitant downregulation of ALDH levels (Fig. 5C). Taken together, these data suggest that FOXD1 and ALDH1A3 are evolutionarily conserved genes that contribute to CNS tumorigenesis (Fig. 5F).

ALDH1A3 expression indicates poor prognosis of post-surgical glioma patients

To interrogate whether ALDH1A3 is a clinically relevant biomarker, we analyzed 40 high-grade glioma patient samples (Fig. 6A). The log rank test showed that the glioma patients with higher ALDH1A3 protein expression has a significantly shorter post-surgical survival periods compared to either the intermediate or low ALDH1A3 protein expression groups (Fig. 6B, P=0.0285, ALDH1A3high vs. ALDH1A3intermediate; and P=0.0016, ALDH1A3high vs. ALDH1A3low). Additionally, the data from Rembrandt database demonstrates a similar pattern (Supplementary Fig. S6). These data indicate the clinical significance of ALDH1A3.

Figure 6
The novel ALDH small molecule inhibitor GA11 attenuates MES GSC growth both in vitro and in vivo

The novel ALDH1 inhibitor GA11 has anti-GBM effects in vitro and in vivo

Based on the inhibitory effects of FOXD1 silencing on the MES GSC-derived mouse brain tumors and of ALDH1A3 silencing on the Drosophila brain cancer model, we sought to design novel, clinically efficacious small molecule inhibitors selectively targeting the ALDH1 activity for GBM therapies. The natural product daidzin and its congeners have been reported to inhibit ALDH1 (38). To design clinically applicable analogs with better pharmacokinetic properties (39), we identified the imidazo [1,2-a] pyrimidine heterocyclic core as the essential scaffold. We added two planar, aromatic and lipophilic areas to this scaffold in positions two and six of the nucleus, thereby generating the novel small molecules GA11 and GA23 (Fig. 6C and Supplementary Fig. S7A). The structural assessment confirmed that the replacement of glucopyranose portion of daidzin with an aromatic ring reduces both hydrogen bond forming potential and rotatable bonds, resulting in smaller molecules with a lower polar surface area and limited flexibility. This modification retained the core structure of daidzin essential for inhibiting ALDH1 activity, while establishing a drug-like profile, i.e. a more favorable prediction of the blood-brain barrier (BBB) penetration. Indeed, an in silico evaluation confirmed that the physical-chemical properties of these novel compounds were fully consistent with those required for BBB penetration (Supplementary Table S3) (40). In addition, the molecular weights of GA11 and GA23 (close to 310, the mean value of marketed CNS drugs) are significantly lower than that of daidzin. Moreover, the H-bonding potential of these novel compounds is commensurate to that of successful CNS drug candidates. Indeed, the sum of heteroatoms capable of hydrogen bonding, like nitrogen (N) and oxygen (O), is less than five, conferring on the molecules a high probability of entering the CNS. Additionally, the rotatable bond count is less than eight, and the limited flexibility of the molecules should promote passive permeation through BBB. Lowering both molecular weight and H-bonding potential have a significant effect on the polar surface area (PSA) of the compounds, which is a key factor in determining the extent of BBB penetration. Unlike daidzin, the novel compounds have PSA values lower than the stringent cut-off 60–70 Å, which characterizes commercial CNS drugs (40).

To validate the inhibitory effects, we first determined the enzymatic activity of yeast ALDH incubated with GA11 and GA23 in an in vitro enzymatic assay (30% of the ALDH sequences in yeast and eukaryotes are conserved (41)). As expected, both GA11 and GA23 inhibited the ALDH enzymatic activity with IC50 values in the micro molar range (Fig. 6D). GA11 was more potent. Further, both GA11 and GA23 inhibited human ALDH1 demonstrated by a marked reduction of ALDHhigh cellular populations in MES83 glioma spheres treated with GA11 and GA23 (Fig. 6E). As a result, both GA11 and GA23 inhibited the growth of MES glioma spheres. In contrast, PN glioma spheres were relatively more resistant to these two compounds (Fig. 6F and Supplementary Fig. S7B–D). More importantly, systemic treatment of MES83 and MES267-based mouse brain tumors with GA11 significantly attenuated tumor growth, thereby extending the survival of tumor bearing mice compared to the vehicle treated counterparts (Fig. 6G, H and Supplementary Fig. S7E).

Discussion

GBMs display intra-tumoral cellular heterogeneity; within the same tumor different cell populations respond differently to therapies (3,42,43). GSCs are at the apex of the cellular hierarchy (3,42). Thus, understanding how GSCs are maintained may improve the efficacies of current therapeutic strategies. Recent evidence suggests that GSCs are subclassified into two subtypes with MES GSCs being more therapy-resistant (4,6). Therefore, identification of MES GSC regulatory molecules has the potential to lead to novel and efficacious GBM therapeutics. Our study (Fig. 5A) is the first demonstrating that ALDH1high GBM cells are more tumorigenic in vivo than the ALDH1low counterparts, indicating that ALDH1 is a specific marker for MES GSCs. This is further supported by analyses of clinical glioma samples (Fig. 6B). To develop chemotherapeutics targeting MES GSC, we synthesized a novel class of imidazo [1,2-a] pyridine derivatives called GA11 and GA23 as ALDH1 inhibitors. These compounds were designed based on the conserved structural traits of the known natural occurring inhibitors including daidzin. In addition to fulfilling pharmacophore needs, these novel compounds possess a good hydrophilic-lipophilic balance, which allows for better penetration of the BBB. In principle, both these features make GA11, in particular, an attractive drug candidate to target GSCs in GBM tumors. This structural prediction was validated in a cell-free kinase assay, which proved the ability of GA11 to inhibit the target enzyme ALDH in yeast (Fig. 6D) and further, an in vitro ALDEFLUOR assay showed the substantial decline of human ALDH1 activity in GA11-treated MES glioma spheres (Fig. 6E). Consistently, GA11 inhibited in vitro glioma sphere proliferation and in vivo xenograft growth in mouse brains (Fig 6F and G, H, respectively). Further pre-clinical evaluation of GA11 and its analogs is currently underway for clinical development of anti-ALDH1 therapeutics for GBM.

Another novel set of findings in this study is that ALDH1A3 is transcriptionally regulated by FOXD1 (Fig. 4E). The ALDH1A3 signaling in cancers is likely evolutionally conserved based on the fact that ALDH1A3 silencing in vivo displayed substantial suppression of the genetically-engineered Drosophila brain cancers (Fig. 5D, E). Interestingly, this FOXD1-mediated ALDH1A3 transcriptional activation was counter-acted by another Forkhead family member, FOXG1 in the MES GSC-enriched cultures (Fig. 4C, D). This data is supported by the presence of a Forkhead TF consensus sequence in the ALDH1A3 promoter region. These data suggest that MES GSCs hijack the molecular mechanism for normal development (e.g. mouse retina) to promote their tumor growth (44). It is not clear, however, whether these two Forkhead TFs compete for the DNA binding in the ALDH1A3 promoter or if FOXG1 indirectly influences FOXD1 signaling in MES GSCs. Additionally, the restoration of ALDH1A3 by exogenous expression did not fully rescue the defects in MES glioma sphere growth caused by FOXD1 silencing (Fig. 4G), suggesting that additional undetermined oncogenic mechanisms are likely involved in MES GBM. Further studies are needed to fully clarify the mechanisms by which FOXD1 maintains the MES GSCs phenotype and thus, GBM tumorigenesis and therapy-resistance.

In conclusion, this study describes the upregulation of ALDH1A3 and FOXD1 in clinical glioma samples and establishes their functional roles in the maintenance of MES GSCs, and therefore MES GBM tumorigenesis. Furthermore, this study provided the first evidence supporting the fact that elevated FOXD1 expression is a negative prognostic factor in glioma, and establishes a role for FOXD1 directly regulating ALDH1A3 transcription in MES GSCs. Taken together, the FOXD1-ALDH1A3 axis is critical for tumor initiation in MES GSCs, therefore providing possible new molecular targets for the treatment of GBM and other ALDH1-activated cancers.

Supplementary Material

1

2

Acknowledgments

Financial support

This study is supported by P01CA163205, R01NS083767, R01NS087913, and R01CA183991 (I. Nakano). P. Cheng was supported by The First Hospital of China Medical University. I. Waghmare was supported in part by the Graduate Program at the University of Dayton. M. Kango-Singh was supported in part by Start-up funds from the University of Dayton.

We thank all the members in the Nakano laboratory for constructive discussion for this study. We also thank Amber K. O’Connor for English language editing for this study.

Footnotes

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed by the authors.

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