Background
The hematopoietic expressed homeobox gene (HHEX), also known as proline-rich homeodomain (PRH), is a transcription factor containing the DNA-binding domain termed the homeodomain. Similarly to the homeobox proteins, HHEX regulates cell development and differentiation, being required for the formation of the vertebrate body axis and the hematopoietic and vascular systems [
1]. HHEX
−/− mice display embryonic lethality due to impaired forebrain, liver, and thyroid development; these mice display also defective vasculogenesis and elevated VEGF-A levels [
2,
3].
HHEX is expressed in areas of the mammalian embryos that mainly contribute to hematopoietic and vascular development [
1]. In particular, HHEX expression is seen very early during embryonic development in the blood islands of the yolk sac [
4]. HHEX is highly expressed in stem cells and myeloid and lymphoid progenitors and its expression is maintained in adult hematopoietic tissues at the level of several blood cell lineages, including hematopoietic progenitors, lymphocytes, and myeloid lineages [
5,
6]. Importantly, HHEX expression was found to be downregulated during terminal differentiation of both B cells [
1] and myeloid cells [
7]. In fact, using Myb-Ets-transformed chicken blastoderm cells (MEPs), it was shown that HHEX RNA and protein levels are downregulated when MEPs differentiate along the myelomonocytic and erythrocytic lineages, while they are maintained when these cells differentiate toward the thrombocytic lineage [
7]. Furthermore, HHEX expression is downmodulated also in the T-cell lineage and this downregulation is physiologically critical since HHEX overexpression in these cells determines the development of T-cell leukemia in mice [
8]. Using various embryonic stem cell differentiation models, it was possible to show that HHEX is required for proliferation and differentiation of definitive HSCs [
9‐
11]. Particularly, Paz and coworkers have shown that HHEX
−/− embryonic stem cells when triggered to hematopoietic differentiation display the accumulation of early hematopoietic progenitors CD41
+c-kit
+ and a reduced capability to generate myeloid hematopoietic colonies, such as BFU-Mix, BFU-E, and CFU-GM [
11].
Few studies have explored the expression and a possible deregulation of HHEX in leukemic cells. HHEX was expressed in the large majority of leukemic cell lines and its expression is usually lost when these cell lines are induced to differentiate [
12]. In some rare AML patients, it was reported that a specific double translocation involving nucleoporin 98 was fused to the DNA-binding domain of the HHEX transcription factor [
13]. The mechanism resulting in leukemia in these patients is not known, but it was proposed that the fusion protein may compete with endogenous HHEX for HHEX targets and may derepress genes normally blocked by HHEX [
13].
Importantly, HHEX was shown to interact with the promyelocytic leukemia protein (PML) in various leukemic cell lines, including the promyelocytic cell line NB4 [
14]. Yeast two-hybrid experiments have shown that HHEX was capable of directly interacting with PML across its ring finger domain, which is required for the protein activity in the control of cell growth [
14]. Furthermore, HHEX was shown to be able to interact also with the PML-RARα oncoprotein that characterizes acute promyelocytic leukemias (APLs) [
14]. According to these observations, it was proposed that disruption not only of PML but also of HHEX functions by PML-RARα fusion protein may play a relevant role in the pathogenesis of APLs [
14]. In an attempt to define the mechanism through which PML-RARα blocks myeloid differentiation at the promyelocytic stage, Wang and coworkers have shown that PML-RARα targets promoter regions containing PU.1 consensus and RARE half sites in APL cells [
15]. Among the various gene promoters displaying these characteristics, there is also the HHEX promoter, seemingly repressed by PML/RARα binding [
15]. PML-RARα-mediated repression of PU.1-mediated transactivation was restored by the addition of all-trans retinoic acid (ATRA). The key functional role of PML-RARα-mediated repression of PU.1 expression and function was carefully confirmed by the same authors in other studies [
16,
17].
Given the key role of HHEX in the control of hematopoietic cell differentiation, the targeting of the HHEX gene by the fusion oncoprotein PML-RARα, and the capacity of the HHEX protein to interact with PML and PML-RARα, we sought to investigate the expression and the possible deregulation of HHEX in APLs. In the present study, we have explored the expression and the deregulation of HHEX in APL. Our results indicate that HHEX expression is clearly downmodulated in APLs, while VEGF-A expression is upregulated. The study of an APL cell line model with inducible PML-RARα expression supports the view that this fusion protein significantly downmodulates HHEX expression. The inhibitory effect exerted by PML-RARα on HHEX expression seems to be physio-pathologically relevant to mediate the inhibitory effect on cell differentiation and the pro-angiogenetic effect induced by this fusion protein.
Discussion
APL is characterized by the PML/RARα fusion gene underlying the
t(15;17) translocation and resulting in the formation of the PML-RARα oncoprotein. The molecular mechanism of leukemic transformation induced by PML-RARα is largely related to the capacity of this fusion oncoprotein to act as a potent transcriptional repressor of RARA and non-RARA target genes, thus interfering with gene expression programs required for hemopoietic progenitor self-renewal and myeloid cell differentiation [
19,
20].
APL blasts are blocked at the promyelocytic stage of myeloid cell differentiation and two unrelated agents, ATRA and arsenic trioxide, are able to induce their differentiation both in vitro and in vivo. ATRA induces granulocytic differentiation through a molecular mechanism mainly involving transcriptional reactivation of PML-RARα-silenced genes [
21], while ATO acts through induction of PML/RARα degradation, the clearance of PML-RARα-bound promoters being sufficient to induce APL differentiation [
22].
In line with these observations, in the present study, we provided evidence that PML-RARα inhibits HHEX expression in APL cells. This downmodulation of HHEX expression, compared to the normal physiological counterpart of APL, was constantly observed in all the 18 primary APLs investigated in the present study. Experiments carried out in U-937 cells expressing inducible PML-RARα strongly support a direct role of this fusion protein in inhibiting HHEX expression. The reduced HHEX expression seems to have two relevant functional consequences in APL cells. First, it seems to be responsible, at least in part, for the elevated expression of angiogenetic factors in APLs. This conclusion is supported by three lines of evidence: in fresh APL blasts, we observed a significant inverse correlation between HHEX and VEGF-A levels; HHEX silencing in PR9 cells mimics the stimulatory effect of PML-RARα on VEGF-A expression; HHEX overexpression in both NB4 and PR9 cells markedly inhibits PML-RARα-induced VEGF-A expression. Our observations on the inverse relationship between HHEX and VEGF-A levels in primary APL cells were corroborated through the analysis of gene expression data available for various AMLs on the TCGA platform. Interestingly, this analysis showed also that among the various AMLs, APLs display the highest VEGF-A expression and the lowest HHEX/VEGF-A ratio. These observations further support numerous other literature data showing a particularly elevated expression of angiogenetic growth factors in APLs [
23‐
25]. The key and a direct involvement of HHEX in transcriptional regulation of multiple genes encoding components of the VEGF signaling pathway, including VEGF, VEGFR-1, and VEGFR-2, was also shown in a chronic myeloid leukemia model [
26]. It is of interest to note that among the various AML subtypes, M5 AMLs are characterized by low HHEX levels and also by increased VEGF-A levels, although less elevated than in M3 AMLs. An inverse relationship between HHEX and VEGF-A was observed also in M5 AMLs. Interestingly, a previous study showed that M5 AMLs were characterized not only by decreased HHEX levels but also by a delocalization of HHEX protein that, at variance with other cell types, is largely confined to the cytoplasm [
27].
The stimulation of an angiogenetic response in APL cells induced by PML-RARα seems to be relevant for disease pathogenesis. This conclusion is supported by recent studies showing that angiogenesis inhibitors [
28] and HIF inhibitors [
29] impair leukemia progression and prolong mice survival in APL mouse models.
Second, the reduced HHEX expression induced by PML/RARα in APL cells seems to play a relevant role in mediating the inhibitory effect of this fusion protein on cell differentiation. This conclusion is supported by experiments carried out on NB4 and PR9 cells. As previously reported, induction of PML-RARα expression in PR9 cells resulted in a marked inhibition of their capacity to differentiate into monocytes in response to 1α25OH-VitD3 [
18]. Interestingly, the silencing in these cells of HHEX expression by siRNA resulted in an inhibitory effect on 1α25OH-VitD3-induced cell differentiation, highly comparable to that induced by PML-RARα. In contrast, HHEX overexpression in these cells resulted in a partial rescue of the inhibitory effect elicited by PML-RARα on monocytic differentiation.
It is of interest to note that recently Kramarzova and coworkers reported a large screening of homeobox cluster A and B expression at the level of various AML FAB subtypes and clearly showed that the lowest levels were observed in FAB M3 AMLs [
30]. Particularly, HOXA1, HOXA5, HOXA6, and HOXB6 expression was low in APLs [
30]. Combined to the findings of the present study, these observations strongly suggest a general repressive role of the PML/RARα fusion gene on HOX gene expression.
A low HHEX expression was recently reported also in AMLs characterized by trisomy 8, the most frequent chromosome numeric aberration observed in AMLs [
31]. The HHEX gene was hypermethylated in trisomy 8 AMLs, a finding in line with the low HHEX expression observed in these AMLs [
31]. In this study, 3 APLs were studied and all three displayed a hypermethylation of HHEX gene [
31].
The present study contributes in part to clarify the complex mechanisms through which PML-RARα exerts its oncogenic potential. In this context, recent studies highlight a relevant role of some long non-coding RNAs (lncRNAs) in the mediation of some key pathogenetic events induced by PML-RARα, particularly at the level of the control of cell proliferation [
32] and of cell differentiation [
33].
The identification and characterization of biomarkers is fundamental in oncology and represents an essential tool for drug development [
34]. At the moment, no data support a potential role of HHEX as a disease biomarker for APLs. However, future studies will address this important issue.
Methods
Human progenitor cell (HPC) purification
Cord blood (CB) was obtained after informed consent from healthy full-term placentas according to institutional guidelines. Human CD34+ cells were purified from CB by positive selection using the midi-MACS immunomagnetic separation system (Miltenyi Biotec, Bergisch Gladabach, Germany) according to the manufacturer’s instructions. The purity of CD34+ cells was assessed by flow cytometry using a monoclonal PE-conjugated anti-CD34 antibody and was routinely over 95 % (range comprised between 92–98 %). Purified human hematopoietic progenitor cells were grown in serum-free medium containing BSA (10 mg/ml), pure human transferrin (1 mg/ml), human low-density lipoproteins (40 μg/ml), insulin (10 μg/ml), sodium pyruvate (10–4 M), l-glutamine (2 × 10−3 M), rare inorganic elements (Sn, Ni, Va, Mo, and Mn) supplemented with iron sulfate (4 × 10−8 M), and nucleosides (10 μg/ml each). HPCs were induced into specific granulopoietic differentiation with IL-3 (1 unit/ml), granulocyte/monocyte CSF (0.1 ng/ml), and saturating amounts of G-CSF (500 units/ml). The differentiation stage of unilineage cultures was evaluated by MayGrunwald–Giemsa staining (Sigma-Aldrich, St. Louis, MO, USA) and cytologic analysis.
Primary leukemic cells
AML fresh leukemic blasts were isolated from diagnostic bone marrows obtained from 18 patients with newly diagnosed APL, using Ficoll–Hypaque density gradient. The rapid diagnosis of APL was based on the detection by immunofluorescence microscopy of a microspeckled nuclear pattern that is characteristic of PML protein delocalization from nuclear bodies in APL. The diagnosis was confirmed by cytogenetic evidence of the t(15;17)(q22;q21) and/or by reverse transcription polymerase chain reaction (RT-PCR) detection of the PML-RARα rearrangement on BM samples. According to the Declaration of Helsinki, informed consent was obtained from all patients and healthy donors and the study was approved by the Institution Review Board (IRB) of the University of Tor Vergata of Rome, Italy.
Cell line culture and differentiation
Fresh leukemic blasts were isolated from either bone marrow (BM) or peripheral blood (PB) by Ficoll–Hypaque density centrifugation obtained after informed consent from 18 APL patients classified as M3 by morphological criteria according to the French–American–British (FAB) classification. APL cells were grown in the above medium in the absence (control) or in the presence of ATRA (1 × 10−6 M; Sigma, St Louis, MO, USA).
The AML cell lines NB4, U937-MT, and U937-PR9 cells (described subclones of the U937 promonocytic cell line [
17]) were grown in RPMI 1640 medium supplemented with 10 % fetal calf serum in 5 % CO
2, 95 % humidified air at 37 °C. In some experiments, NB4 cells were treated for 3 days with 1 μM ATRA (Sigma). U937-PR9 and U937-MT cells were used for induction of PML-RARα expression or as control, respectively. Cells were cultured either with or without varying concentrations of ZnSO
4 (Sigma), ATRA (Sigma), or 50 ng/ml 1α250H-Vitamin D3 (Roche, Basel, Switzerland) for the indicated time points. After different days of in vitro incubation, cells were harvested and assayed for cell vitality (by the trypan blue exclusion test) and cell differentiation. Cell differentiation was assessed by cell morphology and changes in cell surface antigen expression.
Packaging cell line, 293FT, was maintained in DMEM (Life Technologies Corporation, Carlsbad, CA, USA) supplemented with 10 % (v/v) heat-inactivated FBS, 2 mM l-glutamine, 100 U/ml penicillin, and 100 ug/ml streptomycin (Invitrogen).
FACS analysis
Analysis of cell surface antigens was performed by flow cytometry using a FACScan Flow cytometer (Becton Dickinson, Bedford, MA, USA). The cells were stained with a PE-conjugated anti-mouse, CD11b, CD14, CD15, CD18, CD54, and CD64 antibody (Pharmingen, San Diego, CA, USA) as previously reported. Briefly, the cells were incubated for 30 min at 4 °C with an appropriate dilution of antibody; and after three washings in PBS, the cells were fixed in PBS formaldehyde (4 %) and analyzed by flow cytometry. The results were expressed in terms of the percentage of positive cells and of the mean fluorescence intensity (MFI).
Western blot analysis
To prepare total extracts, the cells were washed twice with cold phosphate-buffered saline and lysed on ice for 30 min with 1 % Nonidet P40 lysis buffer (20 mM Tris–HCl pH 8.0, 137 mM NaCl, 10 % glycerol, 2 mM EDTA) in the presence of 1 mM phenymethylsulfonyl fluoride, 1 mM dithiothreitol, 1 mM sodium orthovanadate, 2 μg/ml leupeptin, and 2 μg/ml aprotinin. Cell debris was removed by centrifugation at 10,000 rpm for 10 min at 4 °C, and protein concentration of supernatants was determined by the Bio-Rad protein assay (Richmond, CA, USA). Aliquots of cell extracts containing 30–50 μg of total protein were resolved by 7.5–10 % SDS-PAGE under reducing and denaturing conditions and transferred to nitrocellulose filter. The blots were blocked using 5 % non-fat dry milk in TBST (10 mM Tris–HCl pH 8.0, 150 mM NaCl, 0.1 % Tween 20) for 1 h at room temperature, followed by incubation with primary antibodies. After washing with TBST, the filters were incubated with the appropriate horseradish-peroxidase-conjugated secondary antibodies (Bio-Rad) for 1 h at room temperature. Immunoreactivity was revealed using an ECL detection kit (Pierce, USA). In Western blot experiments, the following antibodies were used: HHEX (Epitomics); VEGFR-2 and RARα (Santa Cruz Biotechnology); actin (Oncogene Research Products, Cambridge, MA),
Cell transfection and proliferation
Transient transfections of U937-PR9 cells with siRNA were carried out using Lipofectamine 2000 (Invitrogen, Carlsbad, CA, USA). Chemically synthesized siRNAs (Silencer Select Pre-designed and Validated siRNA) to HHEX, and scrambled siRNAs were purchased from Ambion and transfected at 10 nM final concentration. After 24 h, the cells were treated with no additives (control) or 50 ng/ml VitD3 or 1 μM Zn2+ or both drugs at the above doses. After 72 h, the expression of HHEX was assayed by Western blot. Cell growth was analyzed using Cell Titer-GloMax assay (Promega, Fitchburg, WI, USA).
Plasmid constructs and lentivirus infection
HHEX complementary DNA (cDNA) (NM_002729) was synthesized and cloned in Xhol-EcoRV restriction sites under CMV promoter into a variant third-generation lentiviral vector, pRRL-CMV-PGK-GFP-WPRE, called Tween [
35], to simultaneously transduce both the reporter GFP and the gene HHEX. Lentiviral particles were produced in 293FT packaging cell line and infection performed as previously described [
35]. After infection, transduced cells were selected with green fluorescent protein (GFP) fluorescence evaluated by FACS analysis.
RNA extraction and analysis
Total RNAs were extracted by the guanidinium isothiocyanate-CsCl method and reverse transcribed using random primers-RT kit (Invitrogen), according to the manufacturer’s procedure. The RT-PCR was normalized for S26 ribosomal protein. The sequences of the oligonucleotide primers used for RT-PCR were as follows: S26 5′-GCCTCCAAGATGACAAAG-3′ and 5′-CCAGAGAATAGCCTGTCT-3-; VEGFR-2 5-GTGACCAACATGGAGTCGTG-3′ and 5′-CCAGAGATTCCATGCCACTT-3′; HHEX 5′-TTCTCCAACGACCAGACCATCG-3′ and 5′-TTTTATCGCCCTCAATGTCCAC-3′; VEGF-A GGCTCTAGATCGGGCCTC and GGCTCTAGAGCGCAGAGT.
Samples were electrophoresed in 1.5 % agarose gel, transferred onto Hybond-N (Amersham Pharmacia Biotech, Uppsala, Sweden) filter and hybridized with an internal oligomer probe.
Quantitative real-time (qRT)-PCR analysis was performed by TaqMan technology, using the ABI PRISM 7700 DNA Sequence Detection System (Applied Biosystems, Foster City, CA, USA). Commercial ready-to-use primers/probe mixes were used (Assays on Demand Products, Applied Biosystems) for GAPDH, HHEX, VEGF-A and VEGFR-2, Tie 2, angiopoietin-1, and FGFR-1.
Chromatin immunoprecipitation (ChIP)
Chromatin immunoprecipitation (ChIP) assay was performed according to Upstate Biotechnology protocol (Lake Placid, NY, USA). Briefly, 5 × 106 cells were crosslinked in vivo, lysed, and immediately sonicated. Chromatin fragments were immunoprecipitated with 4 μg of anti-HHEX (Epitomics) or anti-RARα (SantaCruz) antibody; and after treatment with proteinase K for 2 h at 68 °C, the DNA was purified by phenol/chloroform extraction and amplified by Hot-start PCR (30 cycles of 94 °C for 1 min, 60 °C for 1 min, and 72 °C for 1 min) using the following primers:
VEGFR-2 (FWD) 5′CCTTCTTGGGGCTAGGCAGGTCACTTCA3′ (−671 to −644), and VEGFR-2 (REV), 5′GATCTCCAGCTCCCCAAGCCCATTTA3′ (−148 to −123).
VEGF (FWD) 5′AAAGACCCAACTCAAGTATCATCTSSAGT3′, and VEGF (REV) 5′CACTCACTGTGTGTGGCCTTAGGTTATTCAAC3′.
HHEX (FWD) 5′GGTTCAACAGGTTTGTGCAGT3′ and HHEX (REV) 5′CCGGCTATCAGAAGTCGAGTG3′.
EMSA
The double-stranded HHEX binding sites (400 ng) were labeled with [α32]P dATP using Klenow enzyme. The following sequences in the VEGFR-2 receptor promoter containing a HHEX consensus site (5-ATTA-3′) were used:
A1P1: 5′-GCCATATACATTCATTATATTTCAGCATTAAAATATTTC-3′ and 5′-TATATGTAAGTAATATAAAGTCGTAATTTTATAAAG-3′.
A2P2: 5′-TTCGGGGACCGGCAAGCGATTAAATCTTGGAGTTGCT-3′ and 5-AAGCCCCTGGCCGTTCGCTAATTTAGAACCTCAACGA-3′.
The binding-reaction mixture (20-μl final volume) contained labeled oligonucleotide probes (10,000 cpm) in binding buffer (75 mM KCI, 20 mM Tris–HCl (pH 7.5), 1 mM dithiothreitol) containing 5 μg of bovine serum albumin per milliliter, 14 % (vol/vol) glycerol, and 3 μg of poly(dI-dC). Total cell lysates (10 μg) were added, and the reaction mixture was incubated for 20 min at room temperature. Samples were electrophoresed in a 5 % poly-acrylamide gel in 0.5× Tris-borate-EDTA (TBE) buffer for 2 h at 200 V at 18 °C. The gels were then dried and subjected to autoradiography. Competition studies were performed by adding unlabeled double-stranded oligonucleotides at a 100-, 200-, and 300-fold molar excess over the labeled probe.
Analysis of TCGA data
Gene expression analysis data from AML samples were collected as previously described (The Cancer Genome Atlas Research Network 2008). Processed data sets were obtained directly from the public access data portal (
http://cancergenome.nih.gov/dataportal/data/about); 176 AML samples stratified by leukemia French American British morphology classification (FAB) and a total of 20,319 genes with expression values in the RPKM format were included. The expression levels of HHEX and VEGF-A of the 179 samples from 6 FAB categories were plotted. Data sets were cross-referenced using tumor-specific identification numbers.
Matrigel in vitro HUVEC tube formation assay
PR9 cells treated in different conditions, as reported in the corresponding section “
Results”, have been grown for 48 h, and the conditioned media were collected, centrifuged, transferred to fresh tubes, and stored at −20 °C. Growth factor-reduced Matrigel (100 μl), after being thawed on ice, was plated in a 96-well cell culture plate. The chamber was then incubated at 37 °C for 30 min to allow the Matrigel to polymerize. HUVEC maintained in complete EGM2 medium and starved for 4 h were trypsinized and seeded (2 × 10
4 cells/well) in each well with 100 μl of conditioned medium. EGM2 complete medium, supplemented with endothelial growth factors, was used as a positive control, while EM2 medium without growth factors was used as a negative control. The multiwell plates were incubated for 24 h and then tube formation was analyzed through inspection using and inverted microscope equipped with a digital camera.
Statistical analysis
Data were analyzed using parametric statistics with one-way analysis of variance (ANOVA). Post hoc tests included the Student’s t test and the Tukey multiple comparison tests as appropriate using Prism (GraphPad, San Diego, CA, USA). Data are presented as mean value ± SEM from three independent experiments. Significance was set at p < 0.05.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
ES carried out the molecular studies, contributed to the design of experiments, and helped to draft the manuscript. EP performed at the experiments involving hemopoietic progenitor cell purification. UT conceived the study and was involved in the design and coordination of the study and drafted the manuscript. LP performed all the flow cytometry experiments. FLC supervised the analysis of all data derived from primary leukemic samples. Finally, AP, EC, and RI helped to perform cellular and molecular studies. All authors read and approved the final manuscript.