Expression of HES and HEY genes in infantile hemangiomas
© Adepoju et al; licensee BioMed Central Ltd. 2011
Received: 14 June 2011
Accepted: 11 August 2011
Published: 11 August 2011
Infantile hemangiomas (IHs) are the most common benign tumor of infancy, yet their pathogenesis is poorly understood. IHs are believed to originate from a progenitor cell, the hemangioma stem cell (HemSC). Recent studies by our group showed that NOTCH proteins and NOTCH ligands are expressed in hemangiomas, indicating Notch signaling may be active in IHs. We sought to investigate downstream activation of Notch signaling in hemangioma cells by evaluating the expression of the basic HLH family proteins, HES/HEY, in IHs.
Materials and Methods
HemSCs and hemangioma endothelial cells (HemECs) are isolated from freshly resected hemangioma specimens. Quantitative RT-PCR was performed to probe for relative gene transcript levels (normalized to beta-actin). Immunofluorescence was performed to evaluate protein expression. Co-localization studies were performed with CD31 (endothelial cells) and NOTCH3 (peri-vascular, non-endothelial cells). HemSCs were treated with the gamma secretase inhibitor (GSI) Compound E, and gene transcript levels were quantified with real-time PCR.
HEY1, HEYL, and HES1 are highly expressed in HemSCs, while HEY2 is highly expressed in HemECs. Protein expression evaluation by immunofluorescence confirms that HEY2 is expressed by HemECs (CD31+ cells), while HEY1, HEYL, and HES1 are more widely expressed and mostly expressed by perivascular cells of hemangiomas. Inhibition of Notch signaling by addition of GSI resulted in down-regulation of HES/HEY genes.
HES/HEY genes are expressed in IHs in cell type specific patterns; HEY2 is expressed in HemECs and HEY1, HEYL, HES1 are expressed in HemSCs. This pattern suggests that HEY/HES genes act downstream of Notch receptors that function in distinct cell types of IHs. HES/HEY gene transcripts are decreased with the addition of a gamma-secretase inhibitor, Compound E, demonstrating that Notch signaling is active in infantile hemangioma cells.
Infantile hemangiomas (IHs) are the most common benign tumors of infancy. Despite their prevalence, the pathogenesis of IHs is not well understood. IHs are characterized by three phases: proliferating, involuting and involuted phases. These are defined by a period of rapid proliferation of blood vessels in the first year of life, followed by gradual regression of the vascular component with replacement by fibro-fatty tissue.[1, 2]
The Notch family of proteins function as cell surface receptors, is highly conserved over multiple species, and is involved in cell fate determination during embryogenesis. Notch genes function critically in angiogenesis and arteriogenesis.[4–8] The Notch family of genes consists of four Notch receptors (Notch1, -2, -3, and -4),[9–13] as well as two classes of ligands, Delta-like (Dll-1, -3, -4),[14–16] and Jagged (Jagged-1, -2)[17, 18].
Previous work in our laboratory has shown that members of the Notch signaling pathway, namely receptors and ligands, are expressed in hemangiomas. In particular, we have demonstrated that the endothelial-associated NOTCH1, NOTCH4, and JAGGED-1 genes are highly expressed in involuting hemangiomas and HemECs, while DELTA-LIKE LIGAND-4 (Dll4) showed an intermediate level of expression in HemECs. In contrast, NOTCH3, normally expressed in vascular smooth muscle cells, was found expressed in HemSCs. Moreover, studies by immunofluorescence showed that NOTCH1 and NOTCH4 were expressed in HemECs, whereas NOTCH3 was present in perivascular cells in IH tissue.
The purpose of this study is to evaluate the expression of downstream effector genes of Notch signaling, specifically the HES and HEY family of transcription factors. HES/HEY genes are part of the hairy enhancer of split related (HESR) family of genes and encode basic helix-loop-helix proteins that function as transcriptional repressors. After activation by binding to its ligand, the transmembrane Notch receptor undergoes cleavage and release of the intracellular domain (NICD). This NICD then translocates to the nucleus, where it interacts with the DNA-binding protein RBPJκ. NICD interaction with RBPJκ leads to transcription of Notch target genes. The primary target genes of Notch signaling are the HES/HEY family of transcriptional regulators. 
We asked whether the expression of HES/HEY transcripts and proteins are found in cell types associated with IHs, as further evidence for a role for Notch receptor signaling in IHs.
Preparation of hemangioma specimen
IRB approval for collection of resected hemangiomas was obtained from Columbia University College of Physicians & Surgeons (IRB #AAAA9976). Tissues were either paraffin embedded for sectioning or used immediately for cell isolation for in vitro experiments.
Cell extraction and isolation
HemSC and HemEC isolation was conducted as previously described. Briefly, the hemangioma samples were minced into small pieces with a scalpel and digested using collagenase (Roche). From this single cell suspension, cells expressing CD133 were selected using a magnetic microbead cell sorting system (Miltenyi Biotec), and plated on fibronectin-coated plates in EGM-2 media (Lonza) supplemented with 20% fetal bovine serum (Invitrogen), penicillin, and streptomycin (Invitrogen). CD133 negative cells were plated in the same manner. These cells were later sorted for endothelial cells using CD31 antibody-coated magnetic Dynal beads (Invitrogen). RNA was later isolated from both cell populations (CD133+, or HemSC, and CD31+, or HemEC) as described below. Cells were isolated from 3 different patients (H37, H40, and H41). Furthermore, RNA from isolated, characterized and validated HemSC and HemEC samples was provided by EB and JB.
RNA isolation and RT-PCR
Total RNA isolation was performed with RNeasy Mini Prep Kit (Qiagen). Quantity of RNA was determined with spectrophotometry. cDNA was synthesized by reverse transcription of 1 microgram of total RNA, performed using the Superscript II system (Invitrogen). All reverse transcription (RT) reactions were amplified by PCR to determine expression of beta-actin, to confirm successful generation of cDNA.
Primers used were for HEY1, HEY2, and HEYL, and HES1, HES3, HES5, HES6, and HES7. HEY1, forward: 5' ACG AGA ATG GAA ACT TGA GTT C 3', reverse: 5' AAC TCC GAT AGT CCA TAG CAA G 3'. HEY2, forward: 5' ATG AGC ATA GGA TTC CGA GAG TG 3', reverse: 5' GGC AGG AGG CAC TTC TGA AG 3'. HEYL, forward: 5' CAG GAT TCT TTG ATG CCC GAG 3', reverse: 5' GAC AGG GC5T GGG CAC TCT TC 3'. HES1, forward: 5' CCC AAC GCA GTG TCA CCT TC 3', reverse: 5' TAC AAA GGC GCA ATC CAA TAT G 3'. HES3, forward: 5' CAT CAA TGT GTC ACT GGA GCA G 3', reverse: 5' CAA GGA GTT CTG AAG GCT TCT C 3'. HES5, forward: 5' TCG CCT CGT CGC CTG TTC 3', reverse: 5' CCA CGA GTA GCC TTC GCT GTA G 3'. HES6, forward: 5' AGA ACG CCG AAG TGC TGG AG 3', reverse: 5' GAA CTG AGT CAG GCT CCT GCT G 3'. HES7, forward: 5' GGA ACC CGA AGC TGG AGA AAG 3', reverse: 5' GCC TCG GAT CTA CCG GCT TG 3'.
Survey PCR was performed as follows with the Eppendorf Mastercycler: initiation with heating to 94°C for 2 minutes followed by 95°C for 45 seconds. The program then proceeded to cycle at 60°C (1 minute), annealing at 72°C (1 minute) and extension 72°C (5 minutes) for 40 cycles. Mesenchymal stem cells (MSC, Lonza) were used as controls.
Quantitative Real-time PCR
Real time PCR was performed in triplicate utilizing the Applied Biosystem 7300 and SYBR Green PCR Master Mix reagent (Applied Biosystems). Primers used were described in the above section.
The PCR cycler was programmed as follows: initiation with heating to 50°C for 2 minutes followed by 95°C (2 minutes). The program then proceeded to cycle at 95°C (15 seconds), annealing at 60°C (40 seconds) and extension at 72°C (30 seconds) for 50 cycles. The results were normalized to beta-actin levels and the triplicate results were averaged for each sample.
DNA from human dermal microvascular endothelial cells (HDMEC) (isolated from neonatal foreskin as described for HemECs) and MSCs were used as controls.
Paraffin-embedded hemangioma specimens were stained for NOTCH3, (Abcam 23426, rabbit polyclonal, 1:75), HEY2 (Millipore AB5716, rabbit polyclonal, 1:100) and HEYL (Millipore MAB10094, mouse monoclonal, 1:250). The specimens were also co-stained for Notch3 and HEYL, CD31 (DAKO M0823, mouse monoclonal, 1:20) and HEY1 (Millipore AB5714, rabbit polyclonal, 1:100), and CD31 and HES1 (Santa Cruz SC25392, rabbit polyclonal, 1:50), and visualized by immunofluorescence. Briefly, the slides were de-paraffinized and incubated in blocking solution containing avidin, biotin, and animal serum. The blocking agent was removed and the sections were incubated in primary antibody overnight at 4 degrees. The slides were washed and incubated with secondary antibody at room temperature for 30 minutes. Slides were then incubated with immunofluorescent dye (Alexa green 488 or red 594) and Vectashield with DAPI (Vectorlabs) was applied for visualization of nuclei.
GSI treatment of Hemangioma Stem Cells
HemSCs were grown to confluence in EGM-2 (Lonza EBM-2 with Bullet Kit) supplemented with 20% FBS. 1.5-2 × 106 cells were plated in a 10 cm plate and left overnight at 37°C or until at least 80% confluent. Either 500 nM of compound E (KRICT, South Korea) in 4 microliters, or 4 microliters of vehicle (DMSO) was added to the plate. RNA was harvested from the control and treated cells 24 hours later and used for transcript analysis as described above. Compound E manufactured by KRICT is equivalent to Compound E from Merck.
For qPCR results, gene expression levels were measured in triplicate and normalized to beta-actin for an assigned relative value and standard deviation. Statistical significance between 2 samples was calculated using the unpaired Student's t-test.
Hemangioma stem cell and endothelial cell expression of HES and HEY genes
We surveyed HemSC and HemEC samples by RT-PCR to determine which of the HES/HEY genes are expressed, including use of HemSC/EC 131 and HemSC/EC 133 (characterized and provided by EB and JB). While HES1, -5, and -7 have been shown to be induced by Notch signaling, HES2, -3, and -6 are thought to be Notch independent. Since little is known about HES4 expression patterns, and no published, validated unique primer sets are available for HES2 and HES4, we chose not to evaluate HES2 and HES4.
We used immunoflurescence to evaluate the expression of HES and HEY proteins in IHs and to confirm the expression analysis conducted on transcripts levels in cells derived from IHs. Immunofluorescence studies showed that HES1, HEY1, HEY2, HEYL were expressed in IHs (data not shown). In order to further clarify the staining patterns of these proteins, IH samples were co-stained with antibodies for HES or HEY and either CD31, an endothelial cell marker, or NOTCH3, which is present in non-endothelial peri-vascular cells .
HES/HEYgenes are activated by Notch signaling
Taken together, these results demonstrated that the Notch receptor is active and induces transcription of downstream genes in HemSC. A subset of HES/HEY genes responded to GSI treatment, and suppression of Notch signaling activity reduced the transcript levels of these genes. HES1 transcript levels were consistently down-regulated by GSI, and HEY1 was variably down-regulated. H37 sample, which had higher baseline HES/HEY transcript levels than cells from other hemangioma specimens, showed consistent and dramatic down-regulation of HES/HEY expression in HemSCs.
Notch proteins function as receptors on the cell surface and are activated via interaction with Notch ligands, which are also located on the cell surface. This juxtacrine signaling leads to proteolytic cleavage of the Notch ICD, catalyzed by the γ-secretase complex. The intracellular domain of Notch (NICD) is released and translocates into the cell nucleus, where it binds with the CSL complex, leading to activation of target genes such as the Hes/Hey family of transcription factors.[27–29] As such, expression of Hes and Hey genes is used to indicate that Notch signaling is functional in a cell. Our previous study showed that Notch receptors and ligands were expressed in HemSCs and HemECs and we were interested in determining if Notch target genes were expressed in these cells.
After analysis of eight members of the HES/HEY family, we found that HES1, HEY1, HEY2 and HEYL are expressed in infantile hemangiomas. Moreover, similar to Notch receptors and ligands, there are differences in transcript levels of these genes in the different cell types. This may suggest specificity in Notch receptor downstream signaling. Alternately, this specificity may reflect that distinct HES/HEY genes are inherent to different cell types. Moreover, our immunofluorescence results also parallel the Notch staining patterns previously reported, specifically, that endothelial cells and perivascular cells stain for different Notch receptors. HES1 and HEYL were predominantly present on perivascular cells, however, occasional endothelial cells also expressed these transcription factors. HEY2, an endothelial-associated protein, mostly co-localized to endothelial cells (CD31+) in IHs, however there were occasional non-endothelial, perivascular cells that showed HEY2 staining. Nonetheless, their presence suggests that Notch signaling is active in these cells. Thus we conclude that HES1 and HEYL are expressed in perivascular cells of IHs, whereas HEY2 is specific to the endothelial component of IHs.
Gamma secretase inhibitors (GSI) block proteolytic cleavage by γ-secretase and thus prevent the release of the NICD into the nucleus and subsequent downstream steps. Notch functions are inhibited or attenuated by GSI treatment, and it has been established that GSI treatment can elicit angiogenic phenotypes associated with loss of Notch function [30, 31]. GSI treatment has also been used to evaluate Notch function during wound healing[32, 33]. Moreover, HES1 transcripts in melanoblasts and HEY2 transcripts in human mesenchymal stem cells have been shown to be decreased with gamma secretase inhibitor treatment. However, it was not clear whether they also tested other HES/HEY genes. As expected, treatment of our cells with the GSI Compound E showed a decrease of HES/HEY gene transcription. This suggests that not only are Notch receptors and ligands expressed in HemSCs , but that Notch signaling is occurring in HemSCs.
Since there was variability in transcript levels of the different HES/HEY family proteins when HemSCs and HemECs were compared, there may be target specificity of HES/HEY genes in the Notch signaling pathway. In HemSCs, where NOTCH3 is strongly expressed, HES1, HEY1, and HEYL were expressed at levels 10 to 100 times to that of HEY2. It is conceivable that NOTCH3 specifically induces transcription of HES1, HEY1, and HEYL, whereas transcription of HEY2 is activated by other Notch receptors.
Measurement of these effector genes will provide a tool for exploring Notch functions and relating that expression to the patholobiology of IHs. Moreover, identification of transcription factors critical to hemangioma development may provide multiple specific therapeutic targets along the Notch signaling axis. In a recent study by Boscolo et al, JAGGED-1 was found to be critical in a stem-cell to pericyte differentiation in a murine model of hemangioma. Since JAGGED-1 exerts its activities by signaling through Notch receptors, its crucial role in regulating HemSC differentiation to pericytes suggests that the Notch pathway plays a role in hemangioma pathophysiology. Future studies will concentrate on the effects of functional manipulation of Notch signaling on cell behavior, and more specific inhibition of specific Notch receptors.
In summary, we surveyed HEY1, HEY2, HEYL, and HES1, HES3, HES5, HES6, and HES7 expression in infantile hemangiomas in this study. We showed that HES1, HEY1, and HEYL are highly expressed in HemSCs, whereas HEY2 is expressed in HemECs. Moreover, HES and HEY transcripts are affected by Compound E, a gamma secretase with consistent down-regulation of HES1, showing that NOTCH signaling occurs in infantile hemangiomas.
This study is supported by a Pilot Award from the Irving Institute for Clinical and Translational Research/Clinical Trials Office of Columbia University, and by a Career Development Awards from NHLBI (K08 5K08HL102068-02) for JKW and an R01 from NHLBI (5R01HL096384) for EB and JB.
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