Introduction
The receptor tyrosine kinase-like orphan receptor 2 (ROR2) is a transmembrane protein that belongs to a conserved family of tyrosine kinase receptors involved in many developmental processes, including chondrogenesis [
1], osteoblastogenesis [
2] and neural differentiation [
3]. Accordingly,
ROR2 mutations in humans result in dominant brachydactyly type B and Robinow syndrome [
4], two syndromes of altered development characterised by short stature, brachydactyly, segmental defects of the spine and dysmorphic facial appearance [
5].
ROR2 exerts its role in cell differentiation primarily through the Wnt signalling pathway [
6]. This pathway is comprised of a number of extracellular effectors, membrane proteins, intracellular signal transducers and nuclear gene regulators that transmit extracellular signals to the nucleus as precise instructions for regulating specific genes [
7]. When β-catenin participates in this cascade, the signalling pathway is known as canonical Wnt. Wnt effectors can also induce β-catenin-independent signals that make up the non-canonical Wnt signalling pathway. Within the Wnt signalling pathway, the primary role of ROR2 is to mediate WNT5A signals in a complex manner that is still unclear. ROR2 was initially shown to mediate WNT5A-dependent inhibition of canonical Wnt signalling downstream of β-catenin stabilisation in 293 cells, at the level of TCF-mediated transcription [
8]. ROR2 was subsequently shown to mediate WNT5A-dependent JNK activation in regulating convergent extension movements in
Xenopus gastrulation [
9], and is also known to enhance WNT1 and antagonise WNT3 activities in osteoblastic cells [
10]. In the H441 lung carcinoma cell line, ROR2 positively modulates Wnt3a-activated canonical signalling [
11].
The Wnt signalling pathway is central to cell differentiation and cancer. Genetic and epigenetic alterations of components of the canonical Wnt signalling pathway are a primary mechanism of colon cancer development [
7]. ROR2 is overexpressed in oral [
12] and renal cancer [
13], and in osteosarcoma [
14]. ROR2 overexpression activates JNK, a component of the non-canonical Wnt pathway, and has pro-tumourigenic effects [
13,
14].
ROR2 also mediates inhibition of the β-catenin-dependent Wnt signalling pathway [
8,
10,
15]. Paradoxically, the aberrant epigenetic repression of other Wnt inhibitors such as WIF-1, DKK1, SFRP1 and SFRP2 directly promotes tumourigenesis in colon cancer cells by promoting constitutive Wnt signalling [
7,
16‐
18]. Indeed, the ROR2 extracellular ligand WNT5A, which inhibits the canonical Wnt signalling pathway in certain molecular contexts [
8], is also aberrantly repressed by promoter hypermethylation in acute lymphoblastic leukaemia [
19] and in colon cancer [
20], and its absence is tumourigenic in these tumour types. As ROR2 mediates the inhibition of canonical signalling by WNT5, we hypothesised that this orphan receptor could also be a target of aberrant epigenetic regulation in colon cancer. Here we report that ROR2 is frequently repressed by promoter hypermethylation in colon cancer and that its loss can be protumourigenic in colon cancer.
Discussion
We report that the receptor tyrosine kinase-like orphan receptor 2 (ROR2), a transmembrane protein that participates in Wnt signalling, is frequently repressed by aberrant promoter hypermethylation in human colon cancer. In addition, we demonstrate that the epigenetic-dependent loss of ROR2 can promote tumour growth in colon cancer cells. This effect can be mediated, at least in part, by the canonical Wnt signalling pathway, since restoration of ROR2 activity in DLD1 colon cancer cells induced a decrease in β-catenin/TCF-dependent transcription.
ROR2 is reported to have oncogenic properties in other tumour types such as oral cancer [
12], renal cancer [
13] and osteosarcoma [
14]; it is frequently overexpressed in these tumours, and its suppression in renal cancer cells inhibits cell migration and growth in orthotopic xenograft models [
13]. In osteosarcoma cells, suppressed expression of ROR2 (or its extracellular effector, WNT5A) inhibits cell invasiveness and decreases invadopodium formation [
14].
These lines of evidence suggest that the role of ROR2 in cancer is complex and that it can either promote or suppress tumour formation, depending on tumour type and molecular context. A possible explanation for this complexity could be the multifaceted role of ROR2 in the Wnt signalling pathway. On the one hand, ROR2 can mediate WNT5A-dependent activation of JNK, a member of the non-canonical Wnt pathway in mice [
9,
21], while on the other, ROR2 can govern the WNT5A-dependent inhibition of canonical Wnt signalling downstream of β-catenin stabilisation [
8]. Depending on the tumour type, ROR2 signals can therefore show a preference for β-catenin/TCF-dependent genes or for non-canonical Wnt pathways. Two lines of evidence support these possibilities: first, the pro-tumourigenic role of ROR2 in renal cancer and osteosarcoma is thought to be mediated by activation of the non-canonical Wnt signalling kinase JNK [
13,
14], and second, in colon cancer cells with constitutive Wnt signalling activity, restoration of ROR2 activity increased the inhibition of β-catenin reporter genes (our data and [
15]).
As ROR2 is reported to mediate the WNT5A-dependent inhibition of β-catenin-TCF-regulated genes in human embryonic cells [
8,
15], we tested whether WNT5A mediated the canonical Wnt-dependent role of aberrant epigenetic ROR2 repression in colon cancer. We evaluated the effect of increased or decreased extracellular WNT5A levels on cell growth and canonical Wnt pathway status in ROR2-expressing DLD1 cells and controls. Although modulation of extracellular WNT5A levels affected cell growth, this effect was little influenced by ROR2 expression. The canonical Wnt pathway did not appear to be the effector of WNT5A modulation in these cells. Our data thus suggest that the canonical Wnt-dependent role of ROR2 hypermethylation in colon cancer cells does not necessarily require WNT5A. As WNT5A might have canonical and non-canonical effects, WNT5A/ROR2 could have a role in colon cancer through non-canonical Wnt signalling within certain molecular contexts. This pathway is frequently altered in other tumour types [
12‐
14]. Our results nonetheless do not rule out a role for WNT5A in colon cancer through the canonical Wnt signalling pathway; indeed, WNT5A inhibits β-catenin/TCF-dependent transcription in HCT116 colon cancer cells [
20].
As proposed for ROR2, WNT5A might also have either a tumour-promoting or-suppressing role. Many studies report a tumour-suppressing effect, and it is downregulated in a number of cancers such as colorectal and ductal breast cancer, neuroblastoma and leukaemia (reviewed in [
22]). As we show here for ROR2, WNT5A repression in colon and haematopoietic tumours is mediated by aberrant promoter hypermethylation [
19,
20]. In contrast, WNT5A also has a tumour-promoting role in cases such as non-small-cell lung cancer, melanoma, breast, gastric, pancreatic and prostate cancers (reviewed in [
22]). As ROR2 and WNT5A functions are intimately associated--indeed, ROR2 knockout mice phenocopy most of alterations seen in WNT5A knockout mice [
23]--ROR2 and WNT5A might also have parallel or complementary roles in cancer. ROR2/WNT5A upregulation could be advantageous to cancers driven by non-canonical Wnt signalling, while their epigenetic downregulation would benefit tumours, such as colon and haematopoietic cancers, that are driven by canonical Wnt signalling.
Methods
Human cancer cell lines culture and treatments, and primary tumour samples
The eight human colon cancer cell lines (HCT116, SW480, LOVO, HT29, HCT15, DLD1, COLO205 and RKO) and healthy colonocytes were obtained from the American Type Culture Collection. Cell lines were maintained in appropriate medium and treated with 2 μmol/L 5-aza-2'-deoxycytidine (Sigma) for 3 days for DNA demethylation, anti-WNT5A antibody (2 μg/ml; R&D) or recombinant WNT5A (200 ng/ml; Millipore). We obtained 36 primary colon tumours from the Spanish National Cancer Research Centre Tumour Bank and 20 pairs of same-patient healthy and colon tumour tissue from the Institute of Oncology of Asturias Tumour Bank. The study was approved by the appropriate institutional review boards.
DNA methylation analysis of the ROR2 gene
We established ROR2 CpG island methylation status by PCR analysis of bisulphite-modified genomic DNA. Methylation status was first analysed by bisulphite genomic sequencing of the CpG island using primers: 5'-GGG GTT TTA GTT GTA GTT TTA GT-3' (sense) and 5'-CTC CTC CTT CTC CCT AAC-3' (anti-sense). Twelve independent clones were sequenced for each sample. The second analysis used methylation-specific PCR with primers specific to the methylated or modified unmethylated DNA. Primer sequences for the methylated reaction were 5'- GTT TCG TTT TGT TTA TCG GGG C - 3' (sense) and 5'-ACT AAA AAA ATT CCT TAA CGC GAA -3' (anti-sense), and for the unmethylated reaction 5'-GTT TTG TTT TGT TTA TTG GGG TGG-3' (sense) and 5'-TAA CTA AAA AAA TTC CTT AAC ACA AA-3' (anti-sense). In vitro-methylated DNA (IVD) was used as a positive control for the methylated reaction. PCR products were identified in ethidium bromide-stained 2% agarose electrophoresis gels and viewed under UV light. We designed the bisulphite genomic sequencing and methylation-specific PCR primers using Methyl Primers Express software and confirmed bisulphite sequencing and methylation-specific PCR results using Illumina Infinium Methylation Arrays (Illumina). The two CpG sites analysed were located 465 bp upstream and 322 bp downstream of the ROR2 transcriptional start point. Relative DNA methylation for each CpG site is represented by a scale from 0.0 to 1.0 (corresponding to 0% and 100% likelihood of gene promoter hypermethylation, respectively).
ROR2 RNA and protein analysis by quantitative reverse-transcription PCR and immunohistochemistry
Total RNA (1 μg) extracted with Trizol reagent (Invitrogen) was converted to cDNA using the High Capacity cDNA Reverse Transcription Kit (Applied Biosystems). The PCR reaction was performed by mixing the converted cDNA with the TaqMan Gene Expression Assay for ROR2 (Hs00171695_m1 ROR2) and GAPDH (Hs99999905_m1 GAPDH) and TaqMan Universal PCR Master Mix (Applied Biosystems). qRT-PCR was performed in the Applied Biosystems 7900 HT Fast Real-Time PCR System. Data were normalised using GAPDH as the endogenous control gene. The ROR2 protein was analysed with an anti-ROR2 antibody (from Abcam for immunohistochemistry; from Sigma for WB), following standard procedures.
ROR2 transfection experiments
The pcDNA3.1-ROR2-Flag construct was created using a pcDNA3.1-ROR2 plasmid (kindly provided by Dr J. Billiard) as a template. The construct was confirmed by DNA sequencing. Control cells were transfected using the empty plasmid (mock). DLD1 cells were transfected with the pcDNA3.1-ROR2-Flag plasmid or the empty plasmid using Lipofectamine 2000 (Invitrogen). Stable transfectants were obtained after 2 weeks selection with 1 mg/ml G418 (Calbiochem). ROR2-Flag expression was confirmed by WB using anti-Flag antibody (1:2000, Sigma).
Cell viability assay
Cell viability was determined as described [
24]. Aliquots (10
4 cells) were plated onto 96-well plates. After cell attachment, MTT was added to medium (50 μg; 100 μl/well) and incubated (3 h, 37°C, 5% CO
2). MTT was removed and MTT-formazan crystals were dissolved in DMSO (100 μl/well). Absorbance at 595 nm was determined with an automated microtitre plate reader. Optical density was directly proportional to cell number up to the maximum density measured. Results are expressed as the mean ± SD of at least five replicates and significance was assessed by standard t-test.
Colony formation was assayed on ROR2-Flag-and mock-transfected DLD1 cells. After 14 days selection, stable G418-resistant colonies were fixed, MTT-stained, and the mean number of colonies in each well was determined.
Mouse xenograft model
Five-week-old athymic Nude-Foxn1nu/nu mice (Charles River) were used for tumour xenograft experiments with ROR2-Flag-and mock-transfected DLD1 cells. Twelve mice were used. Both flanks of each animal were injected subcutaneously with 2 × 106 cells in 200 μl PBS; ROR2-Flag-transfected cells were injected into the right flank and empty vector cells into the left flank. Mice were weighed, and tumour width (W) and length (L) were measured every 5 days. Tumour volume was estimated according to the formula V = 0.4. L.W2 (L = maximum length; W = maximum width). Mice were killed 30 days post-injection and tumours from both cell types were excised and weighed. Mean volume and tumour mass ± SEM were calculated for each group, and significant differences were assessed by a two-tailed independent-samples t-test.
Transfection and reporter assays
Subconfluent cultures were transfected in triplicate using JetPEI (PolyPlus Transfection, Illkirch, France) following manufacturer's protocols. pTOPFLASH, and pFOPFLASH reporter plasmids have been described [
25]. A
Renilla luciferase plasmid (pRL-TK) was used in all experiments as an internal control.
Firefly and
Renilla luciferase activities were measured separately using the Dual Luciferase reagent kit (Promega, Madison, WI) and a GloMax 96 microplate luminometer (Promega).
Immunofluorescence
ROR2-Flag-and mock-transfected DLD1 cells were methanol-fixed and stained using mouse anti-β-catenin (Becton Dickinson) and goat anti-α-lamin B (Santa Cruz), followed by Alexa448 donkey anti-goat IgG or Alexa594 donkey anti-mouse IgG (Molecular Probes).
Acknowledgements
The pcDNA3.1-ROR2 plasmid was kindly provided by Dr Julia Billiard. C. Mark provided editorial assistance. AM is funded by the Spanish Ministerio de Ciencia e Innovación (MICINN; SAF2007-60341, ISCIII-RETIC RD06/0020/0009), VC receives a fellowship from the Spanish FPU Research Programme, EL and CH are recipients of fellowships from the Spanish FIS Research Programme. The Instituto Universitario de Oncología is supported by Obra Social Cajastur, Spain. This work was supported by the MICINN (PI061267; PS09/02454; Ref. 200820I172).
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
EL, VC and AFF carried out DNA methylation, expression and transfection experiments and helped to draft the manuscript. CH, AM-P and AJO carried out mouse xenograft experiments. OA and JMG-S carried out reporter assays. LS and AA carried out immunohistochemistry. AM supervised reporter assays and helped to draft the manuscript. CL-O supervised mouse xenograft experiments and helped to draft the manuscript. ME helped to draft the manuscript. MFF conceived the study, participated in its design and wrote the manuscript. All the authors read and approved the final manuscript.