Olfactory receptor 51E1 protein as a potential novel tissue biomarker for small intestine neuroendocrine carcinomas

in European Journal of Endocrinology

Objective

Late diagnosis hinders proper management of small intestine neuroendocrine carcinoma (SI-NEC) patients. The olfactory receptor, family 51, subfamily E, member 1 (OR51E1) has been reported as a potential novel SI-NEC marker, without protein expression recognition. Thus, we further studied whether the encoded protein may be a novel SI-NEC clinical biomarker.

Design

OR51E1 coding sequence was cloned using total RNA from SI-NEC patient specimens. Quantitative real-time PCR analysis explored OR51E1 expression in laser capture microdissected SI-NEC cells and adjacent microenvironment cells. Moreover, immunohistochemistry investigated OR51E1 protein expression on operation and biopsy material from primary SI-NECs, mesentery, and liver metastases from 70 patients. Furthermore, double immunofluorescence studies explored the potential co-localization of the vesicular monoamine transporter 1 (SLC18A1, generally referred to as VMAT1) and OR51E1 in the neoplastic cells and in the intestinal mucosa adjacent to the tumor.

Results

OR51E1 coding sequence analysis showed absence of mutation in SI-NEC patients at different stages of disease. OR51E1 expression was higher in microdissected SI-NEC cells than in the adjacent microenvironment cells. Furthermore, both membranous and cytoplasmic OR51E1 immunostaining patterns were detected in both primary SI-NECs and metastases. Briefly, 18/43 primary tumors, 7/28 mesentery metastases, and 6/18 liver metastases were ‘positive’ for OR51E1 in more than 50% of the tumor cells. In addition, co-localization studies showed that OR51E1 was expressed in >50% of the VMAT1 immunoreactive tumor cells and of the enterochromaffin cells in the intestinal mucosa adjacent to the tumor.

Conclusion

OR51E1 protein is a potential novel clinical tissue biomarker for SI-NECs. Moreover, we suggest its potential therapeutic molecular target development using solid tumor radioimmunotherapy.

Abstract

Objective

Late diagnosis hinders proper management of small intestine neuroendocrine carcinoma (SI-NEC) patients. The olfactory receptor, family 51, subfamily E, member 1 (OR51E1) has been reported as a potential novel SI-NEC marker, without protein expression recognition. Thus, we further studied whether the encoded protein may be a novel SI-NEC clinical biomarker.

Design

OR51E1 coding sequence was cloned using total RNA from SI-NEC patient specimens. Quantitative real-time PCR analysis explored OR51E1 expression in laser capture microdissected SI-NEC cells and adjacent microenvironment cells. Moreover, immunohistochemistry investigated OR51E1 protein expression on operation and biopsy material from primary SI-NECs, mesentery, and liver metastases from 70 patients. Furthermore, double immunofluorescence studies explored the potential co-localization of the vesicular monoamine transporter 1 (SLC18A1, generally referred to as VMAT1) and OR51E1 in the neoplastic cells and in the intestinal mucosa adjacent to the tumor.

Results

OR51E1 coding sequence analysis showed absence of mutation in SI-NEC patients at different stages of disease. OR51E1 expression was higher in microdissected SI-NEC cells than in the adjacent microenvironment cells. Furthermore, both membranous and cytoplasmic OR51E1 immunostaining patterns were detected in both primary SI-NECs and metastases. Briefly, 18/43 primary tumors, 7/28 mesentery metastases, and 6/18 liver metastases were ‘positive’ for OR51E1 in more than 50% of the tumor cells. In addition, co-localization studies showed that OR51E1 was expressed in >50% of the VMAT1 immunoreactive tumor cells and of the enterochromaffin cells in the intestinal mucosa adjacent to the tumor.

Conclusion

OR51E1 protein is a potential novel clinical tissue biomarker for SI-NECs. Moreover, we suggest its potential therapeutic molecular target development using solid tumor radioimmunotherapy.

Introduction

Neuroendocrine carcinomas (NECs) are rare malignancies mainly characterized by an indolent clinical course and a delayed diagnosis. Small intestine NECs (SI-NECs) originate from enterochromaffin (EC) cells in the intestinal mucosa. In general, metastatic disease is present at the time of diagnosis and mainly involves the mesenteric lymph nodes and the liver. The major medical treatment for metastatic SI-NECs comprises biotherapy, using somatostatin analogs (SSAs), interferon α (1), and lately everolimus (2). SSAs relieve the patients' symptoms and also have a major role in controlling tumor growth (3). In addition, treatment with radiolabelled SSAs is a well-established treatment in the management of patients with inoperable SI-NECs (4). Therefore, hormonal receptors, such as somatostatin receptors, became pivotal tissue markers to assess the therapeutic effectiveness of SSAs (5). However, some SI-NEC patients are not responsive to SSA treatment (6). Thus, there is an unmet need of finding novel markers to accelerate and improve the clinical management of SI-NEC patients.

The OR51E1 gene encodes the olfactory receptor, family 51, subfamily E, member 1, and the OR genes belong to the largest receptor superfamily in humans (7). The ORs are mainly expressed in the cilia of the olfactory sensory neurons and play an essential role in the specific recognition of diverse stimuli (8). They are G-protein-coupled receptors, which have been pivotal for drug development and therapeutic approaches against a variety of diseases including cancer (9). As neurons and neuroendocrine cells are phenotypically similar (10), scientists started exploring whether the ORs were also expressed in the EC cells of the normal intestinal mucosa and whether they may be involved in the serotonin release mechanism. Kidd et al. (11) revealed that different olfactants mediate the luminal serotonin release from normal and neoplastic human EC cells. Furthermore, Braun et al. (12) detected the expression of four ORs in the microdissected EC cells of the intestinal mucosa and investigated the downstream signaling pathway.

Leja et al. have previously identified six differentially expressed novel genes, including the OR51E1 gene in SI-NECs. In spite of these findings, OR51E1-encoded protein had not been detected at that time (13). Although the OR51E1 physiological ligand is unknown, Fujita et al. (14) recognized that molecules such as 3-methyl-valeric acid and 4-methyl-valeric acid are able to increase cAMP concentration via a chimeric OR51E1 signaling. OR51E1 mRNA expression was first detected in brain tissues (15) and soon after in normal prostate cells (16). Moreover, OR51E1 has been described as a novel mRNA tissue marker for prostate cancers (16, 17, 18, 19). However, OR51E1 protein expression has never been investigated in either prostate tissue or any tumor material. Thus, in this study, the principal aim was investigating whether OR51E1 protein can be detected in SI-NECs and developed as a new clinical tissue biomarker for these tumors.

Materials and methods

Ethics statement

Patient samples were included in the study after a written consent statement was obtained from each individual. The study was performed in accordance with the regional ethics committee at Uppsala University, Sweden (ref. no. Dnr 2011/426).

Tissue specimens

Snap-frozen tissue specimens (n=9; primary tumors (n=3), mesentery metastases (n=3), and liver metastases (n=3)) from patients with histopathologically confirmed SI-NEC diagnosis were included in the laser capture microdissection (LCM), RNA extraction, and cDNA synthesis studies. Furthermore, formalin-fixed paraffin-embedded tissue specimens (n=90) from patients (n=71) with SI-NEC at different stages of disease (primary tumors (n=43), mesentery metastases (n=28), and liver metastases (n=18)) were included in the immunohistochemical examinations. The median age at operation of SI-NEC patients (men (n=30) and women (n=41)) was 62 years (range 33–82 years). From all the primary tumor cases and liver metastases, sufficient adjacent normal tissue was also available. In addition, one tissue specimen from a prostate carcinoma patient (Gleason grade 5) was also included. Approximately 4 μm-thick sections were cut and attached to positively charged glass slides (Superfrost+, Menzel Gläser, Braunschweig, Germany). Patients' clinicopathological data are summarized in Tables 1 and 2.

Table 1

Frozen tissue specimens used for sequencing and microdissection.

Sex/ageTypeTissueTreatmentApplication
M/80PIleocecalUntreatedLCM, Seq
F/63PIleumUntreatedLCM, Seq
F/63PIleumUntreatedLCM, Seq
F/51MMesenterySSA, IFNLCM
M/56MMesenterySSA, IFNLCM, Seq
F/77MMesenterySSA, IFNLCM, Seq
F/40LLiverSSA, IFNLCM
F/76LLiverSSA, IFNLCM, Seq
F/65LLiverSSA, IFNLCM

P, primary tumor; M, mesentery metastasis; L, liver metastasis; age, age at operation; SSA, somatostatin analog; IFNα, interferon α; LCM, laser capture microdissection; Seq, sequencing.

Table 2

Summary of OR51E1 immunostaining in the tumor cells of SI-NECs.

OR51E1 immunoreactivity
TypeTreatmentC (%)PM (%)CM (%)M (%)Total (%)
MucaNANANANANANA
Pa,bUntreated5820260
PUntreated21013
PUntreated1730320
PUntreated721031385
PUntreated50005
PUntreated4.80.200.25
PUntreated9.50.500.510
PUntreated1.80.200.22
Pa,bUntreated5451660
PUntreated4.50.500.55
PUntreated1.80.200.22
PUntreated3.810.21.25
PUntreated20821030
PUntreated2.90.100.13
PUntreated1.80.200.22
PUntreated9.490.50.010.5110
PUntreated41015
PSSA31125
Pa,bSSA2515153055
Pa,bSSA7320275
PSSA8451690
PSSA30003
PSSA2951635
PSSA60551070
PIFN105606575
PIFN29.90.100.130
PIFN910110
Pa,bIFN27.520.52.530
PIFN441011155
PSSA, IFN2020204060
PSSA, IFN4.80.200.25
PSSA, IFN9.50.500.510
PbSSA, IFN2525305580
PSSA, IFN3035205585
PSSA, IFN401552060
PSSA, IFN6181970
PSSA, IFN381521755
PSSA, IFN552052580
PSSA, IFN9.90.100.110
PSSA, IFN820210
PSSA, IFN551051570
PSSA, IFN2352730
PSSA, IFN5320255
MUntreated4852755
MUntreated0.950.0500.051
MUntreated0.950.0500.051
MUntreated29.450.50.050.5530
MUntreated9.50.500.510
MUntreated32025
MUntreated20002
MaUntreated551051570
MUntreated10001
MUntreated101001020
MUntreated332022255
MUntreated0.90.100.11
MUntreated32025
MUntreated2410125
MUntreated0.90.100.11
MUntreated00000
MSSA2000020
MIFN0.90.100.11
MIFN27.520.52.530
MSSA, IFN342012155
MSSA, IFN1520557590
MSSA, IFN20002
MSSA, IFN0.950.0500.051
MSSA, IFN691011180
MSSA, IFN3280840
MSSA, IFN0.90.100.11
MSSA, IFN41015
MaSSA, IFN651051580
LUntreated41015
LUntreated11123
LUntreated8.810.21.210
LaUntreated5122455
LUntreated8.510.51.510
LUntreated00000
LaUntreated3515203570
LUntreated5221355
LUntreated2550530
LSSA3.510.51.55
LSSA00000
LSSA00000
LIFN4010102060
LIFN45551055
LSSA, IFN253053560
LSSA, IFN550510
LSSA, IFN11001
LSSA, IFN00000

Muc, ileal mucosa adjacent to the primary tumors; P, primary tumor; M, mesentery metastasis; L, liver metastasis; NA, not applicable SSA, somatostatin analog; IFNα, interferon α; C (%), percentage cytoplasmic immunostaining; PM (%), percentage partial membrane immunostaining; CM (%), percentage circumferential membrane immunostaining; M (%), percentage total membrane immunostaining; Total (%), percentage total OR51E1 immunostaining.

Sections also used for co-localization studies.

Sections used for counting OR51E1 immunoreactive cells in the intestinal mucosa adjacent to the tumor.

Laser capture microdissection

Snap-frozen tissue specimens were sliced into ∼10 μm sections using a microtome cryostat (Leica, Solms, Germany) and adhered to polyethylene naphthalate membrane frame slides (Life Technologies). Tumor cells and cells of adjacent tumor microenvironment, from each section, were extracted using the ArcturusXT microdissection system (Life Technologies) according to the manufacturer's instructions.

RNA extraction and cDNA synthesis

Total RNA from snap-frozen SI-NEC specimens was isolated using mirVana miRNA isolation kit (Life Technologies), whereas total RNA from LCM tumor cells and LCM cells of the adjacent tumor microenvironment were prepared using the RNAqueous Micro Kit (Life Technologies). About 1 μg of total RNA from frozen blocks and 3 ng of total RNA from LCM cells were reverse transcribed into cDNA using cDNA synthesis kit (Bio-Rad), after DNase I treatment with DNA-free kit (Life Technologies).

OR51E1 transcript sequencing

OR51E1 coding sequence was amplified using 10 ng cDNA from total RNA of six SI-NEC specimens; forward primer 5′-TCA GCT TCT TCA TGA TGG TGG-3′ and reverse primer 5′-CAC TGA CAC CTA GGG CTC TGA-3′. The PCR products were cloned using the TOPO TA Cloning kit (Life Technologies) and sequenced at the Uppsala Genome Center, Rudbeck Laboratories, Uppsala, Sweden (http://www.genpat.uu.se/facilities/genome_center/). Sequences were then aligned with GenBank published data.

Quantitative real-time PCR

About 400 pg cDNA from LCM cells, as described earlier, were used as template of each reaction. Relative OR51E1 expression was measured using previously described primers (13). The reactions were run using Brilliant II SYBR Green QPCR Master Mix and Agilent Stratagene Mx3005P real-time PCR system (Agilent Technologies, Santa Clara, CA, USA) according to the manufacturer's instructions. The analysis was then normalized with β-actin expression by the 2ΔΔCt method (20).

Immunohistochemistry

The sections were microwave treated for 2×5 min at 700 W and 1×15 min at 380 W using 10 mM citrate buffer, pH 6, as retrieval solution, before immunostaining. The sections were immunostained using a polymer detection system (DakoCytomation, EnVision+System-HRP, cat. no. K4011, Dako, Glostrup, Denmark), then incubated at 4 °C overnight with the rabbit polyclonal anti-human OR51E1 antibody (cat. no. A1854, LSBio, Seattle, WA, USA), and diluted 1:500 in Dako antibody diluent (cat. no. S3022, Dako). Diaminobenzidine was used as chromogen. Nuclear counterstain was performed using Meyer's hematoxylin (Histolab Product, Gothenburg, Sweden). The sections were evaluated under Axiophot light microscope (Carl Zeiss, Oberkochen, Germany) and images were obtained using AxioVision Rel. 4.5 Software (Carl Zeiss). The percentage of the immunoreactive (IR) cells was estimated by a light microscope at a magnification of ×400 using a square grid in one of the oculars. Four randomly selected areas were at least examined on tissue specimens, whereas the entire neoplastic tissue was examined for the smaller lesions. Two methods were used for the immunohistochemical classification of the tumors. In the first method (quantitative), the tumors were considered as ‘positive’ if OR51E1 immunoreactivity appeared in the majority (>50%) of the tumor cells, independently of the cytoplasmic or membranous immunostaining pattern. In the second method (semiquantitative), the subcellular localization of the receptor was taken into consideration as follows: pure cytoplasmic immunoreactivity, membranous immunoreactivity in <50% of the cell circumference (or partial membranous immunoreactivity), and membranous immunoreactivity in over 50% of the cell circumference (or complete circumferential immunoreactivity). In addition, OR51E1 ‘positive’ cells in the intestinal mucosa adjacent to the tumor were counted. The density of IR cells, expressed as the number of IR cells per one high power field (HPF) mucosa area, was calculated using a square grid in one of the oculars at a magnification of ×400. Fifteen HPF/specimen in parallel to the muscularis mucosa were evaluated from sufficient adjacent mucosa on six primary SI-NEC cases.

Prostate carcinoma specimen expressing OR51E1 was used to optimize the method and as a ‘positive’ control in subsequent experiments. In addition, ‘negative’ control studies were performed to validate the sensitivity and specificity of OR51E1 antibody as follows: i) replacement of the primary antibody, on all the tissue specimens, with nonimmune rabbit IgG (cat. no. I-1000, Vector Labs, Burlingame, CA, USA) at the same dilution and in the same diluent, as described earlier; and ii) neutralization test, using 24-h incubation of the primary antibody with the respective blocking peptide (cat. no. P1854, LSBio) before application on the specimens conducted on primary SI-NECs (n=3), mesentery metastasis (n=1), and liver metastasis (n=1).

Co-localization studies were performed in selected sections. The specimens were microwave treated as mentioned earlier and blocked for 30 min with normal donkey serum (cat. no. 017-000-121, Jackson ImmunoResearch, West Grove, PA, USA) at room temperature. Then the specimens were incubated at 4 °C overnight with a mixture of two primary antibodies that included 1:400 diluted goat polyclonal anti-human vesicular monoamine transporter 1 (VMAT1, also known as SLC18A1, solute carrier famuily 18 member1) antibody (C-19, cat. no. sc-7718, Santa Cruz Biotech, Santa Cruz, CA, USA) and 1:100 diluted OR51E1 antibody (cat. no. A1854, LSBio). Then the sections were incubated for 30 min at room temperature with a mixture of fluorescence-labeled secondary antibodies, which included fluorescein (FITC)-donkey anti-goat (cat. no. 705-095-147, Jackson ImmunoResearch) and rhodamine (TRITC)-donkey anti-rabbit (cat. no. 711-025-152, Jackson ImmunoResearch), both diluted 1:100 in PBS. Vectashield (cat. no. H-1000, Vector Labs) was used for the mounting of the specimens. The immunostaining was evaluated under Axioplan2 imaging microscope (Carl Zeiss). Images were acquired with a ×630 magnification and analyzed with AxioVision Rel. 4.8 Software (Carl Zeiss). In addition, the intestinal mucosa adjacent to the tumor was examined in five primary SI-NEC cases with a view to investigate a possible co-expression between VMAT1- and OR51E1-IR cells. Photos were taken from a total of 35–50 randomly selected HPFs per case. The double-immunostained specimens were photographed with the 63× plan-apochromat objective, and the number of cells that co-expressed VMAT1 and the OR51E1 was counted in the merged pictures. Similarly, the possible co-expression of the transporter and the receptor in question in the tumor cells was evaluated in 15–20 HPF/specimen at a magnification of ×630, on five primary SI-NECs, two mesentery metastases, and two liver metastases cases.

Statistical analysis

The paired Student's t-test was used to assess the statistical significance of the OR51E1 transcript expression between the microdissected tumor cells and the microdissected cells of the adjacent tumor microenvironment, from the same patients. The analyses were two tailed and performed using GraphPad Prism 5 (GraphPad Software, La Jolla, CA, USA). All P values <0.05 were considered significant.

Results

Wild-type OR51E1 coding sequence from established NEC cells and SI-NEC specimens

The sequence analysis showed the absence of any mutation in the OR51E1 coding sequence from SI-NEC patient specimens at different stages of disease. Our sequence is identical to the original wild-type OR51E1, which was published on GenBank (accession number: NP_689643.2; data not shown).

OR51E1 transcript expression on microdissected SI-NEC cells and microdissected adjacent tumor microenvironment cells

OR51E1 was differentially expressed in the microdissected SI-NEC cells from the nine patients. Significant lower expression was detected in the microdissected cells of the adjacent tumor microenvironment from all patients (P<0.01).

Immunohistochemistry

About 90% of the prostate carcinoma cells were IR for the OR51E1 (external ‘positive’ control) and showed circumferential immunohistochemical pattern. In the intestinal mucosa adjacent to the tumor cells, 0.9 cells/HPF (median; range 0.2–4 cells/HPF) were IR for the OR51E1 (internal ‘positive’ control). The immunohistochemical pattern detected in the aforementioned cells was either cytoplasmatic or membranous.

Immunohistochemistry results for SI-NECs are shown in Table 2. According to the quantitative evaluation of 89 SI-NEC specimens, 31/89 specimens showed over 50% OR51E1 IR tumor cells, specifically 18/43 of the primary SI-NECs, 7/28 of the mesentery metastases, and 6/18 of the liver metastases (Fig. 1). Cytoplasmic OR51E1 immunostaining pattern was seen in 80% of the IR tumor cells. The strongest signal was clearly detected in the tumor cells facing the fibrovascular stroma. Both partial and complete circumferential OR51E1 membranous immunostaining were detected in 20% of the IR tumor cells at different stages of disease. Perinuclear OR51E1 immunostaining was detected in ∼20% of the tumor cells from patients at different stages of SI-NECs. Representative immunostaining patterns of OR51E1 in SI-NECs are illustrated in Fig. 2. No immunoreactivity was detected either after replacement of the antibody by nonimmune serum or after the neutralization test in SI-NECs. Hepatic cell staining in the evaluated liver metastatic specimen persisted in the neutralization test, suggesting the presence of a nonspecific immunostaining in liver.

Figure 1

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Figure 1

OR51E1 immunoreactivity in SI-NEC specimens at different stages of disease. Numbers of specimens are clearly shown for different proportions of OR51E1 ‘positive’ tumor cells in this figure.

Citation: European Journal of Endocrinology 168, 2; 10.1530/EJE-12-0814

Figure 2

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Figure 2

OR51E1 Immunohistochemistry on 89 SI-NEC specimens. Representative ‘negative’ immunostaining (A) and complete cytoplasmic immunostaining (B1) of OR51E1 are illustrated in primary tumor cells, P. The strongest immunoreactivity was detected in the cells (solid arrows) facing the fibrovascular stroma (hollow arrow) (B2). Partial membranous immunostaining (C) and complete circumferential membranous immunostaining (D) are illustrated in primary tumors (C1 and D1) and mesentery metastasis, M (C2 and D2). Perinuclear localization is depicted in a primary tumor specimen (E, arrows). Liver metastases, L, express OR51E1 (F1). No immunoreactive tumor cells were detected after neutralization with the blocking peptide (F2). Bar=50 μm.

Citation: European Journal of Endocrinology 168, 2; 10.1530/EJE-12-0814

OR51E1 co-localizes with VMAT1 in the majority of EC cells and neoplastic SI-NEC cells

The co-localization studies revealed that 60% of the EC cells recognized by VMAT1, also known as SLC18A1, express OR51E1 (Fig. 3, upper panel, white arrows), whereas 40% of the EC cells are non-IR for OR51E1 (Fig. 3, lower panel, cyan arrows). In addition, 55% of the mucosa cells are IR for OR51E1 but not IR for VMAT1. In the tissue specimens from patients at different stages of SI-NEC, 64% of the VMAT1-IR tumor cells express OR51E1, whereas 91% of OR51E1-IR tumor cells co-localize with VMAT1 (Supplementary Figure 1, see section on supplementary data given at the end of this article). OR51E1 intracellular distribution pattern was predominant in ∼86% of the OR51E1-IR tumor cells. Of these, 14% also displayed membranous immunostaining in addition to cytoplasmic subcellular localization (Fig. 3, white arrows), whereas exclusive membranous immunostaining (Fig. 3, cyan arrows) was detected in about 13% of the OR51E1-IR tumor cells.

Figure 3

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Figure 3

VMAT1 and OR51E1 co-localization studies in intestinal mucosa cells lying adjacent to primary SI-NEC. VMAT1 is visualized as green color (FITC), whereas OR51E1 is red (TRITC). Yellow color in the merged images indicates co-localization of the two proteins. The white arrows in the upper panel show one of the OR51E1-IR EC cells (median 60%; range 44–78%; and 93/160 EC cells). The cyan arrows in the lower panel show two of the OR51E1 non-IR EC cells (median 40%; range 22–56%; and 67/160 EC cells). Bar=20 μm.

Citation: European Journal of Endocrinology 168, 2; 10.1530/EJE-12-0814

Discussion

It has been previously reported that OR51E1 is differentially expressed in SI-NECs (13). This finding prompted us to study whether OR51E1-encoded protein can be developed as a novel SI-NEC biomarker. Indeed, in this study, the absence of OR51E1 coding sequence mutations on SI-NEC patients' tumor material was confirmed. Furthermore, we showed that OR51E1 RNA is predominantly expressed in the microdissected tumor compartments of all the SI-NEC specimens, at different stages of disease, compared with the surrounding microenvironment (Fig. 4). Leja et al. (13) have previously suggested similar results. However, the authors did not use specific microdissection of SI-NEC cells vs the cells of adjacent tumor microenvironment to narrow the cell type.

Figure 4

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Figure 4

OR51E1 and VMAT1 double immunofluorescence in a primary SI-NEC specimen. VMAT1 is visualized as green color (FITC) and OR51E1 as red (TRITC). Merged images show VMAT1 and OR51E1 co-localization in yellow. Both membranous and cytoplasmic immunostaining patterns (white arrows) are shown in about 14% (median; range 8–24%; and ∼2726/26 250 cells) of the OR51E1-IR tumor cells. Exclusive membranous immunostaining (cyan arrows) is shown in about 13% (median; range 9–21%; and ∼1508/26 250 cells) of the OR51E1-IR tumor cells. Bar=20 μm.

Citation: European Journal of Endocrinology 168, 2; 10.1530/EJE-12-0814

The immunohistochemical results of OR51E1, on 89 SI-NEC specimens at different stages of disease, revealed predominant OR51E1 protein expression in the tumor cells of the primaries compared with the metastases. It is plausible that in some cases, the metastatic cells lose the ability to produce OR51E1. However, the number of metastases included in this study was too limited to draw certain conclusions. We also detected the strongest OR51E1 positivity in the stroma-facing tumor cell clusters, which may imply the potential role of OR51E1 in the interaction with the surrounding microenvironment (21). About 80% of OR51E1-IR tumor cells showed cytoplasmic immunostaining. Indeed, it is well known that the GPCR cytoplasmic protein portion might be responsible for receptor internalization (22), and to address this concept, it may be necessary to investigate OR51E1 internalization as well. Moreover, OR51E1 was detected in the perinuclear compartment of about 20% of SI-NEC cells. Although this pattern is not common, the ectopically expressed recombinant G-protein-coupled estrogen receptor (GPER) has been found with perinuclear accumulation shortly after ligand addition (23). Thus, OR51E1 perinuclear immunostaining might reflect an unusual process similar to the GPER one, in which delayed sorting and accumulation of OR51E1 may occur in the perinuclear compartment after endocytosis. Moreover, the OR51E1 protein analysis suggested only OR51E1 nonspecific immunostaining on normal hepatic cells. In addition, we also have an unpublished quantitative real-time PCR analysis of normal human hepatic cells that shows absence of OR51E1 mRNA expression. These data suggest undetectable levels or absence of OR51E1 in normal liver, a major metastatic organ for SI-NEC patients, which would otherwise have been a major concern if OR51E1 was developed as therapeutic target.

OR51E1 immunostaining was depicted in intestinal cells. However, the density of immunostaining was about 1 cell/×400 HPF examined mucosa. We therefore proceeded with our investigation on the potential co-localization of OR51E1 and VMAT1, also known as SLC18A1, which is a well-established specific marker of EC cells (24) and SI-NEC cells (25). Our findings indicated that the majority (60%) of EC cells lying in the mucosa adjacent to the tumor express OR51E1. Taking into consideration that the mRNA levels in the normal EC cells were about ten times lower than those in the SI-NEC cells from most patients (13), OR51E1 levels may be under the detection threshold of immunofluorescence studies in the non-IR EC cells. An alternative explanation is that two subpopulations of EC cells defined by the presence or lack of OR51E1 receptor expression may exist. Similar results were obtained from the co-localization studies in the tumor specimens. Thus, we suggest that the neoplastic cells expressing VMAT1 and the receptor may derive from the EC cells that show immunoreactivity for OR51E1, whereas the tumor cells uniquely expressing VMAT1 may originate from the non-IR OR51E1 EC cells. Although OR51E1 function should be studied further, the abundant expression of the OR51E1 in a fraction of SI-NEC cases makes it an attractive candidate target in future diagnostics and/or therapeutics.

In conclusion, we have detected OR51E1 protein expression in tissue specimens from SI-NEC patients at different stages of disease. Furthermore, 35% (31 out of 89) of the specimens expressed OR51E1 in more than 50% of tumor cells. These novel findings support OR51E1 as a potential clinical tissue biomarker for a subpopulation of SI-NECs. Indeed, OR51E1 may be developed as a target for novel diagnostics such as immuno-positron-emission tomography (26) and radioimmunotherapy (27) and/or novel potential therapeutics. We also plan to investigate OR51E1 function in both EC cells and SI-NECs in the near future.

Supplementary data

This is linked to the online version of the paper at http://dx.doi.org/10.1530/EJE-12-0814.

Declaration of interest

The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.

Funding

This work was supported by funding from Erik, Karin och Gösta Selander Stiftelse (2010 and 2012). The funders had no role in study design, data collection, and analysis; decision to publish; or preparation of the manuscript.

Author contribution statement

T Cui, A V Tsolakis, K Öberg, and V Giandomenico conceived and designed the experiments. T Cui, A V Tsolakis, S-C Li, and J L Cunningham performed the experiments. T Cui, A V Tsolakis, J L Cunningham, and V Giandomenico analyzed the data. A V Tsolakis, J L Cunningham, K Öberg, and V Giandomenico contributed reagents/materials, analysis tools. A V Tsolakis, T Cui, J L Cunningham, and V Giandomenico wrote the paper.

Acknowledgements

The authors thank Dr Jan Grawé, Uppsala University, for technical assistance.

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(T Cui and A V Tsolakis contributed equally to this work.)

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    OR51E1 immunoreactivity in SI-NEC specimens at different stages of disease. Numbers of specimens are clearly shown for different proportions of OR51E1 ‘positive’ tumor cells in this figure.

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    OR51E1 Immunohistochemistry on 89 SI-NEC specimens. Representative ‘negative’ immunostaining (A) and complete cytoplasmic immunostaining (B1) of OR51E1 are illustrated in primary tumor cells, P. The strongest immunoreactivity was detected in the cells (solid arrows) facing the fibrovascular stroma (hollow arrow) (B2). Partial membranous immunostaining (C) and complete circumferential membranous immunostaining (D) are illustrated in primary tumors (C1 and D1) and mesentery metastasis, M (C2 and D2). Perinuclear localization is depicted in a primary tumor specimen (E, arrows). Liver metastases, L, express OR51E1 (F1). No immunoreactive tumor cells were detected after neutralization with the blocking peptide (F2). Bar=50 μm.

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    VMAT1 and OR51E1 co-localization studies in intestinal mucosa cells lying adjacent to primary SI-NEC. VMAT1 is visualized as green color (FITC), whereas OR51E1 is red (TRITC). Yellow color in the merged images indicates co-localization of the two proteins. The white arrows in the upper panel show one of the OR51E1-IR EC cells (median 60%; range 44–78%; and 93/160 EC cells). The cyan arrows in the lower panel show two of the OR51E1 non-IR EC cells (median 40%; range 22–56%; and 67/160 EC cells). Bar=20 μm.

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    OR51E1 and VMAT1 double immunofluorescence in a primary SI-NEC specimen. VMAT1 is visualized as green color (FITC) and OR51E1 as red (TRITC). Merged images show VMAT1 and OR51E1 co-localization in yellow. Both membranous and cytoplasmic immunostaining patterns (white arrows) are shown in about 14% (median; range 8–24%; and ∼2726/26 250 cells) of the OR51E1-IR tumor cells. Exclusive membranous immunostaining (cyan arrows) is shown in about 13% (median; range 9–21%; and ∼1508/26 250 cells) of the OR51E1-IR tumor cells. Bar=20 μm.

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