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Journal of Clinical Endocrinology & Metabolism, doi:10.1210/jc.2005-0271
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The Journal of Clinical Endocrinology & Metabolism Vol. 90, No. 9 5270-5278
Copyright © 2005 by The Endocrine Society

Osteopontin Is Overexpressed in Human Papillary Thyroid Carcinomas and Enhances Thyroid Carcinoma Cell Invasiveness

Valentina Guarino, Pinuccia Faviana, Giuliana Salvatore, Maria Domenica Castellone, Anna Maria Cirafici, Valentina De Falco, Angela Celetti, Riccardo Giannini, Fulvio Basolo, Rosa Marina Melillo and Massimo Santoro

Dipartimento di Biologia e Patologia Cellulare e Molecolare, Università di Napoli Federico II, Istituto di Endocrinologia ed Oncologia Sperimentale del Consiglio Nazionale delle Richerche (V.G., G.S., M.D.C., A.M.C., V.D.F., A.C., R.M.M., M.S.), Naples, Italy; and Dipartimento di Oncologia, Università di Pisa (P.F., R.G., F.B.), Pisa, Italy

Address all correspondence and requests for reprints to: Dr. Massimo Santoro, Dipartimento di Biologia e Patologia Cellulare e Molecolare, Facoltà di Medicina e Chirurgia, Università di Napoli Federico II, via S. Pansini 5, 80131 Naples, Italy. E-mail: masantor{at}unina.it.


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Discussion
 References
 
Context: The transmembrane glycoprotein CD44v6 is overexpressed in most papillary thyroid carcinomas (PTC). We previously reported that osteopontin (OPN), a secreted glycoprotein that functions as a ligand for CD44v6, is overexpressed in thyrocytes transformed by the RET/PTC oncogene.

Objective: In this study we asked whether OPN is overexpressed in human PTC samples, and whether its expression correlates with clinical and histological features of the tumors. Furthermore, we wanted to establish the functional role of the CD44-OPN axis in thyroid tumorigenesis.

Design: Thyroid samples from 117 patients who had undergone surgical resection of the thyroid gland for benign or malignant lesions were collected. OPN and CD44 expressions were evaluated by immunohistochemistry with specific monoclonal antibodies. OPN expression was correlated with different PTC histological variants, lymph node metastasis, and PTC size.

Results: In this study we show that OPN is overexpressed in human PTCs with respect to normal thyroid tissue, follicular adenomas, and multinodular goiters (P < 0.05). The prevalence and intensity of OPN staining were significantly correlated with the presence of lymph node metastases (P = 0.0091) and tumor size (P = 0.0001). We also show that treatment of human PTC cells with recombinant exogenous OPN stimulated Matrigel invasion and activated the ERK and V-AKT murine thymoma viral oncogene homolog 1/protein kinase B; signaling pathways. Blockage of anti-CD44 antibodies prevented these effects.

Conclusions: Given its prevalence and its correlation with aggressive features of human PTCs, we suggest that OPN might be used as a diagnostic and prognostic marker for these tumors. Furthermore, given the role of the OPN-CD44v6 axis in PTC cells, we suggest that CD44 and/or OPN may be molecular targets for therapeutic intervention in aggressive PTCs.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Discussion
 References
 
THYROID TUMORS ARE the most common malignancies of the endocrine system; their annual incidence is estimated to be 122,000 cases worldwide (1). Papillary thyroid carcinomas (PTC) far outnumber the other morphological subtypes (2). The incidence of PTC has increased worldwide. For instance, there were an estimated 22,000 new cases in the United States in 2004 vs. 10,000 cases in 1980 (1). The past decade has witnessed significant advances in our understanding of thyroid carcinogenesis at the molecular level. Hallmarks of PTC are chromosomal translocations or inversions that cause the recombination of the tyrosine kinase domain of the RET receptor to heterologous genes, thereby generating RET/PTC chimeric oncogenes (3). Similar rearrangements of the NTRKA receptor occur in a smaller fraction of PTC (4). The activating V600E mutation in the BRAF serine/threonine kinase, present in 36–69% of PTC cases, is the most frequent genetic change in PTC (5, 6, 7, 8, 9, 10, 11, 12). Very recently, the oncogenic AKAP9-BRAF fusion has been found in radiation-induced PTC (13). RAS point mutations are infrequent in PTC and are restricted to aggressive subtypes (14) and to the follicular variant of PTC (15). Various lines of evidence indicate that the formation of BRAF and RET/PTC oncogenes is the first step of thyroid carcinogenesis: 1) these oncoproteins recreate the disease in transgenic animals; 2) they are activated in the early stages of tumor development; and 3) radiation has been implicated in the oncogenic activation of both of them (16). Activation of the serine/threonine V-AKT murine thymoma viral oncogene homolog 1/protein kinase B (AKT/PKB) kinase is another common feature of human PTC (17).

Despite the link between these oncogenes and PTC, little is known about the molecular mechanisms that control the establishment and maintenance of the PTC neoplastic phenotype. Using oligonucleotide microarrays, we previously found that osteopontin (OPN) is among the transcripts most strongly induced by RET/PTC in thyroid follicular cells (18). OPN, also known as SPP1 (secreted phosphoprotein 1), was first identified as a noncollagenous bone matrix protein. Subsequently, it was shown that OPN is indeed a cytokine, and that it regulates cell trafficking within the immune system (19, 20). OPN binds to the cell surface receptors {alpha}v- or ß1-containing integrins and CD44 (21). CD44 is a cell surface glycoprotein that can be expressed as a standard receptor (CD44s) and as multiple splice isoforms (CD44v) whose expression is altered during tumor growth and progression. Expression of the v6 variant exon is required for efficient OPN binding (21, 22). Under normal conditions, only CD44s is expressed on the cell surface of nonproliferating thyrocytes, whereas CD44v6 is invariably overexpressed in PTC samples (23, 24, 25, 26). OPN is expressed in numerous human tumors, including colon, breast, prostate, gastric, ovarian, and lung carcinomas. In addition, OPN expression often correlates with a poor prognosis (27).

In our previous work we proposed that RET/PTC signaling triggered the formation of an autocrine axis involving OPN and its receptor, CD44. An intact kinase activity and the integrity of tyrosine 1062 of RET were required for the up-regulation of both OPN and CD44. Furthermore, we showed that addition of exogenous OPN or transduction of OPN through a lentiviral vector in RET/PTC1-expressing rat thyroid cells stimulated mitogenesis, survival, and motility (18). To validate these observations and to assess the role of OPN expression in human thyroid tumors, we collected human PTC tumor samples and studied OPN and CD44 expression. Furthermore, we used human PTC cell lines characterized for the presence of the RET/PTC1 rearrangement or BRAF (V600E) mutation. In this study we show that OPN is consistently overexpressed in human PTC samples, and that OPN-induced CD44 stimulation activates the ERK and AKT/PKB signaling pathways, thereby sustaining Matrigel invasion of human PTC cell lines.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Discussion
 References
 
Tumors

Archival thyroid samples from 117 patients were retrieved from the files of the Department of Oncology of University of Pisa (Pisa, Italy). Informed consent was obtained from the patients, and the study was approved by the institutional review board committee. Tumor size, extrathyroid invasion, node metastasis, associated thyroid lesions, and metastatic deposits were recorded. After surgical resection, tissues were fixed in 10% neutral buffered formalin and embedded in paraffin blocks. Sections (4 µm thick) were stained with hematoxylin and eosin for histological examination. The nuclear and architectural features were evaluated to ensure that the samples fulfilled the diagnostic criteria required for the identification of PTC (enlarged nuclei with fine dusty chromatin, nuclear grooves, single or multiple micro/macronucleoli, and intranuclear inclusions) (28, 29). The final histological diagnoses of the carcinomas were: classic papillary (n = 40), follicular variant PTC (PTC-FV; n = 23), and tall cell variant PTC (n = 8). In addition, 34 normal thyroid samples, seven follicular adenomas, and five multinodular goiters were examined.

Immunohistochemistry

Formalin-fixed and paraffin-embedded 4- to 5-µm-thick tumor sections were deparaffinized, placed in a solution of absolute methanol and 0.3% hydrogen peroxide for 30 min, and treated with blocking serum for 20 min. The slides were incubated overnight with anti-OPN or anti-CD44 monoclonal antibodies, with biotinylated anti-IgG, and finally with premixed avidin-biotin complex (Vectostain ABC kits, Vector Laboratories, Inc., Burlingame, CA). Anti-OPN IgG1 mouse monoclonal (10A16) was obtained from Assay Designs (Ann Arbor, MI), and anti-CD44v6 IgG1 mouse monoclonal (NCL-CD44v6, clone VFF-7) was purchased from Novocastra Laboratories Ltd. (Newcastle upon Tyne, UK). The immune reaction was revealed with 0.06 mmol/liter diaminobenzidine (DAB-Dako, DakoCytomation, Carpinteria, CA) and 2 mmol/liter hydrogen peroxide. The slides were counterstained with hematoxylin. As a negative control, tissue slides were incubated with isotype-matched IgG1 control antibodies. The OPN immunostaining was mostly localized in the cytoplasm. Staining intensity was scored semiquantitatively into different grades on an arbitrary scale from 0–3: grade 0, no detectable immunostaining; 1+, weak staining; 2+ moderate staining; and 3+, strong staining intensity. For each sample, the percentage of positive cells for OPN staining was also evaluated.

Cell lines and plasmids

Human primary cultures of thyroid cells (P5) were obtained from F. Curcio (Dipartimento di Patologia e Medicina Sperimentale e Clinica, Università di Udine, Udine, Italy) and cultured as previously described (30). Human RET/PTC1-positive (TPC1, FB2, BHP2-7, BHP7-13, and BHP10-3) and BRAF V600E-positive (NPA and BCPAP) PTC cell lines were described previously (5, 31). BHP5-16, BHP14-9, and BHP17-10 were shown by direct sequencing to harbor the BRAF V600E mutation at the heterozygous level (Salvatore, G., V. Guarino, T. Nappi, F. Carlomagno, R. M. Melillo, and M. Santoro, unpublished observations). PTC cells were maintained in DMEM supplemented with 10% fetal bovine serum, 2 mM L-glutamine, and 100 U/ml penicillin-streptomycin (Invitrogen Life Technologies, Inc., Paisley, PA).

RNA extraction and RT PCR

Total RNA from the indicated cell cultures and from snap-frozen tissue samples was prepared using the RNeasy Mini Kit (Qiagen, Crawley, UK) and subjected to on-column deoxyribonuclease digestion with the ribonuclease-free deoxyribonuclease set (Qiagen) following the manufacturer’s instructions. Only tissue samples containing more than 70% neoplastic cells were used. The quality of RNA from each sample was verified by electrophoresis through 1% agarose gel. Total RNA (2.5 µg) was denatured, and cDNA was synthesized using the GeneAmp RNA PCR Core Kit system (Applied Biosystems, Foster City, CA) following the manufacturer’s instructions. PCR was amplified using 2.5 µl reverse transcriptase product in a reaction volume of 25 µl with primer pairs specific for the gene studied. To exclude DNA contamination, each PCR was also performed with untranscribed RNA. The levels of the housekeeping ß-actin transcript were used as a control for equal RNA loading. Primers were designed with the Primer 3 program (www.genome.wi.mit.edu/cgi-bin/primer/primer3_www.cgi) and synthesized by MWG (Ebersberg, Germany). Primer sequences were as follows: OPN forward, 5'-AGGAGGAGGCAGAGCACA-3'; OPN reverse, 5'-CTGGTATGGCACAGGTGATG-3'; CD44 (exon 2) forward, 5'-GCTTTCAATAGCACCTTGCC-3'; CD44 (exon v6) reverse, 5'-GTTGCCAAACCACTGTTCCT; ß-actin forward, 5'-TGCGTGACATTAAGGAGAAG-3'; and ß-actin reverse, 5'-GCTCGTAGCTCTTCTCCA-3'.

Each RT-PCR product was loaded on 2% agarose gel, stained with ethidium bromide (0.5 µg/ml), and the corresponding image was saved by the Typhoon 8600 laser scanning system (Amersham Biosciences, Little Chalfont, UK). The density and width of each band were quantified using ImageQuant 5.0 (Amersham Biosciences). OPN expression in the different cell lines was expressed as the fold increase with respect to normal P5 thyroid cells (=1) after normalization for ß-actin expression.

Quantitative (real-time) RT-PCR (Q-RT-PCR) was performed by using the SYBR Green PCR MasterMix (Applied Biosystems) in the iCycler apparatus (Bio-Rad Laboratories, Munich, Germany). Amplification reactions (25 µl final reaction volume) contained 200 nM of each primer, 3 mM MgCl2, 300 µM deoxy-NTPs, 1x SYBR Green PCR buffer, 0.1 U/µl AmpliTaq Gold DNA polymerase, 0.01 U/µl Amp Erase, ribonuclease-free water, and 2 µl cDNA samples. We performed 80 cycles of melting to verify the absence of nonspecific products. In all cases, the melting curve confirmed that a single product was generated. Amplification was monitored by measuring the increase in fluorescence caused by SYBR Green binding to double-stranded DNA. Fluorescent threshold values were measured in triplicate, and fold changes were calculated by the formula: 2–(sample 1 {Delta}Ct – sample 2 {Delta}Ct), where {Delta}Ct is the difference between the amplification fluorescent thresholds of the mRNA of interest and the ß-actin mRNA. Primer sequences were as follows: OPN forward, 5'-ATCCATGTGGTCATGGCTTT-3'; OPN reverse, 5'-GAAGGAGCTGAAGGAGCTGA-3'; ß-actin forward, 5'-TGCGTGACATTAAGGAGAAG-3'; and ß-actin reverse, 5'-GCTCGTAGCTCTTCTCCA-3'.

Protein studies

Immunoblotting experiments were performed according to standard procedures. Briefly, cells were harvested in lysis buffer [50 mM HEPES (pH 7.5), 150 mM NaCl, 10% glycerol, 1% Triton X-100, 1 mM EGTA, 1.5 mM MgCl2, 10 mM sodium fluoride, 10 mM sodium pyrophosphate, 1 mM Na3VO4, 10 µg aprotinin/ml, and 10 µg leupeptin/ml) and clarified by centrifugation at 10,000 x g. For protein extraction from human tissues, samples were snap-frozen and immediately homogenized in lysis buffer in the Mixer Mill apparatus (Qiagen). Protein concentration was estimated with a modified Bradford assay (Bio-Rad Laboratories). Antigens were revealed by an enhanced chemiluminescence detection kit (ECL, Amersham Biosciences). Anti-OPN goat polyclonal antibody (K20) and rabbit polyclonal anti-CD44 (H300) were obtained from Santa Cruz Biotechnology, Inc. (Santa Cruz, CA). Monoclonal anti-{alpha}-tubulin was purchased from Sigma-Aldrich Corp. (St. Louis, MO). Anti-phospho-p44/42 MAPK (ERK) and anti-p44/42 MAPK, anti-phospho-AKT, and anti-AKT antibodies were obtained from New England Biolabs (Beverly, MA). Secondary antimouse and antirabbit antibodies coupled to horseradish peroxidase were purchased from Bio-Rad Laboratories. Where indicated, densitometric analysis of the immunoreactive bands was performed by phosphorimager scanning (Typhoon, Amersham Biosciences) and analyzed using ImageQuant 5.0 software. Protein levels were expressed as fold increases with respect to normal thyroid samples (=1) after normalization for tubulin expression.

ELISA

Thyroid cells (3 x 105) were plated in six-well dishes, allowed to grow to 70% confluence, and then serum-deprived for 24 h. Culture media were centrifuged at 2000 rpm at 4 C to remove detached cells and debris. Attached cells were lysed, and total protein concentration was evaluated by a modified Bradford assay (Bio-Rad Laboratories), as described above. OPN levels in culture supernatants were measured using a quantitative immunoassay ELISA kit (QuantiKine assay, R&D Systems, Inc., Minneapolis, MN) following the manufacturer’s instructions. Triplicate samples were analyzed at 490 nm with an ELISA reader (model 550 microplate reader, Bio-Rad Laboratories). OPN levels, expressed in nanograms per milliliter, were adjusted considering total protein levels of the grown cells.

Flow cytometric analysis

Subconfluent cells were detached from culture dishes with a solution of 0.5 mM EDTA, then washed three times in PBS buffer. After saturation with 1 µg human IgG/105 cells, cells were incubated for 20 min on ice with antibodies specific for human CD44v6 (R&D Systems, Inc.) or isotype control antibody. After incubation, unreacted antibody was removed by washing cells twice in PBS buffer. Cells were then incubated (30 min, 4 C) with 100 µl fluorescein-conjugated goat antimouse IgG/M (Jackson ImmunoResearch Laboratories, Inc., West Grove, PA). Cells resuspended in PBS were analyzed on a FACSCalibur cytofluorometer using CellQuest software (BD Biosciences, San Jose, CA). Analyses were performed in triplicate. In each analysis, a total of 104 events were calculated.

Chemoinvasion

In vitro invasiveness through Matrigel was assayed using Transwell cell culture chambers as described previously (18). Briefly, confluent cell monolayers were harvested with trypsin/EDTA and centrifuged at 800 x g for 10 min. The cell suspension (1 x 105 cells/well) was added to the upper chamber of a prehydrated polycarbonate membrane filter with a pore size of 8 µm (Costar, Cambridge, MA) coated with 35 µg Matrigel (Collaborative Research, Inc., Bedford, MA). The lower chamber was filled with complete medium, and when required, purified recombinant OPN (R&D Systems, Inc.) was added at a concentration of 100 ng/ml. To inhibit Matrigel invasion, cells were preincubated with 5 µg/ml CD44-blocking antibodies (KM81 hybridoma, TIB-241, American Type Culture Collection, Manassas, VA) (32). Alternatively, cells were treated for 12 h with U0126 (10 µM) or wortmannin (100 nM; Upstate Biotechnology, Inc., Charlottesville, VA). Cells were then incubated at 37 C in a humidified incubator in 5% CO2 and 95% air for 24 h. Nonmigrating cells on the upper side of the filter and Matrigel were wiped off, and migrating cells on the reverse side of the filter were stained with 0.1% crystal violet in 20% methanol for 15 min and photographed. The stained cells were lysed in 10% acetic acid. Triplicate samples were analyzed at 570 nm with an ELISA reader (model 550 microplate reader, Bio-Rad Laboratories). The results were expressed as the percentage of migrating cells.

Statistical analysis

Statistical analysis (Statistica, StatSoft, Tulsa, OK) was performed using 2 x 2 tables ({chi}2); differences were significant at P < 0.05.


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Discussion
 References
 
Immunohistochemical determination of OPN expression in PTC

We measured OPN expression by immunohistochemistry with an anti-OPN-specific monoclonal antibody in 117 thyroid samples from patients who had undergone surgical resection of the thyroid gland for benign or malignant lesions. Representative immunohistochemical stainings are shown in Fig. 1Go, and the entire dataset is reported in Table 1Go. OPN was virtually undetectable (<10% of cells) in normal thyroid glands (n = 34), follicular adenomas (n = 7), and multinodular goiters (n = 5). In contrast, most of the PTC samples examined (60 of 71), were positive for OPN expression, and positivity was confined to tumor cells (Table 1Go). As shown in Table 2Go, the prevalence and intensity of OPN staining were significantly correlated with the presence of lymph node metastases (P = 0.0091) and tumor size (P = 0.0001). Furthermore, 85% (34 of 40) of the classic PTC tumors and 100% (eight of eight) of the tall cell variant PTC tumors displayed intense (2+/3+) OPN immunoreactivity in more than 70% of the cells, whereas PTC-FV tumors were characterized by less intense or negative staining. Finally, in accordance with previous data (23, 24, 25, 26), classic PTC (n = 40) were invariably positive also to CD44v6-specific monoclonal antibodies (Fig. 1HGo and data not shown).



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  		FIG. 1. Immunohistochemical detection of OPN and CD44 in thyroid samples. Tissue samples from normal thyroid (A and B), primary PTC (C and D), and a PTC node metastasis (F) were incubated with a mouse monoclonal anti-OPN antibody. PTC samples showed intense immunoreactivity for OPN. In PTC samples, intense immunoreactivity was also seen with CD44v6 antibodies (H). Negative controls were performed in all PTC cases using isotype control antibodies (E and G). Magnification: A, C, E, and H, x100; B, D, F, and G, x200.

 

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  		TABLE 1. OPN positivity in thyroid lesions

 

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  		TABLE 2. Correlation between OPN positivity and clinicopathological features in PTC

 
Immunoblot and PCR analysis of OPN expression in PTC

Protein lysates were harvested from a pool of normal human thyroid tissues and from six classic PTC samples and analyzed by immunoblotting. Densitometric analysis of the blots was performed, and OPN levels were normalized to tubulin. As shown in Fig. 2AGo, the OPN protein (molecular mass, ~65,000) was abundantly expressed in all carcinomas, but was barely detectable in normal tissue. To obtain an additional assessment of OPN up-regulation and to determine whether up-regulation occurred at the transcriptional level, we examined a small sample set using Q-RT-PCR. As shown in Fig. 2BGo, the levels of OPN transcripts were significantly higher (8- to 22-fold) in tumor samples than in normal thyroid tissue.



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  		FIG. 2. OPN up-regulation in PTC samples. A, OPN protein levels were evaluated by immunoblot in PTC samples and in a pool of five normal thyroid samples (NT). Equal amounts of proteins (100 µg) were immunoblotted with anti-OPN polyclonal antibodies. Antitubulin monoclonal antibody was used as a control for equal loading. Densitometric analysis was performed with the Typhoon 8600 laser scanning system and the ImageQuant 5.0 software (Amersham Biosciences), and data are shown in the bar graph. Each column represents the relative fold change with respect to the normal thyroid (NT) sample expression (=1). B, Q-RT-PCR was used to calculate OPN mRNA fold induction in six independent PTC tumor samples with respect to a pool of four normal thyroids. The results are the average of three independent experiments ± SD.

 
To verify these findings and to establish a model system with which to study the role of OPN up-regulation, we analyzed OPN mRNA expression in cultured human thyroid cells. In these experiments we used a primary culture of normal thyroid follicular cells (P5) (30) and a panel of PTC cell lines characterized for the presence of RET/PTC rearrangements (TPC1, FB2, BHP2-7, BHP7-13, and BHP10-3) or the V600E mutation in BRAF (BHP5-16, BHP14-9, BHP17-10, NPA, and BCPAP) (5, 31) (Salvatore, G., unpublished observations). As shown by the RT-PCR experiment, all the PTC cell lines analyzed overexpressed OPN by more than 10-fold with respect to normal thyrocytes; BCPAP cells were the least positive (Fig. 3AGo). Thus, OPN up-regulation correlated with the presence of the two most common genetic alterations in PTC, i.e. the RET/PTC and BRAF mutations (16). To verify that the up-regulated OPN was indeed secreted by PTC cells, we used an ELISA to examine conditioned medium harvested from the various cell lines. PTC cells, but not normal P5 cells, secreted abundant OPN in the culture medium (Fig. 3BGo). Again, BCPAP cells were the least positive.



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  		FIG. 3. OPN up-regulation in cultured PTC cells. A, Semiquantitative RT-PCR (25 cycles) was performed to evaluate OPN mRNA levels in the indicated cell lines; ß-actin mRNA detection was used for normalization. Band intensity was calculated by phosphorimaging and expressed as the fold change with respect to P5 normal thyroid cells (=1) in the lower panel. This figure is representative of three independent experiments. B, OPN protein secretion by PTC cells was evaluated by ELISA. Normal thyroid cells (P5) were used as the negative control. The results of three independent determinations performed in triplicate ± SD are reported.

 
CD44 is an OPN cell surface receptor, and it is frequently overexpressed in neoplastic cells. CD44 pre-mRNA is encoded by 20 exons, which are subject to alternative splicing. The constant 5'-terminal five exons encode the N-terminal extracellular domain of the protein, whereas the constant 3'-terminal five exons encode the transmembrane and the short cytosolic tail. An additional 10 exons (variant v1–v10 exons) are alternatively spliced and encode the extracellular membrane-proximal stem structure (21, 22). Cancer cells often overexpress CD44 variants that differ in the number of v exons. The presence of the v6 exon is required for efficient OPN binding (21, 22). Consequently, we screened PTC cell lines for CD44 expression by RT-PCR using primers designed on exons 2 and v6. As shown in Fig. 4AGo, all cancer cells tested, but not normal cells, contained high levels of CD44v6 mRNA. To enable cells to bind OPN, CD44v6 must be expressed on the cell surface. We therefore used flow cytometry to determine whether CD44v6 was expressed on the surface of PTC cells. As shown in Fig. 4BGo, the TPC1 and BCPAP PTC cell lines featured homogeneous cell membrane CD44v6 expression, whereas the others displayed varying expression levels of CD44v6.



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  		FIG. 4. CD44v6 up-regulation in cultured PTC cells. A, Semiquantitative RT-PCR (25 cycles) was performed to evaluate mRNA levels of the exon v6-containing CD44 variant in the indicated cell lines. Amplimers mapping on exons 2 and v6 were used. The exon 2-v6 primer pair amplified one product of 780 bp, containing exons 2–5 and exon v6, as demonstrated by subsequent Southern hybridization with exon-specific probes (not shown). ß-Actin mRNA detection was used for normalization. This figure is representative of three independent experiments. B, Flow cytometric analysis of cell surface expression of CD44v6 receptor in the indicated PTC cell lines. The shadowed curve is the negative control antibody.

 
OPN activates intracellular signaling and invasiveness of PTC cells

OPN protein secretion and cell surface CD44v6 expression reflected the existence of an autocrine/paracrine OPN-CD44 axis that affected PTC cells. To verify that this axis was functional in human PTC cell lines, we examined cell invasion of Matrigel under basal conditions and in the presence of exogenous recombinant OPN. To this aim, we treated normal P5 and PTC-derived cell lines with exogenous OPN and evaluated the number of migrating cells. As shown in Fig. 5AGo, OPN induced a strong migratory response in all PTC, but not in normal cells. Interestingly, the TPC1 and BCPAP cells, which expressed the highest levels of CD44v6, displayed the best migratory response to OPN. To verify whether OPN was able to induce a biochemical response in PTC cells, we selected BCPAP cells, which express high levels of CD44v6 and relatively low levels of OPN. Cells were stimulated with exogenous recombinant OPN and harvested at different time points. Protein lysates were probed with antiphospho-MAPK (ERK) and antiphospho-AKT/PKB antibodies. As shown in Fig. 5BGo, both p44/42 MAPK and AKT were readily activated in OPN-stimulated cells; they peaked at 5–15 min. It has been previously reported that in immortalized liver carcinoma cells (HepG2), OPN up-regulated plasma membrane CD44v6 protein expression in a concentration- and time-dependent fashion (33). To determine whether this was also the case for thyroid cancer cells, we stimulated BCPAP with OPN and evaluated CD44 expression by Western blot analysis. As shown in Fig. 5CGo, OPN treatment significantly increased CD44 protein levels. Such an up-regulation reasonably occurred at a posttranscriptional level, because it was not detected at the mRNA level (not shown).



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  		FIG. 5. OPN-mediated signaling and Matrigel invasion. A, Matrigel invasion of TPC cells in response to normal culture medium or exogenous recombinant OPN. Cells were incubated for 24 h. Thereafter, filters were fixed and stained. The upper surface was wiped clean, and cells on the lower surface were stained with 0.1% crystal violet. The stained cells were lysed in 10% acetic acid. Invasive ability is expressed as the percentage of migrating cells with respect to the total cell number. Quantification was performed in triplicate samples with an ELISA reader. This figure is representative of three independent experiments. B, Total cell lysates were prepared at various time points after stimulation of BCPAP cells with recombinant OPN. Western blots were probed with the indicated phospho-specific antibodies. Anti-MAPK and anti-AKT were used for normalization. C, BCPAP cells were stimulated with recombinant OPN for 12 h. Total cell lysates were then prepared and subjected to immunoblot with anti-CD44 antibodies. OPN stimulated CD44 up-regulation and sustained MAPK activation, as shown by staining of the same filter with anti-pMAPK antibodies. The filter was stripped and reprobed with anti-MAPK antibodies to show equal protein loading.

 
To evaluate whether the CD44 receptor mediated invasiveness of thyroid cancer cells, we treated CD44-blocked BCPAP cells with exogenous OPN (for 5 or 30 min). As shown in Fig. 6AGo, p44/42 MAPK and AKT activation was attenuated by pretreatment with CD44-blocking antibodies. CD44 blockade also prevented migration through Matrigel (Fig. 6BGo). These observations show that CD44 functions as an OPN-signaling receptor in PTC cells. To investigate whether intracellular signaling was involved in Matrigel invasion mediated by the OPN-CD44 axis, we used U0126, an inhibitor of the MAPK pathway (MEK1/2), and wortmannin, an inhibitor of the phosphatidylinositol 3-kinase (PI3-K)/AKT pathway. As shown in Fig. 6BGo, both compounds sharply inhibited (15 ± 5-fold) Matrigel invasion in parallel with MAPK and AKT blockade (Fig. 6AGo).



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  		FIG. 6. CD44 is involved in OPN-mediated cellular effects. A, Where indicated, cells were preincubated (12 h) with U0126 (10 µM), wortmannin (100 nM), or CD44-blocking antibody (KM81 hybridoma; {alpha} CD44). Total cell lysates were prepared 5 and 30 min after stimulation with the cytokine, as indicated. MAPK and AKT activation was assessed by immunoblot. B, Cells were preincubated (12 h) with CD44-blocking antibodies, U0126, or wortmannin. Matrigel invasion was analyzed as described in Fig. 5Go. When required, the lower chamber of Transwells contained the blocking antibody or the inhibitor. The percentage of migrating cells was quantified in triplicate samples with an ELISA reader.

 

   Discussion
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
References
 
OPN is overexpressed in many tumor types (21, 27). The expression of OPN is induced by various signaling proteins that are constitutively active in tumors: growth factors such as the hepatocyte growth factor (34, 35), oncogenes such as RAS (36) and SRC (37), and tumor promoters such as 12-O-tetradecanoyl phorbol 13-acetate (38).

In this study we show that OPN up-regulation is involved in the invasive phenotype of PTC. Overall, as many as 70% of human PTC are estimated to carry mutations at the level of the RET-RAS-BRAF-MAPK signaling cascade (16). We previously reported high levels of OPN and CD44v6 in follicular cells derived from rat thyroid glands and transformed by the RET/PTC oncogene (18). We now show that OPN and CD44v6 up-regulation is a common feature of PTC cells that express the RET/PTC or BRAF V600E oncogenes. This finding suggests that activation of the OPN-CD44v6 axis is one of the end points of the RET-RAS-BRAF-MAPK oncogenic cascade. This model is consistent with the idea that in other cell types, OPN expression is induced by the RAS oncogene (36, 39) and is dependent on the MAPK cascade (40), and that CD44v6 splicing is under control of the RAS-MAPK pathway (41).

Our findings could also clarify the role played by overexpression of CD44v6 in PTC (22, 23, 24, 25, 26). CD44 is able to activate a wealth of signaling proteins, such as ERK (42), RAC (43), and RHO (44), leading to cell adhesion and migration, angiogenesis (45), and the secretion of cytokines (44) and metalloproteinases (46). Given the finding that OPN is able to induce CD44v6 overexpression (33), it is conceivable that in PTC cells, the RET-RAS-BRAF-MAPK oncogenic cascade triggers OPN and CD44v6 up-regulation; this leads to OPN-CD44v6 binding, thereby further increasing CD44v6 up-regulation and enhancing MAPK and AKT signaling. It is noteworthy that AKT activation is a common feature of aggressive thyroid cancers (17). The foregoing functional autocrine loop may sustain the invasive capability of PTC cells. Although our experiments demonstrate that OPN-CD44v6 binding mediates cellular effects in thyroid carcinoma cells, they do not exclude that other interactions between OPN and integrins (21, 27) and between CD44, hyaluronan (22), and other membrane receptors such as MET and ERBB2 (47, 48) may be involved in the effects exerted by the OPN-CD44 axis in PTC. Intriguingly, both MET (49) and ERBB2 (50) are overexpressed in PTC. Normal and cancer thyroid cells have been reported to express several integrin receptors, among which the fibronectin receptors {alpha}3/ß1, {alpha}v/ß3, and {alpha}v/ß5 (51, 52). Furthermore, immunohistochemistry demonstrated altered expression of {alpha}vß3 in human PTC (52). Using real-time RT-PCR, we found expression of the {alpha}v, ß3, and ß5, but not of ß1, integrin in our PTC cell lines, including BCPAP (Guarino, V., A. Celetti, R. M. Melillo, and M. Santoro, unpublished observations).

In this study we show that OPN up-regulation correlates with aggressive clinicopathological features of PTC. Indeed, the presence of lymph node metastases and tumor size both positively correlated with OPN positivity. Thus, OPN might be a diagnostic and prognostic marker for these tumors. Indeed, OPN, which also occurs in blood, has already proven useful as a tumor marker for ovarian (53) and lung (54) carcinomas. Given the low prevalence of OPN overexpression in PTC-FV, our findings anticipate that OPN detection will have a rather low sensitivity in this particular PTC variant. However, our series of PTC-FV is too small to draw firm conclusions. Furthermore, it should be noted that the PTC-FV were all included in the T1-T2 stages, where the positivity for OPN is less prevalent. The MAPK kinase and PI3-K pathways could be targets for thyroid cancer therapy (16, 55). Our experiments with CD44-blocking antibodies and MAPK kinase and PI3-K inhibitors provide proof of principle that the OPN-CD44v6 pathway may be a molecular target for therapeutic intervention in cases of aggressive PTC.


   Acknowledgments
 
We are grateful to F. Curcio for the P5 primary thyroid culture, and to J. M. Hershman for the BHP cell lines. We are grateful to L. Vitiello and L. Racioppi for FACS analysis. We thank Jean Gilder for text editing.


   Footnotes
 
This work was supported by the Associazione Italiana per la Ricerca sul Cancro, the Progetto Strategico Oncologia of the Consiglio Nazionale delle Richerche/Ministero dell’Istruzione dell’Università e della Ricerca, the Italian Ministero della Salute, the Centro Regionale di Competenza Genomic for Applied Research, and the Naples OncoGEnomic Center). M.D.C. and V.G. were recipients of BioGeM scar.l. fellowships.

First Published Online July 5, 2005

Abbreviations: AKT/PKB, V-AKT murine thymoma viral oncogene homolog 1/protein kinase B; FV, Follicular variant; OPN, osteopontin; PI3-K, phosphatidylinositol 3-kinase; PTC, papillary thyroid carcinoma; Q-RT-PCR, quantitative (real-time) RT-PCR.

Received February 7, 2005.

Accepted June 28, 2005.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 References
 

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