Introduction
Sialyl Tn (sTn) antigen, one of the most well-known cancer-associated glycan structures, which is expressed in a large proportion of gastric, colorectal, ovarian, breast, pancreatic and other adenocarcinoma tissues, and is detected in sera and/or other body fluids of patients with these cancers [
1‐
9]. STn expression is correlated with poor prognosis of patients [
1‐
4,
7,
10], and thus it is used as a serum or tissue marker for these tumors. In gastric cancer, sTn expression is also correlated with peritoneal metastasis [
11,
12], a major cause of recurrence, which is thought to originate from intraperitoneal free tumor cells. Thus, sTn is viewed as a favorable target for detection of intraperitoneal free tumor cells and peritoneal micrometastases, as well as prediction of prognosis of patients, although only recently developed, RT-PCR-based detection methods [
13‐
15] are the most sensitive for this purpose to date.
STn is synthesized by transferring a sialic acid in an α2,6-linkage to an
N-acetylgalactosamine linked to a serine or threonine residue (Tn antigen) by ST6GalNAcI [
16]. Since the expression of sTn is correlated with, or induced by, the expression of ST6GalNAcI in some colorectal, gastric, and breast cancer cell lines [
16‐
18], the emergence of sTn is thought to be partly due to aberrant expression of ST6GalNAcI, with or without concomitant decrease or loss of other glycosyltransferases which compete with ST6GalNAcI for their substrate [
19].
Positive correlations of sTn expression with cancer aggressiveness and poor prognosis of the patients have provoked great interest in the functional analyses on sTn. Induction of sTn in a mouse mammary carcinoma cell line led to morphological changes, impaired proliferation, and decreased migration on fibronectin and hyaluronic acid strata [
20]. A human gastric cancer cell line showed decreased cell–cell aggregation, elevated adhesion and migration activity on ECM proteins, and increased invasive capability in in vitro assay using Matrigel, all of which were blocked by an anti-sTn mAb, HB-STn [
21]. Further, transplantation of an ST6GalNAcI-transfected, sTn-positive human breast cancer cell line into mice showed increased tumorigenicity and tumor size in comparison to mock transfectants [
22]. However, the molecular mechanisms underlying these phenotypic changes of the cells still need to be elucidated.
In addition, the roles of sTn and ST6GalNAcI in peritoneal metastasis of gastric cancer are not well understood. Previously, we established a convenient in vivo monitoring system for micrometastases in nude mice using GFP-tagged human gastric cancer cell lines [
23,
24]. In the present study, we applied this in vivo monitoring system to elucidate the roles of ST6GalNAcI and sTn in gastric cancer peritoneal metastasis. We showed that ectopic expression of ST6GalNAcI in a gastric cancer cell line induced surface expression of sTn and resulted in enhanced intraperitoneal metastasis and tumor growth, and shortened survival time, the latter of which was mitigated by administration of anti-sTn mAb. We also found that the major carrier proteins for sTn in these cells are MUC1 and CD44, suggesting the possible involvement of these sTn-carrying glycoproteins in acquisition of this metastatic phenotype by these gastric cancer cells.
Materials and methods
Cell lines
A GFP-tagged human gastric cancer cell line, GCIY-EGFP [
23], a derivative subline of GCIY which had been originally obtained from the RIKEN Cell Bank (Tsukuba, Japan), and its transfectants were maintained in RPMI1640 (Sigma, St. Louis, MO) supplemented with 10% FBS, 100 units/ml penicillin, and 100 μg/ml streptomycin (Invitrogen, Carlsbad, CA) in a humidified 5% CO
2 incubator at 37°C.
DNA transfection and isolation of sTn-expressing cells
To obtain sTn-expressing GCIY-EGFP cells [
23], cells were transfected with an ST6GalNAc I-expressing vector, pXLS [
16], using Lipofectamine 2,000 reagent (Invitrogen). Transfectants were selected with geneticin (G418; 0.6 mg/ml, Sigma) in RPMI1640 supplemented with 10% FBS, 100 units/ml penicillin, and 100 μg/ml streptomycin. Three days after transfection, cells were harvested and stained for sTn with anti-sTn mAb, B72.3, and allophycocyanin (APC)-labeled goat anti-mouse Ig (BD Biosciences, San Diego, CA). Cells positive for both APC and GFP were collected by a FACS (FACSaria; BD Biosciences), as sTn-expressing cells (GCIY/6L) and maintained in RPMI1640 supplemented with 10% FBS, 100 units/ml penicillin, and 100 μg/ml streptomycin. As a negative control, GCIY-EGFP cells were transfected with pXSS, which expressed a truncated inactive form of ST6GalNAc I [
16], and the transfectants were selected with geneticin in RPMI1640 supplemented with 10% FBS, 100 units/ml penicillin, and 100 μg/ml streptomycin (GCIY/6S).
Mice
Seven- to eight-week-old male athymic nude mice of the KSN strain were purchased from Shizuoka Laboratory Animal Center (Hamamatsu, Japan) and kept under specific pathogen-free conditions at Aich Cancer Center Research Institute.
As described previously [
23], exponentially growing cells were harvested with trypsin/EDTA (Invitrogen), washed and resuspended in Hanks’ balanced salt solution. Cell suspensions containing 4 × 10
6 GCIY/6L or GCIY/6S cells were injected into the peritoneal cavity of the recipient mice. Peritoneal metastasis was externally monitored in living mice under illumination of blue light twice a week until the end point. Some of the recipients were sacrificed at 5 weeks after injection to clearly visualize intraperitoneal distribution of cancer cells and the resected tumor masses were measured.
For antibody treatment, the recipient mice were intraperitoneally injected with 4 × 106 GCIY/6L cells. Then, one milligram of purified anti-sTn mAb B72.3 or control mouse IgG (Sigma) was intraperitoneally injected twice a week for 3 weeks (total 6 times) starting from 2 days after injection of tumor cells.
All animal experiments were conducted according to the ethical guidelines for animal experiments of the institutions which met the standard as defined by the UKCCR (UK Co-ordinating Committee on Cancer Research) guidelines and the safety guidelines for DNA manipulation experiments.
Preparation of anti-sialyl Tn mAb, B72.3
The B72.3 hybridoma cell line [
25,
26] was obtained from ATCC. The hybridoma cells were adapted to serum-free medium (Hybridoma-SFM; Invitrogen) to remove proteins from FBS in the normal media. The culture supernatants were then collected and B72.3 mAb was affinity-purified with protein G or protein A Sepharose beads (GE Healthcare, Buckinghamshire, UK).
Immunoprecipitation
For sTn, purified B72.3 mAb was conjugated covalently to the Sepharose beads with CNBr-activated Sepharose 4B (GE Healthcare). For MUC1 and CD44 pull-downs, anti-MUC1 mAb VU4H5, and anti-CD44 polyclonal antibody H-300 (Santa Cruz Biotechnology, Santa Cruz, CA), were immobilized to beads in the Protein G Sepharose 4 Fast Flow kit (GE Healthcare). Cells in culture were grown to near confluency, and culture supernatants were collected and centrifuged to obtain clear supernatants. Cells on dishes or frozen tissue sections were lysed with RIPA buffer (50 mM Tris–HCl, pH7.3, 150 mM NaCl, 2 mM EDTA, 1% Triton X-100, 0.5% sodium deoxycholate, 0.1%SDS) containing protease inhibitor cocktail (Complete Mini, EDTA-free; Roche Diagnostics GmbH, Mannheim, Germany) and centrifuged to prepare clear lysates. These cell culture supernatants and lysates were mixed with antibody-fixed Sepharose beads and rotated overnight at 4°C. After extensive washing with RIPA buffer, bound proteins were extracted from the precipitates by incubating in SDS sample buffer at 98°C for 5 min and then subjected to SDS-PAGE.
Western blotting
Protein samples were applied to SDS-PAGE and separated proteins were transferred onto polyvinylidene difluoride membranes (Immune-Blot PVDF; Bio-Rad Laboratories, Hercules, CA). After blocking with 5% skim milk in Tris-buffered saline containing 0.1% Tween20 (TBS-T), the blot was incubated with the primary antibody appropriately diluted in TBS-T and then with the HRP-conjugated secondary antibody. Detection of the proteins of interest was done by the Western Lightning Chemiluminescence Reagent Plus (PerkinElmer, Boston, MA).
Protein identification by mass spectrometry
Protein samples were applied to SDS-PAGE and the gels were silver-stained with SilverQuest Silver Staining Kit (Invitrogen). Protein bands were excised and destained, followed by reduction and alkylation. Then the proteins were in-gel digested with sequencing grade modified trypsin (Promega, Madison, WI). The tryptic peptides were extracted, absorbed to the C18 resin packed in a ZipTip (Millipore, Bedford, MA), and eluted with elution buffer (50% acetonitrile, 0.1% trifluoroacetic acid (TFA)). After mixing with equal volume of CHCA (α-cyano-4-hydroxycinnamic acid) matrix solution (10 mg/ml CHCA dissolved in 50% acetonitrile, 0.1% TFA), eluted peptides were spotted onto an Anchorchip sample target plate and analyzed by ultraflex MALDI-TOF/TOF mass spectrometer (Bruker Daltonics, Bremen, Germany). The MS-Fit database search engine of the ProteinProspector web site was used for protein identification.
Clinical samples
Gastric cancer tissues were collected from patients with advanced gastric cancer who had completed a written informed consent at Aichi Cancer Center Hospital after Institutional Review Board approval. All experiments using human tissues were conducted after the approval by the institutional ethics committees at Aichi Cancer Center and National Institute of Advanced Industrial Science and Technology (AIST).
Statistical analysis
Survival period were analyzed by the Kaplan–Meier method and compared using the log-rank test. The statistical significance of difference in metastatic tumor weight between groups was determined by applying Student’s t test. Differences in the incidence between the groups were analyzed by Fisher’s exact test. Significant differences were considered as P < 0.05.
Discussion
In this study, we established ST6GalNAcI transfectant of gastric cancer cells with surface expression of sTn and provided the first demonstration of ST6GalNAcI and sTn involvement in intraperitoneal metastasis in a mouse model. Our results suggest that glycoform alteration of carrier proteins to sTn may be involved in the enhanced peritoneal metastasis observed in our animal model. The mechanisms of this enhancement in metastasis are not entirely clear, but may include accelerated cell proliferation, enhanced migratory activity, altered adhesiveness to target matrices or cells, and/or decreased apoptotic activity. These possibilities are supported by the reports showing that expression of sTn induced phenotypic change of the cells in vitro and in vivo [
18‐
22]. In fact, we observed a larger number of metastatic foci in GCIY/6L cell-transplanted mice, indicating enhancement of cancer cell attachment to the peritoneum.
In addition, we found that a large proportion of sTn is carried by MUC1 and CD44 in GCIY/6L cells, suggesting that glycoform alteration of these molecules or unidentified carrier proteins to sTn may be involved in the enhanced peritoneal metastasis observed in our animal model.
MUC1 is a membrane-bound mucin and enhanced expression is detected in many types of epithelial and non-epithelial tumors [
28]. It has been reported that MUC1 expression level or content is positively correlated with the extent of cancer progression or disease stage [
29,
30]. In addition, overexpression of MUC1 confers tumorigenic potential on the cells [
31‐
33]. Although the molecular basis of MUC1 tumorigenicity is not clearly known, phenotypic changes of MUC1-overexpressing cells are thought to be partly due to steric hindrance to the interaction between cell adhesion molecules by its protruding structure above the cell surface and by its dense negative charges from sialic acids on the termini of a large number of
O-glycans [
34,
35]. In addition, glycosylation patterns of MUC1 are also changed during tumor formation. In breast cancer cell lines, MUC1
O-glycans are mostly core 1-based or Tn antigen, in contrast to normal mammary glands which express core 2-based glycans on MUC1 [
36]. Modification of MUC1 with sTn was reported in gastric and breast cancer cell lines transfected with
ST6GalNAcI [
21,
22], and in pancreatic and colon cancer cell lines in which exogenous FLAG-tagged MUC1 was introduced [
37]. In the former two cases [
21,
22], alteration of cellular characteristics was observed, although molecules other than MUC1 were also modified with sTn. In our report, MUC1 modification with sTn was concomitant with enhancement of peritoneal metastatic activity, suggesting that sTn modification of MUC1 was involved in this process. It is not known how sTn modification of MUC1 causes such a phenotypic change, however, two possible mechanisms may be involved. First, glycoform change of MUC1 may alter conformation of the peptide backbone as previously reported [
38,
39]. Second, structural change in glycan may cause changes in interaction with other molecules such as lectins, i.e. loss of interaction with one lectin and gain of interaction with another, although sTn-recognizing endogenous lectins have not yet been identified to date. Although further studies are required to clarify these questions, sTn-MUC1 may be a target molecule for gastric cancer cell detection.
CD44 is a type I transmembrane glycoprotein involved in cell–cell and cell–matrix interactions and cancer metastasis through interaction with extracellular matrix molecules [
40]. Involvement of CD44 in metastasis was first reported by Gunthert & colleagues [
41]. In the report, a variant of CD44 was expressed almost exclusively in metastatic tissues and cancer cell lines, and the expression of this variant converted a non-metastatic cell line to metastatic. It was also reported that the variant-specific anti-CD44 antibody treatment blocked metastasis [
42]. Although the mechanisms by which CD44 variants affect metastasis are not yet fully understood, interacting molecules such as ERM proteins (ezrin, radixin, moesin), which regulate cell motility and shape [
43] and bind to CD44 cytoplasmic tail in active states [
44,
45], are thought to be involved. Besides alternative splicing, aberrant glycosylation of CD44 also affects cellular phenotypes such as tumorigenicity [
46,
47]. It was reported that CD44 carries sTn in a human breast cancer cell line transfected with
ST6GalNAcI, although the roles of sTn-carrying CD44 in enhanced subcutaneous tumor growth have not yet been elucidated [
22]. As also shown by our results, enhancement of intraperitoneal metastatic activity was parallel with sTn-modification of CD44. These results suggest possible roles of sTn-modified CD44 in tumorigenicity and intraperitoneal metastasis in vivo, as well as those of MUC1. However, sTn-modified CD44 was not detected in gastric cancer tissues in the clinical samples used here. This may be because the glycosylation pathway of CD44 may differ from that of MUC1 in tumor cells in gastric cancer tissues or due to the small sample size in our study, thus, extensive studies with much more gastric cancer cases may be necessary to detect sTn-modified CD44 in human tissues.
In this study, we observed prognostic improvement of recipient mice by repeated injection of anti-sTn mAb (Fig.
3), suggesting anti-tumor and anti-metastatic effects of this mAb. Reduced incidence of jaundice by the antibody-treatment may support this idea. However, no significant difference in total tumor weights was observed between anti-sTn mAb-treated and control IgG-treated mice at necropsy in this experimental condition. The results also suggest that administration dose of the mAb and dosing protocol should be improved. Or the role of sTn in the metastatic process may still remain partial, which leads to the observed limited effects of mAb administration. For example unknown functions of ST6GalNAcI may be involved through mechanisms other than sTn synthesis.
In summary, this experimental model will provide a valuable system to study roles of ST6GalNAcI and sTn in gastric cancer cell properties including metastasis.