Background
Endometrial cancer is the most frequent gynaecologic genital malignancy in the western world [
1,
2] and the five-year overall survival rate for patients with advanced stage cancer is about 65% [
3]. Traditional prognostic factors for the disease are histological type, grade, tumor stage, and depth of myometrial invasion. However, even for the patients in the same stage the clinical courses are highly variable. The current diagnostic technology is insufficient to identify endometrial cancer patients with poor prognosis. Since dissemination through lymphatic vessels is the main means of tumor spread, we hypothesized that lymphatic vessel density (LVD) might serve as a prognostic marker for lymph node metastasis and survival.
Lymphangiogenesis has been difficult to study because of the lack of specific lymphatic markers. Recently, this situation has changed with the discovery of lymphangiogenic markers, such as VEGF-C, VEGF-D, VEGFR-3, LYVE-1, PROX1, and podoplanin. Among them, LYVE-1 is a reliable specific marker for lymphatics [
4,
5]. It is a surface endocytic receptor for hyaluronan [
6], which shares 41% homology with the metastasis related CD44 molecule [
7]. CD44 binds to hyaluronic acid (HA), major components of the extracellular matix (ECM) and CD44 is important in tumor progression and metastasis [
8]. In common with CD44, the LYVE-1 molecule binds both soluble and immobilized HA. HA continuously transits through the lymphatic system and is potentially involved in lymph node homing by CD44
+ leukocytes and tumor cells [
9]. However, little is known about the role of LYVE-1 in lymphatic metastasis.
In this study, LYVE-1 staining was used to determine LVD in tissue samples from endometrial carcinoma patients. The findings were analyzed in combination with data regarding lymph node metastasis, lymph vascular space invasion (LVSI), CD44 expression, and other clinicopathological parameters. The potential of intratumoral LVD (I-LVD) and peritumoral LVD (P-LVD) as prognostic factors for lymph node metastasis, progression-free survival and overall survival was investigated.
Methods
The antibody against LYVE-1 was purchased from R&D Systems (USA). Sections were dewaxed and antigen retrieval was carried out by microwaving in retrieval buffer (pH 6.0), three times for four minutes each. Slides were incubated in phosphate buffered saline (PBS) with 5% human serum for 5 minutes. Peroxidase was quenched with methanol and 3% H2O2 for 15 minutes. Then slides were incubated in antibodies to LYVE-1 (monoclonal mouse anti-human antibody 1.25 lg/ml), CD44 (DAKO, Denmark; mouse monoclonal antibody, 1:40). After incubation with the primary antibodies in PBS plus 5% fetal calf serum for 45 minutes and washing with PBS, sections were incubated with a secondary anti-mouse horseradish peroxidase conjugated antibody for 15 minutes and washed in PBS. The color was developed during a 15-minute incubation with diaminobenzidine (DAB) solution (DAKO) and sections were weakly counterstained with haematoxylin. Normal tissue sections from the small intestine were used as positive controls. PBS was substituted for primary antibody as the negative control.
Determination of CD44 levels
Only nuclear staining of cells was considered positive for CD44. The neoplastic cells that were positively stained for CD44 antigen were counted in 10 high power fields (magnification factor of 400) for each tissue section and were scored semi-quantitatively as <10%, between 10% and 70%, or >70% cells positive [
10].
Lymphatic vessel counting
Lymphatic vessel counting was performed in the intratumoral, peritumoral, and normal tissues. As described by Ohno and Weidner [
11,
12], the areas of highest vascularization were chosen at low power (×100) and vessels were counted in three high power (×200) fields. The lymphatic vessel density (LVD) was the mean of the vessel counts obtained in these three fields.
Data analysis
Data are given as mean ± SD. A one-way ANOVA test was used to evaluate the association between I-LVD or P-LVD and clinicopathological parameters. Survival curves were plotted using the method of Kaplan-Meier and compared using the log-rank test by dividing the two groups by their LVD at the mean value to give two similar-sized groups. A multivariate model using Cox stepwise regression analysis was used to assess the effect of tumor variables on survival. Differences were considered statistically significant for P values less than 0.05. Statistical analysis was done using SPSS statistical software (SPSS version 16.0).
Discussion
The study of lymphatics has been facilitated by the identification of the lymphatic marker LYVE-1 [
13‐
17]. LYVE-1 is found in lymphatic endothelium and not in blood vascular endothelium except in the sinusoidal endothelium of liver and spleen where uptake and degradation of HA is known to occur [
17]. There is controversy regarding lymphatics and lymphangiogenesis in the endometrium. There is a report that lymphatic vessels of endometrial adenocarcinomas were located both intra- and peri-tumorally and that vessel density was significantly higher than in normal basalis; moreover, a potential vascular-control feedback loop was indentified [
18]. In contrast, two studies reported an absence of human endometrial lymphatics [
19,
20]. In this study, LYVE-1 positive vessels were identified as lymphatics in endometrial carcinoma tissue. The I-LVD of carcinoma tissue was significantly lower than that of normal tissue, whereas the P-LVD was higher. In our experiment, small, depressed lymph vessels were mostly observed within the carcinoma tissues and the peritumoral lymph vessels were large and dilated. Lymphatic vessels formed anastomoses and possessed frequent blind endings that were occasionally open. Tripp et al. [
21] found inter-connected LYVE-1
+ vessels that went into the deeper dermis in skin. Similarly, a lymphatic network may exist in endometrial tumors in both the peripheral and intratumoral regions. This data contradicts a study by Kouk et al. [
20] that did not find intratumoral lymph vessels or an intratumoral lymphatic network in endometrial carcinomas. It seems clear that lymphatic and vascular systems have numerous connections that allow disseminating cancer cells to pass rapidly from one system to another [
22]. The formation of a lymphatic network within a tumor greatly facilitates tumor growth by draining waste products of metabolism and promoting lymphatic dissemination of tumor cells.
As a tumor grows, the lymphatics inside the tumor are compressed or destroyed and become difficult to detect by immunohistochemistry. This is consistent with our findings that the I-LVD of carcinoma tissues was significantly lower and the P-LVD was significantly higher than that of normal endometrium. Two recent studies in melanoma (using the anti-LYVE-1) confirmed the presence of focal areas exhibiting intratumoral lymphatic proliferation [
23,
24]. In addition, the presence of intratumoral lymphatics in laryngeal carcinoma may assist tumor spread to regional lymph nodes [
25]. However, whether intratumoral lymphangiogenesis is necessary for or enhances lymphatic metastasis is still unclear.
In our study, the P-LVD had the highest correlation with patient survival of the parameter evaluated. The peritumoral lymphatics are likely important for dissemination of endometrial carcinoma cells. In support of this hypothesis, our study demonstrated that high P-LVDs were associated with LVSI, lymph nodes metastasis, and poor likelihood of survival. The results showed that LYVE1 is a useful prognostic tool. The lymphangiogenesis and location of lymphatic vessels relative to a primary tumor may be a determinant of metastatic spread. As reviewed by Achen [
26], the VEGF-C/VEGF-D/VEGFR-3 axis was the best validated signaling system implicated in promoting lymphangiogenesis in solid tumors and the metastatic spread of tumor cells to lymph nodes. These growth factors were also likely candidates for driving lymph node lymphangiogenesis and appear to promote metastatic spread to sentinel lymph nodes and perhaps to more distant sites. Metastasis may also depend on the position of the primary tumor relative to the lymphatic network. P-LVD may be a determinant of metastatic spread as studies in animal models indicate that peritumoral lymphatics are capable of draining fluid and cells from a tumor whereas intratumoral lymphatics are nonfunctional [
27]. It may be that an extensive peritumoral lymphatic network produces high concentrations of VEGF-C and other growth factors that promote angiogenesis and lymphangiogenesis of distant metastases, enhancing further dissemination and growth of cancer. The peritumoral lymphatics may be a target for inhibition of metastasis. Clinically, the patients with high P-LVD should be considered for early chemotherapy and should be re-evaluated often.
In addition to lymphangiogenesis, the interaction between epithelial tumor cells and their surrounding stroma is important in tumor progression and metastasis. As a transmembrane receptor interacting with stromal ECM, CD44 plays an important role in the process through binding to HA. It was reported that with hyperplasia, increasing atypia and adenocarcinoma [
8], the levels of stromal HA, glandular CD44v6, and glandular and stromal CD44s increase. CD44 facilitates breast cancer progression through alterations of tumor cell adhesion characteristics [
28,
29]. CD44 is a mesenchymal stem cell (MSC) marker; the rare epithelial progenitors and MSCs are likely responsible for regenerative capacity that plays a critical role in the development of endometriosis and endometrial cancer. Several versions of CD44v6-specific antibodies have been tested in clinical trials and preliminary results are promising [
30]. It may provide a readily available source of MSCs for cell-based therapies [
31]. The CD44 gene contains at least 20 exons, 10 of which can be alternatively spliced to form a variety of isoforms. Exons 6-15 can be alternatively spliced to generate many CD44 variants (CD44v) mainly expressed on epithelial cells [
32]. CD44v8-10 is overexpressed in various malignant tumors and is considered to be associated with the presence of tumor or with tumor progression. An elevated CD44v8-10-to-standard CD44 ratio in urinary samples may serve as a novel prognostic predictor in patients with urothelial cancer [
33]. CD44v8-v10 can be produced from epithelial cells, but no studies have addressed CD44v8-v10 expression on lymphatics until now. Here we found that CD44 had prognostic significance. Both increased P-LVD and I-LVD were correlated with stronger CD44 staining. Although patients with high levels of CD44 staining did not have significantly reduced overall or progression-free survival in our study, 15 of the 20 patients who developed pelvic local recurrence had strong CD44 staining. Alterations of CD44 expression occurred during local pelvic recurrence. In contrast, the alterations of LYVE1 were associated with the dissemination of the disease. The main physiological functions of LYVE-1 include serving as a receptor for HA and facilitating the transport and metabolism of HA in the extracellular matrix. It has also been shown that LYVE-1 is involved in adherence of tumor cells [
9]. LYVE-1 may promote tumor lymph node metastasis and lymphatic invasion. Based on their similar adhesive functions, LYVE1 and CD44 may both contribute to metastatic spread of cancer cells since HA is more abundant in the surrounding stroma than in the tumor itself. From a clinical point of view, the analysis of LYVE1 and CD44 expression in endometrial carcinomas may help to identify patients with poor prognosis who may benefit from adjuvant therapies.
Conclusions
P-LVD was significantly correlated with lymph node metastases, lymph vascular space invasion (LVSI), tumor stage, and CD44 expression in endometrial carcinoma samples. The P-LVD was an independent risk factor for progression-free survival and overall survival. Therefore, P-LVD may serve as a prognostic factor in endometrial carcinoma and the peritumoral lymphatics might play an important role in lymphatic vessel metastasis.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
YG was the guarantor of integrity of the entire study and designed the experiment, carried out the immunohistochemical method studies and drafted the manuscript. ZL participated in literature research, data analysis and manuscript editing. FG participated in the design of the study and performed the statistical analysis. X.Y-M carried out the immunohistochemical method studies and participated in manuscript preparation. All authors read and approved the final manuscript.