Background
Cancer of the endometrium is the most common malignant tumor of the female genital tract and it typically affects postmenopausal women [
1,
2] The prognosis of endometrial cancer is generally good, since the age-adjusted 5-year overall survival is 82% [
3]. Although most patients are diagnosed at an early stage, i.e. disease confined to the uterus, still 20% of the cancers recur after primary treatment. Adjuvant treatment does not prolong the overall survival, maybe because of inadequate patient selection. Therefore, new prognostic markers are needed. Molecular markers in endometrial cancer are still rather poorly defined [
4].
It has been commonly recognized that development of human neoplasia is accompanied by changes in the extracellular matrix (ECM) which is particularly important in regulating tumor dissemination [
5]. The glycosaminoglycan hyaluronic acid/hyaluronan (HA) is a ubiquitous component of the extracellular matrix (ECM).
Hyaluronan is an independent, unfavorable prognostic factor in another gynaecological malignancy, epithelial ovarian cancer [
6], and a number of other malignancies [
7,
8]. Hyaluronan and its receptor CD44 are both involved in the development and progression of endometrial cancer [
9].
Hyaluronan can be produced in mammals by three hyaluronan synthase isoenzymes: HAS1, HAS2 and HAS3 [
10].
HAS mRNA levels often correspond to the rate of hyaluronan synthesis, and are known to influence the content of hyaluronan in transplanted tumors [
11]. Therefore, upregulation of
HAS expression can contribute to the hyaluronan accumulation in tissues, and promote tumor growth and metastasis in experimental animals, in particular when coexpressed with hyaluronidase [
12,
13].
The catabolism of hyaluronan is more complex process [
14]. Hyaluronan in the extracellular matrix can be partially fragmented by hyaluronidase activity or oxygen free radicals, and diffuse away through lymph. Alternatively, hyaluronan can be taken up by adjacent cells and be subject to lysosomal degradation in the tissue of origin [
15]. The rate of hyaluronan catabolism may therefore be contributed by the formation of oxygen free radicals, access to lymph, local uptake by cells, and hyaluronidases.
There are 6 hyaluronidases in the human genome, two of them (
HYAL1 and
HYAL2) are ubiquitous and characterized at protein level [
16].
HYAL1 and
HYAL2 have been shown to inhibit tumor growth
in vivo, and it has been suggested that these two genes have major roles in the microenvironment of tumor cells [
17]. Recent findings have suggested that depending on its concentration,
HYAL1 can function either as a tumor promoter or as a suppressor [
18].
The major transcript of
HYAL3 is enzymatically inactive and appears to have only a supportive role in
HYAL 1 expression [
19].
HYAL 3 knockout mice do not display any evidence of hyaluronan accumulation [
20]. Very little is known about HYAL4, but its expression is limited, and it might be a chondroitinase rather than hyaluronidase [
16,
21]. The expression of the
SPAM1 gene-encoded PH20 hyaluronidase is almost exclusively detected in testis and sperm, and shows activity in higher pH.
In an invasive bladder cancer cell line, blocking of
HYAL1 expression decreases tumor growth, inhibits tumor infiltration and decreases microvessel density [
22]. Increased hyaluronidase expression has also been reported in prostate [
18] and colon cancer[
14], and in breast tumor metastases [
23]. In contrast, recent findings have shown that the expression of
HYAL1 and
HYAL2 genes is significantly decreased in lung and kidney cancer samples [
17]. Also experimental overexpression of
HYAL1 in a rat colon carcinoma cell line inhibits tumor growth and generates necrotic tumors [
11].
We have found that the median concentration of hyaluronan is increased in malignant ovarian tumors without hyaluronidase activation [
24]. In further studies we have shown that significantly decreased
HYAL1 expression correlates with decreased hyaluronidase activity and elevated hyaluronan content of the tumors, while
HAS expression was not as consistently associated to the accumulation of hyaluronan [
25].
In this study we found that in the most common gynaecological malignancy, endometrial cancer, the accumulation of hyaluronan is also associated with decreased expression of hyaluronidase genes. Blocking the accumulation of hyaluronan might offer a new way of fighting against these diseases
Methods
Patients
A total of 35 endometrial tissue specimens from 35 patients were divided into 5 groups: proliferative and secretory endometrium (n = 10), post-menopausal proliferative endometrium (n = 5), complex atypical hyperplasia (n = 4), grade 1 (n = 8) and grade 2+3 (n = 8) endometrioid adenocarcinomas (Table
1). The normal endometrium tissue specimens were obtained from hysterectomies for nonmalignant diseases (e.g. leiomyoma or prolapse of uterus). The malignant tumors of endometrium were staged according to FIGO. The ethical committee of the Kuopio University Hospital has approved the study protocol and patients signed the informed consent.
Table 1
Clinicopathological data of the tissue samples
Normal endometrium | Proliferating | 4 | 42 (42-45) | | | |
| | Secreting | 6 | 48 (41-52) | | | |
Postmenopausal | Proliferating | 5 | 69 (57-75) | | | |
Hyperplastic endometrium | Complex atypical | 4 | 52 (45-68) | | | |
Malignant endometrium | | | | | | |
Endometrioid adenocarcinoma | Grade I | 8 | 66 (46-76) | 4 | 3 | 1 |
| | Grade II | 4 | 70 (63-81) | 2 | 1 | 1 |
| | Grade III | 4 | 62 (41-81) | 1 | 2 | 2 |
All patients | | 35 | | | | |
Histology
Histological typing and grading were done according to the WHO classification [
26,
27]. Grade 2 and 3 cancers were combined into one subgroup.
Tissue samples
The tissue specimens collected in the operation room were prepared and evaluated by an experienced pathologist (KH). All the samples were collected and handled identically. Aliquots of the tissues were 1) placed in RNAlater® (Ambion, Austin, TX) for mRNA analyses; 2) fixed in 10% buffered formalin and embedded in paraffin.
RNA Extraction and cDNA Preparation
Samples were stored at -80°C until RNA preparation. The samples were frozen by liquid nitrogen and pulverized under pressure using a stainless steel cylinder and a piston. Total RNA was isolated using Trizol® Reagent (Invitrogen) according to manufacturer's protocol, quantified spectrophotometrically and its integrity confirmed by agarose electrophoresis, based on the appearance of the 18S and 28S RNA bands. First strand cDNA was synthesized from 2.5 μg of total RNA using High-Capacity cDNA Archive kit (Applied Biosystems, Foster City, CA) according to manufacturer's protocol in a final volume of 50 μl.
Quantitative real-time RT-PCR
Real-time gene expression analysis of all target genes (
HYAL1,
HYAL2,
HAS1-3) was performed using TaqMan
® Gene Expression Assays (Applied Biosystems) according to manufacturer's instructions and as described previously [
25]. The assay numbers for these genes were as follows: Hs00201046_m1 (HYAL1); Hs00186841_m1 (HYAL2); Hs00758053_m1 (HAS1); Hs00193435_m1 (HAS2); Hs00193436_m1 (HAS3); Hs99999909_m1 (HPRT).
The HPRT1 gene we used for normalization was an accurate reference for the quantitative gene expression assays in clinical tumor samples [
28]. Relative gene expression values were calculated as the ratio between the target gene and HPRT1, obtained for each sample from the standard curves.
Staining of HASs
The HAS immunostainings were performed as described previously [
25]. Shortly, antigen retrieval was performed for HAS1-3 staining by boiling for 3 × 5 min in a citrate buffer. Thereafter sections were treated for 5 min with 1% H
2O
2 to block endogenous peroxidise activity and incubated in 1% bovine serum albumin (BSA) in PBS for 30 min to block nonspecific binding. The sections were incubated overnight at 4°C with polyclonal antibodies for HAS1 (2 μg/ml, sc-34021, Santa Cruz Biotechnology, inc., Santa Cruz, CA), HAS2 (2 μg/ml, sc-34067, Santa Cruz) or HAS3 (2 μg/ml sc-34204, Santa Cruz), diluted in 1% BSA. Sections were incubated for 1 hour with biotinylated antigoat antibody (1:1000, Vector Laboratories). The bound antibodies were visualized with the avidin-biotin peroxidase method (1:200, Vectastain Kit, Vector Laboratories), yielding a brown reaction product. The sections were counterstained with Mayer's hematoxylin. The staining intensity of HAS1, HAS2 and HAS3 was graded into three categories in the epithelium: negative (n.d.), weak and moderate, and into two categories in the stroma: negative (n.d.) or weak. The percentage of positive area for each HAS was estimated both in stroma and epithelium.
Staining of Hyaluronan
Deparaffinized 5-μm tissue sections were stained for hyaluronan with our own preparation of biotinylated hyaluronan-binding complex (bHABC) as described in detail previously [
24]. All samples were scored by an observer unaware of the clinical data (M.A.). The intensity of hyaluronan positivity in epithelium and in stroma was graded into three categories: 1 (weak); 2 (moderate); and 3 (strong) and the area percentage of the strongest hyaluronan expression in the whole tumor section was evaluated separately in epithelium and stroma.
Statistical methods
Statistical analyses were carried out using SPSS 16.0 for Windows (SPSS, Chicago, IL). Differences between the patient groups were first analysed by using a non-parametric Kruskal-Wallis test, and when found significant a non-parametric Mann - Whitney U-test was used for further comparisons between the patient groups. Correlations between gene expression data and hyaluronan staining and immunostaining scores were analysed by using the Spearman's correlation test. A Chi-square test was used to analyse the association of hyaluronan staining and immunostaining scores. We considered p-value ≤ 0.01 as statistically significant.
Acknowledgements
We thank Mrs. Helena Kemiläinen, Mrs. Eija Rahunen and Mr. Kari Kotikumpu for expert technical assistance
This work was supported by grants from The Academy of Finland (MT), Finnish Cancer Foundation (RT, V-MK), The Finnish Cancer Institute (MA), The Finnish Medical Foundation (MA) and EVO Funds of the Kuopio University Hospital (TN, MT, V-MK, MA).
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
TN performed the RNA extraction and RT-QPCR analyses, performed statistical analyses and drafted the manuscript. KR analysed the HAS staining and contributed to the manuscript. MT participated in design of the study and helped to draft the manuscript. KH contributed to pathological analysis of the tissue samples and helped to draft the manuscript. RT, RS, V-MK, and SH participated in design of the study and helped to draft the manuscript. MA conceived of the study, and participated in its design and coordination and helped to draft the manuscript.
All authors read and approved the final manuscript.