Expression and localization of aquaporin-5 in the epithelial ovarian tumors
Introduction
Ovarian carcinoma represents the leading cause of death from gynecologic malignancy [1]. Due to the lack of reliable tumor marker and vague or absent symptoms in the early stages of the diseases, 70% of patients initially present with advanced disease [2]. As ovarian cancer progresses, the peritoneal cavity of patients frequently accumulates malignant ascites fluid. There were two views held on the mechanisms underlying malignant ascites formation. Several authors had suggested that an increase in capillary permeability to osmotically active proteins in the tumor and/or surrounding peritoneal tissues led to excess peritoneal fluid accumulation [3], [4]. However, Feldman and Coates proposed that obliteration of the diaphragmatic lymphatics by tumor cells caused a reduction in absorption of protein normally filtered across peritoneal capillaries, thus resulting in ascites formation [5], [6]. Moreover, Nagy [7], [8] elucidated that vascular permeability factor (VPF) overexpression was an initiating event in the pathogenesis of malignant ascites formation. However, the relationship between the water permeability of ovarian malignant cells and ascites formation is unclear by now.
Changes in water permeability across biological membranes imply the existence of water channels. Identification of molecules of water channels has recently shown that they are members of a new family of membrane proteins termed aquaporins (AQPs) [9]. The family of mammalian AQPs consists of 11 members, AQP0–10, each with a distinct tissue, and it was assumed that alteration in AQP expression or function can be rate-limiting for water transporting certain membranes [9], [10], [11]. AQP5 is a 27-kDa protein that was first cloned from the salivary gland and is known as an exocrine-type water channel with a unique tissue expression [12]. By experiments using Northern blot analysis and in situ hybridization, strong expression of AQP5 mRNA was shown in many exocrine gland tissues, i.e., the salivary gland, eye, lacrimal gland, lung and trachea [12], [13]. AQP5 was thought to play a fundamental role in the water movement for the formation of saliva, tears and other secretions [9]. Recent report had shown that the expression of AQP5 was induced in early-stage disease and maintained through the stages of colon cancer. These findings demonstrate that the expression of AQP5 is associated with an early stage of colorectal cancer development, which suggests that it may be prior to tumorigenesis [14]. However, the distribution and expression of APQ5 in ovarian epithelial tumors and its clinic significance are unclear, which have not been reported until now.
Based upon above observation, we believed that AQP5 play a potential role in ovarian carcinogenesis and ascites formation. To test this hypothesis, we used immunohistochemistry, Western blotting and reverse transcriptase-polymerase chain reaction (RT-PCR) to measure the expression and localization of AQP5 in ovarian epithelial tumors and normal tissue. Our results provide a clear and novel example of AQP5 expression and localization during ovarian carcinogenesis.
Section snippets
Tissue specimens
From February to November 2004, specimen of 65 primary ovarian epithelial tumors (15 benign, 15 borderline and 35 malignant) and 13 normal ovary tissue with uterine myoma were collected from patients (age ranged from 17 to 70, with a median at 49 years) who underwent surgical resection of the ovaries at Women's Hospital of Zhejiang University, Hangzhou, China. Information about FIGO stage, histological type, grade and lymph node metastasis was taken from the patients' clinical records. Of 15
Immunohistochemical localization of AQP5
Immunohistochemical study showed that AQP5 protein was mainly localized in epithelial ovarian tumor cells. Different localization of AQP5 was observed among benign, borderline and malignant tumor cells. AQP5 protein was expressed in the basolateral plasma membranes of benign tumor cells, both the apical and basolateral membrane of borderline cells and scattered in the membranes of malignant cells and almost no or weak staining in normal ovarian epithelium (Figs. 1A–D).
Western blotting analysis
The anti-AQP5 antibody
Discussion
Previous studies found that AQP5 was localized in the apical and lateral membrane of acinar cells and secretory cells, with no signal for AQP5 detectable in the basal membrane [15], [16], [17], [18], [19]. Murdiastuti [20] reported that the submandibular gland of low expressers showed similar results as those reported previously. In the high expressers, AQP5 protein was localized not only in the apical and lateral membrane, but also in the basal membrane of acinar cells. In the present study,
Acknowledgments
The authors thank Dr. Xinchao Wang for his critical review of the manuscript and Huaizeng Chen for helpful technical support.
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