Tight junctions (TJs) are occluding junctions which act as barriers that control paracellular permeability and regulate trans-epithelial water and solute movement. The structures are composed of integral transmembrane proteins that link adjacent cells, with an effect on cell-cell adhesion and epithelial permeability. It also works as a boundary between the apical and basolateral plasma membrane domains to create the polarisation of endothelial and epithelial cells. These junctions are comprised of sets of continuous networking strands in the outwardly facing cytoplasmic leaflet, with complementary grooves in the inwardly facing extra cytoplasmic leaflet.
Figure 1: Tight junction location between the epithelial cells and paracellular transport.
(Singh, Sharma, and Dhawan, 2010)
Claudin in ovarian cancer Claudin proteins consists of 24 transmembrane proteins exhibiting distinct tissue and are the main component of tight junctions that function as selective barriers. Claudins are expressed in both epithelial and endothelial cells in which they form a complex with occluding (Leech A.O et al 2015). Clauidns play important roles in forming and function of the tight junction in normal epithelial and endothelial cells.
Claudins form the backbone of TJs and interrelate with each other in homotypic and heterotypic fashion. There are two transmembrane domains N-termini and C-termini with both ends of the protein located in the cytoplasm. This preparation results in the formation of two extracellular components. The C-terminal region contains a PDZ-domain binding motif that can possibly interact with PDZ- domain containing proteins such as ZO proteins (Morin, 2007).
Figure 2: The structure of claudin proteins and the extracellular loops. The C-terminal is PDZ binding domain. (Morin, 2007)
Ovarian cancer is a type of cancer that begins in tissues of ovaries. The ovaries are the female reproductive organs which produce eggs (ova) as well as female hormones oestrogen and progesterone. Ovarian cancer is the fourth leading cause of cancer among women in Europe and United States, there are more deaths than in any other cancer due to difficulties in diagnosis and therapy. In UK ovarian cancer is the fifth most common cancer in women, around 7000 cases were recorded in 2012. According to (Globocan 2012) approximately 239000 women were to have been diagnosed with ovarian cancer in 2012, and is the eighth most common cause of cancer death in women worldwide.
The ovaries are made up of three main kinds of cells and each type of cell can develop into a different type of tumour, the three types of ovarian cancers (tumours); epithelial ovarian cancer, Germ cell ovarian cancer and stromal ovarian cancer.
• Epithelial ovarian cancer is the most frequent form of ovarian cancer it is derived from cells on the surface of the ovary.
• Germ cell ovarian cancer are much less common and affects children or teenage girls. It is derived from the egg producing cells within the body of the ovary.
• Stromal cell ovarian cancer starts in the cells that hold the ovaries together and produce female hormones.
Page 2: Molecular mechanism of Ovarian Cancer
Major Ovarian cancer signalling pathways are involved in ovarian cancer cell development. The malignant mechanisms and transformation of ovary cells to epithelial ovarian cancer are a path to identify particular ovarian cancer biomarkers.
Most of these genes are linked to cell signalling pathways in which they play an essential role in cancer cell growth, invasion, survival and metastasis. The signalling pathways which are associated with ovarian cancer are;
• Nuclear factor kappa activated B cells (NF-kB)
• Activator of transcription 3 (Jak-STAT 3)
• Mitogen activated protein kinase (MAPK)
• Proto oncogene tyrosine protein kinase Src
• ErbB activation, the lysophosphatidic acid (LPA)
• Phosphatidylinositol 3 kinase (PI3K)
• Vascular Endothelial Growth Factor (VEGF)
• Epidermal Growth Factor Receptor (EGFR)
(Longuespée et al., 2012).
All of the above signalling pathways occur in molecular mechanism of ovarian cancer and correlate with each other however EGFR mutations are rare and not significantly overexpressed. EGFR is a member of ErbB family of receptor tyrosine kinase which includes HER 2, Erb3 and Erb4.
– Epidermal growth factor signalling has been demonstrated to modulate the expression of the claudins in various cell types. In recent study mechanism of the TJ protein regulation in ovarian cancers was explored by treating both ovarian mucinous and serous cystadenocarcinoma cell lines with EGF.
– It’s activated by various ligands such as EGF and TGF and plays role in enhancing and inhibiting tumour survival.
– Following binding of its ligand EGFR is activated and EGFR tyrosine activity phosphorylates tyrosine residues on the EGFR and other proteins causing their activation and causing a sequence of downstream effectors which lead to increased cell growth.
– This than leads to activation of multiple intercellular signalling pathways such as (MAPK) and (PI3K) AKT pathways, which are involved in survival and cell proliferation (Dutta et al., 2010).
– EGF found to downregulate claudin 3 in mucinous ovarian carcinoma cell lines and claudin 4 in ovarian serous cystadenocarinoma by inducing the degradation of these proteins with also changes in the structure and function of tight junction via the MEK/ERK or P13K/AKT signalling pathway (Ogawa et al., 2012).
Claudin overexpression of claudin proteins in ovarian cancer appears to be the most prominent in epithelial tumours due to the sex cord and stromal tumours which do not express the proteins at significant levels.
In recent study showed that the fundamental molecular mechanisms involved in ovarian cancer, a two pathway model for ovarian tumorigenesis with Type 1 pathway involved in the development of low grade tumours and type 2 pathways in the generation of serious high grade tumours (Li et al., 2009).
The observed molecular features in ovarian cancer, often overexpress of several members of the claudin family of tight junction proteins. It has been suggested that claudin 3 and claudin 4 are commonly elevated in ovarian cancer. In various cancers certain caludins to have been as prognostic markers and in ovarian cancer claudin 3 and 7, levels are inversely interrelated with survival (Li et al., 2009).
The mechanism of the increased Claudin 3 and Claudin4 expression in ovarian cancer is thought to be the result of epigenetic modifications of the Claudin promoter regions in the cancer cells which in resulting increased cell survival, invasion and motility (English and Santin, 2013).
In the study microarray analysis have been used in order to identify the molecular changes that occur in claudin 4 overexpressing cells, as cell expressing claudin 4 exhibit elevated angiogenesis properties as measured by both in vitro and vivo assays.
Claudin gene CLDN16 a member of the claudin family, CLDN16 has been suggested to be elevated in ovarian cancer. It plays important part in the maintenance of cell polarity, cellular arrangement, adhesion, paracellular transport and ionic permeability of various epithelial. (English and Santin, 2013)
Table 1: Claudin expression in gynaecologic cancer
How to detect Claudin in ovarian cancer
Researches on claudin in ovarian cancer has been involved both on a basic and clinical level. In basic research involving cell line or animal experiments such as;
– Detection of expression many claudins
– Exploring the mechanism of gene expression
Serial analysis of Gene Expression (SAGE) is techniques used to analyse ovarian cancer gene expression and CLDN3 / CLDN4 found mostly up regulated in the ovarian cancer.
In several studies CLDN7 have been observed that overexpress in ovarian cancer however this change is less consistent than what is detected for CLDN3 and CLDN4. (Morin, 2007)
Due to the overexpression patterns in ovarian cancer claudin proteins may represent useful ovarian cancer markers to detect, diagnose or therapy at an early stage (Morin, 2007). In
Claudin 3, 4 and 7 are very much expressed in ovarian cancer, understanding the exact role of these proteins in ovarian remain poorly defined as well as other human tumours. (Morin, 2007)