Currently, the disruption of claudins expression is regarded as one of the mechanisms responsible for loss of cell adhesion, altered polarity, poor differentiation and increased invasive potential of neoplastic cells
[20–23]. Although the normal ratio of claudins protein has a role in maintaining the structure and function of tight junctions in epithelial cells
, the mechanisms by which claudin expression and destruction of tight junctions induce tumor formation and the effect of these changes on tumor progression have not been studied in detail. It has been postulated that both abnormal up-regulation and down-regulation of claudin proteins would cause the structural and functional disruption of tight junctions, for instance, destruction of tight junction integrity, alteration of intercellular space, and weakening of tight junction cohesion
. In addition, the alteration of claudins protein expression can regulate cellular proliferation, differentiation, survival, and apoptosis through a series of signal transduction pathways, thus, playing an important role in tumorigenesis and tumor metastasis
[26, 27]. Claudin-4 has been shown to activate MMP-2 and claudin-4 expression has been significantly associated with MMP-9 expression, indicating that claudin-mediated increased cancer cell invasion result from activation of MMP proteins
. Phosphorylation of claudin-3 by cAMP-dependent protein kinase and claudin-4 by Ephrin Type-A Receptor 2 can modulate cell-to-cell contact
[29, 30]. Claudin-1 is involved in the β-catenin- T-cell Factor/ Lymphoid Enhancing Factor signaling pathway, and increased expression of claudin-1 may be a component of colorectal tumorigenesis
. It has been reported that Claudin-7 unlike other claudins, has both structural and regulatory functions and may be related to cell differentiation
. Alteration of claudin expression may affect permeability at tight junction, possibly increasing the diffusion of nutrients and other extracellular growth factors to promote cancer cell growth, survival and motility in gastric cancer
. In brief, claudin proteins may participate in regulation of cell proliferation, differentiation and apoptosis directly and indirectly
Recently, claudin-2 has been reported selective up-regulated in colorectal cancer and may be useful as tumor markers and targets for the treatment of colorectal cancer
. Nevertheless, claudin-2 protein expression was significantly down-regulated in tumors compared with corresponding normal breast tissue. Down-regulation of claudin-2 was significantly associated with lymph node metastasis in breast carcinomas by Western blot analysis, and with high clinical stage by immunohistochemistry
. Similarly, claudin-2 were selective down-regulated in gastric cancer compared with corresponding cancer adjacent tissues in our present data. However, the association between claudin-2 protein expression with high clinical stage and lymph node metastasis has not been observed.
We cloned putative mammary cancer suppressor (mes) gene claudin-6 in mammary epithelial cells purified from Cop rat that extremely resistant to mammary cancer reduced by a variety of carcinogen. We have also reported that up-regulation of claudin-6 may induce apoptosis and decrease clone formation, invasiveness and migration of MCF-7 in vitro
. Epigenetic silencing of claudin-6 promoted anchorage-independent growth, cellular invasiveness and transendothelial migration of breast carcinoma cells, accompanied by an increase in matrix metalloproteinase activity
. It is reported that apoptosis signal-regulating kinase 1 is associated with the effect of claudin-6 in breast cancer
. Recent gene expression microarray analyses have indicated that claudin-6 is specifically expressed in atypical teratoid rhomboid tumors (AT/RTs), suggesting a role as a positive diagnostic marker of AT/RTs
. On the contrary, in the present study we found that claudin-6 protein wan expressed at low levels in gastric carcinoma tissues but highly expressed in histologically normal adjacent tissues.
Claudin-11, an oligodendrocyte protein, has been shown to interact with α1-integrin and to regulate the proliferation and migration of oligodendrocytes in culture
. Loss of claudin-11 may be considered to be putative indicators of recurrence and more aggressive behavior of meningiomas
. Accordingly, the overexpression of claudin-11 would decreases the invasive potential of bladder cancer cells in vitro
. However, in our present work the cytoplasmic staining of claudin-11 was strong in gastric cancer tissues and weak in adjacent tissues, reveals that claudin-11 may be a positive diagnostic marker in gastric cancer which was different with claudin-2 and claudin-6. In addition, recent data reveals that claudin-16 and claudin-19 interact and form a tight junction complex generated cation selectivity of the TJ in a synergistic manner
Our present data observes that claudin-2 and claudin-6 were both down-regulated and may be concurrently expressed in gastric cancer, reveals that claudin-2 and claudin-6 may act as synergistic tumor suppressors in gastric cancer. Nevertheless, the correlations between claudin-11 expression with claudin-2 and claudin-6 expression have not been observed.