TSLC1 and DAL-1 are components of the TSLC cascade, which participates in organizing the actin cytoskeleton and constructing stable cell-to-cell adhesion.(16) It is thought that disturbances in the cell adhesion system are critical factors in the growth, invasion and metastasis of cancer cells.(34) Indeed, apoptosis and inhibition of tumor cell growth can be induced by re-expression of TSLC1 in lung cancer cell lines.(35,36) Moreover, re-expression of DAL-1 also increases cellular adhesion and induces apoptosis in a breast cancer cell line.(37,38)
Recently, it was reported that the genes encoding several of the components of the TSLC cascade are inactivated in several human cancers.(13,16–23) As mutations in TSLC1 and DAL-1 are rare in human cancers, aberrant promoter methylation might be a major mechanism of silencing of these genes.(13,16–23) Aberrant promoter methylation of TSLC1 was detected in 44% of non-small cell lung cancers, 32% of prostate cancers, and 27% of pancreatic cancers, in association with loss or reduced expression.(13,17–22) In contrast, aberrant methylation of DAL-1 has also been observed in 55% of primary NSCLC, 33% of NSCLC cell lines, 9% of small cell lung cancer cell lines, and 45% of renal cell carcinomas, and this methylation was also associated with a loss of, or reduction in DAL-1 expression.(16,21–23) Although to our knowledge there are few reports of DAL-1 alterations in HCC, aberrant promoter methylation of TSLC1 has been detected in 29% of HCC.(13,17) Thus, in the present study we investigated the expression levels of Tslc1 and Dal-1 and their DNA methylation status in rat HCC, and showed reduced expression of Tslc1 due to aberrant methylation in 30% of HCC, but no changes in Dal-1 expression. It has been suggested that inactivation of one of these two genes, TSLC1 and DAL-1, might disrupt cell adhesion and links between the membrane and the cytoskeleton, resulting in gain of invasion and metastasis of cancer cells.(22) Furthermore, aberrant methylation of at least one of these two genes occurred at a statistically significant higher frequency in patients with advanced stage disease compared with patients with early stage disease.(22) Therefore, the present study suggests that alterations in the TSLC cascade might have a role in rat hepatocarcinogenesis caused by DEN. Although promoter methylation is often accompanied by complete loss of the TSLC1 expression in cancer cell lines,(13,16–18,20,22) a low amount of Tslc1 expression was observed in three HCC. It may be derived from contaminated non-cancerous tissues.
As epigenetic silencing of TSLC1 and DAL-1 are frequently detected in human lung cancers, it is suggested that alterations in the TSLC cascade are particularly important for lung carcinogenesis.(16,22) Recently, the authors reported that the expression of Tslc1 is significantly reduced in rat lung cancers induced by nitroso-compounds, and that this reduction is associated with aberrant methylation.(24) Moreover, the authors have also detected reduced expression of Dal-1 due to aberrant methylation in the same tumor series of rats.(39) By contrast, alterations of TSLC cascade genes were relatively less frequent in rat HCC than in rat lung tumors. Therefore, it seems that the biologic significance of the TSLC cascade in carcinogenesis might depend on organ and tumor type in rats as well as in humans.
It is well known that liver tumors can be induced by feeding rats a choline-deficient diet without the need for exposure to any chemical carcinogens.(40) This model has advantages for investigating the mechanisms underlying hepatocarcinogenesis due to endogenous factors.(40) So far, we have revealed the possibility that different genetic pathways underlie DEN-induced and choline-deficient diet-induced hepatocarcinogenesis in rats.(9) Therefore, to better understand the involvement of the TSLC cascade in hepatocarcinogenesis, further studies should investigate the alterations to Tslc1 and Dal-1 in rat HCC induced by a choline-deficient diet.