Most studies of post-operative tumor recurrence show that traumatized mesothelial surfaces are preferred sites for tumor cell adhesion. Recently, disassociated cancer cells inside peritoneal cavities, and proteins specifically expressed in peritoneal metastasis of gastric carcinoma were found to be linked to cancer prognoses. While immunogenetic approaches show great promise in the treatment of peritoneal metastasis of gastric carcinoma [13
], the effects of gastric cancer cells on mesothelial cells are poorly understood.
Study showed that mesothelial cells provided protection against peritoneal metastasis of tumor in intact mesothelia [9
]. Paget et al
proposed a “seed and soil” theory: metastasis only occurs when tumor cells live and grow in a favorable environment [18
]. The peritoneum might be such a favorable environment for scirrhous gastric cancer cells; possibly mesothelial cells prevent cancer cells from infiltrating into submesothelial connective tissue. Masakazu et al.
] showed that adjacent confluent mesothelial cells hindered invasion by cancer cells. In addition, Kiyasu et al.
] reported that, prior to peritoneal implantation of cancer cells, mesothelial cells become hemispherical and exfoliate from the peritoneum. Our hypothesis is that after serosa are exposed, free cancer cells shed from primary gastric cancer sites into the abdominal cavity induce apoptosis in peritoneal mesothelial cells [19
]. As a result, mesothelial cells become hemispherical and exfoliation takes place. Naked areas of submesothelial connective tissue are thus exposed to the peritoneal cavity; this injured peritoneal site becomes a favorable environment for peritoneal metastasis [22
We had previously shown gastric cancer cell supernatant to significantly reduce the viability of mesothelial cells, but normal gastric epithelial cell line GES-1 exerts no effect on mesothelial cells [5
]. Our present study also demonstrates that cultured mesothelial cells become hemispherical, and exfoliation occur when serum-free medium conditioned by gastric cancer cells was added, as shown by contrast phase microscopy. Furthermore, cytoplasmic reduction, nuclear condensation, and formation of extracellular and/or intracellular apoptotic bodies were observed under transmission electron microscope. Apoptosis was quantified by two methods: MTT and flow cytometry. We speculate that free gastric cancer cells in the abdominal cavity induce apoptosis of mesothelial cells and cause exfoliation, eventually leading to metastasis. This may be the mechanism by which cancer cells adhere to submesothelial connective tissue, although the mesothelial cells are still well-organized and confluent. Further studies are needed to characterize SF-CM released from gastric cancer cells.
Gastric cancer cells may induce apoptosis through mitochondria- and death receptor-dependent apoptotic pathways. Gastric cancer cells suppress mesothelial cell growth by inhibiting proliferation through the promotion of caspase-dependent apoptosis. Caspases are cytoplasmic aspartate-specific cysteine proteases, and play important roles in apoptosis [25
]. The death receptor-dependent apoptotic pathway is triggered at the cell surface and requires activation of caspase-8, whereas the mitochondrion-dependent pathway is initiated by the release of mitochondrial cytochrome c into the cytoplasm and requires activation of caspase-9. Subsequently, caspase-8 or −9 can activate caspase-3, which in turn targets and degrades specific and vital cellular proteins, ultimately resulting in nuclear DNA degradation and apoptotic cell death [26
]. Bcl-2, an inhibitor of the mitochondrial apoptosis pathway, exerts its action by blocking proapoptotic counterparts, which in turn prevents the release of cytochrome c and the activation of caspases [27
]. Bax is a death promoter, which is neutralized by heterodimerization with Bcl-2. Bax translocates into the outer mitochondrial membrane followed by leakage of cytochrome c from the mitochondria into the cytosol [28
]. Caspase-9 and caspase-3 are activated sequentially, and these events lead to the breakdown of chromosomal DNA. As there is a significant possibility that gastric cancer cell-mediated apoptosis of mesothelial cells is the result of regulation of Bcl-2 and Bax, identification of their target compounds is necessary.
In this study, we utilized a mouse experimental model of peritoneal sclerosis induced by repeated injections of gastric cancer cell SF-CM. Experimental peritoneal fibrosis induced by repeated intraperitoneal injections of gastric cancer cell SF-CM might not completely mimic peritoneal sclerosis observed in patients (diffusely infiltrating carcinoma or Bormann’s Type VI carcinoma). In fact, pathologic findings of peritoneal carcinomatosis and peritoneal sclerosis are not uniform and various factors are involved. In addition, certain common features are observed during the development of peritoneal sclerosis between gastric cancer cell SF-CM-induced experimental animal models and human patients undergoing peritoneal carcinomatosis. These common histological findings include increased accumulation of interstitial collagens such as type I and III collagen, infiltration of monocytes/macrophages, increase in a-SMA+
myofibroblasts, and vascular density in the peritoneum [7
]. These similarities in alterations of the peritoneal membranes between experimental models and human peritoneal carcinomatosis patients suggest that this is an appropriate model for examining the efficacy of various potential therapeutic reagents for regulating peritoneal carcinomatosis.