The earliest event in the formation of peritoneal dissemination is considered through the process of the attachment of intraperitoneal free cancer cells to the submesothelial basement membrane, exposed after contraction of mesothelial cells. We studied the mechanisms of the contraction of mesothelial cells using a. highly metastatic sell line (MKN-45-P) to the peritoneum. Four hours after intraperitoneal inoculation of MKN-45-P, mouse mesothelial cells began to contract, and submesothelial basement membrane was widely exposed after 24 h. The same changes developed four hours after i.p. injection of IL-6, TNF-alpha and IL-8, and were most prominently observed in mice treated with IL-8. However, no significant changes were observed after treatment of HGF, EGF and TGF-beta. Furthermore, IL-1 alpha, IL-6, IL-8, TNF and EGF increased the number of intercellular gaps of a human mesothelial cell monolayer, which was incubated on Matrigel coated dishes. Normal mesothelial cells form a contiguous monolayer of closely apposed polygonal cells, each of which had prominent and peripheral bands of F-actin. After incubation with IL-1 alpha, IL-6, IL-8, TNF and EGF, peripheral actin bands became indistinct and the central stress fibers became numerous. However, no significant changes were found in mesothelial cells, which were treated with TGF-beta and HGF. In addition, the number of attached MKN-45-P cells on a mesothelial cell monolayer after treatment of IL-1 alpha (0.1-1 ng/ml), IL-8 (10-100 ng/ml), and TNF-alpha (100 ng/ml) was significantly larger than that of control and TGF-beta significantly reduced the number of attached cells. Concentration of IL-8 in the serum-free medium of MKN-45-P cells was high (3.4 ng/ml), but IL-1 alpha, IL-6, TNF-alpha, TGF-beta, EGF and HGF could not be detected. None of these cytokines were detected in the conditioning medium of human mesothelial cells. Based on these results, mesothelial cell contraction may be mediated by IL-1 alpha, IL-6, IL-8, TNF-alpha, and EGF, and these cytokines may be produced from cancer cells and/or intraperitoneal inflammatory cells. In contrast, TGF-beta have an inhibitory effect on the mesothelial cell contraction and attachment of cancer cells to a mesothelial monolayer. The attachment of free cancer cells on the peritoneum may be controlled with these cytokines.

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