Adhesion of cancer cells to endothelium is considered an essential step in metastasis. However, we have shown in a previous study that when rat colon cancer cells are administered to the vena portae, they get stuck mechanically in liver sinusoids. Then, endothelial cells retract rapidly and cancer cells bind to hepatocytes. We investigated the molecular nature of these interactions between colon cancer cells and hepatocytes. Cancer cells in coculture with hepatocytes became rapidly activated with distinct morphological changes. Cancer cells formed long cytoplasmic protrusions towards hepatocytes in their close vicinity and these protrusions attached to microvilli of hepatocytes. Then, adhering membrane areas were formed by both cell types. Integrin subunits alphav, alpha6 and beta1 but not alphaL, beta2, beta3 and CD44 and CD44v6 were expressed on the cancer cells. In conclusion, colon cancer cells show an active behaviour to bind to hepatocytes, likely involving the integrin subunits alphav, alpha6 and beta1, indicating that early events in colon cancer metastasis in liver are distinctly different than assumed thus far.

Liver sinusoids harbor populations of 2 important types of immunocompetent cells, Kupffer cells (KCs) and natural killer (NK) cells, which are thought to play an important role in controlling hepatic metastasis in the first 24 hr upon arrival of the tumor cells in the liver. We studied the early interaction of KCs, NK and CC531s colon carcinoma cells in a syngeneic rat model by confocal laser scanning microscopy. Results showed a minority of KCs (19% periportal and 7% pericentral) involved in the interaction with 94% of tumor cells and effecting the phagocytosis of 92% of them. NK cell depletion decreased the phagocytosis of tumor cells by KCs by 33% over a period of 24 hr, leaving 35% of the cancer cells free, as compared to 6% in NK-positive rats. Surviving cancer cells were primarily located close to the Glisson capsule, suggesting that metastasis would initiate from this region.

Investigation of apoptosis is pivotal in searching for mechanisms that eliminate colon cancer cells getting trapped in liver sinusoids at the time of surgical removal of the primary tumor. This study focuses on nitric oxide (NO), Fas/FasL and the involvement of interferon-gamma (IFNgamma) in liver sinusoidal endothelial cells (LSECs) and in the colon carcinoma cell line CC531s.

Apoptosis was quantified and visualized in vitro by specific DNA fragmentation, specific staining and electron microscopy. In vivo experiments were also conducted.

In co-cultures of LSECs with CC531s, apoptosis of CC531s was observed only when they were pre-treated with IFNgamma, and was unaffected by blocking the Fas/FasL pathway. However, LSECs continuously produced NO, and apoptosis was inhibited by NO-inhibitors (NMMA and dexamethasone). When IFNgamma-sensitized CC531s were injected into rats, liver weight was lower, in contrast to control conditions where liver weight was higher.

(i) LSECs induce apoptosis in IFNgamma-sensitized CC531s in vitro; (ii) LSECs express FasL; (iii) Fas on CC531s becomes active after IFNgamma-treatment; however, (iv) blocking the Fas/FasL pathway had no effect; (v) apoptosis was inhibited by NO-inhibitors; (vi) the immune system uses this IFNgamma-activated pathway to support LSECs in killing tumor cells.