In vitro selection of spontaneously transformed sthe cells resistant to the cytotoxic action of macrophages and of hydrogen peroxide

Resistance of spontaneously transformed Syrian hamster embryo cells and their malignant variants to cytotoxic activity of recombinant tumor necrosis factor-alpha

To study the possible role of the host macrophages in the selection of tumor cells and tumor progression, a series of Syrian hamster tumor cell lines all originating from a single spontaneously transformed Syrian hamster embryo cell line (STHE strain) have been established. These STHE tumor cell variants, selected either in vitro with resident and lipopolysaccharide-activated macrophages or in vivo, differ in tumorigenic and metastatic activity. The selected malignant STHE cells become resistant to cytotoxic activity of activated peritoneal macrophages and of exogenous hydrogen peroxide (H2O2). Since activated macrophages are a known source for both cytotoxic agents H2O2 and tumor necrosis factor (TNF), the purpose of the present study was to define the sensitivity of the STHE tumor cell lines to a direct cytotoxic activity mediated by recombinant TNF-alpha in an attempt to understand the role of the cytokine in in vitro selection of a malignant STHE cells by activated macrophages. The spontaneously transformed STHE cells (selected in vivo and in vitro) as well as the hamster embryo cells transformed in vitro by a tumorigenic Rous sarcoma virus (Schmidt-Ruppin strain) were used as targets. TNF-alpha-sensitive mouse L929 cells were included in the study as a positive control. Sensitivity of actinomycin D-pretreated target cells studied for cytotoxic activity of a recombinant TNF-alpha was examined over 21 h with a crystal violet dye assay. It was found that, in contrast to L929 cells, the spontaneously transformed STHE cells as well as tumorigenic Rous sarcoma virus hamster embryo transformants, were all significantly resistant to the TNF-alpha-mediated cytolysis. This indicates that TNF-alpha is not the single factor responsible in in vitro selection of malignant STHE cell variants by activated macrophages. It appears that H2O2 is involved in the selection of the hamster macrophage-resistant STHE tumor cells.

Extremely high production of thymidine by TNF-susceptible L929 cells

We confirmed with the use of crystal violet bioassay the high susceptibility of mouse L929 cells to the cytotoxic action of recombinant tumor necrosis factor-alpha (TNF). However, when a [3H]thymidine release assay was used for the same purpose, we found that [3H]thymidine uptake by the L929 cells, in contrast to low-malignant TNF-resistant spontaneously transformed Syrian hamster embryo cells of the STHE strain, was significantly reduced (P < 0.001). To investigate the mechanism of the low incorporation of [3H]thymidine in L929 cells the culture media from intact L929 cells was used for competition experiments with [3H]thymidine incorporation in the STHE target cells. The undiluted supernatant from L929 cells significantly (up to 83-97%) reduced [3H]thymidine uptake by the STHE cells. Fifty percent inhibition of [3H]thymidine uptake was achieved at L929 supernatant dilutions up to 1:8 (in 4 h incubation), up to 1:16 (in 20-42 h incubation) and even up to 1:32 (in 42 h incubation). The same high level of inhibition of [3H]thymidine uptake by STHE cells was seen with a commercial specimen of a thymidine (Sigma) at a concentration near 500 ng/ml. Thus, we conclude that a resistance of L929 cells to [3H]thymidine uptake is related to their unusually high production of cold thymidine.