Characteristics of in vitro transformed cells essential for their in vivo survival, selection and metastasizing activity.

Abstract

Two questions were investigated: (1) the dynamics of in vivo selection of tumor cells possessing metastasizing activity (MA) at the time preceding the appearance of visible lung metastases, and (2) the possible connection of in vivo selection of metastatic tumor cells with their susceptibility to H2O2 produced by the cytotoxic oxidative burst of activated neutrophils, monocytes and macrophages. Spontaneously in vitro transformed Syrian hamster embryo cells (strain STHE), never selected in vivo and possessing low MA, were used as parental cells. STHE cells were injected i.v. and, after various time intervals (1 to 21 days), isolated from lungs of injected hamsters. A total of 43 STHE cell variants were isolated from individual animals. Their MA, natural-resistance-depressing (NRD) activity and susceptibility to H2O2 were determined. Selection of metastatic tumor cell variants capable of forming experimental metastasis (EM) begins a few hours after injection. The vast majority of injected cells die and the few cells which survive do not proliferate for a long period. The majority of variants isolated at days 5 to 6 after injection and later possess increased MA, depending on the duration of the in vivo selection, and varying among individual hosts. The ability to form spontaneous metastasis (SM) in the lungs and in other organs appears among cell variants isolated at days 10 to 21 after inoculation. This qualitatively new characteristic of cell variants correlates with the restoration of their proliferative activity and in most cases with their NRD activity. With respect to susceptibility to H2O2, parental STHE cells and their 5 in vivo isolated cell variants, all possessing low MA, were highly susceptible to H2O2 injury; 15 highly-metastatic variants were 10 to 200 times more resistant; and 4 variants obtained from s.c. transplants of parental cells were highly resistant to H2O2. Hamster embryo cells in vitro transformed with SV40 or bovine adenovirus type 3, were susceptible to high doses of H2O2 (14-28 mM) but resistant to lower doses. In contrast, the same cells, once passaged in vivo and the cells of tumors induced in vivo by the same viruses were resistant to high doses of H2O2. In vitro transformed cells and their in vivo selected variants thus appear to differ widely in terms of their susceptibility to H2O2. Extremely rare cell variants which are resistant to H2O2 and apparently present in the original populations of in vitro-transformed cells possess an essential selective advantage in vivo, that is resistance to H2O2 appears necessary for in vivo survival of tumor cells and their growth in s.c. nodules and metastases.