Effect of serum on heat response of synchronized mouse neuroblastoma cells: protection of cell cycle progression, protein synthesis and survival

Radiation and/or hyperthermia sensitivity of human melanoma cells after several days of incubation in media lacking serum or certain serum components

PURPOSE

Complete serum and single growth factors have been reported to protect cells against the effects of hyperthermia. The phenomenon is not very well understood, especially with regard to the relative importance of the various serum components. There is also a need to clarify the possible involvement of changes in proliferation. The influence of serum and growth factors on radiation sensitivity has not been studied in detail. The present communication is an attempt to at least partly fill these gaps. Cells were exposed to X-rays and/or heat after incubation in sufficiently supplemented medium and media lacking certain serum components. Differences in clonogenic survival were recorded and related to the proliferative status under the various conditions.

METHODS AND MATERIALS

Human melanoma cells (MeWo) were used throughout. Cells were incubated for 3 or 6 days (a) in Ham's F12 medium with 20% fetal calf serum; (b) in medium without serum, but supplemented with the following components: insulin, transferrin, Na-selenite, and a lipid-protein complex; (c) to (f) in media lacking one or the other of these components; (g) in unsupplemented medium. Colony formation ability served as a measure of cell survival after exposure to radiation (250 kV X-rays) and/or hyperthermia (heating at 43 degrees C). In parallel, cell proliferation was characterized by the bromodeoxyuridine (BrdU) labeling index and the Ki-67 growth fraction, both determined by two-parameter flow cytometry.

RESULTS AND CONCLUSIONS

Sensitivity was hardly influenced by 3 days of incubation in medium containing no supplement at all, whereas after 6 days under the same conditions survival curves both of irradiated and hyperthermia-treated cells had a strongly reduced shoulder or were considerably steeper. Experiments with media lacking only one or the other serum component, showed that deprivation of insulin, transferrin, and sodium selenite led to no more than a modest increase in cell killing, whereas the effects in medium lacking lipid-protein complex were about the same as in medium containing no supplement at all. Sensitivity changes did not seem to be due to the lack of serum or certain of its components per se, but were very well correlated with changes in proliferation as characterized by BrdU labeling index or the Ki-67 growth fraction.

Cell inactivation and cell cycle inhibition as induced by extreme hypoxia: the possible role of cell cycle arrest as a protection against hypoxia-induced lethal damage

Cycling mammalian cells that are rendered extremely hypoxic (less than 4 ppm O2) tend to accumulate in a pre-DNA-synthesis stage. It is not clear whether or not this is the result of an active regulation by the cells. In the present study we have rendered cells, synchronized by mitotic selection, extremely hypoxic over a relatively long period of time (up to 48 h). We have recorded cell cycle progression during hypoxia as well as cell inactivation depending on where in the cell cycle the cells were located when the hypoxic treatment was started. Three main conclusions are drawn: 1 the cell cycle arrest in late-G1 is complete even during a long-lasting (24 h) hypoxic treatment: 2 while cells in early- and mid-S are completely arrested and quickly inactivated under hypoxic conditions, cells in late-S, G2 and mitosis are able to continue cell cycle progression and divide; 3 whether the cells are located in G2, mitosis or early-G1 at the onset of hypoxia, they were able to survive relatively long-lasting hypoxic treatment. The present results are in favour of the view that the pre-DNA-synthetic arrest induced by extreme hypoxia may function to rescue the cells from severely damaging effects that would appear if the cells were able to initiate DNA synthesis.