Growth characteristics of anaerobically treated early and late S-period of Ehrlich ascites tumor cells after reaeration

The retinoblastoma protein-associated cell cycle arrest in S-phase under moderate hypoxia is disrupted in cells expressing HPV18 E7 oncoprotein

We have studied the role of the oxygen-dependent pyrimidine metabolism in the regulation of cell cycle progression under moderate hypoxia in human cell lines containing functional (T-47D) or non-functional (NHIK 3025, SAOS-2) retinoblastoma gene product (pRB). Under aerobic conditions, pRB exerts its growth-regulatory effects during early G1 phase of the cell cycle, when all pRB present has been assumed to be in the underphosphorylated form and bound in the nucleus. We demonstrate that pRB is dephosphorylated and re-bound in the nucleus in approximately 90% of T-47D cells located in S and G2 phases under moderately hypoxic conditions. Under these conditions, no T-47D cells entered S-phase, and no progression through S-phase was observed. Progression of cells through G2 and mitosis seems independent of their functional pRB status. The p21WAF1/CIP1 protein level was significantly reduced by moderate hypoxia in p53-deficient T-47D cells, whereas p16(INK4a) was not expressed in these cells, suggesting that the hypoxia-induced cell cycle arrest is independent of these cyclin-dependent kinase inhibitors. The addition of pyrimidine deoxynucleosides did not release T-47D cells, containing mainly underphosphorylated pRB, from the cell cycle arrest induced by moderate hypoxia. However, NHIK 3025 cells, in which pRB is abrogated by expression of the HPV18 E7 oncoprotein, and SAOS-2 cells, which lack pRB expression, continued cell cycle progression under moderate hypoxia provided that excess pyrimidine deoxynucleosides were present. NHIK 3025 cells express high levels of p16INK4a under both aerobic and moderately hypoxic conditions, suggesting that the inhibitory function of p16(INK4a) would not be manifested in such pRB-deficient cells. Thus, pRB, a key member of the cell cycle checkpoint network, seems to play a major role by inducing growth arrest under moderate hypoxia, and it gradually overrides hypoxia-induced suppression of pyrimidine metabolism in the regulation of progression through S-phase under such conditions.

Hypoxia-induced apoptosis in human cells with normal p53 status and function, without any alteration in the nuclear protein level

We have studied hypoxia-induced inactivation of cells from three established human cell lines with different p53 status. Hypoxia was found to induce apoptosis in cells expressing wild-type p53 (MCF-7 cells), but not in cells where p53 is either mutated (T-47D cells), or abrogated by expression of the HPV18 E6 oncoprotein (NHIK 3025 cells). Apoptosis was demonstrated by DNA fragmentation, using agarose gel electrophoresis of DNA and DNA nick end labeling (TUNEL). We demonstrate that extremely hypoxic conditions (< 4 ppm O2) do not cause any change of expression in the p53 protein level in these three cell lines. In addition, the localization of p53 in MCF-7 cells was found exclusively in the nucleus in only some of the cells both under aerobic and hypoxic conditions. Furthermore, no correlation was found between the p53-expression level and whether or not a cell underwent apoptosis. Flow cytometric TUNEL analysis of MCF-7 cells revealed that initiation of apoptosis occurred in all phases of the cell cycle, although predominantly for cells in S phase. Apoptosis was observed only during a limited time window (i.e., approximately 10 to approximately 24 h) after the onset of extreme hypoxia. While 66% of the MCF-7 cells lost their ability to form visible colonies following 15 h exposure to extreme hypoxia, only approximately 28% were induced to apoptosis, suggesting that approximately 38% were inactivated by other death processes. Commitment to apoptotic cell death was observed in MCF-7 cells even for oxygen concentrations as high as 5000 ppm. Our present results indicate that the p53 status in these three tumor cell lines does not have any major influence on cell's survival following exposure to extremely hypoxic conditions, whereas following moderate hypoxia, cells expressing functional p53 enhanced their susceptibility to cell death. Taken together, although these results suggest that functional p53 might play a role in the induction of apoptosis during hypoxia, other factors seem to be equally important.

Regulation of cell proliferation under extreme and moderate hypoxia: the role of pyrimidine (deoxy)nucleotides

In the present study we have used flow cytometric DNA measurements on synchronised human NHIK 3025 cells to measure cell cycle progression under various conditions of reduced oxygenation. Our data indicate that addition of 0.1 mM deoxycytidine or uridine has no effect on the oxygen-dependent arrest in late G1 or on the inhibition of cell proliferation through S-phase under extremely hypoxic conditions. Following reoxygenation of cells exposed to extremely hypoxic conditions in G2 initiation of DNA synthesis in the subsequent cell cycle is delayed by several hours. This G2-induced delay is completely abolished for approximately 60% of the cell population by addition of deoxycytidine to hypoxic G2 cells. This finding supports our previous proposal that important steps in the preparation for DNA synthesis occur during G2 of the previous cell cycle, and it indicates that this preparation is connected to the de novo synthesis of pyrimidine deoxynucleotide precursors. The results show that cells are able to enter S-phase in the presence of 100-1,300 p.p.m. (0.01-0.13%) oxygen (here denoted 'moderate hypoxia'), but they are not able to complete DNA synthesis under such conditions. However, the cell cycle inhibition induced under moderate hypoxia is partially reversed in the presence of exogenously added deoxycytidine and uridine, while no such reversal is seen in the presence of purine deoxynucleosides (deoxyadenosine and deoxyguanosine). Thus, both deoxycytidine and uridine could replace reoxygenation under these conditions. These results indicate that the reduction of CDP to dCTP by ribonucleotide reductase, an enzyme which requires oxygen as an essential factor for the formation of tyrosyl radicals for its catalytic activity, does not seem to be the limiting step responsible for the reduced dCTP pool observed under moderate hypoxia. We conclude that the oxygen-dependent catalytic activity of the M2 subunit of ribonucleotide reductase is still intact and functional in NHIK 3025 cells even at oxygen concentration as low as 100 p.p.m. Therefore the cell cycle inhibition observed is probably due to inhibition of the respiratory chain-dependent UMP synthesis at the stage of dihydroorotate dehydrogenase.

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.

Cell cycle delays induced by heavy ion irradiation of synchronous mammalian cells

Cell cycle effects of very high LET particles on synchronous V79 Chinese Hamster cells have been studied in a track segment experiment by means of flow cytometric methods. Cells were irradiated with 10 MeV/u Pb-ions (LET = 13500 keV/micrometers) at an average fluence of 2 particles per cell nucleus, corresponding to a survival level of about 25%. Instantaneous drastic reductions of cell proliferation in all cycle phases have been observed, which affect the cell cycle for at least 50 hours after exposure to heavy ions. These findings are in clear contrast to the results from low LET radiation experiments, where significant delays can only be observed in S-phase and G2M-phase and for comparatively short time intervals of a few hours. Additionally, high LET radiation gives rise to prolonged DNA synthesis bypassing cell division, which leads to cells with DNA content greater than that of G2M-cells.

Polyploidization of G2M phase cells separated from aerobically and anaerobically grown Ehrlich ascites tumor cells

G2-enriched fractions of Ehrlich ascites tumor cells (up to 80%-85% G2 cells) separated from anaerobically and aerobically cultured asynchronous populations by centrifugal elutriation revealed the same growth characteristics after recultivation under standard conditions: a significant proportion of cells with increased DNA (DNA content greater than 4C) emerged. Interruption of DNA synthesis by deprivation of oxygen may account for polyploidization (over-replication) of DNA but other mechanisms must be taken into consideration.