Inhibition by N-(phosphonacetyl)-L-aspartate of Ehrlich ascites tumour growth and glucose transport

PALA enhancement of bromodeoxyuridine incorporation into DNA increases radiation cytotoxicity to human ovarian adenocarcinoma cells

PURPOSE

N-(phosphonacetyl)-L-aspartic acid (PALA) is a transition- state inhibitor of L-aspartate transcarbamylase, which catalyses the biosynthesis of carbamyl-L-aspartate in the de novo pyrimidine biosynthetic pathway. 5-Bromodeoxyuridine (BrdUrd) is known to be a potent radiosensitizer of proliferating cells when it is incorporated into DNA. The experiments described herein were performed to test the hypothesis that depletion of cellular pyrimidine precursors by PALA may increase both the incorporation of BrdUrd into DNA and the sensitivity of these cells to the cytotoxic effect of radiation.

METHODS AND MATERIALS

The effect of PALA concentration and exposure time on the incorporation of BrdUrd into the DNA of exponentially growing BG-1 human ovarian carcinoma cells was determined. BG-1 cells exposed to the most effective PALA + BrdUrd treatment schedule were then irradiated to determine if PALA could enhance the radiosensitization already achieved by pretreatment with BrdUrd alone.

RESULTS

A 72-h exposure to PALA (> or = 25 microM) delayed the growth of human ovarian adenocarcinoma BG-1 cells by 40% compared to that of the untreated control cells. Using a clonogenic assay, the IC50 for a 72-h PALA exposure was approximately 25 microM and the cell killing efficiency was dependent on both the concentration and duration of the exposure. A 72-h exposure to 25 microM PALA produced approximately a 90% decrease in the intracellular uridine-5'-triphosphate (UTP) and cytidine-5'-triphosphate (CTP) levels, but had no effect on the intracellular adenosine-5'-triphosphate (ATP) level. This decrease in the UTP and CTP pools promoted a fivefold increase in the incorporation of [3H]BrdUrd into the DNA of BG-1 cells. The most effective treatment schedule involved a 72-h time course, consisting of a 48-h pretreatment with PALA alone, followed by an additional 24-h treatment with both PALA and BrdUrd. The two agent treatments, PALA (25 microM) + BrdUrd (16 microM), PALA (25 microM) + radiation (6 Gy), and BrdUrd (16 microM) + radiation (6 Gy) produced a 2.1-, 7.4-, and 13.2-fold increase in cytotoxicity, respectively, over that expected if the interaction between the two agents was independent and additive. The most effective three-agent treatment schedule consisting of PALA, BrdUrd, and radiation resulted in a greater than 30-fold increase in cytotoxicity over that expected if the interactions and the three agents were additive (p < 0.05).

CONCLUSIONS

These data indicate that PALA alone enhances radiation cytotoxicity and further enhances the radiosensitization already achieved with the halogenated pyrimidines. These effects could be clinically beneficial.

Human tumour necrosis factor-alpha inhibits glucose transport in cultured Ehrlich ascites tumour cells

Human recombinant tumour necrosis factor-alpha (rhTNF-alpha) arrested the growth of Ehrlich ascites tumour (EAT) cells in vitro. It suppressed cellular glucose uptake and decreased the membrane density of glucose transporters as measured by glucose-reversible cytochalasin B binding. The glucose transporters' affinity for substrate was also reduced. However, rhTNF-alpha treatment exerted no effect on the phosphoribosyl pyrophosphate level in EAT cells. The role of rhTNF-alpha on the inhibition of glucose transport of tumour cells is discussed.

Intracellular phosphoribosyl diphosphate level and glucose carrier in Ehrlich ascites tumour cells during tumour growth and diabetes in tumour-bearing mice

The ability of Ehrlich ascites tumour (EAT) cells in mice to take up glucose as well as the density of glucose carriers on the cells increased progressively during the course of tumour development. Simultaneously as the rate of uptake rose, the intracellular phosphoribosyl diphosphate (PRPP) levels dropped responding to the decrease of serum glucose. Hyperglycaemia induced in the host by alloxan or streptozotocin administration increased the serum glucose concentrations and intracellular PRPP levels but decreased the density of glucose carriers of the cells, whereas insulin administration reversed this condition. The physiological significance of these observations are discussed.

Prevention by thymidine against toxicity and glucose uptake inhibition of methotrexate on cultured Ehrlich ascites tumour cells

Methotrexate (MTX) arrested cell growth and inhibited 2-deoxy-D-glucose uptake of Ehrlich ascites tumour cells in vitro. The changes of glucose concentrations in the culture media did not affect the degree of suppression of cell growth by MTX. Thymidine (dThd) protected against the toxic effect of MTX on the cells. It also prevented the inhibition effect on glucose uptake of the cells by MTX. MTX significantly suppressed the maximal uptake rate of glucose (Vmax) while addition of dThd alleviated the suppression. The half-saturation constant of the uptake (Km) remained constant.

Glucose regulates its own transport in Ehrlich ascites tumour cells

The capacity of Ehrlich ascites tumour cells to take up 2-deoxy-D-glucose and to bind cytochalasin B varies adaptatively with the level of glucose in the plasma or culture medium. The effect of glucose is exerted directly on the cells and does not necessarily require the participation of hormones such as insulin, glucagon or corticosterone, although glucagon and the glucocorticoids, but not insulin, can also increase the number of glucose carrier molecules administered in vitro. Cycloheximide suppresses the acute inductive effect of glucose, suggesting that protein synthesis might be required for the increased transport activity.

Effect of tumour necrosis factor on growth of Ehrlich ascites tumour cells in vitro and in vivo

Tumour necrosis factor (TNF) suppressed the growth of Ehrlich ascites tumour (EAT) cells in vitro and in vivo. Exposure of EAT cells to TNF for as little as 4 h totally abolished the transplantability of the tumour. At the same time, the rate of glucose uptake and extent of leucine incorporation were significantly reduced. Prolonged treatment with TNF resulted in extensive cell lysis, as determined by trypan blue exclusion and by release of radioactivity from [3H]thymidine-labelled EAT cells. The significance of these observations are discussed.