Dynamics of modulation of biochemical programs in cancer cells

Abstract

1. 1.|The interpretation that the enzymic imbalance in cancer cells was due to a reprogramming of gene expression was supported by new evidence on the nature of increased CTP synthetase concentration in hepatoma 3924A. 2. 2.|Evidence was provided that the pattern of gene expression in hepatoma tissue culture cells was subject to an integrated modulation. 3. 3.|When resting hepatoma cells were replated, the number of cancer cells remained stationary for 24 hr and then increased sharply, with cells entering into the log phase at 24 and 48 hr and becoming stationary at 72 and 96 hr. 4. 4.|Incorporation of thymidine into DNA increased 3-fold at 6 hr, reaching a peak 16-fold elevation at 12 hr, declining to a 10-fold rise at 24 hr and returning to resting values at 48 hr. The activity of thymidine kinase doubled in 12 hr and reached a peak of a 9-fold increase at 24 hr, decreasing to 8- and 2.5-fold elevations at 48 and 72 hr. Concurrently, at 6 and 12 hr dihydrothymine dehydrogenase activity dropped to 24 and 19% of resting values and then slowly returned to the values of resting hepatoma cells. As a result of the changes in activities of opposing enzymes, the ratio of thymidine kinase/dihydrothymine dehydrogenase activities increased 14-fold at 24 hr. 5. 5.|The earliest alterations that emerged at 6 hr were the 47-fold rise in incorporation of cytidine and the 20-fold rise in that of deoxycytidine. Concurrently, there was a 12-fold increase in dCTP concentration. The CDP reductase activity increased 6-fold and remained at this level for 48 hr, with about 50% of the activity lost at 72 hr. The concentrations of CTP and UTP increased 6-fold and uridine incorporation 7-fold. 6. 6.|At 12 hr the incorporation of uridine reached a peak with a rise of 24-fold and adenosine incorporation peaked at a 46-fold rise. 7. 7.|At 24 hr peaks in concentrations of CTP, UTP, ATP and GTP were reached with increases of 19-, 22-, 5- and 7.4-fold. The concentrations of dCTP, dTTP, dATP and dGTP also reached peaks with increases of 44-, 16-, 14- and 45-fold. The pools were still high at 48 hr, but returned to the resting stage at 72 hr. 8. 8.|The activities of enzymes of pyrimidine biosynthesis showed minor alterations at 6 hr and reached a peak at 24 hr, with increases for uracil phosphoribosyltransferase, CTP synthetase, orotate phosphoribosyltransferase, OMP decarboxylase, and uridine kinase of 4.4-, 3.8-, 2.6-, 1.9- and 1.5-fold, respectively. The lower the activity was in the resting cells, the more marked was the rise in the proliferative phase. 9. 9.|The pattern displayed revealed an integrated program of changes in activities of opposing key enzymes of thymidine metabolism, in activities of various pyrimidine enzymes, in incorporation of precursors and in the concentration of the nucleotides. 10. 10.|The very marked increase in incorporation of nucleosides occurred without any change or in presence of minor increases in the activities of enzymes involved in activating the nucleosides. The activities of such enzymes (uridine-cytidine kinase, thymidine and deoxycytidylate kinases) are far in excess of those required in metabolism of the nucleosides. 11. 11.|The early rise in transport, uptake and incorporation of nucleoside precursors emphasizes the role of this process in the expression of the neoplastic growth program displayed in cancer cells. This observation should have an impact on the design of chemotherapy.