Biochemical commitment to replication in cancer cells

Abstract

The elucidation of the metabolic imbalance that underlies the biochemical commitment to continued replication of cancer cells is essential in the rational targeting of selective anti-cancer chemotherapy. The metabolic strategy of cancer cells entails an ordered pattern of imbalance in the activities of key enzymes and metabolic pathways and in the concentrations of intermediary metabolites, ribonucleotides and deoxyribonucleotides. A meaningful metabolic program was identified which is displayed in neoplastic cells as a result of a reprogramming of gene expression that is manifested in a transformation- and progression-linked fashion. The transition of the non-proliferating, resting cell culture into the proliferating phase provides a particularly suitable biological system of altered gene expression where it is possible to pinpoint the display of enzymic programs that illuminates the biochemical commitment of cancer cells to replication. In the proliferating hepatoma 3924A cells the activity of the pyrimidine synthetic enzymes, ribonucleotide reductase, thymidine kinase, CTP synthetase, uracil phosphoribosyltransferase, OMP decarboxylase, orotate phosphoribosyltransferase and uridine kinase were increased 23-, 13-, 4.9-, 3.1-, 3.5-, 3.6- and 2.1-fold over those of the resting phase cells. Concurrently, the activity of the rate-limiting catabolic enzyme, dihydrothymine dehydrogenase, was decreased to 42%. In consequence, the ratio of the activities of the synthetic to catabolic enzymes of thymidine metabolism increased 31-fold. In purine metabolism, the activity of the rate-limiting enzyme of de novo purine biosynthesis, glutamine PRPP amidotransferase, was increased 1.5-fold and that of the rate-limiting catabolic enzyme, xanthine oxidase, decreased to 75%. In consequence, the ratio of the synthetic to the catabolic enzyme was elevated 2-fold. The activities of the key enzymes of pentose phosphate synthesis and of carbohydrate catabolism rose 1.4- and 1.3-fold, respectively. The extensive rise in the activities of enzymes of pyrimidine and purine biosynthesis indicates that the behavior of these enzymes is more stringently linked with replication than that of other enzymes. The results are in line with earlier ones in this Laboratory on solid tumors indicating an integrated imbalance of the key enzymes of pyrimidine, purine and carbohydrate metabolism that should confer selective advantages to cancer cells. In studying the behavior of purine enzymes that may reveal a relationship with the commitment of cancer cells to replication, the behavior of inosine phosphorylase and guanine deaminase provided such examples. Inosine phosphorylase activity was decreased in all the hepatomas examined irrespective of growth rates of these neoplasms; thus, the decrease of this enzymic activity was transformation-linked. By contrast, the guanine deaminase activity fluctuated in the various lines of the hepatoma spectrum indicating that the behavior of the activity of this enzyme was coincidental to transformation or progression in liver tumors. The relationship of 4 key enzymes with the progression-linked increase in the deoxynucleoside triphosphate pools in hepatomas was examined. The activities of CTP synthetase, thymidine kinase, IMP dehydrogenase and ribonucleotide reductase markedly increased in parallel with tumor growth rates and with the enlargement of the deoxyribonucleotide pools. The applicability of alterations of enzymic programs to 14 different tumor types was shown. There is a shared enzymic program that applies to rodent, avian and human neoplasms. The pattern was observed in carcinomas, sarcomas, leukemias and in human colon tumor xenografts carried in the nude mouse. The biochemical programs displayed were independent from the carcinogenic agent that caused the neoplasms because there was a shared enzymic program in chemically-induced, transplantable rat and mouse tumors, in viral-induced, transplantable avian neoplasms and in primary liver, kidney, lung and colon tumors in human. These observations indicate that strategic aspects of the biochemical commitment of the cancer cells to replication have now been identified. These novel insights should assist in the targeting of selective chemotherapy against cancer cells.