Control of gene expression in carbohydrate, pyrimidine and DNA metabolism

Summary The patterns of behavior of key enzymes in carbohydrate and nucleic acid metabolism were investigated under conditions involving modulation of gene expression. 1. The changes in hepatic cellularity and DNA content were examined in the liver during post-natal differentiation. The cellularity was 650 million cells per g wet weight in the liver of the newborn rat and it decreased during differentiation to 220 million in the adult. The total liver cellularity was 168 million in newborn rats and it increased 14-fold to reach a cell population of over 2 billion in the adult. The DNA content per g liver wet weight and per total liver underwent a similar order of magnitude of change. Thus, the DNA content per cell remained unaltered during differentiation. It was suggested that the enzymatic and metabolic changes in differentiation entail sequential alterations in the read-out of nuclear genes and in their selective transcription and translation. These processes then determine the phenotypic pattern of metabolism in liver cells in different age groups. 2. During post-natal differentiation the key enzymes of liver carbohydrate metabolism follow a characteristic pattern of behavior that is in good agreement with predictions that can be deduced from the Functional Genic Unit Concept. After birth, the activities of the key gluconeogenic enzymes are high whereas the key glycolytic ones are low. During subsequent development for a period of about 30 to 40 days the key gluconeogenic enzymes decrease and the key glycolytic ones increase to achieve the activity levels observed in the adult rat. 3. The opposing enzymes of gluconeogenesis and glycolysis reach a steady state balance in the adult; however, as it has been shown it is possible to markedly modulate gene expression by altering the endocrine status of the organism. When rats were made alloxan diabetic, the liver glucose-6-phosphatase activity increased and the pyruvate kinase activity decreased. Administration of insulin caused the activity of the gluconeogenic enzyme to decrease and that of the glycolytic one to increase to the normal levels of the liver of the adult rat. Thus, insulin coordinates hepatic metabolism, in part, through causing an antagonistic behavior for the opposing gluconeogenic and glycolytic enzymes. 4. In neoplasia a characteristic behavior pattern is also seen which agrees with the predictions of the Functional Genic Unit Concept. In the spectrum of hepatomas of different growth rates, in parallel with the rise in tumor proliferation rate, the key glycolytic enzymes increased, whereas the key gluconeogenic ones decreased. The ratios of the opposing key enzymes, e.g., hexokinase/glucose-6-phosphatase, correlate closely with hepatoma growth rate. Thus, the expression of genes for the key enzymes of carbohydrate metabolism is linked with the expression of the potential for cell proliferation rate in the different lines of hepatomas. 5. The examination of the pattern of gene expression and its regulation revealed that the behavior of the key gluconeogenic and glycolytic enzymes exhibits an antagonistic pattern during differentiation, under hormonal regulatory conditions and in neoplasia. The behavior of the activities of the overall pathways of gluconeogenesis and glycolysis is in close accord with that of the key enzymes of these pathways under the same conditions of gene regulation. 6. In normal liver in UMP metabolism the pathway of catabolism is more active than the synthetic pathway that produces UMP. In the spectrum of hepatomas it was observed that the key enzymes involved in the synthesis of UMP increased whereas dihydrouracil dehydrogenase and the overall degradative pathway decreased in parallel with the increase in hepatoma growth rate. Thus, gene expression in pyrimidine metabolism is also coordinated in terms of the antagonistic behavior of opposing pathways and the enzymes involved. 7. The activities of the enzymes of pyrimidine and DNA metabolism were recalculated from the literature and expressed in a uniform fashion for comparison in a Table. The activities of enzymes in the synthetic processes were very low, especially those that play a role at strategic parts of the pathway, e.g., DNA polymerase, ribonucleotide reductase, dTMP synthase and dTMP kinase. In contrast, the activities of enzymes that function in the degradation of uridine and thymidine were several magnitudes higher than those of the synthetic enzymes. 8. In hepatic thymidine metabolism the behavior of the activity of the synthetic utilization (TdR to DNA) contrasts with that of the degradative pathway (TdR to CO 2 ). In the newborn rat the degradative pathway is active and during development the activity increases 2-fold to the high levels of the adult. The activity of TdR to DNA is very high in the liver of the newborn rat and during differentiation it decreases to less than 10% of this value in the adult. In the regenerating liver the synthetic pathway is markedly increased and the catabolic pathway is decreased. In differentiation and in regeneration the antagonistic behavior of the two opposing pathways of thymidine is closely reflected in that of the activities of the enzymes involved. These results suggest the operation of a coordinated behavior pattern of gene expression manifesting opposing changes in the activities of antagonistic pathways and enzymes in thymidine metabolism. 9. In the hepatomas the TdR to DNA pathway increased and concurrently the TdR to CO 2 pathway decreased in parallel with tumor growth rate. Consequently, there was a 140,000-fold increase in the ratio of the two pathways in the rapidly growing hepatomas. In the neoplastic cells the behavior of the opposing pathways of thymidine utilization and the enzymes involved exhibits a mirror picture. 10. The pattern of the activities of the opposing pathways in carbohydrate, pyrimidine and DNA metabolism suggests a close linkage in the expression of the replicative potential of the genome with the extent of the progressive imbalance in these metabolic pathways. These observations provide further support for the validity of the Molecular Correlation Concept in the hepatoma system. The applicability of this Concept has also been shown for kidney tumors, mammary neoplasms and other tumors of different growth rates indicating that the approaches of the Molecular Correlation Concept are valid not only for the hepatomas, but also for various other types of neoplastic cells.