Biosynthesis of pyrimidines by various organs of the chick during embryogenesis

Modulating pyrimidine ribonucleotide levels for the treatment of cancer

By providing the necessary building blocks for nucleic acids and precursors for cell membrane synthesis, pyrimidine ribonucleotides are essential for cell growth and proliferation. Therefore, depleting pyrimidine ribonucleotide pools has long been considered as a strategy to reduce cancer cell growth. Here, we review the pharmacological approaches that have been employed to modulate pyrimidine ribonucleotide synthesis and degradation routes and discuss their potential use in cancer therapy. New developments in the treatment of myeloid malignancies with inhibitors of pyrimidine ribonucleotide synthesis justify revisiting the literature as well as discussing whether targeting this metabolic pathway can be effective and sufficiently selective for cancer cells to warrant an acceptable therapeutic index in patients.

Regulation of CAD gene expression in mouse fibroblasts during the transition from the resting to the growing state

We have analyzed the steady-state levels of CAD mRNA and ATCase activity in BALB/c 3T3 mouse fibroblasts at quiescence and at various time points following the initiation of serum stimulation. Steady-state levels of CAD mRNA in 3T3 cells following 12 h of serum stimulation increased 10-fold over levels measured at quiescence. In contrast to the observed increase in steady-state levels of CAD mRNA, its rate of transcription increased only 3-fold, suggesting that the expression of CAD gene in these cells is regulated at both the transcriptional and post-transcriptional levels, to a major extent by the latter. These increases in CAD mRNA in serum-stimulated cells were followed by parallel increases in ATCase activity as well. When comparing DNA synthesis [( 3H]thymidine uptake) to the accumulation of CAD mRNA and ATCase activity, it was observed that this accumulation occurred during the mid- to late-G1 phase of the cell cycle. These results suggest that the expression of CAD gene is cell growth dependent and may be a prerequisite to DNA synthesis.

CAD gene expression in serum-starved and serum-stimulated hamster cells

The enzymes in the pathway for de novo pyrimidine biosynthesis, including those associated with the tri-functional CAD protein, show a marked increase in activity in rapidly growing cells and tissues. To learn more about the relationship of this pathway to cellular proliferation, we have studied changes in levels of CAD RNA, rates of CAD protein synthesis, and levels of aspartate transcarbamylase activity in Syrian hamster ts13 cells in response to serum starvation and serum stimulation. The steady-state level of CAD RNA and the synthetic rate of CAD protein decrease by 12- to 15-fold following 24 hr of serum starvation, as compared to exponentially growing cells. Upon serum stimulation of quiescent cells, steady-state CAD RNA levels increase substantially (13-fold), peaking during mid to late G1. Parallel increases occur in the synthesis of new CAD protein and in aspartate transcarbamylase activity. At the same time, the rate of CAD transcription increases only about twofold. These findings indicate that regulation of CAD expression in this system is primarily at the post-transcriptional level. This is in contrast to the transcriptional regulation of CAD previously reported in terminally differentiating HL60 cells (Rao et al., Mol. Cell. Biol. 7, 1961-1966, 1987). While both systems indicate that CAD gene expression is dependent on cell growth, there apparently are alternative mechanisms that can produce the same effect. Evidence is also presented that indicates that the accumulation of CAD transcripts during serum stimulation requires the synthesis of new proteins.

Posttranscriptional regulation of the expression of CAD gene during differentiation of F9 teratocarcinoma cells by induction with retinoic acid and dibutyryl cyclic AMP

We have studied the regulation of expression of the carbamoyl-phosphate synthetase II-aspartate transcarbamylase-dihydroorotase gene in F9 teratocarcinoma cells during their differentiation into parietal endoderm cells by induction with a combination of retinoic acid and dibutyryl cyclic AMP. Steady-state levels of CAD mRNA decreased by 7-fold in F9 cells following 120 h of retinoic acid and dibutyryl cyclic AMP induction as compared to levels in uninduced cells. Conversely, no apparent changes were found in the steady-state levels of beta-actin mRNA between induced and uninduced cells. Despite a 7-fold decrease in the steady-state levels of CAD mRNA, its rate of transcription remained the same between induced and uninduced cells, indicating a role for posttranscriptional mechanisms for its down regulation during retinoic acid- and dibutyryl cyclic AMP-induced differentiation of F9 cells. The cellular growth rate of F9 cells as determined by [3H]thymidine uptake and parallel cell counting decreased markedly during their induction with retinoic acid and dibutyryl cyclic AMP. Taken together, it is apparent that the expression of the CAD gene is cell-growth-dependent and its regulation in this system is at the posttranscriptional level.

Transcriptional regulation of the human CAD gene during myeloid differentiation

CAD codes for a trifunctional protein involved in the catalysis of the first three enzymatic activities in the de novo pyrimidine biosynthetic pathway, namely, carbamoyl-phosphate synthetase II (EC 6.3.5.5), aspartate transcarbamylase (EC 2.1.3.2), and dihydroorotase (EC 3.5.2.3). CAD regulation was studied in the human promyelocyte leukemic line HL-60 as it differentiated into monocytic or granulocytic lineages after induction by 12-O-tetradecanoylphorbol-13-acetate or trans-retinoic acid and dibutyryl cyclic AMP, respectively. Within 12 h of induction of HL-60 cells with either inducer, total cellular levels of CAD RNA essentially disappeared. On the other hand, no apparent decreases in beta-actin RNA levels were seen even 48 h after HL-60 cells were induced, as compared with untreated cells. With nuclear runoff assays, it was clearly shown that the inactivation of CAD gene expression during the induction of HL-60 cells with either inducer was at the transcriptional level. The nuclear runoff experiments also demonstrated that the CAD gene expression was shut down in less than 4 h after induction, well before morphological changes were observed in these cells. At the enzymatic level, the activity of aspartate transcarbamylase, one of the three enzymes encoded by the CAD gene, decreased by about half within 24 h of induction, suggesting a CAD protein half-life of 24 h in differentiating HL-60 cells. Nevertheless, this means that significant levels of aspartate transcarbamylase activity remained even after the cells have stopped proliferating. From the RNA data, it is clear that CAD gene expression is rapidly turned off as promyelocytes begin to terminally differentiate into macrophages and granulocytes. We suspect that the inactivation of the CAD gene in induced HL-60 cells is a consequence of the differentiating cells leaving the cell cycle and becoming nonproliferating.

Transcriptional regulation of the human CAD gene during myeloid differentiation

CAD codes for a trifunctional protein involved in the catalysis of the first three enzymatic activities in the de novo pyrimidine biosynthetic pathway, namely, carbamoyl-phosphate synthetase II (EC 6.3.5.5), aspartate transcarbamylase (EC 2.1.3.2), and dihydroorotase (EC 3.5.2.3). CAD regulation was studied in the human promyelocyte leukemic line HL-60 as it differentiated into monocytic or granulocytic lineages after induction by 12-O-tetradecanoylphorbol-13-acetate or trans-retinoic acid and dibutyryl cyclic AMP, respectively. Within 12 h of induction of HL-60 cells with either inducer, total cellular levels of CAD RNA essentially disappeared. On the other hand, no apparent decreases in beta-actin RNA levels were seen even 48 h after HL-60 cells were induced, as compared with untreated cells. With nuclear runoff assays, it was clearly shown that the inactivation of CAD gene expression during the induction of HL-60 cells with either inducer was at the transcriptional level. The nuclear runoff experiments also demonstrated that the CAD gene expression was shut down in less than 4 h after induction, well before morphological changes were observed in these cells. At the enzymatic level, the activity of aspartate transcarbamylase, one of the three enzymes encoded by the CAD gene, decreased by about half within 24 h of induction, suggesting a CAD protein half-life of 24 h in differentiating HL-60 cells. Nevertheless, this means that significant levels of aspartate transcarbamylase activity remained even after the cells have stopped proliferating. From the RNA data, it is clear that CAD gene expression is rapidly turned off as promyelocytes begin to terminally differentiate into macrophages and granulocytes. We suspect that the inactivation of the CAD gene in induced HL-60 cells is a consequence of the differentiating cells leaving the cell cycle and becoming nonproliferating.

Partial characterisation of aspartate transcarbamylase from the mantle of the mussel Mytilus edulis

The stability and the effect of pH and temperature on the activity of aspartate transcarbamylase from mantle of mussel were studied. The Km values for aspartic acid and carbamylphosphate at 35 degrees C are 1.8 X 10(-2) M and 7 X 10(-3) M respectively, values of Vmax being identical at 17.54 nM carbamylaspartate formed/min/mg protein. Allosteric effectors of ATCase (ATP and CTP) have no effect on the activity of mantle ATCase. PHMB and Cu2+ are strong inhibitors of the ATCase activity, organic solvents (DMF, DMSO) having a strong stimulatory action. ATCase from mantle of mussel has been compared to ATCase from different sources.

Studies of pyrimidine metabolism during chick development: enzymes involved in CMP breakdown

The pattern of cytidylate phosphatase, cytidine and cytosine deaminase has been studied in brain, liver, heart and thigh muscles during chick development. The enzymes involved in CMP catabolism appear in the tissues examined at different developmental periods. In the brain and heart a "salvage pathway" would appear in the pyrimidine metabolism earlier than in the purine one. An attempt has been made to explain the probable physiological role, in relation to differentiation, of the metabolic pathways observed in the tissues examined.

Effects of thyroid hormone on UTP content and uridine kinase activity of rat heart and skeletal muscle

In rats made hyperthyroid by daily intramuscular injections of 250 microgram thyroxine (T4)/100 g body wt for 5 days, uridine kinase activity of extracts of psoas and cardiac muscle was markedly increased Vmax of the enzyme was elevated with no change in the apparent Km for uridine. In animals treated as above, significant increases in UTP and total uracil nucleotide contents were observed in heart and skeletal muscle. Twelve hours after a single intraperitoneal injection of 30 microgram/100 g body wt of 3,5,3'-triiodothyronine (T3), cardiac uridine kinase was significantly increased. Brain uridine kinase was unaffected by thyroid hormone treatment. In thyroidectomized rats, uridine kinase activity was lower than normal. The effect of thyroidectomy on uridine kinase activity was overcome by daily subcutaneous injections of 3 microgram T4/100 g body wt for 7 days. The rise in cardiac uridine kinase activity produced by T3 could be prevented by prior administration of actinomycin D.

The genes for and regulation of the enzyme activities of two multifunctional proteins required for the de novo pathway for UMP biosynthesis in mammals

UMP biosynthesis requires six enzyme activities. Five of these enzyme centers are clustered into two multienzymatic proteins which are known to, or appear to, sequester the intermediates carbamyl approximately P, carbamyl aspartate and orotidylic acid. The advantages of sequestering these intermediates appear to be a conservation of energy, since two intermediates, carbamyl approximately P and orotidylate, might otherwise be rapidly degraded in mammalian cells. Carbamyl-aspartate appears not to be degraded rapidly in mammalian cells but it can pass into the blood and could possible disrupt brain metabolism by action as an acetylaspartate analog, if it passes the blood-brain barrier. For this, and possible for other reasons, there may be advantages to the fact that these intermediates are not other reasons, there may be advantages to the fact that these intermediates are not readily released from Complex A and U. In addition, these multienzymatic proteins may have other kinetic advantages, some of which have been discussed above. Studies with intact cells illustrate that azauridine, a chemical designed originally as an antineoplastic drug, produces a "ripple" effect when it inhibits the last enzyme of this pathway which leads to a sequential accumulation of pools of the various intermediates or their metabolites. This same agent increases the amount of some of the enzymes of this biosynthetic pathway in cells exposed to this drug. Both of these effects can negate the effectiveness of this potential antineoplastic drug. Sophisticated drug design may depend on whole-cell studies, such as those discussed here, in addition to the classic studies on the inhibition of a single enzyme center to select drugs that may be without significant side effects when they are finally tested in animals.

Studies on the female sterile mutant rudimentary of Drosophila melanogaster. II. An analysis of aspartate transcarbamylase and dihydroorotase activities in wild-type and rudimentary strains

The activities of the enzymes aspartate transcarbamylase (ATCase) and dihydroorotase (DHOase) were determined in adult females from a wild-type strain and from eight different alleles of the X-linked mutation rudimentary (r) of Drosophila melanogaster. The alleles chosen span the genetic map of the r locus. The characteristics of the DHOase-catalyzed reaction which converts carbamyl aspartate to dehydroorotate are briefly described. Of all of the r strains tested, only one, r9, has wild-type levels of aspartate transcarbamylase and dihydroorotase activities. The other seven show either intermediate or very low levels of activity for both enzymes. The lowered ATCase and DHOase activities observed in mutants which do not map in the region of the structural gene for these enzymes are interpreted in light of recent evidence that ATCase and DHOase are part of a three-enzyme complex.