Role of pH in Regulating Cancer Pyrimidine Synthesis

Abstract

Replication is a fundamental aspect of cancer, and replication is about reproducing all the elements and structures that form a cell. Among them are DNA, RNA, enzymes, and coenzymes. All the DNA is doubled during each S (synthesis) cell cycle phase. This means that six billion nucleic acids must be synthesized in each cycle. Tumor growth, proliferation, and mutations all depend on this synthesis. Cancer cells require a constant supply of nucleotides and other macromolecules. For this reason, they must stimulate de novo nucleotide synthesis to support nucleic acid provision. When deregulated, de novo nucleic acid synthesis is controlled by oncogenes and tumor suppressor genes that enable increased synthesis and cell proliferation. Furthermore, cell duplication must be achieved swiftly (in a few hours) and in the midst of a nutrient-depleted and hypoxic environment. This also means that the enzymes participating in nucleic acid synthesis must work efficiently. pH is a critical factor in enzymatic efficiency and speed. This review will show that the enzymatic machinery working in nucleic acid synthesis requires a pH on the alkaline side in most cases. This coincides with many other pro-tumoral factors, such as the glycolytic phenotype, benefiting from an increased intracellular pH. An increased intracellular pH is a perfect milieu for high de novo nucleic acid production through optimal enzymatic performance.

Keywords: de novo nucleotide synthesis; intracellular alkalosis; pH deregulation; pyrimidine.

Figure 1

Chemical structure of nitrogen bases.

Figure 2

Signaling pathways that control CAD activity. This diagram is based on references [20,21,22,23]. CAD initiates de novo pyrimidine synthesis. CAD is activated by the activation of growth factors binding growth factor receptors and triggering the MAP kinases pathway. This activation takes place at the beginning of the S phase in the cell cycle. After the S phase is over, CAD is deactivated by protein kinase A (PKA) phosphorylation. The lower panel shows that phosphorylation of the Thr456 residue activates the enzyme, while phosphorylation of Ser1406 acts in the opposite way. The diagram also shows the three enzymes that form CAD. In this regard, the activation and deactivation residues form part of the first enzyme that initiates synthesis, namely, CPS (carbamoyl phosphate synthetase). DHO: dihydroorotase; ATC: aspartate transcarbamoylase.

Figure 3

All the steps of de novo pyrimidines synthesis (left panel). The participating enzymes are in red frames. The right panel shows the origin of the different parts that form the pyrimidine structure. Uracil is being used as an example.

Figure 3

All the steps of de novo pyrimidines synthesis (left panel). The participating enzymes are in red frames. The right panel shows the origin of the different parts that form the pyrimidine structure. Uracil is being used as an example.

Figure 4

Reaction catalyzed by carbamoylphosphate synthase II.

Figure 5

Condensation between aspartic acid and carbamoylphosphate, generating carbamoyl aspartate through the enzymatic action of ATCase (aspartate transcarbamoylase). The structure of uracil is shown in the lower panel for a comparative view of how this structure is being built.

Figure 6

Third step. Conversion of carbamoylaspartate into dihydroorotate through the enzymatic action of dihydroorotase.

Figure 7

Step 4. Chemical reaction catalyzed by dihydroorotate dehydrogenase. The pyrimidine ring is initially formed as orotate. Then, in the next step (step 5), it is attached to ribose phosphate (which is generated in the pentose phosphate pathway) and finally converted to the pyrimidine nucleotides that will be used for DNA and RNA synthesis.

Figure 8

The origin of the sugar molecule of ribonucleotides.

Figure 9

Binding of orotic acid with PPRP generating OMP through the enzymatic activity of OPRT.

Figure 10

Decarboxylation of orotidylate to form UMP (uridine monophosphate, step 6).

Figure 11

Phosphorylation of UMP generating UDP and phosphorylation of UDP generating UTP.

Figure 12

CTP synthesis.

Figure 13

Step 9: The sugar, ribose, is reduced to deoxyribose, forming dUDP.

Figure 14

Dephosphorylation of dUTP yielding dUMP.

Figure 15

Thymidilate synthase as the enzyme and methylentetrahydrofolate as cofactor add a methyl group on the nitrogen base. This converts dUMP to dTMP. Two further phosphorylations generate dTTP.