Cancer Cells Tune the Signaling Pathways to Empower de Novo Synthesis of Nucleotides

Abstract

Cancer cells exhibit a dynamic metabolic landscape and require a sufficient supply of nucleotides and other macromolecules to grow and proliferate. To meet the metabolic requirements for cell growth, cancer cells must stimulate de novo nucleotide synthesis to obtain adequate nucleotide pools to support nucleic acid and protein synthesis along with energy preservation, signaling activity, glycosylation mechanisms, and cytoskeletal function. Both oncogenes and tumor suppressors have recently been identified as key molecular determinants for de novo nucleotide synthesis that contribute to the maintenance of homeostasis and the proliferation of cancer cells. Inactivation of tumor suppressors such as TP53 and LKB1 and hyperactivation of the mTOR pathway and of oncogenes such as MYC, RAS, and AKT have been shown to fuel nucleotide synthesis in tumor cells. The molecular mechanisms by which these signaling hubs influence metabolism, especially the metabolic pathways for nucleotide synthesis, continue to emerge. Here, we focus on the current understanding of the molecular mechanisms by which oncogenes and tumor suppressors modulate nucleotide synthesis in cancer cells and, based on these insights, discuss potential strategies to target cancer cell proliferation.

Keywords: AKT; MYC; RAS; cancer metabolism; de novo nucleotide synthesis; mTORC1; metabolic vulnerability; oncogenes; short term and long-term regulation; tumor suppressors.

Figure 1

The de novo pyrimidine and purine synthesis pathways. (A) Schematic of the de novo pyrimidine synthesis pathway. Pyrimidine synthesis enzymes: CAD: Carbamoyl-Phosphate Synthetase 2, Aspartate Transcarbamylase, And Dihydroorotase; DHODH: Dihydroorotate Dehydrogenase; UMPS: Uridine Monophosphate Synthetase. (B) Schematic of the de novo and purine salvage pathways. Purine synthesis enzymes: PPAT: phosphoribosyl pyrophosphate amidotransferase; GART: Glycinamide Ribonucleotide Transformylase; PFAS: Phosphoribosylformylglycinamidine Synthase; PAICS: Phosphoribosylaminoimidazole Carboxylase And Phosphoribosylamino-imidazolesuccinocarboxamide Synthase; ADSL: Adenylosuccinate Lyase; ATIC: 5-Aminoimidazole-4-Carboxamide Ribonucleotide Formyltransferase; IMPDH: Inosine Monophosphate Dehydrogenase; GMPS: Guanine Monophosphate Synthase; ADSS: Adenylosuccinate Synthase; HPRT: hypoxanthine phosphoribosyltransferase; APRT: adenine phosphoribosyltransferase.

Figure 2

Acute regulation of de novo nucleotide synthesis by oncogenes and tumor suppressors. This schematic highlights our current understanding of how the oncogenic and tumor suppressor signals acutely modulate de novo nucleotide synthesis in cancer cells. In response to growth signals and activated oncogenic RAS, ERK directly phosphorylates CAD on T456 and stimulates de novo pyrimidine synthesis. In addition, mTORC1 activation, downstream of PI3K/Akt signaling, leads to S6K1 mediated-phosphorylation of CAD on S1859, thereby acutely enhancing flux through de novo pyrimidine synthesis. Upstream of mTORC1, Akt phosphorylates TKT on Thr382 and enhances PRPP availability for nucleotide synthesis. Moreover, Akt increases NADP⁺ synthesis through the direct phosphorylation of NAD kinase on S44/S46, thereby increasing the availability of cellular NADPH to sustain NADPH-dependent anabolic production of purine nucleotides. Under metabolic stress, AMPK is activated and directly phosphorylates PRPS1/2 on S180/S183, which inhibits the conversion of ribose 5-phosphate into PRPP reducing its availability for nucleotide synthesis. Tumor suppressors are shown in pink, and key signaling kinases involved in light blue. Metabolic enzymes are shown in orange and small GTPases in dark blue. PRPP, 5’phosphoribosyl-pyrophosphate; PRPS, phosphoribosyl pyrophosphate synthetase; TKT, transketolase; AMPK, AMP-activated protein kinase.

Figure 3

Slow regulation of nucleotide synthesis by oncogenes and tumor suppressors. Growth factor and oncogenic signaling reprogram nucleotide metabolism to increase the biomass essential for cell proliferation. Signaling downstream from PI3K/Akt enhances glucose uptake and glycolysis. The MYC transcription factor, activated downstream from RAS, enhances de novo purine and pyrimidine synthesis. In addition to MYC activation, mTORC1 increases the levels of ATF4 stimulating synthesis of serine, glycine and one carbon formyl units contributing to purine synthesis. Loss of LKB1 and oncogenic activation of K-RAS lead to an increase in CPS1, a key urea cycle enzyme that produces carbamoyl phosphate intermediate supplying de novo pyrimidine synthesis. Tumor suppressors are shown in pink, and key oncogenic signaling kinases in light blue. Metabolic enzymes are shown in orange and small GTPases in dark blue. CPS1- Carbamoyl phosphate synthetase 1, OTC—Ornithine transcarbamoylase, ASS1—Argininosuccinate synthetase 1, ASL—Argininosuccinate lyase, ARG1—Arginase 1.