Purine-Metabolising Enzymes and Apoptosis in Cancer

Abstract

The enzymes of both de novo and salvage pathways for purine nucleotide synthesis are regulated to meet the demand of nucleic acid precursors during proliferation. Among them, the salvage pathway enzymes seem to play the key role in replenishing the purine pool in dividing and tumour cells that require a greater amount of nucleotides. An imbalance in the purine pools is fundamental not only for preventing cell proliferation, but also, in many cases, to promote apoptosis. It is known that tumour cells harbour several mutations that might lead to defective apoptosis-inducing pathways, and this is probably at the basis of the initial expansion of the population of neoplastic cells. Therefore, knowledge of the molecular mechanisms that lead to apoptosis of tumoural cells is key to predicting the possible success of a drug treatment and planning more effective and focused therapies. In this review, we describe how the modulation of enzymes involved in purine metabolism in tumour cells may affect the apoptotic programme. The enzymes discussed are: ectosolic and cytosolic 5'-nucleotidases, purine nucleoside phosphorylase, adenosine deaminase, hypoxanthine-guanine phosphoribosyltransferase, and inosine-5'-monophosphate dehydrogenase, as well as recently described enzymes particularly expressed in tumour cells, such as deoxynucleoside triphosphate triphosphohydrolase and 7,8-dihydro-8-oxoguanine triphosphatase.

Keywords: ADA; CD73; HPRT; IMPDH; MTH1; PNP; SAMHD1; apoptosis; cN-II; purine salvage.

Figure 1

De novo and salvage pathways for purine nucleotide biosynthesis. Cyan background: de novo synthesis; yellow background: salvage synthesis. The figure outlines the central role played by PRPP, needed for both de novo and salvage pathways. Hyp: hypoxanthine; Gua: guanine; Ade: adenine; Rib-5-P: ribose-5-phosphate; PRPP: 5-phosphoribosyl-1-pyrophosphate; Gln: glutamine; Gly: glycine; THF: tetrahydrofolate; Asp: aspartate; S-AMP: succinyl-AMP; XMP: xanthosine-5′-monophosphate; IMP: inosine-5′-monophosphate; GMP: guanosine-5′-monphosphate; AMP: adenosine-5′-monophosphate.

Figure 2

Pathways of purine metabolism. 1: Ectosolic 5′-nucleotidase; 2: Adenosine deaminase; 3: Cytosolic 5′-nucleotidase II; 4: Purine nucleoside phosphorylase; 5: Hypoxanthine-guanine phosphoribosyltransferase; 6: IMP dehydrogenase. Inset: deoxyribonucleoside triphosphates (dNTP) are converted into the respective deoxynucleosides (dNs) by a one-step reaction catalysed by a sterile alpha motif and histidine-aspartate (HD) domain-containing protein 1 (enzyme 7). An increase in reactive oxygen species (ROS) brings about an increase in 8-oxo-dGTP, converted into the monophosphate by human MutT homolog 1 (enzyme 8). Ado: adenosine; Guo: guanosine; Ino: inosine.

Figure 3

Effectors of adenosine receptor-mediated apoptosis. The figure shows the different types of G-proteins associated with the four adenosine receptors and illustrates the apoptosis effectors found in several models by using agonists and antagonists of the receptors, or receptor silencing. The numbers in brackets refer to the respective reference. Note that adenosine receptors can be involved in survival in other cell types [71].

Figure 4

Metabolism of F-araAMP in normal and tumoural cells subjected to Escherichia coli PNP (ePNP)/F-araAMP GDEPT. 9-β-arabinofuranosyl-2-fluoroadenosine 5’-monophoaphate (F-araAMP) is cleaved by plasma phosphatases into 9-β-arabinofuranosyl-2-fluoroadenine (F-araAde), which enters both normal and tumoural cells. Inside the cell, it is activated to the monophosphate by cellular deoxycytidine kinase (dCK). In tumour cells, the presence of ePNP allows for an additional activation pathway, which proceeds through a phosphorolytic cleavage to give 2-fluoroadenine (F-Ade), which is activated by cellular adenine phosphoribosyltransferase (APRT). Through the action of ribonucleotide reductase (RR), F-ADP is converted to the respective deoxynucleotide, thus interfering also on DNA synthesis. GDEPT: gene-directed enzyme prodrug therapy.

Figure 5

Apoptotic pathways triggered by inosine 5′-monophosphate dehydrogenase (IMPDH) inhibitors. The decrease in guanylate pool can trigger apoptosis through multiple pathways: (1): downregulation of the MEK/ERK pathway with inhibition of Bcl-2 and activation of Bax and cytochrome c release [167]; (2): downregulation of Src/PI3K pathway with inhibition of Akt, with downregulation of mammalian target of rapamycin (mTOR) and activation of pro-apoptotic Bak and Bax with Apoptosis Inducing Factor (AIF) and endonuclease G (Endo G) release from mitochondria (caspase-independent apoptosis) [136,162,163]; (3): upregulation of p53 with (a) downregulation of Bcl-2 and Bcl-xL, with consequent inhibition of p27 and survivin, cytochrome c (Cyt c) release, and activation of caspase-9, caspase-3 and polyADP-ribose polymerase (PARP; intrinsic apoptotic pathway) [159,160,161,164,165], (b) upregulation of PUMA and BIM with consequent SMAC/DIABLO release from mitochondria, inhibition of Inhibitor of Apoptosis (IAPs) (a caspase-3 inhibitor) with activation of caspase-3 [164], and (c) activation of caspase-2 with cleavage of Bid into truncated Bid (t-Bid) and AIF/Endo G release from mitochondria [164]; (4): synergistic effect of IMPDH inhibitors with TRAIL through binding with death receptors (DR4 and DR5) which recruit initiator caspase-8 via the adaptor protein FADD. Activated caspase-8 stimulates apoptosis via two parallel cascades: direct cleavage and activation of caspase-3, or cleavage of Bid into t-Bid which translocates to mitochondria, inducing cytochrome c release, with sequential activation of caspase-9 and -3 (extrinsic apoptotic pathway) [164]. TRAIL: tumour necrosis factor-related apoptosis-inducing ligand; FADD: Fas-associated protein with death domain.

Figure 6

Scheme of possible effects of therapeutic use of human MutT homolog 1 (MTH1). Panel 1: Cancer cell proliferation is stimulated by the accumulation of ROS, which favours the formation of 8-oxo-dGTP and 2-hydroxy-dATP. In order to avoid the potentially toxic effects of the incorporation of oxo-derivatives of nucleotides, the cells upregulate MTH1 sanitizing enzyme. Panel 2: downregulation or inhibition of MTH1 leads to an abnormal incorporation of oxidised nucleotides in DNA. Panel 2a: in order to deal with this excessive accumulation of oxo-nucleotides in DNA, the base excision repair (BER) system is, in general, upregulated. If the misincorporation of oxo-nucleotides exceeds the capability of BER, abasic sites (AS) or single strand brakes (SSB) or double strand brakes (DSB) accumulate, leading to several serious consequences. Panel 2b: if BER is downregulated, incorporated oxo-nucleotides are not repaired. In this case no cytotoxic effect is reported, but a promotion of tumourigenesis can occur. Panel 2c: the misincorporation of 8-oxo-dGTP by telomerase into the repeated telomeric sequence TTAGGG leads to the inhibition of the polymerase activity of the enzyme.