Mammalian 5'-nucleotidases

Red cell enzymes

Mutations leading to red cell enzyme deficiencies can be associated with diverse phenotypes that range from hemolytic anemia, methemoglobinemia, polycythemia, and neurological and developmental abnormalities. While most of these mutations occur sporadically, some such as common glucose-6-phosphate dehydrogenase (G6PD) mutants are endemic and rarely cause disease. Common G6PD mutants likely reached their prevalence because they provide some protection against severe malarial complications. In this review G6PD, pyruvate kinase, 5' nucleotidase, and cytochrome b5 reductase deficiencies will be discussed in greater detail. Limitations of commonly used screening tests for detection of these disorders will also be emphasized, as well as emerging knowledge about non-enzymatic function of the glycolytic enzymes.

Enzyme activities controlling adenosine levels in normal and neoplastic tissues

Adenosine is known to be associated with effects such as inhibition of immune response, coronary vasodilation, stimulation of angiogenesis, and inhibition of inflammatory reactions. Some authors suggest that adenosine may also have similar functions in tumor tissues. Tissue levels of adenosine are under close regulation by different enzymes acting at different levels. Adenosine is produced from AMP by the action of 5'-nucleotidase (5'-NT) and is converted back into AMP by adenosine kinase (AK) or into inosine by adenosine deaminase (ADA). Inosine is converted into purine catabolites by purine nucleoside phosphorylase (PNP), whereas AMP is converted into ADP and ATP by adenylate kinase (MK). The aim of this study was to analyze the activities of the above enzymes in fragments of neoplastic and apparently normal mucosa, obtained less than 5 cm and at least 10 cm from tumors, in 40 patients with colorectal cancer. The results showed much higher activities of ADA, AK, 5'-NT, and PNP in tumor tissue than in neighboring mucosa (p > 0.01 for ADA, AK, and PNP; p > 0.05 for 5'-NT), suggesting that the activities of purine metabolizing enzymes increase to cope with accelerated purine metabolism in cancerous tissue. The simultaneous increase in ADA and 5'-NT activities might be a physiological attempt by cancer cells to provide more substrate to accelerate salvage pathway activity.

General enzymatic screens identify three new nucleotidases in Escherichia coli. Biochemical characterization of SurE, YfbR, and YjjG

To find proteins with nucleotidase activity in Escherichia coli, purified unknown proteins were screened for the presence of phosphatase activity using the general phosphatase substrate p-nitrophenyl phosphate. Proteins exhibiting catalytic activity were then assayed for nucleotidase activity against various nucleotides. These screens identified the presence of nucleotidase activity in three uncharacterized E. coli proteins, SurE, YfbR, and YjjG, that belong to different enzyme superfamilies: SurE-like family, HD domain family (YfbR), and haloacid dehalogenase (HAD)-like superfamily (YjjG). The phosphatase activity of these proteins had a neutral pH optimum (pH 7.0-8.0) and was strictly dependent on the presence of divalent metal cations (SurE: Mn(2+) > Co(2+) > Ni(2+) > Mg(2+); YfbR: Co(2+) > Mn(2+) > Cu(2+); YjjG: Mg(2+) > Mn(2+) > Co(2+)). Further biochemical characterization of SurE revealed that it has a broad substrate specificity and can dephosphorylate various ribo- and deoxyribonucleoside 5'-monophosphates and ribonucleoside 3'-monophosphates with highest affinity to 3'-AMP. SurE also hydrolyzed polyphosphate (exopolyphosphatase activity) with the preference for short-chain-length substrates (P(20-25)). YfbR was strictly specific to deoxyribonucleoside 5'-monophosphates, whereas YjjG showed narrow specificity to 5'-dTMP, 5'-dUMP, and 5'-UMP. The three enzymes also exhibited different sensitivities to inhibition by various nucleoside di- and triphosphates: YfbR was equally sensitive to both di- and triphosphates, SurE was inhibited only by triphosphates, and YjjG was insensitive to these effectors. The differences in their sensitivities to nucleotides and their varied substrate specificities suggest that these enzymes play unique functions in the intracellular nucleotide metabolism in E. coli.

Role of estrogen receptor in the regulation of ecto-5'-nucleotidase and adenosine in breast cancer

PURPOSE

The purpose is to understand the expression of ecto-5'-nucleotidase (eN), an adenosine producing enzyme with potential roles in angiogenesis, growth, and immunosuppression, in estrogen receptor (ER)-negative and -positive breast cancer.

EXPERIMENTAL DESIGN

We investigated the regulation of eN expression at the mRNA and protein levels by alpha in a panel of breast cancer cell lines that differ in ER status and invasive and metastatic potential. We also determined rates of adenosine formation in cells with high and low eN expression and in ER+ cells treated with estradiol.

RESULTS

ER-negative cells express high eN protein and mRNA levels and produce up to 104-fold more adenosine from AMP and ATP. Estradiol and antiestrogen treatments confirm that eN mRNA and protein expression and adenosine generation are negatively regulated through the ER. Endogenous expression of eN in ER- cells transfected with ERalpha and phorbol ester-induced eN expression in ER+ cells was strongly suppressed by estradiol, suggesting a dominant function of ER. Finally, an examination of 18 clinical breast cancer samples that were analyzed for both ER status and eN expression by Martin et al. (Cancer Res., 60: 2232-2238, 2000) revealed a significant inverse correlation between ER and eN status.

CONCLUSIONS

Our results show for the first time that eN is negatively regulated by ERalpha in dominant fashion and suggests that eN expression and its generation of adenosine may relate to breast cancer progression. Additionally, increased expression of eN in a subset of ER-negative cells may serve as a novel marker for a subset of more aggressive breast carcinoma.

Characterization of a gemcitabine-resistant murine leukemic cell line: reversion of in vitro resistance by a mononucleotide prodrug

Resistance to cytotoxic nucleoside analogues is a major problem in cancer treatment. The cellular mechanisms involved in this phenomenon have been studied for several years, and some factors have been identified. Various strategies to overcome resistance have been suggested, but none has yet shown efficacy in vivo. We developed a gemcitabine-resistant cell line (L1210 10K) from the murine leukemic L1210 strain (L1210 wt) by continuous exposure to increasing concentrations of gemcitabine. L1210 10K is highly resistant to gemcitabine (14,833-fold), 1-beta-D-arabinofuranosylcytosine (ara-C; 2,100-fold), troxacitabine (>200-fold), and cladribine (160-fold) and slightly resistant to trimidox (7.22-fold), but does not display cross-resistance to fludarabine or nonnucleoside anticancer drugs. Deoxycytidine kinase mRNA was not detected by quantitative real-time reverse transcription-PCR in L1210 10K cells, whereas expression of thymidine kinase 1 and ribonucleotide reductase subunit R2 gene was moderately reduced. L1210 10K cells also demonstrated in vivo resistance to nucleoside analogues: gemcitabine- or ara-C-treated mice carrying L1210 10K had significantly shorter survival than gemcitabine- or ara-C-treated mice carrying L1210 wt (P < 0.05). UA911, a mononucleotide prodrug (pronucleotide) of ara-C was found to significantly sensitize L1210 10K cells in vitro. These results suggest that reduced deoxycytidine kinase expression is a mechanism of resistance to gemcitabine that is relevant in vivo and can be circumvented by a prodrug approach.

Wnt and beta-catenin signaling target the expression of ecto-5'-nucleotidase and increase extracellular adenosine generation

Solid tumors, which routinely experience necrosis and ischemia, release and degrade adenine nucleotides. This process may lead, depending on the expression of enzymes that regulate adenosine, to the generation of extracellular adenosine. Since genes encoding ecto-5'-nucleotidase (eN) and adenosine deaminase (ADA) contain TCF/LEF consensus binding sites, we asked whether Wnt/beta-catenin signaling, a pathway that is deregulated in several human tumors, targets the expression of these genes and thus influence extracellular adenosine generation. Our results show that beta-catenin strongly increased the activity of the 969-bp promoter of eN and this increase depended on the presence of TCF-1 transcription factor. Reciprocally, the eN promoter activity was decreased by co-transfection of APC, a beta-catenin antagonist. The expression of endogenous eN mRNA was increased either in Cos-7 cells transfected with a mutated beta-catenin and TCF-1 or in Rat-1 cells transformed by the Wnt-1 oncogene. In Rat-1 cells, expression of Wnt-1 correlated with increased eN protein levels and enzymatic activity and a concomitant decrease of adenosine deaminase mRNA and enzymatic activity. This expression profile is accompanied by a threefold increase in the generation of extracellular adenosine. Our study demonstrates a link between the Wnt signaling and the regulation of two enzymes that control the metabolism of adenosine.

Mitochondrial deoxyribonucleotides, pool sizes, synthesis, and regulation

We quantify cytosolic and mitochondrial deoxyribonucleoside triphosphates (dNTPs) from four established cell lines using a recently described method for the separation of cytosolic and mitochondrial (mt) dNTPs from as little as 10 million cells in culture (Pontarin, G., Gallinaro, L., Ferraro, P., Reichard, P., and Bianchi, V. (2003) Proc. Natl. Acad. Sci. U. S. A. 100, 12159-12164). In cycling cells the concentrations of the phosphates of thymidine, deoxycytidine, and deoxyadenosine (combining mono-, di-, and triphosphates in each case) did not differ significantly between mitochondria and cytosol, whereas deoxyguanosine phosphates were concentrated to mitochondria. We study the source and regulation of the mt dTTP pool as an example of mt dNTPs. We suggest two pathways as sources for mt dTTP: (i) import from the cytosol of thymidine diphosphate by a deoxynucleotide transporter, predominantly in cells involved in DNA replication with an active synthesis of deoxynucleotides and (ii) import of thymidine followed by phosphorylation by the mt thymidine kinase, predominantly in resting cells. Here we demonstrate that the second pathway is regulated by a mt 5'-deoxyribonucleotidase (mdN). We modify the in situ activity of mdN and measure the transfer of radioactivity from [(3)H]thymidine to mt thymidine phosphates. In cycling cells lacking the cytosolic thymidine kinase, a 30-fold overproduction of mdN decreases the specific radioactivity of mt dTTP to 25%, and an 80% decrease of mdN by RNA interference increases the specific radioactivity 2-fold. These results suggest that mdN modulates the synthesis of mt dTTP by counteracting in a substrate cycle the phosphorylation of thymidine by the mt thymidine kinase.