IMP Dehydrogenase Inhibitors as Immunomodulators

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74880/1/j.1749-6632.1993.tb35869.x.pd

Regulation of the Human Inosine Monophosphate Dehydrogenase Type I Gene: UTILIZATION OF ALTERNATIVE PROMOTERS

Catalysis of guanine nucleotide formation from IMP in the de novo purine synthetic pathway is carried out by two isoforms of the enzyme inosine monophosphate dehydrogenase (IMPDH) that are catalytically indistinguishable but are encoded by separate genes. In order to assess the potential for cell type-specific expression of IMPDH activity, we have characterized the IMPDH type I gene and identified three major RNA transcripts that are differentially expressed from three different promoters. A 4.0-kilobase pair (kb) mRNA containing 1.3 kb of 5'-untranslated region is expressed in activated peripheral blood lymphocytes and to a far lesser extent in cultured tumor cell lines. The P1 promoter that regulates the transcription of this mRNA has a high degree of sequence identity to an Alu repetitive sequence. A transcript of 2.7 kb is found in a subset of the tumor cell lines examined, whereas a 2.5-kb mRNA species is universally expressed and is the prevalent mRNA in most cell lines and tissues. The relative strengths of the three promoter regions and the effects of variable extents of 5'-flanking sequence on the P3 promoter differ in Jurkat T, as compared with Raji B lymphoid cell lines, demonstrating a complex cell type-specific transcriptional regulation of IMPDH type I gene expression

Human Cytosolic 5′-Nucleotidase I: CHARACTERIZATION AND ROLE IN NUCLEOSIDE ANALOG RESISTANCE

Nucleoside analogs are important in the treatment of hematologic malignancies, solid tumors, and viral infections. Their metabolism to the triphosphate form is central to their chemotherapeutic efficacy. Although the nucleoside kinases responsible for the phosphorylation of these compounds have been well described, the nucleotidases that may mediate drug resistance through dephosphorylation remain obscure. We have cloned and characterized a novel human cytosolic 5'-nucleotidase (cN-I) that potentially may have an important role in nucleoside analog metabolism. It is expressed at a high level in skeletal and heart muscle, at an intermediate level in pancreas and brain, and at a low level in kidney, testis, and uterus. The recombinant cN-I showed high affinity toward dCMP and lower affinity toward AMP and IMP. ADP was necessary for maximal catalytic activity. Expression of cN-I in Jurkat and HEK 293 cells conferred resistance to 2-chloro-2'-deoxyadenosine, with a 49-fold increase in the IC(50) in HEK 293 and a greater than 400-fold increase in the IC(50) in Jurkat cells. Expression of cN-I also conferred a 22-fold increase in the IC(50) to 2',3'-difluorodeoxycytidine in HEK 293 cells and an 82-fold increase in the IC(50) to 2',3'-dideoxycytidine in Jurkat cells. These data indicate that cN-I may play an important role in the regulation of physiological pyrimidine nucleotide pools and may also alter the therapeutic efficacy of certain nucleoside analogs

Characterization of the Human Inosine-5′-monophosphate Dehydrogenase Type II Gene

Inosine-5'-monophosphate dehydrogenase (IMPDH) activity and mRNA levels are induced up to 15-fold upon mitogenic or antigenic stimulation of human peripheral blood T lymphocytes. This increase in IMPDH activity is required for cellular proliferation and has been associated with malignant transformation. We have cloned the human IMPDH type II gene and show that it contains 14 exons and is approximately 5.8 kilobases in length. Exons vary in size from 49 to 207 base pairs and introns from 73 to 1065 base pairs. The transcription start site was mapped to a position 50 nucleotides upstream of the translation initiation site. The 5'-flanking region consisting of 463 base pairs upstream of the translation initiation site confers induced transcription and differential regulation upon a chloramphenicol acetyltransferase reporter gene when transfected into Jurkat T cells and human peripheral blood T lymphocytes, respectively. DNase I footprinting analysis using Jurkat T cell nuclear extract identified four protected regions in the promoter which coincide with consensus transcription factor binding sites for the nuclear factors AP2, ATF, CREB, Egr-1, Nm23, and Sp1. These findings suggest that several of these nuclear factors may play a critical role in the regulation of IMPDH type II gene expression during T lymphocyte activation

Tissue-specific Regulation of the Ecto-5′-nucleotidase Promoter: ROLE OF THE cAMP RESPONSE ELEMENT SITE IN MEDIATING REPRESSION BY THE UPSTREAM REGULATORY REGION

We have isolated the 5' region of the ecto-5'-nucleotidase (low K(m) 5'-NT) gene and established that a 969-base pair (bp) fragment confers cell-specific expression of a CAT reporter gene that correlates with the expression of endogenous ecto-5'-NT mRNA and enzymatic activity. A 768-bp upstream negative regulatory region has been identified that conferred lymphocyte-specific negative regulation in a heterologous system with a 244-bp deoxycytidine kinase core promoter. DNase I footprinting identified several protected areas including Sp1, Sp1/AP-2, and cAMP response element (CRE) binding sites within the 201-bp core promoter region and Sp1, NRE-2a, TCF-1/LEF-1, and Sp1/NF-AT binding sites in the upstream regulatory region. Whereas the CRE site was essential in mediating the negative activity of the upstream regulatory region in Jurkat but not in HeLa cells, mutation of the Sp1/AP-2 site decreased promoter activity in both cell lines. Electrophoretic mobility shift assay analysis of proteins binding to the CRE site identified both ATF-1 and ATF-2 in Jurkat cells. Finally, phorbol 12-myristate 13-acetate increased the activity of both the core and the 969-bp promoter fragments, and this increase was abrogated by mutations at the CRE site. In summary, we have identified a tissue-specific regulatory region 5' of the ecto-5'-NT core promoter that requires the presence of a functional CRE site within the basal promoter for its suppressive activity

Depletion of Guanine Nucleotides Leads to the Mdm2-Dependent Proteasomal Degradation of Nucleostemin

Nucleostemin is a positive regulator of cell proliferation and is highly expressed in a variety of stem cells, tumors, and tumor cell lines. The protein shuttles between the nucleolus and the nucleus in a GTP-dependent fashion. Selective depletion of intracellular guanine nucleotides by AVN-944, an inhibitor of the de novo purine synthetic enzyme, IMP dehydrogenase, leads to the rapid disappearance of nucleostemin protein in tumor cell lines, an effect that does not occur with two other nucleolar proteins, nucleophosmin or nucleolin. Endogenous nucleostemin protein is completely stabilized by MG132, an inhibitor of the 26S proteasome, as are the levels of expressed enhanced green fluorescent protein–tagged nucleostemin, both wild-type protein and protein containing mutations at the G1 GTP binding site. Nutlin-3a, a small molecule that disrupts the binding of the E3 ubiquitin ligase, Mdm2, to p53, stabilizes nucleostemin protein in the face of guanine nucleotide depletion, as does siRNA-mediated knockdown of Mdm2 expression and over-expression of a dominant-negative form of Mdm2. Neither Doxorubicin nor Actinomycin D, which cause the release of nucleostemin from the nucleolus, results in nucleostemin degradation. We conclude that nucleostemin is a target for Mdm2-mediated ubiquitination and degradation when not bound to GTP. Because this effect does not occur with other chemotherapeutic agents, the induction of nucleostemin protein degradation in tumor cells by IMP dehydrogenase inhibition or by other small molecules that disrupt GTP binding may offer a new approach to the treatment of certain neoplastic diseases

Regulation of Inosine-5′-monophosphate Dehydrogenase Type II Gene Expression in Human T Cells: ROLE FOR A NOVEL 5′ PALINDROMIC OCTAMER SEQUENCE

Expression of the gene encoding human inosine- 5'-monophosphate dehydrogenase (IMPDH) type II, an enzyme catalyzing the rate-limiting step in the generation of guanine nucleotides, is increased more than 10-fold in activated peripheral blood T lymphocytes and is required for T cell activation. We have examined the 5'-regulatory sequences that are important for the transcriptional regulation of this gene in T cells. DNase I mapping of genomic DNA identified a hypersensitive element near the transcription initiation site. Fine mapping by in vivo footprinting demonstrated five transcription factor binding sites that are occupied in both resting and activated peripheral blood T lymphocytes; these are tandem CRE motifs, a Sp1 site, an overlapping Egr-1/Sp1 site, and a novel palindromic octamer sequence (POS). The tandem CRE and POS sites are of major functional importance as judged by mutational and electrophoretic mobility shift analyses. These data provide evidence that expression of the human IMPDH type II gene is predominantly regulated by the nuclear factors ATF-2 and an as yet unidentified POS-binding protein. Additional major protein-DNA interactions do not occur within the promoter region after T lymphocyte activation, indicating a requirement for additional protein-protein interactions and/or post-translational modifications of pre-bound transcription factors to account for the observed increase in IMPDH type II gene expression

Nucleotide pool imbalance and adenosine deaminase deficiency induce alterations of N-region insertions during V(D)J recombination

Template-independent nucleotide additions (N regions) generated at sites of V(D)J recombination by terminal deoxynucleotidyl transferase (TdT) increase the diversity of antigen receptors. Two inborn errors of purine metabolism, deficiencies of adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP), result in defective lymphoid development and aberrant pools of 2′-deoxynucleotides that are substrates for TdT in lymphoid precursors. We have asked whether selective increases in dATP or dGTP pools result in altered N regions in an extrachromosomal substrate transfected into T-cell or pre–B-cell lines. Exposure of the transfected cells to 2′-deoxyadenosine and an ADA inhibitor increased the dATP pool and resulted in a marked increase in A–T insertions at recombination junctions, with an overall decreased frequency of V(D)J recombination. Sequence analysis of VH-DH-JH junctions from the IgM locus in B-cell lines from ADA-deficient patients demonstrated an increase in A–T insertions equivalent to that found in the transfected cells. In contrast, elevation of dGTP pools, as would occur in PNP deficiency, did not alter the already rich G–C content of N regions. We conclude that the frequency of V(D)J recombination and the composition of N-insertions are influenced by increases in dATP levels, potentially leading to alterations in antigen receptors and aberrant lymphoid development. Alterations in N-region insertions may contribute to the B-cell dysfunction associated with ADA deficiency

Inhibition of Nucleoside Transport by p38 MAPK Inhibitors

While investigating the ability of p38 MAPK to regulate cytarabine (Ara C)-dependent differentiation of erythroleukemia K562 cells, we observed effects that indicated that the imidazoline class of p38 MAPK inhibitors prevented nucleoside transport. Incubation of K562 cells with SB203580, SB203580-iodo, or SB202474, an analogue of SB203580 that does not inhibit p38 MAPK activity, inhibited the uptake of [3H]Ara C or [3H]uridine and the differentiation of K562 cells. Consistent with the effects of these compounds on the nitrobenzylthioinosine (NBMPR)-sensitive equilibrative nucleoside transporter (ENT1), incubation with SB203580 or SB203580-iodo eliminated the binding of [3H]NBMPR to K562 cells or membranes isolated from human erythrocytes. Furthermore, using a uridine-dependent cell type (G9c), we observed that SB203580 or SB203580-iodo efficiently inhibited the salvage synthesis of pyrimidine nucleotides in vivo. Thus these studies demonstrate that the NBMPR-sensitive equilibrative nucleoside transporters are novel and unexpected targets for the p38 MAPK inhibitors at concentrations typically used to inhibit protein kinases

Frequency of 22q11 deletions in patients with conotruncal defects

AbstractObjectives. This study was designed to determine the frequency of 22q11 deletions in a large, prospectively ascertained sample of patients with conotruncal defects and to evaluate the deletion frequency when additional cardiac findings are also considered.Background. Chromosome 22q11 deletions are present in the majority of patients with DiGeorge, velocardiofacial and conotruncal anomaly face syndromes in which conotruncal defects are a cardinal feature. Previous studies suggest that a substantial number of patients with congenital heart disease have a 22q11 deletion.Methods. Two hundred fifty-one patients with conotruncal defects were prospectively enrolled into the study and screened for the presence of a 22q11 deletion.Results. Deletions were found in 50.0% with interrupted aortic arch (IAA), 34.5% of patients with truncus arteriosus (TA), and 15.9% with tetralogy of Fallot (TOF). Two of 6 patients with a posterior malalignment type ventricular septal defect (PMVSD) and only 1 of 20 patients with double outlet right ventricle were found to have a 22q11 deletion. None of the 45 patients with transposition of the great arteries had a deletion. The frequency of 22q11 deletions was higher in patients with anomalies of the pulmonary arteries, aortic arch or its major branches as compared to patients with a normal left aortic arch regardless of intracardiac anatomy.Conclusions. A substantial proportion of patients with IAA, TA, TOF and PMVSD have a deletion of chromosome 22q11. Deletions are more common in patients with aortic arch or vessel anomalies. These results begin to define guidelines for deletion screening of patients with conotruncal defects