Bacterial expression strategies for human angiogenesis proteins

We outline an expression strategy using Escherichia coli to obtain soluble components of a selected group of human proteins implicated in angiogenesis. These targets represent a heterogeneous group of proteins which for expression purposes were separated into cytoplasmic and helical membrane protein categories. Target selection was refined using a bioinformatic approach to generate a list of 50 experimental targets. A group consisting of forty-four cytoplasmic and signal-containing protein targets were amplified and cloned into multiple expression vectors. For this target category, we obtained 48% soluble expression products. In addition, we used a domain expression approach for six high molecular weight proteins predicted to contain membrane spanning helices to obtain soluble domain products. These results validate the utility of a bioinformatically driven high throughput approach to increase the number of soluble proteins or protein domains which can be used for multiple downstream applications.

Efficient recognition of protein fold at low sequence identity by conservative application of Psi-BLAST: application

Based on a study involving structural comparisons of proteins sharing 25% or less sequence identity, three rounds of Psi-BLAST appear capable of identifying remote evolutionary homologs with greater than 95% confidence provided that more than 50% of the query sequence can be aligned with the target sequence. Since it seems that more than 80% of all homologous protein pairs may be characterized by a lack of significant sequence similarity, the experimental biologist is often confronted with a lack of guidance from conventional homology searches involving pair-wise sequence comparisons. The ability to disregard levels of sequence identity and expect value in Psi-BLAST if at least 50% of the query sequence has been aligned allows for generation of new hypotheses by consideration of matches that are conventionally disregarded. In one example, we suggest a possible evolutionary linkage between the cupredoxin and immunoglobulin fold families. A thermostable hypothetical protein of unknown function may be a circularly permuted homolog to phosphotriesterase, an enzyme capable of detoxifying organophosphate nerve agents. In a third example, the amino acid sequence of another hypothetical protein of unknown function reveals the ATP binding-site, metal binding site, and catalytic sidechain consistent with kinase activity of unknown specificity. This approach significantly expands the utility of existing sequence data to define the primary structure degeneracy of binding sites for substrates, cofactors and other proteins.

Genome-scale expression of proteins from Bacillus subtilis

We have applied high throughput methods for cloning and expression of more than 850 genes from the Bacillus subtilis genome. The process uses 96-well plates and is automated from the level of primer design to the detection of soluble protein by a tag detection screen. This process was applied to a set of cytoplasmic targets from Bacillus subtilis to produce clones expressing soluble protein for incorporation into the structure determination pipeline of the Midwest Center for Structural Genomics. We also evaluated the feasibility of these plate-based methods for domain-based cloning and expression of secretory proteins and putative soluble domains of membrane proteins. This approach shows promise for implementation in a high throughput format and could provide additional target resources for structure determination. The continued development of new technologies that can be implemented in an automated format will be essential for continued success in the structural genomic programs.

Express primer tool for high-throughput gene cloning and expression

High-throughput approaches for gene cloning and expression require the development of new nonstandard tools for molecular biologists and biochemists. We introduce a Web-based tool to design primers specifically for the generation of expression clones for both laboratory-scale and high-throughput projects. The application is designed not only to allow the user complete flexibility to specify primer design parameters but also to minimize the amount of manual intervention needed to generate a large number of primers for the simultaneous amplification of multiple target genes.

Differential signatures of bacterial and mammalian IMP dehydrogenase enzymes

IMP dehydrogenase (IMPDH) is an essential enzyme of de novo guanine nucleotide synthesis. IMPDH inhibitors have clinical utility as antiviral, anticancer or immunosuppressive agents. The essential nature of this enzyme suggests its therapeutic applications may be extended to the development of antimicrobial agents. Bacterial IMPDH enzymes show biochemical and kinetic characteristics that are different than the mammalian IMPDH enzymes, suggesting IMPDH may be an attractive target for the development of antimicrobial agents. We suggest that the biochemical and kinetic differences between bacterial and mammalian enzymes are a consequence of the variance of specific, identifiable amino acid residues. Identification of these residues or combination of residues that impart this mammalian or bacterial enzyme signature is a prerequisite for the rational identification of agents that specifically target the bacterial enzyme. We used sequence alignments of IMPDH proteins to identify sequence signatures associated with bacterial or eukaryotic IMPDH enzymes. These selections were further refined to discern those likely to have a role in catalysis using information derived from the bacterial and mammalian IMPDH crystal structures and site-specific mutagenesis. Candidate bacterial sequence signatures identified by this process include regions involved in subunit interactions, the active site flap and the NAD binding region. Analysis of sequence alignments in these regions indicates a pattern of catalytic residues conserved in all enzymes and a secondary pattern of amino acid conservation associated with the major phylogenetic groups. Elucidation of the basis for this mammalian/bacterial IMPDH signature will provide insight into the catalytic mechanism of this enzyme and the foundation for the development of highly specific inhibitors.

Characteristics and crystal structure of bacterial inosine-5'-monophosphate dehydrogenase

IMP dehydrogenase (IMPDH) is an essential enzyme that catalyzes the first step unique to GTP synthesis. To provide a basis for the evaluation of IMPDH inhibitors as antimicrobial agents, we have expressed and characterized IMPDH from the pathogenic bacterium Streptococcus pyogenes. Our results show that the biochemical and kinetic characteristics of S. pyogenes IMPDH are similar to other bacterial IMPDH enzymes. However, the lack of sensitivity to mycophenolic acid and the Km for NAD (1180 microM) exemplify some of the differences between the bacterial and mammalian IMPDH enzymes, making it an attractive target for antimicrobial agents. To evaluate the basis for these differences, we determined the crystal structure of the bacterial enzyme at 1.9 A with substrate bound in the catalytic site. The structure was determined using selenomethionine-substituted protein and multiwavelength anomalous (MAD) analysis of data obtained with synchrotron radiation from the undulator beamline (19ID) of the Structural Biology Center at Argonne's Advanced Photon Source. S. pyogenes IMPDH is a tetramer with its four subunits related by a crystallographic 4-fold axis. The protein is composed of two domains: a TIM barrel domain that embodies the catalytic framework and a cystathione beta-synthase (CBS) dimer domain of so far unknown function. Using information provided by sequence alignments and the crystal structure, we prepared several site-specific mutants to examine the role of various active site regions in catalysis. These variants implicate the active site flap as an essential catalytic element and indicate there are significant differences in the catalytic environment of bacterial and mammalian IMPDH enzymes. Comparison of the structure of bacterial IMPDH with the known partial structures from eukaryotic organisms will provide an explanation of their distinct properties and contribute to the design of specific bacterial IMPDH inhibitors.

Interaction between alpha 5 beta 1 integrin and secreted fibronectin is involved in macrophage differentiation of human HL-60 myeloid leukemia cells

We examined the role of fibronectin (FN) and FN-binding integrins in macrophage differentiation. Increased FN and alpha5beta1 integrin gene expression was observed in phorbol 12-myristate 13-acetate PMA-treated HL-60 cells and PMA- or macrophage-CSF-treated blood monocytes before the manifestation of macrophage markers. After treatment of HL-60 cells and monocytes, newly synthesized FN was released and deposited on the dishes. An HL-60 cell variant, HL-525, which is deficient in the protein kinase Cbeta (PKC-beta) and resistant to PMA-induced differentiation, failed to express FN after PMA treatment. Transfecting HL-525 cells with a PKC-beta expression plasmid restored PMA-induced FN gene expression and macrophage differentiation. Untreated HL-525 cells (which have a high level of the alpha5beta1 integrin) incubated on FN differentiated into macrophages. The percentage of cells having a macrophage phenotype induced by PMA in HL-60 cells, by FN in HL-525 cells, or by either PMA or macrophage-CSF in monocytes was reduced in the presence of mAbs to FN and alpha5beta1 integrin. The integrin-signaling nonreceptor tyrosine kinase, p72Syk, was activated in PMA-treated HL-60 and FN-treated HL-525 cells. We suggest that macrophage differentiation involves the activation of PKC-beta and expression of extracellular matrix proteins such as FN and the corresponding integrins, alpha5beta1 integrin in particular. The stimulated cells, through the integrins, attach to substrates by binding to the deposited FN. This attachment, in turn, may through integrin signaling activate nonreceptor tyrosine kinases, including p72Syk, and later lead to expression of other genes involved in evoking the macrophage phenotype.

Isosteric analogues of nicotinamide adenine dinucleotide derived from furanfurin, thiophenfurin, and selenophenfurin as mammalian inosine monophosphate dehydrogenase (type I and II) inhibitors

Dinucleotides TFAD (6), FFAD (7), and SFAD (8), isosteric NAD analogues derived, respectively, from C-nucleosides 5-beta-d-ribofuranosylthiophene-3-carboxamide (thiophenfurin, 1), 5-beta-d-ribofuranosylfuran-3-carboxamide (furanfurin, 2), and 5-beta-d-ribofuranosylselenophene-3-carboxamide (selenophenfurin, 5), were synthesized as human inosine monophosphate dehydrogenase (IMPDH) type I and II inhibitors. The synthesis was carried out by imidazole-catalyzed coupling of the 5'-monophosphate of 1, 2, and 5 with AMP. These dinucleotides, which are also analogues of thiazole-4-carboxamide adenine dinucleotide (TAD) and selenazole-4-carboxamide adenine dinucleotide (SAD), the active metabolites of the oncolytic C-nucleosides 2-beta-D-ribofuranosylthiazole-4-carboxamide (tiazofurin) and 2-beta-D-ribofuranosylselenazole-4-carboxamide (selenazofurin), were evaluated for their inhibitory potency against recombinant human IMPDH type I and II. The order of inhibitory potency found was SAD > SFAD = TFAD = TAD >> FFAD for both enzyme isoforms. No significant difference was found in inhibition of IMPDH type I and II.

Cloning and characterization of the gene encoding IMP dehydrogenase from Arabidopsis thaliana

We have cloned and characterized the gene encoding inosine monophosphate dehydrogenase (IMPDH) from Arabidopsis thaliana (At). The transcription unit of the At gene spans approximately 1900 bp and specifies a protein of 503 amino acids with a calculated relative molecular mass (M(r)) of 54,190. The gene is comprised of a minimum of four introns and five exons with all donor and acceptor splice sequences conforming to previously proposed consensus sequences. The deduced IMPDH amino-acid sequence from At shows a remarkable similarity to other eukaryotic IMPDH sequences, with a 48% identity to human Type II enzyme. Allowing for conservative substitutions, the enzyme is 69% similar to human Type II IMPDH. The putative active-site sequence of At IMPDH conforms to the IMP dehydrogenase/guanosine monophosphate reductase motif and contains an essential active-site cysteine residue.

Cloning, characterization and sequence comparison of the gene coding for IMP dehydrogenase from Pyrococcus furiosus

We have cloned and characterized the gene encoding inosine monophosphate dehydrogenase (IMPDH) from Pyrococcus furiosus (Pf), a hyperthermophillic archeon. Sequence analysis of the Pf gene indicated an open reading frame specifying a protein of 485 amino acids (aa) with a calculated M(r) of 52900. Canonical Archaea promoter elements, Box A and Box B, are located -49 and -17 nucleotides (nt), respectively, upstream of the putative start codon. The sequence of the putative active-site region conforms to the IMPDH signature motif and contains a putative active-site cysteine. Phylogenetic relationships derived by using all available IMPDH sequences are consistent with trees developed for other molecules; they do not precisely resolve the history of Pf IMPDH but indicate a close similarity to bacterial IMPDH proteins. The phylogenetic analysis indicates that a gene duplication occurred prior to the division between rodents and humans, accounting for the Type I and II isoforms identified in mice and humans.

Interferon-independent activation of (2′-5′) oligoadenylate synthetase in Friend erythroleukemia cell variants exposed to HMBA

To provide evidence for the implication of interferon (IFN)-induced proteins in the regulation of cell growth during differentiation, the activation of (2′-5′) oligoadenylate synthetase (2-5A synthetase) as well as of PKR, two IFN-induced proteins, during differentiation of Friend erythroleukemia cells, was studied. Two cell variants were used. The first (FL) was completely susceptible to hexamethylene bis-acetamide (HMBA)-treatment and responded in both growth-retardation and hemoglobin synthesis. The second (R1) failed to synthesize hemoglobin in response to HMBA although cell growth was still inhibited. In both cell variants, 2–5A synthetase enzyme activity was induced in a similar fashion, reaching a peak at 26 hours after treatment with HMBA. However, the down regulation of activity thereafter was not identical in both cases. In R1 cells, the reduction was much slower compared to FL cells. A similar pattern was observed with the appearance of the 43 kDa isoform of 2–5A synthetase in immunoblots. An analysis of 2–5A synthetase gene expression revealed the presence of 1.7 kb transcripts which peaked at 16 hours after HMBA-treatment in both cell variants. Again, the down-regulation in expression was slower in R1 than in FL cells. Addition of anti-murin alpha/beta-IFN antibodies did not reduce the level of either 2–5A synthetase expression or enzyme activity in either cell variant. Interestingly, the presence of antibodies also did not affect the pattern of pRb phosphorylation in the cell variants exposed to HMBA. In both cell variants, an increase in the amount of the phosphorylated form (ppRb) was observed in immunoblots after 4 hours. This form was gradually transformed to the underphosphorylated molecule (pRb) with time in culture, even in the presence of antibodies. This further substantiates the notion that IFN-induced regulation of pRb phosphorylation is mediated by IFN-induced proteins. The basal level of either expression or ezymatic activity of PKR detected in untreated FL or R1 cells, was relatively high. Treatment with HMBA did not result in further induction of PKR in either cell variant.

Recombinant human inosine monophosphate dehydrogenase type I and type II proteins. Purification and characterization of inhibitor binding

Inosine monophosphate dehydrogenase (IMPDH) activity results from the expression of two separate genes, and the resulting proteins (type I and type II) are 84% identical at the amino acid level. Although the type II mRNA is expressed at higher levels in proliferating cells, both mRNAs, and by extrapolation both proteins, are present in normal and malignant cells. Since IMPDH is an important target for the development of drugs with both chemotherapeutic and immunosuppressive activity, we have compared the kinetic and physical properties of the two human enzymes expressed in and purified from Escherichia coli. Type I and II IMPDH had kcat values of 1.8 and 1.4 sec-1, respectively, with Km values for IMP of 14 and 9 microM and Km values for NAD of 42 and 32 microM. The two enzymes were inhibited competitively by the immunosuppressive agent mizoribine 5'-monophosphate (MMP) with Ki values of 8 and 4 nM and inhibited uncompetitively by mycophenolic acid with Ki values of 11 and 6 nM. The association of MMP to either isozyme, as monitored by fluorescence quenching, was relatively slow with kon values of 3-8 x 10(4) M-1 sec-1 and koff values of 3 x 10(-4) sec-1 (half-lives of 36-43 min). Thus, MMP is a potent, tight-binding competitive inhibitor that does not discriminate between the two IMPDH isozymes.

Heterogeneity in c-jun gene expression in normal and malignant cells exposed to either ionizing radiation or hydrogen peroxide

We investigated the role of reactive oxygen intermediates and protein kinase C in the induction of expression of the c-jun gene in human ML-2 leukemic cells and normal human DET-551 fibroblasts by comparing the effects of exposure to either ionizing radiation or H2O2 in the presence or absence of appropriate inhibitors. In these cell types, the radiation- and H2O2-mediated increase in c-jun mRNA levels could be prevented by pretreatment of the cells with N-acetylcysteine, an antioxidant, or H7, an inhibitor of protein kinase C and protein kinase A, but not by HA1004, a specific inhibitor of protein kinase A and G. These results suggest a role for protein kinase C and reactive oxygen intermediates in the induction of c-jun gene expression in both normal and tumor cells. We also investigated potential differences in c-jun gene expression induced by radiation or H2O2 in normal and tumor cells by examining steady-state c-jun mRNA levels in a number of human fibroblast, leukemia, melanoma, sarcoma and carcinoma cell types. We observed heterogeneity in the steady-state level of c-jun mRNA in both the untreated normal and tumor cells and in such cells exposed to ionizing radiation or to H2O2. Exposure to radiation produced a varied response which ranged from little or no induction to an increase in the steady-state level of the c-jun mRNA of more than two orders of magnitude. Exposure to H2O2 gave a pattern similar to that of ionizing radiation. The basis for the differential induction in response to these agents may be attributable to either cell lineage or genetic heterogeneity or a combination of these two parameters.

Induction of differentiation in human promyelocytic HL-60 leukemia cells activates p21, WAF1/CIP1, expression in the absence of p53

The melanoma differentiation associated gene, mda-6, which is identical to the P53-inducible gene WAF1/CIP1, encodes an M(r) 21,000 protein (p21) that can directly inhibit cell growth by repressing cyclin dependent kinases. mda-6 was identified using subtraction hybridization by virtue of its enhanced expression in human melanoma cells induced to terminally differentiate by treatment with human fibroblast interferon and the anti-leukemic compound mezerein (Jiang and Fisher, 1993). In the present study, we demonstrate that mda-6 (WAF1/CIP1) is an immediate early response gene induced during differentiation of the promyelocytic HL-60 leukemia cell line along the granulocytic or macrophage/monocyte pathway. mda-6 gene expression in HL-60 cells is induced within 1 to 3 h during differentiation along the macrophage/monocyte pathway evoked by 12-0-tetradecanoyl phorbol-13-acetate (TPA) or 1,25-dihydroxyvitamin D3 (Vit D3) or the granulocytic pathway produced by retinoic acid (RA) or dimethylsulfoxide (DMSO). Immunoprecipitation analyses using an anti-p21 antibody indicate a temporal induction of p21 protein following treatment with TPA, DMSO or RA. A relationship between rapid induction of mda-6 gene expression and differentiation is indicated by a delay in this expression in an HL-60 cell variant resistant to TPA-induced growth arrest and differentiation. A similar delay in mda-6 gene expression is not observed in Vit D3 treated TPA-resistant variant cells that are also sensitive to induction of monocytic differentiation. Since HL-60 cells have a null-p53 phenotype, these results demonstrate that p21 induction occurs during initiation of terminal differentiation in a p53-independent manner. In this context, p21 may play a more global role in growth control and differentiation than originally envisioned.

p210 bcr-abl confers overexpression of inosine monophosphate dehydrogenase: an intrinsic pathway to drug resistance mediated by oncogene

The p210 bcr-abl fusion protein tyrosine kinase oncogene has been implicated in the pathogenesis of chronic granulocytic leukemia (CGL). Specific intracellular functions performed by p210 bcr-abl have recently been delineated. We considered the possibility that p210 bcr-abl may also regulate the abundance of inosine 5'-monophosphate dehydrogenase (IMPDH) which is a rate-limiting enzyme for de novo guanylate synthesis. We performed studies of the inhibition of IMPDH by tiazofurin, which acts as a competitive inhibitor through its active species that mimics nicotinamide adenine dinucleotide (NAD), i.e. thiazole-4-carboxamide adenine dinucleotide (TAD). The mean inhibitory concentration (IC50) of tiazofurin for cellular proliferation inhibition was 2.3-2.8-fold greater in cells expressing p210 bcr-abl than in their corresponding parent cells proliferating under the influence of growth factors or in growth factor-independent derivative cells not expressing detectable p210 bcr-abl. IMPDH activity was 1.5-2.3-fold greater within cells expressing p210 bcr-abl than in their parent cells. This increase in enzyme activity was a result of 2-fold increased IMPDH protein as determined by immunoblotting. In addition, an increase in the Km value for NAD utilization by IMPDH was observed in p210 bcr-abl transformed cells, but this increase was within the range of resident NAD concentrations observed in the cells. Increased IMPDH protein in p210 bcr-abl transformed cells was traced to an increased level of IMP dehydrogenase II messenger RNA. Thus, regulation of IMPDH gene expression is mediated at least in part by the bcr-abl gene product and may therefore be indicative of a specific mechanism of intrinsic resistance to tiazofurin.

Regulated expression of the MRP8 and MRP14 genes in human promyelocyticleukemic HL-60 cells treated with the differentiation-inducing agents mycophenolic acid and 1 alpha,25-dihydroxyvitamin D3

The calcium-binding proteins MRP8 and MRP14 present in mature monomyelocytic cells are induced during differentiation and may mediate the growth arrest in differentiating HL-60 cells. We determined the levels of a protein complex (PC) containing MRP8 and MRP14 and investigated the mechanism by which the genes encoding these proteins are regulated in HL-60 cells treated with the differentiation-inducing agent mycophenolic acid (MPA). While the PC was barely detectable in untreated cells, MPA treatment resulted in elevated levels of the PC which were maximal at 3-4 days and were found to directly parallel gains in the steady-state levels of MRP8 and MRP14 mRNA. Transcription studies with the use of nuclear run-on experiments revealed increased transcription initiation at the MRP8 and MRP14 promoters after MPA treatment. 1 alpha,25-Dihydroxyvitamin D3, which induces HL-60 cell differentiation by another mechanism, was also found to increase transcription initiation at the MRP8 and MRP14 promoters. Our results suggest that this initiation is the major control of maturation agent-mediated increases in MRP8 and MRP14 gene expression, and supports a role for the PC in terminal differentiation of human monomyelocytic cells.

Increased inosine-5'-phosphate dehydrogenase gene expression in solid tumor tissues and tumor cell lines

Inosine-5'-phosphate (IMP) dehydrogenase, a regulatory enzyme of guanine nucleotide biosynthesis, may play a role in cell proliferation and malignancy. To assess this role we examined IMP dehydrogenase expression in a series of human solid tumor tissues and tumor cell lines in comparison with their normal counterparts. Increased IMP dehydrogenase gene expression was observed in brain tumors relative to normal brain tissue and in sarcoma cells relative to normal fibroblasts. Similarly, in several B- and T-lymphoid leukemia cell lines, elevated levels of IMP dehydrogenase mRNA and cellular enzyme were observed in comparison with the levels in peripheral blood lymphocytes. These results are consistent with an association between increased IMP dehydrogenase expression and either enhanced cell proliferation or malignant transformation.

Protein kinase C beta gene expression is associated with susceptibility of human promyelocytic leukemia cells to phorbol ester-induced differentiation

To study the signal transduction pathway leading to phorbol 12-myristate 13-acetate (PMA)-induced differentiation in human promyelocytic HL-60 leukemia cells, we examined the expression of protein kinase C (PKC) isozyme genes in HL-60 cells that are susceptible or resistant to PMA-induced differentiation. The PKC-alpha, -beta, -gamma, -delta, epsilon, and -zeta transcript levels were assessed by Northern blotting, and the PKC-alpha, -beta, and -gamma protein levels were examined by immunoblotting. The PMA-resistant cell variants HL-525 and HL-534 were found to be deficient in the PKC-beta isozyme RNA and protein as compared with the PMA-susceptible HL-60 and HL-205 cell lines. In addition, a "delta-like" PKC RNA species identified in these cells demonstrated a reduced abundance in the HL-525 and HL-534 cells. Southern blot analysis indicated that the observed reduction in PKC-beta gene expression does not appear to be due to a gross deletion or rearrangement of the gene. The expression of the early response genes junB, c-fos, and c-jun was attenuated in PMA-treated HL-525 and HL-534 cells as compared to the PMA-treated HL-60 and HL-205 cells. These results suggest that the signal transduction pathway that leads to PMA-induced differentiation in the HL-60 cell system requires PKC-beta and/or delta-like PKC for the proper expression of the early response genes, and ultimately the expression of genes that define the mature state.

Regulation of IMP dehydrogenase gene expression by its end products, guanine nucleotides

To study the regulation of IMP dehydrogenase (IMPDH), the rate-limiting enzyme of guanine nucleotide biosynthesis, we examined the effects of nucleosides, nucleotides, nucleotide analogs, or the IMPDH inhibitor mycophenolic acid (MPA) on the steady-state levels of IMPDH mRNA. The results indicated that IMPDH gene expression is regulated inversely by the intracellular level of guanine ribonucleotides. We have shown that treatment with guanosine increased the level of cellular guanine ribonucleotides and subsequently reduced IMPDH steady-state mRNA levels in a time- and dose-dependent manner. Conversely, MPA treatment diminished the level of guanine ribonucleotides and increased IMPDH mRNA levels. Both of these effects on the steady-state level of IMPDH mRNA could be negated by cotreatment with guanosine and MPA. The down regulation of IMPDH gene expression by guanosine or its up regulation by MPA was not due to major changes in transcriptional initiation and elongation or mRNA stability in the cytoplasm but rather was due to alterations in the levels of the IMPDH mRNA in the nucleus. These results suggest that IMPDH gene expression is regulated by a posttranscriptional, nuclear event in response to fluctuations in the intracellular level of guanine ribonucleotides.

Amplification and molecular cloning of the IMP dehydrogenase gene of Leishmania donovani

A mutant (MPA100) strain of Leishmania donovania was generated from a wild type (D1700) population by virtue of its ability to survive the selective pressure of gradually increasing concentrations of mycophenolic acid (MPA), an inhibitor of IMP dehydrogenase (IMPDH) activity. Comparative growth experiments revealed that the MPA100 strain was 100-fold more resistant to MPA toxicity and cross-resistant to ribavarin, another inhibitor of IMPDH. A direct comparison of IMPDH levels in D1700 and MPA100 cells showed that the latter expressed at least 20-fold higher enzyme activity. In order to evaluate the mechanism by which MPA100 cells overexpressed IMPDH, the leishmanial gene encoding IMPDH was isolated from a genomic library in EMBL3 by cross-hybridization to a mouse IMPDH cDNA, and a 2.3-kilobase EcoRV-PstI fragment was subcloned into a Bluescript vector and sequenced. The EcoRV-PstI fragment contained an open reading frame of 514 amino acids that encompassed the entire leishmanial IMPDH coding sequence. The predicted amino acid sequence showed a 52.5% identity with that of the corresponding human IMPDH. The codon usage of the leishmanial IMPDH gene reflected a strong bias toward codons containing either G or C in the wobble position. The EcoRV-PstI fragment hybridized to a 3.0-kilobase mRNA that was expressed at 10-20-fold greater levels in the MPA100 cells. Using the EcoRV-PstI fragment as a probe, the increased amount of IMPDH activity and IMPDH mRNA in the MPA100 cells could be attributed to an approximately 10-20-fold amplification of the leishmanial IMPDH gene.

Induction of cell differentiation in melanoma cells by inhibitors of IMP dehydrogenase: altered patterns of IMP dehydrogenase expression and activity

To study the induction of differentiation in human melanoma cells, we treated 12 melanoma cell lines with mycophenolic acid and tiazofurin, inhibitors of IMP dehydrogenase (IMPDH). In all cell lines studied, both agents inhibited cell growth and increased melanin content. However, the degree of growth inhibition did not necessarily correspond to the increase in melanin content. A detailed analysis of the HO and SK-MEL-131 cell lines indicated that mycophenolic acid and tiazofurin caused a time- and dose-dependent increase in the expression of a series of other maturation markers, including formation of dendrite-like structures, tyrosinase activity, and reactivity with the CF21 monoclonal antibody. The growth inhibition and melanogenesis induced by the IMPDH inhibitors was abrogated by the addition of exogenous guanosine. No such effect was observed after treatment of the cells with phorbol 12-myristate 13-acetate or retinoic acid, two other inducers of differentiation in these cells. The mycophenolic acid- and tiazofurin-treated cells also showed an increased level of IMPDH mRNA and protein, perhaps because of compensation for the inhibitor-mediated decrease in IMPDH activity. In contrast, treatment with phorbol 12-myristate 13-acetate or retinoic acid resulted in decreased levels of IMPDH mRNA and protein. The lack of a consistent pattern of IMPDH expression in the cells treated with IMPDH inhibitors and phorbol 12-myristate 13-acetate or retinoic acid suggests that the altered expression of IMPDH is not a general requirement for the induction of cell differentiation in these cells. Our results also suggest that IMPDH inhibitors may provide a useful approach to circumvent the differentiation block in melanoma.

Cell differentiation and altered IMP dehydrogenase expression induced in human T-lymphoblastoid leukemia cells by mycophenolic acid and tiazofurin

The IMP dehydrogenase inhibitors mycophenolic acid (MPA) and tiazofurin (TZ) induce a time- and dose-dependent inhibition of cell growth, as well as differentiation in T-lymphoid CEM-2 leukemia cells. The differentiated cells have acquired a suppressor/cytotoxic T-lymphocyte phenotype characterized by reactivity with maturation-specific monoclonal antibodies. Coadministration of guanosine and hypoxanthine reduces the growth inhibition and diminishes the induction of differentiation by either MPA or TZ. No such reduction was observed for differentiation induced by phorbol 12-myristate 13-acetate (PMA), another inducer of a suppressor/cytotoxic phenotype in CEM-2 cells. During the first 2 days of treatment with MPA or TZ, a pattern of stable IMPDH mRNA levels and increased amounts of cellular enzyme was observed, perhaps, because of compensation for the inhibitor-mediated decrease in cellular IMPDH activity or a MPA- or TZ-mediated decrease in proteolysis of IMPDH. PMA treatment decreased the levels of IMPDH mRNA, protein, and activity. In addition, treatment of CEM-2 cells with either IMPDH inhibitors or PMA caused different alterations of the ribonucleotide pools. The lack of a consistent pattern of IMPDH expression in CEM-2 cells treated with IMPDH inhibitors or PMA indicates that no general association exists between the induction of cell differentiation and the expression of IMPDH. Nevertheless, our results indicating that IMPDH inhibitors can induce differentiation in CEM-2 cells suggest that this treatment may provide a useful approach to circumvent the differentiation block in some tumor cells.

Expression of IMP dehydrogenase in differentiating HL-60 cells

Addition of mycophenolic acid to cultures of HL-60 cells results in a decreased cellular level of guanine nucleotides and the induction of cell differentiation. During the early stages of this induction, steady-state levels of cellular IMP dehydrogenase (IMPDH), messenger RNA (mRNA), and protein are increased, perhaps because of cellular compensation for the inhibition of IMPDH activity. The subsequent decrease in IMPH mRNA and protein levels after several days of treatment suggests a change in the control of IMPDH expression. In contrast to the pattern of increased IMPDH expression observed in the mycophenolic acid-treated cells, treatment of HL-60 cells with two other inducers of differentiation, namely retinoic acid and phorbol 12-myristate 13-acetate, resulted in stable or decreased levels of cellular IMPDH mRNA and protein. However, the kinetics of this expression were different. These results suggest that a number of factors influence the regulation of IMPDH expression during the induction of HL-60 cell differentiation, including the nature of the inducer. A decrease in the cellular IMPDH activity was observed for all of the inducers, suggesting that this decreased activity may be a determining factor in the acquisition of a mature phenotype in the HL-60 cells.

A protein containing the cystic fibrosis antigen is an inhibitor of protein kinases

To investigate myeloid cell maturation, we established a panel of monoclonal antibodies that recognize myeloid cell nuclear antigens. One of these monoclonal antibodies was used to purify a specific protein complex (PC) from a human spleen. This PC, which is present at high levels in peripheral blood monocytes and granulocytes, contains a protein that is the cystic fibrosis (CF) antigen. The purified PC was shown to inhibit the activity of casein kinase I and II but not cAMP-dependent protein kinase, protein kinase C, v-abl tyrosine kinase, or insulin receptor tyrosine kinase. The observed Ki values for casein kinases I and II purified from several sources were 1 microM or less. Furthermore, the addition of the purified PC to a nuclear extract from human cells was able to prevent protein kinase-mediated stimulation of RNA polymerase activity. The unique inhibitory character of the PC and its elevated levels in monocytes and granulocytes and of the CF antigen in CF patients implies that this complex may be associated with myeloid cell functions and perhaps with the cause or consequence of the clinical manifestations of CF.

Cloning and sequence analysis of the human and Chinese hamster inosine-5'-monophosphate dehydrogenase cDNAs

Inosine-5'-monophosphate dehydrogenase, a key enzyme in the regulation of guanine nucleotide biosynthesis, was purified to homogeneity; and a polyclonal antibody directed against the purified protein was used to isolate human and Chinese hamster IMP dehydrogenase cDNA clones. These clones were sequenced and found to contain an open reading frame of a protein containing 514 amino acids. A sequence of 35 amino acids obtained by analysis of the purified protein is identical to a segment of the protein sequence deduced from the IMP dehydrogenase cDNA. The molecular mass of the deduced protein is 56 kDa, which is the observed molecular mass of the purified protein and of the immunoprecipitated in vitro translation product. Comparison of the protein sequences deduced from the human and Chinese hamster cDNA clones indicates only eight amino acid differences, suggesting that IMP dehydrogenase is a highly conserved protein.

Amplification of the IMP dehydrogenase gene in Chinese hamster cells resistant to mycophenolic acid

The regulation of IMP dehydrogenase (IMPDH) was analyzed in Chinese hamster V79 cell variants that exhibit different degrees of resistance to the cytotoxic effect of mycophenolic acid, a specific inhibitor of IMPDH. Western blot (immunoblot) analysis with an IMPDH antiserum revealed a 14- to 27-fold increase in the amount of enzyme in the mycophenolic acid-resistant cells. The antiserum was also used to screen for a phage containing the IMPDH cDNA sequence from a lambda gt11 expression library. Northern blot (RNA blot) analyses of total cellular and poly(A)+ RNA showed that an IMPDH cDNA probe hybridized to a 2.2-kilobase transcript, the amount of which was associated with increased resistance. Southern blotting with the probe indicated an amplification of the IMPDH gene in the mycophenolic acid-resistant cells. Our findings suggest that the acquired mycophenolic acid resistance of the V79 cell variants is associated with increases in the amount and activity of IMPDH and the number of IMPDH gene copies.