Feedback inhibition of amidophosphoribosyltransferase regulates the rate of cell growth via purine nucleotide, DNA, and protein syntheses

To clarify the contributions of amidophosphoribosyltransferase (ATase) and its feedback regulation to the rates of purine de novo synthesis, DNA synthesis, protein synthesis, and cell growth, mutated human ATase (mhATase) resistant to feedback inhibition by purine ribonucleotides was engineered by site-directed mutagenesis and expressed in CHO ade (-)A cells (an ATase-deficient cell line of Chinese hamster ovary fibroblasts) and in transgenic mice (mhATase-Tg mice). In Chinese hamster ovary transfectants with mhATase, the following parameters were examined: ATase activity and its subunit structure, the metabolic rates of de novo and salvage pathways, DNA and protein synthesis rates, and the rate of cell growth. In mhATase-Tg mice, ATase activity in the liver and spleen, the metabolic rate of the de novo pathway in the liver, serum uric acid concentration, urinary excretion of purine derivatives, and T lymphocyte proliferation by phytohemagglutinin were examined. We concluded the following. 1) ATase and its feedback inhibition regulate not only the rate of purine de novo synthesis but also DNA and protein synthesis rates and the rate of cell growth in cultured fibroblasts. 2) Suppression of the de novo pathway by the salvage pathway is mainly due to the feedback inhibition of ATase by purine ribonucleotides produced via the salvage pathway, whereas the suppression of the salvage pathway by the de novo pathway is due to consumption of 5-phosphoribosyl 1-pyrophosphate by the de novo pathway. 3) The feedback inhibition of ATase is more important for the regulation of the de novo pathway than that of 5-phosphoribosyl 1-pyrophosphate synthetase. 4) ATase superactivity leads to hyperuricemia and an increased bromodeoxyuridine incorporation in T lymphocytes stimulated by phytohemagglutinin.

The rate of cell growth is regulated by purine biosynthesis via ATP production and G(1) to S phase transition

We recently showed that an increased supply of purine nucleotides increased the growth rate of cultured fibroblasts. To understand the mechanism of the growth rate regulation, CHO K1 (a wild type of Chinese hamster ovary fibroblast cell line) and CHO ade (-)A (a cell line deficient in amidophosphoribosyltransferase, a rate-limiting enzyme of the de novo pathway) were cultured under various conditions. Moreover, a defective de novo pathway in CHO ade (-)A cells was exogenously restored by 5-amino-4-imidazole-carboxamide riboside, a precursor of the de novo pathway. The following parameters were determined: the growth rate of CHO fibroblasts, the metabolic rate of the de novo pathway, the enzyme activities of amidophosphoribosyltransferase and hypoxanthine phosphoribosyltransferase, the content of intracellular nucleotides, and the duration of each cell-cycle phase. We concluded the following: (i) Purine de novo synthesis, rather than purine salvage synthesis or pyrimidine synthesis, limits the growth rate. (ii) Purine nucleotides are synthesized preferentially by the salvage pathway as long as hypoxanthine is available for energy conservation. (iii) The GTP content depends on the intracellular ATP content. (iv) Biosynthesis of purine nucleotides increases the growth rate mainly through ATP production and promotion of the G(1)/S transition.

Different promoter usage and multiple transcription initiation sites of the interleukin-1 receptor-related human ST2 gene in UT-7 and TM12 cells

The ST2 gene encodes receptor-like molecules that are very similar to the type I interleukin-1 receptor. Two distinct types of the ST2 gene products, ST2 (a soluble secreted form) and ST2L (a transmembrane form) are produced by alternative splicing. Here we demonstrate that the human ST2 gene has two alternative promoters followed by distinct noncoding first exons, which are located more than 8 kb apart and are spliced to the common exon 2 containing the translation initiation site. Within 1001 bp upstream of the transcription initiation site of the cloned distal promoter, there are four GATA-1. The main promoter used for the expression of the ST2 gene in UT-7, a human leukaemic cell line, is distinct from that in TM12, a human fibroblastic cell line. Although UT-7 cells use both distal and proximal promoters, the distal promoter is used dominantly for expression of both ST2 and ST2L mRNA. On the other hand, almost all transcription in TM12 cells starts from the proximal promoter. These results contrast with those of former studies on the rat system, in which ST2 and ST2L mRNA were generated by use of the proximal and distal promoters, respectively. Furthermore, UT-7 cells use multiple transcription initiation sites in both the proximal and distal promoters, whereas the transcription of the ST2 gene in TM12 cells starts at a unique site. Intriguingly, these results suggest that ST2 and ST2L proteins have distinct functions in different cells within different biological systems, such as those of growth control, differentiation and immunological responses.

Clonal expansion of CD4+ TCRbetabeta+ T cells in TCR alpha-chain- deficient mice by gut-derived antigens

A population of CD4+ alpha-beta+ T cells increases in the mucosal and peripheral lymphoid tissues of TCRalpha-chain-deficient mice with inflammatory bowel disease. The alpha-beta+ T cells, which produce predominantly IL-4, mediate the proliferation of colonic epithelial crypts and the infiltration of large numbers of IgA-producing plasma cells into the lamina propria of the colon. To examine whether enteric Ags were recognized by a population of monoclonal alpha-beta+ T cells leading to the intestinal inflammation, we examined the usage and clonotypes of TCR expressed by the alpha-beta+ T cells in TCRalpha-chain-deficient mice with inflammatory bowel disease. Analyses of immunoprecipitates by two dimensional electrophoresis and single-cell RT-PCR revealed that TCR of the alpha-beta+ T cells was a homodimer of beta-chains that was capable of recognizing luminal bacterial Ags. PCR single-strand conformation polymorphism analysis of TCR Vbeta transcripts revealed monoclonal accumulation of the alpha-beta+ T cells in the colonic lamina propria of the diseased mice. DNA sequencing revealed the accumulation of the alpha-beta+ T cells with the same CDR3 sequences in the colon. These findings suggest that the pathogenic CD4+ alpha-beta+ T cells expressing a homodimeric form of the TCRbeta-chains can be clonally expanded upon the stimulation with gut-derived Ags.

Molecular cloning of a human cDNA for the 41-kDa phosphoribosylpyrophosphate synthetase-associated protein

A human cDNA encoding 41-kDa phosphoribosylpyrophosphate (PRPP) synthetase (PRS)-associated protein (PAP41) was cloned from two expressed sequence tag (EST) clones having the nucleotide similarity of 61.5 and 70.0% to human PAP39 cDNA. The predicted open reading frame of 1107 base pairs (bp) has the nucleotide identity of 91.8% to rat PAP41 and encodes a protein of 369 amino acids with a calculated molecular weight (MW) of 40,925. The deduced amino acid sequence exhibits the 98.9% identity to rat PAP41 and 72.2, 50.6, and 50.0% identity with human PAP39, PRS I, and PRS II, respectively, but lacks the PRPP binding site. Southern blot analysis suggested that the PAP41 gene exists as a single copy in the human genome. The single PAP41 mRNA of about 2.1 kb was shown to be present in five human cell lines by Northern blot analysis.

Rat genomic structure of amidophosphoribosyltransferase, cDNA sequence of aminoimidazole ribonucleotide carboxylase, and cell cycle-dependent expression of these two physically linked genes

Genomic structure of rat amidophosphoribosyltransferase (ATase; EC 2.4.2.14), which catalyzes the first committed step in de novo purine nucleotide synthesis, was determined by polymerase chain reaction (PCR)-based methods. There are 11 exons and all exon-intron boundaries were conserved among rat, human, and chicken ATase genes. A rat aminoimidazole ribonucleotide carboxylase (AIRC) cDNA encoding a bifunctional enzyme of AIRC (EC 4.1.1.21) at step 6 and SAICAR synthetase (EC 6.3.2.6) at step 7 in de novo purine nucleotide synthesis was cloned and sequenced. The size of the cloned rat AIRC cDNA was 1329 bp, and amino acid identity with human and chicken AIRC was 96 and 85%, respectively. The intergenic sequence using a phage clone and the PCR product disclosed that ATase and AIRC genes are physically linked with the 736 bp sequence between the translation start sites, and the determination of the transcriptional start sites by the primer extension assay for these genes disclosed that distance between the two major transcriptional start sites is 585 bp. The amount of mRNAs of both genes showed approx. 5-6-fold increase in G1/S phase of the cell cycle over those in G0 phase in synchronized rat 3Y1 fibroblasts.

Molecular cloning of a group of mouse pancreatic islet beta-cell-related genes by random cDNA sequencing

To understand the molecular basis of glucose concentration-responsive insulin synthesis and secretion from pancreatic islet beta cells, a group of pancreatic islet beta-cell-related cDNAs was cloned. A pair of cDNA libraries was constructed from a mouse pancreatic islet beta-cell line of MIN6, which was cultured in either high glucose or low glucose media. By applying a random cDNA sequencing approach, 503 and 395 independent species were obtained from a total of 1,011 and 762 clones in the high glucose and low glucose library, respectively. The unknown genes comprised the majority of about 70% independent clones in both libraries. In Northern blot analysis, 311 (69.4%) of 448 independent clones showed positive signals within 72 h of autoradiographic exposure. Surprisingly, 150 (48.2%) out of 311 positive clones showed positive signals to MIN6 cells, but not to NIH/3T3 fibroblasts. The expression level of three unknown clones were glucose-concentration dependent. Combination of a random cDNA sequencing approach and Northern blot analysis is useful to obtain a large number of novel genes and islet beta-cell-related genes.

Multiple fluorescence-based PCR-SSCP analysis with postlabeling

Multiple fluorescence-based PCR single-strand conformation polymorphism (MF-PCR-SSCP) with postlabeling was developed. The target sequence was amplified by PCR using unlabeled primers. Free dNTPs were removed from the amplified products by ethanol precipitation. The dNTPs at the 3' ends of the amplified DNA fragments were exchanged with fluorescent dUTPs or ddNTPs using Klenow fragment of DNA polymerase I. The DNA fragments labeled with fluorescent dUTPs or ddNTPs were heat denatured and applied to a nondenaturing polyacrylamide gel set on an automated DNA sequencer with a gel temperature-controlling system. The image data were analyzed by the computer program Genescan 672. By use of MF-PCR-SSCP with postlabeling, seven different single base mutations of the human K-ras oncogene were detected even under one electrophoresis condition.

PCR with end trimming and cassette ligation: a rapid method to clone exon-intron boundaries and a 5'-upstream sequence of genomic DNA based on a cDNA sequence

We described a method for PCR amplification of unknown flanking genomic DNA fragments. This method is a combination of PCR with "end-trimming method" and "cassettes and cassette-primers method". In this method, genomic DNA was digested with three different groups of restriction enzymes. DNA in group 1 was digested with BamHI, BglII, FbaI, or MboI. DNA in group 2 was digested with BlnI, NheI, SpeI, or XbaI. DNA in group 3 was digested with SalI or XhoI. Digested DNA in each group was end-trimmed with Klenow fragment of DNA polymerase I in the presence of only one dNTP; dGTP, dCTP, and dTTP for group 1, 2, and 3, respectively. The synthesized cassettes, C1, C2, and C3, had 5'protruding sequences of 5'-ATC-3',5'-TAG-3', and 5'-CGA-3', respectively. Each compatible cassette was ligated to the end-trimmed DNAs in group 1-3, respectively. Nested PCR was then performed using an end-trimmed and cassette-ligated DNA as a template. Primers annealing to known sequences and cassettes were used for the nested PCR. The amplified DNA fragments were electrophoresed on a polyacrylamide gel and purified. The sequences of the DNA fragments were determined after cloning into pBluescript.