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

Abstract

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 ceHlular 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.Cellular systems have mechanisms to regulate the expression of genes that mediate the synthesis of necessary metabolites. These mechanisms must respond to internal and external factors in such a way that an appropriate supply of these metabolites is provided for the cell or organism. In prokaryotic systems, some genes are regulated at the transcriptional level by metabolic end products. In Bacillus subtilis, the pur operon encodes enzymes involved in purine biosynthesis, 11 of which catalyze the formation of IMP from 5-phosphoribosyl-1-pyrophosphate (13). This operon is regulated by the purine end products, adenine and guanine nucleotides. Although end-product regulation of genes is common in prokaryotic systems, no parallel has been reported in the regulation of eukaryotic nucleotide biosyntheSiS. The steady-state mRNA level of human IMP dehydrogenase (IMPDH; EC 1.1.1.205), the rate-limiting enzyme in de novo guanine nucleotide biosynthesis, is elevated in human HL-60 and CEM leukemia and HO melanoma cells after they are treated with the competitive IMPDH inhibitor mycophenolic acid (MPA) (11,20,21). This induction is preceded by a decrease in the level of cellular guanine nucleotides as a consequence of MPA-mediated inhibition of IMPDH enzyme activity. These observations suggest that the level of purine end products may influence the expression of the IMPDH gene, perhaps in a manner similar to that observed for the pur operon genes in B. subtilis. To examine the control of IMPDH gene expression, we treated human and Chinese hamster cell lines with nucleosides, nucleotides, nucleotide analogs, or MPA and analyzed the effect on the expression of the IMPDH gene. We provide evidence that the expression of this gene is indeed regulated by guanine ribonucleo...

Overexpression of a cloned IMP dehydrogenase gene of Candida albicans confers resistance to the specific inhibitor mycophenolic acid

“…In the majority of systems investigated, the levels of resistance to MPA are elevated as the gene dosage of IMP dehydrogenase increases (9,20,23,25,54). To determine the effects of increased gene dosage and/or overexpression of IMH3 on the sensitivity of C. albicans to MPA, we constructed the plasmid pGKIII by inserting the entire IMH3 gene and flanking sequences into the replicative plasmid pRC2312 (7) as described in Materials and Methods.…”

“…Thus, the identification of the IMH3 gene in C. albicans suggested to us that mycophenolic acid (MPA), a specific inhibitor of IMP dehydrogenases, might be used to develop a dominant selectable marker system in Candida. In several other organisms, it has been shown that amplification of an IMP dehydrogenase gene can confer MPA resistance in a gene-dosage-dependent manner by overexpression of the enzyme (9,20,23,25,54). We report the cloning and sequencing of a DNA fragment of C. albicans with a coding region for a protein with high amino acid similarity and identity to known IMP dehydrogenases.…”

p53-independent induction of p21waf1/cip1 contributes to the activation of caspases in GTP-depletion-induced apoptosis of insulin-secreting cells

“…2 Its gene expression is regulated inversely by a post-transcriptional nuclear event in response to fluctuations in the intracellular level of GNs. 48 As a specific inhibitor of IMPDH, MPA diminished GNs levels and, therefore, increased the levels of IMPDH mRNA and amounts of the enzyme. 4,48 In addition, IMPDH activity and its protein and mRNA levels could be downregulated by p53 and, thus, this enzyme (or the resultant changes in cellular levels of GNs) might play an important role in mediating p53-dependent suppression of cell growth.…”

“…48 As a specific inhibitor of IMPDH, MPA diminished GNs levels and, therefore, increased the levels of IMPDH mRNA and amounts of the enzyme. 4,48 In addition, IMPDH activity and its protein and mRNA levels could be downregulated by p53 and, thus, this enzyme (or the resultant changes in cellular levels of GNs) might play an important role in mediating p53-dependent suppression of cell growth. 3,6,35,36 The reciprocal effect of IMPDH on p53 has not been reported.…”

Detection of the mycophenolate-inhibited form of IMP dehydrogenase in vivo

“…The two human IMPDH gene products differ by Ϸ5-fold in MPA sensitivity (9). This heterogeneity has potential clinical significance because these two isoforms coexist in the same cell type and both seem MPAinducible (5,(26)(27)(28). Extension of this assay to patient material should facilitate the detection and measurement of the end point of MPA therapy: inhibition of IMPDH.…”