Levels of ornithine aminotransferase messenger RNA under conditions of cyclic AMP induction in cultured hepatocytes

Overexpression of ornithine aminotransferase: consequences on amino acid homeostasis

Ornithine aminotransferase (OAT) is a reversible enzyme expressed mainly in the liver, kidney and intestine. OAT controls the interconversion of ornithine into glutamate semi-aldehyde, and is therefore involved in the metabolism of arginine and glutamine which play a major role in N homeostasis. We hypothesised that OAT could be a limiting step in glutamine–arginine interconversion. To study the contribution of the OAT enzyme in amino acid metabolism, transgenic mice that specifically overexpress human OAT in the liver, kidneys and intestine were generated. The transgene expression was analysed byin situhybridisation and real-time PCR. Tissue (liver, jejunum and kidney) OAT activity, and plasma and tissue (liver and jejunum) amino acid concentrations were measured. Transgenic male mice exhibited higher OAT activity in the liver (25 (sem4)v.11 (sem1) nmol/min per μg protein for wild-type (WT) mice;P < 0·05) but there were no differences in kinetic parameters (i.e.Kmand maximum rate of reaction (Vmax)) between WT and transgenic animals. OAT overexpression decreased plasma and liver ornithine concentrations but did not affect glutamine or arginine homeostasis. There was an inverse relationship between ornithine levels and OAT activity. We conclude that OAT overexpression has only limited metabolic effects, probably due to the reversible nature of the enzyme. Moreover, these metabolic modifications had no effect on phenotype.

Rat colon ornithine and arginine metabolism: coordinated effects after proliferative stimuli

Ornithine decarboxylase (ODC) catalyzes the first step in the polyamine biosynthetic pathway, a highly regulated pathway in which activity increases during rapid growth. Other enzymes also metabolize ornithine, and in hepatomas, rate of growth correlates with decreased activity of these other enzymes, which thus channels more ornithine to polyamine biosynthesis. Ornithine is produced from arginase cleavage of arginine, which also serves as the precursor for nitric oxide production. To study whether short-term coordination of ornithine and arginine metabolism exists in rat colon, ODC, ornithine aminotransferase (OAT), arginase, ornithine, arginine, and polyamine levels were measured after two stimuli (refeeding and/or deoxycholate exposure) known to synergistically induce ODC activity. Increased ODC activity was accompanied by increased putrescine levels, whereas OAT and arginase activity were reduced by either treatment, accompanied by an increase in both arginine and ornithine levels. These results indicate a rapid reciprocal change in ODC, OAT, and arginase activity in response to refeeding or deoxycholate. The accompanying increases in ornithine and arginine concentration are likely to contribute to increased flux through the polyamine and nitric oxide biosynthetic pathways in vivo.

Sequence of the promoter region of the mouse gene encoding ornithine aminotransferase

We have isolated and sequenced the promoter region of the mouse gene (mOAT) encoding ornithine aminotransferase. A comparison of these mOAT sequences with previously reported sequences for the rat and human genes encoding OAT, rOAT and hOAT, respectively, revealed a 256-bp region flanking the transcription start point that is highly conserved between the three genes. This region contains sequence motifs resembling binding sites for general transcription factors, as well as other trans-acting regulatory proteins.

Promoter region of the rat gene encoding ornithine aminotransferase: transcriptional activity, sequence, and DNase-I-hypersensitive sites

In the rat, the gene (rOAT) encoding ornithine aminotransferase (OAT) is expressed in all cell types examined; however, regulation of rOAT expression is complex and cell-type specific. Various regions of the rOAT 5' flanking domain were cloned upstream from the cat reporter gene, and the expression of these OAT::cat fusions was examined following transfection into rat kidney epithelial cells (NRK-52E), human embryonic kidney cells (293), and rat hepatoma cells (H-4-II-E). Although these experiments suggested the presence of one or more positive regulatory elements between nucleotides -661 and -158, and one or more negative elements upstream from nt -897, none of these putative elements appeared to function in a cell-type-specific manner. The nt sequence of 2531 bp of the rOAT domain flanking the promoter revealed several putative promoter/enhancer elements in positions analogous to the human OAT gene, numerous AGGTCA-like motifs related to the binding sites for the estrogen and thyroid hormone receptors, and multiple motifs resembling a putative regulatory element associated with genes encoding enzymes of the urea cycle. Finally, sensitivity of the 5' end of rOAT to cleavage by DNase I was examined, as DNase-I-hypersensitive sites (DHS) are often found in association with cis-acting regulatory elements. Two DHS were identified; one DHS approximately 140 bp upstream, and the second DHS approximately 300 bp downstream, of the transcription start point (tsp). These data provide the foundation upon which to base future studies aimed at elucidating the molecular mechanisms through which rOAT expression is regulated in a cell-type specific manner.

Translational control of ornithine-delta-aminotransferase (OAT) by estrogen

Ornithine-delta-aminotransferase (OAT) catalyzes the reversible transamination of ornithine to glutamate semialdehyde. OAT is abundant in liver, kidney and retina; hereditary deficiency of the enzyme leads to chorioretinal degeneration. Studies of OAT regulation in retinoblastomas have revealed an alternatively spliced OAT mRNA, which contains an additional exon (exon 2) in the 5' untranslated region. Estrogen and thyroid hormone were previously shown to increase OAT mRNA levels approximately 3-fold and 5-fold, respectively, in these cells. To determine the mechanism of hormonal action in retinoblastomas, we performed nuclear transcription assays and analyzed the distribution of OAT mRNAs in individual fractions of a polysome gradient. Thyroid hormone increased the rate of transcription of the OAT mRNA in these cells. Estrogen did not stimulate transcription; it was associated with increased translation, since it resulted in a shift of the major (spliced) OAT mRNA species into denser fractions of the polysome gradient. Cycloheximide treatment suggested that the latter effect was due to increased initiation of translation. The unspliced OAT mRNA, which is inefficiently compared to the spliced mRNA, was insensitive to estrogen in these experiments.

Isolation and characterization of the rat gene encoding ornithine aminotransferase

Herein we describe the isolation and characterization of the rat gene encoding ornithine aminotransferase (rOAT). Six unique genomic clones were characterized and assigned to two nonoverlapping contigs representing approx. 33 kb of the rat genome. The 5' contig contains the rOAT promoter, exons 1 and 3, and a portion of exon 4; an exon corresponding to exon 2 of the human OAT gene (hOAT) was not identified. The rOAT promoter contains several putative regulatory elements in positions similar to hOAT. The 3' contig contains exons 7 through 11 in their entirety. Data presented and discussed herein suggest that approx. 3.0 kb of uncloned genomic DNA, containing the remainder of exon 4 and all of exons 5 and 6, separate the two contigs. Together, these data suggest that rOAT extends over approx. 20 kb and is organized into at least 10 exons, thereby closely resembling hOAT in size and exon/intron organization. Isolation of rOAT provides an important tool for examining the molecular mechanisms through which estrogen and thyroid hormone regulate transcription of this gene in a cell-type specific manner.

Analysis of the human ornithine aminotransferase gene family

Ornithine aminotransferase is a mitochondrial matrix enzyme that is deficient in patients with gyrate atrophy, an autosomal recessive disease of the eye. Southern blots of human DNA probed with a previously characterized OAT cDNA showed a complex pattern of gene fragments, suggesting a gene family. Hybridization of these blots with 5' and 3' OAT cDNA probes indicated that there are at least three to four copies of the OAT (approximately 22 kbp) and OAT-related gene sequence(s). We have isolated and partially characterized human OAT gene clones from total genomic and X-chromosome DNA libraries. Sequence analysis confirmed the following previously reported findings on the functional OAT gene: 11 exons, ten introns, an atypical TATA box (TTTAA), two CCAAT boxes, several GC-rich binding sites, 5' sequence homologous to SV40 enhancer core sequence (GTGGA/GA/GA/GG) and promoter region of three urea cycle enzymes (GTATCCTGCCCTC). In addition, we extended the OAT gene sequence in both the 5' and 3' directions and found its promoter region also contained a sequence homologous to the progesterone receptor (TGTTCA/TCC/T), several of the glucocorticoid responsive element (AGAACA), a cyclic AMP-responsive element (TGACGTCG), and recognition motifs for transcription factors AP-2, NF1 and Sp1. Partial sequence analyses of X-chromosome clones demonstrated an intron-less pseudogene with 77% identity to the functional OAT gene. These results demonstrate that the OAT gene is a gene family that contains both functional and related OAT gene sequence(s).

Inhibition of cyclic AMP-dependent induction of ornithine aminotransferase by simple carbohydrates in cultured hepatocytes

Glucose administration inhibits the induction of ornithine aminotransferase (OAT) in both the whole animal and cultured hepatocytes. We have examined the ability of several hexoses and related molecules to inhibit the cAMP-dependent induction of OAT in primary cultures of adult rat hepatocytes. The hexoses (D-glucose, fructose, sorbitol, sorbose, and mannose) that were effective as inhibitors of OAT induction also resulted in accumulation of lactate in the culture medium, although lactate itself was not effective as an inhibitor. The hexoses and related 6-carbon structures (galactose, L-glucose, 2-deoxyglucose, 3-O-methylglucose, rhamnose, mannitol, and inositol) that were not effective as inhibitors of OAT induction did not result in accumulation of lactate in the culture medium. These results suggest that the carbohydrate repression of hepatic OAT requires metabolism of the carbohydrate by the liver cell. Upon addition to the culture medium of several compounds related to carbohydrate metabolism, many (ribose, xylitol, dihydroxyacetone, and glycerol) exhibited an inhibitory effect, with glycerol exhibiting the greatest effect. Fructose and glycerol inhibit OAT induction in the presence of 2-deoxyglucose, suggesting that the inhibitory effect of nonglucose carbohydrates is not occurring through conversion to glucose. The carbon sources observed to be most effective as inhibitors of OAT induction (glycerol, fructose, sorbitol, and sorbose result in more than 90% inhibition at 25 mM) all enter the glycolytic pathway at the triosephosphate level. The mechanism of the inhibitory effect of simple carbohydrates on OAT induction is not known but may involve an increase in certain glycolytic intermediates. Glucose and the related carbon sources exert their effect by inhibiting the cAMP-dependent increase in OAT synthesis. The cAMP-dependent increase in OAT mRNA was inhibited by fructose. These findings suggest that the carbohydrate inhibition of the cAMP-dependent increase in OAT synthesis occurs at a pretranslational level.

Regulation of lactate dehydrogenase gene expression by cAMP-dependent protein kinase subunits

The studies described in this report suggest a rather complex, albeit incomplete, sequence of molecular events that we believe form part of the cascade of reactions through which a series of hormones, via cAMP, regulates the expression of specific gene products. The majority of our own studies relate to cAMP-mediated induction of LDH. Some, if not all, of the molecular steps discussed in this paper may ultimately be recognized as part of a universal mechanism by which cAMP controls gene expression in higher eukaryotes. The idea of a functional role for cAMP-dependent protein kinase subunits in cAMP-mediated gene control has already had experimental support, but our identification of the regulatory subunit RII as a topoisomerase now more firmly points to a complex function for the kinase in regulating gene function at the DNA level. We look forward to the elucidation of the function of those nuclear proteins that serve as substrate for the catalytic subunit of cAMP-dependent protein kinase. Further studies related to the molecular interaction of RII with chromosomal DNA should be a fruitful area for future research.