Regulation of ornithine aminotransferase mRNA levels in rat kidney by estrogen and thyroid hormone

Adventures in hepatocarcinogenesis

Neoplasia is a heritably altered, relatively autonomous growth of tissue. Hepatocarcinogenesis, the pathogenesis of neoplasia in liver, as modeled in the rat exhibits three distinct, quantifiable stages: initiation, promotion, and progression. Simple mutations and/or epigenetic alterations may result in the irreversible stage of initiation. The stage of promotion results from selective enhancement of cell replication and selective inhibition of cellular apoptosis of initiated cells dependent on the genetic and/or epigenetic alterations of the latter. The irreversible stage of progression results from initial karyotypic alterations that evolve into greater degrees of genomic instability. The initial genomic alteration in the transition from promotion to progression may involve primarily epigenetic mechanisms driven by epigenetic and genetic alterations fixed during the stage of promotion.

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.

Ornithine metabolism in male and female rat kidney: mitochondrial expression of ornithine aminotransferase and arginase II

In the kidney, l-ornithine is reabsorbed along the proximal convoluted tubule (PCT), transported by basolateral carriers, and produced by arginase II (AII). Here, the renal metabolic fate of l-ornithine was analyzed in male and female rats. Kidneys and renal zones were dissected and used for Western blot analysis, immunofluorescence, and electron microscopic studies. Ornithine aminotransferase (OAT) and AII were localized using specific antibodies. Ornithine oxidation was determined by incubating microdissected tubules with l-[1-14C] or l-[U-14C]ornithine in the presence or absence of energy-providing substrates. Ornithine decarboxylase (ODC) mRNAs were localized by in situ hybridization. The 48-kDa OAT protein was detected in male and female kidneys, but its level was fourfold higher in the latter. OAT relative distribution increased from the superficial cortex toward the outer medulla to reach its highest level. Almost all OAT protein was localized in cortical and medullary proximal straight tubules (CPST and OSPST, respectively). In proximal straight tubule (PST), AII protein distribution overlapped that of OAT. No gender difference in AII protein level was found. OAT and AII were colocalized within PST mitochondria. l-[1-14C]ornithine decarboxylation occurred in all tubules, but predominantly in proximal tubules. l-[1-14C]ornithine decarboxylation was enhanced when l-[1-14C]ornithine was given to tubules as the sole substrate. The use of l-[U-14C]ornithine demonstrated the complete oxidation of ornithine. In conclusion, the OAT gene was expressed more in female rat proximal tubules than in male. Because OAT and AII proteins overlapped in PST mitochondria, l-arginine-derived ornithine may be preferentially converted to l-glutamate, as proven by ornithine oxidation. However, the coexpression of ODC, glutamate decarboxylase, and glutamine synthetase in PST suggests that l-ornithine can also be metabolized to putrescine, GABA, and l-glutamine. The fate of l-ornithine may depend on the cellular context.

Molecular cloning and characterization of Drosophila ornithine aminotransferase gene

The cDNA encoding the Drosophila ananassae ornithine aminotransferase (OAT, EC 2.6.1.13) precursor has been cloned and characterized. The predicted OAT protein sequence is 433 amino acids long with a molecular mass of 47,352 Da and is highly homologous to a mammalian OAT, which is a mitochondrial matrix enzyme and is matured by processing of its amino terminal presequence peptide. The Drosophila OAT has characteristics of leader peptides present in mitochondrial proteins. Immunoblotting experiments using polyclonal antibodies against the partial sequence of the OAT protein revealed that the OAT monomer has a molecular mass of 44 kDa. These results suggest that the Drosophila OAT is also processed and localized in the mitochondria. Quantitation of the OAT mRNA and measurement of the OAT activity during fly development show that OAT is expressed at high levels in the fat body of the third instar larvae in both D. ananassae and D. melanogaster.

Ornithine-delta-aminotransferase in lactating bovine mammary glands

The occurrence and subcellular distribution of ornithine-delta-aminotransferase have been studied in lactating bovine mammary glands. The enzyme is localized in the mitochondria and has a unique thermal reaction profile that distinguishes it from putative liver and kidney isozymes. The enzyme concentration in the gland correlates well with a role in the conversion of ornithine into the proline precursor, L-delta 1-pyrroline-5-carboxylate. However, an unusually high Michaelis constant for the mitochondrial enzyme (8.4 mM) raises the question of enzyme efficiency in vivo such that this pathway needs to be considered in estimating barriers to protein secretion into milk.

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.

The ornithine aminotransferase-encoding gene family of rat: cloning, characterization, and evolutionary relationships between a single expressed gene and three pseudogenes

As a first step towards understanding the molecular mechanisms through which the expression of the gene (OAT) encoding ornithine aminotransferase (OAT) is regulated in a tissue-specific manner, we have used a near full length OAT cDNA to isolate related sequences from a rat genomic DNA library. Twenty-one unique clones representing five contigs and spanning approximately 140 kb of genomic DNA were isolated and characterized. From these clones we have identified a single expressed OAT gene and three processed pseudogenes. The comparison of the EcoRI, BamHI, and HindIII fragments contained within these genomic clones with those detected in total genomic DNA by the cDNA probe suggests that essentially all of the OAT-related sequences in the rat genome have been isolated. Thus, the tissue-specific regulation of OAT gene expression appears to be effected through a single expressed gene. Data are presented which suggest that the OAT-1, OAT-2, and OAT-3 pseudogenes arose approximately 28.5, 7.3, and 25.1 Myr ago, respectively. Mutation rates are presented for each codon position of the expressed rat and human OAT genes. The region of the rat genome flanking the boundary of the OAT-3 pseudogene is of additional interest as it shares considerable identity to sequences contained within expressed genes and flanking other processed pseudogenes.

Insulin- and thyroid hormone-independent adaptation of myofibrillar proteolysis to glucocorticoids

Myofibrillar protein breakdown in skeletal muscle progresses through two distinct phases in response to chronic glucocorticoid administration in the rat, i.e., an early phase lasting 4-5 days, during which proteolysis increases followed by a later phase during which proteolysis decreases. The possible involvement of insulin and the iodothyronines in this phenomenon has now been examined. Diabetic, thyroidectomized, and normal rats were treated with corticosteroid for 10-11 days, and at timed intervals muscle proteolysis was evaluated by measuring the release of 3-methyl-L-histidine (3-MH) and tyrosine from the perfused hindquarter as well as the excretion of 3-MH in the urine. Corticosterone (CTC) administration to normal rats increased plasma insulin, whereas plasma 3,5,3'-triiodothyronine responded with an early rise followed by a fall after 4-5 days. However, the biphasic response of myofibrillar proteolysis to chronic glucocorticoid treatment was not abolished in CTC-treated diabetic or thyroidectomized rats. CTC treatment increased release of tyrosine by perfused muscle of diabetic rats but, unlike 3-MH release, did not diminish later. Thus the adaptation of myofibrillar proteolysis to chronic glucocorticoid treatment appears to be independent of insulin and thyroid hormones. However, insulin may play a role in curtailing glucocorticoid-induced breakdown of nonmyofibrillar proteins.

Cellular mechanism of action of thyroid hormones

It has emerged in the last decade that the molecular mechanism of action of thyroid hormones resembles that of steroids; thyroid hormones indeed exert their effects mainly by directly regulating gene expression, on association with specific chromatin-bound receptors. Of the two thyroid hormones, thyroxine (T4) appears to be a sort of prohormone, whereas triiodothyronine (T3) seems to be the active form; in this respect, T4-deiodination, which occurs at the level of the target tissues, may be crucial in the local homeostasis of T3. Moreover, many cellular compartments, other than the nucleus, can bind thyroid hormone, and at least some of these further sites might play some role in modulating T3 supply to the nucleus. The binding of the T3-receptor complex to chromatin is likely to regulate the structural organization of specific genes and, in some instances, of the chromatin as a whole.

Specific gene expression during compensatory renal hypertrophy in the rat

The compensatory growth of the kidney which is induced by unilateral nephrectomy is a highly regulated process resulting principally in hypertrophy of the remaining kidney. The events which regulate this process are unknown. We have examined the levels of transcripts for the proto-oncogenes, myc, H-ras, K-ras, and fos, and the cellular genes, H4 histone, ornithine aminotransferase, and gamma-glutamyl transpeptidase, following unilateral nephrectomy in the rat. The pattern of expression of c-myc, c-H-ras, and c-K-ras during compensatory growth of the kidney differs from the pattern of expression of these proto-oncogenes during liver regeneration, in which, unlike the kidney, hyperplasia rather than hypertrophy predominates. The lack of change in the abundance of these proto-oncogene transcripts following unilateral nephrectomy suggests a primary relationship between the expression of these proto-oncogenes and DNA synthesis and indicates there may be separate signals for cell growth, one to double cell size and one to replicate DNA. Increased mRNA transcripts for the enzymes ornithine aminotransferase and gamma-glutamyl transpeptidase were induced in the contralateral kidney after nephrectomy. The time course of expression for these two enzymes differs. The early expression of the gamma-glutamyl transpeptidase gene may indicate an involvement of this glutathione-metabolizing enzyme during renal compensatory growth, while the function of the delayed increase in ornithine aminotransferase transcripts in the remaining kidney is not apparent.

Comparison of ornithine aminotransferase activities in the pigment epithelium and retina of vertebrates

1. The specific activities of ornithine aminotransferase (OAT) in the pigment epithelia, retinas, and livers from several classes of vertebrates were assayed. 2. The specific activities of OAT were much higher in the pigment epithelia from mammals and birds than in their respective retinas or livers. 3. Pigment epithelium from porcine eyes had the highest specific activity measured. The specific activity of OAT in the pigment epithelium from the pig was five times higher than the OAT activity in its retina and 13 times higher than the OAT activity in its liver.

Synergism of triiodothyronine and corticosterone on muscle protein breakdown

The concerted effect of triiodothyronine (T3) and corticosterone on muscle protein synthesis and breakdown was studied. Thyroidectomized young male rats were treated with T3 (1.5 microgram/100 g body weight per day), corticosterone (10 mg/100 g body weight per day) and both T3 and corticosterone for 4 days. On the 3rd day of the experiment urine was collected to measure N tau-methylhistidine excretion as an index of muscle protein breakdown. On the last day of the experiment, the rates of protein synthesis in skeletal muscles were measured by the large-dose [3H]phenylalanine method. N tau-Methylhistidine excretion was slightly increased by T3 treatment and it was increased about 3-times by corticosterone treatment. When both T3 and corticosterone were administered, it was increased about 6-fold. The rate of muscle protein breakdown calculated from the difference between the rate of protein synthesis and the growth rate was consistent with these findings. The rate of muscle protein synthesis was increased by T3, and it was decreased by corticosterone. The rate was the same as that of the thyroidectomized control group when the animals were given T3 and corticosterone, showing that T3 restrained the inhibiting effect of corticosterone on muscle protein synthesis. The results indicate that a physiological level of T3 enhances the catabolic action of pharmacological doses of glucocorticoids on muscle protein breakdown.