Insulin-mediated regulation of tyrosine aminotransferase in rat hepatoma cells: inhibition of transcription and inhibition of enzyme degradation

Influence of pregnancy and non-fasting conditions on the plasma metabolome in a rat prenatal toxicity study

The current parameters for determining maternal toxicity (e.g. clinical signs, food consumption, body weight development) lack specificity and may underestimate the extent of effects of test compounds on the dams. Previous reports have highlighted the use of plasma metabolomics for an improved and mechanism-based identification of maternal toxicity. To establish metabolite profiles of healthy pregnancies and evaluate the influence of food consumption as a confounding factor, metabolite profiling of rat plasma was performed by gas- and liquid-chromatography-tandem mass spectrometry techniques. Metabolite changes in response to pregnancy, food consumption prior to blood sampling (non-fasting) as well as the interaction of both conditions were studied. In dams, both conditions, non-fasting and pregnancy, had a marked influence on the plasma metabolome and resulted in distinct individual patterns of changed metabolites. Non-fasting was characterized by increased plasma concentrations of amino acids and diet related compounds and lower levels of ketone bodies. The metabolic profile of pregnant rats was characterized by lower amino acids and glucose levels and higher concentrations of plasma fatty acids, triglycerides and hormones, capturing the normal biochemical changes undergone during pregnancy. The establishment of metabolic profiles of pregnant non-fasted rats serves as a baseline to create metabolic fingerprints for prenatal and maternal toxicity studies.

TATN-1 mutations reveal a novel role for tyrosine as a metabolic signal that influences developmental decisions and longevity in Caenorhabditis elegans

Recent work has identified changes in the metabolism of the aromatic amino acid tyrosine as a risk factor for diabetes and a contributor to the development of liver cancer. While these findings could suggest a role for tyrosine as a direct regulator of the behavior of cells and tissues, evidence for this model is currently lacking. Through the use of RNAi and genetic mutants, we identify tatn-1, which is the worm ortholog of tyrosine aminotransferase and catalyzes the first step of the conserved tyrosine degradation pathway, as a novel regulator of the dauer decision and modulator of the daf-2 insulin/IGF-1-like (IGFR) signaling pathway in Caenorhabditis elegans. Mutations affecting tatn-1 elevate tyrosine levels in the animal, and enhance the effects of mutations in genes that lie within the daf-2/insulin signaling pathway or are otherwise upstream of daf-16/FOXO on both dauer formation and worm longevity. These effects are mediated by elevated tyrosine levels as supplemental dietary tyrosine mimics the phenotypes produced by a tatn-1 mutation, and the effects still occur when the enzymes needed to convert tyrosine into catecholamine neurotransmitters are missing. The effects on dauer formation and lifespan require the aak-2/AMPK gene, and tatn-1 mutations increase phospho-AAK-2 levels. In contrast, the daf-16/FOXO transcription factor is only partially required for the effects on dauer formation and not required for increased longevity. We also find that the controlled metabolism of tyrosine by tatn-1 may function normally in dauer formation because the expression of the TATN-1 protein is regulated both by daf-2/IGFR signaling and also by the same dietary and environmental cues which influence dauer formation. Our findings point to a novel role for tyrosine as a developmental regulator and modulator of longevity, and support a model where elevated tyrosine levels play a causal role in the development of diabetes and cancer in people.

Insulin induces expression of adenosine kinase gene in rat lymphocytes by signaling through the mitogen-activated protein kinase pathway

The activity of adenosine kinase (AK) was significantly impaired in splenocytes isolated from diabetic rats. Administration of insulin to diabetic animals restored AK activity, protein, and mRNA levels in diabetic splenocytes. Experiments performed on cultured rat lymphocytes demonstrated that insulin did not change the stability of AK mRNA. Insulin induced AK gene expression in a dose- and time-dependent manner. Maximal increases in AK mRNA (3.9-fold) and activity level (3.7-fold) were observed at the fourth and fifth hours of cell incubation with 10 nM insulin, respectively. The insulin effect on AK expression was not influenced by dibutyryl cAMP (dcAMP). On the other hand dcAMP weakly increased (1.7-fold) basal expression of AK. Exposure of rat lymphocytes to wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3K), or rapamycin, an inhibitor of mTOR, did not affect the ability of insulin to stimulate expression of AK. Prior treatment of the cells with 10 microM PD98059, an inhibitor of mitogen-activated protein kinase (MAPK) kinase (MEK) completely blocked insulin-stimulated expression of AK gene. Insulin produced a significant transient increase in the tyrosine phosphorylation of ERK1/2, and PD98059 inhibited this phosphorylation. Furthermore exposure of cells to insulin has resulted in transient phosphorylation of Elk-1 on Ser-383 and sustained elevation of c-Jun and c-Fos protein. The maximal phosphorylation of Elk-1 was observed at 15 min, and was blocked by PD98059. We concluded that insulin stimulates AK gene expression through a series of events occurring sequentially. This includes activation of the MAPK cascade and subsequent phosphorylation of Elk-1 followed by increased expression of c-fos and c-jun genes.

Involvement of the Ras/extracellular signal-regulated kinase signalling pathway in the regulation of ERCC-1 mRNA levels by insulin

Expression of DNA repair enzymes, which includes ERCC-1, might be under the control of hormonal and growth factor stimulation. In the present study it was observed that insulin increased ERCC-1 mRNA levels both in Chinese hamster ovary cells overexpressing human insulin receptors (HIRc cells) and in fully differentiated 3T3-L1 adipocytes. To investigate the mechanisms underlying the increase in ERCC-1 gene expression in HIRc cells, we used a variety of pharmacological tools known to inhibit distinct signalling pathways. None of these inhibitors affected the amount of ERCC-1 mRNA in unstimulated cells. The pretreatment of cells with two chemically unrelated phosphatidylinositol 3'-kinase inhibitors, wortmannin and LY294002, failed to block the doubling of ERCC-1 mRNA content by insulin. Similarly, inhibition of pp70 S6 kinase by rapamycin had no apparent effects on this insulin response. In contrast, altering the p21(ras)-dependent pathway with either manumycin, an inhibitor of Ras farnesylation, or PD98059, an inhibitor of the mitogen-activated protein kinase/extracellular signal-regulated protein kinase (ERK) kinase, suppressed the induction of ERCC-1 mRNA by insulin (P<0.001). Furthermore inhibition of RNA and protein synthesis negatively regulated the expression of this insulin-regulated gene (P<0.005). These results suggest that insulin enhances ERCC-1 mRNA levels by the activation of the Ras-ERK-dependent pathway without the involvement of the phosphatidylinositol 3'-kinase/pp70 S6 kinase.

Regulation of tyrosine aminotransferase gene expression by glucocorticoids in quiescent and regenerating liver

Following 70% hepatectomy, the induction of tyrosine amino-transferase mRNA by glucocorticoids was marginal at 1.5 h, significantly impaired between 3 and 8 h and, at 16 h post-hepatectomy, reached a value approx. 5-fold the basal level, similar to the level observed in quiescent liver. The fold induction of the mRNA was accounted for by a similar fold activation of transcription of the gene by glucocorticoids in regenerating but not in quiescent liver; in the latter, activation of transcription was marginal in spite of glucocorticoid-induced hypersensitivity to cleavage by DNase I at the glucocorticoid-dependent enhancer of the gene. The possibility that in quiescent liver glucocorticoids act at a transcriptional step beyond initiation, increasing the rate of elongation or overcoming a blockage in elongation, was excluded. However, a similar fold induction was determined for total and nuclear tyrosine aminotransferase mRNA in the presence of glucocorticoids, suggesting that in quiescent liver glucocorticoids promote efficient maturation of the tyrosine aminotransferase primary transcript. Thus a glucocorticoid-induced nuclear post-transcriptional up-regulation apparently compensates for impaired activation of transcription of the tyrosine aminotransferase gene by glucocorticoids in quiescent liver.

Anisoosmotic regulation of hepatic gene expression

The effect of anisoosmolarity on the abundance of various mRNA species was examined in perfused rat liver and H4IIE rat hepatoma cells. Hyperosmotic exposure (385 mosmol/l) of isolated rat livers increased mRNA levels for tyrosine aminotransferase (TAT) by 246% and those for phosphoenolpyruvate carboxykinase (PEPCK) by 186%, whereas hypoosmotic exposure (225 mosmol/l) decreased their levels to 43% and 42%, respectively. mRNA levels for fructose-1,6-bisphosphatase (FBP), argininosuccinate lyase (ASL), argininosuccinate synthetase (ASS), glutamine synthetase (GS), glutaminase (GA) and glucokinase (GK) were largely unaffected. In H4IIE cells the modulation of TAT and PEPCK mRNA levels by anisoosmotic exposure was similar to that found in perfused rat liver. ASL and glutaminase mRNA levels were influenced in an opposite manner. The effects of anisoosmolarity on PEPCK mRNA levels in H4IIE cells were largely abolished in the presence of the protein kinase inhibitors H-7, H-89 and HA-1004. Other protein kinase inhibitors such as Go-6850, KN-62, Rp-8-CPT-cAMPS, rapamycin, wortmannin, genistein or herbimycin did not prevent the osmosensitivity of PEPCK mRNA levels. Also pertussis and cholera toxin, vanadate and colchicine did not affect the osmosensitivity of PEPCK mRNA levels. The data suggest that anisoosmotic exposure acts on the levels of some but not all mRNA species and that this action may involve changes in protein phosphorylation. They further indicate that the recently identified osmosensitive signal transduction pathway which involves a G-protein and tyrosine kinase dependent activation of mitogen-activated protein kinases is apparently not involved in the osmoregulation of PEPCK mRNA levels.

Differential expression of tyrosine aminotransferase by glucocorticoids and insulin

The differential response of the tyrosine aminotransferase (TAT) gene to glucocorticoids and insulin in HTC cells and cell clones derived from Reuber H35 cells (FaO and Fu5.5) have been analyzed by nuclear run-on assay. It has been previously shown that clones of cells from HTC and Reuber H35 cell lines, exhibit different sensitivities for the induction of TAT mRNA and enzyme activity. The purpose of the present study was to determine whether this difference in TAT expression between hepatocytes and hepatoma cell lines occurs at the level of TAT gene transcription or mRNA stability. A study of the TAT mRNA accumulation in all cell types showed that TAT mRNA in the Reuber H35 cell clones and hepatocytes was synthesized at a higher rate than in HTC cells. However, dexamethasone induction of alpha 1 AGP mRNA and glutamine synthetase was comparable to glucocorticoid bound receptors. In addition, cycloheximide decreased the rate at which induced levels of TAT mRNA were degraded. We also show that a heterologous fusion gene constructed from 3.0 kilobases (kb) 5' to the transcription initiation site of the rat TAT gene and the bacterial chloramphenicol acetyltransferase gene (CAT) responds similarly to dexamethasone in Fu5.5 and HTC cells as determined by transient transfection assay; and insulin inhibits dexamethasone mediated transcription in Reuber H35 cells and primary adult hepatocytes. These data indicate that DNA sequences involved in the differential response of the TAT gene to hormone treatments between HTC and Reuber H35 cell lines are not located in the first 3.0 kb fragment.

Insulin-mediated inhibition of the induction of tyrosine aminotransferase by dexamethasone

Insulin-mediated regulation of glucocorticoid-induced expression of the liver-specific gene tyrosine aminotransferase was studied in a clone of the Reuber rat hepatoma cells. Insulin inhibited dexamethasone-induced chloramphenicol acetyltransferase expression from approximately 4 kb of TAT 5' flanking sequence. The degree of this inhibition was comparable to the response of the endogenous gene. A construct of approximately 3 kbp of 5' flanking sequence exhibited no significant basal expression but retained sensitivity to glucocorticoids and to insulin inhibition of the glucocorticoid response. Results of further analysis of the insulin response in deletion constructs and constructs containing glucocorticoid responsive elements ligated to a heterologous promoter suggest that in addition to the glucocorticoid response elements a region close to the start site in the TAT promoter is necessary for insulin to inhibit glucocorticoid-mediated induction of expression.

Insulin-responsive tyrosine aminotransferase transcription requires multiple promoter regions

This study used transient transfection analysis to determine the DNA regions which mediate basal and insulin-sensitive transcription from the gene encoding tyrosine aminotransferase (TAT; EC 2.6.1.5). Basal expression requires at least parts of two regions: a region at -3600 and a region from -208 to + 62. Insulin sensitivity requires at least one region of the promoter not required for basal expression. Thus, insulin cannot act solely by direct modification of any of the components required for basal transcription. Previous results from this laboratory suggest that the insulin effects on basal and glucocorticoid-induced TAT transcription require different regions of the proximal promoter.

Short- and long-term effects of insulin on tyrosine aminotransferase gene expression

In the present study the relationship between changes in tyrosine aminotransferase (TAT) enzyme activity, cytoplasmic mRNA levels, and gene transcription in response to both short- and long-term exposure to insulin was investigated. Insulin acutely inhibited transcription of the TAT gene by 50% in serum-deprived rat H4 hepatoma cells. Following this initial 50% decrease in transcription, there was a 2.5-fold induction in TAT activity that could not be accounted for by a concomitant increase in TAT mRNA levels. Insulin had no effect on the half-life of TAT mRNA. Insulin exposure for short periods of time also inhibited the glucocorticoid- and cAMP-induced transcription of the TAT gene. Like insulin, protein synthesis inhibitors acutely inhibited basal and glucocorticoid-induced TAT transcription. TAT activity gradually returned toward basal levels after 8 h of insulin treatment. A second insulin-induced increase in TAT activity (3.5-fold above basal levels) was observed by 24 h of insulin treatment. This second phase of insulin-induced TAT activity was associated with elevated levels of TAT transcription and TAT mRNA levels, and therefore, unlike the earlier stimulation, could be accounted for by changes in gene expression. Thus, the insulin-mediated regulation of the TAT gene in H4 cells is complex. Different transcriptional and post-transcriptional mechanisms are likely to be involved in the biphasic responses to insulin.

Long-term effects of insulin on the enzyme activity and messenger RNA of glycogen synthase in rat hepatoma H4 cells: an effect of insulin on glycogen synthase mRNA stability

Insulin induced glycogen synthase activity and decreased glycogen synthase mRNA concentrations in rat hepatoma H4 cells. Total enzyme activity measured with glucose 6-phosphate gradually increased during a 24-h insulin incubation. The time course of glycogen synthase activation measured by the activity ratio (low G-6-P/high G-6-P) in response to insulin was biphasic with the first peak at 15 min and the second peak at 4 to 6 h. When cells were incubated with insulin and cycloheximide, the first peak persisted while the second peak was abolished. These data suggest that the first activation peak derives from the classic effect of insulin via dephosphorylation and the second peak from an insulin-induced protein synthesis of a glycogen synthase activator. Ribonuclease protection assays with a cloned rat liver glycogen synthase cDNA were used to quantitate glycogen synthase mRNA. Insulin unexpectedly decreased glycogen synthase mRNA in a time- and a dose-dependent manner. After incubation with the RNA synthesis inhibitor, 5, 6-dichloro-1-beta-D-ribofuranosyl benzimidazole (DRB) without and with insulin, the half time of glycogen synthase mRNA decreased from 6.0 +/- 0.80 to 3.9 +/- 0.75 h, respectively. Nuclear run-off experiments with isolated nuclei showed no change of transcription of glycogen synthase mRNA. These data suggest that insulin in this system affects glycogen synthase mRNA stability rather than transcription.

Mechanism of the inhibition by insulin of the glucagon-dependent activation of the phosphoenolpyruvate carboxykinase gene in rat hepatocyte cultures. Action on gene transcription, mRNA level and -stability as well as hysteresis effect

The mechanism of the antagonistic action of insulin on the glucagon-dependent stimulation of the phosphoenolpyruvate carboxykinase (PEPCK) gene was studied in primary cultures of rat hepatocytes. Gene expression was monitored by the transcriptional activity of the PEPCK gene and the accumulation and degradation of PEPCK mRNA. 1) Insulin in concentrations from 0.1 to 100nM shifted the dose-response curve of the glucagon-dependent accumulation of PEPCK mRNA to the right, increasing the half-maximally effective glucagon concentration gradually from 0.1 to 0.7nM. At saturating 10nM glucagon concentrations insulin was not antagonistic. 2) Glucagon at 0.1nM concentrations increased PEPCK gene transcription and PEPCK mRNA to a transient maximum at 0.5 and 2 h, respectively. Insulin, added at 10nM concentrations simultaneously with glucagon, reduced the maximal increase in PEPCK gene transcription by 70% and in PEPCK mRNA by 45%, respectively. 3) Following the maximal glucagon-induced increase after 2 h PEPCK mRNA declined to half-maximal levels after another 2.3 h. Insulin, added at 2 h at the PEPCK mRNA maximum, accelerated the disappearance of PEPCK mRNA, which reached half-maximal values already after another 1.2 h. 4) The transcriptional inhibitor cordycepin, added at 2 h at the PEPCK mRNA maximum, clearly retarded the normal and the insulin-accelerated decay of PEPCK mRNA so that half-maximal levels were reached only after another 5 h and 3 h, respectively. However, cordycepin did not retard the decay of PEPCK mRNA, when insulin was present from the beginning of induction by glucagon.(ABSTRACT TRUNCATED AT 250 WORDS)