Initial steps in the induction by glucocorticosteroids of rat liver tryptophan oxygenase and tyrosin aminotransferase

Transcriptional induction of hepatic NADPH: cytochrome P450 oxidoreductase by thyroid hormone

Studies were carried out to elucidate the mechanism whereby thyroid hormone (T3) induces NADPH:cytochrome P450 oxidoreductase (P450R) mRNA in rat liver in vivo. Northern blot analysis revealed that T3 treatment increases unspliced liver nuclear P450R RNA 4-fold within 8 h and that this induction precedes the induction of mature, cytoplasmic P450R RNA. Unspliced nuclear P450R RNA was suppressed below basal levels 24 h after T3 treatment, despite the continued presence of elevated circulating T3 levels. To determine whether the T3-stimulated increase in nuclear P450R RNA reflects an increase in P450R transcription initiation, nuclear run-on transcription assays were carried out. T3 induced a 6- to 8-fold increase in P450R transcription rate within 12 h, sufficient to account for the observed increase in nuclear P450R precursor RNA, followed by a decrease back to basal transcription levels at 24 h, consistent with the nuclear RNA profile. Similar transcriptional increases were observed in nuclear run-on transcription studies using hybridization probes corresponding to nine different fragments of the P450R gene, spanning exon 2 to exon 16. Thus, P450R transcription initiation, not transcription elongation, is the T3-regulated event. Similar results were obtained during short (5 min) compared with long (45 min) nuclear run-on transcription assays, suggesting that changes in nuclear RNA processing or regulated degradation do not contribute to the overall RNA induction. This finding was confirmed by the ability of the RNA polymerase inhibitor actinomycin D, administered in vivo, to block T3 induction of P450R transcriptional activity. We conclude that P450R transcription, rather than nuclear RNA processing or mRNA stabilization, is the primary mechanism whereby T3 induces hepatic P450R mRNA.

Effects of light and acetate on the liberation of zoospores by a mutant strain ofChlamydomonas reinhardtii

In light-dark-synchronized cultures of the unicellular green algaChlamydomonas reinhardtii, release of zoospores from the wall of the mother cell normally takes place during the second half of the dark period. The recently isolated mutant 'ls', however, needs light for the liberation of zoospores when grown photoautotrophically under a 12 h light-12 h dark regime. The light-induced release of zoospores was found to be prevented by addition of the photosystem-II inhibitor 3-(3',4'-dichlorophenyl)-1,1-dimethylurea. Furthermore, light dependence of this process was shown to be abolished when the mutant 'ls' was grown either photoautotrophically under a 14 h light-10 h dark regime or in the presence of acetate. Our findings indicate that the light-dependency of zoospore liberation observed in cultures of this particular mutant during photoautotrophic growth under a 12 h light-12 h dark regime might be attributed to an altered energy metabolism. The light-induced release of zoospores was found to be prevented by addition of cycloheximide or chloramphenicol, antibiotics which inhibit protein biosynthesis by cytoplasmic and organellar ribosomes, respectively. Actinomycin D, an inhibitor of RNA synthesis, however, did not affect the light-induced liberation of zoospores.Sporangia accumulate in stationary cultures of the mutant 'ls'. Release of zoospores was observed when these sporangia were collected by centrifugation and incubated in the light after resuspension in fresh culture medium. Since liberation of zoospores was not observed after dilution of the stationary cultures with fresh culture medium, we suppose that components which interfere with the action of the sporangial autolysin are accumulated in the culture medium of the mutant 'ls'.

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.

Immunochemical studies on biosynthesis of rat plasma angiotensinogen and its regulation by cortisol

Glucocorticosteroid hormones increase the level of rat plasma angiotensinogen by increasing its rate of synthesis. Two forms of plasma angiotensinogen have been purified differing with respect to molecular weight and affinity to concanavalin A. Immunochemical studies using antibodies raised against the separated forms of angiotensinogen revealed cross-reactivity with both antigens. Both antibodies were able to quantitatively precipitate the angiotensinogen activity present in rat serum samples. Cortisol increased the total amount of plasma renin substrate without changing the relative amounts of both angiotensinogen forms. mRNA coding for plasma angiotensinogen was determined by in vitro translation of poly(A)-containing RNA and immunochemical analysis of translation products. Angiotensinogen mRNA could be detected in total poly(A)-containing RNA isolated from rat liver, but not in mRNA isolated from brain, although angiotensinogen has been reported to be present in the latter organ. The level of hepatic mRNA coding for plasma angiotensinogen was high in rats treated with cortisol, but not detectable in animals depleted from endogenous glucocorticosteroids by bilateral adrenalectomy.

Effects of cyclic AMP, glucocorticoids and insulin on the activities of phosphatidate phosphohydrolase, tyrosine aminotransferase and glycerol kinase in isolated rat hepatocytes in relation to the control of triacylglycerol synthesis and gluconeogenesis

Rat hepatocytes were incubated in monolayer culture in modified Leibovitz L-15 medium containing either 10% (v/v) newborn-calf serum or 0.2% (w/v) fatty-acid-poor bovine serum albumin. The addition of 100 nM-dexamethasone increased the activities of both phosphatidate phosphohydrolase and tyrosine aminotransferase by about 3.5-fold after 8h, and these activities continued to rise until at least 24h. Incubating the hepatocytes in the albumin-containing medium with 10 microM- or 100 microM-8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate increased the activities of the phosphohydrolase and aminotransferase by 2.6- and 3.4-fold respectively after 8h. These increases were blocked by actinomycin D. The increases in the activities that were produced by the cyclic AMP analogue and dexamethasone were independent and approximately additive. Insulin when added alone did not alter the phosphohydrolase activity, but it increased the aminotransferase activity by 34%. The dexamethasone-induced increase in the phosphohydrolase activity was completely blocked by 7-144 microM-insulin, whereas that of the aminotransferase was only partly suppressed. Insulin had no significant Effects on the increases in the activities of phosphatidate phosphohydrolase and tyrosine aminotransferase that were produced by the cyclic AMP analogue, but this may be because the analogue is fairly resistant to degradation by the phosphodiesterase. The activity of glycerol kinase was not significantly changed by incubating the hepatocytes with insulin, dexamethasone and the cyclic AMP analogue alone or in combinations. It is proposed that high concentrations of cyclic AMP and glucocorticoids increase the total activity of phosphatidate phosphohydrolase in the liver and provide it with an increased capacity for synthesizing triacylglycerols and very-low-density lipoproteins, which is expressed when the availability of fatty acids is high. There appears to be a co-ordinated hormonal control of triacyglycerol synthesis and gluconeogenesis in diabetes and in metabolic stress to enable the liver to supply other organs with energy.

The influence of starvation and tryptophan administration on the metabolism of phenylalanine, tyrosine and tryptophan in isolated rat liver cells

Liver cells from fed Sprague-Dawley rats metabolized phenylalanine, tyrosine and tryptophan at rates consistent with the known kinetic properties of the first enzymes of each pathway. Starvation of rats for 48 h did not increase the maximal activities of phenylalanine hydroxylase, tryptophan 2,3-dioxygenase and tyrosine aminotransferase in liver cell extracts, when results were expressed in terms of cellular DNA. Catabolic flux through the first two enzymes was unchanged; that through the aminotransferase was elevated relatively to enzyme activity. This is interpreted in terms of changes in the concentrations of 2-oxoglutarate and glutamate. Cells from tryptophan-treated animals exhibited significant increases in the catabolism of tyrosine and tryptophan, but not of phenylalanine. The activities of tyrosine aminotransferase and tryptophan 2,3-dioxygenase were also increased, although the changes in flux and enzyme activity did not correspond exactly. These results are discussed with reference to the control of aromatic amino acid catabolism in liver; the role of substrate concentration is emphasized.

No correlation between binding of glucocorticosteroids to specific cytoplasmic proteins in vivo and enzyme induction in the rat liver

Time- and dose-dependence of the formation of the different cytoplasmic hormone-protein complexes were studied in the rat liver after administration in vivo of [3H]cortisol or [3H]dexamethasone and compared with the stimulation of RNA polymerase B and induction of tyrosine aminotransferase and tryptophan oxygenase. No correlation could be found between formation in vivo of any of the five cytoplasmic hormone-protein complexes found and stimulation of RNA polymerase B activity or enzyme induction. After administration of [3H]cortisol, different metabolites of cortisol could be demonstrated in the isolated hormone-protein complexes. No time- or dose-dependence of the metabolite patterns could be observed after application of hormone doses that were in the range of the biologically active doses. After administration of [3H]dexamethasone, the same hormone-protein complexes were observed, which contained, however, the injected steroid instead of metabolites. These results seem to indicate that the cytoplasmic binding components present in the rat liver are enzymes involved in the metabolism of the glucocorticosteroids and that dexamethasone binds to these enzymes as a substrate analogue.

Isolation and characterization of hnRNA-snRNA-protein complexes from Morris hepatoma cells

Of the RNA labelled after incubation of hepatoma cells with radioactive precursors for 20 and 150 min. 35% and 70%, respectively, can be isolated from nuclei by two consecutive extractions with 0.14 M NaCl at pH 8. The isolated RNA is complexed with nuclear proteins forming structures with sedimentation coefficients of less than 30 S to greater than 100 S. Similar complexes from rat liver isolated under the same experimental conditions show coefficients of 30-40 S. The RNA-associated proteins are similar, on the basis of sodium dodecyl sulphate/polyacrylamide gel electrophoresis, to the respective proteins of other cell types. The presence on these RNP complexes of six discrete small nuclear RNAs (snRNA) has been established. Experiments with a reversible inhibitor of RNA synthesis, D-galactosamine, demonstrated, differences in the turnover of hnRNA and snRNA. The half-lives of the six snRNA species has been determined, varying from 32 h for snRNA species a, b and d, to 22 h for snRNA species e and f and to 13 h for snRNA species c. Treatment of the nuclear extracts with 0.7 M and 1 M NaCl results in dissociation of hnRNA from the 'core' and other polypeptides, whereas snRNA remains complexed with polypeptides of Mr 54 000-59 000. Incubation of the nuclear extracts at 0 C with low doses of pancreatic R Nase (up to 1.5 micrograms/ml), which renders approximately 80% of the hnRNA acid-soluble and cleaves most of the snRNA, results in conversion of the high-molecular-weight hnRNPs to 30-S structures, without disrupting the 30-S RNP. Treatment of the nuclear extracts with higher doses of RNase (3 micrograms/ml) leads to disruption of the 30-S RNP and release of the hnRNA-associated proteins, underlining the importance of hnRNA-protein interaction for the retainment of the hnRNP structures.

Further evidence for translational regulation of tyrosine aminotransferase synthesis by dibutyryl cyclic AMP in Reuber H35 hepatoma cells

Cyclic AMP derivatives increase the rate of synthesis of tyrosine aminotransferase in Reuber H35 hepatoma cells. Various studies lend support to the hypothesis that cyclic AMP increases the synthesis of tyrosine aminotransferase by acting at a posttranscriptional site. The presence of a limited non-translatable pool of tyrosine aminotransferase mRNA prior to the formation of the translatable tyrosine aminotransferase mRNA implicates a possible site of action of cyclic AMP. We compared the capacity of N6,O2'-dibutyryl cyclic AMP to induce tyrosine aminotransferase synthesis when untranslatable tyrosine aminotransferase mRNA sequences are present or absent. The transition of a condition in which non-translatable tyrosine aminotransferase mRNA sequences were present to a condition in which they were absent was established by preinduction of Reuber H35 cells with dexamethasone, followed by addition of actinomycin D. In the time period thereafter, the amount of non-translatable mRNA decreased and 1.5-2 h after addition of actinomycin D, only translatable tyrosine aminotransferase mRNA was present. It can be seen that the induction of tyrosine aminotransferase synthesis by dibutyryl cyclic AMP follows the normal decrease of tyrosine aminotransferase mRNA. We present evidence that dibutyryl cyclic AMP in Reuber H35 hepatoma cells regulates tyrosine aminotransferase synthesis at a posttranscriptional site independent of the pool of non-translatable tyrosine aminotransferase mRNA sequences, but influencing the efficiency of translation of active tyrosine aminotransferase mRNA.

Absence of hydrocortisone from cytoplasmic hormone-protein complexes formed in vivo after administration of biologically active doses of [3H]hydrocortisone

After administration of [3H]hydrocortisone to adrenalectomized rats, hormone-protein complexes were isolated from liver cytosol by DEAE-cellulose chromatography. After application of biologically active and inactive doses of hydrocortisone five binding components were detected eluting at the same salt concentrations as the hormone-protein complexes observed after incubation of cytosol with [3H]hydrocortisone in vitro. The isolated hormone-protein fractions were acidified and extracted with ethylacetate and the steroids were analyzed by thin-layer chromatography. No significant amount of hydrocortisone could be detected in any of the complexes formed in vivo 5-60 min after administration of biologically active doses of hydrocortisone. 3 xi, 11 beta, 17 alpha, 20 xi, 21-Pentahydroxypregnane, steroidal carboxy acids, glucuronides and a very polar conjugate of hydrocortisone were found in the different fractions. After an in vivo dose of hydrocortisone of about 1/5000th of the minimal dose required for enzyme induction, hydrocortisone could be found in all the cytoplasmic hormone-protein complexes formed. In contrast to the cytoplasmic hormone-protein complexes, hydrocortisone could be readily demonstrated in nuclei isolated after the administration of biologically active doses of hormone, although acid metabolites were found to represent the main part of the radioactive compounds present in the nuclei. These acid metabolites were located in the nuclear envelope. These results seem to contradict the generally accepted theory that hydrocortisone induces biosynthesis of proteins via a cytoplasmic hydrocortisone-receptor complex: after administration of biologically active doses of hydrocortisone, no such complex could be detected.

Induction of plasma proangiotensin by steroid hormones in nephrectomized rats

The response of plasma proangiotensin to various steroids was studied in bilaterally nephrectomized rats. In male animals, estradiol and testosterone increased the proangiotensin level up to 250% and 180% respectively. In female animals, both hormones lead to an increase of proangiotensin up to 220% of the controls, when given at optimal dose. The dose dependence on estradiol and testosterone was about the same in male rats, whereas in female animals the formation of proangiotensin was stimulated by much lower doses of estradiol as compared to testosterone. Adrenalectomy plus nephrectomy reduced the proangiotensin level to 20-30% of the value measured in animals nephrectomized only. Cortisol caused a rapid increase of plasma proangiotensin up to 1000% in adrenalectomized animals. Independent from the adrenal state, the amounts of cortisol necessary to induce proangiotensin were very low as compared to the dose response of other proteins, biosynthesis of which is regulated by cortisol. 21-Dehydrocortisol and aldosterone induced proangiotensin with the same efficiency as cortisol, whereas several other chemically related steroids were less active or inactive. Comparing the biological activity of the various steroids tested, it has to be concluded that the delta 4-ene-3-one structure of ring A, the 11 beta-OH group of ring C and the carbonyl group at C-20 of the ketol side-chain of cortisol are very important with respect to proangiotensin induction. The response of proangiotensin to cortisol, 21-dehydrocortisol and aldosterone could be inhibited by actinomocin D and cycloheximide, whereas the effects of estradiol and testosterone could be reversed by cycloheximide only.