Messenger RNA content and complexity of euthyroid and hypothyroid rat liver

Effects of metabolic acidosis and diabetes on the abundance of specific renal mRNAs

1. The effects of exogenously (NH4Cl ingestion) and endogenously (streptozotocin-diabetes) generated chronic metabolic acidosis on the abundance of rat renal mRNAs have been examined. 2. Total RNA was translated in vitro and the translation products analyzed by two-dimensional gel electrophoresis. 3. The translation product identified as phosphoenolpyruvate carboxykinase (PEPCK) increased 3.5-fold in both acidosis and diabetes. 4. This increase was not observed in diabetic rats treated with NaHCO3. 5. The abundance of one other translation product increased in acidosis. 6. That of 10 others increased in diabetes, several of which were elevated regardless of acid-base status. 7. The abundance of one translation product decreased in acidosis and diabetes but not in NaHCo3 treated diabetic rats, indicating acid-base regulation of this product. 8. The results establish that the acidosis response is limited to a small number of renal mRNAs and confirm that renal PEPCK is primarily regulated by changes in acid-base status. 9. They also indicate that diabetes affects the abundance of specific renal mRNAs through mechanisms independent of acid-base status.

Triiodothyronine increases translatable albumin messenger RNA in Rana catesbeiana tadpole liver

The effects of both 3,5,3'-triiodo-L-thyronine and spontaneous metamorphosis on Rana catesbeiana liver mRNA were studied using in vitro translation of isolated liver poly(A)+ RNA in a rabbit reticulocyte lysate system. Conventional phenol extraction methods yielded degraded RNA due to high levels of endogenous ribonucleases released upon homogenization of Rana catesbeiana liver. Isolation of intact total RNA was achieved using the potent ribonuclease denaturant, guanidinium thiocyanate. Adult bullfrog serum albumin was purified to homogeneity and a monospecific antibody was elicited against it. A serum protein of 23,000 daltons that migrated near serum albumin on a 6% native gel was also purified to homogeneity. A monospecific antibody was also raised against this protein. Both antibodies were used to quantitatively immunoprecipitate the in vitro translation products of poly(A)+ RNA isolated at intervals following a single injection of triiodothyronine or during various stages of spontaneous amphibian metamorphosis. Triiodothyronine caused a sevenfold increase in translatable albumin mRNA and a threefold increase in translatable mRNA for the 23,000 dalton protein. These increases are consistent with a nuclear initiated mechanism for thyroid hormone action during amphibian metamorphosis.

Hormonal regulation of serum alpha 1-antitrypsin and hepatic alpha 1-antitrypsin mRNA in rats

We evaluated the effects of pituitary dependent hormones on alpha 1-antitrypsin in male rats. Hepatic alpha 1-antitrypsin mRNA was measured by in vitro translation and by specific hybridization with a mouse cDNA alpha 1-antitrypsin probe. Hypophysectomy caused a 50-75% decrease in serum elastase inhibitory capacity (measuring functional alpha 1-antitrypsin) and hepatic alpha 1-antitrypsin mRNA content. In hypophysectomized animals, no increase in elastase inhibitory capacity or alpha 1-antitrypsin mRNA levels by translation was found when met-human growth hormone alone or corticosterone, dihydrotestosterone and thyroxine were given together. Growth hormone increased alpha 1-antitrypsin mRNA by hybridization to a small extent. Addition of growth hormone to the combination of corticosterone, dihydrotestosterone, and thyroxine increased serum elastase inhibitory capacity and alpha 1-antitrypsin mRNA. We conclude that growth hormone acts synergistically with the other pituitary dependent hormones to regulate serum and hepatic mRNA levels of alpha 1-antitrypsin.

Messenger RNA content and complexity in normal and overloaded rat heart

Cardiac overload was studied in rats by abdominal aortic constriction, which increased the left ventricular weight by 59% after 12 days. During the transition, which precedes the compensatory hypertrophy, both total RNA and poly(A)-containing messenger RNA increased. The concentration of these polynucleotides peaked by day 4 after constriction, rising from 1.27 +/- 0.3 to 1.88 +/- 0.2 mg g-1 fresh weight for total RNA, and from 38 +/- 24 to 62 +/- 12 micrograms g-1 fresh weight for poly(A)-containing RNA, and returned to normal by day 12. However, the total amount per ventricle of both RNAs remained high. Poly(A)-containing RNA prepared from normal heart was hybridized to its cDNA copy. These results were expressed as the percentage of hybridization vs. the log 10 of the product of the poly(A)-containing RNA concentration and the time (Rot), and in computer analysis were described by division into three different frequency components. In normal hearts, the Rot 1/2 values of these components were, respectively, 3.98 X 10(-3), 0.338 and 21.380 mol.s.1(-1), which correspond to 2-3, 240 and 12,200 different sequences that were copied 22,000-33,000, 310 and 5 times, respectively. Four and 30 days after banding there was a harmonious enhancement of the number of the copies without any change in the number of different sequences, and the three different hybridization curves were superimposed. In conclusion, cardiac overload raises the poly(A)-containing RNA concentration, probably by stimulating transcription, but no major changes occur in any of the frequency classes.

Thyroid hormone regulation of flavocoenzyme biosynthesis

The means by which thyroid hormone regulates flavocoenzyme biosynthesis was studied in hyper-, eu-, and hypothyroid rats by determining the activities of flavocoenzyme-forming enzymes, viz., flavokinase and FAD synthetase, as well as those of flavocoenzyme-degrading enzymes, viz., FMN phosphatase and FAD pyrophosphatase. Flavokinase activity was increased in hyperthyroid animal and decreased in hypothyroid animals. Correspondence of flavokinase activity with the amount of a high-affinity flavin-binding protein quantitated immunologically in hypo-, eu-, and hyperthyroid rats indicated that the thyroid response is caused by an increased amount of enzyme; moreover, the concomitant decrease in a low-affinity flavin-binding protein suggests an inactive precursor form of flavokinase. FAD synthetase activity showed a similar but less pronounced trend than flavokinase. Activities of FMN phosphatase and FAD pyrophosphatase were not influenced by thyroid hormone. Overall results indicate that the mechanism of thyroid hormone regulation of flavocoenzyme level is in the steps of biosynthesis, especially at flavokinase, rather than in degradation steps.

Early effects of triidothyronine on the complexity of rat heart messenger RNAs

Triiodothyronine (T3), injected daily into rats, induces heart hypertrophy. In a recent work we have shown that a single T3 injection stimulates RNA synthesis and modifies the translational products of myocardial mRNAs in reticulocyte lysates, quantitatively and qualitatively. In this work we show that T3 induces small but significant changes in mRNA size distribution after 4 h and much more important changes after 18 h. It also modifies the size distribution of their poly(A) tails. We studied the early effects of this compound on mRNA complexity, using the nucleic acid hybridization technique with DNA complementary to poly(A)+ RNAs. T3, 4 h after injection, suppresses approximately 15% of the sequences, mostly among rare sequences, and increases the frequency of the abundant sequences and of the sequences of intermediary abundance. A large part of this effect disappears 18 h after the injection. It may be concluded that T3 presents an early effect on gene expression, involving changes either at the gene level or and at some post transcriptional level.

Thyroid hormone action on intermediary metabolism. Part III. Protein metabolism in hyper- and hypothyroidism

In their physiological concentrations, thyroid hormones stimulate the synthesis as well as the degradation of proteins, whereas in supraphysiological doses protein catabolism predominates. In hyperthyroidism skeletal muscle protein stores suffer depletion which is reflected by an increased urinary N- and methylhistidine -excretion. Due to the enhanced skeletal muscle amino acid release, the plasma concentration of glucoplastic amino acids are often enhanced, contributing by means of an elevated substrate supply to the increased hepatic gluconeogenesis. Thyroid hormone excess induces cardiac hypertrophy which is in direct contrast to the hypotroph skeletal muscle in hyperthyroid patients. Thyroid hormones stimulate a series of intracellular and secretory proteins in the liver, although in hyperthyroid liver alcohol dehydrogenase and the enzymes of histidine and tryptophan metabolism show reduced activities. The stimulatory effect is due to thyroid hormone-induced increase in the protein synthesis at a pretranslational level and is supported experimentally for malic enzyme, alpha 2u-globulin and albumin by the measurement of their specific messenger RNA activities. Thyroid hormone action at the cellular level is reflected by a generalized increase in total cellular RNA with a selective increase or decrease in a small population of specific mRNA. The activities of protein catabolizing lysosomal enzymes are stimulated by thyroid hormones; up to now effects of T3 on the degradation of specific enzymes have not been reported. Serum total protein concentration is slightly reduced or even unchanged in hyperthyroidism. The thyroid hormone-induced increase in the turnover of total body protein is part of the hypermetabolism observed in hyperthyroidism.

Filter-binding assay procedure for thyroid hormone receptors

An assay procedure for thyroid hormone receptor activity which used nitrocellulose membrane filters was developed. Receptor proteins, extracted from washed rat liver nuclei with a 0.4 M NaCl solution, were incubated with 125I-labeled thyroid hormone (T3), and filtered on the cellulose ester membranes under suction at 2 degrees C. The filters were subsequently washed with cold buffer and counted for 125I radioactivity. The method allowed an accurate estimation of the receptor activity, satisfying a linear relationship between the activity and the receptor protein concentrations. The usefulness of this filter-binding method became evident when it was compared with the conventional procedure that employs Sephadex G-25 columns. For practical application to routine assays, various filtration conditions were examined, and a standard procedure was established. Using this technique, the isolated receptors were determined to possess an apparent Kd of 1.38 X 10(-10) M and a pH optimum of T3 binding at 8.2-8.4.

Association of thyroid hormone receptors with chromatin

A large body of circumstantial evidence indicates that receptors located in nuclei of T3 responsive tissues represent a site of initiation of thyroid hormone action at the cellular level. Partial characterization of T3 receptors indicates that these proteins are monomeric structures in nuclei and are chromatin-associated non-histone proteins. Treatment of rat liver nuclei with either pancreatic DNase I or micrococcal nuclease releases T3 receptors from nuclei in two forms: a predominant (95 400 Mr; 5.5-6.0S) and a minor (265 000-365 000 Mr; 12.5S) nucleoprotein complex. Similar structures are excised from rat kidney, brain, and heart nuclei and from GH1 pituitary cell nuclei by micrococcal nuclease digestion. These endonuclease-excised receptor-containing complexes are significantly larger than the salt-extracted receptor (50 000 Mr; 3.5S). The presence of DNA and other non-receptor proteins in these structures indicates that T3 receptors probably function within multimeric complexes in vivo. Although T3 receptors appear to be associated with DNA between nucleosomes, i.e. linker DNA, it is not entirely clear whether all or only a fraction of T3 receptors interact with nucleosomal components. The 12.5S receptor-containing nucleoprotein complex may represent T3 receptors in association with linker DNA and nucleosomal components. T3 receptors do not appear to be uniformly distributed to all chromatin fractions, but are associated with structures having characteristics of transcriptionally active chromatin. They are found in a region of chromatin which is enriched in RNA polymerase activity, rapidly labeled RNA and non-histone proteins, and depleted of histone Hl. This region is also highly sensitive to both micrococcal nuclease and pancreatic DNase I digestion. The association of receptors with transcriptionally active chromatin, however, must be considered provisional until additional details of the precise receptor-chromatin interaction have been established. The recent demonstration of a 20-fold increase in a specific hepatic mRNA four hours following administration of T3 to hypothyroid rats indicates that thyroid hormone potentially has very rapid effects on hepatic gene expression. However, significant changes in nuclear protein phosphorylation, nuclear protein composition, and chromatin structure have not been detected within this four-hour period. Thus, effects of T3 on hepatic gene expression are brought about by local and presumably subtle changes in nuclear function.

Association of the thyroid hormone receptor with rat liver chromatin

We have investigated the association of the triiodothyronine (T3) nuclear receptor with rat liver chromatin by the use of selective endonuclease digestion and differential solubilization. The T3 receptor was found in a fraction of chromatin having some of the characteristics of active chromatin: (a) It is highly sensitive to DNase I and micrococcal nuclease digestion; (b) it is enriched in nonhistone proteins and depleted of histone (H-1). Micrococcal nuclease and pancreatic DNase I excised two receptor-containing fragments from chromatin, a minor (12--14 S) form and a major (5.5--6.0 S) form. The latter structure has a Stokes radius of 42 +/- 2 A and an estimated molecular weight of 95400 when a partial specific volume of 0.725 cm3/g for protein is used. It contains DNA but no histones. Similar receptor-containing fragments were excised from chromatin of other rat tissues, including brain, kidney, and heart. Both the 5.5--6 S and the 12--14 S receptor-containing chromatin structures are converted by 0.5 M KCl to the 3.5 S form (R0 35 A molecular weight 50500). Titration with micrococcal nuclease and pancreatic DNase I revealed that the 5.5--6 S form is preferentially excised by endonuclease. Neither receptor occupancy nor thyroidal status had an apparent effect on the susceptibility of chromatin to endonucleolytic digestion nor did they influence the distribution of T3 receptors in chromatin. Our results suggest that T3 receptors are not tightly associated with nucleosomes, the basic subunit of chromatin, but are associated with the DNA-linking nucleosomes in structurally modified regions of chromatin in rat liver nuclei. The T3 receptor-containing fragment may well represent a higher order of structural complexity necessary for T3 action at the cellular level.

Effect of DNA on thyroid-hormone binding by specific receptor proteins from rat-liver nuclei

Influence of double-stranded native DNA on the binding of thyroid hormone, 3,5,3'-triiodo-L-thyronine, by the isolated nuclear receptors was studied and the following results were obtained. (1) The receptor-triiodothyronine complexes bound to DNA with moderate affinities. (2) DNA enhanced the hormone binding of the receptors. (3) The stimulatory DNA effect on triiodothyronine binding of the receptors was dependent on DNA concentration, showing its maximum at 30 microgram/ml. (4) The increase in triiodothyronine binding was observed not only in the initial velocity but also in the plateau level which was attained after sufficient incubation time. (5) There were two types of specific receptors in the rat liver nuclear extract. The dissociation constants and the maximal binding capacities for triiodothyronine, which were determined by Scatchard plot analysis in the presence and absence of DNA, suggested that DNA exerted its effect through increasing binding capacity on one class of the receptors and through enhancing affinity for the hormone on the other class of the receptors. (6) Among various polynucleotides examined, the double-stranded eukaryotic DNA was most effective in enhancing the hormone binding by the receptors. These results indicate that the nuclear thyroid hormone receptors interact with double-stranded DNA in a specific manner and are induced to bind more thyroid hormone. We interpret these results as suggesting that a ternary complex of triiodothyronine, the receptor and DNA is formed in the cell nucleus in vivo, probably representing an intrinsic step in the hormone action. Possible physiological significance of this effect of DNA on the receptors is discussed.

Thyroid hormone receptor-containing fragment released from chromatin by deoxyribonuclease I and micrococcal nuclease

Limited deoxyribonuclease I and micrococcal nuclease digestion of hepatic nuclei from euthyroid rats injected with 125I-labeled triiodothyronine ([125I]T3) releases a discrete [125I]T3-labeled chromatin fragment (5.8S) which is larger than the T3 receptor (3.5S). These results suggest the T3 receptor is associated with a restricted fraction of hepatic chromatin that has a nuclease sensitivity characteristic of transcriptionally active chromatin.

Changes in the hepatic levels of messenger ribonucleic acid for malic enzyme during induction by thyroid hormone or diet

Levels of hepatic messenger ribonucleic acid (mRNA) for malic enzyme [L-malate:NADP oxidoreductase (decarboxylating), EC 1.1.1.40] were quantitated in different dietary and hormonal states of the rat. Polysomal or total cellular poly(A)-containing RNA was translated in the rabbit reticulocyte lysate system, which had been treated to reduce endogenous mRNA activity. The relative level of incorporation of radiolabeled amino acid into malic enzyme was determined by immunoprecipitation with antibody to malic enzyme and formaldehyde-fixed Staphylococcus aureus (Cowens I strain) as an immunoadsorbent. The immunoprecipitated product comigrated with purified malic enzyme on sodium dodecyl sulfate--polyacrylamide gel electrophoresis. No malic enzyme was detected when nonspecific antisera or an excess of unlabeled malic enzyme was added during immunoprecipitation. The level of malic enzyme mRNA was found to markedly increase relative to euthyroid, chow-fed rats when the animal was either fed a high carbohydrate, fat-free diet or made hyperthyroid. Animals receiving both treatments had a further increase in mRNA activity to a level which was approximately 0.2% of the total incorporation of [3H]leucine. Levels of malic enzyme activity and the relative rate of synthesis were found to increase roughly in proportion to mRNA levels in these three states. Thus, the induction of malic enzyme by thyroid hormone or high carbohydrate, fat-free diet is due largely to an increase in the mRNA coding for this enzyme.

Thyroid hormone action at the cellular level

A large body of circumstantial evidence suggests that the basic unit of thyroid hormone action is the triiodothyronine nuclear receptor complex. This complex stimulates the formation, directly or indirectly, of a diversity of messenger RNA (mRNA) sequences. A generalized increase in mRNA as well as a disproportionate increase in a limited number of RNA sequences have been demonstrated. Regulation of thyroid hormone effects may be carried out largely at a local cellular level. Highly selective alterations in sensitivity to the triiodothyronine nuclear receptor complex may occur at specific target genes. Metabolic factors and hormones participate in such regulation. In a given tissue, alterations in the total number of receptor sites has not been shown to be useful as an index of thyroid hormone response, and local modulation of the response to the triiodothyronine receptor complex by a variety of factors other than triiodothyronine may be carried out at a postreceptor level.