Monographs on endocrinology最新文献

The steady accumulation of glycogen in fetal rat liver during the last fifth of gestation is elicited by a transient rise in the level of circulating corticosterone. One effect of glucocorticoids is to induce glycogen synthase. The actual deposition of glycogen, however, depends on the appearance of a small amount of glycogen synthase in the active, dephosphorylated form. Induction of glycogen synthase phosphatase by glucocorticoids may explain the latter crucial process. Insulin enhances further the rate of glycogen deposition. The effect of insulin requires a previous exposure of the fetal liver to glucocorticoids. It is exerted on the enzyme interconversion system and appears not to involve new protein synthesis. Administration of glucocorticoids to adult fed or fasted animals causes within 3 h an intensive deposition of glycogen in the liver. This phenomenon is ultimately explained by both an activation of glycogen synthase and an inactivation of glycogen phosphorylase. The latter process may be due to an enhanced activity of phosphorylase phosphatase, or possibly of phosphorylase kinase phosphatase. The activation of glycogen synthase is explained by an enhanced activity of glycogen synthase phosphatase. The latter enzyme is normally profoundly inhibited by phosphorylase a; glucocorticoids cause the appearance in the liver of a protein factor that decreases and eventually cancels this inhibitory effect of phosphorylase a. It remains to be established whether or not some part of the glucocorticoid effect on adult liver is mediated by insulin.

S49, an established line of mouse lymphoma cells, has several characteristics useful for the genetic analysis of glucocorticoid action: (1) a stable pseudodiploid karyotype; (2) an efficient cytolytic effect of glucocorticoids, which appears to follow the same biochemical pathway as steroid hormone action in other systems; (3) appearance of rare steroid-insensitive clones that exhibit stable, heritable resistance to further glucocorticoid treatment; (4) rapid growth in suspension culture and high cloning efficiency in soft agar, allowing facile isolation of variant clones. Two hundred individual steroid-resistant clones of S49 cells have been isolated and analyzed to determine the origin of their resistance. Most of these variant clones (55 %) fail to bind [3H]dexamethasone at levels above background; 70--75 percent bind less than 30 % of the wild-type level. The remaining clones fall into three general groups with respect to the efficiency with which receptors are translocated to the nucleus following dexamethasone treatment: one class transfers less than 10 percent of labeled receptors to the nucleus, another transfers normal amounts, and a third localizes virtually all of the receptors in the nucleus. The four variant phenotypes have been respectively designated r-, receptor activity deficient; nt-, nuclear transfer deficient; d-, deathless (appears normal in binding and nuclear transfer); and nti, increased nuclear transfer. Physical characterization by sucrose gradient sedimentation and gel permeation chromatography reveals that wild-type receptors are approximately 90,000 daltons and nti receptors about 50,000 daltons. The affinities of variant and wild-type receptors for purified DNA in vitro are consistent with their respective nuclear binding characteristics in vivo. Genetic studies with these and other cell lines, combined with recently developed methods for purification and structural analysis of minute quantities of proteins, can provide the level of biochemical resolution required for a fundamental understanding of the molecular mechanism of steroid hormone action.

HTC cells, an established line of rat hepatoma cells in tissue culture, provide a useful experimental model system for studying the interaction of glucocorticoids and insulin in the regulation of protein metabolism. The actions of insulin and glucocorticoids on amino acid transport and protein degradation are antagonistic in this cell line. In contrast, the actions of these two hormones are additive with regard to the induction of tyrosine aminotransferase. The addition of insulin to HTC cells previously incubated with dexamethasone causes a rapid further doubling in the cellular concentration of this enzyme. The properties of the induction by insulin differ in several respects from the induction by glucocorticoids. The former occurs immediately, without the characteristic lag observed during induction by steroids. Insulin induction of transaminase does not require concomitant RNA synthesis, and does not cause the accumulation of specific mRNA for this enzyme as do glucocorticoids. Using specific immunoprecipitation techniques, we have demonstrated that insulin stimulates a nonselective increase in the rate of total protein synthesis in HTC cells, and a selective decrease in the rate of degradation of tyrosine aminotransferase relative to total protein. Thus the induction of transaminase by insulin involves two distinct actions of the hormone, affecting both synthesis and degradation of protein.

An overview of glucocorticoid action is presented including aspects of historic, clinical, and physiologic interest. Despite the diversity of effects, glucocorticoid hormone action at the cellular level simplifies to a rather universal model that involves binding to a soluble receptor, followed by interaction of the complex with the chromatin and modification of gene expression. This unitary concept has important implications in pathology, pharmacology, and therapeutics. Finally, a definition of a glucocorticoid in terms of its receptor is presented, which we feel is better than the traditional terminology based on specific biological effects. It is hoped that this review will also provide the reader with a framework in which the various contributions in the Monograph can be integrated.

In cultured rat pituitary cells, glucocorticoids regulate growth hormone production by modulating the number of growth hormone messenger RNA molecules. The effect is quite specific, since only a few other mRNAs are affected by the hormones. This response is demonstrated by assays involving cell-free mRNA translation and cDNA-RNA hybridization. Furthermore, the inducibility by the glucocorticoids is regulated by at least one other class of hormones, thyroid hormone. Thus, this system serves as a model for studying not only the glucocorticoid regulation of specific mRNA, but also the control of this regulation by other factors in the target tissue.

Mouse fibroblasts growing in vitro respond to glucocorticoids in a dose-dependent fashion by reduced rates of growth. The inhibition of growth observed in vitro is related to the topical anti-inflammatory action of glucocorticoids and to their capacity to inhibit wound repair. The cells growing in vitro possess a glucocorticoid receptor system that has been studied in some detail using [3H]triamcinolone acetonide as a radiolabeled ligand. The initial binding reaction occurs in the cytosol. The complex is then rapidly taken up in the nucleus of the cell by a temperature-sensitive process. In the nucleus, the complex exists in two forms, one of which is readily extracted by 0.3 M KCl solutions. A small amount of steroid-receptor complex is tightly bound to chromatin. Under normal incubation conditions, there is a constant cycling of steroid-receptor complex, and unbound receptor is generated back into the cytosol from the nucleus with a half-life of about 30 min. Regeneration of unbound receptor does not depend on protein synthesis and is a temperature-sensitive and energy-requiring process. Incubating the steroid-treated cells in the absence of glucose and in the presence of inhibitors such as cyanide or dinitrophenol leads to a loss of cytoplasmic steroid-receptor complexes, and an accumulation of the complex in the nuclear residual form, tightly bound to chromatin. With respect to nuclear effects of steroid treatment, we have found that incubating fibroblasts in vitro with glucocorticoids produces a prompt decrease in the amount of a satellite H1 histone found in these cells.

The major effects of glucocorticoids on white fat are shown in Fig. 1. The glucocorticoid diffuses into the cytosol of fat cells where it binds to a soluble receptor. The steroid-receptor complex then enters the nucleus where RNA synthesis is increased. The next step may be a selective increase in synthesis of protein(s). In any event, there is an inhibition of the membrane-bound glucose transport system and an increase in the ability of lipolytic agents to activate triglyceride lipolysis. There is also a decrease in phosphoenolpyruvate carboxykinase, lipoprotein lipase, and fatty acid synthetase that occurs after a somewhat longer lag period than is required for inhibition of glucose transport or lipolysis activation. Whether these effects are independent or secondary to the glucose transport inhibition and lipolysis activation remains to be established.