Effects of steroid hormones on the level of corticotropin messenger RNA activity in cultured mouse-pituitary-tumor cells

Studies have been made with the mouse pituitary tumor cell line AtT-20 in culture to determine whether or not the suppression of pituitary corticotropin messenger RNA activity observed upon the administration of glucocorticoids to adrenalectomized rats is due to a direct action of these steroid hormones on the pituitary. The levels of corticotropin messenger RNA activity in AtT-20 cells treated with various steroid hormones were measured with the use of the cell-free protein-synthesizing system derived from wheat germ. The addition of dexamethasone to culture medium reduced the level of corticotropin messenger RNA activity to 30-40% of that in untreated cells. Corticosterone and cortisol exhibited a suppressive effect to a lesser extent. In contrast, nonglucocorticoids such as testosterone and 17beta-estradiol were essentially ineffective. These results indicate that at least part of the glucocorticoid action is exerted directly on the pituitary to suppress corticotropin messenger RNA activity.

Glucocorticoid effect on the level of corticotropin messenger RNA activity in rat pituitary

In an attempt to understand the molecular mechanism underlying the depressive effect of glucocorticoids on corticotropin production, the level of corticotropin messenger RNA activity in rat pituitaries was measured with the use of the cell-free protein-synthesizing system derived from wheat germ. The large translation product of corticotropin messenger RNA was identified and quantitated by indirect immunoprecipitation with antibody against corticotropin. The level of corticotropin messenger RNA activity was increased 3- to 6-fold by adrenalectomy. Dexamethasone administration to adrenalectomized rats resulted in a marked suppression of corticotropin messenger RNA activity. Cortisol and corticosterone also exhibited a suppressive effect but were less effective than dexamethasone. In contrast, nonglucocorticoids such as progesterone and aldosterone had no suppressive effect. These results indicate that at least part of the glucocorticoid effect on corticotropin production in the pituitary is exerted at the pretranslational level.

A large product of cell-free translation of messenger RNA coding for corticotropin

Polyadenylate-containing RNA prepared from the membrane fraction of bovine anterior pituitary gland was shown to direct the synthesis of a large translation product related to corticotropin (adrenocorticotropic hormone) in a cell-free system derived from wheat germ. This product was identified by immunoprecipitation with specific antibody against corticotropin, followed by electrophoretic analysis of the dissociated immunoprecipitate on sodium dodecyl sulfate-polyacrylamide gel. Further evidence for the identity of the translation product was provided by the presence of a common peptide in the chymotryptic digest of [35S]methionine-labeled cell-free product and in that of authentic corticotropin. The molecular weight of the translation product was estimated to be approximately 35,000, based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These results indicate that corticotropin messenger RNA directs the cell-free synthesis of a product that contains the amino acid sequence of corticotropin but is much larger than this hormone.

Dietary and hormonal regulation of the content of acetyl coenzyme A carboxylase-synthesizing polysomes in rat liver

Specific polysomes involved the the synthesis of acetyl-CoA carboxylase [acetyl-CoA: carbon-dioxide ligase (ADP-forming), EC 6.4.1.2] have been identified by the binding of 125I-labeled antiacetyl-CoA carboxylase to rat liver polysomes. The binding is highly specific and occurs through the recognition of the nascent peptide chains on polysomes. With the use of the 125I-labeled antibody binding technique, it has been demonstrated that the relative content of acetyl-CoA carboxylase-synthesizing polysomes in the liver correlates well with the rate of hepatic synthesis of the enzyme in rats subjected to different dietary conditions as well as in alloxan-diabetic rats with or without insulin treatment.

Selective effects of MgCl2 and temperature on the initiation of transcription at lac, gal, and lambda promoters

We have studied the effect of Mg2+ on the formation of transcription preinitiation complexes (open complexes) at two adenosine 3':5'-monophosphate (cyclic AMP)-cyclic AMP receptor protein (CRP)-dependent promoters (lac and gal) and two phage lambda promoters, PL and PR. Mg2+ strongly interferes with open complex formation at the lac and gal promoters, partially inhibits the lambda PR promoter, and is without effect on the lambda PL promoter. Mutations in the lac and gal promoters can affect the response of the promoter to Mg2+. Cyclic AMP and CRP specifically reduce the inhibition of the lac and gal promoters by Mg2+. These factors also affect open complexes at lac and gal by (a) increasing the rate at which they are formed and (b) lowering the midpoint of the temperature transition curve for their formation by about 10 degrees. Open complexes at the lac promoter are more unstable to cooling, even in the presence of CRP and cyclic AMP, than open complexes at lambda promoters. Our studies suggest that the DNA of the lac and gal promoter regions is more resistant to denaturation than the DNA of phage promoter regions. Cyclic AMP and CRP act to decrease this stability, stimulating open complex formation under conditions unfavorable for DNA melting, e.g. low temperature and high Mg2+ concentrations.

In vitro repression of the transcription of gas operon by purified gal repressor

We have studied the in vitro repression of gal mRNA synthesis by the gal repressor from Escherichia coli. By use of a four-step purification procedure involving chromatography on phosphocellulose, DEAE-cellulose, and an affinity resin, the gal repressor has been purified about 1600-fold from a crude cell extract. The purification was aided by use of a cell extract made after prophage induction of cells lysogenic for bacteriophage lambda that carries the gal repressor gene (galR). The highly purified gal repressor is an effective and specific repressor of in vitro synthesis of gal mRNA with lambda gal DNA as template. Both D-fucose and D-galactose overcome the action of gal repressor; the half-maximal concentrations of D-fucose and D-galactose for overcoming the action of repressor are 1 mM and 0.5 mM, respectively. The repressor fails to repress gal-specific transcription when the gal DNA contains a cis-dominant operator constitutive (O(c)) mutation. We conclude that the gal repressor recognizes the gal operator site and acts by preventing gal transcription.

Synthesis and degradation of liver acetyl coenzyme A carboxylase in genetically obese mice

The total cytosol activity of acetyl-CoA carboxylase (acetyl-CoA:CO(2) ligase (ADP), EC 6.4.1.2) in the liver is known to be 6- to 10-fold higher in genetically obese hyperglycemic mice (C57BL/6J-ob) than in nonobese mice. The results of immunochemical titrations, Ouchterlony double-diffusion analysis, and kinetic and heat inactivation studies indicated that this rise in the level of carboxylase activity in liver extracts from obese mice was ascribed to an increase in the quantity of the enzyme protein, which was indistinguishable from that derived from nonobese mice. Combined immunochemical and isotopic techniques showed that the rate of synthesis of the carboxylase per liver was 7.7-fold higher in obese than in nonobese mice. The rate of degradation of the carboxylase was found to be 1.7-fold lower in obese than in nonobese mice, the half-life being 115 and 67 hr, respectively. These results indicate that the increase in the acetyl-CoA carboxylase content of the liver in obese mice is due mainly to a rise in the rate of enzyme synthesis, and in a minor degree, to a decrease in the rate of enzyme degradation.