Enhanced transcription by RNA polymerases II and III after inhibition of protein synthesis

Studies on the mechanism of induction of microsomal cytochrome P452 and peroxisomal bifunctional enzyme mRNAs by nafenopin in primary cultures of adult rat hepatocytes

The amount of the two mRNAs although lower in cultured hepatocytes than in freshly isolated cells was found to be rapidly inducible upon the addition of 32 microM nafenopin. The induction of cyt.P452 mRNA always preceded the induction of PBE mRNA, but for both, the maximal induction (10-20-fold over control) was obtained within 24 hr and was achieved by transcriptional activation. At early time points (1 and 2 hr after the addition of nafenopin), in the absence of on-going protein synthesis, the amount of cyt.P452 mRNA (and not of PBE mRNA) was transiently higher in the presence of cycloheximide and nafenopin than in the presence of nafenopin alone.

Phenobarbital induction of cytochrome P-450 b,e genes is dependent on protein synthesis

Phenobarbital induces liver cytochrome P-450 b,e proteins mainly by increasing the rate of transcription of these genes. The mechanism responsible for the phenobarbital increment in the rate of transcription of cytochrome P-450 b,e genes is unknown. The objective of this study was to assess whether active protein synthesis was needed for phenobarbital to induce the liver cytochrome P-450 b,e genes. Cycloheximide (2 mg per kg, i.p.) was administered 90 min prior to a single inductive dose of phenobarbital (80 mg per kg, i.p.) and mRNAS measured at 3, 6 and 12 hr by dot-blot hybridization. While phenobarbital increased cytochrome P-450 b,e mRNAs about 12-fold at 3 hr, this induction was abolished by cycloheximide. To define whether the absence of protein synthesis in hepatocytes inhibited the phenobarbital induction of cytochrome P-450 at the transcriptional level, in vitro transcription rates using isolated nuclei were measured. After phenobarbital administration, there was about a 20-fold increment in transcriptional rate of cytochrome P-450 b,e genes. This increment was abolished by prior injection of cycloheximide. It is proposed that either preexisting regulatory proteins or transacting factors dependent on active protein synthesis participate in the regulation of liver cytochrome P-450 b,e gene transcription after phenobarbital.

Accumulation of heat shock protein 70 RNA and its relationship to protein synthesis after heat shock in mammalian cells

Heat stress induces a set of heat shock proteins (hsps) in a wide variety of species. In response to either a mild (5 min X 45 degrees C) or severe (30 min X 45 degrees C) heat shock, the timing of expression of the hsps and the recovery of general protein synthesis in rat embryonic fibroblasts was dependent on the duration of the hyperthermic exposure. Synthesis of mRNA coding for an hsp of Mr approximately equal to 70,000 (hsp 70) followed immediately after the mild heat shock but was delayed after the severe heat shock. Appearance of the hsps paralleled the synthesis and decay of RNA and was indicative that new RNA synthesis was required for hsp 70 expression. Inhibition of protein synthesis by cycloheximide after the mild heat shock increased the maximal accumulation of hsp 70 encoding mRNA but did not prevent the subsequent decrease in this mRNA species. These results suggest that mammalian cells control the expression of hsp 70 primarily at the level of transcription, and that the normal pattern of hsp 70 mRNA turnover after an inducing heat stress is not dependent on new protein synthesis.

Temporal profiles of proteins responsive to transient ischemia

The responses of long and short half-lived proteins to ischemia were measured in rat brain during 6 days of recovery from 30 min of transient forebrain ischemia produced by four-vessel occlusion. At the end of the ischemic interval, the neocortical activities of four vulnerable enzymes [ornithine (ODC) and S-adenosylmethionine (SAMDC) decarboxylases, and RNA polymerases I and II] were unchanged, but within 30 min of reperfusion, their activities dropped by 25-50%. The loss of substance P in the striatum and substantia nigra was slower, reaching about 50% by 12 h. On the other hand, the activities of 5 long half-lived enzymes did not change in the neocortex at 5 and 15 h of reperfusion and regional protein concentrations were essentially unaffected over 6 days survival. The rate and extent of normalization of the amounts or activities of the vulnerable proteins varied. RNA polymerase II and ODC activities were restored within 4 h, and ODC showed a biphasic increase in activity, with peaks at 10 h and 2-3 days. RNA polymerase I and SAMDC activities were restored by 18 h and 5 days, respectively, whereas substance P concentrations did not completely recover, even at 6-15 days. The greater the regional reduction of blood flow during ischemia, the larger the net change (gain or loss) of SAMDC or ODC activity and the longer the time required to normalize the activities of these enzymes. The average rate of proteolysis, assessed by measuring the rate of clearance of 14C from protein prelabeled with [14C]bicarbonate, was abnormal during the first 2 days of reperfusion. Postischemic changes in both protein synthesis and degradation could affect the amounts of some of the proteins responsive to transient ischemia.

The effect of cycloheximide on nuclear uH2A content

The effect of intraperitonal cycloheximide administration on acid-soluble rat liver chromatin proteins has been investigated by electrophoresis in acetic acid-urea polyacrylamide gels. A nonhistone protein, which migrates between oxidized histone H3 and histone H1 has been found to be increased in amount following cycloheximide treatment. This protein seems to be identical with semihistone protein H24 (uH2A). A possible relationship of uH2A to the inhibition of rRNA synthesis is discussed.

The senescence of postmitotic mammalian cells: a cell-clock hypothesis

A cell-clock hypothesis for the senescence of long-lived postmitotic cells is proposed, based on the concept that the cells cycle regularly between a synthetic and a degradative phase and that the synthetic phase of each cycle is initiated by transcription of a family or families of highly repetitive DNA. The initiator transcripts are conceived to activate other segments of DNA controlling the transcription of mRNAs. The basic senescence mechanism is postulated to be the uncorrected progressive conversion of 5-methylcytosine in repetitive DNA to thymine by hydrolytic deamination, causing an age-related decline in the number of active initiator transcripts and a progressive loss of cell vigor.

Current concepts: III. Molecular aspects of dietary modulation of transcription and enhanced longevity

Dietary restriction is a known means of prolonging the life span of animals. How diet can increase longevity at the molecular level is not yet known. As organisms age, there is a decrease in the ability ot synthesize RNA and a decrease in protein synthesis indicating that there is an overall loss in gene expression. In addition, a decrease in protein turnover is evident indicating a lack of cellular renewal because of the accumulation of tissue protein. Evidence is presented in this review, that certain dietary regimens appear to be capable of enhancing the synthesis of mRNA and probably also produce enhanced turnover of tissue proteins. It is proposed that the physiological "stress" produced by restricted feeding paradigms can enhance gene expression and that this may be a significant factor in the maintenance of cellular homeostasis for a longer period of time.

High salt effect on RNA synthesis in Krebs ascites tumor cells

The incubation of Krebs ascites tumor cells in medium with a high salt concentration resulted in a partial inhibition of nuclear RNA synthesis. The residual RNA polymerase activity in such nuclei was only slightly inhibited by low concentrations (50 nM) of alpha-amanitin. This finding suggested an inhibition of RNA polymerase II activity under conditions of medium hypertonicity. Indeed, RNA polymerase II, isolated from the nuclei of cells exposed to hypertonicity, revealed only half of the control activity. On the other hand, RNA polymerase I was not affected by hypertonicity. Moreover, chromatin fractions isolated from cells incubated in hypertonic or isotonic medium were equally template-active in RNA synthesis.

Transcription in aging: the response of rat liver nuclear RNA polymerases to cycloheximide in vivo

Experiments were performed to address the known relationship between diet and longevity. The acute response of rat liver nuclear RNA polymerases to inhibition of protein synthesis by cycloheximide (10 mg/kg) was quantitated in rats of varying ages. Thirty minutes after cycloheximide administration, there was a "compensatory" 1.5--2-fold increase in RNA polymerase II activity at all ages. Nucleolar RNA polymerase I activity was significantly diminished only up to 4 months, indicating a loss in tight control coupling of protein synthesis to rRNA synthesis with age.

Glucocorticoid-dependent induction of mRNA coding for phosphoenolpyruvate carboxykinase (GTP) in rat kidney. Its inhibition by cycloheximide

The glucocorticoids induce the synthesis of phosphoenolpyruvate carboxykinase (GTP) in rat kidney as a consequence of an increase in the level of the specific enzyme mRNa. The mRNA induction was characterized with respect to its time course after hormone administration and its sensitivity to cycloheximide. The level of rat kidney mRNA directing the synthesis of phosphoenolpyruvate carboxykinase in wheat germ translation system nearly doubled within 2 h of a dexamethasone injection and further increased to four times the initial value at 6 h of treatment and to five times at 10 h. Cycloheximide injected 30 min prior to dexamethasone prevented the mRNA increase. When injected 5 h after dexamethasone, the inhibitor of protein synthesis blocked the rise of phosphoenolpyruvate carboxykinase mRNA occurring normally between 5 h and 10 h after treatment with dexamethasone. Maximal inhibitions of protein synthesis on the one hand and of mRNA induction on the other were achieved at the same dose of cycloheximine, suggesting that the two effect might be related. Dexamethasone caused an increase in the functional level of several as yet unidentified mRNAs in addition to that coding for phosphoenolpyruvate carboxykinase. The main points emerging from this study are: (a) the virtual absence of lag between dexamethasone administration and increase in phosphoenolpyruvate carboxykinase mRNa; (b) the inhibition of phosphoenolpyruvate carboxykinase mRNA induction by cycloheximide, suggesting a possible requirement for ongoing protein synthesis; (c) the existence in the kidney of a glucocorticoid-responsive domain comprising several distinct proteins.