Effects of insulin on messenger RNA activities in rat liver

Insulin and the burned patient

Severe burns lead to insulin resistance, which is associated with hyperglycemia and muscle wasting. Investigators showed relatively recently that control of hyperglycemia with intensive insulin treatment is associated with improved outcomes for those in the intensive care unit, including patients with severe burns. In this article, we review the actions of insulin in terms of glycemic control and muscle metabolism, biochemical and clinical effects of insulin treatment in the severely burned, and the vagaries of glucose control.

Contrasting effects of acute and chronic ethanol administration on rat liver tyrosine aminotransferase

We compared the effects of acute and chronic ethanol administration on the activity and synthesis of tyrosine aminotransferase (TAT) in rat liver. In acute experiments, chow-fed rats received a single dose of either ethanol (6 g/kg body wt.) or saline. In chronic studies, rats were pair-fed liquid diets containing either ethanol (36 % of calories) or isocaloric maltose-dextrin for 6-8 weeks. In rats acutely fed ethanol, the relative rate of TAT synthesis was more than twofold higher than in saline-treated controls. In rats subjected to chronic ethanol administration, both the TAT activity and synthesis rate were the same as in pair-fed controls, but both these parameters in the two groups were equal to those in animals given acute ethanol acutely. These findings indicate that whereas acute ethanol administration was associated with a stimulation of TAT synthesis, long-term ethanol administration was not. The data suggest that ethanol itself does not directly induce TAT. Rather, enzyme synthesis is regulated by one or more endogenous secondary effector(s) whose production is influenced differently by acute or chronic ethanol feeding.

Effects of insulin on total RNA, poly(A)+ RNA, and mRNA in primary cultures of rat hepatocytes

The purpose of this study was to examine mechanisms involved in the regulation of protein synthesis in primary cultures of rat hepatocytes. Hepatocytes were maintained in a chemically defined serum-free medium in the presence or absence of insulin. The rate of protein synthesis in hepatocytes deprived of insulin between days 2 and 5 of culture was reduced to 67% of the rate observed in insulin-maintained controls. The decrease in protein synthetic rate was accompanied by a proportional fall in the content of both total RNA and poly(A)+RNA, suggesting that the capacity for protein synthesis was reduced in the absence of insulin. Both total RNA and poly(A)+ RNA contents and the protein synthetic rate were returned to control values after 3 days of insulin resupplementation. In addition, the effect of insulin on the expression of specific mRNAs was assessed by in vitro translation of total RNA followed by two-dimensional gel analysis of radiolabeled translation products. Only 13 of the greater than 150 spots discernible on the two-dimensional gels were altered in response to insulin. The mRNAs that were altered include examples of repression and stimulation of expression in response to insulin deprivation. Thus, in isolated rat hepatocytes, insulin regulates the capacity of both overall protein synthesis as well as the capacity for the synthesis of specific proteins.

Short- and long-term effects of insulin on tyrosine aminotransferase gene expression

In the present study the relationship between changes in tyrosine aminotransferase (TAT) enzyme activity, cytoplasmic mRNA levels, and gene transcription in response to both short- and long-term exposure to insulin was investigated. Insulin acutely inhibited transcription of the TAT gene by 50% in serum-deprived rat H4 hepatoma cells. Following this initial 50% decrease in transcription, there was a 2.5-fold induction in TAT activity that could not be accounted for by a concomitant increase in TAT mRNA levels. Insulin had no effect on the half-life of TAT mRNA. Insulin exposure for short periods of time also inhibited the glucocorticoid- and cAMP-induced transcription of the TAT gene. Like insulin, protein synthesis inhibitors acutely inhibited basal and glucocorticoid-induced TAT transcription. TAT activity gradually returned toward basal levels after 8 h of insulin treatment. A second insulin-induced increase in TAT activity (3.5-fold above basal levels) was observed by 24 h of insulin treatment. This second phase of insulin-induced TAT activity was associated with elevated levels of TAT transcription and TAT mRNA levels, and therefore, unlike the earlier stimulation, could be accounted for by changes in gene expression. Thus, the insulin-mediated regulation of the TAT gene in H4 cells is complex. Different transcriptional and post-transcriptional mechanisms are likely to be involved in the biphasic responses to insulin.

Effect of diabetes and insulin treatment of diabetic rats on total RNA, poly(A)+ RNA, and mRNA in skeletal muscle

We have assessed the time course of alterations in several biochemical parameters and expression of specific mRNAs in gastrocnemius muscle following both the induction of diabetes and the administration of insulin to diabetic rats. Muscle mass, total RNA, and total protein were reduced, whereas poly(A)+ RNA relative to total RNA was increased following the induction of diabetes. All the above parameters, with the exception of poly(A)+ RNA, were reciprocally and rapidly altered following administration of insulin to 3-day diabetic animals. These changes suggest that during the induction of diabetes 1) total cellular protein is reduced at a rate that is less than the reduction in gastrocnemius mass, whereas RNA is reduced at a rate 1.5 times the reduction in tissue mass, and 2) poly(A)+ RNA is elevated relative to total RNA. After insulin administration, there appears to be coordinate synthesis of both poly(A)+ RNA and ribosomal RNA, assuming 85% of total RNA is ribosomal. Therefore, we conclude that poly(A)+ RNA is more stable than ribosomal RNA during diabetes, whereas the amounts of poly(A)+ RNA and ribosomal RNA are increased at the same rates following insulin administration to diabetic animals. Analysis of expression of specific gene products over the same time course, as assessed by in vitro translation of total RNA followed by two-dimensional gel analysis, suggests that there are a few mRNAs that are very rapidly altered in response to insulin administration. The mRNAs that are altered demonstrate variable temporal patterns of either repression or full or transient expression. These rapid, but limited, alterations in gene expression may prove important in the development of the defects that occur in skeletal muscle in response to diabetes.

Regulation of gene expression by insulin

Insulin after binding to its plasma membrane receptor regulates many cellular processes as well as the expression of several genes. These effects of insulin can be temporarily classified as short-term (minutes) and long-term (hours-days). The different steps of gene expression that may be under insulin control are reviewed. The main focus of the review is on the regulation of gene transcription by insulin. A putative insulin negative regulatory sequence is proposed based on the comparison of the 5'-upstream regions of the phosphoenolpyruvate carboxykinase and protein disulfide isomerase genes and compared with a recently identified positive insulin regulatory element located in the 5'-upstream region of the glyceraldehyde-3-phosphate dehydrogenase gene.

Insulin-mediated regulation of tyrosine aminotransferase in rat hepatoma cells: inhibition of transcription and inhibition of enzyme degradation

Insulin induces the enzyme tyrosine aminotransferase (TAT) in Reuber H-35 rat hepatoma cells. A clone of these cells (KRC-7) was used to study the relationship between changes in enzyme activity and hybridizable mRNA, and rates of transcription for TAT in response to insulin. Our results indicate that enzyme activity is inducible by insulin in the presence of an inhibitor of RNA synthesis, suggesting that insulin functions post-transcriptionally to increase enzyme activity. Unexpectedly, insulin causes a decrease in the level of hybridizable TAT mRNA. Glucocorticoids cause an increase in TAT mRNA and insulin inhibits this increase when added either subsequent to or simultaneous with the addition of this agonist. Transcriptional runoffs demonstrate that insulin inhibits transcription of TAT to account for the aforementioned decrease in hybridizable mRNA. To examine the possibility that a post-translational mechanism is responsible for the increase in TAT activity caused by insulin, the rate of degradation of TAT protein was measured using polyclonal antibody. These experiments indicate that the rate of degradation of TAT is decreased about twofold in the presence of insulin, which suggests that part of the observed increase in TAT activity is due to selective post-translational stabilization of TAT. Therefore, insulin regulates TAT in KRC-7 cells by both transcriptional and post-translational mechanisms, the latter being responsible for the increase in activity.

Insulin antagonism of dexamethasone induction of tyrosine aminotransferase in cultured fetal hepatocytes. A correlation between enzyme activity, synthesis, level of messenger RNA and transcription

Previous studies have shown that insulin depresses the induction of tyrosine aminotransferase by glucocorticoids in cultured fetal rat hepatocytes. However, the site at which this inhibitory effect is exerted was not elucidated, since only enzyme activity was determined in such studies. Therefore, the effect of insulin on tyrosine aminotransferase synthesis, the level of its mRNA as well as the rate of transcription of the gene in isolated nuclei have been determined. The results obtained indicate that in cultures exposed to dexamethasone, Bt2cAMP, insulin and combinations of these additives, there is an excellent correlation between the enzyme activity, enzyme synthesis and the level of mRNA. Run-on transcription experiments indicate that the reduction in the level of mRNA by insulin in dexamethasone-supplemented cultures is the result of a diminished rate of gene transcription.

Insulin-mediated post-transcriptional regulation of hepatic malic enzyme and albumin mRNAs

Livers of insulin-treated diabetic rats accumulate albumin and malic enzyme mRNAs at very different rates. We now report that in normal rats insulin directs a specific increase in malic enzyme mRNA, while albumin mRNA levels remain unaltered. These studies support the contention that insulin regulates the accumulation of hepatic mRNAs in a highly specific manner. To evaluate whether or not albumin and malic enzyme mRNA levels are determined by altered rates of transcription, in vitro transcription assays were performed. The results of these studies demonstrate that increased malic enzyme mRNA levels in insulin-treated normal rats and increased malic enzyme and albumin mRNA levels in insulin-treated diabetic rats do not involve altered rates of transcription of the genetic sequences encoding these proteins. For these two specific proteins, insulin mediates changes in mRNA levels by a post-transcriptional mechanism.

Changes in hepatic differentiation following treatment of rat fetuses with 5-azacytidine

Rat fetuses of 20 days gestational age were treated in utero with 5-azacytidine. Within 14 to 18 h after treatment several significant changes in the fetal livers were observed, including a dramatic maturation of hepatocyte morphology with little alteration in hematopoietic elements. Assessment of mRNA levels by hybridization to cloned cDNAs, together with other measures of gene expression, established that the change in hepatocyte morphology was associated with strong activation of expression of genes normally activated later in development, including those coding for the liver enzymes tyrosine aminotransferase and phosphoenolcarboxykinase and a gene of unknown specificity that is regulated in liver much like the aminotransferase. Rates of transcription of two of these genes, measured in isolated nuclei, were significantly increased after 5-azacytidine treatment. Expression of alpha-fetoprotein, normally declining during the perinatal period of development, was reactivated following treatment with the drug, while albumin expression was somewhat enhanced. For the most part the changes observed reflect temporal advancement of events normally programmed to occur later in differentiation of the liver. These changes appear to be the consequence of multiple effects of 5-azacytidine, including enhanced gene transcription and stabilization of gene products.

Gene-specific acquisition of hormonal responsiveness in rat liver during development

Cloned cDNAs were used in hybridization analyses to assess hormonal responsiveness of two similarly regulated genes in livers of late-term fetal rats. Transcription of the tyrosine aminotransferase gene and of gene 33 (Lee et al.: J Biol Chem 260:16433-16438, 1985) is enhanced by glucocorticoids and by each of the usually antagonistic hormonal agents, insulin and cAMP, in adult liver, and that of both genes is developmentally activated at or just prior to birth. The mRNA of gene 33 was found to be significantly increased by each of the hormonal regulators in livers of fetuses treated in utero. Expression of the nearly silent aminotransferase gene in fetal liver was appreciably increased by cAMP but was refractory to control by either glucocorticoids or insulin; capacity of this gene to respond to insulin was not realized until several days postpartum. The data indicate specificity in the developmental acquisition of the capacity of individual genes to respond to hormonal regulators.

Specific insulin-mediated regulation of glutamine synthetase in cultured chick astroglial cells

The expression of glutamine synthetase (GS; L-glutamate ammonia ligase; EC 6.3.1.2) in primary cultures of chick astroglial cells and neurons grown in a chemically defined medium, with and without insulin added, was investigated. An inhibitory effect of insulin toward GS activity, and specific to chick astroglial cells, was observed. Neurons in culture were not sensitive to the hormone effect. Modulation of the activating effect of hydrocortisone on glial GS by insulin was also observed. The data suggest that insulin contributes to the regulation of the metabolism of amino acid neurotransmitters via its effect on GS.

Insulin enhances transcription of the tyrosine aminotransferase gene in rat liver

The mechanism of insulin-mediated induction of tyrosine aminotransferase in rat liver was investigated using a cloned cDNA probe. The level of aminotransferase mRNA increases about fourfold following administration of the hormone. This induced mRNA accumulation does not require de novo protein synthesis. Nuclear runoff transcription assays in isolated liver nuclei demonstrate that insulin has a rapid and time-dependent stimulatory effect on aminotransferase gene transcription. The magnitude of enhanced transcription can fully account for the increase in the mRNA. We conclude that the induction of tyrosine aminotransferase in rat liver by insulin is primarily a consequence of a selective increase in the rate of transcription of the aminotransferase gene.

Rapid effects of insulin on in vitro translational activity of specific mRNA in diabetic rat heart

We studied the time course of response of specific cardiac mRNA after administration of insulin to diabetic rats. The primary aim was to identify specific cardiac mRNA, which show a rapid response to insulin administration. Diabetic rats were injected with 2 U of regular insulin intravenously, and total cardiac RNA was prepared 0.5, 1.5, 3, 5, 12, and 24 h later. RNA was translated in vitro in the presence of [35S]methionine and the translational products separated by two-dimensional electrophoresis and quantitated by digital matrix photometry. A rapid change in the translational activity of five specific mRNA species was observed within 0.5 h after administration of insulin to the diabetic animal. One translational product exhibits a more delayed response at 1.5 h. The predominance of three of these products was increased, while that of three was decreased. Two specific mRNA coding for translation products designated as spots 97 and 106 show the most significant change, with a dramatic decrease of 15-fold and 6.5-fold, respectively, within 0.5 h after insulin administration. The change in levels of these specific mRNA species could result from effects of insulin at various sites of mRNA synthesis or degradation. However, the rapidity of the response is compatible with a direct effect of insulin on gene expression. The very quick response of these specific mRNA species to insulin could thus serve as a useful model system to examine the molecular mechanisms of insulin action in the heart.

The mechanism of insulin-induced increase of the rate of de novo purine biosynthesis in primary cultured rat hepatocytes

The effect of insulin on the rate of de novo purine biosynthesis was studied in freshly isolated hepatocytes from adult male rats. Insulin started to increase the rate of de novo purine biosynthesis at 1.5 X 10(-10) M and plateaued at 1.5 X 10(-8) M with the magnitude of increase of about 280%. Insulin at 1.5 X 10(-9) M or the higher concentration increased the 5-phosphoribosyl 1-pyrophosphate (PRPP) availability for purine ribonucleotide synthesis to about 230%. Insulin increased ATP concentration to 127% and decreased AMP, ADP, GMP and GDP concentration to 73, 69, 73 and 69% in association with the increased adenylate energy charge of 0.90 in comparison to the control level of 0.83. But the total adenine and guanine nucleotide concentration in the cell was not significantly changed. In addition insulin increased the specific activity of amidophosphoribosyltransferase (ATase) with the increased Vmax and the unchanged Km for PRPP.

Enhancement of (polyA+)RNA synthesis in light in isolated intact photoreceptor cells of the rat

Incorporation of [3H]-cytidine into messenger RNAs (polyadenylated RNAs) was enhanced in light in isolated intact photoreceptor cells of the rat. The increase in polyadenylated (PolyA+)RNAs appeared selective relative to other photoreceptor RNAs since incorporation of [3H]-cytidine into this fraction was up to 10-fold higher while labeling of total cellular RNAs was only two- to three-fold higher in light compared with dark. The photoreceptor cells were isolated in vivo through destruction of inner retina neurons with injections of combinations of neurotoxic substances during early postnatal development. The photoreceptor cells attained normal adult morphology and function: the alpha-wave of the electroretinogram, as well as the thickness of the outer nuclear layer and the length of photoreceptor inner and outer segments, were found to be within the normal range at 4 and 10 weeks of age. In addition to RNA synthesis, such photoreceptor cell preparations when incubated in vitro demonstrated a capacity for regulating light-dependent sodium fluxes comparable to that within the intact retina. The potential usefulness of this model for exploring the molecular biology of photoreceptor cells is discussed.

Insulin mediates the asynchronous accumulation of hepatic albumin and malic enzyme messenger RNAs

We have compared the rate of accumulation of hepatic albumin and malic enzyme mRNAs following insulin treatment of diabetic rats to determine whether insulin coordinately increases mRNA levels or specifically induces the accumulation of individuals mRNAs. Initially, the quantities of both albumin and malic enzyme mRNAs are reduced in diabetic rats compared to normal rats as determined by RNA blot analysis using complementary DNA probes. Following insulin administration for 12 h, albumin and malic enzyme mRNA levels increase at similar rates. However, after 12 h the rate of malic enzyme mRNA accumulation increases dramatically while albumin mRNA continues to increase at its initial rate. This accelerated rate of accumulation of malic enzyme mRNA continued through 60 h of hormone treatment and was associated with the onset of hepatic lipogenesis. Thus, our results suggest that insulin regulates the accumulation of mRNAs encoding these two inducible proteins in an asynchronous manner directly related to the metabolic requirements of the animal.

Essential role of insulin in transcription of the rat 25,000 molecular weight casein gene

Insulin is essential for the accumulation of rat casein messenger RNA (mRNA) in the presence of glucocorticoid and prolactin. The accumulation of certain mRNA's in other tissues has also been linked to insulin action. The present study shows that the accumulation effect on the 25,000 molecular weight rat casein mRNA does not reflect stabilization of the transcript by insulin. Rather, insulin is essential for its synthesis in the presence of glucocorticoid and prolactin.

Transcriptional and posttranscriptional regulation of interferon-induced gene expression in human cells

Eighteen cDNAs, cloned from interferon-treated T98G neuroblastoma cells, correspond to seven different mRNAs induced up to 40-fold by interferon. One codes for metallothionein II and another for a class I HLA. The others do not code for proteins of known sequence. In the continued presence of interferon, accumulation of the mRNAs continues for about 1 day but ceases whenever interferon is removed. Once induced, the mRNAs are stable. Synthesis of new proteins is not required for induction. The rate of transcription of one of the genes doubles 5 min after treatment with interferon and reaches a maximum by 60 min. This rate begins to fall after 4-6 hr, reaching the uninduced level by 8-12 hr. Since the mRNA continues to accumulate after 8-12 hr, posttranscriptional events must also play a role in increasing its level.

Effects of insulin and IGF-I on RNA synthesis in isolated soleus muscle

The effects of insulin and insulin-like growth factor I (IGF-I) on RNA synthesis and the effect of IGF-I on polypeptide chain initiation have been studied in the isolated mouse soleus muscle. Both peptides after a 3 h incubation enhanced net incorporation of the labelled uridine into RNA to a similar extent (40% increase over basal). The maximally effective concentrations were 66 nM and 100 nM for insulin and IGF-I respectively. Actinomycin D prevented the peptides' effect on RNA synthesis without modifying their effect on protein synthesis. Furthermore IGF-I increased the rate of initiation of polypeptide chains. It is suggested that in muscle, IGF-I and insulin stimulate protein synthesis by a dual mechanism: a rapid effect on the rate of polypeptide chain initiation; a slower effect on RNA synthesis.

Early molecular events in rat heart after administration of triiodothyronine and isoproterenol

Triiodothyronine (T3), isoproterenol and aminophylline injected daily into rats induce heart hypertrophy. We have compared the early effects of a single injection of each of these compounds to rats using myocardial RNA synthesis and translational efficiency. In rats injected with T3 4 h before death the synthesis of RNA was increased 2-fold, then the effect of T3 injection decreased with time. Injection of isoproterenol had no effect. Injection of T3 increased the amount of myocardial polysomes, heavy polysomes appeared approx. 15 h after the injection. Neither isoproterenol nor aminophylline modified the polysomal pattern. RNAs were translated in reticulocyte lysates in the presence of [35S]methionine. A small but significant increase in incorporation was observed with RNAs from rats injected with T3 4 and 18 h before death, whereas no modification were observed with RNAs from isoproterenol- and aminophylline-treated rats. Two-dimensional electrophoresis and radioautography showed significant qualitative and quantitative differences between the translational products of RNAs from control, T3-, isoproterenol- and aminophylline-injected rats. These observations are compatible with a mechanism of action of T3 at the transcriptional level and of cAMP on the processing and/or on the stability of various RNA species.

Hormonal regulation of translatable mRNA of glucose-6-phosphate dehydrogenase in primary cultures of adult rat hepatocytes

The quantity of translatable mRNA of glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate: NADP+ 1-oxidoreductase, EC 1.1.1.49) in primary cultures of adult rat hepatocytes subjected to different hormonal conditions was determined with a reticulocyte-lysate, cell-free system. The level of glucose-6-phosphate dehydrogenase mRNA was about 5-fold higher in the presence of insulin than in its absence. This increase of glucose-6-phosphate dehydrogenase mRNA reached a maximum 12 h after the addition of insulin. The maximum level of induction of glucose-6-phosphate dehydrogenase mRNA required 10(-8) M insulin. Glucagon and triiodothyronine had no effect on the glucose-6-phosphate dehydrogenase mRNA level. The increase of glucose-6-phosphate dehydrogenase activity correlated with the increase in level of mRNA of this enzyme. This suggests that the changes in glucose-6-phosphate dehydrogenase activity in response to the above hormonal changes are primarily due to changes in the amount of mRNA coding for this enzyme.

Inhibition of transcription of the phosphoenolpyruvate carboxykinase gene by insulin

Insulin regulates the synthesis of several proteins in a variety of tissues. Before techniques were available to quantify the amount of specific mRNAs, insulin was thought to regulate the synthesis of proteins by influencing the rate of translation of a fixed amount of mRNA. A very different interpretation is called for by experiments which show that insulin alters the amount of several specific mRNAs, but little is known about the mechanism. Insulin decreases the rate of synthesis of the critical gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK) in both liver and H4IIE heptoma cells. We recently showed that insulin acts directly on H4IIE cells to decrease mRNAPEPCK activity without any other hormone intermediaries. This effect is mediated by the insulin receptor and occurs at insulin concentrations which are well within the physiological range range (10(-12)--10(-9) M). Here we extend these studies to show that insulin specifically inhibits transcription of the PEPCK gene. This inhibition results in a rapid decrease in the concentration of nuclear PEPCK transcripts which is followed, in turn, by a proportionate decline in cytoplasmic mRNAPEPCK and synthesis of the protein.

Effect of physiological concentrations of insulin and antidiabetic drugs on RNA release from isolated liver nuclei

The addition of 10(-11) M insulin to a cell-free system from rat liver promotes the release of messengerlike RNA from isolated prelabeled nuclei. The stimulation was similar whether the nuclei were preincubated with insulin, or if insulin was added directly to the cell-free system with or without a protease inhibitor. Dot blot hybridization using cloned cDNA for alpha 2u-globulin mRNA showed that this was one of the messages whose release was enhanced by insulin. Nuclei isolated from rats treated with either of the antidiabetics tolbutamide or tolazamide showed no increase in RNA release in the presence of insulin over the concentration range 10(-5) - 10(-14) M. Furthermore, these nuclei did not release detectable levels of alpha 2u-globulin mRNA.

Development of tyrosine aminotransferase in perinatal rat liver: changes in functional messenger RNA and the role of inducing hormones

Expression of the hepatic enzyme tyrosine aminotransferase was analyzed in the perinatal period of development in the rat, when this expression undergoes significant changes associated with hepatocyte differentiation. In late prenatal liver both enzyme and functional mRNA gene products are present at levels 10- to 15-fold below those in the fully differentiated adult liver. This low level of expression in fetal liver is refractory to induction by glucocorticoids, but both gene products are increased to a limited extent by cyclic AMP. This induction by cyclic AMP (cAMP) does not confer glucocorticoid-responsiveness on expression. By 3 hr after birth both functional mRNA and enzyme levels are significantly increased, an increase which continues until a peak is reached at 12 hr that is appreciably above the adult levels. Both gene products then decline until adult levels are reached by 24 hr. The postnatal shift in aminotransferase expression is accompanied by acquisition of the capacity to respond to glucocorticoids. Treatment of newborns with an antiglucocorticoid steroid or with glucose suppresses the postnatal overshoot of expression, but neither treatment affects the increase from fetal to adult levels of expression. The results indicate that prior to birth, expression of the aminotransferase gene is partially repressed, a repression that is lifted essentially immediately upon birth. The hormones capable of inducing aminotransferase synthesis have no apparent necessary role in this process.

Action of insulin at the nuclear envelope

Insulin binding sites are present on purified nuclear envelopes from liver and other tissues, and EM autoradiographs and other types of studies indicate that insulin can enter intact target cells and interact with several types of intracellular membranes, including the nuclear envelope. More recent studies indicate that insulin has direct effects on both mRNA efflux from isolated nuclei and nuclear envelope NTPase, the enzyme that regulates mRNA efflux. These studies raise the possibility, therefore, that insulin regulates mRNA levels in target cells by directly influencing nuclear membrane functions as NTPase. Since insulin does not dramatically elevate mRNA levels for all proteins, the question arises as to how insulin selectively increases mRNA for specific mRNAs. One possibility is that there is targeting of specific mRNA molecules for specific pore complexes and that insulin may only influence a certain fraction of the nuclear pores. Thus, continued investigation is needed concerning the role of polypeptide hormones such as insulin in nucleocytoplasmic exchange.

Insulin stimulation of nucleoside triphosphatase activity in isolated nuclear envelopes

The activity of nucleoside triphosphatase, an enzyme that regulates nuclear messenger RNA transport, was measured in highly purified nuclear envelopes isolated from rat liver. Addition of picomolar concentrations of insulin to freshly prepared nuclear envelopes directly increased the enzyme activity. The major effect of insulin on this enzyme was to increase the maximum velocity of its activity; no significant effects were seen on the affinity constant. These studies raise the possibility, therefore, that the nuclear envelope is a site where insulin regulates nuclear functions.

Positive and negative cAMP-mediated control of tyrosine aminotransferase synthesis in Reuber H35 hepatoma cells

Induction of L-tyrosine:2-oxoglutarate aminotransferase (EC 2.6.1.5) by N6,O2'-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP) in Reuber H35 hepatoma cells reaches a maximum value between 3-5 h after addition of Bt2cAMP and subsequently decreases in the continuous presence of Bt2cAMP. We have investigated the kinetics of the increase, i.e. induction, and the decrease, i.e. the repressed state, of the tyrosine-aminotransferase-synthesizing system under these conditions. Our experimental results are as follows. 1. The repressed state of the tyrosine-aminotransferase-synthesizing system is not caused by a decrease in the intracellular cAMP concentration. 2. The repressed state is inhibited by actinomycin D (while induction is not inhibited). 3. During the repressed state no effect of dexamethasone on tyrosine aminotransferase synthesis is found, while during induction Bt2cAMP and dexamethasone act synergistically. 4. Longer starvation of the cells in serum-free medium has no influence on the kinetics of the induction/repressed state curve. From these results we have concluded that the mechanism of the transition to the repressed state of the tyrosine-aminotransferase-synthesizing system is essentially different from the mechanism of deinduction which occurs after removal of the inducer. Moreover, the repressed state of the system is a phenomenon which is induced by Bt2cAMP separately from induction at a different level of protein synthesis.

Insulin binding sites on the nuclear envelope: potential relationship to mRNA metabolism

Insulin regulates the growth and metabolism of most tissues. The hormonal potency of insulin results, to a large extent, from its ability to regulate target cells at a variety of subcellular sites. For many years, the effects of insulin on membrane transport, enzyme activity, and protein synthesis have been studied extensively. Less attention, however, was given to how insulin regulates nuclear functions. Recently the presence of specific binding sites for insulin on nuclei and nuclear envelopes have been documented and characterized. These binding sites have biochemical characteristics that are different from insulin binding sites on the plasma membrane. Moreover, direct in vitro effects of insulin on messenger RNA (mRNA) metabolism have recently been reported. These effects include the stimulation of mRNA efflux from intact nuclei, and stimulation of nucleoside triphosphatase activity (NTPase), the enzyme that regulates mRNA efflux. Thus, significant insight is now being gained concerning the action of insulin on the cell nucleus.

Tyrosine aminotransferase activity of frog (Rana esculenta) liver

1. The presence of tyrosine aminotransferase is reported both in particulate and soluble fractions of frog liver. 2. The activity of the soluble enzyme of frog liver was investigated with regard to its dose and time dependence, its substrate specificity and concentration dependence, its thermal sensitivity as well as pH and temperature dependence. 3. It appears that the properties of the soluble tyrosine aminotransferase of frog liver are in close agreement with those reported for the mammalian liver enzyme.