Control of fatty acid synthetase mRNA levels in rat liver by insulin, glucagon, and dibutyl cyclic AMP

Regulation of gene expression by insulin in adipose cells: opposite effects on adipsin and glycerophosphate dehydrogenase genes

Insulin is known to play the role of a positive effector both in vitro on the adipose conversion process and in vivo on the fatty acid synthesis and esterification processes in adipose tissue. The effects of insulin on the expression of two genes activated during adipose conversion, glycerol-3-phosphate dehydrogenase (GPDH) and adipsin genes, have been investigated in 3T3 F442A adipose cells. Within a physiological range of concentrations, insulin exerts opposite effects on the levels of GPDH (EC50 approximately 0.2 nM) and adipsin (EC50 approximately 1 nM) mRNAs. Its negative effect on the abundance of adipsin mRNA involves primarily a rapid inhibition of the transcriptional rate (less than 2 h). Its positive effect on the abundance of GPDH mRNA is due to a stimulation of the transcriptional rate accompanied by a delayed stabilization of GPDH mRNA. In addition, insulin exerts a specific effect on the length of the poly(A) tract of the adipsin mRNA. These results show that a single mechanism for the regulation of adipose-related genes by insulin can be excluded but rather suggest a complex phenomenon in which various levels of regulation take place.

Inhibition of gastrin gene expression by somatostatin

Previous studies performed in this laboratory have demonstrated somatostatin-containing cells in close proximity to gastrin cells in antral mucosa and have shown that somatostatin exerts a local regulatory effect on gastrin release. The present studies were directed to determine whether the effects of somatostatin on the antral gastrin cell involve pretranslational events. The effects of somatostatin on gastrin mRNA were determined by dot blot hybridization using a gastrin antisense RNA probe derived from human gastrin cDNA. Inclusion of somatostatin in the incubation medium caused a dose-dependent inhibition of steady-state gastrin mRNA. Conversely, when antral somatostatin was neutralized by the addition of specific somatostatin antibodies to the incubation medium, gastrin mRNA levels increased by 116 +/- 31% over control values (P less than 0.01). Northern blot hybridization of total antral RNA demonstrated a single major band with a molecular size of approximately 620 nucleotides, closely matching the predicted size of gastrin mRNA. The effect of somatostatin on the rate of gastrin gene transcription was examined using nuclear run-off transcription assays. Inclusion of antibodies to somatostatin in the incubation medium resulted in a 33.8 +/- 3.3% increase in gastrin gene transcriptional activity (P less than 0.01). These studies indicate that, in addition to its established effect on peptide release, somatostatin exerts inhibitory effects on antral gastrin cells at the pretranslational level. Although this inhibition appears to occur in part at the gene transcriptional level, the results also indicate that somatostatin may affect posttranscriptional processing of gastrin mRNA.

Induction of an Alu-sequence containing transcript by insulin in human hepatoma cells

A cDNA clone, designated AF19-1, was isolated from a cDNA library derived from insulin-stimulated hepatoma cells. The nucleotide sequences of AF19-1 showed 83% homology to Alu consensus sequence. It contained a full-length 300-bp Alu family sequence followed in direct tandem by a partial sequence of Alu left monomer. Primer extension analysis confirmed that this Alu transcript was induced even in short-term insulin stimulation. The increase in Alu-transcripts in the early phase of insulin stimulation in hepatoma cells suggests that the Alu sequences may play some important regulatory roles in gene expression.

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.

Insulin regulation of rat growth hormone gene transcription

We have previously shown that insulin suppresses growth hormone (GH) messenger (m) RNA levels in rat pituitary cells. To further delineate the molecular mechanism of insulin action, the effect of insulin treatment on GH gene transcription rates was examined in GH3 pituitary cells grown in serum-free defined medium. A transcriptional run-off assay was performed when intact isolated nuclei were allowed to continue RNA synthesis in an in vitro reaction. Specific incorporation of [32P]GTP into RNA was quantified by hybridization to rat GH complementary (c) DNA. Hybridization efficiency was measured with an internal [3H]cRNA standard and ranged from 30 to 48%. Alpha-amanitin (1 microgram/ml) inhibited total transcription, and excess unlabeled rat pituitary mRNA (250 ng) competitively inhibited GH mRNA hybridization by greater than 80%. Insulin (0.7 nM) inhibited new GH mRNA synthesis, and maximal inhibition (30% of control) was observed with 7 nM insulin after 4 h treatment. The inhibitory effects of insulin on new GH mRNA synthesis were abolished by both insulin-receptor-antiserum and by guinea-pig anti-insulin serum. The results show that insulin exerts a rapid suppression of new GH mRNA synthesis. These data provide evidence for the direct transcriptional regulation of the GH gene by insulin.

Effects of adrenaline on the turnover of lipoprotein lipase in rat adipose tissue

The mechanisms by which adrenaline brings about a reduction in the lipoprotein lipase activity of adipose tissue in vitro were investigated. The incorporation of [3H]leucine into lipoprotein lipase was measured during 1-h pulse incubations of rat epididymal fat bodies that had been preincubated for 4 h in the presence of glucose, insulin and dexamethasone. When adrenaline was added to the incubation medium at the start of the pulse, the incorporation of [3H]leucine was markedly reduced, suggesting that the rate of the enzyme's synthesis had decreased. On the other hand, the degradation of lipoprotein lipase, as measured by the loss of 3H-labelled enzyme protein during pulse-chase incubations of the epididymal fat bodies, was found to be significantly increased by the addition of adrenaline to the incubation medium at the start of the chase period. It is concluded that adrenaline is able both to inhibit the synthesis of lipoprotein lipase and to stimulate its degradation.

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.

Cloning of DNA complementary to rat liver fatty acid synthetase mRNA

Clones, containing DNA complementary (cDNA) to rat liver fatty acid synthetase mRNA, were constructed and identified. cDNA of these clones was then used as a probe to quantify mRNA. The cDNA was synthesized to partially purified rat liver fatty acid synthetase mRNA. Double-stranded cDNA was then prepared and inserted into the PstI site of pBR322 using oligo(dG) X oligo(dC) tailing. Initial selection of the clones was by differential colony hybridization employing [32P]cDNA synthesized from poly(A)-rich mRNA, enriched and non-enriched in fatty acid synthetase mRNA, as probes. Plasmids, containing specific sequences complementary to the fatty acid synthetase mRNA, were identified by hybrid-arrest translation. Cloned cDNA inserts ranged from 300 to 1400 base pairs. Cloned cDNA was employed to probe for mRNA in hybridizations via the dot-blot method. These studies demonstrated an increase in fatty acid synthetase mRNA during dietary induction, which suggests that regulation may involve changes in transcription or changes in post-transcriptional processing of the mRNA.

Kinetics for changes in enzyme synthesis and mRNA content and hormones required for induction of 6-phosphogluconate dehydrogenase in hepatocytes

Rats fasted for 2 days were refed a 60% glucose diet for varying periods of time in order to follow the kinetics for changes in 6-phosphogluconate dehydrogenase synthesis and mRNA content. Hepatocytes isolated from control or induced rats were incubated with actinomycin D and the rate of decline in 6-phosphogluconate dehydrogenase mRNA was determined by translating RNA in a nuclease-treated reticulocyte lysate. The half-life for 6-phosphogluconate dehydrogenase mRNA under both of these conditions was about 2 h. Thus, increases in transcription or the processing of nuclear RNA may increase 6-phosphogluconate dehydrogenase mRNA during the dietary induction of this enzyme. Hepatocytes prepared from fasted rats were cultured with 5% serum and various hormones and energy sources. If hepatocytes were isolated from thyroidectomized rats and cultured in serum from a thyroidectomized calf, the 4-fold induction of 6-phosphogluconate dehydrogenase was primarily dependent upon added insulin. In the presence of optimal insulin concentrations (10(-7) M) triiodothyronine slightly stimulated 6-phosphogluconate dehydrogenase induction. The gut hormones somatostatin and secretin had no effect on 6-phosphogluconate dehydrogenase induction in cultured hepatocytes. Hepatocytes cultured in carbohydrate-free medium and 5% serum required added insulin for maximal induction. 8-Br-cGMP did not significantly affect 6-phosphogluconate dehydrogenase induction in hepatocytes either in the presence or absence of added insulin. Dibutyryl cAMP did not alter the time course or extent of 6-phosphogluconate dehydrogenase induction in cultured hepatocytes. We have concluded that under these conditions insulin is a potent signal regulating the levels of 6-phosphogluconate dehydrogenase mRNA and that this induction is not mediated by cyclic nucleotides.

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.

Insulin regulation of protein phosphorylation in isolated rat liver nuclear envelopes: potential relationship to mRNA metabolism

The direct addition of insulin to highly purified nuclear envelopes prepared from the livers of diabetic rats resulted in a decrease in the incorporation of 32P into trichloroacetic acid-precipitable proteins. Autoradiography of 32P-labeled envelopes, solubilized in sodium dodecyl sulfate and subjected to electrophoresis, revealed that insulin decreased the phosphorylation of all major protein bands. Insulin produced detectable effects at concentrations between 0.1 and 1 pM, maximal effects at 10 pM, and progressively diminished effects at higher concentrations. Two insulin analogs, desdipeptide proinsulin and desoctapeptide insulin, had approximately 10% and 1%, respectively, the activity of native insulin. When nuclear envelopes were first phosphorylated with [gamma-32P]ATP and insulin was then added with an excess of unlabeled ATP, dephosphorylation was enhanced, suggesting that insulin was regulating nuclear envelope phosphatase activity. The direct addition of insulin to isolated rat liver nuclei in the presence of ATP stimulated the release of previously 14C-labeled trichloroacetic acid-precipitable mRNA-like material, and the direct addition of insulin to nuclear envelopes stimulated the activity of nucleoside triphosphatase, the enzyme that participates in mRNA nucleocytoplasmic transport. Moreover, the dose-response curves for these functions mirrored insulin's inhibition of nuclear envelope phosphorylation. These data suggest, therefore, a mechanism whereby insulin directly inhibits the phosphorylation of the nuclear envelope, leading in turn to the regulation of mRNA metabolism.

Induction of fatty acid synthetase and acetyl-CoA carboxylase by isolated rat liver cells

Current studies on the synthesis of long-chain fatty acids by isolated rat liver cells are largely concerned with the regulation of the activity of previously existing acetyl-CoA carboxylase and fatty acid synthetase, and with the regulation of the quantity of these enzymes. These studies have required the development of methods for obtaining high yields of viable hepatocytes that respond to hormonal treatment. Such methods have been developed over the past 10-15 years through the efforts of several laboratories. These studies have also required the development of a method to determine whether a change in the activity of an enzyme is due to a modification of preexisting enzyme or to a change in quantity of that enzyme. The most satisfactory method to use for such studies is immunotitration of enzyme activity. In recent years studies on the regulation of acetyl-CoA carboxylase have largely centered upon the effect of phosphorylation-dephosphorylation on the activity of this enzyme and whether glucagon inhibits the activity of this enzyme through this process. Much data from a number of laboratories have suggested that glucagon regulates the activity of this enzyme through phosphorylation-dephosphorylation. However, several of these studies involved the use of crude systems in which competing enzymes and substrates that can significantly interfere with acetyl-CoA carboxylase activity measurements were still present. Hence, a confirmation of these studies needs to be carried out under conditions in which the effects of competing enzymes and substrates are eliminated. Studies on changes in quantity of acetyl-CoA carboxylase and fatty acid synthetase have shown that these enzymes are induced by the fasting and refeeding of animals. They have also shown that insulin stimulates (10- to 30-fold) the induction of these enzymes. This induction appears to be due to a change in the quantity of translatable mRNA which may, in turn, be due to a change in the rate of transcription of the genes coding for these enzymes.

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.

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.