Location and characterization of two widely separated glucocorticoid response elements in the phosphoenolpyruvate carboxykinase gene

Molecular Mechanisms of Glucocorticoid-Induced Insulin Resistance

Glucocorticoids (GCs) are steroids secreted by the adrenal cortex under the hypothalamic-pituitary-adrenal axis control, one of the major neuro-endocrine systems of the organism. These hormones are involved in tissue repair, immune stability, and metabolic processes, such as the regulation of carbohydrate, lipid, and protein metabolism. Globally, GCs are presented as ‘flight and fight’ hormones and, in that purpose, they are catabolic hormones required to mobilize storage to provide energy for the organism. If acute GC secretion allows fast metabolic adaptations to respond to danger, stress, or metabolic imbalance, long-term GC exposure arising from treatment or Cushing’s syndrome, progressively leads to insulin resistance and, in fine, cardiometabolic disorders. In this review, we briefly summarize the pharmacological actions of GC and metabolic dysregulations observed in patients exposed to an excess of GCs. Next, we describe in detail the molecular mechanisms underlying GC-induced insulin resistance in adipose tissue, liver, muscle, and to a lesser extent in gut, bone, and brain, mainly identified by numerous studies performed in animal models. Finally, we present the paradoxical effects of GCs on beta cell mass and insulin secretion by the pancreas with a specific focus on the direct and indirect (through insulin-sensitive organs) effects of GCs. Overall, a better knowledge of the specific action of GCs on several organs and their molecular targets may help foster the understanding of GCs’ side effects and design new drugs that possess therapeutic benefits without metabolic adverse effects.

Boswellic acids and their derivatives as potent regulators of glucocorticoid receptor actions

Glucocorticoid (GCs) hormones exert their actions via their cognate steroid receptors the Glucocorticoid Receptors (GR), by genomic or non-genomic mechanisms of actions. GCs regulate many cellular functions among them growth, metabolism, immune response and apoptosis. Due to their cell type specific induction of apoptosis GCs are used for the treatment of certain type of cancer. In addition, due to their anti-inflammatory actions, GCs are among the most highly prescribed drug to treat chronic inflammatory disorders, albeit to the many adverse side effects arising by their long term and high doses use. Thus, there is a high need for selective glucocorticoid receptor agonist - modulators (SEGRA- SGRMs) as effective as classic GCs, but with a reduced side effect profile. Boswellic acids (BAs) are triterpenes that show structural similarities with GCs and exhibit anti-inflammatory and anti-cancer activities. In this study we examined whether BA alpha and beta and certain BAs derivatives exert their actions, at least in part, through the regulation of GR activities. Applying docking analysis we found that BAs can bind stably into the deacylcortivazol (DAC) accommodation pocket of GR. Moreover we showed that certain boswellic acids derivatives induce glucocorticoid receptor nuclear translocation, no activation of GRE dependent luciferase gene expression, and suppression of the TNF-α induced NF-κB transcriptional activation in GR positive HeLa and HEK293 cells, but not in low GR level COS-7 cells. Furthermore, certain boswellic acids compounds exert antagonistic effect on the DEX-induced GR transcriptional activation and induce cell type specific mitochondrial dependent apoptosis. Our results indicate that certain BAs are potent selective glucocorticoid receptor regulators and could have great potential for therapeutic use.

Separating the Anti-Inflammatory and Diabetogenic Effects of Glucocorticoids Through LXRβ Antagonism

Synthetic glucocorticoids (GCs), including dexamethasone (DEX), are powerful anti-inflammatory drugs. Long-term use of GCs, however, can result in metabolic side effects such as hyperglycemia, hepatosteatosis, and insulin resistance. The GC receptor (GR) and liver X receptors (LXRα and LXRβ) regulate overlapping genes involved in gluconeogenesis and inflammation. We have previously shown that Lxrβ-/- mice are resistant to the diabetogenic effects of DEX but still sensitive to its immunosuppressive actions. To determine whether this finding could be exploited for therapeutic intervention, we treated mice with GSK2033, a pan-LXR antagonist, alone or combined with DEX. GSK2033 suppressed GC-induced gluconeogenic gene expression without affecting immune-responsive GR target genes. The suppressive effect of GSK2033 on DEX-induced gluconeogenic genes was specific to LXRβ, was liver cell autonomous, and occurred in a target gene-specific manner. Compared with DEX treatment alone, the coadministration of GSK2033 with DEX decreased the recruitment of GR and its accessory factors MED1 and C/EBPβ to the phosphoenolpyruvate carboxykinase promoter. However, GSK2033 had no effect on DEX-mediated suppression of inflammatory genes expressed in the liver or in mouse primary macrophages stimulated with lipopolysaccharides. In conclusion, our study provides evidence that the gluconeogenic and immunosuppressive actions of GR activation can be mechanistically dissociated by pharmacological antagonism of LXRβ. Treatment with an LXRβ antagonist could allow the safer use of existing GC drugs in patients requiring chronic dosing of anti-inflammatory agents for the treatment of diseases such as rheumatoid arthritis and inflammatory bowel disease.

Adrenal-Specific Scavenger Receptor BI Deficiency Induces Glucocorticoid Insufficiency and Lowers Plasma Very-Low-Density and Low-Density Lipoprotein Levels in Mice

Objective—

We determined the physiological consequences of adrenocortical-specific deletion of scavenger receptor BI (SR-BI) function in C57BL/6 wild-type mice.

Methods and Results—

One adrenal from 10-day-old SR-BI knockout (KO) mice or wild-type controls was transplanted under the renal capsule of adrenalectomized C57BL/6 recipient mice. The fasting plasma corticosterone level increased over time in transplanted mice. Corticosterone values in SR-BI KO transplanted mice remained ≈50% lower ( P <0.001) as compared with wild-type transplanted mice, which coincided with adrenocortical lipid depletion. A 6.5-fold higher ( P <0.01) plasma adrenocorticotropic hormone level was present in SR-BI KO transplanted mice reminiscent of primary glucocorticoid insufficiency. On feeding with cholic acid-containing high cholesterol/high fat diet, SR-BI KO transplanted mice exhibited a 26% ( P <0.05) reduction in their liver triglyceride level. Hepatic myosin regulatory light chain interacting protein/inducible degrader of the low-density lipoprotein receptor mRNA expression was 48% ( P <0.01) decreased in adrenal-specific SR-BI KO mice, which was paralleled by a marked decrease (–46%; P <0.01) in proatherogenic very-low-density and low-density lipoprotein levels.

Conclusion—

Adrenal-specific disruption of SR-BI function induces glucocorticoid insufficiency and lowers plasma very-low-density and low-density lipoprotein levels in atherogenic diet-fed C57BL/6 mice. These findings further highlight the interaction between adrenal high-density lipoprotein-cholesterol uptake by SR-BI, adrenal steroidogenesis, and the regulation of hepatic lipid metabolism.

It takes two to tango: dimerisation of glucocorticoid receptor and its anti-inflammatory functions

For a number of years, there has been a widespread view that the adverse side-effects of prolonged glucocorticoid (GC) treatment are a result of glucocorticoid receptor (GR)-mediated gene activation, whilst the beneficial anti-inflammatory effects result from GR-mediated 'transrepression'. Since the introduction of the dimerisation-deficient GR mutant, GR(dim), was apparently unable to activate gene transcription, yet still able to repress pro-inflammatory gene transcription, the search for novel GR modulators has centred on the separation of gene activation from repression by prevention of GR dimerisation. However, recent work has questioned the conclusions drawn from these early GR(dim) studies, with evidence that GR(dim) mutants not only activate gene transcription, but that, in direct contradiction to the initial GR(dim) work, are also capable of forming dimers. This review of the current literature highlights the versatility of the GR in forming homodimer interactions, as well as the ability to bind to alternate nuclear receptors, and investigates the potential implications such varying GR dimer conformations may have for the design of GR ligands with a safer side effect profile.

5α-Reduced glucocorticoids: a story of natural selection

5α-Reduced glucocorticoids (GCs) are formed when one of the two isozymes of 5α-reductase reduces the Δ(4-5) double bond in the A-ring of GCs. These steroids are largely viewed inert, despite the acceptance that other 5α-dihydro steroids, e.g. 5α-dihydrotestosterone, retain or have increased activity at their cognate receptors. However, recent findings suggest that 5α-reduced metabolites of corticosterone have dissociated actions on GC receptors (GRs) in vivo and in vitro and are thus potential candidates for safer anti-inflammatory steroids. 5α-Dihydro- and 5α-tetrahydro-corticosterone can bind with GRs, but interest in these compounds had been limited, since they only weakly activated metabolic gene transcription. However, a greater understanding of the signalling mechanisms has revealed that transactivation represents only one mode of signalling via the GR and recently the abilities of 5α-reduced GCs to suppress inflammation have been demonstrated in vitro and in vivo. Thus, the balance of parent GC and its 5α-reduced metabolite may critically affect the profile of GR signalling. 5α-Reduction of GCs is up-regulated in liver in metabolic disease and may represent a pathway that protects from both GC-induced fuel dyshomeostasis and concomitant inflammatory insult. Therefore, 5α-reduced steroids provide hope for drug development, but may also act as biomarkers of the inflammatory status of the liver in metabolic disease. With these proposals in mind, careful attention must be paid to the possible adverse metabolic effects of 5α-reductase inhibitors, drugs that are commonly administered long term for the treatment of benign prostatic hyperplasia.

Skin elastic fibres: regulation of human elastin promoter activity in transgenic mice

Elastic fibres form an extracellular network which provides elasticity and resilience to tissues such as the skin. To study the regulation of human elastin gene expression, we have developed a line of transgenic mice which harbour 5.2 kb of human elastin gene promoter region in their genome. This promoter is linked to the chloramphenicol acetyltransferase (CAT) reporter gene which allows determination of the expression of human elastin promoter in different tissues. The highest CAT activity was found in the lungs and aorta, tissues rich in elastin, while lower levels were detected in a variety of other tissues, including skin. Assay of CAT activity in the lungs of fetal and newborn animals revealed high activity which progressively declined during the postnatal period up to six months. Thus, there was evidence of tissue-specific and developmentally regulated expression of the human elastin promoter activity in these mice. These animals were then used to examine the expression of the elastin gene by a variety of factors which have previously shown to alter elastin gene expression, as determined at the mRNA or protein levels. First, injection of transforming growth factor beta 1 (100 ng) subcutaneously into the transgenic animals resulted in a time-dependent elevation of the promoter activity up to 10-fold after a single injection. Secondly, enhancement of the human elastin promoter activity by interleukin 1 beta injected subcutaneously resulted in an approximately 10-fold elevation of the CAT activity. Finally, subcutaneous injection of these animals with triamcinolone acetonide or dexamethasone, two glucocorticosteroids in clinical use, resulted in marked enhancement of human elastin promoter activity. Similar changes were noted in fibroblast cultures established from the transgenic animals. These data indicate that the 5.2 kb upstream segment of the human elastin gene contains cis-elements which allow tissue-specific and developmentally regulated expression of the human elastin promoter. Furthermore, this segment of the gene contains responsive elements to a variety of cytokines and pharmacological agents. Collectively, these data indicate that elastin gene expression in the skin in vivo can be regulated at the transcriptional level.

3'-Untranslated region of phosphoenolpyruvate carboxykinase mRNA contains multiple instability elements that bind AUF1

Phosphoenolpyruvate carboxykinase (PEPCK) is regulated solely by alterations in gene expression that involve changes in rates of PEPCK mRNA transcription and degradation. A tetracycline-responsive promoter system was used to quantify the half-life of various chimeric beta-globin-PEPCK (betaG-PCK) mRNAs in LLC-PK -F(+) cells. The control betaG mRNA was extremely stable (t(1/2) = 5 days). However, betaG-PCK-1 mRNA, which contains the entire 3'-UTR of the PEPCK mRNA, was degraded with a half-life of 1.2 h. RNase H treatment indicated that rapid deadenylation occurred concomitant with degradation of the betaG-PCK-1 mRNA. Previous studies indicate that PCK-7, a 50-nucleotide segment at the 3'-end of the 3'-UTR, binds an unidentified protein that may contribute to the rapid decay of the PEPCK mRNA. However, the chimeric betaG-PCK-7 mRNA has a half-life of 17 h. Inclusion of the adjacent PCK-6 segment, a 23-bp AU-rich region, produced the betaG-PCK-6/7 mRNA, which has a half-life of 3.6 h. The betaG-PCK-3 mRNA that contains the 3'-half of 3'-UTR was degraded with the same half-life. Surprisingly, the betaG-PCK-2 mRNA, containing the 5'-end of the 3'-UTR, was also degraded rapidly (t((1/2)) = 5.4 h). RNA gel shift analyses established that AUF1 (hnRNP D) binds to the PCK-7, PCK-6, and PCK-2 segments with high affinity and specificity. Mutational analysis indicated that AUF1 binds to a UUAUUUUAU sequence within PCK-6 and the stem-loop structure and adjacent CU-region of PCK-7. Thus, AUF1 binds to multiple destabilizing elements within the 3'-UTR that participate in the rapid turnover of the PEPCK mRNA.

SHARP-2/Stra13/DEC1 as a potential repressor of phosphoenolpyruvate carboxykinase gene expression

The influence of the enhancer of split- and hairy-related protein-2 (SHARP-2) transcriptional repressor on the expression of rat phosphoenolpyruvate carboxykinase (PEPCK) gene was examined. When H4IIE cells were treated with epigallocatechin gallate, a green tea constituent, an increase in SHARP-2 mRNA levels and a decrease in PEPCK mRNA levels were observed. The adenovirus-mediated overexpression of SHARP-2 in H4IIE cells and primary cultured rat hepatocytes led to a decrease in the levels of PEPCK mRNA. Finally, when a SHARP-2 expression plasmid was transiently transfected with various reporter plasmids into MH1C1 cells, the promoter activity of a PEPCK reporter plasmid was specifically decreased. Based on these findings, we conclude that SHARP-2 is a potential repressor of PEPCK gene expression.

Elements of the glucocorticoid and retinoic acid response units are involved in cAMP-mediated expression of the PEPCK gene

Although many genes are regulated by the concerted action of several hormones, hormonal signaling to gene promoters has generally been studied one hormone at a time. The phosphoenolpyruvate carboxykinase (PEPCK) gene is a case in point. Transcription of this gene is induced by glucagon (acting by the second messenger, cAMP), glucocorticoids, and retinoic acid, and it is dominantly repressed by insulin. These hormonal responses require the presence of different hormone response units (HRUs), which consist of constellations of DNA elements and associated transcription factors. These include the glucocorticoid response unit (GRU), cAMP response unit (CRU), retinoic acid response unit (RARU), and the insulin response unit. HRUs are known to have functional overlap. In particular, the cAMP response element of the CRU is also a component of the GRU. The purpose of this study was to determine whether known GRU or RARU elements or transcription factors function as components of the CRU. We show here that the glucocorticoid accessory factor binding site 1 and glucocorticoid accessory factor binding site 3 elements, which are components of both the GRU and RARU, are an important part of the CRU. Furthermore, we find that the transcription factor, chicken ovalbumin upstream promoter-transcription factor, and two coactivators, cAMP response element-binding protein-binding protein and steroid receptor coactivator-1, participate in both the cAMP and glucocorticoid responses. This provides a further illustration of how the PEPCK gene promoter integrates different hormone responses through overlapping HRUs that utilize some of the same transcription factors and coactivators.

c-Myc is required for the glucose-mediated induction of metabolic enzyme genes

Glucose exerts powerful effects on hepatocyte gene transcription by mechanisms that are incompletely understood. c-Myc regulates hepatic glucose metabolism by increasing glycolytic enzyme gene transcription while concomitantly decreasing gluconeogenic and ketogenic enzyme gene expression. However, the molecular mechanisms by which c-Myc exerts these effects is not known. In this study, the glucose-mediated induction of L-type pyruvate kinase and glucose-6-phosphatase mRNA levels was diminished by maneuvers involving recombinant adenoviral vectors that interfere with (i) c-Myc protein levels by antisense expression or (ii) c-Myc function through a dominant-negative Max protein. These results were obtained using both HL1C rat hepatoma cells and primary rat hepatocytes. Furthermore, a decrease in c-Myc abundance reduced glucose production in HL1C cells, presumably by decreasing glucose-6-phosphatase activity. The repression of hormone-activated phosphoenolpyruvate carboxykinase gene transcription by glucose was not affected by a reduction in c-Myc levels. The basal mRNA levels for L-pyruvate kinase and glucose-6-phosphatase were not altered to any significant degree by adenoviral treatment. Furthermore, adenoviral overexpression of the c-Myc protein induced glucose-6-phosphatase mRNA in the absence of glucose stimulation. We conclude that multiple mechanisms exist to communicate the glucose-derived signal and that c-Myc has a key role in the hepatic glucose signaling pathway.

Insulin inhibits hepatocellular glucose production by utilizing liver-enriched transcriptional inhibitory protein to disrupt the association of CREB-binding protein and RNA polymerase II with the phosphoenolpyruvate carboxykinase gene promoter

Hormones regulate glucose homeostasis, in part, by controlling the expression of gluconeogenic enzymes, such as phosphoenolpyruvate carboxykinase (PEPCK). Insulin and glucocorticoids reciprocally regulate PEPCK expression primarily at the level of gene transcription. We demonstrate here that glucocorticoids promote, whereas insulin disrupts, the association of CREB-binding protein (CBP) and RNA polymerase II with the hepatic PEPCK gene promoter in vivo. We also show that accessory factors, such as CCAAT/enhancer-binding protein beta (C/EBP beta), can recruit CBP to drive transcription. Insulin increases protein levels of liver-enriched transcriptional inhibitory protein (LIP), an inhibitory form of C/EBP beta, in a phosphatidylinositol 3-kinase-dependent manner. LIP concomitantly replaces liver-enriched transcriptional activator protein on the PEPCK gene promoter, which can abrogate the recruitment of CBP and polymerase II, culminating in the repression of PEPCK expression and the attenuation of hepatocellular glucose production.

A point mutation of the AF2 transactivation domain of the glucocorticoid receptor disrupts its interaction with steroid receptor coactivator 1

Glucocorticoids cause a 10-fold increase in hepatic phosphoenolpyruvate carboxykinase (PEPCK) gene transcription through two low affinity glucocorticoid receptor (GR) binding sites and a complex array of accessory factor DNA elements and associated proteins. To analyze how co-activators interact with the GR in this context, we took advantage of the C656G GR mutant that binds ligand with very high affinity. This GR activates PEPCK gene transcription at a 500-fold lower dexamethasone concentration than does wild type GR. Transfected C656G GR containing additional mutations or deletions was tested on PEPCK gene expression in H4IIE hepatoma cells. We found that the AF2 domain is the only one of the three defined transactivation domains in GR that is required for PEPCK gene expression and that mutation of this domain disrupts the direct interaction of GR with steroid receptor coactivator 1 (SRC-1). These data help define the functional interaction between GR and SRC-1 and further define the role of the GR in glucocorticoid-mediated expression of the PEPCK gene.

Phosphoenolpyruvate carboxykinase is induced in growth-arrested hepatoma cells

Phosphoenolpyruvate carboxykinase (PEPCK) mRNA is elevated in H4IIEC3 rat hepatoma cells cultured at high density, suggesting that PEPCK expression and growth arrest may be coordinately regulated. Induction of growth arrest either by contact inhibition (high culture density) or by serum deprivation correlated with significant increases in PEPCK protein and its mRNA. The observation that PEPCK mRNA was induced by contact inhibition in the presence of serum indicates that the effect of high density is independent of insulin or any other serum component. The magnitudes of the changes in PEPCK expression during growth arrest were greatly enhanced in KRC-7 cells, an H4IIEC3 subclone that is much more sensitive to growth arrest than its parental cell line. Restimulation of proliferation in growth-arrested KRC-7 cells, either by addition of serum or insulin to serum-deprived cells or by replating contact-inhibited cells at low density, caused a rapid decrease in PEPCK expression. However, PEPCK mRNA is not always reduced in proliferating cells since treatment of serum-starved cells with epidermal growth factor stimulated entry into the cell cycle but did not affect PEPCK mRNA levels. Finally, dexamethasone induction of PEPCK mRNA was blunted in cells cultured at high density but was unaffected by the presence or absence of serum. Collectively, these data suggest the possibility of cross-talk between the control of PEPCK expression and growth arrest in KRC-7 cells.

Regulation of cAMP-responsive element-binding protein-mediated transcription by the SNF2/SWI-related protein, SRCAP

SRCAP (SNF2-related CPB activator protein) belongs to the SNF2 family of proteins whose members participate in various aspects of transcriptional regulation, including chromatin remodeling. It was identified by its ability to bind to cAMP-responsive-binding protein (CREB)-binding protein (CBP), and it increases the transactivation function of CBP. The phosphoenolpyruvate carboxykinase (PEPCK) promoter was used as a model system to explore the role of SRCAP in the regulation of transcription mediated by factors that utilize CBP as a coactivator. We show that transcription of a PEPCK chloramphenicol acetyltransferase (CAT) reporter gene activated by protein kinase A (PKA) is enhanced 7-fold by SRCAP. In the absence of PKA this SRCAP-mediated enhancement does not occur, suggesting that SRCAP functions as a coactivator for PKA-activated factors such as CREB. Replacing the PEPCK promoter binding site for CREB with a binding site for Gal4 (DeltaCRE (cAMP-responsive element) Gal4 PEPCK-CAT reporter gene) blocks the ability of SRCAP to activate transcription despite the presence of PKA. Expression of a Gal-CREB chimera restores the ability of PKA to regulate transcription of the DeltaCRE Gal4 PEPCK gene and restored the ability of SRCAP to stimulate PKA-activated transcription. In addition, SRCAP in the presence of PKA enhances the ability of the Gal-CREB chimera to activate transcription of a Gal-CAT reporter gene that contains only binding sites for Gal4. SRCAP binds to CBP amino acids 280-460, a region that is important for CBP to function as a coactivator for CREB. Overexpression of a SRCAP peptide corresponding to this CBP binding domain acts as a dominant negative inhibitor of CREB-mediated transcription. Structure-function studies were done to explore the mechanism(s) by which SRCAP regulates transcription. These studies indicate that the N-terminal region of SRCAP, which contains five of the seven regions that comprise the ATPase domain, is not needed for activation of CREB-mediated transcription. SRCAP apparently has several domains that participate in the activation of transcription.

Accessory factors facilitate the binding of glucocorticoid receptor to the phosphoenolpyruvate carboxykinase gene promoter

Glucocorticoid induction of the phosphoenolpyruvate carboxykinase (PEPCK) gene requires a glucocorticoid response unit (GRU) comprised of two non-consensus glucocorticoid receptor (GR) binding sites, GR1 and GR2, and at least three accessory factor elements (gAF1-3). DNA-binding accessory proteins are commonly required for the regulation of genes whose products play an important role in metabolism, development, and a variety of defense responses, but little is known about why they are necessary. Quantitative, real time homogenous assays of cooperative protein-DNA interactions in complex media (e.g. nuclear extracts) have not previously been reported. Here we perform quantitative, real time equilibrium and stopped-flow fluorescence anisotropy measurements of protein-DNA interactions in nuclear extracts to demonstrate that GR binds to the GR1-GR2 elements poorly as compared with a palindromic or consensus glucocorticoid response element (GRE). Inclusion of either the gAF1 or gAF2 element with GR1-GR2, however, creates a high affinity binding environment for GR. GR can undergo multiple rounds of binding and dissociation to the palindromic GRE in less than 100 ms at nanomolar concentrations. The dissociation rate of GR is differentially slowed by the gAF1 or gAF2 elements that bind two functionally distinct accessory factors, COUP-TF/HNF4 and HNF3, respectively.

Role of accessory factors and steroid receptor coactivator 1 in the regulation of phosphoenolpyruvate carboxykinase gene transcription by glucocorticoids

In the liver, glucocorticoids induce a 10-15-fold increase in the rate of transcription of the phosphoenolpyruvate carboxykinase (PEPCK) gene, which encodes a key gluconeogenic enzyme. This induction requires a multicomponent glucocorticoid response unit (GRU) comprised of four glucocorticoid accessory factor (AF) elements and two glucocorticoid receptor binding sites. We show that the AFs that bind the gAF1, gAF2, and gAF3 elements (hepatocyte nuclear factor [HNF]4/chicken ovalbumin upstream promoter transcription factor 1 and HNF3beta) all interact with steroid receptor coactivator 1 (SRC1). This suggests that the AFs function in part by recruiting coactivators to the GRU. The binding of a GAL4-SRC1 chimeric protein completely restores the glucocorticoid induction that is lost when any one of these elements is replaced with a GAL4 binding site. Thus, when SRC1 is recruited directly to gAF1, gAF2, or gAF3, the requirement for the corresponding AF is bypassed. Surprisingly, glucocorticoid receptor is still required when SRC1 is recruited directly to the GAL4 site, suggesting a role for the receptor in activating SRC1 in the context of the GRU. Structural variants of GAL4-SRC1 were used to identify requirements for the basic-helix-loop-helix and histone acetyltransferase domains of SRC1, and these are specific to the region of the promoter to which the coactivator is recruited.

Mapping and functional analysis of an instability element in phosphoenolpyruvate carboxykinase mRNA

Phosphoenolpyruvate carboxykinase (PEPCK) is a key regulatory enzyme of renal gluconeogenesis. The 3'-nontranslated region of the PEPCK mRNA contains an instability element that facilitates its rapid turnover and contributes to the regulation of PEPCK gene expression. Such processes are mediated by specific protein-binding elements. Thus RNA gel shift analysis was used to identify proteins in rat renal cortical cytosolic extracts that bind to the 3'-nontranslated region of the PEPCK mRNA. Deletion constructs were then used to map the binding interactions to two adjacent RNA segments (PEPCK-6 and PEPCK-7). However, competition experiments established that only the binding to PEPCK-7 was specific. Functional studies were performed by cloning similar segments in a luciferase reporter construct, pLuc/Zeo. This analysis indicated that both PEPCK-6 and PEPCK-7 segments were necessary to produce a decrease in luciferase activity equivalent to that observed with the full-length 3'-nontranslated region. Thus the PEPCK-7 segment binds a specific protein that may recruit one or more proteins to form a complex that mediates the rapid decay of the PEPCK mRNA.

NF-kappa B inhibits glucocorticoid and cAMP-mediated expression of the phosphoenolpyruvate carboxykinase gene

Transcription of the phosphoenolpyruvate carboxykinase (PEPCK) gene is regulated by a variety of agents. Glucocorticoids, retinoic acid, and glucagon (via its second messenger, cAMP) stimulate PEPCK gene transcription, whereas insulin, phorbol esters, cytokines, and oxidative stress have an opposing effect. Stimulation of PEPCK gene expression has been extensively studied, and a number of important DNA elements and binding proteins that regulate the transcription of this gene have been identified. However, the mechanisms utilized to turn off expression of this gene are not well-defined. Many of the negative regulators of PEPCK gene transcription also stimulate the nuclear localization and activation of the transcription factor NF-kappaB, so we hypothesized that this factor could be involved in the repression of PEPCK gene expression. We find that the p65 subunit of NF-kappaB represses the increase of PEPCK gene transcription mediated by glucocorticoids and cAMP in a concentration-dependent manner. The mutation of an NF-kappaB binding element identified in the PEPCK gene promoter fails to abrogate this repression. Further analysis suggests that p65 represses PEPCK gene transcription through a protein.protein interaction with the coactivator, CREB binding protein.

Disposition of a mixed meal by conscious dogs after seven days of treatment with cyclosporine A and prednisone

BACKGROUND

Combination immunosuppressive therapy, that often includes prednisone and cyclosporine A (CyA), is commonly used in the treatment of organ transplant patients. We hypothesized that CyA and prednisone treatment would alter the roles of the liver and peripheral tissues in the disposal of carbohydrates from a meal.

METHODS

Using the arteriovenous difference technique, we examined the disposition of an intragastrically delivered mixed meal in eight 24-hour fasted conscious dogs that had received CyA 15 mg x kg(-1) daily and prednisone 5 mg twice daily for 7 consecutive days before study (CyA-prednisone group). The results were compared with those from a group of 13 dogs (control group) receiving the same meal but no drugs.

RESULTS

Neither arterial blood glucose concentrations nor arterial plasma insulin or glucagon concentrations differed significantly between the groups at any time. Cumulative net gut glucose output was equivalent to 43 +/- 9 vs 57% +/- 7% of the glucose in the meal in CyA-prednisone vs control (p = .12). The CyA-prednisone group exhibited greater (p < .05) mean net hepatic glucose uptakes (15.4 +/- 4.6 vs 4.3 +/- 2.2 micromol x kg(-1) x min(-1) and net hepatic fractional extractions of glucose (7.8 +/- 1.6 and 1.5% +/- 1.0%) than the control group. Arterial blood lactate concentrations and net hepatic lactate output were greater in the CyA-prednisone group than the control group (p < .05). Hepatic glycogen content at the end of the study was 2.5-fold greater in the CyA-prednisone group than in the control group (p < .05). The nonhepatic tissues disposed of approximately 91% of the absorbed glucose in the control group but only approximately 26% in the CyA-prednisone group (p < .05).

CONCLUSIONS

CyA-prednisone treatment caused a marked shift in the carbohydrate disposal from a meal, enhancing the hepatic glucose uptake and decreasing peripheral glucose disposal.

Transcription activation by the orphan nuclear receptor, chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI). Definition of the domain involved in the glucocorticoid response of the phosphoenolpyruvate carboxykinase gene

Chicken ovalbumin upstream promoter-transcription factors (COUP-TFs), orphan members of the nuclear receptor superfamily, play a key role in the regulation of organogenesis, neurogenesis, and cellular differentiation during embryogenic development. COUP-TFs are also involved in the regulation of several genes that encode metabolic enzymes. Although COUP-TFs function as potent transcription repressors, there are at least three different molecular mechanisms of activation of gene expression by COUP-TFs. First, as we have previously shown, COUP-TF is required as an accessory factor for the complete induction of phosphoenolpyruvate carboxykinase gene transcription by glucocorticoids. This action is mediated by the binding of COUP-TF to the glucocorticoid accessory factor 1 (gAF1) and 3 (gAF3) elements in the phosphoenolpyruvate carboxykinase gene glucocorticoid response unit. In addition, COUP-TF1 binds to DNA elements in certain genes and transactivates directly. Finally, COUP-TF1 serves as a coactivator through DNA-bound hepatic nuclear factor 4. Here we show that the same region of COUP-TFI, located between amino acids 184 and 423, is involved in these three mechanisms of transactivation by COUP-TFI. Furthermore, we show that GRIP1 and SRC-1 potentiate the activity of COUP-TFI and that COUP-TFI associates with these coactivators in vivo using the same region required for transcription activation. Finally, overexpression of GRIP1 or SRC-1 does not convert COUP-TFI from a transcriptional repressor into a transcriptional activator in HeLa cells.

CCAAT/enhancer-binding protein beta is an accessory factor for the glucocorticoid response from the cAMP response element in the rat phosphoenolpyruvate carboxykinase gene promoter

The cyclic AMP response element (CRE) of the rat phosphoenolpyruvate carboxykinase (PEPCK) gene promoter is required for a complete glucocorticoid response. Proteins known to bind the PEPCK CRE include the CRE-binding protein (CREB) and members of the CCAAT/enhancer-binding protein (C/EBP) family. We took two different approaches to determine which of these proteins provides the accessory factor activity for the glucocorticoid response from the PEPCK CRE. The first strategy involved replacing the CRE of the PEPCK promoter/chloramphenicol acetyltransferase reporter plasmid (pPL32) with a consensus C/EBP-binding sequence. This construct, termed pDeltaCREC/EBP, binds C/EBPalpha and beta but not CREB, yet it confers a nearly complete glucocorticoid response when transiently transfected into H4IIE rat hepatoma cells. These results suggest that one of the C/EBP family members may be the accessory factor. The second strategy involved co-transfecting H4IIE cells with a pPL32 mutant, in which the CRE was replaced with a GAL4-binding sequence (pDeltaCREGAL4), and various GAL4 DNA-binding domain (DBD) fusion protein expression vectors. Although chimeric proteins consisting of the GAL4 DBD fused to either CREB or C/EBPalpha are able to confer an increase in basal transcription, they do not facilitate the glucocorticoid response. In contrast, a fusion protein consisting of the GAL4 DBD and amino acids 1-118 of C/EBPbeta provides a significant glucocorticoid response. Additional GAL4 fusion studies were done to map the minimal domain of C/EBPbeta needed for accessory factor activity to the glucocorticoid response. Chimeric proteins containing amino acid regions 1-84, 52-118, or 85-118 of C/EBPbeta fused to the GAL4 DBD do not mediate a glucocorticoid response. We conclude that the amino terminus of C/EBPbeta contains a multicomponent domain necessary to confer accessory factor activity to the glucocorticoid response from the CRE of the PEPCK gene promoter.

The repression of hormone-activated PEPCK gene expression by glucose is insulin-independent but requires glucose metabolism

Phosphoenolpyruvate carboxykinase (PEPCK) is a rate-controlling enzyme in hepatic gluconeogenesis, and it therefore plays a central role in glucose homeostasis. The rate of transcription of the PEPCK gene is increased by glucagon (via cAMP) and glucocorticoids and is inhibited by insulin. Under certain circumstances glucose also decreases PEPCK gene expression, but the mechanism of this effect is poorly understood. The glucose-mediated stimulation of a number of glycolytic and lipogenic genes requires the expression of glucokinase (GK) and increased glucose metabolism. HL1C rat hepatoma cells are a stably transfected line of H4IIE rat hepatoma cells that express a PEPCK promoter-chloramphenicol acetyltransferase fusion gene that is regulated in the same manner as the endogenous PEPCK gene. These cells do not express GK and do not normally exhibit a response of either the endogenous PEPCK gene, or of the trans-gene, to glucose. A recombinant adenovirus that directs the expression of glucokinase (AdCMV-GK) was used to increase glucose metabolism in HL1C cells to test whether increased glucose flux is also required for the repression of PEPCK gene expression. In AdCMV-GK-treated cells glucose strongly inhibits hormone-activated transcription of the endogenous PEPCK gene and of the expressed fusion gene. The glucose effect on PEPCK gene promoter activity is blocked by 5 mM mannoheptulose, a specific inhibitor of GK activity. The glucose analog, 2-deoxyglucose mimics the glucose response, but this effect does not require GK expression. 3-O-methylglucose is ineffective. Glucose exerts its effect on the PEPCK gene within 4 h, at physiologic concentrations, and with an EC50 of 6.5 mM, which approximates the Km of glucokinase. The effects of glucose and insulin on PEPCK gene expression are additive, but only at suboptimal concentrations of both agents. The results of these studies demonstrate that, by inhibiting PEPCK gene transcription, glucose participates in a feedback control loop that governs its production from gluconeogenesis.

Further characterization of the glucocorticoid response unit in the phosphoenolpyruvate carboxykinase gene. The role of the glucocorticoid receptor-binding sites

Phosphoenolpyruvate carboxykinase (PEPCK) catalyzes the rate-limiting step of gluconeogenesis. The activity of this enzyme is controlled by several hormones, including glucocorticoids, glucagon, retinoic acid, and insulin, that principally affect the rate of transcription of the PEPCK gene. Glucocorticoids induce PEPCK gene transcription through a complex glucocorticoid response unit that consists of, from 5' to 3', accessory factor elements AF1 and AF2; two noncanonical glucocorticoid receptor-binding sites, GR1 and GR2; a third accessory factor element, AF3; and a cAMP-response element, CRE. A complete glucocorticoid response is dependent on the presence of both GR-binding sites, all three accessory elements, and the CRE. In this study we assess the relative roles of GR1 and GR2 in the context of the glucocorticoid response unit and use a combination of binding and function assays to compare GR1 and GR2 to glucocorticoid response elements (GREs) that conform closely to the consensus sequence. The relative binding affinity of GR follows the order: consensus GRE >> GR1 > GR2. Mutations that disrupt the binding of GR to GR1 result in a major reduction of the glucocorticoid response, whereas similar mutations of GR2 have a much smaller effect. Unlike the simple consensus GRE, neither GR1 nor GR2 mediate a glucocorticoid response through a heterologous promoter. The accessory elements appear to have different functional roles. AF2 is still needed for a maximal glucocorticoid response when GR1 is converted to a high-affinity GR-binding element, but AF1 and AF3 are not required.

Activation of the ras mitogen-activated protein kinase-ribosomal protein kinase pathway is not required for the repression of phosphoenolpyruvate carboxykinase gene transcription by insulin

Phosphoenolpyruvate carboxykinase (PEPCK) catalyzes the first committed step in hepatic gluconeogenesis. Glucagon and glucocorticoids stimulate PEPCK gene transcription, whereas insulin has a dominant inhibitory effect. We have shown that inhibitors of 1-phosphatidylinositol 3-kinase (PI 3-kinase) block this action of insulin. In contrast, three distinct agents, all of which prevent activation of p42/p44 mitogen-activated protein (MAP) kinase, have no effect on the regulation of PEPCK transcription by insulin. However, a subsequent report has suggested that this pathway is involved in the inhibition of cAMP-induced PEPCK gene transcription by insulin. To address these conflicting data, we re-examined the Ras MAP kinase pathway, not only with respect to regulation of PEPCK gene transcription, but also for regulation of PI 3-kinase and p42/p44 MAP kinase. Overexpression of constitutively active Ras (V61) (or Raf-1 (RafCAAX)) partially represses PEPCK transcription in hepatoma cells. However, an inhibitor of MAP kinase kinase blocks this action of RafCAAX but has no effect on regulation of PEPCK gene transcription by insulin. Second, the action of a dominant negative Ras (N17Ras) on PEPCK gene transcription correlates more closely with the inhibition of PI 3-kinase than with the inhibition of p42/p44 MAP kinase. Third, insulin cannot activate p42/p44 MAP kinase in the presence of cAMP even though cAMP-induced PEPCK gene transcription is inhibited by insulin. This data confirms that the Ras MAP kinase pathway is not required for the regulation of PEPCK gene transcription by insulin and demonstrates the importance of employing multiple techniques when investigating the function of signaling pathways.

The orphan receptor COUP-TF binds to a third glucocorticoid accessory factor element within the phosphoenolpyruvate carboxykinase gene promoter

The phosphoenolpyruvate carboxykinase (PEPCK) gene promoter contains a glucocorticoid response unit (GRU) that includes, as a linear array, two accessory factor binding sites (AF1 and AF2) and two glucocorticoid receptor binding sites. All of these elements are required for a complete glucocorticoid response. AF1 and AF2 also partially account for the response of the PEPCK gene to retinoic acid and insulin, respectively. A second retinoic acid response element was recently located just downstream of the GRU. In this study we show that mutation of the 3' half-site of this element results in a 60% reduction of the glucocorticoid response of PEPCK promoter-chloramphenicol acetyltransferase (CAT) fusion constructs in transient transfection assays, thus the half-site is now termed AF3. A variety of assays were used to show that chicken ovalbumin upstream promoter transcription factor (COUP-TF) binds specifically to AF3 and that upstream stimulatory factor (USF) binds to an E-box motif located 2 base pairs downstream of AF3. Mutations of AF3 that diminish binding of COUP-TF reduce the glucocorticoid response, but mutation of the USF binding site has no effect. The functional roles of AF1, AF2, and AF3 in the glucocorticoid response were explored using constructs that contained combinations of mutations in all three elements. All three elements are required for a maximal glucocorticoid response, and mutation of any two abolish the response.

Regulatable production of mature insulin from a hepatocyte cell line: insulin production is up-regulated by cAMP and glucocorticoids, and down-regulated by insulin

We engineered a hepatoma cell line that produces an up-regulation of insulin in response to cAMP, dexamethasone, and retinoic acid, and a down-regulation in response to insulin. We devised a regulatory secretion system by placing proinsulin DNA under the regulatable promoter for phosphoenolpyruvate carboxykinase (PEPCK). To assess the ability to regulate insulin secretion, we used the rat hepatoma cell line, H4IIE. The H4IIE cells secreted immunoreactive insulin (IRI) constantly at a level of 1-3 fmol/10(6) cells/h. IRI increased approximately two-fold upon stimulation with 0.5 mM cAMP and five-fold with the addition of the cAMP-dependent phosphodiesterase inhibitor IBMX, as compared to baseline IRI secretion. IRI increased 18-fold by 1-500 nM dexamethasone together with cAMP and IBMX. Addition of exogenous insulin to the culture medium significantly decreased insulin mRNA expression on Northern blot.

Identification and charaterization of the second retinoic acid response element in the phosphoenolpyruvate carboxykinase gene promoter

A previously characterized retinoic acid response element (RARE1) in the phosphoenolpyruvate carboxykinase (PEPCK) gene promoter confers approximately 50% of the response of this gene to retinoic acid (RA). Transient transfection experiments were performed using constructs containing progressive 5' deletions of the PEPCK promoter to locate other elements that contribute to the RA response. A second RARE (RARE2) was located between -402 and -306. Methylation interference and mobility gel shift assays indicated that RAR/RXR bound specifically to a segment of DNA located between -337 and -321. This region contains consensus and degenerate half-sites for receptor binding separated by 5 bp. Mutations in either half-site selectively decreased the RA response and diminished RAR/RXR binding in mobility gel shift assays. When both RARE1 and RARE2 were mutated, 80% of the RA response was lost. Finally, RARE2 conferred a RA response in a heterologous promoter context. We conclude that RAR/RXR binds to RARE2, and that this DR5-type element is a major contributor to the response of the PEPCK gene to RA.

Glucocorticosteroids up-regulate human elastin gene promoter activity in transgenic mice

Recent characterization of the human elastin gene identified three putative glucocorticoid responsive elements (GRE) within the 5'-flanking DNA. To test the functionality of these cis-elements, transgenic mice that express a human elastin promoter-reporter gene (CAT) construct in a tissue-specific manner were injected with triamcinolone acetonide (TMC) or dexamethasone (DEX), two glucocorticosteroids in clinical use. Subcutaneous injection of these glucocorticoids resulted in a marked, up to 28-fold, enhancement of the CAT activity in the skin at the site of injection. Similarly, intraperitoneal injection of DEX resulted in significant increases in the elastin promoter activity in various internal organs. Furthermore, incubation of skin fibroblast and aortic smooth muscle cell cultures established from the transgenic animals with TMC (10 ng/ml) resulted in marked increases in the elastin promoter activity. These studies demonstrate that glucocorticosteroids act as powerful up-regulators of human elastin promoter activity in transgenic mice.

The immediate vicinity of mouse metallothionein-I gene contains two sites conferring glucocorticoid inducibility to the heterologous promoter

Glucocorticoid responsive elements (GREs) located -252 to -209 by upstream and +1011 to +1054 bp downstream of the transcription initiation site of the mouse metallothionein-I (mMT-I) gene were identified in transient experiments. However, the promoter region of the mMT-I gene (-330 to +70 bp) was found to provide low, if any, glucocorticoid induction of the linked CAT gene, while showing strong cadmium regulation, comparable with the in vivo level.

Calcium-mobilizing effectors inhibit P-enolpyruvate carboxykinase gene expression in cultured rat hepatocytes

Incubation of primary cultures of hepatocytes from fed and fasted rats with calcium ionophore strongly decreased glucose production from pyruvate. Like insulin, calcium ionophore A23187, phenylephrine, vasopressin, and prostaglandins E2 and F2 alpha caused a significant reduction (50-60%) in basal concentrations of mRNA for P-enolpyruvate carboxykinase (PEPCK), the main regulatory enzyme of gluconeogenesis. Phenylephrine, prostaglandin E2 and calcium ionophore A23187 were also able to counteract the induction of PEPCK gene expression by Bt2cAMP. These effects were similar to those exerted by both vanadate and phorbol ester TPA. The decrease in extracellular calcium by the addition of the calcium-chelating agent EGTA to the incubation medium caused an increase in PEPCK mRNA levels. This effect was additive to that of Bt2cAMP and was counteracted by vanadate.

The acinar heterotopy of phosphoenolpyruvate carboxykinase activity in rat liver under the influence of the oestrous cycle

The acinar localization of phosphoenolpyruvate carboxykinase was investigated in livers from untreated female rats during the 4-day oestrous cycle. The results were correlated with plasma levels of insulin, glucagon and oestrogens. Total activity was highest in dioestrus and lowest in metoestrus. The highest activities were present in the periportal zone and decreased continuously towards the perivenous zone in all four phases of the cycle. This general pattern was modified under the influence of the cycle. The periportal-perivenous gradient was subject to cycle dependent changes, being steepest in proestrus. In addition to oestrogens the concentrations of plasma insulin also varied during the female cycle. Insulin was highest in oestrus and lowest in proestrus. Glucagon showed only minor variations. Oestrogen concentrations increased continuously from low values in oestrus to high values in proestrus. These changes were directly related to the changes in the slope of the activity gradient.

Epidermal growth factor inhibits phosphoenolpyruvate carboxykinase gene expression in rat hepatocytes in primary culture

Epidermal growth factor (EGF) decreased the basal, and blocked the dibutyryl cyclic AMP (Bt2cAMP)-induced, expression of P-enolpyruvate carboxykinase (GTP) (PEPCK) and tyrosine aminotransferase (TAT) genes in both rat hepatocytes in primary culture and the FTO-2B hepatoma cell line. Treatment of hepatocytes with EGF in combination with phorbol ester (TPA) resulted in an additive decrease of PEPCK mRNA levels. Overnight pretreatment of hepatocytes with TPA, which is known to downregulate protein kinase C, abolished the TPA and reduced the EGF-mediated inhibition of PEPCK gene expression. These results suggested that EGF caused its effect, at least in part, through protein kinase C.

The effects of acute elevations in plasma cortisol levels on alanine metabolism in the conscious dog

The present study was undertaken to determine whether an acute physiological increase in plasma cortisol level had significant effects on alanine metabolism and gluconeogenesis within 3 hours in conscious, overnight-fasted dogs. Each experiment consisted of an 80-minute tracer and dye equilibration period, a 40-minute basal period, and a 3-hour experimental period. A primed, continuous infusion of [3-3H]glucose and continuous infusions of [U-14C]alanine and indocyanine green dye were initiated at the start of the equilibration period and continued throughout the experiment. Dogs were studied with (1) a hydrocortisone infusion ([CORT] 3.0 micrograms.kg-1.min-1, n = 5), (2) hydrocortisone infused as in CORT, but with pancreatic hormones clamped using somatostatin and basal intraportal replacement of insulin and glucagon (CLAMP+CORT, n = 5), or (3) saline infusion during a pancreatic clamp (CLAMP, n = 5). Glucose production and gluconeogenesis were determined using tracer and arteriovenous difference techniques. During CLAMP, all parameters were stable except for a modest 67% +/- 6% increase in gluconeogenic conversion of alanine to glucose and a 53% +/- 26% increase in gluconeogenic efficiency. When plasma cortisol levels were increased fourfold during CLAMP+CORT, there was no change in the concentration, production, or clearance of glucose. Gluconeogenic conversion of alanine to glucose increased 10% +/- 34% and gluconeogenic efficiency increased 65% +/- 43%, while net hepatic alanine uptake (NHAU) increased 60% +/- 19% and hepatic fractional extraction of alanine increased 38% +/- 12%. Cortisol did not cause an increase in the arterial glycerol level or net hepatic glycerol uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple elements regulate phosphoenolpyruvate carboxykinase gene expression in hepatoma hybrid cells

The phosphoenolpyruvate carboxykinase (PEPCK) gene is highly expressed in cultured rat hepatoma cells, but extinguished in hepatoma x fibroblast hybrids. Extinction of PEPCK gene expression in hybrids is a polygenic process that involves several fibroblast loci, only one of which (tissue-specific extinguisher-1, TSE1) has been characterized to date. To identify sequence elements of the PEPCK gene that are involved both in TSE1-mediated extinction and in TSE1-independent processes, we assayed expression of chimeric PEPCK transgenes in transiently and stably transfected hybrid cells. We report that TSE1 responsiveness mapped to the PEPCK CRE (cAMP response element), as shown previously for the tyrosine aminotransferase gene. This was expected from the recent identification of the TSE1 gene product as a regulatory subunit of protein kinase A. However, none of the transgenes we assayed were responsive to TSE1-independent extinction mechanisms, suggesting that these controls require DNA sequences and/or chromatin structures that were not present in the transfected reporters. The implications of these findings are discussed.

Location and characterization of multiple glucocorticoid-responsive elements in the rat serine dehydratase gene

Transcription of the gene coding for serine dehydratase (SDH, EC 4.2.1.13) in the rat in vivo is dramatically increased by glucocorticoid hormones. To identify DNA elements mediating the glucocorticoid-regulated expression of the SDH gene, we transiently transfected 7AD-7 rat hepatoma cells with fusion genes consisting of various regions of the SDH 5' flanking sequence linked to the coding sequence of the gene for chloramphenicol acetyltransferase (CAT). Analysis of the CAT activities from these 5' deletion mutants identified three closely associated glucocorticoid-responsive elements (GREs), located more than 5 kb upstream relative to the cap site. Two distal GREs act synergistically to confer strong glucocorticoid inducibility to the gene, whereas the proximal GRE functions independently of the distal GREs and confers only a weak hormone response to the gene. The purified DNA-binding domain of rat glucocorticoid receptor binds to the sequence of each GRE as shown by footprinting experiments. However, only one of these sequences contains the TGTTCT consensus sequence reportedly associated with many other GREs.

Detection and hormonal regulation of the mRNA for cytosolic phosphoenolpyruvate carboxykinase in rat lung

We have studied the presence of the messenger RNA (mRNA) for the cytosolic enzyme, phosphoenolpyruvate carboxykinase (PEPCK), in rat lung by Northern blot hybridization to a complementary DNA (cDNA) probe. Lung from normal rats contained substantial amounts of this mRNA, although its relative concentration was approximately six times lower than in liver. Fasting produced an eightfold increase in the content of the enzyme mRNA in lung, which could be reverted to normal values by glucose refeeding. Induced diabetes also resulted in a sevenfold increase of the levels of PEPCK mRNA in lung. Dexamethasone, thyroid hormone, dibutyryl cyclic adenosine monophosphate (cAMP), histamine, and serotonin also induced important accumulations of the enzyme mRNA without affecting the concentration of beta-tubulin mRNA measured as reference. Thus, the PEPCK gene appears to be regulated in a similar manner in lung and liver. The results suggest that PEPCK may be involved in lung metabolism in starvation, diabetes, and other specific hormonal situations.

Serotonin increases the cAMP concentration and the phosphoenolpyruvate carboxykinase mRNA in rat kidney, small intestine, and liver

Within 60 min of the administration of serotonin to fasted-refed rats, there was a 5-, 16-, and 20-fold stimulation of the mRNA coding for the cytosolic form of P-enolpyruvate carboxykinase in the kidney, small intestine and liver, respectively. This stimulation was 5-, 1.3-, and 2-fold higher than noted in the same tissue after 24 h of starvation. Dose- and time-response curves to serotonin in the three tissues were similar. The level of PEPCK mRNA in the liver was significantly elevated within 30 min of serotonin administration, whereas 60 min was required in the small intestine and the kidney. The direct effect of serotonin on PEPCK mRNA was also assessed in hepatocytes maintained in primary culture. Serotonin (10(-8) M to 10(-4) M) caused a dose-dependent increase in the level of PEPCK mRNA and a transient increase in cAMP concentration. Within the first min of serotonin (10(-6) M) addition to cells, cAMP concentration increased 4-fold and returned after 10 min to basal level. Therefore, these results provide functional evidence of serotonin action in the rat peripheric tissues and suggest that cAMP is involved in its intracellular signalling.

Binding of glucocorticoid receptors to model DNA response elements

DNA sequences were synthesized that contained the consensus 15-base pair glucocorticoid receptor binding site linked to flanking sequences of various lengths. Binding of these synthetic oligomers to glucocorticoid receptor, employing a reconstituted binding system with purified components, indicated that a minimal size of approximately 45 base pairs was necessary to bind the receptor optimally. Sequences containing multiple receptor binding sites competed more effectively for binding. These findings are consistent with recent demonstrations that multiple control elements act synergistically to affect transcriptional control by glucocorticoids and confirm that regions flanking the consensus GRE binding site are instrumental in optimizing binding interactions.

Glucocorticoid-receptor binding sites at the 5' flanking region of a rat cytochrome CYP2B2 gene predicted with a novel computer method

Using an original computer method to search for potential DNA binding sites for glucocorticoid-receptor complexes (GRC) (Seledtsov, I.A., Solovjev, V.V. and Merkulova, T.I. (1991) Biochim. Biophys. Acta 1089, 367-376), the presence of two such sites in the 5' flanking region of a rat cytochrome CYP2B2 gene has been predicted. This prediction has been confirmed by gel retardation experiments.

Differences in deoxyribonuclease I hypersensitive sites in phenobarbital-inducible and constitutive rabbit P450IIC genes

DNase I hypersensitivity of nuclear chromatin near the rabbit cytochrome P450IIC genes was investigated by indirect end-labeling genomic Southern analysis. Major DNase I hypersensitive sites were observed in proximal (-200 base pairs) and distal (-2000 to -2200 base pairs) regions of the genes. The presence of the proximal site correlated well with the expression states of the individual genes. In contrast, the distal site was present in DNA from both liver and kidney nuclei and in untreated and phenobarbital-treated animals irrespective of the expression state of the genes. However, the distal site was present only in genes that respond to phenobarbital (cytochromes P450IIC1 and P450IIC2) and was not detected in the constitutive cytochrome P450IIC3 gene. The nucleotide sequences of 500 base pairs in the distal site regions of cytochromes P450IIC1 and P450IIC2 were 67% similar, and hepatocyte nuclear factor 1 like motifs were conserved in these two sequences. These results are consistent with the hypothesis that distal regions cooperate with the proximal promoter regions in the regulation of cytochrome P450IIC gene expression.

New elements of glucocorticoid-receptor binding sites of hormone-regulated genes

The structure of the DNA regions recognized by glucocorticoid-receptor complexes (GIRC) was analyzed using frequency matrices and a modified perceptron method. Some complementary conservative elements which may modulate the efficiency of GIRC binding were found at both sides of the previously established conserved nucleotide sequence (core) (Beato, M. et al. (1987) J. Steroid Biochem. 27, 9-14). A criterion based on the concurrent use of several perceptron matrices to search for the potential GIRC binding site sequences has been worked out. By applying this criterion 73 sites were identified in 28 sequences of glucocorticoid regulated genes and 7 sites were identified in 26 sequences independent from glucocorticoid regulation.

Mechanism of the permissive action of dexamethasone on the glucagon-dependent activation of the phosphoenolpyruvate carboxykinase gene in cultured rat hepatocytes

Rat hepatocytes were cultured for 24 h in the presence or absence of 100 nM dexamethasone (DX). After a medium change, phosphoenolpyruvate carboxykinase (PCK) was induced by addition of glucagon at different concentrations, from physiological 0.1 nM to hyperphysiological 10 nM, again in the presence or absence of 100 nM dexamethasone. 1. With dexamethasone addition during the culture and induction phase (DX+/+), 10 nM glucagon increased PCK mRNA abundance (Northern blot analysis) and activity (in vitro translation) synchronously to the same extent with maxima after 2 h and PCK enzyme activity after a time lag with a maximum after 6 h. The total detectable PCK mRNA amount was apparently also translationally active. 10 microM N6,2'-O-dibutyryladenosine 3',5'-(cyclic)phosphate (Bt2cAMP) as the second messenger had essentially the same effect as 10 nM glucagon. 2. In the absence of dexamethasone during the preculture and the induction phase (DX-/-), 10 nM glucagon and 10 microM Bt2cAMP could enhance PCK mRNA only about half-maximally. Glucagon or dexamethasone added alone in physiological concentrations of 0.1 nM and 100 nM, respectively, were unable to increase PCK mRNA. However, treatment of the cells with dexamethasone also enabled 0.1 nM glucagon to enhance PCK mRNA to a maximum after 2 h, independent of the presence of dexamethasone during the induction period (DX+/+ and DX+/- cells). Thus, dexamethasone was a permissive agent in that it shifted the sensitivity of the cells towards glucagon into the physiological concentration range. 3. In the presence of dexamethasone during the culture and induction phase (DX+/+) 0.1 nM glucagon maximally enhanced the transcription of the PCK gene (nuclear run on) fourfold after 30 min; in the absence of dexamethasone during both phases (DX-/-) glucagon was without any effect. The overall transcriptional rate was not significantly different in cells with and without dexamethasone during the culture and induction phase (DX+/+ vs. DX-/-). Thus, dexamethasone acted permissively mainly on the transcription of the PCK gene. 4. With culture in the presence of dexamethasone over decreasing periods of time, 1 nM glucagon could induce submaximal PCK mRNA amounts already after 1-3 h steroid culture. This restitution by dexamethasone of the PCK mRNA inducibility by glucagon was inhibited by cycloheximide. This suggested that ongoing protein synthesis was required for the permissive action of dexamethasone on the expression of the PCK gene. The results allow the following conclusions.(ABSTRACT TRUNCATED AT 400 WORDS)

Insulin and other regulatory factors modulate the growth and the phosphoenolpyruvate carboxykinase (PEPCK) activity of primary rabbit kidney proximal tubule cells in serum free medium

Insulin was observed to modulate the growth and the phosphoenolpyruvate carboxykinase (PEPCK) activity of primary cultures of rabbit renal proximal tubule cells in serum free medium. Insulin was stimulatory to primary proximal tubule cell growth at a concentration of 10(-8) M. In contrast, insulin was inhibitory to a proximal tubule function, PEPCK activity, following a 5-minute incubation period. An insulin dosage as low as 10(-10) M was inhibitory to PEPCK activity, suggesting the involvement of insulin receptors. Although insulin was required at a significantly higher dosage to stimulate the growth of the primary renal proximal tubule cells than to inhibit PEPCK activity, the elevated dosage required in order to observe a growth effect may be explained by the degradation of insulin by the primary renal proximal tubule cells. However the possible involvement of receptors for Insulin-like Growth Factor I (IGF-I) and Insulin-like Growth Factor II (IGF-II) in mediating the effects of insulin cannot be excluded. Other effector molecules were also examined with respect to their effects on PEPCK activity. The possible involvement of cyclic AMP in the control of the PEPCK activity of the primary renal cells was indicated by the stimulatory effects of 8 bromocyclic AMP, isobutyl methylxanthine (a cyclic AMP phosphodiesterase inhibitor), and forskolin (an activator of adenylate cyclase). Phorbol 12-myristate 13-acetate (TPA), which activates protein kinase C, was inhibitory. The actions of these effector molecules and insulin on the PEPCK activity of the primary renal cultures are remarkably similar to their effects on hepatic PEPCK. Several growth factors, fibroblast growth factor (FGF), and transforming growth factor beta (TGF beta) were also examined. FGF was observed to be stimulatory, whereas TGF beta was inhibitory to the PEPCK activity of the primary renal proximal tubule cells.