Egr-1 activation of rat adrenal phenylethanolamine N-methyltransferase gene

Fetal programming of adrenal PNMT and hypertension by glucocorticoids in WKY rats is dose and sex-dependent

Biochemical changes in utero may alter normal fetal development, resulting in disease later in life, a phenomenon known as fetal programming. Recent epidemiological studies link fetal programming to negative health outcomes, such as low birth weight and hypertension in adulthood. Here, we used a WKY rat model and studied the molecular changes triggered by prenatal glucocorticoid (GC) exposure on the development of hypertension, and on the regulation of phenylethanolamine N-methyl transferase (PNMT), the enzyme responsible for biosynthesis of epinephrine, and a candidate gene linked to hypertension. Clinically, high doses of the synthetic GC dexamethasone (DEX) are used to treat infant respiratory distress syndrome. Elevated maternal GCs have been correlated with fetal programming of hypertension. The aim of this study was to determine if lower doses of DEX would not lead to detrimental fetal programming effects such as hypertension. Our data suggests that prenatal stress programs for increased expression of PNMT and altered regulation of PNMT in males and females. Importantly, we identified that DEX mediated programming was more apparent in the male rats, and the lower dose 10μg/kg/day of DEX did not lead to changes in blood pressure (BP) in female rats suggesting that this dose is below the threshold for programming of hypertension. Furthermore, sex-specific differences were observed in regards to programming mechanisms that may account for hypertension in males.

Regulation of Monoamine Oxidase B Gene Expression: Key Roles for Transcription Factors Sp1, Egr1 and CREB, and microRNAs miR-300 and miR-1224

Monoamine oxidase B (MAO-B), a flavoenzyme located in the outer mitochondrial membrane, is involved in the catabolism of monoamines. Altered levels of MAO-B are associated with cardiovascular/neuronal diseases. However, molecular mechanisms of MAO-B gene regulation are partially understood. We undertook a systematic analysis of the MAO-B gene to identify the key transcriptional/post-transcriptional regulatory molecules. Expression of MAO-B promoter-reporter constructs in cultured cells identified the -144/+25-bp domain as the core promoter region. Stringent in silico analysis of this core promoter predicted binding sites for several transcription factors. Over-expression/down-regulation of transcription factors Sp1/Egr1/CREB increased/decreased the MAO-B promoter-reporter activity and endogenous MAO-B protein level. Electrophoretic mobility shift assays and ChIP assays provided evidence for interactions of Sp1/Egr1/CREB with the MAO-B promoter. MAOB transcript level also positively correlated with the transcript level of Sp1/Egr1/CREB in various human tissue samples. Computational predictions using multiple algorithms coupled with systematic functional analysis revealed direct interactions of the microRNAs miR-1224 and miR-300 with MAO-B 3'-UTR. Dopamine dose-dependently enhanced MAO-B transcript and protein levels via increased binding of CREB to MAO-B promoter and reduced miR-1224/miR-300 levels. 8-Bromo-cAMP and forskolin augmented MAO-B expression, whereas inhibition of PKA diminished the gene expression suggesting involvement of cAMP-PKA axis. Interestingly, Sp1/Egr1/CREB/miR-1224 levels correlate with MAO-B expression in rodent models of hypertension/MPTP-induced neurodegeneration, indicating their roles in governing MAO-B gene expression in these disease states. Taken together, this study elucidates the previously unknown roles of the transcription factors Sp1/Egr1/CREB and microRNAs miR-1224/miR-300 in regulating MAO-B gene expression under basal/disease states involving dysregulated catecholamine levels.

Inflammatory Signaling in Hypertension: Regulation of Adrenal Catecholamine Biosynthesis

The immune system is increasingly recognized for its role in the genesis and progression of hypertension. The adrenal gland is a major site that coordinates the stress response via the hypothalamic-pituitary-adrenal axis and the sympathetic-adrenal system. Catecholamines released from the adrenal medulla function in the neuro-hormonal regulation of blood pressure and have a well-established link to hypertension. The immune system has an active role in the progression of hypertension and cytokines are powerful modulators of adrenal cell function. Adrenal medullary cells integrate neural, hormonal, and immune signals. Changes in adrenal cytokines during the progression of hypertension may promote blood pressure elevation by influencing catecholamine biosynthesis. This review highlights the potential interactions of cytokine signaling networks with those of catecholamine biosynthesis within the adrenal, and discusses the role of cytokines in the coordination of blood pressure regulation and the stress response.

Imidacloprid, a neonicotinoid insecticide, facilitates tyrosine hydroxylase transcription and phenylethanolamine N-methyltransferase mRNA expression to enhance catecholamine synthesis and its nicotine-evoked elevation in PC12D cells

Imidacloprid is a neonicotinoid insecticide acting as an agonist of nicotinic acetylcholine receptors (nAChRs) in the target insects. However, questions about the safety to mammals, including human have emerged. Overactivation of mammalian peripheral catecholaminergic systems leads to onset of tachycardia, hypertension, vomiting, etc., which have been observed in acutely imidacloprid-poisoned patients as well. Physiological activation of the nAChRs is known to drive catecholamine biosynthesis and secretion in mammalian adrenal chromaffin cells. Yet, the impacts of imidacloprid on the catecholaminergic function of the chromaffin cells remain to be evaluated. In this study using PC12D cells, a catecholaminergic cell line derived from the medulla chromaffin-cell tumors of rat adrenal gland, we examined whether imidacloprid itself could impact the catecholamine-synthesizing ability. Imidacloprid alone did facilitate tyrosine hydroxylase (TH) transcription via activation of α3β4 nAChR and the α7 subunit-comprising receptor. The insecticide showed the TH transcription-facilitating ability at the concentrations of 3 and 30 μM, at which acetylcholine is known to produce physiological responses, including catecholamine secretion through the nAChRs in adrenal chromaffin cells. The insecticide-facilitated TH transcription was also dependent on PKA- and RhoA-mediated signaling pathways. The insecticide coincidentally raised levels of TH and phenylethanolamine N-methyltransferase (PNMT) mRNA, and as a consequence, increased catecholamine production, although the efficacy of the neonicotinoid was lesser than that of nicotine, indicating its partial agonist-like action. Intriguingly, in cultured rat adrenal chromaffin cells, imidacloprid did increase levels of TH and PNMT protein. When the chromaffin cells were treated with nicotine in the presence of the insecticide, nicotine-elevated adrenaline production was enhanced due to facilitation of nicotine-increased TH and PNMT protein expression, and simultaneous enhancement of nicotine-elevated adrenaline secretion also took place. These findings thus suggest that imidacloprid may facilitate the physiological functions of adrenal glands in mammals.

Phenylethanolamine N-methyltransferase gene expression in adrenergic neurons of spontaneously hypertensive rats

Epinephrine is synthesised by the catecholamine biosynthetic enzyme, phenylethanolamine N-methyltransferase (PNMT), primarily in chromaffin cells of the adrenal medulla and secondarily in brainstem adrenergic neurons of the medulla oblongata. Epinephrine is an important neurotransmitter/neurohormone involved in cardiovascular regulation; however, overproduction is detrimental with negative outcomes such as cellular damage, cardiovascular dysfunction, and hypertension. Genetic mapping studies have linked elevated expression of PNMT to hypertension. Adrenergic neurons are responsible for blood pressure regulation and are the only PNMT containing neurons in the brainstem. The purpose of the current study was to determine whether elevated blood pressure found in adult spontaneously hypertensive rats (SHR) is associated with altered regulation of the PNMT gene in catecholaminergic neurons. C1, C2, and C3 adrenergic regions of 16 week old Wistar Kyoto (WKY) and SHR rats were excised using micropunch microdissection for mRNA expression analyses. Results from the current study confirm high PNMT mRNA expression in all three brainstem adrenergic regions (C1: 2.96-fold; C2: 2.17-fold; C3 1.20-fold) of the SHR compared to normotensive WKY rats. Furthermore, the immediate early gene transcription factor (Egr-1) mRNA was elevated in the C1 (1.84-fold), C2 (8.57-fold) and C3 (2.41-fold) regions in the brainstem of the SHR. Low mRNA expression for transcription factors Sp1 and GR was observed, while no change was observed for AP-2. The findings presented propose that alterations in the PNMT gene regulation in the brainstem contribute to enhanced PNMT production and epinephrine synthesis in the SHR, a genetic model of hypertension.

Prenatal glucocorticoid exposure programs adrenal PNMT expression and adult hypertension

Prenatal exposure to glucocorticoids (GCs) programs for hypertension later in life. The aim of the current study was to examine the impact of prenatal GC exposure on the postnatal regulation of the gene encoding for phenylethanolamine N-methyltransferase (PNMT), the enzyme involved in the biosynthesis of the catecholamine, epinephrine. PNMT has been linked to hypertension and is elevated in animal models of hypertension. Male offspring of Wistar-Kyoto dams treated with dexamethasone (DEX) developed elevated systolic, diastolic and mean arterial blood pressure compared to saline-treated controls. Plasma epinephrine levels were also elevated in adult rats exposed to DEX in utero. RT-PCR analysis revealed adrenal PNMT mRNA was higher in DEX exposed adult rats. This was associated with increased mRNA levels of transcriptional regulators of the PNMT gene: Egr-1, AP-2, and GR. Western blot analyses showed increased expression of PNMT protein, along with increased Egr-1 and GR in adult rats exposed to DEX in utero. Furthermore, gel mobility shift assays showed increased binding of Egr-1 and GR to DNA. These results suggest that increased PNMT gene expression via altered transcriptional activity is a possible mechanism by which prenatal exposure to elevated levels of GCs may program for hypertension later in life.

Cardiac phenylethanolamine N-methyltransferase: localization and regulation of gene expression in the spontaneously hypertensive rat

Phenylethanolamine N-methyltransferase (PNMT) is the terminal enzyme in the catecholamine biosynthetic pathway responsible for adrenaline biosynthesis. Adrenaline is involved in the sympathetic control of blood pressure; it augments cardiac function by increasing stroke volume and cardiac output. Genetic mapping studies have linked the PNMT gene to hypertension. This study examined the expression of cardiac PNMT and changes in its transcriptional regulators in the spontaneously hypertensive (SHR) and wild type Wistar-Kyoto (WKY) rats. SHR exhibit elevated levels of corticosterone, and lower levels of the cytokine IL-1β, revealing systemic differences between SHR and WKY. PNMT mRNA was significantly increased in all chambers of the heart in the SHR, with the greatest increase in the right atrium. Transcriptional regulators of the PNMT promoter show elevated expression of Egr-1, Sp1, AP-2, and GR mRNA in all chambers of the SHR heart, while protein levels of Sp1, Egr-1, and GR were elevated only in the right atrium. Interestingly, only AP-2 protein-DNA binding was increased, suggesting it may be a key regulator of cardiac PNMT in SHR. This study provides the first insights into the molecular mechanisms involved in the dysregulation of cardiac PNMT in a genetic model of hypertension.

A systems approach identifies co-signaling molecules of early growth response 1 transcription factor in immobilization stress

Background

Adaptation to stress is critical for survival. The adrenal medulla, the major source of epinephrine, plays an important role in the development of the hyperadenergic state and increased risk for stress associated disorders, such as hypertension and myocardial infarction. The transcription factor Egr1 plays a central role in acute and repeated stress, however the complexity of the response suggests that other transcription factor pathways might be playing equally important roles during acute and repeated stress. Therefore, we sought to discover such factors by applying a systems approach.

Results

Using microarrays and network analysis we show here for the first time that the transcription factor signal transducer and activator of transcription 3 (Stat3) gene is activated in acute stress whereas the prolactin releasing hormone (Prlh11) and chromogranin B (Chgb) genes are induced in repeated immobilization stress and that along with Egr1 may be critical mediators of the stress response.

Conclusions

Our results suggest possible involvement of Stat3 and Prlh1/Chgb up-regulation in the transition from short to repeated stress activation.

On the origin of information in epigenetic structures in metazoans

Epigenetic inheritance implies the existence of epigenetic information. Great progress has been made in recent years in understanding the role of the changes in epigenetic structures (methylated DNA, histone acetylation/deacetylation and chromatin remodelling) as well as the role of miRNA (MIR) expression patterns in epigenetic processes. However, as of yet, we do not have a satisfactory understanding of the origin of epigenetic information stored in, and conveyed by, these structures. We do not know whether these structures are the ultimate source of the information or whether they are simply media for storing and transmitting epigenetic information for gene expression from upstream sources to the phenotype. Herein an attempt is made to ascertain the ultimate sources of the epigenetic information they contain and transmit by tracing back the causal chain leading to the changes in epigenetic structures.

Physiological and genomic consequences of adrenergic deficiency during embryonic/fetal development in mice: impact on retinoic acid metabolism

Adrenergic hormones are essential for early heart development. To gain insight into understanding how these hormones influence heart development, we evaluated genomic expression changes in embryonic hearts from adrenergic-deficient and wild-type control mice. To perform this study, we used a mouse model with targeted disruption of the Dopamine β-hydroxylase (Dbh) gene, whose product is responsible for enzymatic conversion of dopamine into norepinephrine. Embryos homozygous for the null allele (Dbh(-/-)) die from heart failure beginning as early as embryonic day 10.5 (E10.5). To assess underlying causes of heart failure, we isolated hearts from Dbh(-/-) and Dbh(+/+) embryos prior to manifestation of the phenotype and examined gene expression changes using genomic Affymetrix 430A 2.0 arrays, which enabled simultaneous evaluation of >22,000 genes. We found that only 22 expressed genes showed a significant twofold or greater change, representing ~0.1% of the total genes analyzed. More than half of these genes are associated with either metabolism (31%) or signal transduction (22%). Remarkably, several of the altered genes encode for proteins that are directly involved in retinoic acid (RA) biosynthesis and transport. Subsequent evaluation showed that RA concentrations were significantly elevated by an average of ~3-fold in adrenergic-deficient (Dbh(-/-)) embryos compared with controls, thereby suggesting that RA may be an important downstream mediator of adrenergic action during embryonic heart development.

Epinephrine: a short- and long-term regulator of stress and development of illness : a potential new role for epinephrine in stress

Epinephrine (Epi), which initiates short-term responses to cope with stress, is, in part, stress-regulated via genetic control of its biosynthetic enzyme, phenylethanolamine N-methyltransferase (PNMT). In rats, immobilization (IMMO) stress activates the PNMT gene in the adrenal medulla via Egr-1 and Sp1 induction. Yet, elevated Epi induced by acute and chronic stress is associated with stress induced, chronic illnesses of cardiovascular, immune, cancerous, and behavioral etiologies. Major sources of Epi include the adrenal medulla and brainstem. Although catecholamines do not cross the blood-brain barrier, circulating Epi from the adrenal medulla may communicate with the central nervous system and stress circuitry by activating vagal nerve β-adrenergic receptors to release norepinephrine, which could then stimulate release of the same from the nucleus tractus solitarius and locus coeruleus. In turn, the basal lateral amygdala (BLA) may activate to stimulate afferents to the hypothalamus, neocortex, hippocampus, caudate nucleus, and other brain regions sequentially. Recently, we have shown that repeated IMMO or force swim stress may evoke stress resiliency, as suggested by changes in expression and extinction of fear memory in the fear-potentiated startle paradigm. However, concomitant adrenergic changes seem stressor dependent. Present studies aim to identify stressful conditions that elicit stress resiliency versus stress sensitivity, with the goal of developing a model to investigate the potential role of Epi in stress-associated illness. If chronic Epi over expression does elicit illness, possibilities for alternative therapeutics exist through regulating stress-induced Epi expression, adrenergic receptor function and/or corticosteroid effects on Epi, adrenergic receptors and the stress axis.

Role of reactive oxygen species in the neural and hormonal regulation of the PNMT gene in PC12 cells

The stress hormone, epinephrine, is produced predominantly by adrenal chromaffin cells and its biosynthesis is regulated by the enzyme phenylethanolamine N-methyltransferase (PNMT). Studies have demonstrated that PNMT may be regulated hormonally via the hypothalamic-pituitary-adrenal axis and neurally via the stimulation of the splanchnic nerve. Additionally, hypoxia has been shown to play a key role in the regulation of PNMT. The purpose of this study was to examine the impact of reactive oxygen species (ROS) produced by the hypoxia mimetic agent CoCl₂, on the hormonal and neural stimulation of PNMT in an in vitro cell culture model, utilizing the rat pheochromocytoma (PC12) cell line. RT-PCR analyses show inductions of the PNMT intron-retaining and intronless mRNA splice variants by CoCl₂ (3.0- and 1.76-fold, respectively). Transient transfection assays of cells treated simultaneously with CoCl₂ and the synthetic glucocorticoid, dexamethasone, show increased promoter activity (18.5-fold), while mRNA levels of both splice variants do not demonstrate synergistic effects. Similar results were observed when investigating the effects of CoCl₂-induced ROS on the neural stimulation of PNMT via forskolin. Our findings demonstrate that CoCl₂-induced ROS have synergistic effects on hormonal and neural activation of the PNMT promoter.

Evaluation of biolistic gene transfer methods in vivo using non-invasive bioluminescent imaging techniques

Background

Gene therapy continues to hold great potential for treating many different types of disease and dysfunction. Safe and efficient techniques for gene transfer and expression in vivo are needed to enable gene therapeutic strategies to be effective in patients. Currently, the most commonly used methods employ replication-defective viral vectors for gene transfer, while physical gene transfer methods such as biolistic-mediated ("gene-gun") delivery to target tissues have not been as extensively explored. In the present study, we evaluated the efficacy of biolistic gene transfer techniques in vivo using non-invasive bioluminescent imaging (BLI) methods.

Results

Plasmid DNA carrying the firefly luciferase (LUC) reporter gene under the control of the human Cytomegalovirus (CMV) promoter/enhancer was transfected into mouse skin and liver using biolistic methods. The plasmids were coupled to gold microspheres (1 μm diameter) using different DNA Loading Ratios (DLRs), and "shot" into target tissues using a helium-driven gene gun. The optimal DLR was found to be in the range of 4-10. Bioluminescence was measured using an In Vivo Imaging System (IVIS-50) at various time-points following transfer. Biolistic gene transfer to mouse skin produced peak reporter gene expression one day after transfer. Expression remained detectable through four days, but declined to undetectable levels by six days following gene transfer. Maximum depth of tissue penetration following biolistic transfer to abdominal skin was 200-300 μm. Similarly, biolistic gene transfer to mouse liver in vivo also produced peak early expression followed by a decline over time. In contrast to skin, however, liver expression of the reporter gene was relatively stable 4-8 days post-biolistic gene transfer, and remained detectable for nearly two weeks.

Conclusions

The use of bioluminescence imaging techniques enabled efficient evaluation of reporter gene expression in vivo. Our results demonstrate that different tissues show different expression kinetics following gene transfer of the same reporter plasmid to different mouse tissues in vivo. We evaluated superficial (skin) and abdominal organ (liver) targets, and found that reporter gene expression peaked within the first two days post-transfer in each case, but declined most rapidly in the skin (3-4 days) compared to liver (10-14 days). This information is essential for designing effective gene therapy strategies in different target tissues.

Stress and adrenergic function: HIF1α, a potential regulatory switch

Stress elicits adrenal epinephrine and cortisol release into the bloodstream to initiate physiological and behavioral responses to counter and overcome stress, the classic "fight or flight" response (Cannon and De La Paz, Am J Physiol 28:64-70, 1911). Stress and the stress hormone epinephrine also contribute to the pathophysiology of illness, e.g., behavioral disorders, cardiovascular disease, and immune dysfunction. Epinephrine itself is regulated by stress through its biosynthesis by phenylethanolamine N-methyltransferase (PNMT, EC 2.1.1.28). Single and repeated immobilization (IMMO) stress in rats stimulates adrenal PNMT mRNA and protein expression via the transcription factors, Egr-1 and Sp1. Moderate hypoxic stress increases PNMT promoter-driven gene expression and endogenous PNMT mRNA and protein in PC12 cells. Induction is initiated through cAMP and PLC signaling, with PKA, PKC, PI3K, ERK1/2 MAPK, and p38 MAPK continuing downstream signal transduction, followed by activation of HIF1α, Egr-1, and Sp1. While functional Egr-1 and Sp1 binding sites exist within the proximal PNMT promoter, a putative hypoxia response element is a weak HIF binding site. Yet, HIF1α overexpression increases PNMT promoter-driven luciferase activity and endogenous PNMT. When the Egr-1 or Sp1 sites are mutated, HIF1α does not stimulate the PNMT promoter. siRNA knock down of Egr-1 or Sp1 prevents promoter activation while siRNA knock down of HIF1α inhibits Egr-1 and Sp1 induction. Findings suggest that hypoxia activates the PNMT gene indirectly via HIF1α stimulation of Egr-1 and Sp1. Thus, for stress-induced illnesses where adrenergic dysfunction is implicated, HIF1α may be an "on-off" switch regulating adrenergic responses to stress and a potential target for therapeutic intervention.

Hypoxia and adrenergic function: molecular mechanisms related to Egr-1 and Sp1 activation

Hypoxia is shown to regulate the stress hormone epinephrine through its biosynthesis by phenylethanolamine N-methyltransferase (PNMT) via PNMT gene activation and transcription factors Egr-1 and Sp1 in adrenal medulla-derived PC12 cells. Moderate hypoxia (5% oxygen) markedly stimulates PNMT promoter-driven luciferase activity in the cells. Hypoxia increases Egr-1 and Sp1 mRNA and nuclear protein content and Egr-1 and Sp1 protein-DNA binding complex formation. Subsequent to transcription factor induction, endogenous PNMT mRNA and protein also increase. Egr-1 and Sp1 binding site inactivation or Egr-1 and Sp1 siRNA inhibit PNMT promoter stimulation by hypoxia. Hypoxia elevates protein kinase A (PKA), phospholipase C (PLC), phosphoinositide 3-kinase, protein kinase C, ERK1/2 mitogen-activated protein kinase and p38 mitogen-activated protein kinase expression while selective inhibitors of these signaling enzymes abrogate hypoxic induction of the PNMT promoter and the rise in Egr-1, Sp1 and PNMT mRNA and protein. PC12 cells lacking PKA or PLCgamma-1 show significant reduction in PNMT promoter activation by hypoxia. Signaling inhibitors do not affect these responses or reduce hypoxic induction of the PNMT promoter to a lesser extent. Findings suggest that Egr-1 and Sp1 through synergistic interaction are critical transcriptional activators for hypoxic stress-regulated adrenergic function controlled via cAMP/PKA and PLC signaling. Identification of Sp1 as a mediator of hypoxia-induced transcriptional activation of PNMT has not been previously been shown. The effects of hypoxia on PNMT and thereby epinephrine may have important ramifications for the stress hormone epinephrine, its ability to regulate behavioral and physiological processes associated with stress and stress-elicited illness.

Hypoxic stress-induced changes in adrenergic function: role of HIF1 alpha

Sustaining epinephrine-elicited behavioral and physiological responses during stress requires replenishment of epinephrine stores. Egr-1 and Sp1 contribute by stimulating the gene encoding the epinephrine-synthesizing enzyme, phenylethanolamine N-methyltransferase (PNMT), as shown for immobilization stress in rats in adrenal medulla and for hypoxic stress in adrenal medulla-derived PC12 cells. Hypoxia (5% O(2)) also activates hypoxia inducible factor (HIF) 1alpha, increasing mRNA, nuclear protein and nuclear protein/hypoxia response element binding complex formation. Hypoxia and HIF1alpha over-expression also elevate PNMT promoter-driven luciferase activity in PC12 cells. Hypoxia may be limiting as HIF1alpha over-expression increases luciferase expression to no greater extent than oxygen reduction alone. HIF1alpha inducers CoCl(2) or deferoxamine elevate luciferase as well. PC12 cells harboring a HIF1alpha expression construct show markedly higher levels of Egr-1 and Sp1 mRNA and nuclear protein and PNMT mRNA and cytoplasmic protein. Inactivation of Egr-1 and Sp1 binding sites in the proximal -893 bp of PNMT promoter precludes HIF1alpha stimulation while a potential hypoxia response element (-282 bp) in the promoter shows weak HIF1alpha affinity at best. These findings are the first to suggest that hypoxia activates the proximal rat PNMT promoter primarily via HIF1alpha induction of Egr-1 and Sp1 rather than by co-activation by Egr-1, Sp1 and HIF1alpha. In addition, the rise in HIF1alpha protein leading to Egr-1 and Sp1 stimulation of PNMT appears to include HIF1alpha gene activation rather than simply prevention of HIF1alpha proteolytic degradation.

Morphophysiology of the Zuckerkandl's paraganglion: effects of dexamethasone and aging

The extra-adrenal Zuckerkandl's paraganglion is used as a source of chromaffin cells for transplantation in parkinsonian animals. Aging can affect its viability, and this tissue needs further characterization for improving grafting procedures. The objectives were: (i) to compare the main morpho-functional characteristics of prepubertal and old Zuckerkandl's paraganglion (ZP), and (ii) to discern phenotypic changes after sub-chronic dexamethasone treatment in extra-adrenal tissue of prepubertal rats. For these purposes, immunostaining methods, stereology, voltammetry, cell culture, Western blotting, and ELISA were employed. The findings revealed that all paraganglia were composed of mesenchymal tissue and chromaffin cells. In prepubertal rats, chromaffin cells are arranged as large or small clusters. Large clusters (also known as "cell nests") contain densely packed chromaffin cells, and they are seen as fascicles in longitudinal sections. In old paraganglia, cell nests disappear, and chromaffin cells are found to be arranged as small cell clusters or dispersed throughout the mesenchyma. Paraganglionic chromaffin cells possess a rounded morphology with diameter ranging from 12 to 15 μm, with intracytoplasmic granules (100-500 nm in diameter) containing catecholamines. Prepubertal and old ZP chromaffin cells are mostly noradrenergics, and a few of them are dopaminergics. Aging reduces the amount of chromaffin tissue (28% in adult rats vs. 11% in old animals, both in relation to total volume of the paraganglion), and induces the presence of adrenergic cells and adrenaline. Both prepubertal and old cells express the neurotrophic factors GDNF and TGF-β₁, aging leading to reduced levels of both growth factors. Dexamethasone (50 μg/kg daily, 5 days) leads to the expression of phenylethanolamine-N-methyl-transferase in prepubertal paraganglia, and to a higher content and release of adrenaline.

Adrenergic responses to stress: transcriptional and post-transcriptional changes

Stress effects on adrenergic responses in rats were examined in adrenal medulla, the primary source of circulating epinephrine (Epi). Irrespective of duration, immobilization (IMMO) increased adrenal corticosterone to the same extent. In contrast, Epi changed little, suggesting that Epi synthesis replenishes adrenal pools and sustains circulating levels for the heightened alertness and physiological changes required of the "flight or fight" response. IMMO also induced the Epi-synthesizing enzyme, phenylethanolamine N-methyltransferase (PNMT). The rise in its mRNA and protein was preceded by increases in Egr-1 and Sp1 mRNA, protein, and protein-DNA binding complex formation. With repeated and prolonged stress, PNMT protein did not reflect the magnitude of change in mRNA. The latter suggests that post-transcriptional, in addition to transcriptional mechanisms, regulate PNMT responses to stress. To further reveal molecular mechanisms underlying stress-induced changes in adrenergic function, the effects of hypoxia on PNMT promoter-driven gene expression are being examined in adrenal medulla-derived PC12 cells. Hypoxia activates the PNMT promoter to increase PNMT promoter-driven luciferase reporter gene expression and endogenous PNMT in PC12 cells. Induction of both appear mediated via activation of multiple signaling pathways and downstream activation of hypoxia inducible factor and PNMT transcriptional activators, Egr-1 and Sp1. Hypoxia generates both partially and fully processed forms of PNMT mRNA. The former reportedly is translated into a truncated, nonfunctional protein, and the latter into enzymatically active PNMT. Together, findings suggest that stress increases PNMT gene transcriptional activity but post-transcriptional regulatory mechanisms limit the biological end-point of functional PNMT enzyme and, thereby, Epi.

Expression and distribution of glucocorticoid receptors in the ovine fetal adrenal cortex: effect of long-term hypoxia

This study was designed to determine if long-term hypoxia (LTH) alters adrenal glucocorticoid receptor (GR) expression in the ovine fetal adrenal cortex. Ewes were maintained at 3820 m from approximately 30 to 138 to 140 days' gestation, and fetal adrenals were collected. Western analysis revealed two approximately 94-kDa GR-alpha isoforms and a lower molecular weight (45 kDa) form. A decreasing trend in the ratio of 94-kDa/45-kDa bands following LTH suggested an increase in GR turnover. Immunohistochemistry demonstrated dense GR staining in the zona glomerulosa with minimal staining in the zona fasciculata in the control group, while dense staining was observed throughout the cortex in LTH. Western analysis and reverse transcription polymerase chain reaction confirmed that the GR- beta isoform is not present or expressed at extremely low levels in the fetal adrenal, hypothalamus, pituitary, and placenta. These data indicate that LTH alters GR-alpha function in the fetal adrenal cortex and suggest that GR-beta is not expressed in sheep.

Nicotinic acetylcholine receptors of adrenal chromaffin cells

In the adrenal medulla, acetylcholine released by the sympathetic splanchnic nerves activates neuronal-type nicotinic acetylcholine receptors (nAChRs) on the membrane of chromaffin cells which liberate catecholamines into the bloodstream in preparation for the fight and flight reactions. On adrenal chromaffin cells the main class of nAChRs is a pentameric assembly of alpha3 and beta4 subunits that forms ion channels which produce membrane depolarization by increasing Na+, K+ and Ca2+ permeability. Homomeric alpha7 nicotinic receptors are expressed in a species-dependent manner and do not contribute to catecholamine secretion. Chromaffin cell nAChRs rapidly activate and desensitize with full recovery on washout. nAChR activity is subjected to various types of dynamic regulation. It is allosterically modulated by the endogenous neuropeptide substance P that stabilizes receptors in their desensitized state, thus depressing their responsiveness. The full-length peptide CGRP acts as a negative allosteric modulator by inhibiting responses without changing desensitization, whereas its N-terminal fragments act as positive allosteric modulators to transiently enhance nAChR function. nAChR expression increases when cells are chronically exposed to either selective antagonists or agonists such as nicotine, a protocol mimicking the condition of chronic heavy smokers. In this case, large upregulation of nAChRs occurs even though most of the extra nAChRs remain inside the cells, creating a mismatch between the increase in total nAChRs and increase in functional nAChRs on the cell surface. These findings highlight the plastic properties of cholinergic neurotransmission in the adrenal medulla to provide robust mechanisms for adapting catecholamine release to acute and chronic changes in sympathetic activity.

Regulation of rat dopamine beta-hydroxylase gene transcription by early growth response gene 1 (Egr1)

Egr1, a transcription factor rapidly induced by various stimuli including stress, can elevate transcription of genes for the catecholamine biosynthetic enzymes TH and PNMT. To examine if Egr1 also regulates dopamine beta-hydroxylase (DBH) gene expression, PC12 cells were transfected with expression vector for full length or truncated inactive Egr1 and various DBH promoter-driven luciferase constructs. While Egr1 elevated TH promoter activity, DBH promoter activity was reduced. The reduction occurred as early as 4 h and reached maximal inhibition 16-40 h after transfection. Egr1 also reduced the expression of endogenous DBH mRNA and the induction of DBH promoter activity by cAMP. These effects were not observed with truncated Egr1 lacking the DNA binding domain. The first 247, but not 200, nucleotides of DBH promoter are sufficient for this suppression. Several putative Egr1 motifs were identified, and mutagenesis showed that the motif at -227/-224 is required. Binding of Egr1 to this region of the DBH promoter was verified by chromatin immunoprecipitation and electrophoretic mobility shift assays. This study demonstrates that DBH promoter contains at least one functional Egr1 motif; and indicates, for the first time, that Egr1 can play an inhibitory role in regulation of DBH gene transcription.

Stress-induced catecholaminergic function: transcriptional and post-transcriptional control

This review summarizes knowledge on the effects of stress on two catecholamine biosynthetic enzymes, tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT). Information is presented on differential responses of the enzymes to a variety of stressors as well as differential responses of the enzymes localized to the central nervous system vs. peripheral nervous system and tissues. Changes in mRNA and protein or activity are described, including species- and stressor-specific effects. While temporal changes in these parameters may differ for the particular stressor or enzyme, in general, maximal changes in mRNA and protein content occur at 6-8 and 24 h after stressor exposure, respectively. Elevation of TH and PNMT transcriptional activators prior to mRNA induction and nuclear run-on assays show that stress activates the genes encoding these enzymes. Yet, extents of induction of mRNA, protein and enzyme activity are often discordant depending on the stress, its duration and repetition of exposure. The extremes are concordant changes in mRNA and protein/activity vs. highly elevated mRNA with no change in protein/activity. Post-transcriptional and/or post-translational regulatory influences that may contribute to the complex effects of stress on TH, PNMT and the stress hormone epinephrine are explored.

Stress-induced changes in epinephrine expression in the adrenal medulla in vivo

Immobilization (IMMO) stress was used to examine how stress alters the stress hormone epinephrine (EPI) in the adrenal medulla in vivo. In rats subjected to IMMO for 30 or 120 min, adrenal corticosterone increased to the same extent. In contrast, EPI changed very little, suggesting that EPI synthesis replenishes adrenal pools and sustains circulating levels for the heightened alertness and physiological responses of the 'flight or fight' response. In part, stress activates EPI via the phenylethanolamine N-methyltransferase (PNMT) gene as single or repeated IMMO elevated PNMT mRNA. The rise in PNMT mRNA was preceded by induction of the PNMT gene activator, Egr-1, with increases in Egr-1 mRNA, protein, and protein-DNA binding complex apparent. IMMO also evoked changes in Sp1 mRNA, protein, and Sp1-DNA complex formation, although for chronic IMMO changes were not entirely coincident. In contrast, glucocorticoid receptor and AP-2 mRNA, protein, and protein-DNA complex were unaltered. Finally, IMMO stress elevated PNMT protein. However, with seven daily IMMOs for 120 min and delayed killing, protein stimulation did not attain the highly elevated levels expected based on mRNA changes. The latter may perhaps suggest initiation of adrenergic desensitization to prolonged and repeated IMMO stress and/or dissociation of transcriptional and post-transcriptional regulatory mechanisms.

Adrenal adrenaline- and noradrenaline-containing cells and celiac sympathetic ganglia are differentially controlled by centrally administered corticotropin-releasing factor and arginine-vasopressin in rats

The adrenal glands and sympathetic celiac ganglia are innervated mainly by the greater splanchnic nerves, which contain preganglionic sympathetic nerves that originated from the thoracic spinal cord. The adrenal medulla has two separate populations of chromaffin cells, adrenaline-containing cells (A-cells) and noradrenaline-containing cells (NA-cells), which have been shown to be differentially innervated by separate groups of the preganglionic sympathetic neurons. The present study was designed to characterize the centrally activating mechanisms of the adrenal A-cells, NA-cells and celiac sympathetic ganglia with expression of cFos (a marker for neural excitation), in regard to the brain prostanoids, in anesthetized rats. Intracerebroventricularly (i.c.v.) administered corticotropin-releasing factor (CRF) induced cFos expression in the adrenal A-cells, but not NA-cells, and celiac ganglia. On the other hand, i.c.v. administered arginine-vasopressin (AVP) resulted in cFos induction in both A-cells and NA-cells in the adrenal medulla, but not in the celiac ganglia. Intracerebroventricular pretreatment with indomethacin (an inhibitor of cyclooxygenase) abolished the CRF- and AVP-induced cFos expression in all regions described above. On the other hand, intracerebroventricular pretreatment with furegrelate (an inhibitor of thromboxane A2 synthase) abolished the CRF-induced cFos expression in the adrenal A-cells, but not in the celiac ganglia, and also abolished the AVP-induced cFos expression in both A-cells and NA-cells in the adrenal medulla. These results suggest that centrally administered CRF activates adrenal A-cells and celiac sympathetic ganglia by brain thromboxane A2-mediated and other prostanoid than thromboxane A2 (probably prostaglandin E2)-mediated mechanisms, respectively. On the other hand, centrally administered AVP activates adrenal A-cells and NA-cells by brain thromboxane A2-mediated mechanisms in rats.

Transcriptional regulation of phenylethanolamine N-methyltransferase in pheochromocytomas from patients with von Hippel-Lindau syndrome and multiple endocrine neoplasia type 2

Pheochromocytomas in multiple endocrine neoplasia type 2 (MEN-2) express phenylethanolamine N-methyltransferase (PNMT), the enzyme that catalyzes conversion of norepinephrine to epinephrine, whereas those in von Hippel-Lindau (VHL) syndrome do not. Consequently, pheochromocytomas in MEN-2 produce epinephrine, whereas those in VHL syndrome produce mainly norepinephrine. This study examined whether transcription factors known to regulate expression of PNMT explain the different tumor phenotypes in these syndromes. Quantitative polymerase chain reaction (PCR) and Western blotting were used to assess levels of mRNA and protein for the glucocorticoid receptor, early growth response 1 (Egr-1), the Sp1 transcription factor (Sp1), and MYC-associated zinc finger protein (MAZ) in 6 MEN-2 and 13 VHL tumors. Results were cross-checked with data obtained using microarray gene expression profiling in a further set of 10 MEN-2 and 12 VHL tumors. Pheochromocytomas in MEN-2 and VHL syndrome did not differ in expression of the glucocorticoid receptor, Egr-1, Sp1, or MAZ as assessed by quantitative PCR and Western blotting. Microarray data also indicated no relevant differences in expression of the glucocorticoid receptor, Egr-1, MAZ, and the AP2 transcription factor. Thus, our results do not support a role for the above transcription factors in determining differences in expression of PNMT in pheochromocytomas from patients with VHL syndrome and MEN-2. Microarray analysis, however, did indicate differences in expression of genes involved in neural crest cell lineage and chromaffin cell development, consistent with differential survival of PNMT-expressing cells in the two syndromes.

Nerve Growth Factor Regulates Adrenergic Expression

The mechanism by which nerve growth factor (NGF) regulates adrenergic expression was examined in PC-12 cells transfected with a rat phenylethanolamine N-methyl-transferase (PNMT) promoter-luciferase reporter gene construct pGL3RP893. NGF treatment increased PNMT promoter-driven luciferase activity in a dose- and time-dependent manner. Induction was attenuated by inhibition of the extracellular signal-regulated kinase mitogen-activated protein kinase (MAPK) pathway ( approximately 60%) but not by inhibition of the protein kinase A (PKA), protein kinase C, phosphoinositol kinase, or p38 MAPK pathways. Deletion PNMT promoter-luciferase reporter gene constructs showed that the NGF-responsive sequences lay within the proximal -392 base pairs (bp) of PNMT promoter, wherein binding elements for Egr-1 (-165 bp) and Sp1 (-48 bp) reside. Western analysis further showed that NGF increased nuclear levels of Egr-1, but not Sp1 or the catalytic subunit of PKA. Gel mobility shift assays showed increased potential for Egr-1, but not Sp1, protein-DNA binding complex formation. Mutation of either the Egr-1 or Sp1 binding sites in the PNMT promoter attenuated NGF activation. NGF, combined with pituitary adenylyl cyclase-activating protein (PACAP), another PNMT transcriptional activator, cooperatively stimulated PNMT promoter driven-luciferase activity beyond levels observed with either neurotrophin alone. Finally, post-transcriptional control seems to be another important mechanism by which neurotrophins regulate the adrenergic phenotype. NGF, PACAP, and a combination of the two stimulated both intron-retaining and intronless PNMT mRNA and PNMT protein, but to different extents.

Depletion and recovery of catecholamines in several organs of rats treated with reserpine

Chemical sympathectomy with reserpine depletes catecholamines in every neuronal or nonneuronal cell producing a nonspecific temporal sympathectomy. After reserpine administration, most of the drug is distributed to tissues based on their blood flow and would then either be metabolized or be reversibly bound in lipid depots from where it might be released. Consequently, reserpine concentration and the catecholamine-depleting effect in the various tissues are expected to differ according to the route of administration. This study was designed to compare the effects of intraperitoneal (i.p.) and subcutaneous (s.c.) administration of reserpine on catecholamine depletion and recovery in the liver, portal vein, and adrenal gland on days 1, 4, and 10 after reserpine dosage. Catecholamine determinations were extended to 25 days after the treatment only in s.c. reserpine-treated rats and adding samples of heart and brown adipose tissue to the testing. I.p. and s.c. reserpine administration had the same norepinephrine-depleting effect in the portal vein and liver but full recovery was present in both tissues only in i.p. reserpine-treated rats. In the adrenal gland, both routes of administration produced the same depleting and recovery effect of norepinephrine and epinephrine concentrations. A significant temporary overshoot in epinephrine levels was observed several days after s.c. reserpine treatment. Except for the liver, reserpine injected s.c. depleted norepinephrine concentrations significantly in all other tissues up to the end of the experiment. Our results suggest that chemical sympathectomy caused by reserpine administered s.c. produces a generalized and prolonged decrease in peripheral sympathetic activity that could be compensated by an increase in activity of the adrenal gland.

Gene expression of phenylethanolamine N-methyltransferase in corticotropin-releasing hormone knockout mice during stress exposure

AIMS

Epinephrine (EPI) synthesizing enzyme phenylethanolamine N-methyltransferase (PNMT, EC 2.1.1.28) is primarily localized in the adrenal medulla (AM). We have recently described existence of the PNMT gene expression in cardiac atria and ventricles and in sympathetic ganglia of adult rats and mice. The aim of the present work was to study regulation of the PNMT gene expression in corticotropin-releasing hormone knockout mice (CRH KO) and matched control wild-type mice (WT) under normal and stress conditions.

METHODS

Levels of the PNMT mRNA were determined by RT-PCR; PNMT immunoprotein and protein of transcription factor EGR-1 by Western Blot. Plasma EPI and corticosterone (CORT) levels were determined by radioenzymatic and RIA methods. Immobilization (IMMO) was used as a stressor.

RESULTS

Stress-induced increases in the PNMT mRNA and protein levels observed in WT mice were almost completely absent in CRH KO mouse adrenal medulla, stellate ganglia, and cardiac atria, while ventricular PNMT mRNA elevation was not CRH-dependent. Plasma EPI and CORT levels were markedly reduced in CRH KO compared to WT mice both before and after the stress. Levels of EGR-1, crucial transcription factor for regulation of the PNMT were highly increased in stressed WT and CRH KO mice in cardiac areas, but not in the adrenal medulla.

CONCLUSIONS

Data show that the CRH deficiency can markedly prevent immobilization-triggered induction of the PNMT mRNA and protein levels in the adrenal medulla and stellate ganglia. Reduced plasma epinephrine and corticosterone levels and adrenal medullary EGR-1 protein levels in CRH knockout versus WT mice during stress indicate that the HPA axis plays a crucial role in regulation of the PNMT gene expression in these organs. Cardiac atrial PNMT gene expression with stress is also dependent on intact HPA axis. However, in cardiac ventricles, especially after the single stress exposure, its expression is not impaired by CRH deficiency. Since cardiac EGR-1 protein levels in CRH KO mice are also not affected by the single stress exposure, we propose existence of different regulation of the PNMT gene expression, especially in the cardiac ventricles.Overall, our findings reveal that the PNMT gene expression is regulated through the HPA in both sympathoadrenal system and the heart and also via EGR-1 in the adrenal medulla, but apparently not in the heart. Regulation of the PNMT gene expression in various compartments of heart includes both corticosterone-dependent and independent mechanisms.

Human phenylethanolamine N-methyltransferase pharmacogenomics: gene re-sequencing and functional genomics

Phenylethanolamine N-methyltransferase (PNMT, EC2.1.1.28) catalyzes the N-methylation of norepinephrine to form epinephrine. As a step toward understanding the possible contribution of inheritance to individual variation in PNMT-catalyzed epinephrine formation, we 're-sequenced' the entire human PNMT gene, including the three exons, the introns and approximately 1 kb of the 5'-flanking region (5'-FR), using DNA samples from 60 African-American (AA) and 60 Caucasian-American (CA) subjects. Within the 3.5 kb re-sequenced, 18 single nucleotide polymorphisms (SNPs) were observed, including four non-synonymous coding SNPs (cSNPs) that resulted in the following alterations in encoded amino acid sequence: Asn9Ser, Thr98Ala, Arg112Cys and Ala175Thr. When constructs for the non-synonymous cSNPs were transiently expressed in COS-1 cells, the Ala98 allozyme displayed significantly lower levels of both activity and immunoreactive protein (p < 0.002) than did the wild-type (WT) enzyme due, at least in part, to accelerated protein degradation by a proteasome-mediated process. Luciferase reporter gene constructs were also created for the six common PNMT 5'-FR haplotypes observed. Significant differences were observed among haplotypes in their ability to drive transcription. These observations raise the possibility of inherited variation in the ability to form epinephrine from norepinephrine as a result of variant PNMT polymorphisms and haplotypes.

Increased susceptibility to transcriptional changes with novel stressor in adrenal medulla of rats exposed to prolonged cold stress

The response to stress is influenced by prior experience with the same or different stressor. For example, exposure of cold pre-stressed rats to heterotypic (novel) stressors, such as immobilization (IMO), triggers an exaggerated release of catecholamines and increase in gene expression for adrenomedullary tyrosine hydroxylase (TH), the rate limiting catecholamine biosynthetic enzyme. To study the mechanism, we examined induction or phosphorylation of several transcription factors, which are implicated in IMO-triggered regulation of TH transcription, in rats exposed to cold (4 degrees C) for up to 28 days and then subjected to IMO. Levels of c-fos increased transiently after 2-6 h and returned to basal levels after 1-28 days cold stress. Fra-2, was unaffected by short term cold, but was induced about 2-fold by 28 days continual cold. In contrast, there were no significant changes in CREB phosphorylation or Egr1 induction. Rats, with and without pre-exposure to 28 days cold, were subjected to single IMO for up to 2 h. Phosphorylation of CREB after 30 min IMO was greater in cold pre-exposed rats. Induction of Egr1 was three times higher in cold pre-exposed rats and remained significantly elevated even 3 h after cessation of IMO. Exposure to IMO triggered a 10-20-fold elevation in Fra-2 in both groups, which was even higher 3 h after the IMO. However, Fra-2 was more heavily phosphorylated following IMO stress in cold pre-exposed animals. The results reveal that sensitization to novel stress in cold pre-exposed animals is manifested by exaggerated response of several transcription factors.

Egr-1 modulation of synapsin I expression: permissive effect of forskolin via cAMP

A number of candidate Egr-1 neuronal target genes have been identified including the synapsin I gene. Previous studies have shown that over-expression of Egr-1 in cells transfected with an Egr-1 expression vector is sufficient to activate reporter genes linked to regions of the synapsin I promoter, but any effect on the expression of synapsin I within its genomic context has not been demonstrated. We tested our hypothesis that modulation of synapsin I expression by Egr-1 requires the presence of elevated cAMP which would normally be present during periods of neuronal plasticity. Both the adenyl cyclase activator, forskolin (frsk), and the cAMP analogue, Sp-Adenosine 3',5'-cyclic monophosphorothioate triethylammonium salt (Sp-cAMPS), enhanced the ability of Egr-1 to transactivate a CAT reporter plasmid containing multiple copies of the Egr-1 binding site (EBS). Furthermore, Egr-1 alone had minimal effects on synapsin I expression whereas forskolin treatment of PC12 cells profoundly affected the ability of Egr-1 to regulate synapsin I expression. These results suggest that Egr-1 transactivation during neuronal plasticity may rely on a permissive effect of cAMP.

Genetic mechanisms for adrenergic control during stress

Cortisol and epinephrine released in response to stress are replenished via activation of the hypothalamic-pituitary-adrenal (HPA or stress) axis. Immobilization (IMMO) stress in rats stimulates epinephrine production in part via the gene encoding the epinephrine-synthesizing enzyme phenylethanolamine N-methyltransferase (PNMT). PNMT mRNA rose up to 7.0-fold with acute or chronic stress. Two transcription factors mediating stress induction of the PNMT gene are the glucocorticoid receptor (GR) and Egr-1, which interact with -533, -759, and -773 bp, and -165 bp binding sites in the rat PNMT promoter, respectively. To identify molecular mechanisms involved, effects of hypoxic stress on PNMT promoter activity were examined in PC12 cells transfected with the PNMT promoter-luciferase reporter gene construct pGL3RP893. Oxygen reduction to 5% increased PNMT promoter-driven luciferase expression, with maximum activity at 6 h. Pretreatment of the cells with protein kinase A (PKA) and protein kinase C (PKC) inhibitors, H-89 and GF109203X, respectively, attenuated the rise in luciferase. Similarly, PKA-deficient PC12 cells transfected with pGL3RP893 and exposed to hypoxia also showed attenuated PNMT promoter-driven luciferase expression. Mutation of the Egr-1 binding site completely prevented PNMT promoter activation, indicating that Egr-1 is essential to the stress response. Consistent with this result, hypoxia increased Egr-1 protein. Hypoxia also increased endogenous PNMT mRNA. However, a shift to intron-retaining mRNA from which truncated, nonfunctional protein is produced, occurred, suggesting that posttranscriptional regulation may be an important genetic mechanism controlling adrenergic expression and hence, epinephrine, during stress.

Glucocorticoid activation of the neuronal nicotinic acetylcholine receptor alpha7 subunit gene: involvement of transcription factor Egr-1

The alpha7 subunit is a component of nicotinic acetylcholine receptors expressed in bovine chromaffin cells. The peculiar localization of these receptors at adrenomedullary areas adjacent to the adrenal cortex suggests that factors, probably glucocorticoids, arising from the cortex might diffuse and regulate alpha7 receptor expression. In reporter gene transfection experiments, dexamethasone increased alpha7 promoter activity by up to fivefold in a concentration- and time-dependent manner despite the absence of consensus glucocorticoid receptor elements at the alpha7 promoter. Transcriptional activation induced by glucocorticoids was abolished through simultaneous mutation of at least two of the three sites for the immediate early transcription factor Egr-1, present in the proximal promoter region of the alpha7 subunit gene. Therefore, glucocorticoids activate the alpha7 subunit gene through Egr-1 in an indirect way.

Regulation of the Rat PhenylethanolamineN-Methyltransferase Gene by Transcription Factors Sp1 and MAZ

The rat phenylethanolamine N-methyltransferase (PNMT) gene promoter contains 1-base pair (bp) overlapping consensus sequences for Sp1 and MAZ transcription factors at -48 and -38 bp, respectively. Gel mobility assays using PC-12-derived RS1 cell nuclear extracts or in vitro translated proteins showed that Sp1 and MAZ specifically bind to these elements, that MAZ displaces/prevents Sp1 binding, and that Sp1 and MAZ binding is mutually exclusive, with occupancy dependent on each factor's concentration and affinity for its consensus element. In transfection assays, PNMT promoter activation by Sp1 and MAZ depends on promoter length, with -893 bp of sequence yielding greatest activation. Although MAZ has higher affinity for its binding element, it is a less effective activator. Changes in PNMT promoter activity for the constructs pGL3RP60 or pGL3RP893 using a fixed amount of MAZ expression construct and a variable amount of Sp1 expression construct or vice versa confirmed the latter. Mutation of the MAZ or Sp1 sites in pGL3RP60 attenuated but did not eliminate PNMT promoter activity, even though the proteins no longer bind to their consensus elements. Phosphatase treatment of RS1 cell nuclear extracts prevented MAZ- and Sp1-DNA binding complex formation. Although MAZ and Sp1 elevate endogenous PNMT mRNA in RS1 cells, MAZ preferentially increases intron-retaining whereas Sp1 preferentially increases intronless mRNA. Thus, expression of the PNMT gene seems to be modulated through competitive binding of phosphorylated Sp1 and MAZ to their consensus elements in the promoter. In addition, post-transcriptional regulation seems to be another important mechanism controlling PNMT expression.

Protein kinase A and protein kinase C signaling pathway interaction in phenylethanolamine N-methyltransferase gene regulation

The protein kinase A (PKA) and protein kinase C (PKC) signaling pathways appear to interact in regulating phenylethanolamine N-methyltransferase (PNMT) promoter-driven gene transcription in PC12 cells. Forskolin treatment of cells transfected with the rat PNMT promoter-luciferase reporter gene construct pGL3RP893 increased promoter activity approximately two-fold whereas phorbol-12-myristate-13 acetate (PMA) treatment had no effect. However, simultaneous forskolin and PMA treatment synergistically activated the PNMT promoter approximately four-fold, suggesting that PKC stimulation requires prior induction of the PKA pathway. Consistent with this possibility the adenylate cyclase inhibitor MDL12,330A, and the PKA inhibitor H-89 prevented PNMT promoter stimulation by the combination of forskolin and PMA. PKA and PKC regulation seems to be mediated in part by Egr-1 and Sp1 through their consensus elements in the PNMT promoter. Forskolin and PMA treatment of PC12 cells increased Egr-1 protein and phosphorylated Egr-1/DNA-binding complex formation to the same extent but only increased phosphorylated Sp1/DNA binding complex formation without altering Sp1 protein levels. Mutation of the - 165 bp Egr-1 and - 48 bp Sp1 sites, respectively, attenuated and abolished combined forskolin and PMA-mediated promoter activation. PNMT promoter analysis further showed that synergistic stimulation by PKA and PKC involves DNA sequences between - 442 and - 392 bp, and potentially a GCM binding element lying within this region.

Development of adrenal chromaffin cells is largely normal in mice lacking the receptor tyrosine kinase c-Ret

c-Ret encodes a receptor tyrosine kinase that is essential for normal development of the kidney as well as enteric and sympathetic neurons. Since sympathetic neurons and neuroendocrine chromaffin cells originate from a common progenitor cell, we have examined the relevance of c-Ret for the development of adrenal chromaffin cells by analyzing mouse mutants lacking c-Ret. Adrenal chromaffin cells express c-Ret mRNA at embryonic day (E) 12.5 and 13.5, yet levels of expression decline at later embryonic and postnatal ages. Adrenal medullae of c-Ret deficient mice show normal numbers of tyrosine hydroxylase (TH)-immunoreactive cells at E13.5 and at birth. Ultrastructurally, adrenal chromaffin cells of c-Ret(-/-) mice appear unaltered: chromaffin cells develop typical secretory chromaffin granules, the morphological hallmark of chromaffin cells, and synaptic terminals appear normal. However, adrenaline levels and numbers of chromaffin cells immunoreactive for the adrenaline synthesizing enzyme phenylethanolamine-N-methyltransferase (PNMT) are reduced by about 30% in c-Ret-deficient mice arguing for a direct or indirect role of c-Ret in the regulation of PNMT. Thus, despite expression of c-Ret, adrenal chromaffin cells develop largely normal in mice lacking c-Ret. We therefore conclude that sympathetic neurons and neuroendocrine chromaffin cells profoundly differ in their requirement for c-Ret signaling during development.

The adrenal genetic puzzle: how do the fetal and adult pieces differ?

The basis for the steroidogenic differences between the human fetal adrenal (HFA) and adult adrenal is not well defined. However, gene expression clearly plays a critical role in defining their distinct steroidogenic and structural phenotypes. We used DNA microarrays to compare expression levels of several thousand transcripts between the HFA and adult adrenal gland. Gene profiling was done using seven independent microarrays that contained between 7075 and 9182 cDNA elements. Twenty-five transcripts were found to have a greater than 5-fold difference in expression between HFA and adult adrenals. The largest differences were observed for transcripts that encode insulin-like growth factor-II (IGF-II) (25-fold higher in HFA) and 3beta hydroxysteroid dehydrogenase (3betaHSD) (21-fold higher in adult). The vast majority of the 25 transcripts have not been studied with regard to adrenal function. We also determined the transcripts that had the highest signal intensities, which is an approximate measure of expression level, for both the fetal and adult adrenal RNA samples. The enzyme 24-dehydrocholesterol reductase, which is involved in cholesterol biosynthesis, exhibited the highest signal intensity for fetal adrenal RNA. For adult adrenal mRNA, the expression of 11beta-hydroxylase transcripts was found to have the highest signal intensity ranking. Overall, 10 of the top 20 highest signal intensities were similar for adult and fetal adrenal transcripts. The gene profile data for fetal vs. adult adrenal glands should provide valuable information that could help define mechanisms involved in adrenal growth and development.

Hypoxia activates multiple transcriptional pathways in mouse pheochromocytoma cells

Mouse pheochromocytoma cells (MPCs) provide an excellent model system for investigating the effects of hypoxia on catecholamine enzyme genes and on transcription factors mediating stress responses. RT-PCR detects rapid, transient increases in PNMT mRNA in hypoxic MPC 712 cells. Additionally, elevation of mRNAs encoding transcription factors hypoxia inducible factor 1 (HIF-1) alpha subunit and Egr-1 are evident within 60 min incubation in anoxia. Therefore, hypoxia elicits rapid transcriptional responses in numerous genes expressed by chromaffin cells.

Molecular biology of the chromaffin cell

A large number of molecular biology studies have been performed on chromaffin cells, and many genes involved in catecholamine synthesis, storage, and release have been cloned and their function determined. Catecholamine synthesis takes place in different cellular compartments, and enzymes involved in this process are subject to a fine regulation, as demonstrated by recent studies on their gene promoters. Genes coding for such intravesicular proteins as chromogranin A, B, and secretogranin II (chromogranin C) are also regulated in response to a variety of stimuli. Chromogranin gene promoters and transcription factors involved in their regulation have been elucidated. This review serves as an introduction to the studies described in the chapters to follow.

Cholinergic and peptidergic regulation of phenylethanolamine N-methyltransferase gene expression

The splanchnic nerve, innervating the adrenal medulla, releases a variety of neurotransmitters that stimulate genes involved in catecholamine biosynthesis. In particular, cholinergic agonists have been shown to induce phenylethanolamine N-methyltransferase (PNMT) gene expression through activation of both nicotinic and muscarinic receptors in vivo and in vitro. By contrast, the role of peptidergic neurotransmitters in adrenal medullary PNMT gene expression remains unclear. Using transient transfection assays, we demonstrate that rat PNMT promoter-luciferase reporter gene constructs are markedly activated by 10 nM PACAP when expressed in PC12 cells. PACAP appears to mediate its effects primarily through PAC1 receptors and, subsequently, cAMP-protein kinase A (PKA) and extracellular Ca(2+) signaling mechanisms. Activation of these signal transduction pathways markedly increases nuclear levels of the immediately early gene transcription factor Egr-1 and the developmental factor AP2. A slight decrease in Sp1 expression may also occur, whereas MAZ and glucocorticoid receptor expression remains unaltered. Although PACAP stimulates rapid changes in transcription factor expression and PNMT promoter activity, its effects are long lasting. PNMT promoter induction continues to rise and is sustained for > or=48 hours. By contrast, while muscarine, nicotine, or carbachol (100 micro M) also evoke rapid increases in rat PNMT promoter activity, peak activity is observed at 6 hours, followed by a decline and restoration to basal levels by 24 hours. Cholinergic activation of the PNMT promoter also seems to involve the cAMP-PKA signaling mechanism. However, the magnitude of stimulation and antagonist blockade with H-89 or the polypeptide inhibitor PKI suggests that the extent of activation is much less than that with PACAP.

Glucocorticoid responsiveness of the rat phenylethanolamine N-methyltransferase gene

Two newly identified, overlapping (1 bp) glucocorticoid response elements (GREs) at -759 and -773 bp in the promoter of the rat phenylethanolamine N-methyltransferase (PNMT; EC 2.1.1.28) gene are primarily responsible for its glucocorticoid sensitivity, rather than the originally identified -533-bp GRE. A dose-dependent increase in PNMT promoter activity was observed in RS1 cells transfected with a wild-type PNMT promoter-luciferase reporter gene construct and treated with dexamethasone (maximum activation at 0.1 microM). The type II glucocorticoid receptor antagonist RU38486 (10 microM) fully inhibited dexamethasone (1 microM) activation of the PNMT promoter, consistent with classical glucocorticoid receptors mediating corticosteroid-stimulated transcriptional activity. Relative IC(50) values from gel mobility shift competition assays showed that the -759-bp GRE has a 2-fold greater affinity for the glucocorticoid receptor than the -773-bp GRE. Site-directed mutation of the -533-, -759-, and -773-bp GREs alone or in tandem demonstrated that the -759-bp GRE was also functionally more important, but both the -759- and -773-bp GREs are required for maximum glucocorticoid responses. Moreover, the -533-bp GRE, rather than increasing glucocorticoid sensitivity of the promoter, may limit corticosteroid responsiveness mediated via the -759- and -773-bp GREs. Finally, the glucocorticoid receptor bound to the -759- and -773-bp GREs interacts cooperatively with Egr-1 and/or AP-2 to stimulate PNMT promoter activity in RS1 cells treated with dexamethasone. In contrast, glucocorticoid receptors bound to the -533-bp GRE only seem to participate in synergistic activation of the PNMT promoter through interaction with activator protein 2.

Stress-triggered activation of gene expression in catecholaminergic systems: dynamics of transcriptional events

Stress triggers important adaptive responses that enable an organism to cope with a changing environment. However, when prolonged or repeated, stress can be extremely harmful. The release of catecholamines is a key initial event in responses to stressors and is followed by an increase in the expression of genes that encode catecholamine-synthesizing enzymes. This process is mediated by transcriptional mechanisms in the adrenal medulla and the locus coeruleus. The persistence of transcriptional activation depends on the duration and repetition of the stress. Recent work has begun to identify the various transcription factors that are associated with brief or intermediate duration of a single or repeated stress. These studies suggest that dynamic interplay is involved in converting the transient increases in the rate of transcription into prolonged (potentially adaptive or maladaptive) changes in gene expression.

The genomic action potential

Neurons compute in part by integrating, on a time scale of milliseconds, many synaptic inputs and generating a digital output-the "action potential" of classic electrophysiology. Recent discoveries indicate that neurons also perform a second, much slower, integration operating on a time scale of minutes or even hours. The output of this slower integration involves a pulse of gene expression which may be likened to the electrophysiological action potential. Its function, however, is not directed toward immediate transmission of a synaptic signal but rather toward the experience-dependent modification of the underlying synaptic circuitry. Commonly termed the "immediate early gene" (IEG) response, this phenomenon is often assumed to be a necessary component of a linear, deterministic cascade of memory consolidation. Critical review of the large literature describing the phenomenon, however, leads to an alternative model of IEG function in the brain. In this alternative, IEG activation is not directed at the consolidation of memories of a specific inducing event; instead, it sets the overall gain or efficiency of memory formation and directs it to circuits engaged by behaviorally significant contexts. The net result is a sharpening of the selectivity of memory formation, a recruitment of temporally correlated associations, and an ultimate enhancement of long-term memory retrieval.