Inhibition of corticotrophin-releasing hormone transcription by inducible cAMP-early repressor in the hypothalamic cell line, 4B

Novel insights into biological roles of inducible cAMP early repressor ICER

ICER corresponds to a group of alternatively spliced Inducible cAMP Early Repressors with high similarity, but multiple roles, including in circadian rhythm, and are involved in attenuation of cAMP-dependent gene expression. We present experimental and in silico data revealing biological differences between the isoforms with exon gamma (ICER) or without it (ICERγ). Both isoforms are expressed in the liver and the adrenal glands and can derive from differential splicing. In adrenals the expression is circadian, with maximum at ZT12 and higher amplitude of Icerγ. In the liver, the expression of Icerγ is lower than Icer in the 24 h time frame. Icer mRNA has a delayed early response to forskolin. The longer ICER protein binds to three DNA grooves of the Per1 promoter, while ICERγ only to two, as deduced by molecular modelling. This is in line with gel shift competition assays showing stronger binding of ICER to Per1 promotor. Only Icerγ siRNA provoked an increase of Per1 expression. In conclusion, we show that ICER and ICERγ have distinct biochemical properties in tissue expression, DNA binding, and response to forskolin. Data are in favour of ICERγ as the physiologically important form in hepatic cells where weaker binding of repressor might be preferred in guiding the cAMP-dependent response.

The Effect of Acute and Repeated Stress on CRH-R1 and CRH-R2 mRNA Expression in Pituitaries of Wild Type and CRH Knock-Out Mice

The activation of the HPA axis is the endocrine measure of stress responsiveness that is initiated by corticotropin-releasing hormone (CRH). CRH exerts its effects via CRHR1 and CRH-R2 receptors coupled to the cAMP signaling system and this process involves transcription factor cAMP-responsive element-binding protein (CREB).This study investigated the role of CRH and the possible involvement of CREB in gene regulation of CRH receptor, under basal conditions and after stress application in the pituitary. We used wild type (wt +/+) controls and CRH knock-out (CRH-KO -/-) male mice. Using CRH-deficient mice, we were able to investigate the consequences of the lack of the CRH on the expression of CRH receptors and transcriptional regulation mediated by CREB. We estimated the effect of acute (IMO 1×) and repeated (IMO 7×) restraint stressors lasting 30 and 120 min on the expression of mRNA CREB, CRH-R1, and CRH-R2 by qPCR. We found very significant difference in the expression of these peptides under the effect of single and repeated stress in control and CRH-KO mice. Our results indicate that both CRH receptors and CREB might be involved in the regulation of stress response in the pituitary of mice. We propose that regulation of the stress response may be better understood if more were known about the mechanisms of CRH receptor signal transduction and involvement of CREB system.

Angiotensin II induces membrane trafficking of natively expressed transient receptor potential vanilloid type 4 channels in hypothalamic 4B cells

Transient receptor potential vanilloid family type 4 (TRPV4) channels are expressed in central neuroendocrine neurons and have been shown to be polymodal in other systems. We previously reported that in the rodent, a model of dilutional hyponatremia associated with hepatic cirrhosis, TRPV4 expression is increased in lipid rafts from the hypothalamus and that this effect may be angiotensin dependent. In this study, we utilized the immortalized neuroendocrine rat hypothalamic 4B cell line to more directly test the effects of angiotensin II (ANG II) on TRPV4 expression and function. Our results demonstrate the expression of corticotropin-releasing factor (CRF) transcripts, for sex-determining region Y (SRY) (male genotype), arginine vasopressin (AVP), TRPV4, and ANG II type 1a and 1b receptor in 4B cells. After a 1-h incubation in ANG II (100 nM), 4B cells showed increased TRPV4 abundance in the plasma membrane fraction, and this effect was prevented by the ANG II type 1 receptor antagonist losartan (1 μM) and by a Src kinase inhibitor PP2 (10 μM). Ratiometric calcium imaging experiments demonstrated that ANG II incubation potentiated TRPV4 agonist (GSK 1016790A, 20 nM)-induced calcium influx (control 18.4 ± 2.8% n = 5 and ANG II 80.5 ± 2.4% n = 5). This ANG II-induced increase in calcium influx was also blocked by 1 μM losartan and 10 μM PP2 (losartan 26.4 ± 3.8% n = 5 and PP2 19.7 ± 3.9% n = 5). Our data suggests that ANG II can increase TRPV4 channel membrane expression in 4B cells through its action on AT1R involving a Src kinase pathway.

Regulation of hypothalamic corticotropin-releasing hormone transcription by elevated glucocorticoids

Negative glucocorticoid feedback is essential for preventing the deleterious effects of excessive hypothalamic pituitary adrenal axis axis activation, with an important target being CRH transcription in the hypothalamic paraventricular nucleus. The aim of these studies was to determine whether glucocorticoids repress CRH transcription directly in CRH neurons, by examining glucocorticoid effects on glucocorticoid receptor (GR)-CRH promoter interaction and the activation of proteins required for CRH transcription. Immunoprecipitation of hypothalamic chromatin from intact or adrenalectomized rats subjected to either stress or corticosterone injections showed minor association of the proximal CRH promoter with the GR compared with that with phospho-CREB (pCREB). In contrast, the Period-1 (Per1, a glucocorticoid-responsive gene) promoter markedly recruited GR. Stress increased pCREB recruitment by the CRH but not the Per1 promoter, irrespective of circulating glucocorticoids. In vitro, corticosterone pretreatment (30 minutes or 18 hours) only slightly inhibited basal and forskolin-stimulated CRH heteronuclear RNA in primary hypothalamic neuronal cultures and CRH promoter activity in hypothalamic 4B cells. In 4B cells, 30 minutes or 18 hours of corticosterone exposure had no effect on forskolin-induced nuclear accumulation of the recognized CRH transcriptional regulators, pCREB and transducer of regulated CREB activity 2. The data show that inhibition of CRH transcription by physiological glucocorticoids in vitro is minor and that direct interaction of GR with DNA in the proximal CRH promoter may not be a major mechanism of CRH gene repression. Although GR interaction with distal promoter elements may have a role, the data suggest that transcriptional repression of CRH by glucocorticoids involves protein-protein interactions and/or modulation of afferent inputs to the hypothalamic paraventricular nucleus.

Transcriptional regulation of episodic glucocorticoid secretion

Circadian and ultradian variations of basal glucocorticoid secretion and transient elevations during stress are essential for homeostasis. Using intronic qRT-PCR to measure changes in primary transcript (hnRNA) we have shown that secretory events induced by stress or ACTH injection are followed by episodic increases in transcription of rate limiting steroidogenic proteins, such as steroidogenic acute regulatory protein (StAR), cytochrome P450 side chain cleavage and melanocortin receptor associated protein. These transcriptional episodes imply rapid turnover of steroidogenic proteins and the need of de novo synthesis following each secretory event. In addition to episodic ACTH secretion, it is likely that intracellular feedback mechanisms at the adrenal fasciculata level contribute to the generation of episodes of transcription. The time relationship between activation and translocation of the CREB co-activator, transducer of regulated CREB activity (TORC) to the nucleus preceding transcriptional episodes suggest the involvement of TORC in the transcriptional activation of StAR and other steroidogenic proteins.

Corticotropin-releasing hormone regulates common target genes with divergent functions in corticotrope and neuronal cells

As a key regulator of the neuroendocrine stress axis and as a neuromodulator in the brain, the neuropeptide corticotropin-releasing hormone (CRH) plays an important role in various diseases of the central nervous system. Its cognate receptor CRH receptor type 1 (CRHR1) is a potential novel target for the therapeutic intervention in major depressive disorder. Therefore, a more precise understanding of involved intracellular signaling mechanisms is essential. The objective of this project was to identify specific target genes of CRHR1-mediated signaling pathways in the corticotrope cell line AtT-20 and in the neuronal cell line HN9 using microarray technology and qRT-PCR, respectively. In addition, we assessed the capacity of validated target genes to directly impact on CRHR1-dependent signaling using reporter assays. Thereby, we identified a set of CRHR1 downstream targets with diverging and cell type-specific roles which strengthen the role of CRH and CRHR1 as dynamic modulators of a variety of signal transduction mechanisms and cellular processes.

Six commercially available angiotensin II AT1 receptor antibodies are non-specific

Commercially available Angiotensin II AT1 receptor antibodies are widely employed for receptor localization and quantification, but they have not been adequately validated. In this study, six commercially available AT1 receptor antibodies were characterized by established criteria: sc-1173 and sc-579 from Santa Cruz Biotechnology, Inc., AAR-011 from Alomone Labs, Ltd., AB15552 from Millipore, and ab18801 and ab9391 from Abcam. The immunostaining patterns observed were different for every antibody tested, and were unrelated to the presence or absence of AT1 receptors. The antibodies detected a 43 kDa band in western blots, corresponding to the predicted size of the native AT1 receptor. However, identical bands were observed in wild-type mice and in AT1A knock-out mice not expressing the target protein. Moreover, immunoreactivity detected in rat hypothalamic 4B cells not expressing AT1 receptors or transfected with AT1A receptor construct was identical, as revealed by western blotting and immunocytochemistry in cultured 4B cells. Additional prominent immunoreactive bands above and below 43 kDa were observed by western blotting in extracts from tissues of AT1A knock-out and wild-type mice and in 4B cells with or without AT1 receptor expression. In all cases, the patterns of immunoreactivity were independent of the AT1 receptor expression and different for each antibody studied. We conclude that, in our experimental setup, none of the commercially available AT1 receptor antibodies tested met the criteria for specificity and that competitive radioligand binding remains the only reliable approach to study AT1 receptor physiology in the absence of full antibody characterization.

The molecular physiology of CRH neurons

Corticotropin releasing hormone (CRH) is essential for stress adaptation by mediating hypothalamic-pituitary-adrenal (HPA) axis, behavioral and autonomic responses to stress. Activation of CRH neurons depends on neural afferents from the brain stem and limbic system, leading to sequential CRH release and synthesis. CRH transcription is required to restore mRNA and peptide levels, but termination of the response is essential to prevent pathology associated with chronic elevations of CRH and HPA axis activity. Inhibitory feedback mediated by glucocorticoids and intracellular production of the repressor, Inducible Cyclic AMP Early Repressor (ICER), limit the magnitude and duration of CRH neuronal activation. Induction of CRH transcription is mediated by the cyclic AMP/protein kinase A/cyclic AMP responsive element binding protein (CREB)-dependent pathways, and requires cyclic AMP-dependent nuclear translocation of the CREB co-activator, Transducer of Regulated CREB activity (TORC). This article reviews current knowledge on the mechanisms regulating CRH neuron activity.

Histone deacetylase 1 (HDAC1) participates in the down-regulation of corticotropin releasing hormone gene (crh) expression

The paraventricular nucleus of the hypothalamus (PVH) plays a central role in regulating the hypothalamic-pituitary-adrenal (HPA) axis. Medial parvocellular neurons of the PVH (mpPVH) integrate sensory and humoral inputs to maintain homeostasis. Humoral inputs include glucocorticoids secreted by the adrenals, which down-regulate HPA activation. A primary glucocorticoid target is the population of mpPVH neurons that synthesize and secrete corticotropin-releasing factors, the most potent of which is corticotropin-releasing hormone (CRH). Although CRH gene (crh) expression is known to be down-regulated by glucocorticoids, the mechanisms by which this process occurs are still poorly understood. To begin this study we postulated that glucocorticoid repression of crh involves HDAC recruitment to the region of the crh proximal promoter. To evaluate this hypothesis, we treated hypothalamic cells that express CRH with the HDAC inhibitor trichostatin A (TSA). As predicted, treatment with TSA led to increased CRH mRNA levels and crh promoter activity. Although co-treatment with Dex (10(-7)M) reduced the TSA effect on mRNA levels, it failed to reduce promoter activity; however co-transfection of HDAC1 but not 3 restored Dex inhibition. A distinction between HDAC1 and 3 was also apparent with respect to crh promoter occupancy. Dex led to increased HDAC1 but not HDAC3 occupancy. In vivo studies revealed that CRH-immunoreactive (-ir) neurons contained HDAC1- and HDAC3-ir. Collectively, these data point to a role for HDAC1 in the physiologic regulation of crh.

Stress induces parallel changes in corticotrophin-releasing hormone (CRH) Transcription and nuclear translocation of transducer of regulated cAMP response element-binding activity 2 in hypothalamic CRH neurones

Recent studies in vitro have shown that the cAMP response element-binding (CREB) co-activator, transducer of regulated CREB activity (TORC), is required for transcriptional activation of the corticotrophin-releasing hormone (CRH) gene. To determine the physiological importance of TORC2 regulating CRH transcription during stress, we examined the localisation of TORC2 in CRH neurones, as well as the relationship between changes in CRH heterogeneous nuclear (hn)RNA, nuclear translocation of TORC2 and binding of TORC2 to the CRH promoter. Immunohistochemistry revealed TORC2 immunoreactivity (irTORC2) in the dorsolateral (magnocellular) and dorsomedial (parvocellular) regions of the hypothalamic paraventricular nucleus (PVN). Although staining was mostly cytosolic under basal conditions, there was a marked increase in nuclear irTORC2 in the dorsomedial region after 30 min of restraint, concomitant with increases in CRH hnRNA levels. Levels of nuclear irTORC2 and CRH hnRNA had returned to basal 4 h after stress. Double-staining immunohistochemistry showed TORC2 co-staining in 100% of detected CRH neurones, and nuclear translocation after 30 min of restraint in 61%. Cellular distribution of TORC2 in the dorsolateral PVN was unaffected by restraint. Chromatin immunoprecipitation experiments showed recruitment of TORC2 and phosphorylated CREB (pCREB) by the CRH promoter after 30 min of restraint, but 4 h after stress only pCREB was associated with the CRH promoter. The demonstration that TORC2 translocates to the nucleus of hypothalamic CRH neurones and interacts with the CRH promoter in conjunction with the activation of CRH transcription during restraint stress, provides strong evidence for the involvement of TORC2 in the physiological regulation of CRH transcription.

HPA axis responsiveness to stress: implications for healthy aging

The major neuroendocrine response mediating stress adaptation is activation of the hypothalamic pituitary adrenal axis, with stimulation of corticotropin releasing hormone (CRH) and vasopressin (VP) from parvocellular neurons of the hypothalamic paraventricular nucleus, leading to stimulation of pituitary ACTH secretion and increases in glucocorticoid secretion from the adrenal cortex. Basal production and transient increases during stress of glucocorticoids and its hypothalamic regulators are essential for neuronal plasticity and normal brain function. While activation of the HPA axis is essential for survival during stress, chronic exposure to stress hormones can predispose to psychological, metabolic and immune alterations. Thus, prompt termination of the stress response is essential to prevent negative effects of inappropriate levels of CRH and glucocorticoids. This review addresses the regulation of HPA axis activity with emphasis on the mechanisms of termination of CRH transcription, which is a critical step in this process. In addition, the actions by which glucocorticoids, CRH and VP can affect the aging process will be discussed.

Deducing corticotropin-releasing hormone receptor type 1 signaling networks from gene expression data by usage of genetic algorithms and graphical Gaussian models

BACKGROUND

Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis is a hallmark of complex and multifactorial psychiatric diseases such as anxiety and mood disorders. About 50-60% of patients with major depression show HPA axis dysfunction, i.e. hyperactivity and impaired negative feedback regulation. The neuropeptide corticotropin-releasing hormone (CRH) and its receptor type 1 (CRHR1) are key regulators of this neuroendocrine stress axis. Therefore, we analyzed CRH/CRHR1-dependent gene expression data obtained from the pituitary corticotrope cell line AtT-20, a well-established in vitro model for CRHR1-mediated signal transduction. To extract significantly regulated genes from a genome-wide microarray data set and to deduce underlying CRHR1-dependent signaling networks, we combined supervised and unsupervised algorithms.

RESULTS

We present an efficient variable selection strategy by consecutively applying univariate as well as multivariate methods followed by graphical models. First, feature preselection was used to exclude genes not differentially regulated over time from the dataset. For multivariate variable selection a maximum likelihood (MLHD) discriminant function within GALGO, an R package based on a genetic algorithm (GA), was chosen. The topmost genes representing major nodes in the expression network were ranked to find highly separating candidate genes. By using groups of five genes (chromosome size) in the discriminant function and repeating the genetic algorithm separately four times we found eleven genes occurring at least in three of the top ranked result lists of the four repetitions. In addition, we compared the results of GA/MLHD with the alternative optimization algorithms greedy selection and simulated annealing as well as with the state-of-the-art method random forest. In every case we obtained a clear overlap of the selected genes independently confirming the results of MLHD in combination with a genetic algorithm. With two unsupervised algorithms, principal component analysis and graphical Gaussian models, putative interactions of the candidate genes were determined and reconstructed by literature mining. Differential regulation of six candidate genes was validated by qRT-PCR.

CONCLUSIONS

The combination of supervised and unsupervised algorithms in this study allowed extracting a small subset of meaningful candidate genes from the genome-wide expression data set. Thereby, variable selection using different optimization algorithms based on linear classifiers as well as the nonlinear random forest method resulted in congruent candidate genes. The calculated interacting network connecting these new target genes was bioinformatically mapped to known CRHR1-dependent signaling pathways. Additionally, the differential expression of the identified target genes was confirmed experimentally.

Corticotrophin-releasing factor gene transcription is directly activated after deprivation of glucocorticoids in hypothalamic cells

Corticotrophin-releasing factor (CRF) plays a central role in controlling the hypothalamic-pituitary-adrenal axis during stressful periods. CRF neurones are activated in the hypothalamic paraventricular nucleus (PVN) in response to stress, whereas the activated CRF neurones in the PVN are suppressed by glucocorticoids. Glucocorticoids may act directly on CRF neurones because glucocorticoid receptors are expressed highly on these neurones in the PVN. CRF expression levels in the PVN are also increased by adrenalectomy in vivo. The signalling pathways involved in the control of CRF gene transcription in the hypothalamus when negative feedback by glucocorticoids after adrenalectomy is lost remain undetermined. We investigated whether CRF gene transcription is regulated by both glucocorticoids and glucocorticoid withdrawal in hypothalamic cells. The present study demonstrates that CRF gene transcription activity and mRNA levels in the hypothalamic 4B cells were not modulated by incubation with dexamethasone for a short 2-h period, although they were stimulated by incubation for longer than 5 h. CRF gene transcription activity and mRNA levels were increased after 2 h of dexamethasone deprivation. The cAMP-response element (CRE) on the promoter was the main region that is regulated by both glucocorticoids and glucocorticoid withdrawal. We observed that the intracellular cAMP production levels were transiently increased 30 min after the removal of dexamethasone, whereas they were also increased 2.5 h after incubation with dexamethasone without the removal. Phosphorylated-CRE-binding protein (CREB)/CREB protein levels were also increased rapidly after the deprivation of glucocorticoids via an adenylate cyclase pathway. Therefore, the phosphorylation of CREB contributes to the activation of CRF gene transcription after the deprivation of glucocorticoids in hypothalamic cells.

Involvement of transducer of regulated cAMP response element-binding protein activity on corticotropin releasing hormone transcription

We have recently shown that phospho-cAMP response element-binding protein (CREB) is essential but not sufficient for activation of CRH transcription, suggesting the requirement of a coactivator. Here, we test the hypothesis that the CREB coactivator, transducer of regulated CREB activity (TORC), is required for activation of CRH transcription, using the cell line 4B and primary cultures of hypothalamic neurons. Immunohistochemistry and Western blot experiments in 4B cells revealed time-dependent nuclear translocation of TORC1,TORC 2, and TORC3 by forskolin [but not by the phorbol ester, phorbol 12-myristate 13-acetate (PMA)] in a concentration-dependent manner. In reporter gene assays, cotransfection of TORC1 or TORC2 potentiated the stimulatory effect of forskolin on CRH promoter activity but had no effect in cells treated with PMA. Knockout of endogenous TORC using silencing RNA markedly inhibited forskolin-activated CRH promoter activity in 4B cells, as well as the induction of endogenous CRH primary transcript by forskolin in primary neuronal cultures. Coimmunoprecipitation and chromatin immunoprecipitation experiments in 4B cells revealed association of CREB and TORC in the nucleus, and recruitment of TORC2 by the CRH promoter, after 20-min incubation with forskolin. These studies demonstrate a correlation between nuclear translocation of TORC with association to the CRH promoter and activation of CRH transcription. The data suggest that TORC is required for transcriptional activation of the CRH promoter by acting as a CREB coactivator. In addition, cytoplasmic retention of TORC during PMA treatment is likely to explain the failure of phorbolesters to activate CRH transcription in spite of efficiently phosphorylating CREB.

Cyclic AMP inducible early repressor mediates the termination of corticotropin releasing hormone transcription in hypothalamic neurons

Elevations of inducible cAMP early repressor (ICER), the repressor isoform of the cAMP-responsive element modulator (CREM), are associated with protein binding to the corticotrophin releasing hormone (CRH) promoter and termination of CRH transcriptional responses to stress. To determine whether endogenous ICER production represses CRH transcription, we examined the effect of CREM siRNA on forskolin-stimulated ICER formation and CRH transcription in the hypothalamic cell line, 4B, and in primary cultures of hypothalamic neurons. Cotransfection of 4B cells with CREM siRNA and a CRH promoter-driven luciferase reporter gene markedly reduced the induction of ICER by forskolin and potentiated the stimulatory effect of forskolin on CRH promoter activity, compared with cells cotransfected with a nonspecific oligonucleotide. The role of ICER on endogenous CRH expression was studied in primary cultures of hypothalamic neurons by examining the effect of CREM siRNA on forskolin-induced primary transcript (CRH hnRNA) using intronic real-time PCR. As observed during stress in vivo, forskolin-stimulated CRH hnRNA was transient, increasing up to 60 min and declining to near basal values by 3 h. Transfection of CREM siRNA reduced forskolin-induced ICER by about 45% 48-h later and partially reversed the declining phase of CRH hnRNA production at 3 h. The data provide evidence that endogenous ICER formation is required for termination of CRH transcription and support the hypothesis that ICER is part of an intracellular feedback mechanism limiting the activation of CRH transcription during stress.

Inducible cAMP early repressor (ICER) and brain functions

The inducible cAMP early repressor (ICER) is an endogenous repressor of cAMP-responsive element (CRE)-mediated gene transcription and belongs to the CRE-binding protein (CREB)/CRE modulator (CREM)/activating transcription factor 1 (ATF-1) gene family. ICER plays an important role in regulating the neuroendocrine system and the circadian rhythm. Other aspects of ICER function have recently attracted heightened attention. Being a natural inducible CREB antagonist, and more broadly, an inducible repressor of CRE-mediated gene transcription, ICER regulates long-lasting plastic changes that occur in the brain in response to incoming stimulation. This review will bring together data on ICER and its functions in the brain, with a special emphasis on recent findings highlighting the involvement of ICER in the regulation of long-term plasticity underlying learning and memory.

Direct inhibitory effect of glucocorticoids on corticotrophin-releasing hormone gene expression in neurones of the paraventricular nucleus in rat hypothalamic organotypic cultures

Corticotrophin-releasing hormone (CRH) in the parvocellular neurosecretory neurones of hypothalamic paraventricular nucleus governs neuroendocrine stress cascade and is the major target of the negative feedback effect of corticosteroids. To assess whether glucocorticoids exert their inhibitory effect on CRH expression directly on parvocellular neurones or indirectly through a complex neuronal circuit, we examined the effect of corticosterone (CORT) and dexamethasone (DEX) on CRH mRNA levels in slice explant cultures of the rat hypothalamus. Organotypic slice cultures were prepared from 6 days old rat pups and maintained in vitro for 14 days. CRH mRNA expression was measured by in situ hybridisation histochemistry. Under basal conditions, CRH mRNA expressing cells were exclusively revealed in the paraventricular region along the third ventricle. Inhibition of action potential spike activity by tetrodotoxin (TTX, 1 microm) reduced CRH mRNA signal in the organotypic cultures. CORT (500 nm) or DEX (50 nm) treatment for 24 h significantly inhibited CRH expression in the parvocellular neurones and this effect of corticosteroids was not affected following blockade of voltage dependent sodium channels by TTX. Forskolin-stimulated CRH mRNA levels in the paraventricular nucleus were also inhibited by CORT or DEX in the presence and in the absence of TTX. These studies identify paraventricular CRH neurones as direct target of corticosteroid feedback. Type II corticosteroid receptor agonists act directly on paraventricular neurones to inhibit basal and forskolin-induced CRH mRNA expression in explant cultures of the rat hypothalamus.

Cyclic adenosine 3',5'-monophosphate responsive element binding protein phosphorylation is required but not sufficient for activation of corticotropin-releasing hormone transcription

cAMP is a major regulator of CRH transcription. However, receptors activating CRH neurons (alpha-adrenergic and glutamatergic) do not signal through cAMP, suggesting that calcium phospholipid-dependent signaling synergizes with small elevations of intracellular cAMP. To test this hypothesis, we examined the relationship between activation of CRH transcription, cAMP production, and cAMP response element binding protein (CREB) phosphorylation in neuronal cultures treated with the adenylyl cyclase stimulator, forskolin, the phorbol ester, phorbol-12-myristate-13-acetate (PMA), or their combination. Forskolin, at threshold concentrations for cAMP production and CREB phosphorylation, induced CRH promoter-driven luciferase activity in 4B cells (EC(50) = 0.7 microm) and CRH primary transcript in hypothalamic neurons (EC(50) = 0.6 microm). PMA alone failed to activate CRH transcription despite being as effective as forskolin in phosphorylating CREB (Ser133 and Ser121). Although PMA potentiated the effect of low forskolin concentrations on CRH transcription and CREB phosphorylation, there was no correlation between phosphorylated CREB levels and activation of CRH transcription. Similarly, the calcium/calmodulin-dependent kinase inhibitor, KN-93, enhanced PMA plus forskolin-stimulated CREB phosphorylation and inhibited CRH transcription. Suppression of CREB phosphorylation by the protein kinase A inhibitor, H89, or the CREB dominant negative, A-CREB, did not affect basal but blocked forskolin-stimulated transcription. This study shows that calcium phospholipid-dependent pathways potentiate the ability of small elevations of intracellular cAMP to activate CRH transcription, providing a mechanism by which non-cAMP-dependent regulators induce CRH gene expression. In addition, the data indicate that phosphorylated CREB is essential but not sufficient for activation of CRH transcription, suggesting that full promoter stimulation requires the interaction of phosphorylated CREB with a coactivator.

Effects of estradiol on regulation of corticotropin-releasing factor gene and interleukin-6 production via estrogen receptor type beta in hypothalamic 4B cells

Corticotropin-releasing factor (CRF) is produced in the hypothalamic paraventricular nucleus (PVN) in response to stress and stimulates the release of adrenocorticotropic hormone in the corticotrophs. Estrogens acting centrally are able to modulate the stress responses. In fact, direct estrogenic regulation of CRF gene expression has been demonstrated in various tissues. However, the mechanisms responsible for the actions of estrogens on CRF regulation in the PVN remain undetermined. We investigated whether estradiol (E2) contributes to the regulation of CRF gene and promoter activity in hypothalamic 4B cells. Furthermore, the involvement of E2 in the regulation of interleukin (IL)-6 and its role in hypothalamic 4B cells was explored. We demonstrated the dominant expression of estrogen receptor type beta (ERbeta) and found that a physiologically relevant dose of E2 and an ERbeta agonist stimulated CRF gene transcription in hypothalamic 4B cells. E2 stimulated IL-6 transcriptional activity via ERbeta, and subsequently the levels of IL-6 mRNA and protein. We also found that treatment with IL-6 significantly reduced cell viability. Thus, these data suggest the important effects of E2 on the regulation of CRF gene and IL-6 production via ERbeta in hypothalamic 4B cells.

Negative regulation of corticotropin releasing factor expression and limitation of stress response

Corticotropin releasing factor (CRF) coordinates behavioral, autonomic and hormonal responses to stress. Activation of the hypothalamic pituitary adrenal (HPA) axis with stimulation of CRF and vasopressin (VP) release from hypothalamic parvocellular neurons, and consequent secretion of ACTH from the anterior pituitary and glucocorticoid from the adrenal cortex, is the major endocrine response to stress. Current evidence indicates that the main regulator of ACTH secretion in acute and chronic conditions is CRF, in spite of the fact that the selective increases in expression of parvocellular VP and pituitary VP V1b receptors observed during prolonged activation of the HPA axis have suggested that VP becomes the predominant regulator. Following CRF release, activation of CRF transcription is required to restore mRNA and peptide levels, but termination of the response is essential to prevent pathology associated with chronic elevation of CRF and glucocorticoid production. While glucocorticoid feedback plays an important role in regulating CRF expression, the relative importance of direct transcriptional repression of the CRF gene by glucocorticoids in the overall feedback mechanism is not clear. In addition to glucocorticoids, intracellular feedback mechanisms in the CRF neuron, involving induction of repressor forms of cAMP response element modulator (CREM) limit CRF transcriptional responses by competing with the positive regulator, phospho-CREB. Rapid repression of CRF transcription following stress-induced activation is likely to contribute to limiting the stress response and to preventing disorders associated with excessive CRF production.

Structural and functional conservation of vertebrate corticotropin-releasing factor genes: evidence for a critical role for a conserved cyclic AMP response element

Corticotropin-releasing factor (CRF) plays a central role in neuroendocrine, autonomic, immune, and behavioral responses to stressors. We analyzed the proximal promoters of two Xenopus laevis CRF genes and found them to be remarkably conserved with mammalian CRF genes. We found several conserved cis elements in the frog CRF genes including a cAMP response element (CRE), activator protein 1 binding sites, and glucocorticoid response elements. Exposure to a physical stressor caused a rapid elevation in phosphorylated CRE binding protein (CREB; 20 min) and CRF (1 h) in the anterior preoptic area of juvenile frogs. CREB bound to the putative frog CREs in vitro, which was disrupted by point mutations introduced into the CRE. The frog proximal CRF promoters supported basal transcription in transfection assays, and forskolin caused robust transcriptional activation. Mutagenesis of the CRE or overexpression of a dominant-negative CREB reduced forskolin-induced promoter activation. Using electroporation-mediated gene transfer in tadpole brain, we show that the proximal CRF promoters support cAMP or stressor-dependent transcription in vivo, which was abolished by mutation of the CRE. Using chromatin immunoprecipitation, we found that CREB associated with the proximal frog CRF promoter in vivo in a stressor-dependent manner. These data provide strong support for the hypothesis that stressor-induced CRF gene activation in vivo depends on CREB binding to the CRE in the promoter. Our findings show that the basic regulatory elements of the CRF gene responsible for stressor-induced activation arose early in vertebrate evolution and have been maintained by strong positive selection.

Transcriptional and Epigenetic Regulation of Interleukin-2 Gene in Activated T Cells by Morphine

Chronic morphine inhibits interleukin-2 (IL-2) at both the transcriptional and protein synthesis levels. The molecular mechanisms by which morphine decreases IL-2 are not fully understood. The production of IL-2 is tightly regulated by several transcription factors that bind to the IL-2 promoter. Herein, we show that chronic morphine treatment results in an increase in cAMP levels with a concurrent up-regulation of the cAMP inducible repressor inducible cAMP early repressor (ICER)/cAMP response element modulator (CREM) and down-regulation of p-cAMP-response element-binding protein (CREB) in activated T cells. Furthermore, ICER competes for p-CREB binding to the cAMP-responsive elements (CREs) site. This leads to the uncoupling of CBP/p300 thereby abrogating IL-2 transcription. Overexpression of either antisense CREM or CREB plasmid rescued morphine-induced inhibition of IL-2 promoter activity and protein production. In addition, we also found that chronic morphine treatment inhibited the acetylation and trimethylation of histones and decreased both DNA demethylation and accessibility of the IL-2 promoter. These findings suggest that chronic morphine treatment may function through both transcriptional and epigenetic mechanisms to inhibit IL-2 production.