Glucocorticoid regulation of corticotropin-releasing factor1 receptor expression in pituitary-derived AtT-20 cells

Stress response abnormalities transgenerationally inherited via miR-23 downregulation are restored by a methyl modulator during the lactation period

Low birthweight rats due to fetal undernutrition sustain higher corticosterone levels when exposed to stress. This is due to the upregulated expression of the pituitary-specific Gas5, a long noncoding RNA (lncRNA) that acts as a glucocorticoid receptor decoy and then competitively inhibiting the binding of glucocorticoids to DNA. However, the mechanism of Gas5 lncRNA upregulation remains unclear. Therefore, using the fetal undernourished model, we identified the factors that regulated Gas5 lncRNA expression and examined their effect on subsequent generations. We found that the expression levels of miR-23 was significantly lower in low birth-weight rats compared with controls. The expression of miR-23 was significantly lower and the expression levels of Gas5 lncRNA were significantly higher in the pituitary gland of low birth-weight offspring of the F2 and F3 generations compared with controls. The methyl modulator intervention in lactating F0 maternal rats restored miR-23 and Gas5 lncRNA expressions not only in F1, F2 and F3 offspring. Moreover, the intervention reduced circulating corticosterone levels and gene expressions in the pituitary gland after restraint stress exposure. In conclusion, miR-23-mediated alterations of the stress response are inherited and restored by methyl modulator intervention during lactation.

The vasotocinergic system and its role in the regulation of stress in birds

The regulation of stress in birds includes a complex interaction of neural systems affecting the hypothalamic-pituitary-adrenal (HPA) axis. In addition to the hypothalamic paraventricular nucleus, a structure called the nucleus of the hippocampal commissure likewise affects the output of pituitary stress hormones and appears to be unique to avian species. Within the anterior pituitary, the avian V1a and V1b receptors were found in corticotropes. Based on our studies with central administration of hormones in the chicken, corticotropic releasing hormone (CRH) is a more potent ACTH secretagogue than arginine vasotocin (AVT). In contrast, when applied peripherally, AVT is more efficacious. Co-administration of AVT and CRH peripherally, resulted in a synergistic stimulation of corticosterone release. Data suggest receptor oligomerization as one possible mechanism. In birds, vasotocin receptors associated with stress responses include the V1a and V1b receptors. Three-dimensional, homology-based structural models of the avian V1aR were built to test agonists and antagonists for each receptor that were screened by molecular docking to map their binding sites on each receptor. Additionally, binding affinity values for each available peptide antagonist to the V1aR and V1bR were determined. An anterior pituitary primary culture system was developed to determine how effective each antagonist blocked the function of each receptor in culture when stimulated by a combination of AVT/CRH administration. Use of an antagonist in subsequent in vivo studies identified the V1aR in regulating food intake in birds. The V1aR was likewise found in circumventricular organs of the brain, suggesting a possible function in stress.

Corticotropin releasing hormone can selectively stimulate glucose uptake in corticotropinoma via glucose transporter 1

BACKGROUND

Pre-operative detection of corticotropin (ACTH) secreting microadenomas causing Cushing's disease (CD) improves surgical outcomes. Current best magnetic resonance imaging fails to detect up to 40% of these microadenomas. 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography (PET) is specific, but not sensitive in detecting corticotropinomas. Theoretically, secretagogue stimulation with corticotropin releasing hormone (CRH) could improve detection of adenomas with 18F-FDG PET. Previous attempts with simultaneous CRH stimulation have failed to demonstrate increased 18F-FDG uptake in corticotropinomas. We hypothesized that CRH stimulation leads to a delayed elevation in glucose uptake in corticotropinomas.

METHODS

Clinical data was analyzed for efficacy of CRH in improving 18FDG-PET detection of corticotropinomas in CD. Glucose transporter 1 (GLUT1) immunoreactivity was performed on surgical specimens. Ex-vivo, viable cells from these tumors were tested for secretagogue effects (colorimetric glucose uptake), and for fate of intracellular glucose (glycolysis stress analysis). Validation of ex-vivo findings was performed with AtT-20 cells.

RESULTS

CRH increased glucose uptake in human-derived corticotroph tumor cells and AtT-20, but not in normal murine or human corticotrophs (p < 0.0001). Continuous and intermittent (1 h) CRH exposure increased glucose uptake in AtT-20 with maximal effect at 4 h (p = 0.001). Similarly, CRH and 8-Br-cAMP led to robust GLUT1 upregulation and increased membrane translocation at 2 h, while fasentin suppressed baseline (p < 0.0001) and CRH-mediated glucose uptake. Expectedly, intra-operatively collected corticotropinomas demonstrated GLUT1 overexpression. Lastly, human derived corticotroph tumor cells demonstrated increased glycolysis and low glucose oxidation.

CONCLUSION

Increased and delayed CRH-mediated glucose uptake differentially occurs in adenomatous corticotrophs. Delayed secretagogue-stimulated 18F-FDG PET could improve microadenoma detection.

ATAT1 is essential for regulation of homeostasis-retaining cellular responses in corticotrophs along hypothalamic-pituitary-adrenal axis

The production and secretion of adrenocorticotropin, a proopiomelanocortin (POMC)-derived hormone, by corticotrophs in the anterior pituitary, is regulated by corticotrophin-releasing hormone (CRH) and glucocorticoids. We have previously demonstrated that adrenalectomy induces α-tubulin N-acetyltransferase 1 (ATAT1) expression and α-tubulin acetylation in corticotrophs. However, the regulatory mechanism of ATAT1 expression and the function of acetylated microtubules in corticotrophs are unclear. Here, we analyze the effect of CRH or dexamethasone on Atat1 expression in the mouse corticotroph AtT20 cell line. The expression of Atat1 was increased by CRH and decreased by dexamethasone in AtT20 cells. We examined the effect of Atat1 knockdown on the expression of POMC-associated genes and the dexamethasone-induced nuclear translocation of glucocorticoid receptor (GR) by real-time polymerase chain reaction and Western blot analysis, respectively. Atat1 knockdown resulted in a significant increase in the expression of ACTH-producing genes and decreased the dexamethasone-induced nuclear translocation of GR accompanied with a reduction in α-tubulin acetylation. Atat1 overexpression resulted in a significant increase in α-tubulin acetylation and the dexamethasone-induced nuclear translocation of GR. These results suggest that the acetylated microtubules function as the rail-line for the transportation of GR into the nucleus. We conclude that ATAT1 finely tunes the cellular responses of corticotrophs to hormonal stimulation through an intracellular feedback circuit.

miR-449a contributes to glucocorticoid-induced CRF-R1 downregulation in the pituitary during stress

The hypothalamic-pituitary-adrenal axis is controlled by the feedback of glucocorticoids on the hypothalamus and pituitary. Stress increases CRF, ACTH, and glucocorticoid secretion. The expression of not only CRF mRNA in the hypothalamus and proopiomelanocortin mRNA in corticotrophs, but also CRF type 1 receptor (CRF-R1) mRNA and protein on corticotrophs are downregulated through glucocorticoids. However, the mechanisms underlying the glucocorticoid-induced CRF-R1 downregulation are not fully understood. Short RNA molecules, called microRNAs (miRNAs), are posttranscriptional regulators that usually induce translational repression or gene silencing via binding to complementary sequences within target mRNAs. We hypothesized that glucocorticoids may induce the expression of miRNAs in the pituitary, which are involved in glucocorticoid-induced downregulation of CRF-R1. We found 3 miRNAs with sequences predicted to bind to the CRF-R1 3' untranslated region (3'-UTR) by database search. Expression of 1 of these miRNAs (miR-449a) was significantly higher in the anterior pituitary of restrained rats than in that of unrestrained control rats. Expression of miR-449a was evident in many anterior pituitary cells, including corticotrophs. Although overexpression of miR-449a decreased CRF-R1 mRNA and CRF-R1 protein expression, knockdown of miR-449a attenuated dexamethasone-induced suppression of CRF-R1 mRNA and CRF-R1 protein expression in the monolayer-cultured pituitary cells. Notably, luciferase activity was significantly lower in cells cotransfected with a luciferase vector containing the CRF-R1 3'-UTR and a miR-449a vector. miR-449a expression was significantly increased by dexamethasone. Adrenalectomy attenuated restraint-induced increase in miR-449a expression in the pituitary. These results indicated that miR-449a plays an important role in stress-induced, glucocorticoid-mediated downregulation of CRF-R1 expression.

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.

Severe stress switches CRF action in the nucleus accumbens from appetitive to aversive

Stressors motivate an array of adaptive responses ranging from “fight or flight” to an internal urgency signal facilitating long-term goals¹. However, traumatic or chronic uncontrollable stress promotes the onset of Major Depressive Disorder where acute stressors lose their motivational properties and are perceived as insurmountable impediments². Consequently, stress-induced depression is a debilitating human condition characterized by an affective shift from engagement of the environment to withdrawal³. An emerging neurobiological substrate of depression and associated pathology is the nucleus accumbens, a region with the capacity to mediate a diverse range of stress responses by interfacing limbic, cognitive and motor circuitry⁴. Here we report that corticotropin releasing factor (CRF), a neuropeptide released in response to acute stressors⁵ and other arousing environmental stimuli⁶, acts in the nucleus accumbens of naïve mice to increase dopamine release through co-activation of CRF R1 and R2 receptors. Remarkably, severe stress exposure completely abolished this effect without recovery for at least 90 days. This loss of CRF’s capacity to regulate dopamine release in the nucleus accumbens is accompanied by a switch in the reaction to CRF from appetitive to aversive, indicating a diametric change in the emotional response to acute stressors. Thus, the current findings offer a biological substrate for the switch in affect which is central to stress-induced depressive disorders.

Role and action in the pituitary corticotroph of corticotropin-releasing factor (CRF) in the hypothalamus

Corticotropin-releasing factor (CRF), produced in the hypothalamic paraventricular nucleus (PVN) in response to stress, stimulates the synthesis and secretion of adrenocorticotropin (ACTH) via CRF receptor type 1 (CRF(1) receptor) in the anterior pituitary (AP) of mammals. CRF is critical for the circadian rhythmicity of the hypothalamic-pituitary-adrenal axis and the augmented release of ACTH from the pituitary in response to the stress. A higher molecular weight form of immunoreactive beta-endorphin, putative proopiomelanocortin (POMC), is increased in CRF-knockout mice (CRF KO), suggesting the important role of CRF in the processing of POMC. In fact, CRF is able to modulate the processing of POMC through changes in prohormone convertase (PC)-1 expression levels. Multiple forms of ACTH-related peptides containing unprocessed ones are present in some cases of ACTH-producing tumors, presumably without action of PC-1 under the control of CRF. Following CRF-activated stimulation of the receptor signaling, CRF(1) receptor is down-regulated and desensitized. In fact, CRF facilitates the degradation of CRF(1) receptor mRNA via the protein kinase A pathway. Prolonged agonist activation of CRF(1) receptor leads to a loss of responsiveness, or desensitization of the receptor. G protein-coupled receptor kinase 2 is involved in desensitization of CRF(1) receptor by CRF in the corticotroph.

Selective amplification of glucocorticoid anti-inflammatory activity through synergistic multi-target action of a combination drug

Introduction

Glucocorticoids are a mainstay of anti-inflammatory therapy, but significant adverse effects ultimately limit their utility. Previous efforts to design glucocorticoid structures with an increased therapeutic window have focused on dissociating anti-inflammatory transcriptional repression from adverse effects primarily driven by transcriptional activation. An alternative to this medicinal chemistry approach is a systems biology based strategy that seeks to amplify selectively the anti-inflammatory activity of very low dose glucocorticoid in immune cells without modulating alternative cellular networks that mediate glucocorticoid toxicity.

Methods

The combination of prednisolone and the antithrombotic drug dipyridamole was profiled using in vitro and in vivo models of anti-inflammatory activity and glucocorticoid-induced adverse effects to demonstrate a dissociated activity profile.

Results

The combination synergistically suppresses release of proinflammatory mediators, including tumour necrosis factor-α, IL-6, chemokine (C-C motif) ligand 5 (RANTES), matrix metalloproteinase-9, and others, from human peripheral blood mononuclear cells and mouse macrophages. In rat models of acute lipopolysaccharide-induced endotoxemia and delayed-type hypersensitivity, and in chronic models of collagen-induced and adjuvant-induced arthritis, the combination produced anti-inflammatory activity that required only a subtherapeutic dose of prednisolone. The immune-specific amplification of prednisolone anti-inflammatory activity by dipyridamole did not extend to glucocorticoid-mediated adverse effects, including corticosterone suppression or increased expression of tyrosine aminotransferase, in vivo after repeat dosing in rats. After 8 weeks of oral dosing in mice, treatment with the combination did not alter prednisolone-induced reduction in osteocalcin and mid-femur bone density, which are markers of steroid-induced osteoporosis. Additionally, amplification was not observed in the cellular network of corticotroph AtT-20/D16v-F2 cells in vitro, as measured by pro-opiomelanocortin expression and adrenocorticotropic hormone secretion.

Conclusions

These data suggest that the multi-target mechanism of low-dose prednisolone and dipyridamole creates a dissociated activity profile with an increased therapeutic window through cellular network selective amplification of glucocorticoid-mediated anti-inflammatory signaling.

Expression, binding, and signaling properties of CRF2(a) receptors endogenously expressed in human retinoblastoma Y79 cells: passage-dependent regulation of functional receptors

Endogenous expression of the corticotropin-releasing factor type 2a receptor [CRF2(a)] but not CRF2(b) and CRF2(c) was observed in higher passage cultures of human Y79 retinoblastoma cells. Functional studies further demonstrated an increase in CRF2(a) mRNA and protein levels with higher passage numbers (> 20 passages). Although the CRF1 receptor was expressed at higher levels than the CRF2(a) receptor, both receptors were easily distinguishable from one another by selective receptor ligands. CRF(1)-preferring or non-selective agonists such as CRF, urocortin 1 (UCN1), and sauvagine stimulated cAMP production in Y79 to maximal responses of approximately 100 pmoles/10(5) cells, whereas the exclusive CRF2 receptor-selective agonists UCN2 and 3 stimulated cAMP production to maximal responses of approximately 25-30 pmoles/10(5) cells. UCN2 and 3-mediated cAMP stimulation was potently blocked by the approximately 300-fold selective CRF2 antagonist antisauvagine (IC50 = 6.5 +/- 1.6 nmol/L), whereas the CRF(1)-selective antagonist NBI27914 only blocked cAMP responses at concentrations > 10 microL. When the CRF(1)-preferring agonist ovine CRF was used to activate cAMP signaling, NBI27914 (IC50 = 38.4 +/- 3.6 nmol/L) was a more potent inhibitor than antisauvagine (IC50 = 2.04 +/- 0.2 microL). Finally, UCN2 and 3 treatment potently and rapidly desensitized the CRF2 receptor responses in Y79 cells. These data demonstrate that Y79 cells express functional CRF1 and CRF2a receptors and that the CRF2(a) receptor protein is up-regulated during prolonged culture.

Corticotropin releasing factor (CRF) receptor signaling in the central nervous system: new molecular targets

Corticotropin-releasing factor (CRF) and the related urocortin peptides mediate behavioral, cognitive, autonomic, neuroendocrine and immunologic responses to aversive stimuli by activating CRF(1) or CRF(2) receptors in the central nervous system and anterior pituitary. Markers of hyperactive central CRF systems, including CRF hypersecretion and abnormal hypothalamic-pituitary-adrenal axis functioning, have been identified in subpopulations of patients with anxiety, stress and depressive disorders. Because CRF receptors are rapidly desensitized in the presence of high agonist concentrations, CRF hypersecretion alone may be insufficient to account for the enhanced CRF neurotransmission observed in these patients. Concomitant dysregulation of mechanisms stringently controlling magnitude and duration of CRF receptor signaling also may contribute to this phenomenon. While it is well established that the CRF(1) receptor mediates many anxiety- and depression-like behaviors as well as HPA axis stress responses, CRF(2) receptor functions are not well understood at present. One hypothesis holds that CRF(1) receptor activation initiates fear and anxiety-like responses, while CRF(2) receptor activation re-establishes homeostasis by counteracting the aversive effects of CRF(1) receptor signaling. An alternative hypothesis posits that CRF(1) and CRF(2) receptors contribute to opposite defensive modes, with CRF(1) receptors mediating active defensive responses triggered by escapable stressors, and CRF(2) receptors mediating anxiety- and depression-like responses induced by inescapable, uncontrollable stressors. CRF(1) receptor antagonists are being developed as novel treatments for affective and stress disorders. If it is confirmed that the CRF(2) receptor contributes importantly to anxiety and depression, the development of small molecule CRF(2) receptor antagonists would be therapeutically useful.

Differential regulation of corticotropin-releasing factor receptor type 1 (CRF1 receptor) mRNA via protein kinase A and mitogen-activated protein kinase pathways in rat anterior pituitary cells

Corticotropin-releasing factor (CRF) receptor type 1 (CRF(1) receptor) mRNA levels are down-regulated by CRF via the cyclic AMP-protein kinase A (PKA) pathway. In this study, we focused on the involvement of both the mitogen-activated protein (MAP) kinase pathway and PKA in this regulation. Real-time PCR (RT-PCR) revealed that a MAP kinase, extracellular signal-regulated kinases 1/2, pathway was also involved in the down-regulation of CRF(1) receptor mRNA levels by CRF in the rat anterior pituitary (AP). Down-regulation of CRF(1) receptor mRNA levels was caused by a post-transcriptional system such as mRNA degradation, as incubation with CRF significantly decreased the half-life of CRF(1) receptor mRNA. Furthermore, pre-treatment with a PKA inhibitor completely blocked CRF-induced CRF(1) receptor mRNA destabilization, while pre-treatment with an extracellular signal-regulated kinases 1/2 inhibitor had no inhibitory effect. These results suggested that in the rat AP, down-regulation of CRF(1) receptor mRNA levels is caused by mRNA degradation via PKA, but not by the MAP kinase pathway.

Glucocorticoid regulation of peptide genes in neuroendocrine CRH neurons: a complexity beyond negative feedback

This review will examine our current knowledge of a fundamental property of CRH neuroendocrine neurons: how the major endpoint of the HPA axis--adrenal glucocorticoids--interacts with the mechanisms controlling the expression of the genes that encode ACTH secretogogues. A great deal of work over the past 25 years has led to the notion that this question has an ostensibly simple answer: glucocorticoids inhibit peptide gene expression using "negative feedback" at the CRH neuron and elsewhere. However, closely examining how glucocorticoids act in different physiological circumstances reveals a much more complex set of answers, particularly if we consider how the processes that control peptide synthesis and release are coupled. Out of this examination emerges a more flexible and complex framework for examining the integrative mechanisms controlling the CRH neuron. Although we will mostly focus on the Crh gene, relevant aspects of the vasopressin (Avp) and pro-enkephalin (pEnk) gene regulatory mechanisms will also be discussed.

Chronic cortisol suppresses pituitary and hypothalamic peptide message expression in pigtailed macaques

The effects of chronic elevations in circulating glucocorticoids on the expression of peptides and peptide receptors of the hypothalamic-pituitary-adrenal (HPA) axis have been studied extensively in rodents, but they have not been examined in primates. To determine the responses of the HPA axis in primates to elevated cortisol, hypothalamic and pituitary tissue from normal older pigtailed macaques (Macaca nemestrina) that had received daily oral administration of cortisol or placebo for 1 year were studied. Pro-opiomelanocortin in the anterior pituitary and corticotropin-releasing factor (CRF) mRNA expression in the hypothalamic paraventricular nucleus (PVN) were significantly reduced in cortisol-treated monkeys in comparison with controls. CRF receptor 1 (CRF-R1) expression in the anterior pituitary and arginine vasopressin mRNA expression in the PVN were unchanged by chronic cortisol administration. Sustained elevation of circulating glucocorticoids results in suppression of HPA peptide and peptide receptor expression in the PVN and anterior pituitary similar to those found in rodents. Chronic therapeutic administration of glucocorticoids in humans may have unintended consequences for hypothalamic and pituitary function.

Corticotropin releasing hormone receptors: two decades later

Hypothalamic corticotropin releasing hormone (CRH) regulates pituitary ACTH secretion and mediates behavioral and autonomic responses to stress, through interaction with type 1 plasma membrane receptors (CRHR1) located in pituitary corticotrophs and the brain. Although the CHRI are essential for ACTH responses to stress, their number in the pituitary gland does not correlate with corticotroph responsiveness, suggesting that activation of a small number of receptors is sufficient for maximum ACTH production. CRH binding and hybridization studies in adrenalectomized, glucocorticoid-treated or stressed rats revealed divergent changes in CRH receptors and CRH1 mRNA in the pituitary, with a reduction in receptor binding but normal or elevated expression of CHR1 mRNA levels. Western blot analysis of CRHR1 protein in pituitary membranes from adrenalectomized rats showed unchanged receptor mRNA levels and increased CRHR1 protein, despite binding down-regulation, suggesting that decreased binding is due to homologous desensitization, rather than reduced receptor synthesis. In contrast, decreased CRH binding following glucocorticoid administration is associated with a reduction in CRHR1 protein, suggesting inhibition of CRH1 mRNA translation. The regulation of CRHR1 translation may involve binding of cytosolic proteins, and a minicistron in the 5'-UTR of the CRHR1 mRNA. It is likely that post-transcriptional regulatory mechanisms that permit rapid changes in CRH receptor activity are important for adaptation of corticotroph responsiveness to continuous changes in physiological demands.

Pharmacological characterisation of native somatostatin receptors in AtT-20 mouse tumour corticotrophs

1. The mouse corticotroph tumour cell line AtT-20 is a useful model to investigate the physiological role of native somatostatin (SRIF, Somatotropin release inhibitory factor) receptor subtypes (sst(1) - sst(5)). The objective of this study was to characterise the pharmacological features and the functional effects of SRIF receptors expressed by AtT-20 cells using radioligand binding and cAMP accumulation. 2. [(125)I]LTT-SRIF-28, [(125)I]CGP 23996, [(125)I]Tyr(10)-cortistatin-14 and [(125)I]Tyr(3)-octreotide labelled SRIF receptor binding sites with high affinity and in a saturable manner (B(max)=315, 274, 239 and 206 fmol mg(-1), respectively). [(125)I]LTT-SRIF-28 labels significantly more sites than [(125)I]Tyr(10) -cortistatin-14 and [(125)I]Tyr(3) -octreotide as seen previously in cells expressing pure populations of sst(2) or sst(5) receptors. 3. SRIF analogues displaced the binding of the four radioligands. sst(2/5) receptor-selective ligands showed much higher affinity than sst(1/3/4) receptor-selective ligands. The binding profile of [(125)I]Tyr(3)-octreotide was different from that of [(125)I]LTT-SRIF-28, [(125)I]CGP 23996 and [(125)I]Tyr(10)-cortistatin-14. The sst(5/1) receptor-selective ligand L-817,818 identified two binding sites, one with subnanomolar affinity (sst(5) receptors) and one with micromolar affinity (sst(2) receptors); however, the proportions were different: 70 - 80% high affinity with [(125)I]LTT-SRIF-28, [(125)I]CGP 23996, [(125)I]Tyr(10)-cortistatin-14, but only 20% with [(125)I]Tyr(3)-octreotide. 4. SRIF analogues inhibited the forskolin-stimulated cAMP levels depending on concentration. sst(2/5) receptor-selective ligands were highly potent, whereas sst(1/3/4) receptor-selective ligands had no significant effects. The sst(2) receptor antagonist D-Tyr(8)-CYN 154806 competitively antagonised the effects of SRIF-14 and sst(2) receptor-preferring agonists, but not those of L-817,818. 5. The complex binding properties of SRIF receptor analogues indicate that sst(2) and sst(5) receptors are the predominant SRIF receptors expressed on AtT-20 cell membranes with no or only negligible presence of sst(1), sst(3) and sst(4) receptors. In the functional studies using cAMP accumulation, only sst(2) and sst(5) receptors appear to play a role. However, the "predominant" receptor appears to be the sst(2) receptor, although sst(5) receptors can also mediate the effect, when the ligand is not able to activate sst(2) receptors. This clearly adds flexibility to SRIF-mediated functional effects and suggests that the physiological role of SRIF and its analogues may be mediated preferentially via one subtype over another.

Transcriptional regulation of the GnRH receptor gene by glucocorticoids

Expression of the gonadotropin-releasing hormone (GnRH) receptor gene is stimulated by dexamethasone in GnRH-deficient rodents. In this study we identify a 1226 bp sequence at the 5'-flanking region of the mouse GnRH-R gene that confers dexamethasone responsiveness when expressed in host cells. Further, a glucocorticoid antagonist blocks transcriptional activity of the mGnRHR promoter. Progressive 5'-deletion of the mGnRHR promoter localized the response sequence between the -331/-255 region. Analysis of this region revealed binding sites for the AP-1 transcription factor. Mutation in AP-1 modified the functional activity of the mGnRHR promoter following GnRH agonist or dexamethasone-stimulation. Using an electrophoretic mobility shift assay, a protein complex is shown to bind to the AP-1 site. These results suggest that AP-1 proteins and glucocorticoid receptor regulate transcription of the GnRH-R promoter in a heterologous system.

Multiple sites of control of type-1 corticotropin releasing hormone receptor levels in the pituitary

Hypothalamic corticotropin releasing hormone (CRH) stimulates pituitary ACTH secretion through interaction with type 1 CRH receptors (CRH-R1), the number of which varies during alterations of the hypothalamic-pituitary-adrenal (HPA) axis. CRH-R1 are essential for ACTH responses to stress but CRH receptor content in the pituitary does not correlate with corticotroph responsiveness. This indicates that a small number of receptors is sufficient for full ACTH responses probably through post-receptor interaction with vasopressin (VP) signaling. CRH binding and hybridization studies in adrenalectomized, glucocorticoid-treated or stressed rats revealed divergent levels of CRH receptors and CRH-R1 mRNA in the pituitary, with binding reductions but normal or elevated CRH-R1 mRNA levels during alterations of the HPA axis. Western blot analysis of CRH-R1 protein in pituitary membranes from adrenalectomized rats show unchanged CRH-R1 mRNA levels, but reduced CRH binding associated with significant increases in CRH-R1 protein, suggesting that the decrease in binding is due to homologous desensitization and not to reduced receptor synthesis. In contrast, decreased CRH binding following glucocorticoid administration is associated with reduction in CRH-R1 protein suggesting inhibition of CRH-R1 mRNA translation. Regulation of CRH-R1 translation may involve binding of cytosolic proteins, and a minicistron in the 5'UTR of the CRH-R1 mRNA. Post-transcriptional regulatory mechanisms allowing rapid changes in CRH receptor activity are important for adaptation of corticotroph responsiveness to continuous change in physiological demand.

CRH inhibits cell growth of human endometrial adenocarcinoma cells via CRH-receptor 1-mediated activation of cAMP-PKA pathway

CRH produced by human endometrial cells exerts decidualizing activity via an autocrine mechanism mediated via CRH-R1 receptors. We postulated that such activity exerted by CRH on normal endometrial cells might translate into an antiproliferative action on endometrial-derived malignancies, provided that neoplastic cells maintain the expression of CRH receptors. In this light, here we investigated the possible antiproliferative effects of CRH in an adenocarcinoma cell line derived from human endometrium. CRH induces time- and concentration-dependent inhibition of Ishikawa cell growth, the maximal effect (50% inhibition) being achieved after 3 d of treatment with 10(-7) M CRH. A decrease in telomerase activity, which paralleled tumor growth inhibition, was also observed in CRH-treated samples. The antiproliferative effect was confirmed by colony-formation assay for long-term survival. This effect was counteracted in a concentration-dependent manner by both alpha-helical CRH and astressin; the former also showed intrinsic inhibitory activity. These findings suggested the involvement of CRH-R1 receptor subtype; this hypothesis was confirmed by RNase protection analysis showing the expression of human CRH-R1 mRNA. Experiments with the PKA inhibitor 14-22 amide and forskolin, as well as the measurement of intracellular cAMP, suggested the downstream involvement of cAMP-PKA pathway in CRH-induced inhibition of Ishikawa cell growth.

Regulation of corticotropin-releasing hormone (CRH) transcription and CRH mRNA stability by glucocorticoids

1. The increases in corticotropin-releasing hormone (CRH) mRNA following long-term adrenalectomy are associated with low levels of CRH gene transcription, suggesting that glucocorticoids regulate CRH mRNA at the posttranscriptional level. In this study we determined the time course of transcriptional activation after early adrenalectomy by intronic in situ hybridization, and evaluated the effects of glucocorticoids on CRH mRNA stability. 2. Plasma corticosterone was undetectable 3 h after adrenalectomy, but CRH hnRNA increased only by 12 h, and remained elevated for the next 72 h. CRH mRNA increased 18 h after adrenalectomy and reached a plateau lasting from 2 to 6 days, despite very low CRH hnRNA levels. 3. Assessment of CRH mRNA stability, by incubation of slide-mounted hypothalamic sections in an intracellular-like medium at 37 degrees C, prior to measuring CRH mRNA levels by in situ hybridization, revealed a half-life (t1/2) of 11.5 min in sham-operated rats, and a slower decrease adrenalectomized rats (t1/2--26.3 min). Corticosterone administration for 3 days markedly decreased CRH mRNA t1/2 in both sham-operated and adrenalectomized rats (6.5 and 5.0 min, respectively). 4. The data show that adrenalectomy causes transient increases in CRH mRNA transcription, followed by decreases in the rate of CRH mRNA degradation. This suggests that glucocorticoids regulate CRH mRNA at two sites, by inhibiting transcription and by decreasing mRNA stability.

Persistent corticotropin-releasing factor(1) receptor desensitization and downregulation in the human neuroblastoma cell line IMR-32

Brain corticotropin-releasing factor (CRF) systems integrate various responses to stress. Pathological responses to stress may result from errors in CRF receptor regulation in response to changes in synaptic CRF levels. To establish an in vitro model to study brain CRF receptors, we characterized the CRF-induced modulation of CRF(1) receptors in the human neuroblastoma cell line, IMR-32. Treatment with CRF decreased CRF(1) receptor binding and desensitized CRF-induced increases in cAMP. The decrease in binding had an EC(50) of approximately 10 nM, was maximal by 30 min, and was blocked by the CRF receptor antagonist [D-Phe(12), Nle(21,38), C(alpha)-MeLeu(37)]CRF(12-41). The desensitization was homologous as vasoactive intestinal polypeptide-induced increases in cAMP were unchanged, and elevation of cAMP did not alter CRF(1) receptor binding. Treatment with CRF for up to 24 h did not alter CRF(1) receptor mRNA levels, suggesting that a posttranscriptional mechanism maintains the decrease in receptor binding. Interestingly, recovery of CRF receptor binding and CRF-stimulated cAMP production was only partial following exposure to 100 nM CRF. In contrast, receptor binding recovered to control levels following exposure to 10 nM CRF. These data suggest that exposure to high doses of CRF result in permanent changes characterized by only partial recovery. Identifying the mechanisms underlying this partial recovery may provide insights into mechanisms underlying the acute and chronic effects of stress on CRF receptor regulation.

Postnatal maternal deprivation produces long-lasting modifications of the stress response, feeding and stress-related behaviour in the rat

The hypothalamo-pituitary-adrenal (HPA) axis plays a central role both in the regulation of the stress response, and in the control of feeding behaviour. Sensitivity of the HPA axis to respond to stress varies both during ontogeny and between individuals, and can be altered by neonatal events. The aim of our experiments was to determine whether early events that affect the HPA axis could also induce persistent modifications in food intake (quantitatively and qualitatively), as well as alterations of anxiety-related behaviour. Twenty-four-hour maternal deprivation was introduced at two different periods of HPA maturation, on day 5 (DEP5) or day 14 (DEP14) after birth. Sequential measurements of plasma levels of adrenocorticotropin hormone (ACTH) and corticosterone showed that this deprivation altered the HPA axis of adults; the response to restraint stress was prolonged in DEP5 and a higher ACTH peak appeared in DEP14. The neonatal stress also produced long-lasting modifications of rat behaviour, as DEP14 adults became more anxious. Standard food intake decreased in both groups of deprived rats. Diet preferences also changed, as carbohydrate intake decreased in DEP5 rats. Corticosteroid receptor binding did not vary in the hippocampus of the deprived rats. The modifications of the stress response and the behaviour parameters could be due to the alteration of corticosteroid receptors in the hypothalamic paraventricular nucleus and/or corticotropin-releasing hormone or vasopressin function, but these parameters have yet to be determined. This early stress paradigm altering feeding behaviour could become an interesting model for research into human eating disorders.

Regulation of pituitary corticotropin releasing hormone receptors

Corticotropin releasing hormone (CRH) stimulates pituitary ACTH secretion through type-1 CRH (CRH1) receptors. Stimulation of the hypothalamic pituitary adrenal (HPA) axis as well as increased corticotroph responsiveness during stress and adrenalectomy are associated with marked pituitary CRH binding downregulation. The presence of CRH1 receptors in the pituitary are essential to maintain ACTH secretion. Downregulation of CRH binding is associated with normal or elevated levels of CRH1 receptor mRNA and this may contribute to the maintainence of permissive levels of CRH1 receptors in the pituitary. Injection of either CRH or glucocorticoids in rats in vivo induces CRH binding and CRH1 receptor mRNA downregulation, whereas their simultaneous administration causes only transient CRH1 receptor mRNA loss. Vasopressin increases CRH1 receptor mRNA levels. This suggest that interactions between CRH, vasopressin and glucocorticoids accounts for CRH1 receptor mRNA upregulation during stress. The lack of correlation between CRH binding and CRH1 receptor mRNA indicates that the major sites for pituitary CRH1 receptor regulation are at the post-transcriptional level.

Corticosterone modulation of ACTH secretogogue gene expression in the paraventricular nucleus

This review will describe effects of corticosterone on the temporal dynamics of components within the hypothalamo-pituitary-adrenal (HPA) axis in response to sustained hypovolemia. The characterization of the synthetic and secretory profiles of HPA elements in these rat models reveals the complexities of steroid-mediated regulation of neuroendocrine and corticotrope function during a sustained stress event. Collectively, our data suggest activation of gene transcription and secretion are independently controlled, and that corticosterone affects adrenocorticotropin hormone (ACTH) gene expression in the parvicellular neuroendocrine part of the hypothalamic paraventricular nucleus using two mechanisms: first, an inhibition which contributes to classic negative feedback, and second, a facilitation, which is seen at low plasma concentrations.

Effect of stress-like concentrations of cortisol on the feedback potency of oestradiol in orchidectomized sheep

The effect of stress-like concentrations of cortisol on oestradiol-induced change in LH secretion and GnRH receptor expression was evaluated in orchidectomized sheep (wethers). Twenty-four wethers were assigned at random to one of the four treatment groups in a 2x2 factorial design (n=6 wethers/group). Wethers received cortisol (90 microg/kg/h; groups 2 and 4) or a comparable volume of cortisol delivery vehicle (groups 1 and 3) by continuous infusion for 48 h. During the final 24 h of infusion, wethers received oestradiol (6 ng/kg/h; groups 3 and 4) or oestradiol delivery vehicle (groups 1 and 2). The pattern of LH secretion was assessed during a 3-h period of intensive blood collection beginning 21 h after initiation of oestradiol infusion. Although neither cortisol nor oestradiol alone affected (P>0.05) mean serum concentration of LH or LH pulse frequency, serum LH and the frequency of secretory episodes of LH were significantly reduced (P<0.05) in wethers receiving cortisol and oestradiol in combination. Anterior pituitary tissue was collected at the end of the infusion period. Oestradiol increased (P<0.05) tissue concentrations of GnRH receptor and GnRH receptor mRNA. Although cortisol alone did not affect (P>0.05) basal concentrations of receptor or receptor mRNA, the magnitude of oestradiol-induced increase in GnRH receptor and GnRH receptor mRNA was significantly reduced in wethers receiving cortisol and oestradiol concurrently. Conversely, steady-state concentrations of mRNA encoding the LHbeta and FSHbeta subunits were increased (P<0.05) in wethers receiving cortisol. These observations demonstrate that stress-like concentrations of cortisol act in concert with oestradiol to suppress LH secretion. In addition, cortisol blocks oestradiol-dependent increase in pituitary tissue concentrations of GnRH receptor and GnRH receptor mRNA.

Isolation and characterization of the promoter region of the rat vasopressin V1b receptor gene

Regulation of pituitary vasopressin V1b receptors plays a critical role in regulating pituitary adrenocorticotropic hormone (ACTH) secretion during adaptation to stress. The objective of this study was to isolate the promoter regulatory region of the V1b receptor gene to better understand the molecular mechanisms involved in V1b receptor regulation. Screening of a rat genomic library using probes directed to the coding region and to the 5'UTR of the rat V1b receptor resulted in the isolation of several clones containing the 5'upstream regions of the V1b receptor cDNA. Sequencing of an 11.2 Kb fragment revealed 8.2 Kb upsteam of the reported cDNA sequence, which contains a putative promoter regulatory region. The 3' end of the clone contained 1472 base pairs corresponding to the recognized cDNA sequence, followed by 1506 bp of unknown sequence located at the end of the sixth transmembrane domain, probably corresponding to an intron, characteristic of these family of receptors. An additional 161 bp intron was found in the 5'UTR, similar to that described in the rat oxytocin receptor gene. 5'RACE and RNase protection analysis mapped two major putative transcription start points at -830 and -861 bp from the starting methionine. Analysis of the putative promoter region showed no indication of a proximal TATA box, but the presence of a CACA box, a GAGA box, several AP-1 and AP-2 sites and a cluster of Sp1 sites upstream of the AP-2 sites. A luciferase construct containing a 2.1-kb of putative promoter, and part of the 5'UTR including the first intron, showed promoter activity when transfected into COS-7, CHO and PC12 cell lines but not in AtT-20 cells. A similar construct without the intron and distal 5'UTR sequence has no promoter activity in the same cell lines. In summary, the V1b receptor gene contains at least 3 exons and 2 introns. The 5'flanking sequence contains several potential sites for transcriptional regulation, and induced luciferace activity only in constructs containing intron 1, suggesting that the latter is important for receptor gene activation. The data provide bases for future analysis of the regulatory elements controlling V1b receptor transcription.

Region-specific regulation of RGS4 (Regulator of G-protein-signaling protein type 4) in brain by stress and glucocorticoids: in vivo and in vitro studies

The present study demonstrates that the regulator of G-protein-signaling protein type 4 (RGS4) is differentially regulated in the locus coeruleus (LC) and the paraventricular nucleus (PVN) of the hypothalamus by chronic stress and glucocorticoid treatments. Acute or chronic administration of corticosterone to adult rats decreased RGS4 mRNA levels in the PVN but increased these levels in the LC. Similarly, chronic unpredictable stress decreased RGS4 mRNA levels in the PVN but had a strong trend to increase these levels in the LC. Chronic stress also decreased RGS4 mRNA levels in the pituitary. The molecular mechanisms of RGS4 mRNA regulation were further investigated in vitro in the LC-like CATH.a cell line and the neuroendocrine AtT20 cell line using the synthetic corticosterone analog dexamethasone. Consistent with the findings in vivo, dexamethasone treatment caused a dose- and time-dependent decrease in RGS4 mRNA levels in AtT20 cells but a dose- and time-dependent increase in CATH.a cells. RGS4 mRNA regulation seen in these two cell lines seems to be attributable, at least in part, to opposite changes in mRNA stability. The differential regulation of RGS4 expression in the LC and in key relays of the hypothalamic-pituitary-adrenal axis could contribute to the brain's region-specific and long-term adaptations to stress.

Effect of stress-like concentrations of cortisol on estradiol-dependent expression of gonadotropin-releasing hormone receptor in orchidectomized sheep

The effect of stress-like concentrations of cortisol (C) on estrogen-dependent expression of GnRH receptor was evaluated using orchidectomized sheep (wethers; n = 6 animals per group). C (5.0 mg/50 kg per hour; groups 1-4) or a comparable volume of vehicle (groups 5-8) was delivered by continuous infusion for 48 h. During the final 24 h of infusion, animals received concurrent infusion of estradiol (E2) at rates of 0 (groups 1 and 5), 0.5 (groups 2 and 6), 2.0 (groups 3 and 7), or 8.0 (groups 4 and 8) microg/50 kg per hour. Pituitary tissue was collected at the end of infusion. Although C did not affect (p > 0.05) the basal concentration of GnRH receptor or GnRH receptor mRNA, it reduced (p < 0.05) the increase in receptor and receptor mRNA induced by concurrent administration of 0. 5 microg E2/50 kg per hour. In contrast, the increase in GnRH receptor expression induced by higher levels of estrogen stimulation was not affected (p > 0.05) by concurrent administration of C. The effect of C on the temporal pattern of E2-dependent increase in GnRH receptor expression was assessed using wethers receiving E2 (0.5 microg/50 kg per hour) by continuous infusion for 0 (groups 1 and 5), 24 (groups 2 and 6), 48 (groups 3 and 7), or 72 h (groups 4 and 8). Animals received C (5.0 mg/50 kg per hour; groups 1-4) or vehicle (groups 5-8) beginning 24 h before, and continuing throughout, the E2 delivery period. Stress-like concentrations of C reduced (p < 0. 05) the increase in GnRH receptor and receptor mRNA induced after 24 h of E2 stimulation. However, the suppressive effect of C was transient, and tissue levels of GnRH receptor and receptor mRNA were comparable after 72 h of E2 infusion in animals receiving C or vehicle alone. Collectively these observations demonstrate that C suppresses estrogen-dependent increase in tissue concentrations of GnRH receptor and receptor mRNA. However, this effect of C is transient and not evident in animals receiving moderate to high levels of estrogen stimulation. This transient suppression of GnRH receptor expression may account, at least in part, for the anti-gonadal effect of glucocorticoids.

Corticotropin releasing hormone, receptor regulation and the stress response

Corticotropin releasing hormone (CRH) coordinates behavioral, autonomic and hormonal responses to stress, including activation of the hypothalamic-pituitary-adrenal (HPA) axis with stimulation of adrenocorticotropin (ACTH) and glucocorticoids. Differential changes of expression of CRH and vasopressin(VP) in the parvicellular hypothalamic paraventricular nucleus (PVN), as well as regulation of CRH and VP receptors, are critical for the responsiveness of the HPA axis during stress. Pituitary CRH receptor (CRH-R)expression and content is controlled by the coordinated action of CRH, VP and glucocorticoids. Marked changes in hypothalamic and pituitary CRH-R expression support a key regulatory role for CRH in the HPA axis and the integrated stress response.

Interaction between glucocorticoids and corticotropin releasing hormone (CRH) in the regulation of the pituitary CRH receptor in vivo in the rat

Acute stress causes biphasic changes in corticotropin releasing hormone (CRH) receptor mRNA expression with an early decrease followed by an increase. However, in the absence of glucocorticoids in adrenalectomized rats, stress results in prolonged CRH receptor (CRH-R) mRNA loss, suggesting that interactions between glucocorticoids and hypothalamic factors are critical for regulation of CRH receptor mRNA. To address this question, CRH binding, type-1 CRH-R mRNA, POMC mRNA and POMC hnRNA expression were measured by binding autoradiography and in situ hybridization in pituitaries from intact and adrenalectomized rats. CRH-R mRNA decreased by 59% 5 h after injection of corticosterone (10 mg s.c.) and returned to basal levels by 18 h, a time when plasma corticosterone concentrations were still elevated, and CRH binding and POMC hnRNA were significantly reduced. Elevations in plasma corticosterone in the range of acute stress by injection of 2 mg s.c. caused CRH-R mRNA expression to return to near basal values by 6 h, after a 52% and 39% decrease at 2 h and 4 h. More transient changes were seen after a single injection of CRH (1 microg), with a 44% decrease in CRH-R mRNA and a 175% increase in POMC hnRNA by 2 h, returning to basal values by 4 h. The transient effect of CRH was not due to clearance of CRH from the circulation or receptor desensitization since CRH receptor mRNA expression also recovered after injection of a higher dose (10 microg) or repeated injections of CRH which caused sustained increases in plasma CRH and pituitary POMC hnRNA levels. CRH injection in adrenalectomized rats decreased CRH-R mRNA for up to 6 h, suggesting that glucocorticoids are permissive for the recovery of CRH-R mRNA. Supporting this hypothesis, simultaneous injection of corticosterone and CRH restored CRH-R mRNA expression by 4 h, and increased CRH binding 4 h and 6 h after injection. The data show that interaction between CRH and glucocorticoids counteracts individual inhibitory effects of these regulators alone, and that such effects are likely to contribute to the regulatory pattern of pituitary CRH receptors during acute stress.