Enhanced transcription of the human alpha 2A-adrenergic receptor gene by cAMP: evidence for multiple cAMP responsive sequences in the promoter region of this gene

Opposite regulation by glucocorticoids of the alpha 1B- and alpha 2A-adrenoreceptor mRNA levels in rat cultured anterior hypothalamic slices

In this study we investigated whether the expression of alpha1B- and alpha2A-adrenoreceptor mRNAs is differently modulated by glucocorticoids in rat cultured anterior hypothalamus slices. Using a semi-quantitative reverse transcription-polymerase chain reaction assay, the level of the alpha1B-adrenoreceptor mRNA was significantly reduced in slices cultured in steroid free-medium when compared with that measured in standard medium (i.e. containing basal adrenosteroid plasma concentrations). In contrast, the expression of the alpha2A-adrenoreceptor mRNA was markedly increased. Finally, the ratio of alpha1B- versus alpha2A-mRNA levels was about 1.7 and 0.7 in standard and steroid-free medium, respectively. These responses were completely reversed by supplementation with corticosterone. These findings provide the first evidence that in vitro glucocorticoids may regulate, in an opposite manner, the expression of the alpha1B-and alpha2A-adrenoreceptor mRNAs in the hypothalamus. This kind of regulation could be related to steroid-dependent changes in the noradrenergic control of neuroendocrine secretions.

Pharmacological evidence for the contribution of polyamines as mediators of the transcriptional component involved in smooth muscle relaxation elicited by forskolin

To study whether cAMP-dependent transcriptional effect and polyamines might play a modulatory role on smooth muscle, the effect of forskolin on KCl (60 mM)-induced contractions in isolated rat uterus and its modification by inhibitors of cAMP-dependent protein kinase (PKA) (Rp-cAMPS and TPCK), transcription (actinomycin D), protein synthesis (cycloheximide) and ornithine decarboxylase (alpha-difluoromethyl-ornithine, DFMO), and a polyamine (spermine) have been assayed. Forskolin (0.1 to 6 microM) induced concentration-dependent relaxation on KCl-induced tonic contractions in rat uterus (IC50: 0.55 +/- 0.12 microM) which was antagonized (p<0.05) by Rp-cAMPS (30 microM), TPCK (3 microM), cycloheximide (300 microM), actinomycin D (4 and 12 microM) and TPCK (3 microM) plus actinomycin D (12 microM). The IC50 values of forskolin in the presence of these drugs were 3.75 +/- 1.53 microM, 12.08 +/- 8.18 microM, 6.88 +/- 5.02 microM, 3.80 +/- 2.35 and 5.31 +/- 2.80 microM, and 4.26 +/- 3.65 microM respectively. Furthermore, DFMO (10 mM) also shifted the relaxation curve to forskolin to the right (IC50: 3.06 +/- 2.66 microM, p<0.05) but DFMO (10 mM) plus actinomycin D (12 microM) (IC50: 1.78 +/- 1.33 microM) did not. However, DFMO (10 mM) and actinomycin D (12 microM) did not antagonize the spermine (1-30 mM)-elicited relaxation (IC50s: 7.8 +/- 0.7 mM vs 7.28 +/- 1.4 mM and 4.67 +/- 0.44 mM in the presence of DFMO and actinomycin D, respectively). Moreover, spermine (1 mM) did not decrease the forskolin induced relaxation and counteracted the antagonism produced by actinomycin D and DFMO. Our results suggest that, in rat uterus, forskolin: a) produced cAMP-dependent relaxation, as this is antagonized by Rp-cAMP and TPCK, and b) increased the activity of ornithine decarboxylase, as this is inhibited by DFMO. Therefore, polyamines could be the mediator of the cAMP-dependent transcriptional component involved in forskolin relaxation, since, as mentioned, DFMO antagonized this relaxation and spermine counteracted the displacement produced by DFMO and actinomycin D. Thus, a plasma membrane-nucleus interaction might, at least partially, explain the mechanisms involved in forskolin induced relaxation in smooth muscle of rat uterus under the present experimental conditions.

Molecular cloning, sequencing and functional study of the promoter region of the human alpha2C4-adrenergic receptor gene

Screening of a human foetal brain genomic DNA library allowed us to isolate an EcoRI-EcoRI fragment containing 6 kb of the 5'-flanking region, the open reading frame and 4 kb of the 3'-flanking region of the alpha2C4 gene. Analysis of the sequenced region (4850 bp) revealed that the first 900 bp 5' to the start codon are very rich in GC (84%), contain several Sp1-binding sites and lack a consensus TATA box. The 5'- and 3'-ends of the alpha2C4 transcript were determined by RNase-protection assays carried out with a series of antisense probes. The data obtained with cellular RNA from HepG2 cells demonstrated that transcription is initiated 891 bases upstream of the translation-start site and that the polyadenylation site is located 550 bases downstream of the stop codon. These results are consistent with the existence of a non-conventional TATA box (TTAGAAA) and the presence of a unique polyadenylation signal (AATAAA). They also fit with the size of alpha2C4-RNA found by Northern-blot analysis (2.9 kb). The transcriptional activity of the alpha2C4 promoter region was investigated by transfecting several cell types with chimaeric constructs containing various fragments of the 5'-non-coding region and luciferase as a reporter gene. The activity of the construct containing the entire 5'-non-coding region appeared to depend on the host cell. Removal of the 5'-untranslated region resulted in loss of cell specificity and a concomitant increase in luciferase activity. Transfection of HepG2 and SK-N-MC cells with constructs deleted of additional 5'-flanking fragments permitted the definition of a minimal 200 bp promoter fragment containing the pseudo-TATA box and two putative SP1-binding sites.

The bovine alpha 2D-adrenergic receptor gene: structure, expression in retina, and pharmacological characterization of the encoded receptor

This report describes cloning of the bovine alpha 2D-adrenergic receptor (alpha 2D-AR) gene and determination of the transcription start site, unequivocal presence of the alpha 2D-AR transcript in the retina, and pharmacological characteristics of the encoded product. Furthermore, expression of the gene in selected bovine tissues has also been scrutinized. A genomic clone was isolated from lambda EMBL3 library and a 3 kb fragment was subcloned and sequenced. This fragment contained the putative TATA box and the coding region. The encoded receptor was transiently expressed in COS cells. The recombinant receptor expressed pharmacological characteristics almost identical to the wild-type bovine retinal receptor, which were typical of the alpha 2D-AR subtype. RNase protection analysis confirmed the expression of the gene in the retina. The bovine receptor was structurally close to its rat analogue which also encodes the alpha 2D-AR, but, the highest homology was observed with the porcine receptor expressing alpha 2A-AR pharmacological characteristics. Certain structural features of the bovine gene were unique to itself and not shared by any other alpha2-AR subtype. Among the tissues tested using reverse transcriptase-polymerase chain reaction (RT-PCR), the alpha 2D-AR message was the most abundant in retina, followed by the brain and olfactory lobe. Thus, the availability of the bovine receptor gene probe will become an important additional tool in the elucidation of molecular mechanisms behind the alpha 2D-AR physiology in neurosensory processes such as those occurring in the eye and the brain.

Role of the amino terminus of the third intracellular loop in agonist-promoted downregulation of the alpha2A-adrenergic receptor

A prominent feature of long-term regulation of the alpha2A-adrenergic receptor (alpha2AAR) is a loss of cellular receptors over time (downregulation). The molecular determinants of downregulation were sought by targeting regions of the receptor involved in G protein coupling and phosphorylation. Mutated receptors, consisting of chimeric substitutions of analogous beta2-adrenergic receptor (beta2AR) and serotonin 5-hydroxytryptamine1A (5-HT1A) receptor sequence into the second intracellular loop (ICL2) (residues 113-149), the amino terminus (residues 218-235) and carboxy terminus (residues 355-371) of ICL3, and a deletion of the beta-adrenergic receptor kinase (betaARK) phosphorylation sites in the third intracellular loop (ICL3) (residues 293-304), were expressed in Chinese hamster ovary (CHO) cells. Wild-type alpha2AAR underwent 31% +/- 3% downregulation after 24 h of exposure to 100 microM epinephrine. Loss of downregulation was observed with some mutants, but this was not related to functional coupling to inhibitory or stimulatory guanine nucleotide regulatory binding proteins (Gi or GS) or to phosphorylation. Rather, any mutant with a substitution of the amino terminus of ICL3 (regardless of whether the substitution was with beta2AR or 5-HT1A sequence) resulted in upregulation. Studies with an inhibitor of protein synthesis indicated that the primary mechanism of downregulation of the alpha2AAR is agonist-promoted degradation of receptor protein which requires a destabilization sequence in the amino terminus of ICL3. Thus, in contrast to other G protein-coupled receptors, in which G protein coupling or phosphorylation are critical for long-term agonist regulation, the alpha2AAR has a specific structural domain distinct from these other functional regions that serves to direct agonist-promoted downregulation.

Naloxone-precipitated withdrawal in morphine-dependent rats increases the expression of alpha 2a-adrenoceptor mRNA in brain

Opiate withdrawal has been associated with up-regulation of alpha 2-adrenoceptors (mainly the alpha 2A-subtype) in brain. The modulation of these inhibitory receptors regulating norepinephrine release appears to be a relevant mechanism by which the opiate abstinence syndrome might be counteracted. The aim of this study was to investigate possible changes in alpha 2a-adrenoceptor gene expression as the molecular mechanism underlying the opiate withdrawal-induced up-regulation of alpha 2A-adrenoceptors. In morphine-dependent rats (10-100 mg/kg for 5 days), naloxone (2 mg/kg)-precipitated withdrawal induced a rapid (2 h) and marked up-regulation (111%, P < 0.001) in the expression of alpha 2a-adrenoceptor mRNA (Northern and dot-blot analyses) in the cerebral cortex. Acute and chronic morphine treatments did not alter significantly the expression of cortical alpha 2a-adrenoceptor mRNA. The results indicate that the opiate abstinence syndrome is associated with a transcriptional activation of the alpha 2a-adrenoceptor mRNA which can explain the up-regulation of brain alpha 2A-adrenoceptors during opiate withdrawal.