Decreased expression of the transcription factor NURR1 in dopamine neurons of cocaine abusers

Chronic exposure to cocaine induces long-term adaptations that are likely to involve changes in transcription factor expression. This possibility has not been examined in the cocaine-exposed human brain. The transcription factor nurr1 is highly expressed in rodent midbrain dopamine neurons and is essential for their proper phenotypic development. Here we show that human NURR1 gene expression is robust within control subjects and reduced markedly within the dopamine neurons of human cocaine abusers. NURR1 is known to regulate transcription of the gene encoding the cocaine-sensitive dopamine transporter (DAT). We show here that DAT gene expression also is reduced markedly in the dopamine neurons of NURR1-deficient cocaine abusers, suggesting that NURR1 plays a critical role in vivo in controlling human DAT gene expression and adaptation to repeated exposure to cocaine.

Characterization of RII(beta) and D-AKAP1 in differentiated adipocytes

A-kinase anchoring proteins (AKAPs) have been proposed to regulate cAMP-dependent signaling in the cell by targeting RII subunits of protein kinase A (PKA) to specific subcellular compartments. RII(beta) is the predominant PKA subtype in adipose tissue. In gel overlay assays of C3H/10T1/2 adipocytes and adipose tissue, RII(beta) bound to several proteins including a prominent 132-kDa band, which was strongly induced upon differentiation of C3H/10T1/2 cells into adipocytes. Immunoblotting and nuclease protection analysis of C3H/10T1/2 cellular extracts identified this band as D-AKAP1/S-AKAP84, a putative AKAP. Immunocytochemical analysis of C3H/10T1/2 adipocytes revealed that most of D-AKAP1/S-AKAP84, but not RII(beta), was colocalized with a mitochondrial-selective dye, MitoTracker red. These findings were further confirmed in studies where D-AKAP1/ S-AKAP84, but not RII(beta), were localized in purified mitochondria made from C3H/10T1/2 adipocytes. Moreover, D-AKAP1, which is upregulated after differentiation, did not recruit RII(beta) to membrane fractions enriched in mitochondria. These results demonstrate that D-AKAP1/S-AKAP84 does not interact with PKA in differentiated C3H/10T1/2 adipocytes under the conditions tested.

PPAR delta agonists stimulate oligodendrocyte differentiation in tissue culture

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors of the nuclear hormone receptor superfamily that have been described as master genes that switch cells from an undifferentiated phenotype to a differentiated phenotype. In the present investigation, we examined the possibility that ligands for PPARs are potent activators of oligodendrocyte (OL) differentiation and/or proliferation. Primary glial cultures and enriched OL cultures of neonatal mouse cerebra were treated with three different PPAR agonists: a PPAR gamma-selective agonist, a PPAR delta-selective agonist, and a pan agonist selective for both PPAR gamma and delta. Treatment with PPAR gamma agonist does not have an effect on the differentiation of OLs; however, PPAR delta agonist and the pan agonist treatment accelerates the differentiation of OLs within 24 h of application in mixed glial cultures. The number of OLs with processes and huge membrane sheets increases two- to threefold in both groups. The increase in the size of the sheets is also mirrored by changes in the intensity and distribution of myelin basic protein (MBP) and proteolipid protein (PLP) mRNAs. As compared to controls, the PPAR delta agonist-treated groups contain more OLs that have MBP and PLP mRNA extending into distal processes. These results indicate that PPAR delta plays a significant role in the maturation of OLs and regulates the size of OL sheets. BrdU immunostaining reveals that these agonists do not significantly stimulate proliferation of OLs expressing glycolipids. The studies in enriched OL cultures reproduce the effects of the PPAR agonists seen in the mixed glial cultures, indicating that the effect of the PPAR agonists is directly on the OLs and not via astrocytes. In the enriched cultures, the total number of OLs increases significantly in the PPAR delta agonist-treated groups, but BrdU immunostaining does not show an increased proliferation of cells. These findings suggest that PPAR delta increases the survival of cells and/or prevents cell death in enriched cultures. Although PPAR delta is expressed in various cell types, its role as a factor in the transcriptional regulation of OL differentiation has not been explored. We show for the first time that a ligand that serves as an agonist for PPAR delta activates the program of OL differentiation in primary and enriched OL cultures.

Nurr1 enhances transcription of the human dopamine transporter gene through a novel mechanism

The importance of the nuclear receptor nurr1 for the appropriate development of mesencephalic dopamine-synthesizing neurons has been clearly demonstrated through the targeted disruption of the nurr1 gene. The persistence of nurr1 expression in adult tissue suggests a possible role for this transcription factor in the maintenance, as well as development, of the dopaminergic phenotype. To address this issue, we analyzed the effects of nurr1 on the transcriptional expression of the human dopamine transporter gene (hDAT), one of the most specific phenotypic markers for dopaminergic neurons. Nurr1 enhanced the transcriptional activity of hDAT gene constructs transiently transfected into a newly described cell line (SN4741) that expresses a dopaminergic phenotype, whereas other members of the NGFI-B subfamily of nuclear receptors had lesser or no effects. Nurr1 activation of hDAT was not dependent upon heterodimerization with the retinoid X receptor. Unexpectedly, functional analysis of a series of gene constructs revealed that a region of the hDAT 5'-flanking sequence devoid of NGFI-B response element (NBRE)-like sites mediated nurr1 activation. Additional experiments using a nurr1 mutant construct suggest that nurr1 activates hDAT transcription via a novel NBRE-independent mechanism.

The putative beta4-adrenergic receptor is a novel state of the beta1-adrenergic receptor

The atypical beta3-adrenergic receptor (AR) agonist CGP-12177 has been used to define a novel atypical beta-AR subtype, the putative beta4-AR. Recent evaluation of recombinant beta-AR subtypes and beta-AR-deficient mice, however, has established the identity of the pharmacological beta4-AR as a novel state of the beta1-AR protein. The ability of aryloxypropanolamine ligands like CGP-12177 to independently interact with agonist and antagonist states of the beta1-AR has important implications regarding receptor classification and the potential development of tissue-specific beta-AR agonists.

Aryloxypropanolamine and catecholamine ligand interactions with the beta(1)-adrenergic receptor: evidence for interaction with distinct conformations of beta(1)-adrenergic receptors

Pharmacological responses to aryloxypropanolamines were examined in cells expressing rat or human beta(1)-adrenergic receptors (ARs) using adenylyl cyclase assays. The aryloxypropanolamines CGP 12177 and LY 362884, originally developed as beta(3)-AR agonists, were found to stimulate the beta(1)-AR. Interestingly, both CGP 12177 and LY 362884 exhibited an anomalous biphasic effect on beta(1)-AR. Low concentrations of either CGP 12177 or LY 362884 potently blocked isoproterenol-induced stimulation of beta(1)-AR, whereas higher concentrations of these compounds stimulated the beta(1)-AR. The unusual interaction of these aryloxypropanolamine ligands with the beta(1)-AR was further characterized using beta-AR antagonists. Activation of beta(1)-AR by CGP 12177 or LY 362884 was observed to be significantly more resistant to blockade by beta-AR antagonists compared with activation by catecholamines. These results suggest that catecholamines and aryloxypropanolamines interact with distinct active conformations of the beta(1)-AR: a state that is responsive to catecholamines and is blocked with high affinity by CGP 12177 and LY 362884, and a novel state that is activated by aryloxypropanolamines but is resistant to blockade by standard beta-AR antagonists. Moreover, dependence of antagonist affinity on agonist structure is unprecedented, and its implications on the use of beta-AR agonists such as CGP 12177 in receptor classification are discussed.

beta1-adrenergic receptors mediate beta3-adrenergic-independent effects of CGP 12177 in brown adipose tissue

CGP 12177 is a beta-adrenergic receptor (AR) ligand that has been used to characterize the beta3-AR and the putative beta4-AR. The ability of CGP 12177 to activate beta1-AR when overexpressed in vitro and the presence of beta1-AR in tissues expressing putative beta4-AR prompted us to investigate the actions of CGP 12177 at recombinant and natively-expressed beta-AR. CGP 12177 potently activated recombinant rat and human beta1-AR expressed in Chinese hamster ovary cells. This activation, like that of putative beta4-AR, was resistant to blockade by selective and nonselective beta-AR antagonists. Brown fat has been proposed to contain beta4-AR, as evidenced by the presence of CGP 12177-mediated thermogenesis in mice lacking beta3-AR. Therefore, the identity of the receptors mediating CGP 12177 responses in brown fat was examined using wild-type mice and mice lacking beta1-AR or beta3-AR. In wild-type mice, CGP 12177 activated adenylyl cyclase via high- and low-affinity sites. The high-affinity site, but not the low-affinity site, was blocked by CGP 20712 with potency indicating an interaction with beta1-AR. Moreover, the high-affinity site was absent in mice lacking beta1-AR. In contrast, the low-affinity, CGP 20712-resistant activation by CGP 12177 was absent in mice lacking beta3-AR. Rather, activation occurred exclusively through the high-affinity, CGP 20712-sensitive site. These data indicate that the actions of CGP 12177 in brown fat that have been attributed to novel beta-AR (i.e., beta4-AR) are mediated via an atypical interaction with beta1-AR.

Characterization of the 5'-flanking region of the human dopamine transporter gene

The dopamine transporter (DAT) plays a major role in modulating dopamine (DA) neurotransmission by controlling the levels of this neurotransmitter in the extracellular space. We have isolated 8.3 kb of the 5'-flanking regulatory region of the human DAT (hDAT) gene and identified numerous potential elements involved in transcriptional control of the DAT. A series of hDAT-luciferase reporter constructs encompassing increasing amounts of 5'-flanking sequence was utilized in transient transfection assays assessing basal activity and response to selected stimuli. Our results suggest that the proximal hDAT 5'-flanking region displays a strong, nonselective promoter activity that is silenced through regulatory elements present in the distal portion of the 5'-flanking sequence. Although potential cyclic AMP responsive elements (CRE) were identified on the sequence, hDAT constructs were unresponsive to cyclic AMP induction. The transcription factor nurr1 increases the transcriptional activity of several larger hDAT constructs, consistent with the presence of several putative NGFI-B response elements (NBRE). The cloning and functional analysis of an extensive portion of the 5'-flanking regulatory region of the hDAT gene provides further insights into the factors involved in the regulation of this gene.

A unique mechanism of desensitization to lipolysis mediated by beta(3)-adrenoceptor in rats with thermal injury

Thermal injury causes a hypermetabolic state associated with increased levels of catabolic hormones, but the molecular bases for the metabolic abnormalities are poorly understood. We investigated the lipolytic responses after beta(3)-adrenoceptor (beta(3)-AR) agonists and evaluated the associated changes in beta-AR and its downstream signaling molecules in adipocytes isolated from rats with thermal injury. Maximal lipolytic responses to a specific beta(3)-AR agonist, BRL-37344, were significantly attenuated at post burn days (PBD) 3 and 7. Despite significant reduction of the cell surface beta(3)-AR number and its mRNA at PBD 3 and 7, BRL-37344 and forskolin-stimulated cAMP levels were not decreased. Glycerol production in response to dibutyryl cAMP, a direct stimulant of hormone-sensitive lipase (HSL) via protein kinase A (PKA), was significantly attenuated. Although immunoblot analysis indicated no differences in the expression and activity of PKA or in the expression of HSL, HSL activity showed significant reductions. Finally, beta(3)-AR-induced insulin secretion was indeed attenuated in vivo. These studies indicate that the beta(3)-AR system is desensitized after burns, both in the adipocytes and in beta(3)-AR-induced secretion of insulin. Furthermore, these data suggest a complex and unique mechanism underlying the altered signaling of lipolysis at the level of HSL in animals after burns.

Differential regulation of functional responses by beta-adrenergic receptor subtypes in brown adipocytes

Brown adipose tissue contains both beta(1)- and beta(3)-adrenergic receptors (beta-ARs), and whereas both receptor subtypes can activate adenylyl cyclase, recent studies suggest that these subtypes have different pharmacological properties and may serve different signaling functions. In this study, primary brown adipocyte cultures were used to determine the role of beta-AR subtypes in mediating lipolysis and uncoupling protein-1 (UCP1) gene expression, elicited by the physiological neurohormone norepinephrine (NE). NE increased both lipolysis and UCP1 mRNA levels in brown adipocyte cultures; the beta(1)-receptor-selective antagonist CGP-20712A strongly antagonized the increase in UCP1 gene expression but had little effect on lipolysis. The beta(3)-receptor-selective agonist CL-316243 (CL) also increased lipolysis and UCP1 mRNA levels, yet CL was more potent in stimulating lipolysis than UCP1 gene expression. NE also increased the phosphorylation of cAMP response element-binding protein (CREB) and perilipin (PL), both of which are protein kinase A substrates that are differentially targeted to the nucleus and lipid droplets, respectively. beta(1)-receptor blockade inhibited NE-stimulated phosphorylation of CREB but not PL. The results suggest that beta-AR subtypes regulate different physiological responses stimulated by NE in brown adipocyte cultures in part by differentially transducing signals to subcellular compartments.

RGS mRNA expression in rat striatum: modulation by dopamine receptors and effects of repeated amphetamine administration

Single injections of cocaine, amphetamine, or methamphetamine increased RGS2 mRNA levels in rat striatum by two- to fourfold. The D1 dopamine receptor-selective antagonist SCH-23390 had no effect by itself but strongly attenuated RGS2 mRNA induction by amphetamine. In contrast, the D2 receptor-selective antagonist raclopride induced RGS2 mRNA when administered alone and greatly enhanced stimulation by amphetamine. To examine the effects of repeated amphetamine on RGS2 expression, rats were treated with escalating doses of amphetamine (1.0-7.5 mg/kg) for 4 days, followed by 8 days of multiple daily injections (7.5 mg/kg/2 h x four injections). Twenty hours after the last injection the animals were challenged with amphetamine (7.5 mg/kg) or vehicle and killed 1 h later. In drug-naive animals, acute amphetamine induced the expression of RGS2, 3, and 5 and the immediate early genes c-fos and zif/268. RGS4 mRNA levels were not affected. Prior repeated treatment with amphetamine strongly suppressed induction of immediate early genes and RGS5 to a challenge dose of amphetamine. In sharp contrast, prior exposure to amphetamine did not reduce the induction of RGS2 and RGS3 mRNAs to a challenge dose of amphetamine, indicating that control of these genes is resistant to amphetamine-induced tolerance. These data establish a role for dopamine receptors in the regulation of RGS2 expression and suggest that RGS2 and 3 might mediate some aspects of amphetamine-induced tolerance.

Molecular characterization of human and rat RGS 9L, a novel splice variant enriched in dopamine target regions, and chromosomal localization of the RGS 9 gene

A novel splice variant of RGS 9 was isolated from a rat hypothalamus, human retina, and a human kidney (Wilm's) tumor. This variant, termed RGS 9L, differs from the retinal form (termed RGS 9S) identified previously in that it contains a 211- (rat) or 205- (human) amino acid proline-rich domain on the carboxyl terminus. The pattern of RGS 9 mRNA splicing was tissue specific, with striatum, hypothalamus- and nucleus accumbens expressing RGS 9L, whereas retina and pineal expressed RGS 9S almost exclusively. This pattern of mRNA splicing seemed to be highly conserved between human and rodents, suggesting cell-specific differences in the function of these variants. Transient expression of RGS 9L augmented basal and beta-adrenergic receptor-stimulated adenylyl cyclase activity while suppressing dopamine D2 receptor-mediated inhibition. Furthermore, RGS 9L expression greatly accelerated the decay of dopamine D2 receptor-induced GIRK current. These results indicate RGS 9L inhibits heterotrimeric Gi function in vivo, probably by acting as a GTPase-activating protein. The human RGS 9 gene was localized to chromosome 17 q23-24 by radiation hybrid and fluorescent in situ hybridization analyses. The RGS 9 gene is within a previously defined locus for retinitis pigmentosa (RP 17), a disease that has been linked to genes in the rhodopsin/transducin/cGMP signaling pathway.

Regulators of G protein signaling: rapid changes in mRNA abundance in response to amphetamine

This study examined mRNAs encoding regulators of G protein signaling (RGSs) expressed within the striatum and determined whether their expression in the caudate putamen was altered by amphetamine. RT-PCR techniques were used to clone cDNA probes of RGSs expressed within the rat striatum. Northern blot analysis of caudate putamen and nucleus accumbens RNA determined the relative abundance of RGS mRNA expressed within the caudate putamen and adjacent nucleus accumbens to be RGS 2 > RGS 5 > RGS 16 > RGS 4 = RGS 9 > RGS 8 = RGS 3. A single injection of amphetamine rapidly and transiently induced RGS 2 mRNA. The temporal pattern of induction of RGS 2 strongly resembled that of the immediate early gene c-fos. Levels of mRNAs of RGS 3 and 5 steadily increased over a 4-h interval, as did that of the 6.6-kb transcript of RGS 8. The level of RGS 9 mRNA, which shows strong striatal-specific expression, steadily decreased over a 4-h interval, whereas RGS 4 and 16 and the 3.9-kb transcript of RGS 8 were not significantly affected at any point examined. The ability of amphetamine to alter RGS mRNA expression within the caudate putamen suggests these proteins may play an important role in adaptive processes to psychostimulant exposure.

Agonist interactions with chimeric and mutant beta1- and beta3-adrenergic receptors: involvement of the seventh transmembrane region in conferring subtype specificity

beta1- and beta3-adrenergic receptors (AR) are the predominant beta-AR subtypes in adipocytes, and analysis of native and recombinant beta-AR has revealed several pharmacological and biochemical differences between these subtypes. This study used chimeric and mutated rat beta-AR expressed in Chinese hamster ovary cells to examine the basis of certain characteristic differences in the agonist properties of catecholamines and prototypic beta3-AR agonists. The exchange of sequence beyond transmembrane (TM) region 6 between the beta-AR subtypes had dramatic and reciprocal effects on the affinity and efficacy of the prototypic beta3-AR agonists BRL 37,344 and CL 316,243, without affecting the interactions with catecholamines. Mutation of Phe350 and Phe351 in TM7 of the beta1-AR to Ala and Leu found in the beta3-AR was sufficient to allow activation by prototypic beta3-AR agonists. Interestingly, this mutation did not affect catecholamine action and it did not impair the ability of propranolol to block the actions of isoproterenol or the selective beta3-AR agonists. beta1-AR containing beta3-AR sequence from predicted TM5 through TM6 exhibited reduced affinity for catecholamines without altering agonist potency, suggesting enhanced coupling efficiency. Inclusion of the homologous beta1-AR sequence in the beta3-AR, however, did not produce reciprocal effects. These results are the first to define a major determinant of beta3-AR subtype-selective agonism in TM7 and demonstrate that the determinants of selective phenethanolamines, catecholamines, and propranolol action are distinct.

Regulation of RGS mRNAs by cAMP in PC12 cells

The RGS (regulators of G protein signaling) proteins represent a novel family of proteins which attenuate G protein mediated signaling. Using antisense riboprobes selective for rat RGS4, RGS7, and RGS2, we examined the regulation of these RGS mRNAs in PC12 cells in response to agents which elevate intracellular cAMP. Treatment of the PC12 cells with forskolin, dibutryl cAMP, or 8-CPT-cAMP for three hours decreased RGS4 message by nearly 50%. Actinomycin D, a potent inhibitor of transcription, did not affect the forskolin-induced decrease in RGS4 message, suggesting that forskolin does not alter RGS4 message half-life. RGS7 message is also present in these cells, but was not affected by forskolin. In contrast, RGS2 message is not evident in unstimulated cells but is strongly induced by one hour of treatment with forskolin. Taken together, these data suggest that mRNA levels of different RGS2 family members respond in an idiosynchratic fashion to cAMP challenge.

Effect of hypothyroidism on adenylyl cyclase activity and subtype gene expression in brown adipose tissue

Brown adipose tissue (BAT) expresses several adenylyl cyclase (AC) subtypes, and adrenergic stimulation selectively upregulates AC-III gene expression. Previous studies have described synergistic interactions between the sympathetic nervous system (SNS) and 3,5,3'-triiodothyronine (T3) on the regulation of gene expression in BAT. Because adrenergic stimulation also increases the activity of BAT type II thyroxine 5'-deiodinase (DII) and local T3 generation is important for many functional responses in BAT, we examined the effects of thyroid hormone status on the expression of various AC subtypes. Hypothyroidism selectively increased AC-III mRNA levels in BAT but not in white adipose tissue. Of the other subtypes examined, hypothyroidism did not alter AC-VI mRNA levels and slightly reduced AC-IX mRNA levels in BAT. The increase in AC-III expression was paralleled by an increase in forskolin-stimulated AC activity in BAT membranes. Sympathetic denervation of BAT abolished the increase in both AC activity and AC-III mRNA expression produced by hypothyroidism, but did not affect the expression of other subtypes. Surgical denervation also prevented the induction of AC-III in the cold-stressed euthyroid rat, but injections of T3 failed to alter AC-III expression in intact or denervated BAT. Our results indicate that T3 does not directly affect expression of AC-III. Rather, hypothyroidism increases BAT AC-III expression indirectly via an increase in sympathetic stimulation. Furthermore, our results strongly indicate that the increase in AC activity in hypothyroid BAT is due to increased expression of AC-III.

Selective up-regulation of alpha1a-adrenergic receptor protein and mRNA in brown adipose tissue by neural and beta3-adrenergic stimulation

Previous studies have shown that neural stimulation of brown adipose tissue (BAT) reorganizes the expression and activity of signaling proteins in the beta-adrenergic adenylyl cyclase pathway. Cold stress increases neural stimulation of BAT and increases alpha1-adrenergic receptor number; however, the alpha1 receptor subtype involved and the mechanism of up-regulation by cold stress have not been determined. Using reverse transcription/polymerase chain reaction analysis and nuclease protection assay, BAT was demonstrated to express mRNAs encoding alpha1a and alpha1d, but not alpha1b, receptors. Parallel pharmacologic studies of BAT membranes and recombinant alpha1a and alpha1d receptors expressed in COS-7 cells demonstrated that alpha1a receptors predominate in BAT. Exposure of rats to 4 degrees for 4 days increased alpha1a receptors and mRNA in BAT but did not alter expression of alpha1d receptors or mRNA. The induction of alpha1a receptor and mRNA level by cold stress was prevented by selective surgical denervation of BAT. Furthermore, alpha1a receptor and mRNA expression could be induced in warm-adapted rats by infusions of the selective beta3-adrenergic receptor agonist CL 316,243. These data indicate that neural activation of beta3-adrenergic receptors is an important determinant of alpha1a adrenergic receptor expression in BAT.

Dopamine transporter binding in rat striatum and nucleus accumbens is unaltered following chronic changes in dopamine levels

The effects of chronic treatment with the tyrosine hydroxylase inhibitor alpha-methyltyrosine or the dopamine precursor 1-3,4-dihydroxyphenylalanine on radioligand binding to the dopamine transporter in homogenates of rat striatum and nucleus accumbens were assessed. Chronic alpha-methyltyrosine administration decreased dopamine levels in both the striatum and nucleus accumbens without a resulting change in [3H]mazindol binding to the dopamine transporter in either brain region. In a second experiment, chronic administration of alpha-methyltyrosine or L-3,4-dihydroxyphenylalanine produced significant decreases and increases, respectively, in striatal dopamine levels without altering the binding of [3H]WIN 35,428, a chemically distinct dopamine transporter radioligand. These data suggest that even dramatic fluctuations in brain dopamine levels do not modulate the abundance of the dopamine transporter.

Perinatal expression of adenylyl cyclase subtypes in rat brown adipose tissue

The ability of norepinephrine to stimulate adenylyl cyclase (AC) activity increases during the perinatal period in rat brown adipose tissue (BAT), and this increase is associated with changes in the activities of both GS alpha and AC. The purpose of this study was to determine which AC subtypes are present in neonatal BAT and to examine whether the perinatal increase in AC activity corresponds to an increase in the expression of a particular AC subtype. Analysis of AC mRNAs by nuclease protection assay demonstrated the presence of mRNAs encoding AC-III, AC-IV, AC-VI, and AC-IX in embryonic and postnatal BAT. Of the subtypes detected, only AC-III mRNA levels increased substantially during the perinatal period. The increase in AC-III expression was paralleled by an increase in isoproterenol-stimulated AC activity. Treatment of neonates was the sympathetic neurotoxin 6-hydroxydopamine abolished the perinatal increase in both AC activity and AC-III mRNA levels but had no effect on the expression of other AC subtypes. These results strongly indicate that the increase in AC activity during the perinatal period is due to an increase in the expression of AC-III.

The M(r) 35,000 beta-adrenergic receptor mRNA-binding protein binds transcripts of G-protein-linked receptors which undergo agonist-induced destabilization

The M(r) 35,000 beta-adrenergic receptor mRNA-binding protein, termed beta-ARB protein, is induced by beta-adrenergic agonists and binds to beta 2-receptor mRNAs that display agonist-induced destabilization. Recently a cognate sequence in the mRNA was identified that provides for recognition by beta-ARB protein. In the present work we test the ability of the beta-ARB to discriminate among G-protein-linked receptor mRNAs that either do or do not display agonist-induced destabilization and test the predictive value of the presence of the cognate sequence to identify receptors displaying post-transcriptional regulation. Transcripts of beta 2-, but not rat beta 1-, rat beta 3-, or human beta 3- adrenergic receptors bind beta-ARB protein, linking agonist-induced destabilization of mRNA to transcripts with the cognate sequence. Scanning GeneBank for G-protein-linked receptor transcripts with the cognate sequence revealed several candidates, including the thrombin receptor. We demonstrate that the thrombin receptor mRNA is recognized by beta-ARB protein and like the beta 2-receptor is regulated post-transcriptionally by agonist and cAMP. Thus, the domain of regulation by beta-ARB protein includes transcripts of G-protein-linked receptors other than beta 2-adrenergic receptors.

Expression of adenylyl cyclase subtypes in brown adipose tissue: neural regulation of type III

Previous work has shown that neural stimulation of brown adipose tissue (BAT) increases norepinephrine-stimulated adenylyl cyclase (AC) activity 2- to 3-fold by a mechanism involving postreceptor alterations of the AC system. In view of the recent discovery of multiple AC subtypes, the purposes of this study were to determine which of these subtypes are present in BAT and to examine whether their expression is altered by neural stimulation. Direct hybridization and polymerase chain reaction analyses demonstrated that BAT expresses messenger RNAs (mRNAs) encoding AC-III, AC-IV, AC-V, and AC-VI subtypes. Of the subtypes detected in BAT, neural stimulation increased only AC-III mRNA levels. Activation of beta-adrenergic receptors was sufficient to increase AC-III mRNA levels; however, the potency of beta-receptor agonists was greatly enhanced by simultaneous stimulation of alpha 1-adrenergic receptors. The close correlation between the increases in AC activity and the induction of AC-III mRNA across various physiological and pharmacological treatments indicates that this subtype is responsible for the increase in AC activity that occurs after adrenergic stimulation.

Regulation of mouse beta 3-adrenergic receptor gene expression and mRNA splice variants in adipocytes

This study examined the regulation of murine beta 3-receptor mRNA and determined whether the recently described mRNA splice variants are differentially regulated by agents that alter total beta 3-receptor mRNA levels. In vivo treatment of mice with the beta 3-receptor agonist BRL-26830 reduced total beta 3-transcripts by 64% in white adipose tissue but did not alter the mRNA splicing pattern. Further analysis in cultured 3T3-F442A adipocytes showed that isoproterenol, dexamethasone, or phorbol 12-myristate 13-acetate also greatly reduced beta 3-receptor mRNA levels without selectively altering poly-U-containing transcripts. Blockade of transcription with actinomycin D produced a rapid loss of beta 3-receptor mRNA, which was prevented by blockade of mRNA translation with cycloheximide. However, neither actinomycin D nor cycloheximide altered the splicing pattern of beta 3-receptor mRNA. Analysis of transcription rate by nuclear run-off assay indicated that 8-bromoadenosine 3',5'-cyclic monophosphate and phorbol 12-myristate 13-acetate reduce beta 3-receptor gene transcription and that suppression of transcription is sufficient to account for the reduction in beta 3-receptor mRNA levels by these agents.

Why do adipocytes make the beta 3 adrenergic receptor?

Adipocytes express a mixture of beta-adrenergic receptor subtypes, including the recently characterized beta 3 receptor. The co-expression of these subtypes by fat cells suggest they serve different signalling functions. In this review, the properties of recombinant and natively-expressed beta 3 receptors are detailed and contrasted with those of beta 1 and beta 2 receptors. The beta 3 receptor appears to differ from the other beta receptor subtypes with respect to receptor coupling efficiency, G-protein coupling specificity and regulation by agonist exposure. Lastly, the potential of the beta 3 receptor as a therapeutic target is discussed in view of new data regarding its tissue distribution in humans.

Is the "atypical" beta-receptor in the rat stomach fundus the rat beta 3 receptor?

The rat gastric fundus is known to possess an "atypical" beta-adrenergic receptor that mediates relaxation to isoproterenol. The purpose of this study was to characterize the relationship between this "atypical" beta receptor in the rat stomach and the cloned rat beta 3 receptor by taking advantage of highly selective pharmacological and molecular biological probes of the beta 3 receptor. Nuclease protection analysis of RNA from the rat gastric fundus identified beta 3 receptor mRNA whose levels in the stomach were exceeded only by those in adipose tissue. Pharmacological analysis of the recombinant rat beta 3 receptor expressed in Chinese hamster ovary cells indicated low affinity of propranolol with a Ki value of 2.3 microM. Therefore, 0.3 microM propranolol was chosen as a concentration that would completely block beta 1 and beta 2 receptors (Ki = 1-5 nM) but would leave beta 3 receptors largely intact in the rat stomach fundus. In the presence of propranolol, several beta-adrenergic receptor agonists relaxed the rat stomach fundus with a rank potency order of (R,R)-5-[2-[[2-(3-chlorophenyl)-2-hydroxyethyl]-amino]-propyl]1,3- benzodioxole-2,2-dicarboxylate (CL316,243) > isoproterenol > norepinephrine = epinephrine = dl-4-3[(1,1-dimethylethyl)amino]-2- hydroxylproproy]1,3 dihydro-2H-benzimidazol-2-one hydrochloride (CGP12177) > clenbuterol > terbutaline > pindolol. Isoproterenol, norepinephrine and epinephrine were full agonists, whereas (R,R)-5-[2-[[2-(3-chlorophenyl)-2- hydroxyethyl]-amino]-propyl]1,3-benzodioxole-2,2-dicarboxylate was only a partial agonist with 66% intrinsic activity relative to isoproterenol.(ABSTRACT TRUNCATED AT 250 WORDS)