Long term phorbol ester treatment down-regulates the beta 3-adrenergic receptor in 3T3-F442A adipocytes

Is the β3-Adrenoceptor a Valid Target for the Treatment of Obesity and/or Type 2 Diabetes?

β3-Adrenoceptors mediate several functions in rodents that could be beneficial for the treatment of obesity and type 2 diabetes. This includes promotion of insulin release from the pancreas, cellular glucose uptake, lipolysis, and thermogenesis in brown adipose tissue. In combination, they lead to a reduction of body weight in several rodent models including ob/ob mice and Zucker diabetic fatty rats. These findings stimulated drug development programs in various pharmaceutical companies, and at least nine β3-adrenoceptor agonists have been tested in clinical trials. However, all of these projects were discontinued due to the lack of clinically relevant changes in body weight. Following a concise historical account of discoveries leading to such drug development programs we discuss species differences that explain why β3-adrenoceptors are not a meaningful drug target for the treatment of obesity and type 2 diabetes in humans.

Agonist-induced desensitisation of β3 -adrenoceptors: Where, when, and how?

β₃ -Adrenoceptor agonists have proven useful in the treatment of overactive bladder syndrome, but it is not known whether their efficacy during chronic administration may be limited by receptor-induced desensitisation. Whereas the β₂ -adrenoceptor has phosphorylation sites that are important for desensitisation, the β₃ -adrenoceptor lacks these; therefore, it had been assumed that β₃ -adrenoceptors are largely resistant to agonist-induced desensitisation. While all direct comparative studies demonstrate that β₃ -adrenoceptors are less susceptible to desensitisation than β₂ -adrenoceptors, desensitisation of β₃ -adrenoceptors has been observed in many models and treatment settings. Chimeric β₂ - and β₃ -adrenoceptors have demonstrated that the C-terminal tail of the receptor plays an important role in the relative resistance to desensitisation but is not the only relevant factor. While the evidence from some models, such as transfected CHO cells, is inconsistent, it appears that desensitisation is observed more often after long-term (hours to days) than short-term (minutes to hours) agonist exposure. When it occurs, desensitisation of β₃ -adrenoceptors can involve multiple levels including down-regulation of its mRNA and the receptor protein and alterations in post-receptor signalling events. The relative contributions of these mechanistic factors apparently depend on the cell type under investigation. Which if any of these factors is applicable to the human urinary bladder remains to be determined. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Design and synthesis of aryloxypropanolamine as β3-adrenergic receptor antagonist in cancer and lipolysis

β-adrenergic receptors (β-ARs) are broadly distributed in various tissues and regulate a panel of important physiological functions and disease states including cancer. Above all, β₃-adrenergic receptor (β₃-AR) plays a significant role in regulating lipolysis and thermogenesis in adipose tissue. In this study, we designed and synthesized a series of novel L-748,337 derivatives as selective human β₃-AR antagonists. Among all the tested L-748,337 analogs, compound 23d was found to display 23-fold more potent β₃-AR antagonist activity (EC₅₀ = 0.5117 nM) than L-748,337 (EC₅₀ = 11.91 nM). In vivo, compound 23d could alleviate weight loss and inhibit tumor growth in C26 tumor cachexia animal model.

Activation of the sympathetic nervous system mediates hypophagic and anxiety-like effects of CB₁ receptor blockade

Complex interactions between periphery and the brain regulate food intake in mammals. Cannabinoid type-1 (CB1) receptor antagonists are potent hypophagic agents, but the sites where this acute action is exerted and the underlying mechanisms are not fully elucidated. To dissect the mechanisms underlying the hypophagic effect of CB1 receptor blockade, we combined the acute injection of the CB1 receptor antagonist rimonabant with the use of conditional CB1-knockout mice, as well as with pharmacological modulation of different central and peripheral circuits. Fasting/refeeding experiments revealed that CB1 receptor signaling in many specific brain neurons is dispensable for the acute hypophagic effects of rimonabant. CB1 receptor antagonist-induced hypophagia was fully abolished by peripheral blockade of β-adrenergic transmission, suggesting that this effect is mediated by increased activity of the sympathetic nervous system. Consistently, we found that rimonabant increases gastrointestinal metabolism via increased peripheral β-adrenergic receptor signaling in peripheral organs, including the gastrointestinal tract. Blockade of both visceral afferents and glutamatergic transmission in the nucleus tractus solitarii abolished rimonabant-induced hypophagia. Importantly, these mechanisms were specifically triggered by lipid-deprivation, revealing a nutrient-specific component acutely regulated by CB1 receptor blockade. Finally, peripheral blockade of sympathetic neurotransmission also blunted central effects of CB1 receptor blockade, such as fear responses and anxiety-like behaviors. These data demonstrate that, independently of their site of origin, important effects of CB1 receptor blockade are expressed via activation of peripheral sympathetic activity. Thus, CB1 receptors modulate bidirectional circuits between the periphery and the brain to regulate feeding and other behaviors.

Protein kinase Cβ deficiency attenuates obesity syndrome of ob/ob mice by promoting white adipose tissue remodeling

To explore the role of leptin in PKCβ action and to determine the protective potential of PKCβ deficiency on profound obesity, double knockout (DBKO) mice lacking PKCβ and ob genes were created, and key parameters of metabolism and body composition were studied. DBKO mice had similar caloric intake as ob/ob mice but showed significantly reduced body fat content, improved glucose metabolism, and elevated body temperature. DBKO mice were resistant to high-fat diet-induced obesity. Moreover, PKCβ deficiency increased β-adrenergic signaling by inducing expression of β1- and β3-adrenergic receptors (β-ARs) in white adipose tissue (WAT) of ob/ob mice. Accordingly, p38(MAPK) activation and expression of PGC-1α and UCP-1 were increased in WAT of DBKO mice. Consistent with results of in vivo studies, inhibition of PKCβ in WAT explants from ob/ob mice also increased expression of above β-ARs. In contrast, induction of PGC-1α and UCP-1 expression in brown adipose tissue of DBKO mice was not accompanied by changes in the expression of these β-ARs. Collectively, these findings suggest that PKCβ deficiency may prevent genetic obesity, in part, by remodeling the catabolic function of adipose tissues through β-ARs dependent and independent mechanisms.

Tools to study beta3-adrenoceptors

Beta(3)-adrenoceptors mediate some of the effects of catecholamines on tissues such as blood vessels or the urinary bladder and are putative targets for the treatment of diseases such as the overactive bladder syndrome. Progress in the understanding of the presence, function, and regulation of beta(3)-adrenoceptors has been hampered by a lack of highly specific tools. "Classical" beta(3)-adrenoceptor agonists such as BRL 37,344 [(R*, R*)-(+/-)-4[2-[(3-chlorophenyl)-2-hydroxyethyl) amino] propyl] phenoxyacetic acid] and CGP 12,177 [(+/-)-4-(3-t-butylamino-2-hydroxypropoxy)benzimidazol-2-one] are only partial agonists in many settings, have limited selectivity over other beta-adrenoceptor subtypes, and may additionally act on receptors other than beta-adrenoceptors. More efficacious and more selective agonists have been reported and, in some cases, are in clinical development but are not widely available for experimental studies. The widely used antagonist SR 59,230 [3-(2-ethylphenoxy)-1-[(1,S)-1,2,3,4-tetrahydronapth-1-ylamino]-2S-2-propanoloxalate] is not selective for beta(3)-adrenoceptors, at least in humans, and may actually be a partial agonist. Radioligands, which are suitable either for the selective labeling of beta(3)-adrenoceptors or for the nonselective labeling of all beta-adrenoceptor subtypes, are also missing. beta(3)- and beta(1)/beta(2) double knockout mice have been reported, but their usefulness for extrapolations in humans is questionable based upon major differences between humans and rodents with regard to the ligand recognition and expression profiles of beta(3)-adrenoceptors. While the common availability of more selective agonists and antagonists at the beta(3)-adrenoceptor is urgently awaited, the limitations of the currently available tools need to be considered in studies of beta(3)-adrenoceptor for the time being.

Impaired trafficking of choline transporter-like protein-1 at plasma membrane and inhibition of choline transport in THP-1 monocyte-derived macrophages

The present study investigates choline transport processes and regulation of choline transporter-like protein-1 (CTL1) in human THP-1 monocytic cells and phorbol myristate 13-acetate (PMA)-differentiated macrophages. Choline uptake is saturable and therefore protein-mediated in both cell types, but its transport characteristics change soon after treatments with PMA. The maximal rate of choline uptake intrinsic to monocytic cells is greatly diminished in differentiated macrophages as demonstrated by alterations in Vmax values from 1,973 ± 118 to 380 ± 18 nmol·mg−1·min−1, when the binding affinity did not change significantly ( Km values 56 ± 8 and 53 ± 6 μM, respectively). Treatments with hemicholinim-3 effectively inhibit most of the choline uptake, establishing that a choline-specific transport protein rather than a general transporter is responsible for the observed kinetic parameters. mRNA screening for the expression of various transporters reveals that CTL1 is the most plausible candidate that possesses the described kinetic and inhibitory properties. Fluorescence-activated cell sorting analyses at various times after PMA treatments further demonstrate that the disappearance of CTL1 protein from the cell surface follows the same trend as the reduction in choline uptake. Importantly, the loss of functional CTL1 from the cell surface occurs without significant changes in total CTL1 protein or its mRNA level indicating that an impaired CTL1 trafficking is the key contributing factor to the reduced choline uptake, subsequent to the PMA-induced THP-1 differentiation to macrophages.

Endothelin-1 inhibits TNF alpha-induced iNOS expression in 3T3-F442A adipocytes

1. Endothelin-1 (ET-1) and tumor necrosis factor alpha (TNFalpha) by their action on adipocytes have been independently linked to the pathogenesis of insulino-resistance. In isolated adipocytes, TNFalpha induces the expression of the inducible nitric oxide synthase (iNOS). The purpose of the present work was, in the 3T3-F442A adipocyte cell line, to characterise TNFalpha-induced iNOS expression and to determine whether or not ET-1 could influence TNFalpha-induced iNOS expression and NO production. 2. In differentiated 3T3-F442A, treatment with TNFalpha (20 ng ml(-1)) induced the expression of a functional iNOS as demonstrated by nitrite assay, Western blot, reverse transcription-polymerase chain reaction and Northern blot analysis. TNFalpha-induced iNOS expression requires nuclear factor kappaB activation, but does not necessitate the activation of the PI-3 kinase/Akt and P38-MAP kinase pathways. 3. ET-1, but not ET-3, inhibited the TNFalpha-induced expression of iNOS protein and mRNA as well as nitrite production. The effects of ET-1 were blocked by a specific ETA (BQ123, pA(2) 7.4) but not by a specific ETB receptor antagonist (BQ788). 3T3-F442A adipocytes express the mRNAs for prepro-ET-1 and the ET-A receptor subtype, but not for the ET-B subtype. 4. The inhibitory effect of ET-1 was not affected by bisindolylmaleimide, SB 203580 or indomethacin, inhibitors of protein kinase C, p38-MAP kinase and cyclooxygenase, respectively, and was not associated with cAMP production. However, the effect of ET-1 was partially reversed by wortmannin, suggesting the involvement of PI3 kinase in the transduction signal of ET-1. 5. Differentiated 3T3-F442A adipocytes did not release ET-1 with or without exposure to TNFalpha, although the mRNA for preproET-1 was detected in both pre- and differentiated adipocytes. 6. Thus, these results confirm that adipocytes are a target for circulating ET-1 and demonstrate that the activation of the ETA receptor subtype can prevent TNFalpha-induced iNOS expression.

Epidermal growth factor upregulates beta-adrenergic receptor signaling in a human salivary cell line

The effects of epidermal growth factor (EGF) on the beta-adrenergic receptor-coupled adenylyl cyclase system were studied in a human salivary cell line (HSY). The beta-adrenergic agonist isoproterenol (10(-5) M) stimulated adenylyl cyclase activity by approximately 2-fold, and the isoproterenol response was increased 1.8-fold after prolonged (48 h) exposure to EGF (5 x 10(-10) M). In contrast, enzyme activation via stimulatory prostaglandin receptors and by agents acting on nonreceptor components of the adenylyl cyclase system was not enhanced by EGF. beta-Adrenergic receptor density, assessed by binding of the beta-adrenergic receptor antagonist (-)-[(125)I]iodopindolol, was increased threefold after EGF treatment. Competition binding studies with unlabeled antagonists selective for beta(1)- and beta(2)-adrenergic receptor subtypes indicated that the increase in (-)-[(125)I]iodopindolol binding sites induced by EGF reflected an increased number of beta(2)-adrenergic receptors. Likewise, Northern blot analysis of RNA from EGF-treated cells revealed selective induction of beta(2)-adrenergic receptor mRNA, which was blocked by the RNA synthesis inhibitor actinomycin D. The increase in beta-adrenergic receptor density produced by EGF was unaltered after phorbol ester-induced downregulation of protein kinase C (PKC). Enhancement of isoproterenol-responsive adenylyl cyclase activity and phosphorylation of mitogen-activated protein kinase (MAPK) by EGF were both blocked by the MAPK pathway inhibitor PD-98059. The results suggest that in HSY cells EGF enhances beta-adrenergic responsiveness by upregulating beta(2)-adrenergic receptor expression at the transcriptional level. Moreover, the stimulatory effect of EGF on beta(2)-adrenergic receptor signaling appears to be mediated by the MAPK pathway and independent of PKC activation.

Neuropeptide AF and FF modulation of adipocyte metabolism. Primary insights from functional genomics and effects on beta-adrenergic responsiveness

The presence of a neuropeptide AF and FF receptor (NPFF-R2) mRNA in human adipose tissue (Elshourbagy, N. A., Ames, R. S., Fitzgerald, L. R., Foley, J. J., Chambers, J. K., Szekeres, P. G., Evans, N. A., Schmidt, D. B., Buckley, P. T., Dytko, G. M., Murdock, P. R., Tan, K. B., Shabon, U., Nuthulaganti, P., Wang, D. Y., Wilson, S., Bergsma, D. J., and Sarau, H. M. (2000) J. Biol. Chem. 275, 25965-25971) suggested these peptides, principally recognized for their pain modulating effects, may also impact on adipocyte metabolism, an aspect that has not been explored previously. Our aim was thus to obtain more insights into the actions of these peptides on adipocytes, an approach initially undertaken with a functional genomic assay. First we showed that 3T3-L1 adipocytes express both NPFF-R1 and NPFF-R2 transcripts, and that NPAF binds adipocyte membranes with a nanomolar affinity as assessed by surface plasmon resonance technology. Then, and following a 24-h treatment with NPFF or NPAF (1 microm), we have measured using real-time quantitative reverse transcriptase-PCR the mRNA steady state levels of already well characterized genes involved in key pathways of adipose metabolism. Among the 45 genes tested, few were modulated by NPFF ( approximately 10%) and a larger number by NPAF ( approximately 27%). Interestingly, NPAF increased the mRNA levels of beta2- and beta3-adrenergic receptors (AR), and to a lesser extent those of beta1-ARs. These variations in catecholamine receptor mRNAs correlated with a clear induction in the density of beta2- and beta3-AR proteins, and in the potency of beta-AR subtype-selective agonists to stimulate adenylyl cyclase activity. Altogether, these data show that NPFF-R1 and NPFF-R2 are functionally present in adipocytes and suggest that besides their well described pain modulation effects, NPAF and to a lesser extent NPFF, may have a global impact on body energy storage and utilization.

Protein kinase C-mediated down-regulation of beta(2)-adrenergic receptor and gene expression in rat C6 glioma cells

We investigated the regulation of beta(2)-adrenergic receptors (beta(2)AR) by protein kinase C (PKC) in rat C6 glioma cells at the levels of receptor activity, protein expression and gene expression. Cells exposed to 4beta-phorbol-12-myristate-13-acetate (PMA), a potent activator of PKC, exhibited a time- and concentration-dependent decrease in beta(2)AR binding activity. Maximum down-regulation was approximately 50% by 24 h and western blot analysis revealed a parallel decrease in beta(2)AR protein. In addition, PMA treatment resulted in an acute desensitization of beta(2)AR-stimulated cyclic AMP response prior to any reduction in receptor levels. PMA exposure also affected steady-state beta(2)AR mRNA levels in a time-dependent, biphasic manner. During the first 4 h, levels decreased by approximately 60% and then slowly recovered to approximately 75% of control by 24 h. As the reduction in receptor mRNA was not due to a decrease in its stability, we examined beta(2)AR gene transcription by nuclear run-on assays. Transcriptional activity in nuclei from C6 cells treated with PMA for 2 h was reduced by 70% compared to controls. Thus PKC can regulate beta(2)AR at least two levels: the first being an acute desensitization of receptor function, and the second being a more prolonged repression of receptor gene transcription that in turn results in decreased receptor expression.

Expression of purinergic receptors (ionotropic P2X1-7 and metabotropic P2Y1-11) during myeloid differentiation of HL60 cells

The expression of human purinergic P2 receptors (P2X1-7 and P2Y1-11) as well as the ecto-enzymes apyrase (CD39) and 5'-nucleotidase (CD73) was investigated on the nucleic acid level during granulocytic and monocytic differentiation of HL60 cells and on peripheral human blood leukocytes. RT-PCR and dot-blot hybridization assays indicated that mRNA transcripts of all analyzed P2 receptors apart from the P2X3 receptor were expressed during myeloid development of HL60 cells, showing a distinct regulation during the course of differentiation. In blood leukocytes, transcripts of P2X5, P2X7 and all P2Y receptors, except for P2Y6, receptor were found. CD39 and CD73 showed a marked upregulation during myeloid maturation. Functional analysis of P2 receptor-mediated intracellular Ca(2+)-increase after stimulation with ATP revealed no change during granulocytic differentiation, but showed a strong attenuation in both potency and efficacy during monocytic development of HL60 cells.

Insulin resistance and type 2 diabetes mellitus: its relationship with the beta 3-adrenergic receptor

The beta 3 subtype of adrenaline and noradrenaline receptors has been extensively characterized at structural and functional levels. Ligand binding and adenyl cyclase activation studies have helped to define their unique beta-adrenergic profile. Humans, other larger mammals, and rodents share most of the characteristic beta 3-adrenergic receptor properties, although obvious species-specific differences have been identified. Most studies in animal models have shown a distinct beta 3-adrenergic receptor activity that results in an increase in energy expenditure, decrease of fat mass (especially of intra-abdominal fat), and increased glucose disposal efficiency. It is of interest that mild weight increase was shown to develop in female but not male mice, in whom the beta 3-adrenergic receptor gene was disrupted. Recently, the incidence of a naturally occurring variant of the human beta 3-adrenergic receptor was shown to correlate with hereditary obesity in Pima Indians and Japanese individuals. In Western obese patients, this phenotype increased the capacity to gain weight and develop type 2 diabetes mellitus. Studies of humans with the Trp64Arg variant have shown controversial results. Many studies have failed to show any effect in heterozygous male subjects, and only modest effects in homozygous male subjects. In women, several studies have shown modest-to-significant effects regarding weight gain, intra-abdominal fat, and decreased insulin sensitivity in heterozygous and homozygous women. Other studies have failed to show any effect in heterozygous females. Disruptions in the activity of the beta 3-adrenergic receptor in the homozygous male and the heterozygous or homozygous female appear to have a profound effect in animal models, but a limited consequence in human physiology. Association with obesity or diabetes in humans is still controversial. This difference between animal and human models may be explained by the different quantity and distribution of metabolically active brown adipose tissue in the two.

beta1 to beta3 switch in control of cyclic adenosine monophosphate during brown adipocyte development explains distinct beta-adrenoceptor subtype mediation of proliferation and differentiation

To explain the distinctive pharmacological profiles observed for adrenergic stimulation of cell proliferation (beta1) and cell differentiation (beta3), the adrenergic control of cAMP accumulation was investigated during brown adipocyte development. In preadipocytes, norepinephrine (NE) increased cAMP levels but the beta3-agonists BRL-37344 and CGP-12177 did not; in contrast, when the cells had differentiated into mature brown adipocytes, a large cAMP response to the beta3-agonists had emerged and was now double that to NE (although the affinity of NE had increased 10-fold). Beta1-messenger RNA (mRNA) levels were high in both pre- and mature brown adipocytes; beta3-mRNA did not appear until maturation but then abruptly. Although beta1-receptors remained detectable by [3H]CGP-12177 binding in the mature brown adipocytes, the cAMP response to NE (based on propranolol inhibitory potency) switched from beta1 to beta3. Even the established beta1-agonist dobutamine acted through beta3-receptors in the mature brown adipocytes. The increases in cAMP levels could adequately explain the increased cell proliferation in NE-stimulated preadipocytes and the NE-induced UCP1 gene expression in mature brown adipocytes. The distinctive adrenergic profiles for stimulation of proliferation and of differentiation were thus not due to the existence of additional pathways but to a switch in the type of beta-receptor mediating the NE response, coordinated with an alteration in the nuclear response to increased cAMP levels. The study implies that full recruitment of brown adipose tissue cannot be induced by exclusive beta3-stimulation.

PMA inhibits both spontaneous and glucocorticoid-mediated leptin secretion by human omental adipose tissue explants in vitro

The activation of PKC by the acute administration of the phorbol ester PMA (1 microM, 2h) to omental adipose tissue explants in vitro resulted in a marked (about 75%) and persistent (up to at least 96 h) inhibition of leptin secretion. This PKC-mediated inhibition was not observed after the administration of an inactive phorbol ester (phorbol 12,13-dicecanoate). The inhibition by PMA of leptin secretion was not restricted to the spontaneous secretion, but blocked also effectively the leptin response to a powerful stimulus, such as the glucocorticoid dexamethasone. As the PKC activity has been shown to be elevated during fasting, the negative relation here described between PKC activity and leptin secretion could be of physiological relevance.

Protein kinase C-mediated down-regulation of beta1-adrenergic receptor gene expression in rat C6 glioma cells

In the current study, we investigated the mechanism by which protein kinase C (PKC) regulates the expression of beta1-adrenergic receptor (beta1AR) mRNA in rat C6 glioma cells. Exposure of the cells to 4beta-phorbol-12-myristate-13-acetate (PMA), an activator PKC, resulted in a down-regulation of both beta1AR binding sites and mRNA levels in a time- and concentration-dependent manner. This effect was not observed with phorbol esters that do not activate PKC and was blocked by bisindolylmaleimide, a specific PKC inhibitor. Activation of PKC did not reduce the half-life of beta1AR mRNA but significantly decreased the activity of the beta1AR promoter, as determined by reporter analysis. A putative response element, with partial homology to a consensus cAMP response element, was identified by mutation analysis of the promoter at positions -343 to -336, relative to the translational start site. Mutation of this putative regulatory element, referred to as a beta1AR-PKC response element, completely blocked the PKC-mediated down-regulation of beta1AR promoter activity. Gel mobility shift analysis detected two specific bands when C6 cell extracts were incubated with a labeled DNA probe containing the beta1AR-PKC response element sequence. Formation of one of these bands was inhibited by an oligonucleotide probe containing a consensus CRE and disrupted by an antibody for cAMP response element binding protein. Based on these studies, we propose that the PKC-induced down-regulation of beta1AR gene transcription in C6 cells is mediated in part by a cAMP response element binding protein-dependent mechanism acting on a novel response element.

Endotoxin-induced alteration in the expression of leptin and beta3-adrenergic receptor in adipose tissue

Cytokines, such as tumor necrosis factor (TNF) and interleukin-6, may contribute to the anorexia and cachexia of infection, cancer, and AIDS. The present study tests the hypothesis that endotoxin alters the expression of two key fat cell proteins, leptin and beta3-adrenergic receptor (beta3-AR), through a mechanism involving TNF-alpha. Increasing doses of Escherichia coli endotoxin (lipopolysaccharide, LPS) resulted in dose-dependent elevations of plasma leptin (maximal response approximately 7-fold, half-maximal effective dose of approximately 16 microg/100 g body wt) and white fat leptin mRNA in C3/HeOUJ mice. LPS also produced a large decrease in adipose tissue beta3-AR mRNA and a parallel reduction in beta-agonist-induced activation of adenylyl cyclase. Changes in plasma leptin and beta3-AR mRNA were preceded by an approximately threefold increase in white fat TNF mRNA. TNF administration resulted in changes similar to those seen with LPS. We conclude that endotoxemia results in an induction of leptin mRNA and a decrease in beta3-AR mRNA in adipose tissue, an effect that may be mediated by alterations in TNF-alpha.

Desensitization of beta3-adrenergic receptor-stimulated adenylyl cyclase activity and lipolysis in rats

The beta3-adrenergic receptor is an integral membrane protein consisting of seven transmembrane domains. Unlike the beta1 and beta2 receptors, this subtype lacks the consensus phosphorylation sites required for desensitization by serine kinases. Using the rodent specific beta3 agonist BRL 35135, our initial data indicated that beta3 receptor-mediated glycerol levels progressively decreased following daily oral doses of 5 mg/kg. Therefore, we initiated studies designed to delineate the possible mechanism(s) for this decreased response. Within 3 hours following a single oral dose of BRL 35135, serum glycerol levels and UCP (uncoupling protein) RNA levels were significantly increased whereas beta3 RNA levels were significantly decreased. Rats were dosed daily for 5 days with either vehicle or BRL 35135 (5 mg/kg, p.o.) and blood samples were collected for glycerol analysis. Adipose tissue was excised for lipolysis and adenyl cyclase measurements. In addition, UCP and beta3 receptor RNA levels were assessed. No effect on adipocyte BRL 37344-stimulated adenylyl cyclase activity was observed 3 hours following the initial dose of BRL 35135. Although a slight decrease (approximately 25%) in adenylyl cyclase activity could be observed 24 hours following the initial dose, it wasn't until day 4 of dosing that a significant decrease (50%) was observed. In contrast, beta3- stimulated lipolysis in adipocytes from BRL 35135-treated rats was decreased 85% within 24 hours and this decrease persisted through four days of treatment. These data indicate that the lipolytic response to beta3 receptor activation is decreased after only a single oral dose of BRL 35135, whereas receptor-mediated adenylyl cyclase activation, although initially unaffected, also desensitizes by day four of treatment.

Protein kinase C regulates adenosine A2a receptor mRNA expression in SH-SY5Y cells

Protein kinase C regulates mRNAs encoding several G protein-linked receptors but its role in adenosine A2a receptor expression is not known. We tested the hypothesis that protein kinase C activated by tetradecanoyl phorbol acetate (TPA) regulates adenosine A2a receptor mRNA levels. SH-SY5Y human neuroblastoma cells express adenosine receptors which positively couple to adenylyl cyclase with a pharmacologic profile expected of the A2a subtype. Northern blotting demonstrated an adenosine A2a receptor mRNA species of similar molecular size in SH-SY5Y cells and in human brain. TPA increased adenosine A2a receptor mRNA in a dose- and time-dependent fashion. Transcription or translation inhibition prevented increases in adenosine A2a receptor mRNA. Bisindolylmaleimide blocked TPA effects. Adenosine A2a receptor mRNA stability was unchanged by TPA. This study identifies a human neuroblastoma cell line expressing functional adenosine A2a receptors. Protein kinase C activation appears to enhance transcription of the adenosine A2a receptor gene.

Down-regulation of beta3-adrenergic receptor expression in rat adipose tissue during the fasted/fed transition: evidence for a role of insulin

The beta3-adrenergic receptor (beta3-AR) exerts a central role in the transduction of catecholamine effects in white and brown adipose tissue (WAT and BAT). A recent report has documented that insulin strongly down-regulates beta3-AR expression and catecholamine responsiveness in 3T3-F442A adipocytes [Fève, El Hadri, Quignard-Boulangé and Pairault (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 5677-5681]. In the present report we show that the rise in plasma insulin levels elicited by the fasted/fed transition is associated with a reduction in beta3-AR mRNA levels and beta-adrenergic responsiveness in WAT and BAT. beta3-AR transcripts are also decreased in adipose tissue from animals subjected for 6 h to euglycaemic hyperinsulinaemic glucose clamps. Moreover, insulin acts directly on cultured rat white and brown adipocytes to decrease beta3-AR gene expression and adenylate cyclase activity in response to beta3-AR-selective agonists. These results suggest that there is a close relationship between food intake, plasma insulin levels and beta3-AR expression.

Structure and function of the beta 3-adrenergic receptor

The beta 3 subtype of adrenaline and noradrenaline receptors has now been extensively characterized at the structural and functional levels. Ligand binding and adenylyl cyclase activation studies helped define a beta-adrenergic profile that is quite distinct from that of the beta 1- and beta 2-adrenergic receptors, but strongly reminiscent of most of the "atypical" responses reported in earlier pharmacologic studies. Human, other large mammal, and rodent receptors share most of the characteristic beta 3 properties, although obvious species-specific differences have been identified. Recently, the incidence of a naturally occurring variant of the human beta 3-adrenergic receptor was shown to be correlated with hereditary obesity in Pima Indians and in Japanese individuals, and in Western obese patients with increased dynamic capacity to add on weight and develop non-insulin-dependent diabetes mellitus (NIDDM). A mild weight increase was also shown to develop in female, but not male, mice in which the beta 3 receptor gene was disrupted. Taken together, these results now provide a consistent picture of an important role of the beta 3-adrenoceptor in the regulation of lipid metabolism and as an obvious target for drugs to treat some forms of obesity.

Down-regulation of P2U-purinergic nucleotide receptor messenger RNA expression during in vitro differentiation of human myeloid leukocytes by phorbol esters or inflammatory activators

HL-60 human promyelocytic leukocytes express G protein-coupled P2U-purinergic nucleotide receptors (P2UR or P2Y2R) that activate inositol phospholipid hydrolysis and Ca24 mobilization in response to ATP or UTP. We examined the expression of functional P2UR and P2UR mRNA levels during in vitro differentiation of HL-60 cells by dibutyryl-cAMP (Bt2cAMP), which induces a granulocyte/neutrophil phenotype, or by phorbol-12-myristate-13-acetate (PMA), which induces a monocyte/macrophage phenotype. Both P2UR function and P2UR mRNA levels were only modestly attenuated during granulocytic differentiation by Bt2cAMP. In contrast, P2UR function, as assayed by either Ca2+ mobilization or inositol trisphosphate generation, was greatly reduced in PMA-differentiated cells. This inhibition of P2UR function was strongly correlated with PMA-induced decreases in P2UR mRNA levels, as assayed by Northern blot analysis or reverse transcription-polymerase chain reaction-based quantification. Although PMA induced an early, transient up-regulation of P2UR mRNA, this was rapidly followed by a sustained decrease in P2UR mRNA to a level 5-10-fold lower than that in undifferentiated HL-60 cells. The half-life of the P2UR transcript in HL-60 cells was approximately 60 min, and this was not affected by acute exposure (< or = 4 hr) to Bt2cAMP or PMA. PMA down-regulated P2UR mRNA in THP-1 monocytes and HL-60 granulocytes but not in A431 human epithelial cells or human keratinocytes. P2UR mRNA was also down-regulated in THP-1 monocytes differentiated into inflammatory macrophages by gamma-interferon and endotoxin. These data indicate that myeloid leukocytes possess tissue-specific mechanisms for the rapid modulation of P2UR expression and function during differentiation and inflammatory activation.

Down-regulation of beta3 adrenoreceptor gene expression in brown fat cells is transient and recovery is dependent upon a short-lived protein factor

The regulation of the expression of the beta3 adrenoreceptor gene was examined in the brown adipose tissue of intact mice and in murine brown fat primary cell cultures. Both in vivo and in vitro, high levels of beta3 receptor mRNA were observed. Acute cold exposure of mice resulted in a marked and rapid down-regulation of beta3 gene expression; this down-regulation was, however, transient. Similarly, in brown fat cell cultures, norepinephrine addition led to down-regulation of beta3 gene expression, with a lag phase of 30 min and with an apparent half-life of beta3 mRNA of approximately 30 min. This down-regulation was stimulated via the beta3 receptors themselves and mediated via cAMP; the apparent affinity of norepinephrine was extremely high (<1 nM). The degradation rate after actinomycin was identical to that after norepinephrine and was not affected by the presence of norepinephrine; thus, the down-regulation was due to cessation of transcription but not to an increased rate of degradation. Notably, inhibition of protein synthesis by cycloheximide also led to down-regulation. The norepinephrine-induced down-regulation was transient; spontaneous recovery occurred after approximately 18 h and was not due to depletion of adrenergic agent. Recovery did not occur in the presence of cycloheximide. After recovery, the cells showed a functional desensitization of the down-regulation process itself (EC50 now approximately 10 nM). It is concluded that a down-regulated state cannot explain the functional desensitization of beta3 adrenergic responsiveness observed in brown fat cells isolated from cold-acclimated animals (i.e. physiologically chronically adrenergically stimulated brown fat cells); since the beta3 receptor is not subject to desensitization via phosphorylation processes, no satisfactory explanation for the functional desensitization exists as yet. A model is presented for the down-regulation/recovery process, involving the participation of a phosphorylatable short-lived transcription factor.

Function and regulation of the beta 3-adrenoceptor

The cloning, sequencing and expression in model systems of the previously unidentified beta 3-adrenoceptor recently led to an extensive functional characterization. Ligand binding and adenylate cyclase activation studies helped define a specific profile that is quite distinct from that of the beta 1- and beta 2-adrenoceptors, but strongly reminiscent of most of the 'atypical' beta-adrenoceptor-mediated responses reported in earlier pharmacological studies. More recently, a naturally occurring variation in the human beta 3-adrenoceptor has been correlated with hereditary obesity and with increased dynamic capacity to add on weight and develop non-insulin dependent diabetes in Western obese patients. Donny Strosberg and France Pietri-Rouxel describe how results now provide a consistent picture of an important role for the human beta 3-adrenoceptor in the regulation of lipid metabolism and as an obvious target for drugs to treat some forms of obesity and diabetes.

Inhibition of hormone-sensitive lipase gene expression by cAMP and phorbol esters in 3T3-F442A and BFC-1 adipocytes

Hormone-sensitive lipase (HSL) catalyses the rate-limiting step in adipocyte lipolysis. Short-term hormonal regulation of HSL activity is well characterized, whereas little is known about the control of HSL gene expression. We have measured HSL mRNA content of 3T3-F442A and BFC-1 adipocytes in response to the cAMP analogue 8-(4-chlorophenylthio)-cAMP (8-CPT-cAMP) and to the phorbol ester phorbol 12-myristate 13-acetate (PMA) by Northern blot, using a specific mouse cDNA fragment. Treatment of the cells for 12 or 6 h with, respectively, 0.5 mM 8-CPT-cAMP or 1 microM PMA produced a maximal decrease of about 60% in HSL mRNA. These effects were unaffected by the protein-synthesis inhibitor anisomycin, suggesting that cAMP and PMA actions were direct. The reduction in HSL mRNA was accompanied by a reduction in HSL total activity. The intracellular routes that cAMP and PMA follow for inducing such an effect seemed clearly independent. (i) After desensitization of the protein kinase C regulation pathway by a 24 h treatment of the cells with 1 microM PMA, PMA action was abolished whereas cAMP was still fully active. (ii) Treatment with saturating concentrations of both agents produced an additive effect. (iii) The synthetic glucocorticoid dexamethasone had no proper effect on HSL gene expression but potentiated cAMP action without affecting PMA action. cAMP inhibitory action on HSL is unexpected. Indeed, the second messenger of catecholamines is the main activator of HSL by phosphorylation. We envision that a long-term cAMP treatment of adipocytes induces a counter-regulatory process that reduces HSL content and, ultimately, limits fatty acid depletion from stored triacylglycerols.

Endogenous beta 3- but not beta 1-adrenergic receptors are resistant to agonist-mediated regulation in human SK-N-MC neurotumor cells

Although there is considerable interest in the regulation of the different beta-adrenergic receptor (AR) subtypes, most previous studies have utilized stably transfected cells expressing recombinant receptors under the control of viral promoters. Human SK-N-MC neurotumor cells appear to be novel, since they express both endogenous beta 1AR and beta 3AR based on radioligand binding and on functional response. Saturation binding of either the hydrophilic ligand (-)-[3H]CGP-12177 or the more hydrophobic (-)-[125I]iodocyanopindolol indicated the presence of two populations of binding sites with high and low affinities. With either ligand, the beta 1AR antagonist CGP-20712A preferentially inhibited binding to the high-affinity sites. This is consistent with the latter representing beta 1AR whereas the low-affinity sites represent beta 3AR. Both subtypes appeared to be functional on the basis of isoproterenol stimulation of cyclic adenosine monophosphate (cAMP) in intact cells and adenylyl cyclase activity in cell membranes in the absence and presence of CGP-20712A. SK-N-MC-IXC cells, derived by twice subcloning the parental cells, also expressed both beta AR subtypes, indicating that they co-exist in the same cell. SK-N-MC cells exposed to isoproterenol exhibited a rapid sequestration and a slower downregulation of beta 1AR. The latter subtype also underwent desensitization, as indicated by a rightward shift to less sensitivity in the EC50 for isoproterenol stimulation of adenylyl cyclase activity. In contrast, the beta 3AR subtype was resistant to agonist-mediated sequestration, downregulation, and desensitization. Thus, when endogenously expressed in the same cell line, human beta 1AR and beta 3AR display differences in their ability to be regulated by agonist.

Triiodothyronine regulates beta 3-adrenoceptor expression in 3T3-F442A differentiating adipocytes

The effect of thyroid hormones on the beta 3-adrenoceptor expression was studied in the preadipose 3T3-F442A cell line. As assessed by molecular and pharmacological analyses, triiodothyronine addition to differentiating 3T3-F442A cells caused a 2.3-fold increase in beta 3-adrenoceptor mRNA levels, which was correlated with a parallel induction of beta 3-adrenoceptor number and of beta 3-adrenoceptor coupling to the adenylate cyclase system. Nuclear transcription experiments showed that triiodothyronine did not significantly alter the transcription rate of the beta 3-adrenoceptor gene. By contrast, the hormone increased by 36% the half-life of beta 3-adrenoceptor mRNA. Triiodothyronine exhibited a discrete effect on beta 3-adrenoceptor expression when added to mature 3T3-F442A adipocytes. This study indicates that thyroid hormones exert a differentiation-dependent and post-transcriptional regulation of beta 3-adrenoceptor expression in adipocytes.

Developmental expression and functional activity of beta 1- and beta 3-adrenoceptors in murine 3T3-F442A differentiating adipocytes

Beta 1- and beta 3-adrenoceptor mRNA and protein expression, and contribution of each subtype to the catecholamine-sensitive adenylyl cyclase system were studied during the adipose conversion of the murine 3T3-F442A cell line. Northern and reverse transcriptase-polymerase chain reaction analyses indicated that emergence of beta 3-adrenoceptor transcripts was concomittant with that of the gene encoding adipsin, a very late marker of adipose differentiation. Conversely, the induction of the beta 1-adrenoceptor mRNA occurred early after cell commitment towards adipose conversion. Changes in beta-subtype gene expression were accompanied by parallel modifications in receptor expression and function. 125I-cyanopindolol saturation and competition binding experiments showed a 3-fold increase in beta 1-adrenoceptor density in day 3 post-confluent cells. The beta 3-subtype population became detectable later and represented approximately 95% of total beta-adrenoceptors in day 8 and day 12 post-confluent cells. Adenylyl cyclase activity in response to the beta 3-adrenoceptor-selective agonists CGP12177 (4-(3-t-butylamino-2-hydroxypropoxy)-benzimidazol-2-one), ICI201651 ([(R)-4-(2 hydroxy-3-phenoxypropylamino-ethoxy)-N-(2- methoxyethyl)phenoxy-acetamide]) and cyanopindolol was virtually absent in young adipocytes, but dramatically increased in mature cells. The respective contributions of the beta 1- and the beta 3-subtypes to the production of cAMP were resolved by an Eadie-Hofstee computer analysis of isoproterenol and norepinephrine concentration-response curve of adenylyl cyclase activity. Agonist response curves in the presence of beta 1- and beta 2-adrenoceptor antagonist indicated that the beta 1-subtype accounted for the totality of beta-adrenoceptor-mediated adenylyl cyclase activation in young adipocytes. In mature adipose cells approximately 90% of this response was due to an activation of the beta 3-adrenoceptor. In addition, approximately 84% of the maximal norepinephrine-stimulated lipolysis was mediated by the beta 3-adrenoceptor in fully differentiated adipocytes. The differentiation-dependent expression of beta-subtypes in adipocytes is a biphasic process involving an initial and moderate induction of beta 1-adrenoceptors followed by the emergence of a prominent beta 3-adrenoceptor population. Compared analysis of both receptor occupancy and cAMP production shows that the beta 3-subtype is more efficiently coupled to the adenylyl cyclase system than the beta 1-adrenoceptor. Thus in mature adipose cells this receptor subtype represents the core of cAMP-dependent regulation of the lipolytic, antilipogenic and thermogenic effects of catecholamines.