The alpha1A-adrenoceptor gene is required for the alpha1L-adrenoceptor-mediated response in isolated preparations of the mouse prostate

Adrenoceptors: Receptors, Ligands and Their Clinical Uses, Molecular Pharmacology and Assays

The nine G protein-coupled adrenoceptor subtypes are where the endogenous catecholamines adrenaline and noradrenaline interact with cells. Since they are important therapeutic targets, over a century of effort has been put into developing drugs that modify their activity. This chapter provides an outline of how we have arrived at current knowledge of the receptors, their physiological roles and the methods used to develop ligands. Initial studies in vivo and in vitro with isolated organs and tissues progressed to cell-based techniques and the use of cloned adrenoceptor subtypes together with high-throughput assays that allow close examination of receptors and their signalling pathways. The crystal structures of many of the adrenoceptor subtypes have now been determined opening up new possibilities for drug development.

The pharmacology of α1-adrenoceptor subtypes

This review examines the functions of α₁-adrenoceptor subtypes, particularly in terms of contraction of smooth muscle. There are 3 subtypes of α₁-adrenoceptor, α_1A- α_1B- and α_1D-adrenoceptors. Evidence is presented that the postulated α_1L-adrenoceptor is simply the native α_1A-adrenoceptor at which prazosin has low potency. In most isolated tissue studies, smooth muscle contractions to exogenous agonists are mediated particularly by α_1A-, with a lesser role for α_1D-adrenoceptors, but α_1B-adrenoceptors are clearly involved in contractions of some tissues, for example, the spleen. However, nerve-evoked responses are the most crucial physiologically, so that these studies of exogenous agonists may overestimate the importance of α_1A-adrenoceptors. The major α₁-adrenoceptors involved in blood pressure control by sympathetic nerves are the α_1D- and the α_1A-adrenoceptors, mediating peripheral vasoconstrictor actions. As noradrenaline has high potency at α_1D-adrenceptors, these receptors mediate the fastest response and seem to be targets for neurally released noradrenaline especially to low frequency stimulation, with α_1A-adrenoceptors being more important at high frequencies of stimulation. This is true in rodent vas deferens and may be true in vasopressor nerves controlling peripheral resistance and tissue blood flow. The α_lA-adrenoceptor may act mainly through Ca²⁺ entry through L-type channels, whereas the α_1D-adrenoceptor may act mainly through T-type channels and exhaustable Ca²⁺ stores. α₁-Adrenoceptors may also act through non-G-protein linked second messenger systems. In many tissues, multiple subtypes of α-adrenoceptor are present, and this may be regarded as the norm rather than exception, although one receptor subtype is usually predominant.

What makes the α1A -adrenoceptor gene product assume an α1L -adrenoceptor phenotype?

The α_1A -adrenoceptor is abundantly expressed in the lower urinary tract and is the principal therapeutic target for the symptomatic treatment of lower urinary tract symptoms in men. Prazosin has a lower affinity for the lower urinary tract α_1A -adrenoceptor than α_1A -adrenoceptors found in other parts of the body. This has led to the lower urinary tract α_1A -adrenoceptor being subclassified as an α_1L -adrenoceptor. It was demonstrated that this pharmacologically distinct α_1L -adrenoceptor is a product of the α_1A -adrenoceptor gene, but the mechanism by which this altered phenotype is achieved remains a mystery. Hypotheses for this altered pharmacology include the presence of an interacting protein such as cysteine-rich with EGF-like domain (CRELD) 1 or other GPCRs such as the CXCR2 chemokine or 5-HT_1B receptor. Alternatively, the influence of breast cancer resistance protein (BCRP) efflux transporters on the pharmacology of α_1A -adrenoceptors has also been investigated. These and other hypotheses will be described and discussed in this review. 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.

Factors influencing biased agonism in recombinant cells expressing the human α1A -adrenoceptor

BACKGROUND AND PURPOSE

Agonists acting at GPCRs promote biased signalling via Gα or Gβγ subunits, GPCR kinases and β-arrestins. Since the demonstration of biased agonism has implications for drug discovery, it is essential to consider confounding factors contributing to bias. We have examined bias at human α_1A -adrenoceptors stably expressed at low levels in CHO-K1 cells, identifying off-target effects at endogenous receptors that contribute to ERK1/2 phosphorylation in response to the agonist oxymetazoline.

EXPERIMENTAL APPROACH

Intracellular Ca²⁺ mobilization was monitored in a Flexstation® using Fluo 4-AM. The accumulation of cAMP and ERK1/2 phosphorylation were measured using AlphaScreen® proximity assays, and mRNA expression was measured by RT-qPCR. Ligand bias was determined using the operational model of agonism.

KEY RESULTS

Noradrenaline, phenylephrine, methoxamine and A61603 increased Ca²⁺ mobilization, cAMP accumulation and ERK1/2 phosphorylation. However, oxymetazoline showed low efficacy for Ca⁺² mobilization, no effect on cAMP generation and high efficacy for ERK1/2 phosphorylation. The apparent functional selectivity of oxymetazoline towards ERK1/2 was related to off-target effects at 5-HT_1B receptors endogenously expressed in CHO-K1 cells. Phenylephrine and methoxamine showed genuine bias towards ERK1/2 phosphorylation compared to Ca²⁺ and cAMP pathways, whereas A61603 displayed bias towards cAMP accumulation compared to ERK1/2 phosphorylation.

CONCLUSION AND IMPLICATIONS

We have shown that while adrenergic agonists display bias at human α_1A -adrenoceptors, the marked bias of oxymetazoline for ERK1/2 phosphorylation originates from off-target effects. Commonly used cell lines express a repertoire of endogenous GPCRs that may confound studies on biased agonism at recombinant receptors.

Novel naftopidil-related derivatives and their biological effects as alpha1-adrenoceptors antagonists and antiproliferative agents

Eleven novel naftopidil-related compounds that contain amide and indole groups were designed and synthesized. The biological effects of these compounds on three α1-adrenoceptor subtypes and cancerous human prostate cell lines (PC-3, DU-145, and LNCaP) were determined. Compounds 2, 3, 5, 11, and 12 exhibited an α1-adrenoceptor antagonistic activity, whereas compounds 9, 10, and 12 displayed moderate antiproliferative activities. Compound 3 exhibited a significant α(1D/1A) blocking activity in isolated rat tissues (97.7- and 64.6-fold selective for α(1D) and α(1A) compared with α(1B)) but not a relevant cytotoxic activity. Compound 12 demonstrated a potent and selective α(1D/1A) antagonistic activity (47.9- and 19.1-fold for α(1D) and α(1A) compared with α1B) and a potent antiproliferative activity in PC-3 cells (IC50 = 15.70 μM). Further testing confirmed that compound 12 inhibited the growth of PC-3 cells by inducing apoptosis and G0/G1 cell cycle arrest, which was mediated by α1-adrenoceptor. Therefore, compound 12 is a potential multipotent agent that can act as an effective α1-adrenoceptor subtype antagonist for treating benign prostatic hyperplasia and a preventive medication against human prostate cancer.

Agonist pharmacology at recombinant α1A - and α1L -adrenoceptors and in lower urinary tract α1 -adrenoceptors

BACKGROUND AND PURPOSE

Two distinct α1 -adrenoceptor phenotypes (α1A and α1L ) have recently been demonstrated to originate from a single α1A -adrenoceptor gene. Here, we examined the agonist profiles of recombinant α1A and α1L phenotypes and of lower urinary tract (LUT) α1 -adrenoceptors.

EXPERIMENTAL APPROACH

A series of drugs (A61603, Ro 115-1240, NS-49 , MK017 and ESR1150) originally developed for stress urinary incontinence (SUI) therapy were used to stimulate recombinant α1A - and α1L -adrenoceptor phenotypes, and their potencies and intrinsic activity estimated from Ca(2+) responses. Agonist-induced contractions were also examined in LUT tissues of rats and humans and in human mesenteric artery and rat tail artery.

KEY RESULTS

All the drugs were potent agonists of the α1A -adrenoceptor compared with the α1L -adrenoceptor phenotype. Among them, Ro 115-1240 was shown to be an α1A -specific partial agonist that produced partial contractions through α1A -adrenoceptors in rat prostate and tail artery, but not in the other LUT tissues and human mesenteric artery. In contrast, P-come 102 showed full agonist activity at α1A - and α1L -adrenoceptors, but was less selective than noradrenaline for α1A -adrenoceptors. Like noradrenaline, P-come 102 was highly potent at inducing contractions in all of the LUT tissues tested. However, the potency and intrinsic activity of P-come 102 were significantly lower than those of noradrenaline in human mesenteric artery.

CONCLUSIONS AND IMPLICATIONS

The α1A - and α1L -adrenoceptor phenotypes and LUT α1 -adrenoceptors were demonstrated to have distinct agonist profiles. As adrenergic contractions in LUT are predominantly mediated through α1L -adrenoceptors, the development of α1L -selective agonists may provide clinically useful drugs for SUI therapy.

Development of a P2X1-purinoceptor mediated contractile response in the aged mouse prostate gland through slowing down of ATP breakdown

AIMS

An age-related increase in prostatic smooth muscle tone is partly responsible for the lower urinary tract symptoms associated with benign prostatic hyperplasia (BPH). Changes in the effectors of prostatic smooth muscle contraction with age may play a role in the development of these symptoms. Using a mouse model of prostate contractility, this study investigated the effect of age on the different components of contractility in the prostate gland.

METHODS

The isometric force developed in response to electrical field stimulation or exogenously applied agonists by mouse prostates mounted in organ baths, was evaluated to determine the effect of age on contractile mechanisms. Changes with age in the rate of ATP breakdown and levels of the P2rx1 gene and P2X1-purinoceptor expression in mouse prostate were measured by a modified luciferin-luciferase assay, RT-PCR and western blot, respectively.

RESULTS

Nerve mediated contractile responses containing a component elicited by P2X1-purinoceptors were observed in prostates taken from aged mice, but not in prostates taken from young adult mice. Furthermore, the potency of the endogenous purinoceptor agonist ATP was 50-fold greater in aged mice, whereas the potency of its stable analogue α,β-metATP was unchanged. An age-related decrease in ATP metabolism was also observed.

CONCLUSIONS

With age, a purinergic contractile response to nerve stimulation develops in the mouse prostate gland due to a decrease in the rate of ATP breakdown. This may contribute to the increase in muscular tone observed in BPH and suggests that P2X1-purinoceptors are an additional target for the treatment of BPH.

Stimulation of α1a adrenergic receptors induces cellular proliferation or antiproliferative hypertrophy dependent solely on agonist concentration

Stimulation of α_1aAdrenergic Receptors (ARs) is known to have anti-proliferative and hypertrophic effects; however, some studies also suggests this receptor can increase cell proliferation. Surprisingly, we find the α_1aAR expressed in rat-1 fibroblasts can produce either phenotype, depending exclusively on agonist concentration. Stimulation of the α_1aAR by high dose phenylephrine (>10⁻⁷ M) induces an antiproliferative, hypertrophic response accompanied by robust and extended p38 activation. Inhibition of p38 with SB203580 prevented the antiproliferative response, while inhibition of Erk or Jnk had no effect. In stark contrast, stimulation of the α_1aAR with low dose phenylephrine (∼10⁻⁸ M) induced an Erk-dependent increase in cellular proliferation. Agonist-induced Erk phosphorylation was preceded by rapid FGFR and EGFR transactivation; however, only EGFR inhibition blocked Erk activation and proliferation. The general matrix metalloprotease inhibitor, GM6001, blocked agonist induced Erk activation within seconds, strongly suggesting EGFR activation involved extracellular triple membrane pass signaling. Erk activation required little Ca²⁺ release and was blocked by PLCβ or PKC inhibition but not by intracellular Ca²⁺ chelation, suggesting Ca²⁺ independent activation of novel PKC isoforms. In contrast, Ca²⁺ release was essential for PI3K/Akt activation, which was acutely maximal at non-proliferative doses of agonist. Remarkably, our data suggests EGFR transactivation leading to Erk induced proliferation has the lowest activation threshold of any α_1aAR response. The ability of α_1aARs to induce proliferation are discussed in light of evidence suggesting antagonistic growth responses reflect native α_1aAR function.

Effects of ρ-Da1a a peptidic α(1) (A) -adrenoceptor antagonist in human isolated prostatic adenoma and anaesthetized rats

BACKGROUND AND PURPOSE

ρ-Da1a, a 65 amino-acid peptide, has subnanomolar affinity and high selectivity for the human α(1) (A) -adrenoceptor subtype. The purpose of this study was to characterize the pharmacological effects of ρ-Da1a on prostatic function, both in vivo and in vitro.

EXPERIMENTAL APPROACH

ρ-Da1a was tested as an antagonist of adrenaline-induced effects on COS cells transfected with the human α(1) (A) -adrenoceptor as well as on human isolated prostatic adenoma obtained from patients suffering from benign prostatic hyperplasia. Moreover, we compared the effects of ρ-Da1a and tamsulosin on phenylephrine (PHE)-induced increases in intra-urethral (IUP) and arterial pressures (AP) in anaesthetized rats, following i.v. or p.o. administration.

KEY RESULTS

On COS cells expressing human α(1) (A) -adrenoceptors and on human prostatic strips, ρ-Da1a inhibited adrenaline- and noradrenaline-induced effects. In anaesthetized rats, ρ-Da1a and tamsulosin administered i.v. 30 min before PHE significantly antagonized the effects of PHE on IUP. The pK(B) values for tamsulosin and ρ-Da1a for this effect were similar. With regards to AP, ρ-Da1a only reduced the effect of PHE on AP at the lowest dose tested (10 μg·kg(-1) ), whereas tamsulosin significantly reduced PHE effects at doses between 10 and 150 μg·kg(-1) .

CONCLUSIONS AND IMPLICATIONS

ρ-Da1a exhibited a relevant effect on IUP and a small effect on AP. In contrast, tamsulosin antagonized the effects of PHE on both IUP and AP. We conclude that ρ-Da1a is more uroselective than tamsulosin. ρ-Da1a is the most selective peptidic antagonist for α(1A) -adenoceptors identified to date and could be a new treatment for various urological diseases.

α-Adrenoceptor assays

α-Adrenoceptors mediate responses to activation of both peripheral sympathetic nerves and central noradrenergic neurons. They also serve as autoreceptors that modulate the release of norepinephrine (NE) and other neurotransmitters. There are two major classes of α-adrenoceptors, the α(1)- and α(2). Each class is subdivided into three subtypes: α(1A), α(1B), α(1D), and α(2A), α(2B), α(2C). Described in this unit are in vitro isolated tissue methods used to study α-adrenoceptor functions and to identify novel ligands for these receptors. Detailed protocols describing use of isolated tissues to study the various α(1)- and α(2)-adrenoceptor subtypes are provided.

What makes the α(1A)-adrenoceptor gene express the α(1L)-adrenoceptor functional phenotype?

The α(1A)-adrenoceptor is therapeutically exploited because of its prevalence in the lower urinary tract. The pharmacology shown by this lower urinary tract α(1A)-adrenoceptor is different from that shown by other α(1A)-adrenoceptors, which has led to it being subclassified as an α(1L)-adrenoceptor. Only in the last few years was it shown that this pharmacologically distinct α(1L)-adrenoceptor is a product of the α(1A)-adrenoceptor gene. In this issue of the BJP, Nishimune et al. review the literature on α(1L)-adrenoceptor pharmacology and discuss the possible molecular mechanisms by which the α(1A)-adrenoceptor gene is able to produce two pharmacologically distinct adrenoceptor subtypes. Based primarily from their own research using cell lines transfected with α(1A)-adrenoceptors, they conclude that a protein that interacts with the receptor is the most plausible explanation. The challenge remains to identify any such interacting protein and show how it is able to change the pharmacology of the receptor for different ligands.

Identification and profiling of novel α1A-adrenoceptor-CXC chemokine receptor 2 heteromer

We have provided the first evidence for specific heteromerization between the α(1A)-adrenoceptor (α(1A)AR) and CXC chemokine receptor 2 (CXCR2) in live cells. α(1A)AR and CXCR2 are both expressed in areas such as the stromal smooth muscle layer of the prostate. By utilizing the G protein-coupled receptor (GPCR) heteromer identification technology on the live cell-based bioluminescence resonance energy transfer (BRET) assay platform, our studies in human embryonic kidney 293 cells have identified norepinephrine-dependent β-arrestin recruitment that was in turn dependent upon co-expression of α(1A)AR with CXCR2. These findings have been supported by co-localization observed using confocal microscopy. This norepinephrine-dependent β-arrestin recruitment was inhibited not only by the α(1)AR antagonist Terazosin but also by the CXCR2-specific allosteric inverse agonist SB265610. Furthermore, Labetalol, which is marketed for hypertension as a nonselective β-adrenoceptor antagonist with α(1)AR antagonist properties, was identified as a heteromer-specific-biased agonist exhibiting partial agonism for inositol phosphate production but essentially full agonism for β-arrestin recruitment at the α(1A)AR-CXCR2 heteromer. Finally, bioluminescence resonance energy transfer studies with both receptors tagged suggest that α(1A)AR-CXCR2 heteromerization occurs constitutively and is not modulated by ligand. These findings support the concept of GPCR heteromer complexes exhibiting distinct pharmacology, thereby providing additional mechanisms through which GPCRs can potentially achieve their diverse biological functions. This has important implications for the use and future development of pharmaceuticals targeting these receptors.

Phenotype pharmacology of lower urinary tract α(1)-adrenoceptors

α(1)-Adrenoceptors are involved in numerous physiological functions, including micturition. However, the pharmacological profile of the α(1)-adrenoceptor subtypes remains controversial. Here, we review the literature regarding α(1)-adrenoceptors in the lower urinary tract from the standpoint of α(1L) phenotype pharmacology. Among three α(1)-adrenoceptor subtypes (α(1A), α(1B) and α(1D)), α(1a)-adrenoceptor mRNA is the most abundantly transcribed in the prostate, urethra and bladder neck of many species, including humans. In prostate homogenates or membrane preparations, α(1A)-adrenoceptors with high affinity for prazosin have been detected as radioligand binding sites. Functional α(1)-adrenoceptors in the prostate, urethra and bladder neck have low affinity for prazosin, suggesting the presence of an atypical α(1)-adrenoceptor phenotype (designated as α(1L)). The α(1L)-adrenoceptor occurs as a distinct binding entity from the α(1A)-adrenoceptor in intact segments of variety of tissues including prostate. Both the α(1L)- and α(1A)-adrenoceptors are specifically absent from Adra1A (α(1a)) gene-knockout mice. Transfection of α(1a)-adrenoceptor cDNA predominantly expresses α(1A)-phenotype in several cultured cell lines. However, in CHO cells, such transfection expresses α(1L)- and α(1A)-phenotypes. Under intact cell conditions, the α(1L)-phenotype is predominant when co-expressed with the receptor interacting protein, CRELD1α. In summary, recent pharmacological studies reveal that two distinct α(1)-adrenoceptor phenotypes (α(1A) and α(1L)) originate from a single Adra1A (α(1a)-adrenoceptor) gene, but adrenergic contractions in the lower urinary tract are predominantly mediated via the α(1L)-adrenoceptor. From the standpoint of phenotype pharmacology, it is likely that phenotype-based subtypes such as the α(1L)-adrenoceptor will become new targets for drug development and pharmacotherapy.

Novel drug targets for the pharmacotherapy of benign prostatic hyperplasia (BPH)

Benign prostatic hyperplasia (BPH) is the major cause of lower urinary tract symptoms in men aged 50 or older. Symptoms are not normally life threatening, but often drastically affect the quality of life. The number of men seeking treatment for BPH is expected to grow in the next few years as a result of the ageing male population. Estimates of annual pharmaceutical sales of BPH therapies range from $US 3 to 10 billion, yet this market is dominated by two drug classes. Current drugs are only effective in treating mild to moderate symptoms, yet despite this, no emerging contenders appear to be on the horizon. This is remarkable given the increasing number of patients with severe symptoms who are required to undergo invasive and unpleasant surgery. This review provides a brief background on prostate function and the pathophysiology of BPH, followed by a brief description of BPH epidemiology, the burden it places on society, and the current surgical and pharmaceutical therapies. The recent literature on emerging contenders to current therapies and novel drug targets is then reviewed, focusing on drug targets which are able to relax prostatic smooth muscle in a similar way to the α(1) -adrenoceptor antagonists, as this appears to be the most effective mechanism of action. Other mechanisms which may be of benefit are also discussed. It is concluded that recent basic research has revealed a number of novel drug targets such as muscarinic receptor or P2X-purinoceptor antagonists, which have the potential to produce more effective and safer drug treatments.

The residual nonadrenergic contractile response to nerve stimulation of the mouse prostate is mediated by acetylcholine but not ATP in a comparison with the mouse vas deferens

Neuronal release of noradrenaline is primarily responsible for the contraction of prostatic smooth muscle in all species, and this forms the basis for the use of α(1)-adrenoceptor antagonists as pharmacotherapies for benign prostatic hyperplasia. Previous studies in mice have demonstrated that a residual nonadrenergic component to nerve stimulation remains after α(1)-adrenoceptor antagonism. In the guinea pig and rat prostate and the vas deferens of guinea pigs, rats, and mice, ATP is the mediator of this residual contraction. This study investigates the mediator of residual contraction in the mouse prostate. Whole prostates from wild-type, α(1A)-adrenoceptor, and P2X1-purinoceptor knockout mice were mounted in organ baths, and the isometric force that tissues developed in response to electrical field stimulation or exogenously applied agonists was recorded. Deletion of the P2X1 purinoceptor did not affect nerve-mediated contraction. Furthermore, the P2-purinoceptor antagonist suramin (30 μM) failed to attenuate nerve-mediated contractions in wild-type, α(1A)-adrenoceptor, or P2X1-purinoceptor knockout mice. Atropine (1 μM) attenuated contraction in prostates taken from wild-type mice. In the presence of prazosin (0.3 μM) or guanethidine (10 μM), or in prostates taken from α(1A)-adrenoceptor knockout mice, residual nerve-mediated contraction was abolished by atropine (1 μM), but not suramin (30 μM). Exogenously administered acetylcholine elicited reproducible concentration-dependent contractions of the mouse prostate that were atropine-sensitive (1 μM), but not prazosin-sensitive (0.3 μM). Acetylcholine, but not ATP, mediates the nonadrenergic component of contraction in the mouse prostate. This cholinergic component of prostatic contraction is mediated by activation of muscarinic receptors.

Isoflavones isolated from red clover (Trifolium pratense) inhibit smooth muscle contraction of the isolated rat prostate gland

This study investigated whether red clover contains any bioactive constituents which may affect contractility of rat prostatic smooth muscle in an attempt to determine whether its medicinal use in the treatment of benign prostatic hyperplasia is supported by pharmacological effects. A commercially available red clover extract was chemically fractionated and various isoflavones (genistein, formononetin and biochanin A) were isolated from these fractions and their effects on contractility were examined on preparations of the isolated rat prostate gland. Contractile effects of the isolated fractions were compared with commercially available isoflavones (genistein, formononetin and biochanin A). Pharmacological tools were used to investigate the mechanism of action modifying smooth muscle contraction. Crude red clover extract (Trinovin) inhibited electrical field stimulation induced contractions of the rat prostate across a range of frequencies with an IC(50) of approximately 68 microg/ml. Contractions of the rat prostate elicited by exogenous administration of acetylcholine, noradrenaline or adenosine 5'-triphosphate (ATP) were also inhibited. Chromatographic separation, and final purification by high performance liquid chromatography (HPLC) permitted the isolation of the isoflavones: daidzein, calycosin, formononetin, prunetin, pratensin, biochanin A and genistein. Genistein, formononetin and biochanin A (100 microM) from either commercial sources or isolated from red clover extract inhibited electrical field stimulation induced contractions of the isolated rat prostate. It is concluded that isoflavones contained in red clover are able to inhibit prostatic smooth muscle contractions in addition to their antiproliferative effects. However, the high concentrations required to observe these smooth muscle relaxant effects mean that a therapeutic benefit from this mechanism is unlikely at doses used clinically.

Alpha1-adrenoceptors in the rat cerebral cortex: new insights into the characterization of alpha1L- and alpha1D-adrenoceptors

Among the three alpha(1)-adrenoceptor subtypes (alpha(1A), alpha(1B) and alpha(1D)) a peculiar intracellular localization and poor coupling to membrane signals of cloned alpha(1D)-adrenoceptor have been reported. In addition, the alpha(1L)-adrenoceptor (low affinity for prazosin), a functional phenotype of alpha(1A), has been described. The purpose of this work was to analyze the expression, cellular localization and coupling to membrane signalling (inositol phosphate accumulation) of alpha(1)-adrenoceptor subtypes in a native tissue, the rat cerebral cortex. mRNA for the three subtypes was quantified by real-time RT-PCR (alpha(1D)>alpha(1B)>>alpha(1A)). alpha(1)-Adrenoceptors were also detected by immunoblotting, revealing alpha(1A)- and alpha(1B)-adrenoceptors to be predominantly expressed in the membrane fraction and the alpha(1D)-adrenoceptor to be localized in the cytosolic fraction. Competitive radioligand binding studies revealed the presence of alpha(1D)-adrenoceptor in tissue homogenates, whereas only alpha(1A)- and alpha(1B)-subtypes were detected in membranes. The proportion of alpha(1A)-adrenoceptor increased after treatment with noradrenaline, suggesting differences in agonist-mediated trafficking. Saturation experiments detected high- and low (alpha(1A/L))-prazosin binding sites, the latter of which disappeared on incubation with GppNHp. The alpha(1A/L)-adrenoceptor was heavily implicated in the inositol phosphate response, while the alpha(1D)-subtype did not play a relevant role. These results suggest that the predominant cytosolic localization of alpha(1D)-adrenoceptor lies behind its poor coupling to membrane signalling such as inositol phosphate pathway. The fact that the alpha(1L)-adrenoceptor detected in radioligand binding studies disappeared in the presence of GppNHp implies that it represents a conformational state of the alpha(1A)-adrenoceptor coupled to G-protein.

GPCR theme editorial

This themed section of BJP includes 11 reviews on the biology of G-protein coupled receptors (GPCRs) and the drug targets that these present, 21 research papers on the pharmacology of a range of GPCRs and Commentaries on four of the papers. Areas reviewed include molecular interactions, particular in respect of hetero-dimerisation between receptors and other membrane-located proteins and other key signalling molecules including cAMP and G12/13 proteins and recently de-orphanised receptors including the Neuromedins U & S and the Free Fatty Acid receptors FFA2 & FFA3. The research papers cover the pharmacology of a range of agents acting at GPCRs, including adrenoceptors, purinoceptors, 5HT, opioid, cannabinoid & PAR-2 receptors. A group of papers is concerned with the interesting and rapidly developing pharmacology of drugs acting at beta(2)-adrenoceptors. The reach of GPCRs is illustrated by the range of physiological systems and therapeutic applications involved, including pain, cancer, cardiovascular, gastrointestinal, visual and respiratory and central nervous systems.

The forefront for novel therapeutic agents based on the pathophysiology of lower urinary tract dysfunction: alpha-blockers in the treatment of male voiding dysfunction - how do they work and why do they differ in tolerability?

alpha(1)-Adrenoceptor antagonists are the mainstay of medical treatment of male voiding dysfunction which typically is attributed to benign prostatic hyperplasia. While original concepts have assumed that they relieve voiding dysfunction by relaxing prostatic smooth muscle, newer data indicate that their therapeutic effects at least partly occur independent of prostatic relaxation, perhaps involving direct effects on blood vessels, urothelium, afferent nerves, and/or smooth muscle of the urinary bladder. The adverse event profiles differ among alpha(1)-adrenoceptor antagonists, with tamsulosin having a particularly good cardiovascular tolerability. While this was originally attributed to its selectivity for alpha(1A)-adrenoceptors, it appears that alfuzosin which lacks subtype-selectivity, has a very similar tolerability. In contrast, doxazosin and terazosin, which are chemically and pharmacologically more closely related to alfuzosin than to tamsulosin, appear to have more side effects attributable to the cardiovascular system. More recent data indicate that tolerability differences between alpha(1)-adrenoceptor antagonists may at least partly relate to pharmacokinetic rather than to pharmacodynamic differences. Taken together, these data emphasize the idea that concepts about drug efficacy and tolerability despite being highly plausible may not necessarily be true and always require thorough experimental testing.

Subtypes of functional alpha1-adrenoceptor

In this review, subtypes of functional alpha1-adrenoceptor are discussed. These are cell membrane receptors, belonging to the seven-transmembrane-spanning G-protein-linked family of receptors, which respond to the physiological agonist noradrenaline. alpha1-Adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A fourth alpha1-adrenoceptor, the alpha1L-, represents a functional phenotype of the alpha1A-adrenoceptor. alpha1-Adrenoceptor subtype knock-out mice have refined our knowledge of the functions of alpha-adrenoceptor subtypes, particuarly as subtype-selective agonists and antagonists are not available for all subtypes. alpha1-Adrenoceptors function as stimulatory receptors involved particularly in smooth muscle contraction, especially contraction of vascular smooth muscle, both in local vasoconstriction and in the control of blood pressure and temperature, and contraction of the prostate and bladder neck. Central actions are now being elucidated.

Visualization and tissue distribution of alpha1L-adrenoceptor in human prostate by the fluorescently labeled ligand Alexa-488-silodosin

PURPOSE

Although alpha(1L)-adrenoceptor is recognized as a target of alpha(1) antagonist therapy for benign prostatic hyperplasia, the most common techniques, such as immunohistochemistry and in situ hybridization, are not applicable to examine alpha(1L)-AR vs alpha(1A)-AR tissue distribution because alpha(1L)-AR is now considered another phenotype sharing the alpha(1A)-AR gene and protein molecule. We labeled the alpha(1A) and alpha(1L)-adrenoceptor selective antagonist silodosin (Kissei Pharmaceutical, Matsumoto, Japan) with the fluorophore Alexa Fluor(R) 488 (Alexa-488-silodosin) to visualize alpha(1L)-AR expression.

MATERIALS AND METHODS

Radioligand binding and functional bioassay experiments were done to assess alpha(1)-AR expression in Chinese hamster ovary cells and human prostate tissues. Confocal imaging was subsequently performed.

RESULTS

Although Alexa-488-silodosin had about 10 times lower affinity for all alpha(1)-AR subtypes than silodosin in binding and functional studies, it had high selectivity to alpha(1A) and alpha(1L)-ARs. Confocal imaging revealed clear localization of fluorescence on the membrane of Chinese hamster ovary cells expressing alpha(1A)-AR but not alpha(1B)-and alpha(1D)-ARs, and in the muscle layer of the human prostate. The fluorescent signal in Chinese hamster ovary cells disappeared in the presence of 3 nM prazosin but fluorescence was observed in the human prostate even in the presence of 100 nM prazosin.

CONCLUSIONS

Alexa-488-silodosin is a powerful fluorescent probe with high selectivity to alpha(1A) and alpha(1L)-ARs. Thus, Alexa-488-silodosin successfully visualizes the site of alpha(1L)-ARs in the muscle layer of the human prostate without losing its distinct pharmacological profile.

Isolation and pharmacological characterization of AdTx1, a natural peptide displaying specific insurmountable antagonism of the alpha1A-adrenoceptor

BACKGROUND AND PURPOSE

Venoms are a rich source of ligands for ion channels, but very little is known about their capacity to modulate G-protein coupled receptor (GPCR) activity. We developed a strategy to identify novel toxins targeting GPCRs.

EXPERIMENTAL APPROACH

We studied the interactions of mamba venom fractions with alpha(1)-adrenoceptors in binding experiments with (3)H-prazosin. The active peptide (AdTx1) was sequenced by Edman degradation and mass spectrometry fragmentation. Its synthetic homologue was pharmacologically characterized by binding experiments using cloned receptors and by functional experiments on rabbit isolated prostatic smooth muscle.

KEY RESULTS

AdTx1, a 65 amino-acid peptide stabilized by four disulphide bridges, belongs to the three-finger-fold peptide family. It has subnanomolar affinity (K(i)= 0.35 nM) and high specificity for the human alpha(1A)-adrenoceptor subtype. We showed high selectivity and affinity (K(d)= 0.6 nM) of radio-labelled AdTx1 in direct binding experiments and revealed a slow association constant (k(on)= 6 x 10(6).M(-1).min(-1)) with an unusually stable alpha(1A)-adrenoceptor/AdTx1 complex (t(1/2diss)= 3.6 h). AdTx1 displayed potent insurmountable antagonism of phenylephrine's actions in vitro (rabbit isolated prostatic muscle) at concentrations of 10 to 100 nM.

CONCLUSIONS AND IMPLICATIONS

AdTx1 is the most specific and selective peptide inhibitor for the alpha(1A)-adrenoceptor identified to date. It displays insurmountable antagonism, acting as a potent relaxant of smooth muscle. Its peptidic nature can be exploited to develop new tools, as a radio-labelled-AdTx1 or a fluoro-labelled-AdTx1. Identification of AdTx1 thus offers new perspectives for developing new drugs for treating benign prostatic hyperplasia.

Expression of distinct alpha 1-adrenoceptor phenotypes in the iris of pigmented and albino rabbits

BACKGROUND AND PURPOSE

The expression of multiple pharmacological phenotypes including alpha(1L)-adrenoceptor has recently been reported for alpha(1)-adrenoceptors. The purpose of the present study was to identify alpha(1)-adrenoceptor phenotypes in the irises of pigmented and albino rabbits.

EXPERIMENTAL APPROACH

Radioligand binding and functional bioassay experiments were performed in segments or strips of iris of pigmented and albino rabbits, and their pharmacological profiles were compared.

KEY RESULTS

[(3)H]-silodosin at subnanomolar concentrations bound to intact segments of iris of pigmented and albino rabbits at similar densities (approximately 240 fmol x mg(-1) protein). The binding sites in the iris of a pigmented rabbit were composed of a single component showing extremely low affinities for prazosin, hydrochloride [N-[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha,alpha-dimethyl-1H-indole-3-ethamine hydrochloride (RS-17053)] and 5-methylurapidil, while two components with high and low affinities for prazosin, RS-17053 and 5-methylurapidil were identified in irises from albino rabbits. In contrast, specific binding sites for [(3)H]-prazosin were not clearly detected because a high proportion of non-specific binding and/or low affinity for prazosin occurred. Contractile responses of iris dilator muscle to noradrenaline were antagonized by the above ligands, and their antagonist affinities were consistent with the binding estimates at low-affinity sites identified in both strains of rabbits.

CONCLUSIONS AND IMPLICATIONS

A typical alpha(1L) phenotype with extremely low affinity for prazosin is exclusively expressed in the iris of pigmented rabbits, while two distinct phenotypes (alpha(1A) and alpha(1L)) with high and moderate affinities for prazosin are co-expressed in the iris of albino rabbits. This suggests that a significant difference in the expression of phenotypes of the alpha(1)-adrenoceptor occurs in the irises between the two strains of rabbits.

Subtypes of alpha1-adrenoceptors in BPH: future prospects for personalized medicine

The alpha(1)-adrenoceptors (alpha(1)-ARs) are involved in regulation of prostatic smooth muscle tone, and are a critical mediator of lower urinary tract symptoms and pathophysiology in benign prostatic hyperplasia (BPH). As a result, alpha(1)-AR antagonists are now used as first-line medical treatment for BPH. Three alpha(1)-AR subtypes (alpha(1a)-AR, alpha(1b)-AR, alpha(1d)-AR) have been identified on the basis of results of pharmacological and molecular cloning studies; however, the precise physiological role of individual alpha(1)-AR subtypes remains elusive. The expression levels of alpha(1)-AR subtypes in the prostate differ between patients, and individual differences in the genetic background of patients with BPH might be associated with variation in responses to subtype-selective alpha(1)-AR antagonists. In addition, single nucleotide polymorphism and microarray-based gene expression profiling studies might provide an opportunity to identify markers that predict clinical response and therapeutic tolerance to alpha(1)-AR antagonists. Further genomic studies will refine our knowledge of the functions of alpha(1)-AR subtypes, lead to new strategies for the clinical management of BPH and, perhaps, enable personalized treatment of BPH in the future.

The alpha1L-adrenoceptor is an alternative phenotype of the alpha1A-adrenoceptor

Despite over two decades of research, the molecular identity of the alpha1L-adrenoceptor phenotype has remained elusive. In this issue of the BJP, Gray et al. (2008) provide persuasive evidence that the in vivo alpha1L-adrenoceptor phenotype requires the expression of the alpha1A-adrenoceptor gene. They have shown that in mice lacking the functional alpha1A-adrenoceptor gene, alpha1L-mediated responses to noradrenaline in prostate smooth muscle are substantially attenuated. These findings support earlier evidence that the alpha(1L)-adrenoceptor profile represents a functional phenotype of the alpha(1A)-adrenoceptor gene product, but additional cell background-dependent factors must act in concert with the alpha(1A)-adrenoceptor protein to determine whether an alpha(1L)- or a classical alpha(1A)-adrenoceptor profile is expressed. The challenge remains to establish the nature of these cellular factors and the mechanism(s) by which they influence G-protein-coupled receptor pharmacology.