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

Journal of BJP British Pharmacology DOI:10.1111/j.1476-5381.2011.01663.x www.brjpharmacol.org COMMENTARY bph_1663 Correspondence 1223..1225 What makes the a1Aadrenoceptor gene express the a1L-adrenoceptor functional phenotype? S Ventura ---------------------------------------------------------------- Keywords a-adrenoceptor; benign prostatic hyperplasia (BPH); cysteine-rich epidermal growth factor-like domain (CRELD); lower urinary tract; prostate; receptor interacting proteins; prazosin ---------------------------------------------------------------- Medicinal Chemistry and Drug Action, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, Vic., Australia Sabatino Ventura, Medicinal Chemistry and Drug Action, Monash Institute of Pharmaceutical Sciences, Monash University, 381 Royal Parade, Parkville, Vic. 3052, Australia. E-mail: Received 3 August 2011 Revised 15 August 2011 Accepted 29 August 2011 The a1A-adrenoceptor is therapeutically exploited because of its prevalence in the lower urinary tract. The pharmacology shown by this lower urinary tract a1A-adrenoceptor is different from that shown by other a1A-adrenoceptors, which has led to it being subclassified as an a1L-adrenoceptor. Only in the last few years was it shown that this pharmacologically distinct a1L-adrenoceptor is a product of the a1A-adrenoceptor gene. In this issue of the BJP, Nishimune et al. review the literature on a1L-adrenoceptor pharmacology and discuss the possible molecular mechanisms by which the a1A-adrenoceptor gene is able to produce two pharmacologically distinct adrenoceptor subtypes. Based primarily from their own research using cell lines transfected with a1A-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. LINKED ARTICLE This article is a commentary on Nishimune et al., pp. 1226–1234 of this issue. To view this paper visit http://dx.doi.org/ 10.1111/j.1476-5381.2011.01591.x Abbreviations BPH, benign prostatic hyperplasia; BRET, bioluminescence resonance energy transfer; CRELD, cysteine-rich epidermal growth factor-like domain; FRET, fluorescence resonance energy transfer The most effective and rapidly acting pharmacological treatments for benign prostatic hyperplasia (BPH) are the a1Aadrenoceptor antagonists, such as tamsulosin and alfuzosin (Miano et al., 2008; receptor nomenclature follows Alexander et al., 2011). This class of BPH therapeutic agents makes over US$ 3 billion in worldwide sales (Ventura et al., 2011). Previously, non-selective a1-adrenoceptor antagonists such as prazosin, doxazosin and terazosin were widely used, but these have now been largely superseded by tamsulosin and alfuzosin because of their greater selectivity for the a1Aadrenoceptor subtype over the a1B and a1D-adrenoceptor subtypes. The proportion of the a1A-adrenoceptor subtype expressed in the smooth muscle stroma of the prostate gland is greater than the proportion expressed in vascular smooth muscle, leading to a lower incidence of troublesome vascular side effects such as weakness, fatigue, postural hypotension © 2011 The Author British Journal of Pharmacology © 2011 The British Pharmacological Society and dizziness, which were commonplace with the use of the non-selective a1-adrenoceptor antagonists. a1A-Adrenoceptors are abundant in the male lower urinary tract, and a1A-adrenoceptor antagonists are very effective in relieving lower urinary tract symptoms associated with urethral obstruction caused by prostate enlargement. Despite this, prostate and other lower urinary tract tissues, from all species, do not show typical a1A-adrenoceptor pharmacology (Nishimune et al., 2012). When used in functional isolated tissue experiments, isolated tissues from prostate gland, urethra and bladder, all exhibit a low affinity for prazosin when compared with other a1-adrenoceptor-expressing tissues. A corresponding change in affinity is not seen with tamsulosin. This pharmacological anomaly led to the postulate that a fourth a1-adrenoceptor existed, which was termed the a1L-adrenoceptor. British Journal of Pharmacology (2012) 165 1223–1225 1223 BJP S Ventura Only recently has it been demonstrated with the use of genetically modified adrenoceptor knockout mice that the prostatic a1L-adrenoceptor phenotype requires the expression of the a1A-adrenoceptor gene (Gray et al., 2008; Muramatsu et al., 2008). The term a1L-adrenoceptor is not currently recognized as an official nomenclature term. Rather, the latest edition of the Guide to Receptors and Channels states, ‘Some tissues possess a1A-adrenoceptors that display relatively low affinity in functional and binding assays for prazosin (pKi < 9) that might represent different receptor states (termed a1Ladrenoceptors)’ (Alexander et al., 2011). Further, investigation of this phenomenon is critical to developing a better treatment for BPH as it would seem that men suffering from urethral obstruction resulting from BPH would benefit more from a selective a1L-adrenoceptor antagonist rather than the selective a1A-adrenoceptor antagonists like tamsulosin, which are currently used and show no selectivity between a1A and a1L-adrenoceptors. At present, there are no antagonists showing higher affinity for a1L over a1A-adrenoceptors. Early attempts to explain how the a1L-adrenoceptor phenotype could arise from the a1A-adrenoceptor gene concentrated on whether genetic polymorphisms or splice variants of this gene could give rise to the phenotype. However, a1Aadrenoceptors generated by known polymorphisms and splice variants in cell culture models all showed similar pharmacological characteristics to that of the a1A-adrenoceptor (Shibata et al., 1996; Suzuki et al., 2000; Ramsay et al., 2004), providing evidence that a1A-adrenoceptor polymorphisms and splice variants were not associated with generation of the a1L-adrenoceptor phenotype. Subsequently, a ‘interacting protein’ hypothesis to explain the generation of a1L-adrenoceptors from a1Aadrenocepors has been postulated, following observations from radioligand binding studies. The basis for this hypothesis is that radioligand binding studies of lower urinary tract tissues are almost always carried out using membrane homogenates and yield ligand affinities that fit the pharmacological profile of a1A-adrenoceptor pharmacology. This is despite the findings that isolated intact preparations of prostate, urethra and bladder tissue display a1L-adrenoceptor pharmacology when they have been used in functional studies. In an earlier review, Nishimune et al., (2010a) suggested that this discrepancy was caused by the homogenization process disrupting the cell membrane and thus separating a1Aadrenoceptors from the putative ‘interacting protein’. They hypothesized that only when the a1A-a (...truncated)