Tissue functions mediated by β3-adrenoceptors—findings and challenges

Martin C. Michel 0 Peter Ochodnicky 0 Roger J. Summers 0 0 R. J. Summers Drug Discovery Biology, Monash Institute of Pharmaceutical Sciences, Monash University, 399 Royal Parade , Parkville 3052, Australia As 3-adrenoceptor agonists metamorphose from experimental tools into therapeutic drugs, it is vital to obtain a comprehensive picture of the cell and tissue functions mediated by this receptor subtype in humans. Human tissues with proven functions and/or a high expression of 3-adrenoceptors include the urinary bladder, the gall bladder, and other parts of the gastrointestinal tract. While several other 3-adrenoceptor functions have been proposed based on results obtained in animals, their relevance to humans remains uncertain. For instance, 3adrenoceptors perform an important role in thermogenesis and lipolysis in rodent brown and white adipose tissue, respectively, but their role in humans appears less significant. Moreover, the use of tools such as the agonist BRL 37344 and the antagonist SR59230A to demonstrate functional involvement of 3-adrenoceptors may lead in many cases to misleading conclusions as they can also interact with other -adrenoceptor subtypes or even non-adrenoceptor targets. In conclusion, we propose that many responses attributed to 3-adrenoceptor stimulation may need reevaluation in the light of the development of more selective tools. Moreover, findings in experimental animals need to be extended to humans in order to better understand the potential additional indications and side effects of the 3adrenoceptor agonists that are beginning to enter clinical medicine. - While 1- and 2-adrenoceptor ligands have long assumed key roles in the treatment of various conditions such as coronary heart disease or obstructive airway disease, compounds acting on 3-adrenoceptors are only now undergoing a metamorphosis from experimental tools into therapeutic drugs, e.g., in the treatment of the overactive bladder syndrome (Chapple et al. 2008). The introduction of a new class of drugs is exciting but also generates uncertainty about possible safety and tolerability issues associated with this drug class. The determination of tissues where 3-adrenoceptors play a role has long been hampered by the lack of highly selective agonists and antagonists (Vrydag and Michel 2007). In this issue of the journal, Mori et al. report 3-adrenoceptor-mediated vasodilatation in rat retinal blood vessels in vivo (Mori et al. 2010). Their study expands our knowledge of tissue functions mediated by 3-adrenoceptors but also highlights the methodological challenges in this field. Against this background, we will briefly mention those tissues in which functional 3-adrenoceptors have been demonstrated with various degrees of certainty and discuss the implications for the therapeutic use of agonists acting at these receptors. This discussion will largely be based on examples and does not attempt to be comprehensive. Where possible, we will primarily focus on human tissues. Fields that have been extensively reviewed recently will only be mentioned briefly. At the mRNA level, 3-adrenoceptors have been found in a range of human tissues including brown and white adipose tissue, small and large intestine, gall bladder, urinary bladder, and brain with low levels in heart and colon; no mRNA was detected in quadriceps and abdominal muscle, liver, lung, kidney, thyroid, or lymphocytes (Berkowitz et al. 1995; Krief et al. 1993; Otsuka et al. 2008). Studies in rats have detected 3-adrenoceptor mRNA mainly in brown and white adipose tissue, in various segments of the gastrointestinal tract, and in the urinary bladder (Cohen et al. 1995; Evans et al. 1996; Fujimura et al. 1999; Roberts et al. 1999), but as in humans, it is also present in brain (Summers et al. 1995). Antibodybased detection of 3-adrenoceptor expression at the protein level has been reported in human gall bladder, colon, prostate, right atrium, and gastrocnemius muscle, whereas no labelling was detected in lung, left ventricle, appendix, uterus, or thyroid (Chamberlain et al. 1999). Detection in adipose tissue from breast, perirenal, and axillary sites proved inconclusive due to problems of interpreting labelling of the thin-walled adipocytes (Chamberlain et al. 1999). While this study provided some validation of antibody selectivity, more recent data raise doubts about the validity of many other receptor antibodies (Michel et al. 2009), including those acting on -adrenoceptor subtypes (Hamdani and van der Velden 2009; Pradidarcheep et al. 2009). Based on rodent data, 3-adrenoceptors have long been associated with the promotion of lipolysis in adipocytes, mostly in brown adipose tissue. These findings have prompted drug discovery programmes in the fields of obesity and type 2 diabetes that have yielded disappointing results (Arch 2008) at least partly due to the distinct difference between the rodent and human pharmacophore, which led to the development of several drugs (e.g., BRL 37,344, CL 316,243) that were highly effective and selective in rodents (Arch et al. 1984; Bloom et al. 1992) but with little selectivity or efficacy in humans. The explanation that was adopted initially was that 3adrenoceptors play an important role in rodent lipolysis but have a much smaller role in humans (Arner et al. 1991; Thomas and Liggett 1993). However, recent findings question this assumption and strongly suggest that there is metabolically active brown fat in humans (Nedergaard et al. 2007). Nevertheless, there is still debate as to whether and to what extent the metabolic effects of catecholamines in humans are mediated through 1- or 3-adrenoceptors (Nedergaard and Cannon 2010). If anything, the metabolic effects of 3-adrenoceptor agonists are likely to be beneficial in humans, but whether the extent of such effects is clinically relevant cannot be determined with certainty based upon the present data. In contrast, 3-adrenoceptors play an important role in the urinary bladder of humans, likely to an even greater extent than in some animal species (Michel and Vrydag 2006). Within the urinary bladder, they mediate smooth muscle relaxation (Michel and Parra 2008), but they may also affect the function of the urothelium (Masunaga et al. 2010; Otsuka et al. 2008) and afferent nerves (Aizawa et al. 2010). Accordingly, the 3-adrenoceptor agonist mirabegron, previously known as YM-178, has shown efficacy in a clinical proof of concept study in patients with overactive bladder (Chapple et al. 2008) and is now in the late stages of clinical development for this indication. 3-Adrenoceptors may also play a role in the relaxation of human ureter (Park et al. 2000; Tomiyama et al. 2003; Wanajo et al. 2004), urethra (Yamanishi et al. 2003), and penis smooth muscle (Cirino et al. 2003). 3-Adrenoceptors have also been proposed to play a role in the cardiovascular system, but the evidence for their role remains equivocal, particularly in humans. Thus, s (...truncated)