Cross-inhibition of angiotensin AT 1 receptors supports the concept of receptor oligomerization Eszter Karip a , Ga ´bor Turu a , Katinka Su ¨peki a , La ´szlo ´ Szidonya a , La ´szlo ´ Hunyady a,b, * a Department of Physiology, Semmelweis University, P.O. Box 259, H-1444 Budapest, Hungary b Laboratory of Cellular and Molecular Physiology, Hungarian Academy of Sciences and Semmelweis University, Budapest, Hungary Received 3 April 2007; received in revised form 19 May 2007; accepted 24 May 2007 Available online 8 June 2007 Abstract G protein-coupled receptors are cell surface receptors that mediate the effects of extracellular signals in the endocrine/paracrine and sensory systems. Experimental evidence is accumulating, which suggest that these receptors form dimers or higher order oligomers. The functional relevance of G protein-coupled receptor dimerization or oligomerization has been raised in a number of different processes, including ontogeny, internalization, ligand-induced regulation, pharmacological diversity and signal transduction of these receptors. Agonist-independent homo- and hetero-oligomerization of the angiotensin AT 1 receptor has been reported, and it has been suggested that hetero-oligomerization with b-adrenergic receptors leads to cross-inhibition of these receptors. Much less is known about the functional interactions between AT 1 receptor homo-oligomers. The aim of the present study was to analyze the functional interactions between these homo-oligomers by determining the functions of normal, AT 1 receptor blocker (candesartan) resistant (S109Y) and G protein coupling deficient (DRY/AAY) AT 1 receptors (co-)expressed in COS-7 cells. Although we have found no evidence that stimulation of a G protein coupling deficient receptor could cross-activate co-expressed normal receptors, candesartan binding to a signaling deficient receptor caused cross-inhibition of co-expressed candesartan resistant AT 1 receptors. Since the studied mutations were in the third intracellular helix of the receptor, the observed effects cannot be explained with domain swapping. These data suggest that AT 1 receptor blockers cause cross-inhibition of homo-oligomerized AT 1 receptors, and support the concept that receptor dimers/ oligomers serve as the functional unit of G protein-coupled receptors. # 2007 Elsevier Ltd. All rights reserved. Keywords: Angiotensin II; Receptor dimerization; Receptor oligomerization; Inositol phosphate response; Renin–angiotensin system; G protein 1. Introduction The superfamily of G protein-coupled receptors (GPCRs) is the largest class of signaling molecules in the human genome (Kristiansen, 2004). All GPCRs possess a conserved structure, which is characterized by a heptahelical domain. Mammalian GPCRs are grouped into three families based on sequence homology. The vast majority of receptors (more than 90%) belong to Family A, which include sensory receptors, such as rhodopsin, odorant and taste receptors; receptors for biogenic amines, such as adrenergic receptors (ARs), dopamine, serotonin and muscarinic receptors; receptors for peptides, such as angiotensin II (Ang II), opioid and vasopressin receptors. Family B is a smaller group, which include receptors for larger peptides, such as secretin, cytokines and glucagons, whereas Family C, which is the smallest family, includes GABA B and metabotropic glutamate receptors and the Ca 2+ sensing receptor. The structurally best-characterized GPCR is rhodopsin. The rod outer-segment disc membranes in vertebrate retinal photore- ceptors contain tightly packed rhodopsin molecules causing increased probability of single photon-absorption and subse- quent phototransduction (Liang et al., 2003). Although initially GPCRs were generally considered to function as monomers, an increasing number of recent studies have suggested that dimers or higher order oligomers are the basic functional form of nearly all G protein-coupled receptors (GPCRs) (Prinster et al., 2005). Early studies suggested dimerization or oligomerization of Family A GPCRs (Agnati et al., 1982; Avissar et al., 1983; Cvejic and Devi, 1997; Hebert et al., 1996; Prinster et al., 2005), however, the functional www.elsevier.com/locate/neuint Neurochemistry International 51 (2007) 261–267 * Corresponding author at: Department of Physiology, Semmelweis University, P.O. Box 259, H-1444 Budapest, Hungary. Tel.: +36 1 266 9180; fax: +36 1 266 6504. E-mail address: hunyady@puskin.sote.hu (L. Hunyady). 0197-0186/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuint.2007.05.018