β1-Adrenergic Receptors Mediate β3-Adrenergic-Independent Effects of CGP 12177 in Brown Adipose Tissue

Abstract

CGP 12177 is a β-adrenergic receptor (AR) ligand that has been used to characterize the β3-AR and the putative β4-AR. The ability of CGP 12177 to activate β1-AR when overexpressed in vitro and the presence of β1-AR in tissues expressing putative β4-AR prompted us to investigate the actions of CGP 12177 at recombinant and natively-expressed β-AR. CGP 12177 potently activated recombinant rat and human β1-AR expressed in Chinese hamster ovary cells. This activation, like that of putative β4-AR, was resistant to blockade by selective and nonselective β-AR antagonists. Brown fat has been proposed to contain β4-AR, as evidenced by the presence of CGP 12177-mediated thermogenesis in mice lacking β3-AR. Therefore, the identity of the receptors mediating CGP 12177 responses in brown fat was examined using wild-type mice and mice lacking β1-AR or β3-AR. In wild-type mice, CGP 12177 activated adenylyl cyclase via high- and low-affinity sites. The high-affinity site, but not the low-affinity site, was blocked by CGP 20712 with potency indicating an interaction with β1-AR. Moreover, the high-affinity site was absent in mice lacking β1-AR. In contrast, the low-affinity, CGP 20712-resistant activation by CGP 12177 was absent in mice lacking β3-AR. Rather, activation occurred exclusively through the high-affinity, CGP 20712-sensitive site. These data indicate that the actions of CGP 12177 in brown fat that have been attributed to novel β-AR (i.e., β4-AR) are mediated via an atypical interaction with β1-AR.