Mice expressing human but not murine β3-adrenergic receptors under the control of human gene regulatory elements

Abstract

β3-Adrenergic receptors (ARs) are expressed predominantly in adipose tissue, and β3-selective agonists are effective anti-obesity drugs in rodents. Rodent and human β3-ARs differ with respect to expression in white versus brown adipocytes as well as their ability to be stimulated by β3- AR-selective agonists. Humans express β3-AR mRNA abundantly in brown but not white adipocytes, while rodents express β3-AR mRNA abundantly in both sites. To determine the basis for this difference, we have transgenically introduced 74 kilobases (kb) of human β3-AR genomic sequence into gene knockout mice lacking β3-ARs. Importantly, human β3-AR mRNA was expressed only in brown adipose tissue (BAT) of transgenic mice, with little or no expression being detected in white adipose tissue (WAT), liver, stomach, small intestine, skeletal muscle, and heart. This pattern of expression differed from that observed in mice bearing a murine β3-AR genomic transgene in which β3-AR mRNA was expressed in both WAT and BAT, but not in other sites. Furthermore, we have transgenically introduced smaller human constructs containing -14.5 and -0.6 kb of upstream sequence into β3-AR gene knockout mice. Both -14.5 and -0.6 kb constructs were expressed in BAT but not WAT. Thus, human but not murine cis-regulatory elements direct β3- AR gene expression preferentially to brown adipocytes. Identification of responsible cis-regulatory element(s) and relevant trans-acting factor(s) should provide insight into mechanisms controlling human β3-AR gene expression. In addition, theβ3-AR agonist, CGP-12177, stimulated oxygen consumption in mice expressing human but not murine β3-ARs by 91% compared with only 49% in control β3-AR gene knockout mice, demonstrating that the human β3-AR can functionally couple with energy expenditure. These 'humanized' mice should assist us in the development of drugs that may become effective anti-obesity agents in humans.