Abstract
beta3-Adrenergic receptors (beta3-ARs) are expressed predominantly on white and brown adipocytes, and acute treatment of mice with CL 316,243, a potent and highly selective beta3-AR agonist, produces a 2-fold increase in energy expenditure, a 50-100-fold increase in insulin levels, and a 40-50% reduction in food intake. Recently, we generated gene knockout mice lacking functional beta3-ARs and demonstrated that each of these responses were mediated exclusively by beta3-ARs. However, the tissue site responsible for producing these actions is unknown. In the present study, genetically engineered mice were created in which beta3-ARs are expressed exclusively in white and brown adipocytes (WAT+BAT-mice), or in brown adipocytes only (BAT-mice). This was accomplished by injecting tissue-specific beta3-AR transgenic constructs into mouse zygotes homozygous for the beta3-AR knockout allele. Control, knockout, WAT+BAT, and BAT-mice were then treated acutely with CL, and the effects on various parameters were assessed. As previously observed, all effects of CL were completely absent in gene knockout mice lacking beta3-ARs. The effects on O2 consumption, insulin secretion, and food intake were completely rescued with transgenic re-expression of beta3-ARs in white and brown adipocytes (WAT+BAT-mice), demonstrating that each of these responses is mediated exclusively by beta3-ARs in white and/or brown adipocytes, and that beta3-ARs in other tissue sites were not required. Importantly, transgenic re-expression of beta3-ARs in brown adipocytes only (BAT-mice) failed to rescue, in any way, CL-mediated effects on insulin levels and food intake and only minimally restored effects on oxygen consumption, indicating that any effect on insulin secretion and food intake, and a full stimulation of oxygen consumption required the presence of beta3-ARs in white adipocytes. The mechanisms by which beta3-AR agonist stimulation of white adipocytes produces these responses are unknown but may involve novel mediators not previously known to effect these processes.
Highlights
3-Adrenergic receptors (3-ARs) are expressed predominantly on white and brown adipocytes, and acute treatment of mice with CL 316,243, a potent and highly selective 3-AR agonist, produces a 2-fold increase in energy expenditure, a 50 –100-fold increase in insulin levels, and a 40 –50% reduction in food intake
Transgenic re-expression of 3-ARs in brown adipocytes only (BAT-mice) failed to rescue, in any way, CL-mediated effects on insulin levels and food intake and only minimally restored effects on oxygen consumption, indicating that any effect on insulin secretion and food intake, and a full stimulation of oxygen consumption required the presence of 3-ARs in white adipocytes
Acute treatment of rodents with 3-selective agonists causes a number of diverse metabolic effects including a 2-fold increase in energy expenditure as measured by effects on oxygen consumption, a 10 –100-fold increase in insulin levels, and a 40 –50% reduction in food intake (8 –11)
Summary
(Received for publication, November 22, 1996, and in revised form, March 27, 1997). Danica Grujic‡, Vedrana S. These animals have a slight increase in body fat; but otherwise, they appear to be normal, probably because of adaptations which compensate for the absence of 3-ARs, one example being the observed up-regulation of 1-AR gene expression in brown and white adipose tissue [11] Of significance, these mutant mice are completely resistant to the ability of CL 316,243 [1], a 3-selective agonist, to increase lipolysis, energy expenditure, insulin levels, and to reduce food intake [11]. These mutant mice are completely resistant to the ability of CL 316,243 [1], a 3-selective agonist, to increase lipolysis, energy expenditure, insulin levels, and to reduce food intake [11] Each of these effects is mediated exclusively by 3-ARs. The relative role of 3-ARs in white versus brown adipocytes, as well as 3-ARs in other sites, in mediating each of these responses is unresolved. Knockout, WATϩBAT-, and BAT-mice were used to investigate the relative role of 3-ARs in white versus brown adipose tissue, as well as 3-ARs in other sites, in mediating a number of responses to 3-selective agonists
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