Impaired expression and functional activity of the β3- and β1-adrenergic receptors in adipose tissue of congenitally obese (C57BL/6J ob/ob) mice

Abstract

Adipocytes from genetically obese (ob/ob) mice display an impaired response to β-adrenergic stimulation, but the molecular defects have not been unequivocally identified. The expression and functional activity of the β1-, β2-, and β3-adrenergic receptor (AR) subtypes in white and brown adipose tissue from genetically lean and obese (ob/ob) mice were compared. Three β3AR transcripts of 2.1, 2.6, and 3.5 kilobases were identified in adipose tissue from lean mice by Northern blotting. All three β3AR mRNA species were dramatically reduced (by ∼300-fold) in 12-week-old obese mice compared to those in lean animals. β1AR mRNA levels were also reduced (by ∼4-fold) in obese mice, whereas β2AR mRNA levels were not significantly changed. The functional consequences of these changes in β3AR and β1AR expression were assessed by measuring β-agonist-stimulated adenylyl cyclase activity in adipocyte plasma membranes with subtype-selective β-adrenergic agonists and antagonists. Dose-response curves with epinephrine from lean mice were best fit to a two-component model comprised of 23% high affinity (Kact = 1.42 × 10-7 M) and 77% low affinity (Kact = 1.67 × 10-5 M) components, corresponding to activation of β1AR and β2AR conjointly, and β3AR, respectively. The β1AR-selective antagonist CGP20712A reduced the high affinity component to about 10%, whereas the nonselective β-antagonist propranolol eliminated the high affinity component. The β3AR-selective agonist BRL37344 stimulated adenylyl cyclase activity in lean membranes to a slightly lesser extent than epinephrine, but was more potent (73% high affinity component; Kact = 3.61 × 10-8 M). In obese mice, stimulation of adenylyl cyclase by all agonists was severely blunted and was best fit to a single class of sites. Studies with CGP20712A or the β2AR-selective antagonist ICI118,551 indicated that this residual response was predominantly β2AR in character. Expression of βAR subtypes in both brown and white adipose tissue of weanling obese mice (4-5-weeks of age) was also affected, but to a lesser extent, consistent with the progressive severity of obesity with age. Together the reduction in expression of the β3AR and β1AR impairs the β-agonist-stimulated adenylyl cyclase response over a broad concentration range by greatly lowering the maximum stimulation and shifting the adrenergic sensitivity at low concentrations from a mixed β1AR/β2AR response to predominantly β2AR.