IMPAIRED EXPRESSION AND FUNCTIONAL-ACTIVITY OF THE BETA(3)-ADRENERGIC AND BETA(1)-ADRENERGIC RECEPTORS IN ADIPOSE-TISSUE OF CONGENITALLY OBESE (C57BL/6J OB/OB) MICE

Adipocytes from genetically obese (ob/ob) mice display an impaired response to beta-adrenergic stimulation, but the molecular defects have not been unequivocally identified. The expression and functional activity of the beta(1)-, beta(2)-, and beta(3)-adrenergic receptor (AR) subtypes in white and brown adipose tissue from genetically lean and obese (ob/ob) mice were compared. Three beta(3)AR transcripts of 2.1, 2.6, and 3.5 kilobases were identified in adipose tissue from lean mice by Northern blotting. All three beta(3)AR mRNA species were dramatically reduced (by similar to 300-fold) in 12-week-old obese mice compared to those in lean animals. beta(1)AR mRNA levels were also reduced (by similar to 4-fold) in obese mice, whereas beta(2)AR mRNA levels were not significantly changed. The functional consequences of these changes in beta(3),AR and beta(1)AR expression were assessed by measuring beta-agonist-stimulated adenylyl cyclase activity in adipocyte plasma membranes with subtype-selective beta-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 (K-act = 1.42 x 10(-7) M) and 77% low affinity (K-act = 1.67 x 10(-5) M) components, corresponding to activation of beta(1)AR and beta(2)AR conjointly, and beta(3)AR, respectively. The beta(1)AR-selective antagonist CGP20712A reduced the high affinity component to about 10%, whereas the nonselective beta-antagonist propranolol eliminated the high affinity component. The beta(3)AR-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; K-act = 3.61 x 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 CGP207.12A or the beta(2)AR-selective antagonist ICI118,551 indicated that this residual response was predominantly beta(2)AR in character. Expression of BAR 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 beta(3)AR and beta(1)AR impairs the beta-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 beta(1)AR/beta(2)AR response to predominantly beta(2)AR.