IMPAIRED EXPRESSION AND FUNCTIONAL-ACTIVITY OF THE BETA(3)-ADRENERGICAND 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 res ponse to beta-adrenergic stimulation, but the molecular defects have n ot been unequivocally identified. The expression and functional activi ty of the beta(1)-, beta(2)-, and beta(3)-adrenergic receptor (AR) sub types in white and brown adipose tissue from genetically lean and obes e (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 t o those in lean animals. beta(1)AR mRNA levels were also reduced (by s imilar to 4-fold) in obese mice, whereas beta(2)AR mRNA levels were no t 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 memb ranes 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, c orresponding 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 nonselec tive beta-antagonist propranolol eliminated the high affinity componen t. The beta(3)AR-selective agonist BRL37344 stimulated adenylyl cyclas e activity in lean membranes to a slightly lesser extent than epinephr ine, 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 agoni sts was severely blunted and was best fit to a single class of sites. Studies with CGP207.12A or the beta(2)AR-selective antagonist ICI118,5 51 indicated that this residual response was predominantly beta(2)AR i n character. Expression of BAR subtypes in both brown and white adipos e tissue of weanling obese mice (4-5-weeks of age) was also affected, but to a lesser extent, consistent with the progressive severity of ob esity with age. Together the reduction in expression of the beta(3)AR and beta(1)AR impairs the beta-agonist-stimulated adenylyl cyclase res ponse over a broad concentration range by greatly lowering the maximum stimulation and shifting the adrenergic sensitivity at low concentrat ions from a mixed beta(1)AR/beta(2)AR response to predominantly beta(2 )AR.