Early alterations in the brown adipose tissue adenylate cyclase system of pre-obese Zucker rat fa/fa pups: decreased G-proteins and beta 3-adrenoceptor activities.
This study was undertaken to determine whether receptor and non-receptor components of the adenylate cyclase (AC) cascade were altered in brown adipose tissue (BAT) of 14-day-old pre-obese (fa/fa) rats, before endocrine status is strongly modified by fa gene expression. Activity of the AC catalytic subunit did not differ between the two genotypes. In fa/fa rats compared with control Fa/fa rats, there was a 50% decrease in the activity of alpha Gs (stimulated by NaF or guanosine 5'-[gamma-thio]triphosphate) but no change in protein content (Western blotting). alpha Gi function, assessed by the inhibitory action of low concentrations of guanosine 5'-[beta gamma-imido]triphosphate upon 10(-4) M forskolin-stimulated AC activity, was equally low in both genotypes. Analysis of dose-response curves for different beta-agonists revealed that (i) both the basal and the maximally stimulated activity of AC were 2-fold lower in fa/fa rats than in Fa/fa rats; (ii) BRL37344 and CGP12177 (beta 3 agonists) were less potent in fa/fa than in Fa/fa rats (Kact. multiplied by 2); (iii) noradrenaline and isoprenaline (Iso), at the low-affinity site (beta 3-AR), were less potent in fa/fa than in Fa/fa pups (Kact. increased by 30 and 20% respectively). At the high-affinity site (mainly beta 1) these two agonists were more potent in fa/fa than in Fa/fa rats (Kact. decreased by 40 and 80% respectively). In good agreement with the latter result, the beta 1-adrenergic receptor (beta 1-AR)-selective antagonist CGP20712A had more effect on the Iso-stimulated AC activity in pre-obese than in lean pups (2-fold decreased in IC50). Binding experiments with [3H]CGP12177 show that in BAT of suckling rats, beta 3-ARs represent 80% of the total beta-ARs. Bmax values for the two sites were not affected by the genotype, although the beta 3-AR mRNA concentration in BAT (quantitative reverse-transcriptase PCR) was 3-fold lower in fa/fa rats than in Fa/fa pups. In conclusion, these results provide evidence for alterations in beta 1- and beta 3-AR signalling in BAT of 14-day-old suckling pre-obese Zucker rats with a decreased activity of alpha Gs. The impaired AC responsiveness to catecholamines might be a primary contributor to the development of this genetic obesity.
Charon
C
C
INSERM U 177, Paris, France.
Krief
S
S
Diot-Dupuy
F
F
Strosberg
A D
AD
Emorine
L J
LJ
Bazin
R
R
D000262:Adenylyl Cyclases / Q000378:metabolism*; D002001:Adipose Tissue, Brown / Q000201:enzymology*; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000818:Animals; D000833:Animals, Suckling; D001483:Base Sequence; D005260:Female; D019204:GTP-Binding Proteins / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D007093:Imidazoles / Q000378:metabolism / Q000494:pharmacology; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D008969:Molecular Sequence Data; D009638:Norepinephrine / Q000494:pharmacology; D009765:Obesity / Q000201:enzymology* / Q000235:genetics*; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D011924:Rats, Zucker; D011943:Receptors, Adrenergic, beta / Q000235:genetics / Q000378:metabolism*
D000319:Adrenergic beta-Antagonists; D007093:Imidazoles; D012333:RNA, Messenger; D011943:Receptors, Adrenergic, beta; D006165:Guanylyl Imidodiphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); C049763:CGP 20712A; D019204:GTP-Binding Proteins; D000262:Adenylyl Cyclases; D007545:Isoproterenol; D009638:Norepinephrine