Petersen Pia Steen, Woldbye David P D, Madsen Andreas Nygaard, Egerod Kristoffer L, Jin Chunyu, Lang Manja, Rasmussen Maria, Beck-Sickinger Annette G, Holst Birgitte
Laboratory for Molecular Pharmacology, The Panum Institute, University of Copenhagen, Blegdamsvej 3, DK-2200 Copenhagen, Denmark.
Endocrinology. 2009 Nov;150(11):4920-30. doi: 10.1210/en.2008-1638. Epub 2009 Oct 9.
The receptor for the orexigenic peptide, ghrelin, is one of the most constitutively active 7TM receptors known, as demonstrated under in vitro conditions. Change in expression of a constitutively active receptor is associated with change in signaling independent of the endogenous ligand. In the following study, we found that the expression of the ghrelin receptor in the hypothalamus was up-regulated approximately 2-fold in rats both during 48-h fasting and by streptozotocin-induced hyperphagia. In a separate experiment, to probe for the effect of the high basal signaling of the ghrelin receptor in vivo, we used intracerebroventricular administration by osmotic pumps of a peptide [D-Arg(1), D-Phe(5), D-Trp(7,9), Leu(11)]-substance P. This peptide selectively displays inverse agonism at the ghrelin receptor as compared with an inactive control peptide with just a single amino acid substitution. Food intake and body weight were significantly decreased in the group of rats treated with the inverse agonist, as compared with the groups treated with the control peptide or the vehicle. In the hypothalamus, the expression of neuropeptide Y and uncoupling protein 2 was decreased by the inverse agonist. In a hypothalamic cell line that endogenously expresses the ghrelin receptor, we observed high basal activity of the cAMP response element binding protein, an important signaling transduction pathway for appetite regulation. The activation was further increased by ghrelin administration and decreased by administration of the inverse agonist. It is suggested that the high constitutive signaling activity is important for the in vivo function of the ghrelin receptor in the control of food intake and body weight.
促食欲肽胃饥饿素的受体是已知的组成型活性最强的7次跨膜受体之一，这已在体外条件下得到证实。组成型活性受体表达的变化与不依赖内源性配体的信号传导变化相关。在以下研究中，我们发现，在禁食48小时的大鼠以及链脲佐菌素诱导的食欲亢进大鼠中，下丘脑胃饥饿素受体的表达上调了约2倍。在另一项实验中，为了探究胃饥饿素受体在体内的高基础信号传导作用，我们通过渗透泵向脑室内注射了一种肽[D-精氨酸(1)，D-苯丙氨酸(5)，D-色氨酸(7,9)，亮氨酸(11)]-P物质。与仅单个氨基酸被替换的无活性对照肽相比，该肽在胃饥饿素受体上选择性地表现出反向激动作用。与接受对照肽或赋形剂处理的大鼠组相比，接受反向激动剂处理的大鼠组的食物摄入量和体重显著降低。在下丘脑中，反向激动剂使神经肽Y和解偶联蛋白2的表达降低。在一种内源性表达胃饥饿素受体的下丘脑细胞系中，我们观察到环磷酸腺苷反应元件结合蛋白具有高基础活性，该蛋白是食欲调节的重要信号转导途径。给予胃饥饿素可进一步增强这种激活作用，而给予反向激动剂则可使其减弱。这表明高组成型信号传导活性对于胃饥饿素受体在体内控制食物摄入量和体重的功能很重要。
