Berg K A, Stout B D, Cropper J D, Maayani S, Clarke W P
Department of Pharmacology, University of Texas Health Science Center, San Antonio, Texas 78284-7764, USA.
Mol Pharmacol. 1999 May;55(5):863-72.
In cell systems where ligand-independent receptor activity is optimized (such as when receptors are overexpressed or mutated), acute treatment with inverse agonists reduces basal effector activity whereas prolonged exposure leads to sensitization of receptor systems and receptor up-regulation. Few studies, however, have reported effects of inverse agonists in systems where nonmutated receptors are expressed at relatively low density. Here, we investigated the effects of inverse agonists at human serotonin (5-HT)2C receptors expressed stably in Chinese hamster ovary cells ( approximately 250 fmol/mg protein). In these cells, there is no receptor reserve for 5-HT and 5-HT2C inverse agonists did not reduce basal inositol phosphate (IP) accumulation nor arachidonic acid (AA) release but behaved as simple competitive antagonists, suggesting that these receptors are not overexpressed. Prolonged treatment (24 h) with inverse agonists enhanced selectively 5-HT2C-mediated IP accumulation but not AA release. The enhancing effect occurred within 4 h of treatment, reversed within 3 to 4 h (after 24-h treatment), and could be blocked with neutral antagonists or weak positive agonists. The enhanced responsiveness was not due to receptor up-regulation but may involve changes in the expression of the G protein, Galphaq/11 and possibly Galpha12 and Galpha13. Interestingly, 24-h exposure to inverse agonists acting at 5-HT2C receptors also selectively enhanced IP accumulation, but not AA release, elicited by activation of endogenous purinergic receptors. These data suggest that actions of inverse agonists may be mediated through effects on receptor systems that are not direct targets for these drugs.
在配体非依赖性受体活性得到优化的细胞系统中（例如受体过表达或发生突变时），用反向激动剂进行急性处理可降低基础效应器活性，而长时间暴露则会导致受体系统敏感化和受体上调。然而，很少有研究报道反向激动剂在非突变受体以相对低密度表达的系统中的作用。在此，我们研究了反向激动剂对稳定表达于中国仓鼠卵巢细胞（约250 fmol/mg蛋白）中的人5-羟色胺（5-HT）2C受体的影响。在这些细胞中，5-HT没有受体储备，5-HT2C反向激动剂既不降低基础肌醇磷酸（IP）积累，也不降低花生四烯酸（AA）释放，而是表现为简单的竞争性拮抗剂，这表明这些受体没有过表达。用反向激动剂进行长时间处理（24小时）可选择性增强5-HT2C介导的IP积累，但不增强AA释放。增强作用在处理4小时内出现，在（24小时处理后）3至4小时内逆转，并且可被中性拮抗剂或弱激动剂阻断。增强的反应性不是由于受体上调，而是可能涉及G蛋白Gαq/11以及可能的Gα12和Gα13表达的变化。有趣的是，对作用于5-HT2C受体的反向激动剂进行24小时暴露，也选择性增强了内源性嘌呤能受体激活引起的IP积累，但不增强AA释放。这些数据表明，反向激动剂的作用可能是通过对并非这些药物直接靶点的受体系统的影响来介导的。
