Porzig H, Moudry R, Montandon J B
Department of Pharmacology, University of Bern, Switzerland.
J Cell Physiol. 1991 Jun;147(3):439-46. doi: 10.1002/jcp.1041470309.
In intact reticulocytes, but not in fragmented membranes, the loss of adenylate cyclase activity during cell maturation followed a biphasic time course. A rapid phase (t1/2 approximately 2 h) during which the initial activity was reduced by 40-50% was followed by a slow phase with t1/2 close to 3 days. The fast decay seemed to occur on the adenylate cyclase level since (-)isoprenaline- or forskolin-stimulated activities behaved similarly and bacterial toxin-monitored Gs and Gi proteins remained stable. The mechanism of the initial decrease in hormonal responsiveness was further analysed in hybrid cells prepared by fusing reticulocytes with Friend erythroleukemia (MEL) cells. The hybrids contained reticulocyte-derived beta-adrenoceptors and MEL cell-derived adenylate cyclase and G proteins. Fusion of reticulocytes to native MEL cells caused adenylate cyclase activity to drop by 30% at 2 h and 45% at 18 h after fusion. By contrast, hybrids prepared after dimethylsulfoxide-induced differentiation of MEL cells showed stable or increasing rates of receptor-coupled cAMP formation between 2 and 18 h after fusion, concomitant with the enhanced activity of the Gs protein in these cells. A cyclase-stimulating factor present in the cytosol of MEL cells and of reticulocytes appeared not to be involved in short-term regulation of hormonal responsiveness. We conclude that the strength of beta-adrenergic responses in erythroid progenitor cells is primarily regulated by modulating G protein-mediated receptor cyclase coupling while reticulocytes, during early maturation, seem to rely on direct inactivation of adenylate cyclase, probably via a cytosolic proteolytic pathway.
在完整的网织红细胞中，而非破碎的细胞膜中，细胞成熟过程中腺苷酸环化酶活性的丧失呈现双相时间进程。快速阶段（半衰期约2小时），在此期间初始活性降低40 - 50%，随后是缓慢阶段，半衰期接近3天。快速衰减似乎发生在腺苷酸环化酶水平，因为（-）异丙肾上腺素或福斯高林刺激的活性表现相似，且细菌毒素监测的Gs和Gi蛋白保持稳定。通过将网织红细胞与Friend红白血病（MEL）细胞融合制备的杂交细胞，进一步分析了激素反应性初始下降的机制。杂交细胞含有网织红细胞来源的β - 肾上腺素能受体以及MEL细胞来源的腺苷酸环化酶和G蛋白。网织红细胞与天然MEL细胞融合后，融合后2小时腺苷酸环化酶活性下降30%，18小时下降45%。相比之下，MEL细胞经二甲基亚砜诱导分化后制备的杂交细胞在融合后2至18小时显示受体偶联的cAMP形成速率稳定或增加，同时这些细胞中Gs蛋白的活性增强。MEL细胞和网织红细胞胞质中存在的一种环化酶刺激因子似乎不参与激素反应性的短期调节。我们得出结论，红系祖细胞中β - 肾上腺素能反应的强度主要通过调节G蛋白介导的受体 - 环化酶偶联来调节，而网织红细胞在早期成熟过程中，似乎依赖于腺苷酸环化酶的直接失活，可能通过胞质蛋白水解途径。
