Rozwadowski M, Stephen L L, Goss P M, Bray T M, Nagy L E
Department of Nutritional Sciences, University of Guelph, ON, Canada.
J Nutr. 1995 Mar;125(3):401-9. doi: 10.1093/jn/125.3.401.
Glucagon decreases glutathione synthesis in hepatocytes from well-nourished rats. However, in hepatocytes from malnourished rats, glucagon does not inhibit glutathione synthesis, suggesting a desensitization of cAMP-mediated signal transduction. We investigated the mechanism for this desensitization of cAMP-mediated responsiveness in malnourished rats by comparing the signal transduction pathways in rats fed very low protein diets (0.5 g protein/100 g diet) with those of rats fed diets adequate in protein (15 g protein/100 g diet) for 2 wk. Glucagon receptor and forskolin-stimulated cAMP production were greater in hepatocytes from malnourished rats. Stimulation of adenylyl cyclase with forskolin, guanine nucleotides or manganese in hepatic membranes was also enhanced after malnutrition. Moreover, quantity of the stimulatory guanine nucleotide regulatory protein was 70-80% greater in hepatocytes from malnourished rats but the inhibitory guanine nucleotide regulatory protein was not different. These results suggested that desensitization of cAMP-mediated signal transduction after malnutrition occurred at a site distal to cAMP production. Maximal activity of cAMP-dependent protein kinase was 60% lower in liver homogenates from malnourished rats compared with controls. This difference in activity was confined to the cytosolic compartment, with no difference in activity observed in the particulate fraction. Lower activity of cAMP-dependent protein kinase in the cytosol of malnourished rats was associated with a 43% reduction in the quantity of regulatory subunit type I, with no difference in the regulatory subunit type II. These data indicate that desensitization of cAMP signal transduction in rat liver after malnutrition is due to a decrease in the quantity and activity of cAMP-dependent protein kinase.
胰高血糖素可降低营养良好大鼠肝细胞中的谷胱甘肽合成。然而，在营养不良大鼠的肝细胞中，胰高血糖素并不抑制谷胱甘肽合成，这表明cAMP介导的信号转导出现了脱敏现象。我们通过比较喂食极低蛋白饮食（0.5克蛋白质/100克饮食）2周的大鼠与喂食蛋白质充足饮食（15克蛋白质/100克饮食）2周的大鼠的信号转导途径，来研究营养不良大鼠中cAMP介导反应脱敏的机制。营养不良大鼠肝细胞中的胰高血糖素受体和福斯高林刺激的cAMP产生量更大。营养不良后，用福斯高林、鸟嘌呤核苷酸或锰刺激肝细胞膜中的腺苷酸环化酶也增强。此外，营养不良大鼠肝细胞中刺激性鸟嘌呤核苷酸调节蛋白的量增加了70 - 80%，但抑制性鸟嘌呤核苷酸调节蛋白没有差异。这些结果表明，营养不良后cAMP介导的信号转导脱敏发生在cAMP产生的远端位点。与对照组相比，营养不良大鼠肝脏匀浆中cAMP依赖性蛋白激酶的最大活性降低了60%。这种活性差异仅限于胞质部分，颗粒部分未观察到活性差异。营养不良大鼠胞质中cAMP依赖性蛋白激酶活性降低与I型调节亚基数量减少43%有关，II型调节亚基没有差异。这些数据表明，营养不良后大鼠肝脏中cAMP信号转导脱敏是由于cAMP依赖性蛋白激酶的数量和活性降低所致。
