Kai H, Fukui T, Lassègue B, Shah A, Minieri C A, Griendling K K
Department of Medicine, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
Mol Pharmacol. 1996 Jan;49(1):96-104.
Recent studies have shown that G proteins are a potential regulatory site in the transmembrane signaling cascade. The aim of this study was to examine the effects of prolonged agonist exposure on expression of the Gq class of G protein alpha subunits (G alpha q/G alpha 11) in cultured rat vascular smooth muscle cells (VSMC). Treatment with 100 nM angiotensin II (Ang II) led to a substantial sustained down-regulation of cellular levels of immunologically detectable G alpha q/G alpha 11 by 50% within 6 hr. The effect of Ang II was dose dependent with an EC50 of 2 nM and was specifically blocked by the vascular type-1 Ang II receptor-specific antagonist losartan. The Ang II-induced reduction in cellular levels of G protein alpha subunits was specific for G alpha q/G alpha 11. The calcium ionophore ionomycin or activators of ubiquitous protein kinases (phorbol-12-myristate-13-acetate, forskolin, and 8-bromo-cGMP) did not mimic the effects of Ang II. However, [Arg8]vasopressin also induced a significant loss in cellular G alpha q/G alpha 11 levels. Ang II-induced G alpha q/G alpha 11 down-regulation was reversed by prevention of cellular receptor processing with phenylarsine oxide or chronic potassium depletion. The effects of Ang II on G alpha q/G alpha 11 levels were inhibited when protein kinase C activity was abolished. G alpha q mRNA levels were down-regulated by 30% after 4-hr incubation with Ang II, in part by transcriptional regulation. Although a short term vasopressin pretreatment had no effect on inositol-1,4,5-trisphosphate (IP3) generation in response to subsequent Ang II stimulation, a partial heterologous desensitization of the IP3 response was induced after a long term vasopressin pretreatment, which concurrently down-regulated cellular G alpha q/G alpha 11 levels. Homologous desensitization of IP3 generation on a second Ang II stimulation was observed after both a short and long term Ang II pretreatment. In conclusion, prolonged exposure to Ang II induces down-regulation of cellular G alpha q/G alpha 11 levels in intact VSMC. The effect of Ang II appears to be mediated by the signaling pathway sensitive to inhibition of receptor processing. The present study raises the possibility that agonist-induced G alpha q/G alpha 11 down-regulation participates in the mechanism of long term desensitization of the G alpha q/G alpha 11-mediated signaling system in VSMC.
最近的研究表明，G蛋白是跨膜信号级联反应中的一个潜在调节位点。本研究的目的是检测在培养的大鼠血管平滑肌细胞（VSMC）中，长时间暴露于激动剂对G蛋白α亚基Gq类（Gαq/Gα11）表达的影响。用100 nM血管紧张素II（Ang II）处理导致在6小时内免疫可检测的Gαq/Gα11细胞水平持续大幅下调50%。Ang II的作用呈剂量依赖性，EC50为2 nM，并被血管1型Ang II受体特异性拮抗剂氯沙坦特异性阻断。Ang II诱导的G蛋白α亚基细胞水平降低对Gαq/Gα11具有特异性。钙离子载体离子霉素或普遍存在的蛋白激酶激活剂（佛波醇-12-肉豆蔻酸酯-13-乙酸酯、福斯可林和8-溴-cGMP）不能模拟Ang II的作用。然而，[精氨酸8]加压素也诱导细胞Gαq/Gα11水平显著降低。通过苯胂氧化物预防细胞受体加工或慢性钾缺乏可逆转Ang II诱导的Gαq/Gα11下调。当蛋白激酶C活性被消除时，Ang II对Gαq/Gα11水平的作用受到抑制。与Ang II孵育4小时后，Gαq mRNA水平下调30%，部分是通过转录调节。虽然短期加压素预处理对随后Ang II刺激引起的肌醇-1,4,5-三磷酸（IP3）生成没有影响，但长期加压素预处理后诱导了IP3反应的部分异源脱敏，同时下调了细胞Gαq/Gα11水平。在短期和长期Ang II预处理后，均观察到第二次Ang II刺激时IP3生成的同源脱敏。总之，在完整的VSMC中，长时间暴露于Ang II会诱导细胞Gαq/Gα11水平下调。Ang II的作用似乎是由对受体加工抑制敏感的信号通路介导的。本研究提出了激动剂诱导的Gαq/Gα11下调参与VSMC中Gαq/Gα11介导的信号系统长期脱敏机制的可能性。
