Lustig K D, Conklin B R, Herzmark P, Taussig R, Bourne H R
Department of Pharmacology, University of California School of Medicine, San Francisco 94143.
J Biol Chem. 1993 Jul 5;268(19):13900-5.
Agonists for Gi-coupled receptors augment Gs-stimulated cAMP synthesis in human embryonic kidney (HEK) 293 cells transiently expressing the type II isozyme of adenylylcyclase (AC-II). This augmentation, mediated by beta gamma subunits released from activated Gi, can be blocked by expression of the alpha subunit (alpha t) of retinal transducin (Gt), which presumably sequesters free beta gamma (Federman, A. D., Conklin, B. R., Schrader, K. A., Reed, R. R., and Bourne, H. R. (1992) Nature 356, 159-161). The alpha subunit of Gq, representing a G protein family distinct from both Gs and Gi, mimicked the inhibitory effect of alpha t, suggesting that hormonal stimulation of endogenous Gq might also release beta gamma subunits and thereby augment AC-II activity. Agonists for either of two Gq-coupled receptors did augment Gs-stimulated cAMP synthesis in HEK-293 cells expressing AC-II, but this effect was not blocked by expression of alpha t. The increased stimulation of AC-II was probably not mediated by the release of beta gamma subunits from Gq but rather by activation of protein kinase C (PKC) because of the following. (a) Phorbol esters, which activate PKC directly, elevated cAMP 2-fold in HEK-293 cells transfected with AC-II; this increase was synergistic with Gs-mediated activation of AC-II. (b) Treatments that partially inhibit or down-regulate PKC also partially prevented stimulation of AC-II by phorbol esters or by agonists for Gq-coupled receptors. Taken together, these results indicate that AC-II can integrate regulatory signals transmitted by at least three classes of G proteins; extracellular signals acting through Gs are enhanced synergistically by simultaneous signals transduced by Gi or Gq and mediated via beta gamma or PKC, respectively.
Gi偶联受体的激动剂可增强在瞬时表达腺苷酸环化酶II型同工酶（AC-II）的人胚肾（HEK）293细胞中Gs刺激的cAMP合成。这种增强作用由活化的Gi释放的βγ亚基介导，可被视网膜转导蛋白（Gt）的α亚基（αt）的表达所阻断，αt可能会隔离游离的βγ（费德曼，A.D.，康克林，B.R.，施拉德，K.A.，里德，R.R.，和伯恩，H.R.（1992年）《自然》356，159 - 161）。Gq的α亚基代表了一个不同于Gs和Gi的G蛋白家族，它模拟了αt的抑制作用，这表明内源性Gq的激素刺激也可能释放βγ亚基，从而增强AC-II的活性。两种Gq偶联受体之一的激动剂确实增强了在表达AC-II的HEK - 293细胞中Gs刺激的cAMP合成，但这种作用并未被αt的表达所阻断。AC-II刺激的增加可能不是由Gq释放βγ亚基介导的，而是由蛋白激酶C（PKC）的激活介导的，原因如下：（a）直接激活PKC的佛波酯使转染了AC-II的HEK - 293细胞中的cAMP升高了2倍；这种增加与Gs介导的AC-II激活具有协同作用。（b）部分抑制或下调PKC的处理也部分阻止了佛波酯或Gq偶联受体激动剂对AC-II的刺激。综上所述，这些结果表明AC-II可以整合至少三类G蛋白传递的调节信号；通过Gs起作用的细胞外信号分别由Gi或Gq同时转导并分别通过βγ或PKC介导的信号协同增强。
