Kulik G, Weber M J
Department of Microbiology, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908, USA.
Mol Cell Biol. 1998 Nov;18(11):6711-8. doi: 10.1128/MCB.18.11.6711.
Protein kinase B (PKB)/Akt is implicated in survival signaling in a wide variety of cells including fibroblasts and epithelial and neuronal cells. We and others have described a linear survival signaling cascade used by insulinlike growth factor I (IGF-I) that consists of the IGF-I receptor, phosphoinositide 3-kinase (PI3 kinase), Akt, and Bad. Activation of this pathway can be sufficient to protect cells from apoptosis. However, previous work had not determined whether this pathway is invariably necessary for protection from apoptosis or whether there are alternative survival signaling pathways. In this communication, we report the existence of two survival signaling pathways, one dependent on PI3 kinase and Akt and the other independent of these enzymes. We found that survival signaling initiated by IGF-I treatment of Rat-1 cells could be blocked by overexpression of a dominant negative kinase-deficient Akt (K179A) as well as by wortmannin. This demonstrates a survival signaling pathway dependent on PI3 kinase and Akt. However, when IGF-I receptors were overexpressed in a Rat-1 background (RIG cells), an alternative pathway became apparent, in which survival mediated by IGF-I was no longer sensitive to wortmannin or to overexpression of dominant negative Akt, even though Akt activation and Bad phosphorylation were still wortmannin sensitive. Experiments with inhibitors of RNA synthesis showed that transcriptional activation is dispensable for this alternative PI3 kinase/Akt-independent survival signaling. These findings demonstrate the existence of a new survival signaling pathway independent of PI3 kinase, Akt, and new transcription and which is evident in fibroblasts overexpressing the IGF-I receptor.
蛋白激酶B（PKB）/Akt参与多种细胞（包括成纤维细胞、上皮细胞和神经元细胞）的存活信号传导。我们和其他人已经描述了胰岛素样生长因子I（IGF-I）所使用的线性存活信号级联反应，该级联反应由IGF-I受体、磷酸肌醇3激酶（PI3激酶）、Akt和Bad组成。激活该途径足以保护细胞免于凋亡。然而，先前的研究尚未确定该途径对于防止凋亡是否始终必要，或者是否存在其他存活信号途径。在本通讯中，我们报告了两种存活信号途径的存在，一种依赖于PI3激酶和Akt，另一种独立于这些酶。我们发现，用IGF-I处理大鼠-1细胞引发的存活信号可被显性负性激酶缺陷型Akt（K179A）的过表达以及渥曼青霉素阻断。这证明了一种依赖于PI3激酶和Akt的存活信号途径。然而，当在大鼠-1背景（RIG细胞）中过表达IGF-I受体时，一种替代途径变得明显，其中IGF-I介导的存活不再对渥曼青霉素或显性负性Akt的过表达敏感，尽管Akt激活和Bad磷酸化仍然对渥曼青霉素敏感。用RNA合成抑制剂进行的实验表明，转录激活对于这种替代的不依赖PI3激酶/Akt的存活信号传导是不必要的。这些发现证明了一种新的存活信号途径的存在，该途径独立于PI3激酶、Akt和新的转录，并且在过表达IGF-I受体的成纤维细胞中很明显。
