Yu J T, Foster R G, Dean D C
Division of Molecular Oncology, Departments of Medicine and Cell Biology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Mol Cell Biol. 2001 May;21(10):3325-35. doi: 10.1128/MCB.21.10.3325-3335.2001.
Mutations that lead to anchorage-independent survival are a hallmark of tumor cells. Adhesion of integrin receptors to extracellular matrix activates a survival signaling pathway in epithelial cells where Akt phosphorylates and blocks the activity of proapoptotic proteins such as the BCL2 family member Bad, the forkhead transcription factor FKHRL-1, and caspase 9. Insulin-like growth factor 1 (IGF-1) is a well-established epithelial cell survival factor that also triggers activation of Akt and can maintain Akt activity after cells lose matrix contact. It is not until IGF-1 expression diminishes (~16 h after loss of matrix contact) that epithelial cells deprived of matrix contact undergo apoptosis. This suggests that IGF-1 expression is linked to cell adhesion and that it is the loss of IGF-1 which dictates the onset of apoptosis after cells lose matrix contact. Here, we examine the linkage between cell adhesion and IGF-1 expression. While IGF-1 is able to maintain Akt activity and phosphorylation of proapoptotic proteins in cells that have lost matrix contact, Akt is not able to phosphorylate and inactivate another of its substrates, glycogen synthase kinase 3beta (GSK-3beta), under these conditions. The reason for this appears to be a rapid translocation of active Akt away from GSK-3beta when cells lose matrix contact. One target of GSK-3beta is cyclin D, which is turned over in response to this phosphorylation. Therefore, cyclin D is rapidly lost when cells are deprived of matrix contact, leading to a loss of cyclin-dependent kinase 4 activity and accumulation of hypophosphorylated, active Rb. This facilitates assembly of a repressor complex containing histone deacetylase (HDAC), Rb, and E2F that blocks transcription of the gene for IGF-1, leading to loss of Akt activity, accumulation of active proapoptotic proteins, and apoptosis. This feedback loop containing GSK-3beta, cyclin D, HDAC-Rb-E2F, and IGF-1 then determines how long Akt will remain active after cells lose matrix contact, and thus it serves to regulate the onset of apoptosis in such cells.
导致不依赖贴壁存活的突变是肿瘤细胞的一个标志。整合素受体与细胞外基质的黏附激活上皮细胞中的一条存活信号通路，在该通路中Akt磷酸化并阻断促凋亡蛋白的活性，如BCL2家族成员Bad、叉头转录因子FKHRL-1和半胱天冬酶9。胰岛素样生长因子1（IGF-1）是一种公认的上皮细胞存活因子，它也能触发Akt的激活，并能在细胞失去与基质的接触后维持Akt的活性。直到IGF-1表达减少（失去基质接触后约16小时），失去基质接触的上皮细胞才会发生凋亡。这表明IGF-1的表达与细胞黏附有关，并且正是IGF-1的丧失决定了细胞失去基质接触后凋亡的开始。在这里，我们研究细胞黏附与IGF-1表达之间的联系。虽然IGF-1能够在失去基质接触的细胞中维持Akt活性和促凋亡蛋白的磷酸化，但在这些条件下，Akt无法磷酸化并使其另一个底物糖原合酶激酶3β（GSK-3β）失活。出现这种情况的原因似乎是当细胞失去基质接触时，活性Akt迅速从GSK-3β移位。GSK-3β的一个靶点是细胞周期蛋白D，它会因这种磷酸化而被周转。因此，当细胞被剥夺基质接触时，细胞周期蛋白D会迅速丢失，导致细胞周期蛋白依赖性激酶4活性丧失以及低磷酸化的活性Rb积累。这有利于组装一个包含组蛋白去乙酰化酶（HDAC）、Rb和E2F的阻遏复合物，该复合物会阻断IGF-1基因的转录，导致Akt活性丧失、活性促凋亡蛋白积累以及凋亡。这个包含GSK-3β、细胞周期蛋白D、HDAC-Rb-E2F和IGF-1的反馈环决定了细胞失去基质接触后Akt保持活性的时间，因此它有助于调节此类细胞中凋亡的开始。
