Transcriptional repression by Rb-E2F and regulation of anchorage-independent survival

Mutations that lead to anchorage-independent survival are a hallmark of tum or cells. Adhesion of integrin receptors to extracellular matrix activates a survival signaling pathway in epithelial cells where Akt phosphorylates a nd blocks the activity of proapoptotic proteins such as the BCL2 family mem ber Bad, the forkhead transcription factor FKHRL-1, and caspase 9. Insulin- like growth factor I (IGP-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 diminishe s (similar to 16 h after loss of matrix contact) that epithelial cells depr ived of matrix contact undergo apoptosis. This suggests that IGF-I expressi on is linked to cell adhesion and that it is the loss of IGF-1 which dictat es 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 abl e to maintain Akt activity and phosphorylation of proapoptotic proteins in cells that have lost matrix contact, Akt is not able to phosphorylate and i nactivate another of its substrates, glycogen synthase kinase 3 beta (GSK-3 beta), under these conditions. The reason for this appears to be a rapid t ranslocation of active Akt away from GSK-3 beta when cells lose matrix cont act. One target of GSK-3 beta is cyclin D, which is turned over in response to this phosphorylation. Therefore, cyclin D is rapidly last when cells ar e deprived of matrix contact, leading to a loss of cyclin-dependent kinase 4 activity and accumulation of hypophosphorylated, active Rb. This facilita tes assembly of a repressor complex containing histone deacetylase (HDAC), Rb, and E2F that blocks transcription of the gene for IGF-1, leading to los s of Akt activity, accumulation of active proapoptotic proteins, and apopto sis. This feedback loop containing GSK-3 beta, cyclin D, HDAC-Rb-E2F, and I GF-1 then determines how long Akt will remain active after cells lose matri x contact, and thus it serves to regulate the onset of apoptosis in such ce lls.