| 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 in which Akt phosphorylates and blocks the activity of proapoptotic proteins such as the BCL2 family member Bad and caspase 9. IGF-1 is a well established survival factor for epithelial cells 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 hrs. 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. 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, 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 IGF-1 gene, 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.;In the absence of growth factors, or with cell-cell contact inhibition, epithelial cells become growth arrested. Stably growth arrested epithelial are no longer susceptible to apoptosis after loss of adhesion. This process also involves the maintenance of IGF-1 expression. Cells that have undergone growth arrest are able to maintain IGF-1 expression by regulating the complement and localization of E2F family members available, and thus the interaction of Rb family members with the available E2F proteins. Loss of Rb-mediated repression results in maintained IGF-1 expression and survival of growth arrested cells. |
