Apoptosis

Introduction Objectives Apoptosis Summary Evaluation Extensions Instructional Tools

Apoptosis Induction and Inhibition

Apoptosis Induction

There are a number of mechanisms through which apoptosis can be induced in cells. The sensitivity of cells to any of these stimuli can vary depending on a number of factors such as the expression of pro- and anti-apoptotic proteins (eg. the Bcl-2 p roteins or the Inhibitor of Apoptosis Proteins), the severity of the stimulus and the stage of the cell cycle. Some of the major stimuli that can induce apoptosis are outlined in the illustration below.

In some cases the apoptotic stimuli comprise extrinsic signals such as the binding of death inducing ligands to cell surface receptors called death receptors. These ligands can either be soluble factors or can be expressed on the surface of cells such as cytotoxic T lymphocytes. The latter occurs when T-cells recognise damaged or virus infected cells and initiate apoptosis in order to prevent damaged cells from becoming neoplastic (cancerous) or virus-infected cells from spreading the infection. Apoptosis can also be induced by cytotoxic T-lymphocytes using the enzyme granzyme.

In other cases apoptosis can be initiated following intrinsic signals that are produced following cellular stress. Cellular stress may occur from exposure to radiation or chemicals or to viral infection. It might also be a consequence of growth factor deprivation or oxidative stress caused by free radicals. In general intrinsic signals initiate apoptosis via the involvement of the mitochondria. The relative ratios of the various bcl-2 proteins can often determine how much cellular stress is necessary to induce apoptosis. This initiation of the caspase cascade leads to the proteolysis of a variety of specific target proteins and mediates the coordinate destruction of organelles and macromolecules during apoptosis.

Apoptosis Inhibition

Fas-induced apoptosis can be effectively blocked at several stages by either FLICE-inhibitory protein (FLIP), by Bcl-2, or by the cytokine response modifier A (CrmA). In addition, activation of caspase-3 by caspase-9 can be blocked by inhibitor of apoptosis proteins (IAPs). Moreover, the protein kinase, Akt, can be activated by various growth factors and its activity can be blocked by PTEN. Akt functions to promote cell survival through two distinct pathways. Akt inhibits apoptosis by phosphorylating the Bcl-2 family member Bad, which then interacts with 14-3-3 and dissociates from Bcl-xL allowing for cell survival. Alternatively, Akt activates IKKa that ultimately leads to NF?B activation and cell survival. Proapoptotic Bcl-2 family members, such as Bax and Bak can promote mitochondrial permeability, while Bcl-2 can inhibit their effects. Upon mitochondrial permeability, apoptogenic factors are released from the mitochondrial inter-membrane space and leak into the cytosol. One factor is cytochrome c, which induces the liberation of protease activators (caspases) that ultimately lead to apoptosis through nuclear damage (DNA fragmentation, DNA mutations). In addition, Smac/Diablo is released and can block IAP inhibition of capsase activity. Mitochondrial permeability is also related to the increased generation of reactive oxygen s pecies (ROS), which plays a role in the degradation phase of apoptosis ( i.e. plasma membrane alterations).