Apoptosis - the regulated destruction of a cell - is a complicated process. The decision to die cannot be taken lightly, and the activity of many genes influence a cell's likelihood of activating its self-destruction programme. Once the decision is taken, proper execution of the apoptotic programme requires the coordinated activation and execution of multiple subprogrammes. Here I review the basic components of the death machinery, describe how they interact to regulate apoptosis in a coordinated manner, and discuss the main pathways that are used to activate cell death.

The biochemistry of apoptosis

Apoptosis, an evolutionarily conserved form of cell suicide, requires specialized machinery. The central component of this machinery is a proteolytic system involving a family of proteases called caspases. These enzymes participate in a cascade that is triggered in response to proapoptotic signals and culminates in cleavage of a set of proteins, resulting in disassembly of the cell. Understanding caspase regulation is intimately linked to the ability to rationally manipulate apoptosis for therapeutic gain.

https://science.sciencemag.org/content/sci/281/5381/1312.full.pdf

Caspases: Enemies Within

Beneath the complexity and idiopathy of every cancer lies a limited number of 'mission critical' events that have propelled the tumour cell and its progeny into uncontrolled expansion and invasion One of these is deregulated cell proliferation, which, together with the obligate compensatory suppression of apoptosis needed to support it, provides a minimal 'platform' necessary to support further neoplastic progression Adroit targeting of these critical events should have potent and specific therapeutic consequences

Proliferation, cell cycle and apoptosis in cancer

We have identified two cell types, each using almost exclusively one of two different CD95 (APO‐1/Fas) signaling pathways. In type I cells, caspase‐8 was activated within seconds and caspase‐3 within 30 min of receptor engagement, whereas in type II cells cleavage of both caspases was delayed for ∼60 min. However, both type I and type II cells showed similar kinetics of CD95‐mediated apoptosis and loss of mitochondrial transmembrane potential (ΔΨ m ). Upon CD95 triggering, all mitochondrial apoptogenic activities were blocked by Bcl‐2 or Bcl‐x L overexpression in both cell types. However, in type II but not type I cells, overexpression of Bcl‐2 or Bcl‐x L blocked caspase‐8 and caspase‐3 activation as well as apoptosis. In type I cells, induction of apoptosis was accompanied by activation of large amounts of caspase‐8 by the death‐inducing signaling complex (DISC), whereas in type II cells DISC formation was strongly reduced and activation of caspase‐8 and caspase‐3 occurred following the loss of ΔΨ m . Overexpression of caspase‐3 in the caspase‐3‐negative cell line MCF7‐Fas, normally resistant to CD95‐mediated apoptosis by overexpression of Bcl‐x L , converted these cells into true type I cells in which apoptosis was no longer inhibited by Bcl‐x L . In summary, in the presence of caspase‐3 the amount of active caspase‐8 generated at the DISC determines whether a mitochondria‐independent apoptosis pathway is used (type I cells) or not (type II cells).

/pdf/two-cd95-apo-1-fas-signaling-pathways-4ujtlhag1g.pdf

Two CD95 (APO-1/Fas) signaling pathways

Reactive oxygen species (ROS) are key signaling molecules that play an important role in the progression of inflammatory disorders. An enhanced ROS generation by polymorphonuclear neutrophils (PMNs) at the site of inflammation causes endothelial dysfunction and tissue injury. The vascular endothelium plays an important role in passage of macromolecules and inflammatory cells from the blood to tissue. Under the inflammatory conditions, oxidative stress produced by PMNs leads to the opening of inter-endothelial junctions and promotes the migration of inflammatory cells across the endothelial barrier. The migrated inflammatory cells not only help in the clearance of pathogens and foreign particles but also lead to tissue injury. The current review compiles the past and current research in the area of inflammation with particular emphasis on oxidative stress-mediated signaling mechanisms that are involved in inflammation and tissue injury.

Reactive Oxygen Species in Inflammation and Tissue Injury

The widely expressed protein Fas is a member of the tumour necrosis factor receptor family which can trigger apoptosis1 However, Fas surface expression does not necessarily render cells susceptible to Fas ligand-induced death signals1,2, indicating that inhibitors of the apoptosis-signalling pathway must exist Here we report the characterization of an inhibitor of apoptosis, designated FLIP (for FLICE-inhibitory protein), which is predominantly expressed in muscle and lymphoid tissues The short form, FLIPS, contains two death effector domains and is structurally related to the viral FLIP inhibitors of apoptosis3, whereas the long form, FLIPL, contains in addition a caspase-like domain in which the active-centre cysteine residue is substituted by a tyrosine residue FLIPS and FLIPL interact with the adaptor protein FADD4,5 and the protease FLICE6,7, and potently inhibit apoptosis induced by all known human death receptors1 FLIPL is expressed during the early stage of T-cell activation, but disappears when T cells become susceptible to Fas ligand-mediated apoptosis High levels of FLIPL protein are also detectable in melanoma cell lines and malignant melanoma tumours Thus FLIP may be implicated in tissue homeostasis as an important regulator of apoptosis

/pdf/inhibition-of-death-receptor-signals-by-cellular-flip-1848gtju9y.pdf

Inhibition of death receptor signals by cellular FLIP

Characterization of Fas (Apo-1, CD95)-Fas ligand interaction.

Fas ligand (FasL, Apo-1L) is a member of the tumor necrosis factor protein family and binding to its receptor (Fas, Apo-1, CD95) triggers cell death through apoptosis. Ligand expression is restricted to cells with known cytolytic activity and found on hematopoietic cells of the T cell and natural killer lineage. Here we provide evidence that B lymphocytes can express FasL. Flow cytometric analysis revealed that FasL is expressed on the surface of B cells upon stimulation with either lipopolysaccharide or phorbol 12-myristate 13-acetate/ionomycin. FasL expression on activated B cells was confirmed by western blot and reverse transcriptase polymerase chain reaction analysis. FasL on B cells is functional since lipopolysaccharide-activated B lymphocytes derived from wild type, but not from gld mutant mice, were able to kill Fas-sensitive target cells. Our data suggest that the Fas system may contribute to the control of B cell homeostasis.

Activated B cells express functional Fas ligand

Fas-mediated programmed cell death contributes to the pathology of thyroiditis and hepatitis (pages 409−413).

Jürg Tschopp

Papers

Inhibition of death receptor signals by cellular FLIP

Characterization of Fas (Apo-1, CD95)-Fas ligand interaction.

Activated B cells express functional Fas ligand

Thyroiditis and hepatitis: Fas on the road to disease

Cytotoxic T cells: more weapons for new targets?