Bryan C. Barnhart

The CD95 Type I/Type II model

CD95 (APO-1/Fas) has become the prototype of a death domain containing receptor and is the best studied member of the death receptors that activate the extrinsic apoptosis pathway. This pathway is initiated by recruitment and activation of caspase-8, an initiator caspase, in the death-inducing signaling complex (DISC) followed by direct cleavage of downstream effector caspases. In contrast, the intrinsic apoptosis pathway starts from within the cell either by direct activation of caspases or through intracellular changes such as DNA damage resulting in the release of a number of pro-apoptotic factors from the intermembrane space of mitochondria. The release of these factors results in the activation of another initiator caspase, caspase-9, and ultimately in the activation of effector caspases in a protein complex called the apoptosome. In recent years, it has become apparent that there is cross talk between the extrinsic and intrinsic pathway. In the death receptor pathway of apoptosis induction, the best characterized connection between the two pathways is the Bcl-2 family member Bid which translocates to mitochondria after cleavage by caspase-8 causing pro-apoptotic changes. Cells that die through CD95 without help from mitochondria are called Type I cells, whereas cells in which CD95-mediated death relies mostly on the intrinsic pathway are called Type II. This review focuses on recent developments in the delineation of the biochemistry and the physiological function of the two CD95 pathways.

Molecular Ordering of the Initial Signaling Events of CD95

ABSTRACT Binding of either ligand or agonistic antibodies to the death receptor CD95 (APO-1/Fas) induces the formation of the death-inducing signaling complex (DISC). We now show that signal initiation of CD95 in type I cells can be further separated into at least four distinct steps. (i) The first step is ligand-induced formation of CD95 microaggregates at the cell surface. (ii) The second step is recruitment of FADD to form a DISC. This step is dependent on actin filaments. (iii) The third step involves formation of large CD95 surface clusters. This event is positively regulated by DISC-generated caspase 8. (iv) The fourth step is internalization of activated CD95 through an endosomal pathway. The latter step is again dependent on the presence of actin filaments. The data indicate that the signal initiation by CD95 is a complex process actively regulated at various levels, providing a number of new drug targets to specifically modulate CD95 signaling.

CD95 ligand induces motility and invasiveness of apoptosis‐resistant tumor cells

The apoptosis‐inducing death receptor CD95 (APO‐1/Fas) controls the homeostasis of many tissues. Despite its apoptotic potential, most human tumors are refractory to the cytotoxic effects of CD95 ligand. We now show that CD95 stimulation of multiple apoptosis‐resistant tumor cells by CD95 ligand induces increased motility and invasiveness, a response much less efficiently triggered by TNFα or TRAIL. Three signaling pathways resulting in activation of NF‐κB, Erk1/2 and caspase‐8 were found to be important to this novel activity of CD95. Gene chip analyses of a CD95‐stimulated tumor cell line identified a number of potential survival genes and genes that are known to regulate increased motility and invasiveness of tumor cells to be induced. Among these genes, urokinase plasminogen activator was found to be required for the CD95 ligand‐induced motility and invasiveness. Our data suggest that CD95L, which is found elevated in many human cancer patients, has tumorigenic activities on human cancer cells. This could become highly relevant during chemotherapy, which can cause upregulation of CD95 ligand by both tumor and nontumor cells.

Two CD95 tumor classes with different sensitivities to antitumor drugs.

CD95 type I and II cells differ in their dependence on mitochondria to execute apoptosis, because antiapoptotic members of the Bcl-2 family render only type II cells resistant to death receptor-induced apoptosis. They can also be distinguished by a more efficient formation of the death-inducing signaling complex in type I cells. We have identified a soluble form of CD95 ligand (S2) that is cytotoxic to type II cells but does not kill type I cells. By testing 58 tumor cell lines of the National Cancer Institutes anticancer drug-screening panel for apoptosis sensitivity to S2 and performing death-inducing signaling complex analyses, we determined that half of the CD95-sensitive cells are type I and half are type II. Most of the type I cell lines fall into a distinct class of tumor cells expressing mesenchymal-like genes, whereas the type II cell lines preferentially express epithelium-like markers. This suggests that type I and II tumor cells represent different stages of carcinogenesis that resemble the epithelial–mesenchymal transition. We then screened the National Cancer Institute database of >42,000 compounds for reagents with patterns of growth inhibition that correlated with either type I or type II cell lines and found that actin-binding compounds selectively inhibited growth of type I cells, whereas tubulin-interacting compounds inhibited growth of type II cells. Our analysis reveals fundamental differences in programs of gene expression between type I and type II cells and could impact the way actin- and microtubule-disrupting antitumor agents are used in tumor therapy.

Apoptosis-independent functions of killer caspases

Caspases are well known for their role in the execution of the apoptotic program by cleaving specific target proteins, leading to the dismantling of the cell, as well as for mediating cytokine maturation. Recent work has highlighted novel non-apoptotic activities of apoptotic caspases. These reports indicate that caspases are much more versatile enzymes than we originally expected. In addition to regulating cell survival and cytokine maturation, caspases may be involved in regulating cell differentiation, cell proliferation, spreading and receptor internalization.

The death effector domain protein family

Apoptosis signaling is regulated and executed by specialized proteins that often carry protein/protein interaction domains. One of these domains is the death effector domain (DED) that is predominantly found in components of the death-inducing signaling complex, which forms at the members of the death receptor family following their ligation. Both proapoptotic- and antiapoptotic-DED-containing proteins have been identified, which makes these proteins exquisitely suited to the regulation of apoptosis. Aside from their pivotal role in the control of the apoptotic program, DED-containing proteins have recently been demonstrated to exert their influence on other cellular processes as well, including cell proliferation. These data highlight the multiple roles for the members of this family, suggesting that they are suited to control both life and death decisions of cells. Additionally, because they can act proapoptotically, antiapoptotically, or in the regulation of the cell cycle, this family of proteins may be excellent candidates for cancer therapy targets.

Induction of apoptosis and activation of NF‐κB by CD95 require different signalling thresholds

Mutations in the death domain of the death receptor CD95 (APO‐1/Fas) cause lymphoproliferation and autoimmune disease in both lprcg mice and in patients with autoimmune lymphoproliferative syndrome (ALPS) type Ia. By testing lymphocytes from ALPS type Ia patients, comparing heterozygous with homozygous lprcg mice and coexpressing wild‐type and mutant CD95 receptors, we demonstrate that induction of apoptosis requires two wild‐type alleles of CD95. By contrast, nuclear factor‐κB (NF‐κB) can be fully activated in cells expressing both a mutant and a wild‐type CD95 allele, suggesting different thresholds to activate the two signalling pathways. This was confirmed by testing lymphocytes from heterozygous lpr mice, which showed reduced sensitivity to CD95‐mediated apoptosis but normal activation of NF‐κB when compared with wild‐type mice. Mutations in CD95 may eliminate the tumour‐suppressive function of CD95, at the same time allowing induction of survival or proliferative pathways, which could contribute to the increased risk for lymphoma seen in ALPS type Ia patients.

The TNF Receptor 1: A Split Personality Complex

The tumor necrosis factor receptor 1 (TNFR1), a prototypic member of the death receptor family signals both cell survival and apoptosis. In this issue of Cell, report that apoptotic TNFR1 signaling proceeds via the sequential formation of two distinct complexes. Since the first complex can activate survival signals and influence the activity of the second complex, this mechanism provides a checkpoint to control the execution of apoptosis.

The relevance of NF-?B for CD95 Signaling in Tumor Cells

Most members of the death receptor family including CD95 (APO-1/Fas) have beenshown to induce both apoptosis as well as nonapoptotic pathways depending on thetissue and the circumstances. One of the nonapoptotic pathways emanating from CD95,activation of NF-?B, has recently been demonstrated to regulate invasiveness ofapoptosis resistant tumor cells. In contrast, activation of NF-?B in apoptosing cells isbelieved to be suppressed due to cleavage of various NF-?B pathway components byactive caspases that execute apoptosis. We now present data demonstrating that incertain highly CD95 apoptosis sensitive cells NF-?B is robustly activated. In factoverexpression of apoptosis inhibitors such as Bcl-2 or c-FLIPL in these cells results indecreased activation of NF-?B through CD95. We propose a model in which NF-?B isgenerally activated in certain cells but may have different functions depending onwhether cells are programmed to die or to survive.

CD95 apoptosis resistance in certain cells can be overcome by noncanonical activation of caspase-8

CD95 apoptosis resistance of tumor cells is often acquired through mutations in the death domain (DD) of one of the CD95 alleles. Furthermore, Type I cancer cells are resistant to induction of apoptosis by soluble CD95 ligand (CD95L), which does not induce efficient formation of the death-inducing signaling complex (DISC). Here, we report that tumor cells expressing a CD95 allele that lacks a functional DD, splenocytes from heterozygous lprcg mice, which express one mutated CD95 allele, and Type I tumor cells stimulated with soluble CD95L can all die through CD95 when protein synthesis or nuclear factor kappa B is inhibited. This noncanonical form of CD95-mediated apoptosis is dependent on the enzymatic activity of procaspase-8 but does not involve fully processed active caspase-8 subunits. Our data suggest that it is possible to overcome the CD95 apoptosis resistance of many tumor cells that do not efficiently form a DISC through noncanonical activation of the caspase-8 proenzyme.