Marcus E. Peter, Ph.D., Habil

Apoptosis Signaling Pathways

Research Summary

In the lab of Marcus Peter apoptosis signaling pathways are studied. Apoptosis is a fundamental process to regulate homeostasis of all tissues and to eliminate unwanted cells specifically in the immune system. Various parts of apoptosis signaling pathways have recently been characterized. Specifically in apoptosis pathways initiated by members of the death receptor family such as CD95 (APO-1/Fas) proteins that either contain a death domain (DD) or a death effector domain (DED) have been found to be essential.

The Peter lab is working on various aspects of the signal transduction of the apoptosis inducing death receptors with special emphasis on the CD95 receptor and the function of DED proteins. The following projects are currently studied:

1. Signaling through CD95.
The Peter lab initially described that upon induction of apoptosis CD95 recruits the DD and DED containing adapter molecule FADD and the DED containing cysteine protease caspase-8, caspase-10 and the caspase-8 regulator c-FLIP to the activated receptor forming the death-inducing signaling complex (DISC) (1,2,3). Binding of the proenzyme procaspase-8 to the DISC results in its activation and the release of active caspase-8 into the cytoplasm where it can cleave a number death substrates including caspase-3, BID and proteins of the cytoskeleton such as plectin (4,5,6). Subsequent studies demonstrated that caspase-8 has two ways to activate the downstream apoptosis machinery depending on the cell type (7,8). In Type I cells caspase-8 is activated at the death inducing signaling complex (DISC) in large quantities resulting in direct processing of caspase-3. This step is independent on mitochondria and cannot be blocked by overexpression of Bcl-2. In Type II cells the amount of active caspase-8 generated at the DISC is very small. Apoptosis in Type II cells depends on the apoptogenic activity of mitochondria and is characterized by activation of large quantities of caspase-3 and caspase-8 downstream of the mitochondria. Only in these cells overexpression of Bcl-2 or Bcl-xL blocks activation of both caspases and apoptosis. Recently the Peter lab reported reported that CD95 clusters and internalizes in a caspase-8 and actin dependent fashion (9). However, this activity is restricted to Type I cells (10).

2. CD95 Type I and Type II cell represent mesenchymal and epithelial cells, respectively.
After years of controversy on the physiological function of CD95 ligand CD95L (11) the Peter lab demonstrated recently year that the physiologic CD95 ligand is highly toxic to Type II cells but does not induce apoptosis on Type I cells (12). This allowed to type the 60 tumor cells of the NCI drug screening panel resulting in the discovery that Type I cells represent mesenchymal cells whereas Type II cells correspond to epithelial cells, respectively. It was also found that Type I and Type II tumor cells differ in their sensitivity to anti tumor drugs that target two major cytoskeleton systems. Type I cells are sensitive to all actin binding drugs whereas Type II cells are sensitive to tubulin binding drugs. These findings are relevant for activated T cells since it was previously shown that T cells differentiate from Type II to Type I during long-term T cell activation (13). Recent evidence suggest that the reason for the different signaling through CD95 in short-term versus long-term activated peripheral T-cells could be in the way the DISC is forming. Similar to the TNF-receptor I the DISC in Type II cells seems to form intracellularily (14).

3. The death receptor CD95 can act as a tumorigenic receptor on apoptosis resistant tumor cells through activation of NF-kB and MAP kinases.
Most tumor cells are resistant to CD95 mediated apoptosis but today not a single tumor has been reported that completely lacks CD95 expression. The Peter lab recently showed that on CD95 apoptosis resistant tumor cells and CD95 sensitive Type I tumor cells when treated with soluble CD95L stimulation of CD95 results in activation of NF-kB and MAP kinases inducing increased motility and invasiveness of tumor cells (15). These novel findings underscore the novel role of death pathways and their components outside of apoptosis which are studied in the Peter lab (16,17).

4. The role of the phosphorylation of FADD in cell cycle progression.
FADD is an adaptor molecule that allows recruitment of the initiator caspase-8 to the stimulated death receptor CD95 upon induction of apoptosis (1). However, it has been shown previously that FADD is also important for nonapoptotic processes such as cell cycle progression and T cell activation. The way FADD elicits these activities was unknown. The Peter lab recently reported that it is the specific phosphorylation of FADD on serine 194 they identified previously (18) that regulates its cell regulating activities pointing at the unknown kinase that phosphorylates FADD as important link between death receptor signaling and cell cycle progression (19). Once this kinase is identified the role of the phosphorylation of FADD in T cell activation can be determined.

5. DEDD as a platform protein to regulate activation of caspases.
As members of the death effector domain proteins (20) the Peter lab previously cloned DEDD and DEDD2 (21,22). DEDD was found not only to be monoubiquitinated but also to be a strong ubiquitin binding protein (23). Ubiquitination and especially monoubiquitination has recently been recognized to be an important posttranslational modification found on proteins regulating apoptosis signaling (24). Monoubiquitination of DEDD seems to be important for DEDD to function as a platform protein to recruit and activate caspase-3 (22) and caspase-9 (23). This suggests that DEDD is required for the activation of caspase-9 which so far has been believed to be exclusively activated by the apoptosome, a multi-protein structure that comprises cytochrome c, Apaf-1 procaspase-9 and dATP. DEDD could therefore form an alternative apoptosome making it an important general apoptosis regulator.

Selected Papers

Kischkel FC, Hellbardt S, Behrmann I, Germer M, Pawlita M, Krammer PH and Peter ME. (1995). Cytotoxicity-dependent APO-1(Fas/CD95)-associated proteins form a death-inducing signalling complex (DISC) with the receptor. EMBO J. 14, 5579-5588.

Muzio M, Chinnaiyan AM, Kischkel FC, O'Rourke K, Shevchenko A, Scaffidi C, Zhang M, Ni J, Gentz R, Mann M, Krammer PH, Peter ME* and Dixit VM.* (1996). FLICE, a novel FADD-homologous ICE/CED-3-like protease, is recruited to the CD95 (Fas/APO-1) death-inducing signaling complex (DISC), Cell 85, 817-827. * Authors share senior authorship.

Medema JP, Scaffidi C, Kischkel FC, Shevchenko A, Mann M, Krammer PH and Peter ME. (1997). FLICE is activated by association with the CD95 death-inducing signaling complex (DISC). EMBO J. 16, 2794-2804.

Scaffidi C, Fulda S, Srinivasan A, Li F, Friesen C, Tomasseli KJ, Debatin K-M, Krammer PH and Peter ME. (1998) Two CD95 (APO-1/Fas) signaling pathways. EMBO J., 17, 1675-1687.

Stegh AH, Schickling O, Ehret A, Scaffidi C, Peterhänsel C, Längst G, Hoffmann T, Grummt I, Krammer PH and Peter ME. (1998). DEDD, a novel death effector containing apoptosis-inducing protein targeted to nucleoli. EMBO J. 17, 5974-5986.

Stegh AH, Herrmann H, Lampel S, Weisenberger D, Andrä K, Seper M, Wiche G, Krammer PH and Peter ME. (2000). Identification of the cytolinker plectin as a major early in vivo substrate for caspase-8 during CD95 and TNF-receptor mediated apoptosis. Mol. Cell. Biol., 20, 5665-5679.

Algeciras-Schimnich A, Shen L, Barnhart BC, Murmann AE, Burkhardt J and Peter ME. (2002). Molecular ordering of the initial signaling events of CD95. Mol. Cell. Biol. 22, 207-220.

Lee JC, Schickling O, Stegh AH, Oshima R, Dingsdale D, Cohen GM and Peter ME. (2002). DEDD regulates degradation of intermediate filaments during apoptosis. J. Cell Biol. 158, 1051-1066.

Algeciras-Schimnich A, Pietras E, Barnhart BC, Legembre P, Vijayan S, Holbeck SL and Peter ME. (2003). Two CD95 tumor classes with different sensitivites to antitumor drugs. Proc. Natl. Acad. Sci. USA, 100, 11445-11450.

Dinsdale D, Lee JC, Dewson G, Cohen^, GM and Peter ME. (2004). Intermediate filaments control the intracellular distribution of caspases during apoptosis. Am. J. Pathol., 164, 395-407.

Barnhart BC, Legembre P, Pietras E, Bubici C, Franzoso G and Peter ME. (2004). CD95 ligand induces motility and invasiveness of apoptosis resistant tumor cells. EMBO J. 23, 3175-3185.

Legembre^, P, Barnhart BC, Zheng L, Vijayan S, Straus SE, Puck J, Dale JK, Lenardo M and Peter ME. (2004). Induction of apoptosis and activation of NF-kB by CD95 require different signaling thresholds. EMBO Reports, 5, 1084-1089.

Barnhart BC, Pietras E, Algeciras-Schimnich A, Salmena L, Hakem R and Peter ME. (2005). Overcoming CD5 apoptosis resistance in certain cells through noncanonical activation of caspase-8. Cell Death Differ. 12, 25-37.

Vijayan S, Zhou P, Rajapaksha TW, Alegre ML and Peter ME. (2005). Transplanted islets from lpr mice are resistant to autoimmune destruction in a model of streptozotocin-induced type I diabetes. Apoptosis, 10, 725-730.

Alappat EC, Feig C, Boyerinas B, Volkland J, Samuels M, Murmann AE, Thorburn A, Kidd VJ, Slaughter CA, Osborn S, Winoto A, Tang W-J and Peter ME. (2005). Phosphorylation of FADD at serine 194 by CKIa regulates its nonapoptotic activities. Mol Cell, 19, 321-332.

Lee K-H, Feig C, Tchikov V, Schickel R, Hallas C, Schuetze S, Peter ME* and Chan AC.* (2006). The role of receptor internalization in CD95 signaling. EMBO J., 25, 1009-1023. * shared senior authorship.