Caspase-2 is activated at the CD95 death-inducing signaling complex in the course of CD95-induced apoptosis

Abstract

Caspase-2 was reported to be involved in a number of apoptotic pathways triggered by various stimuli. However, the molecular mechanism of procaspase-2 activation in the course of apoptosis remains poorly defined. In this report, we demonstrate that procaspase-2 is recruited to the CD95 (Fas/APO-1) deathinducing signaling complex (DISC) in human T-and B-cell lines. We show that procaspase-2 is activated at the DISC on CD95 stimulation. Despite its presence at the DISC, caspase-2 does not initiate apoptosis on CD95 stimulation in caspase-8-deficient cell lines. Taken together, our data reveal that caspase-2 is activated at the DISC but does not play an initiating role in the CD95-induced apoptosis. Introduction CD95 (APO-1/Fas) is a member of the death receptor family, a subfamily of the TNF-R superfamily. [1][2][3][4][5] Crosslinking of CD95 with its natural ligand CD95L (FasL/CD178) 6 or with agonistic antibodies such as anti-APO-1 7 induces apoptosis in sensitive cells. The death-inducing signaling complex (DISC) is formed within seconds after CD95 stimulation. 8 The DISC consists of oligomerized, probably trimerized, CD95 receptors; the adaptor molecule FADD; 2 isoforms of procaspase-8 (procaspase-8/a and procaspase-8/b); procaspase-10; and c-FLIP L/S/R . 5,9 The interactions between molecules at the DISC are based on homotypic contacts. The death domain (DD) of the receptor interacts with the DD of FADD, whereas the death effector domain (DED) of FADD interacts with the N-terminal tandem DEDs of procaspase-8, -10, and c-FLIP L/S/R . The binding of procaspase-8 to the DISC results in processing of the zymogen. As a result the active caspase-8 heterotetramer p10 2 -p18 2 is released into the cytosol to propagate the apoptotic signal. 10 Two CD95 signaling pathways were established. 11 Type I cells are characterized by high levels of CD95 DISC formation and increased amounts of active caspase-8. Activated caspase-8 directly leads to the activation of downstream effector caspase-3 and -7. Type II cells are characterized by lower levels of CD95 DISC formation and, thus, lower levels of active caspase-8. 11 In this case, signaling requires an additional amplification loop that involves the cleavage by caspase-8 of the Bcl-2-family protein Bid to generate truncated (t) Bid and subsequent tBid-mediated release of Cytochrome C from mitochondria. The release of Cytochrome C from mitochondria results in apoptosome formation, followed by activation of procaspase-9, which in turn cleaves downstream, effector caspases. 12 Caspase-2 is referred to as an initiator caspase. [13][14][15] It is characterized by the presence of a caspase activation recruitment domain (CARD) and is structurally related to the initiator caspase-9. 16,17 The mechanism of procaspase-2 activation in apoptosis in contrast to other initiator caspases remains poorly defined. It was reported that caspase-2 is implicated in Cytochrome C release and is essential for drug-induced apoptosis in several human cell lines. [18][19][20] On the basis of these dat...

CK2 controls TRAIL and Fas sensitivity by regulating FLIP levels in endometrial carcinoma cells

“…In contradiction to these data, Lavrik and collaborators demonstrated that in T-and B-cell lines, procaspase-2 is recruited and activated at the DISC after Fas engagement. Importantly, authors showed that caspase-2 failed to initiate apoptosis in caspase-8 deficient cells (Droin et al, 2001;Wagner et al, 2004;Lavrik et al, 2006). We have found that CK2 inhibition generates p41/43 fragment of caspase-8 even in absence of TRAIL and, addition of TRAIL or Fas, gives rise to the p18 active fragments and apoptosis.…”

“…Among the caspases activated by CK2 inhibition, the role caspase-2 has been recently studied in death receptor signaling. Caspase-2 is shown to be activated by both stress-induced (Lassus et al, 2002) and death receptor apoptosis (Droin et al, 2001;Wagner et al, 2004;Lavrik et al, 2006). However, its role as initiator caspase in death receptor signaling is controversial.…”

Caspase function in programmed cell death

“…111 Recently, caspase-2 was shown to associate with Fas/CD95 DISC and get activated in that complex, but apparently not required for CD95-induced cell death. 112 Caspase-2 activation occurs rapidly and acute ablation of caspase-2 in a multitude of cell lines inhibits apoptosis in response to a range of stimuli including anticancer drugs. 106,[113][114][115][116][117][118][119][120] Caspase-2 has also been shown to promote MAPK and NFkB activation and may have some functions independent of its protease activity (see review by Lamkanfi et al in this issue).…”

Caspase-2 is required for cell death induced by cytoskeletal disruption

“…Caspase-2 activation has been implicated in apoptosis in response to a range of signals, including DNA damage, withdrawal of trophic support, heat shock, treatment with chemotherapeutic agents and tumor necrosis factor-related apoptosis-inducing ligand (Kumar, 1995;Troy et al, 2001;Lassus et al, 2002;Panaretakis et al, 2005;Shin et al, 2005;Bonzon et al, 2006;Yeung et al, 2006;Mhaidat et al, 2007). Caspase-2 has also been shown to associate with CD95 DISC and is activated within this complex but seems not to be essential for CD95-mediated cell death (Lavrik et al, 2006). The precise function of caspase-2 has remained an enigma as there is no dramatic phenotype observed in Casp2 knockout (KO) mice, apart from a slight increase in the number of oocytes (Bergeron et al, 1998;O'Reilly et al, 2002).…”