Conversion of CD95 (Fas) Type II into Type I signaling by sub-lethal doses of cycloheximide

Constitutive activity of Erk1/2 and NF-κB protects human endometrial stromal cells from death receptor-mediated apoptosis

Apoptosis in the human endometrium plays an essential role for endometrial receptivity and early implantation. A dysbalance of pro- and anti-apoptotic events in the secretory endometrium seems to be involved in implantation disorders and consecutive pregnancy complications. However, little is known about the mechanisms regulating apoptosis-sensitivity in the human endometrium. Therefore this study was performed to identify molecular mechanisms underlying the resistance toward apoptosis in human endometrial stromal cells (ESCs). Human ESCs were isolated from hysterectomy specimens and used as undifferentiated cells or after decidualization in vitro. Cells were incubated with an activating anti-Fas antibody, tumor-necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), TNF-α and inhibitors of protein- and RNA-syntheses, a caspase-inhibitor and inhibitors of extracellular signal regulated kinase (Erk)1/2, nuclear factor (NF)-κB and Akt. Apoptosis was measured by flow cytometric detection of hypodiploid nuclei. Caspase-activity was detected by luminescencent assays. Several pro- and anti-apoptotic molecules and the activation of Erk1/2, NF-κB and Akt were analyzed by in-cell Western assays or flow cytometry. Inhibition of protein- and RNA-syntheses differentially sensitized human ESCs for death receptor-mediated apoptosis in a caspase-dependent manner, based on the up-regulation of the death receptors Fas and TRAIL-R2. The constitutive activity of Erk1/2 and NF-κB could be identified as a reason for the apoptosis-resistance of human ESCs. These results suggest the pro-survival signaling pathways Erk1/2 and NF-κB as key regulators of the sensitivity of human ESCs for death receptor-mediated apoptosis. The modulation of these pathways might play an important role in the physiology of implantation.

Apoptotic potential of Fas-associated death domain on regulation of cell death regulatory protein cFLIP and death receptor mediated apoptosis in HEK 293T cells

Fas-associated death domain (FADD) is a common adaptor molecule which plays an important role in transduction of death receptor mediated apoptosis. The FADD provides DED motif for binding to both procaspase-8 and cFLIP molecules which executes death receptor mediated apoptosis. Dysregulated expression of FADD and cFLIP may contribute to inhibition of apoptosis and promote cell survival in cancer. Moreover elevated intracellular level of cFLIP competitively excludes the binding of procaspase-8 to the death effector domain (DED) of FADD at the DISC to block the activation of death receptor signaling required for apoptosis. Increasing evidence shows that defects in FADD protein expression are associated with progression of malignancies and resistance to apoptosis. Therefore, improved expression and function of FADD may provide new paradigms for regulation of cell proliferation and survival in cancer. In the present study, we have examined the potential of FADD in induction of apoptosis by overexpression of FADD in HEK 293T cells and validated further its consequences on the expression of pro and anti-apoptotic proteins besides initiation of death receptor mediated signaling. We have found deficient expression of FADD and elevated expression of cFLIP(L) in HEK 293T cells. Our results demonstrate that over expression of FADD attenuates the expression of anti-apoptotic protein cFLIP and activates the cascade of extrinsic caspases to execution of apoptosis in HEK 293T cells.

Potential for modulation of the fas apoptotic pathway by epidermal growth factor in sarcomas

One important mechanism by which cancer cells parasitize their host is by escaping apoptosis. Thus, selectively facilitating apoptosis is a therapeutic mechanism by which oncotherapy may prove highly advantageous. One major apoptotic pathway is mediated by Fas ligand (FasL). The death-inducing signaling Ccmplex (DISC) and subsequent death-domain aggregations are created when FasL is bound by its receptor thereby enabling programmed cell death. Conceptually, if a better understanding of the Fas pathway can be garnered, an oncoselective prodeath therapeutic approach can be tailored. Herein, we propose that EGF and CTGF play essential roles in the regulation of the Fas apoptotic pathway in sarcomas. Tumor and in vitro data suggest viable cells counter the prodeath signal induced by FasL by activating EGF, which in turn induces prosurvival CTGF. The prosurvival attributes of CTGF ultimately predominate over the death-inducing FasL. Cells destined for elimination inhibit this prosurvival response via a presently undefined pathway. This scenario represents a novel role for EGF and CTGF as regulators of the Fas pathway in sarcomas.

Mechanisms Conferring Resistance to Pro-Apoptotic Cancer Gene Therapy

Recently, we have described a novel approach to the treatment of cancer that employs a series of vectors that encode surface expressed chimeric proteins in which the cytoplasmic death domain of Fas is fused in-frame to the extracellular domain of one of a number of cell surface receptors that recognize and bind various ligands that are differentially expressed within the tumor microenvironment. Although the majority of tumor cells transduced with such vectors are killed in the presence of the corresponding cognate ligand, a small percentage survive and in vivo may go on to repopulate a treated tumor. In order to understand the mechanisms employed by tumors to escape the cytotoxic effects of pro-apoptotic signals triggered via Fas, we isolated a large number of 293 tumor cell clones that survive following transfection with a plasmid vector encoding Flk-1/Fas, a chimeric receptor that induces tumor cell death in the presence of the pro-angiogenic cytokine VEGF. Characterization of Flk-1/Fas-positive clones revealed that while survival can most often be attributed simply to the down-regulation of VEGF ligand expression, in cells that express both receptor and ligand, other proteins involved in the regulation of apoptosis may be targeted. Specifically, a Flk-1/Fas-positive, VEGF-positive clone was identified in which expression of APAF-1 was almost completely abrogated.

Switch in Fas-activated death signaling pathway as result of keratin 8/18-intermediate filament loss

Fas-induced apoptosis is initiated through the recruitment of FADD and procaspase 8 to form the death-inducing signaling complex (DISC). In some cells (type I cells) the initiator caspase 8 directly activates effector caspases such as procaspase 3, whereas in others (type II cells) the death signal is amplified through mitochondria. In epithelial cells, Fas-induced hierarchic caspase activation is also linked with DEDD, a member of the DED family that binds to keratin (K) intermediate filaments (IFs). Hepatocytes are type II cells and their IFs are made exclusively of K8/K18. We have shown previously that K8-null mouse hepatocytes, lacking K8/K18 IFs, are more sensitive than their wild-type counterparts to Fas-induced apoptosis. Here, by examining the cell-death kinetics and death-signaling ordering, we found that K8-null hepatocytes exhibited prominent DISC formation, higher procaspase 8 activation and direct procaspase 3 activation as reported for type I cells; however they experienced a reduced Bid cleavage and a stronger procaspase 9 activation. In addition, the K8/K18 loss altered the DEDD ubiquitination status and nuclear/cytoplasmic distribution. Together, the results suggest that the K8/K18 loss induces a switch in Fas-induced death signaling, likely through a DEDD involvement.