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Intracellular mechanisms of TRAIL: apoptosis through mitochondrial-dependent and -independent pathways
1Department of Pharmaceutical Sciences, University of Maryland - School of Pharmacy, 20 N Pine Street, Baltimore, Maryland MD 21201, USA.
Abstract:
Tumor necrosis (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF family of cytokines that promotes apoptosis. TRAIL induces apoptosis via death receptors (DR4 and DR5) in a wide variety of tumor cells but not in normal cells. The objectives of this study are to investigate the intracellular mechanisms by which TRAIL induces apoptosis. The death receptor Fas, upon ligand binding, trimerizes and recruits the adaptor protein FADD through the cytoplasmic death domain of Fas. FADD then binds and activates procaspase-8. It is unclear whether FADD is required for TRAIL-induced apoptosis. Here we show that the signaling complex of DR4/DR5 is assembled in response to TRAIL binding. FADD and caspase-8, but not caspase-10, are recruited to the receptor, and cells deficient in either FADD or caspase-8 blocked TRAIL-induced apoptosis. In addition, TRAIL initiates the activation of caspases, the loss of mitochondrial transmembrane potential (Deltapsi(m)), the cleavage of BID, and the redistribution of mitochondrial cytochrome c. Treatment of Jurkat cells with cyclosporin A delayed TRAIL-induced Deltapsi(m), caspase-3 activation and apoptosis. Similarly, Overexpression of Bcl-2 or Bcl-X(L) delayed, but did not inhibit, TRAIL-induced Deltapsi(m) and apoptosis. In contrast, XIAP, cowpox virus CrmA and baculovirus p35 inhibited TRAIL-induced apoptosis. These data suggest that death receptors (DR4 and DR5) and Fas receptors induced apoptosis through identical signaling pathway, and TRAIL-induced apoptosis via both mitochondrial-dependent and -independent pathways.
Insights
Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) triggers programmed cell death in cancer cells. This study reveals TRAIL utilizes death receptors (DR4/DR5) and adaptor protein FADD to initiate apoptosis through caspase activation and mitochondrial pathways.
Area of Science:
- Cell Biology
- Molecular Biology
- Immunology
Background:
- Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a cytokine that induces apoptosis in tumor cells.
- TRAIL signals through death receptors (DR4 and DR5), but its intracellular mechanisms are not fully understood.
- The role of adaptor protein FADD in TRAIL-induced apoptosis is unclear.
Purpose of the Study:
- To investigate the intracellular signaling pathways of TRAIL-induced apoptosis.
- To determine the involvement of FADD and caspases in TRAIL signaling.
- To elucidate the contribution of mitochondrial pathways to TRAIL-mediated cell death.
Main Methods:
- Analysis of signaling complex assembly upon TRAIL binding to DR4/DR5.
- Assessment of apoptosis in cells deficient in FADD or caspase-8.
- Evaluation of mitochondrial events (e.g., loss of potential, cytochrome c release) and caspase activation.
- Inhibition studies using cyclosporin A, Bcl-2, Bcl-X(L), XIAP, CrmA, and p35.
Main Results:
- TRAIL induces the assembly of a signaling complex involving DR4/DR5, FADD, and caspase-8.
- FADD and caspase-8 are essential for TRAIL-induced apoptosis; caspase-10 is not involved.
- TRAIL triggers caspase activation, mitochondrial potential loss, BID cleavage, and cytochrome c release.
- Inhibitors of apoptosis (XIAP, CrmA, p35) block TRAIL-induced cell death, while cyclosporin A and Bcl-2/Bcl-X(L) only delay it.
Conclusions:
- TRAIL-induced apoptosis utilizes a pathway similar to Fas receptor signaling, involving FADD and caspase-8.
- TRAIL-mediated apoptosis involves both mitochondrial-dependent and -independent pathways.
- Understanding these mechanisms can inform cancer therapy strategies targeting TRAIL signaling.