Toll-like Receptor 3 (TLR3) Induces Apoptosis via Death Receptors and Mitochondria by Up-regulating the Transactivating p63 Isoform α (TAP63α)

Abstract

Toll-like receptor 3 (TLR3), a member of the pathogen recognition receptors, is widely expressed in various cells and has been shown to activate immune signaling pathways by recognizing viral double-stranded RNA. Recently, it was reported that the activation of TLR3 induced apoptosis in some cells, but the detailed molecular mechanism is not fully understood. In this study, we found that in endothelial cells polyinosinic-polycytidylic acid (poly(I-C)) induced dose-and time-dependent cell apoptosis, which was elicited by TLR3 activation, as TLR3 neutralization and down-regulation repressed the apoptosis. Poly(I-C) induced the activation of both caspases 8 and 9, indicating that TLR3 triggered the signaling of both the extrinsic and intrinsic apoptotic pathways. Poly(I-C) up-regulated tumor necrosis factor-related apoptosis-inducing ligand and its receptors, death receptors 4/5, resulting in initiating the extrinsic pathway. Furthermore, poly(I-C) down-regulated anti-apoptotic protein, B cell lymphoma 2 (Bcl-2), and up-regulated Noxa, a key Bcl-2 homology 3-only antagonist of Bcl-2, leading to the priming of the intrinsic pathway. A p53-related protein, the transactivating p63 isoform ␣ (TAp63␣), was induced by TLR3 activation and contributed to the activation of both the intrinsic and extrinsic apoptotic pathways. Both the cells deficient in p63 gene expression by RNA interference and cells that overexpressed the N-terminally truncated p63 isoform ␣ (⌬Np63␣), a dominant-negative variant of TAp63␣, by gene transfection, survived TLR3 activation. Taken together, TAp63␣ is a crucial regulator downstream of TLR3 to induce cell death via death receptors and mitochondria.

Preferential involvement of mitochondria in Toll-like receptor 3 agonist-induced neuroblastoma cell apoptosis, but not in inhibition of cell growth

“…Juxtaposition of ER and mitochondria promotes interorganelle crosstalk and mediates cell death, which is tightly controlled by the Bcl-2 family proteins [14,15]. The latter regulate either caspase 8-mediated extrinsic pathway or caspase 9/mitochondria-mediated intrinsic pathway, both being used in TLR3 mediated apoptosis [16,17]. Among the three subgroups of the Bcl-2 family proteins, we found poly(I:C) significantly and preferentially up-regulated BH3-only molecules including BAD, Noxa and tBid in SK-N-AS.…”

“…Juxtaposition of ER and mitochondria promotes inter-organelle crosstalk and mediates cell death. The apoptotic crosstalk between the two organelles is tightly controlled by the Bcl-2 family proteins [14,15], which then regulate either caspase 8-mediated extrinsic pathway or caspase 9/mitochondria-mediated intrinsic pathway, both being used in TLR3 mediated apoptosis [16,17]. Increased production of reactive oxygen species and modulation of cell death by anti-oxidant manganese superoxide dismutase (MnSOD) and/or copper-zinc SOD (CuZnSOD) may be linked to these sequential events [18,19].…”

Poly(I:C) Treatment Leads to Interferon-Dependent Clearance of Hepatitis B Virus in a Hydrodynamic Injection Mouse Model

“…Other studies have suggested that an excessive amount of poly(I:C) may produce a response spectrum unsuitable for viral clearance, such as transactivating p63 (Tap63␣)-dependent apoptosis (27), activation of intracellular negative regulatory molecules, including A20, tumor necrosis factor-related apoptosis-inducing ligand receptor (TRAIL-R), and ubiquitin 1 (28); and type I IFN-dependent exhaustion of memory T cells (29,30) and dendritic cells (DCs) (31). However, the underlying mechanisms remain to be elucidated.…”

Epigenetic Regulation of Toll-Like Receptor Signaling: Implications for Cancer Development

“…It has been shown through multiple studies that TLRs can activate apoptotic cascades. TLR3 can activate apoptosis through both the intrinsic and extrinsic pathways through the binding of the respective synthetic ligand polyinosinic:polycytidylic acid [poly(I:C)] in endothelial cells [24]. TLR2 can activate apoptosis through MyD88 which in turn activates the Fas-associated death domain-caspase-8 pathway in monocytes/macrophages through direct death-domain interactions [25,26,27].…”