Abstract
Toll-like receptor-3 (TLR3), a member of an immune recognition receptor family, is widely expressed in tumour cells and has been shown previously to have the capacity to not only activate immune signalling pathways, but also to exert proapoptotic activity in some cells. We show here that HaCaT human keratinocytes are susceptible to apoptosis induction by the TLR3 ligand poly I:C, and use these cells as a model to analyse the apoptotic signalling pathway. Although the BH3-only protein Noxa was transcriptionally induced by poly I:C and translocated to mitochondria, RNAi experiments showed that the BH3-only proteins Noxa, Bim and Puma were individually dispensable for poly I:C-induced apoptosis. Instead, poly I:C-induced activation of caspase-8 via TLR3 and its adapter TRIF was required for apoptosis. In human melanoma cell lines poly I:C failed to induce apoptosis unless protein synthesis was blocked. Significantly, sensitisation towards poly I:C-dependent caspase-8 activation and apoptosis in melanoma cells was also achieved by the synthetic Smac mimetic/inhibitor of apoptosis protein (IAP) antagonist, LBW242, or by specific downregulation of cIAP1 by siRNA. Inactivation of caspase-8 by CrmA overexpression reduced poly I:C/LBW242-induced apoptosis. These results indicate that the proapoptotic activity of TLR3/TRIF/caspase-8 in melanoma cells is under the control of IAPs, and the use of novel Smac mimetics might be a feasible approach to target melanoma.
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Abbreviations
- Bak:
-
Bcl-2 homologous antagonist/killer
- Bax:
-
Bcl-2associated X protein
- Bcl-2:
-
B-cell lymphoma protein 2
- Bcl-XL:
-
Bcl-2-like protein 1
- Bid:
-
BH3 interacting domain death agonist
- Bim:
-
Bcl-2-like protein 11
- cIAP1/2:
-
cellular IAP1,2
- CHX:
-
cycloheximide
- CrmA:
-
caspase-8 inhibitor from cowpox virus
- DISC:
-
death-inducing signaling complex
- FLIP:
-
CASP8 and FADD-like apoptosis regulator
- GAPDH:
-
Glyceraldehyde 3-phosphate dehydrogenase
- IAP:
-
inhibitor of apoptosis protein
- IFN:
-
interferon
- IL-8:
-
interleukin 8
- MyD88:
-
myeloid differentiation primary response protein-88
- Mcl-1:
-
induced myeloid leukemia cell differentiation protein
- Noxa:
-
phorbol-12-myristate-13-acetate-induced protein 1
- PI:
-
propidium iodide
- poly I:C/pIC:
-
polyinosinic:polycytidylic acid
- Puma:
-
p53 upregulated modulator of apoptosis
- p53:
-
tumor protein 53
- TLR:
-
Toll-like receptors
- TNF:
-
tumor necrosis factor
- TRADD:
-
tumor necrosis factor receptor type 1-associated death domain protein
- TRAF6:
-
TNF receptor associated factor family protein 6
- TRIF:
-
TIR-domain-containing adapter-inducing interferon (IFN)
- XIAP:
-
X-linked inhibitor of apoptosis protein
- zVAD-fmk:
-
(carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]- fluoromethylketone)
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Acknowledgements
We thank Dr Leigh Zawel (Novartis, Cambridge) for generously providing LBW242. We are grateful to Christian Ploner (Medical University of Innsbruck, Innsbruck, Austria) for providing the shScrambled vector and the shRNA construct directed against Bim. We thank Daria Loos for experimental assistance. This work was supported by grants from the Deutsche Forschungsgemeinschaft to GH (Ha2128) and to RB (GK 1202), and from the Deutsche Krebshilfe to RB (grant 107805).
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Weber, A., Kirejczyk, Z., Besch, R. et al. Proapoptotic signalling through Toll-like receptor-3 involves TRIF-dependent activation of caspase-8 and is under the control of inhibitor of apoptosis proteins in melanoma cells. Cell Death Differ 17, 942–951 (2010). https://doi.org/10.1038/cdd.2009.190
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DOI: https://doi.org/10.1038/cdd.2009.190
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