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Inhibition of Daxx-mediated apoptosis by heat shock protein 27
S J Charette1, J N Lavoie, H Lambert
1Centre de Recherche en Cancérologie de l'Université Laval, L'Hôtel-Dieu de Québec, Québec, Canada G1R 2J6.
Abstract:
Heat shock protein 27 (HSP27) confers cellular protection against a variety of cytotoxic stresses and also against physiological stresses associated with growth arrest or receptor-mediated apoptosis. Phosphorylation modulates the activity of HSP27 by causing a major change in the supramolecular organization of the protein, which shifts from oligomers to dimers. Here we show that phosphorylated dimers of HSP27 interact with Daxx, a mediator of Fas-induced apoptosis, preventing the interaction of Daxx with both Ask1 and Fas and blocking Daxx-mediated apoptosis. No such inhibition was observed with an HSP27 phosphorylation mutant that is only expressed as oligomers or when apoptosis was induced by transfection of a Daxx mutant lacking its HSP27 binding domain. HSP27 expression had no effect on Fas-induced FADD- and caspase-dependent apoptosis. However, HSP27 blocked Fas-induced translocation of Daxx from the nucleus to the cytoplasm and Fas-induced Daxx- and Ask1-dependent apoptosis. The observations revealed a new level of regulation of the Fas pathway and suggest a mechanism for the phosphorylation-dependent protective function of HSP27 during stress and differentiation.
Insights
Heat shock protein 27 (HSP27) phosphorylation creates dimers that block apoptosis by interacting with Daxx. This phosphorylation-dependent mechanism regulates the Fas pathway, offering cellular protection during stress.
Area of Science:
- Cellular Biology
- Molecular Biology
- Stress Response
Background:
- Heat shock protein 27 (HSP27) provides cellular protection against cytotoxic and physiological stresses.
- HSP27 activity is modulated by phosphorylation, altering its structure from oligomers to dimers.
- Apoptosis, particularly Fas-induced apoptosis, is a critical cellular process regulated by various protein interactions.
Purpose of the Study:
- To investigate the role of phosphorylated HSP27 dimers in regulating Fas-induced apoptosis.
- To elucidate the mechanism by which HSP27 phosphorylation affects the Fas signaling pathway.
- To identify the specific interactions between HSP27, Daxx, and Ask1 in the context of apoptosis.
Main Methods:
- Investigated the interaction between phosphorylated HSP27 dimers and Daxx using cellular assays.
- Utilized phosphorylation-deficient HSP27 mutants and Daxx mutants to dissect the interaction domains and functional consequences.
- Examined the effect of HSP27 on the translocation of Daxx and the activation of downstream apoptotic factors (FADD, caspase).
Main Results:
- Phosphorylated HSP27 dimers directly interact with Daxx, a key mediator of Fas-induced apoptosis.
- This interaction prevents Daxx from binding to Fas and Ask1, thereby inhibiting Daxx-mediated apoptosis.
- HSP27 did not affect FADD- and caspase-dependent apoptosis but blocked the nuclear-to-cytoplasmic translocation of Daxx induced by Fas.
- Apoptosis induced by a Daxx mutant lacking the HSP27 binding domain was not inhibited by HSP27.
Conclusions:
- Phosphorylation of HSP27 creates dimers that inhibit Fas-induced apoptosis by sequestering Daxx.
- HSP27 phosphorylation represents a novel regulatory mechanism within the Fas signaling pathway.
- This study reveals a phosphorylation-dependent protective function of HSP27 against cellular stress and during differentiation.