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The regulation of apoptosis by toxic shock syndrome toxin-1 and associated mutants Hung, Ryan Wei Yan
Abstract
Toxic shock syndrome toxin-1 (TSST-1), the primary virulence factor in toxic shock syndrome (TSS), is an exotoxin produced by Staphylococcus aureus and a member of the superantigen family of T cell mitogens. Activation-induced cell death (AICD) of superantigen-reactive T cells is thought to play a role in modulating the hyperactive immune response and promoting host survival. While this form of apoptotic cell death occurs with the related staphylococcal enterotoxins (SE) A and B, conflicting reports exist for TSST-1. Therefore, the aim of this project was to investigate the regulation of apoptosis by TSST-1 and site-directed mutants. TSST-1 was found to exert either pro- or anti-apoptotic effects depending on the dose, thus resolving some of the contradictory reports in the literature. At commonly used concentrations (<1 μM), TSST-1 was less potent at inducing apoptosis than SEB. Moreover, there was evidence of early induction of anti-apoptotic pathways. The glycine 31-to-arginine site-directed mutant of TSST-1 (G31R) was serendipitously found to be potently pro-apoptotic at doses as low as 10-nM. This apoptosis was very rapid and broadly directed, affecting T cells, monocytes and T lymphoma-derived cell lines. Surprisingly, high doses of TSST-1 (<1 μM) also exhibited similarly potent, non-specific apoptosis much like G31R. The mechanism behind this novel form of apoptosis was examined by dissecting the involvement of components of the extrinsic and intrinsic apoptotic pathways. When ligated, classical death receptors like Fas receptor utilize the extrinsic pathway. In contrast, conditions unfavourable to cell survival such as growth factor withdrawal, ionizing radiation, and other cell damage can converge at the level of the mitochondria to trigger death through the intrinsic pathway. G31R-induced apoptosis was dependent on the intrinsic, but not the extrinsic, pathway. Because TSST-1 is not known to possess direct enzymatic effects that could account for its cytotoxicity, the most likely site of action for the induction of apoptosis by G31R and high-dose TSST-1 was considered to be a non-classical death receptor coupled to the intrinsic pathway, a phenomenon previously described. However, the upstream pathways remain unclear, so that other possibilities may include direct cell membrane disruption and/or induction of reactive oxygen species. Either way, this previously unknown apoptotic activity of TSST-1 may be relevant in pathogenesis, acting as a local immunosuppressant at infection sites by deleting lymphocytes and monocytes that transit into the region. With identification of the apical mechanism for apoptosis in this system, possibilities exist for the therapeutic use of G31R or its derivatives in the treatment of tumours and autoimmune diseases.
Item Metadata
Title |
The regulation of apoptosis by toxic shock syndrome toxin-1 and associated mutants
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Creator | |
Publisher |
University of British Columbia
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Date Issued |
2004
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Description |
Toxic shock syndrome toxin-1 (TSST-1), the primary virulence factor in toxic shock
syndrome (TSS), is an exotoxin produced by Staphylococcus aureus and a member of the
superantigen family of T cell mitogens. Activation-induced cell death (AICD) of superantigen-reactive
T cells is thought to play a role in modulating the hyperactive immune response and
promoting host survival. While this form of apoptotic cell death occurs with the related
staphylococcal enterotoxins (SE) A and B, conflicting reports exist for TSST-1.
Therefore, the aim of this project was to investigate the regulation of apoptosis by TSST-1
and site-directed mutants. TSST-1 was found to exert either pro- or anti-apoptotic effects
depending on the dose, thus resolving some of the contradictory reports in the literature.
At commonly used concentrations (<1 μM), TSST-1 was less potent at inducing apoptosis
than SEB. Moreover, there was evidence of early induction of anti-apoptotic pathways. The
glycine 31-to-arginine site-directed mutant of TSST-1 (G31R) was serendipitously found to be
potently pro-apoptotic at doses as low as 10-nM. This apoptosis was very rapid and broadly
directed, affecting T cells, monocytes and T lymphoma-derived cell lines. Surprisingly, high
doses of TSST-1 (<1 μM) also exhibited similarly potent, non-specific apoptosis much like
G31R.
The mechanism behind this novel form of apoptosis was examined by dissecting the
involvement of components of the extrinsic and intrinsic apoptotic pathways. When ligated,
classical death receptors like Fas receptor utilize the extrinsic pathway. In contrast, conditions
unfavourable to cell survival such as growth factor withdrawal, ionizing radiation, and other cell
damage can converge at the level of the mitochondria to trigger death through the intrinsic
pathway. G31R-induced apoptosis was dependent on the intrinsic, but not the extrinsic,
pathway.
Because TSST-1 is not known to possess direct enzymatic effects that could account for its
cytotoxicity, the most likely site of action for the induction of apoptosis by G31R and high-dose
TSST-1 was considered to be a non-classical death receptor coupled to the intrinsic pathway, a
phenomenon previously described. However, the upstream pathways remain unclear, so that
other possibilities may include direct cell membrane disruption and/or induction of reactive
oxygen species.
Either way, this previously unknown apoptotic activity of TSST-1 may be relevant in
pathogenesis, acting as a local immunosuppressant at infection sites by deleting lymphocytes and
monocytes that transit into the region. With identification of the apical mechanism for apoptosis
in this system, possibilities exist for the therapeutic use of G31R or its derivatives in the
treatment of tumours and autoimmune diseases.
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Extent |
30174745 bytes
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Genre | |
Type | |
File Format |
application/pdf
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Language |
eng
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Date Available |
2009-12-01
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Provider |
Vancouver : University of British Columbia Library
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Rights |
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.
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DOI |
10.14288/1.0091873
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URI | |
Degree | |
Program | |
Affiliation | |
Degree Grantor |
University of British Columbia
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Graduation Date |
2004-11
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Campus | |
Scholarly Level |
Graduate
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Aggregated Source Repository |
DSpace
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Item Media
Item Citations and Data
Rights
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.