Finlay, B. Brett

Shames, Stephanie R.

English

PubMed

Stephanie R. Shames

B. Brett Finlay

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www.frontiersin.org March 2011 | Volume 2 | Article 38 | 1
OpiniOn Article
published: 01 March 2011
doi: 10.3389/fmicb.2011.00038
Proteolytic cleavage of NF-κB p65: a novel mechanism for 
subversion of innate immune signaling by pathogenic E. coli
Stephanie R. Shames1,2 and B. Brett Finlay1,2,3*
1 Michael Smith Laboratories, University of British Columbia, Vancouver, BC, Canada
2 Department of Microbiology and Immunology, University of British Columbia, Vancouver, BC, Canada
3 Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, BC, Canada
*Correspondence: bfinlay@interchange.ubc.ca
Enteropathogenic and enterohemorrhagic 
Escherichia coli (EPEC and EHEC, respec-
tively) are attaching and effacing (A/E) bac-
terial pathogens that cause severe diarrheal 
disease. An integral mechanism in the viru-
lence strategy of A/E pathogens is the abil-
ity to subvert innate immune signaling by 
down-regulation of interleukin (IL)-8, a 
pro-inflammatory chemokine that func-
tions to recruit neutrophils to sites of infec-
tion (Vallance and Finlay, 2000). Expression 
of IL-8 is stimulated by several pro-inflam-
matory stimuli, which interact with cellu-
lar receptors and initiate signaling cascades 
terminating in the expression of the IL-8 
gene (Hoffmann et al., 2002). Necessary for 
IL-8 gene expression is a nuclear factor-κB 
(NF-κB) dimer containing p65 (Rel-A). In 
resting cells, NF-κB is held inactive in the 
cytosol by an inhibitor of κB (IκB); how-
ever, NF-κB-activating stimuli facilitate 
phosphorylation of IκB by an IκB kinase 
(IKK) and subsequent degradation of IκB 
(Hoffmann et al., 2002). Liberated NF-κB 
dimers translocate to the nucleus where 
they interact with κB-containing enhancers 
and facilitate gene expression (Hoffmann 
et al., 2002).
Enteropathogenic Escherichia coli and 
EHEC rely on a type-III secretion system 
(T3SS) to dampen inflammatory signaling 
(Vallance and Finlay, 2000). Recently, sev-
eral studies have demonstrated a role for 
the non-LEE encoded (Nle) effectors NleB, 
NleC, NleE, NleH1, and NleH2 in dampen-
ing NF-κB-mediated gene expression (Gao 
et al., 2009; Nadler et al., 2010; Newton et al., 
2010; Royan et al., 2010; Yen et al., 2010; 
Baruch et al., 2011). NleB and NleE stabilize 
IκBα and thus limit p65 translocation to the 
nucleus, a mechanism which is shared by the 
T3SS effector OspZ from Shigella flexneri 
(Nadler et al., 2010; Newton et al., 2010). 
NleH1 and NleH2 both dampen NF-κB-
mediated gene expression, but only NleH1 
was shown to mediate this by decreasing 
nuclear abundance of ribosomal protein S3 
(Gao et al., 2009; Royan et al., 2010). Four 
independent studies have recently shown 
that NleC is a metalloprotease that enzy-
matically degrades p65 (Yen et al., 2010; 
Baruch et al., 2011; Muehlen et al., 2011; 
Pearson et al., 2011). All four studies dem-
onstrate that an nleC null (∆nleC) strain of 
EPEC could not fully suppress NF-κB activ-
ity or IL-8 secretion to levels of wild-type 
(WT) EPEC. Ectopic expression of NleC 
could also suppress NF-κB activity and 
IL-8 secretion (Yen et al., 2010; Baruch et al., 
2011; Muehlen et al., 2011; Pearson et al., 
2011). All four studies identified a metal-
loprotease domain within NleC as essential 
for its function within host cells.
NleC is a zinc-dependent protease with 
a canonical HEXXH motif (Yen et al., 2010; 
Baruch et al., 2011; Muehlen et al., 2011). 
Although NleC is the first type-III secreted 
Zn-metalloprotease to be identified, other 
virulence factors have also been shown to 
degrade p65 to dampen the inflammatory 
response in the host. Chlamydia  trachomatis 
and C. pneumoniae both utilize a T3SS to 
secrete chlamydial protease-like activity 
factor (CPAF) from chlamydial inclusions 
into the host cell cytosol (Christian et al., 
2010). CPAF functions to degrade p65 and 
can dampen pro-inflammatory signaling 
in Chlamydia infected cells (Christian 
et al., 2010). Since Chlamydia species are 
obligate intracellular pathogens, it is not 
surprising that they encode virulence fac-
tors to dampen immune signaling. CPAF 
does not exclusively target p65 suggesting 
that NleC may also have other targets in 
the host cell.
The nleC gene is highly conserved 
between EPEC, EHEC, and C. rodentium, 
a natural A/E pathogen of mice, suggest-
ing that it has a similar function in all three 
pathogens. The ability of NleC to dampen 
the release of IL-8 in culture would suggest 
that a C. rodentium ∆nleC strain would 
cause increased inflammation and colitis in 
murine hosts. This is an attractive theory; 
however, NleC has little impact on C. roden-
tium colonization and colon weight in vivo 
(Kelly et al., 2006). Use of a C. rodentium 
nleC/nleE double knockout would likely 
provide additional insight into the impact 
of NF-κB inhibition in vivo. NleC is the first 
p65-targeting metalloprotease to be identi-
fied in the effector arsenal of an extracellular 
pathogen. Recent insights into the function 
of NleC may aid in elucidating functions for 
other effector proteins in other pathogens. 
Understanding the role of effector proteins 
will further our understanding of their role 
in virulence and the pathogenic strategy of 
disease-causing bacteria.
Acknowledgments
We would like to thank Drs. S. D. Auweter 
and K. M. Keeney for helpful discussions 
and critical reading of the manuscript. 
Stephanie R. Shames is an NSERC Post-
Graduate Trainee. B. Brett Finlay holds 
operating grants from the Canadian 
Institute of Health Research (CIHR), the 
Howard Hughes Medical Institute (HHMI), 
and the Canadian Crohn’s and Colitis 
Foundation (CCFC). B. Brett Finlay is an 
HHMI International Research Scholar, and 
the University of British Columbia Peter 
Wall Distinguished Professor.
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Received: 15 February 2011; accepted: 19 February 2011; 
published online: 01 March 2011.
Citation: Shames SR and Finlay BB (2011) Proteolytic 
cleavage of NF-κB p65: a novel mechanism for subversion 
of innate immune signaling by pathogenic E. coli. Front. 
Microbio. 2:38. doi: 10.3389/fmicb.2011.00038
This article was submitted to Frontiers in Cellular and 
Infection Microbiology, a specialty of Frontiers in Microbiology.
Copyright © 2011 Shames and Finlay. This is an open-access 
article subject to an exclusive license agreement between 
the authors and Frontiers Media SA, which permits unre-
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Proteolytic Cleavage of NF-κB p65: A Novel Mechanism for Subversion of Innate Immune Signaling by Pathogenic E. Coli

B. Brett eFinlay

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Proteolytic cleavage of NF-κB p65: a novel mechainsm for subversion of innate immune signaling by pathogenic E. coli

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Proteolytic Cleavage of NF-κB p65: A Novel Mechanism for Subversion of Innate Immune Signaling by Pathogenic E. Coli

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