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Cutting Edge |
Department of Biochemistry and Molecular Biology, Indiana University School of Medicine and Walther Cancer Institute, Indianapolis, IN 46202
| Abstract |
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B activation. However, ectopic
expression of a mouse pellino-2 antisense construct inhibited IL-1 or
LPS-induced activation of NF-
B-dependent IL-8 promoter activity. Our
data reveal that mouse pellino-2 is a tissue-restricted component of a
signaling pathway that couples the mouse pelle-like
kinase/IL-1R-associated kinase protein to IL-1- or LPS-dependent
signaling. | Introduction |
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Pelle is a serine/threonine kinase that is a component of the
Drosophila Toll signaling pathway which controls
dorsal/ventral polarity in the early embryo and is required for
protection against fungal infections in the adult (3).
Four mammalian pelle homologs have been described: the mouse pelle-like
kinase (mPLK)/IL-1R-associated kinase (IRAK)-1, IRAK-2, IRAK-M, and
IRAK-4, and each has been linked to Toll as well as IL-1R signaling
(4, 5, 6, 7). Current models predict that in response to IL-1
binding the type I IL-1R, the IL-1R accessory protein and MyD88 are
recruited to the receptor. The IL-1R accessory protein and/or MyD88
bind mPLK/IRAK-1 (8, 9) and TNFR-associated
factor-6. Through an unknown mechanism,
MyD88-mPLK/IRAK-1-TNFR-associated factor-6 activate the I
B kinase
complex, which in turn phosphorylates I
B
thereby
allowing for activation of Rel (NF-
B)-dependent gene expression
(5, 10, 11). LPS binding to TLR4 is thought to stimulate
activation of NF-
B-dependent transcription in a similar manner (for
review see Ref. 12). The precise role of mPLK/IRAK-1 in
IL-1 and/or Toll-dependent activation of NF-
B-dependent gene
transcription is unclear; in these particular signaling pathways,
mPLK/IRAK-1 protein, but not its cognate catalytic activity, is
required (13).
Although a primarily biochemical approach has provided insight into downstream components of the IL-1 type I receptor and TLR signaling pathways, genetic studies have defined several components of the Drosophila Toll signaling pathway. One of these components is tube, an adapter protein that together with pelle is required for cactus phosphorylation and nuclear relocalization of the Rel family member, dorsal (14, 15). To gain insight into the requirement for pelle in cactus/dorsal complex activation, Grosshans et al. (16) conducted a two-hybrid screen with pelle that resulted in the identification of pellino. Grosshans et al. determined that pellino associates with the kinase domain of pelle; however, a role and/or requirement for pellino in the Toll pathway was not reported. Database searches revealed that pellino is an evolutionarily conserved protein (17, 18). Based on the presumptive identification of pellino homologs in the mouse database, we examined whether a mouse pellino homolog may be involved in modulating mPLK/IRAK-1 activity.
| Materials and Methods |
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Primers were designed (5'-TCTCTAGGTTCGCATGCAGGATCGTGTG-3' and 5'-ACTGGGGTCGGGCTGCATTGATCTCCTG-3') to RT-PCR partial pellino-2 sequence from total RNA isolated from mouse embryonic fibroblasts (C3H10T1/2 cell line). Nucleotide sequence was determined by the Indiana University Biotechnology Facility (Indianapolis, IN) with an Applied Biosystems (Foster City, CA) automated sequencer. Using this sequence, primers were designed (5'-GCTTTCTGTTGACCTGCCCACCTG-3' and 5'-TGCCCTCTATAAGCGGACCTACGCC-3') to isolate the full-length mouse pellino sequence, by RACE with the Marathon cDNA Amplification kit (Clontech Laboratories, Palo Alto, CA) according to manufacturers recommendations. Full-length mouse pellino-2 cDNA was cloned into pCR2.1 vector (Invitrogen, San Diego, CA) and subsequently subcloned into a hemagglutinin (HA)-tagged mammalian expression vector, pHA-CMV (Clontech Laboratories) with the restriction endonucleases BamHI and EcoRI. The pellino-2 antisense construct was subcloned into pcDNA3.1 (Invitrogen).
Northern blot analysis
Mouse embryo and multiple tissue blots were purchased from
Clontech Laboratories. The mouse pellino-2 cDNA probe was radiolabeled
with [
-32P]dCTP using a random primer
labeling kit (Amersham Pharmacia Biotech, Piscataway, NJ).
Hybridization and prehybridization were conducted at 42°C following
the manufacturers recommended procedures.
Cell culture and in vitro assays
The human embryonic kidney (HEK) cells (293-EBNA) and the C3H10T1/2 mouse embryo fibroblast cell line were maintained as described previously (19). Transfections were performed with the indicated DNAs using FuGENE-6 (Roche, Basel, Switzerland) according to the manufacturers recommendations. Luciferase (LUC) activities were quantitated using the dual luciferase assay system (Promega, Madison, WI).
RT-PCR analysis
Total RNA was isolated using TRIzol (Life Technologies, Grand Island, NY), converted to cDNA by reverse transcription and amplified for 30 cycles by PCR using primers from outside of the antisense region. PCR products were analyzed on agarose gel stained with ethidium bromide and visualized by UV illumination.
Immunoprecipitation and Western blot analysis
The immunocomplexing assays and Western analysis were performed as described previously (19).
| Results and Discussion |
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The endogenous pattern of mouse pellino-2 gene expression was
determined by Northern analysis. Two different mRNA species, a 2- and a
6-kb transcript were detected in all tissues examined (Fig. 1
). The size of smaller mRNA species
matches the size of the cDNA isolated by RACE-PCR; whether the 6 kb
corresponds to an alternatively spliced form of pellino-2 is currently
unknown. Pellino-2 transcripts were readily detected in the developing
mouse embryo. In the adult mouse, pellino-2 transcripts were detected
predominantly in the liver, testes, and skin with only low amounts in
other tissues. Intriguingly, pellino-2 transcripts were either not
detected (spleen) or were barely detectable (thymus) in organs that
predominate in the adaptive immune response, but were more abundant in
organs (liver and skin) that predominate in the innate immune
response.
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-dependent activation of NF-
B-dependent gene expression
(19). A search of the Drosophila database
(FlyBase) failed to identify a Drosophila SIMPL homolog, so
it was of interest to determine whether pellino-2 and SIMPL interact
and form complexes. However, immunocomplexes containing
FLAG-tagged SIMPL did not contain pellino-2 (Fig. 2
for 20 min
before harvest of cell lysates and immunocomplexes generated with
antisera that recognize endogenous mPLK/IRAK-1. Western analysis of the
mPLK/IRAK-1-containing immunocomplexes revealed that, under
steady-state conditions, mPLK/IRAK-1 and pellino-2 are not complexed
but mPLK/IRAK-1 and pellino-2 complexes are generated in response to
IL-1 stimulation (Fig. 2
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B-dependent gene expression was
examined. Ectopic expression of pellino-2 in mouse embryonic
fibroblasts did not result in activation of a heterologous reporter
composed of the firefly LUC cDNA under the control of the
NF-
B-dependent IL-8 gene promoter (data not shown). In contrast to
pellino-2, ectopic expression of either mPLK/IRAK-1 or SIMPL leads to a
modest increase in IL-8 promoter activity (13, 19). An
analysis of a mPLK/IRAK-1 nulligenic demonstrated a requirement for
mPLK/IRAK-1 in TNF-
and IL-1-dependent NF-
B DNA binding activity
(20). Work in our laboratory showed that the catalytic
activity of mPLK/IRAK-1, as well as presence of SIMPL protein, is
required for TNF-
-, but not IL-1-, dependent activation of
NF-
B-dependent gene expression (13, 19). Therefore, we
decided to examine whether pellino-2 activity would be required for
TNF-
and/or IL-1-dependent gene expression. Consistent with results
obtained with mPLK/IRAK-1 and SIMPL (13, 19), ectopic
expression of pellino-2 had no effect upon IL-8 promoter activity in
cultures treated with IL-1, LPS, or TNF-
(data not shown).
The lack of known protein motifs meant that mutagenesis could not be
used to investigate a requirement for pellino-2 in IL-1-, LPS-, or
TNF-
-dependent activation of NF-
B activity. Therefore, an
antisense approach was used (21, 22). Previous studies in
our laboratory have demonstrated that ectopic expression of a cDNA
subcloned in the reverse orientation into a mammalian expression vector
will result in a significant drop in the steady-state level of the
protein of interest (19). Therefore, pellino-2 cDNA was
subcloned in the reverse orientation into pcDNA3.1. To determine the
influence of pellino-2 antisense expression, endogenous levels of
pellino-2 transcripts were measured, as antisera to endogenous
pellino-2 are not currently available. Mouse embryo fibroblasts were
transfected with the pellino-2 antisense construct and endogenous
pellino-2 mRNA levels were examined with RT-PCR using primers from 5'
and 3' UTR regions which are present in the pellino-2 mRNA but not
present in the pellino-2 antisense construct (Fig. 3
A). A PCR product for
pellino-2 was detected in fibroblasts transfected with the mammalian
expression vector that did not contain the pellino-2 antisense
construct but this PCR product was dramatically reduced in fibroblasts
transfected with the pellino-2 antisense construct (Fig. 3
B). Amplification of a PCR product using primers specific
to pellino-1 was not affected by expression of the antisense construct
(Fig. 3
B), confirming the specificity of the pellino-2
antisense construct.
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induction of IL-8 promoter activity was examined. Ectopic expression of
the pellino-2 antisense construct inhibited IL-1- or LPS-dependent but
not TNF-
-dependent activation of IL-8 promoter activity (Fig. 3
B-dependent promoter. Several
elements important for regulation of the IL-8 gene promoter have been
identified including NF-
B, AP-1, and C/EBP
elements
(23). To determine whether the requirement for pellino-2
could be linked to a specific cis-acting element, activation
of IL-8 reporter constructs containing mutations in the various
cis-acting elements was examined. Ectopic expression of the
pellino-2 antisense construct inhibited IL-1- or LPS-, but not
TNF-
-, dependent expression of the IL-8 promoter construct
containing the mutated AP-1 element (Fig. 3
B and/or the C/EBP
elements resulted in a loss of
IL-1 or TNF-
inducibility (Fig. 3
B-mutated IL-8 promoter was
equivalent to or slightly greater in the presence of the pellino-2
antisense construct. Taken together, these data reveal that loss of
IL-1- or LPS-dependent activation of the IL-8 promoter that occurs in
the presence of the pellino-2 antisense construct is not dependent upon
a functional AP-1 element; whether the NF-
B and/or C/EBP
elements
are required is less clear.
In summary, the data presented demonstrate that a mouse homolog of the
Drosophila pellino protein is expressed during embryogenesis
and in adult tissues that are involved in the innate immune response.
Consistent with results obtained in Drosophila, an
interaction between pellino-2 and the mouse homolog of pelle was
detected. Further analysis revealed that IL-1 treatment stimulates the
interaction between mPLK/IRAK-1 and pellino-2 and that a decline in the
steady-state level of pellino-2 transcripts (and presumably protein)
compromised IL-1-dependent activation of the NF-
B-dependent IL-8
gene promoter. Taken together, these data extend those previously
reported for the Drosophila pellino protein and suggest that
pellino-2 may be a required component of the type I IL-1R- and TLR
signaling pathways.
| Footnotes |
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2 Address correspondence and reprint requests to Dr. Maureen A. Harrington, Walther Oncology Center R4-302, Indiana University School of Medicine, 1044 West Walnut Street, Indianapolis, IN 46202-5121. E-mail address: mharrin{at}iupui.edu ![]()
3 Abbreviations used in this paper: TLR, Toll-like receptor; mPLK, mouse pelle-like kinase; IRAK, human IL-1R-associated kinase; HA, hemagglutinin; LUC, luciferase; HEK, human embryonic kidney. ![]()
Received for publication May 15, 2002. Accepted for publication August 14, 2002.
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B-dependent innate immune responses. Genes Dev. 15:1900.
B activity. J. Biol. Chem. 276:7859.
B- and cis-regulatory enhancer binding protein-like factor binding elements in activating the interleukin-8 gene by pro-inflammatory cytokines. J. Biol. Chem. 265:21128.This article has been cited by other articles:
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