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Cutting Edge |


Departments of
*
Medicine and
Microbiology, and
Genome Center, College of Physicians and Surgeons, Columbia University, New York, NY 10032
| Abstract |
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| Introduction |
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. These residues then serve as
docking sites for several proteins containing phosphotyrosine
binding or Src homology 2 domains including signal transducer
and activator of transcription family member STAT6. STAT6 is tyrosine
phosphorylated by JAK kinases, which induces its
homodimerization and subsequent translocation into the nucleus, where
it can regulate gene transcription.
IL-4-induced proliferation and survival have been shown to be at least
partially independent of STAT6 (2, 3, 4), whereas
IL-4-induced differentiation appears to be largely dependent on STAT6.
Previous work has shown that B cells from STAT6-deficient mice fail to
produce significant amounts of IgG1 or IgE after infection with the
nematode Nippostrongylus brasiliensis (4).
Furthermore, on stimulation with IL-4, STAT6-deficient B cells fail to
up-regulate Fc
RII/CD23 or MHC class II (2, 3, 4), which
play important roles in Ag capture and presentation, respectively. In
consequence, STAT6-deficient mice show impaired immune responses to
infection with nematodes and diminished allergic responses in murine
models of asthma (4, 5).
STAT6-binding elements were identified in the I
promoter
(6), the CD23 promoter (7), and other
enhancer elements. STAT6 activation alone, however, is not sufficient
to trans-activate these genes in primary cells but in
addition requires de novo protein synthesis which is efficiently
blocked by cycloheximide (6, 8). This finding implies that
IL-4 likely induces the expression of a cascade of essential secondary
transcription factors and proteins. To identify factors that are
regulated in a STAT6-dependent manner in IL-4-stimulated B cells, a
combination of gene targeting and gene expression profiling was
used.
| Materials and Methods |
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Anti-CD24-PE and anti-B220-FITC were from BD PharMingen (San Diego, CA); anti-caspase-6 was from Cell Signaling Technology; anti-B cell lymphoma 6 (BCL-6) and anti-STAT6 were from Santa Cruz Biotechnology (Santa Cruz, CA); anti-extracellular signal-related kinase 1/2 was from Promega (Madison, WI). STAT6-/- mice on the BALB/c background were a gift from Dr. M. Grusby (Boston, MA) (2). Naive splenic B cells were isolated as described previously (9). B cells were >95% B220 positive as confirmed by flow cytometry (data not shown). The murine B cell line M12.4.1 has been described previously (6).
Affymetrix GeneChips analysis
Freshly isolated naive (CD43-B220+) B cells pooled from each two to four BALB/c STAT6+/+ or STAT6 -/- mice were stimulated with LPS (1.5 µg/ml) and murine recombinant IL-4 (1:25, generously provided by W. Paul, National Institutes of Health, Bethesda, MD). After 24 h, total RNA was isolated, processed, and hybridized with Affymetrix U74A GeneChips. Chips were scanned and analyzed using a target intensity normalization of 2500 for all chips as suggested by the manufacturer.
Data analysis
Data obtained from U74A GeneChips version 1 were masked by
Microarray Suite software resulting in a data set of 10,043 probe sets.
Average differences from the different data sets were grouped into
STAT6+/+ vs STAT6-/- or
STAT6-/- LPS vs
STAT6-/- LPS plus IL-4, respectively. Average
differences of <5 were set to 5. Statistical significance was
calculated using Cyber-T software as described previously
(10). A gene was considered differentially expressed
between the compared groups if: 1) p was <0.000099
(1/10,043); 2) the absolute fold difference between the mean average
differences from each group was
2; and 3) the mean average difference
change was
500. This ensures a false positive error rate of <1 of
10,043 genes.
Northern blots, Western blots, and flow cytometry
Total RNA was isolated from B cells using the RNEasy Kit (Qiagen, Valencia, CA), and equal amounts of RNA were used to generate cRNA as described previously (11). Northern blotting of these RNA was performed as described (12). Western blotting was conducted as previously described (13). For flow cytometry, cells were sequentially incubated with Fc block and primary fluorochrome-conjugated Ab, washed, and fixed as suggested (BD PharMingen).
Retroviral transduction
A dominant negative (DN) version of Krox20 (Krox20DN) containing S379R and D380Y point mutations was generated from wild-type (WT) Krox20 (Krox20WT) by site-directed mutagenesis (QuickChange; Stratagene, La Jolla, CA) followed by sequencing. The full length cDNAs of Krox20WT, Krox20DN, or STAT6 were subcloned into pMIG upstream of an internal ribosome entry site and green fluorescent protein (GFP). Generation of ecotropic retroviruses and infection of M12 cells were performed as described (14). After infection, cells were rested for 48 h before additional experiments.
| Results and Discussion |
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To identify genes the expressions of which are modulated by STAT6,
LPS-stimulated B cells were cultured with IL-4 for 24 h, after
which total RNA was isolated and processed to obtain gene expression
profiles. The time point of 24 h was chosen to detect a maximum of
both early and late genes that are altered in response to IL-4. The
mitogen LPS is known to induce activation of NF-
B, which, together
with STAT6, is required for Ig H chain class switching to IgE
(15). Statistical analysis of the expression profiles
identified 114 differentially expressed probe sets at the level of
statistical significance. These probe sets represented 106 distinct
cDNAs, 70 of which were known genes (Fig. 1
) and 36 of which were expressed
sequence tags (Table I
). Differentially
expressed genes were grouped into transcription
factors/transcription-related proteins, kinases/phosphatases, other
enzymes, cytokines/cell surface receptors, Ig H chains, and
miscellaneous. A number of previously described IL-4-inducible genes
were identified as STAT6 dependent by this approach, including the
transcription factor e4bp4/nfil3 (Fig. 1
, Nos. 1 and 54)
(16), as well as the known STAT6 target genes
CD23 and il4r
(Fig. 1
, Nos. 38 and 50). It
should also be noted that STAT6 expression in
STAT6-/- B cells was detectable (Fig. 1
, No.
7), because only the Src homology 2 domain of STAT6 was targeted by
homologous recombination in the STAT6-/- mice,
leaving the 5' portion of the gene intact (2). Thus, a
partial STAT6 mRNA is still expressed in
STAT6-/- mice and detected by GeneChip, which,
however, is not translated into functional STAT6 protein (Fig. 3
d). In STAT6-deficient B cells, 31 of 70 genes were
expressed at lower levels, but surprisingly, expression of 39 genes was
detected at higher levels than in STAT6+/+ B
cells. This finding implies that STAT6 not only induces gene expression
but may also mediate repression of genes in a direct or indirect
manner.
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Importantly, the GeneChip analysis together with Western blot analysis
identified the transcriptional repressor bcl-6 as an
IL-4-inducible, STAT6-dependent gene (Fig. 1
, No. 4, and Fig. 3
b). Furthermore, activation with IL-4 or LPS plus IL-4 for
24 h resulted in a moderate
2-fold, nevertheless highly
reproducible, up-regulation of caspase-6 protein levels in
STAT6+/+, but not in
STAT6-/- B cells (Fig. 3
c).
STAT6-dependent induction of caspase-6 protein levels correlated with
differential caspase-6 enzyme activity in a delayed manner after
prolonged stimulation of at least 72 h (data not shown).
Taken together, these experiments identify several novel IL-4 modulated
STAT6-dependent genes. Surprisingly, there is both STAT6-dependent
induction and repression of gene expression in response to IL-4
stimulation. Interestingly, in STAT6-deficient B cells, only eight
genes were differentially expressed in LPS vs LPS plus IL-4-stimulated
samples (data not shown). None of these eight genes was among those
described in Fig. 1
. This result therefore suggests that expression of
genes was repressed in a STAT6-dependent manner and not selectively
up-regulated by IL-4 only in STAT6-deficient B cells.
Previous work has shown that STAT6 can interfere with NF-
B activity
by competition for overlapping STAT6 and NF-
B DNA-binding elements
(17). Because LPS mediates NF-
B activation, this
mechanism may provide an explanation for down-regulation of gene
expression by STAT6 in LPS plus IL-4-stimulated B cells. Another
potential mechanism for STAT6-mediated negative regulation of gene
expression involves the IL-4 and STAT6-mediated induction of
transcriptional repressors. BCL-6-deficient mice exhibit pathologically
enhanced Th2-type inflammatory responses, suggesting that BCL-6 is a
physiologic repressor of IL-4-mediated effects, including regulation of
I
transcription and class switching to IgE (18, 19).
Some of the genes with mRNA levels that were less abundant in
STAT6+/+ B cells than in
STAT6-/- B cells, including the BCL-6 target
gene cyclin D2 (20), might therefore be targets
of BCL-6-mediated repression. It is striking that a number of genes
that were less abundantly expressed in STAT6+/+ B
cells (i
b-
, irf4, bcl3, stat4, dag kinase
, hck, il-10R
, ifn-
receptor, fcgrIIb, cyclin d2,
gnbp 1 and 2) are normally down-regulated in germinal
center B cells (21). Interestingly, BCL-6 is required for
the formation of germinal centers (18, 19). Our data
therefore suggest that STAT6 may influence B cell differentiation via
transcriptional repressors.
Krox20 modulates LPS plus IL-4-induced CD23 expression in B cells
To test the function of the IL-4-induced transcription factor Krox20 in B cell differentiation, M12.4.1 B cells were stably transduced with STAT6, Krox20WT, or Krox20DN by retroviral infection using a bicistronic vector also encoding GFP. The point mutations in Krox20DN that were used here have been shown to cause dominant congenital neuropathies in humans and alterations of gene transcription in mouse Schwann cells by competing with WT Krox20 (22).
Transduced cells were stimulated with LPS, IL-4, or LPS plus IL-4 for
48 h (Fig. 4
), and levels of CD23
expression in GFP+ cells were assessed by flow
cytometry. Overexpression of STAT6 or Krox20WT resulted in an only
moderate increase in CD23 expression after stimulation with LPS plus
IL-4 (n = 4, p = 0.41 or
p = 0.12, respectively; Fig. 4
). This suggests that
neither STAT6 nor Krox20 limited CD23 expression under these
conditions. Strikingly, expression of Krox20DN resulted in a highly
significant inhibition (between 55 and 81%; n = 4,
p < 0.01) of LPS plus IL-4-induced CD23 expression
(Fig. 4
). The effect of Krox20DN was specific to activation by IL-4,
because this mutant did not significantly alter CD23 expression in
cells that were only stimulated with LPS (Fig. 4
). A potential
Krox20-binding element is present at position -266 of the CD23
promoter, which is
120 bp upstream of the STAT6-binding element
(23). Taken together, these data provide strong evidence
for an important role of Krox20 in IL-4-mediated CD23 expression in B
cells.
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-induced expression of MHC II, which requires
STAT1-mediated induction of the MHC II trans activator CIITA
(24). In summary, our data provide novel insights into the mechanism of action of STAT6 in IL-4-stimulated B cells. The identified genes and their products therefore represent novel targets for the development of therapeutic drugs for the treatment of allergic disease states.
| Acknowledgments |
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| Footnotes |
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2 Abbreviations used in this paper: JAK, Janus kinase; BCL-6, B cell lymphoma 6; WT, wild-type; DN, dominant negative; GFP, green fluorescent protein. ![]()
Received for publication November 12, 2001. Accepted for publication December 10, 2001.
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