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 Previous Article | Table of Contents | Next Article 
Blood, Vol. 91 No. 3 (February 1), 1998:
pp. 940-948
Expression and Activation of the Nonreceptor Tyrosine Kinase Tec
in Human B Cells
By
Akira Kitanaka,
Hiroyuki Mano,
Mary Ellen Conley, and
Dario Campana
From the Departments of Hematology-Oncology and Immunology, St Jude
Children's Research Hospital and University of Tennessee College of
Medicine, Memphis TN; and the Department of Molecular Biology, Jichi
Medical School, Tochigi, Japan.
 |
ABSTRACT |
The tyrosine kinase Tec belongs to a new group of structurally
related nonreceptor tyrosine kinases that also includes Btk and Itk.
Previous studies have suggested that these kinases have lineage-specific roles, with Tec being involved mainly in the regulation of cytokine-mediated myeloid cell growth and
differentiation. In this study, we investigated expression and
activation of Tec in human B-lymphoid cell lines representing different
stages of B-cell maturation, including pro-B (RS4;11, 380, REH), pre-B
(NALM6), and mature B (Ramos, and one Epstein-Barr virus
[EBV]-transformed lymphoblastoid line) cells. Like Btk, Tec protein
was expressed in all B-cell lines tested. Tec was also highly expressed
in two EBV-transformed lymphoblastoid lines derived from patients with X-linked agammaglobulinemia (XLA) lacking Btk expression, as well as in
tonsillar lymphoid cells. In surface immunoglobulin-positive B cells
(Ramos), ligation of the B-cell antigen receptor (BCR) with anti-IgM
antibodies caused marked tyrosine phosphorylation of Tec and increased
Tec tyrosine kinase activity. Likewise, cross-linking of CD19 with a
monoclonal antibody in BCR-negative pro-B (RS4;11, 380) and pre-B
(NALM6) cells induced Tec tyrosine phosphorylation and increased Tec
autophosphorylation, as well as Btk activation. Tyrosine
phosphorylation of Tec, but not of Btk, was detectable in RS4;11 cells
after CD38 ligation, suggesting that these kinases are regulated
differently. We conclude that Tec is expressed and can be stimulated
throughout human B-cell differentiation, implying that this tyrosine
kinase plays a role in B-cell development and activation.
| |
INTRODUCTION |
GROWTH AND differentiation of
hematopoietic cells is regulated by the interaction of surface
receptors with their ligands. While some of these receptors mediate
signaling events via their own tyrosine kinase activity, others use
cytoplasmic tyrosine kinases to transduce signals.1,2
Recently, the Tec/Btk/Itk subfamily has emerged as a new group of
nonreceptor tyrosine kinases that typically have an amino-terminal
pleckstrin-homology domain, a Tec-homology (TH) domain, an Src-homology
(SH)-3 domain, an SH2 domain and a kinase domain.3-9 The
biologic importance of this kinase subfamily is underscored by the
finding that Btk function is essential for B-cell
development,10,11 and that mutations in Btk cause X-linked
agammaglobulinemia (XLA) in humans and the xid B-cell deficient
phenotype in mice.5,6,12,13
It has been suggested that the functions of Btk and Itk are primarily
related to B- and T-lymphoid development,5-7,10-15 while the other prototype molecule of this family, Tec, mainly participates in signaling pathways regulating myeloid growth and
differentiation.8,16-19 In these cells, Tec is tyrosine
phosphorylated following cell stimulation by a variety of hematopoietic
growth factors, such as interleukin-3 (IL-3),17 stem-cell
factor (SCF),18 and granulocyte colony-stimulating factor
(G-CSF).19 Tec tyrosine phosphorylation is accompanied by
enhanced kinase activity and increased association with the adaptor
molecule Vav via the TH domain.20 It has remained unclear
whether Tec participates in signaling in other hematopoietic lineages
(eg, lymphoid cells), or whether it can be activated by stimuli other
than hematopoietic growth factors.
In this study, we investigated whether Tec is a component of signal
transduction in human B-lymphoid cells at different stages of
maturation. We found that Tec was expressed in human pro-B, pre-B, and
B-lymphoid cells, and even in B cells lacking Btk expression. Ligation
of the B-cell antigen receptor (BCR) in mature B cells, and of the
functional surface molecules CD19 and CD38 in immature B cells, induced
Tec tyrosine phosphorylation and enhanced Tec kinase activity. Direct
comparison of Tec and Btk activation by identical stimuli showed
similarities and differences between these tyrosine kinases that
suggest a complementary role in B-cell development.
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MATERIALS AND METHODS |
Reagents and cells.
Rabbit antisera to Tec and Btk were previously
described.17,18,21 Goat antisera to Tec and Btk were
purchased from Santa Cruz Biotechnology (Santa Cruz, CA). Anti-CD38
monoclonal antibodies were T16 (immunoglobulin [Ig] G1; Immunotech
Inc, Westbrook, ME) and THB7 (IgG1; American Type Culture Collection
[ATCC], Rockville, MD). Anti-CD19 monoclonal antibody was HD37 (IgG1;
Dako, Carpinteria, CA). Goat antisera to human and mouse IgM were from
Southern Biotechnology Associates (Birmingham, AL). Goat antiserum to
mouse Ig was from Jackson Immunoresearch Laboratories (West Grove, PA).
For cell stimulation, monoclonal antibodies were used at a
concentration of 5 to 10 µg/mL; goat antisera to human IgM and mouse
Ig were used at 30 µg/mL. Isotype- and species-matched control
antibodies were used at equivalent concentrations. Monoclonal antibody
to phosphotyrosine (4G10) was purchased from UBI (Lake Placid, NY). Rabbit antisera to Lyn, Fyn, Blk, and Lck were from Santa Cruz. All
other reagents were purchased from Sigma (St Louis, MO).
Human immature B-cell lines RS4;11, 380, REH, and NALM6, and the mature
B-cell line Ramos were available from the St. Jude Children's Research
Hospital cell bank or were purchased from ATCC. Epstein-Barr virus
(EBV)-transformed lymphoblastoid cell lines were generated as described
from one normal individual and from two patients with XLA and absent
Btk expression. In one patient, a 4-bp deletion at codons 76 and 77 of
the Btk gene resulted in a frameshift and a premature stop codon at
codon 120. In the other, a base pair substitution caused a premature
stop codon at codon 520.22,23 All cell lines express CD19
and CD38. The RS4;11, 380, and REH cells lack surface and cytoplasmic
Ig (pro-B), NALM6 expresses cytoplasmic µ heavy chains (pre-B), while
the remaining cell lines are surface-Ig positive (mature B). Cell lines
were maintained in RPMI-1640 (Whittaker Bioproducts Inc, Walkersville, MD) with 10% fetal calf serum (Whittaker), L-glutamine, and
antibiotics. Tonsils were from routine tonsillectomies; thymus glands
were removed during cardiac surgery. Cell suspension were prepared by
teasing the tissue with forceps and scalpels. The cells' viability consistently exceeded 90% by trypan-blue dye exclusion assay.
Immunoprecipitation, electrophoresis, Western blotting, and kinase
assay.
Immunoprecipitation was performed as described.21,24
Briefly, after stimulation, cells were lysed in 1 mL of ice-cold lysis buffer (50 mmol/L Tris [pH 7.5], 150 mmol/L NaCl, 1% [vol/vol] Triton X-100, 5 µg/mL aprotinin, 1 mmol/L phenylmethyl sulfonyl fluoride [PMSF], 1 mmol/L EDTA, and 1 mmol/L
Na3VO4); insoluble material was removed by
centrifugation at 20,000g for 20 minutes at 4°C.
Supernatants were cleared with protein A-Sepharose (20 µL of 50%
slurry) for 1 hour. Antibodies were then added before a 1- to 2-hour
incubation at 4°C. The immune-complexes were separated by using
protein A-Sepharose.
For sodium dodecyl sulfate-polyacrylamide gel electrophoresis
(SDS-PAGE), samples were resuspended in sample buffer (10% [vol/vol] glycerol, 5% 2-mercaptoethanol, 3% [wt/vol] SDS, 65 mmol/L Tris-HCl [pH 6.8], and 0.002% [wt/vol] bromophenol blue) and separated on a
7.5% acrylamide gel.21,24 After protein transfer,
nitrocellulose filters were incubated for 2 hours in 5% bovine serum
albumin in TBS-T (20 mmol/L Tris [pH 7.6], 137 mmol/L NaCl, 0.1%
Tween 20), and then with primary antibodies for 1 hour. After washing in TBS-T, the filters were incubated for 1 hour with horseradish peroxidase-conjugated sheep antimouse Ig, donkey antirabbit Ig (Amersham, Arlington Heights, IL), or donkey antigoat Ig (Santa Cruz)
and washed again. Antibody binding was visualized by the enhanced
chemiluminescence detection system (Amersham); densitometric analysis
was performed with an AlphaImager densitometer (Alpha Innotech, San
Leandro, CA). In some experiments, the filters were stripped,
reblocked, washed, and reprobed.
For in vitro kinase assay, proteins immunoprecipitated with anti-Tec or
anti-Btk antibodies were washed with kinase buffer (20 mmol/L Tris [pH
7.5], 150 mmol/L NaCl, 10 mmol/L MgCl2, 10 mmol/L
MnCl2 for Tec; 40 mmol/L HEPES [pH 7.4], 10 mmol/L
MgCl2, 3 mmol/L MnCl2 for Btk) and incubated
for 5 to 20 minutes at 30°C in 30 µL of kinase buffer containing
10 µCi of [ -32P] adenosine triphosphate
(ATP). The reaction was stopped by adding 1 mL of ice-cold
lysis buffer. After extensive washing, proteins were eluted with
SDS-PAGE sample buffer (see above) and separated on 7.5% acrylamide
gel. 32P-labeled proteins were visualized by
autoradiography. All experiments were repeated at least three times.
| |
RESULTS |
Expression of Tec in human B cells at various stages of maturation.
We assessed expression of Tec protein in cell lines representative of
different stages of B-cell differentiation. To determine whether Tec
expression in human B cells is interrelated with that of its family
member Btk, we included two EBV-transformed B-lymphoblastoid cell lines
derived from patients with XLA who had no Btk protein expression.23 From cell lysates containing equal amounts of protein (1 mg), we immunoprecipitated Tec with saturating
concentrations of a rabbit antiserum to Tec. In all of the B-cell
lines, a protein of 70 kD, corresponding to the Tec
protein, reacted with the anti-Tec antisera of either goat or rabbit
origin (Fig 1A), in line with the findings
of Sato et al4 at the mRNA level. Tec expression appeared
to be lower, albeit clearly detectable, in 380 and REH pro-B cells.
Notably, the two XLA cell lines with impaired Btk expression had a
level of Tec similar to that of Btk-positive mature B-cell lines (Fig
1A). By densitometry, we determined that RS4;11 and Ramos cells
expressed levels of Tec identical to those measured in the myeloid cell
line K562, while 380 cells had approximately 30% expression, REH 10%,
and NALM6 70%; EBV-transformed cell lines had 120% to 150% Tec
expression. Levels of Btk expression were nearly identical in all
B-cell lines studied.
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| Fig 1.
Expression of Tec and Btk in human B cells. After
immunoprecipitation with rabbit antisera to Tec or Btk, Western blots
were probed with goat antisera against these proteins. (A) Tec was expressed in all B-cell lines studied representative of different stages of maturation, including two EBV-transformed lymphoblastoid cell
lines derived from patients with XLA and lacking Btk expression. The
myeloid cell line K562 and the T-lymphoid cell line Jurkat were used as
a positive and negative control, respectively. (B) Tec and Btk were
also expressed in tonsillar lymphoid cells, but not in thymocytes.
Molecular mass markers (in kD) are indicated. The intense band of
approximately 50 kD corresponds to the Ig heavy chain of the antibody
used for immunoprecipitation.
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To corroborate results obtained with cell lines, we examined Tec and
Btk expression in human tonsillar and thymic lymphoid cells. Both
kinases were highly expressed in tonsil cells, at levels similar to
those found in Ramos cells, but were not expressed in thymocytes (Fig
1B).
Ligation of the B-cell antigen receptor activates Tec in mature B
cells.
Ligation of the BCR induces tyrosine phosphorylation of several
proteins.25-27 To determine whether Tec was activated by
BCR signaling, we examined Tec tyrosine phosphorylation in the mature human B-cell line Ramos after cross-linking the BCR with a polyclonal antiserum to human IgM. Exposure to anti-IgM antibody caused marked tyrosine phosphorylation of Tec (Fig 2A).
In time-course experiments, Tec tyrosine phosphorylation was rapid and
was maximal 1 minute after ligation of BCR; levels of Tec tyrosine
phosphorylation remained high for at least 10 minutes, but decreased by
30 minutes after BCR cross-linking (Fig 2B). Tyrosine phosphorylation
of Tec was also detected in the murine B-cell line, WEHI-231, after ligation of surface BCR (not shown). In Ramos cells, a tyrosine phosphorylated protein of approximately 62 kD coprecipitated with Tec
after BCR ligation (Fig 2A). This band was the only one consistently present in Tec immunoprecipitates. It is likely to correspond to the
previously described, but as yet unidentified, protein of a similar
molecular weight that coprecipitates with Tec in cells exposed to SCF
or IL-6.18,28
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| Fig 2.
Ligation of BCR induces tyrosine phosphorylation and
activation of Tec. (A) Ramos cells were incubated with anti-IgM
antiserum or control serum for 5 minutes. Proteins immunoprecipitated
with anti-Tec rabbit antibody or normal rabbit serum (NRS) were
separated by SDS-PAGE and transferred to a nitrocellulose membrane. The membrane was probed with antiphosphotyrosine antibody (pTyr; upper panel), then stripped and reprobed with anti-Tec rabbit antibody (lower
panel). Molecular mass markers (in kD) are indicated. The position of
Tec is indicated by an arrow. A tyrosine phosphorylated protein of
approximately 62 kD, coprecipitated with Tec, is also noticeable. The
band above Tec (approximately 74 kD) is likely due to nonspecific
reactivity, as it was also detected after BCR ligation in
immunoprecipitates obtained with NRS. (B) Kinetics of BCR-mediated
tyrosine phosphorylation of Tec. Ramos cells were incubated with
anti-IgM antiserum for the times indicated (minutes). Tyrosine
phosphorylation of Tec was assessed above. (C) BCR
ligation increases Tec kinase activity. Ramos cells were stimulated as above for 5 minutes and Tec kinase activity was assessed by in vitro
kinase assay. The position of Tec is indicated by the arrow.
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BCR ligation in Ramos cells also induced a distinct increase in Tec
autophosphorylation (Fig 2C). In addition, other phosphorylated proteins coprecipitated with Tec, including two prominent proteins of
approximately 56 and 59 kD.18 These proteins had a mobility different than that of the Ig heavy chain of the antibody used for
immunoprecipitation, as assessed by comparisons with the Coomassie blue
stained gel (not shown). Because Tec is associated with Lyn in myeloid
cells,29 we determined whether the 56- and 59-kD proteins
corresponded to Lyn, or to other Src-family kinases known to be
activated by BCR cross-linking, such as Fyn, Blk, and
Lck.25-27 In side-by-side electrophoresis and
autoradiography experiments, none of the four Src-family kinases had
the same mobility that the 56- and 59-kD proteins coprecipitated with
Tec.
CD19 ligation activates Tec in immature B cells.
To determine whether Tec is involved in signaling pathways in immature
B cells that do not express surface BCR, we stimulated cells by
cross-linking CD19, a 95-kD transmembrane molecule expressed throughout
all stages of B-cell differentiation.30,31 We incubated BCR-negative immature B cells (RS4;11, 380 and NALM6) with anti-CD19 monoclonal antibody for 15 minutes at 4°C, followed by
cross-linking with goat antimouse antiserum for 1 minute at 37°C.
This procedure caused marked tyrosine phosphorylation of Tec in all
three cell lines (Fig 3A). As in Ramos
cells after BCR ligation, a 62-kD tyrosine phosphorylated protein
coprecipitated with Tec in RS4;11 and 380 cells after CD19
cross-linking (Fig 3A). CD19 ligation also induced a marked increase in
Tec autophosphorylation (Fig 3B). Thus, Tec kinase is activated by CD19
oligomerization in immature B cells.
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| Fig 3.
Ligation of CD19 in immature B cells induces tyrosine
phosphorylation and activation of Tec. (A) Immature B-cell lines,
RS4;11, 380, and NALM6 were incubated with anti-CD19 monoclonal
antibody or isotype-matched nonreactive Ig for 15 minutes at 4°C
followed by goat antimouse Ig antiserum for 1 minute at 37°C.
Proteins immunoprecipitated with anti-Tec rabbit antibody were
separated by SDS-PAGE and transferred to a nitrocellulose membrane. The membrane was probed with antiphosphotyrosine antibody (pTyr; upper panel), then stripped and reprobed with anti-Tec rabbit antibody (lower
panel). The position of Tec is indicated. (B) CD19 cross-linking in
immature B cells increases Tec kinase activity. RS4;11 cells were
stimulated as above and Tec kinase activity was assessed by in vitro
kinase assay. Molecular mass markers (in kilodaltons) are indicated.
The position of Tec is indicated by the arrow.
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Ligation of CD19 in the BCR-positive cell line Ramos induced tyrosine
phosphorylation of several proteins, including CD19 itself.32 However, we could not detect clear tyrosine
phosphorylation of Tec in these cells after exposure to anti-CD19 (not
shown).
Activation of Btk in immature and mature B cells.
Previous studies have reported that BCR ligation in mature B cells
induces activation of Btk.33-35 In Ramos cells, BCR
ligation did induce tyrosine phosphorylation of Btk
(Fig 4A). To determine whether Btk is
activated in immature B cells after CD19 ligation, we cross-linked CD19
in RS4;11, 380 and NALM6 cells. This procedure caused marked tyrosine
phosphorylation of Btk in RS4;11 and 380 (Fig 4A), indicating that Btk
can be activated by CD19 oligomerization in immature B cells. In NALM6,
Btk appeared to be substantially tyrosine phosphorylated before
stimulation, and there was no net increase in signal after CD19
cross-linking (Fig 4A). In time course experiments, maximal tyrosine
phosphorylation of Btk was observed after 1 minute of CD19
cross-linking (Fig 4B). Cross-linking of CD19 also appeared to enhance
the autophosphorylation of Btk (Fig 4C).
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| Fig 4.
Ligation of CD19 in immature B cells induces tyrosine
phosphorylation and activation of Btk. (A) Immature B-cell lines,
RS4;11, 380, and NALM6 were incubated with anti-CD19 monoclonal
antibody or isotype-matched nonreactive Ig for 15 minutes at 4°C
followed by goat antimouse Ig antiserum for 1 minute at 37°C.
Proteins immunoprecipitated with anti-Btk rabbit antibody were
separated by SDS-PAGE and transferred to a nitrocellulose membrane. The membrane was probed with antiphosphotyrosine antibody (pTyr; upper panel), then stripped and reprobed with anti-Btk goat antibody (lower
panel). The position of Btk is indicated. In RS4;11 and 380 cells,
cross-linking of CD19 induced tyrosine phosphorylation of Btk, as
5-minute exposure to anti-IgM antibody did in Ramos cells.33-35 In NALM6 cells, Btk appeared to be strongly
tyrosine phosphorylated before CD19 ligation and no increase in
phosphorylation was detected. (B) Kinetics of CD19-mediated tyrosine
phosphorylation of Btk. CD19 was cross-linked on RS4;11 cells for the
times indicated (minutes). Tyrosine phosphorylation of Btk was assessed
as above. (C) CD19 ligation increases Btk kinase activity. The 380 cells were stimulated as above for 1 minute and Btk kinase activity was
assessed by in vitro kinase assay. Molecular mass markers (in
kilodaltons) are indicated. The position of Btk is indicated by the
arrow.
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CD38 ligation in RS4;11 immature B cells activates Tec, but not Btk.
CD38 is a 45-kD transmembrane glycoprotein highly expressed on immature
lymphoid cells.36 Ligation of CD38 suppresses cell growth
and induces apoptosis in both normal and leukemic immature B
cells.36 This cellular effect is accompanied by induction of tyrosine kinase activity.21,24,37 To date, the only
identified tyrosine kinase activated by CD38 stimulation in human
immature B cells is Syk.21 In the immature B-cell line,
RS4;11, a 5-minute exposure to anti-CD38 monoclonal antibody (T16 or
THB7) caused distinct tyrosine phosphorylation of Tec
(Fig 5A). As in the case of BCR and CD19
ligation, CD38 stimulation promoted the association of Tec with a
tyrosine phosphorylated protein of approximately 62 kD (Fig 5A), and
caused an increase in Tec autophosphorylation in RS4;11 cells (Fig 5B).
CD38 ligation in the same cells failed to induce tyrosine
phosphorylation of Btk (Fig 5C).
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| Fig 5.
Ligation of CD38 in RS4;11 cells induces tyrosine
phosphorylation and activation of Tec, but not of Btk. (A) RS4;11 cells were incubated with anti-CD38 monoclonal antibodies, T16 and THB7, and
isotype-matched nonreactive Ig for 5 minutes. Proteins
immunoprecipitated with anti-Tec rabbit antibody were separated by
SDS-PAGE and transferred to a nitrocellulose membrane. The membrane was
probed with antiphosphotyrosine antibody (pTyr; upper panel), then
stripped and reprobed with anti-Tec rabbit antibody (lower panel).
Molecular mass markers (in kilodaltons) are indicated. The position of
Tec is indicated by an arrow. (B) CD38 ligation increases Tec kinase
activity. RS4;11 cells were stimulated with THB7 for 5 minutes and Tec
kinase activity was assessed by in vitro kinase assay. (C) RS4;11 cells were stimulated with anti-CD38 monoclonal antibodies as in (A). Tyrosine phosphorylation of Btk was examined as above. The position of
Btk is indicated by an arrow. The presence of Btk in the
immunoprecipitates was confirmed by stripping the blot and reprobing
with anti-Btk goat antibody (lower panel).
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DISCUSSION |
B-cell development proceeds through discrete stages of clonal
selection, proliferation, and differentiation influenced by microenvironmental signals. These events are modulated through ligation
of surface receptors, with specific cellular effects that depend on the
level of cell maturation. These signaling cascades entail early
induction of tyrosine kinase activity, with consequent rapid and
transient tyrosine phosphorylation of intracellular substrates. The
tyrosine kinases most consistently reported to be activated by ligation
of B-cell surface receptors include the Src-family kinases (Lyn, Fyn,
Blk, and Lck), Syk and Btk.21,25-27,30,31,38 We have shown
that Tec is activated in signaling pathways triggered by three
independent B-cell surface receptors. Although previous studies
suggested that Tec is preferentially involved in signaling mechanisms
regulating myeloid development,8,16-19 our results imply
that Tec participates directly in signaling that regulates human B-cell
differentiation. Tec's function in B cells is likely to extend to
other species, as we found that BCR ligation also causes Tec tyrosine
phosphorylation in the murine B-cell line, WEHI-231.
Studies of Tec/Btk/Itk kinases have suggested that these molecules play
a role in transducing stimuli leading to cell differentiation and/or proliferation.8 Our findings indicate that
activation of Tec and Btk may result in either growth-inducing or
growth-suppressing signals, as ligation of either BCR in Ramos and
WEHI-231 cells or CD38 in RS4;11 cells suppresses cell growth and
induces apoptosis.36,39,40 Previous studies have suggested
that Tec tyrosine kinase has a specialized role in signaling pathways
generated by growth factor receptors. Tec activation was detected
following cell exposure to IL-3,17 SCF,18
G-CSF,19 erythropoietin,20 IL-6,28 and thrombopoietin.41 Tec was shown to be associated with
gp130, the signal transducing unit for the IL-6 cytokine family, which includes, among others, IL-6, IL-11, leukemia-inhibitory factor, and
oncostatin M.28 Tec was also shown to be associated with c-kit,18 further implying its involvement in signaling
mechanisms triggered by hematopoietic growth factors. Tec's role in
human B-lymphoid cells, as a component of signaling triggered by
ligation of nonkinase receptors, which do not function as cytokine
receptors, is a novel one for this tyrosine kinase.
The precise molecular interactions that precede and follow Tec
activation in B cells are still unclear. Tec appears to be constitutively bound to Lyn tyrosine kinase through the Tec-homology domain.29,42 Thus, after cross-linking of B-cell surface
receptors, Tec could be tyrosine phosphorylated after activation of
Lyn. It has been reported that tyrosine phosphorylation of Tec promotes its association with the adaptor proteins Shc and Vav.17,20 Thus, events downstream of Tec activation could ultimately lead to
activation of the Ras/mitogen-activated protein (MAP) kinase pathway.
Activation of this pathway is known to follow BCR and CD19
ligation,43,44 and we have observed that CD38 dimerization in immature B cells causes tyrosine phosphorylation of Shc and its
association with Grb2 (A. Kitanaka et al, unpublished observations, September 1996). Therefore, Tec may represent a central link between early signaling events and the Ras/MAP kinase pathway in B-lineage cells.
In summary, we found that Tec is activated in both mature and immature
human B-lymphoid cells by a variety of stimuli, suggesting that this
tyrosine kinase plays an important role in B-cell development. It has
been postulated that expression of Tec in myeloid progenitors could be
responsible for the preservation of normal myeloid development in
patients with XLA.8,45 Although this may be true, it is clear that Tec cannot compensate for defects in Btk in B cells, indicating that the two kinases do not have completely overlapping functions. This hypothesis is supported by our observation that stimulation of the pro-B cell line RS4;11 with CD38 could activate Tec,
but not Btk. Other stimuli that are crucial for B-cell development may
activate Btk, but not Tec. Future studies should clarify the importance
of Tec activation for normal B-cell development and its role in the
pathogenesis of B-cell immunodeficiency.
| |
FOOTNOTES |
Submitted April 23, 1997;
accepted September 23, 1997.
Supported by Grants No. RO1-CA58297 and P30-CA21765 from the National
Cancer Institute and by the American Lebanese Syrian Associated
Charities (ALSAC).
Address reprint requests to Dario Campana, MD, PhD, Department of
Hematology-Oncology, St Jude Children's Research Hospital, 332 N
Lauderdale, Memphis, TN 38105.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. section 1734 solely
to indicate this fact.
| |
ACKNOWLEDGMENT |
We thank Dr Ken Sato for helpful discussions and Sharon Naron for
editorial assistance.
| |
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Abstract
Introduction
Abstract