The Journal of Immunology, 2001, 166: 4273-4277.
Copyright © 2001 by The American Association of Immunologists
Cutting Edge: Differential Signaling Requirements for Activation of Assembled Cyclin D3-cdk4 Complexes in B-1 and B-2 Lymphocyte Subsets1
Debra A. Tanguay2,*,
Thomas P. Colarusso3,
,
,
Cheryl Doughty*,
Sandra Pavlovic-Ewers4,*,
Thomas L. Rothstein
,
,
and
Thomas C. Chiles5,*
*
Department of Biology, Boston College, Chestnut Hill, MA 02467; and Departments of
Medicine and
Microbiology, and
The Evans Memorial Department of Clinical Research, Boston University Medical Center, Boston, MA 02118
 |
Abstract
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B-1 lymphocytes represent a distinct B cell subset with unusual
mitogenic responses. PMA alone promotes proliferation in B-1 cells, but
not in splenic B-2 cells. Although cyclin D2-cyclin-dependent kinase 4
(cdk4) complexes mediate early retinoblastoma gene product (pRb)
phosphorylation in B-1 cells, the transient nature of
their accumulation cannot account for the continued increase in pRb
phosphorylation, which is maximal at 24 h. We show
herein that PMA promotes the accumulation of functional cyclin D3-cdk4
complexes in B-1 cells following loss of cyclin D2. PMA also induces
accumulation of cyclin D3-cdk4 complexes in B-2 cells; however, these
complexes do not phosphorylate pRb. Thus, PMA is sufficient
to induce synthesis and assembly of cyclin D3-cdk4 complexes in B-1 and
B-2 cells; however, PMA triggers cyclin D3-cdk4 activation only in B-1
cells. These results reveal a novel regulatory step that controls
activation of cyclin D3-cdk4 complexes whose function segregates
differentially in B cell subsets.
 |
Introduction
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B-1
cells constitute a unique subset of B lymphocytes, distinguished from
conventional B lymphocytes (B-2) by numerous phenotypic and functional
characteristics (reviewed in Refs. 1, 2, 3). As an example,
B-1 cells localize primarily to the peritoneum, whereas B-2 cells
predominate in spleen and lymph nodes. B-1 cells contribute substantial
proportions of nonimmune (resting) IgM and IgA that is repertoire
restricted. Whether B-1 cells represent a developmental lineage
distinct from B-2 cells or, alternatively, derive from a single B cell
lineage in which B-2 cells differentiate to B-1 cells in response to B
cell Ag receptor
(BCR)6-derived signals
remains a matter of controversy (4, 5, 6). Although B-1 cells
resemble activated B-2 cells in terms of surface expression of IL-5R,
CD44, and nuclear, activated STAT3 (7, 8), many additional
molecular and transcriptional markers associated with B-2 cell
activation are absent in B-1 cells (9, 10).
B-1 cells differ significantly from B-2 cells in the signals required
to induce proliferation. B-1 cells fail to enter S phase in response to
anti-Ig, whereas B-2 cells are mitogenically stimulated by BCR
cross-linking (11, 12). Treatment with phorbol ester alone
is sufficient to stimulate B-1 cells to enter S phase, whereas B-2
cells exit quiescence, but subsequently arrest in
G1 phase of the cell cycle (progression to S
phase requires a second signal provided by calcium ionophore)
(11, 12). The molecular basis underlying the unique
proliferative response of B-1 cells to phorbol esters is not completely
understood.
It is generally considered that growth signals regulate mammalian cell
cycle entry by stimulating the accumulation of D-type cyclins (cyclins
D1, D2, and D3) that function to activate a subset of cyclin-dependent
kinases (cdks) (reviewed in Ref. 13). The retinoblastoma
gene product (pRb) is a target of cdks and acts to suppress
G1-to-S phase progression
(14, 15, 16). pRb is presently the most plausible
candidate for regulating progression through the restriction (R) point
(14, 16). A current model holds that pRb suppression is
alleviated through hyperphosphorylation that is
mediated by both cyclin E and D-type cyclin kinase complexes (15, 16). The proper timing and extent of cdk activation is
controlled by dephosphorylation of inhibitory
sites, phosphorylation of activating sites, the action
of two families of cdk inhibitors (Ink4 and Cip/Kip family), and
by cyclin binding (17). For example, D-type cyclins and
cyclin E function as positive regulatory subunits for cdk4/6 and cdk2,
respectively (13). The requirement of mammalian cyclins D1
and D2 in G1 phase progression has been
definitively established (18, 19). In keeping with this,
distinct phenotypes have been reported in cyclin
D1-/- and cyclin D2-/-
mice (20, 21). Recent studies suggest that cyclin
D3 may function to limit the rate of G1 phase
progression (18, 22). Interestingly, in cyclin
D2-/- mice, B-2 cells appear to remain
responsive to mitogenic signals due to a compensatory induction of
cyclin D3 levels (23).
Cell cycle progression to S phase in normal B-2 cells requires the
accumulation of cyclin D2 and cdk4 and to a lesser extent cdk6
(24, 25, 26). B-2 cells from xid mice, which
exhibit aborted activation in response to BCR cross-linking, do not
up-regulate cyclin D2 protein, suggesting that accumulation of cyclin
D2 in B-2 cells may be linked to passage through the R point
(26). Interestingly, Solvason et al. (27)
recently demonstrated a requirement for cyclin D2 expression in CD5 B
cell development. We have previously demonstrated that phorbol ester
induces unusually early and transient expression of cyclin D2 in B-1,
but not in B-2 cells (28). As such, we proposed that the
early induction of cyclin D2 may account for the rapid entry of B-1
cells into the cell cycle following phorbol ester stimulation. However,
the transient nature of cyclin D2 expression in phorbol
ester-stimulated B-1 cells suggests that it is unlikely to be
responsible for progression through the G1/S
transition. Herein, we report experiments demonstrating that in phorbol
ester-stimulated B-1 cells, cyclin D3-cdk4 complexes assemble and are
active pRb protein kinases in late G1 phase of
the cell cycle. By contrast, phorbol ester stimulation of splenic B-2
cells results in the assembly of cyclin D3-cdk4 complexes; however,
these complexes fail to phosphorylate pRb in vitro. These
findings constitute the first demonstration in a primary mammalian cell
that cyclin D-cdk complex assembly and activation can be dissociated
and regulated by different signals.
 |
Materials and Methods
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Animals
Male BALB/cByJ mice at 814 wk of age were obtained from The
Jackson Laboratory (Bar Harbor, ME). Mice were cared for and handled at
all times in accordance with National Institutes of Health and
institutional guidelines.
B cell purification
B-1 and B-2 lymphocytes were prepared by negative selection from
peritoneal washout cells and from spleen cell suspensions,
respectively, as described (9). The recovered B-1 cell
population was 9096% sIgM+,
CD5/Mac-1+ by flow cytometric analysis. The B
cells were cultured at 37°C with 5% CO2 in
RPMI 1640 medium (BioWhittaker, Walkersville, MD) supplemented with
10% heat-inactivated FBS (Sigma, St. Louis, MO), 10 mM HEPES (pH 7.2),
50 µM 2-ME, 2 mM L-glutamine, 100 U/ml penicillin, and
100 µg/ml streptomycin.
Immunoprecipitation
B cells were solubilized in 1 ml Nonidet P-40 buffer (50 mM
Tris-HCl, pH 7.4, 150 mM NaCl, 20 mM EDTA, 0.5% Nonidet P-40, 1 mM
PMSF, 2.5 µg/ml leupeptin/aprotinin, 1 mM
Na3VO4, and 10 mM
-glycerophosphate) for 30 min (4°C) (28). Insoluble
debris was removed by centrifugation at 15,000 x g for
15 min (4°C). The detergent-soluble cell lysates were incubated for
3 h with 1.5 µg nonimmune IgG or 1.5 µg anti-cdk4 Ab, or
1.5 µg anti-cdk6 Ab, followed by the addition of 50 µl of a 1:1
slurry of protein G-agarose. After 90 min, the immune complexes were
collected, washed several times in Nonidet P-40 buffer, and separated
by electrophoresis through a 10% polyacrylamide SDS gel. The proteins
were transferred to Immobilon-P membrane (Millipore, Bedford, MA) and
immunoblotted with an anti-cyclin D3 mAb (1:500 dilution in TBST)
as described below.
Immune complex kinase assays
B cells were sonicated (4°C) in 1 ml Rb buffer (50 mM HEPES,
pH 7.5, 150 mM NaCl, 1 mM EDTA, 2.5 mM EGTA, 1 mM DTT, 0.1%
Tween20, 10% glycerol, 0.1 mM PMSF, 1 µg/ml
leupeptin/aprotinin, 10 mM
-glycerophosphate, 1 mM NaF, and 0.1 mM
Na3VO4) (29).
Insoluble material was removed by centrifugation, and the supernatant
was incubated with 1.5 µg nonimmune rabbit IgG, 1.5 µg of
anti-cdk4, or 1.5 µg of anti-cdk6 Abs. After 3 h, 50
µl of a 1:1 slurry of protein G-agarose was added and incubated for
1 h. The immune complexes were recovered by centrifugation and
washed six times with Rb buffer and then three times in a buffer of 50
mM HEPES, pH 7.4, and 1 mM DTT. The immune complexes were resuspended
in 30 µl of Rb kinase buffer (50 mM HEPES, pH 7.5, 10 mM
MgCl2, 5 mM MnCl2, 1 mM
DTT, 2.5 mM EGTA, 10 mM
-glycerophosphate, 0.1 mM
Na3VO4, and 10 µCi
[32P]ATP at 6000 Ci/mmol) in the presence of
1 µg of a truncated Rb protein substrate
(p56Rb). After 15 min at 30°C, the reactions
were terminated by the addition of 2x SDS sample buffer, and the
reaction products were separated through a 10% polyacrylamide SDS gel.
Phosphorylated Rb was detected by autoradiography of the
dried gel.
Immunoblot analysis
For the detection of cyclins D2 and D3, B lymphocytes were
solubilized in 100 µl Triton X-100 buffer (50 mM HEPES, pH 7.4, 15 mM
EGTA, 137 mM NaCl, 15 mM MgCl2, 0.1% Triton
X-100, 10 mM
-glycerophosphate, 1 mM
Na3VO4, 1 mM PMSF, and 1
µg/ml aprotinin/leupeptin); for detection of endogenous pRb, B cells
were solubilized in 100 µl Nonidet P-40 buffer containing 20 mM NaF
(28). Insoluble debris was removed by centrifugation at
15,000 x g (15 min), and 1020 µg of total protein was
separated by polyacrylamide SDS gel electrophoresis and transferred to
Immobilon-P membrane. Immune detection was conducted as previously
described (24).
Reagents
F(ab')2 of goat anti-mouse IgM was
obtained from Jackson ImmunoResearch (West Grove, PA). PMA and the
calcium ionophore, ionomycin, were obtained from Sigma. Human pRb mAb
(clone G3-245) was obtained from PharMingen (San Diego, CA).
Anti-rabbit and anti-mouse IgG-conjugated HRP Abs and anti-cdk4
Ab (sc-260) were obtained from Santa Cruz Biotechnology (Santa Cruz,
CA). Anti-cdk6 Ab (13446E) was obtained from PharMingen. Protein
G-agarose was obtained from Life Technologies (Gaithersburg, MD). Mouse
anti-cyclin D2 Ab (DCS-3 and DCS-5) and anti-cyclin D3 Ab
(DCS-22) were a gift from Jiri Bartek (Division of Cancer Biology,
Danish Cancer Society, Copenhagen, Denmark) (19). The
truncated Rb substrate protein (p56Rb) was
obtained from QED Advanced Research Technologies (San Diego, CA).
Enhanced chemiluminescence reagents were obtained from Kirkegaard &
Perry Laboratories (Gaithersburg, MD).
 |
Results and Discussion
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We previously demonstrated that B-1 cell stimulation with the
phorbol ester, PMA, produced cyclin D2 accumulation in a rapid and
transient manner (28). The initial onset of Cdk4-mediated
pRb phosphorylation on Ser780 correlated
with the accumulation and assembly of cyclin D2 into higher order
complexes containing cdk4. However, we noted that
phosphorylation of pRb continued to increase as B-1
cells progressed through the G1-to-S phase
transition. Interestingly, the maximal level of pRb
phosphorylation occurred near the
G1/S transition at a time when cyclin D2 was not
expressed. These findings suggested that PMA stimulation of B-1 cells
promotes the accumulation of additional G1-cyclin
complexes capable of phosphorylating pRb. To investigate the nature of
G1-cyclin complexes that might contribute to the
phosphorylation of endogenous pRb in late
G1 phase, we evaluated the expression of cyclin
D3 in B-1 cells, noting that cyclin D1 is not expressed in murine B
lymphocytes (24, 25, 26). B-1 cells were cultured in medium
alone or stimulated with PMA for various times; cells were then
collected and detergent-solubilized proteins were separated by SDS-PAGE
and immunoblotted with an anti-cyclin D3 mAb (Fig. 1
). Cyclin D3 was not detected in B cells
cultured in medium alone. Stimulation of B-1 cells with PMA induced the
accumulation of cyclin D3 at the 17- and 24-h time points. No cyclin D3
was expressed after PMA stimulation for 4 h, the point at which
cyclin D2 expression peaked (28), indicating that
PMA-stimulated accumulation of cyclin D3 and cyclin D2 are distinct.
The accumulation of cyclin D3 paralleled peak endogenous pRb
phosphorylation in PMA-stimulated B-1 cells (Fig. 2
). PMA also induced the accumulation of
cyclin D3 in B-2 cells; however, in contrast to B-1 cells, PMA
stimulation of B-2 cells at parallel time points (i.e., 4, 16, 24
h) did not lead to increased pRb phosphorylation. As
a positive control for these experiments, pRb phosphorylation was
induced in B-2 cells stimulated with several different mitogens,
including 25 µg/ml LPS, 10 µg/ml anti-Ig and the combination of
PMA (300 ng/ml) plus ionomycin (400 ng/ml) for 24 and 36 h (Fig. 2
).

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FIGURE 1. Cyclin D3 expression is induced late during G1 phase in B-1
lymphocytes treated with PMA. Primary B-1 and B-2 lymphocytes were
cultured in the presence of medium alone (M) or were stimulated with
PMA (300 ng/ml) for 4, 17, and 24 h. At the indicated times, whole
cell lysates were prepared in a 0.1% Trition X-100 buffer, and 10 µg
protein was resolved by 10% SDS-PAGE, transferred to Immobilon-P
membrane, and immunoblotted with a cyclin D3-specific mAb. The position
of cyclin D3 is indicated by the arrow.
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FIGURE 2. PMA treatment of B-1 cells, but not B-2 cells, promotes endogenous pRb
phosphorylation. B-1 lymphocytes were cultured in
medium alone (M) or were stimulated with medium containing PMA at 300
ng/ml for 4, 16, and 24 h as indicated. As a positive control for
pRb phosphorylation, B-1 cells were also stimulated
with 25 µg/ml LPS for 24 h. B-2 cells were cultured in medium
alone (M) or were stimulated with medium containing 25 µg/ml LPS for
24 h or 10 µg/ml anti-Ig (aIg) or 300 ng/ml PMA plus 400
ng/ml ionomycin (P/I) for 24 and 36 h as indicated. Nonidet P-40
detergent lysates were prepared, and 10 µg protein was resolved by
7.5% SDS-PAGE, transferred to Immobilon-P membrane, and immunoblotted
with an anti-human pRb Ab. The position of pRb is indicated by the
arrow. The molecular weight of standard proteins is given in
kilodaltons and indicated on the left.
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The timing of cyclin D3 accumulation in B-1 cells suggests that it may
contribute to the phosphorylation of endogenous pRb. To
test this further, we sought to determine whether PMA promotes the
assembly and activation of cyclin D3-cdk complexes at a time
commensurate with peak PMA-induced phosphorylation of
endogenous pRb. B-1 and B-2 cells were cultured in medium alone or
were stimulated with PMA for 4 and 24 h, at which times
cells were detergent-solubilized and immunoprecipitated with rabbit
anti-mouse cdk4 or anti-mouse cdk6 Abs (Fig. 3
). The immune complexes were
separated by SDS-PAGE and immunoblotted with anti-cyclin D3 mAb.
PMA stimulation of B-1 cells induced the assembly of cyclin D3-cdk4/6
complexes, as evidenced by the appearance of cyclin D3 in cdk4 and cdk6
immunoprecipitates at 24 h. Surprisingly, the same was true of
PMA-stimulated B-2 cells. No detectable cyclin D3 was present in
cdk4/6-immune complexes isolated from unstimulated B-1 or B-2 cells,
and in parallel experiments, cyclin D3 was not detected in any sample
after immunoprecipitation with nonimmune serum (data not shown).
Importantly, cyclin D3-cdk4/6 holoenzyme complexes were not detected in
B-1 cells stimulated with PMA for 4 h, which is consistent with
the lack of detectable cyclin D3 expression at this time (see Fig. 1
).

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FIGURE 3. PMA promotes the assembly of cyclin D3-cdk4 complexes in both B-1 and
B-2 lymphocytes. B-1 and B-2 cells were cultured in medium alone (M) or
were stimulated with PMA at 300 ng/ml for 4 and 24 h as indicated.
Nonidet P-40 detergent extracts were prepared and sequentially
immunoprecipitated (IP) with 1.5 µg of nonimmune serum (data not
shown), 1.5 µg of anti-cdk4 Ab, and 1.5 µg of anti-cdk6 Ab.
The resulting immune complexes were resolved by 10% SDS-PAGE,
transferred to Immobilon-P membrane, and immunoblotted (IB) with
anti-cyclin D3 mAb. The position of cyclin D3 is indicated by the
arrows.
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To determine whether the PMA-induced cdk4- and cdk6-containing
complexes were functional, Rb protein phosphorylation
activity in cdk4 and cdk6 immunoprecipitates was analyzed using a
recombinant COOH-terminal-truncated Rb protein as substrate
(29). In extracts prepared from B-1 cells stimulated with
PMA for 24 h, Rb phosphorylation was produced by
cdk4 immune complexes and to a much lesser extent by cdk6 immune
complexes (Fig. 4
). Nonimmune complexes
were devoid of Rb kinase activity (data not shown). In parallel
experiments, Rb phosphorylation was not stimulated
above control by cdk4 or cdk6 immune complexes isolated from
PMA-stimulated B-2 cells. As a positive control, cdk4 and to a lesser
extent cdk6 immune complexes recovered from B-2 cells stimulated with
the combination of PMA plus calcium ionophore exhibited inducible Rb
kinase activity (Fig. 4
, lane P/I).

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FIGURE 4. Cdk4- and cdk6-containing complexes produced by PMA treatment are
active Rb kinases in B-1, but not B-2 lymphocytes. B-1 and B-2 cells
were cultured in medium alone (Med) or were stimulated with medium
containing PMA at 300 ng/ml for the indicated times. Detergent lysates
were prepared and sequentially immunoprecipitated (IP) with nonimmune
serum (data not shown), anti-cdk4 Ab, and anti-cdk6 Ab, and the
immune complexes were recovered and assayed for in vitro kinase
activity using Rb-GST fusion protein (p56Rb) substrate as
described in Materials and Methods. B-2 cells were also
stimulated with 300 ng/ml PMA plus 400 ng/ml ionomycin (P/I) for
24 h. The position of the phosphorylated Rb-GST fusion
protein is indicated by the arrows.
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We previously reported that cyclin D2 accumulates rapidly and in a
transient manner following stimulation of B-1 cells with PMA alone
(28). Commensurate with cyclin D2 accumulation, B-1 cells
express phosphorylated pRb on Ser780, which increases in a
time-dependent manner. These results suggest that cyclin D2 plays an
important role in early G1 phase progression in
B-1 cells during PMA-mediated proliferation. Although our previous
findings in B-1 cells are consistent with the emerging view that cyclin
D2 is the primary G1 cyclin involved in passage
through the R point during BCR-mediated B-2 cell proliferation
(23, 24, 25, 26), we note that the majority of pRb
phosphorylation occurs in mid-to-late
G1 phase and at a time during which cyclin D2 is
not detected in B-1 cells by Western blot analysis (28).
This observation points to the presence of additional
G1 cyclins that contribute to pRb
phosphorylation following loss of detectable cyclin D2.
The results presented herein indicate that cyclin D3 accumulates in
response to PMA stimulation of B-1 cells. Furthermore, the timing of
cyclin D3 accumulation, which was distinct from that of cyclin D2,
suggests the involvement of cyclin D3 in PMA-induced mid-to-late
G1 phase progression. Consistent with this
conclusion, cyclin D3 assembled into higher order complexes containing
both cdk4 and cdk6 in response to PMA in B-1 cells. Furthermore, cdk4
and to a lesser extent cdk6 immune complexes from PMA-stimulated B-1
cells were capable of phosphorylating Rb in vitro at levels that were
substantially greater than parallel immune complexes recovered from
control cells. Taken together, these results suggest a role for cyclin
D3-cdk4 holoenzyme in passage through the G1/S
phase transition during PMA-mediated B-1 cell proliferation. In keeping
with our findings herein, mounting evidence in several cell types
supports a role for cyclin D3 function in the control of mammalian cell
G1/S transition (19, 22, 30).
The accumulation and assembly of substantial levels of cyclin
D3-containing cdk complexes in B-2 cells following PMA stimulation was
unexpected. The absence of pRb kinase activity associated with these
complexes in B-2 cells suggests that an additional signal(s) is (are)
required for function and points to the existence of a hitherto unknown
regulatory step in B cells that controls the activation of preformed
cyclin D3-cdk complexes. It is well established that formation of
cyclin D holoenzyme complexes relies upon growth factor signals that
act both transcriptionally, to induce accumulation of D-type cyclin and
cdk, and posttranslationally, to promote cyclin D-cdk assembly
(17, 30). For example, ectopically expressed cyclin D1 and
cdk4 subunits are not active in NIH 3T3 cells in the absence of serum
because they fail to assemble in the absence of mitogenic signals. The
mitogenic signal for assembly can be provided by activation of the
Ras/Raf-1/Erk pathway or by ectopic expression of MEK1
(30). Thus, growth factor signals not only function to
induce cyclin D1 transcription, but also to promote assembly of cyclin
D1 into cdk4-containing catalytically active complexes. Our findings in
B-2 cells suggest that accumulation and assembly of cyclin D3-dependent
kinases is not sufficient to induce kinase activation. To our
knowledge, this constitutes the first demonstration in a primary (not
genetically engineered) mammalian cell that cyclin D-cdk
accumulation/assembly and D-type cyclin-cdk activation can be
dissociated and are regulated by different signals.
At present the nature of the regulatory step necessary to promote
activation of assembled cyclin D3-cdk complexes in B-2 cells is
unknown. Given the presence of assembled D-type cyclin-cdk complexes,
it is unlikely that formation of inactive binary cdk/Ink4 complexes or
decreased expression of D-type cyclin or cdk accounts for the observed
inhibition of cyclin D3-cdk complex activity in B-2 cells (13, 17). PMA treatment of B-2 cells does not alter the relative
amount of D-type cyclin-associated p21Cip1 and
p27Kip1 proteins in comparison to control cells (data not
shown). Thus, regulation of cyclin D3-cdk complex activity might depend
on a previously uncharacterized enhancing protein, a previously
uncharacterized inhibitory protein, or a posttranslational modification
of the D-type cyclin and/or cdk4/6 subunits (e.g.,
phosphorylation on an as yet unmapped amino acid
residue) induced by PMA in one B cell population and not the other.
Studies are presently underway to further understand the molecular
step(s) that regulate cyclin D3-cdk complex activation in B
lymphocytes.
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Acknowledgments
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We thank Dr. Jiri Bartek (Division of Cancer Biology, Danish Cancer
Society, Copenhagen, Denmark) for the murine D-type cyclin
mAbs.
 |
Footnotes
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1 This work was supported by Grant MCB-9603784 awarded by the National Science Foundation (to T.C.C.) and U.S. Public Health Service Grant AI29690 awarded by the National Institutes of Health (to T.L.R.). 
2 Current address: Genaissance Pharmaceuticals, Five Science Park, New Haven, CT 06511. 
3 Current address: Genetics Computer Group, 575 Science Drive, Madison, WI 53719. 
4 Current address: Fred Hutchinson Cancer Research Center, Seattle, WA 98109. 
5 Address correspondence and reprint requests to Dr. Thomas C. Chiles, Department of Biology, Boston College, 414 Higgins Hall, Chestnut Hill, MA 02467. 
6 Abbreviations used in this paper: BCR, B cell Ag receptor; pRb, retinoblastoma gene product; cdk, cyclin-dependent kinase; R point, restriction point. 
Received for publication December 8, 2000.
Accepted for publication January 31, 2001.
 |
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