[CANCER RESEARCH 62, 1797–1801, March 15, 2002]
Chfr Regulates a Mitotic Stress Pathway through its RING-Finger Domain with
Ubiquitin Ligase Activity1
Priya Chaturvedi, Valery Sudakin, Matthew L. Bobiak, Paul W. Fisher, Michael R. Mattern, Sandra A. Jablonski,
Mark R. Hurle, Yuan Zhu, Tim J. Yen, and Bin-Bing S. Zhou2
Departments of Oncology Research [P. C., M. L. B., P. W. F., M. R. M., B-B. S. Z.], Bioinformatics [M. R. H.], and Molecular Biology [Y. Z.], GlaxoSmithKline Pharmaceuticals,
King of Prussia, Pennsylvania 19406, and Institute for Cancer Research, Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111 [V. S., S. A. J., T. J. Y.]
ABSTRACT
Resistance to chemotherapy targeting microtubules could be partially
because of the delay in chromosome condensation and segregation during
mitosis. The Chfr pathway has been defined recently, and its activation
causes a delay in chromosome condensation in response to mitotic stress.
Because Chfr contains a RING-finger domain, we tested whether Chfr
inhibits chromosome condensation through an ubiquitin (ubiquitin)dependent pathway. In the presence of purified E1, Ubc4, or Ubc5, and
ubiquitin, Chfr catalyzes its own ubiquitination in vitro, an activity requiring the RING domain. In vivo, overexpressed Chfr but not a RING
domain mutant is spontaneously ubiquitinated. Our studies with DLD1
cells stably expressing wild-type Chfr and Chfr lacking the RING domain
indicated that the RING-finger deletion mutant was defective in inhibiting
chromosome condensation after Taxol treatment. In addition, Chfr expression increases the survival rate after Taxol treatment, an activity
requiring the RING domain. Preliminary studies indicate that Chfr expression is cell cycle regulated and is dependent on its ubiquitin ligase
activity. It is very likely that the Chfr-mediated ubiquitin-dependent
pathway is a critical component of the response to mitotic stress.
densation and nuclear membrane breakdown. Chfr contains a FHA3
domain, which could bind phosphorylated peptides (9, 10); thus,
protein kinases might act upstream of Chfr and regulate its activity.
Chfr also contains a RING-finger domain, which is found in several
ubiquitin ligases (11). In addition to ubiquitin ligase (E3), protein
ubiquitination also involves the action of a ubiquitin-activating enzyme (E1) and a ubiquitin-conjugating enzyme (E2; Ref. 12). The
spindle assembly checkpoint is associated with a ubiquitin-dependent
pathway and is regulated by the ubiquitin ligase activity of the APC/C
(13). Recently, a subunit of APC/C, which contains a RING-finger
domain, APC11, was shown to be the ubiquitin ligase in the APC/C
(14, 15). Because Chfr has a RING-finger domain, it was of interest
to see whether it had a role to play in the ubiquitination process as an
E3 and whether this activity of Chfr mediates its function in the
mitotic stress response.
We report here that Chfr has ubiquitin ligase activity, which may be
required for its role in delaying chromosome condensation in cells
stressed by tubulin poisons such as Taxol. Chfr expression increases
cellular survival after Taxol treatment, consistent with a role for Chfr
in surviving mitotic stress.
INTRODUCTION
The segregation of chromosomes at mitosis involves a series of
steps, including condensation of chromosomes and separation of the
centrosomes in prophase, chromosomal alignment on the spindle in
metaphase, and sister-chromatid separation in anaphase (1– 4). The
surveillance mechanisms activated at various stages of mitosis monitor the fidelity of these processes. Two mitotic checkpoints responding to mitotic stress have been reported. One, the spindle-assembly
checkpoint, ensures that each pair of chromosomes is correctly attached to a bipolar spindle before anaphase (5). A second checkpoint,
identified in yeast, depends on the BIM1/EB1 gene (6); this checkpoint delays the exit from mitosis when the spindle is oriented
abnormally. An additional stress response pathway, involving Chfr,
has been defined recently in mammals. Chfr activation causes a delay
in chromosome condensation in response to mitotic stress, and its
expression increased the cellular ability to survive the stress (7).
How does Chfr delay cell entry into mitosis? Initiation of early
mitotic events, including nuclear membrane breakdown and chromosome condensation, requires the activation of mitotic kinases such as
cyclin B/cdc2 (8). Because cyclin B/cdc2 activity remains high during
the Chfr-dependent delay, its activity might not be the target of the
Chfr-dependent pathway (7). Alternatively, Chfr may be involved in
controlling cyclin B/cdc2 substrates that regulate chromosome conReceived 10/11/01; accepted 1/18/02.
The costs of publication of this article were defrayed in part by the payment of page
charges. This article must therefore be hereby marked advertisement in accordance with
18 U.S.C. Section 1734 solely to indicate this fact.
1
Supported in part by a postdoctoral fellowship from a National Cancer Institute Grant
CA-50771 (to P. C.), by a fellowship from Human Frontiers Science Program (to V. S.),
a NIH core grant (CA06927), and an appropriation from the Commonwealth of Pennsylvania (to T. J. Y.).
2
To whom requests for reprints should be addressed, at Department of Oncology
Research, GlaxoSmithKline, 709 Swedeland Road, King of Prussia, PA 19406. Phone:
(610) 270-6638; Fax: (610) 270-5005; Email: [email protected].
MATERIALS AND METHODS
Constructs, Recombinant Protein Expression, and Purification. The
full-length chfr cDNA were subcloned into baculoviral expression vector,
pAcG2T. The deletion of the RING-finger domain of chfr (Chfr⌬R) was
created by a restriction digestion approach. The pACcG2T vector containing
full-length chfr was digested with BamHI and EcoRI to delete the following
amino acids constituting the RING domain: CIICQDLLHDCVSLQPCMHTFCAACYSGWMERSSLCPTCRCPVE. The RING-fingerless domain fragment was then religated in frame. Both Chfr and Chfr⌬R were also subcloned
into mammalian expression vector pWSgfp (16) for transfection experiments.
Chfr or Chfr⌬R construct in pACcG2T vector was transfected into Sf-9
cells, and the virus from the transfection was harvested for amplification. The
high titer viral stocks were additionally optimized for expression and scaleup.
The harvested cells were lysed, and the protein was purified by sequential
chromatography steps including glutathione Sepharose and Superdex G75
columns at 4°C.
Cell Lines. Human SW480 was cultured in RPMI 1640 containing 10%
FBS, human SAOS2 was maintained in McCoys 5A medium containing 15%
FBS, HeLa, and DLD1 cell lines were maintained in DMEM with 10% FBS
(complete medium). For creating stable cell lines, DLD1 cells were transfected
with pWSgfp-Chfr or pWSgfp-Chfr⌬R using the LipofectAMINE method
(Life Technologies, Inc., Gaithersburg, MD). Clones were selected in G418
and confirmed both by expression of the GFP-Chfr protein using a fluorescent
microscope and by Western blotting with anti-GFP and anti-Chfr antibody.
The stable cell lines including DLD1/Chfr and DLD1/Chfr⌬R were maintained
in complete medium containing 400 ␮g/ml of G418. For synchronization, the
cells were treated with 2 mM thymidine for 18 h, released for 6 h, then treated
with 5 ␮g/ml of aphidicolin for 20 h. To induce mitotic stress, cells were
treated with 300 nM Taxol for 18 –20 h.
3
The abbreviations used are: FHA, fork head-associated; APC/C, anaphase promoting
complex/cyclosome; GST, glutathione S-transferase; FACS, fluorescence-activated cell
sorter; GFP, green fluorescence protein; ubiquitin, ubiquitin; FBS, fetal bovine serum;
XTT, 2,3-bis[2-methoxy-4-nitro-5-sulfophenyl]-2H-tetrazolium-5-carboxanilide inner
salt; HA-ubiquitin, HA-tagged construct of ubiquitin.
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MITOTIC STRESS PROTEIN Chfr IS A UBIQUITIN LIGASE
Antibodies. Rabbit polyclonal antibodies against Chfr were generated
against a NH2-terminal peptide of human Chfr: MERPEEGKQSPPPQPWGRLLRC-COOH and affinity purified. Antibodies against GFP and HA tag
were purchased from Santa Cruz Biotechnology (Santa Cruz, CA).
Western Blotting and Immunoprecipitation. Nuclear lysates were prepared using an extraction kit from Pierce (Rockford, IL). The protein in the
lysates was estimated and an equal amount of protein (150 ␮g) was immunoprecipitated with affinity-purified antibody at 4°C for 2 h followed by incubation with protein A-Sepharose for 1 h at 4°C. The samples were analyzed by
SDS-PAGE. The gel was then transferred onto a nitrocellulose membrane. The
membrane was first blocked in 3% milk followed by addition of primary
antibody for 2 h at room temperature in milk and the secondary antibody for
1 h at room temperature. The blot was visualized by enhanced chemiluminescence (NEN, Boston, MA).
Mitotic Index. Overnight, 2 ⫻ 104 cells/well in 24-well plates were grown
in 37°C, 5% CO2. Taxol (300 nM) dissolved in culture medium was applied to
cells, and cells were incubated for 20 h. Medium was aspirated, and cells were
washed and fixed in 3% paraformaldehyde for 5 min. Permeabilization was
accomplished using 5% Triton X-100 for 5 min. DNA was stained with 100
ng/ml Hoechst dye for 5 min after which the cells were washed and fresh PBS
applied. Condensed chromosome and total cell counts were done manually
using a fluorescent microscope. Mitotic index was calculated based on the
percentage of cells with condensed chromosomes.
In Vitro Ubiquitination Assay. Ubiquitin ligase activity was determined
as described previously (17). Reaction mixture contained the following in a
volume of 10 ␮l: 40 mM Tris-HCl (pH 7.4), 1 mg/ml BSA, 1 mM DTT, 5 mM
MgCl2, 10 mM phospho-creatine, 50 mg/ml creatine phosphokinase, 50 mM
ubiquitin, 1 mM ubiquitin aldehyde, 1 pmol of E1, and 1.4 mg/ml wild-type
Chfr or the Chfr mutants. The E2 enzymes were then added. After incubation
at 30°C for 30 min, samples were subjected to electrophoresis on a 7.5%
polyacrylamide-SDS gel. After transfer the membrane was probed with antiChfr and anti-ubiquitin antibodies. Purified recombinant UbcH5 was purchased from BostonBiochem (Boston, MA); purified recombinant UbcH4 was
a generous gift of Dr. Hongtao Yu.
In Vivo Ubiquitination Assay. HeLa cells (2 ⫻ 106) in Petri dishes were
transiently cotransfected with 10 ␮g GFP-tagged Chfr or Chfr⌬R and HAtagged ubiquitin constructs. HA-tagged ubiquitin construct was a generous gift
of Dr. Dale Haines. Total cell lysates were prepared and immunoprecipitated
with anti-GFP antibody and immunoblotted with anti-Chfr antibody.
Transient Transfection for Immunofluorescence. Dishes of DLD-1 cells
(35 mm) containing coverslips were transfected with 2 ␮g of pWSgfp-Chfr or
pWSgfp-Chfr⌬R using Fugene6 according to manufacturer’s directions
(Roche Molecular, Indianapolis, IN). Coverslips were harvested for staining
20 –24 h after transfection. Cells were first fixed in 3.5% PBS-buffered
paraformaldehyde for 7 min and then solubilized in KB ⫹ 0.5% Triton X-100
for 5 min, followed by a 5-min rinse in KB buffer [10 mM Tris-HCl (pH 7.5),
150 mM NaCl, and 0.1% BSA]. Cells were incubated with CENP-F antibody
for 30 min at 37°C in a humidified chamber, rinsed in KB, followed by
incubation with Alexa-Fluor 594 goat antirabbit IgG (2 ␮g/ml; Molecular
Probes, Eugene, OR) for 30 min at 37°C in a humidified chamber. Cells were
subsequently incubated with 4⬘,6-diamidino-2-phenylindole dihydrochloride
(Molecular Probes; 0.25 ␮g/ml) for 15 min at room temperature, and mounted
onto slides. Coverslips were scanned using a Nikon Microphot.
XTT Assay. Inhibition of growth of various cell lines by Taxol in a 72-h
incubation was assayed by a standard XTT protocol (18). Viability was
calculated from the ratio of the absorbance at 450 nm for the test sample
divided by the absorbance at 450 nm for the standard (untreated control)
sample, measured on a microplate reader (Dynatech, Lorton, VA). Triplicate
determinations were performed, with standard deviation less than 15%.
RING-finger domain within its amino terminus. We initially named
the gene FHAR1 (FHA-RING protein 1); during the course of this
work, the same gene was published as Chfr (checkpoint containing
FHA and RING-finger domain) by Scolnick and Halazonetis (7), and
the latter nomenclature is used in the present work.
Chfr Is a Ubiquitin Ligase Dependent on Its RING-Finger
Domain. The spindle assembly checkpoint involves a ubiquitindependent pathway, but it is unclear whether the Chfr-dependent
mitotic stress response is mediated by ubiquitination. Chfr contains a
RING finger at its COOH terminus. As demonstrated previously
(reviewed in Ref. 11), most RING finger-containing proteins have the
potential for ubiquitin protein ligase activity, so we examined whether
Chfr has intrinsic ubiquitin protein ligase activity dependent on its
RING-finger domain. For these experiments, GST fusion proteins
with full-length Chfr and RING-finger domain deletion mutant
(Chfr⌬R) were expressed and purified from baculovirus. GST-Chfr
and GST-Chfr⌬R were assayed for self-ubiquitination in the presence
of purified recombinant E1, E2, and ubiquitin. Four different human
E2 subfamily members, Ubc10, Ubc2, Ubc4, and Ubc5, were tested in
the assay. As a control, a blank reaction without Chfr was included.
As shown in Fig. 1A, in the presence of two different E2s namely
Ubc4 or Ubc5, high molecular Chfr conjugates were formed (Fig. 1A,
Lane 4 and 5). On the other hand, no Chfr conjugates were found with
two other E2s, Ubc10 and Ubc2, or in the control reaction (Fig. 1A,
Lane 1–3). To confirm that the Chfr conjugates are polyubiquitinated,
the same reaction mixtures were probed with antiubiquitin antibody
(Fig. 1B). These results show that Chfr is a ubiquitin ligase capable of
catalyzing its own ubiquitination, and it prefers the Ubc4/5 family as
its E2. In addition, because no conjugates were formed in the reactions
where Chfr⌬R was used (Fig. 1, A and B, Lanes 6 –10), Chfr requires
an intact RING-finger domain for its ubiquitin ligase activity.
In Vivo Ubiquitination Activity of Chfr. To validate our findings
regarding ubiquitination and Chfr, the ability of Chfr to catalyze its
own ubiquitination in vivo was evaluated using HeLa cells transiently
cotransfected with a HA-ubiquitin and a GFP-tagged Chfr (pSWgfpChfr) or Chfr⌬R (pSWgfp-Chfr⌬R) construct. Negative controls were
the GFP vector (pSWgfp) with ubiquitin construct, GFP-Chfr, or
GFP-Chfr⌬R alone. Lysates from transfected cells were immunoprecipitated with anti-GFP antibody and blotted with anti-Chfr antibody.
GFP-Chfr was conjugated with the HA-ubiquitin to produce a ladder
of ubiquitinated products (Fig. 2, Lane 4). No conjugates were detected when HA-ubiquitin was omitted from the transfection (Fig. 2,
RESULTS
Identification of a Gene Containing Both FHA and RINGFinger Domain. In an effort to find a novel gene containing the FHA
domain, we searched the expressed sequence tag database for cDNAs
with FHA motifs. Two overlapping expressed sequence tags,
AA223499 and AA223601, were found, sequenced, and shown to
encode a 664 amino acid protein with COOH-terminal FHA and
Fig. 1. Chfr has ubiquitin ligase activity dependent on its RING-finger domain. The
ubiquitination reaction was performed in vitro using purified E1, various E2: Ubc10
(Lanes 2 and 7), Ubc2 (Lanes 3 and 8), Ubc4 (Lanes 4 and 9), or Ubc5 (Lanes 5 and 10)
and Chfr (Lanes 1-5), or Chrf⌬R (Lanes 6-10) and ubiquitin at 30°C for 30 min, analyzed
on 7.5% SDS-PAGE, and immunoblotted with anti-Chfr antibody (A) or with antiubiquitin antibody (B).
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MITOTIC STRESS PROTEIN Chfr IS A UBIQUITIN LIGASE
carcinoma cell line SW480 and osteosarcoma cell line SAOS-2 (7);
(b) those that are deficient in Chfr including a colon carcinoma cell
line, DLD1; and (c) a stably transfected clone of DLD1 expressing
Chfr tagged with GFP (DLD1/Chfr), RING-finger deletion mutant of
Chfr tagged with GFP (DLD1/Chfr⌬R), or vector alone (DLD1/
Vector). The cells were treated with 300 nM Taxol for 20 h, fixed in
paraformaldehyde, permeabilized, stained with Hoechst dye, and observed microscopically. The cells that looked clearly rounded (mitotic) also had condensed chromosomes, whereas the cells that were
attached firmly had diffused nuclei. After Taxol treatment, various
cells showed increased mitotic indices as a result of spindle checkpoint arrest at metaphase. As expected, DLD1 cells with vector alone
had high mitotic index (52%) after Taxol treatment, whereas DLD1
expressing wild-type Chfr, SW480, and SAOS-2 cells have somewhat
Fig. 2. In vivo ubiquitination activity of Chfr. HeLa cells were transiently cotransfected
with 10 ␮g of GFP-tagged Chfr (Lane 4) or Chf⌬R (Lane 2) DNA construct and 1 ␮g of
HA-ubiquitin construct. As negative controls, Chfr⌬R construct alone (Lane 1), Chfr
construct alone (Lane 3), and HA-ubiquitin with the GFP vector alone (Lane 5) were
transfected into HeLa cells. Total cell lysates were prepared and immunoprecipitated with
anti-GFP antibody, analyzed on 7.5% SDS-PAGE, and Western blotted with anti-Chfr
antibody.
Fig. 3. Chfr-dependent mitotic delay requires its RING-finger domain. DLD1 cells
alone or those expressing Chfr (DLD1/Chfr), the RING finger-deletion mutant (DLD1/
Chfr⌬R), SW480, and SAOS2 were treated with 300 nM Taxol for 20 h. The cells were
then fixed and DNA stained with Hoechst dye, and observed under the microscope. The
mitotic index before (䡺) and after Taxol treatment (o) were calculated based on the
percentage of cells with condensed chromosomes.
Lane 3). In lysates prepared from GFP-Chfr⌬R and HA-ubiquitin
cotransfections, no significant high molecular weight laddering was
seen (Fig. 2, Lane 2). Lysates from GFP-Chfr and GFP-Chfr⌬R
transfections without HA-ubiquitin or the HA-ubiquitin transfection
alone did not show any dark staining of ubiquitinated products (Fig.
2, Lanes 1, 3, and 5). These results suggest that Chfr exhibits RING
domain-dependent auto-ubiquitination activity in vivo.
RING-Finger Domain Is Essential for the Mitotic Stress Response Function of Chfr. To determine the importance of the ubiquitin ligase activity of Chfr in prolonging a G2 delay on mitotic stress,
the mitotic index (chromosome condensation) was measured in various cells treated with Taxol. For this experiment, three types of cells
were used: (a) those that express wild-type Chfr, including colon
Fig. 4. Role of Chfr in cell survival in the presence of mitotic stress or DNA damage.
DLD1 cells and those expressing either wild type (DLD1/Chfr), the RING finger-deletion
mutant (DLD1/Chfr⌬R), GFP vector alone (DLD1/vector), and SW480 cells were treated
with increasing concentrations of either Taxol (A) or topotecan (B), and their viabilities
were assessed after 72 h by XTT assay as described in “Materials and Methods.” One of
three independent experiments is shown in each case; results were qualitatively similar in
all experiments.
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MITOTIC STRESS PROTEIN Chfr IS A UBIQUITIN LIGASE
Fig. 5. Chfr is expressed in a cell cycle-dependent manner. A, DLD1/Chfr cells were
synchronized at the G1-S boundary by a double block with thymidine and aphidicolin
(Lane 2) and released for 1, 3, 5, 7, and 9 h (Lanes 3–7). Asynchronous cells were
included as control (Lane 1). Nuclear extracts were prepared, resolved on 7.5% SDSPAGE, and immunoblotted with anti-Chfr antibody. B, cell cycle stage for each sample
depicted in A was determined by FACS analysis. G1, f; S, o; G2-M, 䡺. C, Western blot
with anti-Chfr was performed on nuclear lysates prepared from DLD1/Chfr⌬R cells
synchronized as described in A. D, cell cycle stage for each sample depicted in C was
determined by FACS analysis. G1, f; S, o; G2-M, 䡺. E, DLD1 cells were transiently
transfected with GFP-Chfr or GFP-Chfr⌬R construct and counterstained with anti
CENP-F antibody. No mitotic cells were observed among GFP-Chfr transfected cells,
whereas several mitotic cells were observed among GFP-Chfr⌬R-transfected cells.
lower mitotic indices (Fig. 3) consistent with a cell cycle delay or
arrest before mitosis. On the other hand, DLD1 cells stably expressing
Chfr⌬R had a high mitotic index similar to the parental chfr-deficient
DLD1 cells (Fig. 3). These results indicate that in the presence of Chfr
the stressed cells are delayed in mitotic progression and that the
RING-finger domain is required for this function.
Chfr Is Critical for Cell Survival in the Presence of Mitotic
Stress but not for Topotecan-induced DNA Damage Response. To
study the functional consequences of the Chfr-dependent pathway, we
examined the effect of Chfr on cell growth in the presence of Taxol,
comparing Chfr-proficient cells including DLD1/Chfr, SW480, and
Chfr-deficient cells including DLD1 and DLD1/vector cells. In addition, DLD1 cells expressing RING domain-deleted Chfr (DLD1/
Chfr⌬R) were included in the experiments to determine the role of the
RING-finger domain in this mitotic stress response function of Chfr.
Cells were treated with increasing concentrations of Taxol, and their
viabilities were assayed at 72 h. As shown in Fig. 4A, our results
indicate that at as low as 10 nM Taxol, the DLD1/Chfr and SW480
cells were ⬎60% viable compared with the cells expressing the
RING-finger deletion mutant (Chfr⌬R) or the vector alone (DLD1/
vector), or the DLD1 cells, which were at about 20 –30% viability
(Fig. 4A). Three different DLD1/Chfr clones gave very similar results
(data not shown). These findings confirm that the sensitivity of DLD1
cells to Taxol is correlated with their defects in Chfr-dependent
mitotic delay, an activity also requiring RING domain. Also to confirm that Chfr has a role to play specifically in response to mitotic
stress, we repeated the same XTT experiment with cells treated with
increasing concentrations of Topotecan, an anticancer drug that transforms topoisomerase I-DNA-transient intermediates into lethal DNA
lesions and has been suggested to activate a DNA-damage checkpoint
(19 –21). The presence or absence of Chfr made no difference to the
response of cells to DNA damage, indicating that Chfr has a role
limited to mitotic stress response pathway (Fig. 4B).
Cell Cycle-dependent Chfr Expression. To study cell cycledependent Chfr expression, DLD1/Chfr and DLD1/Chfr⌬R cells were
synchronized at the G1-S boundary by a double block with thymidine
and aphidicolin, and released for various times. As indicated in Fig.
5B, FACS analysis was performed to ascertain the phase of the cell
cycle at various times. Because Chfr is a nuclear protein and the
antibody used only recognizes Chfr specifically in the nuclear fraction, nuclear extracts were used to study protein expression. The
Western blot results indicate that cells in G1 express low levels of
Chfr (Fig. 5A, Lanes 2 and 3) as compared with asynchronous cells
(Fig. 5A, Lane 1) and that the protein levels are markedly increased
during late S phase (Fig. 5A, Lanes 4-6). Because Chfr is believed to
function in prophase, this expression pattern is consistent with its role
in the mitotic stress response. Because Chfr has autoubiquitin ligase
activity, we tested whether the RING-finger domain had any role to
play in this cell cycle-dependent expression pattern and whether
ubiquitin ligase activity was required for this process. The same
experiment was repeated with DLD1 cells stably expressing Chfr⌬R
cells; in the absence of the RING-finger domain, Chfr expression
remained unchanged throughout the cell cycle (Fig. 5C). Consistent
with the observation that the Chfr level is low in G1 and high before
mitosis in DLD1 cells stably expressing Chfr, thymidine-treated
SW480 cells with endogenous wild-type Chfr have a lower Chfr level,
whereas Taxol-treated cells have a higher Chfr level in comparison
with that of untreated controls (data not shown).
To validate the results obtained with Western blotting, cell cycledependent expression of Chfr was also measured by fluorescence
imaging. Because the Chfr antibody is not suitable for immunostaining, DLD1 cells were transiently transfected with a GFP-tagged
construct of Chfr. Transfected cells on coverslips were counterstained
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MITOTIC STRESS PROTEIN Chfr IS A UBIQUITIN LIGASE
with anti-CENP-F antibody, which was used as a G2-M marker (22).
Consistent with the finding of optimal Chfr expression in S phase, the
fluorescence imaging showed that protein levels were negligible in
mitotic cells expressing GFP-Chfr (Fig. 5E). Examination of cells
transfected with the GFP-Chfr⌬R showed that it was expressed in
mitotic cells and G1 or S cells that were also CENP-F negative (Fig.
5E). Thus, the loss of GFP-Chfr in mitotic cells correlates with the
Western blot expression pattern. This loss is likely dependent on
ubiquitination activity, because the RING domain mutant is stable in
mitosis and G1 phase.
status of Chfr in various tumors, as well as the role of Chfr in the
sensitivity of human cancers to antimicrotubule agents. Abrogation of
the Chfr-dependent pathway could make current cancer therapy more
effective.
ACKNOWLEDGMENTS
We thank Leo Faucette for his help in setting up the mitotic index assay,
Dale Haines for providing HA-tagged ubiquitin construct and advice on the in
vivo ubiquitination assay, and Hongtao Yu for his-tagged human Ubc4. We
also thank Jim Winkler and Randall Johnson for their generous support during
the course of this work.
DISCUSSION
Chfr has been characterized in the present work as a ubiquitin ligase
capable of catalyzing its own ubiquitination and dependent on an
intact RING-finger domain for activity. The expression of Chfr appears cell cycle dependent. Because in cells expressing RING-fingerdeleted Chfr, protein expression remains constant throughout the cell
cycle, it is possible that the degradation of Chfr at mitosis is mediated
by its ubiquitination activity. However, it cannot be ruled out that
truncation prevents recognition of the protein by an unidentified
ubiquitin ligase. The significance of Chfr degradation is unclear, and
we did not observe obvious differences in cell cycle progression in
cells expressing RING-less Chfr. Nevertheless, Chfr does resemble
many mitotic regulators in that it is expressed in a cyclic fashion.
An important role of a checkpoint protein is to coordinate cell cycle
events and regulate mechanisms that enforce proper order when the
cell perceives stress. When cells are subjected to mitotic stress, Chfr
delays entry into metaphase (7). In cells expressing Chfr, the mitotic
index was lower than that of either cells lacking Chfr or those
expressing the RING-finger deletion mutant (Fig. 3). In cells with
intact Chfr, viability was much greater in the presence of Taxol than
in cells in which Chfr was either absent or presented as a RING-finger
deletion mutant (Fig. 4), whereas no such difference in viability was
observed in cells treated with Topotecan. These results suggest that
Chfr has a role to play specifically on mitotic stress and that the
ubiquitin ligase function is required to enable Chfr to delay mitotic
progression in mitotic stress conditions. However, the RING domain
itself is not sufficient for the function of Chfr, because a FHA domain
deletion with intact RING domain is also functionally defective (7).
Chfr may ultimately resemble another RING domain protein c-Cbl, of
which the RING domain alone has no affinity for platelet-derived
growth factor ␤-receptor but rather requires a SH2 domain to interact
with its substrate (23). In the case of Chfr, substrate binding could
depend on the FHA domain, a phospho-amino acid motif (9, 10). Such
a substrate for Chfr has not yet been identified. It is very likely that
Chfr-mediated ubiquitin-dependent degradation of yet unidentified
proteins contributes to its role in the mitotic stress pathway.
Many cancers show resistance to the current antimitotic drugs
including Taxol and Vinca alkaloids. Thus far very little is known
about the mechanisms which lead to the multidrug resistance-independent resistance in cancer cells. Whereas multidrug resistance accounts for much of the clinical resistance to taxanes and Vinca
alkaloids, other mechanisms are likely important. The present results
suggest an additional basis for drug resistance in cancer cells, because
the loss of Chfr results in increased sensitivity to mitotic poisons. Chfr
function was found to be absent, either because of mutation or loss of
expression, in four of the eight human cancer cell lines examined (7).
Thus, it would seem useful to conduct a thorough examination of the
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Downloaded from cancerres.aacrjournals.org on June 17, 2017. © 2002 American Association for Cancer Research.
Chfr Regulates a Mitotic Stress Pathway through its
RING-Finger Domain with Ubiquitin Ligase Activity
Priya Chaturvedi, Valery Sudakin, Matthew L. Bobiak, et al.
Cancer Res 2002;62:1797-1801.
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