Download Cysteine 230 Modulates Tumor Necrosis Factor

[CANCER RESEARCH 60, 3152–3154, June 15, 2000]
Advances in Brief
Cysteine 230 Modulates Tumor Necrosis Factor-related Apoptosis-inducing
Ligand Activity1
Dai-Wu Seol2 and Timothy R. Billiar
Department of Surgery, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261
Abstract
Biologically active tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) protein is known to form a homotrimer in solution. Unexpectedly, the recombinant active human TRAIL protein purified from
bacteria produced two bands (a Mr 21,000 monomer derived from the
disruption of the trimer in SDS gels and a Mr 42,000 dimer) on nonreducing SDS gels. The treatment of this TRAIL protein with DTT, a
reducing agent, abolished formation of the Mr 42,000 band, suggesting
that the Mr 42,000 band was the result of intermolecular disulfide bridge
formation. Inspection of the amino acid sequence of human TRAIL protein identified a unique cysteine residue at position 230, and subsequent
site-directed mutagenesis revealed that this amino acid residue is responsible for the appearance of the Mr 42,000 dimer. The binding analysis
using the TRAIL protein and a TRAIL receptor (death receptor 5)
revealed that both the dimer and the trimer bind to death receptor 5 with
similar affinity. Interestingly, mutation of cysteine 230 to glycine completely abolished the apoptotic activity of TRAIL protein. The disruption
of the dimer in the mixture of TRAIL dimer and trimer increased the
apoptotic activity slightly, suggesting that the dimer has less apoptotic
activity than the trimer. Therefore, our data indicate that cysteine 230 is
not only required for TRAIL function but also modulates the apoptotic
activity of TRAIL by forming an intermolecular disulfide bridge.
Introduction
Apoptosis is a genetically regulated biological process that plays an
important role in the development and homeostasis of multicellular
organisms (1–3). Thus, aberrations of this process can be detrimental
to organisms. For example, excessive apoptosis causes damage to
normal tissues in certain autoimmune disorders, whereas a failure of
apoptosis allows cells to grow unlimitedly, resulting in cancers.
Members of the TNF3 family such as TNF-␣, FasL, and TRAIL
have been identified as apoptosis inducers. On binding to cognate
receptors, TNF-␣ and FasL recruit cellular components including
Fas-associated death domain-containing protein (4 – 6) and procaspase-8 to signal apoptotic activity (7–9). This initial signaling
event is followed by serial activation of downstream executioner
caspases leading to cleavage of cytosolic, cytoskeletal, and nuclear
proteins as well as DNA, resulting in apoptotic cell death.
TRAIL, a recently identified TNF family member, is a type II
transmembrane protein (10, 11) and a potent inducer of apoptosis. A
recent study (12) on the crystal structure of TRAIL protein revealed
that soluble TRAIL forms a homotrimer similar to other TNF family
members. Currently, relatively little is known about the signaling
Received 11/1/99; accepted 4/28/00.
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 by NIH Grant GM44100.
2
To whom requests for reprints should be addressed, at BST W1503, Department of
Surgery, University of Pittsburgh School of Medicine, 200 Lothrop Street, Pittsburgh, PA
15261. Phone: (412) 624-6740; Fax: (412) 624-1172; E-mail: seold⫹@pitt.edu.
3
The abbreviations used are: TNF, tumor necrosis factor; FasL, Fas ligand; TRAIL,
TNF-related apoptosis-inducing ligand; TRAIL-R, TRAIL receptor; DR, death receptor;
DcR, decoy receptor.
events activated by TRAIL, including the adaptor molecule(s) and
initial caspase(s). Four different TRAIL-Rs have been identified thus
far: (a) DR4/TRAIL-R1 (13); (b) DR5/TRAIL-R2 (14 –16); and (c)
two DcRs [DcR1 and DcR2 (13, 15–17)]. DR4 and DR5 are intact
functional TRAIL-Rs through which the apoptosis-inducing activity
of TRAIL is transmitted into the cytoplasm. DcR1 and DcR2 are
truncated TRAIL-Rs in which the cytoplasmic regions containing the
death domains are deleted. Thus, overexpression of DcR1 and DcR2
blocks the function of DR4 and DR5 (13, 15–17), probably by
competing with DR4 or DR5 for TRAIL.
Although TRAIL is a TNF family member (10, 11), it has some
notable differences compared with TNF-␣ and FasL. First, TRAIL
exhibits much stronger apoptotic activity than other TNF family
members and is able to kill many cell types without potentiating
agents such as actinomycin D or cycloheximide (10, 18, 19). Second,
TRAIL kills tumor cells more effectively than normal cells by virtue
of differential expression of DcRs and DR4 and DR5 (14, 15, 17).
TRAIL has also been shown to kill HIV-1-infected T cells more
effectively than uninfected T cells (20). Third, unlike the restricted
expression of Fas, TRAIL-Rs are constitutively expressed in almost
all tissues (10, 11). Therefore, it seems likely that TRAIL may have
broader cell and tissue targets than the Fas-FasL system. Fourth,
TRAIL activates nuclear factor ␬B only very weakly,4 suggesting that
TRAIL is unlikely to initiate inflammatory cascades on systemic
administration. Thus, unlike FasL or agonistic Fas antibody that
induces fulminant massive liver damage (21, 22) when introduced
systemically, TRAIL exhibited no detectable cytotoxicity in vivo (19,
23). Finally, TRAIL-induced apoptosis does not depend on p53 status,4 which is considered to be a critical factor in cancer therapies
using chemotherapeutic agents or radiation. These features have focused considerable attention on TRAIL as a potential therapeutic to
treat human cancers and AIDS.
Despite the considerable roles of TRAIL in cellular physiology,
little is known about the functional domains and functionally critical
amino acid residues of TRAIL protein. Here we report that cysteine
230 is a required amino acid residue for apoptotic function of the
TRAIL protein.
Materials and Methods
Production of TRAIL Protein. Previously, we described the purification
of human TRAIL protein [amino acids 114 –281 (18)]. TRAIL (C230G)
protein in which cysteine 230 was mutated to glycine was purified in essentially the same manner as described for TRAIL protein (18). To disrupt dimeric
TRAIL, purified TRAIL protein was incubated with DTT (10 mM, final
concentration) for 3 h at 4°C and dialyzed against a dialysis buffer [10 mM
Tris-Cl (pH 7.5), 20 mM NaCl, and 10 mM ZnCl2].
Western Blotting. The samples were fractionated on 15% SDS gels, transferred onto nitrocellulose membrane, and subjected to Western blotting using
an antihistidine antibody (Santa Cruz Biotechnology) as described previously
(18).
4
Unpublished observations.
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MODULATION OF TRAIL ACTIVITY BY CYSTEINE 230
Fig. 1. Identification of dimeric TRAIL protein. The purified TRAIL protein (amino
acids 114 –281) was mixed with the sample buffer with or without DTT (100 mM),
fractionated on 15% SDS gels, and subjected to Coomassie staining or Western blotting
using antihistidine antibody. Arrows and arrowheads, dimeric and monomeric forms of
TRAIL protein, respectively.
human TRAIL protein (amino acids 114 –281) from bacteria and
showed that this TRAIL protein is a potent inducer of apoptosis
(18). Biochemical analysis revealed that the major form of this
recombinant TRAIL protein is trimer (data not shown). Nevertheless, this TRAIL protein generated two bands (Mr 21,000 and Mr
42,000) on nonreducing SDS gels (Fig. 1). Because the estimated
molecular weight of amino acids 114 –281 is Mr 21,000, the
appearance of a Mr 42,000 band suggested the formation of a
dimer. The Mr 42,000 band disappeared after treatment with DTT,
a reducing agent. However, boiling had no effect. These observations suggested that the dimeric TRAIL may be generated by an
intermolecular chemical bond formation.
Binding of Dimeric TRAIL Protein to Its Cognate Receptor. To
examine whether the dimer form of TRAIL protein binds to TRAILRs, TRAIL protein was incubated with TRAIL-R DR5 or a negative
control antibody (FLAG-Ab) and pulled-down with protein (A⫹G)agarose bead. As shown in Fig. 2, the monomer TRAIL protein (this
monomer resulted from trimer disruption by SDS) efficiently bound to
DR5 (Lanes 1 and 2) but did not bind to control FLAG-Ab (Lanes 1
and 6). The dimer TRAIL protein also bound to DR5 (Lanes 1 and 2).
The affinity of trimer and dimer TRAIL proteins to DR5 was similar
(Lanes 1 and 2).
Fig. 2. Binding of dimeric TRAIL protein to TRAIL-R DR5. Purified TRAIL protein
was incubated with Fc-fused TRAIL-R DR5 or anti-FLAG antibody (negative control).
The protein complex was pulled-down with protein (A⫹G)-agarose beads and subjected
to Western blotting using antihistidine antibody. For the input, 10% of the total protein
used in each reaction was loaded. Arrows and arrowheads, dimeric and monomeric forms
of TRAIL protein, respectively.
Pull-down Assay. Ten ␮l of agarose beads (protein A⫹G; Santa Cruz
Biotechnology) were preincubated with buffer A (PBS and 100 ␮g/ml BSA)
for 30 min at room temperature. The combination of purified TRAIL protein
(2 ␮g), purified DR5-Fc protein (2 ␮g; Alexis, CA) and purified monoclonal
anti-FLAG-tag antibody (2 ␮g; Stratagene) was added to the preincubated
agarose beads and incubated for 1 additional h at room temperature. The
agarose beads were collected and washed six times at room temperature with
buffer B [25 mM HEPES, 0.1% NP40, 100 mM NaCl, 1 mM EDTA, 5 mM
MgCl2, 0.1 mM DTT, 100 ␮g/ml phenylmethylsulfonyl fluoride, 2 ␮g/ml
protease inhibitor mixture, and 5% glycerol (adjusted to pH 7.4)]. The input
(10% of total protein used in each reaction) and bound proteins were resolved
on a 15% SDS gel and subjected to Western blotting.
Site-directed Mutagenesis. To mutate cysteine 230 to glycine in the
reported TRAIL expression plasmid (18), the QuikChange site-directed mutagenesis kit (Stratagene) was used. All mutagenesis procedures were performed as described in the manufacturer’s instructions.
Apoptosis Assays. HeLa cells maintained in F-12K culture medium supplemented with 10% (v/v) calf serum and antibiotics (100 ␮g/ml gentamicin
and 100 ␮g/ml penicillin-streptomycin) were incubated with purified TRAIL
or TRAIL (C230G) protein for 6 h. Viable cells were stained with crystal violet
followed by spectrophotometric analysis as described previously (18).
Results and Discussion
Identification of Dimeric TRAIL Protein. Biologically active
soluble human TRAIL protein (amino acids 114 –281) is known to
form a homotrimer (12, 23). Previously, we purified recombinant
Fig. 3. Cysteine 230 is required for both dimer formation and apoptotic function of
TRAIL. A, the TRAIL (C230G) protein was obtained after a specific mutation of cysteine
230 to glycine. Numbers, amino acid positions on TRAIL protein. TM, C, and G,
transmembrane domain, cysteine, and glycine, respectively. B, the mutation of cysteine
230 to glycine disrupts the dimeric form of TRAIL. TRAIL protein (2 ␮g) and TRAIL
(C230G) protein (2, 4, or 6 ␮g) mixed with sample buffer with or without DTT were
resolved on a 15% SDS gel and stained with Coomassie dye. Arrow, a dimeric form of
TRAIL. C, HeLa cells were treated with TRAIL or mutant TRAIL (C230G) for 6 h and
analyzed for apoptosis. The results represent the mean and SE for three separate
experiments.
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MODULATION OF TRAIL ACTIVITY BY CYSTEINE 230
References
Fig. 4. Dimeric TRAIL has decreased apoptotic activity. Purified TRAIL protein was
incubated with DTT (10 mM, final concentration) for 3 h at 4°C, dialyzed, and tested for
apoptotic activity as described in the Fig. 3C legend. The results represent the mean and
SE for three separate experiments.
Cysteine 230 Is Required for Both Dimer Formation and Apoptotic Function of TRAIL. The capacity of the reducing agent DTT
to abolish the dimer form of TRAIL (Fig. 1) suggested that an
intermolecular disulfide bridge might account for dimer formation.
Inspection of the amino acid sequence of human TRAIL protein
identified a unique cysteine residue at position 230 (Fig. 3A). This
cysteine residue was conserved in mouse TRAIL protein (10), pointing to a key functional role for this amino acid residue in the biological function of TRAIL protein. To examine the contribution of this
cysteine residue, the cysteine was mutated to glycine (Fig. 3A), and
TRAIL (C230G) protein was purified (Fig. 3B). The mutant TRAIL
(C230G) protein appeared only as a monomer on SDS gel (Fig. 3B),
indicating that cysteine 230 is responsible for forming dimeric TRAIL
protein.
Next, to examine the role of cysteine 230 in the apoptotic function
of TRAIL, HeLa cells were treated with TRAIL or mutant TRAIL
(C230G) protein. Interestingly, TRAIL (C230G) protein exhibited no
apoptotic activity in a time frame in which wild-type TRAIL induced
massive apoptosis (Fig. 3C). The crystal structure of TRAIL protein
revealed that cysteine 230 is positioned at the upper portion of the
TRAIL trimer, which may form a receptor contact surface (12). If
cysteine 230 is involved in ligand-receptor interaction, mutation of
cysteine 230 to glycine may impair the binding of TRAIL protein to
receptor. Another possibility is that cysteine 230 may function to
promote intermolecular interactions for trimer formation because cysteine 230 appears to face the interface of trimeric TRAIL protein (12).
Failure of TRAIL protein to form a correct trimer may result in loss
of apoptotic activity.
TRAIL Dimer Has Decreased Apoptotic Activity. To assess the
functional role of dimeric TRAIL, the TRAIL protein was treated
with DTT to disrupt the dimer form of TRAIL. The DTT was then
dialyzed off to allow the protein to form a trimer. This conversion
process slightly increased the apoptotic activity of TRAIL protein
(Fig. 4), indicating that cysteine 230 affects the apoptotic activity
of TRAIL protein through oxidation-reduction and that oxidized
dimeric TRAIL protein has lower (or null) apoptotic activity than
the trimeric TRAIL protein.
Here, we demonstrate that cysteine 230 is required for the apoptotic
function of TRAIL protein. Further characterization of the functional
role(s) of cysteine 230 and the consequences of modifications of this
residue in tumorigenesis and other disease models is required.
Note Added in Proof
When this manuscript was under review, studies describing a functional role of
cysteine 230 were published (24, 25). Hymowitz et al. (24, 25) have demonstrated that this
cysteine is required for trimer formation of TRAIL protein.
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Downloaded from cancerres.aacrjournals.org on June 18, 2017. © 2000 American Association for Cancer
Research.
Cysteine 230 Modulates Tumor Necrosis Factor-related
Apoptosis-inducing Ligand Activity
Dai-Wu Seol and Timothy R. Billiar
Cancer Res 2000;60:3152-3154.
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