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Volume 16, Number 12,
Issue of June 15, 1996
pp. 3895-3899
Copyright ©1996 Society for Neuroscience
Nerve Growth Factor (NGF)-Mediated Protection of Neural Crest
Cells from Antimitotic Agent-Induced Apoptosis: The Role of the
Low-Affinity NGF Receptor
Megan H. Cortazzo,
Edmund S. Kassis,
Kari A. Sproul, and
Nina Felice Schor
Departments of Pediatrics, Neurology, and Pharmacology, University
of Pittsburgh, Pittsburgh, Pennsylvania 15213
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
FOOTNOTES
REFERENCES
ABSTRACT
Prevention by nerve growth factor (NGF) of apoptotic death in
neural cells has been variously ascribed to binding of NGF to its
low-affinity (p75) or high-affinity (trkA) receptor or to a cooperative
interaction between the two. In a series of studies using, in turn,
neuroblastoma cell lines that express only p75, mutant NGF species that
bind selectively to either p75 or trkA, and a polyclonal antibody that
binds to the NGF-binding domain of p75, we demonstrate that NGF binding
to p75 is both necessary and sufficient for the abrogation of apoptosis
in neuroblastoma cells treated with antimitotic agents.
Key words:
neuroblastoma;
NGF;
NGF receptors;
p75;
trkA;
apoptosis
INTRODUCTION
The response of cultured cells of neural lineage
to such insults as serum deprivation can be modulated by nerve growth
factor (NGF) (Ibanez et al., 1992 ; Rabizadeh et al., 1993 ). It is not
entirely clear what role each of the two known NGF receptors
[low-affinity (p75) and high-affinity (trkA)] play in this modulatory
activity. It has been suggested that p75 expression is required for the
induction of apoptosis in response to these insults. Furthermore, the
binding of NGF to transfection-derived p75 appears to be protective
against this apoptosis induction in immortalized cerebellar neurons in
culture (Rabizadeh et al., 1993 ). However, other studies have suggested
that trkA binding of NGF is sufficient for ensuring the survival of
sympathetic neurons in culture (Ibanez et al., 1992 ). Finally, more
recent studies indicate that trkA and p75 exhibit a cooperative
relationship, and that both participate in the mediation of the
protective and other effects of NGF on neurons (Barker and Shooter,
1994 ; Huber and Chao, 1995 ; Wolf et al., 1995 ).
We have demonstrated previously that cultured neural-type neural crest
tumor cells uniformly undergo apoptosis in response to mitotic arrest
induced by a variety of agents with different proximate mechanisms of
action (Hartsell et al., 1995 , 1996 ). In all cases, treated cells round
up, detach from the culture surface, and demonstrate Hoechst dye- and
electron microscopy-visible condensation, fragmentation, and
margination of nuclear material accompanied by the formation of
oligonucleosomal-sized DNA fragments (gel electrophoresis and alkaline
elution). Furthermore, the endonuclease inhibitor aurintricarboxylic
acid abrogates the ultimate death of neural crest cells treated with
antimitotic agents.
We also have shown that NGF is protective against antimitotic
agent-induced apoptotic death in some of these cell lines (Falcione et
al., 1993 ; Cortazzo et al., 1995 ). This paper addresses the role of p75
in mediating the protective activity of NGF in neuroblastoma cells
exposed to antimitotic agents.
The use of site-directed mutagenesis to produce NGF species modified at
specific amino acid residues has facilitated identification of those
residues critical for binding of NGF to either trkA or p75. These
studies indicate that residues 32-35 are critical for p75, and not
trkA, binding (Ibanez et al., 1992 ), whereas residues 1-7 are
important for trkA, and not p75, binding (Shih et al., 1994 ) of NGF.
Furthermore, the recognition of the distinct trkA- and p75-binding
regions of NGF has permitted development of antibodies that selectively
block binding of NGF to p75 (Huber and Chao, 1995 ). Using these
reagents and taking advantage of our previous finding that loss of cell
adherence is a marker for apoptosis in this system (Hartsell et al.,
1995 ), we have tested the necessity for and sufficiency of p75 binding
of NGF for protection of neuroblastoma cells from antimitotic
agent-induced apoptosis.
MATERIALS AND METHODS
Chemicals. The antimitotic agent neocarzinostatin
(NCS) was obtained from Kayaku Pharmaceuticals (Tokyo, Japan). NCS was
stored in powder form at 20°C; a 47 µM (0.5 mg/ml) working stock solution in 0.015 M sodium
acetate buffer, pH 5.0, was stored in the dark at 4°C for up to 2 weeks and diluted with medium immediately before each experiment. NGF
was obtained from Boehringer Mannheim (Indianapolis, IN) and stored at
20°C as a stock solution (100 µg/ml) in minimal essential
medium- made 10% in fetal bovine serum. A polyclonal antiserum (Ab
9651) directed against the extracellular domain of the p75 NGF receptor
was obtained from Dr. Moses V. Chao (Cornell University Medical
College, New York, NY) and used for all experiments in a 1:1000
dilution. This antibody blocks NGF binding to p75 (Huber and Chao,
1995 ). All mutant NGF species were obtained from Dr. John W. Winslow
(Genentech, South San Francisco, CA). Mutant 6 (S1Y, S2A, S3E, P5K,
I6S, F7S) is 100-fold p75-selective at 25-37°C (Shih et al., 1995).
Mutants 21 (K32A, K34A, E35A) and 24 (R115A, K116A, R119A) are 100-fold
trkA-selective (Ibanez et al., 1992 ) (J. W. Winslow, Genentech,
personal communication).
Cell lines and culture. Two human neuroblastoma cell lines
were used for these studies. SH-SY5Y cells were obtained from Dr. June
L. Biedler (Memorial-Sloan Kettering Cancer Center, New York, NY) and
maintained as adherent monolayers on plastic tissue culture flasks
(37°C, 5% CO2) with a 1:1 mixture of minimal
essential medium- and Ham's F-12 nutrient mixture supplemented with
10% fetal bovine serum. NGP cells were the kind gift of Dr. Garrett
Brodeur (Children's Hospital of Philadelphia, Philadelphia, PA) and
were maintained as adherent monolayers with minimal essential
medium- made 10% in fetal bovine serum. For all experiments, cells
were transferred from stock flasks to 6-well plates and allowed to
incubate overnight before initiating experimental manipulations.
Effects of native and mutant NGFs on the NCS-induced decrement in
adherent cell number. The association of detachment from the
tissue culture surface with the occurrence of apoptosis has been noted
for many different cell types (Couldwell et al., 1994 ; Martin and
Green, 1994 ; Weller et al., 1994 ; Hartsell et al., 1995 ), although the
mechanism of this detachment is not known (Martin and Green, 1994 ). The
effects of NGF on the decrease in adherent cell number induced by NCS
were determined, as we have described previously (Falcione et al.,
1993 ). Briefly, sister cultures were exposed continuously to NGF or
control medium from 24 hr before NCS treatment through the termination
of the experiment. In studies involving Ab 9651, the antibody was added
to the culture (1:1000 dilution) 1 hr before the addition of NGF and
maintained in the medium for the duration of the experiment. After 24 hr of NGF exposure, cells were treated for 1 hr at 37°C with NCS. The
adherent cell number was determined for each culture immediately after
treatment with NCS and each day thereafter for the duration of the
experiment, as we have described previously (Falcione et al., 1993 ;
Hartsell et al., 1995 , 1996 ).
RESULTS
Sufficiency of binding to p75 for the protective effects of NGF on
NCS-treated neuroblastoma cells
We have demonstrated previously that SH-SY5Y human neuroblastoma
cells undergo apoptosis when treated with antimitotic agents, and that
in cultures of these cells, loss of adherence of cells to the tissue
culture surface inevitably presages the occurrence of apoptosis defined
by Hoechst dye staining, electron microscopy, DNA electrophoresis,
alkaline elution, and response to endonuclease inhibition (Hartsell et
al., 1995 ). SH-SY5Y cells express both p75 (1.3 × 105/cell) and trkA (2.6 × 103/cell), and bind NGF at both of these
receptors (Azar et al., 1991 ). They are protected completely by NGF
from the NCS-induced decrement in adherent cell number (Fig.
1A).
Fig. 1.
Effects of NGF (42 nM) on
antimitotic agent-induced human neuroblastoma cell death. A,
SH-SY5Y cells were pretreated with NGF for 24 hr, then exposed for 1 hr
to 3.2 nM NCS in the presence of NGF.
B, NGP cells were pretreated similarly with NGF, then
exposed for 1 hr to 2.4 nM NCS in the presence of
NGF. In both cases, NGF was maintained in the culture medium for the
duration of the experiment. The relationship between cell adherence and
apoptosis and the methods for determining adherent cell counts have
been described in detail previously (Falcione et al., 1993 ; Hartsell et
al., 1995 ). In this and the following figures, error bars represent the
SEM of three determinations within a single representative experiment
and, where not apparent, are smaller than the size of the symbol. All
experiments were performed twice, and each repetition gave comparable
results to those shown. , NCS alone; , NCS + NGF; , NGF alone;
, control.
[View Larger Version of this Image (18K GIF file)]
To determine whether NGF binding to p75 alone is sufficient to
induce resistance of neuroblastoma cells to antimitotic agent-induced
apoptosis, the effects of NGF on NGP human neuroblastoma cell adherence
were determined. NGP cells express p75 (2.7 × 104 receptors/cell), but not trkA (Azar et al.,
1991 ). NGF (40 nM) completely protected NGP cells
from the effects of NCS (Fig. 1B).
Because the SH-SY5Y and NGP lines undoubtedly differ from one another
in characteristics other than NGF receptor expression, sufficiency of
p75 binding also was tested by treating SH-SY5Y cells continuously with
a mutant NGF that binds to p75 with a 100-fold greater affinity than to
trkA. Figure 2A demonstrates that this
p75-selective NGF mutant 6 (Shih et al., 1995) completely protects
SH-SY5Y cells from the effects of NCS. Mutant 6 has no effect on the
growth or survival of native SH-SY5Y cultures.
Fig. 2.
Effects of p75- and trkA-selective mutant NGF
species (4.2 nM) on antimitotic agent-induced
SH-SY5Y cell death. Mutant NGF species produced by site-directed
mutagenesis were obtained from John W. Winslow (Genentech) and have
been fully characterized as described previously (Shih et al., 1994 ).
Mutant 6 (A) is p75-selective; mutants 21 (B) and
24 (C) are trkA-selective. In all cases, the duration of NCS
(1.6 nM) treatment was 1 hr, and mutant NGF
exposure began 24 hr before NCS treatment and continued throughout the
duration of the experiment. , NCS alone; , NCS + mutant NGF; ,
control; , mutant NGF alone.
[View Larger Version of this Image (13K GIF file)]
Insufficiency of binding to trkA for the protective effects
of NGF
To determine the role of NGF binding to trkA in the protection of
neuroblastoma cells from antimitotic-induced apoptosis, SH-SY5Y cells
were exposed continuously to trkA-specific NGF mutants 21 and 24 (Shih
et al., 1995). These mutants are 100-fold selective for trkA relative
to p75. Neither of these compounds protected SH-SY5Y cells from the
NCS-induced decrement in adherent cell number (Fig.
2B,C). Interestingly, both mutants 21 and 24 alone enhanced the growth rate of SH-SY5Y cultures.
Necessity for p75 binding for the protective effects of NGF
In an effort to establish the requirement for p75 binding for the
protective effects of NGF in this system, SH-SY5Y cells were treated
with NGF and NCS in the presence or absence of Ab 9651. This antibody
recognizes the extracellular domain of p75 and blocks binding of NGF to
this receptor (Huber and Chao, 1995 ). As is demonstrated in Figure
3, Ab 9651 abrogates the protective activity of NGF in
neuroblastoma cells treated with NCS, indicating the necessity for p75
binding in this regard. The antibody itself had no effect on adherent
cell number in either native cultures or cultures treated with NCS.
Fig. 3.
Effects of antibody 9651 (1:1000) on the
protective effects of NGF (42 nM) in antimitotic
agent-treated SH-SY5Y cells. Antibody 9651 was obtained from Moses V. Chao (Cornell University Medical College). This antibody specifically
blocks NGF binding to the extracellular domain of p75 (Huber and Chao,
1995 ). Cells were incubated with antibody for 1 hr before the addition
of NGF, and both antibody and NGF were maintained in the medium
throughout the duration of the experiment. NCS (1.6 nM) treatment was 1 hr in duration, beginning 24 hr after the addition of NGF to the medium. , Control; , NGF
alone; , NGF + antibody 9651; , NCS alone; , NGF + NCS; ,
NGF + antibody 9651 + NCS; , antibody 9651 alone;
+, antibody 9651 + NCS.
[View Larger Version of this Image (21K GIF file)]
DISCUSSION
Apoptosis, the process by which cells orchestrate their own demise
in response to intra- or extracellular events, plays a major role in
normal, pathological, and iatrogenic processes in the nervous system
(Cotter et al., 1990 ; Raff, 1992 ; Wyllie, 1992 ). In non-nervous system
models, several endogenous factors have been shown to protect tumor
and/or normal cells from apoptotic death (Brach et al., 1992 ; Tilly et
al., 1992 ; Harrington et al., 1994 ; Ishizaki et al., 1994 ). These
agents have been referred to collectively as ``survival factors''
(Collins et al., 1994 ). For both normal and neoplastic neural
crest-derived cells, NGF acts as a survival factor that protects
against apoptosis initiated by a variety of exogenous conditions
(Ibanez et al., 1992 ; Rabizadeh et al., 1993 ). We have reported
previously on antimitotic agent-induced apoptosis in neuroblastoma
cells and the protection by NGF of these cells from apoptosis induced
by such agents (Falcione et al., 1993 ; Cortazzo et al., 1995 ; Hartsell
et al., 1995 ). This antimitotic agent-induced apoptosis resembles
apoptosis induced in neural cells by other stimuli; it is prevented by
incubation with NGF, protein synthesis inhibitors, or endonuclease
inhibitors, and is accompanied by all of the changes in morphology and
nuclear chromatin size and configuration that have become the hallmarks
of apoptosis (Bredesen, 1995 ; Hartsell et al., 1995 ).
Two cell surface receptors have been identified for NGF. TrkA binds NGF
with high affinity, whereas p75 binds this peptide with low affinity
(Barbacid, 1993 ). From the functional standpoint, trkA is the starting
point for a protein kinase signal transduction pathway that clearly
mediates the differentiative function of NGF. Many of the intermediate
steps in the pathway from trkA binding of NGF to the induction of
neurite outgrowth have been identified (Cordon-Cardo et al., 1991 ;
Kaplan et al., 1991 , Kremer et al., 1991 , Ohmichi et al., 1991 ; Saltiel
and Ohmichi, 1993 ). The functional role of p75 is, by contrast, much
less well characterized. Studies in some systems suggest that p75
functions in concert with trkA to increase the binding affinity of trkA
for NGF (Hempstead et al., 1991 ). More recently, an independent role
has been suggested for p75 in the induction of apoptosis in neural
cells. Furthermore, the binding of NGF to p75 appears to abrogate this
apoptosis-inducing activity (Rabizadeh et al., 1993 ).
The amino acids that contribute to binding of NGF to each of its
receptors have been determined. Each of these receptors has a unique
binding site on the NGF molecule, and the binding of NGF to each
receptor can be modulated independently by site-directed mutagenesis
(Ibanez et al., 1992 ; Shih et al., 1994 ). In this manner, specific
mutant NGF species have been produced that exhibit selectivity of
binding to trkA or to p75 (Ibanez et al., 1992 ; Shih et al., 1994 ).
Using these mutant NGFs and an antibody that specifically blocks NGF
binding to p75 (Huber and Chao, 1995 ), the present studies demonstrate
that binding of NGF to p75 is both necessary and sufficient for the
protective effects of NGF in neuroblastoma cells treated with
antimitotic agents.
Studies of other cultured neuronal systems indicate some variability
from system to system in the sufficiency of p75 and the requirement for
trkA binding of NGF in this regard. In immortalized cerebellar neurons
deprived of serum, transfection with a p75 expression construct is
required and sufficient for NGF to abrogate the incidence of apoptosis
(Rabizadeh et al., 1993 ). In contrast, early studies in PC12
pheochromocytoma cells and cultured sympathetic neurons suggested the
sufficiency of binding of NGF to trkA for ensuring the survival of
these cells (Ibanez et al., 1992 ). More recent studies in PC12 cells
indicate a cooperative relationship between p75-bound NGF and trkA
(Canossa et al., 1995 ). Specifically, at least one of the activities of
the p75-NGF complex appears to be enhancement of the NGF binding and
autophosphorylation activities of trkA (Barker and Shooter, 1994 ; Verdi
et al., 1994 ). There is some evidence to suggest that variations from
cell line to cell line in the degree of cooperativity between p75 and
trkA are related to the relative abundance of these two receptors on
the cell. Interaction between the two receptors appears to require a 3- to 10-fold excess of p75 (Verdi et al., 1994 ; Greene and Kaplan, 1995 ).
This is in contrast to the 100-fold excess on SH-SY5Y cells (Azar et
al., 1991 ).
Our finding of a proliferative response of SH-SY5Y cells to
trkA-selective mutant NGFs is in contrast to the brief proliferative,
then differentiative, response of normal neural crest cells to NGF.
Defects in the trkA signal transduction pathway and a lack of
differentiative response to NGF have been found in several
neuroblastoma cell lines, including the SK-N-SH line from which SH-SY5Y
cells are derived (Azar et al., 1991 ). It is, perhaps, these defects
that result in the persistence of proliferation seen with trkA binding
of NGF.
Differences in the cellular mediators of the protective effects of NGF
are of more than mechanistic interest. For example, the use of
selective activators of one or the other NGF receptor may have
relevance for targeted therapy in the rapidly expanding list of
neurodegenerative conditions that involve apoptosis in their
pathogeneses (Cotman and Anderson, 1995 ). Furthermore, binding of NGF
to p75 may be of etiological importance in the well-documented clinical
phenomenon of neuroblastoma resistance to chemotherapy (Blatt et al.,
1995 ). If differences in the mediators of the protective activity of
NGF exist among cell types or among apoptosis-inducing insults, it will
be critical to define the nature of such differences before designing
therapeutic approaches to particular conditions affecting particular
nervous system cells. Previous studies have defined a relationship
between trk protein expression and clinical prognosis in neuroblastoma
tumor specimens (Nakagawara et al., 1993 ; Nakagawara et al., 1994 ;
Matsumoto et al., 1995 ; Yamashiro et al., 1996 ). Although the
expression of trkA, trkC, and a truncated transcript for trkB
correlates with a more favorable prognosis, the expression of a
full-length transcript for trkB is associated with more aggressive
tumors. This relationship is thought to reflect differences among
tumors in the degree of cellular maturation. These differences, in
turn, influence malignant potential of the tumors (Lucarelli et al.,
1995 ).
Although the relationship between p75 expression and clinical prognosis
has not been determined, it has been suggested that the spontaneous
regression of neuroblastomas of clinical stage IVS (Evans et al., 1991 )
is related to the occurrence of apoptosis in these tumors (Prichard and
Hickman, 1994 ). Additional studies are required to determine the roles
of apoptosis and p75 expression in the clinical and pathological
regression of neuroblastoma.
FOOTNOTES
Received Feb. 20, 1996; revised March 28, 1996; accepted March 28, 1996.
This study was supported by National Institutes of Health Grant
1F33-CA67421 (N.F.S.) and American Cancer Society Grant DPH-128
(N.F.S.), and by research fellowships from the University of Pittsburgh
School of Medicine (E.S.K., M.H.C.) and the National Institute of
Mental Health (University of Pittsburgh Undergraduate Research Program;
M.H.C.). The authors thank Drs. John W. Winslow and Moses V. Chao for
helpful discussions and for mutant NGF species and antibody 9651, respectively, used in these studies. We also acknowledge the expert
technical assistance of Karen D. Nylander.
Correspondence should be addressed to Dr. Nina Felice Schor, Division
of Child Neurology, Children's Hospital of Pittsburgh, 3705 Fifth
Avenue, Pittsburgh, PA 15213.
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