From the Department of Genetics, Harvard Medical School, and the Howard Hughes Medical
Institute, Boston, Massachusetts 02115
Neu differentiation factor (NDF, also called neuregulin) is a potent inducer of epithelial cell
proliferation and has been shown to induce mammary carcinomas in transgenic mice. Notwithstanding this proliferative effect, we have shown that a novel isoform of NDF can induce
apoptosis when overexpressed. Here we report that this property also extends to other NDF
isoforms and that the cytoplasmic portion of NDF is largely responsible for the apoptotic effect,
whereas the proliferative activity is likely to depend upon the secreted version of NDF. In accordance with these contradictory properties, we find that tumors induced by NDF display extensive apoptosis in vivo. NDF is therefore an oncogene whose deregulation can induce transformation as well as apoptosis.
Key words:
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Introduction |
2b Neu differentiation factor (NDF), a novel isoform
of NDF, was recently isolated in a screen for dominant, apoptosis-inducing genes (1). NDF comprises a gene
family of differentially spliced isoforms. All NDF isoforms
encode membrane-anchored precursor proteins from which
the mature growth factor is proteolytically released (2, 3).
The
2b isoform of NDF can cause apoptosis when overexpressed in tissue culture cells (1). Interestingly, both extra- and intracellular domains of the
2b NDF precursor are required for apoptosis induction. This indicates that
only cells overexpressing the precursor can undergo apoptosis and that this effect is not due to the secreted NDF
molecule. Several other lines of evidence suggest that this
apoptosis is a cell-autonomous effect. Chief among them is
the observation that cells lacking NDF-binding erbB receptors are still sensitive to apoptosis induction (1). In this
report we address the sequence requirements of NDF for
apoptosis induction. We also investigate the apparently contradictory finding that NDF overexpression can lead to
tumor formation in a mouse model (4) as well as induce
apoptosis in cells (1).
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Materials and Methods |
Quantitative Apoptosis Assay.
Quantification of apoptosis induction was performed as previously described (1). In brief, the
indicated amounts of expression plasmid were transfected into
baby hamster kidney (BHK) cells together with 1 µg of a
-galactosidase (
-gal) expression construct. 24 h later the cells were
stained for
-gal activity and the percentage of blue and morphologically apoptotic cells with respect to all blue and transfected
cells was determined.
Cell Transfections.
BHK cells were transfected using calcium
phosphate coprecipitation as previously described (1).
Expression Constructs.
Constructs for expressing the NDF fusion proteins with the signal peptide of the human erbB-3 receptor (residues 1-29) were generated with recombinant PCR. All
other mutants of
2b NDF were likewise engineered by PCR.
For each PCR expression construct, two independently generated clones were used in the transfection experiments. All constructs were control sequenced. For all PCR reactions the thermostable enzyme Pwo (Boehringer Mannheim, Indianapolis, IN)
that has proofreading activity was used. The
2c NDF cDNA was a gift from Amgen Inc. (Thousand Oaks, CA). The human
TGF-
cDNA was a gift from Dr. Merlino (National Cancer Institute, Fredericktown, MD; reference 5). Both cDNAs were
subcloned into the plasmid pcDNA3 (Invitrogen, San Diego,
CA). All constructs were in vitro-translated and yielded proteins
of the correct size. In addition, for each of the inactive constructs
shown in Fig. 1 (
2-262, Sig
2-262, and
2-198), myc-tagged versions were made and tested to ensure that appropriate
protein product was synthesized as a result of the transfection.
Protein of an appropriate mobility on SDS gel was detected by
anti-myc-tag antibody in each case.

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Fig. 1.
Apoptosis induction of NDF deletion mutants with and
without a signal sequence. The domains of wild-type 2b NDF are indicated on top of the diagrammatic representation. IG, immunoglobulin
homology domain; Glyco, glycosylated domain; EGF, epidermal growth
factor domain; , sequences of exon; 2, sequences of exon 2; TM,
transmembrane domain; Cyto, cytoplasmic domain; b, sequences of b
exon; Sig, signal sequence of erbB-3. The names of the deletion constructs denote the deleted amino acids of wild-type 2b NDF. As a control in the case of the constructs that did not induce apoptosis ( 2-262, Sig 2-262, and 2-198), myc-tagged versions were transfected into
BHK cells and the protein products were analyzed on SDS gels and detected with anti-myc-tag antibody. In each case, protein of appropriate
mobility was detected (data not shown). 1.5 µg of each expression construct were transfected into BHK cells with 1 µg of a -gal expression
vector. 24 h after transfection the cells were stained for -gal activity and
the percentage of apoptotic and blue cells of all blue cells was determined
using morphological inspection. Shown are the means and SD of at least
1,000 counted cells of four independent transfections.
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Apoptosis in Tumor Tissue.
Paraffin sections were obtained
from tumors originating in transgenic mice that harbored an
NDF gene, v-Ha-ras, or a myc gene under the control of the
MMTV promoter (4, 6, 7). Subsequently, sections were stained
with the TUNEL (Tdt-mediated dUTP-biotin nick end labeling) technique (Boehringer Mannheim), which marks the DNA
ends generated in apoptosis (8), or the Annexin V stain (PharMingen, San Diego, CA), which detects phosphatidylserine on
the outer membrane of apoptotic cells (9).
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Results |
Previous mapping data (1) indicated that both the extra-
and the intracellular domains of
2c NDF are necessary for
apoptosis. Since NDF does not contain a genuine signal
sequence (2, 3), we speculated that deletions in the NH2-terminal, extracellular domain might cause a mislocalization
of these deletion proteins that could be responsible for their
inactivity. To test this, we fused the signal sequence of the
erbB-3 receptor to two deletion versions of
2b NDF that
were by themselves not able to induce apoptosis: the cytoplasmic domains (Fig. 1, Sig
2-262) and a construct also
containing the transmembrane domain as well as the
and
the 2 exon sequences (Fig. 1, Sig
2-198). A quantitative apoptosis assay revealed that the cytoplasmic domain, even
when fused to the signal sequence (Sig
2-262), remains
inactive, whereas the construct containing the transmembrane domain (Sig
2-198) regains its activity for apoptosis
induction (Fig. 1). Its activity is even higher than that of
the wild-type
2b NDF (24 vs. 13.2% apoptotic cells). A
fusion construct of the complete
2b NDF with the signal
sequence is likewise more efficient than wild-type
2b
NDF (data not shown). To demonstrate that the "inactive" constructs actually synthesized the protein, myc-tagged
versions of each construct (Fig. 1,
2-262, Sig
2-262,
and
2-198) were transfected into BHK cells and analyzed
for protein using SDS gels and anti-myc-tag antibody. Each
transfection produced cross-reacting protein of appropriate
mobility (data not shown).
Since the construct sig
2-198 still contains the combination of the exons
, 2, and b that define the specificity of
this particular isoform, we wanted to test whether other
isoforms could also lead to apoptosis. Fig. 2 shows that exchanging the
exon for the
exon in the intracellular domain of NDF does not alter the extent of cell death. As
shown previously (1), the c isoform of
2 NDF is considerably less efficient in apoptosis induction. A c isoform
containing the
exon instead of the
exon is also slightly
less potent in apoptosis induction (Fig. 2). We also tested
TGF-
in this assay. Like NDF, TGF-
contains an epidermal growth factor homology domain (5) and encodes a
ligand for a receptor that is first synthesized as a membrane-bound precursor protein. Despite this similarity, overexpression of TGF-
is unable to induce apoptosis in this
assay (Fig. 2).

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Fig. 2.
Apoptosis induction by different NDF isoforms. Expression
plasmids (4 µg) of the indicated NDF isoforms or of TGF- were transfected together with a -gal vector (1 µg) into BHK cells. Apoptosis induction was measured as in Fig. 1. The structure of each NDF isoform is
schematically depicted in the diagram. Each subdomain is named as in
Fig. 1. and a denote the or the a exon in the extra- or intracellular
domain, respectively.
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The Fas receptor is another apoptosis inducer that resides
on the membrane. This receptor is activated by overexpression or by cross-linking through its cognate ligand and
concomitant aggregation of its intracellular "death domain"
(10). To test whether
2b NDF is also activated by clustering, we generated a fusion protein with the extracellular sequences of the IL-4 receptor and the cytoplasmic domain
of
2b NDF. However, even when cross-linked with an
antibody against the IL4R, this construct was unable to induce apoptosis (data not shown).
To test whether NDF used the Fas pathway, a dominant
negative version of FADD, one of the downstream molecules in the Fas receptor complex, was used to block Fas
receptor-mediated apoptosis (11). Under these conditions,
we were not able to detect any effect on NDF-induced
apoptosis upon cotransfection of the mutant FADD (data
not shown).
NDF has been shown to lead to tumor formation when
overexpressed in the mammary gland of transgenic mice
(4). Data presented here and previously (1) showed that
overexpressed NDF can also induce cell death. Since these
two effects are seemingly contradictory, we tested the tumors that express high amounts of NDF (4) for apoptosis.
10 advanced adenocarcinomas from 5 different transgenic mice were examined for apoptosis by the TUNEL technique or by staining with Annexin V. Seven tumors (64%)
were strongly positive for apoptosis. The stained cells
showed cytoplasm shrinkage typical of apoptotic cells.
Every transgenic mouse had at least one highly apoptosis-positive tumor. This cell death was not a consequence of
insufficient blood supply since apoptotic cells were evenly distributed in the tumor mass and could also be detected at
the edge of the tumor tissue (Fig. 3). Surrounding normal
tissue was apoptosis free. In contrast, three myc- and three
ras-induced tumors were apoptosis negative. Furthermore,
we have established two cell lines from NDF-induced tumors. These tumor cell lines exhibited strong apoptosis as
evident by Annexin V staining after having been in cell
culture for as long as 25 passages (data not shown).

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Fig. 3.
Apoptosis induction in NDF-caused tumors. Hematoxylin and eosin-stained tissues from an adenocarcinoma in the mammary gland of
NDF-transgenic mice, WT mammary gland, and a myc-induced tumor were probed with the TUNEL technique that detects DNA ends that are generated during apoptosis.
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Discussion |
Previously we described a novel isoform of NDF isolated
in a screen for dominant, apoptosis-inducing genes (1).
Here we show that this property is shared by other isoforms
of NDF and we establish structural requirements for this
activity. For example, the c exon that places a stop codon
at the end of the cytoplasmic domain of
2 NDF results in
decreased apoptosis (1). This is also the case for the
2c isoform, although in a less pronounced manner (Fig. 2). Since
the uncleaved precursor of the c isoform possesses a longer
half-life than the precursor of other isoforms (12), one
could assume that these unprocessed precursors are inactive
with respect to apoptosis and that only cleaved, membrane-bound NDF forms can induce it. This would explain the
reduced apoptotic activities of c exon-containing isoforms.
In addition, extracellular sequences might influence the
kinetics of the processing of NDF precursors, since these are
cleaved at position 228 or 223 in the
or
exons,
respectively (3, 4). This might account for the differences of
apoptotic induction by
2c and
2c NDF (Fig. 2). Furthermore, the efficient apoptosis induction of the construct sig
2-198 NDF (Fig. 1) might be explained by the fact that this
construct mimics an already processed precursor molecule.
We found that attaching a signal sequence to a construct
of NDF lacking its secreted domains was sufficient for induction of cell death (Fig. 1). This suggests that the inactivity of the deletion mutants could be due to mislocalization
on the membrane. This experiment shows that the apoptotic activity of NDF can be separated from its oncogenic
function that appears to be mediated by its growth factor
moiety (4). Therefore, these data also corroborate our notion (1) that NDF causes apoptosis cell autonomously when
overexpressed. As a next step, it will be important to isolate proteins that interact with the cytoplasmic domain of NDF
and that transmit this apoptotic signal. However, we have
also found that the transmembrane domain of NDF is important for its induction of apoptosis. A fusion construct
with only the cytoplasmic domain of NDF and the IL-4R
does not lead to cell death when overexpressed or cross-linked with an antibody. Although this might be due to an
incorrectly folded cytoplasmic domain, it is noteworthy
that the transmembrane domain is the most conserved sequence between NDF and its recently isolated paralog,
NRG-2 (13, 14). Therefore, this might be the crucial element in the loss of function of this construct.
Although we isolated NDF in a screen for dominant,
apoptosis-inducing genes in tissue culture cells (1), NDF
has also been shown to function as an oncogene (4). In this
report we demonstrate that both effects can also be seen in
vivo. NDF overexpression induces tumors in the mammary gland that nevertheless display extensive apoptosis.
Therefore, NDF has a dual role in apoptosis and tumorigenesis.
We would like to suggest a biologic rationale that reconciles NDF's apoptotic and tumorigenic properties (Fig. 4).
A cell might suffer a mutation that activates the endogenous NDF promoter. This would lead to the secretion of
large amounts of NDF that then stimulate erbB receptors
on neighboring cells (or by a paracrine mechanism on the
same cell). This causes these cells to proliferate, which
might be the first step in the transformation process. Thus NDF could use the apoptotic response as an autoregulatory
event, eliminating tumorigenic signals from the organism.
This mechanism might be especially important since secreted NDF could potentially induce proliferation in many
neighboring cells. Furthermore, it has been shown that
NDF-binding erbB receptors cannot be downregulated efficiently (15). Thus, overexpression of NDF would expose
cells permanently to NDF's mitogenic activation. In tumors, NDF's apoptotic activity could be mitigated by secondary events like the activation of Bcl-2-like genes,
which have been found to suppress NDF apoptosis (1).

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Fig. 4.
Model for the apoptotic and tumorigenic activity of NDF. A
cell that overexpresses the NDF-precursor releases high amounts of the
secreted NDF form. Its interaction with erbB receptors can lead to the
subsequent proliferation of neighboring cells and might constitute the first
step towards transformation. NDF's apoptosis-inducing activity is concomitantly activated when NDF is overexpressed and thereby kills this
potentially dangerous cell.
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Address correspondence to Philip Leder, Department of Genetics, Harvard Medical School and Howard
Hughes Medical Institute, 200 Longwood Ave., Boston, MA 02115. Phone: 617-432-7662; Fax: 617-432-7944; E-mail: leder{at}rascal.med.harvard.edu
We thank Drs. Y. Ishida, T. Lane, and K. Fitzgerald for helpful discussions about the manuscript. Thanks
also to Drs. D. Wen (Amgen) and Dr. G. Merlino for providing cDNAs.
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