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Direct Stimulation of Cells Expressing Receptors for Macrophage
Colony-Stimulating Factor (CSF-1) by a Plasma Membrane-Bound
Precursor of Human CSF-1
By Jeremiah Stein, Gary V. Borzillo, and Carl W. Rettenmier
Secreted forms of macrophage colony-stimulating factor
(M-CSF or CSF-1) are generated by proteolytic cleavage of
membrane-bound glycoprotein precursors. Alternatively
spliced transcripts of the human CSF-1 gene encode at
least two different transmembrane precursors that are
differentially processed in mammalian expression systems.
The larger precursor rapidly undergoes proteolysis t o yield
the secreted growth factor and does not give rise to forms
of CSF-1 detected on the cell surface. By contrast, the
smaller human CSF-1 precursor is stably expressed on the
plasma membrane where it is inefficiently cleaved to
release a soluble molecule. To determine whether the
smaller precursor is biologically active on the cell surface,
mouse NIH-3T3 fibroblasts expressing the different forms
of human CSF-1 were killed by chemical fixation and tested
for their ability to support the proliferation of cells that
require this growth factor. Only fixed cells expressing
human CSF-1 precursors on their surface stimulated the
growth in vitro of a murine macrophage cell line or normal
mouse bone marrow-derived mononuclear phagocytes.
The ability of these nonviable fibroblasts t o induce the
proliferation of CSF-1 -dependent cells was not mediated
by release of soluble growth factor, required direct contact
with the target cells, and was blocked by neutralizing
antiserum to CSF-1. These results demonstrate that the
cell surface form of the human CSF-1 precursor is biologically active and indicate that plasma membrane-bound
growth factors can functionally interact with receptorbearing targets by direct cell-cell contact.
0 1990 by The American Society of Hematology.
ACROPHAGE colony-stimulating factor (M-CSF or
CSF-1) is a homodimeric glycoprotein (GP) that
stimulates the proliferation, differentiation, and survival of
mononuclear phagocytes'v2 and may also play a role in
placental
The effects of CSF-1 are mediated
by its binding to a cell surface receptor (CSF-IR), the c-fms
protooncogene product, which is an integral transmembrane
G P that functions as a ligand-activated protein-tyrosine
kinase.' Human CSF-1 is encoded by a single gene that maps
to chromosome 5q33.1'; the primary messenger R N A
(mRNA) transcript of this gene undergoes alternative splicing to generate multiple mature mRNAs encoding different
forms of membrane-bound CSF- 1 precursors. cDNAs encoding three different biologically active CSF- 1 products have
been cloned from human pancreatic carcinoma9-" and
trophoblasticI2 cell lines. All of the deduced translational
products share a common domain structure consisting of an
aminoterminal signal peptide followed, in turn, by the
sequence of the soluble growth factor, a segment of hydrophobic amino acids that serves as a membrane anchor for the
precursor proteins, and a short carboxyterminal tail. The
various CSF-1 precursors are generated by alternative splicing within a single exon that codes for the carboxyterminal
segment of the secreted growth factor, just proximal to the
membrane-anchoring domain." Proteolytic processing within
this variable segment results in the generation of different
forms of the soluble growth factor.
Introduction of the human CSF-1 cDNAs into mammalian expression systems has facilitated direct biochemical
analysis of the posttranslational processing of the CSF-1
precursors. A 554-amino acid precursor encoded by a 4-kilobase (kb) C D N A ' ~ . 'is~ cotranslationally assembled into
disulfide-linked dimers, undergoes both asparagine(N)- and
0-linked glycosylation, and is rapidly cleaved from the
transmembrane domain within the secretory compartment of
the ell.'^.'^ The resulting soluble homodimer of 223 amino
acid subunits is efficiently secreted from the cell, and forms
of the growth factor are not detected on the plasma membrane. By contrast, a 256-amino acid precursor encoded by a
1.6-kb cDNA9 undergoes only N-linked glycosylation and is
not cleaved from the membrane-spanning segment within the
cell; this smaller CSF-1 precursor is transported to the
plasma membrane where it is stably expressed as an integral
transmembrane G P with the growth factor domain exposed
on the cell surface." This precursor is then slowly and
inefficiently cleaved on the plasma membrane to yield a
soluble homodimeric growth factor, the subunits of which
each contain approximately 150 amino
The differential processing of the CSF-1 precursors and
the stable expression of the product encoded by the 1.6-kb
cDNA on the cell surface suggest a physiologic role for the
plasma membrane-bound form of the growth factor. In this
report, we describe the biologic activity of the cell surface
CSF-1 precursor.
From the Departments of HematologylOncology and Tumor Cell
Biology, St Jude Children's Research Hospital, Memphis, TN.
Submitted March I , 1990; accepted June 5,1990.
Supported by National Institutes of Health (NIH)Grant HL40603
(C.W.R.),NIH Training Grant CA09346 (G.V.B.), Cancer Center
(CORE) Grant CA21765, and the American Lebanese Syrian
Associated Charities.
Address reprint requests to Carl W. Rettenmier. MD, PhD.
Department of Pathology. Box 103. Children's Hospital of Los
Angeles. 4650 Sunset Blvd, Los Angeles. CA 90027.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C.section I734 solely to
indicate this fact.
8 I990 by The American Society of Hematology.
0006-4971/90/7607-0012$3.00/0
1308
MATERIALS AND METHODS
Cells and culture conditions. Mouse NIH-3T3 fibroblasts were
grown in Dulbecco's modified Eagle medium (DMEM) (Whittaker
Bioproducts, Walkersville, MD) containing 4.5 g/L D-glucose, 2
mmol/L L-glutamine, 10% fetal bovine serum (FBS) (GIBCO
Laboratories, Grand Island, NY), penicillin G (100 U/mL), and
streptomycin sulfate (100 pg/mL). NIHH3T3 cells transfected with
retroviral vectors expressing the CSF-1 coding sequences of the
1.6-kb9or the 4-kb" human cDNAs have been des~ribed.'~.'~,''
The
murine BACl.2F5 line was derived from simian virus 40-immortalBlood, Vol 76, No 7 (October 1). 1990: pp 1308-1314
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BIOLOGIC ACTIVITY OF CELL SURFACE CSF-1
ized spleen cells and grows as adherent macrophage-like cells that
are completely dependent on CSF-1 for proliferation and survival in
c~1ture.l~
BACl.2F5 cells were passaged in DMEM supplemented
with 15% FBS and 25% L cell-conditioned medium as a source of
CSF- 1.
Murine bone marrow cells were obtained from 4-week-old Balb/c
mice (Charles River Laboratories, Wilmington, MA) and cultured
in Iscove’s modified Dulbecco’s medium (Sigma Chemical Co, St
Louis, MO) supplemented with 36 mmol/L sodium bicarbonate, 5%
FBS, 0.22 mmol/L L-alanine, 0.13 mmol/L L-asparagine, 0.18
mmol/L L-aspartic acid, 0.40 mmol/L L-glutamic acid, 0.28
mmol/L L-proline, 36 nmol/L biotin, 6.5 nmol/L vitamin B,,, and
lo-’ mol/L hydrocortisone (Sigma). Bone marrow cultures were
maintained at 33OC in a 6% CO, environment with weekly removal
of one half of the medium and one half of the nonadherent cells and
replacement with fresh medium. Nonadherent cells were harvested
and grown in identical medium in colony-formation experiments.
Bone marrow-derived macrophages were prepared by culturing
nonadherent bone marrow cells for 2 weeks in DMEM supplemented
with 15% FBS and 25% L-cell conditioned medium.
Colony-formation assay. Human CSF- 1 is biologically active
for the stimulation of murine CSF-1R-bearing cells. NIH-3T3
fibroblasts, with or without introduced human CSF-1 cDNA constructs, were grown to confluence on glass coverslips (22 x 22 mm),
and a quadrant of each coverslip was scraped with a rubber
policeman to remove a portion of the monolayer. The coverslips were
rinsed in phosphate-buffered saline (PBS) and then fixed in either
2% glutaraldehyde, buffered formalin (10% formalin in 0.1 mol/L
phosphate buffer, pH 7.0), or 100% ethanol. The coverslips with the
attached nonviable fibroblast monolayers were washed extensively in
PBS to remove the fixative and were then transferred to fresh 35-mm
tissue culture dishes (Falcon, Oxnard, CA). Viable CSF-1dependent cells ( 5 x lo3 to 3 x lo4) were added in complete
medium lacking CSF-1, and the cultures were incubated for 11 to 14
days. Colonies of proliferating macrophages were visualized against
the background of the fixed fibroblast monolayers by staining either
with hematoxylin and eosin or with a-naphthyl butyrate esterase
(Sigma) and a light green S F (Eastman, Rochester, NY) counterstain. Butyrate esterase is a cytochemical marker for mononuclear
phagocytes, which show a reddish-brown, granular pattern of
cytoplasmic staining2’
Antibody neutralizationexperiments. Coverslipscontaining fixed
monolayers of fibroblasts expressing the plasma membrane-bound
form of human CSF-1 were washed and then incubated for 1 hour at
37OC in 100 p L of PBS containing either normal rabbit serum or
rabbit neutralizing antiserum to CSF-1 at dilutions of l:lO, 150,
1500, or 15,000. Mononuclear phagocytes in medium lacking
CSF-1 were then added without removing the rabbit serum, and
cultures were incubated as described above. The polyvalent antiserum to purified recombinant human CSF- 1 was kindly provided by
R. Halenbeck and K. Koths of Cetus Corporation (Emeryville, CA).
RESULTS
Cell surface CSF-1 stimulates a growth factor-dependent
macrophage cell line. NIH-3T3 fibroblasts producing either membrane-bound or secreted forms of human CSF-1
were killed by chemical fixation and tested for their ability to
support the proliferation of a CSF- ldependant murine
macrophage cell line, BACl.2F5. Fixation of the fibroblast
monolayers was done to prevent their continued synthesis of
CSF-1 and processing of CSF-1 precursors that occurs in
viable cells.I5
Glutaraldehyde-fixed monolayers of parental NIH-3T3
cells and NIH-3T3 cells expressing the 4-kb human CSF-1
1309
cDNA did not support the growth of CSF-l-dependant cells
(Fig 1, a and c), as the fixed fibroblasts do not synthesize
and/or secrete soluble growth factor. Similarly, BAC 1.2F5
cells did not proliferate on fixed monolayers of cells expressing a soluble form of human CSF-1 engineered by the
introduction of a termination codon aminoterminal to the
transmembrane segment of the growth factor encoded by the
1.6-kbcDNA” (data not shown). However, fixed monolayers
of fibroblasts expressing the smaller human CSF- 1 precursor
at their surface stimulated BACl.2F5 cells to proliferate and
form colonies (Fig 1, b and d). The mononuclear phagocyte
lineage of the proliferating cells was confirmed by positive
cytoplasmic staining with butyrate esterase, a cytochemical
marker for macrophages. Direct contact between the fixed
fibroblast monolayer and the BACl.2F5 cells was required
for macrophage proliferation, as evidenced by the formation
of colonies on those portions of the coverslips that contained
fixed monolayers and the absence of colonies on the scraped
quadrants. The results shown in Fig 1 were obtained when
small numbers (approximately 5 x lo’) of macrophages
were added to the preparation. When larger numbers of
BAC1.2F5 cells were added, they proliferated to form a
confluent layer that completely covered, and was limited to,
the portion of the coverslip that contained fixed monolayers
of membrane-bound CSF- l-expressing fibroblasts.
Fibroblast monolayers fixed in buffered formalin, rather
than glutaraldehyde, gave the same results in the macrophage colony assay. However, BACl.2F5 cells plated on
formalin-fixed monolayers were more motile, formed less
compact colonies, and tended to destroy the underlying
monolayer (Fig le) to a greater extent than was seen in
experiments using glutaraldehyde-fixed fibroblasts. Ethanolfixed monolayers did not support BACl.2F5 growth, presumably because of irreversible denaturation of cell surface
CSF-1 by this fixative.
Killing of fibroblast monolayers by chemical fixation was
required to demonstrate the activity of the cell surface CSF- 1
as opposed to its soluble form. Although membrane-bound
CSF-1 is not detected on the surface of parental NIH-3T3
cells by flow cytometry with antibody to the growth factor or
by lactoperoxidase-catalyzed radioiodination and immunoprecipitation,I3.I5these cells do produce low levels of soluble
murine CSF-1 (approximately 50 U/mL/d).1’*21These concentrations were readily detected when the macrophage
colony assay was performed on unfixed fibroblasts, even if
these latter cells were irradiated to prevent their proliferation
(data not shown). Similarly, viable NIH-3T3 cells expressing human CSF-1 cDNAs conditioned the medium with high
concentrations of the human growth factor. However, fixation of the fibroblast monolayers rendered the cells incapable
of CSF-1 secretion. With the addition of soluble murine
CSF-1 to the assay as a control, the BACl.2F5 cells
proliferated on both the fixed fibroblasts and the scraped
quadrant of the coverslips. When grown in medium containing soluble CSF-1, the BAC1.2F5 cells formed loose colonies
of motile cells that rapidly merged with cells of adjacent
colonies.
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STEIN, BORZIUO. AND RElTNMIER
1310
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Fixedfibroblast monolayers do not release soluble CSF-I.
Proliferation of BACl.2F5 cells on fixed monolayers expressing plasma membrane-bound CSF-I was not mediated by
the release of soluble growth factor. Supernatants from
cultures of the various fixed monolayers incubated in the
presence or absence of BAC1.2F5 cells were tested for the
presence of biologically active CSF-I. In all cases these
supernatants failed to stimulate either macrophage proliferation or the incorporation of ['Hlthymidine by these factordependent cells (data not shown). This latter assay reliably
detects CSF-I concentrations as low as 100 U/mL.
To demonstrate that the formation of macrophage colonies
in this assay was specifically dependent on CSF- I , parallel
assays were performed in the presence of neutralizing rabbit
antiserum raised against the human growth factor.'' As
shown in Fig If, normal rabbit serum did not inhibit the
growth of macrophage colonies on fixed monolayers expressing plasma membrane-bound CSF-I . By contrast, incubation
of these fixed monolayers with a 1:lO or 150 dilution of a
neutralizing antiserum completely ablated macrophage colony formation (Fig Ig). More dilute preparations of the
antiserum failed to inhibit the formation of macrophage
colonies (Fig 1 h), presumably because of the reduction of the
neutralizing antibody titer below its ability to saturate the
available CSF-I precursors on the surface of the fixed
"
*,
I
.
Fig 1. Colony formation by SAC12 F 6 cells on
chemically fixed monolayers of NIH-3T3 cells expressing human CSF-1 cDNA constructs. BACl.2F6
cells were plated in medium lacking CSF-1 on
glutaraldehyde (a through d l or formalin (el fixed
m o d a y e r s of NIK3T3 cells transfected with CSF-1
coding sequences of the 1.6-kb (b, d. and el or the
4-kb (cl cDNA. Panel (a1 depicts the same assay
using fixed parental NIH-3T3 cells. The scraped
quadrant of each preparation is a t the t o p of each
panel. (f through hl Glutaraldehydafixed monolayers of NIH-3T3 cells expressing the 1.6-kb CSF-1
cDNA were preincubated in PES containing either
e 1:- dilution of normal rabbit serum (11, or 1:(g) and 1:M)O (h) dilutions of rabbit neutralizing
antiserum t o CSF-1. All panels depict 14-dey cultures established with 6 x l@
BACl.2F6 cells,
stained w i t h hematoxylinleosin. Original magnification x 40 (a through c, f through hl and x 100 ( d
and el.
fibroblast monolayers. Similarly, a 150 dilution of antiserum
failed to inhibit the formation of macrophage colonies in
medium containing IO.000 U/mL of exogenously added
human CSF-I (not shown).
Two observations suggested a concentration dependence
on the plasma membrane-bound CSF-1 precursor in this
assay. First, the fixed fibroblasts stimulated RACl.2F5 cell
growth moreefficiently when they were present as a confluent
monolayer rather than as a sparse population. Second, only
fibroblasts preselected for very high levels of cell surface
CSF- 1 expression were able to promote colony formation in
this assay. Fibroblast cell lines that express lower levels of
cell surface CSF-I, as assayed by flow cytometry with
antibody to the growth factor, were much less efficient in
supporting the growth of macrophage colonies.
Membrane-bound CSF-I stimulates murine bone marrowderived macrophages. We next determined the ability of
the plasma membrane-bound CSF-I precursor to support the
proliferation of primary bone marrow cells obtained using
the long-term culture technique of Dexter et a!*' for maintenance of normal macrophage precursors in vitro. Nonadherent bone marrow cells from these cultures, which contain
precursor and mature cells of myeloid and mononuclear
phagocytic lineages, were added to monolayers of viable
NIH-3T3 fibroblasts, and the nonadherent cell fraction of
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BlCXOGlC ACTIVITY OF CELL SURFACE CSF-1
these cultures was harvested 14 days later. When cultured
with the adherent stromal feeder layer from primary bone
marrow, the nonadherent cell fraction consisted of a mixture
of mature monocytes and macrophages, rare immature
progenitors. and numerous polymorphonuclear leukocytes at
various stages of differentiation. By contrast, the nonadherent cell population was comprised exclusively of butyrate
esterase-positivemature macrophages when viable NIH-3T3
cells, with or without introduced human CSF-I contructs.
were used as the feeder layer. The predominant production of
macrophages under these conditions is presumably due to the
synthesis of CSF-I by the NIH-3T3 fibroblasts.
The efficiency of colony formation by nonadherent bone
marrow cells was compared when viable NIH-3T3 cell lines
expressing human CSF-I cDNAs were used as feeder layers.
Colonies of butyrate esterase-positive mononuclear cells
developed on monolayers of parental NIH-3T3 cells (Fig 2, a
and d) as well as on monolayers of fibroblasts expressing
either the 1.6- or 4-kb CSF-I cDNAs. In triplicate experiments, the efficiency of macrophage colony formation on
cells expressing the plasma membrane-bound CSF-I precursor (Fig 2b) was 0.9% of plated nonadherent cells, compared
with 0.5% on feeder cells producing only the soluble human
CSF-I (Fig 2c) and 0.2% on monolayers of parental NIH3T3 cells (Fig 2a). This demonstrates that viable fibroblasts
which express primarily cell surface CSF-I supported the
proliferation of mononuclear phagocytes fully as efficiently
as cells producing the soluble form of the growth factor. The
sizes of the colonies and the intensity of their staining with
Fig 2. Colony formation by
mononuclear phagocytes from
murine bone marrow cells on
parental NIH-3T3 cells (a and d)
or viable cells expressing 1 . 6 k b
(b and el or 4-kb (c and 1)
human CSF-1 cDNA 11 days
after addition of 2 x lo' nonadherent murine bone marrow
cells. (a through cl: WrightGiemsa stain of 60-mm culture
dishes. (d through 1): Original
magnification x 100 of butyrate esterase-stained preparations.
131 1
butyrate esterase was greater on the monolayers producing
human growth factor (Fig 2, d through f).
The ability of fixed monolayers of the same NIH-3T3 cell
lines to stimulate macrophage colony formation by normal
mouse bone marrow cells was examined. As shown in Fig 3.
butyrate esterase-positive macrophage colonies formed when
nonadherent murine bone marrow cells were plated on
glutaraldehyde-fixed NIH-3T3 cells expressing the plasma
membrane-bound form of CSF-I (Fig 3a), while no colonies
were observed on either fixed cells expressing the rapidly
secreted form of human CSF-I (Fig 3b) or fixed parental
NIH-3T3 cells (not shown). The nonadherent bone marrow
cells formed colonies at an efficiency of 0.7% on the fixed
monolayers expressing the cell surface CSF- 1 precursor.
Highly enriched populations of mouse macrophages were
prepared from bone marrow cultures by the addition of
murine CSF-I for 2 weeks before harvest and plating in the
colony-formation assay. These bone marrow-derived macrophages were added to fixed monolayers of parental NIH3T3 cells and fibroblasts expressing either the membranebound (Fig 3c) or soluble (Fig 3d) forms of human CSF-I.
Again, macrophage colonies formed only on the fixed monolayers containing the cell surface form of CSF-I (Fig 3c),
with an efficiency of 0.9%.
As observed in the experiments with the BACl.2F5 cell
line, the proliferation of macrophage colonies from mouse
bone marrow progenitors was blocked by neutralizingantibodies to CSF-I, appeared to require direct contact between the
fixed fibroblasts and the CSF-I receptor bearing targets, and
was not mediated by the release of soluble growth factor to
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STEIN, BORZILLO, A N D RETTENMIER
1312
Fig 3. Colony formation at 12 days in culture of 3 x lo‘
nonadherent murine bone marrow calls (a and bl, or 1 x lo‘ bone
marrow-derived macrophages (c and dl grown on glutaraldehydefixed NIH-3T3 cells expressing either the 1.6-kb (a and c) or 4-kb (b
and dl human CSF-1 cDNA. Butyrate esterase stain, original
magnification x 40.
condition the medium. Similarly, the macrophage colonies
were less compact and tended to erode the underlying
monolayer when buffered formalin was used to prepare the
feeder layers.
DISCUSSION
The present study shows that a plasma membrane-bound
CSF- I precursor is able to stimulate directly cells bearing the
CSF-I R. To demonstrate biologic activity of the cell surface
form of CSF-I. chemical fixation of the fibroblasts expressing this protein was required. This prevented the cells from
producing soluble CSF-I; without fixation, viable NIH-3T3
cells expressing the plasma membrane-bound form of CSF-I
conditioned the medium with growth factor. The failure to
abrogate CSF-I synthesis and secretion in these cells by
x-irradiation alone confirmed previous observations made
with bone marrow stromal cells.2’.24
After chemical fixation of the fibroblast monolayers,
solublegrowth factor was not detected in the culture medium
by sensitive assays. The inability of the CSF-I-dependent
cells to form colonies on the scraped areas of the coverslips
suggests that direct cell-cell contact was required for stimulation. However, the present experiments do not exclude the
possibility that the mononuclear phagocyte target cells,
which contain active proteases, may cleave the CSF-I
precursor on the surface of the fixed fibroblasts and immediately bind, internalize, and degrade the product. Studies to
directly examine the expression and turnover of the CSF-I
precursor on the fixed cell surface will be required to address
this question.
The presentation of growth factors on the surface of
nonviable feeder cell layers may offer promise as a method
for expanding populations of bone marrow cells in vitro. In
our studies with murine bone marrow-derived cells. fixed
fibroblasts expressing the plasma membrane-bound CSF-I
precursor supported mononuclear phagocyte colony formation at an efficiency nearly equal to that obtained with viable
NIH-3T3 cells producing membrane-bound and soluble
growth factor. Various types of feeder cells apparently differ
with respect to the spectrum of hematopoietic targets that
may be stimulated. The stromal feeder layer from primary
mouse bone marrow supported the proliferation of both
granulocytic and monocytic lineages from bone marrow cells
in vitro. Roberts et al” previously showed that Swiss 3T3
fibroblasts, even after glutaraldehyde fixation, could substitute for normal adherent stromal cells in stimulating granulocyte proliferation. The subclone of 3T3 cells used in our
experiments did not support granulocyte production by the
nonadherent cell population, but rather induced the growth
of mature macrophages. Additionally, the effect was only
observed when the fibroblasts were viable. These findings
may reflect differences in the synthesis of endogenous growth
factors or extracellular matrix components by the two
subclones of 3T3 cells.
Transmembrane precursors have been described for several other growth factors, including epidermal growth factor
(EGF),”.” transforming growth factor (TGF)-a,’*”” and
vaccinia growth factor.” all of which bind the EGF receptor.
Site-directed mutagenesis at the proteolytic processing site of
the TGF-a precursor generates biologically active molecules
that are expressed on the plasma m~mbrane.’~.”Similarly,
forms of the monokines interleukin-la””’ and tumor necrosis factor” have been identified at the cell surface. The
findings of amino acid sequence homology between the EGF
precursor and the products of the homeotic genes norch’9 and
tin-124‘’ have suggested a role for membrane-bound growth
factors in development. The recent observation that cells
expressing mutant forms of the membrane-bound protein
encoded by the Drosophila homeotic gene parched" can
effect changes in the developmental program of neighboring
cells confirms the role of direct cell-to-cell communication in
the process of embryogenesis.
Alternative splicing of the primary CSF- I transcript
generates different mRNAs encoding biologically active
forms of the growth factor that are either directly secreted or
are expressed as membrane-bound molecules at the cell
surface. The I .6-kb human CSF-I cDNA’ was isolated from
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1313
BIOLOGIC ACTIVITY OF CELL SURFACE CSF-1
a phorbol ester-treated pancreatic carcinoma cell line. Although the distribution of cells expressing this smaller
transcript in vivo is presently unknown, forms of the growth
factor similar in size to the soluble product derived from this
precursor have been demonstrated in human serum and
Physiologically, cell surface CSF- 1 might interact
with receptor-bearing targets within the microenvironment
of the bone marrow where islands of hematopoietic cells
proliferate within a supporting stroma, at localized sites of
inflammation to stimulate macrophage function, or in pregnancy during which CSF-1 produced by the uterine glandu-
lar epithelial cells appears to stimulate adjacent placental
trophoblasts that themselves express CSF-1R.5*6
ACKNOWLEDGMENT
We thank Drs Ernest Kawasaki, Martha Ladner, Robert Halenbeck, and Kirsten Koths of Cetus Corporation (Emeryville, CA) for
providing human CSF- 1 cDNA clones and neutralizing antiserum to
the growth factor used in these studies. We are grateful to John
Zacker and the staff of the Biomedical Communications Department at St Jude Children’s Research Hospital (Memphis, TN) for
assistance with photography.
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1990 76: 1308-1314
Direct stimulation of cells expressing receptors for macrophage
colony- stimulating factor (CSF-1) by a plasma membrane-bound
precursor of human CSF-1
J Stein, GV Borzillo and CW Rettenmier
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