(Circulation. 2001;103:2242.)
© 2001 American Heart Association, Inc.
Clinical Investigation and Reports |
From the Molecular/Cancer Biology Laboratory and Ludwig Institute for Cancer Research, Haartman Institute and Biomedicum Helsinki (M.U., J.L., K.A.), and the Department of Clinical Chemistry and Biomedicine and Laboratory Department of Helsinki University Central Hospital (N.H.-K.), University of Helsinki, Finland; and the Ludwig Institute for Cancer Research, Stockholm Branch, Karolinska Institutet, Stockholm, Sweden (X.L., E.B., U.E.).
Correspondence to Kari Alitalo, MD, PhD, Molecular/Cancer Biology Laboratory, Biomedicum Helsinki, POB 63 (Haartmaninkatu 8), 00014, Helsinki, Finland. E-mail kari.alitalo{at}helsinki.fi
| Abstract |
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Methods and ResultsWe used fluorescence in situ hybridization to locate PDGFC and PDGFD in chromosomes 4q32 and 11q22.3 to 23.2, respectively. Exon structures of PDGFC and PDGFD were determined by sequencing from genomic DNA clones. The coding region of PDGFC consists of 6 and PDGFD of 7 exons, of which the last 2 encode the C-terminal PDGF cystine knot growth factor homology domain. An N-terminal CUB domain is encoded by exons 2 and 3 of both genes, and a region of proteolytic cleavage involved in releasing and activating the growth factor domain is located in exon 4 in PDGFC and exon 5 in PDGFD. PDGF-C was expressed predominantly in smooth muscle cells and PDGF-D in fibroblastic adventitial cells, and both genes were active in cultured endothelial cells and in a variety of tumor cell lines. Both PDGF-C and PDGF-D also stimulated human coronary artery smooth muscle cells.
ConclusionsPDGFC and PDGFD have similar genomic structures, which resemble those of the PDGFA and PDGFB genes. Their expression in the arterial wall and cultured vascular cells suggests that they can transduce proliferation/migration signals to pericytes and smooth muscle cells.
Key Words: platelet-derived factors genes muscle, smooth
| Introduction |
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and
PDGFR-ß.1 The
-receptor
binds both the A and B chains of PDGF, but the ß-receptor binds only
the B chain.
Targeted mutagenesis studies of PDGFs and their
receptor genes show evidence of the importance of these genes during
embryonic development. Approximately 50% of the PDGF-Anull mice die
in utero before embryonic day 10, and the surviving mice have a complex
postnatal phenotype affecting the lungs, the skin, and the
nervous system (reviewed by Betsholtz and
Raines2 ). The
PDGFR-
deficient mice have incomplete cephalic closure, impaired
neural crest development, cardiovascular and skeletal
defects, and edemas and die at embryonic days 8 to
16.3 Both PDGF-B and
PDGFR-ßdeficient mice have severely disrupted kidney development
and defective development of blood vessels, with a dilated aorta and
bleeding, leading in most cases to death during embryonic days 17 to
19.4 5 6 7
All muscle cell lineages are severely affected by the lack of PDGFR-ß
in chimeric embryos.8 PDGF
has also been shown to be involved in human diseases, for example, in
arteriosclerosis and cancer (see Betsholtz and
Raines2 for
references).
We have identified 2 novel PDGFs and shown that PDGF-C
binds to PDGFR-
9A and
PDGF-D to PDGFR-ß.9B Here,
we report the structures and chromosomal localizations of the
PDGFC and
PDGFD genes and show that they
are expressed in cells of the vascular wall, suggesting that these
genes can participate in vascular development and
pathology.
| Methods |
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For fluorescence in situ hybridization, genomic DNA clones were nick translated with biotin 11-dUTP (Sigma) and hybridized on metaphase chromosomes derived from normal human peripheral blood lymphocytes, which were treated with 5-bromodeoxyuridine at early replicating phase to induce banding pattern.10 The hybridization was carried out in 50% formamide/10% dextran sulfate in 2x SSC, and the signals were detected as described earlier.11 12 A multicolor image analysis was used for the acquisition, display, and quantification of the hybridization signals.13 The identification of the chromosomes was based on 4',6'-diamino-2-phenylindole banding pattern, which resembles G bands, after 5-bromodeoxyuridine incorporation at the early replicating phase.
Cell Culture and Isolation and Analysis
of RNA
Human umbilical vein and microvascular
endothelial cells and human coronary artery
SMCs (HCASMCs) were purchased from Promocell and cultured in passages 2
to 5, as recommended by the supplier. The Wi-38 fetal lung fibroblasts
and human tumor cell lines were obtained from American Type Culture
Collection. The isolation of polyadenylated RNA was by oligo-dT
cellulose chromatography; 5 µg was electrophoresed in
1.0% agarose gels containing formaldehyde and blotted onto
Hybond-N filters (Amersham), which were used for
hybridization with 32P-labeled
PDGFC and
PDGFD cDNA
fragments.
Cell Proliferation Analysis
The core domain PDGF-CC was generated as previously
described.9A A His-tagged
(6x His) version of full-length PDGF-D was produced in
baculovirus-infected Sf9 insect cells. This protein contained a
specific cleavage site for the factor Xa protease in the hinge region
in front of the core domain (cleavage site IEGRx2, replacing amino
acid residues 251 to 258). The introduced factor Xa cleavage site does
not affect the ability of the PDGF-D chains to form
dimers.9B The factor Xa
cleaved protein product was used in cell proliferation
analysis. HCASMCs were cultured on a 96-well plate for 24
hours. The culture medium was then replaced with 100 µL starvation
medium (HCASMC basal media, Promocell), and either PDGF-BB or PDGF-DD
was added to a final concentration of 100 ng/mL. After 72 hours of
incubation at 37°C, 10 µL of a 5-mg/mL solution of MTT was added,
and cells were incubated at 37°C for another 2 hours, after which the
cells were lysed in 100 µL of 10% SDS/10 mmol/L HCl overnight
at 37°C and absorbance was measured at 540 nm.
Statistical Analysis
For statistical analysis, an unpaired
t test was performed comparing
PDGF-CC, PDGF-DD, and PDGF-BB stimulation against the starvation medium
control. Analysis was done with the
t test calculator at
GraphPad.com.
Antibodies
Rabbit antihuman PDGF-D antiserum was produced in
rabbits by immunization with the MAP peptide SKVDLDRLNDDAKRYS (residues
257 to 272; Genbank accession number AF336376). For affinity
purification, the peptide was coupled to epoxy-activated
Sepharose 6B (Pharmacia). Bound antibodies were eluted with 1% acetic
acid and 20 mmol/L sodium hydroxide, and eluates were pooled and
dialyzed against PBS. Rabbit antihuman PDGF-C antibodies were
produced as
described.9A
Immunohistochemical Staining
Tissues were fixed in 4%
paraformaldehyde at 4°C overnight, dehydrated,
embedded in paraffin, cut at 5 µm, and treated with trypsin for 20
minutes at 37°C. The sections were then stained with the PDGF-C and
PDGF-D specific antibodies by use of the TSA-kit (NEN Life Sciences)
according to the manufacturers instructions. Biotinylated anti-rabbit
IgG (Vector Laboratories, diluted 1:300 before use) was used for
detection. Sections were counterstained with hematoxylin.
Antigen-specific blocking of the antibodies was carried out with a 10x
molar excess of the immunogen for 12 hours at
4°C.
| Results |
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58% identical, as are exon 6 and 7encoded
PDGF-homology domains (57%). As can be seen from the comparisons, exon
4encoded sequences of PDGF-D cannot be identified in PDGF-C, whereas
the exon 5 (PDGFC exon
4)encoded hinge region contains a conserved basic sequence motif,
RKSR/K (arrowheads and shaded area), which may be one target of the
proteolytic cleavage that separates the PDGF domain from the rest of
the propeptide and activates PDGF-C and PDGF-D biological
activity.9A 9B Such cleavage
peptides are located at the N-terminal of the cystine knot growth
factor domain in PDGF-A and PDGF-B as
well.1
|
Chromosomal Locations of the
PDGFC and
PDGFD Genes
In fluorescence in situ hybridization
analysis for the PDGFC
gene, 30 metaphases out of 43 (70%) showed specific hybridization
signals on human chromosome 4q32 and 34 metaphases out of 40 (85%)
showed specific PDGFD
hybridization signals on chromosome 11q22.3 to 23.2
(Figure 2A
).
|
Expression of PDGF-C and PDGF-D in Human
Vascular Cells in Culture
Northern blotting and hybridization analysis
demonstrated signals for PDGF-C mRNA and even stronger signals for
PDGF-D mRNA in human umbilical vein endothelial and
microvascular endothelial cells. In HCASMCs, PDGF-C
mRNA was much more abundant than PDGF-D mRNA
(Figure 2B
). Conversely, in the Wi-38 human fibroblast cell
line, PDGF-D mRNA was considerably more abundant than PDGF-C mRNA
(Figure 2B
and
Table
).
Only very small amounts of PDGF-C mRNA were expressed in the 293T human
kidney cells
(Figure 2B
). Essentially no regulation of either mRNA,
however, was observed in hypoxic conditions or when the human
microvascular endothelial cells were stimulated by
vascular endothelial growth factor-C, fibroblast growth
factor-1, or tumor necrosis factor-
(data not shown). The
Table
shows a summary of the results of mRNA expression analysis in
the human vascular cells.
|
PDGF-C and PDGF-D Expression in Tumor Cell
Lines
Both novel PDGF mRNAs were also expressed in several
tumor cell lines
(Figure 2C
and
Table
).
Specifically, the 3.8- and 2.8-kb transcripts of PDGF-C were
present in fibrosarcoma (HT-1080), breast cancer (BT-474), and
prostate cancer (PC-3) cell lines, whereas only the 3.8-kb transcript
was observed in erythroleukemia cells (HEL). The 4.0-kb PDGF-D mRNA was
present in lung alveolar carcinoma (A-549) and in breast and
prostate carcinoma cells. Interestingly, comparison with the expression
of PDGF-A and PDGF-B genes indicated that all 4 family members are
independently regulated in tumor cell lines
(Table
).
Localization of PDGF-C and PDGF-D Antigens in
the Arterial Wall
Immunoperoxidase staining of sections from the
suprarenal artery showed weak signals for PDGF-C in the SMC layer
(Figure 3A
), and stronger staining for PDGF-D was located in
the adventitial connective tissue layer
(Figure 3C
), consistent with their patterns of
expression in cultured SMCs and fibroblasts,
respectively.
|
PDGF-C and PDGF-D Stimulate Proliferation of
Smooth Muscle Cells
Cultured serum-starved HCASMCs were incubated with
purified recombinant PDGF-CC, PDGF-DD, PDGF-BB, or starvation medium
for 3 days, then lysed, and the numbers of viable cells were
analyzed by use of the MTT assay. The relative MTT values
indicated that the numbers of viable cells were significantly higher in
wells incubated with either PDGF-CC, PDGF-DD, or PDGF-BB than in wells
with only starvation medium
(Figure 4
). Although the detailed dose responses in such
experiments remain to be determined, these results support the
hypothesis that both PDGF-CC and PDGF-DD stimulate SMC
proliferation/survival.
|
| Discussion |
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When overexpressed in the heart, PDGF-C was shown to enhance
myocardial fibroblast
proliferation.9A Because of
the involvement of the PDGF
genes in blood vessel development and in the pathogenesis of
arteriosclerosis,2 14
it was of interest to find expression of the PDGF-C and PDGF-D mRNAs in
cultured microvascular endothelial cells. PDGF-D was
found in the adventitial connective tissue surrounding the suprarenal
artery, suggesting that it provides a paracrine ligand for the
arterial SMCs, which have been shown to express
PDGFR-ß.15 The biological
activity of recombinant PDGF-CC and PDGF-DD was confirmed in HCASMC
cultures, in which both of these factors stimulated cell
proliferation/survival. Thus, although these factors bind to distinct
receptors, PDGFR-
and PDGFR-ß,
respectively,9A 9B they
appear to share at least one redundant effect via these 2 receptors. In
previous studies, both PDGF-AA, which transduces signals via PDGFR-
,
and PDGF-BB, which transduces signals via both PDGFR-
and PDGFR-ß
and via their heterodimers, have been shown to mediate SMC
proliferation stimuli (for references, see Reference 11 ). Several
antagonists specific for PDGF or its receptors have
recently been developed and shown to inhibit intimal hyperplasia
formation in various animal models, predominantly via increased SMC
apoptosis and possibly also via interference with SMC
migration.16 Thus, it can be
speculated that via their ability to stimulate the PDGFRs, PDGF-CC and
PDGF-DD could also play a role in
arteriosclerosis.
The PDGF-C and PDGF-D mRNAs were also expressed along with PDGF-A and PDGF-B mRNAs in several tumor cell lines, but the variable expression patterns seen in a subset of the cell lines indicate that these genes are differently regulated. The role of PDGF-D in tumors needs further study, because the v-sis oncogene of simian sarcoma virus and the dermatofibrosarcoma protuberansassociated fusion gene encode functional PDGF-BB homologues capable of binding all known PDGFRs.1 17 No chromosomal translocations or inherited tumor loci, however, have as yet been mapped to the regions in which we located the novel genes. The PDGFs secreted by tumor cells could be responsible for some of the stromal proliferative or so-called desmoplastic reactions around tumors. Furthermore, because fibroblasts have the ability to contract collagen gels in response to PDGF,18 the novel genes could be involved in modulating the tension between cells and extracellular matrix structures and thus the regulation of interstitial fluid pressure and homeostasis in the connective tissue matrix.
Interaction between endothelial cells and mural cells (pericytes and SMCs) is essential for vascular development and maintenance. Recent results by Yamashita and colleagues19 show that vascular endothelial growth factor receptor-2expressing cells derived from embryonic stem cells can differentiate into both endothelial and mural cells and can reproduce the vascular organization process. In this process, vascular endothelial growth factor promotes endothelial cell differentiation, whereas mural cells are induced by PDGF-BB. A possible role for PDGF-DD in the induction of mural cells remains to be studied. Although the physiological and also possible pathological significance of PDGF-CC and PDGF-DD is unclear, however, at least PDGF-DD could possibly have a role in pericyte migration during angiogenesis,20 and perhaps both PDGF-CC and PDGF-DD could be involved in the intimal SMC accumulation in arteriosclerosis. Such a possibility deserves careful further analysis of these novel genes.
| Acknowledgments |
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Received August 3, 2000; revision received February 8, 2001; accepted February 15, 2001.
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