(Circulation. 1995;92:705-709.)
© 1995 American Heart Association, Inc.
Articles |
From the Department of Internal Medicine, Divisions of Cardiology and Dermatology (W.C.P.), Washington University School of Medicine, St Louis, Mo.
Correspondence to Jeff Rottman, MD, Cardiology Division, Jewish Hospital at Washington University Medical Center, 216 S Kingshighway, St Louis, MO 63110.
| Abstract |
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Methods and Results Hamster hearts were obtained for RNA isolation and analysis and in situ hybridization from two groups: normal control animals (n=4) and untreated cardiomyopathic hamsters (n=5). Osteopontin mRNA was 12-fold greater in cardiomyopathic hearts compared with normal controls (1.76±0.31 versus 0.14±0.04 arbitrary units normalized to GAPDH, mean±SEM, P<.05). In situ hybridization was used to define the origin of osteopontin in the heart. Osteopontin mRNA above background levels was not detected in sections from noncardiomyopathic hamster hearts but was readily detected in sections from cardiomyopathic hamsters, in which it originated in cells morphologically consistent with tissue macrophages.
Conclusions In the hamster, osteopontin is expressed in heritably cardiomyopathic hearts under conditions of chronic injury and repair, and the source of osteopontin message appears to be tissue macrophage-like cells in foci of inflammation. This model could be used to evaluate the biological role of osteopontin in myocardial inflammation and remodeling.
Key Words: osteopontin proteins cardiomyopathy RNA
| Introduction |
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Calcium-mediated necrosis of vascular tissue has been well documented in atherosclerosis. Osteopontin (OPN),a noncollagenous matrix protein, is highly expressed in atherosclerotic lesions.6 This finding has prompted investigation of the cardiovascular role of this protein, which is known to be central to the process of physiological and possibly pathological biomineralization in other tissues.7 OPN is a highly acidic secreted phosphoglycoprotein. Although it was first described in bone, subsequent studies have implicated it in the processes of cell adhesion, cell migration, and signal transduction in nonosseous tissues, including kidney,8 epithelial surfaces,9 and vessel wall.6 10 11 Many of the biological effects of OPN appear to be mediated by binding of a tripeptide arginine-glycine aspartate (RGD) sequence to cell-surface integrin receptors.11 12 13
Recently, OPN expression was examined after experimental cardiac injury in rats and in human myocardial infarction.14 OPN is synthesized by macrophages as part of a generalized reaction to tissue injury; however, levels of OPN in myocardial lesions decreased dramatically over a period of weeks as healing progressed.14 The expression of OPN in conditions of chronic cardiac injury has not been studied previously. We show that OPN expression is increased in Syrian hamster cardiomyopathy, which involves a continuing process of injury and repair. In situ hybridization localizes the sites of expression to foci of acute inflammatory infiltrates. These data suggest that the Syrian hamster cardiomyopathy may serve as a valuable model system in which to study the regulation and consequences of OPN expression in cardiac remodeling and repair.
| Methods |
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RNA Isolation and Analysis
Total RNA was prepared from frozen
tissue by the
guanidiniumacid phenol method (RNAzol B, Test-Test, Inc). Total RNA
(10 µg) was resolved on a 1% formaldehyde-agarose gel and then
transferred and cross-linked to MagnaGraph membranes (Micro Separations
Inc) according to the manufacturer's instructions. The OPN probe,
corresponding to the 1.1-kb BamHI fragment from the plasmid
pGEM4-2arcDNA (gift of Dr Marian F. Young, Bone Research Branch, NIH,
Bethesda, Md), was labeled with [32P]dCTP, random
hexamers, and the Klenow fragment of DNA polymerase I. This cDNA
fragment contains most of the coding sequence of the murine OPN gene.
The exact sequence of the hamster OPN gene is unknown, but comparison
of OPN cDNA nucleotide sequences from a number of other
species (including human, rat, mouse, cow, and chicken) demonstrated
high conservation within this fragment.15 Blots were
prehybridized and hybridized overnight at 42°C in 50% formamide, 5x
Denhardt's solution, 4x SSPE, 2.5 µg/mL herring testis DNA,
and 1%
SDS. The highest-stringency wash was at 50°C in 0.1x SSPE/1% SDS,
followed by autoradiography at -80°C overnight
with Kodak XAR 5 film. The relative intensities of the resultant bands
were quantified in their linear range by computer densitometry. To
assess potential variations in gel loading and transfer, the blots were
then reprobed for GAPDH mRNA under the same conditions.
Once the quality of the RNA and the specificity of the probe were confirmed on Northern blotting, a Minifold II slot-blot system (Schleicher & Schuell) was used to create multiple blots for purposes of quantification. Yeast tRNA was mixed with total sample RNA to produce a 4x range of sample concentrations. After the sample was applied to the slot-blot apparatus, hybridization, wash, and analysis were performed as described above. Data were compiled and analyzed by the Statistical Analysis System procedures (SAS Institute). ANOVA was used to test for significant differences among treatments, and means were separated with Tukey's studentized range test (P<.05).
In Situ Hybridization
The mouse OPN probe was a full-length
antisense transcript
prepared by linearizing pGEM4-2arcDNA (as above) at the Bgl
II site. A 500-nucleotide sense RNA probe transcribed from
a bovine tropoelastin cDNA was used as a control for nonspecific
hybridization. The validity of this probe as a negative control has
been confirmed previously.16 17 The riboprobes were
labeled with 32S-UTP (>1200 Ci/mmol; NEN) by use of
Promega reagents. The manufacturer's instructions were modified as
follows to increase the specific activity of the probes17 :
150 µCi of
-35S-UTP was dried under vacuum with gentle
centrifugation to increase the effective concentration.
Transcription reagents, 5 U of the appropriate RNA polymerase, and 1
µg of linearized DNA template were added to a reaction volume of 20
µL and incubated at 37°C for 1 hour. An additional 5 U of
polymerase was then added, and the incubation was continued for another
3 hours. These modifications typically resulted in a 5- to 10-fold
increase in the specific activity of the RNA probes. Specificity of the
mouse riboprobe was demonstrated with Northern blotting under
conditions similar to those used for in situ hybridization. Heart
samples were fixed in formalin and embedded in paraffin, and normal
bone was processed as a positive control. Five-micrometer
sections were treated with 1 µg/mL nuclease-free proteinase K (Sigma
Chemical Co), washed in freshly prepared 0.1 mol/L triethanolamine
buffer containing 0.25% acetic anhydride, and then covered with
hybridization buffer (50% formamide, 300 mmol/L NaCl, 30 mmol/L sodium
citrate, pH 7.0, 1x Denhardt's solution, 1 mmol/L EDTA, 10%
dextran
sulfate, 100 mmol/L DTT, 0.5 mg/mL yeast tRNA) containing
2.5x104 cpm/µL of 35S-labeled riboprobe.
After hybridization for 18 hours at 55°C, nonspecific background was
reduced by incubation with 20 µg/mL RNase A (Sigma) in 0.5 mol/L
NaCl, 10 mmol/L Tris, pH 8.0, and 1 mmol/L EDTA, followed by washes at
increasing stringency up to 0.1x SSC at 60°C. Slides were then
dipped in Kodak NTB-2 emulsion and processed for
autoradiography. After a 21-day exposure, the
emulsion was developed, and the slides were stained with
hematoxylin-eosin. Areas of calcification were demonstrated with von
Kossa or alizarin staining.18 To quantify calcification,
photomicrographs were digitized with a Javelin CCD camera (Javelin
Electronics) and a NuVista Framebuffer (Truevision, Inc) and
analyzed with NIH Image 1.47 as previously
described.19
| Results |
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A single band of
1.8 kb, the approximate size of the OPN mRNA
in human, rat, mouse, and cow,15 was consistently
observed in the RNA samples from cardiomyopathic hearts
(Fig 1a
, lane 2). A band of the same size was observed
with hamster RNA from kidney and bone, two tissues known to express
OPN, but not in RNA from spleen or liver, two nonexpressing tissues
(data not shown). A corresponding band in RNA prepared from a
nonmyopathic hamster heart was only faintly visible at the same
exposures (Fig 1a
, lane 1). Slot blotting was used to assess
differences in OPN mRNA levels more quantitatively.
Cardiomyopathic hamster hearts consistently
exhibited higher levels of OPN mRNA than did control hamster hearts:
when normalized to GAPDH, there was 12-fold more OPN mRNA in
cardiomyopathic hamster hearts than in the normal
control hearts (Fig 1b
, 1.76±0.31 versus
0.14±0.04 normalized units,
P<.05).
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We performed in situ hybridization to determine the distribution
of OPN mRNA in the heart. Signal for OPN mRNA was not detected (0 of 3)
in the hearts of noncardiomyopathic hamsters (Fig 2a
and
2b
). In contrast, OPN mRNA was readily detected
in all (4 of 4) sections from cardiomyopathic hamsters
(Fig 2c
and 2d
). Grains were localized
exclusively to regions of
inflammatory infiltrate within zones of fibrous scarring (Fig
3A
and 3C
). Not all areas of inflammatory
infiltrate
demonstrated OPN expression, but the areas that did were frequently
more hypercellular than those that did not (see Fig 3A
). Almost
all
areas of infiltrate included or abutted patches of calcification, and
more exuberant calcification was not reliably associated with either
the presence or absence of OPN mRNA signal (see Fig 3A
and
3B
).
Regression analysis disclosed no relation between the level of
OPN mRNA and the cross-sectional extent of calcification (data not
shown). The morphology of the positive cells (Fig 3D
and
3E
) was
consistent with that of tissue macrophages.
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| Discussion |
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As in the recent report by Murry et al,14 the source of OPN mRNA appears to be tissue macrophages in foci of inflammation. Considerable variability in the levels of OPN expression among individual cells was observed in these inflammatory foci, with expression most apparent in the especially hypercellular regions. These regions may represent areas of more recent injury, with expression of OPN constituting an early response, and OPN mRNA decreasing as the scar matures. OPN expression could also result in a chemotactic stimulus that generates the observed hypercellularity. The chemotactic properties of OPN have been demonstrated in other tissues.8 11 20 21
The heart comprises a complex and heterogeneous mixture of
cell types. Thus, to evaluate the significance of localized OPN
expression, it will be necessary to define the distribution of OPN
receptors, including the
vß3
integrin,11 22 23 in different cell
populations in the
heart. The existence of validated, high-resolution ultrasonic
techniques to study the material properties of the matrix in the Syrian
hamster cardiomyopathy model19 may
prove particularly useful in evaluating the microscopic structural
consequences of OPN expression and signaling. In ischemic
cardiomyopathies, OPN could also be expressed in
cells within coronary atherosclerotic
lesions.6 7 10 It is not known whether
OPN from this
source could also affect myocardial remodeling.
Although scar calcification was a prominent and nearly ubiquitous histological finding in the cardiomyopathic hamsters, we could not deduce a clear association between the extent of local calcification and the level of osteopontin expression. Because many areas with the histological appearance of acute inflammation also exhibited adjacent calcification, presumably representative of a more chronic response, repeated waves of acute injury and inflammation may occur at a single location. Tissue calcification may represent a temporally integrated response, whereas OPN expression may or may not occur when sampled at a single time point. The OPN protein may persist in the extracellular matrix in regions in which expression at the RNA level, as detected by in situ hybridization, is only transient. Further experiments may help in defining the temporal course of OPN expression and protein in specific cardiomyopathic lesions.
Received May 10, 1995; revision received June 21, 1995; accepted June 21, 1995.
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