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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
J Mol Cell Cardiol. Author manuscript; available in PMC 2012 July 5.
Published in final edited form as:
PMCID: PMC3390264

Loss of Fibulin-2 Protects Against Progressive Ventricular Dysfunction after Myocardial Infarction


Remodeling of the cardiac extracellular matrix (ECM) is an integral part of wound healing and ventricular adaptation after myocardial infarction (MI), but the underlying mechanisms remain incompletely understood. Fibulin-2 is an ECM protein upregulated during cardiac development and skin wound healing, yet mice lacking fibulin-2 do not display any identifiable phenotypic abnormalities. To investigate the effects of fibulin-2 deficiency on ECM remodeling after MI, we induced experimental MI by permanent coronary artery ligation in both fibulin-2 null and wild-type mice. Fibulin-2 expression was up-regulated at the infarct border zone of the wild-type mice. Acute myocardial tissue responses after MI, including inflammatory cell infiltration and ECM protein synthesis and deposition in the infarct border zone, were markedly attenuated in the fibulin-2 null mice. However, the fibulin-2 null mice had significantly better survival rate after MI compared to the wild-type mice as a result of less frequent cardiac rupture and preserved left ventricular function. Up-regulation of TGF-β signaling and ECM remodeling after MI were attenuated in both ischemic and non-ischemic myocardium of the fibulin-2 null mice compared to the wild type counterparts. Increase in TGF-β signaling in response to angiotensin II was also lessened in cardiac fibroblasts isolated from the fibulin-2 null mice. The studies provide the first evidence that absence of fibulin-2 results in decreased up-regulation of TGF-β signaling after MI and protects against ventricular dysfunction, suggesting that fibulin-2 may be a potential therapeutic target for attenuating the progression of ventricular remodeling.

Keywords: cardiac remodeling, wound healing, extracellular matrix, transforming growth factor-β

1. Introduction

Myocardial infarction (MI) is a major public health problem. The disease prognosis is dictated by the infarct healing process, which can be divided into three overlapping phases: the inflammatory, reparative and remodeling phases [1, 2]. The inflammatory phase occurs immediately after the ischemic myocardial insult, during which inflammatory cells infiltrate the infarcted area and clear the dead tissue. This is followed by the reparative phase with fibroblast proliferation, deposition of collagens and extracellular matrix (ECM) proteins and scar formation. The repair process introduces structural integrity to the damaged myocardium by replacing the dead myocardium with scar tissue. However, alterations in the signaling pathways after a MI, in conjunction with uncompensated mechanical overload, trigger ECM remodeling in the remote non-ischemic ventricular myocardium. This pathological ECM remodeling in the non-ischemic myocardium can lead to progressive functional deterioration, including myocardial fibrosis and ventricular dilatation, and ultimately heart failure or death if untreated. The ECM proteins that play essential roles in cardiac wound healing and ventricular remodeling after MI remain incompletely understood [3].

Fibulin-2 is a 180 kD ECM protein that can interact with a wide range of ECM proteins and be incorporated into various extracellular structures, including the elastin/fibrillin fiber system, fibronectin microfibrils, basement membranes, and proteoglycan aggregates [4]. During embryonic development, high levels of fibulin-2 are detected at sites of epithelial-mesenchymal transformation, including the endocardial cushion and coronary vessels in the heart [5]. In the postnatal period, fibulin-2 expression is down-regulated in most tissues, but remains prominent in the arterial perivascular adventitia and endothelial basement membrane of large- and medium-sized arteries as well as in cardiac valves. The expression of fibulin-2 in adult tissues is induced in pathologic conditions, such as during skin wound repair and in vascular lesions [6, 7]. These observations suggest that fibulin-2 plays significant roles in embryonic development and tissue remodeling.

To investigate the biological role of fibulin-2 in vivo, we have generated a mouse mutant deficient in fibulin-2 (Fbln2−/−) [8]. Surprisingly, the Fbln2−/− mice do not show any obvious phenotypic anomalies throughout their life spans. Here we study the role of fibulin-2 in tissue remodeling by subjecting the Fbln2−/− mice to experimentally induced MI. Unexpectedly, we find that loss of fibulin-2 significantly improves the survival of the mice after MI by attenuating ventricular dysfunction. We show that up-regulation of TGF-β signaling and ECM remodeling after MI are substantially decreased in the Fbln2−/− mice compared to the wild type (WT) animals. Our studies provide novel mechanistic insights into ventricular remodeling and suggest that inhibition of fibulin-2 expression may protect against the development of heart failure and death after MI.

2. Materials and Methods

2.1. Animals

Adult male Fbln2−/− and WT mice (12–20 weeks, weight 25–35 g) were used [8]. The Fbln2−/− mice were backcrossed with C57BL/6 mice over 10 generations. The age-matched C57BL/6 male mice were used as control mice. The investigation conforms to the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No, 85-23, revised 1996). All animal procedures were approved by the IACUC of Thomas Jefferson University and Alfred I. duPont Hospital for Children.

2.2. Experimental Myocardial Infarction (MI)

Non-reperfused experimental MI was performed by procedures described previously [9]. Briefly, under isoflurane anesthesia (2%), the heart was exposed via small left thoracotomy and suture ligation was placed at immediately distal to the bifurcation of the left anterior descending artery with 6.0 silk suture. Sham operation was performed with the suture insertion at the same site of left ventricular free wall without suture ligation.

2.3. Serum troponin-I (cTnI) assay and measurement of area at risk (AAR) and infarct area (Inf)

Blood sample was collected for the measurement of serum troponin-I (cTnI) levels at 24 hours after MI according to the manufacturer’s protocol (Life Diagnostic Inc., West Chester, PA). The area of ischemic myocardium (Area at Risk or AAR) and infarct myocardium (Inf) were measured after injection of Evans Blue into LV cavity as described elsewhere [9]. AAR was defined as the ischemic myocardium that was unperfused by Evans Blue.

2.4. Cardiac physiology

Echocardiogram was obtained before and 14 days after MI with a 30 MHz high-resolution probe (Vevo 770, VisualSonics, Tronto, Canada) under 2% isoflurane ansesthesia. M-mode measurement was obtained via parasternal short axis view. Cardiac catheterization was performed at 2 weeks following MI under intraperitoneal injection of 2.5% avertin (0.02 ml/g, Sigma). A 1.4 Fr Millar Catheter (Millar Instruments, Houston, TX) was inserted retrograde from the carotid artery to the LV cavity. LV pressure (LVP), LV end-diastolic pressure (LVEDP), first derivative of the LVP [(+)dP/dtmax and (−) dP/dtmax], and heart rate (HR) were obtained.

2.5. Northern blot and real-time RT-PCR

Total RNA from heart tissues was isolated using the Totally RNA kit (Ambion, Austin, TX). Northern blot analysis was performed as described previously [8]. Real-time was carried out Superscript III polymerase (Invitrogen, Carlsbad, CA) and the master SYBR Green 1 kit (Bio-Rad Laboratories, Inc., Hercules, CA) using specific primers (Supplement, Table 1S) in a MyiQ Single-Color Real-time PCR Detection System (Bio-Rad). The expression of each gene was determined as the relative expression or cycle threshold (Ct) of the gene of interest to the expression of GAPDH by the formula:

  • ΔCt = 2[(Ct GAPDH) − (Ct gene of interest)].

2.6. Immunoblot analysis

Myocardial tissues were homogenized with RIPA lysis buffer (Santa Cruz Biotechnology, Santa Cruz, CA) and protein concentration of the supernatants was determined by Pierce BCA Assays (Pierce Biotechnology, Rockford, IL). Equal amounts of protein (50 μg) were electrophoresed on a 14% SDS-polyacrylamide gel and electrophoretically transferred to a polyvinylidene difluoride membrane (Millipore, Billerica, MA). The membranes were incubated at 4°C with primary antibodies overnight. Antibodies used were phospho-Smad2, Smad2, phospho-TAK1, and TAK1 (Cell Signaling Technology, Danvers, MA), TGF-β1, β-Actin (Santa Cruz Biotechnology, Santa Cruz, CA), and fibulin-2 [10]. The blots were detected using Pierce ECL Western Blotting Substrate (Pierce Biotechnology) and exposed to Fuji x-ray Film. Signal intensities were analyzed with the NIH ImageJ software (version 1.38x).

2.7. Immunohistochemistry

The protocol for immunohistochemistry has been described elsewhere [5]. Primary antibodies used include fibulin-2 [10], collagens type I and III (Rockland, Gilbertsville, PA), α-SMA, vimentin (Sigma, St. Louis, MO), PECAM-1 (BD PharMigen, San Diego, CA), neutrophils (MCA771G )(AbD Serotec, Raleigh, NC), and macrophages (Mac-3) (BD Bioscience, Franklyn Lakes, NJ). Secondary antibodies used were Cy3 conjugated anti-rabbit antibody, Cy2 conjugated anti-rat antibody (Jackson ImmunoResearch, West Grove, PA), and FITC anti-mouse antibody (Vector Laboratories, Burlingame, CA). For all histology studies, we assessed 3 mice from each group. The histology section was obtained in a consistent fashion, i.e., the section was made perpendicular to the long axis at the level of suture. For quantification of fluorescent-positive cells, we manually counted the number of cells under 20× magnification in 3 random fields within the infarct border zone in one slide from each mouse by one designated observer and obtained a representative number of the group by averaging the average cell numbers in 3 different mice. The infarct border zone is defined as the transitional zone between true infarct and intact myocardium that is observed at the opposite end of the section where myocardial architecture and myocyte morphology are intact.

2.8. In situ hybridization

Non-radioactive mRNA in situ hybridization was conducted as described previously [11]. The sense and antisense riboprobes, corresponding to nucleotide position 1–578 of the fibulin-2 mRNA sequence [10], were labeled with Digoxigenin using T7 polymerase and reagents obtained from Roche Applied Science (Indianapolis, IN). In situ hybridization of cryosections was carried out with the GenPoint kit (DAKO, Carpinteria, CA), in which the positive signals were amplified with biotinyl-tyramide. Hybridization signals were detected by Fast Red Tris tablets (Sigma).

2.9. Gelatin Zymography

Gelatinase activity of MMPs was detected by zymography using methods described previously with some modifications [12]. Total protein from tissue homogenates (5 µg) were mixed with non-reducing Laemmli’s sample buffer and were electrophoresed on 10% SDS-polyacrylamide gels containing l mg/ml of gelatin. After electrophoresis, SDS was eluted and the gels were incubated at 37°C in incubation buffer to allow for degradation of the gelatin substrate at the sites of MMP protein bands. Gels were stained with 10% acetic acid containing 0.5% Coomassie blue and gelatinolytic activity was visualized as a clear band against a blue background.

2.10. Cell Culture

Cardiac fibroblasts (CFs) were isolated from ventricular myocardium after repeated digestion with collagenase IV (100U/ml)/trypsin (0.6mg/ml) (Invitrogen, Carlsbad, CA) solution at 37°C for 30 min. Cells were resuspended in DMEM with 20% FBS (Mediatech Inc., Herndon, VA), and 1% penicillin and streptomycin (Invitrogen) and plated on laminin-coated 60 mm dishes (BD Biosciences, Franklin Lakes, NJ). Weakly adherent cells were removed after incubation for 1 hour. CFs were seeded at 1×105 cells/ cm2 and used at the third passage. After 24 hours serum starvation, Ang II (1×10−7 M) was added to CFs in DMEM without serum and CFs were incubated for 24 hours.

2.11. Statistical Analysis

Data were compared using paired t-tests or ANOVA followed by Student-Newman-Keuls post-hoc test to assess the significance of data values (p < 0.05 is considered as significant). All data were expressed as Mean ± SEM unless otherwise noted.

3. Results

3.1. Fibulin-2 expression was induced in the infarct border zone after MI

We induced MI in WT mice to generate irreversible ischemic injury of the heart [9]. In the ischemic myocardium (AAR, area at risk), fibulin-2 mRNA level was slightly increased at 24 hours after MI but significantly up-regulated at 72 hours and 7 days (Figure 1A). The pattern of fibulin-2 gene expression after MI was very similar to that of α1(I) collagen and lysyl oxidase, two genes involved in collagen fibril formation. In situ hybridization revealed a robust increase in fibulin-2 mRNA in the infarct border zone but not within the infarcted region at 72 hours after MI (Figure 1B, a–b, d–e). Marked up-regulation of fibulin-2 protein is seen in the interstitial tissue of the infarct border zone (Figure 1B, c, f). In situ hybridization showed that fibulin-2 was exclusively synthesized by non-myocytes, which were significantly increased in number after MI. In contrast, fibulin-2 was barely detectable in the sham-operated hearts, and positive signals were restricted to the vascular endothelial basement membrane and perivascular adventitia of middle and large-sized vessels. These findings are consistent with our previous findings in fibulin-2 expression in non-myocytes during embryonic development in which fibulin-2 marks epithelial-mesenchymal transformation [5].

Figure 1
Fibulin-2 is upregulated in the infarct border after experimental MI. A. Northern blot analysis of total RNA (10 µg per lane) from the sham, ischemic myocardium (AAR) and remote non-ischemic myocardium (ANAR) of WT mice at 24 hours, 72 hours and ...

3.2. Loss of fibulin-2 improved the survival of mice after MI

We next induced MI in the Fbln2−/− mice and found that Fbln2−/− survived significantly better than the WT mice at 2 weeks after MI (Figure 2A). In a separate experiment, the survival was followed for 3 weeks after MI and the difference in the survival rate between the two genotypes was even more pronounced (WT: 33% survival, Fbln2−/−: 80%, n = 30 for each genotype). Serum creatine kinase (CK) levels at 24 hours after MI was similar in the Fbln2−/− and WT mice (Figure 2B). The ratio of the infarcted area (Inf) over entire left ventricle (LV) at 24 hours after MI was slightly higher in the Fbln2−/− than WT animals (Figure 2C). In both genotypes, cardiac rupture was observed in almost all mice that died within 5 days after MI but not thereafter (rupture incidence: WT: 17%; Fbln2−/−: 6.2%). Mice that died beyond 6 days after MI had significantly enlarged hearts with some fluid in the thorax, suggesting the presence of pulmonary edema, probably due to severe left ventricular dysfunction.

Figure 2
Survival and tissue injury of the WT and Fbln2−/− mice after MI. A. Survival (%) of the WT and Fbln2−/− mice in two weeks after MI. B. Serum Troponin I (cTnI) levels in the WT and Fbln2−/− mice at 24 hours ...

3.3. Loss of fibulin-2 attenuated acute wound healing after MI

To investigate the mechanisms underlying the improved survival of the fibulin-2 deficient mice, we examined the cardiac wound repair process in the infarct border zone at 72 hours after MI. In the WT mice, fibulin-2 protein was abundantly expressed in the infarct border zone, which was well-demarcated as shown by immunostaining with an antibody against vimentin (Figure 3A, a,c). By contrast, the infarct border in the Fbln2−/− myocardium was not clearly outlined by the vimentin staining, suggesting a delayed wound healing in the absence of fibulin-2 (Figure 3A, b and d). Deposition of collagens I and III (Figure 3B) and the numbers of neutrophils and macrophages (Figure 3C–D) in the ischemic myocardium were substantially reduced in the Fbln2−/− compared to WT mice. In the WT mice, fibulin-2 mRNA was markedly increased after MI (Figure 4A). Proinflammatory cytokines, TNF-α and IL-1β, were markedly up-regulated after MI in both WT and Fbln2−/−, but there was no significant difference between WT-MI and KO-MI (Figure 4A). Increases in mRNAs for TGF-β1, collagen III, MMP-2, MMP-9, TIMP-1 and TIMP-2 after MI were notably higher in the WT than Fbln2−/− mice. Collagen I up-regulation was not statistically different from WT and Fbln2−/− after MI (Figure 4B). Up-regulations of mRNAs for the matricellular proteins, periostin, thrombospondin-1 (Tsp-1), SPARC and tenascin-C after MI were also significantly reduced in the Fbln2−/− mice (Figure 4C). Immunoblotting showed that both TGF-β1 and phosphorylated Smad2 (pSmad2) proteins were substantially up-regulated in the WT mice after MI but not in the Fbln2−/− mice after MI (Figure 4D). In an acute phase, suppressed expression of pro-inflammatory cytokines and matricellular proteins may contribute to the better survival of the Fbln2−/− mice.

Figure 3
Immunohistochemical analysis of the ischemic myocardium in the WT and Fbln2−/− mice at 72 hours after MI. DAPI nuclear stain (blue) was used on each slide. A. Immunostaining of the infarct border zone with antibodies against fibulin-2 ...
Figure 4
Molecular analysis of the ischemic myocardium in the WT and Fbln2−/− mice at 72 hours after MI. A, B, C. Real-time RT-PCR analysis of the mRNA levels normalized to the level of GAPDH in sham (S) and ligated (MI) WT and Fbln2−/− ...

3.4. Left ventricular function was preserved in the fibulin-2−/− mice after MI

To elucidate the possible causes of the better survival of the Fbln2−/− mice, we performed echocardiography and cardiac catheterization at 2 weeks after MI. M-mode echocardiogram (Table 1) revealed that the WT and Fbln2−/− mice developed comparable degrees of LV hypertrophy and dilatation as indicated by LV wall thickness (LVPWd) and LV cavity size (LVIDd), respectively. It should be noted that there was a significant thinning of the LV posterior wall distal to the level where M-mode measurement was performed. Fbln2−/− mice showed significantly better LV systolic function than the WT mice, as indicated by % fractional shortening (%LVFS). Cardiac catheterization under avertin anesthesia revealed that WT mice displayed significantly higher LV end-diastolic pressure (LVEDP) than Fbln2−/− mice and that both (+)dP/dTmax and (−)dP/dTmax, indicators of LV systolic and diastolic function, respectively, were significantly better in Fbln2−/− than WT animals (Table 2). The left ventricle/body weight ratio was similar in the WT and Fbln2−/− mice, supporting the echocardiographic measurement of LV dimension. Together, the physiological studies suggested that the better survival of the Fbln2−/− mice was most likely attributed to preserved LV function of the Fbln2−/− mice after MI.

Table 1
M-mode Echocardiography at 2 weeks after MI.
Table 2
Anatomical and hemodynamic assessment after MI.

3.5. Fibulin-2 deficiency attenuated ECM remodeling and TGF-β signaling in the non-ischemic myocardium

To understand the difference in ventricular function of the two mouse groups, we studied the remote non-ischemic LV myocardium at 2 weeks after MI. Immunohistochemical analyses revealed that fibulin-2 protein expression was diffusely increased in the non-ischemic myocardial interstitial tissue of the WT mice after MI (Figure 5A). The increase was confirmed by immunoblotting (Figure 5A). TGF-β1 mRNA levels in the non-infarcted myocardium of both WT and Fblin2−/− mice were elevated to the same degree after MI (Figure 5B). However, up-regulation of mRNAs for collagen I, collagen III and MMP-2 was significantly lower in the Fblin2−/− than WT mice. Increase in the periostin mRNA after MI was somewhat lower in the Fbln2−/− mice. TIMP-1 and TIMP-2 mRNA levels were not significantly different between Fbln2−/− and WT mice. To determine whether TGF-β signaling is altered, the protein levels of pSmad2 and pTAK1, two independent downstream mediators of the TGF-β pathway, were evaluated by immunoblotting. While both pSmad2 and pTAK1 were significantly up-regulated in the WT mice relative to the sham group after MI, there was no such discernible increase in the Fbln2−/− mice after MI compared with the sham (Figure 5C). Consistent with the mRNA levels, abundance of MMP-2, both active and latent forms, was significantly higher in the WT than Fbln2−/− mice after MI (Figure 5D). The expression of periostin, a TGF-β responsive protein, was also higher in the WT than Fbln2−/− myocardium after MI (Figure 5D).

Figure 5
Analysis of non-ischemic myocardium of the WT and Fbln2−/− mice at 2 weeks after MI. A. Left: Immunohistochemistry showing increased fibulin-2 protein expression in WT myocardium after MI (WT-MI) compared to the sham-operated myocardium ...

3.6. Fibulin-2 modulated angiotensin II-induced TGF-β signaling in vitro

A relationship between fibulin-2 deficiency and TGF-β signaling was investigated by an independent in vitro experiment, in which an increase in TGF-β signaling and ECM remodeling induced by angiotensin II (Ang II) treatment was examined in cardiac fibroblasts (CFs) isolated from WT and Fbln2−/− ventricular myocardium (Figure 6A). When stimulated with recombinant Ang II for 24 hours, fibulin-2 mRNA expression in WT CFs showed over 5-fold increases compared to untreated cells. Up-regulation of mRNAs for collagen I, collagen III, TGF-β1 and MMP-2 by Ang II was significant in the WT CFs, whereas the increases were relatively modest or even abolished (for MMP-2) in the Fbln2−/− CFs. The abundance of MMP-2, both active and latent forms, was notably increased by Ang II in the WT CFs but not in Fbln2−/− CFs (Figure 6B). TGF-β1 protein present in the culture medium and the level of pSmad2 in the cell lysate were both significantly increased by Ang II in the WT but not Fbln2−/− CFs (Figure 6C–D).

Figure 6
Effects of Ang II on cardiac fibroblasts (CFs) from WT and Fbln2−/− ventricular myocardium. A. Real-time RT-PCR analysis of mRNA expression. Values were normalized to mRNA levels of GAPDH. Closed bars: Ang II-treated, open bars: control. ...

4. Discussion

Our studies provide the first evidence that fibulin-2 plays an important role in cardiac ECM remodeling after MI. We show that absence of fibulin-2 attenuated inflammatory responses and ECM synthesis in an acute phase after MI. However, loss of fibulin-2 protected against cardiac rupture and ventricular dysfunction, and significantly improved survival after MI. Importantly, we demonstrate that loss of fibulin-2 resulted in reduced up-regulation of TGF-β signaling and other ECM proteins after MI. TGF-β is a key cytokine known to have multiple and sometimes opposing effects on different phases of infarct healing [13]. Upregulation of fibulin-2 expression has previously been observed in the granulation tissue during skin wound healing [6]. An expression profiling study has also identified fibulin-2 as one of the most highly up-regulated ECM genes in the wound edge after skin injury [14]. Our results extend the previous findings in skin and demonstrate that fibulin-2 is markedly increased in the infarct border and actively participates in the acute cardiac wound repair process. Its absence is associated with decreased deposition of collagens I and III in the infarct border (Figure 3). Many ECM genes are up-regulated after MI and they play essential roles in the formation of granulation tissue and subsequent scar tissue to replace damaged myocardium. While some of the ECM proteins contribute to restore structural integrity of the myocardium, others, such as the matricellular proteins, do not contribute to structural integrity but rather assume functional roles to modulate cellular activity [1518]. Although fibulin-2 co-localizes with elastic and fibronectin fibers, it does not appear to have a specific role in maintaining structural integrity of tissues as mice lacking fibulin-2 have no apparent phenotypic abnormality [8]. Moreover, electron microscopic analysis indicates that collagen and elastic fibers in the fibulin-2 null mice have normal ultrastructure [8]. Here we show that fibulin-2 deficiency attenuates the increase in TGF-β signaling after MI, indicating that fibulin-2 plays a role in modulating cellular functions similar to the matricellular proteins.

An intriguing finding is that loss of fibulin-2 markedly ameliorated ventricular dysfunction and significantly improved survival of the mice after MI (Figure 2, Tables 1 and and2).2). This could be attributed, at least in part, to decreased TGF-β signaling in the non-ischemic myocardium after MI and the resultant reduction in matrix remodeling compared to the WT animals, as demonstrated by decreased up-regulation of mRNAs for collagen I, collagen III and MMP-2, and reduced activation of MMP-2 (Figure 5). Apoptotic cells, analyzed by the TUNEL assay and by immunoblot with antibodies against poly-ADP ribose polymerase (PARP) and cleaved caspase-3, were not detected in the non-infarcted myocardium in either WT or Fbln2−/− mice (data not shown). We suggest that preserved ventricular function in the absence of fibulin-2 could result from a combination of attenuated signaling pathways and subtle changes in the physical properties of the myocardium associated with reduced ECM remodeling. Our results are concordant with ample prior studies. Disruption of TGF-β signals through inactivation of Smad3, an intracellular mediator of TGF-β, can protect mice from developing diastolic dysfunction after MI [19]. The protective effect has been attributed to reduced ECM remodeling, a key pathogenic mechanism for adverse remodeling or cardiac fibrosis. Moreover, blunting TGF-β signaling results in decreased MMP-2 and MMP-9 activities, deletion of MMP-2 leads to attenuation of ventricular remodeling after MI, and pharmacological inhibition of MMP-2 prevents ventricular remodeling [20, 21]. Lastly, conditional activation of myocardial TAK1 results in progressive heart failure and premature death [22].

A relationship between fibulin-2 deficiency and TGF-β signaling is further supported by studies of the cardiac fibroblasts in vitro (Figure 6). Ang II is a principal neurohormone in promoting post-MI ventricular remodeling [23] and cardiac fibroblasts are the primary cells responsible for myocardial fibrosis induced by Ang II [24]. Our results indicate that fibulin-2 deficiency attenuates Ang II-induced ECM remodeling through hampered TGF-β1 signaling. The molecular mechanism by which fibulin-2 regulates TGF-β signaling is currently unknown, but likely to be complex due to the multiple binding interactions of fibulin-2 with other extracellular matrix molecules [25]. One plausible possibility is that fibulin-2 may modulate the storage and/or activation of the latent TGF-β complex in the extracellular milieu. Our recent in vitro study demonstrates that fibulin-2 binds to fibrillin-1 at a site that is very close to the region for the interaction of fibrillin-1 to the latent TGF-β binding proteins (LTBPs) [26]. The binding of fibrillin-1 and LTBPs is important for the extracellular sequestration and bioavailability of TGF-β [27]. Given that fibulin-2 localizes to fibrillin microfibrils in vivo [4], it is possible that fibulin-2 may affect the binding of the latent TGF-β complex to fibrillin microfibrils and thereby may regulate the amount of latent TGF-β complex stored in the extracellular matrix. Fibronectin matrix is another storage site for the latent TGF-β complex and known to regulate TGF-β activation [28]. We have shown previously that fibulin-2 interacts with fibronectin in vitro and localizes to fibronectin microfibrils in vivo [4]. It is noteworthy that active TGF-β can stimulate transcription of the fibulin-2 gene in fibroblasts [29], resulting in an increase in newly synthesized fibulin-2 in the infarct border zone. Thus, the effect of fibulin-2 on TGF-β signaling can be further amplified. In the absence of fibulin-2, the bioavailability or activation of TGF-β is attenuated, leading to reduced ECM remodeling.

A number of ECM proteins have been shown to play critical roles in infarct healing and ventricular remodeling after MI. These include osteopontin, periostin, thromobospondin-1 and -2, tenascin-C, osteonectin/SPARC and biglycan, many of which are matricellular proteins [18, 3032]. Like fibulin-2, these proteins are induced in the infarct border zone and elicit multiple effects on different phases of the healing process. Mice deficient in periostin, biglycan and SPARC all show decreased collagen deposition and reduced ventricular remodeling after MI, like the fibulin-2 null mice. However, these mouse mutants exhibit increased mortality after MI compared to normal mice due to cardiac rupture, in contrast to the observation in the fibulin-2 null mice [3032]. This is likely because collagen I fibrils, which are primarily responsible for the tensile strength of tissue, have altered diameter or shape in the periostin, biglycan and SPARC knockout mice [3234], whereas collagen fibrils in the fibulin-2 null mice are ultrastructurally normal [8]. Our studies demonstrate that absence of fibulin-2 decreases the rate of cardiac rupture and offers substantial protection against adverse ECM remodeling and ventricular dysfunction after MI, which suggests its unique contribution in the development of post-MI adverse remodeling.

In conclusion, we have found that fibulin-2, an ECM protein, played an important role in adverse tissue remodeling after MI and that total deletion of fibulin-2 resulted in significant improvement of survival by reducing incidence of cardiac rupture and attenuation of progressive LV dysfunction. Up-regulation of fibulin-2 appeared to participate in the pathological remodeling process after MI. Fibulin-2 was shown to increase Ang II-induced TGF-β signaling in cardiac fibroblast culture, suggesting its role in modulating TGF-β activation during tissue remodeling. This unique feature of fibulin-2 makes it an attractive potential therapeutic target for preventing progressive ventricular dysfunction after MI.

Supplementary Material



This work was supported by grants from National Institutes of Health (GM55625 to MLC and P20 RR020173 to TT) and Nemours Cardiac Center, Alfred I. duPont Hospital for Children (TT).

Non-standard Abbreviations

area at risk or ischemic myocardium
area not at risk or non-ischemic myocardium
Ang II
angiotensin II
extracellular matrix
infarcted area
left ventricle
myocardial infarction
TGF-β-activated kinase 1





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