Author + information
- Received May 10, 2007
- Revision received December 18, 2007
- Accepted January 3, 2008
- Published online April 8, 2008.
- Marcin Bujak, MD,
- Hyuk Jung Kweon, MD,
- Khaled Chatila, MD,
- Na Li, PhD,
- George Taffet, MD and
- Nikolaos G. Frangogiannis, MD, FACC⁎ ()
- ↵⁎Reprint requests and correspondence:
Dr. Nikolaos G. Frangogiannis, Section of Cardiovascular Sciences, One Baylor Plaza BCM620, Baylor College of Medicine, Houston, Texas 77030.
Objectives The purpose of this study was to study aging-associated alterations in the inflammatory and reparative response after myocardial infarction (MI) and their involvement in adverse post-infarction remodeling of the senescent heart.
Background Advanced age is a predictor of death and ventricular dilation in patients with MI; however, the cellular mechanisms responsible for increased remodeling of the infarcted senescent heart remain poorly understood.
Methods Histomorphometric, molecular, and echocardiographic end points were compared between young and senescent mice undergoing reperfused infarction protocols. The response of young and senescent mouse cardiac fibroblasts to transforming growth factor (TGF)-β stimulation was examined.
Results Senescence was associated with decreased and delayed neutrophil and macrophage infiltration, markedly reduced cytokine and chemokine expression in the infarcted myocardium, and impaired phagocytosis of dead cardiomyocytes. Reduced inflammation in senescent mouse infarcts was followed by decreased myofibroblast density and markedly diminished collagen deposition in the scar. The healing defects in senescent animals were associated with enhanced dilative and hypertrophic remodeling and worse systolic dysfunction. Fibroblasts isolated from senescent mouse hearts showed a blunted response to TGF-β1.
Conclusions Although young mice exhibit a robust post-infarction inflammatory response and form dense collagenous scars, senescent mice show suppressed inflammation, delayed granulation tissue formation, and markedly reduced collagen deposition. These defects might contribute to adverse remodeling. These observations suggest that caution is necessary when attempting to therapeutically target the post-infarction inflammatory response in patients with reperfused MI. The injurious potential of inflammatory mediators might have been overstated, owing to extrapolation of experimental findings from young animals to older human patients.
Mortality due to coronary artery disease is known to increase progressively with age. Older age was associated with a higher risk of in-hospital and post-discharge mortality in the GISSI-2 (Gruppo Italiano per lo Studio della Sopravivenza nell’Infarcto Miocardico 2) trial (1) and was a predictor of death and left ventricular dilation in patients with acute myocardial infarction (MI) enrolled in the SAVE (Survival and Ventricular Enlargement) trial (2). The exponential age-related increase in infarction-related mortality rates was not explained by larger infarcts (1). Although both clinical and experimental studies have demonstrated the adverse effects of senescence on cardiac function and remodeling after MI (2,3), the mechanisms responsible for these effects remain poorly understood.
Post-infarction remodeling is closely intertwined with an inflammatory reaction that ultimately results in fibrous tissue deposition and formation of a scar. Inflammatory mediators regulate key cellular interactions in the infarct, modulating deposition and metabolism of extracellular matrix proteins in the wound. These actions have profound effects on the reparative response and ultimately determine the geometric characteristics of the infarcted ventricle by affecting the tensile strength of the scar (4–6). Both clinical studies and experimental investigations demonstrated aging-associated defects in inflammation and tissue repair. Cutaneous wounds heal more slowly in elderly patients as compared with younger patients (7,8) and show diminished expression of endothelial adhesion molecules (9). Furthermore, healing wounds in aged animals demonstrate a defective response to exogenously administered growth factors (10). We hypothesized that aging-associated alterations in inflammatory mediator expression and impaired responsiveness of senescent cells to growth factors might be important mechanisms responsible for defective infarct healing and adverse remodeling in elderly patients. With a mouse model of reperfused infarction, we compared the inflammatory and fibrotic response between young and old animals. Aging was associated with an attenuated post-infarction inflammatory response, delayed phagocytosis of dead cardiomyocytes, and markedly decreased collagen deposition in the infarct. These defects resulted in increased ventricular dilation and hypertrophy. Fibroblasts isolated from senescent animals exhibited a blunted response to transforming growth factor (TGF)-β stimulation, suggesting that impaired responsiveness to growth factors might mediate defective healing in senescent mouse infarcts.
Murine model of reperfused MI
Young (2 to 3 months of age) and senescent (>2 years of age) C57/BL6 mice underwent reperfused infarction protocols (11,12). A closed-chest mouse model of reperfused MI was used as previously described (12), to avoid the confounding effects of surgical trauma and inflammation, which might influence the baseline levels of chemokines and cytokines. The left anterior descending coronary artery was occluded for 1 h and then reperfused for 6 h to 7 days. At the end of the experiment, the heart was excised, fixed in zinc-formalin, and embedded in paraffin for histological studies or snap frozen for ribonucleic acid (RNA) isolation. Sham animals were prepared identically without undergoing coronary occlusion/reperfusion and were used for histological assessment of the heart (5 mice/group). Animals used for histology underwent 24-h, 72-h, and 7-day reperfusion protocols (8 animals/group). Mice used for RNA extraction underwent 6 h, 24 h, and 72 h of reperfusion (8 animals/group). Additional animals were used for perfusion-fixation after 7 days of reperfusion, in order to assess remodeling-associated parameters.
Immunohistochemistry and quantitative histology
Histological sections were stained immunohistochemically with the following antibodies: anti-α smooth muscle actin antibody (Sigma, St. Louis, Missouri), rat anti-mouse macrophage antibody Mac-2 (Cedarlane, Burlington, North Carolina), and rat anti-neutrophil antibody (Serotec, Raleigh, North Carolina) as previously described (13). Quantitative assessment of neutrophil and macrophage density was performed by counting the number of neutrophils and Mac-2-immunoreactive cells, respectively, in the infarcted area (13). Myofibroblasts were identified as extravascular alpha-smooth muscle actin positive cells and counted in the infarcted myocardium (5). The collagen network was identified with picrosirius red staining (13). The area of collagen staining in the infarcted area was quantitatively assessed with ImagePro software and expressed as the percentage of the area of the infarct.
Short-axis M-mode echocardiography was performed before instrumentation and after 7 days of reperfusion (young: n = 7, senescent: n = 8) with an 8 MHz probe (Sequoia C256, Acuson, Mountain View, California). The following parameters were measured as indicators of function and remodeling: left ventricular end-diastolic diameter (LVEDD), left ventricular end-systolic diameter (LVESD), and fractional shortening (FS) (FS = [LVEDD − LVESD] × 100/LVEDD). Left ventricular mass (LVM) was assessed with the following formula: LVM = 1.05 (septal thickness + LVEDD + posterior wall thickness)3 − LVEDD3 (14). The percent change in these parameters after infarction was quantitatively assessed with the following formulas: ΔLVEDD = (LVEDD7days − LVEDDpre) × 100/LVEDDpre, ΔLVESD = (LVESD7days − LVESDpre) × 100/LVESDpre, ΔFS = (Fspre − FS7days) × 100/FSpre, ΔLVM = (LVM7days − LVMpre) × 100/LVMpre. (“Pre” indicates values prior to instrumentation, whereas “7 days” indicates values after 7 days of reperfusion.)
Perfusion fixation and assessment of ventricular volumes
For assessment of post-infarction remodeling, infarcted hearts after 7 days of reperfusion were used for perfusion-fixation (young, n = 12; senescent, n = 7) as previously described (13). The left ventricular end-diastolic volume was assessed with ImagePro software with methods developed in our laboratory (13). The size of the infarct was expressed as a percentage of the left ventricular volume. Cardiomyocyte size was assessed by measuring the mean cardiomyocyte cross-sectional area for each sham and infarcted heart. Measurements of 50 randomly selected cardiomyocytes, located in the viable remodeling myocardium, were obtained. Only cardiomyocytes cut in cross section and with a central nucleus were included.
RNA extraction and ribonuclease protection assay
Messenger RNA (mRNA) expression levels of the chemokines macrophage inflammatory protein (MIP)-1α, MIP-1β, MIP-2, monocyte chemoattractant protein (MCP)-1, and interferon-γ-inducible protein (IP)-10; the cytokines tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, TGF-β1, TGF-β2, and TGF-β3; and the matricellular protein osteopontin were assessed in the infarcted heart with a ribonuclease protection assay kit (RiboQuant, BD Pharmingen, San Jose, California).
Isolation and stimulation of murine cardiac fibroblasts
Fibroblasts were isolated from young and senescent murine hearts, cultured as previously described (15), and stimulated with recombinant TGF-β1 (10 ng/nl) (R&D Systems, San Diego, California) for 4 to 16 h. At the end of the experiment, the cell lysates were used for protein extraction. Quantitative assessment of Smad2 phosphorylation was used as a measure of activation of Smad2/3 signaling, a pathway with a critical role in mediating the pro-fibrotic effects of TGF-β. Western blotting with rabbit anti-Smad2 (1:1,000) and anti-phosphorylated Smad2 (p-Smad2) (Ser465/467, 1:200) antibodies (Cell Signaling, Beverly, Massachusetts) was carried out as previously described (5). The ratio of p-Smad2/Smad2 expression was quantitatively assessed.
Statistical analysis and comparison of histomorphometric, molecular, and echocardiographic parameters between young and senescent animals was performed with analysis of variance followed by t test corrected for multiple comparisons (Student-Newman-Keuls). Paired t test was used to compare echocardiographic parameters before MI and after 7 days of reperfusion. Comparison of mortality during surgical instrumentation and coronary occlusion between young and senescent animals was performed with Fishers exact test. Data were expressed as mean ± SEM. Statistical significance was set at 0.05.
Senescent mice demonstrated a trend toward increased mortality during coronary occlusion
Eighty-four young and 79 senescent mice were enrolled in the study. The mortality during surgical implantation was 4.8% for the young animals and 10.1% for the senescent mice (p = 0.24). Five young and 5 senescent mice did not undergo ischemia/reperfusion protocols and were used as sham control subjects. The remaining animals underwent 1-h coronary occlusion followed by varying intervals of reperfusion. There was a trend for higher mortality of senescent mice during coronary occlusion (young: 7.5% vs. senescent: 19.7%, p = 0.09). Mortality during reperfusion was 8.5% in young animals and 11.5% in senescent mice.
Senescent mice exhibited impaired phagocytosis of dead cardiomyocytes
In young animals MI resulted in almost complete clearance of dead cardiomyocytes and replacement with granulation tissue after 72 h of reperfusion (Fig. 1A). At the 72-h timepoint senescent mouse infarcts showed persistent presence of nonphagocytosed cardiomyocytes in the infarcted area accompanied by delayed formation of granulation tissue (Fig. 1B).
Senescent mice exhibited reduced neutrophil recruitment in the infarct
Young and senescent sham-operated mice showed no infiltration of the myocardium with neutrophils. Young mice had an intense early infiltration of the infarcted myocardium with neutrophils after 24 h of reperfusion, followed by resolution of the granulocytic infiltrate after 7 days of reperfusion (Figs. 1C to 1E). In comparison with young animals, infarcts in old mice had significantly reduced peak neutrophil density in the infarcted heart (Figs. 1F and 1I) but maintained timely resolution of the neutrophilic infiltrate (Figs. 1H and 1I) after 7 days of reperfusion.
Macrophage infiltration in young and senescent mouse infarcts
The density of Mac-2-immunoreactive macrophages in young mouse infarcts peaked after 72 h of reperfusion (Figs. 2A to 2C and 2G). In comparison with young animals, infarcts in old mice exhibited lower macrophage density after 24 to 72 h of reperfusion (Figs. 2D, 2E, and 2G) but had higher macrophage infiltration after 7 days of reperfusion (Figs. 2F and 2G). Macrophage infiltration in the peri-infarct area and remote remodeling myocardium after 7 days of reperfusion was comparable between young and senescent hearts (peri-infarct young: 57.6 ± 10.22 vs. senescent: 23.73 ± 4.8, p = NS; remote young: 27.2 ± 7.9 vs. senescent: 36.85 ± 7.8, p = NS).
Attenuated chemokine and cytokine induction in senescent mouse infarcts
In young mice, reperfused infarction results in marked chemokine upregulation in the heart peaking after 6 h of reperfusion (12). In comparison with young animals, old mice exhibited markedly reduced MCP-1 (MCP-1/L32 ratio, young 0.55 ± 0.046 vs. senescent 0.37 ± 0.025, p < 0.01), MIP-1β (MIP-1β/L32, young 0.042 ± 0.007 vs. senescent 0.017 ± 0.003, p < 0.05), and MIP-2 mRNA levels (MIP-2/L32, young 0.14 ± 0.003 vs. senescent 0.08 ± 0.01, p < 0.05) in the infarcted heart after 6 h of reperfusion. In contrast, IP-10 (IP-10/L32, young 0.08 ± 0.009 vs. senescent 0.08 ± 0.01, p = NS) and MIP-1α (MIP-1α/L32 0.04 ± 0.009 vs. senescent 0.041 ± 0.01, p = NS) mRNA levels were comparable between groups. At the same timepoint, mRNA expression of the pro-inflammatory cytokines IL-1β (IL-1β/L32, young 0.17 ± 0.03 vs. senescent 0.045 ± 0.007, p < 0.01), TNF-α (TNF-α/L32, young 0.024 ± 0.007 vs. senescent 0.016 ± 0.002, p < 0.05), IL-6 (IL-6/L32, young 0.21 ± 0.04 vs. senescent 0.09 ± 0.01, p < 0.01), and M-colony stimulating factor (CSF) (M-CSF/L32, young 0.3 ± 0.016 vs. senescent 0.2 ± 0.01, p < 0.05) was significantly lower in senescent mouse infarcts. In contrast, expression of the inhibitory cytokine IL-10 was slightly higher in senescent mouse hearts after 72 h of reperfusion (IL-10/L32, young 0.076 ± 0.008 vs. senescent 0.11 ± 0.01, p < 0.05).
Senescent mice showed decreased myofibroblast infiltration and diminished collagen deposition in the healing infarct
Senescent sham-operated hearts had increased collagen content in comparison with young sham hearts (young 4.89 ± 0.18% vs. senescent 7.34 ± 0.49%, p < 0.05), reflecting enhanced aging-associated cardiac fibrosis. Young mouse infarcts exhibited intense accumulation of myofibroblasts in the infarcted heart after 72 h of reperfusion, followed by formation of a collagen-based scar after 7 days of reperfusion. Senescent animals had defective scar formation associated with reduced myofibroblast density (Fig. 3) and markedly attenuated collagen deposition in the infarcted area (Figs. 4A to 4C). In contrast, collagen deposition in the peri-infarct region and in the remote remodeling myocardium was comparable between groups (peri-infarct young: 16.3 ± 1.66% vs. peri-infarct senescent 10.95 ± 1.2%, p = NS; remote young: 10.54 ± 1.65% vs. remote senescent 4.71 ± 0.67%). The mRNA expression of the matricellular protein osteopontin was also significantly diminished in senescent mouse infarcts (Fig. 4D). Because TGF-β is a key regulator of fibrous tissue deposition in healing tissues (16), we assessed expression of TGF-β isoforms in the infarcted heart. The TGF-β1 (Fig. 4E), TGF-β2, and TGF-β3 (Fig. 4F) mRNA levels were comparable in young and senescent mouse infarcts.
Enhanced post-infarction remodeling in senescent mouse hearts
In the absence of injury young and senescent mouse hearts exhibited comparable FS and LVEDD (Table 1), suggesting that aging in mice does not affect cardiac function and chamber dimensions. However, LVM and mean cardiomyocyte cross-sectional area (young sham: 153 ± 6.7 μm2 vs. senescent sham: 293.4 ± 20.4 μm2, p < 0.01) were significantly higher in senescent animals, indicating the development of aging-associated concentric hypertrophy (Table 1). Echocardiographic and quantitative morphometric studies (Table 1) demonstrated that senescent hearts exhibited enhanced dilative and hypertrophic remodeling and worse systolic dysfunction than young hearts after reperfused infarction. After 7 days of reperfusion senescent mice had markedly higher LVM, LVEDD, and left ventricular end-diastolic volume than young mice and showed significantly lower FS (Table 1). Although both young and senescent mouse hearts exhibited hypertrophic remodeling after 7 days of reperfusion, mean cardiomyocyte area was significantly higher in the remodeling myocardium of senescent infarcted hearts when compared with young animals (young 7 days: 270.5 ± 13.5 μm2 vs. senescent 7 days: 376.9 ± 21.37 μm2, p < 0.01) Enhanced remodeling and increased dysfunction in senescent animals was not due to more extensive infarction. No statistically significant difference in scar size was noted between young and senescent animals (Table 1).
Senescent mouse cardiac fibroblasts exhibited a blunted response to TGF-β
Upon stimulation with TGF-β1, cardiac fibroblasts isolated from young mice exhibited markedly increased p-Smad2 expression after 4-h to 16-h stimulation (Figs. 5A and 5B). In contrast, fibroblasts isolated from senescent mouse hearts had a blunted response to TGF-β1, showing no significant Smad2 phosphorylation after 16 h of stimulation (Figs. 5C and 5D).
Experimental studies have suggested that aging results in diverse and sometimes contradictory alterations in inflammatory mediator expression and release. Aged rats had increased constitutive expression of pro-inflammatory cytokines in the coronary arteries (17). In addition, mononuclear cells isolated from elderly subjects exhibited enhanced chemokine expression in response to lipopolysaccharide (18). In contrast, both human and mouse macrophages from senescent subjects had diminished toll-like receptor-1/2 surface expression (19,20) associated with significantly lower cytokine synthesis upon stimulation with known toll-like receptor ligands. Our findings demonstrated that in comparison with young animals, senescent mice exhibited decreased upregulation of chemokines and cytokines in the infarcted myocardium and reduced infiltration with neutrophils and macrophages. Attenuation of the inflammatory response in senescent mouse infarcts resulted in impaired phagocytosis of dead cardiomyocytes and delayed replacement with granulation tissue (Fig. 1). Decreased phagocytotic activity (21) and diminished oxidative response to activating signals (22) displayed by senescent macrophages and neutrophils might contribute to the impaired clearance of dead cardiomyocytes in the infarcted myocardium.
Senescence is associated with defective reparative fibrosis of the infarcted myocardium and markedly decreased collagen deposition in the scar
Suppressed inflammatory response in senescent mouse infarcts was followed by decreased myofibroblast infiltration and reduced expression of matricellular proteins in the senescent infarcted myocardium. In addition, collagen deposition was markedly attenuated in senescent mouse infarcts, resulting in formation of a scar containing loose connective tissue (Fig. 4). Surprisingly, mRNA expression of TGF-β isoforms, key regulators of the fibrotic response (16), was comparable in young and senescent mouse infarcts (Fig. 4), suggesting that reduced reparative fibrosis was not due to attenuation of TGF-β transcription in the infarcted heart. Because of the critical role of the Smad2/3 pathway in mediating fibrogenic TGF-β responses (23), we hypothesized that defective fibrous tissue deposition in senescent infarcted hearts might be due to an impaired response of aged mouse fibroblasts to growth-factor stimulation. Young mouse cardiac fibroblasts exhibited a robust increase in Smad2 phosphorylation after stimulation with TGF-β1. In contrast, fibroblasts isolated from senescent hearts showed a blunted response to TGF-β stimulation (Fig. 5), suggesting that aging results in impaired fibroblast responses to growth factors. These observations are consistent with previous experiments demonstrating that the stimulatory effect of angiotensin II on matrix synthesis is reduced in rat fibroblasts isolated from senescent hearts in comparison with fibroblasts harvested from young hearts (24).
Healing defects in senescent infarcted mice result in markedly enhanced adverse remodeling
Both echocardiographic and morphometric studies demonstrated that senescent mouse hearts exhibited markedly enhanced systolic dysfunction and increased dilative remodeling after infarction (Table 1). This was not associated with a statistically significant difference in infarct size, suggesting that enhanced remodeling in senescent hearts was due at least in part to alterations in the qualitative characteristics of the scar. The marked decrease in collagen deposition in senescent mouse infarcts might decrease the tensile strength of the wound, resulting in increased dilation and dysfunction. In addition, senescent mouse hearts exhibited markedly increased hypertrophic remodeling in comparison with young animals. These findings suggest that impaired responses of the senescent mouse myocardium to volume (25) and pressure overload (26,27) might result in persistent elevation of intracardiac pressures and wall stress, contributing to adverse remodeling of the ventricle.
Clinical implications of the findings
Our findings identify significant aging-associated healing defects that might explain the accentuated adverse remodeling in senescent hearts. Suppressed post-infarction inflammatory response results in delayed replacement of dead cardiomyocytes with granulation tissue. Impaired response of senescent fibroblasts to fibrogenic growth factors markedly decreases collagen deposition in the scar, resulting in decreased tensile strength and enhanced ventricular dilation. These defects have important implications in the design of novel therapeutic targets for the treatment of patients with MI. The findings suggest that aging-associated adverse remodeling of the infarcted ventricle is not due to enhanced inflammatory injury or increased fibrosis but rather results from a defective fibroblast response and impaired formation of the reparative matrix network, necessary to mechanically support the infarcted heart.
These observations suggest that caution is necessary when attempting to target the inflammatory cascade in patients with MI. Although extensive experimental evidence supported the effectiveness of anti-inflammatory strategies in animal models (28,29), the clinical experience with selected interventions targeting the inflammatory response in patients with acute MI has been disappointing (30). Animal studies are almost always performed in young adult animals, which exhibit a robust post-infarction inflammatory response and formation of dense collagenous scars in the infarcted heart. Although experiments in young animals provide valuable insight into the mechanisms involved in infarct healing, they might not accurately reflect the pathology of MI in middle-age or elderly human populations. Thus, the injurious potential of inflammatory mediators in patients with MI might have been overstated, owing to extrapolation from young animals to human patients. Our study indicates that aging is associated with decreased inflammation and attenuated collagen deposition in reperfused infarcts. In addition, senescent hearts show impairment of important cytokine pathways providing cardioprotection to the ischemic heart involving TNF-α (31) and platelet-derived growth factor-AB (32) and have a decreased anti-apoptotic response to administration of granulocyte-colony stimulating factor and stem cell factor (33) in comparison with young animals. Identification of suitable therapeutic targets for treatment of patients with MI is further complicated by species-specific differences in the inflammatory response between rodents and large mammals (12), which raise concerns about distinct responses between mice and humans to selected anti-inflammatory interventions.
Our study has an important limitation. We did not investigate the effects of aging on the response of the remote viable myocardium to overload after MI. Thus, the contribution of specific molecular and functional alterations involving viable cardiomyocytes of senescent animals in the pathogenesis of adverse post-infarction remodeling remains to be elucidated.
Understanding of aging-associated alterations in the cellular responses involved in cardiac repair and remodeling is critical for designing therapeutic strategies for patients with acute MI. Translation of experimental infarction research into therapeutic strategies for patients with acute MI requires extensive knowledge of the effects of senescence on inflammatory and reparative responses.
The authors wish to thank Dr. Mark Entman and the Hankamer Foundation for their continuous support.
This work was supported by National Institutes of Health Grants (R01 HL-76246, HL-85440) (to Dr. Frangogiannis) and the American Heart Association.
- Abbreviations and Acronyms
- colony stimulating factor
- fractional shortening
- interferon-γ-inducible protein
- left ventricular end-diastolic diameter
- left ventricular end-systolic diameter
- left ventricular mass
- monocyte chemoattractant protein
- myocardial infarction
- macrophage inflammatory protein
- messenger ribonucleic acid
- transforming growth factor
- tumor necrosis factor
- Received May 10, 2007.
- Revision received December 18, 2007.
- Accepted January 3, 2008.
- American College of Cardiology Foundation
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