Marcin Bujak

Essential Role of Smad3 in Infarct Healing and in the Pathogenesis of Cardiac Remodeling

Background— Postinfarction cardiac repair is regulated through timely activation and repression of inflammatory pathways, followed by transition to fibrous tissue deposition and formation of a scar. The transforming growth factor-&bgr;/Smad3 pathway is activated in healing infarcts and may regulate cellular events critical for the inflammatory and the fibrotic responses. Methods and Results— We examined the effects of Smad3 gene disruption on infarct healing and the pathogenesis of cardiac remodeling. In the absence of injury, Smad3-null hearts had comparable function to and similar morphology as wild-type hearts. Smad3-null animals had suppressed peak chemokine expression and decreased neutrophil recruitment in the infarcted myocardium but showed timely repression of inflammatory gene synthesis and resolution of the inflammatory infiltrate. Although myofibroblast density was higher in Smad3-null infarcts, interstitial deposition of collagen and tenascin-C in the remodeling myocardium was markedly reduced. Compared with wild-type animals, Smad3−/− mice exhibited decreased dilative remodeling and attenuated diastolic dysfunction; however, infarct size was comparable between groups. Transforming growth factor-&bgr;-mediated induction of procollagen type III and tenascin-C in isolated cardiac fibroblasts was dependent on Smad3, which suggests that decreased fibrotic remodeling in infarcted Smad3-null hearts may be due to abrogation of the profibrotic transforming growth factor-&bgr; responses. Conclusions— Smad3 loss does not alter the time course of resolution of inflammation in healing infarcts, but it prevents interstitial fibrosis in the noninfarcted myocardium and attenuates cardiac remodeling. Thus, the Smad3 cascade may be a promising therapeutic target for the treatment of myocardial infarction.

Smad3 signaling critically regulates fibroblast phenotype and function in healing myocardial infarction

Rationale Cardiac fibroblasts are key effector cells in the pathogenesis of cardiac fibrosis. Transforming growth factor (TGF)-&bgr;/Smad3 signaling is activated in the border zone of healing infarcts and induces fibrotic remodeling of the infarcted ventricle contributing to the development of diastolic dysfunction. Objective The present study explores the mechanisms responsible for the fibrogenic effects of Smad3 by dissecting its role in modulating cardiac fibroblast phenotype and function. Methods and Results Smad3 null mice and corresponding wild-type controls underwent reperfused myocardial infarction protocols. Surprisingly, reduced collagen deposition in Smad3−/− infarcts was associated with increased infiltration with myofibroblasts. In vitro studies demonstrated that TGF-&bgr;1 inhibited murine cardiac fibroblast proliferation; these antiproliferative effects were mediated via Smad3. Smad3−/− fibroblasts were functionally defective, exhibiting impaired collagen lattice contraction when compared with wild-type cells. Decreased contractile function was associated with attenuated TGF-&bgr;–induced expression of &agr;-smooth muscle actin. In addition, Smad3−/− fibroblasts had decreased migratory activity on stimulation with serum, and exhibited attenuated TGF-&bgr;1–induced upregulation of extracellular matrix protein synthesis. Upregulation of connective tissue growth factor, an essential downstream mediator in TGF-&bgr;–induced fibrosis, was in part dependent on Smad3. Connective tissue growth factor stimulation enhanced extracellular matrix protein expression by cardiac fibroblasts in a Smad3-independent manner. Conclusions Disruption of Smad3 results in infiltration of the infarct with abundant hypofunctional fibroblasts that exhibit impaired myofibroblast transdifferentiation, reduced migratory potential, and suppressed expression of fibrosis-associated genes.

Interleukin-1 Receptor Type I Signaling Critically Regulates Infarct Healing and Cardiac Remodeling

The proinflammatory cytokine interleukin (IL)-1 signals exclusively through the type I IL-1 receptor (IL-1RI). IL-1 expression is markedly induced in the infarcted heart; however, its role in cardiac injury and repair remains controversial. We examined the effects of disrupted IL-1 signaling on infarct healing and cardiac remodeling using IL-1RI(-/-) mice. After reperfused infarction IL-1RI-null mice exhibited decreased infiltration of the infarcted myocardium with neutrophils and macrophages and reduced chemokine and cytokine expression. In the absence of IL-1 signaling, suppressed inflammation was followed by an attenuated fibrotic response. Infarcted IL-1RI(-/-) mice had decreased myofibroblast infiltration and reduced collagen deposition in the infarcted and remodeling myocardium. IL-1RI deficiency protected against the development of adverse remodeling; however, infarct size was comparable between groups suggesting that the beneficial effects of IL-1RI gene disruption were not attributable to decreased cardiomyocyte injury. Reduced chamber dilation in IL-1RI-null animals was associated with decreased collagen deposition and attenuated matrix metalloproteinase (MMP)-2 and MMP-3 expression in the peri-infarct area, suggesting decreased fibrotic remodeling of the noninfarcted heart. IL-1beta stimulated MMP mRNA synthesis in wild-type, but not in IL-1RI-null cardiac fibroblasts. In conclusion, IL-1 signaling is essential for activation of inflammatory and fibrogenic pathways in the healing infarct, playing an important role in the pathogenesis of remodeling after infarction. Thus, interventional therapeutics targeting the IL-1 system may have great benefits in myocardial infarction.

The role of IL-1 in the pathogenesis of heart disease

Interleukin (IL)-1 consists of two distinct ligands, IL-1α and IL-1β, with indistinguishable biological activities that signal through the IL-1 type I receptor (IL-1RI). A naturally occurring IL-1 receptor antagonist (IL-1Ra) binds to IL-1RI without initiating signal transduction and prevents IL-1 signaling, competitively inhibiting IL-1-mediated responses. Emerging evidence suggests that the balance between IL-1 agonists and antagonists plays an essential role in a variety of cardiovascular conditions. IL-1 may play a role in atherothrombotic disease by promoting the formation of atheromatous lesions, enhancing vascular inflammation, and triggering plaque destabilization. Following myocardial infarction, IL-1 critically regulates the inflammatory response and is involved in the development of adverse remodeling by enhancing expression of matrix metalloproteinases. IL-1 signaling may also be an essential mediator in the pathogenesis of heart failure by suppressing cardiac contractility, promoting myocardial hypertrophy, and inducing cardiomyocyte apoptosis. The present review summarizes current available data showing the significant role of IL-1 signaling in heart disease and raising the possibility that IL-1 inhibitors (such as anakinra, a nonglycosylated recombinant human IL-1Ra) may be clinically useful agents in patients with certain cardiovascular conditions.

CCR5 Signaling Suppresses Inflammation and Reduces Adverse Remodeling of the Infarcted Heart, Mediating Recruitment of Regulatory T Cells

Myocardial infarction triggers an inflammatory reaction that is involved in cardiac remodeling. Cardiac repair is dependent on regulatory mechanisms that suppress inflammation and prevent excessive matrix degradation. Chemokine induction in the infarcted heart mediates recruitment of leukocyte subsets with distinct properties. We demonstrate that signaling through the CC chemokine receptor 5 (CCR5) prevents uncontrolled postinfarction inflammation and protects from adverse remodeling by recruiting suppressive mononuclear cells. CCR5 and its ligands macrophage inflammatory protein (MIP)−1α and MIP-1β were markedly induced in the infarcted mouse myocardium. In addition, almost 40% of the mononuclear cells infiltrating the infarct expressed CCR5. CCR5−/− mice exhibited marked upregulation of proinflammatory cytokine and chemokine expression in the infarct. In wild-type infarcts CCR5+ mononuclear cells had anti-inflammatory properties, expressing higher levels of IL-10 than CCR5− cells. In contrast, mononuclear cells isolated from CCR5−/− infarcts had reduced IL-10 expression. Moreover, enhanced inflammation in the absence of CCR5 was associated with impaired recruitment of CD4+/foxp3+ regulatory T cells (Tregs). The CCR5+ Treg subset exhibited increased IL-10 expression, reflecting potent anti-inflammatory activity. Accentuated inflammation in CCR5−/− infarcts was associated with increased matrix metalloproteinase (MMP) expression, reduced TIMP levels, and enhanced MMP-2 and MMP-9 activity, resulting in worse cardiac dilation. These results suggest that CCR5-mediated Treg recruitment may restrain postinfarction inflammation, preventing excessive matrix degradation and attenuating adverse remodeling.

Aging-Related Defects Are Associated With Adverse Cardiac Remodeling in a Mouse Model of Reperfused Myocardial Infarction

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)-beta 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-beta1. 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.

Extracellular matrix remodeling in canine and mouse myocardial infarcts

Extracellular matrix proteins not only provide structural support, but also modulate cellular behavior by activating signaling pathways. Healing of myocardial infarcts is associated with dynamic changes in the composition of the extracellular matrix; these changes may play an important role in regulating cellular phenotype and gene expression. We examined the time course of extracellular matrix deposition in a canine and mouse model of reperfused infarction. In both models, myocardial infarction resulted in fragmentation and destruction of the cardiac extracellular matrix, extravasation of plasma proteins, such as fibrinogen and fibronectin, and formation of a fibrin-based provisional matrix providing the scaffold for the infiltration of granulation tissue cells. Lysis of the plasma-derived provisional matrix was followed by the formation of a cell-derived network of provisional matrix composed of cellular fibronectin, laminin, and hyaluronic acid and containing matricellular proteins, such as osteopontin and osteonectin/SPARC. Finally, collagen was deposited in the infarct, and the wound matured into a collagen-based scar with low cellular content. Although the canine and mouse infarcts exhibited a similar pattern of extracellular matrix deposition, deposition of the provisional matrix was more transient in the mouse infarct and was followed by earlier formation of a mature collagen-based scar after 7–14 days of reperfusion; at the same timepoint, the canine infarct was highly cellular and evolving. In addition, mature mouse infarcts showed limited collagen deposition and significant tissue loss leading to the formation of a thin scar. In contrast, dogs exhibited extensive collagen accumulation in the infarcted area. These species-specific differences in infarct wound healing should be taken into account when interpreting experimental infarction studies and when attempting to extrapolate the findings to the human pathological process.

CD44 Is Critically Involved in Infarct Healing by Regulating the Inflammatory and Fibrotic Response

Infarct healing is dependent on an inflammatory reaction that results in leukocyte infiltration and clearance of the wound from dead cells and matrix debris. However, optimal infarct healing requires timely activation of “stop signals” that suppress inflammatory mediator synthesis and mediate resolution of the inflammatory infiltrate, promoting formation of a scar. A growing body of evidence suggests that interactions involving the transmembrane receptor CD44 may play an important role in resolution of inflammation and migration of fibroblasts in injured tissues. We examined the role of CD44 signaling in infarct healing and cardiac remodeling using a mouse model of reperfused infarction. CD44 expression was markedly induced in the infarcted myocardium and was localized on infiltrating leukocytes, wound myofibroblasts, and vascular cells. In comparison with wild-type mice, CD44−/− animals showed enhanced and prolonged neutrophil and macrophage infiltration and increased expression of proinflammatory cytokines following myocardial infarction. In CD44null infarcts, the enhanced inflammatory phase was followed by decreased fibroblast infiltration, reduced collagen deposition, and diminished proliferative activity. Isolated CD44null cardiac fibroblasts had reduced proliferation upon stimulation with serum and decreased collagen synthesis in response to TGF-β in comparison to wild-type fibroblasts. The healing defects in CD44−/− mice were associated with enhanced dilative remodeling of the infarcted ventricle, without affecting the size of the infarct. Our findings suggest that CD44-mediated interactions are critically involved in infarct healing. CD44 signaling is important for resolution of the postinfarction inflammatory reaction and regulates fibroblast function.

Induction of the CXC Chemokine Interferon-γ–Inducible Protein 10 Regulates the Reparative Response Following Myocardial Infarction

Rationale: Interferon-&ggr;–inducible protein (IP)-10/CXCL10, an angiostatic and antifibrotic chemokine with an important role in T-cell trafficking, is markedly induced in myocardial infarcts, and may regulate the reparative response. Objective: To study the role of IP-10 in cardiac repair and remodeling. Methods and Results: We studied cardiac repair in IP-10–null and wild-type (WT) mice undergoing reperfused infarction protocols and examined the effects of IP-10 on cardiac fibroblast function. IP-10–deficient and WT animals had comparable acute infarct size. However, the absence of IP-10 resulted in a hypercellular early reparative response and delayed contraction of the scar. Infarcted IP-10−/− hearts exhibited accentuated early dilation, followed by rapid wall thinning during infarct maturation associated with systolic dysfunction. Although IP-10–null and WT mice had comparable cytokine expression, the absence of IP-10 was associated with marked alterations in the cellular content of the infarct. IP-10−/− infarcts had more intense infiltration with CD45+ leukocytes, Mac-2+ macrophages, and &agr;-smooth muscle actin (&agr;-SMA)+ myofibroblasts than WT infarcts but exhibited reduced recruitment of the subpopulations of leukocytes, T lymphocytes and &agr;-SMA+ cells that expressed CXCR3, the IP-10 receptor. IP-10 did not modulate cardiac fibroblast proliferation and apoptosis but significantly inhibited basic fibroblast growth factor–induced fibroblast migration. In addition, IP-10 enhanced growth factor–mediated wound contraction in fibroblast-populated collagen lattices. Conclusions: Endogenous IP-10 is an essential inhibitory signal that regulates the cellular composition of the healing infarct and promotes wound contraction, attenuating adverse remodeling. IP-10–mediated actions may be due, at least in part, to direct effects on fibroblast migration and function.

The Role of the Thrombospondins in Healing Myocardial Infarcts

The five current members of the thrombospondin (TSP) family can be divided in two subgroups according to their molecular architecture. TSP-1 and -2 (subgroup A) are trimeric matricellular proteins that do not contribute directly to tissue integrity, but influence cell function by modulating cell-matrix interactions, whereas TSP-3, -4 and -5 (subgroup B) are pentameric proteins. TSP-1 and TSP-2 are markedly induced in healing wounds and may regulate cellular responses important for tissue repair. TSP-1 is a crucial activator of TGF-beta, whereas both TSP-1 and TSP-2 inhibit angiogenesis. This manuscript reviews our current knowledge on the expression and role of the TSPs in healing myocardial infarcts. In both canine and murine infarcts, TSP-1 shows a strikingly selective localization in the infarct border zone. In the absence of injury, TSP-1 -/- mice exhibit normal cardiac morphology and show no evidence of myocardial inflammation. Infarcted TSP-1 -/- mice have an enhanced and protracted inflammatory response with subsequent expansion of granulation tissue in the non-infarcted area, resulting in myofibroblast infiltration into the viable myocardium neighboring the infarct. Infarcted TSP-1 -/- animals have enhanced left ventricular remodeling compared with their wildtype littermates. We suggest that TSP-1 is a critical component of the protective mechanisms induced in the infarct border zone in order to limit expansion of fibrosis into the non-infarcted myocardium. Localized TSP-1 expression may suppress expansion of the inflammatory process by activating TGF-beta or by inhibiting local angiogenesis. In addition, TSP-1-mediated inhibition of MMP activity may decrease adverse remodeling. TSP-2, on the other hand, appears to be a crucial regulator of the integrity of the cardiac matrix that is necessary for the myocardium to cope with increased loading. The expression and potential role of the pentameric TSPs in the infarcted heart remain unknown. Understanding the specific mechanisms responsible for the protective effects of TSP-1 and TSP-2 in healing infarcts may lead to novel therapeutic interventions aiming at attenuating adverse left ventricular remodeling.