Cellular and Molecular Mechanisms of Cardiac Fibrosis

Cardiac fibrosis is a common pathway leading to chronic heart failure. Myofibroblasts are a key cell type responsible for cardiac fibrosis. However, the origin of these myofibroblasts remains controversial. In this thesis, we tested a hypothesis that bone marrow (BM)-derived macrophages might contribute to cardiac myofibroblast origin via a process of macrophage-myofibroblast transition (MMT), which is mediated by TGF-beta/Smad signaling. In addition, we also hypothesized that TGF-beta/Smad3 might play a role in cardiac fibrosis via a miR-29-dependent mechanism.;We first examined the origin and role of MMT cells during cardiac fibrosis in both human and animal model in Chapter III. The MMT cells were identified by co-expressing CD68+ (F4/80+) macrophage antigen and alpha-SMA+ myofibroblast phenotype using two-color confocal microscope and flow cytometry. Excitingly, more than 70% of MMT cells were found in patients with surgical removal of scar heart valve tissues. This was confirmed by a cell-tracing study in a mouse model of myocardiac infarction (MI) induced in GFP+ BM chimeric mice. Similarly, about 80% of BM-derived GFP+F4/80+ macrophages co-expressing alpha-SMA and these MMT cells were responsible for cardiac fibrosis since conditional deletion of macrophages in LysM-Cre/DTR mice inhibited MMT and cardiac fibrosis associated with improvement of cardiac dysfunction by increasing ejection fraction (EF).;We then examined the regulatory mechanisms of MMT in a mouse model of MI induced in GFP+ Smad3 WT and GFP+ Smad3 KO BM chimeric mice and in vitro in BM-derived macrophages lacking Smad3 (Chapter IV). Indeed, BM chimeric mice with macrophages lacking Smad3 (F4/80+GFP+Smad3 KO) were protected from MMT and cardiac fibrosis and prevented cardiac dysfunction when compared to those in GFP+Smad3 WT BM chimeric mice. Similarly, BM-derived macrophages lacking Smad3 were also prevented from TGF-beta-induced MMT in vitro, demonstrating a regulatory role for Smad3 in MMT and cardiac fibrosis.;Next, we explored a therapeutic potential for cardiac fibrosis by targeting the TGF-beta/Smad3 pathway using a Smad3 inhibitor (SIS3). As described in Chapter V, treatment with SIS3 prevented MMT and cardiac fibrosis in a mouse model of MI and in vitro in response to TGF-beta1, demonstrating that targeting Smad3 may represent a new therapeutic potential for cardiac fibrosis associated with MMT.;To further explore the molecular mechanisms by which TGF-beta/Smad3 mediates cardiac fibrosis, we studied the role and therapeutic potential of miR-29b, a recently-identified Smad3-dependent miRNA, during hypertensive cardiac remodeling. As described in Chapter VI, Ang II-induced hypertensive cardiac remodeling was associated with a loss of miR-29b, and ultrasound-microbubble-mediated over-expression of cardiac miR-29b was able to prevent and halt cardiac fibrosis in response to chronic Ang II infusion. Importantly, we also identified that miR-29b directly interacted with TGF-beta1 CDS to suppress TGF-beta1 expression, thereby blocking TGF-beta/Smad3-mediated cardiac fibrosis.;In conclusion, this thesis identified that BM-derived MMT is a new pathway of myofibroblast origin during cardiac fibrosis and is mediated by TGF-beta/Smad3 signaling. The ability of targeting TGF-beta/Smad3 with a specific Smad3 inhibitor (SIS3) or by over-expression of miR-29b to inhibit cardiac fibrosis may represent new therapeutic strategies for chronic cardiac disease.