The non-myocyte cells of the healthy heart account for more than 60% of the cardiac cells, include cardiac fibroblasts (CFs) and endothelial cells (ECs), and are actively involved in the remodeling process. 110,111 Fibroblasts, which are responsible for the synthesis of ECM components, account Bicalutamide Androgen Receptor inhibitor for approximately 90% of the non-myocyte cell mass. 110,111 In the stressed

myocardium, fibroblasts differentiate into active myofibroblasts upon a wide range of stimuli (e.g. TGF-β). 89,110 These activated cells can regulate the secretion of ECM components and ECM degrading-enzymes (matrix metalloproteinases, MMPs) and tend to proliferate and migrate, acting to remodel the cardiac interstitium. 89 This process may result in cardiac fibrosis, a hallmark of pathological hypertrophy and HF, which presents with aberrant proliferation of CFs and excessive deposition of ECM proteins in the interstitium and perivascular regions of the myocardium, ultimately impairing

cardiac function. 89 Several lines of evidence indicate that dysregulation of miRNAs during HF occurs in CFs, besides CMCs, thereby contributing to the development of cardiac fibrosis. In particular, the increased miR-21 expression observed in human HF, 70 has been attributed mainly to fibroblasts using the TAC mouse model of HF. 84 Specifically, miR-21 is selectively upregulated in the fibroblasts of the failing heart and has been shown to target Spry1, a negative regulator of ERK-MAPK pathway, which functions to enhance growth factor secretion and fibroblast survival, thus promoting interstitial fibrosis. 84 MiR-21 was also found upregulated in CFs of the infarct zone after ischemia-reperfusion in mice, where it was shown to induce MMP2 (an ECM degrading enzyme) via direct targeting of PTEN, but its role in fibrosis was not further investigated in this model. 112

A more recent study by Liang et al revealed additional evidence supporting a role for mir-21 in fibrosis: miR-21 was upregulated in the border zone of murine hearts after MI, whereas the negative regulator of TGFβ, TGFβRIII, was underexpressed. Further experiments in CFs showed that mir-21 overexpression can enhance collagen production, in part through TGFβRIII suppression, and conversely TGFβRIII overexpression can inhibit mir-21 and reduce collagen production in CFs. 113 Taken together, these studies imply that Carfilzomib mir-21 upregulation under pathologic conditions in the myocardium may impair cardiac function by contributing to cardiac fibrosis. The miR-29 family has also been found deregulated in the failing heart and associated with the pathological mediator of fibrosis TGFβ. The members of the miR-29 family (miR-29a, b, c) are mainly expressed in the CFs of the murine heart and have been found downregulated in response to a variety of remodeling-inducing stresses (TAC, chronic calcineurin signaling, MI). In vitro experiments in cultured CFs showed that this reduction in miR-29 levels may be triggered upon TGFβ stimulus.