Abstract 271: Deletion of Delta-like 1 Homolog Accelerates Fibroblast-myofibroblast Differentiation and Induces Myocardial Fibrosis

Abstract

Myocardial fibrosis is associated with profound changes in ventricular architecture and geometry, resulting in diminished cardiac function. Here we uncover that Delta-like homologue 1 (Dlk1), a paternally imprinted gene encoding a transmembrane protein belonging to the Epidermal Growth Factor (EGF)-like family, orchestrates the process of cardiac fibroblast to myofibroblast differentiation and controls myocardial fibrosis. We first show that cardiomyocytes and cardiac fibroblasts express different Dlk1 mRNA spliced variants and its absence accelerates fibroblast differentiation into myofibroblasts in vitro. Overexpression of Dlk1 in cardiac fibroblasts resulted in inhibition of fibroblast proliferation and differentiation into myofibroblasts. This process appears to be regulated by TGFβ-1 signaling, since fibroblasts lacking Dlk1 exhibited a higher activation of the TGFβ-1/Smad-3 pathway at baseline, leading to an earlier acquisition of the myofibroblast phenotype. Dlk1-null mice myocardium displayed increased TGFβ-1/Smad3 profibrotic activity, resulting in infiltration/accumulation of myofibroblasts, and induction and deposition of the extracellular matrix fibronectin extra domain A isoform and collagen, supporting a role for Dlk1 in cardiac fibrosis. Furthermore, these profibrotic events were associated with reduced myofibril integrity, myocyte hypertrophy and cardiac dysfunction. Interestingly, Dlk1 expression was downregulated in ischemic heart tissue from human patients and in the border and scar-zones of infarcted pigs’ hearts. This phenotype was paralleled by increased expression of the profibrotic markers, collagen I, lysyl oxidase and α-smooth muscle actin. Mechanistically, the inhibitory action of Dlk1 on cardiac fibroblast-myofibroblast differentiation is mediated by miR-370 direct targeting of TGFβ-R2/Smad-3 signaling in the myocardium. Given the deleterious effects of continuous activation of this pathway, we propose Dlk1 as a new potential candidate for therapy in cases where aberrant TGFβ signaling leads to chronic fibrosis.