CCG-1423

Right ventricular (RV) fibrosis is associated with RV dysfunction in a variety of RV pressure-loading conditions in which RV mechanical stress is increased, but the underlying mechanisms driving RV fibrosis are incompletely understood. In pulmonary and cardiovascular diseases characterized by elevated mechanical stress and transforming growth factor-β1 signaling, myocardin-related transcription factor A (MRTF-A) is a mechanosensitive protein critical to driving myofibroblast transition and fibrosis. In this study, we investigated whether MRTF-A inhibition improves RV profibrotic remodeling and function in response to a pulmonary artery banding (PAB) model of RV pressure loading. Rats were assigned into either sham or PAB groups. MRTF-A inhibitor CCG-1423 was administered daily at 0.75 mg/kg in a subset of PAB animals. Echocardiography and pressure-volume hemodynamics were obtained at a terminal experiment 6 weeks later. RV myocardial samples were analyzed for fibrosis, cardiomyocyte hypertrophy, and profibrotic signaling. MRTF-A inhibition slightly reduced systolic dysfunction in PAB rats reflected by increased lateral tricuspid annulus peak systolic velocity, whereas diastolic function parameters were not significantly improved. RV remodeling was attenuated in PAB rats with MRTF-A inhibition, displaying reduced fibrosis. This was accompanied with a reduction in PAB-induced upregulation of Yes-associated protein (YAP) and its paralog transcriptional coactivator with PDZ-binding motif (TAZ). We also confirmed, using a second-generation MRTF-A inhibitor CCG-203971, that MRTF-A is critical in driving RV fibroblast expression of TAZ and markers of myofibroblast transition in response to transforming growth factor-β1 stress and RhoA activation. These studies identify RhoA, MRTF-A, and YAP/TAZ as interconnected regulators of profibrotic signaling in RV pressure loading and as potential targets to improve RV profibrotic remodeling.

Cardiac fibrosis is characterized by aberrant collagen deposition in the heart. Macrophage polarization or infiltration is the main reason to accelerate the collagen deposition. We attempted to investigate the involvement of MKL1 in macrophages during the development of cardiac fibrosis.

Cardiac fibrosis is induced by myocardial infarction (MI). The MKL1^f/f mice were crossed to the Lyz2-cre mice to generate macrophage conditional MKL1 knockout mice (MKL1^ΔMφ). In addition, macrophage conditional MKL1 overexpression mice (MKL1^Mϕ-OE) were constructed by crossing Lyz2-cre mice to MKL1^{ΔN200-Rosa26} mice.

MKL1 expression was significantly increased in macrophages of both ischemic cardiomyopathy (ICM) patients and mice induced to develop myocardial infarction. Deletion of MKL1 in macrophages improved the heart function after MI-induced cardiac fibrosis. Consistently, MKL1^Mϕ-OE mice displayed more severe cardiac fibrosis and worsened heart function than the control mice after MI. Moreover, administration of a small-molecule MKL1 inhibitor CCG-1423 also decreased the collagen deposition after MI.

Our data demonstrate that MKL1 in macrophages contributes to cardiac fibrosis pathogenesis and reinforce the notion that targeting MKL1 may yield effective antifibrotic therapeutics in the heart.