The extracellular matrix proteoglycan fibromodulin is upregulated in clinical and experimental heart failure and affects cardiac remodeling.

Pressure overload of the heart leads to cardiac remodeling that may progress into heart failure, a common, morbid and mortal condition. Increased mechanistic insight into remodeling is instrumental for development of novel heart failure treatment. Cardiac remodeling comprises cardiomyocyte hypertrophic growth, extracellular matrix alterations including fibrosis, and inflammation. Fibromodulin is a small leucine-rich proteoglycan that regulates collagen fibrillogenesis. Fibromodulin is expressed in the cardiac extracellular matrix, however its role in the heart remains largely unknown. We investigated fibromodulin levels in myocardial biopsies from heart failure patients and mice, subjected fibromodulin knock-out (FMOD-KO) mice to pressure overload by aortic banding, and overexpressed fibromodulin in cultured cardiomyocytes and cardiac fibroblasts using adenovirus. Fibromodulin was 3-10-fold upregulated in hearts of heart failure patients and mice. Both cardiomyocytes and cardiac fibroblasts expressed fibromodulin, and its expression was increased by pro-inflammatory stimuli. Without stress, FMOD-KO mice showed no cardiac phenotype. Upon aortic banding, left ventricles of FMOD-KO mice developed mildly exacerbated hypertrophic remodeling compared to wild-type mice, with increased cardiomyocyte size and altered infiltration of leukocytes. There were no differences in mortality, left ventricle dilatation, dysfunction or expression of heart failure markers. Although collagen amount and cross-linking were comparable in FMOD-KO and wild-type, overexpression of fibromodulin in cardiac fibroblasts in vitro decreased their migratory capacity and expression of fibrosis-associated molecules, i.e. the collagen-cross linking enzyme lysyl oxidase, transglutaminase 2 and periostin. In conclusion, despite a robust fibromodulin upregulation in clinical and experimental heart failure, FMOD-KO mice showed a relatively mild hypertrophic phenotype. In cultured cardiac fibroblasts, fibromodulin has anti-fibrotic effects.