FAK (focal adhesion kinase) has been shown to mediate the hypertrophic growth of the left ventricle. Experimental results also suggest that FAK may contribute to the structural and functional deterioration of the chronically overloaded left ventricle. In the present study, we postulated that FAK expression and phosphorylation may be altered in the volume-overloaded heart in humans. FAK expression and phosphorylation at Tyr(397) were detected by Western blotting and immunohistochemistry in samples from endomyocardial biopsies from patients with MR (mitral regurgitation; n=21) and donor subjects (n=4). Hearts from patients with MR had degenerated cardiac myocytes and areas of fibrosis. In this group, the myocardial collagen area was increased (18% in MR hearts compared with 3% in donor hearts respectively) and correlated negatively with left ventricular ejection fraction (r=-0.74; P>0.001). FAK expression and phosphorylation at Tyr(397) (a marker of the enzyme activity) were increased in samples from MR hearts compared with those from donor hearts (3.1- and 4.9-fold respectively). In myocardial samples from donor hearts, anti-FAK staining was almost exclusively restricted to cardiac myocytes; however, in myocardial samples from MR hearts, staining with the anti-FAK antibody was found to occur in myocytes and the interstitium. There was a positive correlation between collagen and the interstitial areas stained with the anti-FAK antibody (r=0.76; P>0.001). Anti-FAK and anti-vimentin staining of the interstitial areas of samples from MR hearts were extensively superimposed, indicating that most of the interstitial FAK was located in fibroblasts. In conclusion, FAK expression and phosphorylation are increased and may contribute to the underlying structural and functional abnormalities in the volume-overloaded heart in humans.
Background: A pathophysiological link exists between dysregulation of MEF2C transcription factors and heart failure (HF), but the underlying mechanisms remain elusive. Alternative splicing of MEF2C exons a, b and g provides transcript diversity with gene activation or repression functionalities.Methods: Neonatal and adult rat ventricular myocytes were used to overexpress MEF2C splicing variants g+ (repressor) or g-, or the inactive MEF2Cg+23/24 (K23T/R24L). Phenotypic alterations in cardiomyocytes were determined by confocal and electron microscopy, flow cytometry and DNA microarray. We used transgenic mice with cardiac-specific overexpression of MEF2Cg+ or MEF2CgÀ to explore the impact of MEF2C variants in cardiac phenotype. Samples of non-infarcted areas of the left ventricle from patients and mouse model of myocardial infarction were used to detect the expression of MEF2Cg+ in failing hearts. Findings: We demonstrate a previously unrealized upregulation of the transrepressor MEF2Cg+ isoform in human and mouse failing hearts. We show that adenovirus-mediated overexpression of MEF2Cg+ downregulates multiple MEF2-target genes, and drives incomplete cell-cycle reentry, partial dedifferentiation and apoptosis in the neonatal and adult rat. None of these changes was observed in cardiomyocytes overexpressing MEF2Cg-. Transgenic mice overexpressing MEF2Cg+, but not the MEF2Cg-, developed dilated cardiomyopathy, correlated to cell-cycle reentry and apoptosis of cardiomyocytes. Interpretation: Our results provide a mechanistic link between MEF2Cg+ and deleterious abnormalities in cardiomyocytes, supporting the notion that splicing dysregulation in MEF2C towards the selection of the MEF2Cg+ variant contributes to the pathogenesis of HF by promoting cardiomyocyte dropout.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.