Outside-in signalling of fibronectin stimulates cardiomyocyte hypertrophy in cultured neonatal rat ventricular myocytes.

Cardiac hypertrophy involves the accumulation of extracellular matrix proteins, such as fibronectin, leading to increasing myocardial stiffness, ventricular dysfunction and heart failure. To better understand the possible role of extracellular matrix-evoked intracellular signalling in ventricular myocytes, we investigated the effect of fibronectin on myocyte hypertrophic responses using cell culture models. Cell size in myocytes cultured on fibronectin-coated dishes was three times larger than that grown on non-coated dishes. However, the number of cells on fibronectin-coated dishes was not changed throughout the experiment. Protein synthesis was significantly increased by fibronectin, as were synthesis of atrial and brain natriuretic peptides. Fibronectin also elicited actin reorganization, co-localization of beta 1 integrin and vinculin, formation of focal adhesions and tyrosine phosphorylation of focal adhesion kinase in myocytes. These fibronectin-mediated effects were inhibited in a dose-dependent manner by GRGDSP, a competitive antagonist of the fibronectin receptors; GRGDSP had no effect on cell number or viability. Blocking antibody for beta 1 and beta 3 integrin significantly suppressed fibronectin-induced secretion of natriuretic peptides. Myocyte hypertrophy was observed in myocyte-nonmyocyte co-culture that reflects more closely the myocyte environment in vivo. GRGDSP may also suppress the myocyte hypertrophic response in the co-culture. These findings demonstrate that the interaction of fibronectin and RGD-dependent integrins is involved in the hypertrophic responses of myocyte in vitro, and suggest that extracellular matrix proteins such as fibronectin are not merely passive adhesive molecules but are active participants in processes leading to myocyte hypertrophy.