Insulin signaling regulates cardiac titin properties in heart development and diabetic cardiomyopathy.
Isoform-switching of the giant elastic protein titin is a main mechanism for adjusting passive myocardial stiffness in perinatal heart development and chronic heart disease. Previous evidence suggested that thyroid-hormone signaling converging onto the phosphoinositol-3-kinase (PI3K)/AKT pathway regulates titin-isoform composition in developing cardiomyocytes. Here we hypothesized that insulin, another activator of PI3K/AKT, alters titin-isoform composition and titin-based stiffness. We also checked for insulin-induced changes in titin phosphorylation. In embryonic rat cardiomyocytes cultured in the presence of insulin for 7 days, the mean proportion of the stiff N2B-titin isoform (M(w), 3000 kDa) significantly increased from 53% in controls to 65% in insulin-treated cells, the remainder being the more compliant N2BA-isoforms (M(w), 3200-3700 kDa). This insulin-dependent titin-isoform shift was blocked by PI3K-inhibitor, LY294002, suggesting that insulin controls the cardiac titin-isoform pattern by activating PI3K/AKT. Titin phosphorylation was substantially increased in insulin-treated compared to control cardiomyocytes. The impact of insulin-deficiency in vivo on titin-isoform expression, titin phosphorylation, and passive myocardial stiffness was studied in streptozotocin-treated (STZ) rats as a model of diabetes mellitus type-1. Within 5 months, STZ rats developed cardiac hypertrophy and mild left ventricular fibrosis, concomitant with elevated glucose levels. The mean proportion of N2B-titin was slightly but significantly decreased from 86% in controls to 79% in STZ hearts. Titin phosphorylation levels remained unchanged. Mechanical measurements on skinned cardiac fibers showed only minor passive stiffness modifications in STZ myocardium. We conclude that insulin signaling regulates titin-isoform composition and titin phosphorylation in embryonic cardiomyocytes and could also contribute to altered diastolic function in diabetic cardiomyopathy.
Krüger
Martina
M
Dept of Cardiovascular Physiology, Ruhr University Bochum, D-44780 Bochum, Germany. martina.krueger@rub.de
Babicz
Kamila
K
von Frieling-Salewsky
Marion
M
Linke
Wolfgang A
WA
D000818:Animals; D009202:Cardiomyopathies / Q000378:metabolism*; D002478:Cells, Cultured; D064211:Connectin; D003921:Diabetes Mellitus, Experimental / Q000150:complications* / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D006321:Heart / Q000502:physiology / Q000503:physiopathology*; D006333:Heart Failure / Q000378:metabolism / Q000503:physiopathology; D007328:Insulin / Q000378:metabolism / Q000494:pharmacology*; D009124:Muscle Proteins / Q000378:metabolism*; D009206:Myocardium / Q000378:metabolism; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D011247:Pregnancy; D011494:Protein Kinases / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*
D064211:Connectin; D007328:Insulin; D009124:Muscle Proteins; D011494:Protein Kinases