Abstract 1621: Phosphorylation of Small Heat Shock Protein 20 (Hsp20) at Serine 16 Modulates Cardiac Contractility
Background We recently reported that cardiac Hsp20 is phosphorylated by β-adrenergic receptor (β-AR) stimulation on Serine (Ser) 16, and the phosphorylated form exhibits increased potency towards cardioprotection from apoptosis, induced by prolonged β-AR stimulation. However, phosphorylation of Ser16 in Hsp20 has been also shown to enhance relaxation in smooth muscle. Thus, this study was designed to elucidate the functional role of Hsp20 and its phosphorylation in cardiac contractility.
Methods and Results Adult rat cardiomyocytes were infected with recombinant adenoviruses encoding the wild-type (WT) Hsp20 and either the constitutively phosphorylated S16D-Hsp20 (substitution of Ser16 with aspartic acid) or non-phosphorylated S16A-Hsp20 (replacing Ser16 with Alanine) mutants. Adenoviral GFP infected cardiomyocytes were used as control. Overexpression of WT-Hsp20 resulted in significant increases in fraction shortening (FS: 19±1%) and the maximal velocities of shortening (+dL/dt: 26±2%) and relengthening (−dL/dt: 29±2%), compared with GFP-infected cells. However, infection with S16D-Hsp20 was associated with further enhancement of FS (52±3%), +dL/dt (36±4%) and −dL/dt (43±3%), compared to GFP-infected cells. On the other hand, these parameters were dramatically decreased by 40±2% (FS), 35±3% (+dL/dt) and 34±3% (−dL/dt) in S16A-Hsp20 infected cells, compared to GPF controls. These findings were extended to in vivo models by generating transgenic mice with cardiac-specific overexpression of wild-type Hsp20 (10-fold) and S16A-Hsp20 (12-fold). Isolated cardiomyocytes from WT-Hsp20 transgenic mice revealed that FS, +dL/dt and −dL/dt were increased by 43±5%, 23±3% and 20±2%, respectively, compared to WT hearts. On the contrary, cardiomyocytes from S16A-Hsp20 hearts exhibited substantial decreases in FS (31± 6%), +dL/dt (67±6%) and −dL/dt (36±9 %), compared to WT cardiomyocytes.
Conclusion Overexpression of Hsp20 enhances cardiomyocyte contractility, which appears to be dependent on its cAMP-phosphorylation at Ser16. Thus, constitutively phosphorylated Hsp20 may represent an attractive therapeutic target for heart disease, providing both cardioprotection against apoptosis and enhancement of cardiac function.