Glycogen synthase kinase 3beta inhibits myocardin-dependent transcription and hypertrophy induction through site-specific phosphorylation.

Cardiomyocyte hypertrophy is transcriptionally controlled and inhibited by glycogen synthase kinase 3beta (GSK3beta). Myocardin is a muscle-specific transcription factor with yet unknown relation to hypertrophy. Therefore, we investigated whether myocardin is sufficient to induce cardiomyocyte hypertrophy and whether myocardin is regulated by GSK3beta through site-specific phosphorylation. Adenoviral myocardin overexpression induced cardiomyocyte hypertrophy in neonatal rat cardiomyocytes, with increased cell size, total protein amount, and transcription of atrial natriuretic factor (ANF). In vitro and in vivo (HEK 293 cells) kinase assays with synthetic peptides and full-length myocardin demonstrated that myocardin was a "primed" GSK3beta substrate, with serines 455 to 467 and 624 to 636 being the major GSK3beta phosphorylation sites. Myocardin-induced ANF transcription and increase in total protein amount were enhanced by GSK3beta blockade (10 mmol/L LiCl), indicating that GSK3beta inhibits myocardin. A GSK3beta phosphorylation-resistant myocardin mutant (8xA) activated ANF transcription twice as potently as wildtype myocardin under basal conditions with GSK3beta being active. Conversely, a GSK3beta phospho-mimetic myocardin mutant (8xD) was transcriptionally repressed after GSK3beta blockade, indicating that GSK3beta phosphorylation at the sites identified inhibits myocardin transcriptional activity. GAL4-myocardin fusion constructs demonstrated that GSK3beta phosphorylation reduced the intrinsic myocardin transcriptional activity. A cell-permeable (Antennapedia protein transduction tag) peptide containing the mapped myocardin GSK3beta motifs 624 to 636 induced hypertrophy of cultured cardiomyocytes, suggesting that the peptide acted as substrate-based GSK3beta inhibitor in cardiomyocytes. Therefore, we conclude that the GSK3beta-myocardin interaction constitutes a novel molecular control of cardiomyocyte hypertrophy. Phosphorylation by GSK3beta comprises a novel post-transcriptional regulatory mechanism of myocardin.