PKA phosphorylation of the small heat-shock protein Hsp20 enhances its cardioprotective effects

The small heat-shock protein Hsp20 (heat-shock protein 20), also known as HspB6, has been shown to protect against a number of pathophysiological cardiac processes, including hypertrophy and apoptosis. Following β-adrenergic stimulation and local increases in cAMP, Hsp20 is phosphorylated on Ser¹⁶ by PKA (protein kinase A). This covalent modification is required for many of its cardioprotective effects. Both Hsp20 expression levels and its phosphorylation on Ser¹⁶ are increased in ischaemic myocardium. Transgenic mouse models with cardiac-specific overexpression of Hsp20 that are subject to ischaemia/reperfusion show smaller myocardial infarcts, and improved recovery of contractile performance during the reperfusion phase, compared with wild-type mice. This has been attributed to Hsp20's ability to protect against cardiomyocyte necrosis and apoptosis. Phosphomimics of Hsp20 (S16D mutants) confer improved protection from β-agonist-induced apoptosis in the heart, whereas phospho-null mutants (S16A) provide no protection. Naturally occurring mutants of Hsp20 at position 20 (P20L substitution) are associated with markedly reduced Hsp20 phosphorylation at Ser¹⁶, and this lack of phosphorylation correlates with abrogation of Hsp20's cardioprotective effects. Therefore phosphorylation of Hsp20 at Ser¹⁶ by PKA is vital for the cardioprotective actions of this small heat-shock protein. Selective targeting of signalling elements that can enhance this modification represents an exciting new therapeutic avenue for the prevention and treatment of myocardial remodelling and ischaemic injury.