Cytosolic Dynamics of Annexin A6 Trigger Feedback Regulation of Hypertrophy via Atrial Natriuretic Peptide in Cardiomyocytes

Background: Altered annexin A6 expression is associated with several cardiovascular disorders, including myocyte hypertrophy, but precise functions of the protein remain elusive. Results: Dynamic association of annexin A6 with intracellular atrial natriuretic peptide precursor protects against adrenergic stimulation-induced hypertrophy of H9c2 cardiomyocytes. Conclusion: Annexin A6 negatively regulates hypertrophic progression of cardiomyocytes. Significance: Annexin A6 exhibits potential for antihypertrophic therapeutics. Malfunctions in regulatory pathways that control cell size are prominent in pathological cardiac hypertrophy. Here, we show annexin A6 (Anxa6) to be a crucial regulator of atrial natriuretic peptide (ANP)-mediated counterhypertrophic responses in cardiomyocytes. Adrenergic stimulation of H9c2 cardiomyocytes by phenylephrine (PE) increased the cell size with enhanced expression of biochemical markers of hypertrophy, concomitant with elevated expression and subcellular redistribution of Anxa6. Stable cell lines with controlled increase in Anxa6 levels were protected against PE-induced adverse changes, whereas Anxa6 knockdown augmented the hypertrophic responses. Strikingly, Anxa6 knockdown also abrogated PE-induced juxtanuclear accumulation of secretory granules (SG) containing ANP propeptides (pro-ANP), a signature of maladaptive hypertrophy having counteractive functions. Mechanistically, PE treatment prompted a dynamic association of Anxa6 with pro-ANP-SG, parallel to their participation in anterograde traffic, in an isoform-specific fashion. Moreover, Anxa6 mutants that failed to associate with pro-ANP hindered ANP-mediated protection against hypertrophy, which was rescued, at least partially, by WT Anxa6. Additionally, elevated intracellular calcium (Ca2+) stimulated Anxa6-pro-ANP colocalization and membrane association. It also rescued pro-ANP translocation in cells expressing an Anxa6 mutant (Anxa6(C)). Furthermore, stable overexpression of Anxa6(T356D), a mutant with superior flexibility, provided enhanced protection against PE, compared with WT, presumably due to enhanced membrane-binding capacity. Together, the present study delivers a cooperative mechanism where Anxa6 potentiates ANP-dependent counterhypertrophic responses in cardiomyocytes by facilitating regulated traffic of pro-ANP.