FOXO3a regulates BNIP3 and modulates mitochondrial calcium, dynamics, and function in cardiac stress

The forkhead box O3a (FOXO3a) transcription factor has been shown to regulate glucose metabolism, muscle atrophy, and cell death in postmitotic cells. Its role in regulation of mitochondrial and myocardial function is not well studied. Based on previous work, we hypothesized that FOXO3a, through BCL2/adenovirus E1B 19-kDa protein-interacting protein 3 (BNIP3), modulates mitochondrial morphology and function in heart failure (HF). We modulated the FOXO3a-BNIP3 pathway in normal and phenylephrine (PE)-stressed adult cardiomyocytes (ACM) in vitro and developed a cardiotropic adeno-associated virus serotype 9 encoding dominant-negative FOXO3a (AAV9.dn-FX3a) for gene delivery in a rat model of HF with preserved ejection fraction (HFpEF). We found that FOXO3a upregulates BNIP3 expression in normal and PE-stressed ACM, with subsequent increases in mitochondrial Ca²⁺, leading to decreased mitochondrial membrane potential, mitochondrial fragmentation, and apoptosis. Whereas dn-FX3a attenuated the increase in BNIP3 expression and its consequences in PE-stressed ACM, AAV9.dn-FX3a delivery in an experimental model of HFpEF decreased BNIP3 expression, reversed adverse left ventricular remodeling, and improved left ventricular systolic and, particularly, diastolic function, with improvements in mitochondrial structure and function. Moreover, AAV9.dn-FX3a restored phospholamban phosphorylation at S16 and enhanced dynamin-related protein 1 phosphorylation at S637. Furthermore, FOXO3a upregulates maladaptive genes involved in mitochondrial apoptosis, autophagy, and cardiac atrophy. We conclude that FOXO3a activation in cardiac stress is maladaptive, in that it modulates Ca²⁺ cycling, Ca²⁺ homeostasis, and mitochondrial dynamics and function. Our results suggest an important role of FOXO3a in HF, making it an attractive potential therapeutic target.