F‐Box and Leucine‐Rich Repeat Protein 4 (FBXL4) Maintains Sarcomere Integrity and Cardiac Function by Enhancing K48‐Linked Ubiquitinated Degradation of Profilin‐1 (PFN1)

ABSTRACT Pathological cardiac hypertrophy is characterized by profound disruptions in protein turnover, a hallmark of maladaptive cardiac remodeling. This study aimed to elucidate the role and underlying molecular mechanisms of an FBP, F‐box and leucine‐rich repeat protein 4 (FBXL4), in pathological cardiac hypertrophy. Transcriptomic analysis of murine heart failure and human dilated cardiomyopathy samples revealed consistent downregulation of FBXL4. Similarly, FBXL4 expression was reduced in failing human hearts, hypertrophic mouse hearts, and angiotensin II (Ang II)‐treated neonatal mouse cardiomyocytes (NMCMs). Inducible ablation of FBXL4 in cardiomyocytes resulted in HF with reduced cardiac function, an enlarged heart chamber, increased fibrosis, and myofibrillar disorganization and sarcomere remodeling. Conversely, cardiac‐specific overexpression of FBXL4 attenuated pressure overload–induced hypertrophy. Mechanistically, FBXL4 interacts with PFN1 and promotes its K48‐linked ubiquitination at lysine 70, leading to its proteasomal degradation and the preservation of sarcomeric integrity. Restoration of FBXL4 expression via AAV9 delivery ameliorated cardiac hypertrophy and dysfunction in FBXL4‐iCKO mice, while AAV9‐mediated PFN1 knockdown or pharmacological inhibition partially reversed these phenotypes. Furthermore, the transcription factor SP1 was found to repress FBXL4 expression during hypertrophy. FBXL4 deficiency also induced hypertrophic features in hiPSC‐derived cardiomyocytes. Together, these findings establish FBXL4 as a key regulator of sarcomere integrity and cardiac function through ubiquitin‐mediated degradation of PFN1.