MLL4 protects cardiomyocytes against ischemia-reperfusion injury through STAT3-mediated mitochondrial function

Myocardial ischemia-reperfusion injury (MIRI) is an inevitable pathophysiological response during the revascularization process following myocardial ischemia. Despite its clinical significance, effective targeted therapies for MIRI remain an unmet medical need. Mixed-lineage leukemia 4 (MLL4), a member of the SET family of histone methyltransferases, exhibits particular methyltransferase action toward histone H3 lysine 4 (H3K4). This study establishes a protective role for MLL4 in MIRI pathogenesis. Utilizing cardiomyocyte-specific Mll4 knockout mice and an in vivo ischemia-reperfusion (I/R) model induced by left anterior descending coronary artery ligation, we observed significant upregulation of MLL4 expression in cardiac tissue following I/R. Genetic ablation of Mll4 in cardiomyocytes markedly exacerbated both acute and chronic phases of MIRI. In vitro, Mll4 knockdown in neonatal rat cardiomyocytes (NRCMs) amplified mitochondrial dysfunction and apoptosis under hypoxia/reoxygenation (H/R) conditions. Integrated analysis of Cleavage Under Targets and Tagmentation sequencing (CUT&Tag-seq) and RNA sequencing (RNA-seq) revealed that Mll4 deficiency induces a pronounced reduction in H3K4 monomethylation (H3K4me1) and histone H3 lysine 27 acetylation (H3K27ac) enrichment at the Stat3 genomic locus. Mechanistically, MLL4 functions as a transcriptional activator of Stat3 by depositing H3K4me1 and H3K27ac, thereby facilitating STAT3 transcription. This regulatory cascade ultimately governs STAT3-dependent mitochondrial homeostasis. Collectively, these findings identify MLL4 as a critical epigenetic regulator of MIRI and suggest its therapeutic targeting may offer a promising strategy for mitigating reperfusion injury.