Melatonin Mitigates Doxorubicin‐Induced Cardiotoxicity by Targeting the USP18/PKM2 Axis Without Compromising Antitumor Efficacy

Doxorubicin (DOX) remains a widely used chemotherapeutic drug, yet its clinical value is constrained by pronounced cardiotoxicity. Although melatonin is known to protect the heart, how it counters DOX-induced cardiotoxicity (DIC), and whether it interferes with chemotherapy, remains unclear. Here we set out to define how melatonin protects the heart during DOX treatment and to test whether this protection holds in tumor-bearing animals. We used acute and chronic DIC mouse models together with a syngeneic melanoma-bearing model, and combined transcriptomic profiling, pharmacological intervention, cardiomyocyte-specific USP18 overexpression and knockout, co-immunoprecipitation, mass spectrometry, ubiquitination assays, and PKM2 inhibitor treatment. Melatonin markedly improved cardiac function, an effect exerted mainly via the MT1/2 receptors. Mechanistically, DOX activated STAT1, which in turn drove myocardial USP18 upregulation. USP18 bound PKM2 and stabilized it through removing K33-linked ubiquitin chains while favoring K27-linked ubiquitination-thereby aggravating cardiac injury. Overexpressing USP18 specifically in cardiomyocytes worsened DIC and abolished the protection conferred by melatonin, whereas deleting USP18 or inhibiting PKM2 reproduced melatonin's benefits. Notably, in bearing melanoma or lymphoma, melatonin relieved DIC without weakening the antitumor activity of DOX. Together, these findings define a previously unrecognized cardioprotective pathway and provide preclinical support for melatonin as a safe adjunct therapy in cardio-oncology.