Background and Purpose

Doxorubicin (DOX) is a highly effective anthracycline, whose clinical application for cancer is limited by cardiotoxicity. The mechanisms underlying doxorubicin-induced toxic cardiomyopathy (DICM) involve electrophysiological remodelling with intracellular Na+ overload because of increased late INa and hyperactivation of CaMKIIδ. Increased [Na+]i contributes to CaMKIIδ activation through Na-dependent Ca2+ overload, and CaMKIIδ can further amplify late INa.
Experimental Approach

We tested whether pharmacological inhibition of the late INa by either ranolazine (RAN, 10 μmol·L−1) or empagliflozin (EMPA, 1 μmol·L−1) is sufficient to attenuate DOX-mediated hyperactivation of CaMKIIδ in isolated wildtype (WT) ventricular cardiomyocytes. The contribution of reciprocal CaMKII-dependent stimulation of late INa was tested in transgenic S571A cardiomyocytes lacking the CaMKII-specific phosphorylation site S571A on NaV1.5. Functional readouts were obtained using patch-clamp technique, as well as confocal and epifluorescence microscopy.
Key Results

DOX acutely increased late INa in WT cardiomyocytes by twofold, which was associated with redox- and phospho-dependent activation of CaMKIIδ. Hyperactivated CaMKIIδ led to acutely impaired Ca2+ handling because of diastolic Ca2+ loss from the sarcoplasmic reticulum (SR) mediated by phosphorylation of the RyR2 at the CaMKII-specific phosphorylation site serine-2814. Pharmacological inhibition of late INa by EMPA or RAN, and genetic deletion of the CaMKII-specific phosphorylation-site serine-571 at NaV1.5 prevented DOX-related stimulation of the late INa and subsequent CaMKIIδ hyperactivation, which functionally preserved intracellular Ca handling.
Conclusion and Implications

Inhibition of late INa protects cardiomyocytes from pathologic CaMKIIδ hyperactivation and impaired Ca2+ handling in the setting of acute DOX cardiotoxicity.