Carfilzomib (Cfz), an irreversible proteasome inhibitor licensed for relapsed/refractory
myeloma, is associated with cardiotoxicity in humans. We sought to establish the optimal
protocol of Cfz-induced cardiac dysfunction, to investigate the underlying molecular-signaling
and, based on the findings, to evaluate the cardioprotective potency of metformin
(Met). Mice were randomized into protocols 1 and 2 (control and Cfz for 1 and 2 consecutive
days, respectively); protocols 3 and 4 (control and alternate doses of Cfz for 6 and
14 days, respectively); protocols 5A and 5B (control and Cfz, intermittent doses on
days 0, 1 [5A] and 0, 1, 7, and 8 [5B] for 13 days); protocols 6A and 6B (pharmacological
intervention; control, Cfz, Cfz+Met and Met for 2 and 6 days, respectively); and protocol
7 (bortezomib). Cfz was administered at 8 mg/kg (IP) and Met at 140 mg/kg (per os).
Cfz resulted in significant reduction of proteasomal activity in heart and peripheral
blood mononuclear cells in all protocols except protocols 5A and 5B. Echocardiography
demonstrated that Cfz led to a significant fractional shortening (FS) depression in
protocols 2 and 3, a borderline dysfunction in protocols 1 and 4, and had no detrimental
effect on protocols 5A and 5B. Molecular analysis revealed that Cfz inhibited AMPKα/mTORC1
pathways derived from increased PP2A activity in protocol 2, whereas it additionally
inhibited phosphatidylinositol 3-kinase/Akt/endothelial nitric oxide synthase pathway
in protocol 3. Coadministration of Met prevented Cfz-induced FS reduction and restored
AMPKα phosphorylation and autophagic signaling. Conclusively, Cfz decreased left ventricular
function through increased PP2A activity and inhibition of AMPKα and its downstream
autophagic targets, whereas Met represents a novel promising intervention against
Cfz-induced cardiotoxicity.