Auxiliary subunits control biophysical properties and response to compound NS5806 of the Kv4 potassium channel complex.

Kv4 pore-forming subunits co-assemble with ß-subunits including KChIP2 and DPP6 and the resultant complexes conduct cardiac transient outward K+ current (Ito). Compound NS5806 has been shown to potentate Ito in canine cardiomyocytes; however, its effects on Ito in other species yet to be determined. We found that NS5806 inhibited native Ito in a concentration-dependent manner (0.1~30 µM) in both mouse ventricular cardiomyocytes and human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), but potentiated Ito in the canine cardiomyocytes. In HEK293 cells co-transfected with cloned Kv4.3 (or Kv4.2) and ß-subunit KChIP2, NS5806 significantly increased the peak current amplitude and slowed the inactivation. In contrast, NS5806 suppressed the current and accelerated inactivation of the channels when cells were co-transfected with Kv4.3 (or Kv4.2), KChIP2 and another ß-subunit, DPP6-L (long isoform). Western blot analysis showed that DPP6-L was dominantly expressed in both mouse ventricular myocardium and hiPSC-CMs, while it was almost undetectable in canine ventricular myocardium. In addition, low level of DPP6-S expression was found in canine heart, whereas levels of KChIP2 expression were comparable among all three species. siRNA knockdown of DPP6 antagonized the Ito inhibition by NS5806 in hiPSC-CMs. Molecular docking simulation suggested that DPP6-L may associate with KChIP2 subunits. Mutations of putative KChIP2-interacting residues of DPP6-L reversed the inhibitory effect of NS5806 into potentiation of the current. We conclude that a pharmacological modulator can elicit opposite regulatory effects on Kv4 channel complex among different species, depending on the presence of distinct ß-subunits. These findings provide novel insight into the molecular design and regulation of cardiac Ito. Since Ito is a potential therapeutic target for treatment of multiple cardiovascular diseases, our data will facilitate the development of new therapeutic Ito modulators.

Upregulation of phosphoinositide 3-kinase prevents sunitinib-induced cardiotoxicity in vitro and in vivo.

Sunitinib (SNT) is a multi-targeted receptor tyrosine kinase inhibitor that has been approved by the FDA for cancer therapy. However, its cardiotoxicity has limited the clinical applicability with no effective therapeutic approach available. As a broadband kinase inhibitor, the function of several kinases that are essential to cardiac function might also be affected by SNT, such as calmodulin-dependent protein kinase (CaMKII), cyclic-AMP-dependent protein kinases (PKA), AMP-activated protein kinase (AMPK), and phosphoinositide 3 kinase (PI3K). In this study, we investigated whether SNT-induced cardiotoxicity could be prevented by blocking SNT-induced alteration in the corresponding signaling pathways. In human induced pluripotent stem cell-derived cardiomyocytes, SNT (0.5-20 µmol/L) inhibited contractility of cardiomyocytes in a concentration-dependent manner, and the inhibitory effect was prevented either by PIP3 (1 µmol/L) application or PI3K overexpression. On the contrary, the CaMKII inhibitor KN-93 (50 nmol/L), PKA inhibitor H89 (1 µmol/L), and AMPK activators metformin (2 mmol/L) and 5-aminoimidazole-4-carboxamide 1-b-D-ribofuranoside (2 mmol/L) presented negligible effects. Oral SNT administration (40 mg/kg/day) in mice progressively decreased the PI3K activity and cardiac function in 2 weeks with a significant decrease in the expression and activity of Cav1.2 and SERCA. Cardiac-specific PI3K overexpression through adeno-associated virus 9-mediated gene delivery in mice prevented SNT-induced reduction in cardiac function, calcium transient, calcium current, and Cav1.2 expression. In summary, our data indicate that increased PI3K activity is protective against SNT-induced calcium mishandling and contractile dysfunction. Cardiac-specific PI3K activation could be an effective therapeutic approach to treat SNT cardiotoxicity in patients with cancer.