PP2 prevents isoproterenol stimulation of cardiac pacemaker activity.

Increasing evidence has demonstrated the potential risks of cardiac arrhythmias (such as prolonged QT interval) using tyrosine kinase inhibitors for cancer therapy. We report here that a widely used selective inhibitor of Src tyrosine kinases, PP2, can inhibit and prevent isoproterenol stimulation of cardiac pacemaker activity. In dissected rat sinus node, PP2 inhibited and prevented isoproterenol stimulation of spontaneous beating rate. In isolated sinus node myocytes, PP2 suppressed the hyperpolarization-activated "funny" current (If) by negatively shifting the activation curve and decelerating activation kinetics, associated with decreased cell surface expression and reduced tyrosine phosphorylation of hyperpolarization-activated cyclic nucleotide-modulated channel 4 (HCN4) channel proteins. In human embryonic kidney 293 cells overexpressing recombinant human HCN4 channels, PP2 reversed isoproterenol stimulation of HCN4 and inhibited HCN4-573x, a cAMP-insensitive human HCN4 mutant. Isoprotenrenol had little effects on HCN4-573x. These results demonstrated that inhibition of presumably tyrosine Src kinase activity in heart by PP2 decreased and prevented the potential β-adrenergic stimulation of cardiac pacemaker activity. These effects are mediated, at least partially, by a cAMP-independent attenuation of channel activity and cell surface expression of HCN4, the key channel protein that controls the heart rate.