Oxidative Inactivation of the Lipid Phosphatase PTEN as a Novel Mechanism of Acquired Long QT Syndrome

The most common cause for cardiac side effects during therapy is drug-induced, acquired long QT syndrome due to direct blockade of the cardiac potassium channel hERG. However, antimony-based antileishmanial compounds and arsenic trioxide (As2O3), an anti-neoplastic drug, are pro-arrhythmic by an increase in cardiac calcium currents. In the present study we investigated the molecular mechanism(s) underlying the increase in calcium currents observed on incubation with As2O3. Calcium currents were recorded in guinea pig ventricular myocytes cultured overnight in the presence of As2O3. We found that arsenic-induced increases in calcium currents could be eliminated by co-incubation with the phosphoinositide 3-kinase (PI3K) inhibitor LY294002. Similarly, As2O3 effects could be abolished by incubation with wortmannin, a structurally different PI3K inhibitor. When isoform-specific PI3K activities were evaluated in cardiomyocytes, we found PI3Kα to be most active. However, PI3Kα activity was not altered on incubation with As2O3. In marked contrast, the lipid phosphatase PTEN which degrades PI3K-produced PIP3, was oxidized on incubation with As2O3 as indicated on Western blots by the appearance of a fast migrating, inactive protein form. In line with this observation, intracellular applied PIP3 was able to increase cardiac calcium currents in guinea pig ventricular myocytes. This effect was specific and could not be observed upon intracellular application of PIP2. Furthermore, As2O3-induced increases of calcium currents were not abolished by inhibition of PKB/Akt, an important downstream kinase activated by PIP3, but instead appeared to require PKC and c-Src function. Taken together, our experiments establish the lipid phosphatase PTEN as a crucial target for oxidative inactivation by As2O3. We propose that an altered gain in PIP3-dependent cell signaling is responsible for the pro-arrhythmic increase in cardiac calcium currents seen with anti-neoplastic As2O3 during therapy.