Abstract 1371: NCS-1 is a Potential Therapeutic Target for Avoiding Bradycardia and Optimizing Cancer Chemotherapy with Paclitaxel
We hypothesize that the inositol 1,4,5-trisphosphate receptor (InsP3R) signaling pathway is crucial for the development of cardiac arrhythmia caused by paclitaxel treatment. Paclitaxel, a well established drug used for cancer chemotherapy, constrains tumor growth through a microtubule stabilizing mechanism. However, use of paclitaxel is limited by serious side effects including bradycardia, which cannot be explained by its effect on microtubules. Recently, a new binding partner of paclitaxel was described - neuronal calcium sensor-1 (NCS-1), a protein that modulates the function of the InsP3R and that we found to be expressed in cardiomoycytes. After treating neonatal ventricular cardiomyocytes from rats with a therapeutic dose of paclitaxel for several hours, NCS-1 levels were markedly elevated. The same effect was also detectable in vivo, in hearts extracted from adult mice chronically injected with paclitaxel. Using live cell imaging, we observed an increase in the peak of the intracellular calcium signal and a significant decrease in the spontaneous calcium oscillation frequency in paclitaxel treated cardiomyocytes compared to control. The changes could not be explained by the function of ryanodine receptors (RyR) because there is no functional interaction between NCS-1 and RyR, as previously shown in bilayer experiments, and no paclitaxel-dependent change in RyR expression. However, co-immunoprecipitation confirmed a direct interaction between NCS-1 and InsP3R type 2, the predominant isoform in cardiomyocytes. To test whether this interaction is functionally relevant, we stimulated cardiomyocytes with endothelin, an activator of the InsP3 signaling cascade and found a difference in calcium transients between paclitaxel-treated and untreated cardiomyocytes. NCS-1 knockdown in cardiomyocytes by shRNA validated our findings. Taken together, our results suggest that paclitaxel-induced cardiac arrhythmia in patients is mediated by the interaction of NCS-1 and the InsP3R. This gives us a new understanding of the pathomechanism and provides a new approach for reducing the side effects of paclitaxel treatment and optimizing cancer chemotherapy.