Major pathways of polymyxin-induced apoptosis in rat kidney proximal tubular cells

Identifying the pathways involved in the apoptotic cell death that is associated with polymyxin-induced nephrotoxicity is crucial for the development of strategies to ameliorate this dose-limiting side effect and for the development of novel safer polymyxins. The primary aim of this study was to identify the major pathways which lead to polymyxin-induced apoptosis in cultured rat kidney proximal tubular cells (NRK-52E). Caspase-3,-8, and-9 were activated by polymyxin B treatment in a concentrationdependent manner. Concentration-and time-dependent expression of FasL and deformation of mitochondrial morphology were revealed following polymyxin B treatment. The proportion of cells with filamentous mitochondria (regular morphology) following an 8-h treatment with 1.0mM polymyxin B was 56.2 ? 9.7 (n=3). This was decreased to 30.7 ? 7.5 when the polymyxin B concentration was increased to 2.0 mM. The mitochondrial membrane potential (??m) decreased to 14.1 ? 2.9 in the cells treated with 1.0mMpolymyxin B for 24 h (n=3) compared to that in the untreated control group. Concomitantly, concentration-and time-dependent production of mitochondrial superoxide was also observed. This study is the first to have demonstrated that polymyxin-induced apoptosis is mediated through both the death receptor and mitochondrial pathways in cultured renal tubular cells. It provides key information not only for the amelioration of polymyxin-induced nephrotoxicity but also for the discovery of novel safer polymyxin-like antibiotics against Gram-negative superbugs