ABCB1 protects kidney proximal tubule cells against cadmium-induced apoptosis: roles of cadmium and ceramide transport.

Cadmium (Cd(2+)) damages the kidney proximal tubule (PT) by ceramide-dependent apoptosis and is also a class I carcinogen. Multidrug resistance P-glycoprotein (MDR1, ABCB1) confers resistance to Cd(2+) apoptosis, and it has been hypothesized that ABCB1 can directly transport Cd(2+) as a mode of cellular protection. Our aim was to investigate the role of ABCB1 in Cd(2+) transport and ceramide apoptosis. In rat PT or Madin-Darby canine kidney (MDCK) cells overexpressing ABCB1, ABCB1-dependent efflux of rhodamine 123(+) (Rh123(+)) or (109)Cd(2+) were determined, and cell death was assayed with MTT, H-33342 nuclear staining, and monolayer integrity by impedance sensing (Electric cell-substrate impedance sensing [ECIS]). ABCB1 inhibitors (PSC833, UIC-2 antibody) did not affect (109)Cd(2+) efflux in PT cells though Rh123(+) transport was blocked. Furthermore, increased ABCB1 expression did not augment (109)Cd(2+) efflux but attenuated apoptosis by 10-50μM Cd(2+) or 5-25μM C(6)-ceramide, which was abolished by PSC833 (1μM). ECIS measurements of ABCB1-MDCK monolayers exhibited similar effects. Moreover, in ABCB1-MDCK cells, Cd(2+)-induced ceramide formation, determined by a diacylglycerol kinase assay, was abolished and increased extrusion of nitro-2-1,3-benzoxadiazol-4-yl (NBD)-C(6)-ceramide, and NBD-C(6)-glucosylceramide was observed compared with MDCK cells. Whereas pharmacological block of sphingomyelin synthase (0.1mM D609) or sphingosine kinase (1μM dimethylsphingosine), which increase the levels of ceramide and its metabolites, augmented Cd(2+)-induced apoptosis, Cd(2+) apoptosis was significantly decreased not only by prevention of de novo ceramide synthesis (0.1μM fumonisin B(1)) but also by inhibition of glucosylceramide synthase (2μM C(9)DGJ). We therefore conclude that Cd(2+) efflux is not the mechanism behind ABCB1-mediated protection from Cd(2+) apoptosis. Rather, the sphingolipid glucosylceramide may be the proapoptotic substrate extruded by ABCB1.