Double impact on p‐glycoprotein by statins enhances doxorubicin cytotoxicity in human neuroblastoma cells

Abstract The development of multidrug resistance (MDR) is a major problem during cancer treatment. Drug efflux via ATP‐binding cassette (ABC) transporters is the main mechanism responsible for resistance to chemotherapeutics. We have recently observed that statins enhance susceptibility to doxorubicin‐induced apoptosis in human rhabdomyosarcoma cells, which is now also observed in human SH‐SY5Y neuroblastoma cells. We have therefore investigated the ABC transporter activity to confirm a possible inhibition by statins in SH‐SY5Y cells. Indeed, simvastatin directly inhibited dye transport at equimolar concentrations of the ABC transporter inhibitor, verapamil. Making use of the fluorescence behavior of doxorubicin the accumulation of anthracycline was monitored in real‐time confocal microscopy. The intracellular doxorubicin accumulation was immediately enhanced by statins in SH‐SY5Y cells and also in a MYCN ‐amplified neuroblastoma cell line STA‐NB‐10. The heavily glycosylated P‐glycoprotein (ABCB1, P‐gp) transporter appeared as a 180‐and 140‐kDa species. Atorvastatin and simvastatin reduced the 180‐kDa form of P‐gp, but not verapamil. Thereby the fully glycosylated species is shifted to the core glycosylated species (140 kDa), which was only seen at statin exposure times longer than 24 hr. The functional importance of glycosylation of the transporter was highlighted by exogenous application of N ‐glycosidase F, which was sufficient to enhance doxorubicin accumulation. Hence, these novel findings of statins' dual impact on P‐gp have clinical implications. The enhanced intracellular accumulation of chemotherapeutics or other ABC transporter substrates in the presence of statins may represent a novel concept to overcome MDR in cancer therapy and improve drug safety.