Dependence of ABCB1 transporter expression and function on distinct sphingolipids generated by ceramide synthases-2 and -6 in chemoresistant renal cancer

Oncogenic multidrug resistance (MDR) is commonly intrinsic to renal cancer based on the physiological expression of detoxification transporters, particularly ABCB1, thus hampering chemotherapy. ABCB1 activity is directly dependent on its lipid microenvironment, localizing to cholesterol- and sphingomyelin-rich domains. As ceramides are the sole source for sphingomyelins, we hypothesized that ceramide synthase (CerS)-derived ceramides regulate ABCB1 activity. Using data from RNA-seq databases, we found patient kidney tumors exhibited increased CerS2 mRNA, which was inversely correlated with CerS6 mRNA in ABCB1+ clear cell carcinomas. Endogenous elevated CerS2 and lower CerS5/6 mRNA and protein resulted in disproportionately higher CerS2 to CerS5/6 activities (∼2-fold) in chemoresistant ABCB1high (A498, Caki-1) compared to chemosensitive ABCB1low (ACHN, HPCT) cells. In addition, lipidomics analyses by HPLC-MS/MS showed bias towards CerS2-associated C20:0/C20:1-ceramides compared to CerS5/6-associated C14:0/C16:0-ceramides (2:1). Sphingomyelins were similarly altered. We demonstrated that chemoresistance to doxorubicin in ABCB1high cells was partially reversed by inhibitors of de novo ceramide synthesis (L-cycloserine) and CerS (fumonisin B1) in cell viability assays. Additionally, downregulation of CerS2/6, but not CerS5, attenuated ABCB1 mRNA, protein, plasma membrane localization, rhodamine 123+ efflux transport activity, and doxorubicin resistance. Similar findings were observed with catalytically-inactive CerS6-H212A. Furthermore, CerS6-targeting siRNA shifted ceramide and sphingomyelin composition to ultra long-chain species (C22-C26). Inhibitors of ER-associated degradation (ERAD) (eeyarestatin I) and the proteasome (MG132, bortezomib) prevented ABCB1 loss induced by CerS2/6 downregulation. We conclude that a critical balance in ceramide/sphingomyelin species is prerequisite to ABCB1 expression and functionalization, which could be targeted to reverse MDR in renal cancers.