Cationic cell-penetrating peptides (CPP) are receiving increasing attention as molecular transporters of membrane-impermeable molecules. Import of cationic CPP occurs both via endocytosis and - at higher peptide concentrations - in an endocytosis-independent manner via localized regions of the plasma membrane. At present, this endocytosis-independent import of cationic CPP is not well understood, but has been shown to be sensitive to various pharmacological inhibitors, suggesting a role of an unidentified enzymatic activity. Here, we demonstrate that the direct translocation of cationic CPP depends on a CPP-induced translocation of acid sphingomyelinase (ASMase) to the outer leaflet of the plasma membrane and ceramide formation. The involvement of ASMase in uptake was confirmed by a pharmacological inhibition of ASMase by imipramine and a subsequent rescue of uptake through external addition of sphingomyelinase, and by using ASMase-deficient cells. We also found that the threshold for direct CPP translocation can be lowered through addition of sphingomyelinase and that sphingomyelinase enhances the translocation of R9 coupled to low-molecular weight cargos, but not high-molecular weight cargos. In conclusion, we show that a previously poorly understood mechanism of cationic CPP import depends on the ASMase-dependent formation of ceramide on the outer leaflet of the plasma membrane. To our knowledge, this is the first illustration that a class of delivery vectors operates through the induction of an enzymatic activity that changes the lipid composition of the plasma membrane.
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