Co-operative membrane disruption between cell-penetrating peptide and cargo: implications for the therapeutic use of the Bcl-2 converter peptide D-NuBCP-9-r8.

Delivering apoptosis inducing peptides to cells is an emerging area in cancer and molecular therapeutics. Here, we have identified an alternative mechanism of action for the proapoptotic chimeric peptide D-NuBCP-9-r8. Integral to D-NuBCP-9-r8 is the Nur-77-derived D-isoform sequence fsrslhsll that targets Bcl-2, and the cell-penetrating peptide (CPP) octaarginine (r8) that is required for intracellular delivery. We find that the N-terminal phenylalanine of fsrslhsll acts in synergy with the cell-penetrating moiety to enhance peptide uptake at low nontoxic levels and cause rapid membrane blebbing and cell necrosis at higher (IC(50)) concentrations. These effects were not observed when a single phenylalanine-alanine mutation was introduced at the N-terminus of D-NuBCP-9-r8. Using primary samples from chronic lymphocytic leukemia (CLL) patients and cancer cell lines, we show that NuBCP-9-r8 induced toxicity, via membrane disruption, is independent of Bcl-2 expression. Overall, this study demonstrates a new mechanism of action for this peptide and cautions its use as a highly specific entity for targeting Bcl-2. For delivery of therapeutic peptides the work emphasizes that key amino acids in cargo, located several residues away from the cell-penetrating sequence, can significantly influence their cellular uptake and mode of action.