Efficient intracellular delivery of a pro-apoptotic peptide with a pH-responsive carrier

A key challenge in developing protein therapeutics or imaging agents that work against cytosolic targets is the intracellular delivery barrier. Here, we show that the pH-responsive, membrane-destabilizing polymer, poly(propylacrylic acid) (PPAA), can strongly enhance target cell killing through the intracellular delivery of a functional pro-apoptotic peptide. The Bak BH3 peptide induces apoptosis via antagonization of suppressor targets such as Bcl-2 and Bcl-xL. A genetically-engineered streptavidin that contains an N-terminal TAT peptide sequence was used to optimize the pinocytotic cell uptake of biotinylated BH3 peptide and end-biotinylated PPAA. Fluorescence microscopic analysis of DAPI-stained HeLa cells was used to quantitate apoptosis. Approximately 30% of cells treated with TAT-SA:BH3 complexes revealed morphologically distinct nuclear condensation, a hallmark of apoptosis. The incorporation of biotinylated PPAA had the effect of markedly enhancing the killing effect of BH3 peptides by an additional 55% (p < 0.001) to a total cell killing efficiency of 85%. Caspase-3 activity was up-regulated in a TAT-SA:BH3:PPAA dose-dependent manner. The induction of apoptosis with the TAT-SA:BH3:PPAA complex was abrogated with the L78A BH3 peptide, that had been previously shown to knock-out antagonization activity. The caspase and L78A peptide results demonstrate that the delivered BH3 is indeed working through the biologically relevant apoptosis signaling pathway. These studies establish the ability of PPAA to strongly enhance the intracellular delivery of a functional pro-apoptotic peptide. Together with the PPAA, the TAT-SA adaptor complex could prove useful as a carrier of peptide/protein cargo to cultured cells.