Abstract C2: Aspartic acid residues drive the membrane translocation of pHLIP, a therapeutic and imaging agent for solid tumors

The pHLIP peptide is a new therapeutic and imaging agent for cancer treatment. The pHLIP (pH-Low-Insertion Peptide) is soluble in solution but interestingly, it is able to interact with the plasma membrane and insert as a monomeric transmembrane helix (pKa=6.0). The insertion requires an acidic extracellular environment, such as that found in most solid tumors and inflammation sites. The pHLIP, which is nontoxic in mice, is specifically distributed to tumors (with minor labeling of kidney), as imaged by PET and near-infrared fluorescence. The insertion of pHLIP is unidirectional, as the C-terminus is translocated across the membrane, while the N-terminus remains exposed to the extracellular medium. The insertion energy can be employed to translocate into the cytoplasm membrane-impermeable molecules. As a proof of principle, we showed that the polar toxin phalloidin (which binds to F-actin, inhibiting its depolymerization) is translocated in a pH-dependent fashion into HeLa, JC, and M4A4 cancer cells, inhibiting cell growth and causing cytoeskeletal immobilization and multinucleation. Here we study the molecular determinants of pHLIP membrane insertion. The pH-mediated translocation is thought to be triggered by the protonation (loss of negative charge) of carboxyl groups present in pHLIP. Accordingly, the four aspartic acid (Asp) residues of pHLIP were sequentially mutated, and the efficacy of the membrane insertion and exit was studied in a liposome system. We found a correlation between the number and location of Asp with the peptides ability in insert into and exit from the membrane in a pH-dependent manner. We also observed that the number of Asp in the peptide determines the insertion properties (pKa and the cooperativity), suggesting that the tumor labeling properties of pHLIP can be specifically tuned to better match the physiological acidity of solid tumors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr C2.