Novel Anti-Cancer Peptides Comprising Three Amino Acids

Background: The NPxY motif common to all β integrin cytoplasmic domains forms part of a canonical recognition sequence for phosphotyrosine-binding domains which are protein modules present in a wide variety of signaling and cytoskeletal proteins. We have recently reported that a non-naturally occurring peptide, RSKAKNPLYR, derived from the β6 integrin cytoplasmic domain inhibits cancer cell growth in vitro and proposed that this may be due, at least in part, to the inhibition of c-Src activity [1]. In the present study we examined the role of the NPLY motif within RSKAKNPLYR in terms of its requirement for inhibition of cancer cell growth. Materials and Methods: The effects of peptide modifications to RSKAKNPLYR on in vitro proliferation of human cancer cell lines (colorectal HT29, prostate DU145, breast MCF-7 and ovarian A2780) were evaluated using the MTT cell growth assay. Passage of peptide across the plasma membrane was assessed by means of confocal microscopy using FITC-labelled peptide. The effect of peptide on kinase activity was assessed in cell-free in vitro kinase assays. Results: The NPLY motif within RSKAKNPLYR was found to be essential for the growth inhibitory effect of this peptide. However, modified forms of this peptide in which all amino acids except the charged residues arginine and lysine were replaced by single non-polar amino acids such as alanine or valine were equally effective at inhibiting cancer cell proliferation. Moreover, these peptides inhibited not only c-Src activity as seen for RSKAKNPLYR but also the activity of members of the PKB/Akt kinase family. Conclusion: Novel decapeptides comprising only three amino acids have anti-cancer effects without the requirement for an integrin-based NPLY motif. These peptides inhibit the activity of not only c-Src but also members of the Akt family of kinases and may be useful as potential anti-cancer agents when used either alone or in combination with compounds previously reported to inhibit c-Src kinase activity.