Epidermal growth factor (EGF) receptor is a membrane-spanning protein with a tyrosine kinases, and its activation is triggered by the binding of the ligand to the ectodomain. Overexpression or unregulated activation of the receptor is known to initiate a cancerous change in various cells, which have been observed in non-small cell lung carcinoma, colorectal carcinoma, etc. In addition, the EGF receptor, which has a property of internalization into cells, is focused as a useful target for the intracellular delivery of bioactive molecules. In development of new anti-cancer therapeutics, antimicrobial peptides (AMPs) have been used as bioactive molecules. Many positively charged AMPs induce cell death by non-selectively disrupting mitochondrial membranes, while they do not show toxicity outside cells. Therefore, selective delivery of AMPs into cancer cells is important for the development of new anti-cancer therapeutic disciplines.
Previously, we found a cyclic decapeptide, CQTPYYMNTC (1), which was captured into the cells and might have potential as an intracellular delivery molecule directed into the EGF receptor-positive cells [1]. In this study, we report that peptide 1 conjugated with a pro-apoptotic domain (PAD) peptide, ([KLAKLAK]2) [2], can function as an intracellular delivery molecule. The cationic PAD peptide has limitation in its use due to deficiency of cell membrane permeability and lack of specificity for cancer cells. The conjugated peptide 2, which was composed of peptide 1, the PAD peptide and a linker, was evaluated by treatment of an EGF receptor positive lung carcinoma cell line A549. According to the results of a cell viability assay, the conjugated peptide 2 suppressed the proliferation. The PAD peptide alone had no effect on the cells. This results suggested that peptide 1 might become a lead for a new intracellular delivery peptide.