Unique membrane-interacting properties of the immunostimulatory cationic peptide KLKL(5)KLK (KLK)

Anticancer effect of rationally designed α-helical amphiphilic peptides

Anticancer peptides (ACPs) have attracted increasing attention in cancer therapy due to their unique mechanism of action on cancer cells. The main challenge is to establish the correlation between their physicochemical properties and their selectivity and anticancer effect, leading to a clear design strategy. In this study, a series of new α-helical short peptides (coded At1-At12) with different anticancer activities were systematically designed with different amphiphilicity based on a natural α-helical antimicrobial peptide (AMP) derived from ant. Three of the designed peptides, At7, At10 and At11, showed considerable anticancer activity with low toxicity to normal skin fibroblasts. The high selectivity of the peptides is attributed to their balanced amphiphilicity and cationic nature which favours binding to the outer membrane of negatively charged cancer cells over the neutral membrane of normal mammalian cells. In addition to rapid membrane penetration, the designed peptides also damaged the mitochondria and induced mitochondrial membrane depolarization. Moreover, these peptides were found to induce apoptosis in cancer cells by up-regulating the expression of apoptotic proteins Bax and Caspase-3, down-regulating the apoptotic protein Bcl-2, and activating the Caspase enzyme-linked reaction. The results of this study reveal the potential of these peptides for clinical applications, and provide a guidance for further development of highly selective anticancer medications.

The toxic alpha-gliadin peptide 31-43 enters cells without a surface membrane receptor

Alpha-gliadin peptide 31-43 is considered to be the main peptide responsible for the innate immune response in celiac disease patients. Recent evidence indicates that peptide 31-43 rapidly enters cells and interacts with the early endocytic vesicular compartment. However, the mechanism of its uptake is not completely understood. Our aim is to characterize, isolate and identify possible cell surface proteins involved in peptide 31-43 internalization by Caco-2 cells. In this study, we used a chemical cross-linker to block peptide 31-43 on cell surface proteins, and pulled-down peptide-proteins complexes using antibodies raised against peptide 31-43. Through this experimental approach, we did not observe any specific complex between cell proteins and peptide 31-43 in Coomassie-stained denaturating gels or by Western blotting. We also found that type 2 transglutaminase was not necessary for peptide 31-43 internalization, even though it had a regulatory role in the process. Finally, we demonstrated that peptide 31-43 did not behave as a classical ligand, indeed the labeled peptide did not displace the unlabeled peptide in a competitive binding assay. On the basis of these findings and of previous evidence demonstrating that peptide 31-43 is able to interact with a membrane-like environment in vitro, we conclude that membrane composition and organization, rather than a specific receptor protein, may have a major role in peptide 31-43 internalization by cells.

The two-component adjuvant IC31® boosts type i interferon production of human monocyte-derived dendritic cells via ligation of endosomal TLRs

The aim of this study was to characterize and identify the mode of action of IC31®, a two-component vaccine adjuvant. We found that IC31® was accumulated in human peripheral blood monocytes, MHC class II positive cells and monocyte-derived DCs (moDCs) but not in plasmacytoid DCs (pDCs). In the presence of IC31® the differentiation of inflammatory CD1a⁺ moDCs and the secretion of chemokines, TNF-α and IL-6 cytokines was inhibited but the production of IFNβ was increased. Sustained addition of IC31® to differentiating moDCs interfered with IκBα phosphorylation, while the level of phospho-IRF3 increased. We also showed that both IC31® and its KLK component exhibited a booster effect on type I IFN responses induced by the specific ligands of TLR3 or TLR7/8, whereas TLR9 ligand induces type I IFN production only in the presence of IC31® or ODN1. Furthermore, long term incubation of moDCs with IC31® caused significantly higher expression of IRF and IFN genes than a single 24 hr treatment. The adjuvant activity of IC31® on the IFN response was shown to be exerted through TLRs residing in the vesicular compartment of moDCs. Based on these results IC31® was identified as a moDC modulatory adjuvant that sets the balance of the NF-κB and IRF3 mediated signaling pathways to the production of IFNβ. Thus IC31® is emerging as a potent adjuvant to increase immune responses against intracellular pathogens and cancer in future vaccination strategies.

Cationic amphipathic peptides accumulate sialylated proteins and lipids in the plasma membrane of eukaryotic host cells

Cationic antimicrobial peptides (CAMPs) selectively target bacterial membranes by electrostatic interactions with negatively charged lipids. It turned out that for inhibition of microbial growth a high CAMP membrane concentration is required, which can be realized by the incorporation of hydrophobic groups within the peptide. Increasing hydrophobicity, however, reduces the CAMP selectivity for bacterial over eukaryotic host membranes, thereby causing the risk of detrimental side-effects. In this study we addressed how cationic amphipathic peptides—in particular a CAMP with Lysine–Leucine–Lysine repeats (termed KLK)—affect the localization and dynamics of molecules in eukaryotic membranes. We found KLK to selectively inhibit the endocytosis of a subgroup of membrane proteins and lipids by electrostatically interacting with negatively charged sialic acid moieties. Ultrastructural characterization revealed the formation of membrane invaginations representing fission or fusion intermediates, in which the sialylated proteins and lipids were immobilized. Experiments on structurally different cationic amphipathic peptides (KLK, 6-MO-LF11-322 and NK14-2) indicated a cooperation of electrostatic and hydrophobic forces that selectively arrest sialylated membrane constituents.

Adjuvating the adjuvant: facilitated delivery of an immunomodulatory oligonucleotide to TLR9 by a cationic antimicrobial peptide in dendritic cells

IC31(®) is a novel bi-component vaccine adjuvant consisting of the peptide KLKL(5)KLK (KLK) and the TLR9 agonist oligonucleotide d(IC)(13) (ODN1a). While membrane-interacting properties of KLK and immuno-modulating capabilities of ODN1a have been characterized in detail, little is known of how these two molecules function together and synergize in interacting with their primary target cells, dendritic cells (DCs). We have found that KLK-triggered aggregates entrapped ODN1a and these complexes readily associated with the DC cell surface. KLK stimulated the uptake and internalization of ODN1a via endocytosis, while the bulk of the peptide remained associated with the cell periphery. ODN1a co-localized with early and late endosomes as well as endoplasmic reticular structures. ODN1a co-localized with TLR9 positive compartments following KLK mediated uptake. These features did not depend on the expression of TLR-9. Our results reveal novel mechanisms that allow KLK to enhance the effects of the TLR-9 ligand ODN1a in immunomodulation.

Therapeutic approaches using host defence peptides to tackle herpes virus infections

One of the most common viral infections in humans is caused by herpes simplex virus (HSV). It can easily be treated with nucleoside analogues (e.g., acyclovir), but resistant strains are on the rise. Naturally occurring antimicrobial peptides have been demonstrated to possess antiviral activity against HSV. New evidence has also indicated that these host defence peptides are able to selectively stimulate the innate immune system to fight of infections. This review will focus on the anti-HSV activity of such peptides (both natural and synthetic), describe their mode of action and their clinical potential.