Antimicrobial peptides (AMPs) have sparked significant interest as potential anti-cancer agents, thereby becoming a focal point in pursuing novel cancer-fighting strategies. These peptides possess distinctive properties, underscoring the importance of developing more potent and selectively targeted versions with diverse mechanisms of action against human cancer cells. Such advancements would offer notable advantages compared to existing cancer therapies. This research aimed to examine the toxicity and selectivity of the nrCap18 peptide in both cancer and normal cell lines. Furthermore, the rate of cellular death was assessed using apoptosis and acridine orange/ethidium bromide (AO/EB) double staining at three distinct incubation times. Additionally, the impact of this peptide on the cancer cell cycle and migration was evaluated, and ultimately, the expression of cyclin-dependent kinase 4/6 (CDK4/6) genes was investigated. The results obtained from the study demonstrated significant toxicity and selectivity in cancer cells compared to normal cells. Moreover, a strong progressive increase in cell death was observed over time. Furthermore, the peptide exhibited the ability to halt the progression of cancer cells in the G1 phase of the cell cycle and impede their migration by suppressing the expression of CDK4/6 genes.

Host defense peptides (HDPs) are naturally occurring polypeptide sequences that, in addition to being active against bacteria, fungi, viruses, and other parasites, may stimulate immunomodulatory responses. Cathelicidins, a family of HDPs, are produced by diverse animal species, such as mammals, fish, birds, amphibians, and reptiles, to protect them against pathogen infections. These peptides have variable C-terminal domains responsible for their antimicrobial and immunomodulatory activities and a highly conserved N-terminal pre-pro region homologous to cathelin. Although cathelicidins are the major components of innate immunity, the molecular basis by which they induce an immune response is still unclear. In this review, we will address the role of the LL-37 domain and its SK-24, IV-20, FK-13 and LL-37 fragments in the immunity response. Other cathelicidins also share structural and functional characteristics with the LL-37 domain, suggesting that these fragments may be responsible for interaction between these peptides and receptors in humans. Fragments of the LL-37 domain can give us clues about how homologous cathelicidins, in general, induce an immune response.

Lipopolysaccharide (LPS) is a toxic and immunogenic agent for human. Additionally, LPS is a good target for some antimicrobial compounds, including antimicrobial peptides (AMPs). LPS-binding peptides (LBPs) can recognize and neutralize LPS. Rabbit and human cathelicidins are AMPs with LPS-binding activity. In this study, we designed and synthesized two new truncated LBPs from rabbit and human CAP18 peptides by in silico methods. After synthesis of peptides, the antimicrobial properties and LPS-binding activity of these peptides were evaluated. The parental rabbit and human CAP18 peptides were selected as positive controls. Next, the changes in the secondary structure of these peptides before and after treatment with LPS were measured by circular dichroism (CD). Human cytotoxicity of the peptides was evaluated by MTT and red blood cells (RBCs) hemolysis assays. Finally, field emission scanning electron microscopy (FE-SEM), confocal microscopy, and flow cytometry were performed to study the action mechanism of these peptides. Results indicated that the hCap18 and rCap18 had antibacterial activity (at a MIC of 4-128 μg/mL). The results of the quantitative LAL test demonstrated that LPS-binding activity of hCap18 peptide was better than rCap18, while rCap18 peptide had better antimicrobial properties. Furthermore, rCap18 had less cytotoxicity than hCap18. However, both peptides were nontoxic for normal human skin fibroblast cell in MIC range. In conclusion, rCap18 has good antibacterial properties, while hCap18 can be tested as a diagnostic molecule in our future studies.

Bacterial resistance to classical antibiotics is emerging worldwide. The number of infections caused by multidrug resistant bacteria is increasing and becoming a serious threat for human health globally. In particular, Gram-negative pathogens including multidrug resistant Escherichia coli are of serious concern being resistant to the currently available antibiotics. All Gram-negative bacteria are enclosed by an outer membrane which acts as an additional protection barrier preventing the entry of toxic compounds including antibiotics and antimicrobial peptides (AMPs). In this study we report that the outer membrane component lipopolysaccharide (LPS) plays a crucial role for the antimicrobial susceptibility of E. coli BW25113 against the cationic AMPs Cap18, Cap11, Cap11-1-18m2, melittin, indolicidin, cecropin P1, cecropin B, and the polypeptide antibiotic colistin, whereas the outer membrane protease OmpT and the lipoprotein Lpp only play a minor role for the susceptibility against cationic AMPs. Increased susceptibility toward cationic AMPs was found for LPS deficient mutants of E. coli BW25113 harboring deletions in any of the genes required for the inner part of core-oligosaccharide of the LPS, waaC, waaE, waaF, waaG, and gmhA. In addition, our study demonstrates that the antimicrobial activity of Cap18, Cap11, Cap11-1-18m2, cecropin B, and cecropin P1 is not only dependent on the inner part of the core oligosaccharide, but also on the outer part and its sugar composition. Finally, we demonstrated that the antimicrobial activity of selected Cap18 derivatives harboring amino acid substitutions in the hydrophobic interface, are non-active against wild-type E. coli ATCC29522. By deleting waaC, waaE, waaF, or waaG the antimicrobial activity of the non-active derivatives can be partially or fully restored, suggesting a very close interplay between the LPS core oligosaccharide and the specific Cap18 derivative. Summarizing, this study implicates that the nature of the outer membrane component LPS has a big impact on the antimicrobial activity of cationic AMPs against E. coli. In particular, the inner as well as the outer part of the core oligosaccharide are important elements determining the antimicrobial susceptibility of E. coli against cationic AMPs.

Cell-penetrating peptides (CPP) have become a widely used tool for efficient cargo delivery into cells. However, one limiting fact is their uptake by endocytosis causing the enclosure of the CPP-cargo construct within endosomes. One often used method to enhance the outflow into the cytosol is the fusion of endosome-disruptive peptide or protein sequences to CPP. But, until now, no studies exist investigating the effects of the fusion peptide to the cellular distribution, structural arrangements and cytotoxic behaviour of the CPP. In this study, we attached a short modified sequence of hemagglutinin subunit HA2 to different CPP and analysed the biologic activity of the new designed peptides. Interestingly, we observed an increased cytosolic distribution but also highly toxic activities in the micromolar range against several cell lines. Structural analysis revealed that attachment of the fusion peptide had profound implications on the whole conformation of the peptide, which might be responsible for membrane interaction and endosome disruption.

The antimicrobial peptide CAP18 has been demonstrated to have a strong in vitro bactericidal effect on Yersinia ruckeri, but its activity in vivo has not been described. In this work, we investigated whether CAP18 protects rainbow trout Oncorhynchus mykiss (Walbaum) against enteric red mouth disease caused by this pathogen either following i.p. injection or by oral administration (in feed). It was found that injection of CAP18 into juvenile rainbow trout before exposure to Y. ruckeri was associated with lowered mortality compared to non-medicated fish although it was less effective than the conventional antibiotic oxolinic acid. Oral administration of CAP18 to trout did not prevent infection. The proteolytic effect of secretions on the peptide CAP18 in the fish gastrointestinal tract is suggested to account for the inferior effect of oral administration.