Antimicrobial peptides can be found in almost all species of plants, invertebrates and vertebrates. Most of the already identified antimicrobial peptides feature less than 100 amino acid residues as well as a myriad of common features, the most notable ones being hydrophobicity, amphipathic structure, and cationicity. The peptides can be found inducibly or constitutively in a variety of tissue and organs that witness constant exposure to microbial pathogens like the cells in the epithelial tissues of the skin, the respiratory tracts, and the gastrointestinal lining. Antimicrobial peptides display a wide range of antimicrobial activities against fungi, bacteria, viruses and protozoa. Presently, they have been heavily studied and their uses in the treatment of multi-drug resistant pathogens have been well-documented.

There are two major mechanisms through which the actions of the function of cationic antimicrobial peptides can take: the first mechanism is through the inhibition of the function and the synthesis of intracellular RNA, DNA and protein. The second is through the disruption of the microbial membrane integrity through interacting with the negatively charged components. Apart from their direct antimicrobial activities, antimicrobial peptides are also known to have immunomodulatory properties like chemotactic activities to the immune cells. The duality in the nature of the functions of antimicrobial peptides have made it nearly impossible or difficult for microbes to develop resistance against antimicrobial peptides. There are several research findings that suggests that antimicrobial peptides are also loaded with anticancer activity. In the past, there have been studies focused on peptides that possesses both anticancer and antimicrobial activities. Owing to the characteristics as well as the observed anticancer activities of the studied peptides, it may be possible to select anticancer peptides or that the anticancer peptides could be designed and used in the development of therapeutic agents for the treatment of various types of cancers.

One of the leading causes of malignant cancer deaths in the world is Hepatocellular carcinoma (HCC). Some of the already known major risks factors responsible for HCC are infections arising from hepatitis B and C viruses, intake of metabolic aflatoxin, the development of non-alcohol fatty liver diseases, and the excessive consumption of alcohol. Though there have been some very promising advances in the diagnosis of this condition, the mortality rates associated with it are still on the rise, and this is primarily due to the absence of effective therapies. Consequently, there is a need for the design and subsequent development of novel HCC treatment strategies.

Antimicrobial peptides that have already displayed dual antimicrobial and anticancer activities are currently considered promising prospects for therapeutic agents that can be used in the treatment, as well as control, of HCC. They can be used as standalone treatments, or in conjunction with other treatments. In this piece, we will briefly discuss some of the new candidates for antimicrobial peptides for the treatment of HCC, the potential action mechanisms of the anticancer peptides, and the designs as well as the modifications of anti-HCC peptides.

Anti-HCC Activity Peptides

In the past few years, the number of anticancer peptides being evaluated for design for treatments against HCC has been on the rise. Most of the anti-HCC peptides are primarily obtained from sources such as terrestrial and marine animals and various types of bacteria. They can be seen as part of the superficially binding peptides that use phage-displayed selection on HCC cancer cell lines. The anticancer peptides have the ability to target ion channels, phospholipid layers, and other types of molecules in certain specific signaling pathways to induce the natural death of cancer cells.

The Mechanisms of the Actions of Anticancer Cells against HCC

Apart from the previously noted functions of the anticancer peptides, peptides obtained from antimicrobial peptides or any other natural source may display the following mechanisms in combating HCC: Immune modulation, direct killing, wound healing, and anti-inflammation activities. Below is a brief look into some of these mechanisms-:

Direct Killing Activities

The antimicrobial activity of antimicrobial peptides is triggered whenever there is an electrostatic interaction between the negatively charged bacterial component and the cationic peptide. This interaction is then followed by the insertion into, as well as the interruption of, the microbial membrane. During this process, it is important to note that the anionic phospholipid components of the cancer cell membranes are not the same as that of the normal cells. It has been observed that the cancer cell membranes usually tend to have a denser negatively charged phosphatidylserine (PS) compared to that of normal cells. This feature is responsible for making them more sensitive to the actions of the anticancer peptides.

Anti-inflammatory Activity

Whenever the permeability of the gut is compromised in any way, there is always the likelihood of the gut microbiota, as well as their products, disseminating directly from the intestines into the liver through the portal vein. Whenever this happens, there are always higher chances for the induction of hepatic inflammatory responses. This may then lead to the development of conditions such as cirrhosis, fibrosis and HCC. Such inflammatory reactions are believed to be a result of the toll-like receptor (TLR) signaling pathways that are normally prominent during the early stages of the development of liver cancer. TLR4 is one of the extracellular pathogen recognition receptors with the ability to bind LPS. As such, they have a significant role to play during the chronic inflammation of HCC. Through hepatocyte TLR4-mediated pathways, it is possible to induce the expression of the inflammatory molecule of Hepcidin. TLR5 present in hepatocytes works as a protection mechanism against high-fat-diet-induced liver disease through the binding actions of bacterial flagellin.

Anticancer peptides that have been derived from antimicrobial peptides may benefit from their strong electrostatic interaction with the negatively charged LPS of gram-negative bacteria and flagellin, but only if they are both used as anti-inflammatory agents. Additionally, TLRs are known to be widely expressed in the bulk of the liver immune cells, including in macrophages, T-cells, B cell and dendritic cells. Consequently, with the modulation activities of TLRs, it may be possible to induce an anti-HCC activity within the liver, and the peptide agonists of TLRs may be vital in the development of novel therapeutics agents for treating HCC.

Immune Modulation

The recognition of tumor-associated antigens (TAA) is possible through the study of antigen-presenting cells which then induces the activation of tumor-responsive T lymphocytes. Several studies have suggested that TAA-derived long peptides may have the ability to elicit Th1, T cells and cytotoxic T lymphocytes (CTL)-mediated anticancer immune response. It is believed that this is possible through cross-presentation. Alpha-fetoprotein (AFP) is one of the most common TAA in HCC. It has been observed that peptides obtained from human AFP may have the ability to stimulate specific T-cell responses both in the T-cells obtained from patients suffering from HCC and in cultured peripheral blood lymphocytes of healthy individuals. A considerable number of tumor-infiltrating lymphocytes inside of a tumor have also been shown to have activity against HCC. Consequently, peptides derived from TAA may be helpful in enhancing anti-HCC treatment therapies.

Additionally, cell-mediated interventions and treatments that feature the adoptive transfer of TILs are also under consideration for use as one of the immunotherapeutic strategies in the management and treatment of cancer. However, it is possible for the cancer cells to evade the immune surveillance through the alteration of their antigen presentation or through the secretion of cytokines and chemokines which will ultimately induce Treg cells to create an immunosuppressive microenvironment. Anticancer peptides that have immunostimulatory activities may be used as a means of enhancing the efficacy of cell-mediated therapy. An example for such an application would be Tyroserleutide which is an immunostimulatory peptide that features three amino acids with the ability to stimulate antitumor effects in macrophages against human HCC cell line.

Wound Healing Activities

Liver fibrosis is one of the known conditions linked to chronic liver injury. It will usually lead to cirrhosis and possibly HCC at the very later stages. Antimicrobial peptides such as endogenous mediators have the ability to enhance the processes of wound healing through antimicrobial activities, angiogenesis, chemotactic activities and LPS neutralization. For instance, in studies involving recombinant human AMP LL-37, it was observed that it enhanced the wound healing process by simply stopping the activation of macrophages with LPS, and also through the induction of the proliferation, as well as the migration, of endothelial cells, re-epithelization and vascularization.

Additionally, it may be possible to modify natural antimicrobial peptides or to combine them with other short peptides to enhance their overall potential or capabilities.

The Modification of Anticancer Peptides

Studies have shown that host antimicrobial peptides have anticancer ability. However, most of their properties may not be directly applicable in the design and the development of anticancer agents since they have very low killing activity. The potential candidates for anticancer peptides may be modified to alter their mode of action. Also through modifications, it is possible for the novel analogs to be developed as a way of increasing the membrane-binding affinity and selectivity to cancer cells through the modulation of their amino acid components. For example, a modified peptide CB1a obtained from antimicrobial peptide Cecropin B has shown to have very promising activity against leukemia with very low cytotoxicity to non-cancer cells. Also, various strategies such as fragmentation, polymerization, hybridization, and cyclization, among others may be used to increase the efficacy as well as the stability while decreasing undesirable collateral cytotoxicity. It is also possible to design anticancer peptides to synthesis in silico with the aim of reducing the associated time and labor.

The Potential Applications of Anticancer Peptides

Anticancer peptides have two main killing mechanisms – modulating immune responses to kill cancer cells and binding with targets to kill cancer cells. These mechanisms make them very attractive in the development of a variety of HCC treatments. To produce peptide based anticancer therapies, it is vital to explore various delivery systems, including nanoparticles, liposomes, and peptide-derived vaccines.

Peptide-Based Vaccines

Most of the peptide-based vaccines are easy to store, are water soluble, and can be customized to be used to target and achieve very specific objectives. For example, Glypican-3 (GPC3) obtained from heparin sulfate proteoglycans is usually overexpressed in HCC and, as such, it is viewed as a potential target for cancer immunotherapeutic. Peptide vaccines derived from GPC3 have demonstrated the ability to lead to an increase in the peptide-specific CTLs and they are currently being evaluated in a variety of clinical trials. There are, however, certain limitations that have been associated with the use of peptide vaccines. For example, peptide vaccines are known to have low efficacy, to be unstable under certain physiological conditions, and to have poor immunogenicity. There are several strategies that have been employed as a means of overcoming certain limitations, with some of the interventions including the use of immunostimulatory adjuvants, multi-epitope approaches, and the use of new delivery systems featuring nano or micro particles. It is also possible to use antimicrobial peptides with anticancer properties as adjuvants for vaccines. Clinical trials for peptide-based vaccines for cancer therapies targeting TAAs have been reviewed in the past few years, and it has been observed that by targeting tumor-specific mutated antigens, it may be possible to reduce the off-tumor cytotoxicity, allowing room for more personalized treatments.

Nanoparticles

Nanoparticles like liposomes and gold nanoparticles have been used in the design and development of various delivery systems because of their properties such as enviable bioactivity, long drug half-life, and good cell selectivity. Another promising strategy in the development of HCC therapies is the possibility of delivering anticancer peptides with the help of nanocarriers. In one study, it was reported that through the use of liposomes to deliver HCC-targeting peptide, there was enhanced therapeutic efficiency as well as proper selection by phage-display in a mouse with the HCC xenograph model. Currently, there is need for more studies to help in the identification of the unique neoantigens that can be used as potential candidates for immunotherapeutic agents against HCC.

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