Cancer is one of the leading causes of death worldwide. It is estimated that nearly eight million people die every year as a result of various forms of cancer, including lung cancer, colon cancer, liver cancer, throat cancer, and pancreatic cancer among others. One of the most pronounced characteristics of cancer is the growth of the abnormal cells that exceeds their usual limit, and over time, they start to invade the adjoining parts of the body before spreading to other organs.
Most of the current cancer treatments require one or more therapeutic modalities such as chemotherapy, radiotherapy, and surgery. Though there has been a lot of progress in cancer care, the use of therapeutic drugs such as in the case of chemotherapy has come with serious drawbacks. Also, a majority of the neoplasm will eventually become resistant to the treatments by chemo drugs owing to the selection of multidrug resistance.
Due to these limitations, there has been a need for the development of new anticancer therapies. One of the most attractive alternatives that the research world is currently focusing on is the use of antimicrobial peptides. These peptides represent a novel group of anticancer agents that can be used to avoid most, if not all of the shortcomings of the current cancer therapies.
Antimicrobial peptides refer to amphipathic molecules that are generated by a wide variety of organisms as one of their first line of defense. They are also produced by a wide variety of molecules as a competition strategy for getting nutrients and space. Presently, it is estimated that there are over2400 antimicrobial peptides already recorded in The Antimicrobial Peptide Database.
The discovery of more antimicrobial peptides is still ongoing, and they are currently being viewed as diverse microorganisms that can be used as natural antibiotics in treating a variety of infectious diseases in both humans and animals. Additionally, antimicrobial peptides display a broad spectrum of biological activities, and since they have demonstrated very low incidences of resistance, they are currently being considered as a potential treatment for various forms of cancers.
Antimicrobial peptides usually feature short peptides, and they mainly exhibit a positive charge, though it is possible to have some which are negatively charged or have a neutral charge. In the past few years, there have been many studies exploring the structure, the mechanisms of action, and the emergence of resistance to antimicrobial peptides. There have also been a number of studies investigating the anticancer activities, as well as the selectivity of these peptides and their efficacy.
The mechanisms, however, which these peptides use to kill cancer cells is not fully understood, though there is some evidence to suggest that both nonmembranolytic and membranolytic mechanisms may be involved. The membranolytic activities of these antimicrobial peptides are seen as a factor of their characteristics and a factor of the target membrane. Additionally, the selectivity of some of the antimicrobial peptides when it comes to cancer cells has been remotely related to the difference in net charges of the membrane, which in most cases has a negative charge. Anionic molecules usually express a net negative charge on the cancer cells, and this typically contrasts with the normal mammalian cell membrane.
Conversely, the nonmembranolytic activities of antimicrobial peptides may feature the inhibition of processes such as angiogenesis which is a vital process when it comes to the formation of tumor-associated vasculature.
Though there have been a lot of promising anticancer characteristics found in antimicrobial peptides, just a handful of them have been successfully tested in comprehensive and conclusive in vivo models. For example, Cecropin B has been found to greatly increase the survival chances of mice infected with ascetic murine colon adenocarcinoma cells.
Similarly, when magainin 2 was tested against murine sarcoma tissues, it was observed that it greatly enhanced the lifespan of the animal. But the information relating to the anticancer effects of antimicrobial peptides is still scarce. In this piece, we will provide an overview of the anticancer activities of plant antimicrobial peptides with the major focus being on their mode of operation, their selectivity as well as their efficacy.
Plant-based antimicrobial peptides
Plants have always been a coveted source of diverse molecules with incredible pharmacological potential. To date, there are over 300 antimicrobial sequences that have already been discovered and carefully described. Most of the time, plants will produce very small amounts of cysteine-rich antimicrobial peptides as part of their natural defenses. These may be expressed constitutively, or they can be released whenever there is a pathogen attack. The majority of plants have expressed an abundance of antimicrobial peptides and just a small portion of them have been shown to produce cysteine-rich antimicrobial peptides.
The antimicrobial peptides from plants are normally produced in all organs, and they are found in abundance in the outer layers compared to the inner plant layers. This is consistent with their role as a defense mechanism against microbial attacks that will mostly come from the outside. The moment an infection is initiated in a plant, the plant will release the plant antimicrobial peptides. Since most of the antimicrobial peptides are expressed by a single gene, they naturally require less biomass and also consume less energy.
The bulk of the plant antimicrobial peptides have a molecular weight of between 2 – 20kDa and they contain between 4 and 12 cysteines that primarily form the disulfide bonds responsible for their structural and thermodynamic stability. The classification of plant-based antimicrobial peptides is normally done based on the identity of the amino acid sequence, and the position and the number of cysteines forming the disulfide bonds. Presently, about twelve families of antimicrobial peptides have been exhaustively described.
The main biological activities of plant antimicrobial peptides are antibacterial, antifungal, and actions against oomycetes as well as the activities of herbivorous insects. Also, antimicrobial peptides have displayed certain enzyme inhibitory activities and have been shown to be crucial in metal tolerance. Besides these, some antimicrobial peptides have cytotoxic activities against mammalian cells and anticancer activities against cancer cells obtained from other sources. Of all the twelve families of antimicrobial peptides already described, about three of them have been found to have potent cytotoxic and anticancer properties – these include thionins, cyclotides, and defensins. Here is a detailed look at each of these categories.
The very first antimicrobial peptides to be isolated from plants were thionins. Thionins are part of a fast-growing group of biologically active peptides in the plant kingdom. They are small cysteine-rich peptides with both toxic and antimicrobial properties. They are further divided into four different groups based on their number of amino acids, their net charge, and the number of disulfide bonds present. The first group is Type 1 thionins – this group has a total of 45 amino acids with four disulfide bonds. Type 2 Thionins have 46 or 47 amino acids with four disulfide bonds. Type 3 thionins have 45 or 46 amino acids peptides with three or four disulfide bonds.
Lastly, Type 4 thionins have 46 amino acid peptides with three disulfide bonds and a neutral net charge.
The major role of thionins is to offer the plant protection against the actions of pathogens. However, they also have the ability to take part in plant physiological processes such as seed maturation, dormancy, seed germination, as well as the packaging of storage proteins into their protein bodies. Additionally, may have a role to play when it comes to altering the cell walls following the penetration of the epidermis by a fungal agent, as well as acting as a secondary messenger in signal transduction.
Apart from the activities of thionins already described above, several plant thionins have shown incredible cytotoxic and anticancer activities. For example, the pyrularia thionin obtained from mistletoe has been discovered to have immense anticancer activity against the cells of cervical cancer and mouse melanoma cells. However, this particular thionin is believed to be cytotoxic due to its ability to cause hemolysis. Its anticancer effects are believed to be a result of cellular response that causes the stimulation of positively charged calcium ions to depolarize the plasma membranes. This leads to the activation of an endogenous phospholipase, leading in turn to the alteration of the membrane before the ultimate death of the cell.
Plant defensins are a group of plant antimicrobial peptides with functional and structural properties that are nearly similar to the peptide defensins produced by vertebrates, fungi, and invertebrates. This group of antimicrobial peptides tends to have diverse amino acid sequences and they also show clear conservation of certain amino acid sequence positions. Most of the plant defensins have the ability to inhibit the growth of a wide variety of fungi and they are also less toxic to both plant and mammalian cells. Their proposed mechanism of action is believed to be either through the destabilization of the cell membrane by inserting themselves into the membranes or coating the outer surface of the membrane to form open pores. Such actions normally allow the vital biomolecules to leak out of the cells.
Apart from the antifungal activities of the plant defensins, they also exhibit anticancer and cytotoxic activities. The very first plant defensin to be reported to have anticancer activities was defensin sesquin isolated from Vigna sesquipedalis. This antimicrobial peptide showed the ability to inhibit the proliferation of leukemia and MCF-7 cells. Also, in a study conducted by Wong and Ng, it was observed that the defensin limenin isolated from Phaseolus limensis had the ability to inhibit the proliferation of leukemia cells, but this study never focused on the effects of the defensin on normal cells. Another plant defensin believed to have potent anticancer activities is lunatusin.
This is a defensin isolated from the seeds of the Chinese lima beans. It was observed that this defensin could inhibit the proliferation of MCF-7 cells. Sadly, it was discovered that lunatusin also had cell-free translation inhibitory activity in the rabbit reticulocyte lysate system. This is an indication that this particular defensin may show a certain degree of cytotoxicity to normal tissues as well as other healthy types of cells. But all the defensins already studied, it has been observed that lunatisin is the only plant defensin that has these effects.
Cyclotides refer to macrocyclic peptides – usually those with less than 30 amino acids that possess incredibly diverse biological activities. They are mostly isolated from the Violaceae and Rubiaceae plant families. They constitute a family of plant antimicrobial peptides that have six conserved cysteines that stabilize the structure through the formation of disulfide bonds. Cyclotides normally have a cysteine knot that has an embedded ring within the structure formed by two disulfide bonds that connect the backbone segment that has been threaded by a third disulfide bond. These features normally lead to the formation of a unique protein fold with complex topology and unique chemical and biological stability as well as enviable medicinal and pharmaceutical importance when it comes to the design of drugs.
Cyclotides have hemolytic activities which only occur in cyclic conditions. Whenever they are linearized, they will lose this activity, and this is a subtle indication that the cyclic backbone in their structure is of immense importance for this activity. Also, there are suggestions that the hemolytic activity of the cyclotides is a factor in their amino acid sequence. The cyclotides O2 and O13 obtained from Viola odorata have been shown to have a variety of hemolytic activities as well. Both of these molecules differ from one another only by one residue. O2 contains a serine residue while O13 has an alanine residue in the same position. With the loss of the hydroxyl group, the hemolytic activity of the molecule also changes by more than three-fold.
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