1. Introduction
Today, finding a suitable, inexpensive, and simple treatment for various cancers is essential [4]. One of these strategies is the use of Antimicrobial Peptides (AMPs), which are part of the innate immune response to microbes [6-8]. AMPs can act against cancer cells through various mechanisms such as apoptosis, activation of the immune system, and so on [7, 9]. In this study, we reviewed some of the most important types of AMPs.
2. Materials and Methods
The papers published in English were searched in Springer, ScienceDirect, PubMed, and Google Scholar databases by using the following keywords: Antimicrobial peptides, Anticancer peptides, Cationic peptides, as well as AMP names.
3. Results
AMPs with α-helical structure
LL-37 peptide is an antimicrobial agent derived from human cathelicidin [10]. Numerous studies have shown that LL-37 and its analogs increase toxicity in cancer cells, and some have pointed to its role in cancer progression and metastasis [11]. Melittin is a water-soluble, linear, cationic, hemolytic, and amphipathic polypeptide containing 26 amino acids [20]. The results suggest that the inhibitory effect of melittin on the migration of breast cancer cells may be related to the inhibition of the mTOR (The mammalian target of rapamycin) pathway [24]. 
Bovine Antimicrobial Peptides (BAMPs) in two types of BMAP-27 and BMAP-28 with one and two amino acid sequences have been isolated from bovine cathelicidin [27, 28]. Treatment with BMAP-28 in human cancer cells of U937 and K562 results in pore opening in the mitochondrial membrane and eventually release of cytochrome c leading to cell death or apoptosis [29]. 
Cecropin A and B are another type of AMPs isolated from the hemolymph of a giant silk moth [31]. The effect of cecropin B on human gastric carcinoma cell lines showed that by decreasing the external currents, it reduced the pore formation of the membrane channels [31]. Magainin, as a specific cytotoxic substance for cancer cells, has been isolated from the skin of African clawed frogs [35]. The findings have indicated that magainin II has specific cytotoxicity against cancer cells and can induce apoptosis [37].
Aurein peptide is another AMP found in the skin secretions of Australian frogs [41]. This peptide has moderate anticancer activity against 52 of the 54 cancer cell lines [41, 43]. Another AMP is gaegurin, which is isolated from the skin of a Korean frog [45], and some studies have shown that gaegurin 5 and gaegurin 6 have specific cytotoxic activity against neoplastic cells [46].
Buforin-I is a 39 amino acid AMP that was first isolated from the stomach tissue of an Asian frog [47]. Numerous studies have shown that buforin II and buforin IIb (a synthetic analog of buforin II), exhibit specific cytolytic activity against 62 cancer cell lines [51].
AMPs with β-sheet structure
Defensins are a group of cysteine- and arginine-rich cationic AMPs [52]. Xu et al. reported that the injection of α-defensin-1 inhibited human lung adenocarcinoma cell growth in nude mice and induced their apoptosis [56]. Lactoferricin is a cationic AMP produced by the hydrolysis of lactoferrin [57]. Some studies have shown that bovine lactoferrin has great anticancer potential by activation of signaling pathways [58].
Another AMP is tachyplesin. In 2018, the combined effect of tachyplesin-I and cisplatin AMPs on tumor cells and normal human cells was studied. Results showed that their combined use reduced the effective cytotoxic dose and thus decreased non-specific toxicity [63].
4. Discussion
Although AMPs have been known for decades, it is only in the last decade that the study of their anticancer activities has increased, and they were referred to as Anticancer Peptides (ACPs). That is why we believe that in the coming years, the use of these peptides, due to their unique properties (influencing cancer cells), will be increased for the treatment of cancer, which is one of the greatest concerns of human society in the world. Another strategy is the combined use of peptides with conventional chemotherapeutic drugs, which reduces the cost of treatment, minimizes the problem of cancer resistance, and prevents its recurrence. Advances in the production of these peptides in a large-scale worldwide have made this treatment cheaper and more accessible to patients. However, there may be some limitations, such as the possible similarity of these peptides to their antigens or the stimulation of the immune system against these peptides. Finally, it can be said that these peptides have gone a long way in optimizing the treatment process for cancer and can provide a novel and less complicated treatment approach.
Ethical Considerations
Compliance with ethical guidelines
This study was extracted from a research proposal approved by the Research Ethics Committee of Arak University of Medical Sciences (Code: IRARAKMU1397.174).
Funding
This study was sponsored by the Deputy for Research and Technology of Arak University of Medical Sciences.
Authors' contributions
Study design, conceptualization, original draft preparation, and editing by Ali Ganji; initial draft preparation and editing by Amir Mohammad Saeedi; Data gathering and editing by Ali Ghazavi; conceptualization and review by  Ghasem Mosayebi. 
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
The authors would like to thank the Deputy for Research and Technology and the Molecular and Medicine Research Center for their financial and spiritual support.
References