Abstract

The rates of cancer recurrence many years after a “cancer-free” status can be a discouraging revelation for patients. Many, if not most cancers, can return and regrow in patients after many cancer-free years. However, the use of antimicrobial peptides in cancer therapeutics has slowly increased in past years and would suggest a promising future. The present report addresses the interaction that appears to exist among:
i. Antimicrobial-like amphipathic peptides,
ii. Cancer stem cells, and
iii. The growth recurrent cancers; taken together, these elements could have promising future implications.

Among the many advantages of using antimicrobial-like peptides, these amphipathic peptides display remarkable cancer cell targeting properties and are capable of transporting and delivering chemo-drug payloads into malignant cells. In addition, the cancer cell targeting abilities of peptides have been found to be dependent on a net negative surface charge displayed on the cancer cell bilayer surface membrane. Thus, one could postulate that the destruction of tumor cells, together with cancer stem cells, could possibly alter and reduce the recurrence rates of cancer patients in future years to come.

Keywords:Alpha-Fetoprotein; Antimicrobial Peptides; Cancer; Recurrence; Stem Cells; Chromosome Instability; Amphipathicity; Cell Membranes; Electrical Change

Abbreviations: AFP: Alpha-Feto Protein; AMLPs: Antimicrobial-Like Peptides; TRP: Transient Receptor Potential; CSC: Cancer stem cells

Introduction

Antimicrobial-like Amphipathic Peptides

The antimicrobial-like peptides (AMLPs) are naturally occurring peptides found in a multitude of organisms [1]. The original AMLPs were found to be broad spectrum antibiotic agents that specifically killed gram-positive and gram-negative bacteria and could further attack enveloped viruses and transformed cancer cells in mammals including man [2,3]. The AMLPs can create pores and/or transmembrane channels within the surface bilayer of cell membranes to translocate into the cell cytoplasmic interior [2].

These newly formed channels are like the already in-place Transient Receptor Potential (TRP) channels, and the two together are associated with other receptors, capable of interacting with invasive AMLPs. The function of the AMLPs is to destabilize, disrupt, and perturb the bilayer membrane to allow cell penetration and entrance into the cancer cell cytoplasmic interior which can subsequently affect other intracellular transduction pathways.

The AMLPs can extend in length from 12 to 50 amino acids, contain two or more arginines, lysines, or histidines, and display several hydrophobic and polar amino acids; overall, these entities are largely zwitterionic and amphipathic peptides [4,5]. The secondary structure of such peptides can consist of alpha-helices, beta-sheets, hairpin loops, and usually contain one or more disulfide bridge linkages [6]. Some AMLPs can also contain random coil (disordered) structures as part of their secondary structure.

The main targeting objective of the AMLPs is the cell surface bilayer membrane in which these peptides are capable of disrupting and destabilizing the bilayer leaflet of the cell membrane [7,8]. In this manner, the AMLPs can permealize (penetrate) into the cell membrane bilayer and form a pore and/or a transmembrane channel to gain cell entry. Such peptides are attracted to and bind to a net negative surface charge on the cell membrane by means of an electrical attraction to the alpha-helical content, amino acid composition, and amphipathic nature of the peptide [8].

It is of interest that a net negative charge on the cell surface membrane is displayed only on bacteria, stem cells, and transformed cancer cells. In contrast, normal non-cancer mammalian cells display only a net positive cell surface membrane charge [9,10]. The net negative charge on cancer and stem cells results from undergoing a phospholipid bilayer flip, in which the innermost negative-charged phospholipid head groups emerge (flip) to the outer bilayer of the cell membrane, while the positive charged phospholipids retreat to the inner membrane leaflet bilayer location [11].

Such bilayer-altered cancer cells subsequently exhibit a net negative cell surface membrane charge. Such a negative charged surface membrane allows the AMLPs to home in on and target only cancer cells, but not normal non-malignant cells [12]. Is it possible that AMLPs presently exist in the toolbox of cancer research biologists? The answer is affirmative in that most AMLPs possess these traits especially in reference to the Growth Inhibitory Peptide, which is derived from the full-length alpha-fetoprotein (AFP) polypeptide. This AFP-derived peptide has long been researched and reported in biomedical literature and is capable of confronting, attacking, and binding to cancer cells in both in vivo and in vitro studies [13-15] (Table 1).

Cancer Stem Cells

Cancer stem cells (CSC) are a small population of cells (2-3%) that reside in tumors and which are self-renewing, transformational, and multipotent. The CSCs are most similar and resemble the stem cells located in hematopoietic tissues; although such stem cells are slow growing, they retain strong long-term stem to-tumor cell-transforming capabilities [16]. The CSCs have been detected and identified in multiple cancer cell types, including breast, brain, intestinal, gastric, liver, and many others [17]. It has been found and reported that such stem cells appear to be the source of tumor recurrence in patients many years after successful medical treatments; thus, even though many patients have been declared “cancer-free” at a previous time, the malignant tumor may return [18]. Thus, CSCs are a specific cell phenotype that can eventually progress to tumor transformation, regrowth, invasiveness, and recurrence many years later.

In lieu of the above discussion, many non-cancer organs have a small reserve of stem cells to replace worn out cells; in addition, many stem cells can further be stored in fatty tissues in other part of the body. In fact, a subset of mesenchymal derived stem cell population (1-3% of total cells) has further been detected in the abdominal fatty storage tissues of the body. In summary, the present treatise should be deemed significant in attempting to explore, study, and understand the main activators, mediators, and regulatory cell mechanisms involved in the historical development of cancer stem cells.

The Origin of Cancer Stem Cells

The origin of CSCs is still in debate and not fully understood. Thus, the precise origin of cancer stem cells has not been definitively established with complete certainty. Nonetheless, three proposed sources of CSCs are thought to be derived from: 1) normal body stem cells; 2) genetically altered adult cells; and 3) transformed fusion cells [19]. However, it appears to be generally accepted in the medical community that most CSCs originate from normal body stem cells and/or precursor cells, which have undergone multiple gene mutations.

Several opinions have been forwarded regarding the traits of CSCs, but one common characteristic appears to be that of genetic instability which is the fundamental basis for the transformation of stem cells or progenitor cells into CSCs [16,20]. One such hypothesis states that normal stem cells have long lifetimes, allowing for accumulation of numerous oncogenes and/or tumor suppressor gene mutations that produce genomic changes, especially the transformation of stem cells into tumor cells [21]. Such events could originate at the chromosome and/or molecular levels in which normal stem cells could differentiate into CSCs; these then exhibit different genotypes and phenotypes from those of normal body stem cells.

The CSCs can acquire increased oncogenes such as C-Myc, RAS, and Notch which manifest the traits of slow-growing and self-renewal abilities, shorter cell cycles, and adaptation to hypoxia environments for ease of cell transformations and transitions [21]. Stem cells with tumor-initiating abilities often display certain cell surface biomarkers on their cell membranes such as CD34+ and CD38+immune-associated markers. The precise origin of CSCs from adult and cell fusions are less understood and will not be further addressed in the present report.

Stem Cells Display a Net Negative Charged Cell Membrane

Stem cells, like all cells, are composed of a cell membrane lipid bilayer composed of two layers of phospholipids with cholesterol interspersed between the two bilayers. The presence of cholesterol ensures that the membrane fluidity (ability to shift movements within the cell membrane) is maintained within the cell membrane. In normal non-malignant cells, the outer layer of the cell membrane is composed of positive-charged phospholipid head groups, while the inner bilayer is composed of negative-charged phospholipid headgroups [22]. Certain cells can undergo a lipid flip in the membrane bilayer which transfers the inside negative charged layer to the outside bilayer leaflet of the cell membrane and shifts the positive charge phospholipids back to the inner membrane leaflet [11]. In this manner, while the outer cell membrane of normal cells displays a net positive surface charge, the stem cells in contrast exhibit a net negative charge on their outer cell surface membrane bilayer. In summation, the net negative surface charge of stem cells is a direct result of the transfer or shift of the negative phospholipid headgroups to the outermost membrane bilayer [11,23].

In contrast, the normal cells of the body carry a net positive surface charge on their outer bilayer due to their positive charged phospholipids. The negative charge of stem cells resulted from the zeta potential that is typically measured as negative (range from -80mV to -25m voltage) depending on the cell type and interstitial space conditions [22]. The cell membrane potential refers to the electrical charge differential across the cell membrane, which is involved in the downstream transmission of transduction signals within cells [24,25]. It is the anionic phospholipids that confer a net negative charge on the outermost bilayer cell membranes of stem cells. Thus, it is the difference in ion concentrations in the cell interior and exterior that gives rise to the electrical potential across the cell membrane.

Cancer Recurrence Rates

The thought of cancer “return” or “recurrence,” after several years in prior cancer-free patients, produces a significant fear in both patients and in their caregivers [26]. Oncologists are encouraged to discuss cancer recurrence rates considering symptoms and practices with their past and present patients; these discussions are regarding the rationale behind various follow-up procedures which can vary widely in patients with different types of cancer. Such procedures are based on disease stage, tumor pathology, patients’ genetic background, and various patient prior treatments.

However, it is important to view recurrence rates only as statistical estimates due to the different cancer cell types, ongoing new developments of anti-cancer therapies, and differences between individual-recovering cancer patients [27]. Summaries of the different recurrence rates in adults are listed in Table 2 presented as three different categories, namely: A) Cancers with high recurrence rates; B) Cancers with medium recurrence rates, and C) Cancers with low recurrence rates. The childhood cancer recurrence rates are further listed in Table 3.

Conclusion and Future Prospects

It can readily be ascertained from the above discussion that the interactions and interplay among the three discussed elements are important. These elements are namely, 1) the AMLPs, 2) cancer stem cells, and 3) cancer reoccurrence rate statistics; knowledge of the three could suggest future encouraging therapeutic possibilities. For example, one could postulate and/or propose that the return of recurring cancers might be preventable under certain combinations of treatment conditions. The encouraging claim by oncologist that a patient is declared cancer-free, following extensive radiation and chemotherapeutic treatments, can eventually be short-lived in the patients future years to come.

Cancer stem cells are very slowly growing and may take many years to manifest and regrow into a newly detectable tumor mass. As shown above, most if not all, tumors surveyed displayed a new cancer regrowth recurrence [16]. However, with the utilization of AMLPs as a therapeutic treatment option, such peptides might specifically be able to target and home onto cancer cells and CSCs while bypassing normal non-malignant dividing body cells. Present chemotherapeutic drugs can home onto normal mitotic dividing body cells together with the cancer mitotic dividing cells, without discrimination between the two different cell types.

In addition, the above discussion further revealed that cancer stem cells display a net negative surface membrane charge, making such cells further vulnerable to AMLP therapeutic treatments. These elements have the potential to simultaneously destroy both cancer cell and cancer stem cell populations. In theory, one could propose that such AMLP treatments could simultaneously destroy both cancer cells and cancer stem cells with the same peptide therapeutic regimen. Such an action could possibly reduce the chance of cancer stem cell transforming into cancer cells and might decrease the chance of both cancer regrowth and recurrence in future years.

Declaration and Acknowledgment

Funding

None; no US federal grants were used in the preparation of this paper.

Conflicts of Interest

The author declares there are no known conflicts of interest in the preparation of this manuscript.

Acknowledgement

The author extends his thanks and gratitude to Ms. Sarah Andres for her commitment and time expenditure in the skilled typing and processing of the manuscript, references, and tables of this report.

References

 

1. Lee HT, Lee CC, Yang JR, Lai JZC, Chang KY (2015) Large-scale structural classification of antimicrobial peptides. Biomed Research International 2015: 475062.
2. Mizejewski GJ (2019) Cell-penetrating versus antimicrobial peptides: comparison of potential use as cancer therapeutics. Journal of Oncology Research Forecast 2: 1013-1015.
3. Mizejewski GJ (2019) Antimicrobial peptides and cancer: potential use of antimicrobial-like peptides in chemotherapy. J Cancer Biol Therap 5: 233-242.
4. Mizejewski GJ (2017) Breast cancer and transient receptor potential (TRP) cation channels: Is there a role for non-selective TRP channels as therapeutic cancer targets: a commentary. Intl Journal of Cancer Res. And Development 2: 4-6.
5. Nguyen LT, Haney EF, Vogel HJ (2011) The expanding scope of antimicrobial peptide structures and their modes of action. Trends Biotechnol 29(9): 464-472.
6. Sitaram N, Nagaraj R (2002) Host-defense antimicrobial peptides: Importance of structure for activity. Cur Pharm Des 8(9): 727-742.
7. Araste F, Abnous K, Hashemi M, Taghdisi SM, Ramezani M, Alibolandi M (2018) Peptide-based targeted therapeutics: focus on cancer therapeutics. J Control Release 292: 141-162.
8. Wang CK, Shih LY, Chang KY (2017) Large-scale analysis of antimicrobial activities in relation to amphipathic and charge reveals novel characterization of antimicrobial peptides. Molecules 22(11): 2211.
9. Henzler Wildman KA, Lee DK, Ramamoorthy A (2003) Mechanism of lipid bilayer disruption by the human antimicrobial peptide, LL-37. Biochemistry 42(21): 6545-6558.
10. Bahar AA, Ren D (2013) Antimicrobial peptides. Pharmaceuticals (Basel) 6(12): 1543-1575.
11. Metakar SS, Wang B, Catalan E (2011) Perforin rapidly induces plasma membrane phospholipid flip-flop. PLOS One 6(9): e24286.
12. Deslouches B, Di YP (2017) Antimicrobial peptides with selective antitumor mechanisms: prospect for anticancer applications. Oncotarget 8(28): 46635-46651.
13. Hui SC, Yu HI, Lu CH, Lee YR (2010) The Effects growth inhibitory peptide on Follicular Thyroid Cancer cell migration and invasion. Tumori Journal 96(3): 448-451.   
14. Muehlemann M, Miller KD, Dauphinee M, Mizejewski GJ (2005) Review of growth inhibitory peptide as a biotherapeutic agent for tumor growth, adhesion, and metastasis. Cancer metastasis Rev 24(3): 441-467.
15. Mizejewski GJ (2023) An alpha-fetoprotein derived peptide suppresses growth in breast cancer and other malignancies: A review & prospectus. Med Res Archives 11(7): 4147.
16. Zhang DY, Monteiro MJ, Sun-Ping L, Wen-Yi G (2021) Mechanisms of cancer stem cell senescence: current understanding and future prospectives. Clin Exp Pharmacol Physiol 48(9): 1185-1202.
17. Petrova DD, Dolgova EV, Proskurina AS, Ritter GS, Ruzanova VS (2022) The new general biological property of stem-like tumor cells (Part II: Surface molecules, which belongs to distinctive groups with particular functions, forms a unique pattern characteristic of a certain type of tumor stem-like cells). Int J Mol Sci 23(24): 15800.
18. Primeau AS (2018) Cancer Recurrence statistics. The Cancer Therapy Advisor.
19. Dolgova EV, Alyamkina EA, Efremov YR, Nikolin VP, Popova NA, et al. (2014) Identification of Cancer stem cells and a strategy for their elimination. Cancer Biol Ther 15(10): 1378-1394.
20. Ritter GS, Dolgova Ev, Petrova DD, Efremov YR (2022) The new general biological property of stem-like tumor cells Part I. peculiarities of the process of the double-stranded DNA fragments internalization into stem-like tumor cells. Front Genet 13: 954395.
21. Potter ES, Dolgova EV, Proskurina AS, Zavylov EL, Taranov OS, et al. (2017) Gene expression profiling of tumor initiating stem cells from mouse krebs-2 carcinoma using a novel marker of poorly differentiated cells. Oncotargets 8(6): 9425-9441.
22. Ribeiro S, Puckert C, Ribeiro C, Gomes AC, Higgins MJ, et al. (2020) Surface charge-mediated cell surface interaction on piezoelectric materials. ACS Appl Mater Interfaces 12(1): 191-199.
23. Kim DY, Kwon JS, Lee JH, Jin LM, Kim JH, et al. (2015) Effects of the surface charge of stem cell membranes and DNA/Polyethyleneimine Nanocomplexes on gene transfection efficiency. J Biomed Nanotechnol 11(3): 522-530.
24. Higuchi Y, Takafuji Y (2021) Controlling cell dynamics by cell surface modification. Yakugaki Zasshi Journal 141(5): 661-665.
25. Huang Z, Zhou Z, Ye Q, Li X, Wang T, et al. (2024) Effects of different surface functionalization of silica nanoparticles on mesenchymal stem cells. ACS Appl Bio Mater 7(5): 3296-3305.
26. Butow P, Sharpe L, Thewes B, Turner J, Gilchrist J, et al. (2018) Fear of cancer recurrence: a practical guide for clinicians. Oncology (Williston Park) 32(1): 32-38.
27. Brookman-May SD, May M, Shariat SF (2013) Time to recurrence is a significant predictor of cancer-specific survival after recurrence in patients with recurrent renal cell carcinoma-results from a comprehensive multi-centre database. BJU Int 112(7): 909-916.