Authors: Mariella Lo Schirico, Neftj Ragusa
Introduction
In human tradition honey has been used since long time both in medical and domestic applications. Its composition is rather variable and depends on the floral source and on external factors, such as seasonal, environmental conditions and processing.
Honey is a supersaturated solution of sugars, mainly composed of fructose (38%) and glucose (31%), but it contains also minerals, proteins, free aminoacids, enzymes, vitamins and polyphenols.
Among polyphenols, flavonoids (such as chrysin, galangin, quercetin, acacetin, etc) are the most abundant and they are closely related to the biological functions of honey.
Honey has positive effects on various chronic diseases. In fact it has been seen to decrease risk factors for cardiovascular diseases by inhibiting inflammation, improving endothelial function, as well as the plasma lipid profile, and increasing low-density lipoprotein (LDL) resistance to oxidation. Honey also modulates the glycemic response in a positive way, by reducing blood glucose, serum fructosamine or glycosylated hemoglobin concentrations and has antibacterial properties due to its high sugar concentration and acid pH (between 3.2 and 4.5). The antibacterial activity of honey is also related to the enzyme glucose oxidase, that becomes active when honey is diluted (e.g. by body fluids). This enzyme catalyses the oxidation of glucose to hydrogen peroxide and gluconic acid:
C 6 H 12 O 6 + H 2 O + O 2 → C 6 H 12 O 7 + H 2 O 2
Honey also has a non-peroxide antibiotic activity due to methylglyoxal (MGO) and bee defensin-1. Most honeys contain very low levels of MGO, but others, such as manuka honey, contain very high levels.
Recently, there are many studies focusing on the use of crude honey for cancer prevention and treatment, that could lead to a new approach for treating cancer instead of chemotherapy and radiotherapy, which themselves are potentially toxic to non-tumoral viable cells of the body. Moreover, some studies demonstrated that simultaneous treatment with a chemotherapeutic drug plus honey led to a highly significant improvement in overall animal survival.
The mechanism of the anti-cancer activity of honey as chemopreventive and therapeutic agent has not been completely understood yet. The possible mechanisms are related to its apoptotic, antiproliferative, antitumor necrosis factor (anti-TNF), antioxidant, anti-inflammatory, estrogenic and immunomodulatory activities.
We examined the role of honey in these anti-tumoral mechanisms.
Honey as a Potential Natural Anticancer Agent: A Review of Its Mechanisms, 2013
Antiproliferative Effects of Honey and of Its Polyphenols: A Review, 2009
Honey and Its Apoptotic Activity
Apoptosis or programmed cell death is now considered as a vital process in the regulation of tissue development and homeostasis. According to this, a dysregulation in its mechanisms can lead to biological alterations, such as in cancer cells, that are characterized by uncontrolled cellular proliferation and inadequate apoptotic pathways. Therefore, many chemotherapeutic drugs are apoptosis inducers.
Apoptosis can be divided into three phases:
1. induction phase, that stimulates proapoptotic signal transduction cascades through death-inducing signals
2. effector phase, that brings cell death through the mitochondrion
3. degradation phase, that consists of nuclear and cytoplasmic events, such as chromatin and nuclear condensation, cell shrinkage, DNA fragmentation, and membrane blebbing; in the cytoplasm, a complex cascade of protein cleaving enzymes, called caspases, is activated.
The cell is finally destined into fragmented apoptotic bodies which are phagocytosed by macrophages or other surrounding cells.
Apoptosis usually follows two pathways: the caspase 8 or extrinsic pathway and caspase 9 or intrinsic pathway.
There are experimental evidences that honey induces programmed cell death in different types of cancer cells (e.g. colon, renal, breast cancer). In fact flavonoids contained in honey (such as acacetin) improve the expression of Fas-ligand and p53, caspase 3, and proapoptotic protein Bax, and downregulate the expression of antiapoptotic protein Bcl2: so both the estrinsic and intrinsic apoptotic pathways are activated, leading to the induction of the cascade of caspases.
Honey-induced apoptosis is also associated with poly ADP-ribose polymerase cleavage, inducing DNA fragmentation.
These effects can be attributed to honey's high tryptophan and phenolic content.
The apoptotic property of honey makes it a possible natural substance as anti-cancer agent as well as many chemotherapeutics currently used.
Honey and Its Antiproliferative Activity
Epithelial cell divides during life. The cell cycle can be divided into four distinguished phases known as G1, S, G2 and M. Quiescent cells that do not divide are in G0 phase.
All the events in the cell cycle are controlled and monitored by several different proteins. The group of factors regulating the cell cycle includes cyclins and cyclin-dependent kinases.
The G1/S phase transition is a vital regulatory point where cell's fate is destined for quiescence, proliferation, differentiation and apoptosis. Overexpression and dysregulation of cell cycle growth factors such as cyclin D1 and cyclin-dependent kinases (CDK) are linked with tumorigenesis. The loss of this regulation is the hallmark of cancer. The nuclear protein Ki-67 is a marker to examine the growth fraction of cell proliferation. It is absent in the resting phase (G0) but expressed during the cell cycle in all the proliferation phases (G1, S, G2, and mitosis).
Honey has been shown to cause cell cycle arrest. Administration of honey mixed with Aloe vera solution showed a significant decrease in expression of Ki-67 in tumor cells in rats: it suggests that honey therapy could lead to lowering tumor cell proliferation by arresting cell cycle. Honey and its several components (like flavonoids and phenolics) are reported to block the cell cycle of colon, glioma, and melanoma cancer cell lines in G0/G1 phase (Honey as a Potential Natural Anticancer Agent: A Review of Its Mechanisms, 2013). Moreover, honey possessing higher phenolic and tryptophan content was more potent in inhibiting the colon cancer cell proliferation. This inhibitory effect on tumor cell proliferation follows the down-expression of different cellular pathways through "ornithine decarboxylase:http://en.wikipedia.org/wiki/Ornithine_decarboxylase and kinase. The results of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) and the trypan blue exclusion assays have confirmed that anti-proliferative effect of honey is a dose- and time - dependent manner (Intravenous Administration of Manuka Honey Inhibits Tumor Growth and Improves Host Survival When Used in Combination with Chemotherapy in a Melanoma Mouse model, 2013).
Honey and Its Effect on Tumor Necrosis Factor (TNF)
Tumor necrosis factor α (TNF-α) have been shown to play a role in tumor initiation and development. The proinflammatory effect of TNF is linked to many diseases because of its capacity to activate NF-kB. Activation of NF-kB leads to the expression of genes involved in inflammatory processes, like cyclooxygenase-2 (COX-2), cell-adhesion molecules, chemokines, inducible nitric oxide synthase (iNOS) and inflammatory cytokines, thus TNF-a takes part in defense mechanisms as a key cytokine.
On the other hand, TNF-α is also involved in death signaling. It could be due to the binding of TNF-R to TNF-α and adaptor protein, such as TNFR-associated-death-domain protein (TRADD), TNF receptor associated factor (*TRAF*), and receptor-interacting protein (*RIP*) that regulate apoptosis and inflammation. However, TNF-induced cell death plays only a minor role compared to its functions in the inflammatory process. Its death-inducing capability is weak compared to other family members (such as Fas), and often masked by the anti-apoptotic effects of NF-κB. Nevertheless, TRADD binds FADD, which then recruits the cysteine protease-caspase-8, activating the extrinsic apoptotic pathways.
Many factors, such as cell type, concurrent stimulation of other cytokines, or the amount of reactive oxygen species (ROS) can shift the balance in favor of one pathway or another.
Royal jelly proteins (such as apalbumin-1 and apalbumin-2) in honey have antitumor properties. In fact these proteins promote the release of TNF-α, interleukin-1(IL-1) and interleukin-6 (IL-6) by macrophages. (The immunostimulatory effect of the recombinant apalbumin 1-major honeybee royal jelly protein-on TNFalpha release, 2005).
This release can play a central role as an important cytokine to modulate important cellular mechanisms such as apoptosis, cell proliferation, and inflammation.
Honey and Its Anti-Inflammatory and Immunomodulatory Activities
Chronic inflammation can prevent healing from cancer by damaging tissues. Honey affect anti-inflammatory response. The literature shows that it reduces inflammation when applied in cell cultures and animal models. (Manuka honey protects middle-aged rats from oxidative damage, 2013; Honey with high levels of antioxidants can provide protection to healthy human subjects, 2003).
The inflammatory process is induced by various types of chemicals and biological agents, including proinflammatory enzymes and cytokines. The enzyme cyclooxygenase-2 (*COX-2*) in inflammatory process catalyses the metabolism of arachidonic acid to prostaglandins. Anomalous arachidonic acid metabolism is involved in carcinogenesis and inflammation. In fact COX-2 is overexpressed in premalignant and malignant conditions.
Phenolic compounds in honey are responsible for anti-inflammatory activity. The mechanism involves the suppression of the proinflammatory activities of COX-2 and/or inducible nitric oxide synthase (*iNOS*) through these phenolic compounds or flavonoids. Honey and its components have been documented to be involved in regulation of proteins such as ornithine decarboxylase, tyrosine kinase, iNOS, and COX-2.
Manuka, Pasture, Nigerian Jungle, and royal jelly honeys are found to increase IL-1β, IL-6, and TNF-α production. This immunomodulatory and immunoprotective activity of honey is often linked to anticancer action. Honey stimulates antibodies, B and T lymphocytes, neutrophils, monocytes, eosinophils, and natural killer cells (*NK-cells*) production during primary and secondary immune responses in tissue culture. It has been shown that honey stimulates macrophages, T-cells, and B-cells to provoke antitumor effect.
Honey and Its Antioxidant Activity
The role of oxidative stress involving free radicals in the carcinogenic process is well established. Reactive oxygen species (*ROS*) and reactive nitrogen species (*RNS*), such as hydroxyl radical (•OH), superoxide (O 2 •−), hydrogen peroxide (H 2 O 2 ), nitric oxide (NO•), peroxynitrite (ONOO−), and others, are oxidative stress agents which damage lipids, proteins, and DNA in cells. Cells exhibit defense system against oxidative damage. This defense system consists of antioxidants or oxidative protective agents such as catalase, superoxide dismutase, peroxidase, ascorbic acid, tocopherol, and polyphenols.
Antioxidants acting as free radical scavengers may inhibit the cancer process in vivo. The exact antioxidant mechanism is unknown, but the proposed mechanism is through hydrogen donation, free radical sequestration, metallic ion chelation, flavonoids substrates for hydroxyl and superoxide radical actions. The antioxidant capacity of honey contributes to the prevention of several acute and chronic disorders such as diabetes, inflammatory disorders, cardiovascular diseases, and cancer. The phenolic acids and flavonoids are responsible for the well-established antioxidant activity of honey.
Daily consumption of 1.2 g/kg body weight of honey has been shown to elevate the amount and the activity of other antioxidant agents such as beta-carotene, vitamin C, glutathione reductase, and uric acid (Effects of daily consumption of honey solution on hematological indices and blood levels of minerals and enzymes in normal individuals, 2003).
Honey and Its Estrogenic Modulatory Activity
Estrogen is involved in various types of cancer. Honey modulates estrogen by its antagonistic action. It may be useful in estrogen-dependent cancers such as breasts and endometrial cancers. Estrogen receptors tie to estrogens to dimerize and then translocate into the nuclei. These complexes then bind to the specific DNA base sequences called estrogen-response elements (EREs) resulting in transcription and translation of the estrogenic effect in the targeted tissue.
This signaling cascade induced by estrogens may be modulated at any stage. Honeys from various floral sources are reported to mediate estrogenic effects via the modulation of estrogen receptor activity. This effect is attributed to its phenolic content. Greek honey extracts exert estrogen agonistic effect at high concentrations and antagonistic effect at low concentrations.
(Honey as a Potential Natural Anticancer Agent: A Review of Its Mechanisms, 2013)
Conclusions
Evidence is growing that honey may have the potential to be anticancer agent through several mechanisms. Though the full mechanism hasn't been fully understood yet, studies have shown that honey has anticancer effect through its interference with multiple cell-signaling pathways, such as inducing apoptosis, antioxidant, antiproliferative pathways, and modulating the body immune system.
However more research is needed to improve our understanding of the positive effect of honey in cancer, and to investigate the real benefits for humans.
