The International Agency for Research on Cancer (IARC) via its GLOBOCAN series estimated that about 12.7 million people were diagnosed with cancer and approximately 7.6 million died from the disease in 2008 [1
]. Hematologic malignancies, consisting of leukemia, Hodgkin lymphoma, non-Hodgkin lymphoma and myeloma, account for about 9% of all cancers and are the fourth most commonly diagnosed cancers in both men (after prostate, lung and colorectum) and women (after breast, lung, and colorectum) in economically advanced nations of the world [2
]. Cancer chemoprevention refers to prevention of cancer using a number of intervention strategies such as administration of either a pharmacologic agent or a chemical constituent of natural diet, the ultimate goal of which is to prevent, halt or reverse the development of cancer (carcinogenesis) at different levels [3
]. Phenolic compounds (polyphenols) are made up of various groups of metabolites commonly found in the human diet and represent different forms of chemo-preventive agents that have been extensively studied in the last decade [3
]. They are widely distributed in the plants and found naturally in a variety of fruits and vegetables [5
] as well as in honey, propolis and royal jelly [6
]. The last few years have witnessed a tremendous research interest in phenolic compounds [3
], this is because there are available evidences from in vitro in vivo
, and epidemiological studies as well as from clinical trials that are supporting their role as antioxidants, anti-inflammatory, and anti-carcinogenic agents as well as in the treatment of heart diseases [9
]. It is now fully established that consumption of an appropriate diet can prevent development of cancer [3
]. So far, the conventional therapeutic and surgical modalities of treatment of cancers have failed to check the increased incidence of most cancers; it is therefore, as a matter of urgency, necessary to establish sound action plans for the purpose of reducing or preventing the rampant incidence of cancers. Use of chemo preventive agents such as polyphenols may be one such strategy. Polyphenols have met the above definition of chemoprevention by virtue of their ability to inhibit the different stages of cancer development i.e.
, tumor initiation, promotion and progression, which could be achieved through inactivation of carcinogen or inhibition of its formation, growth inhibition, cell cycle arrest, induction of apoptosis and differentiation, angiogenesis inhibition, and anti-oxidation or combination of these effects [3
1.1. Cell Cycle
The cell cycle constitutes a series of molecular events of cell division during which cells grow and divide to give rise to new daughter cells. It is generally divided in to four phases, namely G1
(gap 1 phase), S (synthesis phase), G2
(gap 2 phase), and M (mitosis phase). G1
are two breaks with variable duration separating the S and M phases as depicted in and ; this gives the cycle its general illustration of G1
]. Disruption of execution of normal cell cycle plays a vital role in the development of cancer [21
]. The cell cycle is regulated by two vital classes of molecules: The cyclin (cyclin A–cyclin T) and the cyclin-dependent protein kinases (CDK 1–CDK 9). CDK/cyclin complexes are in turn excited by CDK-activating kinase (CAK) also known as cdk7/cyclin H. Growth of cell is necessary before S phase of the cell cycle begins and c-myc in addition to its role in both cell cycle and cell proliferation, is believed to play a key role in this important event of increase in cell volume [19
phase is a resting phase during which cells stop undergoing cell cycle including replication. These nonreplicating cells are either quiescent or senescent cells. It is now believed that anticancer agents that target cell cycle may significantly determine the success of cancer chemotherapy, as well as provide clues in finding a complete cure for many tumors, hence, targeting the different stages of cell cycle could play a vital role in the discovery of new anticancer drugs [18
Four phases of cell cycle (G1-S-G2-M).
Relationship between the different phases of cell cycle.
Apoptosis or programmed cell death is a unique type of cell death observed in both physiological and pathological conditions. The hallmark features of apoptosis include both morphological and biochemical changes in the form of cell shrinkage, DNA fragmentation, membrane blebbings, chromatin condensation and loss of adhesion and rounding [25
]. Apoptosis is regulated by various proteins, notably p53, IAPs, and caspase and Bcl families. It is believed that receptor and mitochondrial mechanisms of apoptosis are triggered in response to cancer treatment. Caspases that regulate apoptosis are classified in to initiator and effector caspases. Bcl-2 family proteins predominantly found in mitochondrial membrane play a vital role in mitochondria mediated apoptotic cell death and are made up of both anti-apoptotic (Bcl-2, Bcl-XL
, and Mcl-1) and pro-apoptotic (Bax, Bck, and Bad) molecules as well as BH3 domain only (Bid, Bim, Puma and Noxa) molecules which are also believed to be pro-apoptotic proteins. IAPs comprise of molecules that exert inhibitory effects on activity of caspase-3, 7 and 9 [27
The p53 gene is very vital in the regulation of both cell cycle and apoptosis; therefore, whenever there are insults to the cells in the form of growth factor deprivation, DNA damage or expression of oncogenes, it will be stimulated and result in either cell cycle arrest or apoptotic cell death [28
]. The cell death induced by p53 upon its activation is believed to be mediated by the release of mitochondrial cytochrome c and Smac/DIABLO. The released cytochrome c appropriates the formation of caspase 9 and adapter protein Apaf-1. The activated caspase 9 in turn activates effector caspases-3 and -7 which eventually cause the cell death. The activation of caspases is aided by the regulatory effects of apoptogenic factors Smac/DIABLO on IAPs. The release of cytochrome c and Smac/DIABLO is mediated by the Bcl-2 family [27
]. Another important molecular event is the inhibition of CDKs by two families of proteins namely: INK4 family consisting of p16INK4a
which specifically inhibits CDK4 and CDK6, and the CIP/KIP family which has stimulatory effect on p21cip1/waf1
is a well-known inhibitor of CDKs [30
Apoptosis yields numerous hints concerning efficient cancer chemotherapy, and many of the reported anticancer agents exert their chemotherapeutic effects by way of inducing apoptotic cell death [25
Honey is a natural sweet substance, produced by honey bees from the nectar of plants or from secretion of living parts of plants or excretion of plant-sucking insects on the living parts of plants, which the bees collect, transform by combining with specific substances of their own, deposit, dehydrate, store and leave in honeycombs to ripen and mature [31
]. It has a long history of human consumption and is commonly used for both nutritional and medicinal purposes. Honey contains carbohydrates, protein and amino acids, vitamins, and phenolic compounds [32
]. Although the anticancer effects of crude honey in various cancer types has been reported [39
], to the author’s knowledge, no similar report is available on the effect of crude honey on leukemia and other hematologic malignancies. We present a review on the evaluation of biological activities of the various phenolic compounds in honey on cell cycle, cell growth and proliferation, as well as induction of apoptosis, with a view that subsequent elucidation of the molecular mechanisms of their actions will provide considerable hint for the development of newer cancer chemotherapeutic and chemo preventive agents. However, this report is not aimed at providing an exhaustive review on this subject matter as the presence of a number of polyphenols in honey is yet to be confirmed, it therefore only concentrates on those phenolic compounds already reported to be found in honey.
1.4. Major Phenolic Compounds Present in Honey
A bulk of the phenolic compounds found in honey is in the form of flavonoids [8
]. However, other classes of phenolic compounds are also present in appreciable amount. summarizes the different classes of phenolic compounds present in honey [3
Phenolic compounds found in honey.