Propolis: a review of its anti-inflammatory and healing actions
Abstract
Tissue healing is an adaptive biological response by which the organism repairs damaged tissue. The initial stage of healing is represented by an acute inflammatory reaction, in which inflammatory cells migrate to damaged tissue and phagocyte debris. At a later stage, fibroblasts and endothelial cells proliferate and generate a scar. The occurrence of inflammatory processes and healing imperfections have been a concern for hundreds of years, especially for individuals with healing difficulties, such as diabetics and carriers of peripheral circulation deficiencies. A wide variety of natural products have been used as anti-inflammatory and healing agents, with propolis being a remarkable option. Propolis has been used in popular medicine for a very long time; however, it is not a drug intended for all diseases. Currently, the determination of quality standards of propolis-containing products is a major problem due to their varying pharmacological activities and chemical compositions. The aim of this review is to discuss the use of propolis with emphasis on its anti-inflammatory and healing properties.
REVIEW ARTICLE
Propolis: a review of its anti-inflammatory and healing actions
Ramos A. F. N.; Miranda J. L.
Laboratory of Pathology, Department of Basic Sciences, Federal University of Jequitinhonha and Mucuri Valleys, Minas Gerais State, Brazil
Correspondence to
ABSTRACT
Tissue healing is an adaptive biological response by which the organism repairs damaged tissue. The initial stage of healing is represented by an acute inflammatory reaction, in which inflammatory cells migrate to damaged tissue and phagocyte debris. At a later stage, fibroblasts and endothelial cells proliferate and generate a scar. The occurrence of inflammatory processes and healing imperfections have been a concern for hundreds of years, especially for individuals with healing difficulties, such as diabetics and carriers of peripheral circulation deficiencies. A wide variety of natural products have been used as anti-inflammatory and healing agents, with propolis being a remarkable option. Propolis has been used in popular medicine for a very long time; however, it is not a drug intended for all diseases. Currently, the determination of quality standards of propolis-containing products is a major problem due to their varying pharmacological activities and chemical compositions. The aim of this review is to discuss the use of propolis with emphasis on its anti-inflammatory and healing properties.
Key words: propolis, inflammation, anti-inflammatory action, healing properties, Apis mellifera.
Propolis is a resinous substance with varying colors and consistencies, collected by Apis mellifera bees from several vegetal sources. The word propolis comes from the Greek pro meaning in defense of and polis city, defense of beehives (3). In fact, bees use propolis to protect themselves from insects and microorganisms, employing it as a cement to seal cracks or open spaces in the hive, to sterilize the queen-bee posture site, and to mummify insect invaders. Commonly, small animals or parts of them are found wrapped within propolis in perfect states of conservation (23).
Propolis is a honeybee product with a very complex chemical composition, made by gummy and balsamic material collected by bees from sprouts, flower-buds, trees and other vegetal-tissue resinous exudates. During propolis collection, bees mix the beeswax and the collected propolis with the 13-glicosidase enzyme found in their saliva, hydrolyzing flavonoids glycosides into flavonoid aglycones (34). Afterwards, the collected material is augmented with enzymatic and salivary secretions.
Along with other honeybee products (honey, royal jelly, pollen), propolis has outstanding therapeutic properties (5), being used since 300 years B.C. in popular medicine in various parts of the world. However, interest in the correlation of propolis chemical composition with its pharmacological activities started only forty years ago (23). In Brazil, little is known about propolis, and only few studies have been conducted. Nonetheless, it is widely used in popular medicine. Propolis is one of the few "natural drugs" being used for a long time by different civilizations (7). Currently, several propolis products are being commercialized worldwide, including candies, chocolate bars, shampoos, skin lotions, antiseptic mixtures, and toothpastes (1). The Persians, Greeks, Romans and Incas also used propolis with therapeutic purposes. In ancient Egypt, it was employed to embalm the dead, while in the Balkans States propolis is widely used. In France, the term propolis is found in the general literature since the sixteenth century. In South Africa, during the Anglo-Boer War, more than 90 years ago, it was used with vaseline for ointment preparation to heal war wounds. This helped save the lives of many soldiers, since antibiotics were not yet available (10). In the Second World War, Soviet medical clinics studied propolis with excellent results.
The color of propolis depends on its origin. It varies from dark-brown to reddish-brown, with a greenish tone. It has a typical odor, which can vary from sample to sample, with some being odorless. Flashing point ranges between 60 and 70º C and, in some cases, may reach up to 100º C. Generally, ethanol is the best solvent for propolis preparation, and other solvents such as ethyl ether, water, methanol and chloroform may also be used for extraction and identification of propolis compounds (24). In addition, glycerin, propylene glycol, and other solutions have been used during propolis preparation for pharmaceutical and cosmetic industries (46). Propolis obtained from beehives, also known as rude propolis, is composed of around 50% balsam resin, 30% wax, 10% essential and aromatic oils, 5% pollen, and 5% other substances, including wood fragments (31).
The resin found in propolis is collected in vegetation nearby beehives, where bees also collect pollen and nectar for feeding. Propolis composition is mainly determined by the phytogeographic characteristics of beehive surroundings (19). However, seasonal variation may occur within the same place (39). Also, variation was observed among samples collected in the same place, but by different A. mellifera subspecies (40). Several contaminants close by beehives can be collected and unexpectedly added to propolis, such as asphalt powder, pesticide, iron excess, copper, magnesium, (12) and even lead (2).
It is known that bees are selective when collecting resin from a specific vegetal source, but factors that guide them are not completely understood (37, 45). The nature of aromatic and terpene compounds found in propolis has a biological importance, allowing bees to determine which vegetal species to visit (23).
The pharmacological activities of propolis are more numerous in tropical regions than in temperate climates, reproducing the richer vegetal diversity observed in the former (4, 28).
More than 300 different compounds have been identified so far in propolis, including aliphatic acids, esters, aromatic acids, fatty acids, carbohydrates, aldehydes, amino acids, ketones, chalcones, dihydrochalcones, terpenoids, vitamins, and inorganic substances (23). Of all, flavonoids are the ones which draw greater research interest (15).
Propolis has several therapeutic properties, such as antibacterial, anti-inflammatory, healing, anesthetic (13), anticariogenic (34), antifungal, antiprotozoan and antiviral activities. The in vitro antibacterial activity was verified against several Gram-positive and Gram-negative bacteria and results from synergism between propolis compounds, mainly pinocembrin and galangin flavonoids. Other flavonoids, such as chrysin and kaempferol, have shown antiviral activity with reduction of intracellular proliferation of some viruses, such as herpes simplex (23).
Many other biological and pharmacological properties of propolis have been noted: cartilage, bone, and dental pulp regeneration; immunological properties; liver defense and antitoxic activity; antioxidant and immunomodulatory actions. Consequently, there has been increasing interest on propolis, with the chemical industry searching for viable commercial formulations. Also, investigation of isolated compounds, such as flavonoids, has grown. These compounds are biologically more active and are responsible for propolis spasmodic (quercetin, kaempferol and pectolinarigenin), anti-inflammatory (acacetin) and antiulcerative (apigenin) activities (11).
Propolis is a low-cost potential anti-inflammatory agent for both acute and chronic stages (6). Its properties are used mainly for muscles and articulations, and also other types of inflammations, infections, rheumatisms and torsions. Mice and rabbit studies have shown that hydroalcoholic solutions of propolis possess anti-inflammatory activity following topical, injectable, or even oral administration (23).
Inflammation is the complex biological response of vascular tissues to harmful stimuli such as cell damage by pathogens. It is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process. The tissue modifications induced by the causal agent are responsible for the release of inflammatory mediators that lead to subsequent inflammatory events. Cytokine release (IL-1, TNF-a) by activated macrophages leads to vessel dilation and results in smooth muscle relaxation and increased local blood flow (hypothermia). Microvascular changes associated with increased vascular permeability take place, leading to accentuated plasmatic exudation, phagocyte accumulation (neutrophils, monocytes, macrophages), and amplification of endogenous chemical mediators. Simultaneously, mast cells, phagocytic cells and endothelial cells use plasma membrane lipids to generate important inflammatory mediators (21). In an immediately posterior stage, several intra and extracellular phospholipases are activated by cytoplasmic membrane phospholipids and activate other enzymes, such as cyclooxygenase (COX) and lipoxygenase (LOX), which in turn act upon arachidonic acid and eicosanoid metabolism, creating important inflammatory mediators (prostaglandins and leukotrienes). These mediators are responsible for the maintenance of the inflammatory process. The fibrinolytic system, kinins, complement, vasoactive amines (histamine and serotonin), and nitric oxide (NO) may lead to inflammation when physiologically altered (47).
Endothelial-leukocyte cellular adhesion occurs in a sequence of events, and specific molecules are expressed in different stages. Selectins (E, P, and L), integrins (VLA-4 and LFA-1), and members of immunoglobulin super-families (ICAM-1 and ICAM-2) transfer leukocytes from the vascular lumen into the tissues (35). In response to several mediators, the vascular endothelium expresses specific glycoproteins on the cell surface, which mediates blood leukocyte connection and extravasation, an important event in tissue repair (3).
According to the frequency and duration of the injurious agent, the inflammatory process can be classified into acute and chronic. The acute-stage response involves serous, fibrinous, suppurative or exudative events as well as microvascular and cellular events; this response to phlogogen occurs within 72h. The chronic stage response includes proliferative events and histological alterations different from those in the acute stage, being characterized by cell emigration and intensive mitosis. Formation of giant multinuclear cells takes place, and all these events are induced by phlogogen (42).
In certain instances, inflammation must be regulated by using specific drugs, since it may lead to toxic consequences to the organism. The damage to the organism during the inflammatory response is induced by free radicals produced by active macrophages and neutrophils. These molecules degrade lipid acids of the plasma membrane, disrupt membrane proteins, and induce DNA mutations. NO is another potent inflammatory mediator produced by endothelial and inflammatory cells that can induce tissue damage if synthesized in large quantities (21).
Some anti-inflammatory substances found in propolis have been isolated. According to Mirzoeva & Calder (29), these substances are caffeic acid, quercetin, naringenin, and caffeic acid phenethyl ester (CAPE). These compounds contribute to the suppression of prostaglandins and leukotrienes synthesis by macrophages and have inhibitory effects on myeloperoxidase activity, NADPH-oxidase, ornithine decarboxylase and tyrosine-protein-kinase (30). Krol et al. (18) attributed propolis anti-inflammatory activity to other compounds, including salicylic acid, apigenin, ferulic acid and galangin.
It is known that macrophages are involved in several body physiology processes, such as phagocytosis, enzyme release, free radical generation, and inflammation. Scheller et al. (38) suggested that propolis immunostimulant activity may be associated with macrophage activation and enhancement of macrophage phagocytic capacity. The results of the study of Orsi et al. (33) corroborated the findings of Schelle, which indicated that macrophages produce high amounts of H 2 O 2 .
While investigating the effects of individual propolis compounds complexes on lysine, Ivanovska et al. (17) found that cinnamic acid tends to inhibit H 2 O 2 release by peritoneal macrophages, whereas caffeic acid induces increased metabolite production.
The inhibition of NO production by macrophages may also be responsible for propolis anti-inflammatory activity (32). Hu et al. (16) evaluated the anti-inflammatory effects of ethanol (EEP) and water (WSD) extracts in ICR mice and Wistar rats on thoracic capillary vessel leakage, carrageenan-induced edema, carrageenan-induced pleurisy, acute lung damage, and Freunds complete adjuvant (FCA)-induced arthritis. In the experiment, EEP and WSD inhibited the increase of PGE 2 and also had a significant inhibitory effect on NO in carrageenan-induced pleurisy exudation. In these models, NO could accelerate an inflammatory reaction by enlarging blood vessels and causing edema. This could increase the expression of inflammatory reactions and accelerate the development of blood poisoning by activating prostaglandin synthesis, as seen in the progression of rheumatism. However, other studies indicated increased NO production by macrophages (32).
Although the mechanism of WSD and EEP on anti-inflammatory performance was apparently similar, there were some differences. In the carrageenan-induced pleurisy and oleic-acid plus LPS-induced acute lung damage studies, WSD not only inhibited the increase of white blood cells (WBC) count, but also inhibited the increase of neutrophils (but not significantly at 5%). This would explain how WSD inhibits WBC and alleviates inflammatory reactions during acute inflammation. The results suggest that additional propolis components, other than flavonoids, possess anti-inflammatory effects. Although EEP did not significantly inhibit WBC, it may possibly alleviate the inflammatory degree synergistically by inhibiting NO. The arthritic rat model induced by FCA is associated with an immune inflammation reaction in these experiments. The main feature of rheumatoid arthritis (RA) is the ongoing damage in arthrosis of cartilage and bone, in addition to a disturbance of the immune function. Cytokines secreted by immune cells (lymphocyte and mononuclear-macrophage), fibroblasts, and endothelial cells play an important role in immune and inflammatory responses in vivo. The results of the present experiment show that EEP and WSD had significant inhibitory effects on the levels of IL-6 in FCA-induced arthritis rats, but not on IFN and IL-2 levels. This could also mean that, in the course of the anti-inflammatory activity of WSD and EEP, the humoral immune system plays an important role inhibiting the activation and differentiation of mononuclear macrophages. This would be a possible mechanism for the anti-inflammatory and immune effects of WSD and EEP (16).
Inflammation can also be related to free radicals increase in the human organism (9). Although oxidative damage is known to be involved in inflammatory-mediated tissue destruction, modulation of oxygen-free radicals production represents a new approach to the treatment of inflammatory diseases (8). Propolis contains polyphenols and a wide range of other compounds capable of removing excessive free radicals from our organism (25). CAPE, a flavonoid-like compound, has been identified as one of the main active ingredients of honeybee propolis and has antioxidant and anti-inflammatory properties (8). For that reason, Celik et al. (8) investigated the efficiency of CAPE administration in preventing oxidative damage due to Escherichia coli-induced pyelonephritis (PYN) in rats. The main hypothesis of events leading to renal scarring has been that bacterial products (e.g. lipopolysaccharide) stimulate the release of proinflammatory cytokines, which initiate an inflammatory response, including chemotaxis with consequent extravasation of polymorphonuclear leukocytes (PNL) (8). PNL release toxic products (e.g. free-oxygen radicals and lysozymes) that seem to be responsible for tissue damage; inhibition of the PNL-produced free radicals can greatly neutralize tissue damage (14).
The levels of lipid peroxidation and NO production, and the activities of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and xanthine oxidase (XO) in E. coli-induced PYN were evaluated in a rat model using kidney homogenates and were significantly increased. However, CAPE administration reduced malondialdehyde (MDA) levels, which is an indicator of free radical generation that increases in final stages of lipid peroxidation. NO levels have been implicated in the mechanisms of cell injury and long-term physiological changes in cellular excitability. This effect may be due to decreased expression of inducible NO (iNOS) (14). In a similar study, Song et al. (41) demonstrated that CAPE was able to inhibit iNOS expression and NO production in macrophages. CAPE significantly increased the activities of the antioxidant enzymes SOD and GSH-Px in the kidneys of infected rats. CAPE administration significantly inhibited the activity of XO, a physiological source of superoxide anions in eukaryotic cells. Histopathologic examination showed that CAPE reduced inflammation induced by E. coli. In summary, CAPE administration decreases the oxidative damage occurring in PYN and thus could be used for medical management of bacterial nephropathy.
The effects of CAPE on cyclooxygenase-2 (COX-2) expression in Harvey-ras-transformed WB-F344 rat live epithelial cells (H-ras WB cells) have also been studied. Several reports have shown that activation of Ras signaling pathways is involved in the induction of COX-2 and matrix metalloproteinase expression. COX catalyzes the critical conversion of arachidonic acid into prostaglandins, which are important mediators of the inflammatory process. Improper up-regulation of COX-2 is relevant to the pathophysiology of inflammatory disorders. The eukaryotic transcription factor nuclear factor kB (NF-kB) plays a central role in general inflammatory as well as immune responses. The 5'-flanking region of the COX-2 promoter contains NF-kB binding sites. In agreement with this concept, NF-kB has been shown to be a critical regulator of COX-2 expression in many cell lines (43). CAPE significantly inhibited the constitutive expression of COX-2 and several studies suggest that CAPE is a potent and specific inhibitor of NF-kB activation. In those studies, histopathological examinations showed that CAPE significantly suppressed inflammation. CAPE has been demonstrated to specifically and completely block the activation of NF-kB induced by a wide variety of inflammatory agents, including TNF and H 2 O 2 . The activation of NF-kB proteins is induced by many factors, such as inflammatory cytokines (IL-1, TNF), bacterial products, and oxidative stress. Others have shown that CAPE not only inhibits transcription factors, but also reduces the production of IL-8 and monocyte chemotactic protein (20).
All the above-mentioned data have demonstrated different mechanisms of inflammatory inhibition of several propolis preparations or its isolated compounds. Nevertheless, the anti-inflammatory effects of propolis depend mainly on the administration mode and dosage (29).
A wide variety of natural products have been used in wounds treatment due to their easy application, innocuity, low cost, and bactericidal/bacteriostatic effect (26). Propolis is remarkably used in dermatology for wounds healing, burn and external ulcers treatment, healing time reduction, wound contraction increase, and tissue repair acceleration. During wound healing, perfect synchronized cellular and molecular interactions occur to repair damaged tissue (22). Healing is a dynamic process involving harmonious biochemical and physiological stages, such as inflammation, fibroblastic and tissue maturation. The wound healing process can be summarized using the healing of a linear skin wound as a prototype. With any wound, the initial events at the site of injury are hemorrhage and formation of a fibrin-rich clot. Fibronectin stabilizes the clot before dehydration takes place and a scab is formed. Macrophages soon follow neutrophils to the site of tissue injury and wound debridement is aided by the opsonization of tissue by fibronectin. The epidermal response to wounding is initiated very rapidly and within a day or so "tongues" of epidermal cells can be seen cleaving a path between the scab and the viable collagenous tissue underneath; two or three days after injury, the wound floor is covered by a sheet of regenerated epidermal cells. The formation of granulation tissue begins at about the same time with an influx and proliferation of fibroblasts and the beginnings of new capillary formation. Four to five days after wounding, stratification of the wound-floor epidermis is readily apparent, fibroblasts are highly active and secretion of extracellular matrix compounds (especially GAGs and type III collagen) and the process of neovascularization are in full swing. To achieve this invasive process, endothelial cells must secrete a battery of proteinases (e.g. type IV collagenase, interstitial collagenase, elastase, stromelysin, and plasminogen activators) to degrade the basement membrane. The diverse components of the epidermis are restored without the reformation of rete ridges, much of the type III collagen fibers orient parallel to lines of mechanical stress (collagen remodeling), and the once highly vascular granulation tissue undergoes a protracted process of vascularization as it matures into relatively avascular scar tissue (27). Sutta et al. (44) used propolis alcoholic solutions at animal wounds treatment in clinical and also experimental cases. Histologically, they observed that propolis treatment induced better healing by reducing the inflammatory response; consequently, epithelial healing was faster with propolis. The authors considered propolis suitable for wound treatment, following elimination of the infection. It is known that healing is directly related to the inflammatory process (36), and if the latter is less pronounced, production of healing molecules and deposition of collagen fiber bundles increase. It is possible that prolonged inflammation unleashes pronounced necrosis, causing more tissue damage and rendering the healing process more difficult. Propolis tissue regeneration properties, including healing, are possibly due to its antioxidant activity. Whenever free radicals are produced, they hamper or even block cells regeneration. Removal of free radicals by propolis flavonoids would allow regeneration of an ill organ or tissue in an ordinary way (23).
The occurrence of the inflammatory process, along with imperfections in healing, has been a problem for centuries. Despite advances on anti-inflammatory, antibiotic and healing treatment, infections continue to be a major reason of concern, especially for individuals that present difficulties in healing, such as diabetics and peripheral circulation deficiency carriers.
It should be stressed that propolis is not a drug for all diseases. A major challenge currently is the determination of which types of propolis are indicated for which medical use, the appropriate dosages, and what effects propolis has on humans and animals, since product quality varies widely. Several researchers have proposed biological assays as well as quantitative analyses of chemical compounds from different propolis samples. Quality-control problems have been confirmed in countries where propolis is commercialized. Hypersensitivity responses induced by propolis, especially those derived from cinnamic acid, have been reported; in the future, an old question will have to be answered: which propolis is best suited for which disease? Therefore, propolis precise pharmacological properties must be determined. With further research and knowledge, development of new drugs will become possible.
Correspondence to: João Luiz de Miranda Laboratório de Patologia, Universidade Federal dos Vales do Jequitinhonha e Mucuri Rua da Glória, 187, Centro 39.100-000, Diamantina, Minas Gerais, Brasil Email:
Received: March 19, 2007
Accepted: May 7, 2007
Abstract published online: May 8, 2007
Full paper published online: November 30, 2007
Conflicts of interest: there is no conflict
Propolis: a new frontier for wound healing? - Burns & Trauma
Characteristics of propolis
Nectar and pollen, the main materials collected by honeybees, are referred to by their botanical names. “Propolis” is a bee-oriented term that does not have a botanical derivation. Bees can use different materials for “manufacturing” of propolis, and these materials are produced in different botanical parts of plants.
These are substances actively secreted by plants and substances exuded from wounds in plants: lipophylic materials on leaves and leaf buds, mucilages, gums, resins, latices, etc. [7].
At elevated temperatures, propolis is soft, pliable, and very sticky; however, when cooled and particularly when frozen or at near freezing, it becomes hard and brittle.
Propolis becomes liquid at 60 to 70 °C, but for some samples, the melting point may be as high as 100 °C. Color of propolis varies from green to brown and reddish, depending on its botanical source.
The propolis’ composition as well as its color and aroma change according to the geographical zones.
Generally, ethanol is the best solvent for propolis preparation, and other solvents such as ethyl ether, water, methanol, and chloroform may be used for extraction and identification of propolis compounds [8].
Other solvents such as glycerin, propylene glycol, and other solutions have been used for propolis preparation for pharmaceutical and cosmetic industries [9].
Propolis obtained from hives, or rude propolis, contains around 50 % balsam resin, 30 % wax, 10 % essential and aromatic oils, 5 % pollen, and 5 % other substances, including wood fragments [5].
More than 300 different compounds have been characterized so far in propolis, including aliphatic acids, esters, aromatic acids, fatty acids, carbohydrates, aldehydes, amino acids, ketones, chalcones, dihydrochalcones, terpenoids, vitamins, and inorganic substances. Of all, flavonoids are the compounds that possess greater research interest [10].
The positive activities of propolis are more numerous in tropical regions than in temperate climates, reproducing the richer vegetal diversity observed in the former [11].
However, such properties can change depending on the composition and polyphenol content that in turn depend on several factors including season, vegetation of the area, geographical origin, and the state of propolis (fresh or aged) [12].
The composition of propolis is guided by the phyto-geographic characteristics of beehive surroundings [13], see also Table 1. Seasonal variations occurring within the same place has been described. Some authors have also reported variations among samples collected in the same area but by different Apis mellifera subspecies [14].
Table 1 Most important propolis types: geographical origin and major constituents Full size table
Propolis collected from many countries (such as China, Korea, Croatia, New Zealand, and Africa) [15–18] showed chemical composition similar to poplar. In fact, poplar tree is a typical tree in Europe and is used to name the common type of propolis. Therefore, the current opinion is that bees collect propolis from resins of poplars and conifers. This propolis is characterized by high level of flavonones, flavones, low phenolic acids, and their esters.
However, in some areas, poplars are not native plants, such as in Australia and South America and bees seek out other plants to produce propolis, which has a similar composition to the poplar propolis.
Propolis from the tropical area, such as Brazilian green and red propolis, are rich respectively in prenylated derivatives of p-coumaric acid and some isoflavonoids that are different from the ones found in poplar propolis [19].
For the propolis of the southeast Brazil, Baccharis dracunculifolia is the main botanical source and the Artepillin C, as the most peculiar component, makes it easy to discriminate from other type of propolis [20, 21].
It has also been reported that propolis from Venezuela, Amazon, and Cuba contains prenylated benzophenones, which are originated from exudates of Clusia flower [22].
Macaranga plants have been established to be the font of propolis in Taiwan and Japan [20]. High concentrations of diterpenoids in Mediterranean basin propolis may originate from Cupressus and Pinus plants [23, 24].
However, some of plant sources are just hypothesized by observing the foraging behaviors of bees and not comparing the chemical composition of secondary metabolites in propolis and in the plant source. So, there is a strong need to compare the chemical composition of propolis and of plants to confirm the exact botanical origin [25].
Some contaminants such as pesticide, copper, iron, magnesium, and even lead can be collected by bees and added to propolis [26].
Biological effects of propolis compounds
An inclusive spectrum of positive biological activity of propolis with respect to the human body largely results from the anti-oxidative effects of polyphenols [27]. Unsettling of the balance between production and deactivation of reactive oxygen species (ROS) leads to many disorders. Free radicals can oxidize cell proteins, nucleic acids, and lipids.
The mechanisms of the anti-oxidative activity of polyphenols are different, such as the ability of inhibiting the appearance of ROS, chelating ions of metals involved in the ROS creation, and scavenging ROS, thus interfering with the cascade of reactions leading to the peroxidation of lipids and synergistic cooperation with other antioxidants.
Intriguingly, propolis displays also anti-inflammatory properties in both acute and chronic inflammatory processes, and this is principally due to its large content of polyphenol compounds.
Propolis contains also active compounds which are known to promote cell proliferation or apoptosis. Among them, there are caffeic acid, caffeic phenyl ester, artepillin C, quercetin, resveratrol, galangin, and genistein [28, 29].
Genistein, quercetin, kaempferol, luteolin, chrysin, and apigenin inhibit cyclins, arresting the cell cycle [30]. Galangin, genistein, and resveratrol display antiproliferative activity with respect to the breast cancer estrogen receptor [31].
Some in vivo tests have demonstrated that some propolis flavonoids inhibit the development of lung cancer and oral cancer, as well as skin, esophagus, stomach, colorectal, prostate, and breast cancers [32].
Flavonoids are considered as valuable natural compounds not only because they avoid rapid blood sugar rises in the serum, but also because they are able to shelter diabetics from the complications of this metabolic disorder.
Matsui et al. [20] observed for Brazilian propolis anti-hyperglycemic effects with respect to caffeoylquinic acids (CQA). Caffeoylquinic acids are powerful inhibitors of β-glucosidase and α-amylase.
Another author [33] demonstrated that, in rat with diabetes, the administration of propolis extracts leads to a reduced level of glucose and a protective effect against lipid peroxidation.
Many works have demonstrated the important antibacterial actions of propolis, where much greater activity was determined with respect to Gram-positive bacteria than to Gram-negative ones [34]. These antibacterial effects may due to the synergistic activity of the many compounds present in propolis. Pinocembrin displays an intense antibacterial activity against Streptococcus sp. Apigenin most powerfully inhibits bacterial glycosyltransferase. p-Coumaric acid, artepillin C, and 3-phenyl-4-dihydrocinnamylocinnamic acid are effective against Helicobacter pylori [35, 36] Fig. 1.
Fig. 1 Molecular mechanisms/targets mediating effects of propolis Full size image
Use of propolis in experiments
Some techniques such as high-performance liquid chromatography (HPLC), gas chromatography (GC), as well as identification techniques, such as mass spectroscopy (MS), gas chromatography, and mass spectroscopy (GC-MS) allowed to increase the number of compounds present in propolis [25].
Therefore, many papers regarding the different roles and different biological properties of propolis’ components have been recently published.
However, a huge part of them are of limited usefulness, for the lack of source for comparison and scientific assessment of the results and because they do not refer to the chemical composition of the studied propolis.
The utilization for research purpose of characterized propolis samples and for comparative study of positive effects of propolis from different geographic areas is the most interesting methodologies in the recent research.
Recently, chemical characterization of the samples tested for antibacterial activity was compared with quantification of the major groups of biologically active substances of the corresponding propolis samples [36]. The results confirm the importance of phenolic acids for propolis antibacterial activity, and the significance of poplar as a propolis source, which offers the most efficient defense for hives against bacteria.
Banksota et al. compared the cytotoxic, hepatoprotective, and free radical scavenging activity of propolis from Brazil, Peru, The Netherlands, and China. The authors found that propolis from The Netherlands and China possessed the strongest cytotoxic activity; while almost all samples possessed significant hepatoprotective activity [37].
The work of Kumazawa et al. [38] is another example of this approach. The authors linked the antioxidant activity of propolis (Argentina, Austria, Brazil, Bulgaria, Chile, China, Hungary, New Zealand, South Africa, Thailand, Ukraine, Uruguay, the USA, and Uzbekistan) and combined these data with chemical analyses.
Major components of propolis were recognized by HPLC analysis with photo-diode array and mass spectrometric detection and quantitatively analyzed. Propolis extracts from Argentina, Australia, China, Hungary, and New Zealand had relatively strong antioxidant activities and were associated with the total polyphenol and flavonoid contents. In fact, the propolis with strongest antioxidant activity contained anti-oxidative compounds such as kaempferol and phenethyl caffeate.
Following a similar approach, Chen et al. related the radical scavenging activity, cytotoxic effects and apoptosis induction in some human melanoma cell lines of propolis from different localities of Taiwan [17].
Wound healing properties of propolis
Along with other honeybee products (honey, royal jelly, pollen), propolis has great therapeutic properties, being used since antiquity in popular medicine in various parts of the world [39, 40].
Propolis is believed to have antiseptic, antibacterial, antimycotic, astringent, spasmolytic, anti-inflammatory, anesthetic, antioxidant, antifungal, antiulcer, anticancer, and immunomodulatory effects [41, 42], see Table 2.
Table 2 Most important properties of propolis Full size table
Propolis, which is well tolerated with rare incidents of allergy and no toxicity, is referred to as an excellent candidate for burn management, enhancing skin cell proliferation, activation, and growth capacity [43].
Some results confirm the propolis therapeutic efficacy, throughout quantitative and qualitative analyses of collagen types I and III expression and degradation in wounds matrix, indicating that propolis could have favorable biochemical environment supporting re-epithelization [44].
Recent discoveries have identified that oxygen is required to disinfect wounds and fuel healing but also that oxygen-dependent redox-sensitive signaling processes plays a pivotal role in the repair cascade.
The interactions of free radicals in skin and in neighboring tissues may be also responsible for some toxic effects and modifications of their structures.
A recent report [45] suggested that propolis is able to quench free radicals in skin. This outcome of propolis on free radicals in the epidermis is the source of safety of its application in the therapy of burn wounds.
Other findings reveal that propolis speed up the burned tissue repair by stimulation of the wound bed matrix remodeling, proposing that the observed changes in extracellular matrix content after propolis application may be connected with the ability of its flavonoid compounds to reduce lipid peroxidation and to prevent necrosis of cells [44].
Biological activities of propolis on wound repair and tissue regeneration might be correlated to its antimicrobial, anti-inflammatory, and immumonodulatory properties [46].
Propolis has demonstrated some in vitro antimicrobial activity, in particular against Gram-positive (Staphylococci and Streptococci spp.) and Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae, Proteus vulgaris, and Pseudomonas aeruginosa), Helicobacter pylori, protozoa (Trypanosoma cruzi), fungi (Candida albicans), and viruses (such as HIV, Herpes viruses, or influenza viruses).
Antimicrobial properties of propolis are essentially due to the flavonoid content and in particular to the presence of pinocembrin, galangin, and pinobanksin. Pinocembrin also exhibits antifungal properties. Other compounds with well-established effects are ester of coumaric and caffeic acids.
Some studies have highlighted the role of propolis as the solvent employed for the extraction of propolis that may influence the potency of its antimicrobial activity [9].
Propolis also shows anti-inflammatory effects against acute and chronic models of inflammation, but how propolis induces this effect is still unclear. Rossi et al. [47] verified that propolis inhibits, in a concentration-dependent manner, COX activity from lung homogenates of saline- or LPS-treated rats [47].
Propolis demonstrates immunostimulatory and immunomodulatory effects in vitro on macrophages; while in vivo it increases the ratio of CD4/CD8 T cells in mice [48].
The results of this study [48] show that application of propolis increases the wound healing rate and re-epithelialization of diabetic wounds in rodents. It has also proposed other roles for propolis in decreasing neutrophil infiltration and normalizing macrophage influx into wounded area.
Wound repair and regeneration proceeds via a finely tuned pattern of integrated phases, such as hemostasis, inflammation, cell proliferation, and tissue remodeling, which all involve a number of cellular and molecular processes [49]. This phenomenon includes migration and proliferation of epidermal cells and keratinocytes, fibroblast adherence, and extracellular matrix (ECM) contraction [50]. Propolis treatment stimulates significant increases in ECM components during the initial phase of wound repair, followed by a reduction in the ECM molecules. It is postulated that this biological effect of propolis is associated with its ability to stimulate the expression of transforming growth factor-β (TGF-β) [51] that participates in the early phases of wound repair such as hemostasis and inflammation.
McLennan and coworkers [52] showed that with a single propolis topical, there is an increase of wound healing in a diabetic rodent model of full-thickness cutaneous wound healing. This was the first systematic study showing that propolis improve wound healing in diabetes.
Some works have explored the effects of propolis solutions at animal wounds treatment in clinical and experimental cases [53]. The results showed that propolis is able to perform a good healing process mainly by reducing the inflammatory response; so, healing process was faster with propolis. The authors considered propolis suitable for wound treatment, following eradication of the infection.
The healing properties of propolis should be also due to its immune stimulating effect. This property was characterized in few clinical studies. Propolis was given, and cytokine secretion capacity was considered during and after treatment. The cytokine secretion capacity increased significantly during the treatment period in a time-dependent manner. The authors concluded that propolis was able to elicit an immune reactivity without side effects [54].
In the last decades, there was a bigger interest in the use of biomaterials, for example biopolymers, in healthcare products, especially as dressing for wounds, a fact that is predominantly associated to the renewable nature, biocompatibility, and biodegradability of these supplies. Tissue engineering needs polymeric membrane for creating the correct environment for cell migration and attachment within the scaffold. Therefore, biocompatible propolis loaded polyurethane nanofibers were successfully prepared using electrospinning of propolis solution [55].
Other properties of propolis
Propolis has been proposed as anti-calculus agents, because it decreases the formation of oral calcium-phosphate precipitate (the main component of dental calculus).
Hidaka and colleagues [56] demonstrated that some types of honeys and propolis are able to reduce the amorphous calcium-phosphate transformation rate into hydroxyapatite.
The activity of propolis against oral bacteria has been explored, suggesting the effectiveness of propolis as an anti-cariogenic product [57].
Samples of saliva of 25 human healthy subjects and 25 patients affected by chronic periodontitis were analyzed for the extent of the microbial inhibition zone, comparing propolis and chlorhexidine efficacy. Propolis exhibited significant antibacterial activity against bacteria in both healthy and pathological saliva [58].
With an increasing incidence rate of cancer worldwide, new anticancer agents are still mandatory. The results of in vitro and in vivo studies suggest that propolis possess important cytotoxic properties against cancer cells through the induction of apoptosis or cell division and cell growth arrest [59–61].
Propolis and its compounds may inhibit cell cycle proliferation or induce apoptosis in cancer cells. They induce the apoptosis pathway by stimulation of Bax, p53, p21 proteins, p38 MAPK, JNK, ERK kinases, cytochrome c release, and activation of caspase cascade [42].
Although many studies have revealed the inhibitory effects of propolis and its compounds on growth and tumor propagation, further investigation is necessary to appreciate the efficiency and mechanism of their beneficial properties [42, 62].
Why Propolis Might Be the Next Big Bee Byproduct
Carly Stein had been getting sick ever since she could remember. The former investment banker grew up plagued by bouts of tonsillitis, sore throat, and constant visits to nose and throat doctors. “I missed a lot of school as a kid,” she says over Zoom. When Stein studied abroad in Italy during college, she came down with tonsillitis. She spoke to the pharmacist in Florence, explaining her sensitivity to medications. They recommended propolis, a sticky mixture created by bees, composed of wax and resin with healing properties. “She gave me this little tincture. I have to put five drops in water and take it a few times a day, and I started using it. In five days, I made a full recovery.” Years later, in 2016, she created her company, Beekeeper’s Natural, specializing in propolis, which comes in forms such as throat sprays and powders.
For centuries, since 300 B.C to be specific, honey-related ingredients like propolis have been touted as a magical cure-all, especially in Eastern Europe, where it has remained a form of alternative medicine. Go to any food bazaar in Eastern Europe and you’re bound to stumble upon one booth that exclusively sells a cornucopia of honey variations. There is most likely a tiny section dedicated to propolis, too, in liquid, powder, or granular form. While honey and propolis are related and have their respective health benefits, the two bee-produced products are different. Propolis is considered “bee glue,” a sticky mix that bees collect from plants and buds. Bees use propolis to repair the hive, seal cracks, and create a protective barrier against predators.
When it comes to the human body, propolis is considered an aid in gut health, which can affect everything from the skin to the immune system. “Flavonoids, contained in the propolis, can help stimulate the growth and activity of digestive microflora,” says nutritionist Mikaela Rueben, “as well as help the body’s natural detoxification process.”
Whitney Bowe, M.D., agrees that those centuries-old rumors of propolis benefits have truth to them, specifically when it comes to our microbiomes, communities of bacteria that help support our metabolism and immune systems. “Propolis seems to enhance our systemic immune system. So it seems to rebalance our microbiome and have a positive effect on our immune health,” Bowe, a dermatologist, tells Vogue. “About 70–80% of our immune system resides in the gut, so it’s no surprise that our diet can impact our overall immune health and the health of distant organs including our skin.”
