Apigenin (4',5,7-trihydroxyflavone) is a flavonoid commonly found in many fruits and vegetables such as parsley, chamomile, celery, and kumquats. In the last few decades, recognition of apigenin as a cancer chemopreventive agent has increased. Significant progress has been made in studying the chemopreventive aspects of apigenin both in vitro and in vivo. Several studies have demonstrated that the anticarcinogenic properties of apigenin occur through regulation of cellular response to oxidative stress and DNA damage, suppression of inflammation and angiogenesis, retardation of cell proliferation, and induction of autophagy and apoptosis. One of the most well-recognized mechanisms of apigenin is the capability to promote cell cycle arrest and induction of apoptosis through the p53-related pathway. A further role of apigenin in chemoprevention is the induction of autophagy in several human cancer cell lines. In this review, we discuss the details of apigenin, apoptosis, autophagy, and the role of apigenin in cancer chemoprevention via the induction of apoptosis and autophagy.

Apigenin, a natural plant flavonoid with antiproliferative activity, is emerging as a promising compound for cancer prevention and therapy, but its mechanism of action remains unclear. High expression of the small heat-shock protein-27 (Hsp27) in leukemia contributes to the resistance of these cells to cancer treatments. Changes in Hsp27 phosphorylation have been associated with heat and metabolic stress, but its role in flavonoid anticancer activity has not been investigated. In this study, we examined the effect of apigenin in the regulation of Hsp27 on leukemia. We showed that apigenin does not affect Hsp27 expression but induces a bimodal phosphorylation on Ser78 and Ser82. The phosphorylation at early times was regulated by p38. At later times, Hsp27 phosphorylation was dependent on p38 activity and for some residues on PKCδ. Silencing of p38 expression reduced apigenin-induced phosphorylation on Ser15, Ser78, and Ser82, whereas silencing of PKCδ expression reduced the phosphorylation on Ser15 and Ser82 without affecting Ser78. In addition, we found that apigenin-induced PKCδ activity is mediated by p38. We also showed that the phosphorylation of Hsp27 significantly increased the susceptibility of leukemia cells to apigenin-induced apoptosis. Together, these results identify a complex signaling network regulating the cytotoxic effect of apigenin through Hsp27 phosphorylation.

BACKGROUND

The flavonoid apigenin exhibits anti-proliferative and anti-angiogenic activities. Our objective was to evaluate the effect of apigenin on hypoxia responsive genes important in pancreatic cancer cell proliferation.

METHODS

Immunohistochemistry for GLUT-1 expression was conducted on human pancreatic cancer samples and adjacent controls. Real-time RT-PCR, Western blot analysis, and enzyme-linked immunosorbent assay (ELISA) were conducted on CD18 and S2-013 human pancreatic cancer cells treated with apigenin (0-50 μM) in normoxic and hypoxic conditions to evaluate HIF-1α, GLUT-1, and VEGF mRNA and protein expression and secretion.

RESULTS

GLUT-1 expression was significantly increased in pancreatic adenocarcinoma samples versus adjacent controls (P < 0.001). Hypoxic conditions induced HIF-1α, GLUT-1, and VEGF protein expression in both CD18 and S2-013 pancreatic cancer cells. Apigenin (50 μM) blocked hypoxia induced up-regulation of all three proteins in both cell lines. Apigenin also impeded hypoxia-mediated induction of GLUT-1 and VEGF mRNA in both cell lines (P < 0.05).

CONCLUSIONS

Apigenin inhibits HIF-1α, GLUT-1, and VEGF mRNA and protein expression in pancreatic cancer cells in both normoxic and hypoxic conditions. This may account for the mechanism of apigenin's anti-proliferative and anti-angiogenic effects and further supports the potential of apigenin as a future chemopreventive agent for pancreatic cancer.

BACKGROUND

Flavones found in plants display various biological activities, including anti-allergic, anti-viral, anti-inflammatory, anti-oxidation, and anti-tumor effects. In this study, we investigated the anti-tumor effects of flavone, apigenin and luteolin on human breast cancer cells.

METHODS

The anti-cancer activity of flavone, apigenin and luteolin was investigated using the MTS assay. Apoptosis was analyzed by Hoechst 33342 staining, flow cytometry and western blot. Cell migration was determined using the culture inserts and xCELLigence real-time cell analyzer instrument equipped with a CIM-plate 16. Real-time quantitative PCR and western blot were used to determine the signaling pathway elicited by flavone, apigenin and luteolin.

RESULTS

Flavone, apigenin and luteolin showed potent inhibitory effects on the proliferation of Hs578T, MDA-MB-231 and MCF-7 breast cancer cells in a concentration and time-dependent manner. The ability of flavone, apigenin and luteolin to inhibit the growth of breast cancer cells through apoptosis was confirmed by Hoechst33342 staining and the induction of sub-G1 phase of the cell cycle. Flavone, apigenin and luteolin induced forkhead box O3 (FOXO3a) expression by inhibiting Phosphoinositide 3-kinase (PI3K) and protein kinase B (PKB)/Akt. This subsequently elevated the expression of FOXO3a target genes, including the Cyclin-dependent kinase inhibitors p21Cip1 (p21) and p27kip1 (p27), which increased the levels of activated poly(ADP) polymerase (PARP) and cytochrome c.

CONCLUSIONS

Taken together, these data demonstrated that flavone, apigenin and luteolin induced cell cycle arrest and apoptosis in breast cancer cells through inhibiting PI3K/Akt activation and increasing FOXO3a activation, which suggest that flavone, apigenin and luteolin will be the potential leads for the preventing and treating of breast cancer.

Natural products are characterized by high chemical diversity and biochemical specificity; therefore, they are appealing as lead compounds for drug discovery. Given the importance of angiogenesis to many pathologies, numerous natural products have been explored as potential anti-angiogenic drugs. Ocular angiogenesis underlies blinding eye diseases such as retinopathy of prematurity (ROP) in children, proliferative diabetic retinopathy (DR) in adults of working age, and age-related macular degeneration (AMD) in the elderly. Despite the presence of effective therapy in many cases, these diseases are still a significant health burden. Anti-VEGF biologics are the standard of care, but may cause ocular or systemic side effects after intraocular administration and patients may be refractory. Many anti-angiogenic compounds inhibit tumor growth and metastasis alone or in combination therapy, but a more select subset of them has been tested in the context of ocular neovascular diseases. Here, we review the promise of natural products as anti-angiogenic agents, with a specific focus on retinal and choroidal neovascularization. The multifunctional curcumin and the chalcone isoliquiritigenin have demonstrated promising anti-angiogenic effects in mouse models of DR and choroidal neovascularization (CNV) respectively. The homoisoflavanone cremastranone and the flavonoid deguelin have been shown to inhibit ocular neovascularization in more than one disease model. The isoflavone genistein and the flavone apigenin on the other hand are showing potential in the prevention of retinal and choroidal angiogenesis with long-term administration. Many other products with anti-angiogenic potential in vitro such as the lactone withaferin A, the flavonol quercetin, and the stilbenoid combretastatin A4 are awaiting investigation in different ocular disease-relevant animal models. These natural products may serve as lead compounds for the design of more specific, efficacious, and affordable drugs with minimal side effects.

More than 50% of the world population is infected with Helicobacter pylori (H. pylori). The bacterium highly links to peptic ulcer diseases and duodenal ulcer, which was classified as a group I carcinogen in 1994 by the WHO. The pathogenesis of H. pylori is contributed by its virulence factors including urease, flagella, vacuolating cytotoxin A (VacA), cytotoxin-associated gene antigen (Cag A), and others. Of those virulence factors, VacA and CagA play the key roles. Infection with H. pylori vacA-positive strains can lead to vacuolation and apoptosis, whereas infection with cagA-positive strains might result in severe gastric inflammation and gastric cancer. Numerous medicinal plants have been reported for their anti-H. pylori activity, and the relevant active compounds including polyphenols, flavonoids, quinones, coumarins, terpenoids, and alkaloids have been studied. The anti-H. pylori action mechanisms, including inhibition of enzymatic (urease, DNA gyrase, dihydrofolate reductase, N-acetyltransferase, and myeloperoxidase) and adhesive activities, high redox potential, and hydrophilic/hydrophobic natures of compounds, have also been discussed in detail. H. pylori-induced gastric inflammation may progress to superficial gastritis, atrophic gastritis, and finally gastric cancer. Many natural products have anti-H. pylori-induced inflammation activity and the relevant mechanisms include suppression of nuclear factor-κB and mitogen-activated protein kinase pathway activation and inhibition of oxidative stress. Anti-H. pylori induced gastric inflammatory effects of plant products, including quercetin, apigenin, carotenoids-rich algae, tea product, garlic extract, apple peel polyphenol, and finger-root extract, have been documented. In conclusion, many medicinal plant products possess anti-H. pylori activity as well as an anti-H. pylori-induced gastric inflammatory effect. Those plant products have showed great potential as pharmaceutical candidates for H. pylori eradication and H. pylori induced related gastric disease prevention.

Flavonoids are becoming popular nutraceuticals. Different flavonoids show similar or distinct biological effects on different tissues or cell types, which may limit or define their usefulness in cancer prevention and/or treatment application. This review focuses on a few selected flavonoids and discusses their functions in normal and transformed pigment cells, including cyanidin, apigenin, genistein, fisetin, EGCG, luteolin, baicalein, quercetin and kaempferol. Flavonoids exhibit melanogenic or anti-melanogenic effects mainly via transcriptional factor MiTF and/or the melanogenesis enzymes tyrosinase, DCT or TYRP-1. To identify a direct target has been a challenge as most studies were not able to discriminate whether the effect(s) of the flavonoid were from direct targeting or represented indirect effects. Flavonoids exhibit an anti-melanoma effect via inhibiting cell proliferation and invasion and inducing apoptosis. The mechanisms are also multi-fold, via ROS-scavenging, immune-modulation, cell cycle regulation and epigenetic modification including DNA methylation and histone deacetylation. In summary, although many flavonoid compounds are extremely promising nutraceuticals, their detailed molecular mechanism and their multi-target (simultaneously targeting multiple molecules) nature warrant further investigation before advancement to translational studies or clinical trials.

1. Chrysin is one of many bioflavonoids with chemopreventive properties in cardiovascular disease and cancer. In an effort better to understand factors that may affect the oral bioavailability of the bioflavonoids from dietary sources, the metabolism of chrysin by cultured intestinal Caco-2 cells and hepatic Hep G2 cells was studied, together modelling human presystemic metabolism. 2. At concentrations that may be achieved in the diet, chrysin was extensively metabolized to two conjugated metabolites, M1 and M2, with no CYP-mediated oxidation. M1 was identified as a glucuronide, and M2 as a sulphate conjugate by LC/MS and other spectroscopic and biochemical techniques. Sulphate conjugation occurred at a rate twice that of glucuronic acid conjugation in both cell types. 3. M1 was catalyzed by UGT1A6 with a Km = 12 microM. M2 was catalyzed both by M- and P-form phenolsulphotransferases (SULT 1A3 and SULT 1A1) with very low Km of 3.1 and 0.05 microM respectively. 4. Pretreatment with 3-methylcholanthrene, interestingly, did not result in oxidation of chrysin but rather in increased glucuronidation. 5. Also, M1 and M2 were the only metabolites formed from chrysin in fresh rat hepatocytes. The metabolism of another flavonoid, apigenin, was very similar to that of chrysin. 6. These observations suggest that both sulphation and glucuronidation are critical determinants of the oral bioavailability of bioflavonoids in humans, although a contribution from CYP-mediated oxidation can not be excluded.

Apigenin has been previously shown to induce G2/M cell-cycle arrest in human colon cancer cell lines. The present study assessed the individual and interactive influence of seven apigenin analogs on cell cycle, cell number, and cell viability in human SW480 and Caco-2 colonic carcinoma cells. Cellular concentration of selected apigenin analogs was further assessed by high-performance liquid chromatography to assess cellular availability. The apigenin analogs studied were acacetin, chrysin, kampherol, luteolin, myricetin, naringenin, and quercetin. DNA flow cytometric analysis indicated that treatment with either chrysin or acacetin at 0 to 80 microM for 48 h resulted in cell-cycle arrest at the G2/M phase in a dose-dependent manner in the SW480 cells but not in the Caco-2 cells. The percentage of SW480 cells at G2/M also increased when cells were treated with kampherol, luteolin, or quercetin between 5 and 30 microM, but the percentage of cells in G2/M decreased at doses greater than 40 microM. Cell number was significantly decreased in a time- and dose-dependent manner following the treatments with each analog except for naringenin and myricetin. The interactive effects of these analogs with apigenin were further assessed by combining each analog at doses from 0 to 80 microM with apigenin at 20 microM, a dose at which apigenin was found to double the proportion of SW480 cells in G2/M. When either acacetin, chrysin, luteolin, kampherol, or quercetin at doses between 5 and 30 microM were combined with apigenin at 20 microM, there was an increase of 22% in the proportion of G2/M cells over that observed with 20 microM apigenin alone (P < 0.05). At doses higher than 40 microM, however, the interaction became antagonistic, and the proportion of cells in G2/M decreased below that observed with apigenin alone. Cell viability, as assessed by Trypan blue exclusion assay, significantly decreased by treatments with high doses of each agent or each agent combined with apigenin. Cellular concentration of apigenin, chrysin, or naringenin in SW480 cells significantly increased at doses of 40 microM or greater, but it was not correlated with their impact on G2/M cell-cycle arrest. The induction of cell-cycle arrest by five of seven tested apigenin analogs and the additive induction by the combination of flavonoids at low doses suggest that apigenin-related flavonoids may cooperatively protect against colorectal cancer through conjoint blocking of cell-cycle progression.

Persuasive epidemiological and experimental evidence suggests that dietary flavonoids have anti-cancer activity. Since conventional therapeutic and surgical approaches have not been able to fully control the incidence and outcome of most cancer types, including colorectal neoplasia, there is an urgent need to develop alternative approaches for the management of cancer. We sought to develop the best flavonoids for the inhibition of cell growth, and apigenin (flavone) proved to be the most promising compound in colorectal cancer cell growth arrest. Subsequently, we found that pro-apoptotic proteins (NAG-1 and p53) and cell cycle inhibitor (p21) were induced in the presence of apigenin, and kinase pathways, including PKCδ and ataxia telangiectasia mutated (ATM), play an important role in activating these proteins. The data generated by in vitro experiments were confirmed in an animal study using APC(MIN+) mice. Apigenin is able to reduce polyp numbers, accompanied by increasing p53 activation through phosphorylation in animal models. Our data suggest apparent beneficial effects of apigenin on colon cancer.

Inappropriate activation of phosphatidylinositol 3-kinase-Akt signaling contributes to the development of several human malignancies. Modulation of Akt activity is a strategy that may be valuable in chemopreventive and chemotherapeutic regimens. We have previously demonstrated that apigenin, a plant flavone, causes decreased survival in human prostate cancer cells. However, the molecular mechanism underlying this observation remains elusive. In the present study, we investigated the mechanisms of apigenin action on human prostate cancer PC-3 cells, which possess constitutively active Akt. Treatment of PC-3 cells with apigenin (5-40 microM) resulted in significant dose- and time-dependent decrease in Akt phosphorylation at Serine473. Apigenin-mediated dephosphorylation of Akt resulted in inhibition of its kinase activity, which was confirmed by reduced phosphorylation of proapoptotic proteins BAD and glycogen synthase kinase-3, essential downstream targets of Akt. Hypophosphorylation of BAD resulted in reduced interaction with 14-3-3beta protein after 20 microM apigenin exposure to PC-3 cells for 24 h. Inactivation of Akt seems to be associated with downregulation of insulin-like growth factor receptor 1 protein level and inhibition of its autophosphorylation upon apigenin treatment. Exposure to apigenin significantly induced caspase-9 activity and decreased the survival of PC-3 cells in a dose-dependent manner. Furthermore, Serine473 phosphorylation of ectopically expressed Akt in DU145 cells was significantly reduced upon 20 microM apigenin treatment. In vivo, apigenin intake through gavage resulted in inactivation of Akt and induction of apoptosis in PC-3 tumors. These results suggest that Akt inactivation and dephosphorylation of BAD is a critical event, at least in part, in apigenin-induced decreased cell survival and apoptosis.

In last couple of decades the use of natural compounds like flavonoids as chemopreventive agents has gained much attention. Our current study focuses on identifying chemopreventive flavonoids and their mechanism of action on human prostate cancer cells. Human prostate cancer cells (PC3), stably transfected with activator protein 1 (AP-1) luciferase reporter gene were treated with four main classes of flavonoids namely flavonols, flavones, flavonones, and isoflavones. The maximum AP-1 luciferase induction of about 3 fold over control was observed with 20 microM concentrations of quercetin, chrysin and genistein and 50 microM concentration of kaempferol. At higher concentrations, most of the flavonoids demonstrated inhibition of AP-1 activity. The MTS assay for cell viability at 24 h showed that even at a very high concentration (500 microM), cell death was minimal for most of the flavonoids. To determine the role of MAPK pathway in the induction of AP-1 by flavonoids, Western blot of phospho MAPK proteins was performed. Four out of the eight flavonoids namely kaempferol, apigenin, genistein and naringenin were used for the Western Blot analysis. Induction of phospho-JNK and phospho-ERK activity was observed after two hour incubation of PC3-AP1 cells with flavonoids. However no induction of phospho-p38 activity was observed. Furthermore, pretreating the cells with specific inhibitors of JNK reduced the AP-1 luciferase activity that was induced by genistein while pretreatment with MEK inhibitor reduced the AP-1 luciferase activity induced by kaempferol. The pharmacological inhibitors did not affect the AP-1 luciferase activity induced by apigenin and naringenin. These results suggest the possible involvement of JNK pathway in genistein induced AP-1 activity while the ERK pathway seems to play an important role in kaempferol induced AP-1 activity.

Ovarian cancer is the number one cause of death from gynaecological malignancy. Platinum-based and taxol-based chemotherapy has been used as a standard therapy, but intrinsic and acquired resistance to chemotherapy is a major obstacle to treat the disease. In the present study, we found that in the chemoresistant ovarian cancer SKOV3/TR cells, interleukin-6 (IL-6), IL-6 receptor and signal transducers and activators of transcription 3 (STAT3) expression as well as STAT3 phosphorylation were upregulated compared to those in parental cells. Silencing of IL-6 using IL-6 siRNA was found to suppress IL-6 production, STAT3 and phosphoSTAT3 levels, which eventually reduced proliferation and clonogenicity of taxol-resistant SKOV3/TR cells. In addition, stattic, a STAT3 inhibitor, was found to result in decrease of cell viability and clonogenicity of these cells, indicating that the elevated IL-6 and STAT3, phosphoSTAT3 levels are associated with the development of taxol resistance. Next, we found anti-proliferative effect of apigenin on both SKOV3 and SKOV3/TR cells. RT-PCR and western blot results showed that apigenin significantly reduced the expression of Axl and Tyro3 receptor tyrosine kinases (RTKs) at mRNA and protein level, which account for its cytotoxic activity. We further found that apigenin decreased Akt phosphorylation and the level of B-cell lymphoma-extra large (Bcl-xl or BCL2-like 1 isoform 1), an inhibitor of apoptosis. On the contrary to these results, apigenin had no effect on IL-6 production, STAT3 and phosphoSTAT3 protein levels, suggesting that apigenin exerts its anti-proliferative activity via downregulation of Axl and Tyro3 expression, Akt phosphorylation and Bcl-xl expression, but not modulation of IL-6/STAT3 axis. Taken together, our data suggest that inhibition of IL-6/STAT3 signaling pathway and downregulation of Axl and Tyro3 RTKs expression might be a therapeutic strategy to overcome taxol resistance in ovarian cancer cells.

Flavonoids (FVs) are an important class of plant compounds postulated to be one of the constituents responsible for the beneficial effects of fruits and vegetables on health, including heart disease and cancer. At pharmacological levels, various naturally-occurring flavonoids have been shown to be cancer-protective in a variety of animal models and flavonoid derivatives, such as flavopyridol, are being assessed as chemotherapy drugs in clinical trials. This report has investigated the effects of the most common dietary FVs on several major signalling pathways in biopsies of human epithelial cells using primary cultures freshly isolated from biopsies and has obtained evidence for the previously unrecognised importance of stress kinase responses induced by kaempferol (KF), apigenin (AP) and luteolin (LU). KF, AP and LU all activated ATM/ATR (mutated in ataxia-telangiectasia and related) kinases and the p38 stress kinase and this was associated with induction of GADD45 and cell cycle arrest in G2, but not induction of apoptosis. These effects were not due to general toxicity since they were reversible on removal of FV. The inductions of ATM/ATR and p38 were functionally important since caffeine, an inhibitor of ATM/ATR, and the p38-specific inhibitor, SB203580, prevented induction of GADD45 and growth arrest by these three flavonoids. In contrast, although quercetin (QU) activated ATM (but not ATR), it did not activate p38 kinase, GADD45 or p53. QU may interfere with one of the lipoxygenase (LOX) pathways since the growth inhibitory effects of QU (but not the other three flavonoids) could be reversed by addition of LOX metabolites, particularly 12- and 15-hydroxyeicostetraenic acids.

In searching for potent cancer chemopreventive agents from synthetic or natural products, 28 randomly selected flavonoids were screened for inhibitory effects against partially purified aromatase prepared from human placenta. Over 50% of the flavonoids significantly inhibited aromatase activity, with greatest activity being demonstrated with apigenin (IC50: 0.9 microg/mL), chrysin (IC50: 1.1 microg/mL), and hesperetin (IC50: 1.0 microg/mL).

In this study, we investigated the mechanistic role of the caspase cascade in extrinsic and intrinsic apoptosis induced by apigenin, which has been targeted as a candidate in the development of noncytotoxic anticancer medicines. Treatment with apigenin (1-100 microM) significantly inhibited the proliferation of MDA-MB-453 human breast cancer cells in a dose- and time-dependent manner with IC(50) values of 59.44 and 35.15 microM at 24 and 72 h, respectively. This inhibition resulted in the induction of apoptosis and the release of cytochrome c in cells exposed to apigenin at its 72 h IC(50). Subsequently, caspase-9, which acts in mitochondria-mediated apoptosis, was cleaved by apigenin. In addition, apigenin activated caspase-3, which functions downstream of caspase-9. The apigenin-induced activation of caspase-3 was accompanied by the cleavage of capases-6, -7, and -8. These results are supported by evidence showing that the activity patterns of caspases-3, -8, and -9 were similar. The present study supports the hypothesis that apigenin-induced apoptosis involves the activation of both the intrinsic and extrinsic apoptotic pathways.

Decrease in expression of the tumor suppressor microRNA-138 (miR-138) correlates well with an increase in telomerase activity in many human cancers. The ability of almost all human cancer cells to grow indefinitely is dependent on presence of telomerase activity. The catalytic component of human telomerase reverse transcriptase (hTERT) regulates telomerase activity in most of the human cancers including malignant neuroblastoma. We observed an indirect increase in the expression of miR-138 after the transfection with hTERT short hairpin RNA (shRNA) plasmid in human malignant neuroblastoma SK-N-DZ and SK-N-BE2 cell lines. Transfection with hTERT shRNA plasmid followed by treatment with the flavonoid apigenin (APG) further increased expression of miR-138. Direct transfection with miR-138 mimic was more powerful than transfection with hTERT shRNA plasmid in potentiating efficacy of APG for decreasing cell viability and colony formation capability of both cell lines. Upregulation of miR-138 was also more effective than down regulation of hTERT in enhancing efficacy of APG for induction of apoptosis in malignant neuroblastoma cells in vitro and in vivo. We delineated that apoptosis occurred with induction of molecular components of the extrinsic and intrinsic pathways in SK-N-DZ and SK-N-BE2 cells both in vitro and in vivo. In conclusion, these results demonstrate that direct miR-138 overexpression is more powerful than hTERT down regulation in enhancing pro-apoptotic effect of APG for controlling growth of human malignant neuroblastoma in cell culture and animal models.

Apigenin, a common dietary flavonoid, has been found to have antitumor properties and therefore poses special interest for the development of chemopreventive and/or chemotherapeutic agent for cancers. Here, we demonstrate that apigenin inhibits expression of focal adhesion kinase (FAK) and migration and invasion of human ovarian cancer A2780 cells. FAK is a non-receptor protein tyrosine kinase downstream of integrins and growth factors. It plays an important role in migration and invasion of cancer cells. We found that apigenin inhibited adhesion, migration and invasion of A2780 cells. Apigenin attenuated FAK expression through reducing its protein stability. FAK plays a critical role in migration and invasion of A2780 cells. Overexpression of FAK could reverse A2780 cell migration and invasion inhibited by apigenin. The in vivo experiments showed that apigenin inhibited spontaneous metastasis of A2780 cells implanted onto the ovary of nude mice. Our results provide a new insight into the mechanisms that apigenin inhibits ovarian cancers. These results suggest that molecular targeting of FAK by apigenin might be a useful strategy for chemoprevention and/or chemotherapeutics of ovarian cancers.

BACKGROUND

Propolis is a resinous hive product collected by honeybees from various plant sources. It is widely used in traditional medicine and is reported to have a broad spectrum of pharmacological effects (antibacterial, antihepatoxic, antioxidative, anti-inflammatory, etc.). Thus the aim of this study was to assess cytotoxic effect of various ethanol propolis extractions on the cervical tumor cell line (HeLa) and compare it with their phenolic acids and flavonoids composition.

METHODS

Twenty samples of raw propolis were collected from 17 localities of Croatia (I-XVII), 2 of Bosnia and Hercegovina (XVIII, XIX) and 1 of Macedonia (XX). Reverse phase HPLC was used to determine the chemical composition of polyphenols. Biological experiments were carried out in vitro on cervix adenocarcinoma cell line (HeLa).

RESULTS

Phenolic acids (ferulic acid, p-coumaric acid, caffeic acid) and flavonoids (tectochrysin, galangin, pinocembrin, pinocembrin-7-methylether, chrysin, apigenin, kaempferol, quercetin) have been determined using HPLC analysis in 20 ethanolic propolis extracts. All samples contain tectochrysin in ranges of 0.1988 mg/g (XVIII) to 1.2004 mg/g (III), while caffeic acid and quercetin have not been found. Flavonoid that is most abundant is galangin in ranges from 0.3706 mg/g (XVII) to 47.4879 mg/g (IX). The samples of propolis numbers I, VI and X applied in the investigated concentration range manifested significant reduction of cell growth. GI(50) value as indicator of cytotoxicity among propolis samples showed that propolis number VII is the most effective (GI(50) =76 μg/ml) followed by propolis nos. XV, XVIII and I.

CONCLUSIONS

Antiproliferative and cytotoxic effect of propolis on the HeLa cells is not correlating with the concentration of particular components but on establishing the possible synergistic antiproliferative activity of individual phenolic acid and flavonoids. Differences in the chemical composition lead to diversity in biological activity of propolis samples.

BACKGROUND

Apigenin, a natural flavone, is widely distributed in plants such as celery, parsley and chamomile. It is present principally as glycosylated in nature. Higher intake of apigenin could reduce the risk of chronic diseases. It has gained particular interest in recent years as a beneficial, health-promoting agent with low intrinsic toxicity. Areas covered: This review summarizes and the absorption, distribution, metabolism and excretion (ADME) properties of apigenin, and drug-drug interaction of apigenin. Expert opinion: Since apigenin is a bioactive plant flavone and is widely distributed in common food, its consumption through the diet is recommended. Apigenin-enriched drugs are better for some chronic diseases, but may affect animal and human health if present in the daily diet. Dietary or therapeutic apigenin has value as a good cellular regulator in cancer, especially cancers of the gastrointestinal tract. Due to apigenin's limitations on absorption and bioavailability, novel carriers would need to be developed to enhance the oral bioavailability of apigenin. Further research about its ADME properties and drug-drug interactions are needed before apigenin can be brought to clinical trials.

Increasing evidence from diverse sources during the past several years has indicated that long-term, low level, chronic inflammation mediates several chronic diseases including cancer, arthritis, obesity, diabetes, cardiovascular diseases, and neurological diseases. The inflammatory molecules and transcription factors, adhesion molecules, AP-1, chemokines, C-reactive protein (CRP), cyclooxygenase (COX)-2, interleukins (ILs), 5-lipooxygenase (5-LOX), matrix metalloproteinases (MMPs), nuclear factor (NF)-kB, signal transducer and activator of transcription 3 (STAT3), tumor necrosis factor (TNF), and vascular endothelial growth factor (VEGF) are molecular links between inflammation and chronic diseases. Thus, suppression of inflammatory molecules could be potential strategy for the prevention and therapy of chronic diseases. The currently available drugs against chronic diseases are highly expensive, minimally effective and produce several side effects when taken for long period of time. The focus of this review is to discuss the potential of nutraceuticals derived from "Mother Nature" such as apigenin, catechins, curcumin, ellagic acid, emodin, epigallocatechin gallate, escin, fisetin, flavopiridol, genistein, isoliquiritigenin, kaempferol, mangostin, morin, myricetin, naringenin, resveratrol, silymarin, vitexin, and xanthohumol in suppression of these inflammatory pathways. Thus, these nutraceuticals offer potential in preventing or delaying the onset of chronic diseases. We provide evidence for the potential of these nutraceuticals from pre-clinical and clinical studies.

Mediterranean-style diets caused a significant decline in cardiovascular diseases (CVDs) in early landmark studies. The effect of a traditional Mediterranean diet on lipoprotein oxidation showed that there was a significant reduction in oxidative stress in the intervention group (Mediterranean diet + Virgin Olive Oil) compared to the low-fat diet group. Conversely, the increase in oxidative stress causing inflammation is a unifying hypothesis for predisposing people to atherosclerosis, carcinogenesis, and osteoporosis. The impact of antioxidants and anti-inflammatory agents on cancer and cardiovascular disease, and the interventive mechanisms for the inhibition of proliferation, inflammation, invasion, metastasis, and activation of apoptosis were explored. Following the Great Oxygen Event some 2.3 billion years ago, organisms have needed antioxidants to survive. Natural products in food preservatives are preferable to synthetic compounds due to their lower volatility and stability and generally higher antioxidant potential. Free radicals, reactive oxygen species, antioxidants, pro-oxidants and inflammation are described with examples of free radical damage based on the hydroxyl, nitric oxide and superoxide radicals. Flavonoid antioxidants with 2- or 3-phenylchroman structures such as quercetin, kaempferol, myricetin, apigenin, and luteolin, constituents of fruits, vegetables, tea, and wine, which may reduce coronary disease and cancer, are described. The protective effect of flavonoids on the DNA damage caused by hydroxyl radicals through chelation is an important mechanism, though the converse may be possible, e.g., quercetin. The antioxidant properties of carotenoids, which are dietary natural pigments, have been studied in relation to breast cancer risk and an inverse association was found with plasma concentrations: higher levels mean lower risk. The manipulation of primary and secondary human metabolomes derived especially from existing or transformed gut microbiota was explored as a possible alternative to single-agent dietary interventions for cancer and cardiovascular disease. Sustained oxidative stress leading to inflammation and thence to possibly to cancer and cardiovascular disease is described for spices and herbs, using curcumin as an example of an intervention, based on activation of transcription factors which suggest that oxidative stress, chronic inflammation, and cancer are closely linked.

Apigenin is a dietary flavonoid possessing therapeutic potential against cancers. This study was designed to investigate whether combination therapy with gemcitabine and apigenin enhanced anti-tumor efficacy in pancreatic cancer. In vitro, the combination treatment resulted in more growth inhibition and apoptosis through the down-regulation of NF-kappa B activity with suppression of Akt activation in pancreatic cancer cell lines (MiaPaca-2, AsPC-1). In vivo, the combination therapy augmented tumor growth inhibition through the down-regulation of NF-kappa B activity with the suppression of Akt in tumor tissue. The combination of gemcitabine and apigenin enhanced anti-tumor efficacy through Akt and NF-kappa B activity suppression and apoptosis induction.

We have shown that exposure of the HER2/neu-overexpressing breast cancer cells to apigenin resulted in induction of apoptosis by depleting HER2/neu protein and, in turn, suppressing the signaling of the HER2/HER3-PI3K/Akt pathway. Here, we examined whether inhibition of this pathway played a role in the anti-tumor effect. The results revealed that treatment with apigenin induced apoptosis through cytochrome c release and caused a rapid induction of caspase-3 activity and stimulated proteolytic cleavage of DFF-45. Furthermore, apigenin downregulated cyclin D1, D3 and Cdk4 and increased p27 protein levels. Colony formation in the soft agar assay, a hallmark of the transformation phenotype, was preferentially suppressed in HER2/neu-overexpressing breast cancer cells in the presence of apigenin. In addition, a structure-activity relationship study indicated that (1) the position of B ring; and (2) the existence of the 3', 4'-hydroxyl group on the 2-phenyl group were important for the depletion of HER2/neu protein by flavonoids. These results provided new insights into the structure-activity relationship of flavonoids.