Cancer remains to be one of the leading causes of death in the United States and around the world. The advent of modern drug-targeted therapies has undeniably improved cancer patients’ cares. However, advanced metastasized cancer remains untreatable. Hence, continued searching for a safer and more effective chemoprevention and treatment is clearly needed for the improvement of the efficiency and to lower the treatment cost for cancer care. Cancer chemoprevention with natural phytochemical compounds is an emerging strategy to prevent, impede, delay, or cure cancer. This review summarizes the latest research in cancer chemoprevention and treatment using the bioactive components from natural plants. Relevant molecular mechanisms involved in the pharmacological effects of these phytochemicals are discussed. Pharmaceutical developmental challenges and opportunities in bringing the phytochemicals into the market are also explored. The authors wish to expand this research area not only for their scientific soundness, but also for their potential druggability.
Keywords: anti-cancer, chemoprevention, druggability, mechanism, natural compounds, pathway, phytochemical’
Natural plants have been used to prevent and to treat various diseases for thousands of years.
There are excellent sources of bioactive components exerting their health beneficial effects, and very often, these sources are materials for gourmet food consumptions. Certain bioactive components from the plants have been confirmed for their anti-cancer activities. There is an estimate that approximately 50-60% of cancer patients in the United States utilize agents derived from different parts of plants or nutrients (complementary and alternative medicine), exclusively or concomitantly with traditional therapeutic regimen such as chemotherapy and/or radiation therapy . These include curcumin from tumeric, genistein from soybean, tea polyphenols from green tea, resveratrol from grapes, sulforaphane from broccoli, isothiocyanates from cruciferous vegetables, silymarin from milk thistle, diallyl sulfide from garlic, lycopene from tomato, rosmarinic acid from rosemary, apigenin from parsley, and gingerol from gingers, just to name a few.
Various review articles summarized natural phytochemicals and their anti-cancer effects. In recent years, some of these reviews touched the general overview for the bioactive aspect for phytochemical compounds , or specific compounds such as Vitamin E from plant oil , boron-rich natural compound , hydroxytyrosol from virgin olive oil , resveratrol from grapes , phytoestrogens most notably from soybean , or EGCG from green tea polyphenols , while the others are more specific for certain cancers, e.g., colorectal cancer , breast cancer , head and neck cancer , pancreatic cancer , prostate cancer , or protein targets and pathway mechanisms, such as Nrf2 , COX-2 , PLK1 , angiogenesis . In this review, we will provide a comprehensive summary for the current status of the research and challenges in this area .
The researchers reviewed the pharmacological properties of the tropical plants reportedly used for cancer, and selected seven promising plant species for further investigation: bandicoot berry, sabah snake grass, fool’s curry leaf, seven star needle, black face general, South African leaf, and simple leaf chastetree.
The experiments involved preparing extracts of fresh, healthy and mature leaves of the seven plants, and testing the extracts with the cell lines of seven different types of cancers—breast, cervical, colon, leukemia, liver, ovarian, and uterine. The team opted to examine leaves as they can regrow without harming the plants—making it a sustainable choice, unlike using the bark or roots.
Among the seven plants, the researchers found the extracts of the leaves of the bandicoot berry, South African Leaf, and simple leaf chaste tree promising in fighting against the seven types of cancers. The leaf extracts of the seven star needle performed well against cervical, colon, liver, ovarian, and uterine cancer cells. The leaf extracts of two other plants—fool’s curry leaf and black face general—demonstrated efficacy against some cancer cell lines, too.
“What we did not expect is that the leaf extract of the sabah snake grass was not very effective in inhibiting growth of cancer cells. In our earlier study, this plant was frequently reported to be used by cancer patients in the region. One possibility could be that it may be helping cancer patients in other ways, rather than killing the cancer cells directly,”
Phytochemicals used as cancer chemopreventive and treatment agents
1 Apigenin from parsley
Apigenin is a flavone present in vegetables such as parsley, celery, chamomile, and Egyptian plant Moringa peregrina. It demonstrates cytotoxic activities against breast cancer cell lines (MCF 7), colon cell line (HCT 116), and its cytotoxic activity is comparable to that of doxorubicin. Apigenin is also being considered as a mediator for chemoprevention in the cancerous process and induces a process of autophagia but may induce resistance against chemotherapy. It induces apoptosis in human colon cancer cells , reduces azoxymethane (AOM) induced aberrant crypt foci (ACF) formation in male Sprague-Dawley rats, and increases apoptosis which may contribute to the colon cancer prevention . Apigenin affects leptin/leptin receptor pathway, and induces cell apoptosis in lung adenocarcinoma cell line . It also increases melanogenesis in B16 cells by activating the p38 MAPK pathway at least partially and suggests that apigenin or its derivatives may potentially be used for treating hypopigmentation disorders . Apigenin has been shown to be one of the beneficial compounds in various stages of carcinogenesis. In a recent review by Clere et al, the preventive and therapeutic effects of Apigenin and other flavonoids was summarized to facilitate the extrapolation from animal studies to human.
2 Curcumin from turmeric
Curcumin (diferuloylmethane) is the major components of popular Indian spice turmeric, Curcuma longa L., a member of the ginger family. Its anti-cancer effects have been studied for colon cancer, breast cancer , lung metastases, and brain tumor .
Curcumin’s anticancer effect is attributed to its ability to induce apoptosis in cancer cells without cytotoxic effects on healthy cells, which is very attractive to cancer research scientists. Curcumin interferes with NF-κB , which connects with inflammatory diseases including cancer . Curcumin was able to dissociate raptor from mTOR, inhibit mTOR complex I and might represent a new class of mTOR inhibitor . Ravindran et al suggested that curcumin modulates growth of tumor cells through regulation of multiple cell signaling pathways including cell proliferation pathway (cyclin D1, c-myc), cell survival pathway (Bcl-2, Bcl-x, cFLIP, XIAP, c-IAP1), caspase activation pathway (caspase-8, 3, 9), tumor suppressor pathway (p53, p21), death receptor pathway (DR4, DR5), mitochondrial pathways, and protein kinase pathway (JNK, Akt, and AMPK). Curcumin inhibits p65 and cell invasion by downregulation of COX-2 and MMP-2 expression ; by suppression of gene expression of EGFR and modulation of Akt/mTOR signaling, and inhibition of cell growth . It has also been reported that curcumin suppresses p38 mitogen-activated protein kinase (MAPK) activation, reduces IL-1 beta and matrix metalloproteinase-3, and enhances IL-10 in the mucosa of children and adults with inflammatory bowel disease . Epstein and co-workers had a thorough review on in vitro, animal and clinical studies . In that review, curcumin is cited as non-toxic to human subjects at a high oral dose of up to 12 g/day, and it has anti-inflammatory, antioxidant and anti-cancer properties, however, under some circumstances, its effects can be contradictory as the first clinical trial failed to show benefit, which may be due to an unexpected lack of cognitive decline in placebo group. In our lab, curcumin was studied for modulating AP-1 in human colon HT-29 cancer cell line and was found increasing AP-1-luciferase activity dose-dependently from 1 to 25 μM, and the expression of endogenous cyclin D1 protein was well correlated with those of AP-1-luciferase assay . It inhibited NF-κB stimulator lipopolysaccharide (LPS)-induced inflammation, reduced LPS-induced IκB phosphorylation, and potently inhibited cell growth in MTS assay. Caspase-3 activity was also induced by curcumin . Among our other studies, Affymetrix mouse genome 430 array (45K) was used to analyze mouse liver and intestine mRNA after oral dose of curcumin at 1,000 mg/kg. Our results showed that 822 (664 induced and 158 suppressed) and 222 (154 induced and 68 suppressed) genes in the liver and small intestine, respectively, were curcumin-regulated Nrf2 dependent, which can be classified as ubiquitination and proteolysis, electron transport, detoxification, transport, apoptosis and cell cycle control, cell adhesion, kinase and phosphatase, and transcription factor . Another study from our lab found curcumin inhibited the phosphorylation of Akt, mTOR, and their downstream substrate in human prostate cancer PC-3 cells concentration- and time-dependently. And the inhibition of Akt/mTOR signaling by curcumin resulted from calyculin A-sensitive protein phosphatase-dependent dephosphorylation . We have also investigated combination of curcumin with sulforaphane , with PUFA, with PEITC in inhibiting the growth of human PC-3 prostate xenografts in immunodeficient mice and in inhibiting EGFR signaling in human prostate cancer PC-3 cells and these studies demonstrated various levels of synergistric effects.
3 Crocetin from saffron
Saffron is a spice from the flower of the Saffron crocus and a food colorant present in the dry stigmas of the plant Crocus sativus L.. In a recent review article, saffron is listed as a potential agent for a novel anti-cancer drug against hepatocellular carcinoma . Saffron and its ethanolic extracts are also reported for the studies on human lung cancer, pancreatic cancer cell line , skin carcinoma , colorectal cancer cells , and breast cancer . Its applications and mechanism of actions are reviewed by Bathaie and Mousavi , and more recently, by Gutheil and Reed . Yet, it has been concluded that the exact mechanism of action is still not clear. In general, crocetin affects the growth of cancer cells by inhibiting nucleic acid synthesis, enhancing anti-oxidative system, inducing apoptosis and hindering growth factor signaling pathways . Nam’s study has shown that crocetin is effective for the inhibition of LPS-induced nitric oxide release, for the reduction of the produced TNF-α, IL-1β, and intracellular reactive oxygen species, for the activation of NF-κB, and blockage of the effect of LPS on hippocampal cell death . Although some studies beyond those mentioned above are successfully conducted, more thorough understanding of the mechanism on crocetin and its effects are needed.
Cyanidin is an extract of pigment from red berries such as grapes, blackberry, cranberry, raspberry, or apples and plums, red cabbage and red onion. It possesses antioxidant and radical-scavenging effects which may reduce the risk of cancer. It is reported to inhibit cell proliferation, and iNOS and COX-2 gene expression in colon cancer cells . Another study shows that cyanidin-3-glucoside (C3G) attenuated the benzo[a]pyrene-7,8-diol-9,10-epoxide-induced activation of AP-1 and NF-κB and phosphorylation of MEK, MKK4, Akt, and MAPKs, blocked the activation of the Fyn kinase signaling pathway, which may contributed to its chemopreventive potential. C3G blocks ethanol-induced activation of the ErbB2/cSrc/FAK pathway in breast cancer cells and may prevent/reduce ethanol-induced breast cancer metastasis. Cyanidin-3-O-glucoside, cyanidin-3-O-rutinoside, and the ethanol extract of their source of freeze-dried black raspberries selectively caused significant growth inhibition and induction of apoptosis in a highly tumorigenic rat esophagus cell line (RE-149 DHD) but not in a weakly tumorigenic line (RE-149) . Cyanidin markedly inhibited UVB-induced COX-2 expression and PGE2 secretion in the epidermal skin cell line by suppressing NF-κB and AP-1 which are regulated by MAPK. In that study, MKK-4, MEK1 and Raf-1 are targets of cyanidin for the suppression of UVB-induced COX-2 expression. Cyanidin-3-galactoside and cyanidin-3-glucoside are found to be BCRP substrates, and cyanidin, cyanidin-3,5-diglucoside, and cyanidin-3-rutinoside are potential BCRP inhibitors but their effects on MDR1 were weak . This finding may be helpful for the further development of these compounds for clinical studies and may explain their pharmacokinetic performance in vivo.
5 Diindolylmethane (DIM) /Indole-3-carbinol (I3C) from Brassica vegetables
Indole-3-carbinol (I3C) is found in Brassica vegetables, such as broccoli, cauliflower, collard greens. Diindolylmethane (DIM) is a digestion derivative of indole-3-carbinol via condensation formed in the acidic environment of the stomach. Both are studied for their anticarcinogenic effects
I3C has been studied for cancer prevention and therapy for years for tobacco smoke carcinogen-induced lung adenocarcinoma in A/J mice and it was found that the lung cancer preventive effects are mediated via modulation of the receptor tyrosine kinase/PI3K/Akt signaling pathway, at least partially . I3C and DIM demonstrated exceptional anti-cancer effects against hormone responsive cancers like breast, prostate and ovarian cancers . In a recent study, it is concluded that DIM rather than I3C is the active agent in cell culture studies
DIM showed anti-cancer properties and is currently in clinical trials for various forms of cancers. DIM transduces signaling via aryl hydrocarbon (Ah) receptor, NF-κB/Wnt/Akt/mTOR pathways, impinging on cell cycle arrest, modulated key CYP enzymes, altering angiogenesis, invasion, metastasis and epigenetic behavior of cancer cells . DIM, along with I3C were found to induce Nrf2-mediated phase II drug metabolizing (GSTm2, UGT1A1, and NQO1) and antioxidant (HO-1 and SOD1) genes and also shown synergism with isothiocyanates, such phenethyl isothiocyanate (PEITC) and sulforaphane (SFN) . Lubet et al found that I3C acts as AhR agonist in mammary cancers while DIM does not, and DIM is not analogous to I3C in exerting their anticarcinogenesis effects. DIM and I3C may act more effectively at earlier stage of prostate carcinogenesis and likely through a combination of effects on steroid hormones and/or xenobiotic metabolism pathway .
6 Epigallocatechin gallate from green tea
EGCG is the most abundant catechin compounds in green tea. Increasing evidences show that EGCG can be beneficial in treating brain, prostate , cervical, and bladder] cancers. Yang et al reviewed tea and cancer prevention on molecular mechanisms, molecular targets and human relevance of tea constituents. Among numerous mechanism studies, EGCG binds and inhibits the anti-apoptotic protein Bcl-xl, a protein involved in both cancer cell and normal cell survival . EGCG suppressed AOM-induced colonic premalignant lesions in mice, interfered with EGFR signaling, and inhibited hepatocyte growth factor-induced cell proliferation in human colon cancer cells . EGCG has shown inhibition of mitogen-activated protein kinases (MAPK), cyclin-dependent kinases, growth factor-related cell signaling, activation of activator protein 1 and NF-κB, topoisomerase I and matrix metalloproteinases. In human, the pharmacological concentration are typically at least 10 μmol/L .
Our lab studied EGCG induced stress signals in HT-29 human colon adenocarcinoma cells and found that EGCG inhibited HT-29 cell growth with an IC50 of approximately 100 μM, and the dose levels higher than that showed apparent nuclear condensation and fragmentation, and the study concluded that EGCG caused damage to mitochondria and JNK mediated EGCG-induced apoptotic cell death . EGCG was also found to increase AP-1 luciferase activity dose-dependently up to 100 μM , reduce LPS-induced IκB alpha phosphorylation. Additional study in our group demonstrated that combining sulforaphane and EGCG exerted synergistic effects in HT-29 AP-1 human colon carcinoma cells . To investigate possible Nrf2-mediation, EGCG were orally dosed to C57BL/6J and C57BL/6J/Nrf2(−/−) mice. The liver and small intestine were analyzed using Affymetrix mouse genome 430 2.0 array. Gene expression showed that 671 Nrf2-dependent and 256 Nrf2-independent genes were regulated by EGCG in liver, and 228 Nrf2-dependent and 98 Nrf2-independent genes are regulated by EGCG in intestine. This study pointed out that the EGCG chemopreventive effects may be mediated by Nrf2, at least partially .
7 Fisetin from strawberries, apples
Fisetin is a flavone found in various plants such as Acacia greggii, Acacia berlandieri, Euroasian smoketree, parrot tree, strawberries, apple, persimmon, grape, onion, and cucumber. Fisetin has been found to alleviate aging effects in the yeast or fruit fly , exert anti-inflammatory effect in LPS-induced acute pulmonary inflammation and anti-carcinogenesis effects in HCT-116 human colon cancer cells]. Fisetin is also a potent antioxidant and modulates protein kinase and lipid kinase pathways . Fisetin, along with other flavonoids such as luteolin, quercetin, galangin and EGCG, induced the expression of Nrf2 and the phase II gene product HO-1 in human retinal pigment epithelial (RPE) cells which could protect RPE cells from oxidative-stress-induced death with a high degree of potency and low toxicity and reduced hydrogen peroxide (H2O2)-induced cell death . A recent study by Khan et al found dual inhibition of PI3K/Akt and mTOR signaling in human non-small cell lung cancer cells by fisetin . Fisetin inhibited Wnt signaling through the modulation of beta-catenin expression, transcriptional activity and of the subsequent expression of Wnt target genes . Other studies found fisetin decreased cell viability with G1-phase arrest and disrupted Wnt/β-catenin signaling , exhibited an inhibitory effect on the abilities of adhesion, migration, and invasion, and significantly decreased the nuclear levels of nuclear factor kappa B (NF-κB) and activator protein-1 (AP-1). Fisetin was also found to help to overcome the multidrug resistance caused by the high expression of the plasma membrane drug transporter P-glycoprotein (P-gp), which is associated with an elevated intracellular glutathione (GSH) content in various human tumors .
8 Genistein from soybean
Genistein is an isoflavone originates from a number of plants such as lupine, fava beans, soybeans, kudzu, and psoralea, Flemingia vestita, and coffee. Functioning as antioxidant and anthelmintic, genistein has been found to have antiangiogenic effects (blocking formation of new blood vessels), and may block the uncontrolled cell growth associated with cancer, most likely by inhibiting the enzymes that regulate cell division and cell survival (growth factors). Genistein’s activity was chiefly functioned as a tyrosine kinase inhibitor by inhibiting DNA topoisomerase II . In vitro and in vivo studies show that genistein has been found to be useful in treating leukemia .
Estrogen receptors are over-expressed in around 70% of breast cancer cases (ER-positive). Binding of estrogen to the ER stimulates proliferation of mammary cells, with the resulting increase in cell division and DNA replication. Estrogen metabolism produces genotoxic waste, which may cause disruption of cell cycle, apoptosis, DNA repair, and forms tumor. Genistein can compete with 17β-estradiol (estrogen) to bind to estrogen receptor and shows higher affinity towards estrogen receptor β than towards estrogen receptor α , where estrogen receptor functions as a DNA-binding transcription factor that regulates gene expression. Genistein was confirmed to increase the rate of growth of some estrogen receptor in breast cancer and the rate of proliferation of estrogen-dependent breast cancer when not co-treated with an estrogen antagonist. In colon cancer, genistein is thought to contribute to reduced colonic inflammation in 2,4,6-trinitrobenzenesulfonic acid (TNBS)-induced colitis . Our lab previously investigated genistein and found that genistein possibly involved in JNK pathway in inducing AP-1 activity .
9 Gingerol from gingers
Gingerol is the active component of fresh ginger with distinctive spicyness. Gingerol has been studied for its anticancerous effects for the tumors in colon , breast and ovarian , and pancreas A recent review by Oyagbemi et al summarized the mechanisms in the therapeutic effects of gingerol . In short, gingerol has demonstrated antioxidant, anti-inflammation, and antitumor promoting properties, decreases iNOS and TNF-alpha expression via suppression of IκBα phosphorylation and NF-κB nuclear translocation . Treating K562 cells and MOLT4 cells with gingerol, the ROS levels were significantly higher than control groups, inducing apoptosis of leukemia cells by mitochondrial pathway . On human hepatocarcinoma cells, gingerol, along with 6-shogaol were found to exert anti-invasive activity against hepatoma cells through regulation of MMP-9 and TIMP-1, and 6-shogaol further regulated urokinase-type plasminogen activity . Topical application of 6-shogaol, another active component from ginger is more effective than 6-gingerol and curcumin in inhibiting 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced transcription of iNOS and COX-2 mRNA expression in mouse skin, which may justify further in vitro and in vovo studies.
10 Kaempferol from tea, broccoli, grapefruit
Kaempferol is a natural flavonol isolated from tea, broccoli, Witch-hazel, grapefruit, Brussels sprouts, apples, etc . Kaempferol has been studied for pancreatic cancer , and lung cancer . It has been investigated for its antiangiogenic, anticancer, and radical scavenging effects . Kaempferol, displayed moderate cytostatic activity of 24.8 – 64.7 μM in the cell lines of PC3, HeLa and K562 human cancer cells . To et al studied kaempferol as aryl hydrocarbon receptor (AhR) antagonist showing inhibition of ABCG2 upregulation, thereby reversing the ABCG2-mediated multi-drug resistance, which may be useful for esophageal cancer treatment . Luo et al found that kaempferol induces apoptosis in ovarian cancer cells through the activation of p53 in the intrinsic pathway . Yang et al reported that kaempferol inhibited quinine reductase 2 with an IC (50) value of 33.6 μM for NF-κB activity . In a study by Niestroy et al, kaempferol was studied on benzo[a]pyrene (BaP) mediated effects on Caco-2 cells on concerted effects on the expression of AhR and Nrf2 pathway components . In that study, BaP, quercetin and kaempferol activated Nrf2 pathway by induction of Nrf2, and its target genes NQO1, GSTP1, GSTA1, and GCLC. However, in spite of their own induction potential for Nrf2, both quercetin and kaempferol counteract the effects of BaP on expression of AhR, AhRR, Nrf2, GSTP1 and NQO1.
Kaempferol showed very low bioavailability of approximately 2% in earlier study . Using Madin-Darby canine kidney (MDCK) cell monolayers, kaempferol was shown to be a breast cancer resistance protein (Bcrp, Abcg2) inhibitor and may also be a Bcrp substrate, which may represent one possible mechanism for the low bioavailability of kaempferol.
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11 Lycopene from tomato
Lycopene is a bright red pigment and phytochemical from tomatoes, red carrots, watermelons, and red papayas. It demonstrates antioxidant activity and chemopreventive effects in many studies, especially for prostate cancer. Poorly solube in water, lycopene has high solubility in organic solvents. Its anti-cancer property is attributed to activating cancer preventive enzymes such as phase II detoxification enzymes . Lycopene was found to inhibit human cancer cell proliferation, and to suppress insulin-like growth factor-I-stimulated growth. This may open new avenues for lycopene study on the role of the prevention or treatment of endometrial cancer and other tumors . Lycopene also possesses inhibitory effects on breast and endometrial cancer cells , prostate cancer cells , and colon cancer cells . However, in a study conducted by Erdman and group using xenocraft prostate tumors into rats, it was found that the tumors grew more slowly in those given whole dried tomato powder but not in those given lycopene, which may indicate that lycopene may be an important component in tomato but not the only component in tomato that actively suppressing the growth of the prostate cancer .
12 Phenethyl Isothiocyanate (PEITC) from cruciferous vegetable
PEITC, along with sulforaphane from cruciferous vegetables, such as watercress, broccoli, cabbage, etc., have been studied for induction of apoptosis in cell lines. PEITC has shown very strong potency against melanoma. It has been intensively studied for chemoprevention against breast cancer cells, non-small cell lung cancer , cervical cancer, osteogenic sarcoma U-2 OS, prostate cancer , and myeloma cell lines . PEITC induces apoptosis in some cell lines that are resistant to some currently used chemotherapeutics drugs.
PEITC induced apoptosis in highly metastatic human non-small cell lung cancer L9981 cells via Caspase-3 activation , leading to cell cycle arrest at the G2/M phase by modulation of cyclin B1 expression, where MAPK/AP-1 pathway was the target . In vitro and in vivo data support that PEITC, as well as sulforaphane, induced G2/M cell cycle arrest, apoptosis of cell death of myeloma cells . In cervical cancer cells, PEITC was found to increase the expression of the death receptors (DR4 and DR5), cleaved caspase-3, induced caspase-8 and truncated BID, down-regulated the ERK1/2 and MEK phosphorylation while maintaining the expression of JNK and phospho-p38 MAPK . PEITC was also studied for cytotoxicity in a human liver hepatoma cell line (HepG2-C8) along with I3C, DIM, and sulforaphane, and it turned out that PEITC was more toxic than I3C and DIM. In human prostate cancer DU 145 cells, PEITC induced apoptosis mediated by the activation of caspase-8, -9, and -3-dependent pathways. PEITC induced substential increase in the activation of caspase-3, -8, -9, cleavage and degradation of PARP, and apoptosis dose- and time-dependently, accompanied by the caspase-independent downregulation of Mcl-1, Akt inactivation, and activation of JNK. Using human osteogenic sarcoma U-2 OS cells, PEITC, along with benzyl isothiocyanates (BITC), caused growth inhibition, inhibited cell cycle regulatory proteins, promoted Chk1 and p53, induced apoptosis and poly(ADP-ribose)polymerase (PARP) cleavage . Wang et al found that cells with mutant p53 are more sensitive to cytotoxicity induced by PEITC than those with wild-type protein, which may be a novel target for cancer chemoprevention.
13 Resveratrol from grapes
Resveratrol is a natural phenol and can be found in the red grapes skin, peanuts and in other fruits. Jang et al reported cancer chemopreventive activity of resveratrol . In that study, resveratrol was found to possess anti-initiation activity by inducing phase II drug metabolizing enzymes, anti-promotion activity by mediating anti-inflammatory effects and inhibiting cyclooxygenase and hydroperoxidase functions, and anti-progression activity by inducing cell differentiation in human promyelocytic leukemia. However, poor oral bioavailability caused by rapid metabolism limited its effectiveness in animal cancer models and in human studies . However, with direct contact, resveratrol has demonstrated anti-carcinogenesis effects in skin tumor and gastrointestinal tract tumor, such as N-nitrosomethylbenzylamine (NMBA)-induced esophageal tumors in rats . Resveratrol was found to inhibit metastasis via reducing hypoxia inducible factor-1α and MMP-9 expression in colon cancer cells ; to suppress dextran sulfate sodium (DSS) – induced colitis through downregulation of p38, prostaglandin E synthase-1, iNOS, and COX-2 in mice ; to inhibit Wnt signaling and beta-catenin localization in colon-derived cells . Another study found that resveratrol at a concentration of 10 μM or more induces apoptosis in normal cells as well as cancer cells which demonstrated a potential cytotoxic effect on normal cells .
Our lab studied resveratrol’s modulation of AP-1 in human colon HT-29 cancer cell line and reported that resveratrol increased AP-1-luciferase activity dose-dependently and induced cell death in a dose-dependent manner . Resveratrol increased activation of LPS-induced NF-κB-luciferase activity at lower dose, but inhibited activation at higher dose, reduced LPS-induced IκB alpha phosphorylation, and induced caspase-3 activation . Our another toxicogenomics study of resveratrol in rat liver showed that at the high doses (3 gm/kg/day for 28 days) the modulation of liver genes may implicate the potential toxicity .
14 Rosmarinic acid from rosemary
Rosmarinic acid (RA) is a natural antioxidant found in culinary spice and medicinal herbs such as lemon balm, peppermint, sage, thyme, oregano, and rosemary to treat numerous ailments. Rosemary extracts play important roles in anti-inflammation, anti-tumor, and anti-proliferation in various in vitro and in vivo studies. Study in Ls174-T human colon carcinoma cells found that rosmarinic acid inhibits migration, adhesion, and invasion dose-dependently . In another study, rosmarinic acid may inhibit bone metastasis from breast carcinoma mainly via the pathway of the NF-κB and by simultaneous suppression of interleukin-8 (IL-8) . Moon et al investigated TNF-α mediated anti-cancer therapy mechanism. In human leukemia U938 cells, rosmarinic acid significantly sensitized TNF-α-induced apoptosis through the suppression of NF-κB and reactive oxygen species (ROS), and suppressed NF-κB activation through inhibition of phosphorylation and degradation of IκBα . Rosmarinic acid reduced 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced COX-2 promoter activity and protein levels in colon cancer HT-29 cells, repressed binding of the activator protein-1 (AP-1) in a nonmalignant breast epithelial cell line (MCF10A), and antagonized the stimulatory effects of TPA on COX-2 protein expression.
Sulforaphane is an organosulfur compound obtained from cruciferous vegetables such as broccoli, Brussels sprouts and cabbages. The enzyme myrosinase in GI tract transforms glucoraphanin into sulforaphane upon damage to the plant such as from chewing. Broccoli sprouts and cauliflower sprouts are rich in glucoraphanin.
Sulforaphane has shown induction of phase II drug metabolism enzymes of xenobiotic transformation, such as quinine reductase and glutathione S-transferase, and enhances the transcription of tumor suppression proteins. Sulforaphane downregulated the Wnt/beta-catenin self-renewal pathway in breast cancer stem cells ; protected skin against UV radiation damage , and inhibited histone deacetylase (HDAC) activity . In Apc(Min/+) mice, sulforaphane reduced the number of polyps by inhibiting Akt, ERK signaling, COX-2, and cyclin D1 protein expression and also inhibited cancer cell growth by inducing apoptosis in SW620 cells . In a recent study, sulforaphane induced cytotoxicity and lysosome- and mitochondria-dependent cell death in colon cancer cells with deleted p53. It also increased Bax in the presence of JNK-mediated Bcl-2 inhibition followed by mitochondrial release of cytochrome c and activation of apoptosis .
In our lab, sulforaphane has been studied for its chemoprevention activities and its involvement in anti-inflammation. In human colon HT-29 cancer cells, sulforaphane increased AP-1-luciferase activity dose-dependently and then decreased at higher doses, and induced JNK activity . Sulforaphane also strongly inhibited LPS-induced NF-κB-luciferase activations. In MTS assay, sulforaphane potently inhibited cell growth and induced caspase-3 activity . In HepG2 human hepatoma cells, sulforaphane strongly induced Nrf2 protein expression and ARE-mediated transcription activation, retarded degradation of Nrf2 through inhibiting Keap1, and activated transcriptional expression of antioxidant enzyme HO-1 . In human prostate cancer PC-3 cells, sulforaphane suppressed NF-κB and NF-κB-regulated gene expression through IκB-alpha, and IKK pathway . Sulforaphane was found to be unable to disrupt the cytosolic distribution of Nrf2 zip which indicates that the importance of Keap1 retention as a key rate-limiting step in Nrf2 activation . Study in HepG2 cells also found that transcriptional activation of Nrf2/ARE is critical in sulforaphane-mediated induction of HO-1, which can be modulated in part by the blockade of p38 MAPK signaling pathway. In addition, p38 MAPK can phosphorylate Nrf2 and enhances the association between Nrf2 and Keap1 proteins, thereby potentially inhibiting Nrf2 translocation into nuclear to initiate antioxidant gene transcription . Pretreatment of sulforaphane in primary peritoneal macrophages of wild type mice potently inhibited LPS-stimulated mRNA expression, protein expression of TNF-alpha, IL-1beta, COX-2 and iNOS. HO-1 expression was significantly augmented as well. The anti-inflammatory effects was attenuated in Nrf2 (−/−) primary peritoneal macrophages and therefore, the anti-inflammatory activity was mainly exerted by Nrf2 pathway in mouse peritoneal macrophages .
In the liver of C57BL/6J and C57BL/6J/Nrf2(−/−) mice, sulforaphane induced Nrf2-dependent detoxification phase I, II drug metabolizing enzymes and phase III transporters, using Affymetrix 39K oligonucleotide microarray. This study indicates that sulforaphane increases the expression of genes through the Nrf2 signaling pathway that directly detoxify exogenous toxins/carcinogens or endogenous reactive oxygen species, and genes involved in the recognition and repair/removal of damaged proteins . In the ApcMin/+ mice, when fed with SFN supplemented diet, the mice developed significantly less and smaller polyps with higher apoptotic and lower proliferative indices in their small intestine in a dose-dependent manner. SFN also found to suppress the expression of phosphorylated c-Jun N-terminal kinase (p-JNK), phosphorylated extracellular signal-regulated kinases (p-ERK) and phosphorylated-Akt (p-Akt). However, the biomarkers of the Wnt pathway, beta-catenin and cyclin-D1 were unaffected by sulforaphane treatment. This study also found that a diet of 3 to 30 nmol/g is required to prevent or retard adenoma formation in the ApcMin/+ gastrointestinal tract . In our another study, sulforaphane was found to inhibit 7,12-dimethylbenz(a)anthracene-induced skin tumorigenesis in C57BL/6 mice mediated by Nrf2 . In ApcMin/+ mice, the effects of sulforaphane on the gene expression profile in small intestinal polyps were studied using Affymetrix microarray. While SFN is a strong phase II drug metabolizing enzyme inducer, apoptosis genes MBD4, TNFR-7 and TNF (ligand)-11 were up-regulated, cell growth and maintenance genes, pro-survival genes cyclin-D2, integrin-beta1 and Wnt-9A were down-regulated, where the predicted phase II genes were less modulated. Genes potentially involved in colorectal carcinogenesis, 15-LOX was found increased and COX-2 decreased . In C57BL/6J wild type and C57BL/6J/Nrf2(−/−) knock-out mice, UVB exposure (300mJ/cm2) resulted in skin inflammation in both groups, however, WT mice returned to basal level to a greater extent; and mice treated with sulforaphane restored sunburn cells by 8 days but KO mice did not, which indicates functional Nrf2 confers a protective effect against UVB-induced inflammation, and sulforaphane mediates photoprotective effects in the mice .
Sulforaphane demonstrated synergistic effects when combined with EGCG in HT-29 AP-1 human colon carcinoma cells , or with dibenzoylmethane in ApcMin/+ mice for reducing intestinal adenomas , or with phenethyl isothiocyanate in down-regulating inflammation markers TNF, IL-1, NO, PGE2 and inducing phase II/antioxidant enzymes like HO-1, NQO1 in RAW 264.7 cells .
After fed with dietary broccoli sprouts for 16 weeks, TRAMP mice were sacrificed and analyzed for sulforaphane and sulforaphane-GSH conjugate in the prostate tumor. TRAMP mice with high broccoli diet showed significant retardation of prostate tumor growth and elevated expression levels of Nrf2, HO-1, cleaved-Caspase-3, cleaved-PARP and Bax proteins and decreased expression levels of Keap1 and Bcl-xL proteins; and the Akt and its downstream kinase and target proteins such as mTOR, 4E-BP1 and cyclin D1 were also reduced. All of these indicate that sulforaphane has significant inhibitory effects on prostate tumorigenesis .
Many other laboratories have been very active in the research on sulforaphane. There are currently eighteen clinical studies registered with and sulforaphane is a promising compound for its druggability.
16 Triterpenoids from wax-like coatings of fruits and medicinal herbs
Triterpenoids are biosynthesized in plants by cyclization of squalene, a triterpene hydrocarbon and precursor of all steroids . This group of phytochemicals are sub-classified into cucurbitanes, dammaranes, ergostanes, friedelanes, lanostanes, limonoids, lupanes, oleananes, tirucallanes, ursanes , and the list is still growing. The diversity and regulation of terpenoids are appreciated by Tholl review . Various in vitro and in vivo studies have been conducted for chemoprevention and therapy of breast cancer , and pancreatic cancer using triterpenoids. This group of phytochemicals exert their chemopreventive and anti-cancer activities via enhancing apoptosis, NO, stimulating DR4, DR5, caspase-3/7, caspase 8, Bax, JNK, MAPK, p38, decreasing phosphor-STAT3, PARP cleavage, suppressing COX-2, IL-1β, NF-κB, IKKα/β, cyclin D1, cyclin A, cyclin B1, ERα protein and mRNA, HER2 phosphorylation, caveolin-1, Akt, JAK1, STAT 3, Bcl2, c-Jun, c-Fos, JNK, mTOR, blocking cell cycle at G1, G1-S, G2-M, etc .
Through these studies, triterpenoids have been shown to possess pleiotropic mode of effects for cancers in in vitro and in vivo models. More studies are needed to validate their promises in their chemopreventive and anti-cancer activities in clinical stage.
17 Vitamin D from mushroom
After exposed to ultraviolet B light, vertebrate can generate Vitamin D in their skins. Light exposed mushroom could also be an excellent source of Vitamin D. Vitamin D has been involved in breast cancer , colon cancer , ovarian cancer, and pancreatic cancer . The mechanism is still not quite clear. However, vitamin D receptor (VDR) appears playing an important role. For example, women with mutations in the VDR gene had an increased risk of breast cancer and VDR may be a mediator of breast cancer risk which could represent a target for cancer prevention efforts .
Two physiologically relevant Vitamin Ds are vitamin D2 (ergocalciferol) and D3 (cholecalciferol). D3 is produced after exposure to ultraviolet B light from the sun or artificial sources. Numerous studies have linked vitamin D and cancer but opposite conclusion were also presented by the conflicting study results. Vitamin D’s anti-cancer effect may be mediated via vitamin D receptors (VDR) in cancer cells . Increased risk of breast cancer has been linked with the polymorphisms of VDR gene. Kovalenko et al using VDR KO and WT mice and showed that low diet vitamin D or VDR deletion provided a prostate environment that is permissive to early pro-carcinogenic events that enhance prostate cancer risk . Stefanska et al reported that vitamin D3 possess high efficacy in the reduction of PTEN promoter methylation and it was associated with PTEN induction as well as DNA methyltransferase down-regulation and p21 up-regulation after treatments with vitamin D3, suggesting a complex regulation of the DNA methylation machinery . However, a literature conducted through June 2010, Hypovitaminosis D seems to be associated with a worse prognosis in some cancers, but vitamin D supplementation failed to demonstrate a benefit in prostate cancer patients and the available evidence is insufficient to recommend vitamin D supplementation in cancer patients in clinical practice . And study also suggested that genetic polymorphisms in vitamin D-related genes do not play a major role in breast cancer risk in Chinese women . Therefore, vitamin D’s skin cancer and prostate cancer prevention are still inconclusive .
18 Vitamin E from plant oil
Vitamin E represents a family of compounds comprising both tocopherols and tocotrienols and is a fat-soluble antioxidant that exists in many foods including wheat germ oil, sunflower oil, and safflower oils. Alphatocopherol is the most bioactive form of vitamin E that stops the production of reactive oxygen species when fat undergoes oxidation. There are reports that both tocopherols and tocotrienols have anti-tumor effects due to their antioxidant properties, and tocotrienols show stronger bioactivity and both show antiproliferative, proapoptotic and COX-2 inhibiting effects in in vitro studies . Review by Viola et al discussed the hypomethylated forms of tocotrienols in their high in vitro and in vivo metabolism and their potency in cytoprotection, cancer prevention and even chemotherapeutic effects . Chen et al reported that vitamin E supplementation could evidently inhibit or reverse the cytotoxic effects of cigarette smoke extract in a dose- and time-dependent manner in mouse embryonic lung cells . A recent review by Nesaretnam and Meganathan linked tocotrienols and their roles in inflammation and cancer, and in this review, mechanism of the cellular signaling pathways of NF-κB, STAT3, and COX-2 were discussed . In a meta-analysis and meta-regression study, although vitamin A, dietary vitamin E, and total vitamin E intake all reduced breast cancer risk significantly when data from all studies were pooled, the results became non-significant when data from cohort studies were pooled .
Tocotrienols are members of the vitamin E family. Unlike tocopherols, tocotrienols possess an unsaturated isoprenoid side chain that confers superior anti-cancer properties and they inhibit AKT and ERK activation and suppress pancreatic cancer cell proliferation by suppressing the ErbB2 pathway . In pancreatic cancer cell lines, tocotrienols selectively inhibit the HMG-CoA reductase pathway through posttranslational degradation and suppress the activity of transcription factor NF-κB. γ- and δ-tocotrienol treatment of cells reduced the activation of ERK MAP kinase and that of its downstream mediator ribosomal protein S6 kinase (RSK) in addition to suppressing the activation of protein kinase AKT. Tocotrienols reduced apoptosis in pancreatic cancer cells through the suppression of vital cell survival and proliferative signaling pathways such as those mediated by the PI3-kinase/AKT and ERK/MAP kinases via downregulation of Her2/ErbB2 expression . Sylvester et al discussed the approach to combine tocotrienols with agents that have complementary anticancer mechanisms of action to achieve synergistic anticancer response, e.g., combination with traditional cancer chemotherapy, with statins, with receptor tyrosine kinase inhibitors, and with COX-2 inhibitors .