Skin cancer chemopreventive effects of a flavonoid antioxidant silymarin are mediated via impairment of receptor tyrosine kinase signaling and perturbation in cell cycle progression

Role of Silymarin in Cancer Treatment: Facts, Hypotheses, and Questions

The flavonoid silymarin extracted from the seeds of Sylibum marianum is a mixture of 6 flavolignan isomers. The 3 more important isomers are silybin (or silibinin), silydianin, and silychristin. Silybin is functionally the most active of these compounds. This group of flavonoids has been extensively studied and they have been used as hepato-protective substances for the mushroom Amanita phalloides intoxication and mainly chronic liver diseases such as alcoholic cirrhosis and nonalcoholic fatty liver. Hepatitis C progression is not, or slightly, modified by silymarin. Recently, it has also been proposed for SARS COVID-19 infection therapy. The biochemical and molecular mechanisms of action of these substances in cancer are subjects of ongoing research. Paradoxically, many of its identified actions such as antioxidant, promoter of ribosomal synthesis, and mitochondrial membrane stabilization, may seem protumoral at first sight, however, silymarin compounds have clear anticancer effects. Some of them are: decreasing migration through multiple targeting, decreasing hypoxia inducible factor-1α expression, inducing apoptosis in some malignant cells, and inhibiting promitotic signaling among others. Interestingly, the antitumoral activity of silymarin compounds is limited to malignant cells while the nonmalignant cells seem not to be affected. Furthermore, there is a long history of silymarin use in human diseases without toxicity after prolonged administration. The ample distribution and easy accessibility to milk thistle-the source of silymarin compounds, its over the counter availability, the fact that it is a weed, some controversial issues regarding bioavailability, and being a nutraceutical rather than a drug, has somehow led medical professionals to view its anticancer effects with skepticism. This is a fundamental reason why it never achieved bedside status in cancer treatment. However, in spite of all the antitumoral effects, silymarin actually has dual effects and in some cases such as pancreatic cancer it can promote stemness. This review deals with recent investigations to elucidate the molecular actions of this flavonoid in cancer, and to consider the possibility of repurposing it. Particular attention is dedicated to silymarin's dual role in cancer and to some controversies of its real effectiveness.

NADPH Oxidase and Epidermal Growth Factor Receptor Are Promising Targets of Phytochemicals for Ultraviolet-Induced Skin Carcinogenesis

The skin acts as the primary defense organ that protects the body from the external environment. Skin cancer is one of the most common cancers in the world. Skin carcinogenesis is usually caused by cell degeneration due to exposure to ultraviolet (UV) radiation, which causes changes in various signaling networks, disrupting the homeostasis of single skin cells. In this review, we summarize the roles of nicotinamide adenine dinucleotide phosphate oxidase (NOX) and epidermal growth factor receptor (EGFR) in UV-induced skin carcinogenesis. Furthermore, we describe the crosstalk that exists between NOX, EGFR, and protein tyrosine phosphatase κ and its oncogenic downstream signaling pathways. Chemoprevention is the use of chemical compounds to recover the healthy status of the skin or delay cancer development. Current evidence from in vitro and in vivo studies on chemopreventive phytochemicals that target NOX, EGFR, or both, as major regulators of skin carcinogenesis will also be discussed.

Effect of lemon peel flavonoids on UVB-induced skin damage in mice

By establishing an effective ultraviolet B (UVB) radiation model of skin damage in mice, the effect of lemon peel flavonoids (LPF) on skin damage was explored. UVB skin damage in UV-irradiated mice was simulated, and animal models were established. Serum parameters were measured using kits, skin sections were stained with hematoxylin-eosin (H&E) and Masson, and quantitative polymerase chain reaction (qPCR) was used to detect the expression of skin tissue-related mRNA. The experimental results showed that LPF increased the activity of catalase (CAT) and superoxide dismutase (SOD) oxidases in serum of mice with UVB-induced skin damage and decreased MDA, interleukin-1β (IL-1β), IL-6, IL-10, and tumor necrosis factor-alpha (TNF-α) levels. Pathological observation indicated that LPF alleviated the skin tissue lesions caused by UVB. LPF upregulated the mRNA expression of SOD1, SOD2, CAT, nuclear factor erythroid-2 related factor 2 (Nrf2), heme oxygenase-1 (HO-1), and inhibitor of NF-κB alpha (IκB-α) and downregulated the expression of nuclear factor kappa B (NF-κB), p38 MAPK, and cyclooxygenase-2 (COX-2) in the skin tissue of skin-damaged mice. There was a greater protective effect of LPF on the skin as compared to vitamin C (VC) at the same application concentration, and the effect of LPF was positively correlated with the concentration. High performance liquid chromatography (HPLC) analysis showed that LPF contained five flavonoid compounds, namely isomangiferin, rutin, astragalin, naringin, and quercetin. We demonstrated that flavonoids from LPF exhibit an excellent skin protection effect with satisfactory application value.

Cancer chemoprevention through dietary flavonoids: what's limiting?

Flavonoids are polyphenols that are found in numerous edible plant species. Data obtained from preclinical and clinical studies suggest that specific flavonoids are chemo-preventive and cytotoxic against various cancers via a multitude of mechanisms. However, the clinical use of flavonoids is limited due to challenges associated with their effective use, including (1) the isolation and purification of flavonoids from their natural resources; (2) demonstration of the effects of flavonoids in reducing the risk of certain cancer, in tandem with the cost and time needed for epidemiological studies, and (3) numerous pharmacokinetic challenges (e.g., bioavailability, drug–drug interactions, and metabolic instability). Currently, numerous approaches are being used to surmount some of these challenges, thereby increasing the likelihood of flavonoids being used as chemo-preventive drugs in the clinic. In this review, we summarize the most important challenges and efforts that are being made to surmount these challenges.

Chemopreventive Efficacy of Silymarin in Skin and Prostate Cancer

Prevention and therapeutic intervention by phytochemicals are newer dimensions in the arena of cancer management. In this regard, the cancer chemopreventive role of silymarin (Silybum marianum) has been extensively studied and has shown anticancer efficacy against various cancer sites, especially skin and prostate. In skin cancer, silymarin treatment inhibits ultraviolet B radiation or chemically initiated or promoted carcinogenesis. These effects of silymarin against skin carcinogenesis have been attributed to its strong antioxidant and anti-inflammatory action as well as its inhibitory effect on mitogenic signaling. Similarly, silymarin treatment inhibits 3, 2-dimethyl-4-aminobiphenyl—induced prostate carcinogenesis and retards the growth of advanced prostate tumor xenograft in athymic nude mice. In prostate cancer, silymarin treatment down-regulates androgen receptor—, epidermal growth factor receptor—, and nuclear factor-κB— mediated signaling and induces cell cycle arrest. Extensive preclinical findings have supported the anticancer potential of silymarin, and now its efficacy is being evaluated in cancer patients.

Size restricted silymarin suspension evokes integrated adaptive response against acute hypoxia exposure in rat lung

Despite its extraordinary antioxidant capacity, the clinical usage of silymarin has remained restricted to amelioration of hepatic pathology. Perhaps its low bioavailability and uneven bio-distribution, owing to its poor aqueous solubility, are two main causes that have dampened the clinical applicability and scope of this preparation. We took these two challenges and suggested an unexplored application of silymarin. Apart from liver, two of the most susceptible vital organs at the highest risk of oxidative stress are brain and lung, especially during reduced oxygen saturation (hypoxia) at anatomical level. Hypoxia causes excess generation of radicals primarily in the lungs as it is the first organ at the interphase of atmosphere and organism making it the most prone and vulnerable to oxidative stress and the first responder against hypobaric hypoxia. As our first objective, we improved the silymarin formulation by restricting its size to the lower threshold and then successfully tested the prophylactic and therapeutic action in rat lung challenged with simulated hypobaric hypoxia. After dose optimization, we observed that 50mg/kg BW silymarin as size restricted and homogenous aqueous suspension successfully minimized the reactive oxygen species and augmented the antioxidant defense by significant upregulation of catalase and superoxide dismutase and reduced glutathione. Moreover, the well-established hypoxia markers and proteins related to hypoxia adaptability, hif1a and VEGF were differentially regulated conferring significant reduction in the inflammation caused by hypobaric hypoxia. We therefore report,the unexplored potential benefits of silymarin for preventing high altitude associated pathophysiology further paving its road to clinical trials.

Supercritical CO₂ extraction of oil, fatty acids and flavonolignans from milk thistle seeds: Evaluation of their antioxidant and cytotoxic activities in Caco-2 cells

The optimal conditions of supercritical carbon dioxide (SC-CO2) (160-220 bars, 40-80 °C) technology combined with co-solvent (ethanol), to recover oil, flavonolignans (silychristin, silydianin and silybinin) and fatty acids from milk thistle seeds, to be used as food additives and/or nutraceuticals, were studied. Moreover, the antioxidant and cytotoxic activities of the SC-CO2 oil seeds extracts were evaluated in Caco-2 carcinoma cells. Pressure and temperature had a significant effect on oil and flavonolignans recovery, although there was not observed a clear trend. SC-CO2 with co-solvent extraction at 220 bars, 40 °C was the optimum treatment to recover oil (30.8%) and flavonolignans from milk thistle seeds. Moreover, linoleic (47.64-66.70%), and oleic (19.68-24.83%) acids were the predominant fatty acids in the oil extracts recovered from milk thistle under SC-CO2. In addition, SC-CO2 extract showed a high antioxidant activity determined by DPPH and ABTS tests. Cytotoxic activities of silychristin, silydianin and silybinin and the obtained SC-CO2 extract (220 bars, 40 °C) were evaluated against Caco-2 cells. The SC-CO2 extract inhibited the proliferation of Caco-2 cells in a dose-responsive manner and induced the highest percentage of mortality of Caco-2 cells (from 43 to 71% for concentrations from 10 up to 100 μg/ml of SC-CO2 oil seeds).

Restoration of the anti-proliferative and anti-migratory effects of 1,25-dihydroxyvitamin D by silibinin in vitamin D-resistant colon cancer cells

Colorectal carcinoma (CRC) is the third most common cancer in developed countries. A large fraction of cases are linked to chronic intestinal inflammation, with concomitant increased TNF-α release and elevated Snail1/Snail2 levels. These transcription factors in turn suppress vitamin D receptor (VDR) expression, resulting in loss of responsiveness to the protective anti-proliferative and anti-migratory effects of 1,25-dihydroxyvitamin D (1,25D). Experimental and epidemiologic evidence support the use of natural products to target CRC. Here we show that the flavonolignan silibinin reverses the TNF-α-induced upregulation of Snail1 and Snail2 in the 1,25D-resistant human colon carcinoma cells HT-29. These silibinin effects are accompanied by an increase in VDR levels; Snail1 overexpression reverses these silibinin effects. Silibinin also restores promoter activity from a vitamin D-response element (VDRE) reporter construct. While 1,25D had no significant effect on HT-29 and SW480-R cell proliferation and migration, co-treatment with silibinin restored 1,25D responsiveness. In addition, co-treatment with silibinin plus 1,25D decreased proliferation and migration at doses where silibinin alone had no effect. These findings demonstrate that this combination may present a novel approach to target CRC in conditions of chronic colonic inflammation.

The binding of silibinin to ERp57

The flavonoid silibinin is known to intervene in many cellular processes involved in a variety of pathologies, thus appearing a promising therapeutic tool. The molecular mechanisms responsible for these activities, however, have not been clearly defined, and although some of its interactions with proteins have been identified, the relative affinities are often too low to appear relevant in vivo. Here we describe the interaction of silibinin with the protein disulfide isomerase ERp57, characterized by a submicromolar dissociation constant. This interaction enhances the formation of a ERp57/REF-1 complex, and furthermore appears to affect the intracellular distribution of ERp57. This protein is involved in signaling pathways which are also affected by silibinin. This suggests that the ERp57-silibinin interaction might explain at least some of the biological effects caused by the flavonoid.

Skin photoaging and the role of antioxidants in its prevention

Photoaging of the skin depends primarily on the degree of ultraviolet radiation (UVR) and on an amount of melanin in the skin (skin phototype). In addition to direct or indirect DNA damage, UVR activates cell surface receptors of keratinocytes and fibroblasts in the skin, which leads to a breakdown of collagen in the extracellular matrix and a shutdown of new collagen synthesis. It is hypothesized that dermal collagen breakdown is followed by imperfect repair that yields a deficit in the structural integrity of the skin, formation of a solar scar, and ultimately clinically visible skin atrophy and wrinkles. Many studies confirmed that acute exposure of human skin to UVR leads to oxidation of cellular biomolecules that could be prevented by prior antioxidant treatment and to depletion of endogenous antioxidants. Skin has a network of all major endogenous enzymatic and nonenzymatic protective antioxidants, but their role in protecting cells against oxidative damage generated by UV radiation has not been elucidated. It seems that skin's antioxidative defence is also influenced by vitamins and nutritive factors and that combination of different antioxidants simultaneously provides synergistic effect.

Flavonoids as RTK inhibitors and potential anticancer agents

Tyrosine kinase receptors (RTKs) play a crucial role in the regulation of the cell division cycle. Currently more than 50 RTKs divided into several subfamilies have been described. The inhibition of these enzymes has emerged as an important research-area. Compounds able to inhibit the activity of these enzymes are expected to display antiproliferative properties. Flavonoids are representative of various small molecules acting as RTK inhibitors. These naturally occurring compounds are able to bind to the ATP-binding site of several kinases. The most plausible current hypothesis explaining the action of these substances on kinases is that the chromenone moiety of the flavonoid acts as a mimetic of the adenine moiety of ATP, the receptor co-factor. In this review, we report recent results on the activity of natural and synthetic derivatives of flavonoids as inhibitors of RTKs. Mechanistic aspects, the therapeutic usefulness, and the potential clinical use are discussed.

Evaluation of the cancer chemopreventive efficacy of silibinin in genetic mouse models of prostate and intestinal carcinogenesis: relationship with silibinin levels

Silibinin, a flavonolignan from milk thistle seeds, possesses cancer chemopreventive properties in rodent models of carcinogenesis. We tested the hypotheses that silibinin or silipide, silibinin formulated with phospholipids, delays tumour development in TRAMP or Apc(Min) mice, genetic models of prostate or intestinal malignancies, respectively. Mice received silibinin or silipide with their diet (0.2% silibinin equivalents) from weaning. Intervention with silipide reduced the size of well differentiated TRAMP adenocarcinomas by 31%. Silipide and silibinin decreased the incidence of poorly differentiated carcinomas by 61% compared to mice on control diet. Silipide decreased plasma levels of insulin-like growth factor (IGF)-1 by 36%. Levels of circulating IGF binding protein (IGFBP)-3 in mice on silipide or silibinin were 3.9- or 5.9-fold, respectively, elevated over those in control TRAMP mice. In Apc(Min) mice silibinin, but not silipide, had only a marginal adenoma number-reducing effect. The results cautiously support the advancement of silipide to the stage of clinical investigation in prostate cancer.

Therapeutic value of orally administered silibinin in renal cell carcinoma: manipulation of insulin-like growth factor binding protein-3 levels

OBJECTIVES

To investigate if the feeding of silibinin (an anticancer flavonoid) to mice inhibits in vivo renal cell carcinoma (RCC) growth via changes in insulin-like growth factor binding protein-3 (IGFBP-3) levels.

MATERIALS AND METHODS

Male severe combined immunodeficiency disease (SCID) mice (7 weeks old), with left kidneys injected with 1 million SN12K1 cells, were fed a silibinin-containing diet (0.1%, 0.2% and 0.4% w/w) or control AIN-93G diet for 39 days from 1 day after tumour engraftment.

RESULTS

There was a reduction in tumour deposits and tumour kidney weight in SCID mice fed with a 0.4% silibinin-containing diet compared to those fed the control diet. Mice with tumour injection (silibinin or control-diet group) had constant total body weight and food consumption. The mean plasma and tumourous kidney silibinin concentrations, as measured by high-pressure liquid chromatography-tandem mass spectrometry, increased with escalating doses of silibinin. Using real-time polymerase chain reaction and enzyme-linked immunosorbent assay, the mean tissue IGFBP-3 mRNA (in SN12K1-implanted kidney) and plasma IGFBP-3 levels increased in mice fed with 0.1% silibinin (tumour IGFBP-3 mRNA levels, 156% higher vs control-diet group, P = 0.007; and plasma IGFBP-3 levels, 61% higher vs control-diet group, P = 0.002) but not in mice fed with the higher silibinin pellet strengths.

CONCLUSION

Oral administration of silibinin suppressed local and metastatic tumour growth in vivo in an orthotopic xenograft model of RCC. This anti-neoplastic action of silibinin might involve IGFBP-3. The exact mechanism through which IGFBP-3 promotes silibinin's anticancer effects warrants further investigation.

Silymarin and epithelial cancer chemoprevention: how close we are to bedside?

Failure and high systemic toxicity of conventional cancer therapies have accelerated the focus on the search for newer agents, which could prevent and/or slow-down cancer growth and have more human acceptability by being less or non-toxic. Silymarin is one such agent, which has been extensively used since ages for the treatment of liver conditions, and thus has possibly the greatest patient acceptability. In recent years, increasing body of evidence has underscored the cancer preventive efficacy of silymarin in both in vitro and in vivo animal models of various epithelial cancers. Apart from chemopreventive effects, other noteworthy aspects of silymarin and its active constituent silibinin in cancer treatment include their capability to potentiate the efficacy of known chemotherapeutic drugs, as an inhibitor of multidrug resistance-associated proteins and as an adjunct to the cancer therapeutic drugs due to their organ-protective efficacy specifically liver, and immunostimulatory effects. Widespread use of silymarin for liver health in humans and commercial availability of its formulations with increased bioavailability, further underscore the necessity of carrying out controlled clinical trials with these agents in cancer patients. In this review, we will briefly discuss the outcomes of clinical trials being conducted by us and others in cancer patients to provide insight into the clinical relevance of the observed chemopreventive effects of these agents in various epithelial cancer models.

Chemoprevention of nonmelanoma skin cancer

Skin cancer is the most common cancer in human beings. The increased incidence of skin cancer has brought much attention to the process by which these tumors develop and how they can be prevented. Efforts have been made to educate the public about the importance of protecting skin from excessive ultraviolet light. Despite this work, the incidence of skin cancer continues to increase. Available compounds may be useful in the chemoprevention of skin cancer. Chemoprevention is defined as oral or topical use of dietary or pharmacologic agents to inhibit or reverse the development of cancer. Potential agents included are the retinoids; difluoromethylornithine; T4 endonuclease V; polyphenolic antioxidants, such as (-)-epigallocatechin gallate, found in green tea and grape seed extract; silymarin; isoflavone genestein; nonsteroidal anti-inflammatory drugs; curcumin; lycopene; vitamin E; beta-carotene; and selenium. Many of these agents are available over the counter as topical or oral preparations.

LEARNING OBJECTIVE

At the conclusion of this activity, participants should be familiar with the chemopreventive agents and their efficacy, as well as any significant side effects associated with them.

Electron-transfer capacity of catechin derivatives and influence on the cell cycle and apoptosis in HT29 cells

Galloylated and nongalloylated catechin conjugates with cysteine derivatives have been synthesized and evaluated for their capacity to scavenge free radicals and to influence crucial functions (cell cycle, apoptosis) in HT29 colon carcinoma cells. We show that the nonphenolic part of the molecule modified the capacity of catechins to donate hydrogen atoms and to transfer electrons to free radicals. Nongalloylated derivatives did not significantly influence either the cell cycle or apoptosis. Among the galloylated species, 4beta-[S-(O-ethyl-cysteinyl)]epicatechin 3-O-gallate, which showed a high electron-transfer capacity (5 e- per molecule), arrested the cell cycle and induced apoptosis as expected for galloylated catechins such as tea (-)-epigallocatechin 3-O-gallate. 4beta-[S-(N-Acetyl-O-methyl-cysteinyl)]epicatechin 3-O-gallate, which showed the highest hydrogen-donating capacity (10 H per molecule) while keeping the electron-transfer capacity low (2.9 e- per molecule), did not trigger any significant apoptosis. The gallate moiety did not appear to be sufficient for the pro-apoptotic effect of the catechin derivatives in HT29 cells. Instead, a high electron-transfer capacity is more likely to be behind this effect. The use of stable radicals sensitive exclusively to electron transfer may help to design molecules with either preventive scavenging action (high hydrogen donation, low electron transfer) or therapeutic pro-apoptotic activity (high electron transfer).

Pilot study of oral silibinin, a putative chemopreventive agent, in colorectal cancer patients: silibinin levels in plasma, colorectum, and liver and their pharmacodynamic consequences

Silibinin, a flavonolignan from milk thistle, has intestinal cancer chemopreventive efficacy in rodents. It is a strong antioxidant and modulates the insulin-like growth factor (IGF) system by increasing circulating levels of IGF-binding protein 3 (IGFBP-3) and decreasing levels of IGF-I. Here, the hypothesis was tested that administration of oral silibinin generates agent levels in human blood and colorectal and hepatic tissues consistent with pharmacologic activity. Patients with confirmed colorectal adenocarcinoma received silibinin formulated with phosphatidylcholine (silipide) at dosages of 360, 720, or 1,440 mg silibinin daily for 7 days. Blood and biopsy samples of normal and malignant colorectum or liver were obtained before dosing, and blood and colorectal or hepatic tissues were collected at resection surgery after the final silipide dose. Levels of silibinin were quantified by high-pressure liquid chromatography-UV, and plasma metabolites were identified by liquid chromatography-mass spectrometry. Blood levels of IGFBP-3, IGF-I, and the oxidative DNA damage pyrimidopurinone adduct of deoxyguanosine (M1dG) were determined. Repeated administration of silipide was safe and achieved levels of silibinin of 0.3 to 4 micromol/L in the plasma, 0.3 to 2.5 nmol/g tissue in the liver, and 20 to 141 nmol/g tissue in colorectal tissue. Silibinin monoglucuronide, silibinin diglucuronide, silibinin monosulfate, and silibinin glucuronide sulfate were identified in the plasma. Intervention with silipide did not affect circulating levels of IGFBP-3, IGF-I, or M1dG. The high silibinin levels achieved in the human colorectal mucosa after consumption of safe silibinin doses support its further exploration as a potential human colorectal cancer chemopreventive agent.

New derivatives of silybin and 2,3-dehydrosilybin and their cytotoxic and P-glycoprotein modulatory activity

Large series of O-alkyl derivatives (methyl and benzyl) of silybin and 2,3-dehydrosilybin was prepared. Selective alkylation of the silybin molecule was systematically investigated. For the first time we present here, for example, preparation of 19-nor-2,3-dehydrosilybin. All prepared silybin/2,3-dehydrosilybin derivatives were tested for cytotoxicity on a panel of drugs sensitive against multidrug resistant cell lines and the ability to inhibit P-glycoprotein mediated efflux activity. We have identified effective and relatively non-cytotoxic inhibitors of P-gp derived from 2,3-dehydrosilybin. Some of them were more effective inhibitors at concentrations lower than a standard P-gp efflux inhibitor cyclosporin A. Another group of 2,3-dehydrosilybin derivatives also had better inhibitory effects on P-gp efflux but a cytotoxicity comparable with that of parent 2,3-dehydrosilybin. Structural requirements for improving inhibitory activity and reducing toxicity of 2,3-dehydrosilybin were established. Effect of E-ring substitution as well as an influence of the substituent size at the C-7-OH position of A-ring on P-gp-inhibitory activity was evaluated for the first time in this study.

Structure prerequisite for antioxidant activity of silybin in different biochemical systems in vitro

Structural analogues (flavanone: 2-4 and flavone: 5 and 6, respectively) of silybin (1a) were synthesized and tested for inhibitory activity on O(2)(-) release and PKC translocation in PMA-stimulated neutrophils as well as xanthine oxidase activity in order to identify the molecular structures responsible for the antioxidant property of silybin. Concerning the prevention of hem-mediated oxidative modification of LDL by silybin, the hydroxyl radical scavenging activity of its structural analogues was also determined. We demonstrated that the basic skeleton of 1a (4) is responsible for its inhibitory activity on O(2)(-) release in PMA-stimulated neutrophils via inhibition of PKC translocation, since introduction of a double bound and hydroxyl groups at C-5 and C-7 position (5 and 6) did not result in further increase in inhibition of O(2)(-) release. It has been shown that the presence of the phenolic hydroxyl group at C-5 and C-7 of 1a is essential for the inhibition of xanthine oxidase activity. Moreover, introduction of a double bond into the C-ring of 2 and 3, resulting in flavone derivatives (5 and 6), markedly enhanced the antioxidant effect in all the tested systems. Finally, silybin (1a) and its flavon derivatives (5 and 6) directly scavenged hydroxyl radicals as well. On the basis of these results it might be concluded that different moiety of silybin is responsible for inhibition of overproduction of O(2)(-) in stimulated neutrophils, xanthine oxidase activity, and for prevention of hem-mediated oxidative modification of LDL.

Silibinin inhibits angiogenesis via Flt-1, but not KDR, receptor up-regulation

BACKGROUND

Our previous study found that silymarin (SM) and its major pure component silibinin (SB) have anti-angiogenic effects via decreased vascular endothelium growth factor (VEGF) secretion of LoVo cells (colon cancer). We designed this consecutive study to evaluate the anti-angiogenic effects of SM/SB in vivo, and on VEGF receptor (VEGFR) gene expression.

MATERIALS AND METHODS

We used LoVo cells exposed to SM/SB in a modified chicken chorioallantoic membrane assay (CAM) to evaluate anti-angiogenic effects. We used EA.hy 926 cells (endothelial cells) exposed to SM/SB to evaluate the effect on VEGFR-1 (Flt-1) and VEGFR-2 (KDR), with 1-step, real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

In CAM, SM/SB showed a dose-dependent decrease on the vascular density index (VDI) induced by LoVo cells, as did thalidomide in a concentration of 10 microg/ml. Adding escalating dosages of VEGF successfully reversed this inhibitory effect. RT-PCR revealed that SB up-regulated Flt-1 mRNA expression of EA.hy 926 cells. SM had a similar trend, although the effect was not statistically significant (P = 0.19). Neither drug effected KDR mRNA expression.

CONCLUSION

We conclude that anti-angiogenic effects of SM/SB are associated with the up-regulation of VEGFR-1 (Flt-1) gene expression and that they are good candidates for combination therapy to treat colorectal cancer.

Conjugation of catechins with cysteine generates antioxidant compounds with enhanced neuroprotective activity

Antioxidant compounds derived from the conjugation of (-)-epicatechin and (-)-epicatechin 3-O-gallate with cysteine and cysteine derivatives protected HT-22 nerve cells (EC50 between 36 and 65 microM) from death triggered by glutamate while underivatized (-)-epicatechin was almost inactive (EC50=610 microM). Differences in free radical scavenging capacity (DPPH assay) could not account for the improvement in neuroprotective activity upon derivatization of (-)-epicatechin with thiols. Moreover, while the gallate-containing compounds are more efficient radical scavengers than their non-galloylated counterparts, they are only equally or less potent as neuroprotective agents. Although all of the conjugates were able to scavenge mitochondrially generated reactive oxygen species (ROS) inside the cells, the majority of their neuroprotective activity appeared to be dependent upon their ability to maintain glutathione levels. These results suggest that a mechanism other than ROS scavenging is involved in the neuroprotective action exerted by the epicatechin conjugates.

Effects of silymarin flavonolignans and synthetic silybin derivatives on estrogen and aryl hydrocarbon receptor activation

Silymarin, a standardized mixture of flavonolignans, or its major constituents could be effective for prevention and treatment of hepatic damage or skin cancer. However, their potential side effects, such as modulation of endocrine functions via the disruption of estrogen receptor (ER) and/or aryl hydrocarbon receptor (AhR) activation, are largely unknown. In the present study, we investigated impact of silymarin, its constituents and a series of their synthetic derivatives on ER- and AhR-mediated activities using in vitro reporter gene assays. We found that none of the compounds under study affected the AhR-mediated activity in rat hepatoma cells. Contrary to that, several compounds behaved as either partial or full ER agonists. Silymarin elicited partial ER activation, with silybin B being probably responsible for a majority of the weak ER-mediated activity of silymarin; silybin A and other flavonolignans were found to be inactive and potent ER agonist taxifolin is only a minor constituent of silymarin. To our knowledge, this is probably the first time, when receptor-specific in vitro effects of separated diastereomers have been demonstrated. In contrast to silymarin constituents, the synthetic silybin derivatives, potentially useful as chemoprotective agents, did not modulate the ER-mediated activity, with exception of 23-O-pivaloylsilybin. Interestingly, 7-O-benzylsilybin potentiated ER-mediated activity of 17beta-estradiol despite possessing no estrogenic activity. In conclusion, our data suggest that estrogenicity of some silymarin constituents should be taken in account as their potential side effect when considered as chemopreventive compounds. These results also stress the need to study biological activities of purified or synthesized diastereomers of silybin derivatives.

Effect of new antioxidant cysteinyl-flavanol conjugates on skin cancer cells

Novel catechin derivatives obtained from grape procyanidins and l-cysteine scavenge free radicals by hydrogen atom donation, rather than electron transfer, and reduce cell viability in A375 and M21 melanoma cells. In particular, 4beta-(S-cysteinyl)epicatechin 3-O-gallate has a free radical scavenging capacity as strong as that of tea (-)-epigallocatechin gallate and causes a significant S-phase cell-cycle arrest in both cell lines at doses higher than 100 microM. The other cysteinyl compounds do not affect normal cell cycle distribution. The gallate derivative also induces apoptosis in melanoma cells more strongly than the other derivatives and the parent (-)-epicatechin do. The gallate compound seems to trigger nuclear condensation and fragmentation, which is confirmed by DNA laddering. Interestingly, they do not induce apoptosis in keratinocytes (HaCaT).

Dietary Supplementation with Silymarin Inhibits 3,2'-Dimethyl-4-Aminobiphenyl-Induced Prostate Carcinogenesis in Male F344 Rats

PURPOSE

Silymarin has been shown to be a potent anticarcinogenic agent. Here, we investigated the modifying effects of dietary feeding with a naturally occurring polyphenolic antioxidant flavonoid silymarin on 3,2'-dimethyl-4-aminobiphenyl (DMAB)-induced prostatic carcinogenesis in male F344 rats.

EXPERIMENTAL DESIGN

Male F344 rats were given s.c. injections of DMAB (25 mg/kg body weight) every other week for 20 weeks. They also received the experimental diet containing 100 or 500 ppm silymarin for 40 weeks, starting 1 week after the last dosing of DMAB. All of the rats were sacrificed 60 weeks after the start of the experiment. Histopathology and immunohistochemistry for proliferative cell nuclear antigen, cyclin D1, and apoptotic indices were done in the prostatic lesions, including invasive adenocarcinomas, intraepithelial neoplasms, and nonlesional glands.

RESULTS

Dietary feeding with 500 ppm silymarin significantly inhibited the incidence of prostatic adenocarcinoma when compared with the DMAB-alone group (17.6% versus 50.0%, P < 0.05). The proliferative cell nuclear antigen- and cyclin D1-positive indices in adenocarcinomas, prostatic intraepithelial neoplasm, and nonlesional glands in rats treated with DMAB and silymarin were slightly lower than that of the DMAB-alone group. Also, dietary administration of silymarin increased apoptotic index in prostatic adenocarcinoma by measuring immunohistochemically positive nuclei for ssDNA.

CONCLUSIONS

Our results indicate that silymarin exerts chemopreventive ability against chemically induced prostatic carcinogenesis through apoptosis induction and modification of cell proliferation.

Procyanidin fractions from pine (Pinus pinaster) bark: radical scavenging power in solution, antioxidant activity in emulsion, and antiproliferative effect in melanoma cells

Pine (Pinus pinaster) bark is a rich source of procyanidin oligomers. From a total polyphenolic extract, we have generated fractions of different procyanidin composition. The mixtures, devoid of gallate esters, were active as free radical scavengers against ABTS(*+), DPPH, and HNTTM. Pine bark fractions were tested for antioxidant activity in solution (hydrogen donation and electron transfer) and emulsion (inhibition of lipid peroxidation) and compared with their galloylated counterparts from grape origin. While galloylation clearly influenced the free radical scavenging efficiency in solution, it did not seem to play a determinant role in protection against lipid peroxidation in emulsion. The fractions were very mild inhibitors of cell proliferation. Because gallate esters appear to interfere with crucial cell functions, gallate free pine procyanidins may be the innocuous chemopreventative agents of choice for many applications in food and skin protection.

Silybin and silymarin--new effects and applications

This article aims to review critically literature published mainly within this millennium on the new and emerging applications of silymarin, the polyphenolic fraction from the seeds of Silybum marianum and its main component silybin. Silymarin and silybin used so far mostly as hepatoprotectants were shown to have other interesting activities as e.g., anticancer and canceroprotective. These activities were demonstrated in a large variety of illnesses of different organs as e.g., prostate, lungs, CNS, kidneys, pancreas and others. Besides the cytoprotective activity of silybin mediated by its antioxidative and radical-scavenging properties also new activities based on the specific receptor interaction were discovered--e.g., inhibition and modulation of drug transporters, P-glycoproteins, estrogenic receptors, nuclear receptors and some others. New derivatives of silybin open new ways to its therapeutic applications. Pharmacology dealing with optically pure silybin diastereomers may suggest new mechanisms of its action.

Grape-seed derived procyanidins interfere with adipogenesis of 3T3-L1 cells at the onset of differentiation

OBJECTIVE

Our group's previous results on the effects of a grape seed procyanidin extract (GSPE) on adipose metabolism showed that peroxisome proliferator-activated receptor-gamma (PPARgamma) plays a central role in the lipolytic effects of GSPE on adipocytes. Since PPARgamma2 is a main regulator of the differentiation process of adipocytes, we investigated whether GSPE affects the adipogenesis of 3T3-L1 cells.

DESIGN

We performed a time point screening by treating 3T3-L1 cells with GSPE during the differentiation process for 24 h.

MEASUREMENTS

Differentiation markers and differential gene expression due to GSPE treatment (using the microarray technique).

RESULTS

Twenty four hour-GSPE treatment at the onset of differentiation reduces adipose-specific markers and maintains the expression of preadipocyte marker preadipocyte factor-1 (Pref-1) significantly elevated. These effects were not found in other time points. Microarray analysis of gene expression after GSPE treatment at the early stage of differentiation showed a modified gene expression profile in which cell cycle and growth-related genes were downregulated by GSPE.

CONCLUSION

These results suggest that GSPE affects adipogenesis, mainly at the induction of differentiation, and that procyanidins may have a new role in which they impede the formation of adipose cells.

Anti-arthritic effect and subacute toxicological evaluation of Baccharis genistelloides aqueous extract

This work studies the potential subacute toxicological effects of the aqueous extract of Baccharis genistelloides (AEBg) and demonstrates a new anti-arthritic therapeutic effect. The treatment of the collagen-induced arthritis (CIA) group with 4.2 mg/kg AEBg induced an important decrease (75%) in CIA severity in all animals, while the 42 mg/kg dose treated only 50% of animals. After AEBg treatment, no significant differences were observed in body weight, aspect, color and relative weight of liver, kidneys, thymus or lungs between CIA groups. CIA and healthy AEBg groups treated with both doses did not show genotoxic effects to liver and kidney cells by the Comet assay, compared to its own control group. The augmented AST in the CIA group, compared to healthy control one was regularized by the AEBg treatment with 4.2 mg/kg but not with 42 mg/kg. No other significant difference was found on serum biochemical parameters, as well as on spontaneous or stimulated lymphocyte proliferation between CIA groups. The treatment of healthy animals with AEBg 4.2 mg/kg did not change the aspect, color or relative weight of kidneys, liver or lungs but reduced the body weight, the thymus and popliteal lymph node (PLN) relative weight and serum glucose and triglyceride levels. Concluding, our results indicate an anti-arthritic effects of AEBg without liver and kidney subacute toxicity and hypoglycemic and hypotriglyceridemic actions on healthy animals.

New advances on the functions of epidermal growth factor receptor and ceramides in skin cell differentiation, disorders and cancers

Recent advances in understanding of the biological functions of the epidermal growth factor and epidermal growth factor receptor (EGF-EGFR) system and ceramide production for the maintenance of skin integrity and barrier function are reported. In particular, the opposite roles of EGFR and ceramide cascades in epithelial keratinocyte proliferation, migration and terminal differentiation are described. Moreover, the functions of ceramides in the epidermal permeability barrier are reviewed. The alterations in EGFR signaling and ceramide metabolism, which might be involved in the etiopathogenesis of diverse skin disorders and cancers, are described. New progress in understanding of skin organization, which might provide the basis for the design of new transcutaneous drug delivery techniques as well as for the development of new therapies of skin disorders and cancers, are reported.

Effect of silybin on phorbol myristate actetate-induced protein kinase C translocation, NADPH oxidase activity and apoptosis in human neutrophils

Mechanism of the action of silybin (1) and its derivatives (2-4), possessing different lipid solubility in PMA-stimulated neutrophils was evaluated. Silybin (1) inhibited the calcium, phosphatidylserine- and diacylglycerol-dependent protein kinase C translocation and the NADPH oxidase activity in PMA-stimulated neutrophils and resulted in decreased apoptosis. Furthermore, silybin (1) inhibited xanthine oxidase activity and hem-mediated oxidative degradation of low-density lipoprotein, as well. Its derivatives (2-4), possessing different lipid-solubility, affected all the studied parameters. The lipid solubility of silybin (1) was enhanced by methylation (5'7'4''trimethylsilybin: 2), whereas a decrease in lipid-solubility by acetylation of compound 2 (5',7,'4"-trimethylsilybin-acetate: 3) or all the hydroxyl groups of silybin (peracetyl-silybin: 4) attenuated the antioxidant capacity by decreasing the inhibition in PKC translocation and NADPH oxidase activation. All the derivatives of silybin (2-4) showed no inhibition in cell free systems; e.g. did not alter the xanthine oxidase activity and the hem-mediated oxidative degradation of LDL. In conclusion, the antioxidant activity of (1) might be due to its ability to inhibit PKC translocation and NADPH oxidase activation in PMA-stimulated neutrophils. The increase of lipid solubility of silybin (1) supports its penetration through cell membrane and enhances its inhibitory effects. This structural modification of (1) might have pharmacological consequences.

Effects of dietary flavonoids on major signal transduction pathways in human epithelial 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.

Silibinin upregulates the expression of cyclin-dependent kinase inhibitors and causes cell cycle arrest and apoptosis in human colon carcinoma HT-29 cells

Silymarin, a defined mixture of natural flavonoid, has recently been shown to have potent cancer chemopreventive efficacy against colon carcinogenesis in rat model; however, the mechanism of such efficacy is not elucidated. Here, using pure active agent in silymarin, namely silibinin, we show its antiproliferative and apoptotic effects, and associated molecular alterations in human colon carcinoma HT-29 cells. Silibinin treatment of cells at 50-100 microg/ml doses resulted in a moderate to very strong growth inhibition in a dose- and a time-dependent manner, which was largely due to a G0/G1 arrest in cell cycle progression; higher dose and longer treatment time also caused a G2/M arrest. In mechanistic studies related its effect on cell cycle progression, silibinin treatment resulted in an upregulation of Kip1/p27 and Cip1/p21 protein as well as mRNA levels, and decreased CDK2, CDK4, cyclin E and cyclin D1 protein levels together with an inhibition in CDK2 and CDK4 kinase activities. In other studies, we observed that G2/M arrest by silibinin was associated with a decrease in cdc25C, cdc2/p34 and cyclin B1 protein levels, as well as cdc2/p34 kinase activity. In the studies assessing biological fate of silibinin-treated cells, silibinin-induced cell cycle arrest and growth inhibition were not associated with cellular differentiation, but caused apoptotic death. The quantitative apoptosis analysis showed up to 15% apoptotic cell death after 48 h of silibinin treatment. Interestingly, silibinin-induced apoptosis in HT-29 cells was independent of caspases activation, as all caspases inhibitor did not reverse silibinin-induced apoptosis. This observation was further confirmed by the findings showing a lack in caspases activity increase and caspases and PARP cleavage as well as a lack in cytochrome c release in cytosol following silibinin treatment of HT-29 cells. Additional studies conducted in mice showed that silibinin doses found effective in HT-29 cells are achievable in plasma, which increases the significance of the present findings and their possible translation in in vivo anticancer efficacy of silibinin against colon cancer. Together, these results identify molecular mechanisms of silibinin efficacy as a cell cycle regulator and apoptosis inducer in human colon carcinoma HT-29 cells, and justify further studies to investigate potential usefulness of this nontoxic agent in colon cancer prevention and intervention.

Anti-angiogenic effect of silymarin on colon cancer LoVo cell line

OBJECTIVE

This study was designed to evaluate the anti-angiogenic effect of silymarin (SM) and its major pure component silibinin (SB), and also thalidomide (TH).

MATERIALS AND METHODS

A modified in vitro system using a coculture of endothelial (EA.hy 926) and colon cancer (LoVo) cell lines was adopted in this study.

RESULTS

In cytotoxicity assay, SM/SB/TH concentrations causing 20% (IC(20)) inhibition of cellular growth were 41.8 microg/ml/0.22 mM/0.088 mM for EA.hy 926 cells, and 16.1 microg/ml/0.12 mM/0.099 mM for LoVo cells, respectively. All 3 drugs showed concentration dependent inhibition of migration and differentiation assay. The IC(50) inhibiting chemotaxis migration of EA.hy 926 towards LoVo by SM/SB/TH was 1.15 microg/ml/0.66 microM/1.98 microM, respectively. In differentiation assay, SM/SB/TH inhibited in vitro capillary tube formation by 50% at 1.25 microg/ml/2.6 micro/6.3 microM, respectively. In an analysis of vascular endothelial growth factor secreted by LoVo cells, SM/SB/TH decreased 50% secretion at 6.52 microg/ml/6.6 microM/131.7 microM, respectively.

CONCLUSION

SM/SB has a strong anti-angiogenesis effect on the colon cancer cell line, and this might provide an alternative treatment option for anti-cancer treatment.

The role of nutrition in preventing prostate cancer: a review of the proposed mechanism of action of various dietary substances

BACKGROUND

Dietary modifications to prevent prostate cancer (PCa) continue to gain attention as research demonstrates that various dietary nutrients/supplements are related to decreased risk of developing prostate cancer (PCa). Several studies have focused on the antioxidant and nonantioxidant effects of various dietary substances in the prevention of PCa. Research into the mechanisms by which PCa is prevented, or its disease severity is reduced by dietary micronutrients and vitamins continues to enrich our understanding of the mechanisms by which PCa is initiated and progresses.

METHODS

We reviewed the literature on dietary nutrients with antioxidant properties that have been shown to have a positive effect in reducing the incidence or preventing the occurrence of PCa including carotenoids (e.g., lycopene), retinoids (e.g., vitamin A), vitamin E, vitamin C, selenium, and polyphenols. Other nutrients examined included vitamin D and calcium.

RESULTS

Many dietary micronutrients have demonstrated significant and complex effects on PCa cell proliferation, differentiation, and signaling related to the initiation, progression, and regression of PCa.

CONCLUSION

Understanding the mechanisms by which various dietary nutrients exert their effects on PCa may make it possible to design effective drugs for treating PCa and to promote better nutrition and lifestyle changes in those at risk for PCa.

Valorization of grape (Vitis vinifera) byproducts. Antioxidant and biological properties of polyphenolic fractions differing in procyanidin composition and flavonol content

Many byproducts and wastes generated by agroindustries contain polyphenols with potential application as food antioxidants and preventive agents against skin cancer and other diseases. The performance of polyphenolic fractions from Parellada grape (Vitis vinifera) pomace as antioxidants in different physicochemical environments was tested. Fractions containing oligomers with mean degree of polymerization between 3 and 4 and percentage galloylation ca. 30% were the most potent free radical scavengers and efficient antioxidants in an oil-in-water emulsion. A fraction including glycosylated flavonols was also efficient in the emulsion. All the fractions showed low aquatic toxicity and weak influence on proliferation of human melanoma cells.

Flavonoid antioxidant silymarin and skin cancer

Oxidative stress is one of the key players in skin carcinogenesis, and therefore identifying nontoxic strong antioxidants to prevent skin cancer is an important area of research. In both animal and cell culture studies, we have shown that silymarin, a naturally occurring polyphenolic flavonoid antioxidant, exhibits preventive and anticancer effects against skin cancer. For example, silymarin strongly prevents both photocarcinogenesis and skin tumor promotion in mice, in part, by scavenging free radicals and reactive oxygen species and strengthening the antioxidant system. We also found that this effect of silymarin is by inhibiting endogenous tumor promoter tumor necrosis factor alpha in mouse skin, a central mediator in skin tumor promotion. In mechanistic studies, silymarin inhibits mitogenic and cell survival signaling and induces apoptosis. Furthermore, silymarin effectively modulates cell-cycle regulators and check points toward inhibition of proliferation, and growth arrest in G0-G1 and G2-M phases of the cell cycle. Thus, due to its mechanism-based chemopreventive and anticancer effects in experimental models, silymarin is an important candidate for the prevention and/or therapy of skin cancer, as well as other cancers of epithelial origin in humans.

Cysteinyl-flavan-3-ol conjugates from grape procyanidins. Antioxidant and antiproliferative properties

New bio-based antioxidant compounds have been obtained by depolymerisation of grape polymeric flavanols in the presence of cysteine. Their preparation and purification, as well as their antiradical/antioxidant and antiproliferative properties are reported. 4beta-(S-cysteinyl)epicatechin 5, 4beta-(S-cysteinyl)catechin 6 and 4beta-(S-cysteinyl)epicatechin 3-O-gallate 7 were efficiently purified from the crude depolymerised mixture by cation-exchange chromatography and preparative reversed-phase chromatography. The new compounds were more efficient than the underivatised (-)-epicatechin 1 as scavengers of the 1,1-diphenyl-2-picrylhydrazyl free radical (DPPH) and weak growth inhibitors of human colon carcinoma HT29 cells. The order of antiradical and antiproliferative efficiency was 7 >5 approximately 6 >1, the same for both assays.

Differential responses of skin cancer-chemopreventive agents silibinin, quercetin, and epigallocatechin 3-gallate on mitogenic signaling and cell cycle regulators in human epidermoid carcinoma A431 cells

Silibinin, quercetin, and epigallocatechin 3-gallate (EGCG) have been shown to be skin cancer-preventive agents, albeit by several different mechanisms. Here, we assessed whether these agents show their cancer-preventive potential by a differential effect on mitogenic signaling molecules and cell cycle regulators. Treatment of human epidermoid carcinoma A431 cells with these agents inhibited the activation of the epidermal growth factor receptor and the downstream adapter protein Shc, but only silibinin showed a marked inhibition of mitogen-activated protein kinase-extracellular signal-regulated kinase-1 and -2 activation. In terms of cell cycle regulators, silibinin treatment showed an induction of Cip1/p21 and Kip1/p27 together with a significant decrease in cyclin-dependent kinase (CDK)-4, CDK2, and cyclin D1. Quercetin treatment, however, resulted in a moderate increase in Cip1/p21 with no change in Kip1/p27 and a decrease in CDK4 and cyclin D1. EGCG treatment also led to an induction of Cip1/p21 but no change in Kip1/27, CDK2, and cyclin D1 and a decrease in CDK4 only at low doses. Treatment of cells with these agents resulted in a strong dose- and time-dependent cell growth inhibition. A high dose of silibinin and low and high doses of quercetin and EGCG also led to cell death by apoptosis, suggesting that a lack of their inhibitory effect on mitogen-activated protein kinase-extracellular signal-regulated kinase-1 and -2 activation possibly "turns on" an apoptotic cell death response associated with their cancer-preventive and anticarcinogenic effects. Together, these results suggest that silibinin, quercetin, and EGCG exert their cancer-preventive effects by differential responses on mitogenic signaling and cell cycle regulators.

Silibinin inhibits constitutive and TNFalpha-induced activation of NF-kappaB and sensitizes human prostate carcinoma DU145 cells to TNFalpha-induced apoptosis

Prostate cancer (PCA) is one of the most common invasive malignancies of men in the US, however, there have been limited successes so far in its therapy. Even most potent agents (e.g. TNFalpha) are ineffective in killing human PCA cells possibly due to constitutive activation of NF-kappaB that subsequently activates a large number of anti-apoptotic genes. In such a scenario, strong apoptotic agent TNFalpha, further induces NF-kappaB activation rather than inducing apoptosis. In several recent studies, we have demonstrated both cancer preventive and anti-cancer efficacy of silymarin and its constituent silibinin in a variety of experimental tumor models and cell culture systems. Here we examined whether silibinin is effective in inhibiting constitutive NF-kappaB activation in human PCA cells, which would help in overcoming TNFalpha-insensitivity. Our studies reveal that silibinin effectively inhibits constitutive activation of NF-kappaB in advanced human prostate carcinoma DU145 cells. Consistent with this, nuclear levels of p65 and p50 sub-units of NF-kappaB were also reduced. In the studies assessing molecular mechanism of this effect, silibinin treatment resulted in a significant increase in the level of IkappaBalpha with a concomitant decrease in phospho-IkappaBalpha. Kinase assays revealed that silibinin dose-dependently decreases IKKalpha kinase activity. The effect of silibinin on IKKalpha seemed to be direct as evidenced by the in vitro kinase assay, where immunoprecipitated IKKalpha was incubated with silibinin. This shows that silibinin does not necessarily need an upstream event to bring about its inhibitory effect on IKKalpha and downstream effectors. Additional studies showed that silibinin also inhibits TNFalpha-induced activation of NF-kappaB via IkappaBalpha pathway and subsequently sensitizes DU145 cells to TNFalpha-induced apoptosis. These results indicate that silibinin could be used to enhance the effectiveness of TNFalpha-based chemotherapy in advanced PCA.

Silymarin inhibits growth and causes regression of established skin tumors in SENCAR mice via modulation of mitogen-activated protein kinases and induction of apoptosis

This study reports in vivo therapeutic efficacy of silymarin against skin tumors with mechanistic rationale. 7,12-Dimethylbenz[a]anthracene-12-O-tetradecanoyl-phorbol-13-acetate (DMBA-TPA)-induced established skin papilloma (tumor)-bearing SENCAR mice were fed with 0.5% silymarin in AIN-93M-purified diet (w/w), and both tumor growth and regression were monitored during 5 weeks of feeding regimen. Silymarin feeding significantly inhibited (74%, P < 0.01) tumor growth and also caused regression (43%, P < 0.01) of established tumors. Proliferating cell nuclear antigen and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling immunohistochemical staining of tumors showed that silymarin decreases proliferation index by 48% (P < 0.001) and increases apoptotic index by 2.5-fold (P < 0.001), respectively. Skin tumor growth inhibition and regression by silymarin were also accompanied by a strong decrease (P < 0.001) in phospho-ERK1/2 levels in tumors from silymarin-fed mice compared with controls. In the studies evaluating bioavailability and physiologically achievable level of silymarin (as silibinin) in plasma, skin tumor, skin, liver, lung, mammary gland and spleen, we found 10, 6.5, 3.1, 13.7, 7.7, 5.9 and 4.4 microg silibinin/ml plasma or per gram tissue, respectively. In an attempt to translate these findings to human skin cancer and to establish biological significance of physiologically achievable level, effect of plasma concentration of silibinin was next examined in human epidermoid carcinoma A431 cells. Silibinin treatment of cells in culture at 12.5, 25 (plasma level) and 50 microM doses resulted in 30-74% (P < 0.01-0.001) growth inhibition and 7-42% death of A431 cells in a dose- and time-dependent manner; apoptosis was identified as a cell death response by silibinin. Similar silibinin treatments also resulted in a significant decrease in phospho-mitogen-activated protein kinase/extracellular signal-regulated protein kinase 1/2 (MAPK/ERK1/2) levels, but an up-regulation of stress-activated protein kinase/jun NH(2)-terminal kinase (SAPK/JNK1/2) and p38 mitogen-activated protein kinase (p38 MAPK) activation in A431 cells. The use of MEK1 inhibitor, PD98059, showed that inhibition of ERK1/2 signaling, in part, contributes to silibinin-caused cell growth inhibition. Together, the data suggest that an inhibition of ERK1/2 activation and an increased activation of JNK1/2 and p38 by silibinin could be possible underlying molecular events involved in inhibition of proliferation and induction of apoptosis in A431 cells. These data suggest that silymarin and/or its major active constituent silibinin could be an effective agent for both prevention and intervention of human skin cancer.

Baicalin induces apoptosis via mitochondrial pathway as prooxidant

Baicalin is a flavonoid and a major component of a herbal medicine, Sho-saiko-to, which is commonly used for treatment of chronic hepatitis in Japan and China. Flavonoids including baicalin have been reported to not only function as anti-oxidants but also cause cytotoxic effect. We investigated the mechanism of baicalin-induced cytotoxicity in leukemia-derived T cell line, Jurkat cells. When cells were cultured with 50-200 microg/ml baicalin for 6h, caspase-3 was activated and then cells fell into apoptosis. Induction of apoptosis by baicalin was accompanied with the marginal generation of intracellular reactive oxygen species (ROS), the increase of the cytosolic fractions of cytochrome c, and the disruption of mitochondrial transmembrane potential (DeltaPsi(m)) prior to the activation of caspase-3. The pre-culture with 5 mM of buthionine sulfoximine (BSO), an inhibitor of glutathione (GSH) synthesis, facilitated baicalin-induced disruption of DeltaPsi(m) and induction of apoptosis. The pre-culture with N-benzyloxycarbonyl-valyl-alanyl-aspartyl fluoromethylketone (Z-VAD-fmk), a pan-caspase inhibitor, partially suppressed the induction of apoptosis. On the other hand, baicalin showed little toxic effect on peripheral blood mononuclear cells (PBMCs) from healthy volunteers. These results indicate that baicalin acts as a prooxidant and induces caspase-3 activation and apoptosis via mitochondrial pathway.

Baicalein, a novel apoptotic agent for hepatoma cell lines: a potential medicine for hepatoma

This study has demonstrated that baicalein has anticancer effectiveness against human hepatoma cells. The dose response of baicalein in Hep G2 and Hep J2 cells indicates that baicalein decreased viability >90%. In comparison, baicalein had only minimal effects on the viability of control Chang liver cells. Flow cytometric analysis showed that baicalein inhibited the cell cycle of Hep G2 cells in the S phase. In addition, baicalein treatment resulted in a decreased mitochondrial transmembrane potential and damaged the integrity of the cell membrane. The TdT-mediated dUTP-biotin nick end labeling assay results indicated that baicalein elicited a significant increase of DNA fragmentation in Hep G2 cells after incubation for 48 hours. These results indicate that baicalein is an effective antihepatoma agent with minimal influence on noncancer cells. The effects of baicalein on Hep G2 cells include inhibition of the S phase of the cell cycle, dysfunction of mitochondria, and initiation of apoptosis.