The inhibition of phosphatidylinositol 3-kinase by quercetin and analogs

Quercetin enhances susceptibility to NK cell-mediated lysis of tumor cells through induction of NKG2D ligands and suppression of HSP70

It is known that treatments with heat shock, some anticancer drugs, and ionizing radiation increase the expression of heat-shock proteins (HSPs) and natural killer group 2D (NKG2D) ligands in tumor cells. The increased HSPs may make the tumor cells resistant to apoptosis and reduction of HSPs may make the tumor cells more susceptible to natural killer (NK)-cell mediated lysis of tumor cells. In this study, we investigated whether quercetin which has inhibitory activities against heat-shock factor, protein kinase C, nuclear factor-kappaB, and phosphatidyl inositol 3-kinase, can modulate the expression of NKG2D ligands and suppress the HSPs in tumor cells. The results of this study showed that quercetin significantly induced the expression of several NKG2D ligands including major histocompatibility complex class I-related chain B, UL16-binding protein 1, and UL16-binding protein 2 in K562, SNU1, and SNU-C4 cells. The quercetin-treated K562, SNU1, and SNU-C4 cells showed an enhanced susceptibility to NK-92 cells through induction of NKG2D ligands. This increased expression of NKG2D ligands seemed to be due to the inhibition of the nuclear factor-kappaB and phosphatidyl inositol 3-kinase pathways. The findings of this study suggest that the induced NKG2D ligands with the decrease of HSP70 protein by quercetin may provide an attractive strategy to improve the effectiveness of NK cell-based cancer immunotherapy.

Flavonoids as protein kinase inhibitors for cancer chemoprevention: direct binding and molecular modeling

Protein kinases play crucial roles in the regulation of multiple cell signaling pathways and cellular functions. Deregulation of protein kinase function has been implicated in carcinogenesis. The inhibition of protein kinases has emerged as an important target for cancer chemoprevention and therapy. Accumulated data revealed that flavonoids exert chemopreventive effects through acting at protein kinase signaling pathways, more than as conventional hydrogen-donating antioxidants. Recent studies show that flavonoids can bind directly to some protein kinases, including Akt/protein kinase B (Akt/PKB), Fyn, Janus kinase 1 (JAK1), mitogen-activated protein kinase kinase 1 (MEK1), phosphoinositide 3-kinase (PI3K), mitogen-activated protein (MAP) kinase kinase 4 (MKK4), Raf1, and zeta chain-associated 70-kDa protein (ZAP-70) kinase, and then alter their phosphorylation state to regulate multiple cell signaling pathways in carcinogenesis processes. In this review, we report recent results on the interactions of flavonoids and protein kinases, especially their direct binding and molecular modeling. The data suggest that flavonoids act as protein kinase inhibitors for cancer chemoprevention that were thought previously as conventional hydrogen-donating antioxidant. Moreover, the molecular modeling data show some hints for creating natural compound-based protein kinase inhibitors for cancer chemoprevention and therapy.

A structural basis for the inhibition of collagen-stimulated platelet function by quercetin and structurally related flavonoids

BACKGROUND AND PURPOSE

Molecular mechanisms underlying the links between dietary intake of flavonoids and reduced cardiovascular disease risk are only partially understood. Key events in the pathogenesis of cardiovascular disease, particularly thrombosis, are inhibited by these polyphenolic compounds via mechanisms such as inhibition of platelet activation and associated signal transduction, attenuation of generation of reactive oxygen species, enhancement of nitric oxide production and binding to thromboxane A(2) receptors. In vivo, effects of flavonoids are mediated by their metabolites, but the effects and modes of action of these compounds are not well-characterized. A good understanding of flavonoid structure-activity relationships with regard to platelet function is also lacking.

EXPERIMENTAL APPROACH

Inhibitory potencies of structurally distinct flavonoids (quercetin, apigenin and catechin) and plasma metabolites (tamarixetin, quercetin-3'-sulphate and quercetin-3-glucuronide) for collagen-stimulated platelet aggregation and 5-hydroxytryptamine secretion were measured in human platelets. Tyrosine phosphorylation of total protein, Syk and PLCgamma2 (immunoprecipitation and Western blot analyses), and Fyn kinase activity were also measured in platelets. Internalization of flavonoids and metabolites in a megakaryocytic cell line (MEG-01 cells) was studied by fluorescence confocal microscopy.

KEY RESULTS

The inhibitory mechanisms of these compounds included blocking Fyn kinase activity and the tyrosine phosphorylation of Syk and PLCgamma2 following internalization. Principal functional groups attributed to potent inhibition were a planar, C-4 carbonyl substituted and C-3 hydroxylated C ring in addition to a B ring catechol moiety.

CONCLUSIONS AND IMPLICATIONS

The structure-activity relationship for flavonoids on platelet function presented here may be exploited to design selective inhibitors of cell signalling.

Inhibitors of phosphatidylinositol-3-kinase in cancer therapy

The phosphatidylinositol-3-kinase (PI3K) signaling pathway is implicated in multiple aspects of tumorigenesis and tumor maintenance, and recent years have seen significant efforts towards developing agents to inhibit the pathway. However, the development of such agents raises issues such as what specific member or members in the PI3K family should be inhibited to achieve maximal therapeutic benefit, and can specific inhibitors be developed with the necessary pharmacologic properties to allow them to proceed to clinical trials? The number of PI3K inhibitors has gone from a handful of archetypal inhibitors which largely determined how the pathway was initially defined through their inhibition of PI3K, but also due to their off target properties, to a much larger number of inhibitors of not only PI3K but also other members of the PI3K family. The question remains to be answered whether greater therapeutic efficacy will be obtained through the use of inhibitors with increased specificity, or through inhibitors that target a spectrum of targets within the pathway. This review will cover the development of agents targeting the pathway, and will discuss current issues surrounding the development of such agents.

Beyond antioxidants: the cellular and molecular interactions of flavonoids and how these underpin their actions on the brain

The consumption of flavonoid-rich foods and beverages has been suggested to limit the neurodegeneration associated with a variety of neurological disorders and to prevent or reverse normal or abnormal deteriorations in cognitive performance. Flavonoids mediate these effects via a number of routes, including a potential to protect neurons against injury induced by neurotoxins, an ability to suppress neuroinflammation and a potential to promote memory, learning and cognitive function. Originally, it was thought that such actions were mediated by the antioxidant capacity of flavonoids. However, their limited absorption and their low bioavailability in the brain suggest that this explanation is unlikely. Instead, this multiplicity of effects appears to be underpinned by three separate processes: first, through their interactions with important neuronal and glial signalling cascades in the brain, most notably the phosphatidylinositol 3-kinase/Akt and mitogen-activated protein kinase pathways that regulate pro-survival transcription factors and gene expression; second, through an ability to improve peripheral and cerebral blood flow and to trigger angiogenesis and neurogenesis in the hippocampus; third, by their capacity to directly react with and scavenge neurotoxic species and pro-inflammatory agents produced in the brain as a result of both normal and abnormal brain ageing. The present review explores the potential inhibitory or stimulatory actions of flavonoids within these three systems and describes how such interactions are likely to underlie neurological effects.

Small molecule inhibitors of the PI3-kinase family

The PI3-K family is one of the most intensely pursued classes of drug targets. This chapter reviews some of the chemical and structural features that determine the selectivity of PI3-K inhibitors, by focusing on a few key compounds that have been instrumental in guiding our understanding of how to design drugs against this family.

Modulating the tumor microenvironment to increase radiation responsiveness

Radiosensitivity can be influenced both by factors intrinsic and extrinsic to the cancer cell. One of the factors in the tumor microenvironment (TME) extrinsic to the cancer cell that can affect radiosensitivity is oxygenation. Severely hypoxic cells require a 2-3 fold higher dose of radiation to achieve the same level of cell killing as do well-oxygenated cells. Other elements in the microenvironment that may influence tumor radiosensitivity are the response of stromal cells to radiation and the expression of factors such as vascular endothelial growth factor (VEGF) and hypoxia inducible factor-1 (HIF-1). There are currently several classes of agents that may increase tumor radiosensitivity by modulating the TME. Pre-clinical evidence indicates that inhibition of VEGF may increase local control after radiation. Several mechanisms have been postulated to explain this including radiosensitization of tumor endothelial cells, prevention of the establishment of new vasculature post-radiation, and increased oxygenation secondary to vascular normalization. Agents targeting HIF-1 also increase local control after radiation in pre-clinical models. This may occur via indirect inhibition of VEGF, which is a downstream target of HIF-1, or by VEGF-independent means. When combined with radiation, the EGFR inhibitor cetuximab improves local control and survival in patients with head and neck cancer. Pre-clinical data indicate that EGFR inhibitors can increase the intrinsic radiosensitivity of cancer cells. They can also improve tumor blood flow and oxygenation, which may increase extrinsic radiosensitivity. One of the pathways downstream of EGFR that may contribute to this effect is the PI3K/Akt pathway. Agents that directly inhibit this pathway improve blood flow and increase tumor oxygenation in pre-clinical models. The challenge remains to obtain clinical data from patients showing that modulation of the TME is an important mechanism by which biological agents can radiosensitize tumors and then to utilize this information to optimize therapy.

Flavonoids and cognition: the molecular mechanisms underlying their behavioural effects

Evidence suggests that a group of phytochemicals known as flavonoids are highly effective in reversing age-related declines in neuro-cognitive performance through their ability to interact with the cellular and molecular architecture of the brain responsible for memory and by reducing neuronal loss due to neurodegenerative processes. In particular, they may increase the number of, and strength of, connections between neurons, via their specific interactions with the ERK and Akt signalling pathways, leading to an increase in neurotrophins such as BDNF. Concurrently, their effects on the peripheral and cerebral vascular system may also lead to enhancements in cognitive performance through increased brain blood flow and an ability to initiate neurogenesis in the hippocampus. Finally, they have also been shown to reduce neuronal damage and losses induced by various neurotoxic species and neuroinflammation. Together, these processes act to maintain the number and quality of synaptic connections in the brain, a factor known to be essential for efficient LTP, synaptic plasticity and ultimately the efficient working of memory.

The impact of flavonoids on spatial memory in rodents: from behaviour to underlying hippocampal mechanisms

Emerging evidence suggests that a group of dietary-derived phytochemicals known as flavonoids are able to induce improvements in memory, learning and cognition. Flavonoids have been shown to modulate critical neuronal signalling pathways involved in processes of memory, and therefore are likely to affect synaptic plasticity and long-term potentiation mechanisms, widely considered to provide a basis for memory. Animal dietary supplementation studies have further shown that flavonoid-rich foods are able to reverse age-related spatial memory and spatial learning impairments. A more accurate understanding of how a particular spatial memory task works and of which aspects of memory and learning can be assessed in each case, are necessary for a correct interpretation of data relating to diet-cognition experiments. Further understanding of how specific behavioural tasks relate to the functioning of hippocampal circuitry during learning processes might be also elucidative of the specific observed memory improvements. The overall goal of this review is to give an overview of how the hippocampal circuitry operates as a memory system during behavioural tasks, which we believe will provide a new insight into the underlying mechanisms of the action of flavonoids on cognition.

Luteolin, quercetin and ursolic acid are potent inhibitors of proliferation and inducers of apoptosis in both KRAS and BRAF mutated human colorectal cancer cells

KRAS and BRAF mutations are frequent in colorectal carcinoma (CRC) and have the potential to activate proliferation and survival through MAPK/ERK and/or PI3K signalling pathways. Because diet is one of the most important determinants of CRC incidence and progression, we studied the effects of the dietary phytochemicals quercetin (Q), luteolin (L) and ursolic acid (UA) on cell proliferation and apoptosis in two human CRC derived cell lines, HCT15 and CO115, harboring KRAS and BRAF activating mutations, respectively. In KRAS mutated HCT15 cells, Q and L significantly decreased ERK phosphorylation, whereas in BRAF mutated CO115 cells the three compounds decreased Akt phosphorylation but had no effect on phospho-ERK. Our findings show that these natural compounds have antiproliferative and proapoptotic effects and simultaneously seem to act on KRAS and PI3K but not on BRAF. These results shed light on the molecular mechanisms of action of Q, L and UA and emphasize the potential of dietary choices for the control of CRC progression.

Inhibition of mammalian S6 kinase by resveratrol suppresses autophagy

Resveratrol is a plant-derived polyphenol that promotes health and disease resistance in rodent models, and extends lifespan in lower organisms. A major challenge is to understand the biological processes and molecular pathways by which resveratrol induces these beneficial effects. Autophagy is a critical process by which cells turn over damaged components and maintain bioenergetic requirements. Disruption of the normal balance between pro- and anti-autophagic signals is linked to cancer, liver disease, and neurodegenerative disorders. Here we show that resveratrol attenuates autophagy in response to nutrient limitation or rapamycin in multiple cell lines through a pathway independent of a known target, SIRT1. In a large-scalein vitro kinase screen we identified p70 S6 kinase (S6K1) as a target of resveratrol. Blocking S6K1 activity by expression of a dominant-negative mutant or RNA interference is sufficient to disrupt autophagy to a similar extent as resveratrol. Furthermore, co-administration of resveratrol with S6K1 knockdown does not produce an additive effect. These data indicate that S6K1 is important for the full induction of autophagy in mammals and raise the possibility that some of the beneficial effects of resveratrol are due to modulation of S6K1 activity.

Failure of dietary quercetin to alter the temporal progression of insulin resistance among tissues of C57BL/6J mice during the development of diet-induced obesity

AIMS/HYPOTHESES

High-fat diets produce obesity and glucose intolerance by promoting the development of insulin resistance in peripheral tissues and liver. The present studies sought to identify the initial site(s) where insulin resistance develops using a moderately high-fat diet and to assess whether the bioflavonoid, quercetin, ameliorates progression of this sequence.

METHODS

Four cohorts of male C57BL/6J mice were placed on diets formulated to be low-fat (10% of energy from fat), high-fat (45% of energy from fat) or high-fat plus 1.2% quercetin (wt/wt). After 3 and 8 weeks, cohorts were evaluated using euglycaemic-hyperinsulinaemic clamps, metabolomic analysis of fatty acylcarnitines and acute in vitro assessments of insulin signalling among tissues.

RESULTS

After 3 and 8 weeks, the high-fat diet produced whole-body insulin resistance without altering insulin-dependent glucose uptake in peripheral tissues. The primary defect was impaired suppression of hepatic glucose production by insulin at both times. Quercetin initially exacerbated the effect of high-fat diet by further increasing hepatic insulin resistance, but by 8 weeks insulin resistance and hepatic responsiveness to insulin were similarly compromised in both high-fat groups. The high-fat diet, irrespective of quercetin, increased short-chain fatty acylcarnitines in liver but not in muscle, while reciprocally reducing hepatic long-chain fatty acylcarnitines and increasing them in muscle.

CONCLUSIONS/INTERPRETATION

Failure of insulin to suppress hepatic glucose output is the initial defect that accounts for the insulin resistance that develops after short-term consumption of a high-fat (45% of energy) diet. Hepatic insulin resistance is associated with accumulation of short- and medium-, but not long-chain fatty acylcarnitines. Dietary quercetin does not ameliorate the progression of this sequence.

Take your PIK: phosphatidylinositol 3-kinase inhibitors race through the clinic and toward cancer therapy

The phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway is currently one of the most exciting drug targets in oncology. However, only a short time ago, the paradigm existed that drugs targeted to the four PI3K class I isoforms would be too toxic for use in cancer therapy due to effects on physiologic signaling. Since that time, studies have delineated the roles of these four isoforms in nonpathologic signaling as well as their roles in cancer. An extensive effort has gone into developing agents that inhibit one or more PI3K isoforms, as well as closely related proteins implicated in cancer. These agents have proved to be tolerable and therapeutically beneficial in animal studies, and a number are in clinical testing. The agents, their properties, and their molecular targets are discussed in this review.

Quercetin mediated lifespan extension in Caenorhabditis elegans is modulated by age-1, daf-2, sek-1 and unc-43

The nematode Caenorhabditis elegans responds to flavonoid-rich diets with improved health and longevity. The precise mechanism(s) responsible for this remains to be identified, but is believed to be linked to the highly antioxidative properties of flavonoids. This study provides a dissection of lifespan modulation by the flavonoid quercetin. In detail, quercetin was shown not to act as a simple antimicrobial agent or exclusively via radical scavenging capacities. Likewise, lifespan extension had no effect on reproduction and body length. Furthermore, neither a caloric restriction mimetic nor a sirtuin (sir-2.1) dependence was identified as a likely mode of action. However, four genes were pinpointed to be required for the quercetin derived lifespan extension, namely age-1, daf-2, unc-43 and sek-1. The latter two have, to date, not been linked to quercetin-mediated lifespan extension.

Food for thought: the role of dietary flavonoids in enhancing human memory, learning and neuro-cognitive performance

Emerging evidence suggests that dietary-derived flavonoids have the potential to improve human memory and neuro-cognitive performance via their ability to protect vulnerable neurons, enhance existing neuronal function and stimulate neuronal regeneration. Long-term potentiation (LTP) is widely considered to be one of the major mechanisms underlying memory acquisition, consolidation and storage in the brain and is known to be controlled at the molecular level by the activation of a number of neuronal signalling pathways. These pathways include the phosphatidylinositol-3 kinase/protein kinase B/Akt (Akt), protein kinase C, protein kinase A, Ca-calmodulin kinase and mitogen-activated protein kinase pathways. Growing evidence suggests that flavonoids exert effects on LTP, and consequently memory and cognitive performance, through their interactions with these signalling pathways. Of particular interest is the ability of flavonoids to activate the extracellular signal-regulated kinase and the Akt signalling pathways leading to the activation of the cAMP-response element-binding protein, a transcription factor responsible for increasing the expression of a number of neurotrophins important in LTP and long-term memory. One such neurotrophin is brain-derived neurotrophic factor, which is known to be crucial in controlling synapse growth, in promoting an increase in dendritic spine density and in enhancing synaptic receptor density. The present review explores the potential of flavonoids and their metabolite forms to promote memory and learning through their interactions with neuronal signalling pathways pivotal in controlling LTP and memory in human subjects.

Inhibitory mechanisms of flavonoids on insulin-stimulated glucose uptake in MC3T3-G2/PA6 adipose cells

We assessed the effects of different classes of flavonoids on insulin-stimulated 2-deoxy-D-[1-(3)H]glucose uptake by mouse MC3T3-G2/PA6 cells differentiated into mature adipose cells. Among the flavonoids examined, the flavones, apigenin and luteolin, the flavonols, kaempferol, quercetin and fisetin, an isoflavone, genistein, a flavanonol, silybin, and the flavanols, (-)-epigallocatechin gallate (EGCG) and theaflavins, significantly inhibited insulin-stimulated glucose uptake. Key structural features of flavonoids for inhibition of insulin-stimulated glucose uptake are the B-ring 4'- or 3',4'-OH group and the C-ring C2-C3 double bond of the flavones and flavonols, the A-ring 5-OH of isoflavones, and the galloyl group of EGCG and theaflavins. Luteolin significantly inhibits insulin-stimulated phosphorylation of insulin receptor-beta subunit (IR-beta), and apigenin, kaempferol, quercetin and fisetin, also tended to inhibit the IR-beta phosphorylation. On the other hand, isoflavones, flavanols or flavanonols did not affect insulin-stimulated IR-beta phosphorylation. Apigenin, luteolin, kaempferol, quercetin and fisetin also appeared to inhibit insulin-stimulated activation of Akt, a pivotal downstream effector of phosphatidylinositol 3-kinase (PI3K), and suppressed insulin-dependent translocation of a glucose transporter, (GLUT)4, into the plasma membrane. Although genistein, silybin, EGCG and theaflavins had no effect on the insulin-stimulated activation of Akt, they blocked insulin-dependent GLUT4 translocation. These results provide novel insights into the modulation by flavonoids of insulin's actions, including glucose uptake in adipocytes.

Flavonoids: modulators of brain function?

Emerging evidence suggests that dietary phytochemicals, in particular flavonoids, may exert beneficial effects on the central nervous system by protecting neurons against stress-induced injury, by suppressing neuroinflammation and by improving cognitive function. It is likely that flavonoids exert such effects, through selective actions on different components of a number of protein kinase and lipid kinase signalling cascades, such as the phosphatidylinositol-3 kinase (PI3K)/Akt, protein kinase C and mitogen-activated protein kinase (MAPK) pathways. This review explores the potential inhibitory or stimulatory actions of flavonoids within these pathways, and describes how such interactions are likely to underlie neurological effects through their ability to affect the activation state of target molecules and/or by modulating gene expression. Future research directions are outlined in relation to the precise site(s) of action of flavonoids within signalling pathways and the sequence of events that allow them to regulate neuronal function.

The interactions of flavonoids within neuronal signalling pathways

Emerging evidence suggests that dietary phytochemicals, in particular flavonoids, may exert beneficial effects in the central nervous system by protecting neurons against stress-induced injury, by suppressing neuroinflammation and by promoting neurocognitive performance, through changes in synaptic plasticity. It is likely that flavonoids exert such effects in neurons, through selective actions on different components within a number of protein kinase and lipid kinase signalling cascades, such as phosphatidylinositol-3 kinase (PI3K)/Akt, protein kinase C and mitogen-activated protein kinase. This review details the potential inhibitory or stimulatory actions of flavonoids within these pathways, and describes how such interactions are likely to affect cellular function through changes in the activation state of target molecules and/or by modulating gene expression. Although, precise sites of action are presently unknown, their abilities to: (1) bind to ATP binding sites on enzymes and receptors; (2) modulate the activity of kinases directly; (3) affect the function of important phosphatases; (4) preserve neuronal Ca(2+) homeostasis; and (5) modulate signalling cascades lying downstream of kinases, are explored. Future research directions are outlined in relation to their precise site(s) of action within the signalling pathways and the sequence of events that allow them to regulate neuronal function in the central nervous system.

Structure-activity relationship of flavonoids for inhibition of epidermal growth factor-induced transformation of JB6 Cl 41 cells

We found that quercetin, myricetin, quercetagetin, fisetin, (-)-epigallocatechin gallate (EGCG), and theaflavins, among 24 flavonoids examined, markedly inhibited epidermal growth factor (EGF)-induced cell transformation of mouse epidermal JB6 Cl 41 cells. The six flavonoids suppressed the EGF-induced activation of activator protein 1 (AP-1). In addition, myricetin, quercetagetin, EGCG, and theaflavins directly inhibited EGF-induced phosphatidylinositol 3-kinase (PI3K) activation. The important structural features of flavonoids for cell transformation-inhibitory activity are 3'- and 4'-OH on the B-ring, 3-OH on the C-ring, C2=C3 double bond in the C-ring, and the phenylchromone (C6-C5-C6) skeleton in the flavonols, and the galloyl group in EGCG and theaflavins. Our results provide new insight into possible mechanisms of the anti-carcinogenic effects of flavonoids, and could help to provide a basis for the design of novel cancer chemopreventive agents.

Inhibitors of the heat shock response: biology and pharmacology

A number of human diseases can be linked to aberrations in protein folding which cause an imbalance in protein homeostasis. Molecular chaperones, including heat shock proteins, act to assist protein folding, stability and activity in the cell. Attention has begun to focus on modulating the expression and/or activity of this group of proteins for the treatment of a wide variety of human diseases. This review will describe the progress made to date in developing pharmacological modulators of the heat shock response, including both agents which affect the entire heat shock response and those that specifically target the HSP70 and HSP90 chaperone families.

Proteomics-based identification of biomarkers for predicting sensitivity to a PI3-kinase inhibitor in cancer

To identify biomarkers for predicting sensitivity to phosphatidylinositol 3-kinase (PI3K) inhibitors, we have developed a proteomics-based approach. Using surface-enhanced laser desorption-ionization time-of-flight mass spectrometry (SELDI-TOF MS), we measured the expression of 393 proteins in 39 human cancer cell lines (JFCR-39), and combined it with our previously established chemosensitivity database to select for proteins whose expressions show significant correlations to drug sensitivities. This integrated approach allowed us to identify peaks from two proteins, 11.6 and 11.8 kDa, that showed significant correlations with the sensitivity to a PI3K inhibitor, LY294002. We found that the 11.8 kDa protein was a phosphorylated form of the 11.6 kDa protein. While the 11.8 kDa protein showed a positive correlation with the sensitivity to LY294002, the 11.6 kDa protein showed a negative correlation with that of the LY294002. The 11.6 kDa protein was purified chromatographically, and was identified by SELDI-TOF MS as the ribosomal P2 protein, which possesses two prospective phosphorylation sites. These results suggested that the phosphorylation status of the ribosomal P2 was responsible for determining the sensitivity to LY294002, and that the ribosomal P2 could be a potential biomarker for predicting chemosensitivity.

Design, synthesis, and antiproliferative and CDK2-cyclin a inhibitory activity of novel flavopiridol analogues

The design and synthesis of a small library of 8-amidoflavone, 8-sulfonamidoflavone, 8-amido-7-hydroxyflavone, and heterocyclic analogues of flavopiridol is reported. The potential activity of these compounds as kinase inhibitors was evaluated by cytotoxicity studies in MCF-7 and ID-8 cancer cell lines and inhibition of CDK2-Cyclin A enzyme activity in vitro. The antiproliferative and CDK2-Cyclin A inhibitory activity of these analogues was significantly lower than the activity of flavopiridol. Molecular docking simulations were carried out and these studies suggested a different binding orientation inside the CDK2 binding pocket for these analogues compared to flavopiridol.

Dietary flavonoids attenuate tumor necrosis factor alpha-induced adhesion molecule expression in human aortic endothelial cells. Structure-function relationships and activity after first pass metabolism

Flavonoids have been suggested to exert human health benefits by anti-oxidant and anti-inflammatory mechanisms. In this study, we investigated whether and by what mechanisms dietary flavonoids inhibit expression of cellular adhesion molecules, which is relevant to inflammation and atherosclerosis. We found that the capacity of flavonoids to inhibit tumor necrosis factor alpha-induced adhesion molecule expression in human aortic endothelial cells was dependent on specific structural features of the flavonoids. The 5,7-dihydroxyl substitution of a flavonoid A-ring and 2,3-double bond and 4-keto group of the C-ring were the main structural requirements for inhibition of adhesion molecule expression. In striking contrast, hydroxyl substitutions of the B- and C-rings but not the A-ring were essential for antioxidant activity. Hence, only hydroxyl flavones, such as apigenin and chrysin, and flavonols, such as galangin, kaempferol, and quercetin, were able to inhibit endothelial adhesion molecule expression, whereas flavone, chromone, the flavanone, naringenin, and the flavanol, (-)-epicatechin, were ineffectual. At low concentrations, the active flavonoids significantly attenuated expression of E-selectin and intercellular adhesion molecule 1 but not vascular cell adhesion molecule 1. In addition, exposure of apigenin and kaempferol to cultured hepatocytes, mimicking first pass metabolism, greatly diminished the inhibitory effect of flavonoids on endothelial intercellular adhesion molecule 1 expression. We conclude that the effect of dietary flavonoids on endothelial adhesion molecule expression depends on their molecular structure, concentration, and metabolic transformation but not their antioxidant activity.

Flavonoid compounds in maintenance of prostate health and prevention and treatment of cancer

Compounds based on a flavonoid (di-phenolic) ring structure are emerging as a potentially important new class of pharmaceutical compounds with a broad range of biological activities, most prominent of which are their potential role as anticancer agents. These compounds exert a wide range of upregulating and downregulating effects on signal transduction processes within cells in both plants and animals. The observation that human communities, which consume large quantities of these compounds (legume-based vegetarian diets), have a lower incidence of many degenerative diseases and some cancers has led to the speculation that these compounds, or synthetic analogs, may be of therapeutic value. This article reviews the evidence supporting this hypothesis and provides some examples of attempts to develop new therapeutics based on dietary isoflavones or novel isoflavonoid structures in maintaining prostate health and in cancer treatment and management. One of these compounds, phenoxodiol, is now in human clinical trials and has shown promise in patients with recurrent ovarian cancer where the cancer is refractory or resistant to standard chemotherapy, and in patients with hormone-refractory prostate cancer.

Neisseria gonorrhoeae PLD directly interacts with Akt kinase upon infection of primary, human, cervical epithelial cells

Neisseria gonorrhoeae secrets a phospholipase D (NgPLD), which augments complement receptor 3 (CR3)-mediated invasion of cervical epithelial cells. To elucidate the signalling pathways triggered with gonococcus CR3-engagement and the putative function of NgPLD in these events, we analysed the contribution of the phosphoinositide-Akt pathway to cervical infection. Our data indicated that Akt plays a critical role in cervical infection. Inhibition of myosin light chain kinase, PtdIns(4,5)P2, and Akt functions resulted in decreased gonococcus invasion of primary, human, cervical epithelial cells as well as Akt kinase activity. Akt activity was similarly impaired when cervical cells were challenged with NgPLD-mutant gonococci. Conversely, the PI3-kinase inhibitor, LY294002, enhanced gonococcal invasion of, and Akt activity within, primary cervical cells. We demonstrated that NgPLD directly binds to the Akt PH domain and can compete with a natural Akt ligand, PtdIns(3,4,5)P3, for Akt binding. Collectively, our data suggested that NgPLD augments gonococcus invasion of cervical epithelia by interacting with Akt kinase in a PI3-kinase-independent manner, which results in subversion of normal cervical cell signalling.

Quercetin inhibits eNOS, microtubule polymerization, and mitotic progression in bovine aortic endothelial cells

Quercetin (QRN), one of the most abundant flavonoids in the human diet, is a known antioxidant and inhibitor of cancer cell cycle progression. Here, we provide the first evidence that QRN inhibits angiogenesis via a mechanism involving both suppression of endothelial nitric oxide synthase (eNOS) and early M-phase cell cycle arrest. Bovine aortic endothelial (BAE) cells were exposed to doses of up to 100 micromol/L QRN and assayed for eNOS activity and phosphorylation status. Phosphorylation of eNOS at Ser 617 (bovine sequence) is thought to occur in response to Akt stimulation and to be required for eNOS activity. Together with basal eNOS activity, eNOS phosphorylation at Ser 617 and Akt Ser 473 phosphorylation were dose dependently and concomitantly suppressed by QRN within 30 min. Furthermore, although the significant (P < 0.05) inhibitory effect of a single 100 micromol/L QRN dose on eNOS activity was overcome within approximately 24 h, chronic QRN exposures (24-48 h) led to early M-phase arrest and disruption of mitotic microtubule polymerization. In vivo, QRN administered i.p. to female Balb/C mice bearing both syngeneic mammary tumors and Matrigel implants suppressed angiogenesis as measured by endothelial cell immunohistochemistry and hemoglobin concentration. Taken together, these findings suggest a dual mechanism by which QRN suppresses endothelial cell proliferation, both acutely via inhibition of eNOS Ser 617 phosphorylation, and chronically via perturbation of mitotic microtubule polymerization. This novel mechanism of QRN in endothelial cells may in part explain its inhibitory action on angiogenesis and further discern a potential role of QRN as a chemopreventive agent.

A Robust Screen for Inhibitors and Enhancers of Phosphoinositide-3 Kinase (PI3K) Activities by Ratiometric Fluorescence Superquenching

Aberrant regulation of phosphoinositide 3-kinase (PI3K) activity is implicated in various diseases such as cancer and diabetes. Thus, high-throughput screening (HTS) of small-molecule inhibitors for PI3 kinases is an appealing strategy for drug development. Despite the attractiveness of lipid kinases as drug targets, screening for inhibitors for PI3K activities has been hampered by limited assay formats adaptable for HTS. The authors describe a homogeneous, direct, and nonradioactive assay for highly sensitive detection of PI3Kα, β, δ, and γ activities, which is suitable for HTS. The assay is based on fluorescence superquenching of a conjugated polymer upon metal-ion-mediated association of phosphorylated and dye-labeled substrates. As a result of phosphorylation, quencher and polymer are brought into proximity, and fluorescent energy transfer occurs. This event can be monitored as either fluorescence quench of the polymer or as enhanced emission from the quencher. Ratiometric analysis of the wavelengths eliminates interferences from autofluorescing compounds, which are present in HTS libraries. The platform has been adapted for the 384-well microplate format and delivers Z factors of > 0.6 at substrate conversions as low as 7%. Using this assay platform, several unreported inhibitors and activators of PI3Ks were identified in an 84- compound screen.

Heat shock proteins protect both MPP+ and paraquat neurotoxicity

The exposure of immortalized rat neuroblast cells to MPP(+) and paraquat results in cell death. Heat shock pre-treatment prior to the addition of MPP(+) and paraquat significantly reduced cell death and led to an increase in the synthesis of Hsp 27 and Hsp70 proteins. Quercetin inhibits the synthesis of heat shock proteins (Hsp) and prevents their protective effect, which suggests that this protection was dependent on the Hsps synthesis. These data indicate that heat shock protects cells from the toxic effect of MPP(+) and paraquat. These results and the structural similarity between paraquat and MPP(+) support the role of paraquat as a putative risk factor in the etiology of Parkinson's disease.

The stimulation of cell proliferation by quercetin is mediated by the estrogen receptor

Quercetin causes biphasic modulation of the proliferation of specific colon and mammary cancer cells. In this study, the possible involvement of the estrogen receptor (ER) in the stimulation of cell proliferation by quercetin was investigated. For this purpose, the effect of quercetin on cell proliferation was tested in ER-positive MCF-7 and T47D cells, and in ER-negative HCC-38 and MDA-MB231 cells. Quercetin stimulated proliferation of ER-positive cells only, suggesting this effect to be ER-dependent. In support of these results, quercetin induced ER-ERE-mediated gene expression in a reporter gene assay using U2-OS cells transfected with either ERalpha or ERbeta, with 10(5)-10(6) times lower affinity than 17beta-estradiol (E2) and 10(2)-10(3 )times lower affinity than genistein. Quercetin activated the ERbeta to a 4.5-fold higher level than E2, whereas the maximum induction level of ERalpha by quercetin was only 1.7 fold that of E2. These results point at the relatively high capacity of quercetin to stimulate supposed 'beneficial' ERbeta responses as compared to the stimulation of ERalpha, the receptor possibly involved in adverse cell proliferative effects. Altogether, the results of this study reveal that physiologically relevant concentrations of quercetin can exert phytoestrogen-like activity similar to that observed for the isoflavonoid genistein.

Targeting the gatekeeper residue in phosphoinositide 3-kinases

A single residue in the ATP binding pocket of protein kinases-termed the gatekeeper-has been shown to control sensitivity to a wide range of small molecule inhibitors (Chem. Biol.2004, 11, 691; Chem. Biol.1999, 6, 671). Kinases that possess a small side chain at this position (Thr, Ala, or Gly) are readily targeted by structurally diverse classes of inhibitors, whereas kinases that possess a larger residue at this position are broadly resistant. Recently, lipid kinases of the phosphoinositide 3-kinase (PI3-K) family have become the focus of intense research interest as potential drug targets (Chem. Biol.2003, 10, 207; Curr. Opin. Pharmacol.2003, 3, 426). In this study, we identify the residue that corresponds structurally to the gatekeeper in PI3-Ks, and explore its importance in controlling enzyme activity and small molecule sensitivity. Isoleucine 848 of p110alpha was mutated to alanine and glycine, but the mutated kinase was found to have severely impaired enzymatic activity. A structural bioinformatic comparison of this kinase with its yeast orthologs identified second site mutations that rescued the enzymatic activity of the I848A kinase. To probe the dimensions of the gatekeeper pocket, a focused panel of analogs of the PI3-K inhibitor LY294002 was synthesized and its activity against gatekeeper mutated and wild-type p110alpha was assessed.

Phytoestrogen exposure elevates PTEN levels

Epidemiological data suggest that consumption of phytoestrogens can be protective against the development of breast cancer. It may be logical to postulate that phytoestrogens may regulate proteins that control cellular division, such as the tumor suppressor PTEN. Germline, and more significantly, somatic PTEN mutations have been observed in a broad range of human cancers, especially those of the breast. Active PTEN results in decreased phosphorylation of Akt and MAPK, the up-regulation of p27 and down-regulation of cyclin D1 protein levels resulting in decreased proliferation and an increase in apoptosis. We hypothesized that phytoestrogen exposure regulates PTEN protein expression in the breast cancer cell line, MCF-7. When MCF-7 cells were stimulated with resveratrol, quercetin or genistein, there was an increase in PTEN protein levels. Concomitantly, phytoestrogen stimulation resulted in decreased Akt phosphorylation and an increase in p27 protein levels, indicating active PTEN lipid phosphatase activity. In contrast, we found that MAPK phosphorylation and cyclin D1 levels, which are regulated by PTEN's protein phosphatase activity, were not altered. Using semi-quantitative RT-PCR, we found that mRNA levels were slightly increased in cells stimulated by phytoestrogens, suggesting that the mechanism for increased PTEN protein expression is dependent upon transcription. Concurrently, our data provide evidence that a mechanism for phytoestrogens' protective nature is partially through increased PTEN expression. More importantly, it provides a novel target for the regulation of PTEN expression and suggests that dietary changes may be adjunctive to traditional preventive and therapeutic strategies against breast cancer.

Flavonoids and alkenylbenzenes: mechanisms of mutagenic action and carcinogenic risk

The present review focuses on the mechanisms of mutagenic action and the carcinogenic risk of two categories of botanical ingredients, namely the flavonoids with quercetin as an important bioactive representative, and the alkenylbenzenes, namely safrole, methyleugenol and estragole. For quercetin a metabolic pathway for activation to DNA-reactive species may include enzymatic and/or chemical oxidation of quercetin to quercetin ortho-quinone, followed by isomerisation of the ortho-quinone to quinone methides. These quinone methides are suggested to be the active alkylating DNA-reactive intermediates. Recent results have demonstrated the formation of quercetin DNA adducts in exposed cells in vitro. The question that remains to be answered is why these genotoxic characteristics of quercetin are not reflected by carcinogenicity. This might in part be related to the transient nature of quercetin quinone methide adducts, and suggests that stability and/or repair of DNA adducts may need increased attention in in vitro genotoxicity studies. Thus, in vitro mutagenicity studies should put more emphasis on the transient nature of the DNA adducts responsible for the mutagenicity in vitro, since this transient nature of the formed DNA adducts may play an essential role in whether the genotoxicity observed in vitro will have any impact in vivo. For alkenylbenzenes the ultimate electrophilic and carcinogenic metabolites are the carbocations formed upon degradation of their 1'-sulfooxy derivatives, so bioactivation of the alkenylbenzenes to their ultimate carcinogens requires the involvement of cytochromes P450 and sulfotransferases. Identification of the cytochrome P450 isoenzymes involved in bioactivation of the alkenylbenzenes identifies the groups within the population possibly at increased risk, due to life style factors or genetic polymorphisms leading to rapid metaboliser phenotypes. Furthermore, toxicokinetics for conversion of the alkenylbenzenes to their carcinogenic metabolites and kinetics for repair of the DNA adducts formed provide other important aspects that have to be taken into account in the high to low dose risk extrapolation in the risk assessment on alkenylbenzenes. Altogether the present review stresses that species differences and mechanistic data have to be taken into account and that new mechanism- and toxicokinetic-based methods and models are required for cancer risk extrapolation from high dose experimental animal data to low dose carcinogenic risks for man.

Cloning, expression, purification, and characterization of the human Class Ia phosphoinositide 3-kinase isoforms

The Class I phosphoinositide 3-kinases (PI3Ks) are lipid kinases that phosphorylate the 3-hydroxyl group of the inositol ring of phosphatidylinositides. Although closely related, experimental evidence suggests that the four Class I PI3Ks may be functionally distinct. To further study their unique biochemical properties, the three human Class Ia PI3K (alpha, beta, and delta) p110 catalytic domains were cloned and co-expressed with the p85alpha regulatory domain in Sf9 cells. None of the p110 subunits were successfully expressed in the absence of p85alpha. Successful expression and purification of each p85alpha/p110 protein required using an excess of the p110 vector over the p85 vector during co-infection of Sf9 cells. Proteins were purified as the p85alpha/p110 complex by nickel affinity chromatography through an N-terminal His-tag on the p110 subunit using an imidazole gradient. The purification yields were high using the optimized ratio of p85/p110 vector and small culture volumes, with 24mg/L cell culture media for p85alpha/p110alpha, 17.5mg/L for p85alpha/p110delta, and 3.5mg/L for p85alpha/p110beta. The identity of each purified isoform was confirmed by mass spectral analysis and immunoblotting. The activities of the three p85alpha/p110 proteins and the Class Ib p110gamma catalytic domain were investigated using phosphatidylinositol 4,5-bisphosphate (PIP2) as the substrate in a PIP2/phosphatidylserine (PS) liposome. All four enzymes exhibited reaction velocities that were dependent on the surface concentration of PIP2. The surface concentrations that gave maximal activity for each human isoform with 0.5mM PIP2 were 2.5mol% PIP2 for p110gamma, 7.5mol% for p85alpha/p110beta, and 10mol% PIP2 for p85alpha/p110alpha and p85alpha/p110delta. The specific activity of p85alpha/p110alpha was three to five times higher than that of the other human isoforms. These kinetic differences may contribute to the unique roles of these isoforms in cells.

Antiproliferative plant and synthetic polyphenolics are specific inhibitors of vertebrate inositol-1,4,5-trisphosphate 3-kinases and inositol polyphosphate multikinase

Inositol-1,4,5-trisphosphate 3-kinases (IP3K) A, B, and C as well as inositol polyphosphate multikinase (IPMK) catalyze the first step in the formation of the higher phosphorylated inositols InsP5 and InsP6 by metabolizing Ins(1,4,5)P3 to Ins(1,3,4,5)P4. In order to clarify the special role of these InsP3 phosphorylating enzymes and of subsequent anabolic inositol phosphate reactions, a search was conducted for potent enzyme inhibitors starting with a fully active IP3K-A catalytic domain. Seven polyphenolic compounds could be identified as potent inhibitors with IC50 < 200 nM (IC50 given): ellagic acid (36 nM), gossypol (58 nM), (-)-epicatechin-3-gallate (94 nM), (-)-epigallocatechin-3-gallate (EGCG, 120 nM), aurintricarboxylic acid (ATA, 150 nM), hypericin (170 nM), and quercetin (180 nM). All inhibitors displayed a mixed-type inhibition with respect to ATP and a non-competitive inhibition with respect to Ins(1,4,5)P3. Examination of these inhibitors toward IP3K-A, -B, and -C and IPMK from mammals revealed that ATA potently inhibits all kinases while the other inhibitors do not markedly affect IPMK but differentially inhibit IP3K isoforms. We identified chlorogenic acid as a specific IPMK inhibitor whereas the flavonoids myricetin, 3',4',7,8-tetrahydroxyflavone and EGCG inhibit preferentially IP3K-A and IP3K-C. Mutagenesis studies revealed that both the calmodulin binding and the ATP [corrected] binding domain in IP3K are involved in inhibitor binding. Their absence in IPMK and the presence of a unique insertion in IPMK were found to be important for selectivity differences from IP3K. The fact that all identified IP3K and IPMK inhibitors have been reported as antiproliferative agents and that IP3Ks or IPMK often are the best binding targets deserves further investigation concerning their antitumor potential.

Activation of protein kinase C βII/ε-c-Jun NH2-terminal kinase pathway and inhibition of mitogen-activated protein/extracellular signal-regulated kinase 1/2 phosphorylation in antitumor invasive activity induced by the polymethoxy flavonoid, nobiletin

Flavonoids from medicinal plants have been therapeutically administered for cancer therapy. We recently reported that nobiletin (5,6,7,8,3′,4′-hexamethoxy flavone) exhibits novel antitumor invasive activities by suppressing the production of pro-matrix metalloproteinases (proMMPs) and augmenting the expression of tissue inhibitor of metalloproteinases-1 (TIMP-1) in vivo and in vitro. In the present study, intracellular target molecules associated with the actions of nobiletin against tumor invasion were identified. Nobiletin inhibited the phosphorylation of mitogen-activated protein/extracellular signal-regulated kinase (MEK) 1/2, but not the activity of Ras or the phosphorylation of Raf. Moreover, a MEK1/2 inhibitor, U0126, mimicked nobiletin's ability to decrease the production of proMMPs-1 and 9 in human fibrosarcoma HT-1080 cells stimulated by 12-O-tetradecanoyl phorbol-13-acetate (TPA). In addition, neither the activity of phosphatidylinositol 3-kinase (PI3K) nor the phosphorylation of Akt was influenced by nobiletin. However, nobiletin was found to augment the phosphorylation of c-Jun NH2-terminal kinase (JNK), a downstream signal factor of the PI3K-Akt pathway, in TPA-treated HT-1080 cells. A similar augmentation of JNK phosphorylation was observed on treatment with a PI3K inhibitor, LY-294002. Furthermore, nobiletin enhancement of TIMP-1 production in TPA-stimulated HT-1080 cells was found to be diminished by adding a JNK inhibitor, SP600125. Moreover, protein kinase C (PKC) inhibitor experiments showed that PKCβII/ε were associated with the nobiletin-mediated augmentation of JNK phosphorylation. Therefore, these results introduce novel evidence that the antitumor effects of nobiletin are finely regulated by the following intracellular mechanisms: (1) the inhibition of MEK1/2 activity is involved in the suppression of MMP expression and (2) the activation of the novel PKCβII/ε-JNK pathway is associated with the augmentation of TIMP-1 expression.

PDE2 inhibition by the PI3 kinase inhibitor LY294002 and analogues

Synthetic 2-morpholinochromones, including the known PI3-kinase inhibitor LY294002, have been evaluated in vitro as inhibitors of isolated human platelet phosphodiesterases. Inhibition of the cAMP-phosphodiesterases, PDE2 and PDE3 by LY294002 is reported for the first time. Preliminary screening across a range of 2-morpholinochromones has revealed structural features for optimised PDE2 inhibition.

Flavonoids: antioxidants or signalling molecules?

Many studies are accumulating that report the neuroprotective, cardioprotective, and chemopreventive actions of dietary flavonoids. While there has been a major focus on the antioxidant properties, there is an emerging view that flavonoids, and their in vivo metabolites, do not act as conventional hydrogen-donating antioxidants but may exert modulatory actions in cells through actions at protein kinase and lipid kinase signalling pathways. Flavonoids, and more recently their metabolites, have been reported to act at phosphoinositide 3-kinase (PI 3-kinase), Akt/protein kinase B (Akt/PKB), tyrosine kinases, protein kinase C (PKC), and mitogen activated protein kinase (MAP kinase) signalling cascades. Inhibitory or stimulatory actions at these pathways are likely to affect cellular function profoundly by altering the phosphorylation state of target molecules and by modulating gene expression. A clear understanding of the mechanisms of action of flavonoids, either as antioxidants or modulators of cell signalling, and the influence of their metabolism on these properties are key to the evaluation of these potent biomolecules as anticancer agents, cardioprotectants, and inhibitors of neurodegeneration

Neuroprotection by flavonoids

The high morbidity, high socioeconomic costs and lack of specific treatments are key factors that define the relevance of brain pathology for human health and the importance of research on neuronal protective agents. Epidemiological studies have shown beneficial effects of flavonoids on arteriosclerosis-related pathology in general and neurodegeneration in particular. Flavonoids can protect the brain by their ability to modulate intracellular signals promoting cellular survival. Quercetin and structurally related flavonoids (myricetin, fisetin, luteolin) showed a marked cytoprotective capacity in in vitro experimental conditions in models of predominantly apoptotic death such as that induced by medium concentrations (200 M) of H2O2 added to PC12 cells in culture. Nevertheless, quercetin did not protect substantia nigra neurons in vivo from an oxidative insult (6-hydroxydopamine), probably due to difficulties in crossing the blood-brain barrier. On the other hand, treatment of permanent focal ischemia with a lecithin/quercetin preparation decreased lesion volume, showing that preparations that help to cross the blood-brain barrier may be critical for the expression of the effects of flavonoids on the brain. The hypothesis is advanced that a group of quercetin-related flavonoids could become lead molecules for the development of neuroprotective compounds with multitarget anti-ischemic effects.

Biphasic modulation of cell proliferation by quercetin at concentrations physiologically relevant in humans

Optimal in vitro conditions regarding quercetin solubility and stability were defined. Using these conditions, the effect of quercetin on proliferation of the colon carcinoma cell lines HCT-116 and HT29 and the mammary adenocarcinoma cell line MCF-7 was investigated. For the colon carcinoma cell lines, at relatively high concentrations, a significant decrease in cell proliferation was observed, providing a basis for claims on the anti-carcinogenic activity of quercetin. However, at lower concentrations, a subtle but significant stimulation of cell proliferation was observed for all cell lines tested. These results point at a dualistic influence of quercetin on cell proliferation that may affect present views on its supposed beneficial anti-proliferative effect.

Binding of quercetin with human serum albumin: A critical spectroscopic study

Flavonols are plant pigments that are ubiquitous in nature. Quercetin (3,3',4',5,7-pentahydroxyflavone) and other related plant flavonols have come into recent prominence because of their usefulness as anticancer, antitumor, anti-AIDS, and other important therapeutic activities of significant potency and low systemic toxicity. Quercetin is intrinsically weakly fluorescent in aqueous solution, showing an emission maximum at approximately 538 nm. Upon binding to human serum albumin (HSA), quercetin undergoes dramatic enhancement in its fluorescence emission intensity, along with the appearance of dual emission behavior, consisting of normal and excited-state proton transfer (ESPT) fluorescence. In addition, the occurrence of a third emitting species has been noted for the first time. This is attributed to a electronic ground-state complex formed in the protein environment. High values of the fluorescence anisotropy (r) are obtained in the presence of HSA for the ESPT tautomer (r = 0.18), as well as the complex species (r = 0.37) of quercetin, indicating that the precursor ground-state molecules for both these emitting species of quercetin molecules are located in the motionally constrained sites of HSA. The steady-state emission data suggest that quercetin binds to two distinct sites in HSA from which the emissions from the normal tautomer and complex species take place. The preliminary results of studies on emission decay kinetics are also reported herein. Studies by far-UV circular dichroism spectroscopy reveal that binding of quercetin induces no significant perturbation in the secondary structure of HSA.