Biomedical subjects

Lai K Leung

Publications and source records attributed to Lai K Leung.

12 recordsLinked to original sources

PubMed2006

The red wine polyphenol resveratrol displays bilevel inhibition on aromatase in breast cancer cells.

Estrogen plays a crucial role in the development of breast cancer, and the inhibition of estrogen synthesis has been an important target for the prevention and treatment of this disease. The rate-limiting reaction of the hormone biosynthesis is catalyzed by cytochrome P450 (CYP) 19 enzyme or aromatase. It has been of genuine interest to uncover an aromatase-inhibitory compound from a dietary source. Resveratrol is a polyphenolic compound that can be isolated from grape peel. Because of its structural resemblance to estrogen, resveratrol's agonistic and antagonistic properties on estrogen receptor have been examined and demonstrated. In the present study, the effect of resveratrol on the expression and enzyme activity of aromatase was investigated. By assaying on MCF-7 cells stably transfected with CYP19 (MCF-7aro cells), resveratrol inhibited the aromatase activity with an IC(50) value of 25 microM. Kinetic analysis indicated that both competitive and noncompetitive inhibition might be involved. The administration of 10 nmol/l testosterone-a substrate of aromatase-produced a 50% increase in the MCF-7aro cell number. This cell proliferation specifically induced by testosterone was significantly reduced by 10 microM resveratrol. In addition, 50 microM resveratrol significantly reduced the CYP19-encoding mRNA abundance in SK-BR-3 cells. The transcriptional control of CYP19 gene is tissue specific, and promoter regions I.3 and II have previously been shown to be responsible for CYP19 expression in breast cancer cells. Luciferase reporter gene assays revealed that resveratrol could repress the transcriptional control dictated by the promoter regulation. The present study illustrated that pharmacological dosage of resveratrol inhibited aromatase at both the enzyme and mRNA levels.

Aromatase↗

PubMed2006

Soya isoflavones suppress phorbol 12-myristate 13-acetate-induced COX-2 expression in MCF-7 cells.

Epidemiological studies indicate that Asian women have a lower incidence of breast cancer compared with their counterparts in the West, and soya consumption has been suggested as a contributory factor. Clinical and animal studies have revealed that cyclooxygenase-2 (COX-2) expression is associated with a risk of breast cancer. In the present study, we investigated the effect of soya isoflavones on the expression of COX-2 in the breast cell line MCF-7. Genistein, daidzein and equol were found to inhibit COX-2 expression induced by phorbol 12-myristate 13-acetate (PMA). Similar findings were observed in the COX-2 protein analysis. In order to study transcriptional control, a fragment of the 5'-flanking region of the hCOX-2 gene was amplified and inserted into a firefly luciferase reporter plasmid. The reporter assay indicated that the transactivation of the hCOX-2 promoter was induced by PMA, and activity was inhibited with the co-administration of genistein, daidzein or equol. An activator protein-1 (AP-1)/cyclic AMP response element binding protein (CREB) binding site (-59/-53) was identified in hCOX-2 promoter, and this could be critical in PMA-induced COX-2 expression. Truncation reporter plasmids with (-70/-36) and without (-51/-36) AP-1/CREB were constructed for subsequent analysis. The results revealed that the hCOX-2 promoter transactivation suppressed by isoflavone could be dependent on AP-1/CREB binding. Nonetheless, this study illustrated that the soya isoflavones reduced COX-2 expression, which could be important in the post-initiation events of breast carcinogenesis.

Anticarcinogenic Agents↗

PubMed2005

Polycyclic aromatic hydrocarbon-induced CYP1B1 activity is suppressed by perillyl alcohol in MCF-7 cells.

Perillyl alcohol (POH) is a dietary monoterpene with potential applications in chemoprevention and chemotherapy. Although clinical trials are under way, POH's physiological and pharmacological properties are still unclear. In the present study, the effect of POH on polycyclic aromatic hydrocarbon (PAH)-induced genotoxicity, and the related expression were examined in MCF-7 cells. Exposure to environmental toxicant increases the risk of cancer. Many of these compounds are pro-carcinogens and are biotransformed into their ultimate genotoxic structures by xenobiotic metabolizing enzymes. CYP1A1 and 1B1 are enzymes that catalyze the biotransformation of dimethylbenz[a]anthracene (DMBA). Our data revealed that 0.5 microM of POH was effective in blocking DMBA-DNA binding. Ethoxyresorufin-O-deethylase (EROD) assay indicated that the administration of POH inhibited the DMBA-induced enzyme activity in MCF-7 cells. Enzyme kinetic analysis revealed that POH inhibited CYP1B1 but not CYP1A1 activity. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) assay also demonstrated that the monoterpene reduced CYP1B1 mRNA abundance induced by DMBA. The present study illustrated that POH might inhibit and downregulate CYP1B1, which could protect against PAH-induced carcinogenesis.

9,10-Dimethyl-1,2-benzanthracene↗

PubMed2005

The plant polyphenol butein inhibits testosterone-induced proliferation in breast cancer cells expressing aromatase.

Chalcones are precursor compounds for flavonoid synthesis in plants, and they can also be synthesized in laboratory. Previous study has documented some of the pharmacological applications of these compounds. Estrogen has long been associated with the initiation and promotion of breast cancer. Inhibiting estrogen synthesis can be effective in the prevention and treatment of the disease. Since most breast cancers received estrogen supplied from local tissues, we employed a breast cancer cell line expressing aromatase to screen for the inhibitory potentials of five hydroxychalcones, i.e. 2-hydroxychalcone, 2'-hydroxychalcone, 4-hydroxychalcone, 4,2',4'-trihydroxy-chalcone (isoquiritigenin), 3,4,2',4'-tetrahydroxychalcone (butein). In the preliminary results, butein was found to be the strongest inhibitor among the tested compounds, and its IC(50) value was 3.75 microM. Subsequent enzyme kinetic study revealed that butein acted on aromatase with a mixed type of inhibition and the K(i) value was determined to be 0.32 microM. Cell proliferation assay indicated that the cell number increased by 10 nM-testosterone treatment was significantly reduced by 5 microM butein, and the administration of flutamide could not reverse the effect. The present study illustrated that butein was an aromatase inhibitor and a potential natural alternative for the chemoprevention or therapy of breast cancer.

Aromatase↗

PubMed2005

Hydroxychalcones exhibit differential effects on XRE transactivation.

Chalcones are phenolic compounds that can be isolated from plants. Previous studies have described some pharmacological applications for these compounds. Making use of our established reporter gene system, we determined the effect of five hydroxychalcones--2-hydroxychalcone, 2'-hydroxychalcone, 4-hydroxychalcone, 4,2',4'-trihydroxychalcone, and 3,4,2',4'-tetrahydroxychalcone--on the cellular xenobiotic responsive element (XRE)-transactivation. The interference of chalcones acting against polycyclic aromatic hydrocarbon (PAH)-DNA binding was also examined. Enzyme inhibition assays of cytochrome P450 (CYP) 1A1 and CYP1B1 were initially performed on recombinant protein expressed in insect microsomes. 2'-Hydroxychalcone and 2-hydroxychalcone were the most effective among the tested hydroxychalcones. The two hydroxychalcones had comparable IC50 values for CYP1A1 and CYP1B1, which were determined to be at the micromolar and submicromolar range, respectively. However, reporter gene assays indicated that 2'-hydroxychalcone suppressed XRE-transactivation, whereas 2-hydroxychalcone induced it when 7,12-dimethylbenz[a]anthracene (DMBA) was co-administered. In the absence of DMBA, 10 microM 2-hydroxychalcone and 2'-hydroxychalcone increased XRE-transactivation by 18- and 2.5-fold, respectively, while other chalcones did not significantly alter the response. Cultures treated with the two hydroxychalcones also displayed separate trends in ethoxyresorufin-O-deethylase (EROD) activity and DMBA-DNA covalent binding. In summary, the present study illustrated that the inhibition of hydroxychalcone on CYP1 enzymes and XRE-transactivation was affected by the position and number of hydroxyl groups in its structure.

9,10-Dimethyl-1,2-benzanthracene↗

PubMed2004

Effect of dietary flavonols on oestrogen receptor transactivation and cell death induction.

Consumption of fruits and vegetables has been associated with cancer prevention; flavonoids are widely distributed in plant foods and considered to be the active ingredients. Quercetin and kaempferol are two of the most commonly found dietary flavonols, and have been reported to prevent cancer. We have previously reported that the isoflavone genistein and the flavone baicalein exert differential actions on the oestrogen receptor (OR) alpha in HepG2 cells. Because of the structural resemblance to both isoflavone and flavone, we examined the effects of these dietary flavonols on ORalpha- and ORbeta-specific transactivations and their subsequent involvement in inducing MCF-7 cell death. In the present study, both quercetin and kaempferol were able to compete for OR binding in a cell-free system and were agonistic to ORalpha and -beta expressed in HepG2 cells, while some additive effect was observed in the oestrogen response element (ORE)-driven transcription when 17beta-oestradiol was co-administered. Since the bcl-2 promoter contained two ORE, and ORE-driven transcriptional activity and Bcl-2 mRNA expression were increased by treatment with 10 microm-quercetin or kaempferol, it is possible that quercetin and kaempferol might up-regulate Bcl-2 expression through OR transactivation in MCF-7 cells. Cell death ELISA assay performed on MCF-7 cells indicated that an increase of apoptosis occurred at 25 microm-, but not 10 microm-, quercetin or kaempferol. Indirectly the results suggest that OR activation is not sufficient to induce apoptosis and that apoptosis is induced despite an increase in Bcl-2 expression.

Anticarcinogenic Agents↗

PubMed2003

The red clover (Trifolium pratense) isoflavone biochanin A modulates the biotransformation pathways of 7,12-dimethylbenz[a]anthracene.

Several flavonoids have shown their anti-carcinogenic effects in various models. The soyabean isoflavone genistein was demonstrated earlier in our laboratory to be an effective inhibitor of dimethylbenz[a]anthracene (DMBA)-induced DNA damage in MCF-7 cells by curbing cytochrome P450 (CYP) 1 enzymes. The red clover (Trifolium pratense) isoflavone biochanin A is a methylated derivative of genistein, and its anti-mutagenic effect in bacterial cells has been shown previously. Because of its protection against chemical carcinogenesis in an animal model, biochanin A was selected for testing in our established MCF-7 cell system. From the results obtained in the semi-quantitative reverse transcription-polymerase chain reaction and xenobiotic response element (XRE)-luciferase reporter assays, biochanin A could reduce xenobiotic-induced CYP1A1 and -1B1 mRNA abundances through the interference of XRE-dependent transactivation. Enzyme kinetic studies also indicated that biochanin A inhibited both CYP1A1 and -1B1 enzymes with inhibition constant (Ki) values 4.00 and 0.59 microm respectively. Since the biotransformation of DMBA was dependent on CYP1 enzyme activities, biochanin A was able to decrease the DMBA-DNA lesions. The present study illustrated that the red clover isoflavone could protect against polycylic aromatic hydrocarbon-induced DNA damage.

9,10-Dimethyl-1,2-benzanthracene↗

PubMed2003

A potential protective mechanism of soya isoflavones against 7,12-dimethylbenz[a]anthracene tumour initiation.

Epidemiological studies indicate that Asian women have a lower breast cancer incidence compared with their counterparts in the West, and the difference has been related to soya consumption. Animal studies have suggested that soya may prevent dimethylbenz[a]anthracene (DMBA)-induced carcinogenesis in the breast. In the present study a cell culture model was developed to address the effect of soya isoflavones on the DMBA-induced DNA damage. DMBA is metabolized into a DNA-attacking moiety by two phase I cytochrome P450 (CYP) enzymes CYP1A1 and CYP1B1. DNA mutation caused by this genotoxic agent is a crucial step in cancer initiation. Substances that interfere with the CYP1 enzyme activities can affect the initiation. In the present study, genistein was found to be an effective inhibitor of recombinant human CYP1A1 and CYP1B1 with Ki of 15.35 and 0.68 micromol/l. The other soya isoflavone daidzein, on the other hand, did not demonstrate any significant inhibition of the enzyme activities. At the transcriptional level, DMBA induced the CYP1 enzyme expressions by stimulating the xenobiotic response element (XRE)-dependent transactivation pathway. When genistein (25 micromol/l) was co-administered with DMBA, the XRE-Luc activity the CYP1 mRNA abundances were significantly suppressed. The present study illustrated that the soya isoflavone genistein, but not daidzein, protected against DMBA genotoxicity.

9,10-Dimethyl-1,2-benzanthracene↗

PubMed2003

Screening of chemopreventive tea polyphenols against PAH genotoxicity in breast cancer cells by a XRE-luciferase reporter construct.

Polycyclic aromatic hydrocarbons (PAHs) are established cancer initiators that can be found in our food and environment. Some dietary plant polyphenols are strong inhibitors to PAH-induced mutagenesis, whereas others may not be as effective. To identify the chemopreventive compounds from a huge volume of dietary components, the development of an efficient screening method is required. In this study, a xenobiotic response element (XRE)-luciferase reporter plasmid was constructed to screen for some potential chemopreventive agents in tea against PAH-induced DNA damage. Tea is one of the most consumed beverages worldwide, and its beneficial effects on health have been documented. Previous studies have claimed that tea polyphenols could be protective against various cancers, and the rich database can be a source for comparison. Among the green and black tea polyphenols, the XRE-luciferase reporter assays suggested that only epigallocatechin gallate (EGCG) was effective in reducing XRE-driven luciferase assay in MCF-7 cells at the concentrations tested. Further study indicated EGCG could reduce CYP1A1 and CYP1B1 mRNA abundances and decrease the DMBA-DNA lesions. The results of DNA covalent binding of all tea polyphenols tested were consistent with the XRE-reporter assays. This study illustrated that the XRE-reporter assay was a viable screening test for dietary chemopreventive agents against PAH-initiated breast mutagenesis. It has the advantages of shorter sample processing time and producing no radioactive waste over directly measuring the CYP1A1/1B1 expressions, DNA lesion, or gel mobility shift assay.

Analysis of Variance↗

PubMed2002

Baicalein and genistein display differential actions on estrogen receptor (ER) transactivation and apoptosis in MCF-7 cells.

Genistein is a soy isoflavone that has attracted much attention for its chemopreventive properties. Baicalein is a flavone found in Scutellaria species, and its structure is analogous to that of genistein. We examined the two flavones' antiestrogenicity by using a transient transfection model. Our results indicated that baicalein could suppress 17beta-estradiol-induced transactivation in cells expressing estrogen receptor alpha, whereas no antagonistic activity was seen in genistein. Cell death assay and flow cytometry analysis demonstrated that baicalein was a stronger apoptosis inducing agent than genistein. Considering all the criteria tested, baicalein could be a better chemopreventive agent than genistein.

Antineoplastic Agents↗

PubMed2002

Baicalein inhibits DMBA-DNA adduct formation by modulating CYP1A1 and CYP1B1 activities.

Flavonoids are phenolic compounds isolated from plants, and several of them like genistein and quercetin, have been documented to be effective in preventing cancer. Baicalein, a flavonoid extracted from the root of Scutellaria species, is widely used as a health supplement and herbal medicine in Asian countries. In this study, the chemopreventive effect of baicalein on 7,12-dimethylbenz[a]anthracene (DMBA)-induced DNA damage was evaluated in an established cell culture model. In a preliminary screening, baicalein was identified to be a strong inhibitor to EROD activities induced by DMBA in MCF-7 cells. Subsequent enzyme kinetic analysis revealed that baicalein was a competitive inhibitor to EROD, and CYP1A1 and CYP1B1 gene expressions were also determined. Baicalein could reduce the CYP1A1/1B1 mRNA expressions induced by DMBA, and the mRNA abundance of CYP1A1 appeared to be more responsive than that of CYP1B1. A XRE-luciferase gene reporter assay indicated that AhR transactivation was suppressed. Since CYP1A1/1B1 were responsible for the biotransformation of polycyclic aromatic hydrocarbons, baicalein also demonstrated its ability to reduce DMBA-DNA adduct formation in MCF-7 cells. This study suggested that the natural occurring baicalein could be an agent preventing carcinogen-DNA adduct formation.

9,10-Dimethyl-1,2-benzanthracene↗

PubMed2002

Genistein-induced apoptosis in MCF-7 cells involves changes in Bak and Bcl-x without evidence of anti-oestrogenic effects.

South-east Asian women have a lower rate of breast cancer compared with their counterparts in western countries and the difference in soyabean consumption has been claimed to be a major contributing factor. Genistein is the most studied phytochemical in the soyabean. An anti-oestrogenic effect is believed to play a crucial part in its chemopreventive mechanism. In the present study, we expressed oestrogen receptor (OR) in an OR-negative cell line, HepG2, to investigate the pro- and anti-oestrogenic effect of genistein on the OR transcriptional activity. Genistein by itself had an estimated concentration that induced 50 % of the maximum response (EC50) of 2.5 microm for the binding to OR-alpha. In these experiments, genistein concentration as high as 50 microm could not reduce the oestrogen response element-driven luciferase activities initiated by oestradiol. Instead, genistein potentiated the OR transactivational activity while cell death was detected. On the other hand, an increased Bak and a reduced Bcl-x(L) was observed at 50 microm-genistein by Western analysis. The combined effect of these two proteins could be important in the apoptotic process. Since plasma genistein >50 microm has never been documented following consuming of soyabean or soyabean products, the present study does not support the notion that dietary soyabean exerts its chemopreventive effect through antagonizing OR.

Anticarcinogenic Agents↗