This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Ciolino, H. P. Articles by Yeh, G. C. Search for Related Content PubMed PubMed Citation Articles by Ciolino, H. P. Articles by Yeh, G. C.

Inhibition of Carcinogen-activating Enzymes by 16-Fluoro-5-androsten-17-one

Cellular Defense and Carcinogenesis Section, Basic Research Laboratory, Center for Cancer Research, National Cancer Institute at Frederick, NIH, Frederick, Maryland 21702-1201

	ABSTRACT

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
In the present study, we examined the effect of a synthetic analogue of the chemopreventive hormone dehydroepiandrosterone, 16-fluoro-5-androsten-17-one, also known as fluasterone, on the activity and expression of carcinogen-activating enzymes in MCF-7 cells. The increase in cytochrome P450 (CYP) 1A1 and 1B1 activity, as measured by ethoxyresorufin-O-deethylase activity, in cells treated with the carcinogens dimethylbenzanthracene (DMBA) or 2,3,5,7-tetrachlorodibenzo-p-dioxin (TCDD), was inhibited by cotreatment with fluasterone. However, treatment of the cells with fluasterone after induction with DMBA or TCDD failed to decrease enzyme activity, indicating that inhibition was not the result of direct enzyme inhibition. Therefore, we examined the effect of fluasterone on gene expression at the mRNA level. Both DMBA and TCDD caused a dramatic increase in the amount of CYP1A1 and CYP1B1 mRNA, the two major isoforms involved in carcinogen activation in these cells. In cells cotreated with fluasterone, however, there was a dose-dependent decrease in CYP1A1 and CYP1B1 mRNA. Fluasterone also inhibited the basal level of CYP1A1 mRNA but not CYP1B1. Fluasterone inhibited the rate of CYP1A1 promoter-controlled transcription, indicating that it affects the transcriptional regulation of the gene. Actinomycin D chase experiments showed that fluasterone also caused an increase in the degradation of CYP1A1 mRNA, while leaving CYP1B1 mRNA unaffected. These results indicate that fluasterone inhibits the increase in the expression of CYP1A1 normally caused by exposure to carcinogens by both transcriptional and post-transcriptional mechanisms and that CYP1B1 expression is not susceptible to the same post-transcriptional mechanism.

	INTRODUCTION

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Many environmental compounds are carcinogenic only after metabolic activation. Exposure to carcinogens, such as PAH,² causes an increase in the expression of the enzymes responsible for this activation. These enzymes consist of members of the CYP 1A and 1B subfamilies. They generate genotoxic epoxide metabolites of the parent aryl hydrocarbon, which can bind DNA, forming adducts (1) . These adducts, if not repaired, can cause specific mutations leading to cellular transformation. Therefore, the activity and expression of carcinogen-activating enzymes are key components in chemically induced carcinogenesis, and the inhibition of their activity, either by direct enzyme inhibition or through modulation of their expression, is thought to be an important mechanism in the prevention of carcinogenesis (2) .

DHEA is the most abundant steroid hormone in humans (3) . Although its physiological function remains unclear, it is associated with a number of beneficial health effects in humans (4) . A considerable body of evidence indicates that DHEA is also associated with a decrease in the incidence of a number of different types of cancer in humans (5 , 6) . In animal models, DHEA has been shown to inhibit both spontaneous and chemically induced carcinogenesis in rodents (7, 8, 9) . Specifically, DHEA inhibits both skin and mammary tumorigenesis caused by the PAH DMBA (10, 11, 12, 13) . DHEA has been shown to inhibit DMBA activation in vitro (14) and DMBA-DNA binding in vivo (15, 16, 17, 18) . The dramatic decline of DHEA levels in humans with advancing age is, therefore, of substantial concern. However, DHEA supplementation is problematic, because it can be converted to both testosterone and estrone (19) , and it exhibits considerable liver toxicity (20) and hepatocarcinogenicity (21) . Thus, DHEA’s clinical usefulness is limited. Recently, a synthetic analogue of DHEA, 16-fluoro-5-androsten-17-one (fluasterone), was developed that lacks the liver toxicity and hormone-related side effects of DHEA (22) . Fluasterone has been shown to be chemopreventive against DMBA-induced carcinogenesis (23) , and, like DHEA, it prevents DMBA activation and DMBA-DNA adduct formation (10 , 22) . However, the mechanism of this activity is unknown. Therefore, we have tested the capacity of fluasterone to modulate the effects of DMBA and TCDD on carcinogen-activating enzyme activity and expression in vitro. DMBA is a classic model aryl hydrocarbon. TCDD is a widespread environmental contaminant produced during trash incineration, bleaching of paper pulp, synthesis of pesticides, and as a by-product of combustion during various industrial processes (24 , 25) .

	MATERIALS AND METHODS

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Materials.
Human breast cancer MCF-7 and human liver carcinoma HepG2 cells were from the American Type Culture Collection (Rockville, MD). RPMI 1640, glutamine, fetal bovine serum, trypsin/EDTA, and PBS were from BioFluids (Rockville, MD). Actinomycin D, -NF, DMBA, EDTA, ethoxyresorufin, resorufin, Tris-HCl, and DMSO were from Sigma (St. Louis, MO). [³²P]dATP was from DuPont NEN (Boston, MA). TCDD was from the Midwest Research Institute (Kansas City, MO). RT-PCR was performed with an Omniscript kit from Qiagen (Valencia, CA). Tris/borate/EDTA gels, running buffer, and high-density sample buffer were from Novex (San Diego, CA). Primers for GPDH PCR and the ß-galactosidase-containing reporter vector were from Clontech (Palo Alto, CA). TRIzol reagent and LipofectAmine were from Life Technologies, Inc. (Gaithersburg, MD). CAT ELISA assay kit was from Boehringer Mannheim (Indianapolis, IN). Fluasterone was a gift from Dr. Thomas Wang (Phytonutrients Laboratory, USDA, Beltsville, MD).

Cell Culture.
MCF-7 and HepG2 were grown in RPMI 1640 with 2 mM glutamine and 10% fetal bovine serum and subcultured weekly using 0.25% trypsin/0.05% EDTA. All experiments were carried out on confluent cultures in growth medium, unless otherwise noted.

Assay of EROD Activity.
Confluent MCF-7 or HepG2 cells in 24-well plates were treated with 1 ml of growth medium containing 1 µM DMBA or 1 nM TCDD in the presence of DMSO (vehicle control) or fluasterone for 24 h. The final DMSO concentration in both control and treated cultures for this and all other experiments was 0.1%. The medium was removed, and the wells were washed two times with fresh growth medium. EROD activity was determined in intact cells using 1 µM ethoxyresorufin in growth medium as a substrate in the presence of 1.5 mM salicylamide to inhibit conjugating enzymes. The assay was carried out at 37°C. The fluorescence of resorufin generated from the conversion of ethoxyresorufin by CYP1A1/CYP1B1 was measured every 10 min for 60 min with a CytoFluor II multiwell fluorescence plate reader (PerSeptive Biosystems, Framingham, MA), with an excitation wavelength of 530 nm and emission at 590 nm.

To determine the direct effect of fluasterone on EROD activity, MCF-7 cells were incubated with 1 µM DMBA or 1 nM TCDD for 24 h to induce enzyme expression. The cells were then washed extensively and incubated with DMSO (vehicle control), 10 µM fluasterone, or 5 µM -NF as a positive control for 3 h, and EROD activity was determined. In addition, the effect of fluasterone on EROD activity in microsomes isolated from TCDD-treated MCF-7 cells was measured as described (26) .

RT-PCR.
MCF-7 cells were grown in six-well plates and treated with DMBA (1 µM) or TCDD (1 nM) in the presence of DMSO (control) or fluasterone for 6 h. Isolation of total RNA; cDNA synthesis; semiquantitative RT-PCR for CYP1A1, CYP1B1, and GPDH mRNA; and analysis of results were performed as described previously (27) . Primer sequences for CYP1A1 and CYP1B1 were described in Dohr et al. (28) . cDNA was synthesized from 2 µg of total RNA using a Omniscript RT-PCR kit as instructed. A cycle number that fell within the linear range of response for CYP1A1 (27 cycles for the determination of CYP1A1 mRNA stability and basal mRNA levels, 24 cycles otherwise), CYP1B1 (24 cycles for mRNA stability and basal mRNA levels, 22 cycles otherwise), and GPDH (17 cycles) was used.

Transient Transfections.
XRE-controlled CAT transcription was determined as described previously (29) .

Determination of mRNA Stability.
MCF-7 cells were pretreated with 1 µM DMBA for 24 h to induce CYP1A1 and CYP1B1 expression. The cells were then washed extensively and incubated with media containing 5 µg/ml the transcription inhibitor actinomycin D in the presence of DMSO (control) or fluasterone. After 4 h, the total RNA was isolated, and RT-PCR for CYP1A1 and CYP1B1 was performed as described above.

Statistical Analysis.
Statistical analyses were performed using StatView Statistical Analysis software (SAS Institute). Differences between group mean values were determined by a one-factor ANOVA, followed by Fisher’s protected least-significant difference post hoc analysis for pairwise comparison of means.

	RESULTS

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Fluasterone Inhibits CYP1A1/1B1 Enzyme Activity in Intact Cells.
Carcinogen-activating enzyme activity was measured in intact MCF-7 cells by the EROD assay, which is specific for CYP1A1/1B1. The treatment of MCF-7 cells with 1 µM DMBA for 24 h resulted in an increase in activity from undetectable levels to 1.38 ± 0.13 pmol/min/10⁵ cells. In cells coincubated with DMBA and fluasterone, there was a dose-dependent decrease in EROD activity, with a IC₅₀ of 0.5 µM (Fig. 1A) . In cells treated with 1 nM TCDD, the most potent inducer of these enzymes, there was an increase in EROD activity to 16.72 ± 0.43 pmol/min/10⁵ cells. This was also decreased in a dose-dependent fashion by coincubation with fluasterone, with a IC₅₀ of 2.5 µM (Fig. 1B) . Fluasterone and DHEA also inhibited TCDD-induced EROD activity in human liver HepG2 cells (Fig. 1C) .

View larger version (20K): [in this window] [in a new window]   Fig. 1. Effect of fluasterone on cellular EROD activity induced by DMBA (A), TCDD (B), or TCDD in HepG2 cells (C). MCF-7 or HepG2 cells were incubated with 1 µM DMBA or 1 nM TCDD for 24 h in the presence of the indicated concentrations of fluasterone, and cellular EROD activity was determined. n = 4 ± SE. There was a significant decrease of EROD activity in the presence of 0.25 µM fluasterone in A and 1 µM in B (P < 0.05).

View larger version (21K): [in this window] [in a new window]   Fig. 2. Effect of fluasterone on CYP1A1 mRNA induced by DMBA (A) or TCDD (B). MCF-7 cells were treated for 6 h with DMSO (Control), 1 µM DMBA, or 1 nM TCDD in the presence of DMSO or fluasterone at the indicated concentrations. Total RNA was isolated, cDNA was synthesized, and the amount of mRNA was determined by PCR. The results were visualized and quantified by phosphoimaging. For graph, the levels of CYP1A1 were normalized to GPDH level. n = 3 ± SE. There was a significant decrease in CYP1A1 mRNA in cells treated with fluasterone (P < 0.05).

View larger version (21K): [in this window] [in a new window]   Fig. 3. Effect of fluasterone on CYP1B1 mRNA induced by DMBA (A) or TCDD (B). Cells were treated and analyzed for CYP1B1 mRNA as described in the legend for Fig. 2 . For graph, the level of CYP1B1 was normalized to GPDH level. n = 3 ± SE. There was no significant decrease in CYP1B1 in cells treated with fluasterone.

View larger version (35K): [in this window] [in a new window]   Fig. 4. Inhibition of basal CYP1A1 mRNA levels by fluasterone. MCF-7 cells were treated with the indicated concentrations of fluasterone for 24 h. CYP1A1, CYP1B1, and GPDH mRNA were determined by RT-PCR. n = 3 ± SE. There was a significant decrease in basal CYP1A1 mRNA in cells treated with all concentrations of fluasterone compared with controls (P < 0.05) and no significant difference in CYP1B1 mRNA.

View larger version (21K): [in this window] [in a new window]   Fig. 5. Fluasterone inhibits DMBA- (A) or TCDD-induced and basal CAT transcription (B) mediated by the XRE. MCF-7 cells were transfected with a CAT reporter vector controlled by the XRE and with a vector containing ß-Gal. Transfected cells were treated with 1 nM TCDD (solid bars) or 1 µM DMBA (hatched bars) for 6 h in the presence of the indicated concentrations of fluasterone. CAT transcription was normalized to ß-galactosidase transcription. n = 4 ± SE. There was a significant difference in transcription in cells treated with 5 or 10 µM fluasterone (P < 0.05).

View larger version (36K): [in this window] [in a new window]   Fig. 6. Effect of fluasterone on the stability of CYP1A1 and CYP1B1 mRNA. MCF-7 cells were incubated with 1 µM DMBA for 24 h to induce CYP1A1 and CYP1B1 expression, then washed three times in growth medium. The cells were then incubated for 4 h in growth medium without DMBA in the presence of 5 µg/ml actinomycin D and the indicated concentrations of fluasterone. CYP1A1, CYP1B1, and GPDH mRNA were determined by RT-PCR. n = 3 ± SE. For graph, the levels of CYP1A1 and CYP1B1 were normalized to GPDH level. There was a significant decrease in CYP1A1 mRNA in cells treated with fluasterone (P < 0.05) but no change in CYP1B1 mRNA.

	DISCUSSION

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

Treatment of MCF-7 cells with DMBA or TCDD results in a profound increase in CYP enzyme activity, as measured by EROD assay. Both CYP1A1 and CYP1B1 contribute to EROD activity, although the specific activity of CYP1A1 is 10-fold higher than CYP1B1 (37) . In cells cotreated with fluasterone, there was a concentration-dependent decrease in EROD activity (Fig. 1A) . The IC₅₀ toward DMBA-induced EROD activity was 0.5 µM, which is 5-fold higher than for DHEA (33) . The IC₅₀ for the inhibition of TCDD-induced EROD activity was 2.5 µM (Fig. 1B) , compared with 1 µM for DHEA. Thus, the parent molecule DHEA is a more effective inhibitor of carcinogen-induced EROD activity than fluasterone. However, in the HepG2 human hepatic cancer cell line, fluasterone is more effective than DHEA in inhibiting TCDD-induced EROD activity (Fig. 1C) , although both compounds have higher IC₅₀s in HepG2 cells than in MCF-7 cells. This may be attributable to the presence of sulfating enzymes in cells of liver origin, such as HepG2 cells, because the sulfated form of DHEA does not inhibit EROD activity (33) . Thus, the relative effectiveness of both DHEA and fluasterone may be cell type dependent. When fluasterone was added to MCF-7 cells that had been pretreated with either DMBA or TCDD, it did not inhibit EROD activity (Table 1) . Nor could it inhibit the EROD activity present in microsomes isolated from treated cells. Table 1 indicates that fluasterone, like DHEA, does not directly inhibit CYP enzyme activity, as do other compounds, such as -NF. Therefore, we investigated the effect of fluasterone on CYP expression.

DMBA or TCDD treatment of cells causes an increase in the levels of CYP1A1 mRNA, as determined by RT-PCR (Fig. 2) . Cotreatment with fluasterone inhibited this increase. As for EROD activity, fluasterone was more effective in inhibiting DMBA-induced CYP1A1 mRNA than in inhibiting TCDD-induced mRNA. This may be because TCDD, being a more potent inducer of CYP1A1 expression, causes much higher amounts of CYP1A1. We also measured the mRNA of the other major carcinogen-activating enzyme in MCF-7 cells, CYP1B1. The level of induction of CYP1B1 mRNA by DMBA or TCDD (Fig. 3) was not as profound as CYP1A1. Cotreatment with fluasterone also inhibited the increase in CYP1B1 mRNA, although not as dramatically as for CYP1A1. These results demonstrate that the inhibition of carcinogen-induced CYP enzyme activity is attributable to the inhibition of CYP expression. The level of CYP1A1 and CYP1B1 in MCF-7 cells is low but measurable in the absence of carcinogen treatment. Fluasterone, like DHEA, caused a decrease in the amount of basal CYP1A1. Basal CYP1B1 levels, on the other hand, were unaffected (Fig. 4) .

CYP1A1 and CYP1B1 expression are controlled at the transcriptional level by the aryl hydrocarbon receptor, a cytosolic protein that binds carcinogens and activates CYP transcription by binding to the XRE contained in the promoter region of the CYP1A1 and CYP1B1 genes. We demonstrated previously that DHEA does not affect XRE-mediated transcription, whether it was induced by TCDD or DMBA (33) . We tested the effect of fluasterone on XRE-mediated transcription and found that it, in contrast to DHEA, caused a modest inhibition of both TCDD- or DMBA-induced transcription (Fig. 5A) and inhibited the basal rate of transcription (Fig. 5B) . Structurally, fluasterone differs from DHEA only in the presence of a single fluorine atom, which prevents the metabolism of the compound to other steroids. This difference may account for the inhibitory effect on aryl hydrocarbon receptor-mediated transcription. Another possibility is that DHEA itself would inhibit transcription but that it is quickly metabolized by MCF-7 cells. But although fluasterone at 1 µM caused a significant decrease in DMBA- or TCDD-induced CYP1A1 mRNA, it caused only a slight decrease in transcription. Thus, the inhibition of transcription, although it may contribute to the overall inhibition of CYP expression by fluasterone, is not the only mechanism. Therefore, we also examined the effect of fluasterone on the stability of CYP mRNA. As shown in Fig. 6 , fluasterone caused a concentration-dependent decrease in CYP1A1 mRNA in the presence of the RNA polymerase II inhibitor actinomycin D after induction by DMBA. This agrees with our previous study (33) , which demonstrated that DHEA decreased CYP1A1 mRNA stability. Interestingly, fluasterone had no effect on CYP1B1 mRNA stability. These results, along with Fig. 4 , suggest that there is a fundamental difference in the regulation of CYP1A1 and CYP1B1 mRNA at the post-transcriptional level, although the nature of the mechanism involved is unknown. The combination of transcriptional and post-transcriptional inhibition may be reason that fluasterone is more effective in inhibiting CYP1A1 than CYP1B1 expression.

These data demonstrate that fluasterone inhibits carcinogen-activating enzyme activity and expression in vitro by both transcriptional and post-transcriptional mechanisms. These results suggest that the chemopreventive properties of fluasterone toward PAH-induced carcinogenesis may be attributable, in part, to its effect on the expression of CYP enzymes. These results are particularly important because fluasterone is now in Phase II clinical trials to measure its effectiveness in vivo. This study does not exclude other chemopreventive mechanisms of action, e.g., fluasterone, in addition to inhibiting the initiation of DMBA-induced mammary and skin tumorigenesis, has also been shown to inhibit 12-O-tetradecanoylphorbol-13-acetate-promoted skin papilloma formation when administered after initiation by DMBA (10) , suggesting that it also affects biochemical mechanisms involved in the promotion phase.

	FOOTNOTES

 
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom requests for reprints should be addressed, at Basic Research Laboratory, Building 560/Room 12-05, National Cancer Institute at Frederick, NIH, Frederick, MD 21702-1201. Phone: (301) 846-5160; Fax: (301) 846-6709; E-mail: hciolino{at}ncifcrf.gov

² The abbreviations used are: PAH, polycyclic aromatic hydrocarbon; CAT, chloramphenicol acetyltransferase; CYP, cytochrome P450; DHEA, dehydroepiandrosterone; DMBA, dimethylbenzanthracene; EROD, ethoxyresorufin-O-deethylase; G6PDH, glucose-6-phosphate dehydrogenase; GPDH, glyceraldehyde-3-phosphate dehydrogenase; -NF, -naphthoflavone; RT-PCR, reverse transcription-PCR; TCDD, 2,3,5,7-tetrachlorodibenzo-p-dioxin; XRE, xenobiotic-responsive element.

Received 1/11/02. Accepted 5/ 2/02.

	REFERENCES

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

1. Peltonen K., Dipple A. Polycyclic aromatic hydrocarbons: chemistry of DNA adduct formation. J. Occup. Environ. Med., 37: 52-58, 1995.[Medline]
2. Wattenberg L. An overview of chemoprevention: current status and future prospects. Proc. Soc. Exp. Biol. Med., 16: 133-141, 1997.
3. Ebeling P., Koivisto V. A. Physiological importance of dehydroepiandrosterone. Lancet, 343: 1479-1481, 1994.[Medline]
4. Svec F., Porter J. R. The actions of exogenous dehydroepiandrosterone in experimental animals and humans. Proc. Soc. Exp. Biol. Med., 218: 174-191, 1998.[Abstract]
5. Regelson W., Kalimi M. Dehydroepiandrosterone (DHEA)–the multifunctional steroid. II. Effects on the CNS, cell proliferation, metabolic and vascular, clinical and other effects. Mechanism of action?. Ann. N. Y. Acad. Sci., 719: 564-575, 1994.[Medline]
6. Rose D. P., Stauber P., Thiel A., Crowley J. J., Milbrath J. R. Plasma dehydroepiandrosterone sulfate, androstenedione and cortisol, and urinary free cortisol excretion in breast cancer. Eur. J. Cancer, 13: 43-47, 1977.
7. Hursting S. D., Perkins S. N., Haines D. C., Ward J. M., Phang J. M. Chemoprevention of spontaneous tumorigenesis in p53-knockout mice. Cancer Res., 55: 3949-3953, 1995.[Abstract/Free Full Text]
8. Nyce J. W., Magee P. N., Hard G. C., Schwartz A. G. Inhibition of 1, 2-dimethylhydrazine-induced colon tumorigenesis in Balb/c mice by dehydroepi-androsterone. Carcinogenesis (Lond.), 5: 57-62, 1984.[Abstract/Free Full Text]
9. McCormick D. L., Rao K. V., Johnson W. D., Bowman-Gram T. A., Steele V. E., Lubet R. A., Kellof G. J. Exceptional chemopreventive activity of low-dose dehydroepiandrosterone in the rat mammary gland. Cancer Res., 56: 1724-1726, 1996.[Abstract/Free Full Text]
10. Schwartz A. G., Fairman D. K., Polansky M., Lewbart M. L., Pashko L. L. Inhibition of 7, 12-dimethylbenz[a]anthracene-initiated and 12-O-tetradecanoyl-phorbol-13-acetate-promoted skin papilloma formation in mice by dehydro-epiandrosterone and two synthetic analogs. Carcinogenesis (Lond.), 10: 1809-1813, 1989.[Abstract/Free Full Text]
11. Pashko L. L., Hard G. C., Rovito R. J., Williams J. R., Sobel E. L., Schwartz A. G. Inhibition of 7, 12-dimethylbenz(a)anthracene-induced skin papillomas and carcinomas by dehydroepiandrosterone and 3-ß-methylandrost-5-en-17-one in mice. Cancer Res., 45: 164-166, 1985.[Abstract/Free Full Text]
12. Li S, Yan X., Belanger A., Labrie F. Prevention by dehydroepiandrosterone of the development of mammary carcinoma induced by 7, 12-dimethylbenz(a)anthracene (DMBA) in the rat. Breast Cancer Res. Treat., 29: 203-217, 1994.[Medline]
13. Luo S., Labrie C., Belanger A., Labrie F. Combined effects of dehydroepiandrosterone and EM-800 on bone mass, serum lipids, and the development of dimethylbenz(a)anthracene-induced mammary carcinoma in the rat. Endocrinology, 138: 3387-3394, 1997.[Abstract/Free Full Text]
14. Schwartz A. G., Perantoni A. Protective effect of dehydroepiandrosterone against aflatoxin B1- and 7, 12-dimethylbenz(a)anthracene-induced cytotoxicity and transformation in cultured cells. Cancer Res., 5: 2482-2487, 1975.
15. Kim S. H., Han H. M., Kang S. Y., Jung K. K., Kim T. G., Oh H. Y., Lee Y. K, Rheu H. M. Modulation of chemical carcinogen-induced unscheduled DNA synthesis by dehydroepiandrosterone (DHEA) in the primary rat hepatocytes. Arch. Pharm. Res., 22: 474-478, 1999.[Medline]
16. Ikegwuonu F. I., Jefcoate C. R. Evidence for the involvement of the fatty acid and peroxisomal beta-oxidation pathways in the inhibition by dehydroepiandrosterone (DHEA) and induction by 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and benz(a)anthracene (BA) of cytochrome P4501B1 (CYP1B1) in mouse embryo fibroblasts (C3H10T1/2 cells). Mol. Cell. Biochem., 198: 89-100, 1999.[Medline]
17. Prasanna H. R., Hart R. W., Magee P. N. Differential effects of dehydro-epiandrosterone and clofibrate on the binding of 7, 12-dimethylbenz(a)anthracene to hepatic DNA in vivo–a preliminary study. Drug Chem. Toxicol., 12: 327-335, 1989.[Medline]
18. Pashko L. L., Schwartz A. G. Effect of food restriction, dehydroepiandrosterone, or obesity on the binding of [³H]-7, 12-dimethylbenz(a)anthracene to mouse skin DNA. J. Gerontol., 38: 8-12, 1983.[Medline]
19. Longcope C. Dehydroepiandrosterone metabolism. J. Endocrinol., 150: 125-127, 1996.
20. Bellei M., Battelli D., Fornieri C., Mori G., Muscatello U., Lardy H., Bobyleva V. Changes in liver structure and function after short-term and long-term treatment of rats with dehydroepiandrosterone. J. Nutr., 122: 967-976, 1992.
21. Rao M. S., Subbarao V., Yeldandi A. V., Reddy J. K. Hepatocarcinogenicity of dehydroepiandrosterone in the rat. Cancer Res., 52: 2977-2979, 1992.[Abstract/Free Full Text]
22. Schwartz A. G., Lewbart M. L., Pashko L. L. Novel dehydroepiandrosterone analogues with enhanced biological activity and reduced side effects in mice and rats. Cancer Res., 48: 4817-4822, 1988.[Abstract/Free Full Text]
23. Schwartz A. G., Pashko L. L Cancer prevention with dehydroepiandrosterone and non-androgenic structural analogs. J. Cell. Biochem., 22: 210-217, 1995.
24. Dyke P. H., Foan C., Wenborn M., Coleman P. J. A review of dioxin releases to land and water in the UK. Sci. Total Environ., 207: 119-131, 1997.[Medline]
25. Quass U., Fermann M. W., Broker G. Steps towards a European dioxin emission inventory. Chemosphere, 40: 1125-1129, 2000.[Medline]
26. Ciolino H. P., Yeh G. C. The flavonoid galangin is an inhibitor of CYP1A1 activity and an agonist/antagonist of the aryl hydrocarbon receptor. Br. J. Cancer, 79: 1340-1346, 1999.[Medline]
27. MacDonald C. J., Ciolino H. P., Yeh G. C. Dibenzoylmethane modulates aryl hydrocarbon receptor function and expression of cytochromes P50 1A1, 1A2, and 1B1. Cancer Res., 61: 3919-3924, 2001.[Abstract/Free Full Text]
28. Dohr O., Vogel C., Abel J. Different response of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD)-sensitive genes in human breast cancer MCF-7 and MDA-MB 231 cells. Arch. Biochem. Biophys., 321: 405-412, 1995.[Medline]
29. Ciolino H. P., Daschner P. J., Yeh G. C. Dietary flavonols quercetin and kaempferol are ligands of the aryl hydrocarbon receptor that affect CYP1A1 transcription differentially. Biochem. J., 340: 715-722, 1999.
30. Perkins S. N., Hursting S. D., Haines D. C., James S. J., Miller B. J., Phang J. M. Chemoprevention of spontaneous tumorigenesis in nullizygous p53-deficient mice by dehydroepiandrosterone and its analog 16--fluoro-5-androsten-17-one. Carcinogenesis (Lond.), 18: 989-994, 1997.[Abstract/Free Full Text]
31. Casazza J. P., Schaffer W. T., Veech R. L. The effect of dehydroepiandrosterone on liver metabolites. J. Nutr., 116: 304-310, 1986.
32. Cleary M. P. Effect of dehydroepiandrosterone treatment on liver metabolism in rats. Int. J. Biochem., 22: 205-210, 1990.[Medline]
33. Ciolino H. P., Yeh G. C. The steroid hormone dehydroepiandrosterone inhibits CYP1A1 expression in vitro by a post-transcriptional mechanism. J. Biol. Chem., 274: 35186-35190, 1999.[Abstract/Free Full Text]
34. Schwartz A. G., Pashko L. L. Potential therapeutic use of dehydroepiandrosterone and structural analogs. Diabetes Technol. Ther., 3: 221-224, 2001.[Medline]
35. Wang X., Thomsen J. S., Santostefano M., Rosengren R., Safe S., Perdew G. H. Comparative properties of the nuclear aryl hydrocarbon (Ah) receptor complex from several human cell lines. Eur. J. Pharmacol., 293: 191-205, 1995.[Medline]
36. Moore M., Wang X., Lu Y. F., Wormke M., Craig A., Gerlach J. H, Burghardt R., Barhoumi R., Safe S. Benzo[a]pyrene-resistant MCF-7 human breast cancer cells. A unique aryl hydrocarbon-nonresponsive clone. J. Biol. Chem., 269: 11751-11759, 1994.[Abstract/Free Full Text]
37. Doostdar H., Burke M. D., Mayer R. T. Bioflavonoids: selective substrates and inhibitors for cytochrome P450 CYP1A and CYP1B1. Toxicology, 144: 31-38, 2000.[Medline]

This article has been cited by other articles:

				D. C. Spink, B. H. Katz, M. M. Hussain, B. T. Pentecost, Z. Cao, and B. C. Spink Estrogen regulates Ah responsiveness in MCF-7 breast cancer cells Carcinogenesis, December 1, 2003; 24(12): 1941 - 1950. [Abstract] [Full Text] [PDF]

				P. Gibson, J. H. Gill, P. A. Khan, J. M. Seargent, S. W. Martin, P. A. Batman, J. Griffith, C. Bradley, J. A. Double, M. C. Bibby, et al. Cytochrome P450 1B1 (CYP1B1) Is Overexpressed in Human Colon Adenocarcinomas Relative to Normal Colon: Implications for Drug Development Mol. Cancer Ther., June 1, 2003; 2(6): 527 - 534. [Abstract] [Full Text] [PDF]

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Ciolino, H. P. Articles by Yeh, G. C. Search for Related Content PubMed PubMed Citation Articles by Ciolino, H. P. Articles by Yeh, G. C.