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Environmental Endocrinology Laboratory, Center for Bioenvironmental Research, Tulane and Xavier Universities, * Molecular and Cellular Biology Program, Tulane Cancer Center, Department of Pharmacology, Tulane University School of Medicine, New Orleans, LA 70112
2To whom correspondence should be addressed. E-mail: mburow{at}tulane.edu.
Phytochemicals bind to and regulate the human estrogen receptors (ER
and ERß), mimicking actions of the endogenous estrogen, 17ß-estradiol, and known antiestrogens such as ICI 182,780. Recently, however, some of these estrogenic phytochemicals have been shown to affect other signal transduction pathways, such as receptor tyrosine kinases and mitogen-activated protein kinases (MAPK). Previously, we found that certain phytochemicals, such as flavone, apigenin, kaempferide and chalcone, have potent antiestrogenic activity. However, the antiestrogenicity of these compounds does not correlate with their ER binding capacity, suggesting alternative signaling as a mechanism for their antagonistic effects. In this study, we examined the effects of these compounds on the transcription factor activator protein-1 (AP-1). Using AP-1-luciferase stable human endometrial adenocarcinoma Ishikawa and human embryonic kidney (HEK) 293 cells, chalcone, flavone and apigenin all stimulated AP-1 activity. Additionally, we determined the effects of the phytochemicals on transcription factors that are downstream targets of various MAPK pathways. To test this, we used HEK 293 cells stably cointegrated with GAL4 transcriptional activation systems of Elk-1, c-Jun or C/EBP homologous protein (CHOP). Chalcone was the only phytochemical that activated all three transcription factors [Elk-1, 2.7-fold (P < 0.001); c-Jun, 2.7-fold (P = 0.025); CHOP, 3.0-fold (P = 0.002)], whereas apigenin stimulated CHOP (3.9-fold; P < 0.001), but inhibited phorbol myristoyl acetate–induced c-Jun activity (71%;P = 0.006). This work suggests that phytochemicals affect multiple signaling pathways that converge at the level of transcriptional regulation. The ability of flavonoids to regulate MAPK-responsive pathways in a selective manner indicates a mechanism by which phytochemicals may influence human health and disease.
KEY WORDS: • flavonoid phytochemicals • estrogen receptor • mitogen-activated protein kinase • activator protein-1
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