Molecular mechanism of emodin action: transition from laxative ingredient to an antitumor agent

Anthraquinones represent a large family of compounds having diverse biological properties. Emodin (1,3,8-trihydroxy-6-methylanthraquinone) is a naturally occurring anthraquinone present in the roots and barks of numerous plants, molds, and lichens, and an active ingredient of various Chinese herbs. Earlier studies have documented mutagenic/genotoxic effects of emodin, mainly in bacterial system. Emodin, first assigned to be a specific inhibitor of the protein tyrosine kinase p65lck, has now a number of cellular targets interacting with it. Its inhibitory effect on mammalian cell cycle modulation in specific oncogene overexpressed cells formed the basis of using this compound as an anticancer agent. Identification of apoptosis as a mechanism of elimination of cells treated with cytotoxic agents initiated new studies deciphering the mechanism of apoptosis induced by emodin. At present, its role in combination chemotherapy with standard drugs to reduce toxicity and to enhance efficacy is pursued vigorously. Its additional inhibitory effects on angiogenic and metastasis regulatory processes make emodin a sensible candidate as a specific blocker of tumor-associated events. Additionally, because of its quinone structure, emodin may interfere with electron transport process and in altering cellular redox status, which may account for its cytotoxic properties in different systems. However, there is no documentation available which reviews the biological activities of emodin, in particular, its growth inhibitory effects. This review is an attempt to analyze the biological properties of emodin, a molecule offering a broad therapeutic window, which in future may become a member of anticancer armamentarium. 2006 Wiley Periodicals. Inc.