Phytochemicals as Anticancer and Chemopreventive Topoisomerase II Poisons

Abstract

Phytochemicals are a rich source of anticancer drugs and chemopreventive agents. Several of these chemicals appear to exert at least some of their effects through interactions with topoisomerase II, an essential enzyme that regulates DNA supercoiling and removes knots and tangles from the genome. Topoisomerase II-active phytochemicals function by stabilizing covalent protein-cleaved DNA complexes that are intermediates in the catalytic cycle of the enzyme. As a result, these compounds convert topoisomerase II to a cellular toxin that fragments the genome. Because of their mode of action, they are referred to as topoisomerase II poisons as opposed to catalytic inhibitors. The first sections of this article discuss DNA topology, the catalytic cycle of topoisomerase II, and the two mechanisms (interfacial vs. covalent) by which different classes of topoisomerase II poisons alter enzyme activity. Subsequent sections discuss the effects of several phytochemicals on the type II enzyme, including demethyl-epipodophyllotoxins (semisynthetic anticancer drugs) as well as flavones, flavonols, isoflavones, catechins, isothiocyanates, and curcumin (dietary chemopreventive agents). Finally, the leukemogenic potential of topoisomerase II-targeted phytochemicals is described.

Keywords: Bioflavonoids; Catechins; Curcumin; Demethyl-epipodophyllotoxins; Isothiocyanates.

Figure 1

Catalytic cycle of type II topoisomerases. Adapted from (Ketron and Osheroff, 2013). The homodimeric enzyme is shown in blue, the DNA double helix that is cleaved and acts as the DNA gate (G-segment) is shown in green, and the double helix that is transported through the DNA gate (T-segment) is shown in yellow. Details of the individual reaction steps are described in the text.

Figure 2

Topoisomerase II-DNA cleavage complex equilibrium. Adapted from (Deweese and Osheroff, 2009). The formation of covalent DNA cleavage complexes is required for topoisomerases to perform their critical cellular functions. If the level of topoisomerase II-DNA cleavage complexes falls below threshold levels (left arrow), cells are unable to segregate their chromosomes and ultimately die of mitotic failure. If the level of cleavage complexes becomes too high (right arrow) the actions of DNA tracking systems can convert these transient complexes to permanent double-stranded breaks. The resulting DNA breaks, as well as the inhibition of essential DNA processes, initiate recombination/repair pathways and generate mutations, chromosome translocations, and other DNA aberrations. If the strand breaks overwhelm the cell, they can trigger apoptosis. This is the basis for the actions of several widely prescribed anticancer drugs that target topoisomerase II. However, if the increase in enzyme-mediated DNA strand breaks does not kill the cell, mutations or chromosomal aberrations may persist in surviving populations. In some cases, exposure to topoisomerase II-targeted agents has been associated with the formation of acute myeloid leukemias that involve the MLL (mixed lineage leukemia) gene at chromosome band 11q23 (lower right arrow).

Figure 3

Distinguishing characteristics of interfacial vs. covalent topoisomerase II poisons. Details are provided in the text.

Figure 4

Structures of podophyllotoxin and demethylated epimers. Podophyllotoxin and etoposide are shown. C-4 (*) substitutions of the glycosidic moiety on etoposide are shown for teniposide, TOP-53 and F14512.

Figure 5

Summary of etoposide substituents that interact with human topoisomerase IIα. Adapted from (Pitts et al., 2011). Protons that interact with the enzyme (as determined by saturation transfer difference NMR spectroscopy) are shown in red. The blue region on etoposide, including portions of the A–, B– and E–rings, has been proposed to interact with topoisomerase II in the binary drug-enzyme complex. E–ring substituents highlighted with yellow boxes are important for drug function and interact with the enzyme, but did not appear to contribute significantly to binding. It has been proposed that interactions between etoposide and DNA in the ternary complex (shaded in gray) are driven primarily by the D-ring, with additional contributions from the C-4 sugar.

Figure 6

Structures of selected bioflavonoids. Adapted from (Bandele and Osheroff, 2007). Flavones, flavonols, and isoflavones are shown, and the ability of each to enhance topoisomerase II-mediated DNA cleavage is indicated as >8-fold (+++), 6- to 8-fold (++), 3- to 6-fold (+), 2- to 3-fold (+/−), or <2-fold (−) over baseline.

Figure 7

Structures of EGCG and related catechins.

Figure 8

Structures of selected isothiocyanate-based topoisomerase II poisons.

Figure 9

Oxidative transformation of curcumin. Adapted from (Griesser et al., 2011)