Why do flavonoid topoisomerase II poisons preferentially kill cancer cells?

Fisetin, quercetin, genistein and some other plant-derived flavonoids have been shown to exhibit antiproliferative effects in several tumor cell lines primarily through activation of the programmed cell death pathways. For example fisetin mechanisms are inhibition of Cdk1/2/4/6, nuclear factor-kappa B and Topoisomerase II activities. The topimerase inhibition (or alternatively induction) has however also raised concerns about carcinogenic side effects.

So called Topoisomerase poisons are widely used as chemotherapy treatments for certain cancers. These cancer drugs either induce or inhibit topo-DNA complexes with either topo I or topo II that eventually trigger cell death. However the poisoning can also induce DNA alterations that may lead to carcinogenic effects, an increased risk for developing leukemia has been observed in patients treated with some topo II inhibitor drugs.

Therefore considering that all cells in the human body contain topoisomerases an obvious question is: why would flavonoid topoisomerase posions preferentially kill cancer cells without adversely impacting normal cells?

Topoisomerases are enzymes that regulate the overwinding or underwinding of DNA. Topoisomerase I is an essential enzyme that relaxes DNA supercoiling during replication and transcription. Topo I makes DNA single-strand breaks that allows rotation of the cleaved strand around the double helix axis. Topo I also religates the cleaved strand to reestablish intact duplex DNA. Treatment with Topo I inhibitors stabilize the cleaved complexes and prevent DNA religation leading to permanent lethal DNA strand breaks. Topoisomerase II cuts both strands of one DNA double helix, passes another unbroken DNA helix through it, and then reanneals the cut strands. The so called Topo II poisons and inducers encourage the forward cleavage reaction, if the number of DNA breaks overwhelms the repair process, it can initiate cell death pathways. Other poisons prevent the re-ligation of DNA. Then there are the Topo II inhibitors which disturb the critical balance of cleavage complexes. If Topo II cleavage drops below threshold levels, daughter chromosomes remain entangled following replication. As a result, chromosomes cannot segregate properly, and cells die as a result of catastrophic mitotic failure. If cells are not killed, malfunctions can be converted to permanent chromosomal translocation that lead to specific forms of leukemia.

In studies several flavonoids have been shown to interact with purified topo I and topo II which has led to the suggestion that these compounds may possess both anticancer and carcinogenic activity. Because the activity of a drug on purified topoisomerases does not always represent the activity in the cell, some studies have been setup over the years to evaluate the effects of several common dietary flavonoids on these enzymes in cells.

In this study the effects of topoisomerase poisoning by genistein in the intestine of rats was studied. The results showed that in case of a lifelong dietary isoflavone exposure, no effects on the stabilization of cleavage complexes was observed, except a slight increase of topoisomerase IIα-DNA-complexes in the colon. However injection of genestein clearly showed topoisomerase poisoning properties in vivo. The researchers reasoned that this might be attributed to the higher serum concentration of the genistein aglycon due to circumvention of first-pass metabolism compared to oral consumption of an isoflavone-rich diet.

Another (in-vitro) study into the anti-cancer effects of Fisetin noted that:
“Interestingly, the non-carcinogenic MCF-10A cells were less sensitive to the fisetin-induced cell killing, which proposes that cell line-specific variations exists either in the mode of action of fisetin, in its pharmacokinetics, or in cellular processes of the drug response pathways.”

In this article the authors saw the following mechanisms as basis for the selectivity of topoisomerase II poisons:

– The mechanism in topo II poisons is the generation of DNA strand breaks and the higher the cellular level of topo II, the more lethal they become. Because most cancer cells proliferate rapidly as compared to healthy cells they contain higher levels of type II enzymes. Therefore, treatment with topo II poisons generates more DNA strand breaks and induces greater toxicity in cancer cells.

– The DNA breaks generated by topo II are converted to permanent strand breaks upon collision with DNA tracking machinery, such as replication and transcription complexes. Since cancer cells are distinguished by high rates of metabolism and frequent replication, cleavage complexes stabilized by topo II poisons in these cells are more likely to be converted to permanent (and potentially lethal) DNA strand breaks.

– Because cancer cells already display characteristics of genomic instability and impaired DNA damage response pathways they are more susceptible than normal cells to the effects of DNA damaging agents, such as topoisomerase II poisons.

While this does not entirely answer the question whether or not flavonoids that have topoisomerase poison characteristics may cause carcinogenic effects it does suggest that they may preferentially affect cells that are progressing towards malignancy. Overall it also suggests a dose dependency and that mega dosing is wise to be avoided till more about the selectivity mechanisms is known. A special note and warning wrt supplementation during pregnancy: epidemiological studies indicate that the risk of developing infant leukemias increases >3-fold by the maternal consumption (during pregnancy) of foods that are rich in bioflavonoids and other naturally occurring topoisomerase II poison.

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