Inhibition of DNA Topoisomerase Ι by Flavonoids and Polyacetylenes Isolated from <i>Bidens pilosa</i> L.

Abstract

Human DNA topoisomerase I (Topo I) is an essential enzyme in regulating DNA supercoiling during transcription and replication, and it is an important therapeutic target for anti-tumor agents. <i>Bidens pilosa</i> L. is a medicinal herb that is used as a folk medicine for cancers in China. A new flavonoid (<b>1</b>) and a new polyacetylene (<b>20</b>), along with eighteen flavonoids (<b>2</b>-<b>19</b>) and nine polyacetylenes (<b>21</b>-<b>29</b>), were isolated and identified from the methanol extract of the whole plant of <i>B. pilosa</i>, and some of the compounds (<b>4</b>, <b>5</b>, <b>6</b> and <b>7</b>) exhibited potent cytotoxicity against a panel of five human cancer cell lines. The DNA relaxation assay revealed that some flavonoids and polyacetylenes exerted inhibitory activities on human DNA Topo I, among them compounds <b>1</b>, <b>2</b>, <b>5</b>, <b>6</b>, <b>7</b>, <b>8</b>, <b>15</b>, <b>19</b>, <b>20</b>, <b>22</b>, and <b>24</b> were the most active ones, with IC₅₀ values of 393.5, 328.98, 145.57, 239.27, 224.38, 189.84, 89.91, 47.5, 301.32, 178.03, and 218.27 μM, respectively. The structure-activity analysis of flavonoids was performed according to the results from the Topo I inhibition assay. The DNA content analysis revealed that <b>5</b>, <b>6</b>, and <b>7</b> potently arrested cell cycle at the G₁/S and G₂/M phases in human colon cancer cell DLD-1 depending on the concentration of the inhibitors. The levels of protein expression related to the G₁/S and G₂/M cell cycle checkpoints were in accordance with the results from the DNA content analysis. These findings suggest that flavonoids are one of the key active ingredients accounting for the anti-tumor effect of <i>B. pilosa</i>.