We demonstrate for the first time that TR and AtPAP1 root extracts of Leonurus sibiricus containing high amounts of phenolic acid may activate specific molecular signalling pathways and significantly interfere with the survival and proliferation of grade IV glioma cells and U87MG cancer cells by two routes: firstly, by inhibition of PARP activity, which increases the susceptibility of cells to DNA damaging agents, possibly by preventing DNA strand break rejoining; secondly, by decreasing UHRF1 and DNMT1 expression, thus influencing epigenetic regulation. Here, DNMT1 is linked to cancer.