Ovarian cancer has an enrichment of cancer stem cells (CSCs) which contribute to the treatment resistant tumor's high rate of recurrence and metastasis. Here we investigated 2 plant extracts from the medicinal plants Pao Pereira (Pao) and Rauwolfia vomitoria (Rau) each for their activities against ovarian CSCs. Both Pao and Rau inhibited overall proliferation of human ovarian cancer cell lines with IC50 ranging from 210 to 420 μg/mL and had limited cytotoxicity to normal epithelial cells. Ovarian CSC population was examined using cell surface markers and tumor spheroid formation assays. The results showed that both Pao and Rau treatment significantly reduced the ovarian CSC population. Pao and Rau had similar activities in inhibiting ovarian CSCs, with IC50s of ~120 μg/mL for 24 hours treatment, and ~50 μg/mL for long-term tumor spheroid formation. Nuclear β-catenin levels were decreased, suggesting suppression of Wnt/β-catenin signaling pathway. Taken together, data here showed that Pao and Rau both inhibited ovarian cancer stem cells, probably in preference to the bulk of tumor cells. Further mechanistic studies and in vivo investigation validating these findings are warranted, given that inhibition of cancer stem cells holds the promise of comprehensively inhibiting cancer metastasis, drug resistance and recurrence.
Keywords: Pao Pereira; Rauwolfia vomitoria; cancer stem cells; medicinal plant; natural product; ovarian cancer.