Abstract

Ethnopharmacological relevance: The search for effective herbal medicines for complementary treatments is on the rise due to the high incidence of recurrence and mortality rate in human oral cancer. Rhinacanthus nasutus KURZ., an annual herb found mostly in Southeast Asia including Thailand, has been wildly used as a traditional folk medicine for the treatment of several diseases including cancer. However, the anti-cancer effect of Rhinacanthin-C (Rh–C) as a major naphthoquinone compound found in R. nasutus and the underlying mechanism of its action on human oral cancer cells remain unknown. Aim of the study: To investigate the anti-cancer mechanism of Rh–C extracted from R. nasutus in human oral cancer cells. Materials and methods: The anti-proliferative effect of Rh–C on human oral squamous cell carcinoma (HSC4) was determined and compared to normal oral cells (human gingival fibroblasts, HGF, and normal oral keratinocytes, NOK) using the SRB colorimetric method. The molecular mechanism of Rh–C was explored using flow cytometry, colorimetric assay, in vitro human topoisomerase II assay, and Western blotting. Results: Rh–C displayed a time- and concentration-dependent growth inhibition on HSC4 and was much less effective on both tested normal oral cells. Rh–C inhibited Akt phosphorylation whereas over-activated p38 MAPK phosphorylation in HSC4 but not in HGF. Rh–C also inhibited topoisomerase II activity. As a result, the cell cycle was arrested in S-phase as the expression of CDK1/2 and Cyclin A2 was decreased. Eventually, the induction of HSC4 cell apoptosis was mediated by increased caspase 3 activity. Conclusions: Rh–C isolated from R. nasutus possesses anti-cancer properties on human oral cancer cells by causing the S arrest and the apoptotic induction via modulating Akt/p38 signaling pathways. The results provide molecular bases for further developing Rh–C as a potential drug candidate or a complementary treatment for oral cancer.