Abstract

Due to their RNA-N-glycosidase activity, ribosome-inactivating proteins (RIPs) are attractive candidates as antitumor and antiviral agents in biomedical and agricultural research. We have isolated and characterized two such proteins, foetidissimin II and texanin, from two Cucurbitaceae species. Foetidissimin II, obtained from the roots of Cucurbita foetidissima, was identified as a type-2 RIP, with a molecular weight of 61 kDa, as estimated by gel electrophoresis. It is composed of two chains, a 29-kDa chain A, and a 32-kDa chain B. Texanin, isolated from the fruits of Cucurbita texana, is a type-I RIP, with a single chain of molecular weight 29.7 kDa, as estimated by MALDI-TOF-MS. Both proteins exhibit RNA-N-glycosidase activity, with aniline playing a critical role in rRNA cleavage. The IC₅₀ value of foetidissimin II, determined by cell-free protein-synthesis inhibition, was 0.251 μM. In an in vitro cytotoxicity assay, foetidissimin II exhibited IC₅₀ values of ca. 70 nM to both adenocarcinoma and erythroleukemia cells. Texanin exhibited a weaker anticancer activity against erythroleukemia cells, with an IC₅₀ value of 95 μM, but no activity against adenocarcinoma cells. The N-terminal sequences of both proteins were compared with those of reported RIPs.

Doxorubicin is an anticancer drug that causes apoptosis in cells, but cardiotoxicity limits the cumulative dose that can remain in the blood. Echinacea extracts have been prescribed to supplement cancer chemotherapy. In a recent study, it was reported that Echinacea purpurea extracts protected noncancerous cells from apoptosis. Our study aimed to determine interference with doxorubicin chemotherapy, and if fractions and compounds from Echinacea angustifolia roots protected the cells. Cervical and breast cancer cells were treated with the Echinacea samples and doxorubicin. At 0.05 and 0.5 μM doxorubicin concentration, cynarine increased HeLa cell growth by 48–125% and 29–101%, respectively (p<0.01). At 0.05 μM doxorubicin concentration, chicoric acid increased cell growth by 23–100% (p<0.01). When MCF-7 cells were treated with Echinacea and doxorubicin, the ethyl acetate fraction increased cell growth by 20–25%, and chicoric acid increased cell growth by 10–15%. Cynarine showed proliferative activity on HeLa cells, but showed antiproliferative activity on MCF-7 cells. Results indicate that phenolic compounds are responsible for proliferative activity. Studies with individual compounds show that chicoric acid and cynarine interfered with cells treated with 0.5 μM doxorubicin. The results of this study show that Echinacea herbal medicines affect cell proliferation despite cancer treatment, and that herbal medicines require further study with respect to anticancer drugs.