Caspase independent apoptosis induced by Methanolic extract of Hibiscus sabdariffa L.

Abstract

Hibiscus sabdariffa L. is a polyphenol rich plant that is native to Africa and extract of hibiscus calyces is known for its health beneficial effects in various area inculding antioxidant, antiobesity and anticancer. It is an edible plant that is actually taken as beverages in daily life. Recent novel findings have shown that multiple polyphenols found in extract of hibiscus possess anticancer effect in various cancer cells. It is however still remained unclear how the extract of hibiscus can orchestrate complecate intertwining pathways to induce selective cell death. In the present study, the anticancer activity of methanolic extract obtained from calyces of hibiscus sabdariffa L. (HME) was demonstrated with bio-assays. The apoptotic cell death was selectively induced only for cancer cells but not in normal cells both in vitro and in vivo. Furthermore, HME induced cell death did not exhibit activation of caspase 3, despite the fact that morphological and biochemical characteristics such as DNA fragmentation and mitochondrial dysfunction showed that cells clearly undergo apoptosis.

The mechanisms of which HME mediate apoptosis is through mitochondrial death pathway. Herein, the expression of various pathways were examined in time dependent manner in order to see how HME induced apoptosis is progressed. The activation of both proapoptotic and antiapoptotic proteins in MAP kinases, Akt and Bcl-2 families were evaluated through the course of study. HME induced apoptosis via dual regulation of accumulating proapoptotic proteins such as JNK, p38MAPK, Bim while attenuating antiapoptotic ones. Subsequently, these cascade of signaling activation induced the release of cytochrome c from mitochondria accompanied by mitochondrial dysfunction, which in turn upregulated translocaton of apoptosis inducing factor (AIF) into nucleus. Taken together, these findings indicated that signaling cascades induced from MAPK and Akt pathways altered mitochondrial membrane potential via mediating Bcl-2 family protein activation and releasing cytochrome c. As a consequence, the translocation of AIF played crucial role in HME-induced caspase independent apoptosis in human leukemic U937 cell.