Protein kinase Cα-mediated cytotoxic activity of ineupatorolide B from Inula cappa DC. in HeLa cells

There is an ongoing search for plant-derived sesquiterpenes, particularly for those with anticancer activity against human cancer cells. The sesquiterpene ineupatorolide B (InB), isolated from the Inula cappa, showed potent growth-inhibitory activity against HeLa cells but less activity against MM1-CB melanoma cells. Staining by terminal deoxynucleotidyl transferase dUTP nick-end labeling method revealed that this activity was, at least in part, due to the induction of apoptosis. The activities of major transcription factors were examined by using a luciferase reporter assay. The results showed that the transactivation ability of nuclear factor of activated T-cell (NFAT) was enhanced. The activation of NFAT by InB was largely suppressed by preincubation with protein kinase C (PKC) inhibitors such as staurosporine and K252a. Western blot analysis revealed that the the levels of phosphorylated PKCα, but not other subtypes, increased after treatment with InB. Knockdown of PKCα using siRNA attenuated the cytotoxic activity of InB. Thus, InB may exhibit growth-inhibitory activity through the activation of PKCα, followed by an increase in NFAT transactivation ability.