Luteolin induces intrinsic apoptosis via inhibition of E6/E7 oncogenes and activation of extrinsic and intrinsic signaling pathways in HPV-18-associated cells

Luteolin, a flavonoid extracted from a number of plants with recognized anticancer, anti-inflammatory and anti-oxidative activities, inhibits angiogenic processes and modulates multidrug resistance. However, the efficacy and mechanisms of action of this flavonoid agent are still undergoing study. In order to elucidate whether luteolin exhibits an anticancer effect in cervical cancer cells, HeLa cells were incubated with luteolin and apoptosis was assessed by observing nuclear morphological changes, and performing Annexin V-fluorescein isothiocyanate (FITC)/propidium iodide (PI) staining. Cell cycle analysis, western blotting, RT-PCR and mitochondrial membrane potential measurements were also carried out. Luteolin showed a significant dose-dependent cytotoxic effect only in human papillomavirus (HPV)-positive cervical cancer cells, when compared to its effect on HPV-negative cervical cancer C33A cells. Expression levels of human papilloma virus E6 and E7 oncogenes were suppressed, those of related factors pRb and p53 were recovered and E2F5 was increased by luteolin treatment. Furthermore, luteolin enhanced the expression of death receptors and death receptor downstream factors such as Fast FasL, DR5/TRAIL and FADD in HeLa cells, and activated caspase cascades. In particular, luteolin enhanced the activity of caspase-3 and -8 in a dose-dependent manner. Activation of caspase-3 induced caspase-8 activity and vice versa. Luteolin also induced mitochondrial membrane potential collapse and cytochrome c release, and inhibited Bcl-2 and Bcl-xL expression. In conclusion, luteolin exerts anticarcinogenic activity through inhibition of E6 and E7 expression and cross-activation of caspase-3 and -8. Taken together, these results suggest that luteolin induces inactivation of HPV-18 oncogene expression and apoptosis by activating the intrinsic and extrinsic pathways.