Human papilloma virus 16 E6 protein as a target for curcuminoids, curcumin conjugates and congeners for chemoprevention of oral and cervical cancers.

Curcumin (diferuloyl methane) and its naturally occurring analogs viz. demethoxy, bisdemethoxy and cyclocurcumin, present in rhizomes of curcuma species turmeric, have been shown to inhibit the proliferation of a wide variety of tumor cells. Target nuclear protein HPV 16 E6 (PDB ID: 2fk4) is the major protein actively participating in oral and cervical cancers. In silico studies indicate that curcumin and its natural analogs have effective binding with different active sites on HPV 16 E6 protein, ideal target for restoring the tumor suppressor function of p53 and thus allowing the apoptosis of infected cells. The main limitation in the use of curcuminoids as therapeutic agents is their low bioavailability. Since piperine is known to enhance curcumin bioavailability to more than two thousand times by inhibiting its efflux, a conjugate of curcumin-piperic acid was also used. Although curcumin has been found to have strongest binding with this target and the two curcuminoids, demethoxy and bisdemethoxy curcumin have lower but comparable affinity, chlorogenic acid amongst the naturally occurring analogs has been found to have best binding affinity amongst all the analogs. Although curcumin-piperoyl conjugate does not show very encouraging results, it is likely to have potential activity in vitro and in vivo. These results throw light on the SAR of curcuminoids, leading to future designing of potent, non-toxic drugs for oral and cervical cancers.