Honokiol inhibits DNA polymerases β and λ and increases bleomycin sensitivity of human cancer cells
A. S. Prakasha Gowda, Zucai Suo, and Thomas E Spratt
Chem Res Toxicol (2107) ASAP article

Proposed mechanisms by which honokiol sensitizes cancer cells to bleomycin

Proposed mechanisms by which honokiol sensitizes cancer cells to bleomycin

Abstract.

A major concept to sensitize cancer cells to DNA damaging agents is by inhibiting proteins in the DNA repair pathways.  X-Family DNA polymerases play critical roles in both base excision repair (BER) and non-homologous end joining (NHEJ).   In this study, we examined the effectiveness of honokiol to inhibit human DNA polymerase β (pol β), which is involved in BER, and DNA polymerase λ (pol λ), which is involved in NHEJ.  Kinetic analysis with purified polymerases showed that honokiol inhibited DNA polymerase activity.  The inhibition mode for the polymerases was a mixed-function noncompetitive inhibition with respect to the substrate, dCTP.  The X-family polymerases, pol β and pol λ were slightly more sensitive to inhibition by honokiol based on the Ki value of 4.0 µM for pol β, and 8.3 µM for pol λ, while the Ki values for pol h and Kf were 20 and 26 mM, respectively.  Next we extended our studies to determine the effect of honokiol on the cytotoxicity of bleomycin and temozolomide in human cancer cell lines A549, MCF7, PANC-1, UACC903 and normal blood lymphocytes (GM12878).  Bleomycin causes both single strand DNA damage that is repaired by BER and double strand breaks that are repaired by NHEJ, while temozolomide causes methylation damage repaired by BER and O6-alkylguanine-DNA alkyltransferase.  The greatest effects were found with the honokiol and bleomycin combination in MCF7, PANC-1, UACC903 cells, in which the EC50 values were decreased 10-fold.  The temozolomide and honokiol combination was less effective; the EC50 values decreased 3-fold due to the combination.  It is hypothesized that the greater effect of honokiol on bleomycin is due to inhibition of the repair of the single strand and double strand damage.  The synergistic activity shown by the combination of bleomycin and honokiol suggests that they can be used as combination therapy for treatment of cancer, which will decrease the therapeutic dosage and side effects of bleomycin.