title: Honokiol Inhibits DNA Polymerases β and λ and Increases Bleomycin Sensitivity of Human Cancer Cells.
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 nonhomologous 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 Kivalue of 4.0 μM for pol β, and 8.3 μM for pol λ, while the Kivalues for pol η and Kf were 20 and 26 μM, 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, and UACC903 cells, in which the EC50values were decreased 10-fold. The temozolomide and honokiol combination was less effective; the EC50values decreased three-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.
authors.affiliation: Department of Biochemistry and Molecular Biology, Milton S. Hershey Medical Center, Pennsylvania State University College of Medicine , Hershey, Pennsylvania 17033, United States.
authors.forename: A S Prakasha
authors.initials: AS
authors.lastname: Gowda
authors.affiliation: Department of Chemistry and Biochemistry, The Ohio State University , Columbus, Ohio 43210, United States.
authors.forename: Zucai
authors.identifier: 0000-0003-3871-3420
authors.initials: Z
authors.lastname: Suo
authors.affiliation: Department of Biochemistry and Molecular Biology, Milton S. Hershey Medical Center, Pennsylvania State University College of Medicine , Hershey, Pennsylvania 17033, United States.
authors.forename: Thomas E
authors.identifier: 0000-0002-6805-3729
authors.initials: TE
authors.lastname: Spratt
mesh_terms: D000903:Antibiotics, Antineoplastic / Q000494:pharmacology*; D001713:Biphenyl Compounds / Q000494:pharmacology*; D001761:Bleomycin / Q000494:pharmacology*; D045744:Cell Line, Tumor; D019951:DNA Polymerase beta / Q000037:antagonists & inhibitors*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D017705:Lignans / Q000494:pharmacology*; D000498:Allyl Compounds; D010636:Phenols
chemical_list: D000903:Antibiotics, Antineoplastic; D001713:Biphenyl Compounds; D001761:Bleomycin; D019951:DNA Polymerase beta; D004791:Enzyme Inhibitors; D017705:Lignans; C005499:honokiol; D000498:Allyl Compounds; D010636:Phenols; C415367:DNA polymerase beta2