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Honokiol Inhibits DNA Polymerases β and λ and Increases Bleomycin Sensitivity of Human Cancer Cells
A S Prakasha Gowda1, Zucai Suo2, Thomas E Spratt1
1Department of Biochemistry and Molecular Biology, Milton S. Hershey Medical Center, Pennsylvania State University College of Medicine , Hershey, Pennsylvania 17033, United States.
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 Ki value of 4.0 μM for pol β, and 8.3 μM for pol λ, while the Ki values 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 EC50 values were decreased 10-fold. The temozolomide and honokiol combination was less effective; the EC50 values 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.
Insights
Honokiol inhibits DNA repair polymerases, enhancing cancer cell sensitivity to DNA damaging agents like bleomycin. This combination therapy shows promise for reducing cancer treatment dosage and side effects.
Area of Science:
- Molecular Biology
- Cancer Research
- Pharmacology
Background:
- Cancer cells resist DNA damaging agents by employing DNA repair pathways.
- X-family DNA polymerases are crucial for base excision repair (BER) and nonhomologous end joining (NHEJ).
Purpose of the Study:
- To investigate honokiol's inhibitory effects on human DNA polymerase beta (pol β) and DNA polymerase lambda (pol λ).
- To evaluate honokiol's impact on the cytotoxicity of bleomycin and temozolomide in various cancer cell lines.
Main Methods:
- Kinetic analysis of purified pol β and pol λ inhibition by honokiol.
- Cytotoxicity assays combining honokiol with bleomycin or temozolomide in human cancer cell lines (A549, MCF7, PANC-1, UACC903) and normal lymphocytes (GM12878).
Main Results:
- Honokiol exhibited mixed-function noncompetitive inhibition of pol β (Ki = 4.0 μM) and pol λ (Ki = 8.3 μM).
- The combination of honokiol and bleomycin significantly enhanced cytotoxicity in MCF7, PANC-1, and UACC903 cells (10-fold decrease in EC50).
- The honokiol and temozolomide combination showed a less pronounced effect (3-fold decrease in EC50).
Conclusions:
- Honokiol effectively inhibits X-family DNA polymerases involved in DNA repair.
- The synergistic activity of honokiol with bleomycin suggests potential for combination cancer therapy.
- This approach may allow for reduced therapeutic doses and minimized side effects of bleomycin treatment.
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