Tatsuo Nakahara

Inhibitory effects of α-mangostin on mammalian DNA polymerase, topoisomerase, and human cancer cell proliferation.

We found that the ethanol extract of mangosteen (Garcinia mangostana L.) fruit rind had a strong inhibitory effect on mammalian DNA polymerase (pol) activity and isolated α-mangostin as a potent pol inhibitor from the extract. In this study, the inhibitory activities against mammalian pols by α-mangostin and its related five compounds, 3-isomangostin, xanthone, 9,10-anthraquinone, 9-anthracenecarboxylic acid, and anthracene, were investigated. α-Mangostin was the most potent inhibitor of the mammalian pol species among the tested compounds, with IC₅₀ values of 14.8-25.6 μM. This compound also inhibited human DNA topoisomerases (topos) I and II activities with IC₅₀ values of 15.0 and 7.5 μM, respectively, but did not inhibit the activities of other DNA metabolic enzymes tested. α-Mangostin also did not directly bind to double-stranded DNA as determined by thermal transition analysis. α-Mangostin was found to suppress human colon HCT116 carcinoma cell proliferation with an LC₅₀ of 18.5 μM, inhibit the activity of cellular topos, halt cell cycle in the G2/M phase, and induce apoptosis. These results suggest that decreased proliferation by α-mangostin may be a result of the inhibition of cellular topos rather than pols, and α-mangostin might be an anticancer chemotherapeutic agent.

The Establishment of an Assay to Measure DNA Polymerase-Catalyzed Repair of UVB-Induced DNA Damage in Skin Cells and Screening of DNA Polymerase Enhancers from Medicinal Plants

An in vitro assay method was established to measure the activity of cellular DNA polymerases (Pols) in cultured normal human epidermal keratinocytes (NHEKs) by modifying Pol inhibitor activity. Ultraviolet (UV) irradiation enhanced the activity of Pols, especially DNA repair-related Pols, in the cell extracts of NHEKs. The optimal ultraviolet B (UVB) exposure dose and culture time to upregulate Pols activity was 100 mJ/cm2 and 4-h incubation, respectively. We screened eight extracts of medicinal plants for enhancement of UVB-exposed cellular Pols activity using NHEKs, and found that rose myrtle was the strongest Pols enhancer. A Pols’ enhancement compound was purified from an 80% ethanol extract of rose myrtle, and piceatannol was isolated by spectroscopic analysis. Induction of Pol activity involved synergy between UVB irradiation and rose myrtle extract and/or piceatannol. Both the extract and piceatannol reduced UVB-induced cyclobutane pyrimidine dimer production, and prevented UVB-induced cytotoxicity. These results indicate that rose myrtle extract and piceatannol, its component, are potential photo-protective candidates for UV-induced skin damage.