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Full-Time

Tsai-Kun Li Professor

聯絡電話:886-2-2312-3456 ext. 88287or 88294

E-mail: tsaikunli@ntu.edu.tw

Personal File

Field:

DNA topology, topoisomerase and its targeting agents

Education:

美國羅格斯大學藥理學系

Title:

台大醫學院微生物學科助理教授 (2011.08)
大醫學院微生物學科副教授 (2007.08-2011.07)
台大醫學院微生物學科助理教授 (2003.02-2007.08)
UMDNJ Pharmacology Research Teaching Specialist III
(2000/ 01-2003/ 01)
UMDNJ Pharmacology Postdoc. (1999/ 05-1999/12)

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Current Research

Topoisomerases, Topoisomerase-targeting drugs, DNA damage and repair.

Research Fields:

Ø DNA topology and its biological implications

Ø Topoisomerases and their targeting drugs

Ø Repair and signaling pathways for DNA damage

Research Description:

The research conducted in my laboratory combines both molecular and biochemical approaches toward investigating the following interrelated programs of research:  

1. Roles of DNA topoisomerases in DNA organization: Topoisomerases are ubiquitous, essential nuclear enzymes that participate in the regulation of DNA different topological states by transient breakage and rejoining of DNA. Cellular functions of different topoisomerases are currently under investigation. 

a. Regulation of R-loop: Activation-induced deaminase (AID) and transcription have been tightly associated with the recombination events that lead to antibody diversity. In addition, ssDNA in R-loop has been demonstrated to be the substrate for AID. We have developed an E. coli model system to study the roles of DNA topoisomerases and recombination enzymes in regulating the R-loop-associated, AID-mediated DNA recombination.  

b. DNA topoisomerase II (TOP2) in higher-order nucleoid structure: Combining genetic and molecular approaches, our recent results have indicated that bacterial nucleoids are also organized into form of DNA loops (~ 50-100 kb in size) similar to that of mammalian chromosomes. Both of TOP2 isozymes, gyrase and Topo IV, participate in the supramolecular looped arrangement of nucleoid DNA with gyrase plays a more significant role. Furthermore, RNA interference (RNAi) has been utilized to study the relative contribution of TOP2 isoforms in chromatin loops organization of human cells. 

c. Functional studies of DNA topoisomerase III (TOP3): RNAi, Yeast 2-hybrid screen and biochemical purification have been employed to investigate the functional roles of hTOP3a and hTOP3b. We currently focus our effect on the potential roles of TOP3 in cell cycle checkpoint and chromatin maintenance. We have underscored checkpoint deficiencies and aneuploidy phenotype in hTOP3a RNAi-knockdown cell lines. The molecular mechanisms underlying these phenotypes are being actively pursued.  

2. Studies on topoisomerase-mediated DNA damage: Due to its delicate act of breaking/rejoining DNA, topoisomerase is highly vulnerable while performing its enzymatic reaction on DNA. In agreement with this notion, DNA topoisomerases have been firmly established as highly effective molecular targets for antibiotics (e.g. quinolones) and anti-tumor drugs (e.g. camptothecins). Our lab also interested in understanding the molecular determinants for topoisomerase-targeting conditions. 

a. Processing events downstream of protein-capped DNA damage: Topoisomerase- targeting drugs kill cells by trapping a key covalent enzyme-DNA reaction intermediate named the reversible cleavable complex. Unknown cellular processing pathways are needed for this unique type of protein-capped DNA breakage needs to expose the real damage. We have recently identified two molecularly distinct pathways, transcription (TIP)- and replication (RIP)-initiated processing, downstream of cleavable complexes. The roles of these two novel pathways in the repair and signaling of cleavable complexes are being investigated.

b. Bacterial cell killing mechanism of hydrogen peroxide: Oxidative stress, such as generation of hydrogen peroxide (H2O2), has been implicated to play important role in many cellular processes, such as macrophage-mediated phagocytic effect. We sought to determine the involvement of gyrase and Topo IV in H2O2-meidated cell death both at DNA and signaling levels. Pulsed-field gel electrophoresis (PFGE) reveals that H2O2 induced loop-sized, high-molecular-weight (HMW) nucleoid DNA fragments. Both bacterial TOP2s play roles in the generation of HMW DNA fragments induced by H2O2. 

c. Topoisomerases in ischemia acidosis: Acidic pH, possibly through protonization/deprotonization of TOP2, can induce DNA breaks. We employed different approaches, such as comet assay, pulsed-field gel electrophoresis (PFGE) and Western blot analysis, coupled with RNAi-knockdown cell lines to determine the differential roles of two TOP2 isoforms in ischemia-related acidotic apoptosis.  

3. Post-translational modification of DNA topoisomerases: Modification and proteolytic pathways have been suggested to participate in regulating functions and activities of DNA topoisomerases. For example, ubiqutin/26S proteasome pathway has been implicated to degrade topoisomerases from cleavable complexes.  

a. Calpain-mediated proteolytic processing of DNA topoisomerases: Divalent cations are required for many enzymatic activities. We found that calcium (Ca2+) activates calpain proteases, which then cleave hTOP1, hTOP2b, but not hTOP2a. Our studies not only provide the first demonstration of calpain-mediated, preferential proteolysis on topoisomerases, but might also provide a system for probing the activation pathway(s) of calpains to cleave nuclear proteins.  

b. Modulation of cellular location of DNA topoisomerases by calcium and their SUMOylation states: SUMOylation status has been associated with nucleolus location of DNA TOP1. We have demonstrated that calcium influx causes general deSUMOylation of cellular proteins. The change in SUMOylation status of DNA topoisomerases also correlated with their cellular localizations. We speculate that the dynamic shuttling of DNA topoisomerase might closely dependent on its SUMOylation status which is associated with intracellular calcium concentration.

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Staff members

博士班研究生:謝美儀、葉彥秀、李玠樺、楊育臻
碩士班研究生:林允飛、李煦羚、鞏馬克、劉至堯、許凰鈴
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Selected publications

五年內著作目錄 李財坤 (Tsai-Kun Li)
1. Wang YW, Hong TW, Tai YL, Wang YJ, Tsai SH, Lien PT, Chou TH, Lai JY, Chu R, Ding ST, Irie K, Li TK.*, Tzean SS*, Shen TL.* Evaluation of an Epitypified Ophiocordyceps formosana (Cordyceps s.l.) for Its Pharmacological Potential. Evid Based Complement Alternat Med. 2015;2015:189891.
2. Chou SM, Lai WJ, Hong T, Tsai SH, Chen YH, Kao CH, Chu R, Shen TL*, Li TK.* Involvement of p38 MAPK in the Anticancer Activity of Cultivated Cordyceps militaris. Am J Chin Med. Am J Chin Med. 2015;43(5):1043-57.
3. Hsieh MH, Tsai CH, Lin CC, Li TK, Hung TW, Chang LT, Hsin LW, Teng SC.* Topoisomerase II inhibition suppresses the proliferation of telomerase-negative cancers. Cell Mol Life Sci. 2015 May;72(9):1825-37.
4. Chou SM†, Lai WJ†, Hong TW, Lai JY, Tsai SH, Chen YH, Yu SH, Kao CH, Chu R, Ding ST, Li TK.*, Shen TL.* Synergistic property of cordycepin in cultivated Cordyceps militaris-mediated apoptosis in human leukemia cells. Phytomedicine. 2014 Oct 15;21(12):1516-24. (†, equal contribution)
5. Hsieh MY, Fan JR, Chang HW, Chen HC, Shen TL, Teng SC, Yeh YH, Li TK.* DNA topoisomerase III alpha regulates p53-mediated tumor suppression. Clin Cancer Res. 2014 Mar 15;20(6):1489-501.
6. Wu CC, Li YC, Wang YR, Li TK.*, Chan NL.* On the structural basis and design guidelines for type II topoisomerase-targeting anticancer drugs. Nucleic Acids Res. 2013 Dec 1;41(22):10630-40.
7. Yeh YH, Yang YC, Hsieh MY, Yeh YC, Li TK.* A negative feedback of the HIF-1α pathway via interferon-stimulated gene 15 and ISGylation. Clin Cancer Res. 2013 Nov 1;19(21):5927-39.
8. Lin BC, Su LH, Weng SC, Pan YJ, Chan NL, Li TK, Wang HC, Sun CH. DNA topoisomerase II is involved in regulation of cyst wall protein genes and differentiation in Giardia lamblia. PLoS Negl Trop Dis. 2013 May 16;7(5):e2218.
9. Yang YC, Chou HY, Shen TL, Chang WJ, Tai PH, Li TK.* Topoisomerase II-mediated DNA cleavage and mutagenesis activated by nitric oxide underlie the inflammation-associated tumorigenesis. Antioxid Redox Signal. 2013 Apr 1;18(10):1129-40.
10.Chen YJ, Chen CC, Li TK, Wang PH, Liu LR, Chang FY, Wang YC, Yu YH, Lin SP, Mersmann HJ, Ding ST. Docosahexaenoic acid suppresses the expression of FoxO and its target genes. J Nutr Biochem. 2012 Dec;23(12):1609-16.
11.Wang JS, Chang YL, Yu YH, Chen CY, Kao MC, Li TK, Lin WW. Cell type-specific effects of Adenosine 5'-triphosphate and pyrophosphate on the antitumor activity of doxorubicin. Cancer Sci. 2012 Oct;103(10):1811-9.
12.Hsu JL, Ho YF, Li TK, Chen CS, Hsu LC, Guh JH. Rottlerin potentiates camptothecin-induced cytotoxicity in human hormone refractory prostate cancers through increased formation and stabilization of topoisomerase I-DNA cleavage complexes in a PKCδ-independent pathway. Biochem Pharmacol. 2012 Jul 1;84(1):59-67. Epub 2012 Apr 2.
13.Wang LY, Lin SS, Hung TH, Li TK, Lin NC, Shen TL. Multiple domains of the tobacco mosaic virus p126 protein can independently suppress local and systemic RNA silencing. Mol Plant Microbe Interact. 2012 May;25(5):648-57.
14.Lee CH, Hsieh MY, Hsin LW, Chen HC, Lo SC, Fan JR, Chen WR, Chen HW, Chan NL*, Li TK.* Anthracenedione-methionine conjugates are novel topoisomerase II-targeting anticancer agents with favorable drug resistance profiles. Biochem Pharmacol. 2012 May 1;83(9):1208-16.
15.Chen MC, Pan SL, Shi Q, Xiao Z, Lee KH, Li TK, Teng CM. QS-ZYX-1-61 induces apoptosis through topoisomerase II in human non-small-cell lung cancer A549 cells. Cancer Sci. 2012 Jan;103(1):80-7.
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