title: Enhanced 7-ethyl-10-hydroxycamptothecin (SN-38) lethality by methylselenocysteine is associated with Chk2 phosphorylation at threonine-68 and down-regulation of Cdc6 expression.
abstract: Methylselenocysteine (MSC) is an organic selenium compound in preventative clinical trials involving prostate, lung, and colon carcinoma. We found that methioninase-activated MSC potentiates 7-ethyl-10-hydroxycamptothecin (SN-38)-induced cell lethality in vitro in the p53-defective human head and neck carcinoma A253 cells. Activated MSC increases chk2 phosphorylation at threonine-68 induced by SN-38, with no significant effect on chk1 phosphorylation. Cell cycle arrest induced by SN-38, however, was not abrogated or potentiated by MSC. These results suggest that the enhanced cellular lethality of SN-38 by MSC was not associated with cell cycle regulation pathways. Because chk2, in addition to its role in cell cycle arrest, can induce apoptosis by phosphorylation/activation, we examined whether increased chk2 phosphorylation could induce preapoptotic DNA fragmentation. DNA damage analysis showed that megabase DNA fragmentation is decreased, accompanied by the increased 30 to 300 kilobase pairs of DNA fragmentation after exposure to SN-38 with MSC, compared with SN-38 alone. No significant changes in the amount of DNA fragments were observed in cells treated with SN-38 or MSC alone. Moreover, proteolytic destruction of DNA replication-associated proteins cdc6, MCM2, and cdc25A may induce a DNA damage checkpoint response. The observed down-regulation of DNA replication proteins cdc6, MCM2, and cdc25A after exposure to SN-38 with MSC further indicates a relationship between drug response and DNA damage. Exposure to SN-38 with MSC resulted in a significant increase of poly(ADP-ribose) polymerasecleavage and caspase 3 activation. All together, the data support the hypothesis that enhanced lethality of this combination is associated with increased chk2 phosphorylation at Thr68 and down-regulation of specific DNA replication-associated proteins, which result in poly(ADP-ribose) polymerase cleavage, caspase 3 activation, and the induction of 30 to 300 kilobase pairs of DNA fragmentation.
authors.affiliation: Department of Pharmacology and Therapeutics, Grace Cancer Drug Center, Roswell Park Cancer Institute, Elm and Carlton Streets, Buffalo, NY 14263, USA.
authors.forename: Ming-Biao
authors.initials: MB
authors.lastname: Yin
authors.forename: Zhan-Rong
authors.initials: ZR
authors.lastname: Li
authors.forename: Shousong
authors.initials: S
authors.lastname: Cao
authors.forename: Farukh A
authors.initials: FA
authors.lastname: Durrani
authors.forename: Rami G
authors.initials: RG
authors.lastname: Azrak
authors.forename: Cheryl
authors.initials: C
authors.lastname: Frank
authors.forename: Youcef M
authors.initials: YM
authors.lastname: Rustum
mesh_terms: D000972:Antineoplastic Agents, Phytogenic / Q000494:pharmacology*; D002166:Camptothecin / Q000031:analogs & derivatives* / Q000494:pharmacology*; D013437:Carbon-Sulfur Lyases / Q000378:metabolism; D053148:Caspase 3; D020169:Caspases / Q000378:metabolism; D002453:Cell Cycle / Q000187:drug effects; D018797:Cell Cycle Proteins / Q000378:metabolism; D002455:Cell Division / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects; D000071877:Checkpoint Kinase 1; D064447:Checkpoint Kinase 2; D003545:Cysteine / Q000031:analogs & derivatives* / Q000494:pharmacology*; D053938:DNA Fragmentation / Q000187:drug effects; D004347:Drug Interactions; D004789:Enzyme Activation; D015870:Gene Expression / Q000187:drug effects; D006801:Humans; D000077146:Irinotecan; D064111:Minichromosome Maintenance Complex Component 2; D009687:Nuclear Proteins / Q000378:metabolism; D016566:Organoselenium Compounds / Q000494:pharmacology*; D010766:Phosphorylation / Q000187:drug effects; D011065:Poly(ADP-ribose) Polymerases / Q000378:metabolism; D011494:Protein Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins; D017279:Selenocysteine / Q000031:analogs & derivatives; D020687:cdc25 Phosphatases / Q000378:metabolism
chemical_list: D000972:Antineoplastic Agents, Phytogenic; C066503:CDC6 protein, S cerevisiae; D018797:Cell Cycle Proteins; D009687:Nuclear Proteins; D016566:Organoselenium Compounds; D029701:Saccharomyces cerevisiae Proteins; D017279:Selenocysteine; D000077146:Irinotecan; C028098:10-hydroxycamptothecin; D011065:Poly(ADP-ribose) Polymerases; D011494:Protein Kinases; D064447:Checkpoint Kinase 2; C000605867:CHEK1 protein, human; C578634:CHEK2 protein, human; D000071877:Checkpoint Kinase 1; D017346:Protein Serine-Threonine Kinases; C517004:CDC25A protein, human; D020687:cdc25 Phosphatases; C505407:CASP3 protein, human; D053148:Caspase 3; D020169:Caspases; C084859:MCM2 protein, human; D064111:Minichromosome Maintenance Complex Component 2; D013437:Carbon-Sulfur Lyases; C021558:L-methionine gamma-lyase; D003545:Cysteine; C002979:selenomethylselenocysteine; D002166:Camptothecin