title: Wild-type and mutant p53 mediate cisplatin resistance through interaction and inhibition of active caspase-9.
abstract: The p53 gene has been implicated in many cancers due to its frequent mutations as well as mutations in other genes whose proteins directly affect p53's functions. In addition, high expression of p53 [wild-type (WT) or mutant] has been found in the cytoplasm of many tumor cells, and studies have associated these observations with more aggressive tumors and poor prognosis. Cytoplasmic mis-localization of p53 subsequently reduced its transcriptional activity and this loss-of-function (LOF) was used to explain the lack of response to chemotherapeutic agents. However, this hypothesis seemed inadequate in explaining the apparent selection for tumor cells with high levels of p53 protein, a phenomenon that suggests a gain-of-function (GOF) of these mis-localized p53 proteins. In this study, we explored whether the direct involvement of p53 in the apoptotic response is via regulation of the caspase pathway in the cytoplasm. We demonstrate that p53, when present at high levels in the cytoplasm, has an inhibitory effect on caspase-9. Concurrently, knockdown of endogenous p53 caused an increase in the activity of caspase-9. p53 was found to interact with the p35 fragment of caspase-9, and this interaction inhibits the caspase-9 activity. In a p53-null background, the high-level expression of both exogenous WT and mutant p53 increased the resistance of these cells to cisplatin, and the data showed a correlation between high p53 expression and caspase-9 inhibition. These results suggest the inhibition of caspase-9 as a potential mechanism in evading apoptosis in tumors with high-level p53 expression that is cytoplasmically localized.
authors.affiliation: Deparment of Biological Sciences, National University of Singapore, Singapore. cheej@gis.a-star.edu.sg
authors.forename: Jacqueline L Y
authors.initials: JL
authors.lastname: Chee
authors.forename: Suzan
authors.initials: S
authors.lastname: Saidin
authors.forename: David P
authors.initials: DP
authors.lastname: Lane
authors.forename: Sai Mun
authors.initials: SM
authors.lastname: Leong
authors.forename: Jacqueline E
authors.initials: JE
authors.lastname: Noll
authors.forename: Paul M
authors.initials: PM
authors.lastname: Neilsen
authors.forename: Yi Ting
authors.initials: YT
authors.lastname: Phua
authors.forename: Hani
authors.initials: H
authors.lastname: Gabra
authors.forename: Tit Meng
authors.initials: TM
authors.lastname: Lim
mesh_terms: D017209:Apoptosis / Q000235:genetics*; D053453:Caspase 9 / Q000378:metabolism*; D061945:Caspase Inhibitors / Q000378:metabolism / Q000494:pharmacology; D045744:Cell Line, Tumor; D002945:Cisplatin; D003593:Cytoplasm / Q000378:metabolism; D004351:Drug Resistance / Q000235:genetics*; D006801:Humans; D015151:Immunoblotting; D009154:Mutation / Q000235:genetics; D016159:Tumor Suppressor Protein p53 / Q000235:genetics* / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D061945:Caspase Inhibitors; D016159:Tumor Suppressor Protein p53; D053453:Caspase 9; D002945:Cisplatin