title: Flavopiridol induces p53 via initial inhibition of Mdm2 and p21 and, independently of p53, sensitizes apoptosis-reluctant cells to tumor necrosis factor.
abstract: Flavopiridol (FP) inhibits gene expression and causes apoptosis, and these effects cannot be explained by inhibition of cyclin-dependent kinases that govern cell cycle. The simple and established notion that FP is an inhibitor of transcription predicts its effects. Because Mdm-2 targets p53 for degradation, FP, as predicted, dramatically induced p53 by inhibiting Mdm-2. Once p53 was induced, restoration of transcription (by removal of FP) resulted in superinduction of p21 and Mdm-2. Similarly, low concentrations of FP (50 nm) induced p21 and Mdm-2 because of their initial down-regulation. A sustained decrease of Mdm-2/p21 expression and accumulation of p53 coincided with near-maximal cytotoxicity of FP at concentrations >100 nm. Induction of p53 was a marker, not a cause, of cytotoxicity. FP caused rapid apoptosis (caspase-dependent cell death) in p53-null leukemia cells. In these cells, FP-induced apoptosis was converted to growth arrest by inhibitors of caspases. In apoptosis-reluctant A549 and PC3M cancer cells, FP inhibited cell proliferation but did not cause apoptosis. Like typical inhibitors of transcription, FP sensitized cells to apoptotic stimuli, allowing tumor necrosis factor to cause rapid and massive apoptosis in otherwise apoptosis-reluctant cells. We discuss that, as a reversible inhibitor of transcription, FP can be used clinically in novel rational drug combinations.
authors.affiliation: Brander Cancer Research Institute and Department of Medicine, New York Medical College, Valhalla, New York, USA.
authors.forename: Zoya N
authors.initials: ZN
authors.lastname: Demidenko
authors.forename: Mikhail V
authors.initials: MV
authors.lastname: Blagosklonny
mesh_terms: D017209:Apoptosis / Q000187:drug effects*; D045744:Cell Line, Tumor; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins / Q000037:antagonists & inhibitors* / Q000096:biosynthesis / Q000235:genetics; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D005419:Flavonoids / Q000494:pharmacology*; D045325:HCT116 Cells; D018922:HL-60 Cells; D006801:Humans; D019169:Jurkat Cells; D008175:Lung Neoplasms / Q000188:drug therapy / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D008297:Male; D009687:Nuclear Proteins / Q000037:antagonists & inhibitors*; D010880:Piperidines / Q000494:pharmacology*; D011471:Prostatic Neoplasms / Q000188:drug therapy / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D011518:Proto-Oncogene Proteins / Q000037:antagonists & inhibitors*; D051736:Proto-Oncogene Proteins c-mdm2; D014158:Transcription, Genetic / Q000187:drug effects; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology*; D016159:Tumor Suppressor Protein p53 / Q000096:biosynthesis* / Q000235:genetics; D020298:U937 Cells
chemical_list: C493956:CDKN1A protein, human; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D005419:Flavonoids; D009687:Nuclear Proteins; D010880:Piperidines; D011518:Proto-Oncogene Proteins; D014409:Tumor Necrosis Factor-alpha; D016159:Tumor Suppressor Protein p53; C077990:alvocidib; C497206:MDM2 protein, human; D051736:Proto-Oncogene Proteins c-mdm2