Flavopiridol, a cyclin-dependent kinase inhibitor, enhances radiosensitivity of ovarian carcinoma cells.
Flavopiridol, a cyclin-dependent kinase (cdk) inhibitor, can cause cell cycle arrest, induce apoptosis in cancer cells, and inhibit tumor cell growth in vivo. The present study investigated the in vitro radiosensitizing effect of flavopiridol and the underlying molecular mechanisms in a murine ovarian cancer cell line, OCA-I. Flavopiridol inhibited cell growth in a dose-dependent manner and enhanced cell radiosensitivity assessed by the clonogenic cell survival assay. A flavopiridol dose of 300 nM, given for 1 day, enhanced radiosensitivity by a factor of 2.1. Clonogenic cell survival after split-dose radiation showed that flavopiridol inhibited repair from radiation damage. In addition, flavopiridol treatment (300 nM, 1 day) resulted in decreased levels of Ku70 and Ku86 proteins that play a role in DNA repair processes, suggesting that DNA repair processes may have been disrupted by this agent. Flow cytometry analysis showed that flavopiridol (300 nM, 1 day) accumulated the cells in G(1) and G(2) phases, with a significant reduction in the S phase component. This cell cycle redistribution is likely another mechanism underlying flavopiridol-induced cell radiosensitivity. Flavopiridol down-regulated cyclin D1 and cyclin E protein levels and also inhibited phosphorylation of retinoblastoma protein, which is inconsistent with the observed cell cycle arrest. Among the cdks tested, cdk-9, the catalytic subunit of positive transcription elongation factor b, was significantly down-regulated by flavopiridol, suggesting that flavopiridol may modulate cellular transcription processes. Furthermore, flavopiridol on its own induced apoptosis in the OCA-I cells, whereas in combination with radiation, exerted no additional increase in apoptosis. Taken together, our data show that flavopiridol strongly augmented the response of ovarian carcinoma cells to radiation and that the underlying mechanisms included inhibition of sublethal DNA damage repair and cell cycle redistribution. At the molecular level, transcriptional regulation by flavopiridol may have been involved.
Raju
Uma
U
Department of Experimental Radiation Oncology, The University of Texas, M. D. Anderson Cancer Center, Houston, Texas 77030, USA. uraju@mdanderson.org
Nakata
Eiko
E
Mason
Kathy A
KA
Ang
K Kian
KK
Milas
Luka
L
D000818:Animals; D034961:Antigens, Nuclear / Q000032:analysis; D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects / Q000528:radiation effects; D053148:Caspase 3; D020169:Caspases / Q000378:metabolism; D002453:Cell Cycle / Q000187:drug effects; D019938:Cyclin D1 / Q000096:biosynthesis / Q000235:genetics; D019927:Cyclin E / Q000096:biosynthesis / Q000235:genetics; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors*; D004265:DNA Helicases; D004260:DNA Repair / Q000187:drug effects; D004268:DNA-Binding Proteins / Q000032:analysis; D004354:Drug Screening Assays, Antitumor; D004789:Enzyme Activation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005260:Female; D005419:Flavonoids / Q000494:pharmacology*; D005720:Gamma Rays; D000072200:Ku Autoantigen; D051379:Mice; D009363:Neoplasm Proteins / Q000032:analysis / Q000037:antagonists & inhibitors* / Q000096:biosynthesis / Q000235:genetics; D010051:Ovarian Neoplasms / Q000473:pathology*; D010880:Piperidines / Q000494:pharmacology*; D011836:Radiation Tolerance / Q000187:drug effects; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*
D034961:Antigens, Nuclear; D000970:Antineoplastic Agents; D019927:Cyclin E; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; D005419:Flavonoids; D009363:Neoplasm Proteins; D010880:Piperidines; D011838:Radiation-Sensitizing Agents; D019938:Cyclin D1; C077990:alvocidib; D018844:Cyclin-Dependent Kinases; C505407:CASP3 protein, human; C505409:Casp3 protein, mouse; D053148:Caspase 3; D020169:Caspases; D004265:DNA Helicases; C106893:XRCC5 protein, human; C000606512:Xrcc5 protein, mouse; C000606509:Xrcc6 protein, human; C000606510:Xrcc6 protein, mouse; D000072200:Ku Autoantigen