Therapeutic efficacy of seliciclib in combination with ionizing radiation for human nasopharyngeal carcinoma.
OBJECTIVE
Seliciclib is a small-molecule cyclin-dependent kinase inhibitor, which has been reported to induce apoptosis and cell cycle arrest in EBV-negative nasopharyngeal carcinoma cell lines. Because most nasopharyngeal carcinoma patients harbor EBV, we proceeded to evaluate the cytotoxic effects of seliciclib in EBV-positive nasopharyngeal carcinoma models.


METHODS
Cytotoxicity of seliciclib was investigated in the EBV-positive cell line C666-1 and the C666-1 and C15 xenograft models. Caspase activities and cell cycle analyses were measured by flow cytometry. Efficacy of combined treatment of seliciclib with radiation therapy was also evaluated.


RESULTS
Seliciclib caused significant cytotoxicity in the C666-1 cells in a time- and dose-dependent manner, with accumulation of cells in both sub-G(1) and G(2)-M phases, indicative of apoptosis and cell cycle arrest, respectively. Caspase-2, -3, -8, and -9 activities were all increased, with caspase-3 being the most significantly activated at 48 h after treatment. These cells also showed a reduction of Mcl-1 mRNA and protein levels. Combined treatment of seliciclib with radiation therapy showed a synergistic interaction with enhanced cytotoxicity in C666-1 cells and delayed repair of double-strand DNA breaks. For in vivo models, significant delays in tumor growth were observed for both C666-1 and C15 tumors, which were associated with enhanced apoptosis as determined by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling and immunohistochemistry analyses.


CONCLUSIONS
Seliciclib enhanced the antitumor efficacy of radiation therapy in EBV-positive nasopharyngeal carcinoma, characterized by G(2)-M arrest, and apoptosis, associated with an induction in caspase activity. This process is mediated by reduction in Mcl-1 expression and by attenuation of double-strand DNA break repair.
Hui
Angela B Y
AB
Division of Applied Molecular Oncology, Ontario Cancer Institute, University Health Network, University of Toronto, Canada.
Yue
Shijun
S
Shi
Wei
W
Alajez
Nehad M
NM
Ito
Emma
E
Green
Simon R
SR
Frame
Sheelagh
S
O'Sullivan
Brian
B
Liu
Fei-Fei
FF
D000818:Animals; D000970:Antineoplastic Agents / Q000627:therapeutic use; D017209:Apoptosis / Q000187:drug effects / Q000528:radiation effects; D015153:Blotting, Western; D053143:Caspase 2 / Q000378:metabolism; D053148:Caspase 3 / Q000378:metabolism; D053181:Caspase 8 / Q000378:metabolism; D053453:Caspase 9 / Q000378:metabolism; D002453:Cell Cycle / Q000187:drug effects / Q000528:radiation effects; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects / Q000528:radiation effects; D003131:Combined Modality Therapy; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors; D053903:DNA Breaks, Double-Stranded / Q000187:drug effects / Q000528:radiation effects; D004305:Dose-Response Relationship, Drug; D006801:Humans; D051379:Mice; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D009303:Nasopharyngeal Neoplasms / Q000378:metabolism / Q000473:pathology / Q000628:therapy*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000235:genetics / Q000378:metabolism; D011687:Purines / Q000627:therapeutic use*; D011839:Radiation, Ionizing; D020133:Reverse Transcriptase Polymerase Chain Reaction; D000077546:Roscovitine; D013997:Time Factors; D016896:Treatment Outcome; D023041:Xenograft Model Antitumor Assays
D000970:Antineoplastic Agents; C579094:Mcl1 protein, mouse; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D019253:Proto-Oncogene Proteins c-bcl-2; D011687:Purines; D000077546:Roscovitine; D018844:Cyclin-Dependent Kinases; D053143:Caspase 2; D053148:Caspase 3; D053181:Caspase 8; D053453:Caspase 9