title: The role of p16 in the E2F-dependent thymidine kinase regulation.
abstract: The role of alterations of the MTS1 tumor suppressor gene on chromosome 9p21, which encodes p16, the inhibitor of cyclin-dependent-kinase-4 and 6, in tumorigenesis is not yet clear. Phosphorylation of the retinoblastoma protein by cyclin-dependent kinases 4 and 6 prevents its interaction with the transcription factor E2F, which subsequently promotes the expression of S phase regulated genes, such as thymidine kinase. Although a role of p16 in this regulation has been presumed, there is no proof so far that loss of this tumor suppressor gene really affects E2F-mediated regulations. We investigated the regulation of thymidine kinase in phytohemagglutinin-stimulated normal human lymphocytes and in the p16-negative human acute lymphoblastic leukemia cell lines, MOLT-4 and CEM. Compared to normal lymphocytes, MOLT-4 and CEM cells exhibited an altered cell cycle regulation of thymidine kinase, a much higher intracellular activity of this enzyme, and higher thymidine kinase mRNA expression. Transient expression of p16 in normal human lymphocytes caused arrest in G1, but was without effect on the cell growth of MOLT-4 and CEM cells, although all of them express functional retinoblastoma protein. Nevertheless, in the two leukemia cell lines transient overexpression of p16 reestablished the normal regulation of thymidine kinase, paralleled by an increase of the underphosphorylated form of retinoblastoma protein and decrease of free E2F bound to its motif in the thymidine kinase promoter. We demonstrate that loss of p16 causes upregulation of this DNA precursor pathway enzyme via activation of E2F by a mechanism involving retinoblastoma protein.
authors.affiliation: University of Vienna, Department of Prenatal Diagnosis and Therapy, Austria.
authors.forename: M
authors.initials: M
authors.lastname: Hengstschläger
authors.forename: E
authors.initials: E
authors.lastname: Hengstschläger-Ottnad
authors.forename: O
authors.initials: O
authors.lastname: Pusch
authors.forename: E
authors.initials: E
authors.lastname: Wawra
mesh_terms: D002352:Carrier Proteins / Q000502:physiology*; D002453:Cell Cycle / Q000235:genetics; D018797:Cell Cycle Proteins; D002455:Cell Division / Q000235:genetics; D002478:Cells, Cultured; D019941:Cyclin-Dependent Kinase Inhibitor p16; D004268:DNA-Binding Proteins; D003842:Deoxycytidine Kinase / Q000096:biosynthesis; D015536:Down-Regulation; D050684:E2F Transcription Factors; D016193:G1 Phase; D015971:Gene Expression Regulation, Enzymologic; D015972:Gene Expression Regulation, Neoplastic; D006367:HeLa Cells; D006801:Humans; D008214:Lymphocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D010766:Phosphorylation; D010835:Phytohemagglutinins / Q000378:metabolism / Q000494:pharmacology; D054198:Precursor Cell Lymphoblastic Leukemia-Lymphoma / Q000235:genetics / Q000473:pathology; D016160:Retinoblastoma Protein / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D056465:Retinoblastoma-Binding Protein 1; D013937:Thymidine Kinase / Q000096:biosynthesis* / Q000235:genetics; D050680:Transcription Factor DP1; D014157:Transcription Factors / Q000235:genetics / Q000502:physiology*; D014158:Transcription, Genetic; D014162:Transfection; D014407:Tumor Cells, Cultured; D015854:Up-Regulation
chemical_list: D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D019941:Cyclin-Dependent Kinase Inhibitor p16; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D010835:Phytohemagglutinins; D016160:Retinoblastoma Protein; D056465:Retinoblastoma-Binding Protein 1; D050680:Transcription Factor DP1; D014157:Transcription Factors; D013937:Thymidine Kinase; D003842:Deoxycytidine Kinase