title: Limited redundancy in phosphorylation of retinoblastoma tumor suppressor protein by cyclin-dependent kinases in acute lymphoblastic leukemia.
abstract: Cyclin-dependent kinases (CDKs) successively phosphorylate the retinoblastoma protein (RB) at the restriction point in G1 phase. Hyperphosphorylation results in functional inactivation of RB, activation of the E2F transcriptional program, and entry of cells into S phase. RB unphosphorylated at serine 608 has growth suppressive activity. Phosphorylation of serines 608/612 inhibits binding of E2F-1 to RB. In Nalm-6 acute lymphoblastic leukemia extracts, serine 608 is phosphorylated by CDK4/6 complexes but not by CDK2. We reasoned that phosphorylation of serines 608/612 by redundant CDKs could accelerate phospho group formation and determined which G1 CDK contributes to serine 612 phosphorylation. Here, we report that CDK4 complexes from Nalm-6 extracts phosphorylated in vitro the CDK2-preferred serine 612, which was inhibited by p16INK4a, and fascaplysin. In contrast, serine 780 and serine 795 were efficiently phosphorylated by CDK4 but not by CDK2. The data suggest that the redundancy in phosphorylation of RB by CDK2 and CDK4 in Nalm-6 extracts is limited. Serine 612 phosphorylation by CDK4 also occurred in extracts of childhood acute lymphoblastic leukemia cells but not in extracts of mobilized CD34+ hemopoietic progenitor cells. This phenomenon could contribute to the commitment of childhood acute lymphocytic leukemia cells to proliferate and explain their refractoriness to differentiation-inducing agents.
authors.affiliation: Department of Clinical Research, University of Bern, and the Department of Paediatrics, University Children's Hospital Inselspital, Switzerland. N.M.Schmitz@bham.ac.uk
authors.forename: Nicole M R
authors.initials: NM
authors.lastname: Schmitz
authors.forename: Andreas
authors.initials: A
authors.lastname: Hirt
authors.forename: Markus
authors.initials: M
authors.lastname: Aebi
authors.forename: Kurt
authors.initials: K
authors.lastname: Leibundgut
mesh_terms: D018952:Antigens, CD34; D002051:Burkitt Lymphoma / Q000378:metabolism*; D002454:Cell Differentiation / Q000187:drug effects; D045744:Cell Line, Tumor; D002474:Cell-Free System / Q000378:metabolism; D051357:Cyclin-Dependent Kinase 2 / Q000378:metabolism*; D051358:Cyclin-Dependent Kinase 4 / Q000378:metabolism*; D019008:Drug Resistance, Neoplasm / Q000187:drug effects; D016193:G1 Phase / Q000187:drug effects; D006412:Hematopoietic Stem Cells / Q000378:metabolism; D006801:Humans; D007211:Indoles / Q000494:pharmacology; D046912:Multiprotein Complexes / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D054198:Precursor Cell Lymphoblastic Leukemia-Lymphoma / Q000378:metabolism*; D011499:Protein Processing, Post-Translational / Q000187:drug effects; D050720:Retinoblastoma-Like Protein p107 / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds; D002243:Carbolines; D007212:Indolizines
chemical_list: D018952:Antigens, CD34; D051357:Cyclin-Dependent Kinase 2; D051358:Cyclin-Dependent Kinase 4; D007211:Indoles; D046912:Multiprotein Complexes; D050720:Retinoblastoma-Like Protein p107; C069346:fascaplysine; D000644:Quaternary Ammonium Compounds; D002243:Carbolines; D007212:Indolizines; C495894:CDK2 protein, human; C495900:CDK4 protein, human