title: MyoD prevents cyclinA/cdk2 containing E2F complexes formation in terminally differentiated myocytes.
abstract: Withdrawal from the cell cycle of differentiating myocytes is regulated by the myogenic basic helix-loop-helix (bHLH) protein MyoD and the pocket proteins pRb, p107 and pRb2/p130. Downstream effectors of 'pocket' proteins are the components of the E2F family of transcription factors, which regulate the G1/S-phase transition. We analysed by EMSA the composition of E2F complexes in cycling, quiescent undifferentiated and differentiated C2C12 skeletal muscle cells. An E2F complex containing mainly E2F4 and pRb2/p130 (E2F-G0/G1 complex) appears when DNA synthesis arrests, replacing the cyclinA/cdk2 containing E2F complex of proliferating myoblasts (E2F-G1/S complex). Serum stimulation reinduces DNA synthesis and the re-appearance of E2F-G1/S complexes in quiescent myoblasts but not in differentiated C2C12 myotubes. In differentiating C2C12 cells, E2F complexes switch and DNA synthesis in response to serum are prevented when MyoD DNA binding activity and the cdks inhibitor MyoD downstream effector p21 are induced. Thus, during myogenic differentiation, formation of E2F4 and pRb2/p130 containing complexes is an early event, but not enough on its own to prevent the reactivation of DNA synthesis. Using a subclone of C3H10T1/2 mouse fibroblasts stably expressing Estrogen Receptor-MyoD (ER-MyoD) chimerae, we found that estrogen directed MyoD activation prevents the reassociation of cyclinA/cdk2 to the E2F4 containing complex following serum stimulation and this correlates with suppression of E2F activity and the inability of cells to re-enter the cell cycle. Our data indicate that, in differentiating myocytes, one mechanism through which MyoD induces permanent cell cycle arrest involves p21 upregulation and suppression of the proliferation-associated cdks-containing E2F complexes formation.
authors.affiliation: Fondazione A Cesalpino and I Clinica Medica, University of Rome La Sapienza, Roma, Italy.
authors.forename: P L
authors.initials: PL
authors.lastname: Puri
authors.forename: C
authors.initials: C
authors.lastname: Balsano
authors.forename: V L
authors.initials: VL
authors.lastname: Burgio
authors.forename: P
authors.initials: P
authors.lastname: Chirillo
authors.forename: G
authors.initials: G
authors.lastname: Natoli
authors.forename: L
authors.initials: L
authors.lastname: Ricci
authors.forename: E
authors.initials: E
authors.lastname: Mattei
authors.forename: A
authors.initials: A
authors.lastname: Graessmann
authors.forename: M
authors.initials: M
authors.lastname: Levrero
mesh_terms: D000818:Animals; D042846:CDC2-CDC28 Kinases; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D002454:Cell Differentiation / Q000502:physiology; D002460:Cell Line; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D016213:Cyclins / Q000378:metabolism*; D004247:DNA / Q000378:metabolism; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D050704:E2F4 Transcription Factor; D016193:G1 Phase / Q000502:physiology; D051379:Mice; D008809:Mice, Inbred C3H; D018482:Muscle, Skeletal / Q000166:cytology / Q000378:metabolism*; D017570:MyoD Protein / Q000235:genetics / Q000502:physiology*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D016192:Resting Phase, Cell Cycle / Q000502:physiology; D056465:Retinoblastoma-Binding Protein 1; D050680:Transcription Factor DP1; D014157:Transcription Factors / Q000378:metabolism*; D015854:Up-Regulation
chemical_list: C533431:Arid4a protein, mouse; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D016213:Cyclins; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D050704:E2F4 Transcription Factor; C493763:E2f4 protein, mouse; D017570:MyoD Protein; D056465:Retinoblastoma-Binding Protein 1; D050680:Transcription Factor DP1; D014157:Transcription Factors; D004247:DNA; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495893:Cdk2 protein, mouse; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases