Modulation of E2F activity in primary mouse B cells following stimulation via surface IgM and CD40 receptors.
Since signals via CD40 and the B cell receptor are known to synergize to induce B cell activation, we have analyzed the pocket protein/E2F complexes in mouse B lymphocytes following stimulation by anti-IgM, anti-CD40, alone or together. We find that E2F4 and DP1 form the predominant E2F heterodimers in the G0 and G1 phases of the cell cycle, complexed with hypophosphorylated p130. During late G1 and S phase this complex is replaced by at least three different E2F complexes, one of which is an E2F complex containing p107 or pRB as well as two "free" E2F complexes consisting of E2F4/DP1 and E2F1-3/DP1. These effects were mirrored by the levels and phosphorylation status of the three pocket proteins. We also observed an increase in electrophoretic mobility of DP1 and E2F4 as B cells progressed from G0 into early G1, resulting from their dephosphorylation. This is known to correlate with a decrease in DNA binding capacity of these proteins and could also be important for derepression of genes negatively regulated through E2F sites in their promoters. These results therefore indicate that the pRB/E2F pathway integrates proliferative signals emanating from the sIgM and CD40 receptors.
Lam
E W
EW
Ludwig Institute for Cancer Research and Section of Virology and Cell Biology Imperial College School of Medicine at St Mary's, London, GB. eric.lam@ic.ac.uk
Glassford
J
J
van der Sman
J
J
Banerji
L
L
Pizzey
A R
AR
Shaun
N
N
Thomas
B
B
Klaus
G G
GG
D000818:Animals; D001402:B-Lymphocytes / Q000276:immunology* / Q000378:metabolism; D019013:CD40 Antigens / Q000378:metabolism*; D002352:Carrier Proteins; D018797:Cell Cycle Proteins / Q000032:analysis / Q000096:biosynthesis / Q000378:metabolism; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000032:analysis / Q000096:biosynthesis / Q000378:metabolism; D050684:E2F Transcription Factors; D050687:E2F1 Transcription Factor; D050699:E2F3 Transcription Factor; D050704:E2F4 Transcription Factor; D007075:Immunoglobulin M / Q000378:metabolism*; D008213:Lymphocyte Activation; D046911:Macromolecular Substances; D051379:Mice; D008810:Mice, Inbred C57BL; D008808:Mice, Inbred CBA; D010766:Phosphorylation; D011947:Receptors, Antigen, B-Cell / Q000502:physiology*; D011961:Receptors, Fc / Q000502:physiology*; D056465:Retinoblastoma-Binding Protein 1; D050680:Transcription Factor DP1; D014157:Transcription Factors / Q000032:analysis / Q000096:biosynthesis / Q000378:metabolism*
C533431:Arid4a protein, mouse; D019013:CD40 Antigens; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D050687:E2F1 Transcription Factor; D050699:E2F3 Transcription Factor; D050704:E2F4 Transcription Factor; C493754:E2f1 protein, mouse; C490840:E2f3 protein, mouse; C493763:E2f4 protein, mouse; D007075:Immunoglobulin M; D046911:Macromolecular Substances; D011947:Receptors, Antigen, B-Cell; D011961:Receptors, Fc; D056465:Retinoblastoma-Binding Protein 1; C493736:Tfdp1 protein, mouse; D050680:Transcription Factor DP1; D014157:Transcription Factors; C035311:immunoglobulin M receptor