TI  - <prot>MyoD</prot> prevents  <prot>cyclinA</prot> / <prot>cdk2</prot>  containing <prot>E2F</prot> complexes formation in
      terminally differentiated myocytes.
PG  - 1171-84
AB  - Withdrawal from the cell cycle of differentiating myocytes is regulated by
      the <prot>myogenic basic helix-loop-helix</prot> (<prot>bHLH</prot>) protein <prot>MyoD</prot> and the pocket
      proteins <prot>pRb</prot>, <prot>p107</prot> and <prot>pRb2</prot>/<prot>p130</prot>. Downstream effectors of 'pocket'
      proteins are the components of the <prot>E2F</prot> family of transcription factors,
      which regulate the G1/S-phase transition. We analysed by EMSA the
      composition of <prot>E2F</prot> complexes in cycling, quiescent undifferentiated and
      differentiated C2C12 skeletal muscle cells. An <prot>E2F</prot> complex containing
      mainly  <prot>E2F4</prot>  and   <prot>pRb2</prot> / <prot>p130</prot>   (<prot>E2F</prot>-G0/G1 complex) appears when DNA synthesis
      arrests, replacing the  <prot>cyclinA</prot> / <prot>cdk2</prot>  containing <prot>E2F</prot> complex of
      proliferating myoblasts (<prot>E2F</prot>-G1/S complex). Serum stimulation reinduces
      DNA synthesis and the re-appearance of <prot>E2F</prot>-G1/S complexes in quiescent
      myoblasts but not in differentiated C2C12 myotubes. In differentiating
      C2C12 cells, <prot>E2F</prot> complexes switch and DNA synthesis in response to serum
      are prevented when <prot>MyoD</prot> DNA binding activity and the cdks inhibitor <prot>MyoD</prot>
      downstream effector <prot>p21</prot> are induced. Thus, during myogenic
      differentiation, formation of <prot>E2F4</prot> and  <prot>pRb2</prot> / <prot>p130</prot>  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 <prot>Estrogen Receptor</prot>-<prot>MyoD</prot> (<prot>ER</prot>-<prot>MyoD</prot>) chimerae, we found that
      estrogen directed <prot>MyoD</prot> activation prevents the reassociation of
      <prot>cyclinA</prot>/<prot>cdk2</prot> to the <prot>E2F4</prot> containing complex following serum stimulation
      and this correlates with suppression of <prot>E2F</prot> activity and the inability of
      cells to re-enter the cell cycle. Our data indicate that, in
      differentiating myocytes, one mechanism through which <prot>MyoD</prot> induces
      permanent cell cycle arrest involves <prot>p21</prot> upregulation and suppression of
      the proliferation-associated cdks-containing <prot>E2F</prot> complexes formation.
AD  - Fondazione A Cesalpino and I Clinica Medica, University of Rome La
