TI  - Discrete protein interactions with the  <prot>Grb2</prot> / <prot>c-Cbl</prot>  complex in <prot>SCF</prot>- and
      <prot>TPO</prot>-mediated myeloid cell proliferation.
PG  - 2067-76
AB  - Hemopoietic cell proliferation is mediated by non-tyrosine and tyrosine
      kinases that signal via uncommon and common sets of downstream effector
      molecules including the <prot>Grb2</prot>/<prot>c-Cbl</prot>. In the present study we evaluated
      tyrosine phosphorylation of <prot>c-Cbl</prot> and the interaction of the  <prot>Grb2</prot> / <prot>c-Cbl</prot> 
      complex with signaling proteins upon activation of non-tyrosine (<prot>c-Mpl</prot>)
      and tyrosine kinase (<prot>c-Kit</prot>) receptors leading to myeloid cell
      proliferation. By using the growth factor dependent M-07e cell line, we
      found that both <prot>c-Mpl</prot> and <prot>c-Kit</prot> ligands, namely:  <prot>SCF</prot>  and  <prot>TPO</prot> , induce   <prot>c-Cbl</prot>  
      tyrosine phosphorylation. In these cells the adaptor protein  <prot>Grb2</prot> 
      constitutively binds a substantial fraction of  <prot>c-Cbl</prot>  through the
      N-terminal SH3 domain. In vitro experiments showed that the stable
        <prot>Grb2</prot> / <prot>c-Cbl</prot>   complex interacts, through the <prot>Grb2</prot> SH2 domain, with the
      <prot>SCF</prot>-activated  <prot>c-Kit</prot> . By contrast stimulation with <prot>TPO</prot> leads to the
      formation of a <prot>Grb2</prot> complex containing <prot>JAK2</prot>. In vitro and in vivo
      experiments support the hypothesis that <prot>Grb2</prot> mediates the association of
       <prot>c-Kit</prot>  with  <prot>c-Cbl</prot> . Moreover we found that, upon <prot>SCF</prot> stimulation, the
       <prot>Grb2</prot> / <prot>c-Cbl</prot>  complex recruits <prot>Shc</prot>, probably via <prot>Grb2</prot>. By contrast the <prot>Ras</prot>
      exchanger factor (<prot>Sos1</prot>) was not detected in anti-<prot>c-Cbl</prot> immunoprecipitates
      suggesting that <prot>Grb2</prot>/<prot>Sos1</prot> and <prot>Grb2</prot>/<prot>c-Cbl</prot> are present in different
      complexes. Taken together our results demonstrate that <prot>c-Cbl</prot> plays an
      important role in coupling both tyrosine and non-tyrosine kinase receptors
      to downstream effector molecules and that different signaling molecules
      interact with <prot>Grb2</prot>/<prot>c-Cbl</prot> complex when non-tyrosine or tyrosine kinase
      receptors are activated.
AD  - Dipartimento di Medicina Interna, Universita di Torino, Italy.
