Recent studies have demonstrated that Cbl, the 120-kDa protein product of the c-cbl proto-oncogene, serves as a substrate of a number of receptor- coupled tyrosine kinases and forms complexes with SH3 and SH2 domain- containing proteins, pointing to its role in signal transduction. Based on genetic evidence that the Caenorhabditis elegans Cbl homolog, SLI-1, functions as a negative regulator of the LET-23 receptor tyrosine kinase and our demonstration that Cbl's evolutionarily conserved N-terminal transforming region (Cbl-N; residues 1 to 357) harbors a phosphotyrosine binding (PTB) domain that binds to activated ZAP-70 tyrosine kinase, we examined the possibility that oncogenie Cbl mutants may activate mitogenic signaling by deregulating cellular tyrosine kinase machinery. Here, we show that expression of Cbl-N and two other transforming Cbl mutants (CblY368Δ and Cbl366-382Δ or Cbl70Z), but not wild-type Cbl, in NIH 3T3 fibroblasts leads to enhancement of endogenous tyrosine kinase signaling. We identified platelet-derived growth factor receptor a (PDGFRα) as one target of mutant Cbl-induced deregulation. In mutant Cbl transfectants, PDGFRα was hyperphosphorylated and constitutively complexed with a number of SH2 domain- containing proteins. PDGFRα hyperphosphorylation and enhanced proliferation of mutant Cbl-transfected NIH 3T3 cells were drastically reduced upon serum starvation, and PDGF-AA substituted for the maintenance of these traits. PDGF-AA stimulation of serum-starved Cbl transfectants induced the in vivo association of transfected Cbl proteins with PDGFRα. In vitro, Cbl-N directly bound to PDGFRα derived from PDGF-AA-stimulated cells but not to that from unstimulated cells, and this binding was abrogated by a point mutation (G306E) corresponding to a loss-of-function mutation in SLI-1. The Cbl-N/G306E mutant protein, which failed to induce enhanced growth and transformation of NIH 3T3 cells; also failed to induce hyperphosphorylation of PDGFRα. Altogether, these findings identify a novel mechanism of Cbl's physiological function and oncogenesis, involving its PTB domain-dependent direct interaction with cellular tyrosine kinases.
|Original language||English (US)|
|Number of pages||14|
|Journal||Molecular and cellular biology|
|Publication status||Published - Aug 1 1997|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology