Protein kinase C activation inhibits tyrosine phosphorylation of Cbl and its recruitment of Src homology 2 domain-containing proteins

One of the major proteins that is rapidly tyrosine phosphorylated upon stim ulation of the TCR/CD3 complex is the 120-kDa product of the c-cbl protoonc ogene (Cbl), Upon activation, tyrosine-phosphorylated Cbl interacts with th e Src homology 2 (SH2) domains of several signaling proteins, e.g., phospha tidylinositol 3-kinase (PI3-K) and CrkL, In the present study, we report th at pretreatment of Jurkat T cells with PMA reduced the anti-CD3-induced tyr osine phosphorylation of Cbl and, consequently, its activation-dependent as sociation with PI3-K and CrkL, A specific protein kinase C (PKC) inhibitor (GF-109203X) reversed the effect of PMA on tyrosine phosphorylation of Cbl and restored the activation-dependent association of Cbl with PI3-K and Crk L, We also provide evidence that PKC alpha and PKC theta can physically ass ociate with Cbl and are able to phosphorylate it in vitro and in vivo. Furt hermore, a serine-rich motif at the C terminus of Cbl, which is critical fo r PMA-induced 14-3-3 binding, is also phosphorylated by PKC alpha and PKC t heta in vitro. These results suggest that, by regulating tyrosine and serin e phosphorylation of Chi, PKC is able to control the association of Cbl wit h signaling intermediates, such as SH2 domain-containing proteins and 14-3- 3 proteins, which may consequently result in the modulation of its function .