Tumour induction by activated abl involves tyrosine phosphorylation of the product of the cbl oncogene.

Abstract

v‐cbl is the transforming gene of a murine retrovirus which induces pre‐B cell lymphomas and myelogenous leukaemias. It encodes 40 kDa of a gag fusion protein which is localized in the cytoplasm and nucleus of infected cells. The c‐cbl oncogene encodes a 120 kDa cytoplasmic protein and its overexpression is not associated with tumorigenesis. The c‐cbl sequence has shown that v‐cbl was generated by a truncation that removed 60% of the C‐terminus. In this study, we carried out experiments to identify the position within cbl where the transition occurs between non‐tumorigenic and tumorigenic forms. These experiments focused attention on a region of 17 amino acids which is deleted from cbl in the 70Z/3 pre‐B lymphoma due to a splice acceptor site mutation. This mutation activates cbl's tumorigenic potential and induces its tyrosine phosphorylation. We also show that the expression of the v‐abl and bcr‐abl oncogenes results in the induction of cbl tyrosine phosphorylation, and that abl and cbl associate in vivo. These findings demonstrate that tyrosine‐phosphorylated cbl promotes tumorigenesis and that cbl is a downstream target of the bcr‐abl and v‐abl kinases.

Tyrosine kinase activity of the EGF receptor is enhanced by the expression of oncogenic 70Z-Cbl

“…Cbl lacks a de®ned catalytic domain but is rapidly phosphorylated on tyrosine residues following the stimulation of a wide range of cell surface receptors which include growth factor receptors, immunoglobulin receptors, antigen receptors and integrin receptors (Donovan et al, 1994;Tanaka et al, 1995;Galisteo et al, 1995;Bowtell and Langdon, 1995;Odai et al, 1995;Marcilla et al, 1995;Wang et al, 1996;Cory et al, 1995;Kontani et al, 1996;Ojaniemi et al, 1997). Indeed in many cell types Cbl appears to be one of the most prominent and rapidly phosphorylated substrates of protein tyrosine kinases (Andoniou et al, 1994;Donovan et al, 1994;. This suggests a pivotal role in early events following signal transduction across the cell membrane into the cytoplasm.…”

“…This suggests a pivotal role in early events following signal transduction across the cell membrane into the cytoplasm. Furthermore, Cbl has been found to associate constitutively or inductively with many signalling proteins such as the Grb2, Crk and Nck adaptor proteins, members of the Src, Abl and Syk tyrosine kinase families, the p85 regulatory subunit of PI 3-kinase and 14-3-3 proteins (Donovan et al, 1994;de Jong et al, 1995;Buday et al, 1996;RiveroLezcano et al, 1994;Ribon et al, 1996;Andoniou et al, 1994Andoniou et al, , 1996Smit et al, 1996;Tanaka et al, 1996;Tsygankov et al, 1996;Fournel et al, 1996;Kim et al, 1995;Meisner et al, 1995;Solto and Cantley, 1996;Liu et al, 1996). To date however a de®nite function for Cbl has not emerged from these studies.…”

“…Cbl can be converted to an oncogenic protein either by a large carboxy truncation that generated v-Cbl, or by a small internal deletion at the amino terminus of the Ring ®nger that was identi®ed in a mutant allele of Cbl from the mouse pre-B cell lymphoma line, 70Z/3 ( Figure 1a) (Langdon et al, 1989;Blake et al, 1991;Andoniou et al, 1994). The v-Cbl truncation removes a large proline-rich SH3-binding region and the Ring ®nger domain to reveal a novel phosphotyrosine binding (PTB) domain that forms a direct association with the ZAP-70 tyrosine kinase and the EGF receptor (Lupher et al, 1996;Thien and Langdon, 1997).…”

“…In contrast, transformation by 70Z-Cbl appears to involve a positive signalling mechanism. This protein exhibits a markedly enhanced level of tyrosine phosphorylation under conditions of minimal growth factor stimulation, and has the capability to transform ®broblasts at protein levels where v-Cbl transformation is not evident (Andoniou et al, 1994;Bowtell and Langdon, 1995). Recently the e ects of 70Z-Cbl on the transcriptonal activation of nuclear factor of activated T cells (NFAT) were investigated in Jurkat T cells.…”

“…However the elevated level of tyrosine phosphorylation of 70Z-Cbl protein compared to c-Cbl has provided direction for further investigation. We originally hypothesized that the enhanced tyrosine phosphorylation was due to an intrinsic property of 70Z-Cbl which increased its accessibility to tyrosine kinases (Andoniou et al, 1994). In this study we have further investigated the mechanism of 70Z-Cbl tyrosine phosphorylation using NIH3T3 ®broblasts expressing the human epidermal growth factor receptor (EGFR).…”