Comparison of baculovirus-expressed c-Abl and BCR/ABL protein tyrosine kinases

Novel BCR-ABL transcript containing an intronic sequence insert in a patient with Philadelphia-positive acute lymphoblastic leukaemia

In a patient with Philadelphia chromosome-positive acute lymphoblastic leukaemia (ALL), a novel variant of the chimaeric BCR-ABL mRNA transcript was detected by reverse transcription polymerase chain reaction (RT-PCR). Sequencing revealed the novel transcript to be a chimaeric mRNA produced by fusion of the BCR exon 14 (b3) to the ABL exon a2 with a 49-base pair (bp) insertion of an ABL intron 1b sequence between them. The insertion of the 49 bp introduced a stop codon. These data show that this variant of the chimaeric mRNA would not be translated into the p210 BCR-ABL protein. This could be one of the explanations as to why clinically the patient has responded well to therapy and continues to follow a mild clinical course.

An intramolecular SH3-domain interaction regulates c-Abl activity

The ABL1 proto-oncogene encodes a cytoplasmic and nuclear protein tyrosine kinase (c-Abl) that has been implicated in processes of cell differentiation, cell division, cell adhesion and stress response. Alterations of ABL1 by chromosomal rearrangement or viral transduction can lead to malignant transformation. Activity of the c-Abl protein is negatively regulated by its SH3 domain through an unknown mechanism, and deletion of the SH3 domain turns ABL1 into an oncogene. We present evidence for an intramolecular inhibitory interaction of the SH3 domain with the catalytic domain and with the linker between the SH2 and catalytic domain (SH2-CD linker). Site-directed mutations in each of these three elements activate c-Abl. Mutations in the linker cause a conformational change of the molecule and increase binding of the SH3 domain to peptide ligands. Individual mutation of two charged residues in the SH3 and catalytic domain activates c-Abl, while inhibition is restored in the double reciprocal mutant. We propose that regulators of c-Abl will have opposite effects on its activity depending on their ability to favour or disrupt these intramolecular interactions.

Indolocarbazoles: potent and selective inhibitors of platelet-derived growth factor receptor autophosphorylation

A quantitative assay for measuring the autophosphorylation of platelet-derived growth factor (PDGF) receptors in intact vascular smooth muscle cells has been developed and used to screen for novel tyrosine kinase (TK) inhibitors. Several novel inhibitors of PDGF receptor autophosphorylation have been identified from the indolocarbazole series, including the 3,9 dimethoxy derivative, 3744W (IC50 = 14.5+/-2 nM). Tested against a panel of tyrosine and serine/threonine kinases, 3744W is at least 1,000 fold selective for the PDGF receptor tyrosine kinase and was found to inhibit autophosphorylation of both the alpha and beta isoforms of the PDGF receptor in human smooth muscle cells. PDGF-BB-stimulated DNA synthesis in quiescent cultures of human smooth muscle cells was blocked in a concentration-dependent manner by 3744W, IC50 = 10 nM. Binding studies showed that 3744W did not block the binding of PDGF-BB to cell surface receptors on human airway smooth muscle cells. Furthermore, inhibition of bone marrow stem cell proliferation by 3744W was only observed at concentrations 100-1,000 times greater than those needed to block PDGF-driven DNA synthesis in human smooth muscle cells. 3744W represents a novel, potent and selective inhibitor of PDGF receptor autophosphorylation and a powerful biochemical probe for investigating PDGF-dependent responses in vitro.