The inadvertent activation of the Abelson tyrosine kinase (Abl) causes chronic myelogenous leukemia (CML). A small-molecule inhibitor of Abl (STI-571) is effective in the treatment of CML. We report the crystal structure of the catalytic domain of Abl, complexed to a variant of STI-571. Critical to the binding of STI-571 is the adoption by the kinase of an inactive conformation, in which a centrally located "activation loop" is not phosphorylated. The conformation of this loop is distinct from that in active protein kinases, as well as in the inactive form of the closely related Src kinases. These results suggest that compounds that exploit the distinctive inactivation mechanisms of individual protein kinases can achieve both high affinity and high specificity.

c-Abl is normally regulated by an autoinhibitory mechanism, the disruption of which leads to chronic myelogenous leukemia. The details of this mechanism have been elusive because c-Abl lacks a phosphotyrosine residue that triggers the assembly of the autoinhibited form of the closely related Src kinases by internally engaging the SH2 domain. Crystal structures of c-Abl show that the N-terminal myristoyl modification of c-Abl 1b binds to the kinase domain and induces conformational changes that allow the SH2 and SH3 domains to dock onto it. Autoinhibited c-Abl forms an assembly that is strikingly similar to that of inactive Src kinases but with specific differences that explain the differential ability of the drug STI-571/Gleevec/imatinib (STI-571) to inhibit the catalytic activity of Abl, but not that of c-Src.

Persistently activated or tyrosine-phosphorylated STAT3 (pSTAT3) is found in 50% of lung adenocarcinomas. pSTAT3 is found in primary adenocarcinomas and cell lines harboring somatic-activating mutations in the tyrosine kinase domain of EGFR. Treatment of cell lines with either an EGFR inhibitor or an src kinase inhibitor had no effect on pSTAT3 levels, whereas a pan-JAK inhibitor (P6) blocked activation of STAT3 and inhibited tumorigenesis. Cell lines expressing these persistently activated mutant EGFRs also produced high IL-6 levels, and blockade of the IL-6/gp130/JAK pathway led to a decrease in pSTAT3 levels. In addition, reduction of IL-6 levels by RNA interference led to a decrease in tumorigenesis. Introduction of persistently activated EGFR into immortalized breast epithelial cells led to tumorigenesis, IL-6 expression, and STAT3 activation, all of which could be inhibited with P6 or gp130 blockade. Furthermore, inhibition of EGFR activity in multiple cell lines partially blocked transcription of IL-6 and concurrently decreased production and release of IL-6. Finally, immunohistochemical analysis revealed a positive correlation between pSTAT3 and IL-6 positivity in primary lung adenocarcinomas. Therefore, mutant EGFR could activate the gp130/JAK/STAT3 pathway by means of IL-6 upregulation in primary human lung adenocarcinomas, making this pathway a potential target for cancer treatment.

No abstract

Amyloid-␤ (A␤) peptides, consisting mainly of 40 and 42 aa (A␤40 and A␤42, respectively), are metabolites of the amyloid precursor protein and are believed to be major pathological determinants of Alzheimer's disease. The proteolytic cleavages that form the A␤ N and C termini are catalyzed by ␤-secretase and ␥-secretase, respectively. Here we demonstrate that ␥-secretase generation of A␤ in an N2a cell-free system is ATP dependent. In addition, the Abl kinase inhibitor imatinib mesylate (Gleevec, or STI571), which targets the ATP-binding site of Abl and several other tyrosine kinases, potently reduces A␤ production in the N2a cell-free system and in intact N2a cells. Both STI571 and a related compound, inhibitor 2, also reduce A␤ production in rat primary neuronal cultures and in vivo in guinea pig brain. STI571 does not inhibit the ␥-secretase-catalyzed S3 cleavage of Notch-1. Furthermore, production of A␤ and its inhibition by STI571 were demonstrated to occur to similar extents in both Abl ؊/؊ and WT mouse fibroblasts, indicating that the effect of STI571 on A␤ production does not involve Abl kinase. The efficacy of STI571 in reducing A␤ without affecting Notch-1 cleavage may prove useful as a basis for developing novel therapies for Alzheimer's disease.