The epidermal growth factor receptor (EGFR) has an essential role in multiple signaling pathways, including cell proliferation and migration, through extracellular ligand binding and subsequent activation of its intracellular tyrosine kinase (TK) domain. The non-small cell lung cancer (NSCLC)-associated EGFR mutants, L858R and G719S, are constitutively active and oncogenic. They display sensitivity to TK inhibitors, including gefitinib and erlotinib. In contrast, the secondary mutation of the gatekeeper residue, T790M, reportedly confers inhibitor resistance on the oncogenic EGFR mutants. In this study, our biochemical analyses revealed that the introduction of the T790M mutation confers gefitinib resistance on the G719S mutant. The G719S/T790M double mutant has enhanced activity and retains high gefitinib-binding affinity. The T790M mutation increases the ATP affinity of the G719S mutant, explaining the acquired drug resistance of the double mutant. Structural analyses of the G719S/T790M double mutant, as well as the wild type and the G719S and L858R mutants, revealed that the T790M mutation stabilizes the hydrophobic spine of the active EGFR-TK conformation. The Met790 side chain of the G719S/T790M double mutant, in the apo form and gefitinib- and AMPPNP-bound forms, adopts different conformations that explain the accommodation of these ligands. In the L858R mutant structure, the active-site cleft is expanded by the repositioning of Phe723 within the P-loop. Notably, the introduction of the F723A mutation greatly enhanced the gefitinib sensitivity of the wild-type EGFR in vivo, supporting our hypothesis that the expansion of the active-site cleft results in enhanced gefitinib sensitivity. Taken together, our results provide a structural basis for the altered drug sensitivities caused by distinct NSCLC-associated EGFR mutations.

We identified AATYK2 ( A poptosis-A ssociated Ty rosine K inase 2) through a database search as a kinase specifically expressed in the brain. After characterization, we renamed it BREK (Brain-Enriched Kinase). Mouse BREK mRNA is expressed predominantly in brain, especially in olfactory bulb, olfactory tubercle, hippocampus, striatum, cerebellum, and cerebral cortex. Levels of expression and phosphorylation of BREK were high at 0-2 weeks after birth, suggesting that BREK is involved in neural development and functions during the early postnatal period. Phosphoamino acid analysis following in vitro kinase reaction revealed that BREK is a catalytically active, serine/ threonine kinase. In PC12 cells, BREK was phosphorylated rapidly upon stimulation with nerve growth factor (NGF) in a protein kinase C-dependent pathway. In differentiated PC12 cells, BREK was enriched in cell bodies and growth cones, and also present along neurites. Introduction of a kinase-defective mutant of BREK into PC12 cells enhanced both ERK phosphorylation and neurite outgrowth in response to NGF, suggesting that BREK is a negative regulator of NGF-induced neuronal differentiation. Thus, we conclude that BREK is a new member of the family of protein serine/threonine kinases and that it plays important roles in NGF-TrkA signalling.

Tumor microenvironments present significant barriers to penetration by antibodies, immunoconjugates, and other immunotoxins. In this report, we illustrate a novel strategy to increase tumor cell uptake of immunotoxin by combination with Taxol. SS1P is an immunotoxin composed of the Fv portion of a mesothelinspecific antibody fused to a bacterial toxin that is presently undergoing phase II testing in mesothelioma. Using novel flow cytometry and gel filtration methods, we quantified SS1P uptake in individual tumor cells along with levels of shed mesothelin (sMSLN), a barrier of SS1P therapy. The validity of our flow cytometric method was confirmed by the ability to similarly quantitate tumor cell uptake of Herceptin and an immunotoxin targeting HER2/neu. SS1P uptake peaked several hours after SS1P was cleared from the blood, reflecting an intratumor distribution process of SS1P that is independent of blood supply. Using the methods developed, we demonstrated that Taxol could improve SS1P penetration into tumors in parallel with an associated reduction of sMSLN in tumor extracellular fluid. Our findings offer a mechanistic rationale to combine SS1P with Taxol or another cytotoxic drug as a strategy to increase immunotoxin uptake by tumor cells. Further, we suggest one basis to understand why chemotherapy and antibody-based therapies cooperate when combined in cancer treatment.