摘要：

Receptor tyrosine kinases (RTKs) comprise a family of cell surface receptors that control many critical cellular processes. Various human diseases are caused by dysfunction in RTKs or in their intracellular signaling pathways. Stem Cell Factor (SCF) initiates its multiple cellular responses by binding to the extracellular region of the RTK KIT resulting in receptor dimerization and tyrosine kinase activation. We have determined the crystal structure of the entire extracellular region of KIT before and after SCF stimulation. The structures show that KIT dimerization is driven by SCF binding whose sole role is to bring two KIT molecules together. We have used X-ray crystallography, electron microscopy (EM) and biochemical experiments to determine the three dimensional structure of intact, SCF-stimulated KIT dimers. Several forms of dimeric KIT molecules with different asymmetric arrangements of the two tyrosine kinase domains were identified. These asymmetric contacts may represent specific interactions occurring between two KIT tyrosine kinase domains poised towards trans autophosphorylation. We propose that cooperative interactions mediated by multiple weak homotypic contacts between the extracellular, transmembrane and cytoplasmic regions of KIT are responsible for tyrosine kinase activation and cell signaling in normal and transformed cells. Sutent is a drug that blocks the tyrosine kinase activities of several RTKs including KIT and VEGFR2. Sutent has been approved by the FDA for the treatment of advanced kidney cancers, gastrointestinal tumors and for endocrine pancreatic cancers. A scaffold-based drug discovery approach was developed enabling the development of new families of inhibitors for protein kinases and other enzymes that play a role in cell signaling. V600E B-RAF mutant is an oncogenic mutation identified in approximately 50% of melanomas and in 4% of all solid tumors. By using a scaffold-based drug discovery approach, a potent inhibitor of V600E B-RAF mutant has been discovered designated PLX 4032 (Vemurafenib). Phase-I and-II clinical trials in multiple medical centers revealed 80% responses such as tumor shrinkage, delay in tumor progression and survival of melanoma patients harboring the B-RAF mutation. Primary endpoints of phase-III clinical trials in multiple medical centers were achieved to support Vemurafenib registration.

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