Discoidin domain receptor 1 is activated independently of beta(1) integrin

Various types of collagen have been identified as potential ligands for the two mammalian discoidin domain receptor (DDR) tyrosine kinases, DDR1 and D DR2, It is presently unclear whether collagen-induced DDR receptor activati on, which occurs with very slow kinetics, involves additional proteins with kinase activity or membrane-anchored proteins serving as coreceptors. In p articular, the role of the collagen-binding integrins alpha(1)beta(1) or al pha(2)beta(1) in the DDR activation process is undefined. Here, we provide three lines of evidence suggesting that DDR1 signaling is distinct from int egrin activation. First we demonstrate that the enzymatic activity of DDR1 is essential for receptor tyrosine phosphorylation, Collagen-induced DDR re ceptor autophosphorylation can be blocked either by a dominant negative mut ant or by a preparation of recombinant extracellular domain, Second, we sho w DDR1 signals independent of the epidermal growth factor (EGF) receptor. I n cells that endogenously express both DDR1 and the EGF receptor, stimulati on with EGF does not induce DDR activation. Third, we detected full DDR1 ac tivation after collagen stimulation in cells that have been treated with bl ocking antibodies for alpha(2)beta(1) integrin or in cells with a targeted deletion of the beta(1) integrin gene. Finally, we show that overexpression of dominant negative DDR1 in the myoblast cell line C2C12 blocks cellular differentiation and the formation of myofibers.