The LAR protein tyrosine phosphatase enables PDGF β-receptor activation through attenuation of the c-Abl kinase activity

The receptor tyrosine phosphatase (RPTP) LAR negatively regulates the activity of several receptor tyrosine kinases. To investigate if LAR affects the platelet-derived growth factor (PDGF) receptor signaling, mouse embryonic fibroblasts (MEFs) from mice where the LAR phosphatase domains were deleted (LAR Delta P), and wt littermates, were stimulated with 20 ng/ml PDGF-BB. In LAR phosphatase deficient MEFs, the phosphorylation of the PDGF beta-receptor was surprisingly reduced, an event that was rescued by re-expression of wt LAR. The decreased phosphorylation of the PDGF beta-receptor was observed independent of ligand concentration and occurred on all tyrosine residues, as determined by immunoblotting analysis using site-selective phosphotyrosine antibodies. This suggests that LAR is required for full PDGF beta-receptor kinase activation. Downstream of receptor activation, phosphorylation of Akt and PLC gamma were decreased in LAR Delta P MEFs, whereas Src and Erk MAP kinase pathways were less affected. The proliferation of LAR Delta P MEFs in response to PDGF-BB was also reduced. The inhibitory effect on the PDGF beta-receptor in LAR Delta P cells was exerted via increased basal activity of c-Abl, since inhibition of c-Abl, by AG957 or siRNA, restored PDGF beta-receptor phosphorylation. These observations suggest that LAR reduces the basal c-Abl activity thereby allowing for PDGF beta-receptor kinase activation. (C) 2011 Elsevier Inc. All rights reserved.