Activated PDGF receptors phosphorylate a large number of substrates (more than 20), including themselves, initiating a complex network of signaling cascades, which utltimately regulate transcription factors and gene expression.
Many cellular effects of PDGF are mediated by gene regulation. Using microarrays, we have identified a large number of genes regulated by PDGF on various cell types, including fibroblasts and neural stem cells. We are now interested in the transcritpion factors that control the expression of these genes (Demoulin, 2004, Kallin 2007, Essaghir 2009). In particular, activation of SREBP and inhibition of FOXO seem to play an important role in cell growth.
Function of the PDGF receptor C-terminus:
NHERF binding and inhibition of the receptor activity
Proteins interacting with the human PDGF receptor were isolated using immobilized peptides derived from the receptor C-terminus as a bait. We identified two PDZ domain proteins, namely NHERF/EBP50 and NHERF2. The recruitment of these proteins to the PDGF receptor was induced by PDGF treatment. Although NHERF was initially characterized as a factor required for intracellular pH regulation by adrenergic receptors, we observed that it was not involved in pH regulation by PDGF, but it played a role in actin filament reorganization (Demoulin et al, 2003).
NHERF initiates a new type of signaling cascade that we will continue to explore in the future.
We also observed that upstream the NHERF-binding site in the PDGF receptor, a pro/glu-rich motif inhibits the activity of the receptor kinase. We hope that these results will help us to design new specific inhibitors of the PDGF receptor (Chiara et al, 2004).
We are analyzing the degradation of the receptor by the ubiquitin ligase complex that contains the c-Cbl oncogene.
Role of Gab1 in PDGF signaling
We and others have shown that PDGF stimulation of cells induces the phosphorylation of the PH-domain containing adapter protein Gab1. After PDGF-induced activation, Gab1 interacts with the tyrosine phosphatase SHP2 and the adapter Grb2, which acts as a bridge between Gab1 and the PDGF receptor.
Using a tetracycline inducible system to over-express Gab1 in porcine aortic endothelial cells stably transfected with PDGF beta receptor, we could demontrate that Gab1 controls the level of activation of p38 and ERK by PDGF. Gab1 overexpression resulted in a dramatic reorganization of the cytoskeleton (Kallin et al, 2004). Other signaling pathways are currently being analysed in our lab.
