PDGF-induced internalisation promotes proteolytic cleavage of PDGFRβ in mesenchymal cells

Platelet-derived growth factor (PDGF)-induced signalling via PDGF receptor β (PDGFRβ) leads to activation of downstream signalling pathways which regulate multiple cellular responses. It is unclear how PDGFRβ is degraded; both lysosomal and proteasomal degradation have been suggested. In this study, we have characterised the proteolytic cleavage of ligand-activated PDGFRβ, which results in two fragments: a larger fragment containing the extracellular domain, the transmembrane segment, and a part of the intracellular juxtamembrane region with a molecular mass of ∼130 kDa, and an intracellular ∼70 kDa fragment released into the cytoplasm. The proteolytic processing did not take place without internalisation of PDGFRβ. In addition, chelation of intracellular Ca²⁺ inhibited proteolytic processing. Inhibition of the proteasome affected signal transduction by increasing the phosphorylation of PDGFRβ, PLCγ, and STAT3 while reducing it on Erk1/2 and not affecting Akt. The proteolytic cleavage was observed in fibroblasts or cells that had undergone epithelial-mesenchymal transition.