Role of the Interaction of proHB - EGF with Heparan Sulfate Proteoglycans

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) exhibits activity as a juxtacrine, paracrine, and autocrine ligand for the epidermal growth factor receptor (EGFR), and possesses the ability to bind heparan sulfate proteoglycans (HSPGs). The interaction of HB-EGF with HSPGs has been previously studied only with the soluble (autocrine/paracrine) form of the protein (sHB-EGF), produced after proteolytic cleavage of the transmembrane form (proHB-EGF) from the cell surface. It was hypothesized that HSPGs interact with proHB-EGF in ways that could alter behavior of the transmembrane form of this ligand and consequent processes. Using an engineered form of proHB-EGF that allowed for independent tracking of the extracellular domain and the C-terminal tail, proHB-EGF was observed primarily at sites of cell-cell contact. However, a dramatic change in this localization was observed upon the addition of exogenous heparin, heparan sulfate, heparinase III or mutation of the heparin-binding domain of proHB-EGF, suggesting that an interaction with HSPGs is responsible for localizing proHB-EGF to sites of cell-cell contact. Further studies in wild-type CHO-Ki cells and heparan sulfate deficient CHOpgsD-677 cells demonstrated that a trans interaction between proHB-EGF and HSPGs on neighboring cells was responsible for this localization. Additionally, this interaction inhibited proteolytic processing of the ligand, as heparin and mutation of the heparin-binding domain increased the amount of sHB-EGF accumulated in the media.