2008 Volume 21 Issue 2 Pages 126-133
Heparin-binding EGF-like growth factor (HB-EGF) is a member of the EGF family of growth factors and is first synthesized as membrane associated precursor molecule (proHB-EGF). ProHB-EGF is cleaved enzymatically, resulting in the shedding of the mature secreted form (soluble HB-EGF). While soluble HB-EGF as a mitogenic signals bind to and activate the EGF receptor (EGFR) of neighboring cells as autocrine/paracrine manner, a considerable amount of proHB-EGF remains on the cell surface under normal physiological conditions, and it is biologically active as juxtacrine growth factor that signals to neighboring cells (juxtacrine activation). Juxtacrine activation plays a role to cell survival and epithelial integrity through EGFR. As epithelial cells attach and spread, they may make contact with neighboring cell, and focal complex processes including lamellipodia are considered premature cell-cell contact sites. In MDCK cells, stable expression of a non-cleavable and membrane-anchored human HB-EGF construct (5AA cell) results in enhanced cell spreading and formation of significantly extended lamellipodia and filopodia compared to wild type (WT) cells. Furthermore, there was increased tyrosine phosphorylation of Paxillin and FAK in the 5AA cells compared to the control cells, which Src tyrosine phosphorylation was not different between the two cell lines. In addition, there was a marked increase in phosphorylation of Akt in 5AA cells compared to WT cells. Cells expressing a non-cleavable HB-EGF construct mutated to prevent EGFR activation (m5AA cell) showed late attachment and had no change in cell spreading compared to WT. Western blot analysis also showed decreased expression levels of phosphorylation of FAK and Paxillin. These findings suggest that proHB-EGF can not only precursor for soluble forms but also important roles for biological functions of EGFR activation as juxtacrine manner.
