THE MEMBRANE-PROTEIN CDB DRAP-27 POTENTIATES THE JUXTACRINE GROWTH-FACTOR ACTIVITY OF THE MEMBRANE-ANCHORED HEPARIN-BINDING EGF-LIKE GROWTH-FACTOR/

The membrane-anchored heparin-binding EGF-like growth factor precursor (proHB-EGF)/diphtheria toxin receptor (DTR) belongs to a class of tra nsmembrane growth factors and physically associates with CD9/DRAP27 wh ich is also a transmembrane protein. To evaluate the biological activi ties of proHB-EGF/DTR as a juxtacrine growth factor and the biological significance of its association with CD9/DRAP27, the mitogenic activi ty of proHB-EGF/DTR was analyzed using stable transfectants of mouse L cells expressing both human proHB-EGF/DTR and monkey CD9/DRAP27, or e ither one alone. Juxtacrine activity was assayed by measuring the abil ity of cells in co-culture to stimulate DNA synthesis in an EGF recept or ligand dependent cell line, EP170.7. LH-2 cells expressing human pr oHB-EGF/DTR stimulated EP170.7 cell growth moderately. However, LCH-1 cells, a stable co-transfectant expressing both human proHB-EGF/DTR an d monkey CD9/DRAP27 cDNAs, dramatically unregulated the juxtacrine gro wth factor activity of proHB-EGF/DTR approximately 25 times over that of LH-2 cells even though both cell types expressed similar levels of proHB-EGF/DTR on the cell surface. Anti-CD9/DRAP27 antibodies which we re not able to neutralize the mitogenic activity of soluble HB-EGF sup pressed LCH-1 cell juxtacrine growth activity to the same extent as di d anti-HB-EGF neutralizing antibodies and CRM 197, specific inhibitors of human HG-EGF. These findings suggest that optimal expression of th e juxtacrine growth activity of proHB-EGF/DTR requires co-expression o f CD9/DRAP27. These studies also indicate that growth factor potentiat ion effects which have been observed previously for soluble growth fac tors also occurs at the level of cell surface associated growth factor s.