The ephrinA1 ligand exerts antioncogenic effects in tumor cells through 
activation and downregulation of the EphA2 receptor and has been described as a 
membrane-anchored protein requiring clustering for function. However, while 
investigating the ephrinA1/EphA2 system in the pathobiology of glioblastoma 
multiforme (GBM), we uncovered that ephrinA1 is released from GBM and breast 
adenocarcinoma cells as a soluble, monomeric protein and is a functional form of 
the ligand in this state. Conditioned media containing a soluble monomer of 
ephrinA1 caused EphA2 internalization and downregulation, dramatic alteration of 
cell morphology and suppression of the Ras-MAPK pathway. Moreover, soluble 
monomeric ephrinA1 was functional in a physiological context, eliciting collapse 
of embryonic neuronal growth cones. We also found that ephrinA1 is cleaved from 
the plasma membrane of GBM cells, an event which involves the action of a 
metalloprotease. Thus, the ephrinA1 ligand can, indeed, function as a soluble 
monomer and may act in a paracrine manner on the EphA2 receptor without the need 
for juxtacrine interactions. These findings have important implications for 
further deciphering the function of these proteins in pathology and physiology, 
as well as for the design of ephrinA1-based EphA2-targeted antitumor 
therapeutics.