Autocrine, paracrine, and juxtacrine are recognized modes of action for 
mammalian EGFR ligands including EGF, TGF-α (TGFα), amphiregulin (AREG), 
heparin-binding EGF-like growth factor (HB-EGF), betacellulin, epiregulin, and 
epigen. We identify a new mode of EGFR ligand signaling via exosomes. Human 
breast and colorectal cancer cells release exosomes containing full-length, 
signaling-competent EGFR ligands. Exosomes isolated from MDCK cells expressing 
individual full-length EGFR ligands displayed differential activities; AREG 
exosomes increased invasiveness of recipient breast cancer cells 4-fold over 
TGFα or HB-EGF exosomes and 5-fold over equivalent amounts of recombinant AREG. 
Exosomal AREG displayed significantly greater membrane stability than TGFα or 
HB-EGF. An average of 24 AREG molecules are packaged within an individual 
exosome, and AREG exosomes are rapidly internalized by recipient cells. Whether 
the composition and behavior of exosomes differ between nontransformed and 
transformed cells is unknown. Exosomes from DLD-1 colon cancer cells with a 
mutant KRAS allele exhibited both higher AREG levels and greater invasive 
potential than exosomes from isogenically matched, nontransformed cells in which 
mutant KRAS was eliminated by homologous recombination. We speculate that EGFR 
ligand signaling via exosomes might contribute to diverse cancer phenomena such 
as field effect and priming of the metastatic niche.