Intercellular Adhesion Molecule (ICAM)-1, But Not ICAM-2, Activates RhoA and Stimulates c-fos and rhoA Transcription in Endothelial Cells1

ICAM-1 and -2 are integrin-binding Ig superfamily adhesion molecules that are important for leukocyte transmigration across endothelial monolayers. ICAM-1 cross-linking is known to activate the small GTPase RhoA and induce stress fiber formation in endothelial cells, but ICAM-2 signaling has not been investigated. In this study, we compare ICAM-1 and ICAM-2 signaling and localization in HUVECs. Although ICAM-1 and ICAM-2 both localize with the actin-binding protein moesin in apical microvilli, only ICAM-1 colocalizes with moesin after cross-linking. Unlike ICAM-1, ICAM-2 does not activate RhoA or alter actin cytoskeletal organization. Interestingly, ICAM-1 stimulates transcription of c-fos, a known early response gene. In addition, it up-regulates rhoA expression, suggesting that it activates a positive feedback pathway after RhoA activation. These results indicate that in endothelial cells, ICAM-1, but not ICAM-2, rapidly stimulates signaling responses involving RhoA.