ICAM-1 is an Ig-like cell adhesion molecule expressed by several cell types, including the endothelium. Cross-linking of ICAM-1 on the surface of different cell types has previously been shown to cause an increase in cellular activation within the cytoplasm. In this study, we have compared signaling events following ligation of ICAM-1 by cross-linking with mAbs with events after activation of HUVEC by TNF. ICAM-1 cross-linking caused activation of Erk-1 and the AP-1 transcription factor complex, without any increase in NF-kappaB activity, in contrast to TNF stimulation. Transcription of VCAM-1 mRNA was observed by reverse-transcriptase PCR after ICAM-1 cross-linking, with no associated transcription of E-selectin. This was reflected by the presence of VCAM-1 protein after immunoprecipitation, without E-selectin expression, in ICAM-1 cross-linked cells. In contrast, mRNA and protein for both VCAM-1 and E-selectin were observed in TNF-treated HUVEC, as expected. Addition of the MEK (MAP/Erk kinase) inhibitor PD98059 reduced expression of VCAM-1 after ICAM-1 cross-linking, suggesting that the Erk pathway is involved in ICAM-1-mediated VCAM-1 expression. In conclusion, ICAM-1-induced expression of VCAM-1 represents a pathway for adhesion molecule up-regulation that is distinct from the TNF-induced pathway. It may be similar to the IL-4 pathway or it may represent a novel pathway.