PECAM-1 shedding during apoptosis generates a membrane-anchored truncated molecule with unique signaling characteristics

Shedding of cell surface molecules, including growth factor receptors, prov ides a mechanism by which cells regulate signal transduction events. Here w e show that platelet-endothelial cell adhesion molecule (PECAM)-1 is shed f rom the endothelial cell surface during apoptosis and accumulates in the cu lture medium as a similar to 100 kDa soluble protein. The cleavage mediatin g the shedding is matrix metalloproteinase (MMP) dependent, as GM6001, a br oad-spectrum MMP inhibitor, inhibits PECAM-1 accumulation in the culture me dium in a dose-responsive manner. In addition to the 100 kDa soluble fragme nt, PECAM-1 cleavage generates the formation of a truncated (Tr.) similar t o 28 kDa molecule, composed of the transmembrane and the cytoplasmic PECAM- 1 domains. Transfections of the full-length (F1) and the Tr. PECAM-1 gene c onstructs into endothelial and nonendothelial cells were performed. We foun d 1) significantly more gamma -catenin and SHP-2 bound to the truncated tha n to the full-length PECAM-1; 2) stable expression of the truncated PECAM-1 in SW480 colon carcinoma cells resulted in a dramatic decrease in cell pro liferation, whereas expression of comparable levels of the full-length PECA M-1 had no effect; 3) the decrease observed in cell proliferation is due, i n part, to an increase in programmed cell death (apoptosis) and correlated with continuous caspase 8 cleavage and p38/JNK phosphorylation. These resul ts support the intimate involvement of PECAM-1 in signal transduction casca des and also suggest that caspase substrates (e.g., PECAM-1) may possess di stinct and unique functions on cleavage.