Internalization and processing of human angiogenin by cultured aortic smooth muscle cells

Human angiogenin is a 14-kDa plasma protein with angiogenic and ribonucleol ytic activities, Angiogenin binds specifically to aortic smooth muscle cell s, activates second messenger pathways, and inhibits their proliferation, H uman and bovine aortic smooth muscle cells were used to study the internali zation and intracellular fate of human angiogenin at 37 degrees C, Using a specific antibody against angiogenin, we found that the internalized native protein was localized in the perinuclear region at 30 min and then dispers ed throughout the cytoplasm. In conditions favoring receptor-mediated endoc ytosis, internalization of iodinated angiogenin showed a first peak at 5 mi n and then further increased for up to 24 h. The half-life of the molecule, calculated as 12 h in chase experiments, could contribute to its intracell ular accumulation. In cell extracts, in addition to the 14-kDa protein, a 8 .7-kDa fragment was observed at 24 h, and three fragments with molecular ma ss of 10.5, 8.7, and 6.1 kDa were detected at 48 h. Our data point to a spe cific internalization and processing of human angiogenin by aortic smooth m uscle cells. (C) 1999 Academic Press.