Beta-amyloid peptide (Abeta), which plays a central role in the pathogenesis of Alzheimer's disease, is derived from the transmembrane beta-amyloid precursor protein (APP) by proteolytic processing. Although mechanisms associated with Abeta generation are not fully understood, it is known that Abeta can be generated within endosomal compartments upon internalization of APP from the cell surface. The low density lipoprotein receptor-related protein (LRP) was previously shown to mediate the endocytosis of APP isoforms containing the Kunitz proteinase inhibitor domain (Kounnas, M. Z., Moir, R. D., Rebeck, G. W., Bush, A. I., Argraves, W. S., Tanzi, R. E., Hyman, B. T., and Strickland, D. K. (1995) Cell 82, 331-340; Knauer, M. F., Orlando, R. A., and Glabe, C. G. (1996) Brain Res. 740, 6-14). The objective of the current study was to test the hypothesis that LRP-mediated internalization of cell surface APP can modulate APP processing and thereby affect Abeta generation. Here, we show that long term culturing of cells in the presence of the LRP-antagonist RAP leads to increased cell surface levels of APP and a significant reduction in Abeta synthesis. Further, restoring LRP function in LRP-deficient cells results in a substantial increase in Abeta production. These findings demonstrate that LRP contributes to Abeta generation and suggest novel pharmacological approaches to reduce Abeta levels based on selective LRP blockade.