Cellular cholesterol levels alter the processing of the amyloid precursor protein (APP) to produce A. Activation of liver X receptors (LXRs), one cellular mechanism to regulate cholesterol homeostasis, has been found to alter A levels in vitro and in vivo. To identify genes regulated by LXR, we treated human neuroblastoma cells with an LXR agonist (TO-901317) and examined gene expression by microarray. As expected, TO-901317 upregulated several cholesterol metabolism genes, but it also decreased expression of a metalloprotease inhibitor, TIMP-3. We confirmed this finding using real-time PCR and by measuring TIMP-3 protein in glia, SY5Y cells, and COS7 cells. TIMP-3 is a member of a family of metalloproteinase inhibitors and blocks A disintegrin and metalloproteinase-10 (ADAM-10) and ADAM-17, two APP ␣-secretases. We found that TIMP-3 inhibited ␣-secretase cleavage of APP and an apolipoprotein E (apoE) receptor, ApoER2. TIMP-3 decreased surface levels of ADAM-10, APP, and ApoER2. These changes were accompanied by increased APP -Cterminal fragment and A production. These data suggest that TIMP-3 preferentially routes APP and ApoER2 away from the cell surface and ␣-secretase cleavage and encourages endocytosis and -secretase cleavage. In vivo, TO-901317 decreased brain TIMP-3 levels. TIMP-3 protein levels were increased in human Alzheimer's disease (AD) brain and in APP transgenic mice, suggesting that increased levels of TIMP-3 in AD may contribute to higher levels of A.