The metalloprotease inhibitor TIMP-3 regulates amyloid precursor protein and apolipoprotein E receptor proteolysis

Cellular cholesterol levels alter the processing of the amyloid precursor protein ( APP) to produce A beta. Activation of liver X receptors ( LXRs), one cellular mechanism to regulate cholesterol homeostasis, has been found to alter A beta 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 alpha-secretases. We found that TIMP-3 inhibited alpha-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 beta-C-terminal fragment and A beta production. These data suggest that TIMP-3 preferentially routes APP and ApoER2 away from the cell surface and alpha-secretase cleavage and encourages endocytosis and beta-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 beta.