THE EFFECT OF TACRINE AND LEUPEPTIN ON THE SECRETION OF THE BETA-AMYLOID PRECURSOR PROTEIN IN HELA-CELLS

The senile plaque in Alzheimer's disease (AD) consists mainly of the a myloid beta-peptide (A beta) derived from a larger beta-amyloid precur sor protein (beta APP). The majority of BAPP is processed by either a secretory or lysosomal/ endosomal pathway. Soluble derivatives of beta APP (sAPP) and A beta generated by the proteolytic processing of full -length BAPP are normally secreted into the conditioned medium of cult ured cells. Tacrine, a centrally active potent cholinesterase inhibito r that has been shown to improve cognitive functions in some patients with AD, inhibits the secretion of sAPP. Here we have investigated whe ther leupeptin, a lysosomal protease inhibitor, could influence this e ffect of tacrine. We analyzed levels of beta APP derivatives in cultur ed HeLa cells by immunoblotting cell lysates and conditioned media usi ng the monoclonal antibody 22C11. Levels of sAPP normally present in c onditioned media were severely reduced by treating cells with tacrine. The treatment of cells with tacrine resulted in a small decrease in t he intracellular levels of beta APP. The effect of treating the cells with tacrine did not depend upon the growing state of the cells as a s imilar effect was observed when the drug was added either during initi al plating of the cells or after the attachment of the cells. The effe ct of tacrine was not affected by preincubating the cells with low ser um in the culture medium. The treatment of cells with tacrine plus leu peptin reduced the secretion of sAPP in the medium to the same degree as did the treatment with tacrine alone, suggesting that the tacrine-m ediated inhibition of sAPP release may not involve leupeptin-sensitive proteolytic pathways. The results suggest that the inhibitory effect of tacrine on sAPP secretion is not due to the proteolytic cleavage of the holoprotein in the medium.