THROMBIN RECEPTOR ACTIVATION INDUCES SECRETION AND NONAMYLOIDOGENIC PROCESSING OF AMYLOID BETA-PROTEIN PRECURSOR

The amyloid beta-protein (A beta) and protease nexin-2/ amyloid beta-p rotein precursor (PN-2/A beta PP) are major constituents of senile pla ques and cerebrovascular deposits in individuals with Alzheimer's dise ase and related disorders. It has been suggested that the coagulation protease thrombin may process A beta PP in a manner leading to the for mation of A beta. Here we investigated the effects of thrombin on the secretion and processing of PN-2/A beta PP and the production of A bet a in a cellular system. Incubation of glioblastoma cells with thrombin (1-5 nM) resulted in the accumulation of abnormally processed, carbox yl-terminal-truncated forms of secreted PN-2/ A beta PP (approximate t o 85 kDa) in the culture medium. Higher concentrations of thrombin (>1 0 nM) also increased the levels of secreted PN-2/A beta PP in cultured untransfected glioblastoma cells and glioblastoma cells that were sta bly transfected to overproduce the 695 isoform of A beta PP. Increased secretion of PN-2/A beta PP required the proteolytic activity of thro mbin, was induced by activation of the thrombin receptor by agonist pe ptides, and required activation of protein kinase C. Incubation of the untransfected and transfected glioblastoma cells with thrombin led to decreased levels of soluble A beta in the culture medium consistent w ith previously suggested mechanisms regarding the secretion of PN-2/A beta PP. Although the present studies suggest that thrombin does not d irectly contribute to A beta formation, its proteolysis of secreted PN -2/A beta PP may disrupt regions near the carboxyl terminus of the sec reted proteins that account for their neuroprotective and cell adhesiv e properties.