FIBROBLASTS OF PATIENTS AFFECTED BY DOWNS-SYNDROME OVERSECRETE AMYLOID PRECURSOR PROTEIN AND ARE HYPORESPONSIVE TO PROTEIN-KINASE-C STIMULATION

The present study investigates the ability of the pharmacologic activa tion of protein kinase C (PKC) to modulate amyloid precursor protein ( APP) secretion in human skin fibroblasts from patients affected by Dow n's syndrome (DS). We assessed DS subjects at the Hospital Institute o f Sospiro, Cremona, and at the Alzheimer's Disease Unit of the Sacred Heart Hospital in Brescia, and we subdivided them into nondemented (ND S) and demented (DDS) patients. All DS patients were trisomy 21 karyot ype. DS fibroblasts had an increased content of APP immunoreactive mat erial as revealed by immunocytochemistry analysis. The basal secretion of soluble APP was higher (+94.6%) in Down's cells with respect to co ntrols. The observation on the fibroblasts prepared from DS is consist ent with these patients' possessing an extra copy of the APP gene (map ped on chromosome 21) leading to increased APP expression. Phorbol-12, 13-dibutyrate (PdBu, 9 to 150 nM) treatment promoted a dose-dependent increase of secreted APP in the conditioned medium of control fibrobla sts. The peak response (+102.2%) was attained using 150 nM PdBu. In Do wn's fibroblasts, PdBu stimulated APP secretion already maximally at l ow concentrations (9 nM), but the peak response, due to the higher bas al release, was lower on a percentage basis (+16.4%) than in control f ibroblasts. The results indicate that in Down's fibroblasts the mechan isms controlling APP release are at least quantitatively altered. In a ddition, these results suggest caution when using information obtained from Down's patients to model Alzheimer's disease biochemical defects .