In Alzheimer's disease (AD), enhancing alpha-secretase processing of amyloid precursor protein (APP) is an important pathway to decrease neurotoxic amyloid beta (A beta) secretion. The alpha-secretase is reported to be regulated by protein kinase C (PKC) and various endogenous proteins or cell surface receptors. In this report, we first examined whether A beta reduces alpha-secretase activity, and showed that A beta peptide 1-40 (0.001 and 0.01 mu M) reduced the secretion of soluble amyloid precursor protein alpha (sAPP alpha) in carbachol-stimulated SH-SY5Y neuroblastoma cells. E-64-d (3 mu M), which is a potent calpain inhibitor that prevents PKC degradation, ameliorated the A beta-induced reduction of sAPP alpha secretion. In addition, we observed that A beta significantly enhanced ceramide production by activating neutral sphingomyelinase. The cell-permeable ceramide analog, C-2-ceramide (1 mu g/mL), also reduced sAPP alpha secretion, and in addition, E-64-d eliminated the observed decrease of sAPP alpha secretion. C-2-ceramide induced down-regulation of PKC-alpha, -beta(1), and -beta(2) isozymes in SH-SY5Y cells. These findings suggest that ceramide may play an important role in sAPP alpha processing by modulating PKC activity. (C) 2013 Elsevier Inc. All rights reserved.