Calpain inhibitor I increases β-amyloid peptide production by inhibiting the degradation of the substrate of γ-secretase -: Evidence that substrate availability limits β-amyloid peptide production

The calpain inhibitor N-acetyl-leucyl-leucyl-norleucinal (ALLN) has been reported to have complex effects on the production of the beta-amyloid peptide (A beta), In this study, the effects of ALLN on the processing of the amyloid precursor protein (APP) to A beta were examined in 293 cells expressing APP or the C-terminal 100 amino acids of APP (C100), In cells expressing APP or low levels of C100, ALLN increased A beta 40 and A beta 42 secretion at low concentrations, decreased A beta 40 and A beta 42 secretion at high concentrations, and increased cellular levels of C100 in a concentration-dependent manner by inhibiting C100 degradation. Low concentrations of ALLN increased A beta 42 secretion more dramatically than A beta 40 secretion. ALLN treatment of cells expressing high levels of C100 did not alter cellular C100 levels and inhibited A beta 40 and A beta 42 secretion with similar IC50 values. These results suggest that C100 can be processed both by gamma-secretase and by a degradation pathway that is inhibited by low concentrations of ALLN, The data are consistent with inhibition of gamma-secretase by high concentrations of ALLN but do not support previous assertions that ALLN is a selective inhibitor of the gamma-secretase producing A beta 40. Rather, A beta 42 secretion may be more dependent on C100 substrate concentration than A beta 40 secretion.