Calpain inhibitor I increases beta-amyloid peptide production by inhibiting the degradation of the substrate of gamma-secretase - Evidence that substrate availability limits beta-amyloid peptide production

The calpain inhibitor N-acetyl-leucyl-leucyl-norleucinal (ALLN) has been re ported to have complex effects on the production of the beta-amyloid peptid e (A beta), In this study, the effects of ALLN on the processing of the amy loid precursor protein (APP) to A beta were examined in 293 cells expressin g 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-d ependent manner by inhibiting C100 degradation. Low concentrations of ALLN increased A beta 42 secretion more dramatically than A beta 40 secretion. A LLN treatment of cells expressing high levels of C100 did not alter cellula r 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 concentrat ions of ALLN, The data are consistent with inhibition of gamma-secretase by high concentrations of ALLN but do not support previous assertions that AL LN is a selective inhibitor of the gamma-secretase producing A beta 40. Rat her, A beta 42 secretion may be more dependent on C100 substrate concentrat ion than A beta 40 secretion.