Presenilin‐directed inhibitors of γ‐secretase trigger caspase 3 activation in presenilin‐expressing and presenilin‐deficient cells

The amyloid β peptide (Aβ) is generated by subsequent cleavages by β‐ and γ‐secretases. Therefore, these two enzymes are putative therapeutic targets to prevent Aβ production, and hopefully to slow down or even stop the Alzheimer's disease (AD) neurodegenerative process. Several studies have revealed that γ‐secretase hydrolyses other important substrates besides β‐amyloid precursor protein (βAPP) thus adding another level of complexity to designing fully AD‐specific interfering drugs. Here we demonstrate that three distinct presenilin‐directed γ‐secretase inhibitors as well as JLK compounds indirectly potentiate caspase 3 activity, the effector caspase of the apoptotic cascade. Thus, inhibitors were shown to drastically stimulate caspase 3 activity in wild‐type mice blastocyst‐derived and fibroblast cells. Interestingly, some of these inhibitors known to interact with presenilins also trigger caspase activation in presenilin‐deficient cells. However, inhibitors do not affect recombinant caspase 3 activity, indicating that the effect on this enzyme was indirect. Furthermore, we established that caspase 3 activation was not due to an effect of γ‐secretase inhibitors on calpains, a family of proteolytic enzymes able to modulate caspase 3 activity. Altogether, our data demonstrate that presenilin‐directed γ‐secretase inhibitors affect caspase 3 activity in a presenilin‐independent manner. Therefore, as presenilin‐dependent γ‐secretase activity is not specific for βAPP and because its inhibitors clearly affect other vital cell functions, care should be taken in considering ‘γ‐secretase’ inhibitors as putative therapeutic tools to interfere with AD pathology.