Modulation of inositol 1,4,5‐triphosphate concentration by prolyl endopeptidase inhibition

Prolyl endopeptidase (PEP) is a proline‐specific oligopeptidase with a reported effect on learning and memory in different rat model systems. Using the astroglioma cell line U343, PEP expression was reduced by an antisense technique. Measuring different second‐messenger concentrations revealed an inverse correlation between inositol 1,4,5‐triphosphate [Ins(1,4,5)P3] concentration and PEP expression in the generated antisense cell lines. However, no effect on cAMP generation was observed. In addition, complete suppression of PEP activity by the specific inhibitor, Fmoc‐Ala‐Pyrr‐CN (5 µm) induced in U343 and other cell lines an enhanced, but delayed, increase in Ins(1,4,5)P3 concentration. This indicates that the proteolytic activity of PEP is responsible for the observed effect. Furthermore, the reduced PEP activity was found to amplify Substance P‐mediated stimulation of Ins(1,4,5)P3. The effect of reduced PEP activity on second‐messenger concentration indicates a novel intracellular function of this peptidase, which may have an impact on the reported cognitive enhancements due to PEP inhibition.