The peptidyl-proline isomerase, protein never in mitosis gene A interacting-1 (PIN1) binds and isomerizes proteins phosphorylated on serine/threonine before a proline. It was previously found that depletion of PIN1 greatly increased induction of cyclooxygenase-2 and inducible nitric oxide synthase by lowering calpain activity in murine aortic endothelial cells (MAEC). Here we investigated the effect of PIN1 on the endogenous inhibitor of heterodimeric mu- and m-calpains, calpastatin. MAEC were transduced with small hairpin (sh) RNA to knock down PIN1 (KD) or an inactive Control shRNA. Cells were also treated with non-targeted double stranded small inhibitory RNA (siRNA) or siRNA designed to deplete calpastatin. Despite reducing calpain activity, PIN1 KD did not significantly affect the expression of mu- and m-calpains, or calpastatin, compared to Control shRNA. Instead, depletion of PIN1 increased the inhibitory activity of calpastatin. Calpastatin co-immunoprecipitated with endogenous PIN1 and was pulled down with glutathione-S-transferase (GST)-PIN1 fusion protein. Adding GST-PIN1 to KD cell extracts lacking PIN1 reduced calpastatin inhibitory activity. Substrate binding and catalytic domain mutants of PIN1 failed to do so. These results suggest that protein interaction and the proline isomerase functions of PIN1 are required for it to inhibit calpastatin. Furthermore, depletion of calpastatin raised calpain activity and reduced calpain inhibitory activity to similar levels in KID and Control MAEC, indicating that calpastatin is required for PIN1 depletion to lower calpain activity. Thus. PIN1 apparently restrains the ability of calpastatin to inhibit calpain, maintaining calpain activity in endothelial cells. PIN1 may act directly via phosphorylated serine/threonine-proline motifs in calpastatin, or indirectly via other PIN1 substrates that control calpastatin. (C) 2011 Elsevier Inc. All rights reserved.