Protein Never in Mitosis Gene A Interacting-1 regulates calpain activity and the degradation of cyclooxygenase-2 in endothelial cells

Protein Never in Mitosis Gene A Interacting-1 (PIN1) is an enzyme that regulates transcription, and turnover of mRNA and proteins. PIN1 is a cis-trans peptidyl-prolyl isomerase that contains an amino-terminal domain, the tryptophan-tryptophan (WW) domain, which is characterized by two tryptophan residues separated by 22 amino acids that can bind to phosphorylated serine- or threonine-proline sequences in substrate proteins. PIN1 also isomerizes this motif with its carboxy-terminal catalytic domain [1]. Isomerization of the phosphorylated serine- or threonine-proline motif has a significant effect on conformation of many phospho-proteins. The conformational switching catalyzed by PIN1 allows it to regulate transcription factors, mRNA stabilization factors, and the susceptibility of a growing list of proteins to post-translational modifications and proteases [1‐5].

Previously, we found that depletion of PIN1 and treatment with a calpain inhibitor each reduced the degradation of inducible nitric oxide synthase (iNOS) in murine aortic endothelial cells (MAEC) stimulated with E. coli endotoxin (LPS) and interferon-γ (IFN). PIN1 bound to iNOS suggesting that it might directly regulate the sensitivity of iNOS to calpain [6]. PIN1 may also regulate expression of inflammatory proteins by an effect on calpain.

Cyclooxygenase (COX)-2 is induced by LPS, IFN, and other factors in endothelial cells cultured from various organs and species [7‐14]. Elevated endothelial COX-2 may contribute to vascular pathogenesis [15, 16]. This enzyme is also significant for endotoxin action as COX-2 knockout mice are resistant to LPS-induced inflammation and death [17]. COX-2 has a relatively short half-life, indicating that turnover may effectively control its expression [8]. While COX-2 and iNOS can be degraded by several processes [6, 8, 18‐20], calpain inhibitors are known to suppress cleavage of iNOS [6] and COX-2 [18].

The purpose of this investigation was to determine whether PIN1 regulates the expression of COX-2, which is induced by LPS and IFN in MAEC. It was hypothesized that PIN1 would associate with COX-2 and that depletion of PIN1 would enhance its induction in MAEC. The impact of PIN1 depletion on calpain activity was also determined.

Endothelial cell growth supplement, heparin, phenylmethylsulfonyl fluoride, Bradford reagent, E. coli LPS, serotype 0111:B4, and arachidonic acid were obtained from Sigma Chemical Co. (St Louis, MO). Recombinant mouse IFN was from R&D Systems (Minneapolis, MN). Cycloheximide, carbobenzoxy-valinyl-phenylalaninal (zVF, MDL-28170 or calpain inhibitor III), PD150606, porcine μ-calpain, [4-((4-(dimethylamino)phenyl)azo)benzoic acid, succinimidyl ester]-threonine-proline-leucine-lysine~serine-proline-proline-proline-serine-proline-arginine-[5-((2-aminoethyl)amino)naphthalene-1-sulfonic acid], and carboxybenzyl-phenylalanine-arginine-7-amido-4-methylcoumarin were obtained from Calbiochem (La Jolla, CA). Fetal bovine serum was from Hyclone Laboratories (Logan, UT). Agarose, ethidium bromide, ethylenediamine tetraacetic acid, sodium dodecyl sulfate, NaCl, Na₃VO₄, NaF, tris-base and tween 20 were obtained from Fisher Scientific (Fair Lawn, NJ). Triton X-100 was from Pierce (Rockford, IL). Dulbecco's minimum essential medium, trypsin, Trizol, Superscript Reverse Transcriptase Taq DNA polymerase, RNAse-free DNAse, deoxynucleotides, and protein G agarose were purchased from Invitrogen (Carlsbad, CA). Glutathione-sepharose was purchased from Amersham Biosciences (Uppsala, Sweden). A prostaglandin E2 competition enzyme-linked immunosorbent assay kit was obtained from R and D Systems, Minneapolis, MN. Anti-COX-2 antibody directed against a 16 amino acid sequence, ending 7 residues from the C-terminus of the protein, SC-560 and NS-398 were purchased from Cayman Chemical (Ann Arbor, MI). Anti-PIN1 was from R&D Systems (Minneapolis, MN). Horseradish peroxidase-conjugated goat anti-mouse and goat anti-rabbit secondary antibodies were from Jackson Immunoresearch Laboratories, Inc. (West Grove, PA). Renaissance Enhanced Chemiluminescence Reagent was purchased from New England Nuclear Life Sciences (Boston, MA).

Previously, KD shRNA was shown to reduce PIN1 by 85% compared with Control shRNA in MAEC [6]. COX-2 protein was very low in vehicle-treated KD and Control MAEC (figure 1), and incubation with either LPS or IFN alone did not induce it (data not shown). However, stimulation with 10 μg LPS plus 20 ng IFN per ml increased COX-2 expression. The protein appeared to increase as early as 1 h after treatment, and induction persisted through 24 h. Differences between KD and Control cells were qualitatively noticeable by 4 h after treatment and became greater with time (figure 1A). After 24 h, the signal for COX-2 protein was increased 10-fold in Control shRNA MAEC (figure 1B). The COX-2 signal was significantly more induced in PIN1 KD cells (47-fold). Similar results were obtained in 2 other independent pairs of cultures selected for the KD and Control shRNA (data not shown). COX-2-mediated prostaglandin E2 production increased 3-fold in KD MAEC, but not in the Control cells (figure 2). LPS/IFN induced COX-2 mRNA in KD and Control shRNA cells as indicated by RT-PCR. The message increased within 1 h and remained elevated at 24 h. However, there was no difference between KD and Control shRNA MAEC at any time (figure 3).

PIN1 KD and Control shRNA MAEC were pretreated with vehicle or the calpain inhibitors, zVF or PD150606, for 1 h, and then treated with LPS and IFN for 24 h. Again, COX-2 increased more in KD than Control cells (figure 4). In Control cells, COX-2 was induced 5.5-fold more in the presence of zVF than in its absence. zVF also increased the induction of COX-2 from its elevated level in KD MAEC by a factor of two (figure 4A). PD 150606, which is more selective than zVF for calpain relative to cathepsin activities [29, 30], also increased the induction of COX-2 in KD and Control cells. zVF did not increase the induction of COX-2 mRNA in cells treated with LPS/IFN for 1 or 24 h (figure 5).

The effect of PIN1 depletion and zVF on degradation of COX-2 was assessed. Cells were induced with LPS/IFN and then treated with 90 μg cycloheximide/ml to block translation. The level of COX-2 protein fell to 44% of its initial value 2 h after addition of cycloheximide to Control shRNA cells (figure 6). However, a similar decrease to 47% was delayed until 4 h in KD MAEC. COX-2 protein fell only to 78% of initial 2 h after cycloheximide in zVF-treated Control cells. zVF also inhibited the loss of COX-2 in KD cells at 4 h.

To confirm that COX-2 is a potential substrate for calpain, its digestion in vitro was examined. Addition of porcine μ-calpain to extracts of LPS/IFN-treated Control cells caused a concentration-dependent loss of COX-2 signal (figure 7).

Since PIN1 is known to bind its substrate proteins, interaction with COX-2 was investigated. Immunoprecipitation of PIN1 from extracts of vehicle- or LPS/IFN-treated Control cells did not produce any COX-2 detectable on western blots. COX-2 was not pulled down with glutathione-S-transferase-PIN1 fusion protein or glutathione-S-transferase (not shown).

Given the effect of calpain inhibitors on COX-2, calpain and cathepsin activities were measured. LPS/IFN increased calpain activity 6.0-fold in KD cells, and 5.9-fold in Control MAEC. Calpain activity in vehicle-treated KD cells was approximately 45% of the activity in the Control cells with or without treatment with LPS/IFN (figure 8A). Cathepsin activity was measured since zVF might also inhibit it. There were no differences in cathepsin activity in extracts of KD and Control MAEC, with our without treatment with LPS/IFN (figure 8B).

PIN1 regulates the levels and activity of factors that can affect COX-2 synthesis in various cell types [2, 4, 31‐36]. Here, suppression of PIN1 in endothelial cells increased the induction of COX-2, and COX-2-dependent production of prostaglandin E2 by LPS/IFN (figures 1 and 2). Despite a nearly 5-fold greater induction of COX-2 protein in KD compared with Control MAEC, there was no difference in the induction of COX-2 mRNA (figure 3). This suggests that PIN1 regulates COX-2 by a post-transcriptional mechanism. Consistent with a post-transcriptional effect, PIN1 depletion reduced the turnover of COX-2 (figure 6). Since COX-2 has a relatively short half-life, inhibition of turnover could lead to large, cumulative, post-transcriptional increases after induction with LPS/IFN [8].

One prior study revealed that cleavage of COX-2 was reduced by the inhibitor, E-64d, in human synovial fibroblasts [18]. Here, the calpain inhibitor, zVF, increased the induction of COX-2 (figure 4) and reduced its degradation (figure 6), without increasing its mRNA (figure 5). As for E-64d, zVF can also inhibit cathepsin activity at concentrations similar to those that inhibit calpain [29, 37, 38]. Therefore, PD 150606, which is more selective for calpain compared with cathepsin [30], was tested. Like zVF, PD 150606 increased the induction of COX-2 in KD and Control MAEC (figure 4B), further suggesting that calpain is responsible for restraining the induction of COX-2 in MAEC.

In support of this idea, it was shown here for the first time that PIN1 depletion reduced calpain activity in endothelial cells. In contrast, cathepsin activity was not affected by PIN1 depletion (figure 8). This result, combined with the effects of zVF and PD150606 on COX-2 induction and turnover, suggests again that calpain limits the expression of COX-2 in MAEC. Indeed, COX-2 was degraded by μ-calpain in vitro, indicating that it is a potential substrate in cells (figure 7). The reduced calpain activity in KD extracts (figure 8) could be due to an increase in expression or function of calpastatin or other unrecognized endogenous calpain inhibitors in KD cells, or to a reduction in expression or function of calpains [39]. Nevertheless, the results suggest that PIN1 depletion reduces calpain activity, consequently reducing the turnover of COX-2 in MAEC.

zVF further reduced the loss of COX-2 in cycloheximide-treated KD cells (figure 6). This may be due to the partial 55% reduction of calpain activity in KD MAEC (figure 8). The partially reduced calpain activity could account for the intermediate loss of COX-2 in the cycloheximide-treated KD cells, allowing zVF to further suppress turnover. It may also explain the ability of calpain inhibitors to increase induction of COX-2 in both KD and Control MAEC (figure 4). The partial reduction of calpain activity may be due to the incomplete (85%) suppression of PIN1 by the shRNA [6]. PIN1 may also function as a modulator of calpain activity and not as an absolute requirement.

Previously, we observed that PIN1 depletion and zVF each increased the induction of iNOS, and reduced its degradation. PIN1 physically interacted with iNOS. The WW and catalytic domains of PIN1 appeared to contribute to the association [6]. This suggested that PIN1 depletion might alter the susceptibility of its targets to digestion by calpain. For example, PIN1 could associate with these substrates and catalyze proline isomerization, affecting protease sensitivity. In contrast to iNOS, however, interaction between COX-2 and PIN1 was not detected here. A role for direct interaction between COX-2 and PIN1 cannot be completely excluded, however, since association of the proline isomerase with its putative substrate may be weak or transient. PIN1 could also affect association of COX-2, or iNOS, with other proteins that may indirectly regulate proteolysis. Thus, effects of PIN1 on calpain activity and/or COX-2, or associated factors, could affect the sensitivity of COX-2 to digestion with calpain.

Overall, the results indicate that PIN1 regulates the induction of COX-2, and iNOS, by a previously unknown effect on calpain-mediated turnover in MAEC. The mechanisms by which PIN1 regulates calpain activity are under investigation. In particular, PIN1 could affect the expression or activity of calpain subunits, and the endogenous inhibitor of heterodimeric calpains, calpastatin [39].

The effects of COX-2 in acute and chronic inflammatory responses in the vasculature are complicated by multiple primary and secondary stimuli that may be present, and by cellular factors, such as supply of arachidonic acid, complement of various prostaglandin synthases, and expression of prostaglandin receptors [16]. Here, depletion of PIN1 and inhibition of calpain each caused over-induction of both COX-2 and iNOS. The consequences of co-induction of these two particular enzymes may be significant. Peroxynitrite from NO increases prostaglandin synthesis [40], and S-nitrosylation of COX-2 activates the enzyme and contributes to cell injury [41, 42]. The impact of the co-induction of iNOS and COX-2 in endothelium requires further investigation.

The role of calpain activity may also be complex. The most well-studied calpains, heterodimeric μ- and m-calpain, can cleave numerous protein substrates, and enhance or down-regulate different signal transduction processes. Excessive calpain activity can also cause cell injury and death in several organs, which can be reduced with calpain inhibitors [39, 43]. Thus, it remains to be determined whether PIN1 or specific calpains in endothelial cells can be exploited to manipulate inflammatory activation in a therapeutically useful manner. In any case, the results here indicate that COX-2 is degraded by calpain, and that PIN1 regulates its expression via effects on calpain activity in MAEC.

Depletion of PIN1 increased induction of COX-2 by LPS/IFN by a post-transcriptional mechanism associated with reduced calpain activity. Consistent with the short lifespan of COX-2 in MAEC, suppression of PIN1 and calpain inhibitors increased its induction. This previously unknown connection suggests that PIN1 may normally function to maintain calpain activity and, consequently, restrain the induction COX-2, iNOS, and perhaps other substrates in MAEC. PIN1 is likely to regulate a range of calpain-dependent endothelial activities.