BACKGROUND Both tissue plasminogen activator (tPA) in the circulation and urokinase (uPA) in tissues cleave plasminogen (PLG) to plasmin to promote clot lysis. Tranexamic acid (TXA) blocks both the tPA-dependent generation of plasmin on blood clots as well as active plasmin binding to polymerized fibrin, and is commonly administered for bleeding in trauma to limit fibrinolysis. In addition to lysing clots, however, active plasmin also cleaves complement proteins, potentially enhancing inflammation. Because TXA does not block uPA-dependent plasmin generation from PLG and instead augments it, we hypothesized that administration of TXA could enhance or inhibit proinflammatory C5a formation in a PLG activator–dependent manner. METHODS Citrate platelet-poor plasma (PPP) and PPP depleted of complement protein C3 or PLG were obtained from healthy donors and commercial sources. Platelet-poor plasma was treated ex vivo with or without TXA and either with or without tPA or with or without uPA. Clotting was then induced by calcium and thrombin in clotted PPP experiments, while unclotted PPP experiments were treated with vehicle controls. C5a levels were measured via enzyme–linked immunosorbent assay. Data were expressed as mean ± SEM. RESULTS Plasmin-mediated fibrinolysis by tPA in clotted PPP led to an approximately threefold increase in C5a production (p < 0.0001), which was significantly inhibited by TXA (p < 0.001). Paradoxically, when fibrinolysis was induced by uPA, TXA treatment led to further increases in C5a production beyond uPA alone (p < 0.0001). Furthermore, clotting was not required for C5a generation from uPA + TXA. C3 depletion had no effect on C5a production, while depletion of PLG eliminated it. CONCLUSIONS Tranexamic acid administration can have proinflammatory or anti-inflammatory effects through regulating C5a generation by plasmin, depending on the predominating PLG activator. Tranexamic acid may cause significant inflammatory C5a elevations in injured tissues by augmenting uPA-mediated plasmin generation in a fibrin-independent manner. In contrast, TXA reduces C5a generation during tPA-mediated fibrinolysis that may reduce inflammatory responses. In vivo validation of these novel ex vivo findings is warranted and may have important clinical consequences.

ABSTRACTBinding of polo‐like kinase 1 (Plk1) polo‐box domains (PBDs) to phosphothreonine (pThr)/phosphoserine (pSer)‐containing sequences is critical for the proper function of Plk1. Although high‐affinity synthetic pThr‐containing peptides provide starting points for developing PBD‐directed inhibitors, to date the efficacy of such peptides in whole cell assays has been poor. This potentially reflects limited cell membrane permeability arising, in part, from the di‐anionic nature of the phosphoryl group or its mimetics. In our current article we report the unanticipated on‐resin N(τ)‐alkylation of histidine residues already bearing a N(π)‐ alkyl group. This resulted in cationic imidazolium‐containing pThr peptides, several of which exhibit single‐digit nanomolar PBD‐binding affinities in extracellular assays and improved antimitotic efficacies in intact cells. We enhanced the cellular efficacies of these peptides further by applying bio‐reversible pivaloyloxymethyl (POM) phosphoryl protection. New structural insights presented in our current study, including the potential utility of intramolecular charge masking, may be useful for the further development of PBD‐binding peptides and peptide mimetics. © 2014 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 102: 444–455, 2014.

The polo-box domain (PBD) of polo-like kinase 1 (Plk1) is essentially required for the function of Plk1 in cell proliferation. The availability of the phosphopeptide-binding pocket on PBD provides a unique opportunity to develop novel protein–protein interaction inhibitors. Recent identification of a minimal 5-residue-long phosphopeptide, PLHSpT, as a Plk1 PBD-specific ligand has led to the development of several peptide-based inhibitors, but none of them is cyclic peptide. Through the combination of single-peptoid mimics and thio-ether bridged cyclization, we successfully demonstrated for the first time two cyclic peptomers, PL-116 and PL-120, dramatically improved the binding affinity without losing mono-specificity against Plk1 PBD in comparison with the linear parental peptide, PLHSpT. These cyclic peptomers could serve as promising templates for future drug designs to inhibit Plk1 PBD.

Binding of polo-like kinase 1 (Plk1) polo-box domains (PBDs) to phosphothreonine (pThr)/phosphoserine (pSer)-containing sequences is critical for the proper function of Plk1. Although high-affinity synthetic pThr-containing peptides may be used to disrupt PBD function, the efficacy of such peptides in whole cell assays has been poor. This potentially reflects limited cell membrane permeability arising in part from the di-anionic nature of the phosphoryl group. We report five-mer peptides containing mono-anionic pThr phosphoryl esters that exhibit single-digit nanomolar PBD binding affinities in extracellular assays and improved antimitotic efficacies in whole cell assays. The cellular efficacies of these peptides have been further enhanced by the application of bio-reversible pivaloyloxymethyl (POM) phosphoryl protection to a pThr-containing polypeptide. Our findings may redefine structural parameters for the development of PBD-binding peptides and peptide mimetics.

In an effort to develop improved binding antagonists of the polo-like kinase 1 (Plk1) polo-box domain (PBD), we optimized interactions of the known high affinity 5-mer peptide PLHSpT using oxime-based post solid-phase peptide diversification of the N-terminal Pro residue. This allowed us to achieve up to two orders of magnitude potency enhancement. An X-ray crystal structure of the highest affinity analogue in complex with Plk1 PBD revealed new binding interactions in a hydrophobic channel that had been occluded in X-ray structures of the unliganded protein. This study represents an important example where amino acid modification by post solid-phase oxime ligation can facilitate the development of protein-protein interaction inhibitors by identifying new binding pockets that would not otherwise be accessible to coded amino acid residues.

We replaced the amino terminal Pro residue of the Plk1 polo‐box‐domain‐binding pentapeptide (PLHSpT) with a library of N‐alkyl‐Gly “peptoids”, and identified long‐chain tethered phenyl moieties giving greater than two‐orders‐of‐magnitude affinity enhancement. Further simplification by replacing the peptoid residue with appropriate amides gave low‐nanomolar affinity N‐acylated tetrapeptides. Binding of the N‐terminal long‐chain phenyl extension was demonstrated by X‐ray co‐crystal data.

We obtained unanticipated synthetic byproducts from alkylation of the δ(1) nitrogen (N3) of the histidine imidazole ring of the polo-like kinase-1 (Plk1) polo-box domain (PBD)-binding peptide PLHSpT. For the highest-affinity byproduct, bearing a C(6)H(5)(CH(2))(8)- group, a Plk1 PBD cocrystal structure revealed a new binding channel that had previously been occluded. An N-terminal PEGylated version of this peptide containing a hydrolytically stable phosphothreonyl residue (pT) bound the Plk1 PBD with affinity equal to that of the non-PEGylated parent but showed markedly less interaction with the PBDs of the two closely related proteins Plk2 and Plk3. Treatment of cultured cells with this PEGylated peptide resulted in delocalization of Plk1 from centrosomes and kinetochores and in chromosome misalignment that effectively induced mitotic block and apoptotic cell death. This work provides insights that might advance efforts to develop Plk1 PBD-binding inhibitors as potential Plk1-specific anticancer agents.

Nat. Struct. Mol. Biol. 16, 876–882 (2009); published online 13 July 2009; corrected after print 21 September 2010 In the version of this article initially published, two of the numbers shown in Table 1 had the wrong sign. The error has been corrected in the HTML and PDF versions of the article.

Polo-like kinase-1 (Plk1) is activated before mitosis by Aurora A and its cofactor Bora. In mitosis, Bora is degraded in a manner dependent on Plk1 kinase activity and the E3 ubiquitin ligase SCF-beta TrCP. Here, we show that Plk1 is also required for the timely destruction of its activator Aurora A in late anaphase. It has been shown that Aurora A destruction is controlled by the auxiliary subunit Cdh1 of the Anaphase-Promoting Complex/Cyclosome (APC/C). Remarkably, we found that Plk1-depletion prevented the efficient dephosphorylation of Cdh1 during mitotic exit. Plk1 mediated its effect on Cdh1, at least in part, through direct phosphorylation of the human phosphatase Cdc14A, controlling the phosphorylation state of Cdh1. We conclude that Plk1 facilitates efficient Aurora A degradation through APC/C-Cdh1 activation after mitosis, with a potential role for hCdc14A.

Cell division: PLK1 and aurora A at the checkpoint Polo-like kinase-1 (PLK1) is an essential mitotic kinase regulating multiple aspects of the cell division process. Here, activation of PLK1 is shown to occur before mitosis and to depend on phosphorylation by aurora-A kinase, facilitated by a cofactor Bora. The initial activation of PLK1 seems to be a primary function of aurora-A.

Polo-like kinase-1 (Plk1) phosphorylates a number of mitotic substrates, but the diversity of Plk1-dependent processes suggests the existence of additional targets. Plk1 contains a specialized phosphoserine-threonine binding domain, the Polo-box domain (PBD), postulated to target the kinase to its substrates. Using the specialized PBD of Plk1 as an affinity capture agent, we performed a screen to define the mitotic Plk1-PBD interactome by mass spectrometry. We identified 622 proteins that showed phosphorylation-dependent mitosis-specific interactions, including proteins involved in well-established Plk1-regulated processes, and in processes not previously linked to Plk1 such as translational control, RNA processing, and vesicle transport. Many proteins identified in our screen play important roles in cytokinesis, where, in mammalian cells, the detailed mechanistic role of Plk1 remains poorly defined. We go on to characterize the mitosis-specific interaction of the Plk1-PBD with the cytokinesis effector kinase Rho-associated coiled-coil domain-containing protein kinase 2 (Rock2), demonstrate that Rock2 is a Plk1 substrate, and show that Rock2 colocalizes with Plk1 during cytokinesis. Finally, we show that Plk1 and RhoA function together to maximally enhance Rock2 kinase activity in vitro and within cells, and implicate Plk1 as a central regulator of multiple pathways that synergistically converge to regulate actomyosin ring contraction during cleavage furrow ingression.