Tissue transglutaminase inhibition

Structure-Activity Relationships of Potent, Targeted Covalent Inhibitors That Abolish Both the Transamidation and GTP Binding Activities of Human Tissue Transglutaminase

Human tissue transglutaminase (hTG2) is a multifunctional enzyme. It is primarily known for its calcium-dependent transamidation activity that leads to formation of an isopeptide bond between glutamine and lysine residues found on the surface of proteins, but it is also a GTP binding protein. Overexpression and unregulated hTG2 activity have been associated with numerous human diseases, including cancer stem cell survival and metastatic phenotype. Herein, we present a series of targeted covalent inhibitors (TCIs) based on our previously reported Cbz-Lys scaffold. From this structure-activity relationship (SAR) study, novel irreversible inhibitors were identified that block the transamidation activity of hTG2 and allosterically abolish its GTP binding ability with a high degree of selectivity and efficiency (k_inact/K_I > 10⁵ M^-1 min^-1). One optimized inhibitor (VA4) was also shown to inhibit epidermal cancer stem cell invasion with an EC₅₀ of 3.9 μM, representing a significant improvement over our previously reported "hit" NC9.

Transglutaminase is a therapeutic target for oxidative stress, excitotoxicity and stroke: a new epigenetic kid on the CNS block

Transglutaminases (TGs) are multifunctional, calcium-dependent enzymes that have been recently implicated in stroke pathophysiology. Classically, these enzymes are thought to participate in cell injury and death in chronic neurodegenerative conditions via their ability to catalyze covalent, nondegradable crosslinks between proteins or to incorporate polyamines into protein substrates. Accumulating lines of inquiry indicate that specific TG isoforms can shuttle into the nucleus when they sense pathologic changes in calcium or oxidative stress, bind to chromatin and thereby transduce these changes into transcriptional repression of genes involved in metabolic or oxidant adaptation. Here, we review the evidence that supports principally a role for one isoform of this family, TG2, in cell injury and death associated with hemorrhagic or ischemic stroke. We also outline an evolving model in which TG2 is a critical mediator between pathologic signaling and epigenetic modifications that lead to gene repression. Accordingly, the salutary effects of TG inhibitors in stroke may derive from their ability to restore homeostasis by removing inappropriate deactivation of adaptive genetic programs by oxidative stress or extrasynaptic glutamate receptor signaling.

Dihydroisoxazole analogs for labeling and visualization of catalytically active transglutaminase 2

We report the synthesis and preliminary characterization of "clickable" inhibitors of human transglutaminase 2 (TG2). These inhibitors possess the 3-halo-4,5-dihydroisoxazole warhead along with bioorthogonal groups such as azide or alkyne moieties that enable subsequent covalent modification with fluorophores. Their mechanism for inhibition of TG2 is based on halide displacement, resulting in the formation of a stable imino thioether. Inhibition assays against recombinant human TG2 revealed that some of the clickable inhibitors prepared in this study have comparable specificity as benchmark dihydroisoxazole inhibitors reported earlier. At low micromolar concentrations they completely inhibited transiently activated TG2 in a WI-38 fibroblast scratch assay and could subsequently be used to visualize the active enzyme in situ. The potential use of these inhibitors to probe the role of TG2 in celiac sprue as well as other diseases is discussed.

An in situ and in vitro investigation for the transglutaminase potential in tissue engineering

Transglutaminases (TGases) constitute a family of enzymes that stabilize protein assemblies by gamma-glutamyl-epsilon-lysine crosslinks. The role of tissue transglutaminase (TGase 2) in several pathophysiologies, wound healing applications, biomaterials functionalization, and drug delivery systems provides grounds for its use in tissue engineering. Herein, we initially studied the endogenous TGase activity and expression under normal (skin, duodenum, colon, and small bowel) and pathophysiological (keloid scar) conditions on cadaveric human tissues. Successful inhibition was achieved using low concentrations of BOC-DON-QIV-OMe (0.1 mM and 1 mM for normal skin and keloid scar, respectively), iodoacetamide (0.1 mM and 1 mM for normal skin and keloid scar, respectively), and cystamine dihydrochloride (1 mM and 10 mM for normal skin and keloid scar, respectively), whilst di-BOC-cystamine was found ineffective even at 100 mM concentration. Secondly, the addition of exogenous guinea pig liver transglutaminase (gpTGase) onto the inhibited tissues and collagen scaffolds was studied, and results presented advocate its use as potential tissue adhesive and drug delivery tool. However, the investigation of its crosslinking extent using second harmonic generation microscopy and differentially scanning calorimetry revealed rather poor stabilization function. Overall, our study indicates that TGase 2 has a role as a biological glue to consolidate various micro-structural components of tissues and biomaterials.

Photolabeling of tissue transglutaminase reveals the binding mode of potent cinnamoyl inhibitors

We have recently developed a new class of cinnamoyl derivatives as potent tissue transglutaminase (TG2) inhibitors. Herein, we report the synthesis of a diazirine derivative of these inhibitors and its application to the photolabeling of its binding site on guinea pig liver transglutaminase. Two novel homology models were generated for this commonly studied TG2, which differ in the conformational state they represent. Tryptic digest and mass spectrometric analysis of the photolabeling experiment showed that only residue Cys230 was labeled, and our homology models were used to visualize these results. This visualization suggested that Cys230 is somewhat more solvent-exposed in the "closed" conformation of TG2, compared to the "open" conformation. Docking experiments suggested binding modes consistent with the labeling pattern that would block access to the tunnel leading to the active site, consistent with the observed mode of inhibition. However, while these modeling simulations favored the closed conformation as the target of our cinnamoyl inhibitors, native PAGE experiments indicated the open conformation of the enzyme in fact predominates in the presence of our photolabeling derivative. These results are important for understanding the binding modes of TG2 inhibitors in general and will be critical for the structure-based design of future inhibitors.

The bioorganic chemistry of transglutaminase — from mechanism to inhibition and engineering

Through a multidisciplinary approach comprising organic synthesis, molecular biology, and physical organic kinetic studies, we have studied the mechanism of transglutaminase-mediated transamidation. More recently, we have applied our understanding of the mechanism to the design of reversible inhibitors and affinity labels for biological application. We have also undertaken the engineering of transglutaminase for its application as a “peptide synthase”. Herein, we present a brief overview of previously published work as well as recent results presented at the 2007 Merck–Frosst Centre for Therapeutic Research Award Lecture.Key words: transglutaminase, enzyme mechanism, enzyme inhibition, protein engineering

Synthesis and evaluation of peptidic maleimides as transglutaminase inhibitors

A series of novel transglutaminase inhibitors was prepared, based on the scaffold of a commonly used peptide substrate and bearing an electrophilic maleimide group. These compounds were evaluated in vitro and shown to lead to irreversible inactivation of tissue transglutaminase. Comparison with inhibitors studied previously provides insight into the steric environment of the enzyme active site.

Synthesis and evaluation of peptidic irreversible inhibitors of tissue transglutaminase

Herein we report the synthesis and the evaluation of eight novel compounds as irreversible inhibitors of transglutaminase (TGase). These compounds are based on a minimal peptidic scaffold shown previously [Chem. Biol.2005, 12, 469-475] to confer affinity for the TGase active site and bear electrophilic groups such as alpha,beta-unsaturated amide, chloroacetamide or maleimide; their general structure being Cbz-Phe-spacer-electrophile. The affinity conferred by the Cbz-Phe scaffold was determined by comparison to N-propylacrylamide and the length of the spacer was also varied to evaluate its importance. The inhibitory efficiencies (k(inact)/K(I)) of these compounds vary up to 10(5)M(-1)min(-1), among the highest reported for derivatives based on this simple Cbz-Phe peptidic scaffold.