Sonia Melino

Histatins: salivary peptides with copper(II)- and zinc(II)-binding motifs Perspectives for biomedical applications

Natural antimicrobial peptides represent a primordial mechanism of immunity in both vertebrate and nonvertebrate organisms. Among them, histatins belong to a family of human salivary metal‐binding peptides displaying potent antibacterial, antifungal and wound‐healing activities. These properties, along with the ability of histatins to inhibit collagenases and cysteine proteases, have attracted much attention for their potential use in the treatment of several oral diseases. This review critically assesses the studies carried out to date in order to provide a comprehensive and systematic vision of the information accumulated so far. In particular, the relationship between metal‐binding and peptide activity is extensively analysed. The review provides important clues for developing possible therapeutic applications of histatins and their synthetic peptide analogues by creating a set of necessary resource materials to support investigators and industries interested in exploiting their unique properties.

Progress for dengue virus diseases

Transmitted by the Aedes aegypti mosquito, the dengue virus is the etiological agent of dengue fever, dengue hemorrhagic fever and dengue shock syndrome, and, as such, is a significant factor in the high death rate found in most tropical and subtropical areas of the world. Dengue diseases are not only a health burden to developing countries, but pose an emerging problem worldwide. The immunopathological mechanisms appear to include a complex series of immune responses. A rapid increase in the levels of cytokines and chemical mediators during dengue disease plays a key role in inducing plasma leakage, shock and hemorrhagic manifestations. Currently, there are no vaccines available against dengue virus, although several tetravalent live‐attenuated dengue vaccines are in clinical phases I or II, and prevention through vaccination has become a major priority on the agendas of the World Health Organization and of national ministries of health and military organizations. An alternative to vaccines is found in therapeutic‐based approaches. Understanding the molecular mechanisms of viral replication has led to the development of potential drugs, and new molecular viral targets for therapy are emerging. The NS3 protease domain of the NS3 protein is responsible for processing the viral polyprotein and its inhibition is one of the principal aims of pharmacological therapy. This review is an overview of the progress made against dengue virus; in particular, it examines the unique properties – structural and functional – of the NS3 protease for the treatment of dengue virus infections by the inhibition of viral polyprotein processing.

Rhodanese–thioredoxin system and allyl sulfur compounds

Sodium 2‐propenyl thiosulfate, a water‐soluble organo‐sulfane sulfur compound isolated from garlic, induces apoptosis in a number of cancer cells. The molecular mechanism of action of sodium 2‐propenyl thiosulfate has not been completely clarified. In this work we investigated, by in vivo and in vitro experiments, the effects of this compound on the expression and activity of rhodanese. Rhodanese is a protein belonging to a family of enzymes widely present in all phyla and reputed to play a number of distinct biological roles, such as cyanide detoxification, regeneration of iron–sulfur clusters and metabolism of sulfur sulfane compounds. The cytotoxic effects of sodium 2‐propenyl thiosulfate on HuT 78 cells were evaluated by flow cytometry and DNA fragmentation and by monitoring the progressive formation of mobile lipids by NMR spectroscopy. Sodium 2‐propenyl thiosulfate was also found to induce inhibition of the sulfurtransferase activity in tumor cells. Interestingly, in vitro experiments using fluorescence spectroscopy, kinetic studies and MS analysis showed that sodium 2‐propenyl thiosulfate was able to bind the sulfur‐free form of the rhodanese, inhibiting its thiosulfate:cyanide‐sulfurtransferase activity by thiolation of the catalytic cysteine. The activity of the enzyme was restored by thioredoxin in a concentration‐dependent and time‐dependent manner. Our results suggest an important involvement of the essential thioredoxin–thioredoxin reductase system in cancer cell cytotoxicity by organo‐sulfane sulfur compounds and highlight the correlation between apoptosis induced by these compounds and the damage to the mitochondrial enzymes involved in the repair of the Fe–S cluster and in the detoxification system.

Allyl sulfur compounds and cellular detoxification system: effects and perspectives in cancer therapy

Natural organosulfur compounds (OSCs) have been shown to have chemopreventive effects and to suppress the proliferation of tumor cells in vitro through the induction of apoptosis. The biochemical mechanisms underlying the antitumorigenic and anti-proliferative effects of garlic-derived OSCs are not fully understood. Several modes of action of these compounds have been proposed, and it seems likely that the rate of clearance of allyl sulfur groups from cells is a determinant of the overall response. The aim of this review is to focus attention on the effects of natural allyl sulfur compounds on the cell detoxification system in normal and tumor cells. It has been already reported that several natural allyl sulfur compounds induce chemopreventive effects by affecting xenobiotic metabolizing enzymes and inducing their down-activation. Moreover, different effects of water- and oil-soluble allyl sulfur compounds on enzymes involved in the detoxification system of rat tissues have been observed. A direct interaction of the garlic allyl sulfur compounds with proteins involved in the detoxification system was studied in order to support the hypothesis that proteins possessing reactive thiol groups and that are involved in the detoxification system and in the cellular redox homeostasis, are likely the preferential targets of these compounds. The biochemical transformation of the OSCs in the cell and their adducts with thiol functional groups of these proteins, could be considered relevant events to uncover the anticancer properties of the allyl sulfur compounds. Although additional studies, using proteomic approaches and transgenic models, are needed to identify the molecular targets and modes of action of these natural compounds, the allyl sulfur compounds can represent potential ideal agents in anticancer therapy, either alone or in association with other antitumor drugs.

Glutathione-Garlic Sulfur Conjugates: Slow Hydrogen Sulfide Releasing Agents for Therapeutic Applications

Natural organosulfur compounds (OSCs) from Allium sativum L. display antioxidant and chemo-sensitization properties, including the in vitro inhibition of tumor cell proliferation through the induction of apoptosis. Garlic water- and oil-soluble allyl sulfur compounds show distinct properties and the capability to inhibit the proliferation of tumor cells. In the present study, we optimized a new protocol for the extraction of water-soluble compounds from garlic at low temperatures and the production of glutathionyl-OSC conjugates during the extraction. Spontaneously, Cys/GSH-mixed-disulfide conjugates are produced by in vivo metabolism of OSCs and represent active molecules able to affect cellular metabolism. Water-soluble extracts, with (GSGaWS) or without (GaWS) glutathione conjugates, were here produced and tested for their ability to release hydrogen sulfide (H2S), also in the presence of reductants and of thiosulfate:cyanide sulfurtransferase (TST) enzyme. Thus, the TST catalysis of the H2S-release from garlic OSCs and their conjugates has been investigated by molecular in vitro experiments. The antiproliferative properties of these extracts on the human T-cell lymphoma cell line, HuT 78, were observed and related to histone hyperacetylation and downregulation of GAPDH expression. Altogether, the results presented here pave the way for the production of a GSGaWS as new, slowly-releasing hydrogen sulfide extract for potential therapeutic applications.

The active essential CFNS3d protein complex

The NS2B–NS3 protease complex is essential for the replication of dengue virus, which is the etiologic agent of dengue and hemorrhagic fevers, diseases that are a burden for the tropical and subtropical areas of the world. The active form of the NS3 protease linked to the 40 residues of the NS2B cofactor shows highly flexible and disordered region(s) that are responsible for its high propensity to aggregate at the concentrations necessary for NMR spectroscopy studies or for crystallization. Limited proteolysis of this active form of the protease enabled us to obtain a folded and new essential form of the NS2B–NS3 protease complex. We found that the region from residues D50 to E80 of NS2B interacts directly and strongly with the NS3 protease domain. The proteolytic activity of the noncovalently binding complex was determined by a rapid and continuous fluorescence resonance energy transfer activity assay using a depsipeptide substrate. The new protein–cofactor complex obtained, encompassing the NS2B fragment (D50–E80) and the NS3 protease, shows proteolytic activity. The 1H‐15N‐heteronuclear single quantum coherence spectrum of the isotopically enriched protein complex shows good cross‐peak dispersion; this is indicative of a stable folded state. Our results significantly complement the X‐ray structure of the NS2B–NS3pro complex published recently. Moreover, these results open the way to performing direct structural and interaction studies in solution on a new active NS2B–NS3pro complex with libraries of substrates and inhibitors in order to identify new drugs that prevent viral polyprotein processing.

Characterization of toad liver glutathione transferase

The major form of glutathione transferase from the toad liver previously designed as Bufo bufo liver GST-7.6 (A. Aceto, B. Dragani, T. Bucciarelli, P. Sacchetta, F. Martini, S. Angelucci, F. Amicarelli, M. Miranda and C. Di Ilio, Biochem. J. 289 (1993) 417-422) has been characterized. According to its partial amino acid sequence, the toad enzyme may be included in the pi class GST and named bbGST P2-2. However, bbGST P2-2 appears to be immunologically, structurally and kinetically distinct from any other members of pi family, including bbGST P1-1, suggesting that it may constitute a subset of pi class GST. The data support the hypothesis that the transition from aquatic to terrestrial life causes a switch of the GST amphibian pattern promoting the expression of a GST form (bbGST P2-2) able to counteract, with higher efficiency, the toxic effects of reactive metabolites of oxidative metabolism and those of hydrophobic xenobiotics.

Recognition mechanism of p63 by the E3 ligase Itch: Novel strategy in the study and inhibition of this interaction

The HECT-containing E3 ubiquitin ligase Itch mediates the degradation of several proteins, including p63 and p73, involved in cell specification and fate. Itch contains four WW domains, which are essential for recognition on the target substrate, which contains a short proline-rich sequence. Several signaling complexes containing these domains have been associated with human diseases such as muscular dystrophy, Alzheimer’s or Huntington’s diseases. To gain further insight into the structural determinants of the Itch-WW2 domain, we investigated its interaction with p63. We assigned, by 3D heteronuclear NMR experiments, the backbone and side chains of the uniformly ¹³C-¹⁵N-labeled Itch-WW2. In vitro interaction of Itch-WW2 domain with p63 was studied using its interactive p63 peptide, pep63. Pep63 is an 18-mer peptide corresponding to the region from 534–551 residue of p63, encompassing the PPxY motif that interacts with the Itch-WW domains, and we identified the residues involved in this molecular recognition. Moreover, here, a strategy of stabilization of the conformation of the PPxY peptide has been adopted, increasing the WW-ligand binding. We demonstrated that cyclization of pep63 leads to an increase of both the biological stability of the peptide and of the WW-ligand complex. Stable metal-binding complexes of the pep63 have been also obtained, and localized oxidative damage on Itch-WW2 domain has been induced, demonstrating the possibility of use of metal-pep63 complexes as models for the design of metal drugs to inhibit the Itch-WW-p63 recognition in vivo. Thus, our data suggest a novel strategy to study and inhibit the recognition mechanism of Itch E3-ligase.

Oxidative species and S-glutathionyl conjugates in the apoptosis induction by allyl thiosulfate

Natural allyl sulfur compounds show antiproliferative effects on tumor cells, but the biochemical mechanisms underlying the antitumorigenic properties of the organ sulfur compounds are not yet fully understood. Sodium 2‐propenyl‐thiosulfate is a garlic water‐soluble organo‐sulfane sulfur compound able to promote apoptosis in cancer cells, affecting the ‘managing’ of the redox state in the cell. Our studies show that sodium 2‐propenyl‐thiosulfate reacts spontaneously with reduced glutathione at physiological pH, leading to the formation of S‐allyl‐mercapto‐glutathione, radicals and peroxyl species, which are able to induce inhibition of enzymes with cysteine in the catalytic site, such as sulfurtransferases. S‐Allyl‐mercapto‐glutathione was purified and characterized by NMR and MS, and its cytotoxic effect at 500 μm on HuT 78 cells was analyzed, showing activation of the p38–MAPK pathway. Many allyl sulfur compounds are also able to promote chemoprevention by induction of xenobiotic‐metabolizing enzymes, inducing down‐activation or detoxification of the carcinogens. Thus, the effects of the S‐allyl‐mercapto‐glutathione on proteins involved in the cellular detoxification system, such as glutathione S‐transferase, have been evaluated both in vitro and in HuT 78 cells. Although the antitumor properties of water‐soluble sulfur compounds may arise from several mechanisms and it is likely that more cellular events occur simultaneously, a relevant role is played by the formation of both reduced glutathione conjugates and radical species that affect the activity of the thiol‐proteins involved in fundamental cellular processes.

Recognition of p63 by the E3 ligase ITCH: Effect of an ectodermal dysplasia mutant

The E3 ubiquitin ligase Itch mediates the degradation of the p63 protein. Itch contains four WW domains which are pivotal for the substrate recognition process. Indeed, this domain is implicated in several signalling complexes crucially involved in human diseases including Muscular Dystrophy, Alzheimers Disease, and Huntington Disease. WW domains are highly compact protein-protein binding modules that interact with short proline-rich sequences. The four WW domains present in Itch belong to the Group I type, which binds polypeptides with a PY motif characterized by a PPxY consensus sequence, where x can be any residue. Accordingly, the Itch-p63 interaction results from a direct binding of Itch-WW2 domain with the PY motif of p63. Here, we report a structural analysis of the Itch-p63 interaction by fluorescence, CD and NMR spectroscopy. Indeed, we studied the in vitro interaction between Itch-WW2 domain and p63(534-551), an 18-mer peptide encompassing a fragment of the p63 protein including the PY motif. In addition, we evaluated the conformation and the interaction with Itch-WW2 of a site specific mutant of p63, I549T, that has been reported in both Hay–Wells syndrome and Rapp–Hodgkin syndrome. Based on our results, we propose an extended PPxY motif for the Itch recognition motif (P-P-P-Y-x(4)-[ST]-[ILV]), which includes these C-terminal residues to the PPxY motif.