In vitro study of proteolytic degradation of rat histidine decarboxylase

Cysteine 180 Is a Redox Sensor Modulating the Activity of Human Pyridoxal 5'-Phosphate Histidine Decarboxylase

Histidine decarboxylase is a pyridoxal 5'-phosphate enzyme catalyzing the conversion of histidine to histamine, a bioactive molecule exerting its role in many modulatory processes. The human enzyme is involved in many physiological functions, such as neurotransmission, gastrointestinal track function, cell growth, and differentiation. Here, we studied the functional properties of the human enzyme and, in particular, the effects exerted at the protein level by two cysteine residues: Cys-180 and Cys-418. Surprisingly, the enzyme exists in an equilibrium between a reduced and an oxidized form whose extent depends on the redox state of Cys-180. Moreover, we determined that (i) the two enzymatic redox species exhibit modest structural changes in the coenzyme microenvironment and (ii) the oxidized form is slightly more active and stable than the reduced one. These data are consistent with the model proposed by bioinformatics analyses and molecular dynamics simulations in which the Cys-180 redox state could be responsible for a structural transition affecting the C-terminal domain reorientation leading to active site alterations. Furthermore, the biochemical properties of the purified C180S and C418S variants reveal that C180S behaves like the reduced form of the wild-type enzyme, while C418S is sensitive to reductants like the wild-type enzyme, thus allowing the identification of Cys-180 as the redox sensitive switch. On the other hand, Cys-418 appears to be a residue involved in aggregation propensity. A possible role for Cys-180 as a regulatory switch in response to different cellular redox conditions could be suggested.

What We Know and What We Need to Know about Aromatic and Cationic Biogenic Amines in the Gastrointestinal Tract

Biogenic amines derived from basic and aromatic amino acids (B/A-BAs), polyamines, histamine, serotonin, and catecholamines are a group of molecules playing essential roles in many relevant physiological processes, including cell proliferation, immune response, nutrition and reproduction. All these physiological effects involve a variety of tissue-specific cellular receptors and signalling pathways, which conforms to a very complex network that is not yet well-characterized. Strong evidence has proved the importance of this group of molecules in the gastrointestinal context, also playing roles in several pathologies. This work is based on the hypothesis that integration of biomedical information helps to reach new translational actions. Thus, the major aim of this work is to combine scientific knowledge on biomolecules, metabolism and physiology of the main B/A-BAs involved in the pathophysiology of the gastrointestinal tract, in order to point out important gaps in information and other facts deserving further research efforts in order to connect molecular information with pathophysiological observations.

Histamine synthesis and lessons learned from histidine decarboxylase deficient mice

This chapter summarizes the information about the transcriptional regulation of histidine decarboxylase (HDC), which is the catabolic enzyme of histamine synthesis, and the activity of histamine in vivo as clarified using HDC gene deficient mice (HDC-KO). The research of the regulatory mechanism of histamine synthesis has been focused on transcriptional and posttranslational aspects. The generation ofHDC-KO mice clarified several new pathophysiological functions of histamine. It is now recognized that the activity of histamine is not limited to allergic, peptic and neurological functions as in the old paradigm, but extends to other fields such as cardiology, immunology and infectious diseases. Therefore, this chapter will focus on these newly revealed functions of histamine. For example, histamine was known to be involved in the effector phase of allergic responses, but a role has now been shown in the sensitization phases and in innate immunity. In the allergic bronchial asthma model using HDC-KO mice it was found that histamine positively controls eosinophilia, but not bronchial hypersensitivity. The effect on eosinophils was afterwards shown to be mediated through the activity of the histamine H4 receptor. The recent advances in the understanding of histamine synthesis and the activity of HDC have dramatically expanded our understanding of the scope of histamine function.

Polyamines affect histamine synthesis during early stages of IL-3-induced bone marrow cell differentiation

Mast cells synthesize and store histamine, a key immunomodulatory mediator. Polyamines are essential for every living cell. Previously, we detected an antagonistic relationship between the metabolisms of these amines in established mast cell and basophilic cell lines. Here, we used the IL-3-driven mouse bone marrow-derived mast cell (BMMC) culture system to further investigate this antagonism in a mast cell model of deeper physiological significance. Polyamines and histamine levels followed opposite profiles along the bone marrow cell cultures leading to BMMCs. alpha-Difluoromethylornithine (DFMO)-induced polyamine depletion resulted in an upregulation of histidine decarboxylase (HDC, the histamine-synthesizing enzyme) expression and activity, accompanied by increased histamine levels, specifically during early stages of these cell cultures, where an active histamine synthesis process occurs. In contrast, DFMO did not induce any effect in either HDC activity or histamine levels of differentiated BMMCs or C57.1 mast cells, that exhibit a nearly inactive histamine synthesis rate. Sequence-specific DNA methylation analysis revealed that the DFMO-induced HDC mRNA upregulation observed in early bone marrow cell cultures is not attributable to a demethylation of the gene promoter caused by the pharmacological polyamine depletion. Taken together, the results support an inverse relationship between histamine and polyamine metabolisms during the bone marrow cell cultures leading to BMMCs and, moreover, suggest that the regulation of the histamine synthesis occurring during the early stages of these cultures depends on the concentrations of polyamines.

Exploring polyamine regulation by nascent histamine in a human-transfected cell model

There are multiple lines of evidence suggesting interplay between histamine and polyamines in several mammalian cell types. However, the complex metabolic context makes it difficult to elucidate the mechanisms involved. Histamine's effects can be elicited after its binding to any of the four subtypes of G-protein coupled histamine membrane receptors. In addition, intracellular histamine can also interfere with polyamine metabolism, since there are several metabolic connections between the synthesis and degradation pathways of both types of amines. In order to dissect the metabolic effects of intracellular histamine on polyamine metabolism, we chose a well-known cell culture line, i.e., the human embryonic kidney 293 cells (HEK-293 cells). Initially, we show that HEK-293 cells lack a polyamine metabolic response to extracellular histamine, even over a wide range of histamine concentrations. HEK-293 cells were transfected with active and inactive versions of human histidine decarboxylase, and changes in many of the overlapping metabolic factors and limiting steps were tested. Overall, the results indicate a regulatory effect of histamine on the post-transcriptional expression of ornithine decarboxylase and suggest that this effect is primarily responsible for the decrease in polyamine synthesis and partial blockade of cell-cycle progression, which should affect cell proliferation rate.

Structural features of mammalian histidine decarboxylase reveal the basis for specific inhibition

For a long time the structural and molecular features of mammalian histidine decarboxylase (EC 4.1.1.22), the enzyme that produces histamine, have evaded characterization. We overcome the experimental problems for the study of this enzyme by using a computer-based modelling and simulation approach, and have now the conditions to use histidine decarboxylase as a target in histamine pharmacology. In this review, we present the recent (last 5 years) advances in the structure-function relationship of histidine decarboxylase and the strategy for the discovery of new drugs.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.

Mammalian histidine decarboxylase: from structure to function

Histamine is a multifunctional biogenic amine with relevant roles in intercellular communication, inflammatory processes and highly prevalent pathologies. Histamine biosynthesis depends on a single decarboxylation step, carried out by a PLP-dependent histidine decarboxylase activity (EC 4.1.1.22), an enzyme that still remains to be fully characterized. Nevertheless, during the last few years, important advances have been made in this field, including the generation and validation of the first three-dimensional model of the enzyme, which allows us to revisit previous results and conclusions. This essay provides a comprehensive review of the current knowledge of the structural and functional characteristics of mammalian histidine decarboxylase.

Recombinant Listeria Vaccines Containing PEST Sequences Are Potent Immune Adjuvants for the Tumor-Associated Antigen Human Papillomavirus-16 E7

Previous work in our laboratory has established that the fusion of tumor-associated antigens to a truncated form of the Listeria monocytogenes virulence factor listeriolysin O (LLO) enhances the immunogenicity and antitumor efficacy of the tumor antigen when delivered by Listeria or by vaccinia. LLO contains a PEST sequence at the NH(2) terminus. These sequences, which are found in eukaryotic proteins with a short cellular half-life, target proteins for degradation in the ubiquitin-proteosome pathway. To investigate whether the enhanced immunogenicity conferred by LLO is due to the PEST sequence, we constructed new Listeria recombinants that expressed the HPV-16 E7 antigen fused to LLO, which either contained or had been deleted of this sequence. We then compared the antitumor efficacy of this set of vectors and found that Listeria expressing the fusion protein LLO-E7 or PEST-E7 were effective at regressing established macroscopic HPV-16 immortalized tumors in syngeneic mice. In contrast, Listeria recombinants expressing E7 alone or E7 fused to LLO from which the PEST sequence had been genetically removed could only slow tumor growth. Because CD8(+) T cell epitopes are generated in the ubiquitin-proteosome pathway, we also investigated the ability of the vaccines to induce E7-specific CD8(+) T cells in the spleen and to generate E7-specific tumor-infiltrating lymphocytes. A strong correlation was observed between CD8(+) T-cell induction and tumor homing and the antitumor efficacy of the Listeria-E7 vaccines. These findings suggest a strategy for the augmentation of tumor antigen-based immunotherapeutic strategies that may be broadly applicable.

Local changes in the catalytic site of mammalian histidine decarboxylase can affect its global conformation and stability

Mature, active mammalian histidine decarboxylase is a dimeric enzyme of carboxy-truncated monomers (approximately 53 kDa). By using a biocomputational approach, we have generated a three-dimensional model of a recombinant 1/512 fragment of the rat enzyme, which shows kinetic constants similar to those of the mature enzyme purified from rodent tissues. This model, together with previous spectroscopic data, allowed us to postulate that the occupation of the catalytic center by the natural substrate, or by substrate-analogs, would induce remarkable changes in the conformation of the intact holoenzyme. To investigate the proposed conformational changes during catalysis, we have carried out electrophoretic, chromatographic and spectroscopic analyses of purified recombinant rat 1/512 histidine decarboxylase in the presence of the natural substrate or substrate-analogs. Our results suggest that local changes in the catalytic site indeed affect the global conformation and stability of the dimeric protein. These results provide insights for new alternatives to inhibit histamine production efficiently in vivo.

Biogenic amines and polyamines: similar biochemistry for different physiological missions and biomedical applications

Biogenic amines are organic polycations derived from aromatic or cationic amino acids. All of them have one or more positive charges and a hydrophobic skeleton. Nature has evolved these molecules to play different physiological roles in mammals, but maintains similar patterns for their metabolic and intracellular handling. As deduced from this review, many questions still remain to be solved around their biochemistry and molecular biology, blocking our aims to control the relevant pathologies in which they are involved (cancer and immunological, neurological, and gastrointestinal diseases). Advances in this knowledge are dispersed among groups working on different biomedical areas. In these pages, we put together the most relevant information to remark how fruitful it can be to learn from Nature and to take advantage of the biochemical similarities (key protein structures and their regulation data on metabolic interplays and binding properties) to generate new hypothesis and develop different biomedical strategies based on biochemistry and molecular biology of these compounds.

The PEST sequence does not contribute to the stability of the cystic fibrosis transmembrane conductance regulator

BACKGROUND

Endoplasmic reticulum retention of misfolded cystic fibrosis transmembrane conductance regulator (CFTR) mutants and their rapid degradation is the major cause of cystic fibrosis (CF). An important goal is to understand the mechanism of how the misfolded proteins are recognized, retained, and targeted for degradation.

RESULTS

Using a web-based algorithm, PESTFind, we found a PEST sequence in the regulatory (R) domain of CFTR. The PEST sequence is found in many short-lived eukaryotic proteins and plays a role in their degradation. To determine its role in the stability and degradation of misprocessed CFTR, we introduced a number of site-directed mutations into the PEST sequence in the cDNA of DeltaF508 CFTR, the most prevalent misprocessed mutation found in CF patients. Analysis of these mutants showed that the disruption of the PEST sequence plays a minor role in the degradation of the CFTR mutants. Multiple mutations to the PEST sequence within the R domain of CFTR inhibit maturation of CFTR and prevent the formation of a 100 kDa degradation product. The mutations, however, do not improve the stability of the mutant DeltaF508 CFTR.

CONCLUSION

These observations show that disruption of the structure of the R domain of CFTR can inhibit maturation of the protein and that the predicted PEST sequence plays no significant role in the degradation of CFTR.