In vitro modification of betaine-homocysteine S-methyltransferase by tissue-type transglutaminase

Identification of hepatic protein-protein interaction targets for betaine homocysteine S-methyltransferase

Protein-protein interactions are an important mechanism for the regulation of enzyme function allowing metabolite channeling, crosstalk between pathways or the introduction of post-translational modifications. Therefore, a number of high-throughput studies have been carried out to shed light on the protein networks established under different pathophysiological settings. Surprisingly, this type of information is quite limited for enzymes of intermediary metabolism such as betaine homocysteine S-methyltransferase, despite its high hepatic abundancy and its role in homocysteine metabolism. Here, we have taken advantage of two approaches, affinity purification combined with mass spectrometry and yeast two-hybrid, to further uncover the array of interactions of betaine homocysteine S-methyltransferase in normal liver of Rattus norvegicus. A total of 131 non-redundant putative interaction targets were identified, out of which 20 were selected for further validation by coimmunoprecipitation. Interaction targets validated by two different methods include: S-methylmethionine homocysteine methyltransferase or betaine homocysteine methyltransferase 2, methionine adenosyltransferases α1 and α2, cAMP-dependent protein kinase catalytic subunit alpha, 4-hydroxyphenylpyruvic acid dioxygenase and aldolase b. Network analysis identified 122 nodes and 165 edges, as well as a limited number of KEGG pathways that comprise: the biosynthesis of amino acids, cysteine and methionine metabolism, the spliceosome and metabolic pathways. These results further expand the connections within the hepatic methionine cycle and suggest putative cross-talks with additional metabolic pathways that deserve additional research.

Quantitative proteomics analysis by iTRAQ in human nuclear cataracts of different ages and normal lens nuclei

PURPOSE

The goal of this study was to quantitatively identify the differentially expressed proteins in nuclear cataracts of different ages and normal lens nuclei in humans.

EXPERIMENTAL DESIGN

Forty-eight human lens nucleus samples with hardness grades III, IV were obtained during cataract surgery by extracapsular cataract extraction. Seven normal transparent human lens nuclei were obtained from fresh normal cadaver eyes during corneal transplantation surgery. Lens nuclei were divided into seven groups according to age and optic axis: Group A (average age 80.8 ± 1.2 years), Group B (average age 57.0 ± 4.0 years), Group C average age 80.3 ± 4.5 years), Group D (average age 56.9 ± 4.2 years), Group E (average age 78.1 ± 2.5 years), Group F (average age 57.6 ± 3.3 years) and Group G (seven normal transparent human lenses from normal cadaver eyes, average age 34.7 ± 4.2 years). Water-soluble, water-insoluble, and water-insoluble-urea-soluble protein fractions were extracted from samples. The three-part protein fractions from the individual lenses were combined to form the total proteins of each sample. The proteomic profiles of each group were further analyzed using 8-plex iTRAQ labeling combined with 2D-LC-MS/MS. The data were analyzed with the ProteinPilot software for peptide matching, protein identification, and quantification. Differentially expressed proteins were validated by Western blotting.

RESULTS

We employed biological and technical replicates and selected the intersection of the two results, which included 80 proteins. Nine proteins were differentially expressed among the 80 proteins identified using proteomic techniques. In age-related nuclear cataracts (ARNC), the expression levels of fatty acid-binding protein and pterin-4-alpha-carbinolamine dehydratase were upregulated, whereas the levels of alpha-crystallin B chain (CRYAB), GSH synthetase, phakinin, gamma-crystallin C, phosphoglycerate kinase 1, betaine-homocysteine S-methyltransferase 1 (BHMT1), and spectrin beta chain were downregulated. These proteins may be associated with abnormal protein aggregation and oxidative stress. GSH synthetase and CRYAB expression levels in the nuclear cataract decreased with age. The mass spectrometric analysis results were consistent with the Western blot validation.

CONCLUSION AND CLINICAL RELEVANCE

The results indicate that CRYAB and GSH synthetase may be involved in ARNC pathogenesis. iTRAQ combined with 2D-LC-MS/MS provides new methods for future studies of pathological mechanisms and protective drug development for ARNC.

Betaine homocysteine methyltransferase (BHMT)-dependent remethylation pathway in human healthy and tumoral liver

Carcinogenesis is a multi-step and multifactorial process. It includes genetic, epigenetic, nutritional and environmental factors, which are closely interconnected. Human hepatocellular carcinoma (HCC) is among the most frequent and lethal cancers. Imbalance in the S-adenosylmethionine (SAM) concentration, the main methyl group donor, strongly influences the development of HCC. Key enzymes of carbon metabolism are greatly reduced in patients with cirrhosis and HCC. These alterations play a role in genetic instability and epigenetic modifications (DNA methylation, and histone modifications), however, the molecular underlying mechanisms are still poorly understood. We aimed to investigate betaine homocysteine methyltransferase (BHMT) expression in HepG2 cells and human hepatocarcinoma tissues. Tumor and surrounding healthy tissue were compared. HepG2 cells and tumor samples showed a strong decrease in BHMT transcripts resulting from the transcription of a splicing variant that contained a frameshift mutation generating a premature termination codon and gene loss of function. This splicing variant, not detected in normal adult and fetal liver, cannot be explained by any mechanism involving the known splicing consensus sequences. BHMT activity was abolished in HepG2 cells and protein expression was detected neither in HepG2 cells nor in five of the six tumor samples investigated. Further investigation is needed to elucidate whether this abnormal BHMT transcription is part of cause or consequence of liver carcinogenesis.

A splicing variant leads to complete loss of function of betaine-homocysteine methyltransferase (BHMT) gene in hepatocellular carcinoma

The remethylation of homocyteine into methionine is catalyzed either by methionine synthase (MTR) or by betaine-homocysteine methyltransferase (BHMT), in the liver. Choline/betaine deficiency and impaired BHMT pathway have been associated with hepatocellular carcinogenesis, in animal models. The molecular mechanisms that impair the BHMT pathway are unknown. We aimed to investigate BHMT, BHMT2, and MTR expression in HepG2 cells and human hepatocarcinoma tissues. Transcripts were quantified by RT-qPCR and splicing was assessed by analysis of exon junctions and sequencing of variants. Protein expression was studied by Western Blot, immunohistochemistry and enzyme activity. Tumor tissue was compared with surrounding healthy tissue. RT-qPCR of HepG2 cells and of tumor samples showed a strong decrease of transcripts of BHMT and BHMT2, compared to normal. MTR transcript levels were not different. The decreased BHMT expression resulted from the transcription of a splicing variant that produced a frameshift in exon 4, with a premature termination codon in exon 5 and a loss of function of the gene. This splicing variant did not fit with any mechanism resulting from known splicing consensus sequences and was not detected in normal adult and fetal liver. Consistently, BHMT activity was abolished in HepG2 and protein expression was not detectable in HepG2 and in 5 of the 6 tumor samples, compared to normal tissues. In conclusion, a transcription variant of exon 4 produces a loss of function of BHMT in human hepatocarcinoma. Whether this abnormal transcription of BHMT is part or consequence of liver carcinogenesis should deserve further investigations.

S-adenosylmethionine and its products

S-adenosylmethionine is involved in many processes, mainly methylation, polyamine synthesis and radical-based catalysis. It is synthesised through the catalysis of differently regulated enzyme forms. When it is used, the compounds formed are reutilized in different ways: in case of methylation, its end product is homocysteine, which can be remethylated to methionine, give rise to cysteine in the so-called transsulphuration pathway, or be released; in the case of polyamine synthesis, the methylthioadenosine formed is cleaved and gives rise to compounds which can be reutilized; during radical-based catalysis, 5-deoxyadenosine is formed and this, too, is cleaved and reutilized.

Mammalian transglutaminases. Identification of substrates as a key to physiological function and physiopathological relevance

Transglutaminases form a large family of intracellular and extracellular enzymes that catalyse the Ca2+-dependent post-translational modification of proteins. Despite significant advances in our understanding of the biological role of most mammalian transglutaminase isoforms, recent findings suggest new scenarios, most notably for the ubiquitous tissue transglutaminase. It is becoming apparent that some transglutaminases, normally expressed at low levels in many tissue types, are activated and/or overexpressed in a variety of diseases, thereby resulting in enhanced concentrations of cross-linked proteins. As applies to all enzymes that exert their metabolic function by modifying the properties of target proteins, the identification and characterization of the modified proteins will cast light on the functions of transglutaminases and their involvement in human diseases. In this paper we review data on the properties of mammalian transglutaminases, particularly as regards their protein substrates and the relevance of transglutaminase-catalysed reactions in physiological and disease conditions.