Protein-polyamine conjugates by transglutaminase 2 as potential markers for antineoplastic screening of natural compounds

Biological Implications and Functional Significance of Transglutaminase Type 2 in Nervous System Tumors

Transglutaminase type 2 (TG2) is the most ubiquitously expressed member of the transglutaminase family. TG2 catalyzes the transamidation reaction leading to several protein post-translational modifications and it is also implicated in signal transduction thanks to its GTP binding/hydrolyzing activity. In the nervous system, TG2 regulates multiple physiological processes, such as development, neuronal cell death and differentiation, and synaptic plasticity. Given its different enzymatic activities, aberrant expression or activity of TG2 can contribute to tumorigenesis, including in peripheral and central nervous system tumors. Indeed, TG2 dysregulation has been reported in meningiomas, medulloblastomas, neuroblastomas, glioblastomas, and other adult-type diffuse gliomas. The aim of this review is to provide an overview of the biological and functional relevance of TG2 in the pathogenesis of nervous system tumors, highlighting its involvement in survival, tumor inflammation, differentiation, and in the resistance to standard therapies.

Tissue transglutaminase: a multifunctional and multisite regulator in health and disease

Tissue transglutaminase (TG2) is a widely distributed multifunctional protein involved in a broad range of cellular and metabolic functions carried out in a variety of cellular compartments. In addition to transamidation, TG2 also functions as a Gα signaling protein, a protein disulfide isomerase (PDI), a protein kinase, and a scaffolding protein. In the nucleus, TG2 modifies histones and transcription factors. The PDI function catalyzes the trimerization and activation of heat shock factor-1 in the nucleus and regulates the oxidation state of several mitochondrial complexes. Cytosolic TG2 modifies proteins by the addition of serotonin or other primary amines and in this way affects cell signaling. Modification of protein-bound glutamines reduces ubiquitin-dependent proteasomal degradation. At the cell membrane, TG2 is associated with G protein-coupled receptors (GPCRs), where it functions in transmembrane signaling. TG2 is also found in the extracellular space, where it functions in protein cross-linking and extracellular matrix stabilization. Of particular importance in transglutaminase research are recent findings concerning the role of TG2 in gene expression, protein homeostasis, cell signaling, autoimmunity, inflammation, and hypoxia. Thus, TG2 performs a multitude of functions in multiple cellular compartments, making it one of the most versatile cellular proteins. Additional evidence links TG2 with multiple human diseases including preeclampsia, hypertension, cardiovascular disease, organ fibrosis, cancer, neurodegenerative diseases, and celiac disease. In conclusion, TG2 provides a multifunctional and multisite response to physiological stress.

The Role of Tissue Transglutaminase in Cancer Cell Initiation, Survival and Progression

Tissue transglutaminase (transglutaminase type 2; TG2) is the most ubiquitously expressed member of the transglutaminase family (EC 2.3.2.13) that catalyzes specific post-translational modifications of proteins through a calcium-dependent acyl-transfer reaction (transamidation). In addition, this enzyme displays multiple additional enzymatic activities, such as guanine nucleotide binding and hydrolysis, protein kinase, disulfide isomerase activities, and is involved in cell adhesion. Transglutaminase 2 has been reported as one of key enzymes that is involved in all stages of carcinogenesis; the molecular mechanisms of action and physiopathological effects depend on its expression or activities, cellular localization, and specific cancer model. Since it has been reported as both a potential tumor suppressor and a tumor-promoting factor, the role of this enzyme in cancer is still controversial. Indeed, TG2 overexpression has been frequently associated with cancer stem cells’ survival, inflammation, metastatic spread, and drug resistance. On the other hand, the use of inducers of TG2 transamidating activity seems to inhibit tumor cell plasticity and invasion. This review covers the extensive and rapidly growing field of the role of TG2 in cancer stem cells survival and epithelial–mesenchymal transition, apoptosis and differentiation, and formation of aggressive metastatic phenotypes.

Tissue transglutaminase (TG2) is involved in the resistance of cancer cells to the histone deacetylase (HDAC) inhibitor vorinostat

Vorinostat demonstrated preclinical and clinical efficacy in human cancers and is the first histone deacetylase inhibitor (HDACi) approved for cancer treatment. Tissue transglutaminase (TG2) is a multifunctional enzyme that catalyzes a Ca²⁺ dependent transamidating reaction resulting in covalent cross-links between proteins. TG2 acts also as G-protein in trans-membrane signaling and as a cell surface adhesion mediator. TG2 up-regulation has been demonstrated in several cancers and its expression levels correlate with resistance to chemotherapy and metastatic potential. We demonstrated that the anti-proliferative effect of the HDACi vorinostat is paralleled by the induction of TG2 mRNA and protein expression in cancer cells but not in ex vivo treated peripheral blood lymphocytes. This effect was also shared by other pan-HDACi and resulted in increased TG2 transamidating activity. Notably, high TG2 basal levels in a panel of cancer cell lines correlated with lower vorinostat antiproliferative activity. Notably, in TG2-knockdown cancer cells vorinostat anti-proliferative and pro-apoptotic effects were enhanced, whereas in TG2-full-length transfected cells were impaired, suggesting that TG2 could represent a mechanism of intrinsic or acquired resistance to vorinostat. In fact, co-treatment of tumor cells with inhibitors of TG2 transamidating activity potentiated the antitumor effect of vorinostat. Moreover, vorinostat-resistant MCF7 cells selected by stepwise increasing concentrations of the drug, significantly overexpressed TG2 protein compared to parental cells, and co-treatment of these cells with TG2 inhibitors reversed vorinostat-resistance. Taken together, our data demonstrated that TG2 is involved in the resistance of cancer cells to vorinostat, as well as to other HDACi.

Transglutaminases: recent achievements and new sources

Transglutaminases are a family of enzymes (EC 2.3.2.13), widely distributed in various organs, tissues, and body fluids, that catalyze the formation of a covalent bond between a free amine group and the γ-carboxamide group of protein or peptide-bound glutamine. Besides forming these bonds, that exhibit high resistance to proteolytic degradation, transglutaminases also form extensively cross-linked, generally insoluble, protein biopolymers that are indispensable for the organism to create barriers and stable structures. The extremely high cost of transglutaminase of animal origin has hampered its wider application and has initiated efforts to find an enzyme of microbial origin. Since the early 1990s, many microbial transglutaminase-producing strains have been found, and production processes have been optimized. This has resulted in a rapidly increasing number of applications of transglutaminase in the food sector. However, applications of microbial transglutaminase in other sectors have also been explored, but in a much lesser extent. Our group has identified a transglutaminase in the oomycete Phytophthora cinnamomi, which is able to induct defense responses and disease-like symptoms. In this mini-review, we report the achievements in this area in order to illustrate the importance and the versatility of transglutaminases.

Positional cloning and next-generation sequencing identified a TGM6 mutation in a large Chinese pedigree with acute myeloid leukaemia

An inherited predisposition to acute myeloid leukaemia (AML) is exceedingly rare, but the investigation of these families will aid in the delineation of the underlying mechanisms of the more common, sporadic cases. Three AML predisposition genes, RUNX1, CEBPA and GATA2, have been recognised, but the culprit genes in the majority of AML pedigrees remain obscure. We applied a combined strategy of linkage analysis and next-generation sequencing (NGS) technology in an autosomal-dominant AML Chinese family with 11 cases in four generations. A genome-wide linkage scan using a 500K SNP genotyping array was conducted to identify a previously unreported candidate region on 20p13 with a maximum multipoint heterogeneity LOD (HLOD) score of 3.56 (P=0.00005). Targeted NGS within this region and whole-exome sequencing (WES) revealed a missense mutation in TGM6 (RefSeq, NM_198994.2:c.1550T>G, p.(L517W)), which cosegregated with the phenotype in this family, and was absent in 530 healthy controls. The mutated amino acid was located in a highly conserved position, which may be deleterious and affect the activation of TGM6. Our results strongly support the candidacy of TGM6 as a novel familial AML-associated gene.

Antineoplastic activity of strawberry (Fragaria×ananassa Duch.) crude extracts on B16-F10 melanoma cells

The antiproliferative and differentiation potential of anthocyanin-rich strawberry fruit crude extracts (SE) were investigated on B16-F10 murine melanoma cells. Treatment of melanoma cells with SE produced a remarkable reduction of cell proliferation, paralleled with both the lowering of the intracellular levels of polyamine, and the enhancement of tissue transglutaminase (TG2, EC 2.3.2.13) activity (used as a differentiation marker). To gain further insight into profiling altered protein expression as a potential biomarker of the SE action on melanoma cells, analysis of the proteomic profile was performed on the treated B16-F10 cells, compared to the control. Following SE treatment, 30 proteins resulted up-regulated, and 87 proteins were down-regulated. In particular proteins overexpressed in cancer cells, involved in tumor progression and metabolism, were down-regulated. The possibility that SE may affect the Warburg effect in B16-F10 melanoma cells is discussed.

Tissue transglutaminase activity protects from cutaneous melanoma metastatic dissemination: an in vivo study

The role of tissue transglutaminase (TG-2, TGase-2) in cancer development is still a fascinating field of research. The available reports do not elucidate fully its mechanism of action, due to the limitations of in vitro approaches. Therefore, to understand TG-2 role in cancer, we carried out an in vivo study with a more direct approach. TG-2 was in vivo overexpressed in a murine model of melanoma (intravenous injection of B16 melanoma cells in C57BL/6N mice) by means of a plasmid carrying the TG-2 cDNA. The evaluation of the frequency and size of the metastases indicated that the number of melanoma lung foci was more markedly reduced by TG-2 overexpression than the metastatic size. Then, TG-2 overexpressing mice showed a prolonged survival with respect to control mice. Further analyses were carried by means of proteomic analysis of melanoma cell lysates and meta-analysis of published transcriptomic datasets. Proteomic analysis of cell lysates from a human melanoma cell line compared to human keratinocytes showed significant differences in the expression of TG-2 substrates known to be involved in proliferation/differentiation and cancer progression. Taken together, these findings indicate a protective role of TG-2 enzymatic activity in melanoma progression in vivo.

γ-Tocopherol inhibits human prostate cancer cell proliferation by up-regulation of transglutaminase 2 and down-regulation of cyclins

To establish a system to study differentiation therapy drugs, we used the androgen-independent human prostate PC-3 tumor cell line as a target and α- and γ-tocopherol as inducers. Effects of α- and γ-tocopherol on the cell cycle, proliferation and differentiation, were examined. A more significant growth inhibition activity for γ- than for α-tocopherol was observed. Flow cytometry analysis of α- and γ-tocopherol-treated prostate carcinoma PC3 cells showed decreased progression into the S-phase. This effect, particularly evident for γ-tocopherol, was associated with an up-regulation and increased activity of transglutaminase 2 (TG2), a reduced DNA synthesis and a remarkable decreased levels of cyclin D1 and cyclin E. Activation of TG2 suggests that γ-tocopherol has an evident differentiative capacity on PC3 cells, leading to an increased expression of TG2, and reduced cyclin D1 and cyclin E levels, affecting cell cycle progression. It is feasible that up-regulation and activation of TG2, associated with a reduced proliferation, are parts of a large-scale reprogramming that can attenuate the malignant phenotype of PC3 cells in vitro. These data suggest further investigation on the potential use of this γ-form of vitamin E as a differentiative agent, in combination with the common cytotoxic treatments for prostate cancer therapy.

Effects of novel C-methylated spermidine analogs on cell growth via hypusination of eukaryotic translation initiation factor 5A

The polyamines, putrescine, spermidine, and spermine, are ubiquitous multifunctional cations essential for cellular proliferation. One specific function of spermidine in cell growth is its role as a butylamine donor for hypusine synthesis in the eukaryotic initiation factor 5A (eIF5A). Here, we report the ability of novel mono-methylated spermidine analogs (α-MeSpd, β-MeSpd, γ-MeSpd, and ω-MeSpd) to function in the hypusination of eIF5A and in supporting the growth of DFMO-treated DU145 cells. We also tested them as substrates and inhibitors for deoxyhypusine synthase (DHS) in vitro. Of these compounds, α-MeSpd, β-MeSpd, and γ-MeSpd (but not ω-MeSpd) were substrates for DHS in vitro, while they all inhibited the enzyme reaction. As racemic mixtures, only α-MeSpd and β-MeSpd supported long-term growth (9-18 days) of spermidine-depleted DU145 cells, whereas γ-MeSpd and ω-MeSpd did not. The S-enantiomer of α-MeSpd, which supported long-term growth, was a good substrate for DHS in vitro, whereas the R-isomer was not. The long-term growth of DFMO-treated cells correlated with the hypusine modification of eIF5A by intracellular methylated spermidine analogs. These results underscore the critical requirement for hypusine modification in mammalian cell proliferation and provide new insights into the specificity of the deoxyhypusine synthase reaction.

Aloin enhances cisplatin antineoplastic activity in B16-F10 melanoma cells by transglutaminase-induced differentiation

Aloin, a natural anthracycline from aloe plant, is a hydroxyanthraquinone derivative shown to have antitumor properties. This study demonstrated that aloin exerted inhibition of cell proliferation, adhesion and invasion abilities of B16-F10 melanoma cells under non-cytotoxic concentrations. Furthermore, aloin induced melanoma cell differentiation through the enhancement of melanogenesis and transglutaminase activity. To improve the growth-inhibiting effect of anticancer agents, we found that the combined treatment of cells with aloin and low doses of cisplatin increases the antiproliferative activity of aloin. The results suggest that aloin possesses antineoplastic and antimetastatic properties, exerted likely through the induction of melanoma cell differentiation.

Tissue transglutaminase: a new target to reverse cancer drug resistance

Cancer resistance mechanisms, which result from intrinsic genetic alterations of tumor cells or acquired genetic and epigenetic changes, limit the long-lasting benefits of anti-cancer treatments. Tissue transglutaminase (TG2) has emerged as a putative gene involved in tumor cell drug resistance and evasion of apoptosis. Although some reports have indicated that TG2 can suppress tumor growth and enhance the growth inhibitory effects of anti-tumor agents, several studies have presented both pro-survival and anti-apoptotic roles for TG2 in malignant cells. Increased TG2 expression has been found in several tumors, where it was considered a potential negative prognostic marker, and it is often associated with advanced stages of disease, metastatic spread and drug resistance. TG2 mediates drug resistance through the activation of survival pathways and the inhibition of apoptosis, but also by regulating extracellular matrix (ECM) formation, the epithelial-to-mesenchymal transition (EMT) or autophagy. Because TG2 knockdown or inhibition of TG2 enzymatic activity may reverse drug resistance and sensitize cancer cells to drug-induced apoptosis, many small molecules capable of blocking TG2 have recently been developed. Additional insight into the multifunctional nature of TG2 as well as translational studies concerning the correlation between TG2 expression, function or location and cancer behavior will aid in translating these findings into new therapeutic approaches for cancer patients.

Aloe-emodin as antiproliferative and differentiating agent on human U937 monoblastic leukemia cells

AIMS

Aloe-emodin (AE), a plant derived anthraquinone, has been shown to have anticancer activity in various human cancer cell lines. We have recently reported that AE possesses a differentiative potential on melanoma cells. The purpose of this study was to investigate the possible modulation of defined markers of monocytic differentiation of AE on human U937 cell line.

MAIN METHODS

U937 cells differentiation has been confirmed unequivocally by Griess and nitroblue tetrazolium reduction assays, protoporphyrin IX accumulation, expression of CD14 and CD11b surface antigens, phagocytic activity, migration and attachment ability. The effect on polyamine metabolism, apoptosis and cytokine production was also investigated.

KEY FINDINGS

We showed that AE-treated U937 cells exhibit a noticeably rise in transglutaminase activity. This enhanced enzyme activity correlates with AE-induced growth arrest and differentiation to functionally mature monocytes.

SIGNIFICANCE

Taken together, the results reported here show that AE can promote the macrophage differentiation of U937 cells, suggesting that this anthraquinone could be a potential candidate as a differentiation-inducing selective agent for therapeutic treatment of leukemia.

Synergic effect of α-tocopherol and naringenin in transglutaminase-induced differentiation of human prostate cancer cells

Prostate cancer is the second most common cancer in men worldwide. Its prevention and treatment remain a challenge to clinicians. Thus, there is an urgent need to discover novel, less toxic, and more effective therapies for patients. Many vitamins and related chemicals, including vitamin E, (tocopherols) have shown their anti-cancer activities as anti-oxidants, activators of transcription factors or factors influencing epigenetic events. Although laboratory tests including the use of animal models showed that this vitamin may have anticancer properties, whether it can effectively prevent the development and/or progression of prostate cancer in humans remains to be intensively studied. This review provides up-to-date information regarding the recent outcomes of laboratory, epidemiology and/or clinical trials on the effects of tocopherols on prostate cancer development, along with our last observations on a combined treatment of a prostate cancer cell line (PC-3) with two natural antineoplastic compounds, naringenin (NG) and α-tocopherol (α-TOC). We report the synergic effect of α-TOC and NG in transglutaminase-induced differentiation of human PC-3 prostate cancer cells. While our results are based on one histological class of tumor, the most significant implication of this observation is that establishes a new way in the screening for detecting new differentiative antineoplastic agents.

Similar antineoplastic effects of nimesulide, a selective COX-2 inhibitor, and prostaglandin E1 on B16-F10 murine melanoma cells

There is now increasing evidence that a constitutive expression of cyclooxygenase 2 (COX-2) plays a role in the development and progression of malignant ectodermal tumours. In this study, we investigated whether the selective inhibition of COX-2 would be beneficial in melanoma treatment. Nimesulide, a selective inhibitor of COX-2 that causes the breakdown of proinflammatory 2-series prostaglandins (PG2), adversely affected the growth of B16-F10 melanoma cells through the induction of differentiation. The intracellular levels of polyamine, as a proliferation marker, were reduced by the treatment; at the same time, transglutaminase activation and increase in melanin content, as differentiation indicators, were observed. The potential antimetastatic activity of the drug was further shown by means of the Boyden invasion assay and gelatin zymography for metalloproteinase activity. Comparable results were obtained after the treatment of cells with one of the 1-series PGs (PGE1). Therefore, our hypothesis is that the antineoplastic activity observed for nimesulide may be ascribed to intracellular changes in alterations in PG levels.

Antitumor properties of aloe-emodin and induction of transglutaminase 2 activity in B16-F10 melanoma cells

AIMS

Aloe-emodin (AE), a natural hydroxyanthraquinone compound, has been reported as a potential anticancer agent. We studied the antineoplastic properties of AE on highly metastatic B16-F10 melanoma murine cells.

MAIN METHODS

Cell proliferation was assessed by cell counting and viability was investigated using MTT and Trypan Bleu exclusion tests. As a growth marker, we determined intracellular polyamine levels by high performance liquid chromatography. Then, we evaluated transglutaminase 2 (TG2) activity, protoporphyrin IX accumulation and melanin content as differentiative markers. Tyrosinase activity was checked by DOPA-staining assay. The antimetastatic effect of AE was evaluated by means of a series of in vitro metastatic assays, including aggregation, wound healing migration, adhesion, 3D-invasion, circular invasion and the Boyden chamber invasion assays. Gelatin zymography was performed to evaluate metalloproteinase activities.

KEY FINDINGS

Our results demonstrated inhibitory effects of AE on melanoma cell proliferation and invasion power, accompanied by the stimulation of cell differentiation parameters. Cell differentiation correlated with a remarkable increase of the activity of the transamidating form of TG2, with a significative enhancement of cell adhesion and aggregation. Impaired invasion was paralleled by the decrease of the secretion of matrix metalloproteinase-9.

SIGNIFICANCE

The overall data confirm a remarkable antiproliferative, antimetastatic and differentiative capability of this anthraquinone. Results suggest that AE appears particularly promising for its potential application in the newborn differentiation therapy of cancer.