The transglutaminase type 2 and pyruvate kinase isoenzyme M2 interplay in autophagy regulation

Pyruvate Kinase M2: A Potential Regulator of Cardiac Injury Through Glycolytic and Non-glycolytic Pathways

ABSTRACT

Adult animals are unable to regenerate heart cells due to postnatal cardiomyocyte cycle arrest, leading to higher mortality rates in cardiomyopathy. However, reprogramming of energy metabolism in cardiomyocytes provides a new perspective on the contribution of glycolysis to repair, regeneration, and fibrosis after cardiac injury. Pyruvate kinase (PK) is a key enzyme in the glycolysis process. This review focuses on the glycolysis function of PKM2, although PKM1 and PKM2 both play significant roles in the process after cardiac injury. PKM2 exists in both low-activity dimer and high-activity tetramer forms. PKM2 dimers promote aerobic glycolysis but have low catalytic activity, leading to the accumulation of glycolytic intermediates. These intermediates enter the pentose phosphate pathway to promote cardiomyocyte proliferation and heart regeneration. Additionally, they activate adenosine triphosphate (ATP)-sensitive K + (K ATP ) channels, protecting the heart against ischemic damage. PKM2 tetramers function similar to PKM1 in glycolysis, promoting pyruvate oxidation and subsequently ATP generation to protect the heart from ischemic damage. They also activate KDM5 through the accumulation of αKG, thereby promoting cardiomyocyte proliferation and cardiac regeneration. Apart from glycolysis, PKM2 interacts with transcription factors like Jmjd4, RAC1, β-catenin, and hypoxia-inducible factor (HIF)-1α, playing various roles in homeostasis maintenance, remodeling, survival regulation, and neovascularization promotion. However, PKM2 has also been implicated in promoting cardiac fibrosis through mechanisms like sirtuin (SIRT) 3 deletion, TG2 expression enhancement, and activation of transforming growth factor-β1 (TGF-β1)/Smad2/3 and Jak2/Stat3 signals. Overall, PKM2 shows promising potential as a therapeutic target for promoting cardiomyocyte proliferation and cardiac regeneration and addressing cardiac fibrosis after injury.

Dapagliflozin protects heart function against type-4 cardiorenal syndrome through activation of PKM2/PP1/FUNDC1-dependent mitophagy

Dapagliflozin (DAPA) confers significant protection against heart and kidney diseases. However, whether DAPA can alleviate type 4 cardiorenal syndrome (CRS-4)-related cardiomyopathy remains unclear. We tested the hypothesis that DAPA attenuates CRS-4-related myocardial damage through pyruvate kinase isozyme M2 (PKM2) induction and FUN14 domain containing 1 (FUNDC1)-related mitophagy. Cardiomyocyte-specific PKM2 knockout (PKM2CKO) and FUNDC1 knockout (FUNDC1CKO) mice were subjected to subtotal (5/6) nephrectomy to establish a CRS-4 model in vivo. DAPA enhanced PKM2 expression and improved myocardial function and structure in vivo, and this effect was abrogated by PKM2 knockdown. A significant improvement in mitochondrial function was observed in HL-1 cells exposed to sera from DAPA-treated mice, as featured by increased ATP production, decreased mtROS production, improved mitochondrial membrane potential, preserved mitochondrial complex activity, and reduced mitochondrial apoptosis. DAPA restored FUNDC1-dependent mitophagy through post-transcriptional dephosphorylation in a manner dependent on PKM2 whereas ablation of FUNDC1 abolished the defensive actions of DAPA on myocardium and mitochondria under CRS-4. Co-IP and molecular docking assays indicated that PKM2 directly interacted with protein phosphatase 1 (PP1) and FUNDC1, leading to PP1-mediated FUNDC1 dephosphorylation. These results suggest that DAPA attenuates CRS-4-related cardiomyopathy through activating the PKM2/PP1/FUNDC1-mitophagy pathway.

The Transglutaminase-2 Interactome in the APP23 Mouse Model of Alzheimer's Disease

Amyloid-beta (Aβ) deposition in the brain is closely linked with the development of Alzheimer’s disease (AD). Unfortunately, therapies specifically targeting Aβ deposition have failed to reach their primary clinical endpoints, emphasizing the need to broaden the search strategy for alternative targets/mechanisms. Transglutaminase-2 (TG2) catalyzes post-translational modifications, is present in AD lesions and interacts with AD-associated proteins. However, an unbiased overview of TG2 interactors is lacking in both control and AD brain. Here we aimed to identify these interactors using a crossbreed of the AD-mimicking APP23 mouse model with wild type and TG2 knock-out (TG2^−/−) mice. We found that absence of TG2 had no (statistically) significant effect on Aβ pathology, soluble brain levels of Aβ_1–40 and Aβ_1–42, and mRNA levels of TG family members compared to APP23 mice at 18 months of age. Quantitative proteomics and network analysis revealed a large cluster of TG2 interactors involved in synaptic transmission/assembly and cell adhesion in the APP23 brain typical of AD. Comparative proteomics of wild type and TG2^−/− brains revealed a TG2-linked pathological proteome consistent with alterations in both pathways. Our data show that TG2 deletion leads to considerable network alterations consistent with a TG2 role in (dys)regulation of synaptic transmission and cell adhesion in APP23 brains.

A transglutaminase 2-like gene from sea cucumber Apostichopus japonicus mediates coelomocytes autophagy

Transglutaminases (TGases) are widely known to play critical roles in innate immunity, in particular, TGase2, which involves in autophagy process to help degrade protein aggregates under stressful conditions in mammals. Nevertheless, the function of the TGase2 counterpart whether involves in invertebrate autophagy is largely unknown. In this study, a novel TGase2-like homologous gene from the sea cucumber Apostichopus japonicus (named as AjTGase2-like) was cloned using RACE technology and its biological functions were also investigated. The AjTGase2-like gene encoded a peptide of 750 amino acids with the representative domains of Transglut_N domain, TGc domain, and two Transglut_C domains, which exhibited highly conservative with vertebrate TGase2. Multiple sequence alignments and phylogenetic analysis both supported that AjTGase2-like belonged to a new member of TGase2 subfamily. AjTGase2-like was pervasively expressed in all examined tissues, with the largest transcription in muscle, followed by respiratory trees, and intestine. After immersion infection with Vibrio splendidus, the mRNA and protein levels of AjTGase2-like were both significantly induced and reached the highest levels at 24 h, indicating AjTGase2-like plays a key role in immune response. Further functional analysis showed that the ubiquitinated protein level was significantly increased by 1.65-fold (p < 0.01) after silencing of AjTGase2-like, and the protein levels of AjLC3-II/I and AjBeclin1 were both obviously decreased by 0.49-fold (p < 0.01) and 0.64-fold (p < 0.01) at the same time, while the authophagy receptor of Ajp62 was signally up-regulated by 1.40-fold (p < 0.01) under same condition. Moreover, the immunofluorescence signals of AjLC3 and Ajp62 were consistent with their protein levels, suggesting knockdown of AjTGase2-like causes a blockage in autophagy. More importantly, the AjLC3 positive signal was not increased after adding with chloroquine in the case of AjTGase2-like interference, indicating AjTGase2-like might play pivotal role in the early step of autophagosome formation. Besides, our results showed that the fluorescence signal of AjTGase2-like was largely co-localized with Ajp62 around the cytoplasm in vivo, and rAjp62 could directly combine with rAjTGase2-like in vitro, indicating AjTGase2-like interacts with Ajp62 during autophagy. Overall, our findings supported that AjTGase2-like served as a positive regulator in sea cucumber authophay.

The Multifaceted Role of HSF1 in Pathophysiology: Focus on Its Interplay with TG2

The cellular environment needs to be strongly regulated and the maintenance of protein homeostasis is crucial for cell function and survival. HSF1 is the main regulator of the heat shock response (HSR), the master pathway required to maintain proteostasis, as involved in the expression of the heat shock proteins (HSPs). HSF1 plays numerous physiological functions; however, the main role concerns the modulation of HSPs synthesis in response to stress. Alterations in HSF1 function impact protein homeostasis and are strongly linked to diseases, such as neurodegenerative disorders, metabolic diseases, and different types of cancers. In this context, type 2 Transglutaminase (TG2), a ubiquitous enzyme activated during stress condition has been shown to promote HSF1 activation. HSF1-TG2 axis regulates the HSR and its function is evolutionary conserved and implicated in pathological conditions. In this review, we discuss the role of HSF1 in the maintenance of proteostasis with regard to the HSF1-TG2 axis and we dissect the stress response pathways implicated in physiological and pathological conditions.

Transglutaminase 2 Regulates Innate Immunity by Modulating the STING/TBK1/IRF3 Axis

We have recently shown that type 2 transglutaminase (TG2) plays a key role in the host's inflammatory response during bacterial infections. In this study, we investigated whether the enzyme is involved in the regulation of the STING pathway, which is the main signaling activated in the presence of both self- and pathogen DNA in the cytoplasm, leading to type I IFN (IFN I) production. In this study, we demonstrated that TG2 negatively regulates STING signaling by impairing IRF3 phosphorylation in bone marrow-derived macrophages, isolated from wild-type and TG2 knockout mice. In the absence of TG2, we found an increase in the IFN-β production and in the downstream JAK/STAT pathway activation. Interestingly, proteomic analysis revealed that TG2 interacts with TBK1, affecting its interactome composition. Indeed, TG2 ablation facilitates the TBK1-IRF3 interaction, thus indicating that the enzyme plays a negative regulatory effect on IRF3 recruitment in the STING/TBK1 complex. In keeping with these findings, we observed an increase in the IFNβ production in bronchoalveolar lavage fluids from COVID-19-positive dead patients paralleled by a dramatic decrease of the TG2 expression in the lung pneumocytes. Taken together, these results suggest that TG2 plays a negative regulation on the IFN-β production associated with the innate immunity response to the cytosolic presence of both self- and pathogen DNA.

Combined proteomic and lipidomic studies in Pompe disease allow a better disease mechanism understanding

Pompe disease (PD) is caused by deficiency of the enzyme acid α-glucosidase resulting in glycogen accumulation in lysosomes. Clinical symptoms include skeletal myopathy, respiratory failure, and cardiac hypertrophy. We studied plasma proteomic and lipidomic profiles in 12 PD patients compared to age-matched controls. The proteomic profiles were analyzed by nLC-MS/MS SWATH method. Wide-targeted lipidomic analysis was performed by LC-IMS/MS, allowing to quantify >1100 lipid species, spanning 13 classes. Significant differences were found for 16 proteins, with four showing the most relevant changes (GPLD1, PON1, LDHB, PKM). Lipidomic analysis showed elevated levels of three phosphatidylcholines and of the free fatty acid 22:4, and reduced levels of six lysophosphatidylcholines. Up-regulated glycolytic enzymes (LDHB and PKM) are involved in autophagy and glycogen metabolism, while down-regulated PON1 and GPLD1 combined with lipidomic data indicate an abnormal phospholipid metabolism. Reduced GPLD1 and dysregulation of lipids with acyl-chains characteristic of GPI-anchor structure suggest the potential involvement of GPI-anchor system in PD. Results of proteomic analysis displayed the involvement of multiple cellular functions affecting inflammatory, immune and antioxidant responses, autophagy, Ca²⁺ -homeostasis, and cell adhesion. The combined multi-omic approach revealed new biosignatures in PD, providing novel insights in disease pathophysiology with potential future clinical application.

Autophagy activation and photoreceptor survival in retinal detachment

We assess the effect of autophagy inhibition on photoreceptor (PR) survival during experimental retinal detachment (RD) and examine the and examine the relationship between autophagy and the expression of glycolytic enzymes HK2 and PKM2 in the retina. We find that inhibiting autophagy by genetic knock out of the autophagy activator Atg5 in rod PRs resulted in increased apoptotic and necroptotic cell death during RD, demonstrated by elevated terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive cells, caspase 8 activity, transcript levels of Fas receptor and RIPK3 as compared to controls. The absence of autophagy in rods resulted in downregulation of hexokinase 2 and pyruvate kinase muscle isozyme 2 levels. More than 460 proteins were identified by mass spectroscopy in autophagosomes isolated from detached retinas compared with less than 150 proteins identified in autophagosomes from attached retinas. Among various cellular compartments, proteins from cytoskeleton, cytoplasm and intracellular organelles constituted a large portion of increased autophagosome contents. These proteins represent numerous biological processes, including phototransduction, cell-cell signaling, metabolism and inflammation. Our findings suggest that competent autophagy machinery is necessary for PR homeostasis and improving PR survival during periods of nutrient deprivation.

Genistein antagonizes gliadin-induced CFTR malfunction in models of celiac disease

In celiac disease (CD), an intolerance to dietary gluten/gliadin, antigenic gliadin peptides trigger an HLA-DQ2/DQ8-restricted adaptive Th1 immune response. Epithelial stress, induced by other non-antigenic gliadin peptides, is required for gliadin to become fully immunogenic. We found that cystic-fibrosis-transmembrane-conductance-regulator (CFTR) acts as membrane receptor for gliadin-derived peptide P31-43, as it binds to CFTR and impairs its channel function. P31-43-induced CFTR malfunction generates epithelial stress and intestinal inflammation. Maintaining CFTR in an active open conformation by the CFTR potentiators VX-770 (Ivacaftor) or Vrx-532, prevents P31-43 binding to CFTR and controls gliadin-induced manifestations. Here, we evaluated the possibility that the over-the-counter nutraceutical genistein, known to potentiate CFTR function, would allow to control gliadin-induced alterations. We demonstrated that pre-treatment with genistein prevented P31-43-induced CFTR malfunction and an epithelial stress response in Caco-2 cells. These effects were abrogated when the CFTR gene was knocked out by CRISP/Cas9 technology, indicating that genistein protects intestinal epithelial cells by potentiating CFTR function. Notably, genistein protected gliadin-sensitive mice from intestinal CFTR malfunction and gliadin-induced inflammation as it prevented gliadin-induced IFN-γ production by celiac peripheral-blood-mononuclear-cells (PBMC) cultured ex-vivo in the presence of P31-43-challenged Caco-2 cells. Our results indicate that natural compounds capable to increase CFTR channel gating might be used for the treatment of CD.

Identifying transglutaminase reaction products via mass spectrometry as exemplified by the MUC2 mucin - Pitfalls and traps

In order to demonstrate transglutaminase activity in biological samples immunological as well as glutamine- and amine-donor based assays are commonly used. However, the identification of the transglutaminase reaction product, i. e. the isopeptide cross-linked peptides/proteins or the deamidated protein/peptide are often neglected. This article describes a workflow for the detection of the products of transglutaminase-catalyzed reactions. In particular, possible pitfalls and traps that can arise during the mass spectrometry-based identification of isopeptide cross-links are addressed and characterised on actual samples.

Infection-driven activation of transglutaminase 2 boosts glucose uptake and hexosamine biosynthesis in epithelial cells

Transglutaminase 2 (TG2) is a ubiquitously expressed enzyme with transamidating activity. We report here that both expression and activity of TG2 are enhanced in mammalian epithelial cells infected with the obligate intracellular bacteria Chlamydia trachomatis. Genetic or pharmacological inhibition of TG2 impairs bacterial development. We show that TG2 increases glucose import by up-regulating the transcription of the glucose transporter genes GLUT-1 and GLUT-3. Furthermore, TG2 activation drives one specific glucose-dependent pathway in the host, i.e., hexosamine biosynthesis. Mechanistically, we identify the glucosamine:fructose-6-phosphate amidotransferase (GFPT) among the substrates of TG2. GFPT modification by TG2 increases its enzymatic activity, resulting in higher levels of UDP-N-acetylglucosamine biosynthesis and protein O-GlcNAcylation. The correlation between TG2 transamidating activity and O-GlcNAcylation is disrupted in infected cells because host hexosamine biosynthesis is being exploited by the bacteria, in particular to assist their division. In conclusion, our work establishes TG2 as a key player in controlling glucose-derived metabolic pathways in mammalian cells, themselves hijacked by C. trachomatis to sustain their own metabolic needs.

Glycolysis regulated transglutaminase 2 activation in cardiopulmonary fibrogenic remodeling

The pathophysiology of pulmonary hypertension (PH) and heart failure (HF) includes fibrogenic remodeling associated with the loss of pulmonary arterial (PA) and cardiac compliance. We and others have previously identified transglutaminase 2 (TG2) as a participant in adverse fibrogenic remodeling. However, little is known about the biologic mechanisms that regulate TG2 function. We examined physiological mouse models of experimental PH, HF, and type 1 diabetes that are associated with altered glucose metabolism/glycolysis and report here that TG2 expression and activity are elevated in pulmonary and cardiac tissues under all these conditions. We additionally used PA adventitial fibroblasts to test the hypothesis that TG2 is an intermediary between enhanced tissue glycolysis and fibrogenesis. Our in vitro results show that glycolytic enzymes and TG2 are upregulated in fibroblasts exposed to high glucose, which stimulates cellular glycolysis as measured by Seahorse analysis. We examined the relationship of TG2 to a terminal glycolytic enzyme, pyruvate kinase M2 (PKM2), and found that PKM2 regulates glucose-induced TG2 expression and activity as well as fibrogenesis. Our studies further show that TG2 inhibition blocks glucose-induced fibrogenesis and cell proliferation. Our findings support a novel role for glycolysis-mediated TG2 induction and tissue fibrosis associated with experimental PH, HF, and hyperglycemia.

Transglutaminase Type 2 Regulates ER-Mitochondria Contact Sites by Interacting with GRP75

Transglutaminase type 2 (TG2) is a multifunctional enzyme that plays a key role in mitochondria homeostasis under stressful cellular conditions. TG2 interactome analysis reveals an enzyme interaction with GRP75 (glucose-regulated protein 75). GRP75 localizes in mitochondria-associated membranes (MAMs) and acts as a bridging molecule between the two organelles by assembling the IP3R-GRP75-VDAC complex, which is involved in the transport of Ca²⁺ from the endoplasmic reticulum (ER) to mitochondria. We demonstrate that the TG2 and GRP75 interaction occurs in MAMs. The absence of the TG2-GRP75 interaction leads to an increase of the interaction between IP3R-3 and GRP75; a decrease of the number of ER-mitochondria contact sites; an impairment of the ER-mitochondrial Ca²⁺ flux; and an altered profile of the MAM proteome. These findings indicate TG2 is a key regulatory element of the MAMs.

Transglutaminase type 2 in the regulation of proteostasis

The maintenance of protein homeostasis (proteostasis) is a fundamental aspect of cell physiology that is essential for the survival of organisms under a variety of environmental and/or intracellular stress conditions. Acute and/or persistent stress exceeding the capacity of the intracellular homeostatic systems results in protein aggregation and/or damaged organelles that leads to pathological cellular states often resulting in cell death. These events are continuously suppressed by a complex macromolecular machinery that uses different intracellular pathways to maintain the proteome integrity in the various subcellular compartments ensuring a healthy cellular life span. Recent findings have highlighted the role of the multifunctional enzyme type 2 transglutaminase (TG2) as a key player in the regulation of intracellular pathways, such as autophagy/mitophagy, exosomes formation and chaperones function, which form the basis of proteostasis regulation under conditions of cellular stress. Here, we review the role of TG2 in these stress response pathways and how its various enzymatic activities might contributes to the proteostasis control.

Transglutaminase Activity Determines Nuclear Localization of Serotonin Immunoreactivity in the Early Embryos of Invertebrates and Vertebrates

Serotonin (5-HT) is a key player in many physiological processes in both the adult organism and developing embryo. One of the mechanisms for 5-HT-mediated effects is covalent binding of 5-HT to the target proteins catalyzed by transglutaminases (serotonylation). Despite the implication in a variety of physiological processes, the involvement of serotonylation in embryonic development remains unclear. Here we tested the hypothesis that 5-HT serves as a substrate for transglutaminase-mediated transamidation of the nuclear proteins in the early embryos of both vertebrates and invertebrates. For this, we demonstrated that the level of serotonin immunoreactivity (5-HT-ir) in cell nuclei increases upon the elevation of 5-HT concentration in embryos of sea urchins, mollusks, and teleost fish. Consistently, pharmacological inhibition of transglutaminase activity resulted in the reduction of both brightness and nuclear localization of anti-5-HT staining. We identified specific and bright 5-HT-ir within nuclei attributed to a subset of different cell types: ectodermal and endodermal, macro- and micromeres, and blastoderm. Western blot and dot blot confirmed the presence of 5-HT-ir epitopes in the normal embryos of all the species examined. The experimental elevation of 5-HT level led to the enhancement of 5-HT-ir-related signal on blots in a species-specific manner. The obtained results demonstrate that 5-HT is involved in transglutaminase-dependent monoaminylation of nuclear proteins and suggest nuclear serotonylation as a possible regulatory mechanism during early embryonic development. The results reveal that this pathway is conserved in the development of both vertebrates and invertebrates.

A pathogenic role for cystic fibrosis transmembrane conductance regulator in celiac disease

Intestinal handling of dietary proteins usually prevents local inflammatory and immune responses and promotes oral tolerance. However, in ~ 1% of the world population, gluten proteins from wheat and related cereals trigger an HLA DQ2/8‐restricted T_H1 immune and antibody response leading to celiac disease. Prior epithelial stress and innate immune activation are essential for breaking oral tolerance to the gluten component gliadin. How gliadin subverts host intestinal mucosal defenses remains elusive. Here, we show that the α‐gliadin‐derived LGQQQPFPPQQPY peptide (P31–43) inhibits the function of cystic fibrosis transmembrane conductance regulator (CFTR), an anion channel pivotal for epithelial adaptation to cell‐autonomous or environmental stress. P31–43 binds to, and reduces ATPase activity of, the nucleotide‐binding domain‐1 (NBD1) of CFTR, thus impairing CFTR function. This generates epithelial stress, tissue transglutaminase and inflammasome activation, NF‐κB nuclear translocation and IL‐15 production, that all can be prevented by potentiators of CFTR channel gating. The CFTR potentiator VX‐770 attenuates gliadin‐induced inflammation and promotes a tolerogenic response in gluten‐sensitive mice and cells from celiac patients. Our results unveil a primordial role for CFTR as a central hub orchestrating gliadin activities and identify a novel therapeutic option for celiac disease.

TG2 regulates the heat-shock response by the post-translational modification of HSF1

Heat-shock factor 1 (HSF1) is the master transcription factor that regulates the response to proteotoxic stress by controlling the transcription of many stress-responsive genes including the heat-shock proteins. Here, we show a novel molecular mechanism controlling the activation of HSF1. We demonstrate that transglutaminase type 2 (TG2), dependent on its protein disulphide isomerase activity, triggers the trimerization and activation of HSF1 regulating adaptation to stress and proteostasis impairment. In particular, we find that TG2 loss of function correlates with a defect in the nuclear translocation of HSF1 and in its DNA-binding ability to the HSP70 promoter. We show that the inhibition of TG2 restores the unbalance in HSF1-HSP70 pathway in cystic fibrosis (CF), a human disorder characterized by deregulation of proteostasis. The absence of TG2 leads to an increase of about 40% in CFTR function in a new experimental CF mouse model lacking TG2. Altogether, these results indicate that TG2 plays a key role in the regulation of cellular proteostasis under stressful cellular conditions through the modulation of the heat-shock response.

miR-361-5p modulates metabolism and autophagy via the Sp1-mediated regulation of PKM2 in prostate cancer

Prostate cancer (PCa) is a leading cause of death among men. The dysregulation of metabolism and autophagy contributes to the progression of PCa. The transcription factor specificity protein 1 (Sp1) is implicated in the regulation of metabolism and autophagy. We confirmed that Sp1 is overexpressed in castration-resistant prostate cancer (CRPC) cells. However, the roles of Sp1 in PCa metabolism and autophagy remain unclear. Thus, in the present study, we retrieved the GSE35988 dataset from Gene Expression Omnibus (GEO) database to reinvestigate Sp1 expression and its role in PCa.We found that in PCa, Sp1 knockdown significantly inhibited cell growth, aerobic glycolysis, and hypoxia-induced autophagy, which were accompanied by an increased G1 cell cycle arrest. Pearson correlation indicated that pyruvate kinase isoenzyme type M2 (PKM2) is positively correlated with Sp1 expression. Western blot analysis demonstrated that Sp1 directly regulates PKM2; therefore, Sp1 modulates metabolism and autophagy in CRPC. Western blot analysis and luciferase reporter assay also indicated that the tumor suppressor miR-361-5p inversely regulates Sp1 by directly targeting the binding site in the 3'UTR of Sp1. miR-361-5p overexpression presented effects that are similar to Sp1 depletion in PCa. In summary, this study is the first to demonstrate that miR-361-5p suppresses the Sp1/PKM2 axis, consequently affecting the progression of PCa and the metabolism and autophagy of PCa cells. Therefore, targeting the miR-361-5p/Sp1/PKM2 pathway has considerable clinical significance in preventing the malignant progression of PCa.

Assessing the Catalytic Activity of Transglutaminases in the Context of Autophagic Responses

The human transglutaminases (TGases) are a widely distributed and peculiar group of enzymes that catalyze the posttranslational modification of proteins by the formation of isopeptide bonds. Tissue or type 2 transglutaminase (TG2) represents the most ubiquitous isoform belonging to TGases family. The vast array of biochemical functions catalyzed by TG2 distinguishes it from the other members of the TGase family. In the presence of high calcium levels TG2 catalyzes a vast array of protein posttranslational modifications, including protein-protein cross-linking, incorporation of primary amines into proteins, as well as glutamine deamination. In the last few years, it has become evident that TG2 is involved in the final maturation of autolysosomes. The TG2 regulation of autophagy occurs by its transamidating activity and its inhibition results in the intracellular increase of ubiquitinated protein aggregates. In this chapter, we describe the methods used in our laboratories to assess the catalytic activity of TG2 in the autophagic process.

G9a inhibition induced PKM2 regulates autophagic responses

Epigenetic regulation by histone methyltransferase G9a is known to control autophagic responses. As the link between autophagy and metabolic homeostasis is widely accepted, we investigated whether G9a affects metabolic circuitries to affect autophagic response in glioma cells. Both pharmacological inhibition and siRNA mediated knockdown of G9a increased autophagy marker LC3B in glioma cells. G9a inhibitor BIX-01294 (BIX) induced Akt-dependent increase in HIF-1α expression and activity. Inhibition of Akt-HIF-1α axis reversed BIX-mediated (i) increase in LC3B expression and (ii) decrease in Yes-associated protein 1 (YAP1) phosphorylation. YAP1 over-expression abrogated BIX induced increase in LC3B expression. Interestingly, BIX induced increase in metabolic modelers TIGAR (TP53-induced glycolysis and apoptosis regulator) and PKM2 (Pyruvate kinase M2) were crucial for BIX-mediated changes, as transfection with TIGAR mutant or PKM2 siRNA reversed BIX-mediated alterations in pYAP1 and LC3B expression. Coherent with the in vitro observation, BIX had no significant effect on the tumor burden in heterotypic xenograft glioma mouse model. Elevated LC3B and PKM2 in BIX-treated xenograft tissue was accompanied by decreased YAP1 levels. Taken together, our findings suggest that Akt-HIF-1α axis driven PKM2-YAP1 cross talk activates autophagic responses in glioma cells upon G9a inhibition.

Changing perspective on oncometabolites: from metabolic signature of cancer to tumorigenic and immunosuppressive agents

During tumorigenesis, the shift from oxidative phosphorylation to glycolysis in ATP production accounts for the dramatic change in the cellular metabolism and represents one of the major steps leading to tumour formation. The so-called Warburg effect is currently considered something more than a mere modification in the cellular metabolism. The paradox that during cancer cell proliferation the increase in energy need is supplied by glycolysis can be only explained by taking into account the many roles that intermediates of glycolysis or TCA cycle play in cellular physiology, besides energy production. Recent studies have shown that metabolic intermediates induce changes in chromatin structure or drive neo-angiogenesis. In this review, we present some of the latest findings in the study of cancer metabolism with particular attention to how tumour metabolism and its microenvironment can favour tumour growth and aggressiveness, by hijacking and dampening the anti-tumoral immune response.

Ribose 5-phosphate isomerase inhibits LC3 processing and basal autophagy

Autophagy and cellular metabolism are tightly linked processes, but how individual metabolic enzymes regulate the process of autophagy is not well understood. This study implicates ribose-5-phosphate isomerase (RPIA), a key regulator of the pentose phosphate pathway, in the control of autophagy. We used a dual gene deletion strategy, combining shRNA-mediated knockdown studies with CRISPR/Cas9 genome editing. Knockdown of RPIA by shRNA or genomic deletion by CRISPR/Cas9 genome editing, results in an increase of ATG4B-mediated LC3 processing and in the appearance of LC3-positive autophagosomes in cells. Increased LC3 processing upon knockdown of RPIA can be reversed by treatment with the antioxidant N-acetyl cysteine. The results are consistent with a model in which RPIA suppresses autophagy and LC3 processing by modulation of redox signaling.

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Ribose-5-phosphate isomerase links autophagy with the pentose phosphate pathway.

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Generation of a CRISPR/Cas9 genome edited RPIA knockout cell line

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RPIA isomerase suppresses cellular LC3 processing and autophagosome formation.