Modulation of intracellular Ca(2+) via alpha(1B)-adrenoreceptor signaling molecules, G alpha(h) (transglutaminase II) and phospholipase C-delta 1

A Multidisciplinary Approach Establishes a Link between Transglutaminase 2 and the Kv10.1 Voltage-Dependent K+ Channel in Breast Cancer

Since the multifunctionality of transglutaminase 2 (TG2) includes extra- and intracellular functions, we investigated the effects of intracellular administration of TG2 inhibitors in three breast cancer cell lines, MDA-MB-231, MDA-MB-436 and MDA-MB-468, which are representative of different triple-negative phenotypes, using a patch-clamp technique. The first cell line has a highly voltage-dependent a membrane current, which is low in the second and almost absent in the third one. While applying a voltage protocol to responsive single cells, injection of TG2 inhibitors triggered a significant decrease of the current in MDA-MB-231 that we attributed to voltage-dependent K⁺ channels using the specific inhibitors 4-aminopyridine and astemizole. Since the Kv10.1 channel plays a dominant role as a marker of cell migration and survival in breast cancer, we investigated its relationship with TG2 by immunoprecipitation. Our data reveal their physical interaction affects membrane currents in MDA-MB-231 but not in the less sensitive MDA-MB-436 cells. We further correlated the efficacy of TG2 inhibition with metabolic changes in the supernatants of treated cells, resulting in increased concentration of methyl- and dimethylamines, representing possible response markers. In conclusion, our findings highlight the interference of TG2 inhibitors with the Kv10.1 channel as a potential therapeutic tool depending on the specific features of cancer cells.

Cystamine reduces vascular stiffness in Western diet-fed female mice

Consumption of diets high in fat, sugar, and salt (Western diet, WD) is associated with accelerated arterial stiffening, a major independent risk factor for cardiovascular disease (CVD). Women with obesity are more prone to develop arterial stiffening leading to more frequent and severe CVD compared with men. As tissue transglutaminase (TG2) has been implicated in vascular stiffening, our goal herein was to determine the efficacy of cystamine, a nonspecific TG2 inhibitor, at reducing vascular stiffness in female mice chronically fed a WD. Three experimental groups of female mice were created. One was fed regular chow diet (CD) for 43 wk starting at 4 wk of age. The second was fed a WD for the same 43 wk, whereas a third cohort was fed WD, but also received cystamine (216 mg/kg/day) in the drinking water during the last 8 wk on the diet (WD + C). All vascular stiffness parameters assessed, including aortic pulse wave velocity and the incremental modulus of elasticity of isolated femoral and mesenteric arteries, were significantly increased in WD- versus CD-fed mice, and reduced in WD + C versus WD-fed mice. These changes coincided with respectively augmented and diminished vascular wall collagen and F-actin content, with no associated effect in blood pressure. In cultured human vascular smooth muscle cells, cystamine reduced TG2 activity, F-actin:G-actin ratio, collagen compaction capacity, and cellular stiffness. We conclude that cystamine treatment represents an effective approach to reduce vascular stiffness in female mice in the setting of WD consumption, likely because of its TG2 inhibitory capacity.NEW & NOTEWORTHY This study evaluates the novel role of transglutaminase 2 (TG2) inhibition to directly treat vascular stiffness. Our data demonstrate that cystamine, a nonspecific TG2 inhibitor, improves vascular stiffness induced by a diet rich in fat, fructose, and salt. This research suggests that TG2 inhibition might bear therapeutic potential to reduce the disproportionate burden of cardiovascular disease in females in conditions of chronic overnutrition.

Oláh A, Fésüs L, Mádi A. Transglutaminase 2 Has Metabolic and Vascular Regulatory Functions Revealed by In Vivo Activation of Alpha1-Adrenergic Receptor

The multifunctional tissue transglutaminase has been demonstrated to act as α1-adrenergic receptor-coupled G protein with GTPase activity in several cell types. To explore further the pathophysiological significance of this function we investigated the in vivo effects of the α1-adrenergic receptor agonist phenylephrine comparing responses in wild type and TG2-/- mice. Injection of phenylephrine, but not a beta3-adrenergic agonist (CL-316,243), resulted in the long-term decline of the respiratory exchange ratio and lower lactate concentration in TG2-/- mice indicating they preferred to utilize fatty acids instead of glucose as fuels. Measurement of tail blood pressure revealed that the vasoconstrictive effect of phenylephrine was milder in TG2-/- mice leading to lower levels of lactate dehydrogenase (LDH) isoenzymes in blood. LDH isoenzyme patterns indicated more damage in lung, liver, kidney, skeletal, and cardiac muscle of wild type mice; the latter was confirmed by a higher level of heart-specific CK-MB. Our data suggest that TG2 as an α1-adrenergic receptor-coupled G protein has important regulatory functions in alpha1-adrenergic receptor-mediated metabolic processes and vascular functions.

G Protein-Coupled Receptors (GPCRs)-Mediated Calcium Signaling in Ovarian Cancer: Focus on GPCRs activated by Neurotransmitters and Inflammation-Associated Molecules

G-coupled protein receptors (GCPR) involve several signaling pathways, some of them being coupled with intracellular calcium (Ca²⁺) mobilization. GPCRs were involved in migration, invasion and metastasis of different types of cancers, including ovarian cancer. Many studies have discussed the essential contribution of GPCRs activated by steroid hormones in ovarian cancer. However, ovarian cancer is also associated with altered signals coming from the nervous system, the immune system or the inflammatory environment, in which GPCRs are ‘sensing’ these molecular signals. Many studies have been oriented so far on ovarian cell lines (most of them being of human cell lines), and only few studies based on animal models or clinical studies have been devoted to the expression changes or functional role of GPCRs in ovarian cancer. In this paper, we review the alterations of GPCRs activated by neurotransmitters (muscarinic receptors, serotonin receptors, dopamine receptors, adrenoceptors) or inflammation-associated molecules (bradykinin receptors, histamine receptors, chemokine receptors) in ovarian cancer and we discuss their potential as histological biomarkers.

Induction of GD3/α1-adrenergic receptor/transglutaminase 2-mediated erythroid differentiation in chronic myelogenous leukemic K562 cells

The disialic acid-containing glycosphingolipid GD3 recruited membrane transglutaminase 2 (TG2) as a signaling molecule for erythroid differentiation in human chronic myelogenous leukemia (CML) K562 cells. The α1-adrenergic receptor (α1-AR)/TG2-mediated signaling pathway regulated GD3 functions, including gene expression and production, to differentiate CML K562 cells into erythroid lineage cells. Epinephrine, an AR agonist, increased membrane recruitment as well as GTP-photoaffinity of TG2, inducing GD3 synthase gene expression. Epinephrine activated PI3K/Akt signaling and GTPase downstream of TG2 activated Akt. The coupling of TG2 and GD3 production was specifically suppressed by prazosin (α1-AR antagonist), but not by propranolol (β-AR antagonist) or rauwolscine (α2-AR antagonist), indicating α1-AR specificity. Small interfering RNA (siRNA) experiment results indicated that the α1-AR/TG2-mediated signaling pathway activated PKCs α and δ to induce GD3 synthase gene expression. Transcription factors CREB, AP-1, and NF-κB regulated GD3 synthase gene expression during α1-AR-induced differentiation in CML K562 cells. In addition, GD3 synthase gene expression was upregulated in TG2-transfected cells via α1-AR with expression of erythroid lineage markers and benzidine-positive staining. α1-AR/TG2 signaling pathway-directed GD3 production is a crucial step in erythroid differentiation of K562 cells and GD3 interacts with α1-AR/TG2, inducing GD3/α1-AR/TG2-mediated erythroid differentiation. These results suggest that GD3, which acts as a membrane mediator of erythroid differentiation in CML cells, provides a therapeutic avenue for leukemia treatment.

Genomic variants reveal differential evolutionary constraints on human transglutaminases and point towards unrecognized significance of transglutaminase 2

Transglutaminases (TGMs) catalyze Ca2+-dependent transamidation of proteins with specified roles in blood clotting (F13a) and in cornification (TGM1, TGM3). The ubiquitous TGM2 has well described enzymatic and non-enzymatic functions but in-spite of numerous studies its physiological function in humans has not been defined. We compared data on non-synonymous single nucleotide variations (nsSNVs) and loss-of-function variants on TGM1-7 and F13a from the Exome aggregation consortium dataset, and used computational and biochemical analysis to reveal the roles of damaging nsSNVs of TGM2. TGM2 and F13a display rarer damaging nsSNV sites than other TGMs and sequence of TGM2, F13a and TGM1 are evolutionary constrained. TGM2 nsSNVs are predicted to destabilize protein structure, influence Ca2+ and GTP regulation, and non-enzymatic interactions, but none coincide with conserved functional sites. We have experimentally characterized six TGM2 allelic variants detected so far in homozygous form, out of which only one, p.Arg222Gln, has decreased activities. Published exome sequencing data from various populations have not uncovered individuals with homozygous loss-of-function variants for TGM2, TGM3 and TGM7. Thus it can be concluded that human transglutaminases differ in harboring damaging variants and TGM2 is under purifying selection suggesting that it may have so far not revealed physiological functions.

Tissue Transglutaminase Modulates Vascular Stiffness and Function Through Crosslinking-Dependent and Crosslinking-Independent Functions

Background

The structural elements of the vascular wall, namely, extracellular matrix and smooth muscle cells (SMCs), contribute to the overall stiffness of the vessel. In this study, we examined the crosslinking‐dependent and crosslinking‐independent roles of tissue transglutaminase (TG2) in vascular function and stiffness.

Methods and Results

SMCs were isolated from the aortae of TG2−/− and wild‐type (WT) mice. Cell adhesion was examined by using electrical cell–substrate impedance sensing and PicoGreen assay. Cell motility was examined using a Boyden chamber assay. Cell proliferation was examined by electrical cell–substrate impedance sensing and EdU incorporation assays. Cell micromechanics were studied using magnetic torsion cytometry and spontaneous nanobead tracer motions. Aortic mechanics were examined by tensile testing. Vasoreactivity was studied by wire myography. SMCs from TG2−/− mice had delayed adhesion, reduced motility, and accelerated de‐adhesion and proliferation rates compared with those from WT. TG2−/− SMCs were stiffer and displayed fewer cytoskeletal remodeling events than WT. Collagen assembly was delayed in TG2−/− SMCs and recovered with adenoviral transduction of TG2. Aortic rings from TG2−/− mice were less stiff than those from WT; stiffness was partly recovered by incubation with guinea pig liver TG2 independent of crosslinking function. TG2−/− rings showed augmented response to phenylephrine‐mediated vasoconstriction when compared with WT. In human coronary arteries, vascular media and plaque, high abundance of fibronectin expression, and colocalization with TG2 were observed.

Conclusions

TG2 modulates vascular function/tone by altering SMC contractility independent of its crosslinking function and contributes to vascular stiffness by regulating SMC proliferation and matrix remodeling.

Physiological, pathological, and structural implications of non-enzymatic protein-protein interactions of the multifunctional human transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed member of an enzyme family catalyzing Ca(2+)-dependent transamidation of proteins. It is a multifunctional protein having several well-defined enzymatic (GTP binding and hydrolysis, protein disulfide isomerase, and protein kinase activities) and non-enzymatic (multiple interactions in protein scaffolds) functions. Unlike its enzymatic interactions, the significance of TG2's non-enzymatic regulation of its activities has recently gained importance. In this review, we summarize all the partners that directly interact with TG2 in a non-enzymatic manner and analyze how these interactions could modulate the crosslinking activity and cellular functions of TG2 in different cell compartments. We have found that TG2 mostly acts as a scaffold to bridge various proteins, leading to different functional outcomes. We have also studied how specific structural features, such as intrinsically disordered regions and embedded short linear motifs contribute to multifunctionality of TG2. Conformational diversity of intrinsically disordered regions enables them to interact with multiple partners, which can result in different biological outcomes. Indeed, ID regions in TG2 were identified in functionally relevant locations, indicating that they could facilitate conformational transitions towards the catalytically competent form. We reason that these structural features contribute to modulating the physiological and pathological functions of TG2 and could provide a new direction for detecting unique regulatory partners. Additionally, we have assembled all known anti-TG2 antibodies and have discussed their significance as a toolbox for identifying and confirming novel TG2 regulatory functions.

Regulation of Calcium-Independent Phospholipase A2 Expression by Adrenoceptors and Sterol Regulatory Element Binding Protein-Potential Crosstalk Between Sterol and Glycerophospholipid Mediators

Calcium-independent phospholipase A2 (iPLA2) is an 85-kDa enzyme that releases docosahexaenoic acid (DHA) from glycerophospholipids. DHA can be metabolized to resolvins and neuroprotectins that have anti-inflammatory properties and effects on neural plasticity. Recent studies show an important role of prefrontal cortical iPLA2 in hippocampo-prefrontal cortical LTP and antidepressant-like effect of the norepinephrine reuptake inhibitor (NRI) antidepressant, maprotiline. In this study, we elucidated the cellular mechanisms through which stimulation of adrenergic receptors could lead to increased iPLA2 expression. Treatment of SH-SY5Y neuroblastoma cells with maprotiline, another tricyclic antidepressant with noradrenaline reuptake inhibiting properties, nortriptyline, and the adrenergic receptor agonist, phenylephrine, resulted in increased iPLA2β mRNA expression. This increase was blocked by inhibitors to alpha-1 adrenergic receptor, mitogen-activated protein (MAP) kinase or extracellular signal-regulated kinase (ERK) 1/2, and sterol regulatory element-binding protein (SREBP). Maprotiline and phenylephrine induced binding of SREBP-2 to sterol regulatory element (SRE) region on the iPLA2 promoter, as determined by electrophoretic mobility shift assay (EMSA). Together, results indicate that stimulation of adrenoreceptors causes increased iPLA2 expression via MAP kinase/ERK 1/2 and SREBP, and suggest a possible mechanism for effect of CNS noradrenaline on neural plasticity and crosstalk between sterol and glycerophospholipid mediators, that may play a role in physiological or pathophysiological processes in the brain and other organs.

Transglutaminase regulation of cell function

Transglutaminases (TGs) are multifunctional proteins having enzymatic and scaffolding functions that participate in regulation of cell fate in a wide range of cellular systems and are implicated to have roles in development of disease. This review highlights the mechanism of action of these proteins with respect to their structure, impact on cell differentiation and survival, role in cancer development and progression, and function in signal transduction. We also discuss the mechanisms whereby TG level is controlled and how TGs control downstream targets. The studies described herein begin to clarify the physiological roles of TGs in both normal biology and disease states.

PLCδ1 protein rescues ischemia-reperfused heart by the regulation of calcium homeostasis

Myocardial Ca(2+) overload induced by ischemia/reperfusion (I/R) is a major element of myocardial dysfunction in heart failure. Phospholipase C (PLC) plays important roles in the regulation of the phosphoinositol pathway and Ca(2+) homeostasis in various types of cells. Here, we investigated the protective role of PLCδ1 against myocardial I/R injury through the regulation of Ca(2+) homeostasis. To investigate its role, PLCδ1 was fused to Hph1, a cell-permeable protein transduction domain (PTD), and treated into rat neonatal cardiomyocytes and rat hearts under respective hypoxia-reoxygenation (H/R) and ischemia-reperfusion conditions. Treatment with Hph1-PLCδ1 significantly inhibited intracellular Ca(2+) overload, reactive oxygen species generation, mitochondrial permeability transition pore opening, and mitochondrial membrane potential elevation in H/R neonatal cardiomyocytes, resulting in the inhibition of apoptosis. Intravenous injections of Hph1-PLCδ1 in rats with I/R-injured myocardium caused significant reductions in infarct size and apoptosis and also improved systolic and diastolic cardiac functioning. Furthermore, a small ions profile obtained using time-of-flight secondary ion mass spectrometry showed that treatment with Hph1-PLCδ1 leads to significant recovery of calcium-related ions toward normal levels in I/R-injured myocardium. These results suggest that Hph1-PLCδ1 may manifest as a promising cardioprotective drug due to its inhibition of the mitochondrial apoptotic pathway in cells suffering from I/R injury.

Cellular functions of tissue transglutaminase

Transglutaminase 2 (TG2 or tissue transglutaminase) is a highly complex multifunctional protein that acts as transglutaminase, GTPase/ATPase, protein disulfide isomerase, and protein kinase. Moreover, TG2 has many well-documented nonenzymatic functions that are based on its noncovalent interactions with multiple cellular proteins. A vast array of biochemical activities of TG2 accounts for its involvement in a variety of cellular processes, including adhesion, migration, growth, survival, apoptosis, differentiation, and extracellular matrix organization. In turn, the impact of TG2 on these processes implicates this protein in various physiological responses and pathological states, contributing to wound healing, inflammation, autoimmunity, neurodegeneration, vascular remodeling, tumor growth and metastasis, and tissue fibrosis. TG2 is ubiquitously expressed and is particularly abundant in endothelial cells, fibroblasts, osteoblasts, monocytes/macrophages, and smooth muscle cells. The protein is localized in multiple cellular compartments, including the nucleus, cytosol, mitochondria, endolysosomes, plasma membrane, and cell surface and extracellular matrix, where Ca(2+), nucleotides, nitric oxide, reactive oxygen species, membrane lipids, and distinct protein-protein interactions in the local microenvironment jointly regulate its activities. In this review, we discuss the complex biochemical activities and molecular interactions of TG2 in the context of diverse subcellular compartments and evaluate its wide ranging and cell type-specific biological functions and their regulation.

Nonmuscle myosin IIA (myosin heavy polypeptide 9): a novel class of signal transducer mediating the activation of G alpha h/phospholipase C-delta 1 pathway

The dimeric Gh protein is comprised of alpha (tissue transglutaminase) and beta (Calreticulin) subunits and known to be associated with FSH-, oxytocin-, or epinephrine-receptors/functions in their respective target cells. After establishing the FSH-induced activation of G alpha h/phospholipase C (PLC)-delta 1 pathway in rat Sertoli cells (SCs), we have attempted to identify a possible G alpha h-coupled novel FSH receptor (FSH-R). Remarkably, a protein with approximately 240-kDa molecular mass was coimmunoprecipitated with G alpha h in the fractionated membrane proteins of rat SCs. The protein was identified as myosin heavy polypeptide 9 (MyH9) by mass spectrometric analysis and immunoblotting. In addition, immunoprecipitation analysis reveals that MyH9 is constitutively associated with classical Gs-coupled FSH-R and inactive GDP-bound G alpha h at resting state of rat SCs, but did not interact with FSH directly as judged by Far-Western analysis. Upon the stimulation of higher levels of extracellular FSH (>1000 IU/liter), classical FSH-R induces the phosphorylation of MyH9, the dissociation of active GTP-bound G alpha h from FSH-R:MyH9 complexes, and the elicitation of G alpha h/PLC-delta 1 pathway-dependent Ca(2+)-influx in rat SCs. Furthermore, the specific inhibition of MyH9 ATPase activity with Blebbistatin dose-dependently suppressed FSH-induced G alpha h/PLC-delta 1 signaling and Ca(2+)-influx, but not intracellular cAMP accumulation in rat SCs, implying that MyH9 mediates FSH-induced activation of G alpha h/PLC-delta 1/IP(3)/Ca(2+)-influx pathway in rat SCs. This is the first to demonstrate that the filament protein MyH9 constitutively forms a ternary complex with FSH-R and inactive GDP-bound G alpha h. At higher FSH levels, this ternary complex executes an alternative signaling of classical Gs-coupled FSH-R through activating a Gs/cAMP-independent, G alpha h/PLC-delta 1 pathway in rat SCs.

Functional linkage as a direction for studies in oxidative stress: α-adrenergic receptorsThis review is one of a selection of papers published in a Special Issue on Oxidative Stress in Health and Disease

The α-adrenergic receptors (adrenoceptors) are activated by the endogenous agonists epinephrine and norepinephrine. They are G protein-coupled receptors that may be broadly classified into α1 (subclasses α1A, α1B, α1D) and α2 (subclasses α2A, α2B, α2C). The α1-adrenoceptors act by binding to Gαq subunits of the G proteins, causing activation of phospholipase C (PLC). PLC converts phosphatidylinositol 4,5-bisphosphate into inositol trisphosphate (IP3) and diacylglycerol (DAG), which have downstream effects on cytosolic Ca2+ concentration. The α2-adrenoceptors bind to Gαi thus inhibiting adenylyl cyclase and decreasing cAMP levels. DAG alters protein kinase C activity and cAMP activates protein kinase A. The downstream pathways of the two receptors may also interact. Activation of α1- and α2-adrenoceptors in vascular smooth muscle results in vasoconstriction. However, the densities of individual receptor subclasses vary between vessel beds or between vessels of various sizes within the same bed. In vasculature, the densities of adrenoceptor subclasses differ between conduit arteries and arterioles. These differences, along with differences in coupling mechanisms, allow for fine regulation of arterial blood flow. This diversity is enhanced by interactions resulting from homo- and heterodimer formation of the receptors, metabolic pathways, and kinases. Reactive oxygen species generated in pathologies may alter α1- and α2-adrenoceptor cascades, change vascular contractility, or cause remodeling of blood vessels. This review emphasizes the need for understanding the functional linkage between α-adrenoceptor subtypes, coupling, cross talk, and oxidative stress in cardiovascular pathologies.

Disialoganglioside GD3 synthase expression recruits membrane transglutaminase 2 during erythroid differentiation of the human chronic myelogenous leukemia K562 cells

By employing proteomics analysis tool, we examined the effects of GD3 synthase expression on the differentiation properties of chronic myelogenous leukemia (CML)-derived leukemia cells K562. Forced expression of GD3 synthase induced erythroid differentiation as determined by an increase in glycophorin A expression and synthesis of hemoglobins. The proteomic analysis revealed that 15 proteins were increased by GD3 synthase. In contrast, we observed three protein gel spots decreased in contents in the cell membranes of GD3 synthase-transfected K562 cells. Among the increased proteins, membrane transglutaminase 2 (TG2) was specifically increased in the cell membrane of GD3 synthase-transfected K562 cells. Then, we generated the GD3 synthase-transfected cells in the K562 cells. Interestingly, the TG2 level was increased in GD3 synthase-transfected cells compared with vector- and plasma membrane-associated ganglioside sialidase (Neu3)-transfected cells. In addition, its ability to be photoaffinity-labeled with [alpha-(32)P]GTP was also increased in the GD3 synthase- and TG2-transfected cells. Moreover, small interfering RNA (siRNA) analysis for the GD3 synthase showed the decrease or abolishment of the membrane TG2. Finally, GD3 synthase-transfected cells accelerated the erythroid differentiation. Therefore, we propose that the recruitment of TG2 into membranes by GD3 might play an important role in the erythroid differentiation in K562 cells.

Hormone effects on specific and global brain functions

The first demonstration of how biochemical changes in neurons in specific parts of the brain direct a complete mammalian behavior derived from the effects of estrogens in hypothalamic neurons that facilitate lordosis behavior, the primary reproductive behavior of female quadrupeds (Pfaff. Estrogens and Brain Function. 1980; Pfaff. Drive: Neurobiological and Molecular Mechanisms of Sexual Motivation. 1999). Sex behaviors depend on sexual arousal that in turn depends on a primitive function: generalized CNS arousal (Pfaff. Brain Arousal and Information Theory. 2006). Here we summarize one of the ways in which a generalized arousal transmitter, norepinephrine, can influence the electrical excitability of ventromedial hypothalamic cells in a way that will foster female sex behavior.

Tissue transglutaminase inhibits autophagy in pancreatic cancer cells

Elevated expression of tissue transglutaminase (TG2) in cancer cells has been implicated in the development of drug resistance and metastatic phenotypes. However, the role and the mechanisms that regulate TG2 expression remain elusive. Here, we provide evidence that protein kinase Cdelta (PKCdelta) regulates TG2 expression, which in turn inhibits autophagy, a type II programmed cell death, in pancreatic cancer cells that are frequently insensitive to standard chemotherapeutic agents. Rottlerin, a PKCdelta-specific inhibitor, and PKCdelta small interfering RNA (siRNA) down-regulated the expression of TG2 mRNA and protein and induced growth inhibition without inducing apoptosis in pancreatic cancer cells. Inhibition of PKCdelta by rottlerin or knockdown of TG2 protein by a TG2-specific siRNA resulted in a marked increase in autophagy shown by presence of autophagic vacuoles in the cytoplasm, formation of the acidic vesicular organelles, membrane association of microtubule-associated protein 1 light chain 3 (LC3) with autophagosomes, and a marked induction of LC3-II protein, important hallmarks of autophagy, and by electron microscopy. Furthermore, inhibition of TG2 by rottlerin or by the siRNA led to accumulation of green fluorescent protein (GFP)-LC3-II in autophagosomes in pancreatic cancer cells transfected with GFP-LC3 (GFP-ATG8) expression vector. Knockdown of Beclin-1, a specific autophagy-promoting protein and the product of Becn1 (ATG6), inhibited rottlerin-induced and TG2 siRNA-induced autophagy, indicating that Beclin-1 is required for this process. These results revealed that PKCdelta plays a critical role in the expression of TG2, which in turn regulates autophagy. In conclusion, these results suggest a novel mechanism of regulation of TG2 and TG2-mediated autophagy in pancreatic cancer cells.

Transglutaminase 2 kinase activity facilitates protein kinase A-induced phosphorylation of retinoblastoma protein

Transglutaminase 2 (TG2, tissue transglutaminase) is a multifunctional protein involved in cross-linking a variety of proteins, including retinoblastoma protein (Rb). Here we show that Rb is also a substrate for the recently identified serine/threonine kinase activity of TG2 and that TG2 phosphorylates Rb at the critically important Ser780 residue. Furthermore, phosphorylation of Rb by TG2 destabilizes the Rb.E2F1 complex. TG2 phosphorylation of Rb was abrogated by high Ca2+ concentrations, whereas TG2 transamidating activity was inhibited by ATP. TG2 was itself phosphorylated by protein kinase A (PKA). Phosphorylation of TG2 by PKA attenuated its transamidating activity and enhanced its kinase activity. Activation of PKA in mouse embryonic fibroblasts (MEF) with dibutyryl-cAMP enhanced phosphorylation of both TG2 and Rb by a process that was inhibited by the PKA inhibitor H89. Treatment with dibutyryl-cAMP enhanced Rb phosphorylation in MEFtg2+/+ cells but not in MEFtg2-/- cells. These data indicate that Rb is a substrate for TG2 kinase activity and suggest that phosphorylation of Rb, which results from activation of PKA in fibroblasts, is indirect and requires TG2 kinase activity.

Signaling through a G Protein-coupled receptor and its corresponding G protein follows a stoichiometrically limited model

The bradykinin receptor is a G protein-coupled receptor (GPCR) that is coupled to the Galpha(q) family of heterotrimeric G proteins. In general, a GPCR can exert intracellular signals either by transiently associating with multiple diffusing G protein subunits or by activating a G protein that is stably bound to the receptor, thus generating a signal that is limited by the stoichiometry of the complex. Here we have distinguished between these models by monitoring the association of type 2 bradykinin receptor (B(2)R) and the Galpha(q)/Gbetagamma heterotrimer in living human embryonic kidney 293 cells expressing fluorescent-tagged proteins. Stable B(2)R-Galpha(q) x Gbetagamma complexes are observed in resting cells by fluorescence resonance energy transfer from either Galpha(q)-eCFP or eCFP-Gbetagamma to B(2)R-eYFP. Stimulating the cells with bradykinin causes detachment of B(2)R from the G protein subunits as the receptor internalizes into early endosomes, with a corresponding elimination of B(2)R-G protein fluorescence resonance energy transfer because Galpha(q) and its associated Gbetagamma remain on the plasma membrane. Single point and scanning fluorescence correlation spectroscopy measurements show that a portion of B(2)R molecules diffuses with a mobility corresponding to dimers or small oligomers, whereas a second fraction diffuses in higher order molecular assemblies. Our studies support a model in which receptors are pre-coupled with their corresponding G proteins in the basal state of cells thereby limiting the response to an external signal to a defined stoichiometry that allows for a rapid and directed cellular response.

CNS arousal mechanisms bearing on sex and other biologically regulated behaviors

It now seems possible to move beyond analyzing only the mechanisms for specific sexual behaviors to the analysis of 'generalized arousal' that underlies all motivated behaviors. Our science has advanced sufficiently to attack mechanisms linking specific motivations to these general arousal mechanisms that intrinsically activate all biologically-regulated behaviors including ingestive behaviors. Learning from the well-developed reproductive behavior paradigm, we know that sex hormone effects on hypothalamic neurons have been studied to a point where receptor mechanisms are relatively well understood, a neural circuit for a sex steroid-dependent behavior has been worked out, and several functional genomic regulations have been discovered. Here we focus for the first time on three chemical systems that signal 'generalized arousal' and which impact hormone-dependent hypothalamic neurons of importance to sexual arousal: histamine, norepinephrine and enkephalin. Progress in linking generalized arousal to specific motivational mechanisms is reviewed.

Functional genomics of sex hormone-dependent neuroendocrine systems: specific and generalized actions in the CNS

Sex hormone effects on hypothalamic neurons have been worked out to a point where receptor mechanisms are relatively well understood, a neural circuit for a sex steroid-dependent behavior has been determined, and several functional genomic regulations have been discovered and conceptualized. With that knowledge in hand, we approach deeper problems of explaining sexual arousal and generalized CNS arousal. After a brief summary of arousal mechanisms, we focus on three chemical systems which signal generalized arousal and impact hormone-dependent hypothalamic neurons of behavioral importance: histamine, norepinephrine and enkephalin.

Characterization of purified rat testicular transglutaminase and age-dependent changes of the enzyme activities

The Ca2+-dependent tissue transglutaminase is widely distributed in various tissues and has been reported to participate in many cellular growth and differentiation processes. In the past decade, tissue transglutaminase is also identified as a G protein, G(alphah), for intercellular signaling. To further characterize testicular transglutaminase, the rat testicular transglutaminase was purified by ammonium sulfate precipitation, DEAE ion-exchange, heparin-agarose, and GTP-agarose affinity chromatographies. This purification protocol resulted in a 8400-fold enrichment of the enzyme with a reproducible 15% yield. The purified enzyme showed as a single band of 78kDa on SDS-polyacrylamide gel. Western blot analysis using anti-liver tissue transglutaminase monoclonal antibody also recognized the enzyme, indicating it is a t-TGase in nature. The Km values of purified testicular transglutaminase for putrescine and N,N-dimethylcasein were determined to be 35 and 17 microM, respectively. Its transglutaminase cross-linking activity was strongly inhibited by EGTA, GTP, polyamines, and cystamine, as well as moderately by ATP and NaCl. The enzyme exhibited a magnesium-dependent GTP-hydrolyzing capacity, but its GTP-binding activity did not require magnesium. Furthermore, the enzyme activity was found to be closely related with the first wave of spermatogenesis. Thus, testicular transglutaminase is speculated to participate in the event of spermatogenesis. In conclusion, the purified testicular transglutaminase displays property of either the tissue-type transglutaminase, or the GTP-binding and hydrolyzing characteristics. The activity of testicular transglutaminase is age-dependent, greatly stimulated during the first wave of spermatogenesis.

Activation of PLC-delta1 by Gi/o-coupled receptor agonists

The mechanism of phospholipase (PLC)-delta activation by G protein-coupled receptor agonists was examined in rabbit gastric smooth muscle. Ca(2+) stimulated an eightfold increase in PLC-delta1 activity in permeabilized muscle cells. Treatment of dispersed or cultured muscle cells with three G(i/o)-coupled receptor agonists (somatostatin, delta-opioid agonist [D-Pen(2),D-Pen(5)]enkephalin, and A(1) agonist cyclopentyl adenosine) caused delayed increase in phosphoinositide (PI) hydrolysis (8- to 10-fold) that was strongly inhibited by overexpression of dominant-negative PLC-delta1(E341R/D343R; 65-76%) or constitutively active RhoA(G14V). The response coincided with capacitative Ca(2+) influx and was not observed in the absence of extracellular Ca(2+), but was partly inhibited by nifedipine (16-30%) and strongly inhibited by SKF-96365, a blocker of store-operated Ca(2+) channels. Treatment of the cells with a G(q/13)-coupled receptor agonist, CCK-8, caused only transient, PLC-beta1-mediated PI hydrolysis. Unlike G(i/o)-coupled receptor agonists, CCK-8 activated RhoA and stimulated RhoA:PLC-delta1 association. Inhibition of RhoA activity with C3 exoenzyme or by overexpression of dominant-negative RhoA(T19N) or Galpha(13) minigene unmasked a delayed increase in PI hydrolysis that was strongly inhibited by coexpression of PLC-delta1(E341R/D343R) or by SKF-96365. Agonist-independent capacitative Ca(2+) influx induced by thapsigargin stimulated PI hydrolysis (8-fold), which was partly inhibited by nifedipine ( approximately 25%) and strongly inhibited by SKF-96365 ( approximately 75%) and in cells expressing PLC-delta1(E341R/D343R). Agonist-independent Ca(2+) release or Ca(2+) influx via voltage-gated Ca(2+) channels stimulated only moderate PI hydrolysis (2- to 3-fold), which was abolished by PLC-delta1 antibody or nifedipine. We conclude that PLC-delta1 is activated by G(i/o)-coupled receptor agonists that do not activate RhoA. The activation is preferentially mediated by Ca(2+) influx via store-operated Ca(2+) channels.

Overexpression of transglutaminase 2 accelerates the erythroid differentiation of human chronic myelogenous leukemia K562 cell line through PI3K/Akt signaling pathway

Transglutaminase 2 (TG2) is a GTP-binding protein with transglutaminase activity. Despite advances in the characterization of TG2 functions and their impact on cellular processes, the role of TG2 in Human chronic myelogenous leukemia K562 cell line is still poorly understood. To understand the biological significance of TG2 during the differentiation of K562 cells, we established and characterized K562 cells that specifically express TG2. Non-transfected K562 cells showed the increase of membrane-bound-TG2 level after 3 days in the response to Hemin and all trans-retinoic acid (tRA), indicating that membrane recruitment of TG2 is occurred during the erythroid differentiation. However, membrane recruitment of TG2 in TG2-transfected cells revealed within earlier time period, compared with that in vector-transfected cells. The ability of membrane-bound-TG2 to be photoaffinity-labeled with [alpha-32P]GTP was also increased in TG2-transfected cells. TG2-transfected cells activated Akt phosphorylation and inactivated ERK1/2 phosphorylation, compared with vector-transfected cells. Furthermore, phosphorylation of CREB, one of the Akt substrates, was increased in TG2-transfected cells and this phenomenon was confirmed by RT-PCR analysis of several marker genes related with erythroid lineage in the absence of PI3K specific inhibitor, Wortmannin, indicating that PI3K/Akt signaling pathway also involved in the differentiation of the cell. Finally, as results of benzidine positive staining as well as hemoglobinization analysis, overexpression of TG2 revealed acceleration of the erythroid differentiation of K562 cells. Taken together, there was no increased TG2 expression level in the response of Hemin/tRA and delayed differentiation in vector transfected cells than in TG2-transfected cells, suggesting that suppression of TG2 expression may retard the erythroid differentiation of K562 cells. Therefore, our study may give a new insight for another aspect of the development of this disease.

Alpha1B-adrenoceptor signaling and cell motility: GTPase function of Gh/transglutaminase 2 inhibits cell migration through interaction with cytoplasmic tail of integrin alpha subunits

A multifunctional enzyme, G(h), is a GTP-binding protein that couples to the alpha(1B)-adrenoreceptor and stimulates phospholipase C-delta1 but also displays transglutaminase 2 (TG2) activity. G(h)/TG2 has been implicated to play a role in cell motility. In this study we have examined which function of G(h)/TG2 is involved in this cellular response and the molecular basis. Treatment of human aortic smooth muscle cell with epinephrine inhibits migration to fibronectin and vitronectin, and the inhibition is blocked by the alpha(1)-adrenoreceptor antagonist prazosin or chloroethylclonidine. Up-regulation or overexpression of G(h)/TG2 in human aortic smooth muscle cells, DDT1-MF2, or human embryonic kidney cells, HEK 293 cells, results in inhibition of the migratory activity, and stimulation of the alpha(1B)-adrenoreceptor with the alpha(1) agonist further augments the inhibition of migration of human aortic smooth muscle cells and DDT1-MF2. G(h)/TG2 is coimmunoprecipitated by an integrin alpha(5) antibody and binds to the cytoplasmic tail peptide of integrins alpha(5), alpha(v), and alpha(IIb) subunits in the presence of guanosine 5'-3-O-(thio)triphosphate (GTPgammaS). Mutation of Lys-Arg residues in the GFFKR motif, present in the alpha(5)-tail, significantly reduces the binding of GTPgammaS-G(h)/TG2. Moreover, the motif-containing integrin alpha(5)-tail peptides block G(h)/TG2 coimmunoprecipitation and reverse the inhibition of the migratory activity of HEK 293 cells caused by overexpression G(h)/TG2. These results provide evidence that G(h) function initiates the modulation of cell motility via association of GTP-bound G(h)/TG2 with the GFFKR motif located in integrin alpha subunits.

Tissue transglutaminase has intrinsic kinase activity: identification of transglutaminase 2 as an insulin-like growth factor-binding protein-3 kinase

Tissue transglutaminase (TG2) is a ubiquitous enzyme that cross-links glutamine residues with lysine residues, resulting in protein polymerization, cross-linking of dissimilar proteins, and incorporation of diamines and polyamines into proteins. It has not previously been known to have kinase activity. Recently, insulin-like growth factor-binding protein-3 (IGFBP-3) has been reported to be phosphorylated by breast cancer cell membranes. We purified the IGFBP-3 kinase activity from solubilized T47D breast cancer cell membranes using gel filtration, ion-exchange chromatography, and IGFBP-3 affinity chromatography. The fractions containing kinase activity were further purified by high pressure liquid chromatography and analyzed by tandem mass spectroscopy. TG2 was detected in fractions containing kinase activity. Antisera to TG2 and protein A-Sepharose were used to immunoprecipitate TG2 from membrane fractions. The immunoprecipitates retained IGFBP-3 kinase, whereas immunoprecipitation deleted kinase activity in the membrane supernatant. The inhibitors of TG2, cystamine and monodansyl cadaverine, abolished the ability of the T47D cell membrane preparation to phosphorylate IGFBP-3. Both TG2 purified from guinea pig liver and recombinant human TG2 expressed in insect cells were able to phosphorylate IGFBP-3. TG2 kinase activity was inhibited in a concentration-dependent fashion by calcium, which has previously been shown to be important for the cross-linking activity of TG2. These data provide compelling evidence that TG2 has intrinsic kinase activity, a function that has not previously been ascribed to TG2. Furthermore, we provide evidence that TG2 is a major component of the IGFBP-3 kinase activity present on breast cancer cell membranes.

Functional coupling of rat myometrial alpha 1-adrenergic receptors to Gh alpha/tissue transglutaminase 2 during pregnancy

Gh alpha protein, which exhibits both transglutaminase and GTPase activities, represents a new class of GTP-binding proteins. In the present study, we characterized Gh alpha in rat uterine smooth muscle (myometrium) and followed its expression during pregnancy by reverse transcription-PCR and Western blot. We also measured transglutaminase and GTP binding functions and used a smooth muscle cell line to evaluate the role of Gh alpha in cell proliferation. The results show that pregnancy is associated with an up-regulation of Gh alpha expression at both the mRNA and protein level. Gh alpha induced during pregnancy is preferentially localized to the plasma membrane. This was found associated with an increased ability of plasma membrane preparations to catalyze Ca(2+)-dependent incorporation of [(3)H]putrescine into casein in vitro. In the cytosol, significant changes in the level of immunodetected Gh alpha and transglutaminase activity were seen only at term. Activation of alpha1-adrenergic receptors (alpha1-AR) enhanced photoaffinity labeling of plasma membrane Gh alpha. Moreover, the level of alpha1-AR-coupled Gh alpha increased progressively with pregnancy, which parallels the active period of myometrial cell proliferation. Overexpression of wild type Gh alpha in smooth muscle cell line DDT1-MF2 increased alpha1-AR-induced [(3)H]thymidine incorporation. A similar response was obtained in cells expressing the transglutaminase inactive mutant (C277S) of Gh alpha. Together, these findings underscore the role of Gh alpha as signal transducer of alpha1-AR-induced smooth muscle cell proliferation. In this context, pregnant rat myometrium provides an interesting physiological model to study the mechanisms underlying the regulation of the GTPase function of Gh alpha

Structure and dynamics of the phospholipase C-delta1 pleckstrin homology domain located at the lipid bilayer surface

Despite the importance of signal transduction pathways at membrane surfaces, there have been few means of investigating their molecular mechanisms based on the structural information of membrane-bound proteins. We applied solid state NMR as a novel method to obtain structural information about the phospholipase C-delta1 (PLC-delta1) pleckstrin homology (PH) domain at the lipid bilayer surface. NMR spectra of the alanine residues in the vicinity of the beta5/beta6 loop in the PH domain revealed changes in local conformations due to the membrane localization of the protein. We propose that these conformational changes originate from a hydrophobic interaction between the amphipathic alpha-helix located in the beta5/beta6 loop and the hydrophobic layer of the membrane and contribute to the membrane binding affinity, interdomain interactions and intermolecular interactions of PLC-delta1.