Evidence that the Gh protein is a signal mediator from alpha 1-adrenoceptor to a phospholipase C. I. Identification of alpha 1-adrenoceptor-coupled Gh family and purification of Gh7 from bovine heart

Cardiac and Vascular α1-Adrenoceptors in Congestive Heart Failure: A Systematic Review

As heart failure (HF) is a devastating health problem worldwide, a better understanding and the development of more effective therapeutic approaches are required. HF is characterized by sympathetic system activation which stimulates α- and β-adrenoceptors (ARs). The exposure of the cardiovascular system to the increased locally released and circulating levels of catecholamines leads to a well-described downregulation and desensitization of β-ARs. However, information on the role of α-AR is limited. We have performed a systematic literature review examining the role of both cardiac and vascular α1-ARs in HF using 5 databases for our search. All three α1-AR subtypes (α1A, α1B and α1D) are expressed in human and animal hearts and blood vessels in a tissue-dependent manner. We summarize the changes observed in HF regarding the density, signaling and responses of α1-ARs. Conflicting findings arise from different studies concerning the influence that HF has on α1-AR expression and function; in contrast to β-ARs there is no consistent evidence for down-regulation or desensitization of cardiac or vascular α1-ARs. Whether α1-ARs are a therapeutic target in HF remains a matter of debate.

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.

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.

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.

Increased potency of α1-adrenergic receptors to induce inositol phosphates production correlates with the up-regulation of α1d/Ghα/phospholipase Cδ1 signaling pathway in term rat myometrium

In the present study, we studied the potential regulation by rat myometrial α1-adrenergic receptors (α1-AR) of the newly identified Ghα protein/phospholipase Cδ1 (PLCδ1) signaling pathway and compared myometrial inositol phosphates (InsP) production and activity of the uterine circular muscle in response to α1-AR activation between mid-pregnancy and term. For this, we quantified the level of rat myometrial α1-AR coupling to Ghα protein by photoaffinity-labeling, the cytosolic amount of PLCδ1 enzyme by immunoblotting, and the expression level of α1-AR subtypes by RT-PCR. The results showed an increased level of α1-AR/Ghα protein coupling and the amount of PLCδ1 at term (+147 and +65% respectively, versus mid-pregnancy). This was correlated with an up-regulation of α1d-AR subtype (+70% versus mid-pregnancy). Incubation of myometrial strips with phenylephrine (Phe), a global α1-agonist, increased InsP production in a dose-dependent manner at both mid-pregnancy and term, but with an enhanced potency (tenfold decrease in EC50value) at term. Phe also dose-dependently induced contraction of the circular muscle at both mid-pregnancy and term. However, unlike InsP response, no amelioration of potency was observed at term. Similar results were obtained with the endogenous agonist norepinephrine. Our results show, for the first time, that rat myometrial α1d-AR/Ghα/PLCδ1 signaling pathway is up-regulated at term. This is associated with an increased potency of α1-AR to elicit InsP production but not uterine contraction at this period. It is thus hypothesized that α1-AR, through activation of Ghα/PLCδ1 system, are not primarily involved in the initiation of labor but may rather regulate responses such as myometrial cell proliferation or hypertrophy.

Signal transduction and regulation: are all alpha1-adrenergic receptor subtypes created equal?

The current manuscript reviews the evidence whether and how subtypes of alpha(1)-adrenergic receptors, i.e. alpha(1A)-, alpha(1B)- and alpha(1D)-adrenergic receptors, differentially couple to signal transduction pathways and exhibit differential susceptibility to regulation. In both regards studies in tissues or cells natively expressing the subtypes are hampered because the relative expression of the subtypes is poorly controlled and the observed effects may be cell-type specific. An alternative approach, i.e. transfection of multiple subtypes into the same host cell line overcomes this limitation, but it often remains unclear whether results in such artificial systems are representative for the physiological situation. The overall evidence suggests that indeed subtype-intrinsic and cell type-specific factors interact to direct alpha(1)-adrenergic receptor signaling and regulation. This may explain why so many apparently controversial findings have been reported from various tissues and cells. One of the few consistent themes is that alpha(1D)-adrenergic receptors signal less effectively upon agonist stimulation than the other subtypes, most likely because they exhibit spontaneous internalization.

Mutation of a critical arginine in the GTP-binding site of transglutaminase 2 disinhibits intracellular cross-linking activity

Transglutaminase type 2 (TG2; also known as G(h)) is a multifunctional protein involved in diverse cellular processes. It has two well characterized enzyme activities: receptor-stimulated signaling that requires GTP binding and calcium-activated transamidation or cross-linking that is inhibited by GTP. In addition to the GDP binding residues identified from the human TG2 crystal structure (Liu, S., Cerione, R. A., and Clardy, J. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 2743-2747), we have previously implicated Ser171 in GTP binding, as binding is lost with glutamate substitution (Iismaa, S. E., Wu, M.-J., Nanda, N., Church, W. B., and Graham, R. M. (2000) J. Biol. Chem. 275, 18259-18265). Here, we have shown that alanine substitution of homologous residues in rat TG2 (Phe174 in the core domain or Arg476, Arg478, or Arg579 in barrel 1) does not affect TG activity but reduces or abolishes GTP binding and GTPgammaS inhibition of TG activity in vitro, indicating that these residues are important in GTP binding. Alanine substitution of Ser171 does not impair GTP binding, indicating this residue does not interact directly with GTP. Arg579 is particularly important for GTP binding, as isothermal titration calorimetry demonstrated a 100-fold reduction in GTP binding affinity by the R579A mutant. Unlike wild-type TG2 or its S171E or F174A mutants, which are sensitive to both trypsin and mu-calpain digestion, R579A is inherently more resistant to mu-calpain, but not trypsin, digestion, indicating reduced accessibility and/or flexibility of this mutant in the region of the calpain cleavage site(s). Basal TG activity of intact R579A stable SH-SY5Y neuroblastoma cell transfectants was slightly increased relative to wild-type transfectants and, in contrast to the TG activity of the latter, was further stimulated by muscarinic receptor-activated calcium mobilization. Thus, loss of GTP binding sensitizes TG2 to intracellular calcium concentrations. These findings are consistent with the notion that intracellularly, under physiological conditions, TG2 is maintained largely as a latent enzyme, its calcium-activated cross-linking activity being suppressed allosterically by guanine nucleotide binding.

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

Neural connections between the hypothalamus and the liver

After receiving information from afferent nerves, the hypothalamus sends signals to peripheral organs, including the liver, to keep homeostasis. There are two ways for the hypothalamus to signal to the peripheral organs: by stimulating the autonomic nerves and by releasing hormones from the pituitary gland. In order to reveal the involvement of the autonomic nervous system in liver function, we focus in this study on autonomic nerves and neuroendocrine connections between the hypothalamus and the liver. The hypothalamus consists of three major areas: lateral, medial, and periventricular. Each area has some nuclei. There are two important nuclei and one area in the hypothalamus that send out the neural autonomic information to the peripheral organs: the ventromedial hypothalamic nucleus (VMH) in the medial area, the lateral hypothalamic area (LHA), and the periventricular hypothalamic nucleus (PVN) in the periventricular area. VMH sends sympathetic signals to the liver via the celiac ganglia, the LHA sends parasympathetic signals to the liver via the vagal nerve, and the PVN integrates information from other areas of the hypothalamus and sends both autonomic signals to the liver. As for the afferent nerves, there are two pathways: a vagal afferent and a dorsal afferent nerve pathway. Vagal afferent nerves are thought to play a role as sensors in the peripheral organs and to send signals to the brain, including the hypothalamus, via nodosa ganglia of the vagal nerve. On the other hand, dorsal afferent nerves are primary sensory nerves that send signals to the brain via lower thoracic dorsal root ganglia. In the liver, many nerves contain classical neurotransmitters (noradrenaline and acetylcholine) and neuropeptides (substance P, calcitonin gene-related peptide, neuropeptide Y, vasoactive intestinal polypeptide, somatostatin, glucagon, glucagon-like peptide, neurotensin, serotonin, and galanin). Their distribution in the liver is species-dependent. Some of these nerves are thought to be involved in the regulation of hepatic function as well as of hemodynamics. In addition to direct neural connections, the hypothalamus can affect metabolic functions by neuroendocrine connections: the hypothalamus-pancreas axis, the hypothalamus-adrenal axis, and the hypothalamus-pituitary axis. In the hypothalamus-pancreas axis, autonomic nerves release glucagon and insulin, which directly enter the liver and affect liver metabolism. In the hypothalamus-adrenal axis, autonomic nerves release catecholamines such as adrenaline and noradrenaline from the adrenal medulla, which also affects liver metabolism. In the hypothalamus-pituitary axis, release of glucocorticoids and thyroid hormones is stimulated by pituitary hormones. Both groups of hormones modulate hepatic metabolism. Taken together, the hypothalamus controls liver functions by neural and neuroendocrine connections.

GTP is required to stabilize and display transamidation activity of transglutaminase 2

Transglutaminase 2 (TGase 2) is a bifunctional enzyme that catalyzes calcium-dependent transamidation and GTP binding/hydrolysis. The transamidation activity is proposed to be associated with several neurodegenerative disorders such as Alzheimer's and Hungtinton's disease. However, the regulation mechanism by which TGase 2 causes neurodegeneration is unknown. In this study, we show that two activities of TGase 2 have a differential stability; transamidation activity is less stable than GTP hydrolytic activity, and that GTP was required to stabilize and to display transamidation activity. Moreover, GTP binding-defective mutant of TGase 2 did not show any transamidation activity in transfection experiments. These results indicate that GTP binding is crucial for transamidation activity of TGase 2, suggesting that protein cross-linking by TGase 2 might be associated with G-protein coupled receptor signaling system. Thus, our data could contribute to understand the regulation of TGase 2 activity and TGase 2-associated pathogenesis.

Structural basis for the guanine nucleotide-binding activity of tissue transglutaminase and its regulation of transamidation activity

Tissue transglutaminase (TG) is a Ca2+-dependent acyltransferase with roles in cellular differentiation, apoptosis, and other biological functions. In addition to being a transamidase, TG undergoes a GTP-binding/GTPase cycle even though it lacks any obvious sequence similarity with canonical GTP-binding (G) proteins. Guanine nucleotide binding and Ca2+ concentration reciprocally regulate TG's transamidation activity, with nucleotide binding being the negative regulator. Here we report the x-ray structure determined to 2.8-A resolution of human TG complexed with GDP. Although the transamidation active site is similar to those of other known transglutaminases, the guanine nucleotide-binding site of TG differs markedly from other G proteins. The structure suggests a structural basis for the negative regulation of transamidation activity by bound nucleotide, and the positive regulation of transamidation by Ca2+.

Multidrug-resistant MCF-7 breast cancer cells contain deficient intracellular calcium pools

Emergence of resistance to antineoplastic drugs poses a major impediment to the successful treatment of breast cancer. We previously reported that human breast carcinoma MCF-7 cells selected for resistance against doxorubicin (MCF-7/DOX cells) expressed high levels of tissue-type transglutaminase (tTGase), a calcium-dependent protein cross-linking enzyme that plays a role in apoptosis. The purpose of this study was to determine the mechanisms by which MCF-7/DOX cells survive and proliferate despite high levels of tTGase expression. Our results demonstrate that the MCF-7/DOX cells contain deficient intracellular calcium pools, which may explain their ability to survive and tolerate the high levels of tTGase expression. Treatment with thapsigargin failed to induce any significant killing of MCF-7/DOX cells. Similar treatment of the drug-sensitive MCF-7 wild-type (MCF-7/WT) cells, however, induced significant apoptosis. Treatment with the ionophore A23187, on the other hand, killed a large percentage of both the MCF-7/DOX and the MCF-7/WT cells. We also established a revertant cell line, MCF-7/RT, from MCF-7/DOX cells to rule out the involvement of P-glycoprotein (P-gp) in these phenomena. Unlike the MCF-7/DOX cells, the MCF-7/RT cells showed no detectable P-gp expression; the MCF-7/RT cells, however, continued to express high levels of tTGase. Moreover, like MCF-7/DOX cells, the MCF-7/RT cells were highly resistant to thapsigargin-induced apoptosis but were sensitive to the ionophore A23187-induced apoptosis. These results suggest that the resistance of MCF7/DOX cells to thapsigargin is linked to their defective intracellular Ca2+ stores, a notion that was directly confirmed by single-cell spectrofluorometric analysis.

Targeted inactivation of Gh/tissue transglutaminase II

The novel G-protein, G(h)/tissue transglutaminase (TGase II), has both guanosine triphosphatase and Ca(2+)-activated transglutaminase activity and has been implicated in a number of processes including signal transduction, apoptosis, bone ossification, wound healing, and cell adhesion and spreading. To determine the role of G(h) in vivo, the Cre/loxP site-specific recombinase system was used to develop a mouse line in which its expression was ubiquitously inactivated. Despite the absence of G(h) expression and a lack of intracellular TGase activity that was not compensated by other TGases, the Tgm2(-/-) mice were viable, phenotypically normal, and were born with the expected Mendelian frequency. Absence of G(h) coupling to alpha(1)-adrenergic receptor signaling in Tgm2(-/-) mice was demonstrated by the lack of agonist-stimulated [alpha-(32)P]GTP photolabeling of a 74-kDa protein in liver membranes. Annexin-V positivity observed with dexamethasone-induced apoptosis was not different in Tgm2(-/-) thymocytes compared with Tgm2(+/+) thymocytes. However, with this treatment there was a highly significant decrease in the viability (propidium iodide negativity) of Tgm2(-/-) thymocytes. Primary fibroblasts isolated from Tgm2(-/-) mice also showed decreased adherence with culture. These results indicate that G(h) may be importantly involved in stabilizing apoptotic cells before clearance, and in responses such as wound healing that require fibroblast adhesion mediated by extracellular matrix cross-linking.

Distinct characteristic of Galpha(h) (transglutaminase II) by compartment: GTPase and transglutaminase activities

Galpha(h) (transglutaminase II) is a bifunctional enzyme possessing transglutaminase and GTPase activities. To better understand the factors affecting these two functions of Galpha(h), we have examined the characteristics of purified Galpha(h) from membrane and cytosol. GTP binding activity of mouse heart Galpha(h) was higher in membrane than that from cytosol. Furthermore, phospholipase C-delta1 (PLC-delta1) activity and coimmunoprecipitation of Galpha(h)-coupled PLC-delta1 in the alpha(1)-adrenoceptor-Galpha(h)-PLC-delta1 complex preparations were increased by phenylephrine in the presence of membranous Galpha(h). On the other hand, transglutaminase activity of cytosolic Galpha(h) was higher than that from membrane Galpha(h). These results demonstrate that bifunctions of Galpha(h) are regulated by its localization that can reflect the cellular functions of Galpha(h).

Phospholipase Cdelta1 is a guanine nucleotide exchanging factor for transglutaminase II (Galpha h) and promotes alpha 1B-adrenoreceptor-mediated GTP binding and intracellular calcium release

Effectors involved in G protein-coupled receptor signaling modulate activity of GTPases through GTPase-activating protein or guanine nucleotide exchanging factor (GEF). Phospholipase Cdelta1 (PLCdelta1) is an effector in tissue transglutaminase (TGII)-mediated alpha1B-adrenoreceptor (alpha(1B)AR) signaling. We investigated whether PLCdelta1 modulates TGII activity. PLCdelta1 stimulated GDP release from TGII in a concentration-dependent manner, resulting in an increase in GTPgammaS binding to TGII. PLCdelta1 also inhibited GTP hydrolysis by TGII that was independent from the alpha(1B)AR. These results indicate that PLCdelta1 is GEF for TGII and stabilizes the GTP.TGII complex. When GEF function of PLCdelta1 was compared with that of the alpha(1B)AR, the alpha(1B)AR-mediated GTPgammaS binding to TGII was greater than PLCdelta1-mediated binding and was accelerated in the presence of PLCdelta1. Thus, the alpha(1B)AR is the prime GEF for TGII, and GEF activity of PLCdelta1 promotes coupling efficacy of this signaling system. Overexpression of TGII and its mutants with and without PLCdelta1 resulted in an increase in alpha(1B)AR-stimulated Ca2+ release from intracellular stores in a TGII-specific manner. We conclude that PLCdelta1 assists the alpha(1B)AR function through its GEF action and is primarily activated by the coupling of TGII to the cognate receptors.

Phospholipase C Isoforms, Cytoskeletal Organization, and Vascular Smooth Muscle Differentiation

The function of differentiated vascular smooth muscle cells (VSMC) in vivo is the regulation of contractility. Following injury or disease, however, VSMC lose their contractile function and take on a synthetic, proliferative phenotype. This dedifferentiation is generally accompanied by a change in the expression profile of phospholipase C isoforms.

Alpha 1-adrenoceptors: subtypes, signaling, and roles in health and disease

Alpha 1-adrenoceptors mediate some of the main actions of the natural catecholamines, adrenaline, and noradrenaline. They participate in many essential physiological processes, such as sympathetic neurotransmission, modulation of hepatic metabolism, control of vascular tone, cardiac contraction, and the regulation of smooth muscle activity in the genitourinary system. It is now clear that alpha 1-adrenoceptors mediate, in addition to immediate effects, longer term actions of catecholamines such as cell growth and proliferation. In fact, adrenoceptor genes can be considered as protooncogenes. Over the past years, considerable progress has been achieved in the molecular characterization of different alpha 1-adrenoceptor subtypes. Three main subtypes have been characterized pharmacologically and in molecular terms. Splice variants, truncated isoforms, and polymorphisms have also been detected. Similarly, it is now clear that these receptors are coupled to several classes of G proteins that, therefore, are capable of modulating different signaling pathways. In the present article, some of these aspects are reviewed, together with the distribution of the subtypes in different tissues and some of the known roles of these receptors in health and disease.

Phospholipase C-delta1 is activated by capacitative calcium entry that follows phospholipase C-beta activation upon bradykinin stimulation

To characterize the regulatory mechanism of phospholipase C-delta1 (PLC-delta1) in the bradykinin (BK) receptor-mediated signaling pathway, we used a clone of PC12 cells, which stably overexpress PLC-delta1 (PC12-D1). Stimulation with BK induced a significantly higher Ca(2+) elevation and inositol 1,4,5-trisphosphate (IP(3)) production with a much lower half-maximal effective concentration (EC(50)) of BK in PC12-D1 cells than in wild type (PC12-W) or vector-transfected (PC12-V) cells. However, BK-induced intracellular Ca(2+) release and IP(3) generation was similar between PC12-V and PC12-D1 cells in the absence of extracellular Ca(2+), suggesting that the availability of extracellular Ca(2+) is essential to the activation of PLC-delta1. When PC12-D1 cells were treated with agents that induce Ca(2+) influx, more IP(3) was produced, suggesting that the Ca(2+) entry induces IP(3) production in PC12-D1 cells. Furthermore, the additional IP(3) production after BK-induced capacitative calcium entry was detected in PC12-D1 cells, suggesting that PLC-delta1 is mainly activated by capacitative calcium entry. When cells were stimulated with BK in the presence of extracellular Ca(2+), [(3)H]norepinephrine secretion was much greater from PC12-D1 cells than from PC12-V cells. Our results suggest that PLC-delta1 is activated by capacitative calcium entry following the activation of PLC-beta, additively inducing IP(3) production and Ca(2+) rise in BK-stimulated PC12 cells.

Cardiac specific overexpression of transglutaminase II (G(h)) results in a unique hypertrophy phenotype independent of phospholipase C activation

Tissue type transglutaminase (TGII, also known as G(h)) has been considered a multifunctional protein, with both transglutaminase and GTPase activity. The role of the latter function, which is proposed as a coupling mechanism between alpha(1)-adrenergic receptors and phospholipase C (PLC), is not well defined. TGII was overexpressed in transgenic mice in a cardiac specific manner to delineated relevant signaling pathways and their consequences in the heart. Cardiac transglutaminase activity in the highest expressing line was approximately 37-fold greater than in nontransgenic lines. However, in vivo signaling to PLC, as assessed by inositol phosphate turnover in [(3)H]myoinositol organ bath atrial preparations, was not increased in the TGII mice at base line or in response to alpha(1)-adrenergic receptor stimulation; nor was protein kinase Calpha (PKCalpha) or PKCepsilon activity enhanced in the TGII transgenic mice. This is in contrast to mice moderately (approximately 5-fold) overexpressing G(alphaq), where inositol phosphate turnover and PKC activity were found to be clearly enhanced. TGII overexpression resulted in a remodeling of the heart with mild hypertrophy, elevated expression of beta-myosin heavy chain and alpha-skeletal actin genes, and diffuse interstitial fibrosis. Resting ventricular function was depressed, but responsiveness to beta-agonist was not impaired. This set of pathophysiologic findings is distinct from that evoked by overexpression of G(alphaq). We conclude that TGII acts in the heart primarily as a transglutaminase, and modulation of this function results in unique pathologic sequelae. Evidence for TGII acting as a G-protein-like transducer of receptor signaling to PLC in the heart is not supported by these studies.

Differential signaling by the thromboxane receptor isoforms via the novel GTP-binding protein, Gh

Thromboxane A2 acts via G protein-coupled receptors; two splice variants of the thromboxane A2 receptor (TPalpha and TPbeta) have been cloned. It is unknown whether they differ in their capacity to activate intracellular signaling pathways. Recently, a high molecular weight G protein, Gh, that can also function as a tissue transglutaminase, has been described. We investigated whether Gh functions as a signaling protein in association with thromboxane receptors. First, we sought Gh expression in cells known to express TPs. Reverse transcription-polymerase chain reaction and immunoblotting demonstrated Gh expression in platelets, megakaryocytic cell lines, and endothelial and vascular smooth muscle cells. Second, immunoprecipitation of both TPalpha and TPbeta in transfected COS-7 cells resulted in the co-immunoprecipitation of Gh, indicating that TPs may associate Gh in vivo. Finally, agonist activation of TPalpha, but not of TPbeta, resulted in stimulation of phospholipase C-mediated inositol phosphate production in cells cotransfected with Gh. By contrast, agonist activation of both TP isoforms resulted in Gq-mediated inositol phosphate signaling. Gh is expressed in platelets and vascular cells and may associate with both TP isoforms. However, stimulation of TP isoforms results in differential activation of downstream signaling pathways via this novel G protein.

The left ventricular contractility of the rat heart is modulated by changes in flow and alpha 1-adrenoceptor stimulation

Myocardial contractility depends on several mechanisms such as coronary perfusion pressure (CPP) and flow as well as on alpha 1-adrenoceptor stimulation. Both effects occur during the sympathetic stimulation mediated by norepinephrine. Norepinephrine increases force development in the heart and produces vasoconstriction increasing arterial pressure and, in turn, CPP. The contribution of each of these factors to the increase in myocardial performance needs to be clarified. Thus, in the present study we used two protocols: in the first we measured mean arterial pressure, left ventricular pressure and rate of rise of left ventricular pressure development in anesthetized rats (N = 10) submitted to phenylephrine (PE) stimulation before and after propranolol plus atropine treatment. These observations showed that in vivo alpha 1-adrenergic stimulation increases left ventricular developed pressure (P < 0.05) together with arterial blood pressure (P < 0.05). In the second protocol, we measured left ventricular isovolumic systolic pressure (ISP) and CPP in Langendorff constant flow-perfused hearts. The hearts (N = 7) were perfused with increasing flow rates under control conditions and PE or PE + nitroprusside (NP). Both CPP and ISP increased (P < 0.01) as a function of flow. CPP changes were not affected by drug treatment but ISP increased (P < 0.01). The largest ISP increase was obtained with PE + NP treatment (P < 0.01). The results suggest that both mechanisms, i.e., direct stimulation of myocardial alpha 1-adrenoceptors and increased flow, increased cardiac performance acting simultaneously and synergistically.

Identification of a distinct molecular mass G alpha(h) (transglutaminase II) coupled to alpha1-adrenoceptor in mouse heart

Our previous studies on alpha1-adrenoceptor signaling suggested that G alpha(h) family is a signal mediator in different species. To elucidate the species-specificity of G alpha(h) family in molecular mass, we used the solubilized membranes from mouse heart and the ternary complex preparations containing alpha1-agonist/receptor/G-protein. Binding of [35S]GTPgammaS and the intensity of the [alpha-32P]GTP photoaffinity labeled protein resulting from activation of the alpha1-adrenoceptor were significantly attenuated by the antagonist, phentolamine. The molecular mass of the specific GTP-binding protein was approximately 72-kDa; homologous with G alpha(h) (transglutaminase II) family. Furthermore, immunological cross-reactivity of ternary complex from mouse heart and purified G alpha(h) from rat, guinea pig, and bovine using anti-G alpha(h7) antibody showed that their molecular masses were distinctly different and approximately 72-kDa G alpha(h) from mouse heart was the lowest molecular mass. Consistent with these observations, in co-immunoprecipitation and co-immunoadsorption of the alpha1-adrenoceptor in the ternary complex preparation by anti-G alpha(h7) antibody, the G alpha(h) family protein tightly coupled to alpha1-adrenoceptor. These results demonstrate the species-specificity of G alpha(h) family in molecular mass, especially the lowest molecular mass in mouse.

Regulation of eukaryotic phosphatidylinositol-specific phospholipase C and phospholipase D

This review focuses on two phospholipase activities involved in eukaryotic signal transduction. The action of the phosphatidylinositol-specific phospholipase C enzymes produces two well-characterized second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. This discussion emphasizes recent advances in elucidation of the mechanisms of regulation and catalysis of the various isoforms of these enzymes. These are especially related to structural information now available for a phospholipase C delta isozyme. Phospholipase D hydrolyzes phospholipids to produce phosphatidic acid and the respective head group. A perspective of selected past studies is related to emerging molecular characterization of purified and cloned phospholipases D. Evidence for various stimulatory agents (two small G protein families, protein kinase C, and phosphoinositides) suggests complex regulatory mechanisms, and some studies suggest a role for this enzyme activity in intracellular membrane traffic.

Alpha 1-adrenergic receptor coupling with Gh in the failing human heart

BACKGROUND

We recently demonstrated that Gh, which transfers the signal from the alpha 1-adrenergic receptor to the 69-kD phospholipase C, is the previously identified tissue-type transglutaminase (TGase II). The alpha 1-adrenergic receptor mediates actions of the sympathetic nervous system, including cardiac, arteriolar, and smooth muscle contractions. In human cardiac tissue, the expression of the alpha 1-adrenergic receptor is increased under pathophysiological conditions, but changes in the physiological response are small. Therefore, it has been suggested that the other components involved in the alpha 1-adrenergic receptor-mediated signaling pathway are probably altered.

METHODS AND RESULTS

Immunological and biochemical studies with nonfailling and failing human heart tissues revealed that the GTP-binding and TGase activities of human heart TGase II (hhG alpha n) are downregulated in both ischemic and dilated cardiomyopathic human heart. In ischemic cardiomyopathy, the alpha 1-adrenergic receptor number increased twofold (27.0 fmol/mg) compared with the nonfailing (12.8 fmol/mg) and the dilated cardiomyopathic (15.6 fmol/mg) heart tissues, but the coupling of hhG alpha h with the alpha 1-adrenergic receptor did not increase. The intrinsic activity of hhG alpha h, was greatly decreased in membrane fractions, whereas the cytosolic TGase activity was not changed. In the dilated cardiomyopathic human heart, these intrinsic enzyme activities of hhG alpha h were also downregulated in the membrane fraction, whereas the amount of hhG alpha h protein was greatly increased (2.8-fold) compared with the nonfailing heart.

CONCLUSIONS

The results of the present study clearly demonstrate that the alpha 1-adrenergic receptor in human heart couples with Gh (TGase II) and indicate that downregulation of hhG alpha h activity is associated with human cardiac failure but that the mechanism differs between ischemic and dilated cardiomyopathies.

Evidence that phospholipase delta1 is the effector in the Gh (transglutaminase II)-mediated signaling

A new class of GTP-binding protein transglutaminase II (Gh) couples to a 69-kDa phospholipase C (PLC). An 8-amino acid region (Leu665-Lys672) of the alpha-subunit of Gh (Galphah) is involved in interaction and activation of PLC, an observation that has now been used to characterize the 69-kDa PLC further. A 20-amino acid peptide corresponding to Leu654-Leu673 of Galphah was used to prepare an affinity resin. On incubation with a partially purified PLC preparation from rat liver membranes, the affinity resin-bound approximately69- and 85-kDa proteins were recognized by an antibody to the 69-kDa PLC. Both purified 69-kDa PLC and PLC-delta1 bound to the affinity resin; moreover, antibodies to PLC-delta1 recognized the 69-kDa PLC, and antibodies to the 69-kDa PLC recognized PLC-delta1. A synthetic peptide corresponding to Leu661-Lys672 of Galphah inhibited the binding of PLC-delta1 to the affinity resin and also stimulated PLC-delta1. Reconstitution of PLC-delta1 with GTPgammaS (guanosine 5'-3-O-(thio)triphosphate)-activated Gh resulted in activation of PLC-delta1. Antibodies to Galphah also coimmunoprecipitated PLC-delta1 upon activation of Gh. These findings indicate that PLC-delta1 is the effector of Gh-mediated signaling.

Interaction site of GTP binding Gh (transglutaminase II) with phospholipase C

The GTP binding G alpha h (transglutaminase II) mediates the alpha 1B-adrenoreceptor signal to a 69-kDa phospholipase C (PLC). Thus, G alpha h possesses both GTPase and transglutaminase activities with a signal transfer role. The recognition sites of this unique GTP binding protein for either the receptor or the effector are completely unknown. A site on human heart G alpha h (hhG alpha h) has been identified that interacts with and stimulates PLC. Expressed mutants of hhG alpha h with deleted C-terminal regions lost the response to (-)-epinephrine and GTP and failed to coimmunoprecipitate PLC by the specific Gh7 alpha antibody. The interaction regions were further defined by studies with synthetic peptides of hhG alpha h and a chimera in which residues Val665-Lys672 of hhG alpha h were substituted with Ile707-Ser714 residues of human coagulation factor XIIIa. Thus, eight amino acid residues near the C terminus of hhG alpha h are critical for recognition and stimulation of PLC.

Gh: a GTP-binding protein with transglutaminase activity and receptor signaling function

The alpha 1-adrenergic receptors activate a phospholipase C enzyme by coupling to members of the large molecular size (approximately 74 to 80 kilodaltons) G alpha h family of guanosine triphosphate (GTP)-binding proteins. Rat liver G alpha h is now shown to be a tissue transglutaminase type II (TGase II). The transglutaminase activity of rat liver TGase II expressed in COS-1 cells was inhibited by the nonhydrolyzable GTP analog guanosine 5'-O-(3-thiotriphosphate) or by alpha 1-adrenergic receptor activation. Rat liver TGase II also mediated alpha 1-adrenergic receptor stimulation of phospholipase C activity. Thus, G alpha h represents a new class of GTP-binding proteins that participate in receptor signaling and may be a component of a complex regulatory network in which receptor-stimulated GTP binding switches the function of G alpha h from transglutamination to receptor signaling.