Adenylate cyclase regulation via proteasome-mediated modulation of Galphas levels

Gαs-Protein Kinase A (PKA) Pathway Signalopathies: The Emerging Genetic Landscape and Therapeutic Potential of Human Diseases Driven by Aberrant Gαs-PKA Signaling

Many of the fundamental concepts of signal transduction and kinase activity are attributed to the discovery and crystallization of cAMP-dependent protein kinase, or protein kinase A. PKA is one of the best-studied kinases in human biology, with emphasis in biochemistry and biophysics, all the way to metabolism, hormone action, and gene expression regulation. It is surprising, however, that our understanding of PKA's role in disease is largely underappreciated. Although genetic mutations in the PKA holoenzyme are known to cause diseases such as Carney complex, Cushing syndrome, and acrodysostosis, the story largely stops there. With the recent explosion of genomic medicine, we can finally appreciate the broader role of the Gαs-PKA pathway in disease, with contributions from aberrant functioning G proteins and G protein-coupled receptors, as well as multiple alterations in other pathway components and negative regulators. Together, these represent a broad family of diseases we term the Gαs-PKA pathway signalopathies. The Gαs-PKA pathway signalopathies encompass diseases caused by germline, postzygotic, and somatic mutations in the Gαs-PKA pathway, with largely endocrine and neoplastic phenotypes. Here, we present a signaling-centric review of Gαs-PKA-driven pathophysiology and integrate computational and structural analysis to identify mutational themes commonly exploited by the Gαs-PKA pathway signalopathies. Major mutational themes include hotspot activating mutations in Gαs, encoded by GNAS, and mutations that destabilize the PKA holoenzyme. With this review, we hope to incite further study and ultimately the development of new therapeutic strategies in the treatment of a wide range of human diseases. SIGNIFICANCE STATEMENT: Little recognition is given to the causative role of Gαs-PKA pathway dysregulation in disease, with effects ranging from infectious disease, endocrine syndromes, and many cancers, yet these disparate diseases can all be understood by common genetic themes and biochemical signaling connections. By highlighting these common pathogenic mechanisms and bridging multiple disciplines, important progress can be made toward therapeutic advances in treating Gαs-PKA pathway-driven disease.

The complex of TRIP-Br1 and XIAP ubiquitinates and degrades multiple adenylyl cyclase isoforms

Adenylyl cyclases (ACs) generate cAMP, a second messenger of utmost importance that regulates a vast array of biological processes in all kingdoms of life. However, almost nothing is known about how AC activity is regulated through protein degradation mediated by ubiquitination or other mechanisms. Here, we show that transcriptional regulator interacting with the PHD-bromodomain 1 (TRIP-Br1, Sertad1), a newly identified protein with poorly characterized functions, acts as an adaptor that bridges the interaction of multiple AC isoforms with X-linked inhibitor of apoptosis protein (XIAP), a RING-domain E3 ubiquitin ligase. XIAP ubiquitinates a highly conserved Lys residue in AC isoforms and thereby accelerates the endocytosis and degradation of multiple AC isoforms in human cell lines and mice. XIAP/TRIP-Br1-mediated degradation of ACs forms part of a negative-feedback loop that controls the homeostasis of cAMP signaling in mice. Our findings reveal a previously unrecognized mechanism for degrading multiple AC isoforms and modulating the homeostasis of cAMP signaling.

DOI:http://dx.doi.org/10.7554/eLife.28021.001

Peripheral inflammatory injury alters the relative abundance of Gα subunits in the dorsal horn of the spinal cord and in the rostral ventromedial medulla of male rats

Abstract

A diverse array of G protein-coupled receptors (GPCRs) is implicated in the modulation of nociception. The efficacy and potency of several GPCR agonists change as a consequence of peripheral inflammatory injury. Whether these changes reflect alterations in expression of the G proteins themselves is not known. This study examined the expression of transcripts and proteins for the α subunits of three classes of heteromeric G proteins in the dorsal horn of the spinal cord and the rostral ventromedial medulla (RVM) of male rats four days and two weeks after intraplantar injection of complete Freund’s adjuvant (CFA) or saline. Levels of Gα transcript in the dorsal horn or RVM were unchanged by CFA treatment. However, in the dorsal horn, Gαi protein decreased in cytosolic and membrane fractions four days after CFA treatment. Levels of Gαz protein decreased in the membrane fraction. Levels of the other Gα subunits did not differ. Levels of the Gα subunits were unchanged two weeks after CFA treatment. In the RVM, Gαz protein levels decreased in the cytosolic fraction four days after CFA treatment. No other differences were observed. Two weeks after CFA, the levels for all Gα subunits trended higher in the RVM. These data indicate that peripheral inflammatory injury induces subtle changes in the abundance of Gα subunits that is specific with respect to class, subcellular compartment, tissue, and time after injury. These changes have the potential to alter the balance of the different subcellular signaling pathways through which GPCR agonists act to modulate nociception.

Chapter Two - Heterotrimeric G Protein Ubiquitination as a Regulator of G Protein Signaling

Ubiquitin-mediated regulation of G proteins has been known for over 20 years as a result of discoveries made independently in yeast and vertebrate model systems for pheromone and photoreception, respectively. Since that time, several details underlying the cause and effect of G protein ubiquitination have been determined-including the initiating signals, responsible enzymes, trafficking pathways, and their effects on protein structure, function, interactions, and cell signaling. The collective body of evidence suggests that Gα subunits are the primary targets of ubiquitination. However, longstanding and recent results suggest that Gβ and Gγ subunits are also ubiquitinated, in some cases impacting cell polarization-a process essential for chemotaxis and polarized cell growth. More recently, evidence from mass spectrometry (MS)-based proteomics coupled with advances in PTM bioinformatics have revealed that protein families representing G protein subunits contain several structural hotspots for ubiquitination-most of which have not been investigated for a functional role in signal transduction. Taken together, our knowledge and understanding of heterotrimeric G protein ubiquitination as a regulator of G protein signaling-despite 20 years of research-is still emerging.

Increased ubiquitination and the crosstalk of G protein signaling in cardiac myocytes: involvement of Ric-8B in Gs suppression by Gq signal

Hyperactivation of Gq signaling causes cardiac hypertrophy, and β-adrenergic receptor-mediated Gs signaling is attenuated in hypertrophic cardiomyocytes. Here, we found the increase in a global ubiquitination in hypertrophic mouse heart. The activation of Gq signaling resulted in the ubiquitination of Gαs in neonatal rat cardiomyocytes, reduced Gαs expression, and suppressed cAMP response to β-adrenergic receptor stimulation. Ectopic expression of Gαq induced a similar suppression, which is due to the degradation of Gαs by a ubiquitin-proteasome pathway. Co-expression of Ric-8B, a positive regulator of Gαs, effectively canceled the Gαq-induced ubiquitination of Gαs and recovered the cAMP accumulation. In vitro, Gαq competes for the binding of Gαs to Ric-8B. These data show a new role of Ric-8B in the crosstalk of two distinct G protein signaling pathways, which are possibly involved in a part of mechanisms of chronic heart failure.

Minireview: ubiquitination-regulated G protein-coupled receptor signaling and trafficking

G protein-coupled receptors (GPCRs) are the largest and most diverse superfamily of membrane proteins and mediate most cellular responses to hormones and neurotransmitters. Posttranslational modifications are considered the main regulators of all GPCRs. In addition to phosphorylation, glycosylation, and palmitoylation, increasing evidence as reviewed here reveals that ubiquitination also regulates the magnitude and temporospatial aspects of GPCR signaling. Posttranslational protein modification by ubiquitin is a key molecular mechanism governing proteins degradation. Ubiquitination mediates the covalent conjugation of ubiquitin, a highly conserved polypeptide of 76 amino acids, to protein substrates. This process is catalyzed by 3 enzymes acting in tandem: an E1, ubiquitin-activating enzyme; an E2, ubiquitin-carrying enzyme; and an E3, ubiquitin ligase. Ubiquitination is counteracted by deubiquitinating enzymes that deconjugate ubiquitin-modified proteins and rescue the substrate from proteasomal degradation. Although ubiquitination is known to target many GPCRs for lysosomal or proteasomal degradation, emerging findings define novel roles for the basal status of ubiquitination and for rapid deubiquitination and transubiquitination controlling cell surface expression and cellular responsiveness of some GPCRs. In this review, we highlight the classical and novel roles of ubiquitin in the regulation of GPCR function, signaling, and trafficking.

HIV-1 Nef impairs heterotrimeric G-protein signaling by targeting Gα(i2) for degradation through ubiquitination

The HIV Nef protein is an important pathogenic factor that modulates cell surface receptor trafficking and impairs cell motility, presumably by interfering at multiple steps with chemotactic receptor signaling. Here, we report that a dominant effect of Nef is to trigger AIP4 E3 ligase-mediated Gα(i2) ubiquitination, which leads to Gα(i2) endolysosomal sequestration and destruction. The loss of the Gα(i2) subunit was demonstrable in many cell types in the context of gene transfection, HIV infection, or Nef protein transduction. Nef directly interacts with Gα(i2) and ternary complexes containing AIP4, Nef, and Gα(i2) form. A substantial reversal of Gα(i2) loss and a partial recovery of impaired chemotaxis occurred following siRNA knockdown of AIP4 or NEDD4 or by inhibiting dynamin. The N-terminal myristoyl group, (62)EEEE(65) motif, and (72)PXXP(75) motif of Nef are critical for this effect to occur. Nef expression does not affect a Gq(i5) chimera where the five C-terminal residues of Gq are replaced with those of Gα(i2). Lysine at position 296 of Gα(i2) was identified as the critical determinant of Nef-induced degradation. By specifically degrading Gα(i2), Nef directly subverts leukocyte migration and homing. Impaired trafficking and homing of HIV Nef-expressing lymphocytes probably contributes to early immune dysfunction following HIV infection.

Expression of the GTP-binding protein Gαs in human myometrial cells is regulated by ubiquitination and protein degradation: involvement of proteasomal inhibition by trichostatin A

In this study, we show that myometrial transcriptional complexes consisting of Sp1, Sp3, histone deacetylase (HDAC)1/2, RbAp48, and mSin3A are recruited to 4 out of the 6 Sp1-4 sites within the Gαs promoter. Moreover disruption in the binding of these complexes via mithramycin administration results in a substantial decrease in expression of Gαs proteins in myometrial cell cultures. In many instances, these transcriptional regulatory complexes repress expression of genes having a high CG content within their promoter region. This repression can be attenuated by inhibition of HDAC activity by the class I/II HDAC inhibitor trichostatin A (TSA) resulting in increased gene transcription. However, although a substantial increase in Gαs protein levels was observed upon administration of TSA to primary cultures of human myometrial cells, this was not preceded by an increase in messenger RNA (mRNA) and thus an elevation in gene transcription. Importantly the increase in Gαs protein levels occurred via ubiquitination and inhibition of proteasomal activity, indicating that this pathway is also involved in regulating Gαs protein expression during pregnancy and parturition.

Pheromone- and RSP5-dependent ubiquitination of the G protein beta subunit Ste4 in yeast

Ste4 is the β subunit of a heterotrimeric G protein that mediates mating responses in Saccharomyces cerevisiae. Here we show that Ste4 undergoes ubiquitination in response to pheromone stimulation. Ubiquitination of Ste4 is dependent on the E3 ligase Rsp5. Disrupting the activity of Rsp5 abolishes ubiquitination of Ste4 in vivo, and recombinant Rsp5 is capable of ubiquitinating Ste4 in vitro. We find also that Lys-340 is a major ubiquitination site on Ste4, as pheromone-induced ubiquitination of the protein is prevented when this residue is mutated to an arginine. Functionally, ubiquitination does not appear to regulate the stability of Ste4, as blocking ubiquitination has no apparent effect on either the abundance or the half-life of the protein. However, when presented with a concentration gradient of pheromone, Ste4(K340R) mutant cells polarize significantly faster than wild-type cells, indicating that ubiquitination limits pheromone-directed polarized growth. Together, these findings reveal a novel stimulus-dependent posttranslational modification of a Gβ subunit, establish Ste4 as a new substrate of the E3 ligase Rsp5, and demonstrate a role for G protein ubiquitination in cell polarization.

Differential regulation of proteasome function in isoproterenol-induced cardiac hypertrophy

RATIONALE

Proteasomal degradation is altered in many disease phenotypes including cardiac hypertrophy, a prevalent condition leading to heart failure. Our recent investigations identified heterogeneous subpopulations of proteasome complexes in the heart and implicated multiple mechanisms for their regulation.

OBJECTIVE

The study aimed at identification of molecular mechanisms changing proteasome function in the hypertrophic heart.

METHOD AND RESULTS

Proteasome function, expression, and assembly were analyzed during the development of cardiac hypertrophy induced by β-adrenergic stimulation. The analysis revealed, for the first time, divergent regulation of proteasome function in cardiac hypertrophy. Proteasome complexes have 3 different proteolytic activities, which are ATP-dependent for 26S complexes (19S assembled with 20S) and ATP-independent for 20S core particles. The 26S activities were enhanced in hypertrophic hearts, partially because of increased expression and assembly of 19S subunits with 20S core complexes. In contrast, caspase- and trypsin-like 20S activities were significantly decreased. Activation of endogenous cAMP-dependent protein kinase (PKA) rescued the depressed 20S functions, supporting the notion that PKA signaling is a positive regulator of protein degradation in the heart. Chymotrypsin-like 20S activity was stably maintained during cardiac remodeling, indicating a switch in proteasome subpopulations, which was supported by altered expression and incorporation of inducible β subunits.

CONCLUSIONS

Three novel mechanisms for the regulation of proteasome activities were discovered in the development of cardiac hypertrophy: (1) increased incorporation of inducible subunits in 20S proteasomes; (2) enhanced 20S sensitivity to PKA activation; and (3) increased 26S assembly. PKA modulation of proteasome complexes may provide a novel therapeutic avenue for restoration of cardiac function in the diseased myocardium.

Signalling pathway alterations in pituitary adenomas: involvement of Gsalpha, cAMP and mitogen-activated protein kinases

Despite extensive research on sporadic pituitary adenomas, it is not yet possible to assign one protein alteration to one specific type of pituitary adenomas. Nevertheless, alterations of the cAMP pathway appear to be molecular hallmarks of most growth hormone (GH)-secreting adenomas. However, these alterations do not confer specific phenotypes to patients carrying these alterations. In this review, we summarise the literature regarding signalling alterations observed in GH-secreting adenomas. We focus on Gsalpha alterations and their possible cross-talk with the extracellular signal-related kinase (ERK)1/2 pathway. In the light of results obtained on human somatotroph adenoma cells in primary culture and on models of murine somatotroph cell lines, we postulate a crucial role for ERK1/2 in GH-secreting adenomas downstream of cAMP pathway alterations that might impact the tumoural phenotype.

Protein kinase A as a biological target in cancer therapy

BACKGROUND

cAMP is a second messenger that plays a role in intracellular signal transduction of various stimuli. a major function of cAMP in eukaryotes is activation of cAMP-dependent protein kinase (PKA). PKA is the best understood member of the serine-threonine protein kinase superfamily, and is involved in the control of a variety of cellular processes. since it has been implicated in the initiation and progression of many tumors, PKA has been suggested as a novel molecular target for cancer therapy.

OBJECTIVE/METHODS

Here, after describing some features of cAMP/PKA signaling that are relevant to cancer biology, we review targeting of PKA in cancer therapy, also discussing PKA as a biomarker for cancer detection and monitoring of therapy.

RESULTS/CONCLUSIONS

PKA is an increasingly relevant biological target in the therapy and management of cancer.

G Protein Mono-ubiquitination by the Rsp5 Ubiquitin Ligase

Emerging evidence suggests that ubiquitination serves as a protein trafficking signal in addition to its well characterized role in promoting protein degradation. The yeast G protein alpha subunit Gpa1 represents a rare example of a protein that undergoes both mono- and poly-ubiquitination. Whereas mono-ubiquitinated Gpa1 is targeted to the vacuole, poly-ubiquitinated Gpa1 is directed instead to the proteasome. Here we investigate the structural requirements for mono- and poly-ubiquitination of Gpa1. We find that variants of Gpa1 engineered to be unstable are more likely to be poly-ubiquitinated and less likely to be mono-ubiquitinated. In addition, mutants that cannot be myristoylated are no longer mono-ubiquitinated but are still polyubiquitinated. Finally, we show that the ubiquitin ligase Rsp5 is necessary for Gpa1 mono-ubiquitination in vivo and that the purified enzyme is sufficient to catalyze Gpa1 mono-ubiquitination in vitro. Taken together, these data indicate that mono- and poly-ubiquitination have distinct enzyme and substrate recognition requirements; whereas poly-ubiquitination targets misfolded protein for degradation, a distinct ubiquitination apparatus targets the fully mature, fully myristoylated G protein for mono-ubiquitination and delivery to the vacuole.

Galphaq reduces cAMP production by decreasing Galphas protein abundance

The heterotrimeric guanine nucleotide-binding protein Galphaq transduces signals from heptahelical transmembrane receptors (e.g., alpha(1)-adrenergic, endothelin 1A, and angiotensin II) to stimulate generation of inositol-1,4,5-trisphosphate and diacylglycerol. In addition, Galphaq decreases cAMP production, through unknown mechanisms, and thus affects physiological responsiveness of cardiac myocytes and other cells. Here, we provide evidence that Galphaq expression increases Galphas ubiquitination, decreases Galphas protein content, and impairs basal and beta(1)-adrenergic receptor-stimulated cAMP production. These biochemical and functional changes are associated with Akt activation. Expression of constitutively active Akt also decreases Galphas protein content and inhibits basal and beta(1)-adrenergic receptor-stimulated cAMP production. Akt knockdown inhibits Galphaq-induced reduction of Galphas protein. In addition, MDM2, an E3 ubiquitin ligase, binds Galphas and promotes its degradation. Therefore, increased expression of Galphaq decreases cAMP production through Akt-mediated Galphas protein ubiquitination and proteasomal degradation.

Degradation of the non-palmitoylated invertebrate visual guanine-nucleotide binding protein, iGq alpha(C3,4A), by the ubiquitin-proteasomal pathway is regulated by its activation and translocation to the cytoplasm

Light-dependent translocation of invertebrate visual guanine-nucleotide binding protein, iGq alpha, from rhabdomeric membranes to the cytoplasm is one of many mechanisms that contribute to light adaptation in the invertebrate eye. We have previously cloned iGq alpha from a Loligo pealei photoreceptor cDNA library and shown that when expressed in HEK 293T cells it is palmitoylated. In this study we compared the activation, cytoplasmic translocation, and turnover of iGq alpha with that of a non-palmitoylated mutant, iGq alpha(C3,4A). In the HEK 293T cells, muscarinic M1 receptors coupled equally well to iGq alpha and iGq alpha(C3,4A) to activate phospholipase C. Activation of iGq alpha(C3,4A), but not iGq alpha, induced translocation of the alpha subunit from the membrane to cytosol with rapid degradation of the soluble protein resulting in a decreased half-life for iGq alpha(C3,4A) of 10 hours compared to 20 hours for iGq alpha. Degradation of iGq alpha(C3,4A) was inhibited by proteasomal inhibitors but not by inhibitors of lysosomal proteases or calpain. The presence of the proteasomal inhibitor led to the accumulation of polyubiquitinated species of either iGq alpha or iGq alpha(C3,4A). Our results suggest that palmitoylation of iGq alpha is required to maintain membrane association of the protein in its active conformation, and whereas membrane-bound and soluble iGq alpha can be polyubiquitinated, membrane association protects the protein from rapid degradation by the proteasomal pathway.

Seven-transmembrane receptors and ubiquitination

Regulation of protein function by posttranslational modification plays an important role in many biological pathways. The most well known among such modifications is protein phosphorylation performed by highly specific protein kinases. In the past decade, however, covalent linkage of the low-molecular-weight protein ubiquitin to substrate proteins (protein ubiquitination) has proven to be yet another widely used mechanism of protein regulation playing a crucial role in virtually all aspects of cellular functions. This review highlights some of the recently discovered and provocative roles for ubiquitination in the regulation of the life cycle and signal transduction properties of 7-transmembrane receptors that serve to integrate many biological functions and play fundamental roles in cardiovascular homeostasis.

Adenylate cyclase/cAMP pathway downmodulation counteracts apoptosis induced by IFN-alpha in human epidermoid cancer cells

We have reported previously that interferon-alpha (IFN-alpha) induces apoptosis that is counteracted by an epidermal growth factor (EGF) --> Ras --> extracellular signal-regulated kinase (ERK)-dependent survival response in human epidermoid cancer KB cells. We have studied the effects of the cytokine on the cAMP-dependent pathway in these cells. A decrease in the intracellular cAMP levels was recorded in KB cells treated with IFN-alpha, whereas forskolin induced an increase in the production of cAMP that was reduced in the presence of IFN-alpha, suggesting a reduction in the activity of adenylate cyclase (AC) induced by IFN-alpha. These effects were paralleled by significant change in the expression of some AC catalytic subunit(s) and by reduction in the activity of protein kinase A (PKA). 8-Br-cAMP completely antagonized the reduction of PKA activity induced by IFN-alpha, whereas PKA inhibitor KT5720 enhanced the reduction of the enzyme activity induced by IFN-alpha. We have found that IFN-alpha induced a decrease in cAMP response element binding protein (CREB) phosphorylation without changes in its total expression. The concomitant treatment with IFN-alpha and 8-Br-cAMP potentiated and KT5720 counteracted apoptosis induced by IFN-alpha alone. In conclusion, these data suggest that the decrease in AC/cAMP pathway activity is a survival response to the apoptosis induced by IFN-alpha. Therefore, this pathway could represent a target to enhance the antitumor activity of IFN-alpha.

Regulation of G protein and mitogen-activated protein kinase signaling by ubiquitination: insights from model organisms

Guanine nucleotide binding proteins (G proteins) and mitogen-activated protein kinases are highly conserved signaling molecules engaged in a wide variety of cellular processes. The strength and duration of signaling mediated by G proteins and mitogen-activated protein kinases are well known to be regulated via phosphorylation of pathway components. Over the past few years, it has become evident that many of the same signaling proteins also undergo ubiquitination, a posttranslational modification that typically leads to protein degradation. Consequently the strength and duration of signaling can also be modulated by regulating the abundance of signaling proteins. This article describes G protein- and mitogen-activated protein kinase-mediated signaling pathways that are known to be regulated by ubiquitination. The focus is on studies performed in the budding yeast Saccharomyces cerevisiae, as many principles governing this new regulatory mechanism were initially discovered in this model organism. Similar mechanisms uncovered in other model systems are also briefly discussed to illustrate the importance and universality of signaling regulation by ubiquitination.

Inorganic phosphate inhibits growth of human osteosarcoma U2OS cells via adenylate cyclase/cAMP pathway

In order to elucidate how phosphate regulates cellular functions, we investigated the effects of inorganic phosphate (Pi) on adenylate cyclase (AC)/cyclic AMP (cAMP) axis. Here we describe that Pi treatment of human osteosarcoma U2OS cells results in a decrease of both intracellular cAMP levels and AC activity, and in a cell growth inhibition. The phosphate-triggered effects observed in U2OS cells are not a widespread phenomenon regarding all cell lines, since other cell lines screened respond differently to parallel Pi treatments. In U2OS cell line, the AC activity/cAMP downregulation is accompanied by significant variations in the levels of some membrane proteins belonging to the AC system. Remarkably, the above effects are blunted by pharmacological inhibition of sodium-dependent phosphate transport. Moreover, 8-Br-cAMP and other cAMP-elevating agents, such as IBMX and forskolin, interestingly, prevent the cell growth inhibition in response to phosphate. Our results enforce the increasing evidences of phosphate as a signaling molecule, identifying in U2OS cell line the AC/cAMP axis, as a novel-signaling pathway modulated by phosphate to ultimately affect cell growth.