AMP-activated protein kinase (AMPK) is tightly regulated by the cellular AMP:ATP ratio and plays a central role in regulation of energy homeostasis and metabolic stress. Metformin has been shown to activate AMPK. We hypothesized that metformin may prevent nuclear factor kappaB (NF-kappaB) activation in endothelial cells exposed to inflammatory cytokines. Metformin was observed to activate AMPK, as well as its downstream target, phosphoacetyl coenzyme A carboxylase, in human umbilical vein endothelial cells (HUVECs). Metformin also dose-dependently inhibited tumor necrosis factor (TNF)-alpha-induced NF-kappaB activation and TNF-alpha-induced IkappaB kinase activity. Furthermore, metformin attenuated the TNF-alpha-induced gene expression of various proinflammatory and cell adhesion molecules, such as vascular cell adhesion molecule-1, E-selectin, intercellular adhesion molecule-1, and monocyte chemoattractant protein-1, in HUVECs. A pharmacological activator of AMPK, 5-amino-4-imidazole carboxamide riboside (AICAR), dose-dependently inhibited TNF-alpha- and interleukin-1beta-induced NF-kappaB reporter gene expression. AICAR also suppressed the TNF-alpha- and interleukin-1beta-induced gene expression of vascular cell adhesion molecule-1, E-selectin, intercellular adhesion molecule-1, and monocyte chemoattractant protein-1 in HUVECs. The small interfering RNA for AMPKalpha1 attenuated metformin or AICAR-induced inhibition of NF-kappaB activation by TNF-alpha, suggesting a possible role of AMPK in the regulation of cell inflammation. In light of these findings, we suggest that metformin attenuates the cytokine-induced expression of proinflammatory and adhesion molecule genes by inhibiting NF-kappaB activation via AMPK activation. Thus, it might be useful to target AMPK signaling in future efforts to prevent atherogenic and inflammatory vascular disease.

Compelling evidence has been gathered indicating that pro-opiomelanocortin peptides, alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropic hormone (ACTH), through the cyclic AMP pathway, play a pivotal role in melanocyte differentiation and in the regulation of melanogenesis. Recently, the molecular events linking cAMP to melanogenesis up-regulation have been elucidated. This cascade involves the activation of protein kinase A and CREB transcription factor, leading to the up-regulation of the expression of Microphthalmia associated transcription factor (MITF). MITF has been found mutated in patients with Waardenburg syndrome 2A, and plays a crucial role in melanocyte development. MITF binds and activates melanogenic gene promoters, thereby increasing their expression which results in an increased melanin synthesis. Beyond this simplified scheme, It appears that melanogenic gene expression is controlled by a complex network of regulation involving other transcription factors such as Brn2, TBX2, PAX3 and SOX10. Further studies are required to better understand the respective roles of these factors in the regulation of melanin synthesis. In addition, other intracellular signaling pathways, like the phosphatidyl inositol 3-kinase pathway, as well as the molecular cascade of events governed by the small GTP-binding protein Rho, seem to be involved in the regulation of melanogenesis and melanocyte dendricity. Finally, it should be mentioned that cAMP activates a melanocyte-specific pathway leading to MAP kinase activation. MAP kinase, ERK2, phosphorylates MITF, thereby targeting the transcription factor to proteasomes for degradation. Thus, in addition to the complex transcriptional regulation, melanogenesis is also subjected to a post-translational regulation that controls MITF or tyrosinase function. Taken together, these complex molecular processes would finally allow a fine tuning of melanocyte differentiation leading to melanin synthesis.

Glucose- or nitrogen-starved cultures of Escherichia coli exhibited enhanced resistance to heat (57 degrees C) or H2O2 (15 mM) challenge, compared with their exponentially growing counterparts. The degree of resistance increased with the time for which the cells were starved prior to the challenge, with 4 h of starvation providing the maximal protection. Protein synthesis during starvation was essential for these cross protections, since chloramphenicol addition at the onset of starvation prevented the development of thermal or oxidative resistance. Starved cultures also demonstrated stronger thermal and oxidative resistance than did growing cultures adapted to heat, H2O2, or ethanol prior to the heat or H2O2 challenge. Two-dimensional gel electrophoresis of 35S-pulse-labeled proteins showed that subsets of the 30 glucose starvation proteins were also synthesized during heat or H2O2 adaptation; three proteins were common to all three stresses. Most of the common proteins were among the previously identified Pex proteins (J.E. Schultz, G. I. Latter, and A. Matin, J. Bacteriol. 170:3903-3909, 1988), which are independent of cyclic AMP positive control for their induction during starvation. Induction of starvation proteins dependent on cyclic AMP was not important in these cross protections, since a delta cya strain of E. coli K-12 exhibited the same degree of resistance to heat or H2O2 as the wild-type parent did during both growth and starvation.

Mutants of Escherichia coli K12 have been isolated that grow on media containing pyruvate of proline as sole carbon sources despite the presence of 10 or 50 mM-sodium fluoroacetate. Such mutants lack either acetate kinase [ATP: acetate phosphotransferase; EC 2.7.2.1] or phosphotransacetylase [acetyl-CoA: orthophosphate acetyltransferase; EC 2.3.1.8] activity. Unlike wild-type E. coli, phosphotransacetylase mutants do not excrete acetate when growing aerobically or anaerobically on glucose; their anaerobic growth on this sugar is slow. The genes that specify acetate kinase (ack) and phosphotransacetylase (pta) activities are cotransducible with each other and with purF and are thus located at about min 50 on the E. coli linkage map. Although Pta- and Ack- mutants are greatly impaired in their growth on acetate, they incorporate [2-14C]acetate added to cultures growing on glycerol, but not on glucose. An inducible acetyl-CoA synthetase [acetate: CoA ligase (AMP-forming); EC 6.2.1.1] effects this uptake of acetate.

Efficient control of energy metabolic homeostasis, enhanced stress resistance, and qualified cellular housekeeping are the hallmarks of improved healthspan and extended lifespan. AMPK signaling is involved in the regulation of all these characteristics via an integrated signaling network. Many studies with lower organisms have revealed that increased AMPK activity can extend the lifespan. Experiments in mammals have demonstrated that AMPK controls autophagy through mTOR and ULK1 signaling which augment the quality of cellular housekeeping. Moreover, AMPK-induced stimulation of FoxO/DAF-16, Nrf2/SKN-1, and SIRT1 signaling pathways improves cellular stress resistance. Furthermore, inhibition of NF-κB signaling by AMPK suppresses inflammatory responses. Emerging studies indicate that the responsiveness of AMPK signaling clearly declines with aging. The loss of sensitivity of AMPK activation to cellular stress impairs metabolic regulation, increases oxidative stress and reduces autophagic clearance. These age-related changes activate innate immunity defence, triggering a low-grade inflammation and metabolic disorders. We will review in detail the signaling pathways of this integrated network through which AMPK controls energy metabolism, autophagic degradation and stress resistance and ultimately the aging process.

For unknown reasons, humans infected with the bacterium Bordetella pertussis are exceptionally vulnerable to secondary infections. Bordetella species elaborate a soluble, heat-stable, and highly active adenylate cyclase. This enzyme is internalized by phagocytic cells and catalyzes the unregulated formation of adenosine 3',5'-monophosphate (cyclic AMP), thereby disrupting normal cellular function. This unusual phenomenon may explain Bordetella-induced aphylaxis and may prove to be useful for investigating a variety of cyclic AMP-governed processes.

Studies in a variety of model organisms indicate that nutrient signaling is tightly coupled to longevity. In nutrient replete conditions, organisms develop, grow, and age quickly. When nutrients become sparse as with dietary restriction, growth and development decline, stress response pathways become induced and organisms live longer. Considerable effort has been devoted to understanding the molecular events mediating lifespan extension by dietary restriction. One central focus has been on nutrient-responsive signal transduction pathways including insulin/IGF-1, AMP kinase, protein kinase A and the TOR pathway. Here we describe the increasingly prominent links between TOR signaling and aging in invertebrates. Longevity studies in mammals are not published to date. Instead, we highlight studies in mouse models, which indicate that dampening the TOR pathway leads to widespread protection from an array of age-related diseases.

The LKB1 tumour suppressor phosphorylates and activates AMPK (AMP-activated protein kinase) when cellular energy levels are low, thereby suppressing growth through multiple pathways, including inhibiting the mTORC1 (mammalian target of rapamycin complex 1) kinase that is activated in the majority of human cancers. Blood glucose-lowering Type 2 diabetes drugs also induce LKB1 to activate AMPK, indicating that these compounds could be used to suppress growth of tumour cells. In the present study, we investigated the importance of the LKB1-AMPK pathway in regulating tumorigenesis in mice resulting from deficiency of the PTEN (phosphatase and tensin homologue deleted on chromosome 10) tumour suppressor, which drives cell growth through overactivation of the Akt and mTOR (mammalian target of rapamycin) kinases. We demonstrate that inhibition of AMPK resulting from a hypomorphic mutation that decreases LKB1 expression does not lead to tumorigenesis on its own, but markedly accelerates tumour development in PTEN(+/-) mice. In contrast, activating the AMPK pathway by administration of metformin, phenformin or A-769662 to PTEN(+/-) mice significantly delayed tumour onset. We demonstrate that LKB1 is required for activators of AMPK to inhibit mTORC1 signalling as well as cell growth in PTEN-deficient cells. Our findings highlight, using an animal model relevant to understanding human cancer, the vital role that the LKB1-AMPK pathway plays in suppressing tumorigenesis resulting from loss of the PTEN tumour suppressor. They also suggest that pharmacological inhibition of LKB1 and/or AMPK would be undesirable, at least for the treatment of cancers in which the mTORC1 pathway is activated. Most importantly, our results demonstrate the potential of AMPK activators, such as clinically approved metformin, as anticancer agents, which will suppress tumour development by triggering a physiological signalling pathway that potently inhibits cell growth.

The mechanism of cross talk between the Wnt signaling and cyclic AMP (cAMP)-dependent protein kinase (protein kinase A [PKA]) pathways was studied. Prostaglandin E(1) (PGE(1)), isoproterenol, and dibutyryl cAMP (Bt(2)cAMP), all of which activate PKA, increased the cytoplasmic and nuclear beta-catenin protein level, and these actions were suppressed by a PKA inhibitor and RNA interference for PKA. PGE(1) and Bt(2)cAMP also increased T-cell factor (Tcf)-dependent transcription through beta-catenin. Bt(2)cAMP suppressed degradation of beta-catenin at the protein level. Although PKA did not affect the formation of a complex between glycogen synthase kinase 3beta (GSK-3beta), beta-catenin, and Axin, phosphorylation of beta-catenin by PKA inhibited ubiquitination of beta-catenin in intact cells and in vitro. Ser675 was found to be a site for phosphorylation by PKA, and substitution of this serine residue with alanine in beta-catenin attenuated inhibition of the ubiquitination of beta-catenin by PKA, PKA-induced stabilization of beta-catenin, and PKA-dependent activation of Tcf. These results indicate that PKA inhibits the ubiquitination of beta-catenin by phosphorylating beta-catenin, thereby causing beta-catenin to accumulate and the Wnt signaling pathway to be activated.

Cyclooxygenease-2 (COX-2) is the key enzyme regulating the production of prostaglandins, central mediators of inflammation. The expression of cyclooxygenease-2 is induced by several extra cellular signals including pro-inflammatory and growth-promoting stimuli. All signals converge to the activation of mitogen-activated protein kinases (MAPK) that regulate cyclooxygenease-2 mRNA levels both at the transcriptional and post-transcriptional level. The machinery appears to be highly specialized involving activation of distinct signalling molecules depending on the type of extracellular stimulus. Expression of cyclooxygenease-2 mRNA is regulated by several transcription factors including the cyclic-AMP response element binding protein (CREB), nuclear factor kappa B (NFkB) and the CCAAT-enhancer binding protein (C/EBP). Cyclooxygenease-2 is also affected post-transcriptionaly, at the level of mRNA stability. Finally, cyclooxygenease-2 can be affected directly at its enzymatic activity by nitric oxide and nitric oxide synthase (iNOS). Each step of cyclooxygenease-2 regulation can be used as potential therapeutic target.

The Tax protein of human T-lymphotropic virus (HTLV)-1 activates expression of the HTLV-1 long terminal repeat through a DNA element that resembles the cellular cyclic AMP-regulated enhancer (CRE). Tax contains a transcriptional activation domain, but its ability to activate gene expression depends on interactions with cellular CRE-binding proteins such as CREB. Whether Tax can activate the expression of cellular CRE-containing genes has been controversial. Here we show that Tax can activate both the HTLV-1 and consensus cellular CREs, and propose that this activation may occur through mechanisms that are differentially dependent on CREB phosphorylation. Tax not only increases the binding of CREB to the viral CRE but also recruits the transcriptional co-activator CBP in a manner independent of CREB phosphorylation. In contrast, association of Tax with the cellular CRE occurs through CBP which, in turn, is recruited only in the presence of phosphorylated CREB.

It was recently reported that the plant polyphenol resveratrol, found, e.g., in grape berry skins, extended lifespan in the fruit fly Drosophila melanogaster and the nematode worm Caenorhabditis elegans. This lifespan extension was dependent on an NAD(+)-dependent histone deacetylase, Sir2 in Drosophila and SIR-2.1 in C. elegans. The extension of lifespan appeared to occur through a mechanism related to dietary restriction (DR), the reduction of available nutrients without causing malnutrition, an intervention that extends lifespan in diverse organisms from yeast to mammals. In Drosophila, lifespan extension by DR is associated with a reduction in fecundity. However, a slight increase in fecundity was reported upon treatment with resveratrol, suggesting a mode of action at least partially distinct from that of DR. To probe this mechanism further, we initiated a new study of the effects of resveratrol on Drosophila. We saw no significant effects on lifespan in seven independent trials. We analysed our resveratrol and found that its structure was normal, with no oxidative modifications. We therefore re-tested the effects of resveratrol in C. elegans, in both wild-type and sir-2.1 mutant worms. The results were variable, with resveratrol treatment resulting in slight increases in lifespan in some trials but not others, in both wild type and sir-2.1 mutant animals. We postulate that the effect of resveratrol upon lifespan in C. elegans could reflect induction of phase 2 drug detoxification or activation of AMP kinase.

Synaptic plasticity in response to changes in physiologic state is coordinated by hormonal signals across multiple neuronal cell types. Here, we combine cell-type-specific electrophysiological, pharmacological, and optogenetic techniques to dissect neural circuits and molecular pathways controlling synaptic plasticity onto AGRP neurons, a population that regulates feeding. We find that food deprivation elevates excitatory synaptic input, which is mediated by a presynaptic positive feedback loop involving AMP-activated protein kinase. Potentiation of glutamate release was triggered by the orexigenic hormone ghrelin and exhibited hysteresis, persisting for hours after ghrelin removal. Persistent activity was reversed by the anorexigenic hormone leptin, and optogenetic photostimulation demonstrated involvement of opioid release from POMC neurons. Based on these experiments, we propose a memory storage device for physiological state constructed from bistable synapses that are flipped between two sustained activity states by transient exposure to hormones signaling energy levels.

Microtubules prepared from chick brain homogenates by successive cycles of assembly-disassembly were found to contain two high-molecular-weight proteins, designated microtubule-associated protein1 and microtubule-associated protein2. Microtubule-associated protein2 (apparent molecular weight 300,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) was the preferred substrate for an endogenous cyclic AMP-dependent protein kinase which appeared to be an integral component of the microtubules. The initial rate of phosphorylation of microtubule-associated protein2 was enhanced 4- to 6-fold by cyclic AMP, with half-maximal stimulation occurring at 2 times 10-7 M cyclic AMP. Under optimal conditions, a total of 1.0 and 1.9 mol of phosphate was incorporated per mole of microtubule-associated protein2, in the absence and presence of cyclic AMP, respectively. Cyclic AMP also stimulated the phosphorylation of tubulin, but the rate of phosphate incorporation per mol of tubulin was only 0.15% that of microtubule-associated protein2. The data raise the possibility that the cyclic AMP-dependent phosphorylation of microtubule-associated protein 2 may play a role in microtubule assembly or function.

Amino acids positively regulate signaling through the mammalian target of rapamycin (mTOR). Recent work demonstrated the importance of the tuberous sclerosis protein TSC2 for regulation of mTOR by insulin. TSC2 contains a GTPase-activator domain that promotes hydrolysis of GTP bound to Rheb, which positively regulates mTOR signaling. Some studies have suggested that TSC2 also mediates the control of mTOR by amino acids. In cells lacking TSC2, amino acid withdrawal still results in dephosphorylation of S6K1, ribosomal protein S6, the eukaryotic initiation factor 4E-binding protein, and elongation factor-2 kinase. The effects of amino acid withdrawal are diminished by inhibiting protein synthesis or adding back amino acids. These studies demonstrate that amino acid signaling to mTOR occurs independently of TSC2 and involves additional unidentified inputs. Although TSC2 is not required for amino acid control of mTOR, amino acid withdrawal does decrease the proportion of Rheb in the active GTP-bound state. Here we also show that Rheb and mTOR form stable complexes, which are not, however, disrupted by amino acid withdrawal. Mutants of Rheb that cannot bind GTP or GDP can interact with mTOR complexes. We also show that the effects of hydrogen peroxide and sorbitol, cell stresses that impair mTOR signaling, are independent of TSC2. Finally, we show that the ability of energy depletion (which impairs mTOR signaling in TSC2+/+ cells) to increase the phosphorylation of eukaryotic elongation factor 2 is also independent of TSC2. This likely involves the phosphorylation of the elongation factor-2 kinase by the AMP-activated protein kinase.

The fuel-sensing enzyme 5'-AMP-activated protein kinase (AMPK) has a major role in the regulation of cellular lipid and protein metabolism in response to stimuli such as exercise, changes in fuel availability and the adipocyte-derived hormones leptin and adiponectin. Recent studies indicate that abnormalities in cellular lipid metabolism are involved in the pathogenesis of the metabolic syndrome, possibly because of dysregulation of AMPK and malonyl-CoA, a closely related molecule. As we discuss in this article, several findings also point to a link between AMPK and the growth and/or survival of some cancer cells. Thus, it has been demonstrated recently that the tumor suppressor LKB1 is a kinase that has a major role in phosphorylating and activating AMPK, and that another tumor suppressor, tuberous sclerosis complex 2, is phosphorylated and activated by AMPK. In addition, other studies indicate that mammalian homolog of target of rapamycin (mTOR), which has been implicated in the pathogenesis of insulin resistance and many types of cancer, is inhibited by AMPK.

A novel member of the serine/threonine protein kinase gene family, designated sgk, for serum and glucocorticoid-regulated kinase, was identified in a differential screen for glucocorticoid-inducible transcripts expressed in the Con8.hd6 rat mammary tumor cell line. sgk encodes a protein of 49 kDa which has significant sequence homology (45 to 55% identity) throughout its catalytic domain with rac protein kinase, the protein kinase C family, ribosomal protein S6 kinase, and cyclic AMP-dependent protein kinase. sgk mRNA is expressed in most adult rat tissues, with the highest levels in the thymus, ovary, and lung, as well as in several rodent and human cell lines. sgk mRNA was stimulated by glucocorticoids and by serum within 30 min, and both inductions were independent of de novo protein synthesis. The transcriptional regulation by glucocorticoids is a primary response, since the promoter of sgk contains a glucocorticoid response element consensus sequence 1.0 kb upstream of the start of transcription which is able to stimulate chloramphenicol acetyltransferase reporter gene activity in a dexamethasone-dependent manner. Antibodies that specifically recognize sgk-encoded protein on an immunoblot were generated. This protein was shown to increase in abundance with glucocorticoid treatment in a manner which paralleled the mRNA accumulation. This is the first report of a presumed serine/threonine protein kinase that is highly regulated at the transcriptional level by glucocorticoid hormones and suggests a novel interplay between glucocorticoid receptor signalling and a protein kinase of the second messenger family.

The Snf1/AMP-activated protein kinase (AMPK) family is important for metabolic regulation and is highly conserved from yeast to mammals. The upstream kinases are also functionally conserved, and the AMPK kinases LKB1 and Ca2+/calmodulin-dependent protein kinase kinase activate Snf1 in mutant yeast cells lacking the native Snf1-activating kinases, Sak1, Tos3, and Elm1. Here, we exploited the yeast genetic system to identify members of the mammalian AMPK kinase family by their function as Snf1-activating kinases. A mouse embryo cDNA library in a yeast expression vector was used to transform sak1Delta tos3Delta elm1Delta yeast cells. Selection for a Snf+ growth phenotype yielded cDNA plasmids expressing LKB1, Ca2+/calmodulin-dependent protein kinase kinase, and transforming growth factor-beta-activated kinase (TAK1), a member of the mitogen-activated protein kinase kinase kinase family. We present genetic and biochemical evidence that TAK1 activates Snf1 protein kinase in vivo and in vitro. We further show that recombinant TAK1, fused to the activation domain of its binding partner TAB1, phosphorylates Thr-172 in the activation loop of the AMPK catalytic domain. Finally, expression of TAK1 and TAB1 in HeLa cells or treatment of cells with cytokines stimulated phosphorylation of Thr-172 of AMPK. These findings indicate that TAK1 is a functional member of the Snf1/AMPK kinase family and support TAK1 as a candidate for an authentic AMPK kinase in mammalian cells.

Activation of the stimulator of interferon genes (STING) pathway by microbial or self-DNA, as well as cyclic dinucleotides (CDNs), results in the induction of numerous genes that suppress pathogen replication and facilitate adaptive immunity. However, sustained gene transcription is rigidly prevented to avoid lethal STING-dependent proinflammatory disease by mechanisms that remain unknown. We demonstrate here that, after autophagy-dependent STING delivery of TANK-binding kinase 1 (TBK1) to endosomal/lysosomal compartments and activation of transcription factors interferon regulatory factor 3 (IRF3) and NF-κB, STING is subsequently phosphorylated by serine/threonine UNC-51-like kinase (ULK1/ATG1), and IRF3 function is suppressed. ULK1 activation occurred following disassociation from its repressor AMP activated protein kinase (AMPK) and was elicited by CDNs generated by the cGAMP synthase, cGAS. Thus, although CDNs may initially facilitate STING function, they subsequently trigger negative-feedback control of STING activity, thus preventing the persistent transcription of innate immune genes.

OBJECTIVE

Autophagy is a critical cellular system for removal of aggregated proteins and damaged organelles. Although dysregulated autophagy is implicated in the development of heart failure, the role of autophagy in the development of diabetic cardiomyopathy has not been studied. We investigated whether chronic activation of the AMP-activated protein kinase (AMPK) by metformin restores cardiac function and cardiomyocyte autophagy in OVE26 diabetic mice.

METHODS

OVE26 mice and cardiac-specific AMPK dominant negative transgenic (DN)-AMPK diabetic mice were treated with metformin or vehicle for 4 months, and cardiac autophagy, cardiac functions, and cardiomyocyte apoptosis were monitored.

RESULTS

Compared with control mice, diabetic OVE26 mice exhibited a significant reduction of AMPK activity in parallel with reduced cardiomyocyte autophagy and cardiac dysfunction in vivo and in isolated hearts. Furthermore, diabetic OVE26 mouse hearts exhibited aggregation of chaotically distributed mitochondria between poorly organized myofibrils and increased polyubiquitinated protein and apoptosis. Inhibition of AMPK by overexpression of a cardiac-specific DN-AMPK gene reduced cardiomyocyte autophagy, exacerbated cardiac dysfunctions, and increased mortality in diabetic mice. Finally, chronic metformin therapy significantly enhanced autophagic activity and preserved cardiac functions in diabetic OVE26 mice but not in DN-AMPK diabetic mice.

CONCLUSIONS

Decreased AMPK activity and subsequent reduction in cardiac autophagy are important events in the development of diabetic cardiomyopathy. Chronic AMPK activation by metformin prevents cardiomyopathy by upregulating autophagy activity in diabetic OVE26 mice. Thus, stimulation of AMPK may represent a novel approach to treat diabetic cardiomyopathy.

Activating transcription factor 4 (ATF4) belongs to the ATF/CREB (activating transcription factor/cyclic AMP response element binding protein) family of basic region-leucine zipper (bZip) transcription factors, which have the consensus binding site cAMP responsive element (CRE). ATF4 has numerous dimerization partners. ATF4 is induced by stress signals including anoxia/hypoxia, endoplasmic reticulum stress, amino acid deprivation, and oxidative stress. ATF4 expression is regulated transcriptionally, translationally via the PERK pathway of eIF2alpha phosphorylation, and posttranslationally by phosphorylation, which targets ATF4 to proteasomal degradation. ATF4 regulates the expression of genes involved in oxidative stress, amino acid synthesis, differentiation, metastasis and angiogenesis. Transgenic studies have demonstrated ATF4 to be involved in hematopoiesis, lens and skeletal development, fertility, proliferation, differentiation, and long-term memory. ATF4 expression is upregulated in cancer. Since ATF4 is induced by tumour microenvironmental factors, and regulates processes relevant to cancer progression, it might serve as a potential therapeutic target in cancer.

Mitochondria are well appreciated for their role as biosynthetic and bioenergetic organelles. In the past two decades, mitochondria have emerged as signaling organelles that contribute critical decisions about cell proliferation, death, and differentiation. Mitochondria not only sustain immune cell phenotypes but also are necessary for establishing immune cell phenotype and their function. Mitochondria can rapidly switch from primarily being catabolic organelles generating ATP to anabolic organelles that generate both ATP and building blocks for macromolecule synthesis. This enables them to fulfill appropriate metabolic demands of different immune cells. Mitochondria have multiple mechanisms that allow them to activate signaling pathways in the cytosol including altering in AMP/ATP ratio, the release of ROS and TCA cycle metabolites, as well as the localization of immune regulatory proteins on the outer mitochondrial membrane. In this Review, we discuss the evidence and mechanisms that mitochondrial dependent signaling controls innate and adaptive immune responses.

A cyclic AMP binding protein has been purified over 100-fold from E. coli extracts. Protein purified from wild-type strains binds cyclic AMP with an apparent dissociation constant of 1-2 x 10(-6)M. Two mutant strains that are unresponsive to exogenous cyclic AMP have altered binding activity; the protein purified from one of these mutants has a decreased affinity for cyclic AMP (apparent dissociation constant = 2 x 10(-5)M). Extracts of this mutant are deficient in their ability to support beta-galactosidase synthesis in vitro. The addition of purified, wild-type binding protein to these extracts restores enzyme synthesis toward normal. Because this binding protein appears to be required for cyclic AMP action, we suggest it be called the cyclic AMP receptor protein (CR protein).

Huntington's disease is caused by an expanded CAG repeat in the gene encoding huntingtin (HTT), resulting in loss of striatal and cortical neurons. Given that the gene product is widely expressed, it remains unclear why neurons are selectively targeted. Here we show the relationship between synaptic and extrasynaptic activity, inclusion formation of mutant huntingtin protein (mtHtt) and neuronal survival. Synaptic N-methyl-D-aspartate-type glutamate receptor (NMDAR) activity induces mtHtt inclusions via a T complex-1 (TCP-1) ring complex (TRiC)-dependent mechanism, rendering neurons more resistant to mtHtt-mediated cell death. In contrast, stimulation of extrasynaptic NMDARs increases the vulnerability of mtHtt-containing neurons to cell death by impairing the neuroprotective cyclic AMP response element-binding protein (CREB)-peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) cascade and increasing the level of the small guanine nucleotide-binding protein Rhes, which is known to sumoylate and disaggregate mtHtt. Treatment of transgenic mice expressing a yeast artificial chromosome containing 128 CAG repeats (YAC128) with low-dose memantine blocks extrasynaptic (but not synaptic) NMDARs and ameliorates neuropathological and behavioral manifestations. By contrast, high-dose memantine, which blocks both extrasynaptic and synaptic NMDAR activity, decreases neuronal inclusions and worsens these outcomes. Our findings offer a rational therapeutic approach for protecting susceptible neurons in Huntington's disease.