Signal Transduction Crosstalk Research Articles

Severe hypoleptinaemia associated with insulin resistance in patients with common variable immunodeficiency

Common variable immunodeficiency (CVI) is a primary immunodeficiency syndrome characterized by impaired production of antibodies and recurrent infections. Delay in diagnosis leads to metabolic wastage and low body weight. Leptin, a hormone produced by white adipose tissue, modulates insulin action by signal transduction cross-talk and by direct action on pancreatic beta-cells. We hypothesized that patients with CVI might present a defective regulation of leptin production and insulin resistance. Thirteen CVI patients (39 +/- 11 years) under gammaglobulin replacement were evaluated in parallel with 13 gender-, age-, body weight- and body mass index (BMI)-matched healthy voluntaries, and with data from two large population series, the Bruneck and the Hoorn Studies. Serum leptin and insulin levels, homeostasis model assessment - insulin resistance (HOMA-IR), body composition, haematological, biochemical and immunoglobulin measurements were obtained. Data were analysed by a one-way analysis of variance (anova) and by Pearson's rank analysis. The institutional ethics committee approved the study, and informed consent was obtained from patients and controls. No differences were found between CVI and the control group when comparing gender distribution, age, body weight, BMI, waist/hip ratio, relative body fat and fasting glucose levels. Leptin levels were lower (P < 0.05) in CVI patients than in controls and lower than fasting leptin levels detected in a large population study. CVI patients' serum leptin levels did not correlate with BMI (r = 0.074, P = 0.8) and their high HOMA-IR indicated insulin resistance. CVI patients are relatively hypoleptinaemic and insulin resistant, and their serum leptin levels are not correlated to their BMI.

Functional Characterization in Vitro of All Two-component Signal Transduction Systems from Escherichia coli

Bacteria possess a signal transduction system, referred to as a two-component system, for adaptation to external stimuli. Each two-component system consists of a sensor protein-histidine kinase (HK) and a response regulator (RR), together forming a signal transduction pathway via histidyl-aspartyl phospho-relay. A total of 30 sensor HKs, including as yet uncharacterized putative HKs (BaeS, BasS, CreC, CusS, HydH, RstB, YedV, and YfhK), and a total of 34 RRs, including putative RRs (BaeR, BasR, CreB, CusR, HydG, RstA, YedW, YfhA, YgeK, and YhjB), have been suggested to exist in Escherichia coli. We have purified the carboxyl-terminal catalytic domain of 27 sensor HKs and the full-length protein of all 34 RRs to apparent homogeneity. Self-phosphorylation in vitro was detected for 25 HKs. The rate of self-phosphorylation differed among HKs, whereas the level of phosphorylation was generally co-related with the phosphorylation rate. However, the phosphorylation level was low for ArcB, HydH, NarQ, and NtrB even though the reaction rate was fast, whereas the level was high for the slow phosphorylation species BasS, CheA, and CreC. By using the phosphorylated HKs, we examined trans-phosphorylation in vitro of RRs for all possible combinations. Trans-phosphorylation of presumed cognate RRs by HKs was detected, for the first time, for eight pairs, BaeS-BaeR, BasS-BasR, CreC-CreB, CusS-CusR, HydH-HydG, RstB-RstA, YedV-YedW, and YfhK-YfhA. All trans-phosphorylation took place within less than 1/2 min, but the stability of phosphorylated RRs differed, indicating the involvement of de-phosphorylation control. In addition to the trans-phosphorylation between the cognate pairs, we detected trans-phosphorylation between about 3% of non-cognate HK-RR pairs, raising the possibility that the cross-talk in signal transduction takes place between two-component systems.

Retinoid target genes in acute promyelocytic leukemia.

All-trans-retinoic acid (RA)-based differentiation therapy induces clinical remissions in acute promyelocytic leukemia (APL). This has propelled interest in elucidating the molecular mechanisms responsible for these remissions. The t(15;17) rearrangement results in the expression of the PML/RARalpha fusion transcript that is paradoxically linked to the etiology and clinical retinoid response in APL. PML/RARalpha expression blocks terminal myeloid differentiation in APL. Treatment with pharmacological RA dosages overcomes the dominant-negative effects of PML/RARalpha to activate transcription of retinoid target genes. This regulation is linked directly to RA effects in APL, including PML/RARalpha degradation and induction of differentiation. Identifying retinoid target genes is an important step in developing a mechanistic understanding of RA effects in APL. RA target genes have been uncovered through the use of molecular genetic approaches as well as unique cellular and transgenic APL models. Recent developments in the proteomic and functional genomic fields are providing useful tools for elucidating mechanisms of RA response or resistance in APL. These target genes represent potential therapeutic targets in APL and other retinoid-responsive diseases. Previous spotlights in Leukemia have highlighted the importance of cytokine effects and signal transduction crosstalk in retinoid response in APL and in normal hematopoiesis. This review builds on prior work by addressing the role of retinoid target genes in mediating retinoid response or resistance in APL.

Cyclin D1 repression of peroxisome proliferator-activated receptor gamma expression and transactivation.

The cyclin D1 gene is overexpressed in human breast cancers and is required for oncogene-induced tumorigenesis. Peroxisome proliferator-activated receptor gamma (PPAR gamma) is a nuclear receptor selectively activated by ligands of the thiazolidinedione class. PPAR gamma induces hepatic steatosis, and liganded PPAR gamma promotes adipocyte differentiation. Herein, cyclin D1 inhibited ligand-induced PPAR gamma function, transactivation, expression, and promoter activity. PPAR gamma transactivation induced by the ligand BRL49653 was inhibited by cyclin D1 through a pRB- and cdk-independent mechanism, requiring a region predicted to form an helix-loop-helix (HLH) structure. The cyclin D1 HLH region was also required for repression of the PPAR gamma ligand-binding domain linked to a heterologous DNA binding domain. Adipocyte differentiation by PPAR gamma-specific ligands (BRL49653, troglitazone) was enhanced in cyclin D1(-/-) fibroblasts and reversed by retroviral expression of cyclin D1. Homozygous deletion of the cyclin D1 gene, enhanced expression by PPAR gamma ligands of PPAR gamma and PPAR gamma-responsive genes, and cyclin D1(-/-) mice exhibit hepatic steatosis. Finally, reduction of cyclin D1 abundance in vivo using ponasterone-inducible cyclin D1 antisense transgenic mice, increased expression of PPAR gamma in vivo. The inhibition of PPAR gamma function by cyclin D1 is a new mechanism of signal transduction cross talk between PPAR gamma ligands and mitogenic signals that induce cyclin D1.

Molecular mechanisms of transcriptional regulation by nuclear receptors. Perspectives for therapeutic implications

Nuclear receptors are ligand-regulated transcription factors that evolved from an ancestral orphan receptor into a highly diverse family present throughout the entire animal kingdom. They encompass receptors for steroid and non-steroid hormones, vitamins and metabolic intermediates. These receptors signal through endocrine, paracrine, autocrine and intracrine networks to regulate multiple aspects of animal physiology, including homeostasis, development and reproduction. They exert genomic effects via direct binding as monomers, homo- or heterodimers on cognate DNA elements (hormone response elements). They also participate in signal transduction cross-talk to indirectly modulate other gene expression programmes. By coordinating expression of genetic programmes, nuclear receptors contribute to cell fate-determining processes, thereby shaping and sustaining the organism. All these actions result from one fundamental interaction: receptor binding of a cognate ligand, which induces a major allosteric change in the ligand-binding domain. This conformational alteration is transformed into cascades of protein-protein recognitions, culminating in the establishment of coregulator/cointegrator complexes on gene promoters. Coregulators induce chromatin remodelling and acetylation, thus enabling the targeted recruitment and activation of the basal transcription machinery. This review discusses the molecular infrastructure of nuclear receptor signalling. Emphasis is given to determinants of signalling specificity, especially since they highlight prominent targets for novel drug discovery.

Specificity and cross-talk in plant signal transduction: January 2002 Keystone Symposium.

More than 200 researchers convened in Tahoe City, CA, for this excellent symposium, which was organized by Julian Schroeder, Mark Estelle, and Masaki Furuya and sponsored by Monsanto Co. Many of the symposium speakers are coauthors of reviews presented in this Special Issue of The Plant Cell . In

Cross-talk between IL-1 and IL-6 signaling pathways in rheumatoid arthritis synovial fibroblasts.

The balance between pro- and anti-inflammatory cytokines plays an important role in determining the severity of inflammation in rheumatoid arthritis (RA). Antagonism between opposing cytokines at the level of signal transduction plays an important role in many other systems. We have begun to explore the possible contribution of signal transduction cross-talk to cytokine balance in RA by examining the effects of IL-1, a proinflammatory cytokine, on the signaling and action of IL-6, a pleiotropic cytokine that has both pro- and anti-inflammatory actions, in RA synovial fibroblasts. Pretreatment with IL-1 suppressed Janus kinase-STAT signaling by IL-6, modified patterns of gene activation, and blocked IL-6 induction of tissue inhibitor of metalloproteases 1 expression. These results suggest that proinflammatory cytokines may contribute to pathogenesis by modulating or blocking signal transduction by pleiotropic or anti-inflammatory cytokines. The mechanism of inhibition did not require de novo gene activation and did not depend upon tyrosine phosphatase activity, but, instead, was dependent on the p38 stress kinase. These results identify a molecular basis for IL-1 and IL-6 cross-talk in RA synoviocytes and suggest that, in addition to levels of cytokine expression, modulation of signal transduction also plays a role in regulating cytokine balance in RA.

Emotional stress induces immediate-early gene expression in rat heart via activation of alpha- and beta-adrenoceptors.

We have studied the adrenergic mechanisms of immediate-early gene (IEG) induction in the discrete types of cardiac cells with the use of in situ hybridization histochemistry in an immobilization-stress model in conscious rats. Expression of c-fos, fos B, c-jun, jun B, NGFI-A, and NGFI-B mRNA was rapidly upregulated in the endothelial, myocardial, and smooth muscle cells of coronary vessels by 15-45 min after the onset of immobilization. Simultaneous blockade of both alpha- and beta-adrenoceptors completely abolished expression of IEGs in these cardiac cells. Application of an alpha-agonist or beta-agonist alone to the perfused rat heart under constant pressure elicited the upregulation of IEGs in a fashion similar to that of emotional stress. These data suggest that activation of either alpha- or beta-adrenoceptor is sufficient to evoke expression of these genes and that there may be cross talk in signal transduction downstream from alpha- and beta-adrenoceptors in cardiac cells.

Cholesterol Efflux-mediated Signal Transduction in Mammalian Sperm: β-CYCLODEXTRINS INITIATE TRANSMEMBRANE SIGNALING LEADING TO AN INCREASE IN PROTEIN TYROSINE PHOSPHORYLATION AND CAPACITATION

Sperm capacitation in vitro is highly correlated with an increase in protein tyrosine phosphorylation that is regulated by cAMP through a unique mode of signal transduction cross-talk. The activation of this signaling pathway, as well as capacitation, requires bovine serum albumin (BSA) in the incubation medium. BSA is hypothesized to modulate capacitation through its ability to remove cholesterol from the sperm plasma membrane. Here we demonstrate that the cholesterol-binding heptasaccharides, methyl-beta-cyclodextrin and OH-propyl-beta-cyclodextrin, promote the release of cholesterol from the mouse sperm plasma membrane in media devoid of BSA. Both of these beta-cyclodextrins were also demonstrated to increase protein tyrosine phosphorylation in the absence of BSA in both mouse and bull sperm, and the patterns of phosphorylation were similar to those induced by media containing BSA. The potency of the different beta-cyclodextrins to increase protein tyrosine phosphorylation in sperm was correlated with their cholesterol binding efficiencies, and preincubation of the beta-cyclodextrins with cholesterol-SO4- to saturate their cholesterol-binding sites blocked the ability of these compounds to stimulate protein tyrosine phosphorylation. The beta-cyclodextrin effect on protein tyrosine phosphorylation was both NaHCO3 and protein kinase A-dependent. The beta-cyclodextrins were also able to capacitate mouse sperm in the absence of BSA, as measured by the ability of the zona pellucida to induce the acrosome reaction and by successful fertilization in vitro. In summary, beta-cyclodextrins can completely replace BSA in media to support signal transduction leading to capacitation. These data further support the coupling of cholesterol efflux to the activation of membrane and transmembrane signaling events leading to the activation of a unique signaling pathway involving the cross-talk between cAMP and tyrosine kinase second messenger systems, thus defining a new mode of cellular signal transduction initiated by cholesterol release.

Glucose and ethylene signal transduction crosstalk revealed by an Arabidopsis glucose-insensitive mutant.

Glucose is an essential signaling molecule that controls plant development and gene expression through largely unknown mechanisms. To initiate the dissection of the glucose signal transduction pathway in plants by using a genetic approach, we have identified an Arabidopsis mutant, gin1 (glucose-insensitive), in which glucose repression of cotyledon greening and expansion, shoot development, floral transition, and gene expression is impaired. Genetic analysis indicates that GIN1 acts downstream of the sensor hexokinase in the glucose signaling pathway. Surprisingly, gin1 insensitivity to glucose repression of cotyledon and shoot development is phenocopied by ethylene precursor treatment of wild-type plants or by constitutive ethylene biosynthesis and constitutive ethylene signaling mutants. In contrast, the ethylene insensitive mutant etr1-1 exhibits glucose hypersensitivity. Epistasis analysis places GIN1 downstream of the ethylene receptor, ETR1, and defines a new branch of ethylene signaling pathway that is uncoupled from the triple response induced by ethylene. The isolation and characterization of gin1 reveal an unexpected convergence between the glucose and the ethylene signal transduction pathways. GIN1 may function to balance the control of plant development in response to metabolic and hormonal stimuli that act antagonistically.

Stimulation of P 2Y-purinoceptors on astrocytes results in immediate early gene expression and potentiation of neuropeptide action

The action of adenosine-5′-O-(2-thiodiphosphate), a non-hydrolysable purine analogue and potent P 2Y1-purinoceptor agonist, was studied on immediate early gene expression in rat astrocyte cultures. A rapid and transient increase in c-fos, junB, c-jun and Tis11 messenger RNA was observed in cultured astrocytes after treatment with adenosine-5′-O-(2-thiodiphosphate). Maximal induction of immediate early gene expression was obtained within 30 min of stimulation and c-fos was the most sensitive indicator of P 2Y-purinoceptor activation. Calcitonin gene-related peptide has also been shown to be a potent inducer of c-fos messenger RNA in cultured astroglial cells. The combined stimulation of astrocytes with calcitonin gene-related peptide and adenosine-5′-O-(2-thiodiphosphate) resulted in the potentiated expression of c-fos messenger RNA. The superinduction of immediate early gene expression by calcitonin gene-related peptide and extracellular ATP in cultured astrocytes might result from intracellular signal transduction cross-talk, since adenosine-5′-O-(2-thiodiphosphate) was found to increase calcitonin gene-related peptide-induced cyclic AMP accumulation by 35%. Phorbol 12-myristate 13-acetate also increased calcitonin gene-related peptide-evoked cyclic AMP accumulation and led to the induction of immediate early gene expression, suggesting that protein kinase C might be at least in part involved in purinergic cross-talk. Our results demonstrate synergistic roles for extracellular ATP and calcitonin gene-related peptide in the transcriptional activation of astroglial cells.

Cross-talk in signal transduction: Ras-dependent induction of cAMP-responsive transcriptional repressor ICER by nerve growth factor.

The CREM gene encodes both activators and repressors of cAMP-induced transcription. By virtue of an alternative, intronic promoter within the gene, the ICER (Inducible cAMP Early Repressor) isoform is generated. ICER acts as a dominant negative regulator and is cAMP-inducible in various neuroendocrine cells and tissues. ICER negatively autoregulates its own expression, and appears to participate in the molecular events governing oscillatory hormonal regulations. Here we report that ICER is inducible with nerve growth factor (NGF). This is the first example of cAMP-independent induction of ICER expression. Importantly, induction by NGF occurs via a subset of the CREs present in the ICER promoter which were previously shown to direct cAMP-inducibility. ICER induction correlates with a NGF-mediated phosphorylation of CREB. Both CREB phosphorylation and ICER inducibility require an intact Ras-dependent signalling pathway. We show that increased ICER levels result in the attenuation of c-fos expression. The activation of a powerful repressor of cAMP-responsive transcription by NGF, whose transduction signalling is cAMP-independent, constitutes a notable example of nuclear cross-talk and thus is likely to have relevant physiological implications.

Retinoic acid attenuates phospholipase C-mediated signaling in HaCaT keratinocytes.

Release of inositol(1,4,5)trisphosphate (Ins(1,4,5)P3) generated by phospholipase C (PLC) upon receptor stimulation plays an important role in the regulation of cell growth and differentiation. A second, completely different, signal transduction system involves retinoic acid (RA) and related derivatives. Binding to intracellular receptor sites can modulate keratinocyte growth and inhibits differentiation. The present study was aimed at characterizing possible interactions between the two signalling pathways in HaCaT keratinocytes. As determined by anion exchange chromatography and HPLC analysis, HaCaT keratinocytes treated with 1 microM RA for up to 72 h showed a marked decrease in Ins(1,4,5)P3 release upon stimulation with 10 microM bradykinin or 10 microM ionomycin. Thin-layer chromatography of phosphatidylinositol phosphates, the substrates of PLC, revealed no differences between RA-treated and untreated cells. Western blot analysis of the PLC isozymes present in HaCaT cells, PLC beta 3 and PLC gamma 1, showed no alterations in the expression of these proteins in RA-treated cells as compared to vehicle-treated controls. In addition, expression of the PLC-activating G protein G alpha q was not affected by RA treatment. Our results show that RA downregulates the PLC-mediated signaling system. The point of interference of this signal transduction crosstalk has yet to be elucidated. Our results suggest, furthermore, that RA-induced attenuation of keratinocyte differentiation might be mediated at least in part by the downregulation of Ins(1,4,5)P3 release.

Signal transduction crosstalk in the endocrine system: pancreatic β-cells and the glucose competence concept

Crosstalk between intracellular signalling systems is recognized as the principal means by which a cell orchestrates coordinate responses to stimulation by neurotransmitters, hormones or growth factors. The functional consequences of crosstalk are evident at multiple levels within a given signalling cascade, including the regulation of receptor-ligand interactions, guanine nucleotide-binding proteins, enzyme activities, ion channel function and gene expression. Here we focus on the pancreatic β-cells of the islets of Langerhans to illustrate the important role crosstalk plays in the regulation of glucose-induced insulin secretion. Recent studies indicating a synergistic interaction in β-cells between the glucose-regulated ATP-dependent signalling system and the hormonally regulated cAMP-dependent signalling system are emphasized. This interaction gives β-cells the ability to match the ambient concentration of glucose to an appropriate insulin secretory response, a process we refer to as the induction of glucose competence. The glucose competence concept may provide new insights into the etiology and treatment of non-insulin-dependent diabetes mellitus (Type II diabetes).